Research Highlights

This page features highlights of publications from STAR/ALICE Collaborations in high impact journals like Physical Review Letters or Nature or Science or Papers which has crossed 200 citations or Paper has been highlighted by the Journal (e.g. Editor’s suggestion or cover page) where Indian colleagues are the primary authors.

2020

Title Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

For the first time, evidence has been found for a significant spin alignment of vector mesons in heavy-ion collisions. The effect is strongest at low pT with respect to a vector perpendicular to the reaction plane and for midcentral (10–50%) collisions. These observations are qualitatively consistent with expectations from the effect of large initial angular momentum in noncentral heavy-ion collisions, which leads to quark polarization via spin-orbit coupling, subsequently transferred to hadronic degrees of freedom by hadronization via recombination.

Journal Phys. Rev. Lett. 125, 012301 (2020)
Collaboration ALICE
Full list of primary authors Ajay Kumar Dash, Sourav Kundu, Bedangadas Mohanty, Ranbir Singh
Primary author institutes NISER
News
  1. Editor's suggestion
  2. CERN Courier
  3. LHC seminar

2017

Title Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program

We have studied the bulk properties of matter in the Beam Energy Scan program (BES) at RHIC. The BES program covers the energy range from 7.7 GeV to 39 GeV which along with top RHIC energy corresponds to the baryon chemical potential region of 20-400 MeV. The mid-rapidity yields of identified hadrons have been presented. They show the expected signatures of a high-baryon stopping region at lower energies. At high energies, the pair production mechanism dominates the particle production. At intermediate energies there is clearly a transition between these two regions, which is explored by the BES Program. The data have been used to analyse both chemical and kinetic freeze-out parameters. The results indicate in heavy-ion collisions a clear centrality dependence of the baryon chemical potential at lower energies. The centrality dependence of the freeze-out parameters provides an opportunity for the BES program at RHIC to enlarge the \((T, \mu_B)\) region of the phase diagram to search for the QCD critical point. The difference between chemical and kinetic freeze-out increases with increasing energy suggesting increasing hadronic interactions after chemical freeze-out at higher energies.


Journal Phys. Rev. C 96, 044904 – Published 13 October 2017
Citations 275+
Collaboration STAR
Full list of primary authors Sabita Das, Declan Keane, Lokesh Kumar, Bedangadas Mohanty
Primary author institutes IOP, Kent State University, Panjab University, NISER
News
  1. CERN Courier
  2. Editor's suggestion

2016

Title Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program

We have reported high-statistics elliptic flow measurements for multi-strange hadrons (\(\Sigma\) and \(\Omega\)) and \(\phi\) meson with other light and strange hadrons \((\pi,K,K^0_S,p\) and \(\Lambda)\) in Au + Au collisions at \(\sqrt{s_\mathrm{NN}}= 200\) GeV for different centralities. The \(p_T\) dependence of \(\phi\) and \(\Omega\) \(v_2\) is observed to be similar to that of \(\pi\) and \(p\), indicating that a large amount of collectivity is developed in the initial partonic phase for light and strange hadrons. NCQ scaling holds within the statistical uncertainty for both 0-30% and 30-80% centralities, suggesting collective motion of quarks prior to hadronization.


Journal Phys. Rev. Lett. 116, 062301 – Published 10 February 2016
Collaboration STAR
Full list of primary authors Hiroshi Masui, Bedangadas Mohanty, Md. Nasim, Shusu Shi, Xu Sun
Primary author institutes LBNL, NISER, CCNU
Corresponding author Md. Nasim (NISER)
Title \(K^{∗0}(892)\) and \(\phi(1020)\) production in Pb-Pb collisions at \(\sqrt{s_\mathrm{NN}}=2.76\) TeV

The \(K^*\) yields relative to charged kaons in Pb-Pb collisions show a suppression with respect to pp collisions, which increases with the system size. In contrast, no such suppression is observed for the \(\phi\) mesons. The lack of suppression for the \(\phi\) meson can be attributed to the fact that most of them decay outside the fireball because of its longer lifetime (\(46.3 \pm 0.4\) fm/c). Because of a shorter lifetime (\(K^* = 4.16 \pm 0.05\) fm/c), a significant number of produced \(K^*\) decays in the hadronic medium. The decay product(s) undergo interactions with other hadrons in the medium resulting in a significant change in their momentum, and no longer contributing to the \(K^*\) signal reconstructed in the experiment. Although both rescattering and regeneration are possible, the results presented here represent an experimental demonstration of the predominance of rescattering effects in the hadronic phase of the system produced in heavy-ion collisions.


Journal Phys. Rev. C 91, 024609 – Published 17 February 2015
Citations 230+
Collaboration ALICE
Full list of primary authors Anders Knospe, Christina Markert, Bedangadas Mohanty, Subhash Singha (PhD thesis paper)
Primary author institutes University of Texas, NISER

2014

Title Energy Dependence of Moments of Net-proton Multiplicity Distributions at RHIC

We report the measurements of the higher moments and their products (Sσ and κσ2) of the net-proton distributions at midrapidity \((|y|\lt 0.5)\) within \(0.4\lt p_T \lt 0.8GeV\) in Au+Au collisions over a wide range of \(\sqrt{s_\mathrm{NN}}\) and \(\mu_B\) have been presented to search for a possible Critical Point (CP) and signals of a phase transition in the collisions. These observables show a centrality and energy dependence, which are neither reproduced by non-CP transport model calculations, nor by a hadron resonance gas model. For \(\sqrt{s_\mathrm{NN}}\gt 39\) GeV, \(S\sigma\) and \(\kappa\sigma^2\) values are simi-lar for central, peripheral Au+Au collisions and p+p collisions. Deviations for both \(\kappa\sigma^2\) and \(S\sigma\) from HRG and Skellam expectations are observed for \(\sqrt{s_\mathrm{NN}}\leq 27\) GeV. The data presented here also provides information to extract freeze-out conditions in heavy-ion collisions using QCD based approaches .


Journal Phys. Rev. Lett. 112, 032302 – Published 23 January 2014
Citations 440+
Collaboration STAR
Full list of primary authors Lizhu Chen, Bill Llope, Xiaofeng Luo, Bedanga Mohanty, Daniel McDonald
Primary author institutes CCNU, Wayne State University, Rice University, NISER
Coresponding author Bedangadas Mohanty (NISER)
Title Beam Energy Dependence of Moments of the Net-Charge Multiplicity Distributions in Au+Au Collisions at RHIC

We report the first results of the moments of net-charge multiplicity distributions for\(|\eta|\lt 0.5\) as a function of centrality for Au+Au collisions at seven collision energies from \(\sqrt{s_\mathrm{NN}}=7.7\) to \(200\) GeV are presented. These data can be used to explore the nature of the QCD phase transition and to locate the QCD critical point. The measured moments of net-charge multiplicity distributions provide unique information about the freeze-out parameters by directly comparing with theoretical model calculations. Future measurements with high statistics data will be needed for precise determination of freeze-out conditions and to make definitive conclusions regarding the critical point.


Journal Phys. Rev. Lett. 113, 092301 – Published 26 August 2014
Citations 300+
Collaboration STAR
Full list of primary authors W. J. Llope, D. McDonald, Bedangadas Mohanty, T. K. Nayak, N. R. Sahoo, G. Westfall.
Primary author institutes MSU, VECC, Rice University, NISER

2013

Title Net-Charge Fluctuations in Pb-Pb collisions at \(\sqrt{s_\mathrm{NN}} = 2.76\) TeV

We report the first measurement of the net-charge fluctuations in Pb-Pb collisions at \(\sqrt{s_\mathrm{NN}} = 2.76\) TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions. An additional reduction in the amount of fluctuations is seen in comparison to the results from lower energies. We examine the dependence of fluctuations on the pseudorapidity interval, which may account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations at the LHC are smaller compared to the measurements at the BNL Relativistic Heavy Ion Collider, and as such, closer to what has been theoretically predicted for the formation of a quark-gluon plasma.


Journal Phys. Rev. Lett. 110, 152301 – Published 10 April 2013
Collaboration ALICE
Full list of primary authors Panagiotis Christakoglou, Tapan Nayak, Satyajit Jena, Basanta Nandi
Primary author institutes NIKHEF, VECC, IIT-B
Title Observation of an Energy-Dependent Difference in Elliptic Flow between Particles and Antiparticles in Relativistic Heavy Ion Collisions

We report the first observation of a beam-energy-dependent difference in elliptic flow versus transverse momentum between particles and corresponding antiparticles for minimum bias \(\sqrt{s_\mathrm{NN}}= 7.7-62.4\) GeV Au+Au collisions at midrapidity. The difference increases with decreasing beam energy. Baryons show a larger difference compared to mesons. The relative values of \(v_2\) for charged pions have the opposite trend to the values of charged kaons. It is concluded that, at the lower energies, particles and anti-particles are no longer consistent with the single NCQ scaling that was observed for \(\sqrt{s_\mathrm{NN}}=200\) GeV. However, for the group of particles the NCQ scaling holds within 10% while for the group of antiparticles the difference between baryon and meso \(v_2\) continues to decrease to lower energies. We further observed that the phi‑meson v2 atthe highest measured mT-m0 value is low compared to other hadrons at \(\sqrt{s_\mathrm{NN}}=7.7\)and \(11.5\) GeV with \(1.8\sigma\) and \(2.3\sigma\), respectively.


Journal Phys. Rev. Lett. 110, 142301 – Published 2 April 2013
Collaboration STAR
Full list of primary authors Xin Dong, Chitrasen Jena, Hiroshi Masui, Bedanga Mohanty, Md Nasim, Art Poskanzer, Hans Georg Ritter, Alex Schmah, Shusu Shi, Xiaoping Zhang, Jie Zhao
Primary author institutes LBNL, NISER, IOP, VECC, CCNU, TU

2011

Title Observation of the antimatter helium-4 nucleus

High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus (\(\bar{^4\mathrm{He}}\)), also known as the anti-\(\alpha\) (\(\bar{\alpha}\)), consists of two antiprotons and two antineutrons (baryon number B = −4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level. Antimatter nuclei with \(B  \lt −1\) have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon. Here we report the observation of \(\bar{^4\mathrm{He}}\), the heaviest observed antinucleus to date. In total, 18 \(\bar{^4\mathrm{He}}\) counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) in \(10^9\) recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of \(\bar{^4\mathrm{He}}\) in cosmic radiation.


Journal Nature 473 (2011) 353 – Published 24 April 2011
Collaboration STAR
Full list of primary authors J. Chen, H. Crawford, Y. Fisyak, F. Geurts, Q. Hao, B. Huang , C. Jena, H. Ke, D. Keane, T. Kollegger, J. Landgraf, T. Ljubicic, M. Naglis, X. Sun, A. Tang, G. Van Buren, Z. Xu , L. Xue, J. Zhao.
Primary author institutes LBNL, UCB, SNIP, Rice University, BNL, VECC, IOP, KSU
Chair of paper committee Bedangadas Mohanty (VECC)

2010

Title Identified particle production, azimuthal anisotropy, and interferometry measurements in Au+Au collisions at \(\sqrt{s_\mathrm{NN}}=9.2\) GeV

We present the first measurements of identified hadron production, azimuthal anisotropy, and pion interferometry from Au+Au collisions below the nominal injection energy at the Relativistic Heavy-Ion Collider (RHIC) facility. The data were collected using the large acceptance STAR detector at \(\sqrt{s_\mathrm{NN}}=9.2\) GeV from a test run of the collider in the year 2008. Midrapidity results on multiplicity density (\(dN/dy\)) in rapidity (\(y\)), average transverse momentum, particle ratios, elliptic flow, and HBT radii are consistent with the corresponding results at similar \(\sqrt{s_\mathrm{NN}}\)​ from fixed target experiments. Directed flow measurements are presented for both midrapidity and forward rapidity regions. Furthermore the collision centrality dependence of identified particle \(dN/dy\), mean \(p_T\) , and particle ratios are discussed. These results also demonstrate the readiness of the STAR detector to undertake the proposed QCD critical point search and the exploration of the QCD phase diagram at RHIC.


Journal Phys. Rev. C 81, 024911 – Published 26 February 2010
Citation 210+
Collaboration STAR
Full list of primary authors Madan M. Aggarwal, Jiyaun Chen, Debasish Das, Lokesh Kumar, Hiroshi Masui , Bedanga Mohanty, Yadav Pandit and Shusu Shi
Primary author institutes PU, VECC, PU, LBNL, KSU
Chair of paper committee Bedangadas Mohanty (VECC/LBNL)

2009

Title \(K/\pi\) Fluctuations at Relativistic Energies

We report \(K/\pi\) fluctuations from Au+Au collisions at \(\sqrt{s_\mathrm{NN}}= 19.6, 62.4, 130\) and \(200\) GeV using the STAR detector at the Relativistic Heavy Ion Collider. \(K/\pi\) fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at \(\sqrt{s_\mathrm{NN}}= 12.3\) and \(17.3\) GeV. We report results for the collision centrality dependence of \(K/\pi\) fluctuations and results for charge-separated fluctuations. We observe that the \(K/\pi\) fluctuations scale with the charged particle multiplicity density. These results may indicate that, due to later stage hadronic rescattering, the decay products of resonances are less likely to survive in central collisions than in peripheral collisions, or may be due to the onset of radial flow combined with our acceptance in \(p_T\).


Journal Phys. Rev. Lett. 103, 092301 – Published 24 August 2009
Collaboration STAR
Full list of primary authors G. Westfall, S. Das, S. Chattopadhyay, C. Pruneau, S. Voloshin, A. Vander Molen.
Primary author institutes MSU, VECC, WSU

2007

Title Energy dependence of \(\pi^\pm, p\) and anti-proton transverse momentum spectra for collisions at \(\sqrt{s_\mathrm{NN}} = 62.4\) and \(200\) GeV

We have presented a study of the energy dependence of \(\pi^\pm, p\) and \(\bar{p}\) production for Au+Au at \(\sqrt{s_\mathrm{NN}} = 62.4\) and \(200\) GeV. These measurements provide new experimental data for investigating the production of quarks, gluons and their interactions with the medium formed in heavy-ioncollisions and the interplay between coalescence of thermal partons and jet fragmentation.


Journal Physics Letters B Volume 655, Issues 3–4, 1 November 2007, Pg 104-113
Citations 200+
Collaboration STAR
Full list of primary authors Olga Barannikova, Haidong Liu, Pawan Kumar Netrakanti, Bedangadas Mohanty, Levente Molnar, Lijuan Ruan, Ming Shao, Zhangbu Xu
Primary author institutes Purdue University, VECC, USTC, BNL
Corresponding author Bedangadas Mohanty

2006

Title Identified hadron spectra at large transverse momentum in p+p and d+Au collisions at \(\sqrt{s_\mathrm{NN}} = 200\) GeV

We have presented transverse momentum spectra for identified charged pions, protons and anti-protons from p+p and d+Au collisions at√sNN= 200 GeV. The transverse momentum spectra are measured around midrapidity \((|y|\lt 0.5)\) over the range of \(0.3 \lt pT \lt 10\) GeV/c with particle identification from the ionization energy loss and its relativistic rise in the Time ProjectionChamber, as well as the Time-of-Flight in STAR. The following conclusions can be drawn from the present study: (a) The nuclear modification factor around midrapidity is enhanced in d+Au collisions to about 1.5 for pions and to about 2 for protons and antiprotons at intermediate \(p_T\) \((2 \lt p_T \lt5 GeV/c)\).(b) Identified particle ratios are shown to be sensitive to the relative contributions from quark and gluon fragmentation as well as to their fragmentation functions. (c) The NLOpQCD calculations describe the high pT data for charged pions reasonably well in p+p collisions and d+Au collisions. In general, baryon production has historically been difficult to describe by pQCD and hadronization. (d) The proton and pion spectra in p+p collisions follow \(x_T\)-scaling with a beam-energy dependent factor \(\sim\sqrt{s_\mathrm{NN}}6.5\) above \(p_T\sim 2\) GeV/c. The pion and proton spectra follow transverse mass scaling for \(m_T \lt 2\) GeV/\(c^2\) in both p+p and d+Au collisions, suggesting the transition region from soft to hard process domination occurs at \(p_T\sim 2\) GeV/c in these collision systems. The measurements presented in this paper provide better constraints on jet quenching and quark recombination models which are presently the best candidates for explaining particle production in the intermediate pT region.


Journal Physics Letters B Volume 637, Issue 3, 8 June 2006, Pg. 161-169
Citations 250+
Collaboration STAR
Full list of primary authors Pawan K. Netrakanti, Bedangadas Mohanty, Zhangbu Xu
Primary author institutes VECC, BNL
Corresponding author Bedangadas Mohanty

2005

Title Multiplicity and Pseudorapidity Distributions of Photons in Au+Au Collisions at \(\sqrt{s_\mathrm{NN}} = 62.4\) GeV

We have presented the first results of photon multiplicity measurements at RHIC in the pseudorapidity region \(2.3\leq \eta\leq 3.7\). Photon production per participant pair is found to be approximately independent of centrality in this pseudorapidity region. Comparison with photon and charged pion data at RHIC and SPS energies shows, for the first time in heavy ion collisions, that photons and pions follow an energy independent limiting fragmentation behavior, as previously found for inclusive charged particles. Furthermore, photons are observed to follow a centrality independent limiting fragmentation scenario.


Journal Phys. Rev. Lett. 95, 062301 – Published 5 August 2005
Collaboration STAR
Full list of primary authors Pawan K. Netrakanti, Bedangadas Mohanty, Subhasis Chattopadhyay
Primary author institutes VECC
Corresponding author Bedangadas Mohanty