Probes In Heavy-ion Collisions Research Articles

Hard probes of heavy ion collisions with ATLAS

This proceeding gives an overview of the latest hard process measurements in heavy ion collision systems with the ATLAS detector at the LHC, utilizing the high statistics of 5.02 TeV Pb+Pb data collected in 2015 and 2018. These include multiple measurements of jet production and structure, which probe the dynamics of the hot, dense quark-gluon plasma (QGP) formed in relativistic nucleus-nucleus collisions. New results sensitive to the role of path length, color charge, parton mass, and parton fragmentation in the jet quenching will also be shown. Furthermore, new measurements of quarkonia and heavy flavor production to probe the QGP medium properties will be discussed. A particular focus of the measurements is the systematic comparison of fully unfolded data to state-of-the-art theoretical models.

Intermediate mass dileptons as pre-equilibrium probes in heavy ion collisions

The production of dileptons with an invariant mass in the range 1GeV<M<5 GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory. We argue that the dilepton mass spectrum provides a measure of the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors.

Probes of the quark-gluon plasma and plasma instabilities

Penetrating probes in heavy-ion collisions, like jets and photons, are sensitive to the transport coefficients of the produced quark-gluon plasma, such as shear and bulk viscosity. Quantifying this sensitivity requires a detailed understanding of photon emission and jet-medium interaction in a nonequilibrium plasma during the hydrodynamic stages of heavy-ion collisions. Up to now, such an understanding has been hindered by plasma instabilities. These instabilities arise out of equilibrium and lead to spurious divergences when evaluating the rate of interaction of hard probes with the plasma. In this paper, we show that taking into account the time evolution of an unstable plasma cures these divergences. Specifically, we calculate the time evolution of gluon two-point correlators in a setup with a small initial momentum anisotropy and show that the gluon occupation density at first grows exponentially. Based on this calculation, we argue for a phenomenological prescription where instability poles are subtracted. Finally, we show that in the Abelian case instability fields do not affect medium-induced photon emission to our order of approximation.

Jet momentum broadening in the pre-equilibrium Glasma

Jets are important probes of heavy ion collisions as they can provide information on the interaction of a highly energetic parton with the medium it traverses. In the hydrodynamic stage of dense, strongly interacting matter, these interactions can be explained in terms of scattering processes, soft gluon emission and collinear parton splittings. However, jets originate even before the hydrodynamic stage. Here we report on the first numerical simulation of transverse momentum broadening of jets stemming from the interaction of partons with boost-invariantly expanding Glasma flux tubes. The Glasma stage is a pre-hydrodynamic stage based on the Color Glass Condensate framework. Our calculation shows strong time-dependence and an intrinsic anisotropy of momentum broadening in the directions transverse to the jet propagation direction.

Recent results on hard probes with ALICE and STAR

This paper reviews recent experimental results on hard probes in heavy-ion collisions from the ALICE and STAR Collaboration. These studies include various observables characterizing jet properties like nuclear modification factors, recoil jet yields, di-jet and photon-jet energy imbalance, and the observables characterizing jet properties like jet fragmentation function and jet shapes; and measurements of high-pT charged hadrons from jet fragmentation and triggered particle correlations will be highlighted.

Tests of the CBM Rich readout and Daq prototype

The CBM RICH detector is an integral component of the future CBM experiment at FAIR, providing efficient electron identification and pion suppression necessary for the measurement of rare dileptonic probes in heavy ion collisions. An overview of the CBM RICH readout and DAQ system prototype is given, consisting of the PADIWA preamplifier-discriminator board, the TDC-HUB board TRBv3, and DAQ and analysis code in the CbmRoot framework. The laboratory setup built for studying the timing characteristics of the readout chain and the analysis results obtained using the laboratory measurements are presented. The fine time calibration and inter-channel delay correction techniques and their implementation and effect are discussed.

Towards tomography of quark\u2013gluon plasma using double inclusive forward-central jets in Pb\u2013Pb collision

We propose a new framework, merging High Energy Factorization with final-state jet quenching effects due to interactions in a quark–gluon plasma, to compute di-jet rates at mid-rapidity and forward rapidity. It allows one to consistently study the interplay of initial-state effects with medium interactions, opening the possibility for understanding the dynamics of hard probes in heavy-ion collisions and the QGP evolution in rapidity.

Electromagnetic probes of heavy ion collisions: recent developments

The current status of photon and dilepton emission in ultra-relativistic heavy ion collisions is reviewed, and recent developments are highlighted. The importance of emissions at early, intermediate and late times is emphasized.

Jet and heavy flavor production in heavy-ion collisions

We review recent progress in the theoretical description of hard probes in heavy-ion collisions within the framework of Soft Collinear Effective Theory (SCET). Firstly, we consider the inclusive production of heavy flavor mesons and jets with a small radius parameter in proton-proton collisions. Secondly, we describe how in-medium effects can be incorporated consistently with the proton-proton baseline calculations. The in-medium interactions are described by Glauber gluon interactions. We present results for the nuclear modification factor RAA which is most commonly used to study the quenching of hadron or jet production yields in heavy-ion collisions and we compare to recent data from the LHC.

W and Z bosons with CMS in pp, pPb and PbPb collisions

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. W and Z bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. The final results on the W and Z production in pPb collisions at 5.02 TeV, combining both the muon and electron channels, will be presented. When compared to theory calculations that include nuclear modifications to the parton distributions, data show a clear sensitivity to this type of effects. The final results in PbPb collisions at 2.76 TeV, compared to pp collisions at the same centre of mass energy, will also be presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions, while the differential cross sections points to initial state effects small compared to the statistical precision of the available data.

Differential measurements of dielectron and direct virtual photon productions at STAR

Dileptons and photons have been serving as penetrating probes in heavy-ion collisions for quite a long time since the strongly interacting matter predicted by QCD theory. We present the mid-rapidity dielectron results in 0-80% Au+Au collisions at sNN=19.6,39,62.4, and 200 GeV, and the direct virtual photon results in 0-80% Au+Au collisions at sNN=200 GeV at STAR. The excess yields of dielectron with respect to hadronic sources at low mass range are presented, which are consistently described by a broadened ρ spectral function plus thermal radiations. Furthermore, the direct virtual photon yield has been measured and found it is consistent with a model that includes thermal radiation and initial hard parton scattering.

CMS results on electroweak probes from LHC Run 1

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. The Z and W bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. Final results on Z boson production in PbPb collisions compared to pp collisions are presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions and the differential cross sections show that initial state effects are small compared to the statistical precision of the available data. Measurements of the W and Z boson production in pPb collisions, combining both the muon and electron decay channels are also presented. The data are compared to theory predictions for nuclear modifications of the parton distributions and show a clear sensitivity to these effects.

The CBM RICH project

The CBM RICH detector is an integral component of the future CBM experiment at FAIR, providing efficient electron identification and pion suppression necessary for the measurement of rare dileptonic probes in heavy ion collisions. The RICH design is based on CO2 gas as radiator, a segmented spherical glass focussing mirror with Al+MgF2 reflective coating, and Multianode Photomultipliers for efficient Cherenkov photon detection. Hamamatsu H12700 MAPMTs have recently been selected as photon sensors, following an extensive sensor evaluation, including irradiation tests to ensure sufficient radiation hardness of the MAPMTs. A brief overview of the detector design and concept is given, results on the radiation hardness of the photon sensors are shown, and the development of a FPGA-TDC based readout chain is discussed.

Electroweak probes in heavy-ion collisions at the LHC with ATLAS

Measurements of vector boson production in lead-lead collisions provide essential control data for studies of jets and jet quenching in the quark gluon plasma. Because the electroweak bosons do not interact strongly in the plasma, measurements of their production rates can be predicted using standard high-energy event generators. In addition, the vector boson spectra are potentially sensitive to nuclear parton distribution functions. The ATLAS detector has proven to be an excellent apparatus in measurements involving photons, electrons and muons in the high occupancy environment produced in heavy-ion collisions. The experiment has recorded 158 μb−1 of lead-lead and 29.85 nb−1 of proton-lead data, both of which have similar integrated partonic luminosities. In this letter, measurements of γ, Z and W± production in lead-lead and proton-lead collisions are shown and compared to predicted rates from JETPHOX (for γ) and PYTHIA/POWHEG (for Z and W±).

Resonances as Probes of Heavy-Ion Collisions at ALICE

Hadronic resonances serve as unique probes in the study of the hot and dense nuclear matter produced in heavy-ion collisions. Properties of the hadronic phase of the collision can be extracted from measurements of the suppression of resonance yields. A comparison of the transverse-momentum spectra of the ϕ(1020) meson and the proton (which have similar masses) can be used to study particle production mechanisms. Resonance measurements in pp collisions provide input for tuning QCD-inspired particle production models and serve as reference measurements for other collision systems. Measurements of resonances in p-Pb collisions allow nuclear effects in the absence of a hot and dense final state to be studied. The ALICE Collaboration has measured resonances in pp, p-Pb, and Pb-Pb collisions. These measurements will be discussed and compared to results from other experiments and to theoretical models.

Present and future of QGP research in high energy heavy ion collision experiments

from p-p and p-Pb collisions at LHC at different centre of mass energies have been used as references to the results from Pb-Pb collisions. These results are further compared with detailed and elaborate measurements performed with Au-Au collisions at p sNN = 200 GeV at RHIC. The basic properties of the matter created at LHC have been found to be similar to that at RHIC, e.g., the matter in Pb-Pb collisions at LHC also behaves like a strongly coupled partonic liquid as inferred for the Au-Au collisions at top RHIC energy. However, with higher collision energy at LHC, a new set of probes in heavy ion collisions like jets and higher states of quarkonia open up new avenues in detailed understanding of the properties of the matter. Recent findings of the collective behaviour at p-Pb collisions at LHC however questions the existing theoretical understanding of the medium formation in p-Pb and Pb-Pb collisions. RHIC and LHC explore the high temperature and nearly zero net-baryon density region of the phase diagram, another extremes are being explored at the beam energy scan (BES) programme at RHIC and the upcoming CBM experiment at FAIR. The BES-I results demonstrate that at lower p sNN , several findings that were interpreted as the signatures of the formation of de-confined partonic matter at top RHIC energy were found to be absent. The interpretation of these results require thorough understanding of the role of baryons in the matter created at lower p sNN . The CBM experiment at FAIR aims to explore the matter created at low collision energy by the use of bulk and rare probes.

Hard Probes in Heavy Ion Collisions: Current Status and Prospects for Application of QCD Evolution Techniques

In the past decade the observation of cross section modification for leading hadrons, heavy flavor and two particle correlations in heavy ion collisions has provided important insights into the dynamics of parton propagation in dense strongly-interacting matter. The development of the theory of reconstructed jets and related experimental measurements have further shed light on the characteristics of in-medium parton showers. So far, experimental results from ultra-relativistic nuclear collisions at RHIC and LHC have been analyzed in the framework of parton energy loss, where the precision of the theoretical predictions cannot be systematically improved. Only recently have higher order calculations and applications of resummation and evolution to heavy ion collisions begun to emerge. Several examples of such advances are discussed in these proceedings.

Electromagnetic probes in heavy-ion collisions

Due to their penetrating nature, electromagnetic probes, i.e., lepton-antilepton pairs (dileptons) and photons are unique tools to gain insight into the nature of the hot and dense medium of strongly-interacting particles created in relativistic heavy-ion collisions, including hints to the nature of the restoration of chiral symmetry of QCD. Of particular interest are the spectral properties of the electromagnetic current-correlation function of these particles within the dense and/or hot medium. The related theoretical investigations of the in-medium properties of the involved particles in both the partonic and hadronic part of the QCD phase diagram underline the importance of a proper understanding of the properties of various hadron resonances in the medium.

Measurements of high- [formula omitted] probes in heavy ion collisions at CMS

The capabilities of the CMS detector at the LHC will be described for measuring high- p T hadrons, photons and jets in heavy ion collisions. Detailed simulations of various studies planned with the CMS apparatus, including charged particle tracking, jet reconstruction using calorimetry, dimuon and isolated photon detection and the measurement of in-medium fragmentation functions using high- p T photon-jet correlations will be discussed.

From RAA via correlations to jets – the long road to tomography

The main motivation to investigate hard probes in heavy ion collisions is to do tomography, i.e. to infer medium properties from the in-medium modification of hard processes. Yet while the suppression of high P T hadrons has been measured for some time, solid tomographic information is slow to emerge. This can be traced back to theoretical uncertainties and ambiguities in modelling both medium evolution and parton-medium interaction. Ways to overcome these difficulties are to constrain models better and to focus on more differential observables. Correlations of high P T hadrons offer non-trivial information beyond what can be deduced from single hadron suppression. They reflect not only the hard reaction being modified by the medium, but also the back reaction of the medium to the hard probe. Models for hard back-to-back correlations are now very well constrained by a wealth of data and allow insights into the nature of the parton-medium interaction as well as first true tomographic results. Models of full in-medium jet evolution are being actively developed, but have yet to make substantial contact with data. Progress is slower in the understanding of low P T correlations, the ridge and the cone, although a qualitative understanding of the nature of the physics behind these correlations starts to emerge.