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Register a new userEnergy independent scaling of ridge and final state description of high multiplicity p+p collisions at $sqrt{s}$ = 7 and 13 TeV
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Debojit Sarkar
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An energy independent scaling of the near-side ridge yield at a given multiplicity has been observed by the ATLAS and the CMS collaborations in p+p collisions at s = 7 and 13 TeV. Such a striking feature of the data can be successfully explained by approaches based on initial state momentum space correlation generated due to gluon saturation. In this paper, we try to examine if such a scaling is also an inherent feature of the approaches that employ strong final state interaction in p+p collisions. We find that hydrodynamical modeling of p+p collisions using EPOS 3 shows a violation of such scaling. The current study can, therefore, provide important new insights on the origin of long range azimuthal correlations in high multiplicity p+p collisions at the LHC energies.
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In inelastic $p+p$ collisions, the interacting objects are quarks and gluons (partons). It is believed that there are multiple interactions between the partons in a single $p+p$ event. Recent studies of multiplicity dependence of particle production in $p+p$ collisions have gathered considerable interest in the scientific community. According to several theoretical calculations, multiple gluon participation in hadronic collisions is the cause of high-multiplicity events. If the interaction is hard enough (large $p_{rm T}$ transfer), the semi-hard processes of multiple interactions of partons might also lead to production of heavy particles like J/$psi$. At the LHC, an approximately linear increase of the relative J/$psi$ yield with charged particle multiplicity is observed in $p+p$ collisions. In the present work, we have studied the contribution of quarks and gluons to the multiplicity dependence of J/$psi$ production using pQCD inspired event generator, PYTHIA8 tune 4C, in $p+p$ collisions at $sqrt{s} =$13 TeV by investigating relative J/$psi$ yield and relative $langle p_{rm T} rangle$ of J/$psi$ as a function of charged particle multiplicity for different hard-QCD processes. We have estimated a newly defined ratio, $r_{pp} = {langle p_{rm T}^{2} rangle}_{i}/{langle p_{rm T}^{2} rangle}_{rm MB}$, to understand J/$psi$ production in high-multiplicity $p+p$ collisions. For the first time we attempt to study the nuclear modification factor like observables ($R_{rm pp}$ and $R_{rm cp}$) to understand the QCD medium formed in high-multiplicity $p+p$ collisions.
An analysis is made of the particle composition in the final state of $pp$ collisions at 7 TeV as a function of the charged particle multiplicity ($dN_{ch}/deta$). The thermal model is used to determine the chemical freeze-out temperature as well as the radius and strangeness suppression factor $gamma_s$. Three different ensembles are used in the analysis. The grand canonical ensemble, the canonical ensemble with exact strangeness conservation and the canonical ensemble with exact baryon number, strangeness and electric charge conservation. It is shown that for the highest multiplicity class the three ensembles lead to the same result. This allows us to conclude that this multiplicity class is close to the thermodynamic limit. It is estimated that the final state in $pp$ collisions could reach the thermodynamic limit when $dN_{ch}/deta$ is larger than twenty per unit of rapidity, corresponding to about 300 particles in the final state when integrated over the full rapidity interval.
We have used the dynamically constrained phase space coalescence model to study the production of the exotic state $X(3872)$ based on the hadronic final states generated by the parton and hadron cascade model (PACIAE) with $|y| < 7.5$ and $p_T < 15.5$ GeV/c in $pp$ collisions at $sqrt{s}=7$ and 13 TeV, respectively. Here the $X(3872)$ is assumed to consist of bound state $Dbar {D^*}$, which can form three possible structures for the tetraquark state, the nucleus-like state, and the molecular state. The yields of three different structures $X(3872)$ were predicted. The transverse momentum distribution and the rapidity distribution of three different structures $X(3872)$ are also presented. Sizable difference can be found in the transverse momentum and rapidity distributions for the three different $X(3872)$ structures.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section of $phi$(1020)-meson production at forward rapidity in $p$$+$$p$ collisions at $sqrt{s}=$510 GeV via the dimuon decay channel. The partial cross section in the rapidity and $p_T$ ranges $1.2<|y|<2.2$ and $2<p_T<7$ GeV/$c$ is $sigma_phi=[2.28 pm 0.09,{rm (stat)} pm 0.14,{rm (syst)} pm 0.27, {rm (norm)}] times 10^{-2}$~mb. The energy dependence of $sigma_phi$ ($1.2<|y|<2.2, ; 2<p_T<5$ GeV/$c$) is studied using the PHENIX measurements at $sqrt{s}=$200 and 510 GeV and the Large-Hadron-Collider measurements at $sqrt{s}=$2.76 and 7 TeV. The experimental results are compared to various event generator predictions ({sc pythia6, pythia8, phojet, ampt, epos3,} and {sc epos-lhc}).
The ALICE data on light flavor hadron production obtained in $p-Pb$ collisions at $sqrt{s_{NN}} $ = 5.02 TeV are studied in the thermal model using the canonical approach with exact strangeness conservation. The chemical freeze-out temperature is independent of centrality except for the lowest multiplicity bin, with values close to 160 MeV but consistent with those obtained in $Pb-Pb$ collisions at $sqrt{s_{NN}}$ = 2.76 TeV. The value of the strangeness non-equilibrium factor $gamma_s$ is slowly increasing with multiplicity from 0.9 to 0.96, i.e. it is always very close to full chemical equilibrium.
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