Mid Rapidity Research Articles

Quarkonia as probes of initial and final states in small systems with ALICE

The multiple parton−parton interactions (MPIs) are an important element to describe the observed collective flow and strangeness enhancement in high multiplicity pp and p−Pb collisions, the so-called small systems. At LHC energy, MPIs affect both soft and hard scales of the event. Since quarkonium production involves both, it can be an excellent tool to understand the role of MPIs in small systems. Study of multiplicity dependent quarkonium production provides an indirect probe of MPIs in hadronic collisions. Also, relative production of excited-to-ground quarkonium states as a function of multiplicity are sensitive to final state effects. Cross section measurements of different quarkonium states are also important to understand their production mechanisms. In these proceedings, first preliminary results of ψ(2S)-over-J/ψ cross section measurements at mid and forward rapidity (y) from Run 2 and Run 3 pp collisions at √S and 13.6 TeV, respectively will be presented. Preliminary results on ϒ(nS) states (n = 1,2) cross sections and final multiplicity dependent measurements of excited-to-ground states quarkonium yields, at forward y, in pp collisions at √S TeV will be shown. In addition, few performance plots from Run 3 in the quarkonium sector will be presented and discussed.

Isolated photon-hadron production in high energy pp and pA collisions at RHIC and LHC

We compute the isolated photon production in association with a charged hadron at mid rapidity in $pp$ and $pA$ based on the Color Glass Condensate (CGC) framework of high energy QCD where, for the first time, we incorporate the Sudakov effect of soft gluon emissions. Our results are based on the leading order $q g \to q \gamma$ channel in the CGC framework and confronted with the recent data from RHIC and LHC concerning the angular distributions and out-of-plane transverse momentum distributions. We find that, while the CGC computation alone results in too narrow distributions, with the help of the Sudakov effect, we can get a satisfactory description of the data. With this as a benchmark, we provide predictions for the magnitude of the nuclear effect brought by the phenomena of gluon saturation in the CGC.

Study of Strange Particle Production in Central Pb–Pb Collisions at

We study here the transverse momentum spectra of Ks 0 – mesons, A − hyperons and multi–strange (Ξ–, Ω–) particles in the most central Pb–Pb collisions at √SNN = 2.76 TeV for the mid rapidity interval |y| < 0.5 by using two different Monte Carlo simulation models: EPOS–1.99 and EPOS–LHC. The validity of simulation codes of these models is checked for Pb–Pb collisions at √SNN = 2.76 TeV by comparing the simulation data with ALICE experimental data for the same collisions at √SNN = 2.76 TeV. Strange particles ratio i.e. and nuclear modification factors for multi–strange particles have been constructed as function of pT to study these strange particle production and their energy loss mechanism. The simulation models show suppression for the multi–strange particles in the transverse momentum range of 3–7 GeV/c. The outcome agrees with a recent finding by ALICE collaboration where it suggested a possibility that the kinetic freeze–out conditions for strange particles are different from those for non–strange particles

Anisotropic flow fluctuations of charged and identified hadrons in Pb-Pb collisions with the ALICE detector

Anisotropic flow fluctuations can be used to constrain the initial conditions and probe the properties and evolution of the system created in heavy-ion collisions. In this proceeding, the pT-differential flow coefficient vn with 2-, 4-, 6- and 8-particle cumulants for charged hadrons are investigated, which allows to study the higher order moments, including skewness and kurtosis, of v2 probability density function (p.d.f.). In addition, the v2 fluctuations of π±, K±, p+p¯, ϕ, KS0, Λ+Λ¯ and Ξ−+Ξ¯+ are also measured at mid rapidity in Pb-Pb collisions at sNN=5.02TeV cumulants. The implications of the results for understanding the properties of the medium are discussed.

Recent quarkonium measurements in small systems with the ALICE detector at the LHC

ALICE has measured the quarkonium production in mid and forward rapidity in pp and p–Pb collisions. Recent nuclear modification factor measurements at mid-rapidity in p–Pb collisions at sNN=5.02TeV are shown for both prompt and non-prompt J/ψ mesons. The status of mid-rapidity J/ψ and forward rapidity ϒ analyses in p–Pb collisions at sNN=8.16TeV are also presented. Regarding pp collisions, recent J/ψ cross-section measurements at s=5and13TeV performed at central and forward rapidity, including the status of the analysis for the separation between prompt and non-prompt J/ψ components at mid-rapidity, are presented. Comparisons of the results in both pp and p–Pb collisions with the available theoretical model calculations are provided.

PHENIX results on polarization in p+p collisions.

One of the big questions in quarkonia production is how heavy-quark pairs are produced in the initial hard scattering and hadronize into final quarkonia states. This question also affects the estimation of quarkonia breakup cross-sections and color screening in heavy ion collisions. Many existing models describe well general features of quarkonia production like cross-section and transverse momentum distributions. In order to differentiate between various models one has to study quarkonia production in more details. One of such tools is a study of angular distributions of decay leptons, usually called polarization. The PHENIX experiment has measured inclusive polar and azimuthal angular decay coefficients in the mid and forward rapidity regions in p+p collisions at 200 GeV and 510 GeV. This talk will present the analysis strategy as well as the results in different polarization frames.

Dependence of Temperatures and Kinetic Freeze-Out Volume on Centrality in Au-Au and Pb-Pb Collisions at High Energy

Centrality-dependent double-differential transverse momentum spectra of negatively charged particles (π−, K−, and p¯) at the mid(pseudo)rapidity interval in nuclear collisions are analyzed by the standard distribution in terms of multicomponent. The experimental data measured in gold-gold (Au-Au) collisions by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC) and in lead-lead (Pb-Pb) collisions by the ALICE Collaboration at the Large Hadron Collider (LHC) are studied. The effective temperature, initial temperature, kinetic freeze-out temperature, transverse flow velocity, and kinetic freeze-out volume are extracted from the fitting to transverse momentum spectra. We observed that the mentioned five quantities increase with the increase of event centrality due to the fact that the average transverse momentum increases with the increase of event centrality. This renders that larger momentum (energy) transfer and further multiple scattering had happened in central centrality.

THE ONSET OF JET QUENCHING PHENOMENON

The aim of this study is to set a baseline for the jet quenching measurements of the Quark Gluon Plasma (QGP) formed in the large system size Nucleus-Nucleus (A-A) at top central collisions, via studying simulated small system size, Nucleon-Nucleon (N-N) collisions. The proton-proton (p-p) collisions were simulated using PYTHIA, at center of mass energies $\sqrt{s_{_{NN}}} = 200$ $GeV$ and $\sqrt{s_{_{NN}}} = 13$ $TeV$ corresponding to the available energies at the current collider experiments; the Relativistic Heavy Ion Collider (RHIC), and the Large Hadron Collider (LHC). At both energies, the two-particle azimuthal correlation functions have been considered, and the yield associated with the high transverse momentum ($p_{_{T}}$) particles were extracted at its near-side ($\Delta\phi \approx 0$) and away-side ($\Delta\phi \approx \pi$) at mid pseudo rapidity ($|\eta| \le 2$). The ratio between the near-side yields in the high multiplicity events to these of the low multiplicity events ($I_{_{HL}}^{^{N}}$), as well as, the ratio of the away-side yields ($I_{_{HL}}^{^{A}}$) were calculated at both energies as a function of the hadron fractional energy $z_{_{T}}$ of the high-$p_{_{T}}$ particle. At both energies, the values of $I_{_{HL}}^{^{N}}$ and $I_{_{HL}}^{^{A}}$ were less than unity, and of trivial dependence on $z_{_{T}}$. The values of $I_{_{HL}}^{^{A}}$ are always less than these of $I_{_{HL}}^{^{N}}$ at the same multiplicity and energy, and both quantities show a pattern of systematic decreases with the multiplicity. Such multiplicity dependence cannot be used neither to exclude the jet quenching nor to prove it in the high multiplicity events in p-p collisions, as the suppressions have been found at both sides, near and away of the high-$p_{_{T}}$ particle.

Onset of the Jet Quenching Phenomenon

The aim of this study is to set a baseline for the jet quenching measurements of the Quark Gluon Plasma (QGP) formed in the large system size Nucleus-Nucleus (A-A) at top central collisions, via studying simulated small system size, Nucleon-Nucleon (N-N) collisions. The proton-proton (p-p) collisions were simulated using PYTHIA, at center of mass energies $\sqrt{s_{_{NN}}} = 200$ $GeV$ and $\sqrt{s_{_{NN}}} = 13$ $TeV$ corresponding to the available energies at the current collider experiments; the Relativistic Heavy Ion Collider (RHIC), and the Large Hadron Collider (LHC). At both energies, the two-particle azimuthal correlation functions have been considered, and the yield associated with the high transverse momentum ($p_{_{T}}$) particles were extracted at its near-side ($\Delta\phi \approx 0$) and away-side ($\Delta\phi \approx \pi$) at mid pseudo rapidity ($|\eta| \le 2$). The ratio between the near-side yields in the high multiplicity events to these of the low multiplicity events ($I_{_{HL}}^{^{N}}$), as well as, the ratio of the away-side yields ($I_{_{HL}}^{^{A}}$) were calculated at both energies as a function of the hadron fractional energy $z_{_{T}}$ of the high-$p_{_{T}}$ particle. At both energies, the values of $I_{_{HL}}^{^{N}}$ and $I_{_{HL}}^{^{A}}$ were less than unity, and of trivial dependence on $z_{_{T}}$. The values of $I_{_{HL}}^{^{A}}$ are always less than these of $I_{_{HL}}^{^{N}}$ at the same multiplicity and energy, and both quantities show a pattern of systematic decreases with the multiplicity. Such multiplicity dependence cannot be used neither to exclude the jet quenching nor to prove it in the high multiplicity events in p-p collisions, as the suppressions have been found at both sides, near and away of the high-$p_{_{T}}$ particle.

Hadron production models\u2019 prediction for pT distribution of charged hadrons in pp interactions at 7 TeV

The yield of transverse momentum (pT) spectra of integrated hadrons and their ratios produced in pp collisions at 7 TeV are reported using DPMJET-III, EPOS1.99, EPOS-LHC, HIJING1.383, QGSJETII-04, and Sibyll2.3c models. The models’ predictions are compared with the ALICE measurements obtained at mid rapidity and in the pT range from 0.3–6 GeV/c, 0.2–6 GeV/c, and 0.1–3 GeV/c for protons, kaons and pions respectively. Compared to the experimental data, the EPOS1.99 and EPOS-LHC models predict the yield of pion very well while reproducing yields of kaon and proton at low pT only. The DPMJET-III model predicts the yield of pions, kaons and protons only at intermediate value of pT. The HIJING and Sibyll models describe the integrated yield of pion well at high pT only. The QGSJETII model predicts the yield of kaon for almost the whole pT range. EPOS1.99 and Sibyll models reproduce excellent prediction of the K/π ratio over the entire range of pT whereas the EPOS-LHC, HIJING and DPMJET models underpredict with increasing order of discrepancy respectively. The EPOS1.99, EPOS-LHC, and Sibyll models predict the p/π ratio for low values of pT, but overpredict at high values, while the DPMJET-III and HIJING models underpredict over the entire range of pT. Although the models’ predictions are mostly consistent with the ALICE measurements but none of the models completely describe the entire distribution for the integrated yield of hadron and their ratios.

J/ψ -meson production within improved color evaporation model with the kT -factorization approach for cc¯ production

We use a new approach to color evaporation model (CEM) for quarkonium production. The production of $c\bar c$ pairs is performed within $k_T$-factorization approach using different unintegrated gluon distribution functions (UGDF) from the literature. We include all recent improvements to color evaporation model. We cannot describe simultaneously mid and forward rapidity data measured at the LHC when using the KMR UGDF with the same normalization parameter. Furthermore we get somewhat too hard distribution in $J/\psi$ transverse momentum. Correcting the standard KMR distributions for saturation effects at small values of $x$ improves $J/\psi$ rapidity distributions, while taking correction for emissions hidden in the KMR UGDF in the evaluation of $x$ values improves a bit $J/\psi$ transverse momentum distributions. The modifications improve also description of the LHCb $D$-meson data.

Quarkonium production in p-Pb collisions with ALICE

ALICE has measured quarkonium production in p-Pb collisions at backward (−4.46 <ycms < −2.96), mid (−1.37 <ycms < 0.43) and forward (2.03 <ycms < 3.53) rapidity (y) regions down to zero transverse momentum (pT). The inclusive J/ψ production has been studied at mid-y in p-Pb interactions at sNN=5.02TeV and at forward and backward y in p-Pb collisions at sNN=5.02TeV and8.16TeV. The comparison of the J/ψ production to the one of the loosely bound ψ(2S) state is discussed, together with new results on the nuclear modification factors of the ϒ(1S) and ϒ(2S) states measured at forward and backward y. All the results will be compared to those obtained at lower energies and with available theoretical calculations.

Leading cluster approach to simulations of hadron collisions with GHOST generator

The model HDPM of CORSIKA has been updated and developed on the base of the recent measurements by ALICE, CMS, TOTEM, LHCb, LHCf... The new model, GHOST, involving a four-source production reproduces correctly the pseudo-rapidity distributions of charged secondaries and has been tested with the data in the mid and forward rapidity region, especially in the complex case of TOTEM, and also with the recent measurements of CMS, up to $ \sqrt s = 13\,{\rm{TeV}} $ (9.0 1016 eV in laboratory system). Special calculations have been devoted to the semi-inclusive data playing an important role in the cosmic ray simulation (fluctuations in earliest collisions, individual cascades measured at high altitude with high energy emulsion chambers). Taking into account the violation of KNO scaling, the negative binomial distribution (NegBin-expressed in terms of scaled elements) $ z = {n \mathord{\left/ {\vphantom {n {\bar {n}}}} \right. \kern-\nulldelimiterspace} {\bar {n}}} $ (n is the number of charged secondaries) has been used pointing out a possible asymptotic behaviour of total charged multiplicities at primary energies exceeding 40 TeV (8.5 1017 eV). Thus, larger reduction of the energies devoted to the leading cluster and very large multiplicity of secondary particles could suggest for EAS generated by primary protons a larger production of muons and a shower maximum at higher altitude.

Double and triple inclusive gluon production at mid rapidity: quantum interference in p-A scattering

We compute double and triple inclusive gluon production in p-A scattering beyond the so-called “glasma graph” approximation. We consider quantum interference effects and identify in this general setup the terms responsible for the gluon HBT and initial wave function Bose enhancement which lead to correlations in particle production. Both of these terms originate from the factorizable part of the quadrupole and sextupole terms in the production cross section. We also show that the target Bose enhancement in this regime is suppressed at large number of colors.

Neutral meson production measurements with the ALICE at the LHC

Identified hadron spectra are considered to be sensitive to the transport properties of strongly interacting matter produced in high-energy nucleus-nucleus collisions. π0 and η mesons in ALICE are identified via their two-photon decays by using calorimeters and the central tracking system. In the latter, photons are measured via their conversion to electron-positron pairs in the material of the inner ALICE barrel tracking detectors. The measured production spectra in pp, p–Pb and Pb–Pb collisions at mid–rapidity and over a wide pT range will be presented in the available Large Hadron Collider (LHC) energies of Run I. The resulting nuclear modification factor RAA at different centrality classes shows a clear pattern of strong suppression in the hot QCD medium with respect to pp collisions. Comparison of the ALICE results on neutral mesons with lower-energy experiments is also discussed.

Multiple production up to √s = 13 TeV with the generator GHOST adapted to cosmic ray simulation

The model HDPM of CORSIKA has been updated and developed on the base of the recent measurements by LHCb, CMS, TOTEM... The new model GHOST involving a 4-source production reproduces correctly the pseudorapidity distributions of charged secondaries and has been testified with the data in the mid and forward rapidity region, especially in the complex case of TOTEM and also with the recent measurements of CMS up to FX2 (9.2 · 1016 eV). Special calculations have been devoted to the semi-inclusive data playing an important role in the cosmic ray simulation (fluctuations in the earliest collisions, individual cascades and remarkable events). Taking into account the violation of KNO scaling, the negative binomial distribution has been used pointing out an asymptotic behaviour of total charged multiplicities at primary energies exceeding 40 TeV (7 · 1017 eV). The interpretation of the most recent measurements suggests for EAS generated by primary protons a larger production of muons and a cascade maximum at higher altitude than previously assumed.

Multiple production up to √s = 13 TeV with the generator GHOST adapted to cosmic ray simulation

The model HDPM of CORSIKA has been updated and developed on the base of the recent measurements by LHCb, CMS, TOTEM... The new model GHOST involving a 4-source production reproduces correctly the pseudorapidity distributions of charged secondaries and has been testified with the data in the mid and forward rapidity region, especially in the complex case of TOTEM and also with the recent measurements of CMS up to FX2 (9.2 · 10 16 eV). Special calculations have been devoted to the semi-inclusive data playing an important role in the cosmic ray simulation (fluctuations in the earliest collisions, individual cascades and remarkable events). Taking into account the violation of KNO scaling, the negative binomial distribution has been used pointing out an asymptotic behaviour of total charged multiplicities at primary energies exceeding 40 TeV (7 · 10 17 eV). The interpretation of the most recent measurements suggests for EAS generated by primary protons a larger production of muons and a cascade maximum at higher altitude than previously assumed.

Latest results of charged hadron flow measurements in CuAu collisions at RHIC-PHENIX

Measurements of azimuthal anisotropic flow vn for inclusive charged hadrons and identified particles at mid rapidity in Cu+Au collisions at √sNN = 200GeV are presented. The data were recorded by the PHENIX experiment at Relativistic Heavy Ion Collider(RHIC). Directed, elliptic and triangular flow as a function of transverse momentum pT are measured with respect to event planes. The inclusive charged hadron vi shows the negative value at high pT. The v2 and v3 are compared to those in Au+Au and Cu+Cu collisions. We find the v2 and v3 follow an empirical scaling with 1/(ϵnN1/3part). We also compare the v2 and v3 to hydrodynamical predictions. The identified particles v2 and v3 show a mass ordering in low pT region and baryon and meson splitting in high pT region. However the identified hadron v1 only shows mass ordering in mid pT region.

Measurements of strange hadrons KS0, Λ and Ξ from Au+Au collisions at √SNN = 14.5 GeV in STAR

We report the measurements of the production of strange hadrons KS0, Λ and Ξ at mid rapidity in Au+Au collisions at √SNN = 14.5 GeV from the Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). The collision energy dependence of strange hadron yields are also presented. To understand the recombination and part.on energy loss mechanisms, nuclear modification factors and baryon to meson ratios are measured and also compared with the statistical and thermal model.

Open heavy-flavor measurements with ALICE at the LHC

ALICE is well equipped to reconstruct heavy-flavor particles down to low transverse momentum pT at mid and forward rapidity. An overview of the ALICE results obtained with the Run 1 data in pp (√5=2.76TeV and √s=7TeV), Pb-Pb (√Snn=2.76TeV) and p-Pb (√sNN=5.02 TeV) collisions is presented. In pp collisions the measured cross sections are well described by pQCD calculations. The charged-particle multiplicity dependence of heavy-flavor yields indicates that Multi-Parton Interactions contribute to the high-multiplicity pp collisions and affect charm and beauty production in a similar way. In p-Pb and Pb-Pb collisions the measured nuclear modification factors indicate a final-state energy loss of heavy- quarks in central Pb-Pb collisions. Furthermore, the observed positive heavy-flavor elliptic flow in semi-central Pb-Pb collisions gives a hint that charm quarks participate in the collective expansion of the medium at low pT. In high-multiplicity p-Pb collisions, a double-ridge structure is observed in the heavy-flavor decay electron-hadron azimuthal correlations at low pT similar to what is measured in the light-flavor sector. Such long-range correlations in ŋ could originate from a collective expansion of the system, as well as from gluon saturation in the initial state (color-glass condensate) or other mechanisms.