Measurement of the branching fraction of $J/\psi \rightarrow \omega \eta' \pi^{+}\pi^{-}$ and search for $J/\psi \rightarrow \omega X(1835), \, X(1835) \rightarrow \eta' \pi^{+}\pi^{-}$ decay

TL;DR

This study measures the branching fraction of a specific J/psi decay and searches for the X(1835) state, setting an upper limit on its production, using a large dataset from BESIII.

Contribution

First measurement of the branching ratio for J/psi to omega eta' pi+ pi- and the first search for X(1835) in this decay channel.

Findings

Branching ratio B(J/psi → ω η' π+ π−) = (1.12 ± 0.02 ± 0.13) × 10^{-3}

No evidence for X(1835) in the decay channel

Upper limit on X(1835) production set at 6.2 × 10^{-5}

Abstract

Using a sample of $1.31 \times 10^{9}$ $J/\psi$ events collected by the BESIII detector at BEPCII during 2009 and 2012, we study the $J/\psi \rightarrow \omega \eta' \pi^{+}\pi^{-}$ hadronic process. For the first time, we measure the branching ratio $B(J/\psi \rightarrow \omega \eta' \pi^{+}\pi^{-}) = (1.12 \pm 0.02 \pm 0.13) \times 10^{-3}$. We search for the $X(1835)$ state in the $\eta' \pi^{+}\pi^{-}$ invariant mass spectra. No evidence is found and we estimate the upper limit on the branching fraction at 90% confidence level to be $B(J/\psi \rightarrow \omega X(1835), \, X(1835) \rightarrow \eta' \pi^{+}\pi^{-}) < 6.2 \times 10^{-5}$.

Tables

Table 1. Table 1: Summary of systematic uncertainties. Those items marked with “-” has been taken into account in obtaining the UL on the number of X ​ ( 1835 ) 𝑋 1835 X(1835) signal events.

Sources	$ℬ\left(J/\psi \to \omega {\eta }^{\prime }{\pi }^{+}{\pi }^{-}\right)$	UL
	($\%$)	($\%$)
Number of $J/\psi$	0.5	0.5
${ℬ}_{\mathrm{int}}\left(\omega \to {\pi }^0{\pi }^{+}{\pi }^{-}\right)$	0.78	0.78
${ℬ}_{\mathrm{int}}\left({\pi }^0\to \gamma \gamma \right)$	0.03	0.03
${ℬ}_{\mathrm{int}}\left({\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}\right)$	1.63	1.63
${ℬ}_{\mathrm{int}}\left(\eta \to \gamma \gamma \right)$	0.51	0.51
Tracking efficiency	6	6
Photon detection	4	4
Selection efficiency	3.2	negligible
Angular distribution	1.0	-
Kinematic fit	5	5
$\omega /{\eta }^{\prime }$ signal function	4.8	-
Fit range	2.6	-
Background shape	4.3	-
Total	11.8	9.0

Equations

$$\mathcal{B}(J/\psi \to \omega {\eta }^{\prime }{\pi }^{+}{\pi }^{-})=\frac{N_{\mathrm{sig}}}{N_{J/\psi }\cdot ϵ\cdot {\mathcal{B}}_{\mathrm{int}}},$$

$$\mathcal{B}$$

Full text

One of the main topics of the BESIII physics program is the search for unconventional hadronic states. Among the light hadrons, the $X(1835)$ state has caught the attention both from an experimental and a theoretical point of view. It was observed first in the $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass spectra at BES in the $J/\psi\rightarrow\gamma\eta^{\prime}\pi^{+}\pi^{-}$ radiative decay Ablikim et al. (2005), and confirmed later with much higher statistics by BESIII Ablikim et al. (2011). Its mass and width were measured to be $M=1836.5\pm 3.0^{+5.6}_{-2.1}$ ${\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$ and \Gamma=190\pm 9^{+38}_{-36}$$\mathrm{\,Me\kern-1.00006ptV}, with the product of branching fractions $\mathcal{B}(J/\psi\rightarrow\gamma X(1835))\cdot\mathcal{B}(X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-})=(2.87\pm 0.09^{+0.49}_{-0.52})\times 10^{-4}$  Ablikim et al. (2011). The $X(1835)$ state was also seen in the process $J/\psi\rightarrow\gamma K_{S}^{0}K_{S}^{0}\eta$ Ablikim et al. (2015); its mass and width were found to be in agreement with those measured in Ref. Ablikim et al. (2011), and he quantum numbers $J^{PC}$ were determined to be $0^{-+}$ from a partial wave analysis.

Just a few years before the observation of the $X(1835)$ state, an anomalous enhancement close to the $p\bar{p}$ mass threshold, called $X(1860)$, has been observed by BES in the $J/\psi\rightarrow\gamma p\bar{p}$ decay Bai et al. (2003), and confirmed by BESIII Ablikim et al. (2010a) and CLEO Alexander et al. (2010), while no evidence has been seen in other channels, such as $J/\psi\rightarrow\omega p\bar{p}$ M. Ablikim et al. (2008); Ablikim et al. (2013) or $J/\psi\rightarrow\phi p\bar{p}$ Ablikim et al. (2016a). A partial wave analysis of the $p\bar{p}$ mass-threshold enhancement was performed Ablikim et al. (2012), and the $J^{PC}$ quantum number were determined to be the same as for the $X(1835)$. The discovery of these new states has stimulated many theoretical speculations on their nature, such as a $p\bar{p}$ bound state Dedonder et al. (2009); Liu et al. (2009); Ding et al. (2006), a pseudo-scalar glueball A. Li (2006); Kochelev and Min (2006); Hao et al. (2006), a radial excitation of the $\eta^{\prime}$ meson Huang and Zhu (2006), etc. Thanks to the world’s largest $e^{+}e^{-}\to J/\psi$ data set collected by BESIII, it has been possible to study in detail the significant abrupt change in the line shape of the $X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-}$ in correspondence of the $p\bar{p}$ mass threshold Ablikim et al. (2016b), which could be originated from the opening of the $p\bar{p}$ additional decay channel (threshold effect) or by the interference between two different resonances. However, none of the hypotheses could be excluded and no final conclusion has been made. In order to extract additional information about the states around $1.85$ ${\mathrm{\,Ge\kern-1.00006ptV\!/}c^{2}}$  with the present BESIII statistics, additional decay modes must be investigated.

In this paper, we report on the search for $X(1835)$ in the $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ process. The comparison of the production rates between $J/\psi\rightarrow\omega X(1835)$ and $J/\psi\rightarrow\gamma X(1835)$ could also help to get information on the $q\overline{q}$ or gluon component of $X(1835)$ A. Li (2006); Ding et al. (2006), i.e. if $X(1835)$ contains substantial $q\overline{q}$ components, like the $\eta^{\prime}$ meson, it should be observed in $J/\psi\rightarrow\omega X(1835)$. Using the branching fraction of $J/\psi\rightarrow\omega(\phi)\eta^{\prime}$, the branching fraction of $J/\psi\rightarrow\omega(\phi)X(1835)$ is estimated to be in the order of $10^{-5}$ A. Li (2006). On the other hand, a very small branching fraction is expected for larger gluon component. Another estimation was done in Ref. Ding et al. (2006), where $\mathcal{B}(J/\psi\rightarrow\omega X(1835))$ is expected to be two orders of magnitude less than that of $J/\psi\rightarrow\gamma X(1835)$ decay.

This analysis is based on $1.31\times 10^{9}$ $J/\psi$ events collected by BESIII during 2009 and 2012. The BESIII detector Ablikim et al. (2010b) is a magnetic spectrometer operating at BEPCII, a double-ring $e^{+}e^{-}$ collider with center-of-mass energies ranging from $2.0$ to $4.6$ $\mathrm{\,Ge\kern-1.00006ptV}$. The geometrical acceptance covered is $93\%$ of a $4\pi$ solid angle. From the inner to the outer side, it consists of a helium-based main drift chamber (MDC), a time-of-flight system (TOF) and a CsI(Tl) electromagnetic calorimeter (EMC), all enclosed in a superconducting solenoidal magnet providing a magnetic field of $1$ T ($0.9$ T in 2012). The solenoid is surrounded by an octagonal flux-return yoke with resistive plate chambers interleaved with steel.

A GEANT4-based Monte Carlo (MC) simulation package Agostinelli et al. (2003) is used to optimize selection criteria, estimate background processes, and determine detection efficiency. The production of the $J/\psi$ resonance is simulated with KKMC event generator Jadach et al. (2000, 2001), while the decays are generated with EvtGen Lange (2001); Ping (2008). Simulated inclusive $J/\psi$ events of approximatively the equivalent luminosity of data are used to study background processes. The known decays of $J/\psi$ are modeled with branching fractions being set to the world average values from Particle Data Group (PDG) Tanabashi et al. (2018), while the remaining decays are generated with LUNDCHARM Chen et al. (2000). We simulate 700,000 MC events using phase space model for the processes $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ and $J/\psi\rightarrow\omega X(1835),\,X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-}$, which are used to optimize the event selection and to determine the selection efficiency. For the $J/\psi\rightarrow\omega X(1835)$ signal simulation we also take into account the $J^{PC}=0^{-+}$ quantum numbers.

For each candidate event, we select charged tracks well reconstructed in the MDC detector with the polar angle $\theta$ satisfying the condition $|\cos\theta|<0.93$. The tracks are required to pass the interaction point within $\pm 10$ cm along the beam direction and within $1$ cm in the plane perpendicular to the beams. Photon candidates are reconstructed using clusters of energy deposited in the EMC. The energy deposited in the TOF is also included in EMC measurements in order to improve the reconstruction efficiency and the energy resolution. Good photon candidates are required to have a deposited energy larger than $25$ MeV in the barrel region ($|\cos\theta|<0.8$) and $50$ MeV in the end caps ($0.86<|\cos\theta|<0.92$). To eliminate those clusters associated with charged tracks, the angle between the direction of any charged track and the photon candidate must be larger than $5^{\circ}$. Clusters due to the electronic noise and energy deposit unrelated to the event are suppressed by requiring the shower time to be within $700$ ns of the event start time. Events with six charged tracks, net charge equal to zero, and at least four photon candidates that satisfy the above requirements are retained for further studies.

In the reconstruction of $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$, the $\omega$ meson is reconstructed in its dominant $\pi^{+}\pi^{-}\pi^{0}$ decay mode and $\eta^{\prime}$ via $\eta^{\prime}\rightarrow\eta\pi^{+}\pi^{-}$, while both $\eta$ and $\pi^{0}$ are reconstructed from $\gamma\gamma$ pairs after applying the corresponding mass constrained kinematic fit. To improve momentum resolution, for each $\pi^{0}\eta\pi^{+}\pi^{-}\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ combination a four constraints ($4C$) energy-momentum kinematic fit is performed. We select only events with $\chi^{2}_{4C}<60$. In order to determine the $\pi^{+}\pi^{-}$ pairs produced in $\omega/\eta^{\prime}$ decays, we select the combination which minimize the quantity $\sqrt{(M_{\pi^{+}\pi^{-}\pi^{0}}-m_{\omega})^{2}+(M_{\eta\pi^{+}\pi^{-}}-m_{\eta^{\prime}})^{2}}$, where $m_{\omega}$ and $m_{\eta^{\prime}}$ are the nominal masses of $\omega$ and $\eta^{\prime}$ Tanabashi et al. (2018), respectively, while $M_{\pi^{+}\pi^{-}\pi^{0}}$ ($M_{\eta\pi^{+}\pi^{-}}$) is the $\pi^{+}\pi^{-}\pi^{0}$ ($\eta\pi^{+}\pi^{-}$) invariant mass. Then, we require $M_{\pi^{+}\pi^{-}\pi^{0}}$ and $M_{\eta\pi^{+}\pi^{-}}$ to be within $3\sigma$ the fitted widths: $|M_{\pi^{+}\pi^{-}\pi^{0}}-m_{\omega}|<22$ ${\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$ and $|M_{\eta\pi^{+}\pi^{-}}-m_{\eta^{\prime}}|<12$ ${\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$. Figure 1 shows the $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass distribution ($M_{\eta^{\prime}\pi^{+}\pi^{-}}$) from data sample for those events that satisfy the selection criteria. No clear enhancement is visible.

In Fig. 1, the $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass spectrum from inclusive MC sample is also reported. Since there are some discrepancies between the two distributions, more pronounced for $M_{\eta^{\prime}\pi^{+}\pi^{-}}>1.9$ ${\mathrm{\,Ge\kern-1.00006ptV\!/}c^{2}}$, we cannot use the inclusive MC sample to model the background contribution. As an alternative, a two-dimensional fit to the $\pi^{+}\pi^{-}\pi^{0}$ and $\eta\pi^{+}\pi^{-}$ distributions will be used to get the number of $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ signal events. The scatter plot of $M_{\eta\pi^{+}\pi^{-}}$ as a function of $M_{\pi^{+}\pi^{-}\pi^{0}}$ is reported in Fig. 2. The $\omega$ signal is parametrized by a Breit-Wigner (BW) function convolved with a double Gaussian and the $\eta^{\prime}$ signal by a double Gaussian function, while third-order polynomial functions are used for both $\omega$ and $\eta^{\prime}$ backgrounds. All the parameters are treated as free with the exception of the $\omega$ width, which is fixed to the world average value Tanabashi et al. (2018).

The one-dimensional projections of the fit result are shown in Fig. 3. The branching fraction of the $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ process is calculated with

[TABLE]

where $N_{\rm{sig}}=14151\pm 287$ is the number of $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ signal events from the fit to the data sample, $N_{J/\psi}$ the number of $J/\psi$ events Ablikim et al. (2017), $\epsilon=6.48\%$ the detection efficiency calculated from signal simulation, and $\mathcal{B}_{\rm{int}}$ the product of the decay branching fractions for the $\omega\rightarrow\pi^{+}\pi^{-}\pi^{0}$, $\pi^{0}\rightarrow\gamma\gamma$, $\eta^{\prime}\rightarrow\eta\pi^{+}\pi^{-}$, and $\eta\rightarrow\gamma\gamma$ intermediate states quoted from PDG Tanabashi et al. (2018). The branching fraction is then determined to be $\mathcal{B}(J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-})=(1.12\pm 0.02)\times 10^{-3}$, where the uncertainty is statistical only.

As can been seen from Fig. 1, no significant $X(1835)$ signal is observed in the $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass spectrum, and hence we extract the upper limit (UL) on the number of $X(1835)$ signal events. As stated before, we cannot use inclusive MC sample to parametrize the background shape. Also a polynomial function may not be appropriate to describe a large background component under a very small signal fraction of a broad resonance. As an alternative in order to extract the background-corrected distribution, the two-dimensional fit to the $\pi^{+}\pi^{-}\pi^{0}$ and $\eta\pi^{+}\pi^{-}$ invariant mass spectra is performed in eight slices of $\eta^{\prime}\pi^{+}\pi^{-}$ mass spectrum from $1.4$ ${\mathrm{\,Ge\kern-1.00006ptV\!/}c^{2}}$ to $2.2$ ${\mathrm{\,Ge\kern-1.00006ptV\!/}c^{2}}$. The background-subtracted $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass is shown in Fig. 4. The UL on the number of $X(1835)$ signal events is extracted by means of a $\chi^{2}$-fit. In this fit, all processes other than $J/\psi\rightarrow\omega X(1835)$ are considered as background, and we assume there is no interference between $X(1835)$ and non-$X(1835)$ components. Both the $X(1835)$ signal and background yields are fitted as free parameters, and we associate to the signal yield a Gaussian distribution with mean and width equal to the number of signal events and the corresponding uncertainty resulting from the $\chi^{2}$-fit. Then, the UL at $90\%$ confidence level (C.L.) is obtained by finding the point where the cumulative probability of this Gaussian distribution is equal to 0.9.

In the $\chi^{2}$-fit, two different signal functions are taken into account: a BW function with $X(1835)$ mass and width fixed to values from Ref. Ablikim et al. (2011), and a Flatté function with fixed parameters from Ref. Ablikim et al. (2016b), both weighted by the efficiency, while a third-order polynomial function is used for the background. Systematic effects are evaluated by changing the $\eta^{\prime}\pi^{+}\pi^{-}$ fit range and the bin size, as well as by varying the fit parameters within one standard deviation. Since the $\eta^{\prime}\pi^{+}\pi^{-}$ background-corrected distribution is extracted from a two-dimensional fit to the $\pi^{+}\pi^{-}\pi^{0}$ and $\eta\pi^{+}\pi^{-}$ invariant mass spectra, we need to evaluate its systematic contribution. On this purpose, three different signal functions are used to parametrize the $\omega$ and $\eta^{\prime}$ signal: (1) a BW convolved with a double Gaussian for $\omega$ and a double Gaussian for $\eta^{\prime}$, (2) the $\omega$ and $\eta^{\prime}$ MC shapes, and (3) the convolution of the $\omega$ and $\eta^{\prime}$ MC shapes with a double Gaussian. The resulting $\eta^{\prime}\pi^{+}\pi^{-}$ background-corrected distribution are then fitted using a $\chi^{2}$-fit, as described before. The fit that gives the largest result is then used to extract the UL on the number of $X(1835)$ signal events at $90\%$ C.L., which amounts to $N^{UL}=582$. The corresponding UL on the branching fraction of the $J/\psi\rightarrow\omega X(1835),\,X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-}$ decay at $90\%$ C.L. is calculated as

[TABLE]

where $\epsilon^{\prime}=5.26\%$ is the $X(1835)$ selection efficiency in the $\omega-\eta^{\prime}$ signal region, and $\sigma_{\rm{sys}}$ is the total systematic uncertainty reported in Table 1 and discussed below.

Several sources of systematic uncertainties are considered: uncertainty due to the total number of $J/\psi$ events Ablikim et al. (2017), intermediate branching fractions Tanabashi et al. (2018), data-MC differences in tracking efficiency, photon detection efficiency, selection efficiencies, angular distributions, kinematic fit, signal and background functions and fit range. Uncertainties due to the tracking efficiency for charged tracks are determined using control samples of $J/\psi\rightarrow\pi^{+}\pi^{-}p\bar{p}$ and $J/\psi\rightarrow K_{S}^{0}K^{\pm}\pi^{\mp}$. The difference between the tracking efficiency in data and MC simulations is $1\%$ for each charged track. However, since we have six charged pions in the final state, and hence pions with very low momentum, we check for possible tracking efficiency underestimation. We correct our MC simulations according to the data, also taking into account possible difference in the polar angle distributions, and we find a tracking efficiency consistent with $6\%$. For the neutral candidates, control samples of $J/\psi\rightarrow\rho\pi^{0}$ and $e^{+}e^{-}\rightarrow\gamma\gamma$ are used to study the photon detection efficiency, which amount to $1\%$ for each photon candidate.

The systematic contributions related to the selection efficiency used to calculate both branching fraction and upper limit are evaluated by means of additional MC samples, in which also different intermediate states are considered. However, since no obvious structures are observed in the different combinations of two- or three-particles invariant mass distributions, we simulate a MC sample, without intermediate resonances, taking into account the spin-parity of the initial and final states, and the difference in the efficiency is taken as systematic contribution. Additional contribution can arise from differences between the angular distribution of data and simulation in the $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ process. We simulate a new MC sample following the same angular dependence as in the data. The difference in the efficiency amount to $1\%$, which is taken as systematic uncertainty.

A control sample of $\psi(3686)\rightarrow\pi^{+}\pi^{-}J/\psi,\,J/\psi\rightarrow\pi^{+}\pi^{-}\pi^{0}\eta^{\prime},\,\eta^{\prime}\rightarrow\eta\pi^{+}\pi^{-}$ is used to determine the systematic uncertainty related to the kinematic fit. We perform a 2-dimensional fit to the $J/\psi$ and $\eta^{\prime}$ invariant mass spectra in order to extract the number of signal events and calculate the efficiency as a function of $\chi^{2}_{4C}$. The difference between data and MC in correspondence of the $\chi^{2}_{4C}$ cut used in this analysis is taken as systematic uncertainty.

Systematic contributions associated only with the branching fraction are those related to the two-dimensional fit of the $\pi^{+}\pi^{-}\pi^{0}$ and $\eta\pi^{+}\pi^{-}$ invariant masses. In particular, the systematic related to the signal functions are evaluated by means of MC shape distributions for both the $\omega$ and $\eta^{\prime}$ invariant mass spectra. For the background, instead, we change the order of the polynomial function. In both cases, the difference in the number of signal events is taken as systematic uncertainty. We also change the fit range by a step of $5$ ${\mathrm{\,Me\kern-1.00006ptV\!/}c^{2}}$, and the difference in the signal yield is taken as systematic uncertainty.

Table 1 summarizes all sources of systematic uncertainties, for which the total contribution is obtained as sum of them in quadrature.

Using a sample of $1.31\times 10^{9}$ $J/\psi$ events collected with the BESIII detector, we measure for the first time the branching fraction for the decay $J/\psi\rightarrow\omega\eta^{\prime}\pi^{+}\pi^{-}$ to be $(1.12\pm 0.02\pm 0.13)\times 10^{-3}$, where the first uncertainty is statistical and the second systematic. We also search for the $X(1835)$ state in the hadronic $J/\psi$ decay $J/\psi\rightarrow\omega X(1835)$, with $X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-}$. No significant signal is observed and the upper limit at $90\%$ C.L. on the branching fraction is determined to be $\mathcal{B}(J/\psi\rightarrow\omega X(1835),\,X(1835)\rightarrow\eta^{\prime}\pi^{+}\pi^{-})<6.2\times 10^{-5}$. Since the $X(1835)$ state is observed only in radiative $J/\psi$ decays and the branching fraction is measured to be of the order of $10^{-4}$ Ablikim et al. (2011, 2015, 2016b), authors of Ref. A. Li (2006) suggest that a smaller branching fraction measured in hadronic $J/\psi$ decays could be an indication of a large gluon component. Authors of Ref. Ding et al. (2006) treat $X(1835)$ as a baryonium with sizable gluon content, and estimate a branching ratio of the order of $10^{-6}$. Unfortunately, our upper limit result is too large to confirm or distinguish among several theoretical interpretations, but it provides the first search for the $X(1835)$ state in $J/\psi$ hadronic decays, which can be further investigated by studying additional hadronic decay modes.

Acknowledgment

The BESIII collaboration thanks the staff of BEPCII and the IHEP computing center for their strong support. This work is supported in part by National Key Basic Research Program of China under Contract No. 2015CB856700; National Natural Science Foundation of China (NSFC) under Contracts Nos. 11335008, 11425524, 11625523, 11635010, 11735014; the Chinese Acade my of Sciences (CAS) Large-Scale Scientific Facility Program; the CAS Center for Excellence in Particle Physics (CCEPP); Joint Large-Scale Scientific Facility Funds of the NSFC and CAS under Contracts Nos. U1532257, U1532258, U1732263; CAS Key Research Program of Frontier Sciences under Contracts Nos. QYZDJ-SSW-SLH003, QYZDJ-SSW-SLH040; 100 Talents Program o f CAS; INPAC and Shanghai Key Laboratory for Particle Physics and Cosmology; German Research Foundation DFG under Contracts Nos. Collaborative Research Center CRC 1044, FOR 2359; I stituto Nazionale di Fisica Nucleare, Italy; Koninklijke Nederlandse Akademie van Wetenschappen (KNAW) under Contract No. 530-4CDP03; Ministry of Development of Turkey under Contra ct No. DPT2006K-120470; National Science and Technology fund; The Swedish Research Council; U. S. Department of Energy under Contracts Nos. DE-FG02-05ER41374, DE-SC-0010118, DE-SC- 0010504, DE-SC-0012069; University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt.