Research and development in cylindrical triple-GEM detector with $\mu$TPC readout for the BESIII experiment

TL;DR

This paper presents the development, optimization, and testing of a cylindrical triple-GEM detector with a novel $mbda$TPC readout for the BESIII experiment, enhancing spatial resolution and efficiency in high radiation environments.

Contribution

It introduces a new $mbda$TPC readout technique for triple-GEM detectors and provides a comprehensive simulation and testing framework for the BESIII inner tracker upgrade.

Findings

Achieved spatial resolution better than 150 μm

Maintained high detection efficiency close to 1

Validated detector performance through beam tests and simulations

Abstract

The third generation of the Beijing Electron Spectrometer, BESIII, is an apparatus for high energy physics research. The hunting of new particles and the measurement of their properties or the research of rare processes are sought to understand if the measurements confirm the Standard Model and to look for physics beyond it. The detectors ensure the reconstruction of events belonging to the sub-atomic domain. The operation and the efficiency of the BESIII inner tracker is compromised due to the the radiation level of the apparatus. A new detector is needed to guarantee better performance and to improve the physics research. A cylindrical triple-GEM detector (CGEM) is an answer to this need: it will maintain the excellent performance of the inner tracker while improving the spatial resolution in the beam direction allowing a better reconstruction of secondary vertices. The technological challenge of the CGEM is related in its spatial limitation and the needed cylindrical shape. At the same time the detector has to ensure an efficiency close to 1 and a stable spatial resolution better than 150 $\mu$m, independently from the track incident angle and the presence of 1 T magnetic field. In the years 2014-2018 the CGEM-IT has been designed and built. Through several test beam and simulations the optimal configuration from the geometrical and electrical points of view has been found. This allows to measure the position of the charged particle interacting with the CGEM-IT. Two algorithms have been used for this purpose, the charge centroid and the $\mu$TPC, a new technique introduced by ATLAS in MicroMegas and developed here for the first time for triple-GEM detector. A complete triple-GEM simulation software has been developed to improve the knowledge of the detection processes. The software reproduces the CGEM-IT behavior in the BESIII offline software.

Figures

Tables

Table 1. Table 1.1 : Main parameters of the BEPCII storage ring [ 2 ] .

Parameter	Unit	Value
Energy	$\mathrm{GeV}$	1.89
Circumference	$m$	237.53
RF frequency	$\mathrm{MHz}$	499.8
Harmonic	-	396
RF voltage	$\mathrm{MV}$	1.5
Transverse tunes	-	6.53/7.58
Damping time	$\mathrm{ms}$	25/25/12.5
Beam current	$A$	0.91
Bunch number	-	93
SR loss per turn	$\mathrm{keV}$	121
SR power	$\mathrm{kW}$	110
Energy spread	-	5.16 $\times$ 10-4
Compact factor	-	0.0235
Bunch length	$\mathrm{cm}$	1.5
Emittance	$\mathrm{nm}\cdot \mathrm{rad}$	144/2.2
$\beta$ function at IP	$m$	1/0.015
Crossing angle	$\mathrm{mrad}$	11 $\times$ 2
Bunch spacing	$m$	2.4
Beam-beam Parameter	-	0.04/0.04
Luminosity	${\mathrm{cm}}^{-2}{s}^{-1}$	1.0$\cdot$1033

Table 2. Table 1.2 : Main parameters of the BESIII spectrometer sub-systems.

Sub-detector	Parameter	Value
MDC	Single wire ${\sigma }_{r\varphi }$	$130\mu m$
MDC	${\sigma }_z$	$2\mathrm{mm}$
MDC	${\sigma }_p$/p ($1\mathrm{GeV}/c$)	0.5%
MDC	$\sigma$ ($dE/dx$)	6%
TOF	${\sigma }_t$ barrel	$100\mathrm{ps}$
TOF	${\sigma }_t$ end-cap	$110\mathrm{ps}$
EMC	${\sigma }_E/E$ ($1\mathrm{GeV}$)	2.5%
EMC	Position resolution ($1\mathrm{GeV}$)	$0.6\mathrm{cm}$
Muon	N∘ layers barrel	9
Muon	N∘ layers end-cap	8
Muon	Cut-off momentum	$0.5\mathrm{MeV}/c$
Solenoid magnet	Magnetic field	$1T$

Table 3. Table 1.3 : Analysis of TOF time resolution for 1 GeV/c muon

Item	Contribution (ps)
Counter intrinsic time resolution	80-90
Uncertainty from $15\mathrm{mm}$ bunch length	25
Uncertainty from clock system	20
Uncertainty from $\theta$ angle	25
Uncertainty from electronics	25
Uncertainty in expected flight time	30
Uncertainty from time walk	10
Total time resolution, one layer barrel	100
Total time resolution, two layers barrel	80

Table 4. Table 1.4 : Geometrical parameters of EMC.

Parameter	Value
Crystal length	$28\mathrm{cm}$
Front size	$5.2\times \mathrm{\hspace{0.17em}5.2}{\mathrm{cm}}^2$
Read size	$6.4\times \mathrm{\hspace{0.17em}6.4}{\mathrm{cm}}^2$
Number of $\varphi$ sectors	120
Barrel number of $\theta$ rings	44
Barrel number of crystals	5280
Barrel inner radius	$94\mathrm{cm}$
Barrel $\theta$ coverage
Barrel total weight	$21.56\mathrm{tons}$
end-caps number of $\theta$ rings	6
end-caps number of crystals	960
end-caps distance to IP	$138\mathrm{cm}$
end-caps $\theta$ coverage
end-caps total weight	$21.56\mathrm{tons}$

Table 5. Table 1.5 : Different theories used and their energy scale validity.

Theory Name	Energy Scale
The chiral-symmetry theory	E $\ll {\Lambda }_{QCD}$
Perturbative QCD	E $\gg {\Lambda }_{QCD}$
Global SU(3) flavour symmetry	E $\ll {\Lambda }_{QCD}$
Non-relativistic QCD	E $>$ 1 GeV

Table 6. Table 1.6 : Inner tracker requirements

Parameter	Value
Rate capability	${10}^4\mathrm{Hz}/{\mathrm{cm}}^2$
${\sigma }_{r-\varphi }$	$150\mu m$
${\sigma }_z$	$1\mathrm{mm}$
${\sigma }_{p_t}/p_t$	0.5% $@$ 1GeV
Efficiency	98%
Material budget	$\le$ 1.5 X0
$\Delta \Omega$	93% 4$\pi$
Inner radius	$78\mathrm{mm}$
Outer radius	$178\mathrm{mm}$

Table 7. Table 2.1 : CGEM-IT geometrical details.

	Inner radius	Length	Stereo angle	N∘ strips $\varphi$	N∘ strips $V$
Layer 1	$76.9\mathrm{mm}$	$532\mathrm{mm}$	43.3∘	846	1177
Layer 2	$121.4\mathrm{mm}$	$690\mathrm{mm}$	-31.1∘	1282	2194
Layer 3	$161.9\mathrm{mm}$	$847\mathrm{mm}$	33.0∘	1692	2838

Table 8. Table 5.1 : Drift parameters from Garfield++ are shown as a function of the z 𝑧 z , the distance of the generated electron from the cathode (in cm). The magnetic field influences the spatial shift only. The shift and spread are applied to the electron generated in the drift gap and Ar+10%iC 4 H 10 to extrapolate its time and spatial position at the anode.

Spatial shiftB=0T	[cm]	$0$
Spatial shiftB=1T	[cm]	$-0.317z+\mathrm{\hspace{0.17em}0.271}$
Spatial spread	[cm]	$\sqrt{{\left(-0.057z^2+\mathrm{\hspace{0.17em}0.028}\right)}^2+\mathrm{\hspace{0.17em}0.001}}$
Temporal shift	[ns]	$-272z+\mathrm{\hspace{0.17em}307}$
Temporal spread	[ns]	$\sqrt{(-3.34z^2-\mathrm{\hspace{0.17em}1.77}z+\mathrm{\hspace{0.17em}5.00}}+\mathrm{\hspace{0.17em}13.29}$

Table 9. Table 6.1 : Summary cuts on the J / ψ → π + ​ π − ​ π 0 → 𝐽 𝜓 superscript 𝜋 superscript 𝜋 superscript 𝜋 0 J/\psi\rightarrow\pi^{+}\pi^{-}\pi^{0} benchmark channel.

Variables	Value
N∘ Charged tracks	$\ge$ 2
N∘ Photons	$\ge$ 2
track vertex distance from
the interaction point	$10\mathrm{cm}$ along z
	$1\mathrm{cm}$ along r
Photon energy	$\ge$ $25\mathrm{MeV}$ in barrel
	$\ge$ $50\mathrm{MeV}$ in end caps
Invariant mass of ${\pi }^{+}{\pi }^{-}{\pi }^0$
before 5C	$J/\psi$ mass $\pm$ $150\mathrm{MeV}$
Di-photons invariant mass
before 5C	${\pi }^0$ mass $\pm$ $75\mathrm{MeV}$
Invariant mass of ${\pi }^{+}{\pi }^{-}{\pi }^0$
after 5C	$J/\psi$ mass
Di-photons invariant mass
after 5C	${\pi }^0$ mass
Total tri-momentum
after 5C	0
Kalman chi-square cut	$\le$ 200

Table 10. Table A.1 : Physical constants and approximated values of energy loss and ion pair production [ 1 ] .

Gas	Wi	NP [cm-1]	NT [cm-1]
He	41	5.9	7.8
Ar	25	25	106
Xe	22	41	312
CH4	30	37	54
C2H6	26	48	112
iC4H10	26	90	220
CO2	34	35	100
CF4	54	63	120

Equations

$$L(c{m}^{-2}{s}^{-1})=2.17\times 10^{34}(1+r){ϵ}_y\frac{E(GeV)k_b{I}_b(A)}{{\beta }_y^{\ast }(cm)}$$

$$\frac{{\sigma }_{p_t}}{p_t}=\sqrt{{\left(\frac{{\sigma }_{p_t}^{wire}}{p_t}\right)}^2+{\left(\frac{{\sigma }_{p_t}^{ms}}{p_t}\right)}^2}$$

$$\sigma =\sqrt{\frac{N_1{\sigma }_1^2+N_2{\sigma }_2^2}{N_1+N_2}}$$

$$V_0^{c\overline{c}}(r)=-\frac{4}{3}\frac{{\alpha }_s}{r}+br.$$

$$V_{spin-dep}=\frac{32\pi {\alpha }_s}{9m_c^2}{\mathbf{S}}_{\mathbf{c}}\cdot {\mathbf{S}}_{\overline{\mathbf{c}}}\delta (\mathbf{x})+\frac{1}{m_c^2}\left[\left(\frac{2{\alpha }_s}{r^3}-\frac{b}{2r}\right)\mathbf{L}\cdot \mathbf{S}+\frac{4{\alpha }_s}{r^3}T\right]$$

$${\Gamma }_{c\overline{c}}^{e^{+}{e}^{-}}\left({}^3{S}_1\right)=\frac{16}{9}{\alpha }^2\frac{{\left|\psi (0)\right|}^2}{M_{c\overline{c}}^2}.$$

$${\Gamma }_{c\overline{c}}^{e^{+}{e}^{-}}\left({}^3{D}_1\right)=\frac{50}{9}{\alpha }^2\frac{{\left|\psi (0)\right|}^n}{M_{c\overline{c}}^2{m}_c^4}.$$

$${\alpha }_s\left(M_{J/\psi }\right)=\frac{\Gamma (J/\psi \to {\gamma }_{direct}X)}{{\Gamma }_{strong}(J/\psi \to X)}$$

$${\sigma }_{\mathrm{charm}}=\frac{N_{\mathrm{charm}}}{\mathcal{L}}.$$

$$\Gamma \left(D^{+}\to l^{+}\nu \right)=\frac{G_F^2}{8\pi }{f}_{D^{+}}^2{m}_l^2{M}_{D^{+}}\left(1-\frac{m_l^2}{M_{D^{+}}^2}\right)|V_{cd}{|}^2.$$

$$\mathcal{A}=\frac{G_F}{\sqrt{2}}{V}_{cq}^{\ast }\overline{\nu }{\gamma }_{\text{upmu}}(1-{\gamma }_5)l⟨X|\overline{q}{\gamma }^{\text{upmu}}(1-{\gamma }_5)c|D⟩.$$

$$Q(t)=Q_0+\frac{Q_{\max }}{1+\exp \left(-\frac{t-t_{\mathrm{FD}}}{{\sigma }_{\mathrm{FD}}}\right)}$$

$$x_{\mathrm{CC}}=\frac{{\sum }_i^{N_{\mathrm{hit}}}{Q}_{\mathrm{hit},i}{x}_{\mathrm{hit},i}}{{\sum }_i^{N_{\mathrm{hit}}}{Q}_{\mathrm{hit},i}}$$

$$z_{\mathrm{hit}}=t_{\mathrm{hit}}{v}_{\mathrm{drift}}$$

$$d{z}_{\mathrm{hit}}=d{t}_{\mathrm{FD}}{v}_{\mathrm{drift}}$$

$$\Delta x_{\mathrm{tracker}}=x_{\mathrm{detector}}-x_{\mathrm{expected}}$$

$$\Delta x_{1,2}=x_{\mathrm{detector},1}-x_{\mathrm{detector},2}$$

$${\sigma }_{\mathrm{residual}}^2={\sigma }_{\mathrm{detector},1}^2+{\sigma }_{\mathrm{detector},2}^2$$

$${\epsilon }_{1\&2}={\epsilon }_1{\epsilon }_2=\frac{N_{\epsilon }}{D_{\epsilon }}$$

$$x_{\text{upmu}\mathrm{TPC}}^{B_{\mathrm{field}}}=\frac{gap/2-b}{a}\pm \sqrt{{\left(\frac{gap}{2\cos ({\theta }_{\mathrm{Lorentz}})}\right)}^2-{\left(\frac{gap}{2}\right)}^2}$$

$$v_{drift}^{\mathrm{measured}}=\frac{gap}{t_{\mathrm{last}}-t_0}$$

$$x_{\mathrm{merge}}=w_{\mathrm{cc}}(x_{\mathrm{cc}}-{\Delta }_{\mathrm{cc}})+(1-w_{\mathrm{cc}})x_{\mathrm{tpc}}$$

$$G_{\mathrm{eff}}=T{G}_{\mathrm{intr}}$$

$$P(G)=C_0\frac{(1+\theta {)}^{1+\theta }}{\Gamma (1+\theta )\left(\frac{G}{\overline{G}}\right)\exp \left[-(1+\theta )\frac{G}{\overline{G}}\right]}$$

$$I_k=-q\mathbf{v}\cdot {\mathbf{E}}_k^{\mathrm{weight}}$$

$$\lambda =\frac{1}{N_e{\sigma }_i}$$

$$P_k^{N_i}=\frac{(N_i{)}^k}{k!}{e}^{-N_i}$$

$$ϵ=1-P_0^{N_i}=1-e^{N_i}$$

$$N_T=\frac{\Delta E}{W_i}$$

$$N_T^{Ar:i{C}_4{H}_{10}}=0.9\cdot N_T^{Ar}+0.1\cdot N_T^{i{C}_4{H}_{10}}=117c{m}^{-1}$$

Full text

University of Ferrara

Physics and Earth Sciences Department

Degree of Doctor of Philosophy in subject of Physics

**Research and development in cylindrical triple-GEM detector with $\mu$TPC readout for the BESIII experiment

**

Supervisor:

Dr. Gianluigi Cibinetto

Examiners:

Dr. Erika De Lucia

Prof. Theodoros Alexopoulos

PhD Coordinator:

Prof. Vincenzo Guidi

*Candidate:                   *

Riccardo Farinelli

Academic years 2015-2018

XXXI course - FIS/04

Abstract

The third generation of the Beijing Electron Spectrometer, BESIII, is an apparatus for high energy physics research. The hunting of new particles and the measurement of their properties or the research of rare processes are sought to understand if the measurements confirm the Standard Model and to look for physics beyond it. The detectors ensure the reconstruction of events belonging to the sub-atomic domain. The operation and the efficiency of the BESIII inner tracker is compromised due to the the radiation level of the apparatus. A new detector is needed to guarantee better performance and to improve the physics research. A cylindrical triple-GEM detector (CGEM) is an answer to this need: it will maintain the excellent performance of the inner tracker while improving the spatial resolution in the beam direction allowing a better reconstruction of secondary vertices. The technological challenge of the CGEM is related in its spatial limitation and the needed cylindrical shape. At the same time the detector has to ensure an efficiency close to 1 and a stable spatial resolution better than $150\,\mathrm{\upmu m}$, independently from the track incident angle and the presence of $1\,\mathrm{T}$ magnetic field.

In the years 2014-2018 the CGEM-IT has been designed and built. Through several test beam and simulations the optimal configuration from the geometrical and electrical points of view has been found. A new electronics has been developed to readout charge amplitude and time information from the detector signal. This allows to measure the position of the charged particle interacting with the CGEM-IT. Two algorithms have been used for this purpose, the charge centroid and the $\upmu$TPC, a new technique introduced by ATLAS in MicroMegas and developed here for the first time for triple-GEM detector.

A complete triple-GEM simulation software has been developed to improve the knowledge of the detection processes. The software reproduces the CGEM-IT behavior in the BESIII offline software. The simulation of the CGEM-IT in the BESIII apparatus validates the improvements of the detector thought the study of some physics analysis.

Contents

1. 1 The Beijing Electron Positron Collider II and the Beijing Spectrometer III

2. 1.1 BEPCII

3. 1.2 The BESIII detector

4. 1.2.1 The multi-layer drift chamber

5. 1.2.2 The time of flight

6. 1.2.3 The electromagnetic calorimeter

7. 1.2.4 The muon counter

8. 1.2.5 The trigger system

9. 1.3 The BESIII physics

10. 1.3.1 Hadron Spectroscopy

11. 1.3.2 Charmonium physics

12. 1.3.3 Charm physics

13. 1.4 The needs of a new inner tracker for BESIII

14. 2 CGEM-IT

15. 2.1 The KLOE-2 IT

16. 2.2 Innovation of the BESIII CGEM-IT

17. 2.3 The new ASIC for the BESIII CGEM-IT

18. 3 The GEM technology

19. 3.1 Gaseous electron multiplier

20. 3.2 Design and construction

21. 3.3 Electrical configuration

22. 3.4 Multistage multiplication

23. 3.5 Innovations and performance of triple-GEM detectors

24. 4 Characterization and reconstruction of a triple-GEM signal

25. 4.1 Experimental setup

26. 4.1.1 The facilities

27. 4.1.2 Readout electronics

28. 4.2 The offline software: GRAAL

29. 4.2.1 Hit digitization with APV25

30. 4.2.2 Hit digitization with TIGER

31. 4.2.3 Hit clusterization

32. 4.2.4 Time reference for $\upmu$TPC reconstruction

33. 4.2.5 Residual measurement

34. 4.2.6 The alignment procedures

35. 4.3 Analysis results

36. 4.3.1 Detector efficiency and signal characterization

37. 4.3.2 Performance of a triple-GEM in magnetic field

38. 4.3.3 Performance of a triple-GEM with sloped tracks

39. 4.3.4 Temporal studies with sloped tracks

40. 4.3.5 Merging algorithm

41. 4.4 Performance at high rate

42. 4.5 Test of Cylindrical GEM

43. 4.6 TIGER with triple-GEM performance

44. 5 Triple-GEM simulation

45. 5.1 Primary ionization simulation

46. 5.2 Electron diffusion

47. 5.3 Transparency and gain of a GEM

48. 5.4 Signal induction on the readout

49. 5.5 Capacitive effect

50. 5.6 Comparison of the simulation and the test beam data

51. 5.6.1 Cross checks for $\upmu$TPC

52. 5.7 From GTS to a full simulation

53. 6 Impact of the CGEM-IT on physics analysis

54. 6.1 The BOSS and CGEMBOSS environments

55. 6.2 Study of the reaction $e^{+}e^{-}\rightarrow J/\psi\pi^{+}\pi^{-}\pi^{0}$ to mark the CGEM-IT improvements

56. 7 Conclusion and outlook

57. A Review on the gaseous detectors

58. A.1 Ionization in gaseous detectors

59. A.2 Drift of electrons and ions in gases

60. A.3 Amplification of ionization

61. A.4 Gas choice

62. A.5 Creation of signal

63. A.6 Brief history of the gaseous detectors

64. A.6.1 Single wire chamber

65. A.6.2 Proportional counter

66. A.6.3 Multi-wire proportional counter

67. A.6.4 Drift chamber

68. A.6.5 Time projection chamber

69. A.6.6 Resistive plate chamber

70. A.7 Micro pattern gaseous detectors

71. A.7.1 Micro Strip Gas Chamber

72. A.7.2 Micro-mesh gaseous structure

73. A.7.3 Gas Electron Multiplier

74. A.7.4 Micro Resistive Well

75. B Diffusion and capacitive corrections in $\upmu$TPC

76. B.1 The correction algorithms

77. B.1.1 Data driven corrections based on the strip index

78. B.1.2 Data driven corrections based on the charge ratio

79. B.1.3 Data driven corrections based on the mean position

80. B.2 Diffusion and capacitive correction on real and simulated data

List of Figures

1. 1.1 BEPCII.
2. 1.2 BESIII detector
3. 1.3 MDC performance
4. 1.4 TOF performance
5. 1.5 RPC internal structure
6. 1.6 MDC aging effect in hit efficiency
7. 2.1 Jagged anode layout
8. 2.2 TIGER calibration curves
9. 3.1 Gem Holes
10. 3.2 Gem electric field
11. 3.3 Induction field in a GEM
12. 3.4 Drift field in a GEM
13. 3.5 Triple-GEM representations
14. 3.6 Gain and discharge probability in GEM
15. 4.1 Test beam setup
16. 4.2 Gas gain in a triple-GEM
17. 4.3 Offline software block diagram
18. 4.4 Fermi-Dirac and linear fit in time measurement
19. 4.5 Time distribution with TIGER
20. 4.6 $\upmu$TPC reconstruction
21. 4.7 Time distribution with APV25
22. 4.8 Alignment plots
23. 4.9 Efficiency and CC resolution results
24. 4.10 Cluster size and charge results
25. 4.11 CC and $\upmu$TPC in magnetic field
26. 4.12 Cluster size and charge results
27. 4.13 Cluster charge distribution
28. 4.14 Cluster time distribution
29. 4.15 CC, $\upmu$TPC and merge resolution with sloped tracks
30. 4.16 Simulated and measured drift velocity
31. 4.17 CC weighting functions
32. 4.18 Spatial resolution for different incident angle with magnetic field
33. 4.19 Cluster charge dependency on beam rate in triple-GEM
34. 4.20 Drift velocity dependency on beam rate in triple-GEM
35. 4.21 GEM and CGEM signal comparison
36. 4.22 CC resolution in CGEM
37. 4.23 TIGER with triple-GEM results
38. 5.1 Ionization distributions from Garfield++ and GTS
39. 5.2 Electron drift distributions from Garfield++
40. 5.3 GEM gain in Garfield++ and GTS
41. 5.4 Induced current and readout charge in GTS
42. 5.5 Experimental measurement of capacitive effects
43. 5.6 GTS and real data comparison
44. 5.7 Drift velocity and time resolution comparison
45. 6.1 BOSS and CGEMBOSS comparison
46. A.1 Electron diffusion and drift velocity.
47. A.2 HV regime in proportional counter
48. A.3 Single wire electric field
49. A.4 Space charge effect in proportional chamber
50. A.5 MWPC electric field.
51. A.6 Time projection chamber representation
52. A.7 Resistive plate chamber
53. A.8 Micro Strip Gas Chamber electric field.
54. A.9 MicroMegas.
55. A.10 $\upmu$-RWELL.
56. B.1 The $\upmu$TPC points residual
57. B.2 First diffusion and capacitive correction
58. B.3 First diffusion and capacitive corrections parameters
59. B.4 Second diffusion and capacitive correction and its parameters
60. B.5 Third diffusion and capacitive corrections
61. B.6 $\upmu$TPC resolution with and without corrections

List of Tables

1. 1.1 Main parameters of the BEPCII storage ring
2. 1.2 Main parameters of the BESIII
3. 1.3 TOF time resolution
4. 1.4 Geometrical parameters of EMC
5. 1.3.1 Hadron Spectroscopy
6. 1.6 Inner tracker requirements
7. 2.1 CGEM-IT geometrical details
8. 5.1 Drift parameters from Garfield++
9. 6.1 Summary cuts on the benchmark channel
10. A.1 Physical constants and approximated values of energy loss and ion pair production

Chapter 1 The Beijing Electron Positron Collider II and the Beijing Spectrometer III

High energy collisions between particles create new states of matter as ruled by the nature and described by the physics laws. As the number of collisions increases then it is possible to perform more precise measurements of the particle properties or the transitions from a state to another. Around the world there are several experiments and scientists that approach the knowledge of the physics laws looking at the collision results of high energy particles, one of those experiments is the BEijing Spectrometer (BESIII) at the Institute of High Energy Physics (IHEP) in People’s Republic of China (PRC): an apparatus composed of several sub-detectors that measure the properties of the particles such as momentum, energy, mass and path. The leptons are provided by the Beijing Electron Positron Collider (BEPCII). They are injected by a linear accelerator in two accumulation rings where electrons and positrons are disposed in bunches. The collision happens in the interaction point (IP). The center of mass energy range varies from $2\,\mathrm{GeV}$ to $4.6\,\mathrm{GeV}$ in order to explore the $\tau$-charm region to continue the research about light hadrons, charmonium, exotics and other topics explained in Sect. 1.3

In this Chap. a review about the experimental layout will be reported and the physics research of the BESIII experiment in order to understand the scenario and the background needed by the new particle detector that has been studied in this work.

1.1 BEPCII

BEPCII is the upgrade of the pre-existing BEPC machine and it is a double ring electron positron collider that acts also as a synchrotron radiation (SR) source [1, 2]. The beam energy ranges from 1 to $2.3\,\mathrm{GeV}$ with a design luminosity of $10^{33}\,\mathrm{cm^{-2}s^{-1}}$ at $1.89\,\mathrm{GeV}$ that has been reached in 2016 data taking on April 14th. The facility is exploited in high energy physics experiment and synchrotron radiation. The acceleration of $e^{+}e^{-}$ is performed by an injector consisting in a $202\,\mathrm{m}$ electron linac with 16 Radio-Frequency (RF) power sources and 56 S-band RF structures. Then two super-conducting cavities are used with one cavity in each ring to provide the RF voltage. BEPCII requires 363 magnets of different types: bending magnets, quadrupole and sextupole. The details of BEPCII are shown in the Tab. 1.1. The luminosity of an electron-positron collider is expressed as:

[TABLE]

where r is the beam aspect ratio at the IP, $\epsilon_{y}$ the vertical beam-beam parameter, $\beta^{*}_{y}$ the vertical envelope function at IP, $k_{b}$ the bunch number in each beam and $I_{b}$ the bunch current. A double accumulation ring scheme allows to reach a higher bunch number than an single one. The inner and outer rings (represented in Fig. 1.1 ) cross each other in the northern and the southern IP. In order to have sufficient separation between the rings without a significant degradation of the luminosity, the horizontal crossing angle between the two beams is 11 mrad $\times$ 2 in the southern IP, where the BESIII experiment is placed. In the northern IP a vertical bump is used to separate the beams. The layout of BEPCII minimizes the machine errors such as misalignment or problems in the multipole field of magnets. This tunes the beta function and reduces the background due to Touschek effect. The mean beam lifetime is around three hours and its limits are due to beam-beam bremsstrahlung, Touschek effect and beam-gas interaction.

1.2 The BESIII detector

BESIII measures trajectories and momenta of charged and neutral particles except $\nu$, produced in $e^{+}e^{-}$ collisions in the energy region between 2 and 4.6 GeV by means of a set of detectors and a solenoid producing a 1 T magnetic field [4]. A schematic drawing of BESIII is shown in Fig. 1.2. Around the beryllium beam pipe the Multi-layer Drift Chamber (MDC) is placed and around its conical shape in the end-cap regions there are two superconducting quadrupoles (SCQs). The time-of-flight (TOF) system is located outside the MDC: it is composed by two layers of plastic scintillator counters in the barrel and a multigap resistive plate chamber in the end-caps. The CsI(Tl) ElectroMagnetic Calorimeter (EMC) is placed between the TOF system and the Superconducting Solenoid Magnet (SSM). The MUon Counter (MUC) is built up by several layers of resistive plate chambers (RPCs) placed in the gaps between steel plates of the flux return yoke. The coil of the superconducting magnet is placed outside the electromagnetic calorimeter. It has a mean radius of $1.482\,\mathrm{m}$ and length of 3.52 m. The covered solid angle is $\Delta\Sigma/4\pi$ = 0.93, where the azimutal angle is totally covered while the polar angle is limited between 21∘ and 159∘. The main parameters of BESIII sub-detectors are summarized in Tab. 1.2.

1.2.1 The multi-layer drift chamber

The design of the MDC has been optimized to detect low momentum particles with performance shown in Tab. 1.2. Moreover the MDC provides the level 1 (L1) trigger as described in Sect. 1.2.5 to select good events and to reject the background. The radius of the MDC varies from 59 mm to 810 mm and the entire detector is composed by 43 sense wire layers. The first 8 define the inner MDC (IDC) and the others the outerMDC (ODC). The gas mixture is He-C3H8 (60:40) to minimize the multiple scattering effect. The measured position in the r-$\phi$111$r$ and $\phi$ are defined in Fig. 1.2. plane is given by each single wire while the measurement along the beam direction is performed by layers with different stereo angles (-3.4*∘* and + 3.9*∘*). The average gas gain of the MDC is about $3\times 10^{4}$ at the reference operating high voltage of about $2200\,V$.

Each layer is divided in several cells with an almost square section. The cell size varies from $12$ to $16.2\,mm$. The cells have a trapezoidal shape with eight field wire on the perimeter and the sense wire in the middle. Each cell of a layer has the same shape and length. The resolution of a single wire is dominated by the electron diffusion and a proper balance is needed between the cell size, the readout channels and the multiple scattering given by the wires [5]. The $25\,\mathrm{\upmu m}$ wires are made of gold-plated tungsten with 3% rhenium. The 43 layers of the MDC are divided in 11 superlayers and the field wires within a superlayer are shared between neighboring cells. Track segments reconstructed in superlayers are linked and used in the L1 trigger.

The momentum resolution of the MDC is determined by the resolution of a single wire and the multiple scattering.

[TABLE]

The single wire resolution is a function of the drift distance and includes contribution from primary ionization statistic, longitudinal electron diffusion and the jitter in the time measurement. The distribution has been fitted to two Gaussian functions and the $\sigma$ has been calculated by:

[TABLE]

where $N_{1}$ and $N_{2}$ are the amplitudes and $\sigma_{1}$ and $\sigma_{2}$ are the standard deviations of the two Gaussian functions. The average spatial resolution of drift cell with a pion beams at 12 GeV/c is shows in Fig. 1.3.

The $\mathrm{d}E/\mathrm{d}x$ performance is determined mainly by the fluctuation in the number of primary ionizations along the track. A minimum ionizing particle (MIP) track in helium creates about 8 ion pair per centimeter. Monte-Carlo simulation shows that the $\mathrm{d}E/\mathrm{d}x$ is about 6% and the corresponding momentum resolution is better than $\sigma_{p_{t}}/p_{t}\,=\,0.32\%\,p_{t}\oplus 0.37\%\,/\beta$, where the first term is related to the trajectory measurement and the second one to the multiple scattering. It allows to separate $\pi$ and $K$ within 3 $\sigma$ up to momenta of $770\,\mathrm{MeV/c}$.

The signals from the sense wired are first amplified by fast trans-impedance preamplifiers located close to the wire then output signal is sent outside BESIII thought 18 m cables. Then it is further amplified and split in three branches for timing, charge measurements and the L1 trigger. The wire spatial resolution of $130\,\mathrm{\upmu m}$ corresponds to a time resolution of $3.5\,\mathrm{ns}$ and the charge produced by a MIP on a sense wire after the amplification is about $450\,\mathrm{fC}$.

1.2.2 The time of flight

The time-of-flight system is segmented in a barrel and two end-caps. The barrel is built up by two layers of 88 scintillating bars with a thickness of about $5\,\mathrm{cm}$ and a trapezoidal cross section. The signal is collected by two photomultiplier tubes (PMT) attached to the bars. The end-caps have been recently upgraded with Multi-gap Resistive Plate Chambers (MRPC) and each end-cap station has 36 trapezoidal shaped modules arranged in a circular double layers [6].

The PMTs chosen to readout the TOF counters in the barrel are Hamamatsu R5924-70 and they match the size of the scintillator bars. The average quantum efficiency is about 23%, the rise time is $2.5\,\mathrm{ns}$ and the signal transition time is $9.5\,\mathrm{ns}$ with $0.44\,\mathrm{ns}$ rms spread. A fast preamplifier with a gain factor of 10 is used to boost the signals and to extend the lifetime of the PMT. The signal is received by the front-end electronics that splits the signals into two branches for time and charge measurements. The required dynamic range is $60\,\mathrm{ns}$ and the time resolution of the electronics is less than 25 ps. The charge amplitude signal measured ranges from 200 mV to 4 V.

A different technology is used in the end-cap where the time measurement is performed by MRPC. Each module is divided into 12 readout strips and their length changes from $9.1\,\mathrm{cm}$ to $14.1\,\mathrm{cm}$ and the width is $2.4\,\mathrm{cm}$. The strips are readout from both ends. The internal structure is composed by 14 pieces of thin glass sheet, arranged into two stacks. The stacks are sandwiched in between two layers of readout strips. The time measurement is given by 12 active gas gaps of $0.22\,\mathrm{mm}$ thick. The MRPC module is placed in a gas-tight aluminum box with a gas mixture of Freon-SF6-iC4H10 90:5:5.

The time resolution is about 100 ps in the barrel and 65 ps in the end-caps and it allows to separate $\pi/K$ within 3 $\sigma$ up to $900\,\mathrm{MeV/c}$ at 90∘. The precise time resolution of the TOF is also used in the trigger logic for charged particles. The solid angle coverage of the barrel is $|\cos(\theta)|<0.83$ while in the end-caps is $0.85<|\cos(\theta)|\,<\,0.95$. The dead gaps are needed for the mechanical support of the MDC and service line. The inner radius of the first barrel TOF layer is $81\,\mathrm{cm}$ and the second $86\,\mathrm{cm}$, while in the end-caps the flight path is about $140\,\mathrm{cm}$.

The time resolution of the TOF is given by several contribution as shown in Tab. 1.3, while the intrinsic time resolution is determine by the rise time of the scintillation light, the fluctuation of the photon arrival time at the PMT and the transition time spread of the PMT.

The $\pi/K$ separation capability depends on the polar angles of the tracks and its limit is reached at momenta of about 0.7 GeV/c at 90 ∘ where the particle flight is the shortest in the barrel, the limit moves to about 1.0 GeV/c in the end-cap systems. The particle identification is calculated by a likelihood analysis. The probability to identify a kaon or to misidentify it as a function of the kaon momentum is shown in Fig. 1.4. A separation efficiency of 95% is reached up to 0.9 GeV/c.

1.2.3 The electromagnetic calorimeter

The electromagnetic calorimeter measures the energy of photons above $20\,\mathrm{MeV}$ and provides a trigger signal. It is mandatory to measure precisely invariant mass of particles with radiative decay, $e.g$ $\pi^{0}$, $\eta$, $\rho$, etc. In radiative decay the photon has to be distinguished from $\pi^{0}$ and $\eta$ decays. The minimum detectable opening angle between two photons is about 10∘ and for momenta higher than $200\,\mathrm{MeV/c}$ it can discriminate $e/\pi$.

The EMC consists of 6240 CsI(Tl) crystal with a length of $28\,\mathrm{cm}$, about 15 radiation length (X0), and a surface of $5.2\,\mathrm{cm}$ $\times$ $5.2\,\mathrm{cm}$. The crystals are arranged as 56 rings and each one covers an angle of about 3∘ in both polar and azimuthal directions. The calorimeter is divided in three parts: the barrel and two end-caps. The total weight of the EMC is about 24 tons. The design energy resolution is $\sigma_{E}/E$ = 2.5%$\sqrt{E}$ and the position resolution is $\sigma$ = $0.6\,\mathrm{cm}/\sqrt{E}$ at $1\,\mathrm{GeV}$. The angular coverage is $|\cos(\theta)|<0.83$ while in the end-caps is $0.85<\cos(\theta)<0.95$. Details of the EMC are shown in Table 1.4.

The energy resolution of EMC depends by the crystal quality, dead material between the IP and the crystal, photodiode and amplifier noise, fluctuation of shower energy, etc.

The signal readout is performed by two photodiodes of 1 cm $\times$ 2 cm and a light guide of $2\,\mathrm{mm}$ thick glued directly at the center of the crystal surface. Only 10% of the surface is covered but a larger fraction of light is collected thanks to the light reflection performed by reflective material covering the crystal surface. The signal is readout by the electronics that measures the energy deposited and provides a fast energy trigger.

1.2.4 The muon counter

Resistive plate counters are active detector used within the gaps of the flux return steel to identify the muons surviving the EMC with respect other charged particles such as pions. It is divided in three pieces: a barrel and two end-caps. The system is composed by nine layers of steel plates with a total thickness of $41\,\mathrm{cm}$ and nine layers of RPCs in the barrel. The end-caps have eight layers of counters. The minimum muon momentum at which the RPC starts to become effective is about 0.4 GeV/c due to the energy loss in the EMC and the bending in the magnetic field. The hits reconstructed in the RPC are associated to the tracks reconstructed in the MDC and the energy measured in the EMC. The readout of the RPC is performed by strip of about $4\,\mathrm{cm}$ in both direction $\theta$ and $\phi$. The space resolution is modest because of the multiple scattering of low momentum muons in the EMC.

The RPC are constructed by two plates of high resistivity material, the Bakelite, of $2\,\mathrm{mm}$ thickness around a gas gap of $2\,\mathrm{mm}$. Figure 1.5 shows the cross-sectional view of the detector. The RPCs work in streamer mode and the signal is collected on the strips outside the gas gap. The gas mixture is Ar-C2F4H2-C4H10 50:42:8 and the working voltage is of 8 kV. This provide a single gap efficiency of about 96%. In order to improve the detection efficiency a double-gap layout has been used with the readout sandwiched in between the two. Each layer can measure only one coordinate and the orientation of the strips changes layer by layer. Due to the size of the detector, the number of readout channel is about 10000.

1.2.5 The trigger system

The trigger, data acquisition and online computing system are designed to sustain an acquisition with multi-beam bunches separated by 8 ns. The trigger system has two levels: a hardware level (L1) and a software level (L3). The L1 trigger signal is generated by the TOF, MDC and EMC. Conditions on the number of minimum tracks are required on the MDC, TOF and EMC. The clock of the trigger system is $41.65\,\mathrm{MHz}$, it is synchronized with the accelerator RF and it is distributed to the readout electronics crate to synchronize every operation in the BESIII data acquisition. The maximum L1 rate expected is about $4\,\mathrm{kHz}$ at $3.097\,\mathrm{GeV}$. The data in the front-end buffers is read once L1 trigger is validated, $6.4\,\mathrm{\upmu s}$ after the collision. Then the event reconstruction takes place and the background events are suppressed by the L3 trigger.

1.3 The BESIII physics

The BESIII physics plan is to accumulate a significant data sample in the center of mass energy between 2 and 4.6 GeV: energy point at the mass of charmonium states, above $D\overline{D}$ threshold, energy points at the mass of new resonances and in the continuum region. Up to now it has collected $10^{10}$ $J/\psi$, $10^{9}$ $\psi(2S)$, $10^{6}$ $D\overline{D}$. The aim is to be sensitive to the observation of new physics concerning hot topics like $D^{0}\overline{D^{0}}$ mixing, the charmonium-like $XYZ$ states and CP violation in charmed-quark and $\tau$ sectors. The high precision measurements at BESIII are compared with QCD calculations. The huge data sample allows to search for rare decays like lepton-number violating or flavour violating processes.

1.3.1 Hadron Spectroscopy

BESIII investigates the structure of matter and the nature of the interactions between its constituent components [3]. To access a small object it is needed to increase the energy scale but increasing the energy the theoretical description of the phenomena changes. At momentum transfers around the MeV scale, below $\Lambda_{QCD}$ $\sim$ $200\,\mathrm{MeV}$, the fundamental scale of Quantum Chromodynamics (QCD), the chiral symmetry theory can provide solutions to study the low-energy dynamics as QCD becomes non-perturbative [7]. Increasing the energy in a range that is greater than $\Lambda_{QCD}$, the partonic and gluonic behavior is shown in the deep inelastic scattering. In this energy region it is allowed to study QCD processes through perturbative expansion [7]. Between these two energy regions there is no theory that can describe the phenomena so meson and baryon resonances are studied to know the constituent behavior. The QCD calculations in this region are described with a naive quark model [8, 9]:

• •

spontaneous breaking of chiral symmetry leads to the presence of massive constituent quarks within a hadron as effective degrees of freedom;

• •

hadrons can be viewed as a quark system in which the gluon fields generate effective potentials that depend on the relative positions and spins of the massive quarks.

These pieces compose the QCD puzzle and the hadron spectroscopy plays an important role in its interpretation. In the light quark sector ($u,d,s$) the mass differences are relatively small ($m_{u}$ $\sim$ $3\,\mathrm{MeV}$, $m_{d}$ $\sim$ $5\,\mathrm{MeV}$ and $m_{s}$ $\sim$ $95\,\mathrm{MeV}$ [10]) then the strong interaction can be approximated with a global SU(3) flavour symmetry. This approach allows to predict the masses of mesons and baryons composed by the quarks of the symmetry. Different theories and their validity ranges are summarized in Tab. 1.5.

A non-relativistic approach can be used with the naive method for quarks that have constituent masses comparable or greater than the $\Lambda_{QCD}$. For quarks with an excitation energy of the excited hadron states comparable to their masses this approach is meaningless but it works in a wide range of empirical tests [7]. Several theories can be found in literature: the semi-relativistic flux-tube model [11, 12], the instanton model [13], the Goldstone boson exchange model [14], the diquark model [15], etc.

The ultimate goal of the study of the hadron spectroscopy is to understand the dynamics of the constituent interactions. Conventional $q\overline{q}$ meson can be constructed in the quark model and their spectrum is studied to be described in an empirically efficient way. QCD also does not forbid the existence of other bound states that are made completely of gluons ($e.g.$ so-called “glueball”), multiquark mesons, such as $qq\overline{qq}$ and so-called “hybrid” mesons which contain both $q\overline{q}$ and gluon. Glueballs are bound states of at least 2 or 3 gluons in a color singlet. Gluons inside glueballs could be massive. The abundance of isoscalar scalars in the 1-2 GeV, mass region, $e.g.$ $f_{0}$(1370), $f_{0}$(1500), $f_{0}$(1710), $f_{0}$(1790) and $f_{0}$(1810) should be confirmed in further experiments. They are the natural scalar glueball candidates and they are studied at BESIII as decay products. Hybrid mesons are hypothesized to be formed by a $q\overline{q}$ pair plus one explicit gluon field. Evidence for the existence of hybrid mesons would be direct proof of the existence of the gluonic degree of freedom and the validity of QCD. In addition, the lightest hybrid multiplet includes at least one JPC exotic state. Multiquark are states with a number of quark-antiquark greater than three. They are always colourless. An example of a recent discovery of $c\overline{c}q\overline{q}$ states is the $Y$(4260) [16]. Hybrid states are a wide topic and they are connected to Charmonium and Charm physics, further description will be given in next sections.

1.3.2 Charmonium physics

Charmonia are charm-anticharm ($c\overline{c}$) bound states, like the $J/\psi$ resonance or its excitation. Due to their heavy mass ($c$ quark mass is $\sim$ 1.29 GeV) they probe the QCD at different energy scales: from the hard region, where the coupling constant is small and can be expanded, to the soft region, where non-perturbative effects dominate.

To predict the properties of the charmonia states in these regions, a Non-Relativistic QCD (NRQCD) expansion can be used because $m_{c}$ is large and the motion velocity of the c quark is smaller than the speed of light ($v^{2}$ $\sim$ 0.3 by potential model and Lattice simulation [7]). This method allows to expand the theory in power of $v$ and describe the mesons annihilation decays. It is named as perturbative NRQCD (pNRQCD) if $mv~{}\gg~{}\Lambda_{QCD}$. The system is weakly coupled and the potential of the meson is calculated. The next challenge is to interpret the newly discovered charmonia states. The spectrum of the charmonium states has been measured and the narrow resonances below the open-charm threshold are experimentally clear. On the other hand above the $c\overline{c}$ threshold many charmonia states have been theorized but only few of them have been observed. The spectrum of the charmonium states can be described by a $c\overline{c}$ potential model that combines a Coulomb-like term and a linear one for the confinement: the Cornell potential.

[TABLE]

A spin dependent term is added to better describe the scalar nature of the potential:

[TABLE]

Precision measurements are needed to answer some questions related to the low-mass charmonium spectrum and BESIII is playing its role to determine the masses and the widths of all charmonium states.

Some charmonium-like mesons not fitting in the previous discussed $c\overline{c}$ model were observed, while others, which are foreseen by the theory, like charmonium hybrids, have not been observed. Particles of interest are called <$XYZ$ mesons: multiquarks candidates. On April 26th, 2013 BESIII announces a new mystery particle: $Z_{c}^{+}$(3900) the first observation of a tetra-quark or a charm molecule confirmed later by other experiments. Later on several decays of the $Z$ state has been observed in decay channels with charmonium or open charm state [17, 18, 19, 20]. $Y$ states are well investigated by BESIII thanks to the high precision measurement. $Y$ are connected to the $Z$ states and their nature is still puzzling. An abundance of those states have been observed such as $Y$(4220) and $Y$(4360) in several charmonium decays and open charm mesons [21, 22].

Charmonium spectroscopy is studied with the hadronic and radiative transitions. Hadronic transitions are decay modes of heavy quarkonia to lighter $c\overline{c}$ state and a light hadrons. The typical mass difference between the initial and final meson is around a few MeV, so that the typical momentum of the emitted hadron is low. The light hadron is produced from gluons and it is emitted in S,D and P-Wave mode. Information about the wave-function of the $1^{--}$ charmonium states is accessible from the leptonic partial widths. In the non-relativistic limit of an S-wave and D-wave quarkonium system the coupling $e^{+}e^{-}$ through a virtual photon involves the wave function and the partial width is given by [23]:

[TABLE]

[TABLE]

Radiative decay can be used to extract the coupling constant $\alpha_{s}\left(M_{J/\psi}\right)$ from:

[TABLE]

where $X$ stands for light hadrons and $\gamma_{direct}$ is a photon produced by the $J/\psi$. At BESIII huge data samples of vector charmonium states as $J/\psi$, $\psi^{\prime}$ and $\psi^{\prime\prime}$ are produced and it allowed to understand the light hadron spectroscopy and charmonium decay dynamics. Hadronics decays are the 85-98% in charmonia mesons, the basic process is the annihilation of the $c\overline{c}$ inside the charmonium to light quarks, gluons or leptons. The energy released in the process is 2mc and the space-time distance is $\sim$ $\dfrac{1}{2m_{c}}$. $J/\psi$, $\psi$’, $\eta_{c}$ and $\chi_{c}$ decays to light hadrons are used to test theories as NPQCD and extract the variables of these or to access the gluonic behavior.

1.3.3 Charm physics

$D$ mesons are the lightest particle containing charm quark and due to Standard Model (SM) prediction the $D$ system is a good environment to test the theory, such as CKM matrix and $D^{0}\overline{D^{0}}$ mixing to the CP asymmetries, through their leptonic and semi-leptonic decay. The QCD potential used for $D$ meson system needs precise theoretical control to describe branching ratios, spectrum and quantum numbers. Meanwhile SM predicts small CP-violating asymmetries in charmed particle decay because the mode is Cabibbo suppressed and a large data sample is required with a complex final states to be analyzed. A $c\overline{c}$ resonance above the $D\overline{D}$ threshold has to be created to produce $D$ mesons pairs via $e^{+}e^{-}$. There are four broad resonance states that decay into pair of charmed meson final states: the initial $c\overline{c}$ meson decays via the production of light $q\overline{q}$ pair and forms $c\overline{q}$-$\overline{c}q$ system that separate in two charmed mesons. These states are the $\psi$(3770), $\psi$(4040), $\psi$(4160) and $\psi$(4415). They are easily produced at $e^{+}e^{-}$ collider because their quantum number JPC 1*–*. To study this mechanism the charmed cross section is measured

[TABLE]

where $\mathcal{L}$ is the integrated luminosity and $N_{charm}$ is the number of charmed meson obtained using tagging technique. The cross section of the $c\overline{c}$ states above the open-charm threshold can be understood as the successive onset of the charmed meson channels: $D\overline{D}$,$D^{*}\overline{D}$,$D^{*}\overline{D^{*}}$,$D_{s}\overline{D}_{s}$, etc. Each charm decay of these charmonium state is studied to determinate amplitude and width and to compare with the theory that describe these decays [7]. Pure leptonic decays $D^{+}\rightarrow l^{+}\nu$ are the cleanest decay modes of $D^{+}$ meson: the $c$ and $\overline{d}$ quarks annihilate into a virtual $W^{+}$ boson with a decay width given by the SM:

[TABLE]

where $M_{D}^{+}$ is the $D^{+}$ mass, $m_{l}$ is the lepton mass, $V_{cd}$ is the CKM matrix element and $f_{D+}$ is the decay constant that contains the information about the dynamics of the quarks inside the meson. Studying this kind of decay it determines $f_{D+}|V_{cd}|$. In the literature, $f_{D+}$ has been extensively studied in a variety of theoretical approaches [24, 25]. In similar way $D_{s}^{+}\rightarrow l^{+}\nu$ mode is used to access to $|V_{cs}|$ and $f_{D_{s}}$. BESIII physics program include high precision measurement of $f_{D}$ and $f_{D_{s}}$ mesons. Semileptonic decays occur when a charm meson decay via weak interaction by emitting a $l^{+}\nu$ lepton pair: $c\rightarrow ql^{+}\nu$, where $q$=$d$,$s$. The $q$ quark is bound to the initial light quark of the charm meson by strong interaction ($X$ meson in Eq. 1.11). In semileptonic decay the leptons do not interfere with the strong interaction then they can factored out the hadronic process. The process amplitude is:

[TABLE]

The process depends on the quark-mixing parameter $V_{cq}$ so the semileptonic decay is a good system to access to the CKM matrix and test the technique that calculate the hadronic matrix element developed by several method [26, 27, 28, 29, 30, 31]. These decay are sensitive to $V_{cd}$ and $V_{cs}$ by the determination of the absolute branching fractions, $e.g.$ for $V_{cs}$ in $D\rightarrow\pi l^{+}\nu$, $D\rightarrow Kl^{+}\nu$, $D\rightarrow\eta(\eta^{\prime})l^{+}\nu$, $D\rightarrow\rho l^{+}\nu$, $D\rightarrow K^{*}l^{+}\nu$, $D_{s}\rightarrow\varphi l^{+}\nu$, etc. These parameters are used together with the other CKM element to measure the mixing and the CP violating parameters, to test the self-consistency of the CKM picture and to search possible new physics beyond the CKM mechanism.

Meson-antimeson oscillation is another topic accessible at BESIII experiment. In these oscillation weak interaction effect build up over macroscopic distance and makes tiny mass differences measurable. It is mandatory to probe $D^{0}-\overline{D^{0}}$ oscillation as sensitively as possible to provide proof of New Physics and CP violation.

$\Lambda_{c}$ physics is another branch of interest in charm studies. The lightest charmed baryon $\Lambda_{c}^{+}$ ($udc$) decays only through the c weak decay, $c\rightarrow W^{+}s$,$W^{+}d$, at the leading order and it is the simpler theoretical environment to describe with non-perturbative models. This particle allows to measure the absolute branching fraction of $\Lambda_{c}^{+}\rightarrow\Lambda l^{+}\nu$ and 12 Cabibbo-favored decays, studies of singly-Cabibbo-suppressed decays and the observation of the modes with a neutron in the final state.

1.4 The needs of a new inner tracker for BESIII

BESIII started to operate in 2009. In about the past 10 years the detectors suffered of aging due to radiation effect. The integrated charge on the innermost layer is about $170\,\mathrm{mC}/\mathrm{cm}$. The drift chamber is the system closest to the beam pipe. After several years of operation the performance of the drift chamber decreased due to the integrated charge. The aging in the anode is given by gas polymers that condense on the sense wire surface as thin films, whiskers and powder due to the high electric field. Those accumulation could be conductive or insulating; both contaminations cause a gain loss due to the increase of the effective diameter of the sense wire. Reduction of the gain is due also to the reduction of the electric field caused by the accumulation of charges in the insulating layer. The cathode suffers the aging too. A polymer deposits on the cathode surface and insulate it. This effect prevents the neutralization of the positive charge that modifies the detector electric field around the cathode up to a value that extracts electrons from the electrode. Those electrons can drift to the sense wire and create avalanches. The avalanche positive ions come back to the cathode, enhance the electric field around the cathode and thus feeds continuously this self-sustaining local discharge. This effect is called the Malter effect [32] and some water vapor has been added to the MDC gas mixture to slow down this issue. In order to reduce the dark currents of the sense wires and to use the detector in a safe configuration the voltage of the first four layers has been decreased with respect to the normal values, which result in the decrease of the performance of the detector. The gain with respect to 2009 status has been reduced up to 60% and this affects the event reconstruction worsening the detection efficiency up to 50% [34]. The gain of the cells closer to the IP shows a sizable decrease, as shown in Fig. 1.6. This worsening does not allow the MDC to maintain the required performance in the next years. The BESIII collaboration decided to replace the inner part of the MDC with a new and more performing inner tracker[33] with required performance described in Tab. 1.6.

A three layers cylindrical triple-GEM inner tracker has been proposed to replace the first eight layers of the MDC. A GEM (Gas Electron Multiplier) is a Micro-Pattern Gas Detector, a particular class of gas detectors that profits from modern photolitography and deposition of polyamide thin-layer. This allows to reach unprecedented performance with gas detectors as it will be shown in Chap. 4.

Chapter 2 CGEM-IT

Despite various improvements, the detection technique based on wire structures suffers by limitation due to diffusion processes, space charge effects and aging of the anode and cathode electrodes. Modern construction technique allows to solve many of those problems, $e.g.$ by using pitch size of a few hundred of $\mathrm{\upmu m}$ to increase the granularity, or by photolithographic processes and the polyamide depositions of a layers of few $\mathrm{\upmu m}$ to define new amplification geometries and detection techniques as reported in App. A.7. The GEM technology is the one chosen to replace the inner MDC: thanks to a thin pierced foil of copper-insulator-copper can magnify the primary ionization and measure the particle position within approximately $1\,\mathrm{cm}$ of gas volume. The resolution achieved depends on gas mixture, gain and front-end electronics. In literature spatial resolution below hundred $\mathrm{\upmu m}$ are reported [1] for orthogonal tracks without magnetic field. The GEM foils can be shaped to the needed surface and thanks to their thickness and formability are suitable to be used as inner trackers with cylindrical surface. In this Chap. a general overview about the components, the construction technique and the electronics of CGEM-IT will be provided. The details of GEM technology will be explained in detail in Chap. 3.

2.1 The KLOE-2 IT

The KLOE-2 inner tracker is an innovative detector built up by four coaxial layers of cylindrical triple-GEM: each layer exploits three GEM foils to amplify the signal from the particles that interact with it and it is shaped around the beam pipe. The inner radius of the layers ranges from 130 to $205\,\mathrm{mm}$ and the length is about $700\,\mathrm{mm}$. Each layer is made by five foils: three GEMs, a cathode and a segmented anode. The layers provide a bi-dimensional measurement due to longitudinal and stereo strips of $650\,\mathrm{\upmu m}$ pitch.

GEM foils of the needed dimension can not be produced then they have been obtained through a gluing process with vacuum bag technique of two or three GEM foils together by means of epoxy glue on a $3\,\mathrm{mm}$ peripheral region of kapton, the insulator material between the copper foils of the GEM. Then the large GEM foils are wrapped on an aluminum mould coated with Teflon film and glued in a $3\,\mathrm{mm}$ overlap region. During the curing of the epoxy, a vacuum bag system is used to constrain the foil to the cylindrical shape given by the mould [2]. Anode and cathode electrodes are shaped with the same technique then on a Honeycomb structure and, together with two permaglass rings glued at the edges of those two electrodes provide the structural support to the detector. The five electrodes are inserted one onto each other through a precise machine named Vertical Insertion System.

The front-end electronics used by KLOE-2 IT is GASTONE-ASIC [4]: an analogue preamplifier integrates the signal that it is filtered from noise and then discriminated with a threshold range from 0 to $200\,\mathrm{fC}$. A digital output from each strip is sent out for the position measurement.

2.2 Innovation of the BESIII CGEM-IT

The BESIII IT starts from the KLOE-2 experience to develop a new tracker with GEM technology and cylindrical shape. The configuration used by KLOE-2 could not match the BESIII requirements due to its dimension, its total material budget and its spatial resolution (see Sect. 1.4) then a series of improvement have been introduced. Due to the dimension of the inner MDC in BESIII, the volume left free from its removal permits to insert three cylindrical layers instead of four. To keep reasonable the cost of the IT and for mechanical constrains, a pitch of $650\,\mathrm{\upmu m}$ has been chosen despite it would not reach the needed resolution if the digital readout was kept. It was mandatory to develop a new ASIC to measure charge and time information needed in the reconstruction algorithms described in Sect. 4.2. More details about the new ASIC will be discussed in Sect. 2.3. This is one of the most important innovation with respect to KLOE-2.

A new anode design has been implemented to reduce the inter-strip capacitance: longitudinal and stereo strips cross each other many times in the anode and each overlap of the two surfaces introduce a capacitance between the two strips that increases the noise in the readout channel. A jagged layout has been used in the BESIII IT to reduce the overlapping area. Simulations [5] show that it allows to reduce the inter-strip capacitance of about 30% with respect to the regular layout used by KLOE-2. Figure 2.1 shows the differences between the two design.

The sensitive volume in a triple-GEM detector has about $1\,\mathrm{cm}$ of thickness. A signal bigger than 1 fC is collected mostly from a fraction of this sensitive volume: the region between the cathode and the first GEM because only the primary electrons generated in this region cross three timesthe GEM foils. In KLOE-2 this region, named $drift~{}gap$, has $3\,\mathrm{mm}$ thickness while in the BESIII IT has been increased to $5\,\mathrm{mm}$ to increase the number of primary ionizations and to improves the reconstruction performances that will be explained in Sect. 4.3: larger gap correspond to better spatial resolution for the $\upmu$TPC technique.

The mechanical structure of the CGEM-IT in the KLOE-2 experiment is given by a Honeycomb foil on the anode and the cathode electrodes and the permaglass ring at the edges of the detector. BESIII uses a different material: the Rohacell that it allows to reduce the thickness of the structure, keeping the same mechanical properties. This reduces the material budged of the detector then its interaction length $X_{0}$. The last design innovation is in the reduction of the copper thickness on the faces of the GEM foil from 5 to $3\,\mathrm{\upmu m}$. This reduces the radiation length of the entire detector. The entire CGEM-IT will have a total material budget below 1.5% of $X_{0}$.

In Tab. 2.1 are reported the values of the inner radius of each CGEM-IT layer, the active area length and the number of strips and the angle between longitudinal and stereo strips.

2.3 The new ASIC for the BESIII CGEM-IT

TIGER (Torino Integrated Gem Electronics for Readout) electronics, is a mixed-signal 64-channel ASIC developed to readout the CGEM detector of the BESIII Experiment [6]. The ASIC is installed on a Front-End Board (FEB) with two chips to measure the signals from 128 strips. The ASIC measures time and charge of the signal. The chip uses two threshold, a first one to measure the rise up edge of the signal and a second one to discriminate signal from noise. The second threshold depends on the strip capacitance and chip temperature. Due to the variable length of the stereo strips, different threshold are used for each channel. A chiller stabilizes the chip temperature to avoid threshold variation. The time is measured with a lower threshold that send out the time information when the signal overstep the discrimination one. The charge can be measured with two methods: sampling and hold (S/H) mode and time-over-threshold (ToT) mode. S/H measures the signal amplitude in a dynamic range from 0 to $50-60\,\mathrm{fC}$. The ToT measures the leading edge of the signal and it convert the time length of the signal into charge information. Both methods need a calibration that has to be performed for each channel of the TIGER. Figure 2.2 shows the calibration curve of a TIGER for S/H and ToT methods [7]. The output of the chip is fully digital. The advantage of S/H is the linearity between the charge and the output but the operating range is limited while ToT has an unlimited range but the output is not linear and it needs calibrations.