Effect of Rope Hadronisation on Strangeness Enhancement in p$-$p collisions at LHC energies

TL;DR

This paper investigates how rope hadronization influences strangeness enhancement in high-multiplicity proton-proton collisions at LHC energies, using Pythia8 simulations to replicate experimental observations.

Contribution

It demonstrates that the microscopic rope hadronization model can successfully describe strangeness enhancement and related phenomena in high-energy p-p collisions.

Findings

Rope hadronization explains strangeness enhancement in high-multiplicity p-p collisions.

Model reproduces the shape and hardening of $p_T$ spectra with multiplicity.

Predictions align with experimental data for strange hadron yields at 7 and 13 TeV.

Abstract

The p$-$p collisions at high multiplicity at LHC show small scale collective effects similar to that observed in heavy ion collisions such as enhanced production of strange and multi-strange hadrons, long range azimuthal correlations, etc. The observation of strangeness enhancement in p$-$p collisions at at $\sqrt{s}$ = 7 TeV and 13 TeV as measured by ALICE experiment is explored using Pythia8 event generator within the framework of microscopic rope hadronization model which assumes the formation of ropes due to overlapping of strings in high multiplicity environment. The transverse momentum ($p_{T}$) spectra shape and its hardening with multiplicity is well described by the model. The mechanism of formation of ropes also described the observed experimental strangeness enhancement for higher multiplicity classes in p$-$p collisions at 7 TeV and 13 TeV. The enhancement with multiplicity is further investigated by studying the mean $p_{T}$ ($<p_{T}>$) and the integrated yields ($<dN/dy>$ ) of strange and multi-strange hadrons and comparing the predictions to the measured data at LHC for 7 TeV and 13 TeV.

Tables

Table 1. Table 1: The values of ⟨ d ​ N c ​ h / d ​ η ⟩ η < 0.5 subscript delimited-⟨⟩ 𝑑 subscript 𝑁 𝑐 ℎ 𝑑 𝜂 𝜂 0.5 {\langle dN_{ch}/d\eta\rangle}_{\eta<0.5} for different multiplicity classes as used by ALICE experiment for p − - p collisions at s 𝑠 \sqrt{s} = 7 TeV nature .

Multiplicity	$⟨d{N}_{ch}/d\eta ⟩$	Multiplicity	$⟨d{N}_{ch}/d\eta ⟩$
class		class
I	21.5 $\pm$ 0.6	VI	8.45 $\pm$ 0.25
II	16.5 $\pm$ 0.5	VII	6.72 $\pm$ 0.21
III	13.5 $\pm$ 0.4	VIII	5.40 $\pm$ 0.17
IV	11.5 $\pm$ 0.3	IX	3.90 $\pm$ 0.14
V	10.1 $\pm$ 0.3	X	2.26 $\pm$ 0.12

Full text

I Introduction

The recent observation of enhanced production of strange and multi-strange hadrons in p$-$p collisions at at $\sqrt{s}$ = 7 TeV with high final state multiplicity as measured by ALICE experiment nature and long range azimuthal correlations measured by CMS and ATLAS experiment cms1 ; cms2 ; atlas have generated a lot of interest in small systems. These observations mimic features present in deconfined matter formed in heavy ion collisions and are manifestations of Quark gluon Plasma (QGP) dynamics. The QGP observables are studied as a function of charged particle density ($dN_{ch}/d\eta$) in smaller systems and the size of the effects seem to agree with heavy ion collisions for similar charged particle density. Therefore, the possibility of formation of a mini-QGP in small systems for high multiplicity is also being predicted.

An enhanced production of strange and mutli-strange hadrons in heavy ion collisions has long been predicted to be a signature of the formation of the QGP medium rafelski . The strange valence quark is not present in the initial state of colliding nuclei and hence are produced by hard partonic scattering processes such as flavor excitation and flavor creation in initial stages of collision and therefore the strange hadrons are predominantly produced in high $p_{T}$ region. However, in the low $p_{T}$ region, where the perturbative description fails, the production of strange hadrons is suppressed compared to the light quark ($u$ and $d$) hadrons owing to the relatively heavier mass of strange quarks. In heavy ion collisions, the enhanced production of strange particles in central and mid-central collisions have been attributed to the abundance of strange and anti-strange quarks in the deconfined QGP medium koch ; rafelski . The heavy ion data was also successfully described using statistical thermal models assuming a grand canonical ensemble approach.In peripheral collisions, the strangeness production is similar to that what observed in p$-$p collisions and was attributed to strangeness canonical suppression. The measurement by ALICE experiment might indicate towards the formation of mini-QGP like system in p$-$p collisions nature . Many alternative explanations based on increased and complex interactions among partons in the fragmentation phase of the Quantum Chromodynamics (QCD) based string hadronization models were put forward to explain some of the experimental observations. The description of these dynamic interactions among partons do not assume a formation of deconfined and thermalized plasma state. One of the mechanism in QCD based String fragmentation is rope hadronization which is quite successful in describing the enhancement bielrich1 ; dipsy . In this work, we study the strangeness enhancement observed in p$-$p collisions at $\sqrt{s}$ = 7 TeV and 13 TeV using rope hadronization mechanism of PYTHIA bielrich1 ; bielrich2 and compare with the ALICE measurementnature . The predictions of some observables like mean transverse momentum ($\langle p_{T}\rangle$) and mean integrated yields($\langle dN/dy\rangle$) of strange hadrons have also been studied to understand the dependence of strange particle production on beam energy.

II Rope Hadronization

In high energy p$-$p collisions, the particle production can be simplified into two broad steps namely initial hard scattering leading the production of partons and the subsequent hadronization of the initial parton configuration lund1 ; lund2 . The generation of partons and the partonic level activity involving multiple parton-parton interactions, the initial and final state radiations, and the activity of beam remnants can be approximately described by perturbative QCD while the fragmentation of the final parton configuration to observable hadrons is completely non-perturbative in nature. The understanding of the later part depends on statistical parametrization of experimental data, realistic modeling, parameter tuning etc. One of the fragmentation models namely The Lund string model has been extremely successful in describing the hadronization process which envisages stretched color flux tubes between two partons leading to linear confinement via massless relativistic string. As the potential energy in the string increases, the quark-antiquark pair move apart and the string breaks producing a new quark-antiquark pair. The hadrons are formed by combining these quarks. In high energy collisions, one can have many such overlapping strings in a small transverse area due to multi-parton interactions leading to description of underlying event activity within the framework of rope hadronization rope1 ; rope2 . Within this framework, the overlapping colored strings act coherently to form a color rope which subsequently hadronize with a higher effective string tension. The breakup of strings with higher string tension produce more strange quarks and di-quarks resulting in enhanced production of baryons and strange hadrons.

III PYTHIA Event generator and Analysis

PYTHIA is a standalone Monte-Carlo event generator for high energy e$-$e, p$-$p and $\mu-\mu$ collisions and has been extensively used to study LHC physics and compare its data. The routines are completely written in C++ (compared to its predecessor PYTHIA6 which was in Fortran) with some introduction of new physics and improvisations in the existing processes at both partonic and hadronic level. The details of the physics processes and its implementation can be found in reference pythia8 . In this present work, the Monash 2013 tune of Pythia8.2 monash based on larger and recent set of LHC data has been used to generate events for p$-$p collisions. The multi-parton interactions are enabled with QCD-based color reconnections where one obtains different quark junctions due to reconnections of hadronizing strings rope3 ; rope2 . The analysis was also checked with MPI based CR scheme where one obtains smaller string length due to fusion of color flow between partons belonging to different MPI system. The subsequent hadronization is studied within the framework of formation of color ropes. The results (only shown for QCD-based CR scheme) are also compared to the scenario where the mechanism of rope hadronization is not included.

The analysis is done by generating 50 Million inelastic non-diffractive events with soft QCD processes for p$-$p collisions at $\sqrt{s}$ = 7 TeV and 13 TeV with rope hadronization and without it. As the aim of this work is to see whether the strangeness enhancement as observed by ALICE experiment can be explained with the mechanism of rope hadronization, the multiplicity classes were determined by dividing the event sample into ten different event classes based on the total charged multiplicity within the acceptance of ALICE V0 detectors. The yield of strange ($K^{0}_{S}$ and $\Lambda+\overline{\Lambda}$) and multi-strange hadrons ($\Xi^{-}+{\overline{\Xi}}^{+}$ and $\Omega^{-}+{\overline{\Omega}}^{+}$) was obtained for $|y|<0.5$ for different multiplicity classes. The mean pseudorapidity density of charged particles, $\langle dN_{ch}/d\eta\rangle$ was also estimated for $|\eta|<0.5$. The multiplicity class and the corresponding $<dN_{ch}/d\eta>$ range as used by ALICE experiment for the strangeness enhancement study is shown in Table 1.

IV RESULTS AND COMPARISON TO DATA

The analysis was carried out with the generated events using PYTHIA generator and the estimates of various observables were studied and compared to the existing data to understand the mechanism of strange particle production in high multiplicity p$-$p collisions. The $p_{T}$ spectra of $K^{0}_{S}$, $\Lambda$, $\Xi^{-}$ and $\Omega^{-}$) for three different multiplicity classes are shown for p$-$p collisions at $\sqrt{s}=$ 7 TeV (left panel) and 13 TeV (right panel) with rope hadronization mechanism in Figure 11. One can observe that the $p_{T}$ spectra becomes harder with an increase in multiplicity as observed in data measured by ALICE experiment. It is also important to note that the hardening is conspicuous for higher mass particles. This feature is consistent with the onset of collectivity in Pb$-$Pb collisions. However, it is important to note that the hardening of the spectra as well the hardening becoming more for higher mass particles is well reproduced by rope hadronization mechanism where the formation of plasma or collectivity is not assumed. The rate of hardening for various multiplicity classes can be compared by estimating the mean transverse momentum ($<p_{T}>$) as a function of multiplicity. Figure 2 compares the $<p_{T}>$ of $K^{0}_{S}$ and $\Omega^{-}$ as measured by ALICE experiment with the predictions of Pythia8 with (and without ) rope hadronization. The estimations for $\Lambda$ and $\Xi$ is also shown. It is worth noting that the evolution of mean $<p_{T}>$ of strange hadrons with event multiplicity is well reproduced by rope hadronization for both 7 TeV and 13 TeV. The PYTHIA tune where there is no formation of color ropes do not describe the evolution satisfactorily and the discrepancy becomes more for strange hadrons with higher mass.

The enhanced production of strange hadrons is quantified by measuring the ratio of strange hadron yield with respect to the yield of pion (non-strange particle) in the same acceptance. Figure 3 compares the enhancement of strange and multi-strange hadrons as a function of multiplicity as measured by ALICE experiment to the values obtained with PYTHIA generator. As can be seen in the figure, the mechanism of rope hadronization (with color reconnections) in PYTHIA describes the observed experimental strangeness enhancement in p$-$p collisions at 7 TeV while the estimations without rope hadronization do not show the same. A similar result where the charged particle multiplicity was measured in the forward region with rope hadronization mechanism can be found here. The predicted enhancement for p$-$p collisions at 13 TeV and its comparison with 7 TeV is shown in the lower panel of figure 3. There is no significant difference in enhancement obtained for 13 TeV for similar multiplicity classes. In a previous study, it was shown that the yield ratios $\Lambda/K^{0}_{S}$ did not vary much as a function of multiplicity for p$-$p collisions at 7 TeV nature and the observation was not explained by PYTHIA 8 predictions. However, one observes that the variation of ratio, $\Lambda/K^{0}_{S}$ with multiplicity is described well when one incorporates the mechanism of rope formation as shown in Fig 4. The observation also indicated that the observed strangeness enhancement is not due to the hadronic mass or specie of the particle but can be attributed to enhanced production of strange particles due to possible formation of color ropes. The enhancement was further investigated by comparing the multiplicity dependence of the integrated yields of strange hadrons ($K^{0}_{S}$, $\Lambda$, $\Xi^{-}$ and $\Omega^{-}$) in p$-$p collisions at 7 TeV and 13 TeV . This PYTHIA8 predictions are compared with the available data in Fig. 5. One observes that the mechanism of rope hadronization essentially captures the evolution of enhancement with multiplicity and the estimates are in good agreement with the data. One can also conclude that the enhancement is driven by the event activity rather than the variation in centre of mass energies. It can be seen that the evolution of integrated yield with $<dN_{ch}/d\eta>$ for $K^{0}_{S}$ can be described by both PYTHIA tunes without the formation of ropes while they under predict the data for strange baryons. However, there is a good agreement with the data (within the experimental errors ) when one includes the rope hadronization scenario.

V Summary

The recent observation of enhanced production of strange and multi-strange hadrons in p$-$p collisions at $\sqrt{s}=$ 7 TeV and 13 TeV as measured by ALICE experiment is well described by the microscopic model of rope hadronization implemented in Pythia8. The $p_{T}$ spectra of the strange hadrons ($K^{0}_{S}$, $\Lambda+\bar{\Lambda}$ , $\Xi^{-}+\bar{\Xi^{+}}$ and $\Omega^{-}+\Omega^{+}$) for different multiplicity classes were observed to harden with an increase in multiplicity and the hardening was conspicuous for particles with higher mass. This was further investigated by studying the multiplicity dependence of $\langle p_{T}\rangle$ and $\langle dN/dy\rangle$ of the strange hadrons. The evolution of these observables was nicely described by the mechanism of rope hadronization while the one without this mechanism did not describe the data. The rope hadronization which do not assume the formation of any thermalized plasma state also described the strangeness enhancement (which is quantified by the ratio of strange hadrons to pions ) observed in data for p$-$p collisions at $\sqrt{s}$ = 7 TeV and 13 TeV. The observed strangeness enhancement seems to saturate at higher multiplicities for both the collision energy. It will be interesting to observe the application of rope hadronization mechanism to other particle production scenario involving higher mass particles such as heavy flavor hadrons and resonance particles.

VI Acknowledgements

The authors would like to thank Christian Bierlich for his valuable suggestions on rope hadronisation. Sadhana Dash would like to thank the Department of Science and Technology (DST), India for supporting the present work.