Minimum Bias Collisions Research Articles

Features of hadronic and deconfined matter in a range of collision energies spanning from the BNL Alternating Gradient Synchrotron to the CERN Large Hadron Collider

Previous extensive studies on the dependence of the average transverse momentum, its slope as a function of the hadron mass, and the average transverse expansion on the particle multiplicity per unit rapidity and unit transverse overlap area of the colliding partners are extended to the ratio of the energy density to the entropy density. The behavior of the ratio between the average transverse momentum and the square root of the particle multiplicity per unit rapidity and unit transverse overlap area $\ensuremath{\langle}{p}_{T}\ensuremath{\rangle}/\sqrt{\ensuremath{\langle}dN/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}}$ as a function of collision energy for a given centrality or as a function of centrality for a given collision energy supports the predictions of color glass condensate and percolation based approaches. The dependence of the ratio of the energy density $\ensuremath{\langle}d{E}_{T}/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}$ to the entropy density $\ensuremath{\langle}dN/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}$ at different collision centralities for $A\text{\ensuremath{-}}A$ collisions from the Alternating Gradient Synchrotron, the Super Proton Synchrotron, the BNL Relativistic Heavy Ion Collider, and the CERN Large Hadron Collider energies is presented. The trend of this ratio towards a plateau at the highest RHIC energies followed by a steep rise at LHC energies is in agreement with theoretical predictions made 40 years ago that indicate this behavior as a signature of a phase transition. This pattern strongly depends on the collision geometry, converging towards the dependence that characterizes the $pp$ minimum bias collisions for the most peripheral $A\text{\ensuremath{-}}A$ collisions. Expected similarities between $pp$ and Pb-Pb collisions at LHC energies are confirmed.

Projections of two-particle correlations onto transverse rapidity in Au+Au collisions at sNN=200 GeV at STAR

Two-particle correlation measurements projected onto two-dimensional, transverse rapidity coordinates (yT1,yT2) provide an independent, orthogonal view of the multidimensional correlation distribution that is most often studied via angular projections. As such, these independent transverse projections allow access to manifestations of dynamical fluctuations in relativistic heavy-ion collisions that angular-correlation measurements may not be sensitive to. We report nonidentified charged-particle correlations for Au+Au minimum-bias collisions at sNN=200 GeV taken by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Correlations are presented as two-dimensional functions of transverse rapidity for like-sign, unlike-sign, and all charged-particle pairs, as well as for particle pairs whose relative azimuthal angles lie on the near-side, the away-side, or at all relative azimuth. The correlations are constructed using charged particles with transverse momentum pT≥0.15 GeV/c, pseudorapidity from −1 to 1, and azimuthal angles from −π to π. The significant correlation structures that are observed evolve smoothly with collision centrality. The major correlation features include a saddle shape plus a broad peak with maximum near yT≈3, corresponding to pT≈1.5 GeV/c. The broad peak is observed in both like- and unlike-sign charge combinations and in near- and away-side relative azimuthal angles. The all-charge, all-azimuth correlation measurements are compared with the predictions of hijing and epos to provide theoretical context for these new measurements. The results indicate that the correlations for peripheral to mid-central collisions can be approximately described as a superposition of nucleon + nucleon collisions with minimal effects from the quantum chromodynamics medium. Strong medium effects are indicated in mid- to most-central collisions.1 MoreReceived 22 April 2022Accepted 28 September 2022DOI:https://doi.org/10.1103/PhysRevC.106.044906©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle correlations & fluctuationsRelativistic heavy-ion collisionsNuclear Physics

Azimuthal anisotropy measurements of strange and multistrange hadrons in U+U collisions at sNN=193 GeV at the BNL Relativistic Heavy Ion Collider

We present systematic measurements of azimuthal anisotropy for strange and multistrange hadrons (Ks0, Λ, Ξ, and Ω) and ϕ mesons at midrapidity (|y|< 1.0) in collisions of U+U nuclei at sNN=193 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. Transverse momentum (pT) dependence of flow coefficients (v2, v3, and v4) is presented for minimum bias collisions and three different centrality intervals. Number of constituent quark scaling of the measured flow coefficients in U+U collisions is discussed. We also present the ratio of vn scaled by the participant eccentricity (ɛn2) to explore system size dependence and collectivity in U+U collisions. The magnitude of v2/ɛ2 is found to be smaller in U+U collisions than that in central Au+Au collisions contradicting naive eccentricity scaling. Furthermore, the ratios between various flow harmonics (v3/v23/2, v4/v24/2) are studied and compared with hydrodynamic and transport model calculations.9 MoreReceived 18 March 2021Accepted 24 May 2021DOI:https://doi.org/10.1103/PhysRevC.103.064907©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective flowRelativistic heavy-ion collisionsNuclear Physics

J/ψ production dynamics: event shape, multiplicity and rapidity dependence in proton+proton collisions at LHC energies using PYTHIA8

High-multiplicity pp collisions at the Large Hadron Collider (LHC) energies have created special importance in view of the underlying event (UE) observables. The recent results of the LHC, such as long range angular correlation, flow-like patterns, strangeness enhancement etc. in high-multiplicity events are not yet completely understood. In the same direction, the understanding of multiplicity dependence of J/ψ production is highly necessary. Transverse spherocity, which is an event shape variable, helps to investigate the particle production by isolating the hard and the soft components. In the present study, we have investigated the multiplicity dependence of J/ψ production at mid-rapidity and forward rapidity through the transverse spherocity analysis and tried to understand the role of jets by separating the isotropic and jetty events from the minimum bias collisions. We have analyzed the J/ψ production at the mid-rapidity and forward rapidities via dielectron and dimuon channels, respectively, using a 4C tuned PYTHIA8 event generator. The analysis has been performed in two different center-of-mass energies: and 13 TeV, to see the energy dependence of jet contribution to the multiplicity dependence study of J/ψ production. Furthermore, we have studied the production dynamics through the dependence of thermodynamic parameters on event multiplicity and transverse spherocity.

QCD Soft and Forward Physics at CMS

We present recent results on minimum bias collisions, underlying event activity and double parton scattering using data recorded by the CMS detector at the LHC. The results on the measurement of the underlying event (UE) using leading tracks, jets, and Drell-Yan processes are presented. Double parton scattering is investigated in several final states including vector bosons, and the results are compared to other experiments and to multi parton interaction (MPI) models tuned to recent underlying event measurements at CMS.

PHENIX results on open heavy flavor production

PHENIX measures the open heavy flavor productions in p + p, Cu+Au, and Au+Au collisions at [see formula in PDF] = 200 and 510 GeV using the silicon tracking detectors for mid- and forward rapidities. In Au+Au collisions, the nuclear modification of single electrons from bottom and charm hadron decays are measured for minimum bias and most central collisions. It is found that bottoms are less suppressed than charms in pT=3-5 GeV/c and charms in most central collisions are more suppressed than that in minimum bias collisions. In p + p and Cu+Au collisions, J/ψ from B meson decays are measured at forward and backward rapidities. The nuclear modification of B mesons in Cu+Au collisions is consistent with unity.

Charged-particle multiplicity and transverse-energy distribution using the Weibull-Glauber approach in heavy-ion collisions

The charged-particle multiplicity distribution and the transverse-energy distribution measured in heavy-ion collisions at top RHIC and LHC energies are described using the two-component model approach based on a convolution of the Monte Carlo Glauber model with the Weibull model for particle production. The model successfully describes the multiplicity and transverse-energy distribution of minimum-bias collision data for a wide range of energies. The Weibull-Glauber model can be used to determine the centrality classes in heavy-ion collisions as an alternative to the conventional negative binomial distribution (NBD)-Glauber approach.

Centrality dependence of D0 elliptic and triangular flow in Au+Au collisions at [formula omitted] at STAR

Due to their large masses, heavy quarks are predominantly produced through initial hard scatterings in heavy-ion collisions and, as such, they experience the entire evolution of the hot and dense medium created in those collisions. They can therefore provide important insights into the properties of the strongly-coupled Quark-Gluon Plasma. For instance, the measurement of the elliptic flow (v2) of charm quarks can provide information on the degree of thermalization for charm quarks in the medium and thus the bulk properties of the system. Furthermore, several models have predicted that fluctuations in the initial conditions, together with strong charm-medium interactions, could lead to a finite triangular flow (v3) for charm quarks, providing another handle to study the dynamics at the early stage of heavy-ion collisions. In these proceedings, we present the measurements of the v2 and v3 of D0 mesons in Au+Au collisions at sNN=200GeV with the Heavy Flavor Tracker at STAR. Compared to previously reported D0v2 in minimum-bias collisions, the precision of the new results is improved by a factor of 2–4 depending on D0 transverse momentum (pT). The D0v3 as a function of pT is also reported for the first time. The results are compared with the measurements of other particle species and a series of model calculations.

Correlated wounded hot spots in proton-proton interactions

We investigate the effect of non-trivial spatial correlations between proton constituents, considered in this work to be gluonic hot spots, on the initial conditions of proton-proton collisions from ISR to LHC energies, i.e. $\sqrt s\!=\!52.6,7000,13000$ GeV. The inclusion of these correlations is motivated by their fundamental role in the description of a recently observed new feature of $pp$ scattering at $\sqrt s\!=\!7$ TeV, the hollowness effect. Our analysis relies on a Monte-Carlo Glauber approach including fluctuations in the hot spot positions and their entropy deposition in the transverse plane. We explore both the energy dependence and the effect of spatial correlations on the number of wounded hot spots, their spatial distribution and the eccentricities, $\varepsilon_n$, of the initial state geometry of the collision. In minimum bias collisions we find that the inclusion of short range repulsive correlations between the hot spots reduces the value of the eccentricity ($\varepsilon_2$) and the triangularity ($\varepsilon_3$). In turn, upon considering only the events with the highest entropy deposition i.e. the ultra-central ones, the probability of having larger $\varepsilon_{2,3}$ increases significantly in the correlated scenario. Finally, the eccentricities show a quite mild energy dependence.

Study of the bulk properties of the system formed in Au + Au collisions at [formula omitted] using the STAR detector at RHIC

We present the first measurements of the transverse momentum spectra and azimuthal anisotropy of the identified hadrons (π+,π−,K+,K−,p(p‾),Ks0,Λ(Λ‾) and ϕ) at mid-rapidity in Au+Au collisions at sNN=14.5 GeV for various collision centralities. These measurements are compared to corresponding results from other BES energies. The bulk properties of the system, like the chemical and kinetic freeze-out conditions and the collectivity extracted from the measured yields of the produced particles are presented. The difference between baryon and anti-baryon elliptic flow for minimum bias collisions previously reported by STAR is also observed in the new data taken at sNN=14.5 GeV. Furthermore, the new data at 14.5 GeV are consistent with the trend established by the results at lower and higher beam energies. The energy and centrality dependence of the baryon chemical potential (μB), radial flow velocity (〈β〉), and chemical and kinetic freeze-out temperatures is discussed systematically.

Centrality and pseudorapidity dependence of the transverse energy flow in pPb collisions at [formula omitted

The almost hermetic coverage of CMS is used to measure the distribution of transverse energy as a function of pseudo-rapidity for pPb collisions at sNN=5.02 TeV. For minimum bias collisions (1/N)dET/dη reaches 23 GeV which implies an ET per participant pair comparable to that of peripheral PbPb collisions at sNN=2.76 TeV. The centrality dependence of transverse energy production has been studied using centrality measures defined in three different angular regions. There is a strong auto-correlation between (1/N)dET/dη and the η range used to define centrality for data and the EPOS-LHC and HIJING event generators. The centrality dependence of the data is much stronger for η values on the lead side than the proton side and shows significant differences from that predicted by either event generator.

Centrality dependence of identified particle elliptic flow in relativistic heavy ion collisions atsNN=7.7–62.4GeV

Elliptic flow (v_2) values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at sqrt{s_{NN}}= 7.7--62.4 GeV are presented for three centrality classes. The centrality dependence and the data at sqrt{s_{NN}}= 14.5 GeV are new. Except at the lowest beam energies we observe a similar relative v_2 baryon-meson splitting for all centrality classes which is in agreement within 15% with the number-of-constituent quark scaling. The larger v_2 for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with A Multiphase Transport Model and fit with a Blast Wave model.

Dielectron production in Au + Au collisions atsNN=200GeV

We present measurements of e+e- production at midrapidity in Au+Au collisions at sNN=200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass (mee<5 GeV/c2) and pair transverse momentum (pT<5 GeV/c) for minimum bias and for five centrality classes. The e+e- yield is compared to the expectations from known sources. In the low-mass region (mee=0.30-0.76 GeV/c2) there is an enhancement that increases with centrality and is distributed over the entire pair pT range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to 2.3±0.4(stat)±0.4(syst)±0.2(model) or to 1.7±0.3(stat)±0.3(syst)±0.2(model) for minimum bias collisions when the open heavy-flavor contribution is calculated with pythia or mc@nlo, respectively. The inclusive mass and pT distributions, as well as the centrality dependence, are well reproduced by model calculations where the enhancement mainly originates from the melting of the ρ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region (mee=1.2-2.8 GeV/c2), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.

Measurements of dielectron production in Au + Au collisions atsNN=200GeV from the STAR experiment

We report on measurements of dielectron ($e^+e^-$) production in Au$+$Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at RHIC. Systematic measurements of the dielectron yield as a function of transverse momentum ($p_{\rm T}$) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region ($M_{ee}<$ 1 GeV/$c^2$). This enhancement cannot be reproduced by the $\rho$-meson vacuum spectral function. In minimum-bias collisions, in the invariant-mass range of 0.30 $-$ 0.76 GeV/$c^2$, integrated over the full $p_{\rm T}$ acceptance, the enhancement factor is 1.76 $\pm$ 0.06 (stat.) $\pm$ 0.26 (sys.) $\pm$ 0.29 (cocktail). The enhancement factor exhibits weak centrality and $p_{\rm T}$ dependence in STAR's accessible kinematic regions, while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 $\pm$ 0.10. Models that assume an in-medium broadening of the $\rho$ meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of $\omega$ and $\phi$-meson production through their $e^+e^-$ decay channel. These measurements show good agreement with Tsallis Blast-Wave model predictions as well as, in the case of the $\phi$-meson, results through its $K^+K^-$ decay channel. In the intermediate invariant-mass region (1.1$<M_{ee}<$ 3 GeV/$c^2$), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed.

Systematic study of the elliptic flow parameter using a transport approach

Elliptic flow parameter, v2 is considered as a sensitive probe for early dynamics of the heavy-ion collision. In this work, we have discussed the effect of detector efficiency, procedure of centrality determination, effect of resonance decay and the procedure to correct event plane resolution on the measured v2 by standard event plane method within the framework of a transport model. The measured value of v2 depends on the efficiency in particle number counting by the detector. The effect of centrality determination is found to be negligible. The new method of event plane resolution correction for wide centrality bin yields results in v2 values closer to the true value of the v2. The contributions from the resonance decay seems to decrease the value of v2 of π and K within the Ultra relativistic Quantum Molecular Dynamics (UrQMD) model based calculation. We also propose a procedure to correct for an event bias effect on v2 arising while comparing the minimum bias collision centrality v2 values for different multi-strange hadrons. Finally, we have presented a model based confirmation of the recently proposed relation between v2 obtained using event plane method and scalar product method to the true value of v2.

Blast wave fits to elliptic flow data atsNN=7.7–2760GeV

We present blast wave fits to elliptic flow [${v}_{2}({p}_{\mathrm{T}})$] data in minimum bias collisions from $\sqrt{{s}_{\mathit{\text{NN}}}}=7.7--200$ GeV at the BNL Relativistic Heavy Ion Collider, and also at the CERN Large Hadron Collider energy of 2.76 TeV. The fits are performed separately for particles and corresponding antiparticles. The mean transverse velocity parameter $\ensuremath{\beta}$ shows an energy-dependent difference between particles and corresponding antiparticles, which increases as the beam energy decreases. Possible effects of feed down, baryon stopping, antiparticle absorption, and early production times for antiparticles are discussed.

Open charm production in p + p and Pb + Pb collisions at the CERN Large Hadron Collider

Effects of strong longitudinal color electric fields, shadowing, and quenching on the production of prompt open charm mesons (D0, D+, D, D) in central Pb + Pb collisions at = 2.76 TeV are investigated within the framework of the HIJING/B v2.0 model. We compute the nuclear modification factor RPbPbD, and show that the above nuclear effects constitute important dynamical mechanisms in the description of experimental data. The strength of color fields (as characterized by the string tension κ), partonic energy loss and jet quenching process lead to a suppression factor consistent with recent published data. Predictions for beauty mesons are presented. In addition, ratios of strange to non-strange prompt charm mesons in central Pb + Pb and minimum bias (MB) collisions at 2.76 TeV are also discussed. MB collisions which constitute a theoretical baseline in our calculations are studied at centre of mass energies = 2.76 TeV and 7 TeV.

Hot medium effects on [formula omitted] production in [formula omitted] collisions at [formula omitted

Based on a kinetic description of J/ψ dissociation and production in an expanding quark–gluon plasma that is described by a 2+1 dimensional ideal hydrodynamics, we have studied the hot medium effects on J/ψ production in p+Pb collisions at sNN=5.02 TeV. Including also the cold nuclear matter effects, we are able to reproduce recent experimental results on the nuclear modification factor RpPb(J/ψ) measured by the ALICE Collaboration. We have also made predictions for the RpPb of J/ψ and the double ratio RpPbpro(ψ′)/RpPbpro(J/ψ) of prompt quarkonia produced in the most central 10% p+Pb collisions. We find that different from the cold nuclear matter effects, the RpPb(J/ψ) is slightly smaller than that in the minimum bias collisions, and the double ratio is significantly less than one at backward rapidity.

First results from p–Pb collisions at the LHC

The first results from p-Pb collisions at sqrt(sNN) = 5.02 TeV are discussed.

The Underlying Event in Hadronic Collisions

I review studies of the underlying event (UE) in hadronic collisions, dating from the first CDF studies in 2000 to the latest LHC findings and surprises. I explain the CDF quantum chromodynamics (QCD) Monte Carlo model tunes and describe how well the Tevatron tunes did at predicting the behavior of the UE at the LHC. In a very short time, the LHC experiments collected a large amount of data at 900 GeV and 7 TeV that can be used to study the UE in great detail. I review the LHC UE results and compare them with one another, with the Tevatron results, and with some of the LHC QCD Monte Carlo model tunes. I also explain the relationship between minimum bias collisions and the UE and discuss new techniques for studying the UE (i.e., techniques beyond the traditional approach).