AA Collisions Research Articles

Charge-dependent azimuthal correlations in pPb collisions with CMS experiment

Charge-dependent azimuthal correlations relative to the event plane in AA collisions have been suggested as providing evidence for the chiral magnetic effect (CME) caused by local strong parity violation. However, the observation of the CME remains inconclusive because of several possible sources of background correlations that may account for part or all of the observed signals. This talk will present the first application of three-particle, charge-dependent azimuthal correlation analysis in proton-nucleus collisions, using pPb data collected with the CMS experiment at the LHC at sNN=5.02 TeV. The differences found in comparing same and opposite sign correlations are studied as a function of event multiplicity and the pseudorapidity gap between two of the particles detected in the CMS tracker detector. After selecting events with comparable charge-particle multiplicities, the results for pPb collisions are found to be similar to those for PbPb collisions collected at the same collision energy. With a reduced magnetic field strength and a random field orientation in high multiplicity pPb events, the CME contribution to any charge separation signal is expected to be much smaller than found in peripheral PbPb events. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

Probing transverse momentum broadening via dihadron and hadron-jet angular correlations in relativistic heavy-ion collisions

Dijet, dihadron, hadron-jet angular correlations have been reckoned as important probes of the transverse momentum broadening effects in relativistic nuclear collisions. When a pair of high-energy jets created in hard collisions traverse the quark–gluon plasma produced in heavy-ion collisions, they become de-correlated due to the vacuum soft gluon radiation associated with the Sudakov logarithms and the medium-induced transverse momentum broadening. For the first time, we employ the systematical resummation formalism and establish a baseline calculation to describe the dihadron and hadron-jet angular correlation data in pp and peripheral AA collisions where the medium effect is negligible. We demonstrate that the medium-induced broadening 〈p⊥2〉 and the so-called jet quenching parameter qˆ can be extracted from the angular de-correlations observed in AA collisions. A global χ2 analysis of dihadron and hadron-jet angular correlation data renders 〈p⊥2〉∼13−4+5 GeV2 for a quark jet at RHIC top energy. Further experimental and theoretical efforts along the direction of this work shall significantly advance the quantitative understanding of transverse momentum broadening and help us acquire unprecedented knowledge of jet quenching parameter in relativistic heavy-ion collisions.

Absence of jet quenching in peripheral nucleus–nucleus collisions

Medium effects on the production of high-pT particles in nucleus–nucleus (AA) collisions are generally quantified by the nuclear modification factor (RAA), defined to be unity in absence of nuclear effects. Modeling particle production including a nucleon–nucleon impact parameter dependence, we demonstrate that RAA at midrapidity in peripheral AA collisions can be significantly affected by event selection and geometry biases. Even without jet quenching and shadowing, these biases cause an apparent suppression for RAA in peripheral collisions, and are relevant for all types of hard probes and all collision energies. Our studies indicate that calculations of jet quenching in peripheral AA collisions should account for the biases, or else they will overestimate the relevance of parton energy loss. Similarly, expectations of parton energy loss in light–heavy collision systems based on comparison with apparent suppression seen in peripheral RAA should be revised. Our interpretation of the peripheral RAA data would unify observations for lighter collision systems or lower energies where significant values of elliptic flow are observed despite the absence of strong jet quenching.

Phenomenology of collinear photon emission from quark–gluon plasma in AA collisions

We study the role of running coupling and the effect of variation of the thermal quark mass on contribution of the collinear bremsstrahlung and annihilation to photon emission in $AA$ collisions in a scheme similar to that used in our previous jet quenching analyses. We find that for a scenario with the thermal quark mass $m_q\sim 50-100$ MeV contribution of the higher order collinear processes summed with the $2\to 2$ processes can explain a considerable part ($\sim 50$\%) of the experimental photon spectrum at $k_T\sim 2-3$ GeV for Au+Au collisions at $\sqrt{s}=0.2$ TeV. But for $m_q=300$ MeV and for the thermal quark mass predicted by the HTL scheme the theoretical predictions underestimate considerably the experimental spectrum.

Photon and Pomeron-induced Production of Dijets in pp, pA and AA Collisions

In this paper we present a detailed comparison of the dijet production by photon–photon, photon–pomeron and pomeron–pomeron interactions in pp, pA and mathrm{AA} collisions at the LHC energy. The transverse momentum, pseudo-rapidity and angular dependencies of the cross sections are calculated at LHC energy using the Forward Physics Monte Carlo (FPMC), which allows one to obtain realistic predictions for the dijet production with two leading intact hadrons. We see that the gamma {I!P} channel is dominant at forward rapidities in pp collisions and in the full kinematical range in the nuclear collisions of heavy nuclei. Our results indicate that the analysis of dijet production at the LHC can be useful to test the resolved pomeron model as well as to constrain the magnitude of the absorption effects.

Probing transverse momentum broadening via dihadron and hadron-jet angular decorrelations

In the recent paper [1], we carry out the numerical calculations for back-to-back dihadron and hadron-jet angular correlations within the Sudakov resummation framework. This framework allows us to compute transverse momentum broadening and away-side angular correlations in pp collisions for the first time and use it as the baseline to study the medium effects in AA collisions. It paves the way for the quantitative study of jet-medium interaction and the transport properties of QGP. This proceeding is mainly a brief summary of that paper.

Jet Spectra in pp, pA, and AA Collisions

It is commonly believed that a new state of matter known as Quark-Gluon Plasma (QGP) is created in high-energy collisions of heavy elements. As high momentum particles traverse the hot and dense medium, they lose energy via a variety of processes, including radiative and collisional energy loss. Fully reconstructed jets are used as a proxy to measure the parton energy, though the connection between the jet and the incident parton is not as straightforward as originally believed. To combat this, recent jet measurements compare to a wide variety of other “baseline” processes, including back-to-back jets, photons, and Z bosons, among others. These proceedings summarize recent jet spectra results in both large and small systems and describe the latest advances in experimental jet techniques.

Extracting jet quenching parameters from large pT hadron suppression at RHIC/LHC

The large pT hadron suppression in high-energy heavy-ion collisions at RHIC/LHC is studied in a NLO pQCD parton model in which the fragmentation function is modified due to jet quenching. The energy loss of the energetic parton jets is incorporated in the modified fragmentation functions which utilize two important parameters to characterize the jet transport properties, the jet transport parameter qˆ0 and the mean free path λ0, both at the initial time τ0 in the center of the fireball created in central AA collisions. We performed a phenomenological study of simultaneous χ2/d.o.f fits to RHIC as well as LHC data to constrain the two parameters for energetic quarks, qˆ0≈1.1±0.2 GeV2/fm and λ0≈0.4±0.03 fm in central Au+ Au collisions at sNN=200 GeV, while qˆ0≈1.7±0.3 GeV2/fm, and λ0≈0.5±0.05 fm in central Pb + Pb collisions at sNN=2.76 TeV. Numerical results for the best fit give a multiple scattering picture for the hard jets with a moderate averaged number of gluon emissions. Based on the best constraints for λ0 and τ0, the estimated value for the mean-squared transverse momentum broadening is moderate which implies that the hard jets go through the medium with small reflection.

Bulk viscous corrections to photon production in the quark–gluon plasma

Photons radiated in heavy-ion collisions are a penetrating probe, and as such can play an important role in the determination of the quark-gluon plasma (QGP) transport coefficients. In this work we calculate the bulk viscous correction to photon production in two-to-two scattering reactions. Phase-space integrals describing the bulk viscous correction are evaluated explicitly in order to avoid the forward scattering approximation which is shown to be poor for photons at lower energies. We furthermore present hydrodynamical simulations of AA collisions focusing on the effect of this calculation on photonic observables. Bulk corrections are shown to reduce the elliptic flow of photons at higher pT.

Corona effect in AA collisions at the LHC

Following our earlier finding based on RHIC data on the dominant jet production from nucleus corona region, we reconsider this effect in nucleus-nucleus collisions at LHC energy. Our hypothesis was based on experimental data, which raised the idea of a finite formation time for the produced medium. At RHIC energy and in low density corona region this time reaches about 2 fm/c. All observed high p_t particles are produced in the corona region and have a chance to escape during this 2 fm/c. After that, the formed high density matter absorbs all jets. Following this hypothesis, the nuclear modification factor R_{AA} should be independent on particle momentum and be flat versus p_t. At the same time, we describe the finite azimuthal anisotropy of high p_t particles, v_2. A separate prediction held that, at LHC energy, the formation time in the corona region should be two times smaller, about 1 fm/c. New data at LHC show that R_{AA} is not flat and is rising with p_t. We add to our original hypothesis an assumption that a fast parton traversing the produced medium loses the fixed portion of its energy. A shift of about 7 GeV (I would call it as a parton binding energy) from the original power law p^{-6} production cross section in pp explains well all the observed R_{AA} dependencies. The shift of about 7 GeV is also valid at RHIC energy, where the cross section follows a power law with about p^{-8} and this shift explains a very slow rise of R_{AA} seen for neutral pions with p_t above 15 GeV/c. We also show that the observed at LHC dependence of v_2 at high p_t and our previous predictions agree.

Multiple Scattering with fully coherent scattering in pA and AA collisions

A new energy loss mechanism model is presented. Medium multiple scattering is modified by the interaction with a classical non-abelian background field. It is shown that the obtained gluon radiation spectrum has a symmetric structure in x (the outgoing radiation energy over incident parton energy) and (1 − x) and the appropriate sign.

Azimuthal anisotropy of long-range correlations at LHC energy in Monte Carlo model with string fusion

Long-range multiplicity correlations in intervals separated in pseudorapidity and azimuth are studied in the framework of string fusion approach. We applied a Monte Carlo model, in which the string configurations in the transverse plane and rapidity are simulating event-by-event. The string interaction is realized in the lattice string fusion approach with introduction of a grid in the transverse plane. We assumed that the azimuthal anisotropy of particle production is caused by parton energy loss traveling trough the media formed by clusters of fused strings : Δpt /Δx = −α (pt √η )2/3 , where η is a string density. In the cellular approach the Bresenham’s line algorithm has been applied. It is obtained that in AA collisions, the parton energy loss seems to play considerable role, in particular, by providing large contribution to the correlation of mean transverse momentum with multiplicity. The developed approach provides non-zero values flows in p-Pb collisions at LHC energies and produces the pattern similar to the one of the experimental di-hadron analysis.

Overview of ALICE results in pp, pA and AA collisions

The ALICE experiment at the Large Hadron Collider (LHC) at CERN is optimized for recording events in the very large particle multiplicity environment of heavy-ion collisions at LHC energies. The ALICE collaboration has taken data in Pb-Pb collisions in Run I and Run II at nucleon-nucleon center-of-mass energies $\sqrt{s_{\text{NN}}}$ = 2.76 and \mbox{5.02 TeV}, respectively, and in pp collisions at center-of-mass energies $\sqrt{s}$ = 0.9, 2.76, 5.02, 7, 8 and 13 TeV. The asymmetric system p-Pb was measured at a center-of-mass energy $\sqrt{s_{\text{NN}}}$ = 5.02 TeV. Selected physics results from the analysis of these data are presented, and an outline of the ALICE prospects for Run III is given.

Measurements of beauty-decay electrons in ALICE at the LHC

Heavy quarks are essential probes of the evolution of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. In particular, the dependencies of the medium-induced parton energy loss on the parton mass and colour charge can be investigated via measurements of open heavy-flavour hadrons. To quantify medium effects in AA collisions, one needs to study p-A collisions in order to disentangle possible cold nuclear matter effects. Thanks to the excellent vertex and impact parameter resolution of ITS and electron-identification capability provided by TPC and TOF in the ALICE experimental setup, measurements of beauty production in pp, p-Pb and Pb-Pb collisions could be done via electrons from semi-leptonic decays of beauty hadrons. The recent measurements of beauty-decay electrons in p-Pb collisions at √s NN = 5.02 TeV and in Pb-Pb collisions at √s NN = 2.76 TeV in ALICE are presented.

Initial conditions in AA and pA collisions

A full understanding of the spacetime evolution of the QCD matter created in a heavy ion collision requires understanding the properties of the initial stages. In the weak coupling picture these are dominated by classical gluon fields, whose properties can also be studied via the scattering of dilute probes off a high energy hadron or nucleus. A particular challenge is understanding small systems, where LHC data is also showing signs of collective behavior. We discuss some recent results of on the initial matter production and thermalization in heavy ion collisions, in particular in the gluon saturation framework.

Short path length pQCD corrections to energy loss in the quark gluon plasma.

Recent surprising discoveries of collective behaviour of low-pT particles in pA collisions at LHC hint at the creation of a hot, fluid-like QGP medium. The seemingly conflicting measurements of non-zero particle correlations and RpA that appears to be consistent with unity demand a more careful analysis of the mechanisms at work in such ostensibly minuscule systems. We study the way in which energy is dissipated in the QGP created in pA collisions by calculating, in pQCD, the short separation distance corrections to the well-known DGLV energy loss formulae that have produced excellent predictions for AA collisions. We find that, shockingly, the large formation time (compared to the 1/μ Debye screening length) assumption that was used in the original DGLV calculation, results in a highly non-trivial cancellation of correction terms. We investigate the effect of relaxing the large formation time assumption in the final stages of the calculation and find that, not only is the effect of the small separation distance correction important even in large (~ 5 fm) systems, but also that the correction term dominates over the leading term at high energies.

Energy dependence of resonance production in relativistic heavy ion collisions**Supported by National Natural Science Foundation of China (11575100, 11305076, 11505104)

The production of the hadronic resonances K*0(892), ϕ(1020), Σ*(1385), and Ξ*(1530) in central AA collisions at , 200, and 2760 GeV is systematically studied. The direct production of these resonances at system hadronization is described by the quark combination model and the effects of hadron multiple-scattering stage are dealt with by a ultra-relativistic quantum molecular dynamics model (UrQMD). We study the contribution of these two production sources to final observation and compare the final spectra with the available experimental data. The pT spectra of K*0(892) calculated directly by quark combination model are explicitly higher than the data at low pT ≲ 1.5 GeV, and taking into account the modification of rescattering effects, the resulting final spectra well agree with the data at all three collision energies. The rescattering effect on ϕ(1020) production is weak and including it can slightly improve our description at low pT on the basis of overall agreement with the data. We also predict the pT spectra of Σ*(1385) and Ξ*(1530), to be tested by the future experimental data.

EPOSHQ–a new approach to describe charmed mesons in pp, pA and AA collisions

We present first results of a new approach, EPOSHQ, which combines the EPOS3 event generator with the heavy quarks physics. In this approach light and heavy quarks are simultaneously created in the elementary collisions. The heavy quarks interact by elastic and radiative collisions with the plasma constituents, given by the EPOS3 approach, employing the full Boltzmann collision integral. This approach will allow for the description of correlations between light and heavy mesons.

Nuclear collisions at the Future Circular Collider

The Future Circular Collider is a new proposed collider at CERN with centre-of-mass energies around 100 TeV in the pp mode. Ongoing studies aim at assessing its physics potential and technical feasibility. Here we focus on updates in physics opportunities accessible in pA and AA collisions not covered in previous Quark Matter contributions, including Quark-Gluon Plasma and gluon saturation studies, novel hard probes of QCD matter, and photon-induced collisions.

The unreasonable effectiveness of hydrodynamics in heavy ion collisions

Event-by-event hydrodynamic simulations of AA and pA collisions involve initial energy densities with large spatial gradients. This is associated with the presence of large Knudsen numbers (Kn≈1) at early times, which may lead one to question the validity of the hydrodynamic approach in these rapidly evolving, largely inhomogeneous systems. A new procedure to smooth out the initial energy densities is employed to show that the initial spatial eccentricities, εn, are remarkably robust with respect to variations in the underlying scale of initial energy density spatial gradients, λ. For sNN=2.76 TeV LHC initial conditions generated by the MCKLN code, εn (across centralities) remains nearly constant if the fluctuation scale varies by an order of magnitude, i.e., when λ varies from 0.1 to 1 fm. Given that the local Knudsen number Kn≈1/λ, the robustness of the initial eccentricities with respect to changes in the fluctuation scale suggests that the vn's cannot be used to distinguish between events with large Kn from events where Kn is in the hydrodynamic regime. We use the 2+1 Lagrangian hydrodynamic code v-USPhydro to show that this is indeed the case: anisotropic flow coefficients computed within event-by-event viscous hydrodynamics are only sensitive to long wavelength scales of order 1/ΛQCD≈1 fm and are incredibly robust with respect to variations in the initial local Knudsen number. This robustness can be used to justify the somewhat unreasonable effectiveness of the nearly perfect fluid paradigm in heavy ion collisions.