Signature Of Quark-gluon Plasma Formation Research Articles

Multiplicity-dependent strangeness enhancement in small systems at LHCb

The phenomenon of strangeness enhancement, originally proposed as a signature of quark-gluon plasma formation[1, 2], has received considerable new interest following recent observations in small collision systems[3]. LHCb is uniquely well suited to study such effects in the heavy quark sector and in the forward region. One such advantage is the ability to measure event activity using estimators that are geometrically inclusive and geometrically distinct from the signal region, thus providing insight into local particle density dependencies. Here we present results on multiplicity-dependent to B 0 cross-section ratios in pp collisions at TeV[4]. The ratio is measured to increase with local particle density for low momenta ( PT < 6 GeV/c).

Recent Quarkonia Measurements in Small Systems at RHIC and LHC Energies

Heavy-ion research at the Relativistic Heavy Ion Collider (RHIC) during the first decade of data collection, approximately during the years 2000–2010, was primarily focused on the study of Au+Au collisions. The search for evidence of quark-gluon plasma (QGP), a state of matter where quarks and gluons become unbound within a high energy density environment, which was at the forefront of research efforts. However, studies of the azimuthal anisotropy parameter v2 in p/d+Pb collisions from the Large Hadron Collider (LHC) yielded results consistent with the hydrodynamic flow, one of the signatures of quark-gluon plasma formation in heavy-ion collisions. Since the publication of these findings, the field of heavy-ion physics has made subsequent measurements in small system collisions to study cold nuclear matter effects as well as look for additional evidence of hot nuclear matter effects. Quarkonia, a bound state of a cc¯ or bb¯ pair, has often been used to probe a wide range of nuclear effects in both large and small collision systems. Here we will review recent quarkonia measurements in small system collisions at RHIC and LHC energies and summarize the experimental conclusions.

Multiplicity-dependent production of heavy mesons with strangeness in small systems at LHCb

The phenomenon of strangeness enhancement, originally proposed as a signature of quark-gluon plasma formation, has received considerable new interest following recent observations in small collision systems. LHCb is uniquely well suited to study such effects in the heavy quark sector, down to very low transverse momentum. Here we will present new LHCb results on the production rates of B0s relative to B0 mesons versus multiplicity in pp collisions. Potential implications for the hadronization mechanism of heavy quarks and our understanding of the factorization of fragmentation functions will be discussed.

Investigating the origin of strangeness enhancement in small systems through multi-differential analyses

The main goal of the ALICE experiment is to study the physics of strongly interacting matter, including the properties of the quark-gluon plasma (QGP). The relative production of strange hadrons with respect to non-strange hadrons in heavy-ion collisions was historically considered as one of the signatures of QGP formation. However, measurements at the LHC in pp and p-Pb collisions have shown similar features to those observed in Pb-Pb, measuring an increase in the production of strange hadrons relative to pions with the charged-particle multiplicity in the event. In order to better understand the role of the event multiplicity to the observed enhancement in pp, two new complementary analyses have been performed. The first exploits the angular correlation between strange and high-pT hadrons in the event to classify in-jet and out-of-jet strangeness production. The second uses the concept of the effective energy available in the event for particle production which is estimated by an anti-correlation with the energy deposited in ALICE’s Zero Degree Calorimeters. The results suggest that strangeness enhancement emerges from the growth of the underlying event and is not connected to initial state properties.

Light Meson Production in Heavy Ion Collisions in the PHENIX Experiment

Production of a strongly interacting quark−gluon plasma (QGP) has been established in central heavy ion collisions during the studies carried out at the RHIC collider. The experiments on interactions of asymmetric nuclei (Cu + Au, U + U) provide the means to study systematically a jet quenching effect, which is a signature of QGP formation. In this paper, the factors of nuclear modification of π0 and η mesons in Cu + Au and U + U collisions at 200 GeV measured in the PHENIX experiment at RHIC are presented.

Quarkonium cross sections and polarizations in pp collisions with CMS

According to NRQCD, quarkonium production starts with the production of a pre-resonant quark-antiquark (QQ‾) pair that then binds into the observed quarkonium state through a possible interaction with surroundings. Understanding this interaction would play a crucial role in the interpretation of the quarkonium suppression patterns as signatures of quark-gluon plasma formation. Unlike the measurements of quarkonium differential cross sections, which reflect partonic processes and are sensitive to initial-state effects, the polarization measurements are unambiguous probes of the final-state quarkonium formation process, determined by the quantum properties of the QQ‾: a change in polarization would be a direct signal, virtually unaffected by initial-state phenomena, of a change in binding mechanism. This report presents the ψ(nS) and ϒ(nS) cross sections and polarizations measured by CMS, using dimuon samples collected in pp collisions at s=7 TeV. The cross sections of five S-wave states extend up to or exceed pT=100 GeV, probing kinematical windows where the theory calculations are supposed to be the most reliable. The polarizations are measured up to high pT at midrapidity, and the Upsilon polarizations are also shown as a function of the number of charged particles produced in the pp collision.

Thermodynamically anomalous regions as a mixed phase signal

Using the most advanced model of the hadron resonance gas we reveal, at chemical freeze-out, remarkable irregularities such as an abrupt change of the effective number of degrees of freedom and plateaus in the collision-energy dependence of the entropy per baryon, total pion number per baryon, and thermal pion number per baryon at laboratory energies 6.9-11.6 AGeV. On the basis of the generalized shock adiabat model we show that these plateaus give evidence for the thermodynamic anomalous properties of the mixed phase at its boundary to the quark-gluon plasma (QGP). A new signal for QGP formation is suggested and justified.

Quarkonium measurements in the STAR experiment

Abstract Calculations of Quantum Chromodynamics on lattice showed that under conditions of high energy density or high temperature nuclear matter undergoes a phase transition from a state of quarks and gluons confined in hadrons to a deconfined state, the Quark–Gluon Plasma (QGP). Such conditions were present in first moments after the Big Bang and can be created in laboratory by colliding of heavy ions with sufficient energy. One of the most prominent signatures of QGP formation is the quarkonium suppression in central heavy-ion collisions arising from the Debye screening of the quark–antiquark potential in hot and dense nuclear matter. However, cold nuclear matter effects and heavy quark recombination could influence the measured quarkonium yields. Measurements of J / ψ at different collision energies can shed new light on understanding the interplay of these mechanisms for J / ψ production. The production mechanism of J / ψ is not well understood yet and the simultaneous measurement of J / ψ transverse momentum spectra and polarization in p + p collisions can significantly constrain possible production scenarios. In this proceedings, we report the recent STAR measurements of J / ψ production at mid-rapidity in p + p , d + Au , Au + Au at s NN = 200 GeV and in Au + Au collisions at s NN = 39 GeV and s NN = 62 GeV .

Formula omitted] production and absorption in p + A and d+Au collisions

The level of “anomalous” charmonium suppression in high-energy heavy-ion collisions and its interpretation as a signal of quark-gluon plasma formation requires a robust understanding of charmonium production and absorption in proton-nucleus collisions. In a previous study we have shown that, contrary to common belief, the so-called J / ψ “absorption cross section”, σ a b s J / ψ , is not a “universal constant” but, rather, an effective parameter that depends very significantly on the charmonium rapidity and on the collision energy. Here we present ugraded Glauber calculations with the EPS09 parameterization of nuclear modifications of the parton densities. We confirm that the effective “absorption cross section” depends on the J / ψ kinematics and the collision energy. We also make further steps towards understanding the physics of the mechanisms behind the observed “cold nuclear matter” effects.

Photon and Dilepton Production at LHC

We review various production mechanisms of photons and small mass dileptons at large transverse momentum at the LHC. Their relevance as a signal for quark-gluon plasma formation is discussed.

Pion Interferometry of square root of (s(NN)) =130 GeV Au + Au collisions at RHIC.

Two-pion correlation functions in Au+Au collisions at square root of [s(NN)] = 130 GeV have been measured by the STAR (solenoidal tracker at RHIC) detector. The source size extracted by fitting the correlations grows with event multiplicity and decreases with transverse momentum. Anomalously large sizes or emission durations, which have been suggested as signals of quark-gluon plasma formation and rehadronization, are not observed. The Hanbury Brown-Twiss parameters display a weak energy dependence over a broad range in square root of [s(NN)].

Can doubly strange dibaryon resonances be discovered at RHIC?

The baryon-baryon continuum invariant mass spectrum generated from relativistic nucleus + nucleus collision data may reveal the existence of doubly-strange dibaryons not stable against strong decay if they lie within a few MeV of threshold. Furthermore, since the dominant component of these states is a superposition of two color-octet clusters which can be produced intermediately in a color-deconfined quark-gluon plasma (QGP), an enhanced production of dibaryon resonances could be a signal of QGP formation. A total of eight, doubly-strange dibaryon states are considered for experimental search using the STAR detector (Solenoidal Tracker at RHIC) at the new Relativistic Heavy Ion Collider (RHIC). These states may decay to Lambda-Lambda and/or proton-Cascade-minus, depending on the resonance energy. STAR's large acceptance, precision tracking and vertex reconstruction capabilities, and large data volume capacity, make it an ideal instrument to use for such a search. Detector performance and analysis sensitivity are studied as a function of resonance production rate and width for one particular dibaryon which can directly strong decay to proton-Cascade-minus but not Lambda-Lambda. Results indicate that such resonances may be discovered using STAR if the resonance production rates are comparable to coalescence model predictions for dibaryon bound states.

Energy Loss in Perturbative QCD

▪ Abstract We review the propagation of energetic partons in hot or cold QCD matter, as known from recent work. We summarize advances in the understanding of both collisional and radiative energy loss. Our emphasis is on radiative energy loss, which has very interesting properties that may help to detect the quark-gluon plasma produced in heavy-ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated with the aim of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly greater in hot matter than in cold. This makes the magnitude of the radiative energy loss a remarkable signal for quark-gluon plasma formation.

Hadron production in nuclear collisions from the NA49 experiment at 158GeV/ c · A

With its large acceptance and particle identification coverage the NA49 experiment (Fig. 1) can study hadron production in a wide range of high energy reactions [1]. Originally aimed at examining central Pb+Pb collisions for signatures of quark-gluon plasma formation, the scope of the experiment has been enhanced with a systematic study of impact parameter and projectile size dependence, as well as the inclusion of the more elementary p+p and p+A interactions. The question is: are predicted signals of the quark-gluon plasma observed and are there discontinuities which would support the concept of hadronic phase transition?

Hadron production in nuclear collisions from the NA49 experiment at 158 a A

With its large acceptance and particle identification coverage the NA49 experiment (Fig. 1) can study hadron production in a wide range of high energy reactions [1]. Originally aimed at examining central Pb+Pb collisions for signatures of quark-gluon plasma formation, the scope of the experiment has been enhanced with a systematic study of impact parameter and projectile size dependence, as well as the inclusion of the more elementary p+p and p+A interactions. The question is: are predicted signals of the quark-gluon plasma observed and are there discontinuities which would support the concept of hadronic phase transition?

Multipion correlations as a signal for quark-gluon plasma formation in high energy heavy ion collisions

The multiparticle Bose-Einstein correlations for quark-gluon plasma (QGP) formation in high energy heavy ion collisions are analyzed. The average correlation intensities of multipions as a function of pion multiplicity are investigated. It is shown that the average correlation intensities of multipions have obvious enhancements in the high multiplicity region. This signature can be used to detect the existence of QGP in rare high pion multiplicity events in high energy heavy ion collisions.

Signatures of quark-gluon plasma formation in nuclear collisions: News from QM96

Three generic signatures of quark-gluon plasma production in nuclear collisions are reviewed. First, we discuss whether the recent J Ψ suppression data in Pb + Pb are the “smoking gun” for the deconfinement transition. Second, claims that the excess production of low mass dileptons provides evidence for chiral restoration are reviewed. Finally, the pion interferometry time delay signature of a rapid cross-over region in the QCD equation of state is analyzed.

Estimates of relative yields of strange baryons and antibaryons from pp and pA interactions

Relative production rates of hyperons and antihyperons from heavy ion collisions are expected to be sensitive signals of quark-gluon plasma formation. They have to be confronted to corresponding quantities from pp and pA collisions which are evaluated here.

Radiative energy loss and p⊥-broadening of high energy partons in nuclei

The medium-induced p ⊥-broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that − dE/ dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton p 2 ⊥ w . We find −dE/dz = 1 8 α sN cp 2 ⊥w holds independent of the details of the parton-nucleon scatterings so long as L is large and so long as perturbative QCD applies at short distances. Numerical estimates suggest that p ⊥-broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.

The time-delay signature of quark-gluon plasma formation in relativistic nuclear collisions

The hydrodynamic expansion of quark-gluon plasmas with spherical and longitudinally boost-invariant geometries is studied as a function of the initial energy density. The sensitivity of the collective flow pattern to uncertainties in the nuclear matter equation of state is explored. We concentrate on the effect of a possible finite width, ΔT ∼ 0.1 T c, of the transition region between quark-gluon plasma and hadronic phase. Although slow deflagration solutions that act to stall the expansion do not exist for ΔT > 0.08 T c, we find, nevertheless, that the equation of state remains sufficiently soft in the transition region to delay the propagation of ordinary rarefaction waves for a considerable time. We compute the dependence of the pion-interferometry correlation function on ΔT, since this is the most promising observable for time-delayed expansion. The signature of time delay, proposed by Pratt and Bertsch, is an enhancement of the ratio of the inverse width of the pion correlation function in out-direction to that in side-direction. One of our main results is that this generic signature of quark-gluon plasma formation is rather robust to the uncertainties in the width of the transition region. Furthermore, for longitudinal boost-invariant geometries, the signal is likely to be maximized around RHIC energies