Quark Energy Loss Research Articles

Light and heavy flavor dijet production and dijet mass modification in heavy ion collisions

Back-to-back light and heavy flavor dijet measurements are promising experimental channels to accurately study the physics of jet production and propagation in a dense QCD medium. They can provide new insights into the path length, color charge, and mass dependence of quark and gluon energy loss in the quark-gluon plasma produced in reactions of ultra-relativistic nuclei. To this end, we perform a comprehensive study of both light and heavy flavor dijet production in heavy ion collisions. We propose the modification of dijet invariant mass distributions in such reactions as a novel observable that shows enhanced sensitivity to the QGP transport properties and heavy quark mass effects on in-medium parton showers. This is achieved through the addition of the jet quenching effects on the individual jets as opposed to their subtraction. The latter drives the subtle effects on more conventional observables, such as the dijet momentum imbalance shifts, which we also calculate here. Results are presented in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for comparison to data at the Large Hadron Collider and in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV to guide the future sPHENIX program at the Relativistic Heavy Ion Collider.

Quarkonium production in heavy ion collisions: coupled Boltzmann transport equations

We develop a set of coupled Boltzmann equations to describe the dynamical evolution of heavy quarks and quarkonia inside the quark-gluon plasma. The quarkonium dissociation and recombination terms are calculated from pNRQCD. Their interplay drives the system to a detailed balance. The heavy quark energy loss term is necessary for the system to reach kinematic thermalization. By coupling the transport equations with initial particles' momenta generated by Pythia and hydrodynamic medium evolutions, we can describe the RAA of ϒ family at both RHIC and LHC energies. The transverse momentum azimuthal anisotropy of ϒ(1S) in 2.76 TeV peripheral Pb-Pb collisions is also studied.

Measurement of azimuthal correlations of D mesons with charged particles in pp collisions at √s=13$\sqrt s = 13$ TeV with ALICE at the LHC

The ALICE (A Large Ion Collider Experiment) detector at the LHC (Large Hadron Collider) is dedicated to the study of the properties of the hot and dense QCD matter (Quark–Gluon Plasma) produced in the nucleus-nucleus collisions at high energy. Heavy quarks (charm and beauty), having large masses, are produced in the hard-parton scattering in the early stages of the collision. Therefore, they experience the whole evolution of the hot and dense medium, representing an important tool for its characterization. The study of angular correlations between D mesons and charged particles in Pb–Pb collisions gives insight about the energy loss of charm quarks and the medium-induced modification of its fragmentation into jets. Moreover, pp collisions help to understand the production mechanisms, fragmentation and hadronization of charm quarks and acts as a reference for p–Pb and Pb–Pb measurements. In this article, the measurement of azimuthal correlations between D0 meson and charged particles in pp collisions at $\sqrt s = 13$ TeV is presented. The collisional energy dependence of the correlations is extracted from the comparison with the results at $\sqrt s = 7$ TeV. The data are also compared with simulations performed with different event generators.

Heavy quarks and chromo-electromagnetic field fluctuations: The Drag, Diffusion coefficient and Nuclear modification factor

We have calculated the temperature dependent drag and diffussion coefficients of heavy quarks and nuclear modification factor (RAA) of heavy mesons (D and B mesons) by considering the energy gain due to chromo-electromagnetic field fluctuations along with collisional and radiative energy loss of charm and beauty quarks in the hot and dense deconfined medium of quarks and gluons created in relativistic heavy ion collisions. Our results are in good agreement with the experimentally measured RAA of D and B mesons by ALICE and CMS experiments at and .

Revisiting heavy quark radiative energy loss in nuclei within the high-twist approach

We revisit the calculation of multiple parton scattering of a heavy quark in nuclei within the framework of recently improved high-twist factorization formalism, in which gauge invariance is ensured by a delicate setup of the initial partons' transverse momenta. We derive a new result for medium modified heavy quark fragmentation functions in deeply inelastic scattering. It is consistent with the previous calculation of light quark energy loss in the massless limit, but leads to a new correction term in the heavy quark case, which vanishes in the soft gluon radiation limit. We show numerically the significance of the new correction term in the calculation of heavy quark energy loss as compared to previous studies and with soft gluon radiation approximation.

Radiative heavy quark energy loss in an expanding viscous QCD plasma

We study viscous effects on heavy quark radiative energy loss in a dynamically screened medium with boost-invariant longitudinal expansion. We calculate, to first order in opacity, the energy loss by incorporating viscous corrections in the single-particle phase-space distribution function within relativistic dissipative hydrodynamics. We consider the Grad's 14-moment and the Chapman-Enskog-like methods for the nonequilibrium distribution functions. Our numerical results for the charm quark radiative energy loss show that, as compared to static fluid, an expanding ideal (nonviscous) fluid causes a much smaller energy loss. Viscosity in the evolution lead to somewhat enhanced energy loss which is insensitive to the underlying viscous hydrodynamic models used. Further inclusion of viscous correction induces larger energy loss, the magnitude and pattern of this enhancement crucially depend on the form of viscous corrections used.

Measurement of Prompt D^{0} Meson Azimuthal Anisotropy in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The prompt D^{0} meson azimuthal anisotropy coefficients, v_{2} and v_{3}, are measured at midrapidity (|y|<1.0) in Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with data collected by the CMS experiment. The measurement is performed in the transverse momentum (p_{T}) range of 1 to 40 GeV/c, for central and midcentral collisions. The v_{2} coefficient is found to be positive throughout the p_{T} range studied. The first measurement of the prompt D^{0} meson v_{3} coefficient is performed, and values up to 0.07 are observed for p_{T} around 4 GeV/c. Compared to measurements of charged particles, a similar p_{T} dependence, but smaller magnitude for p_{T}<6 GeV/c, is found for prompt D^{0} meson v_{2} and v_{3} coefficients. The results are consistent with the presence of collective motion of charm quarks at low p_{T} and a path length dependence of charm quark energy loss at high p_{T}, thereby providing new constraints on the theoretical description of the interactions between charm quarks and the quark-gluon plasma.

The energy loss and nuclear absorption effects in semi-inclusive deep inelastic scattering on nucleus

Semi-inclusive deep inelastic lepton-nucleus scattering provides a good opportunity to investigate the cold nuclear effects on quark propagation and hadronization. Considering the nuclear modification of the quark energy loss and nuclear absorption effects in final state, the leading-order computations on hadron multiplicity ratios for both hadronization occurring outside and inside the medium are performed with the nuclear geometry effect of the path length L of the struck quark in the medium. By fitting the HERMES two-dimensional data on the multiplicity ratios for positively and negatively charged pions and kaons produced on neon, the hadron–nucleon inelastic cross section for different identified hadrons is determined, respectively. It is found that our predictions obtained with the analytic parameterizations of quenching weights based on BDMPS formalism and the nuclear absorption factor are in good agreement with the experimental measurements. This indicates that the energy loss and nuclear absorption are the main nuclear effects inducing a reduction of the hadron yield for quark hadronization occurring outside and inside the nucleus, respectively.

Event-by-event correlations between soft hadrons and D0 mesons in 5.02 TeV PbPb collisions at the CERN Large Hadron Collider

In this paper heavy quark energy loss models are embedded in full event-by-event viscous hydrodynamic simulations to investigate the nuclear suppression factor and azimuthal anisotropy of D$^0$ mesons in PbPb collisions at 5.02 TeV in the $p_T$ range 8-40 GeV. In our model calculations, the $R_\text{AA}$ of D$^0$ mesons is consistent with experimental data from the CMS experiment. We present the first calculations of heavy flavor cumulants $v_2\{2\}$ and $v_3\{2\}$ (and also discuss $v_2\{4\}$), which is also consistent with experimental data. Event-shape engineering techniques are used to compute the event-by-event correlation between the soft hadron $v_n$ and the heavy meson $v_n$. We predict a linear correlation between these observables on an event-by-event basis.

Measurement of the B^{±} Meson Nuclear Modification Factor in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The differential production cross sections of B^{±} mesons are measured via the exclusive decay channels B^{±}→J/ψK^{±}→μ^{+}μ^{-}K^{±} as a function of transverse momentum in pp and Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with the CMS detector at the LHC. The pp(Pb-Pb) data set used for this analysis corresponds to an integrated luminosity of 28.0 pb^{-1} (351 μb^{-1}). The measurement is performed in the B^{±} meson transverse momentum range of 7 to 50 GeV/c, in the rapidity interval |y|<2.4. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from pp reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Heavy and light hadron production and D-hadron correlation in relativistic heavy-ion collisions

We establish a linear Boltzmann transport (LBT) model coupled to hydrodynamical background to study hard parton evolution in heavy-ion collisions. Both elastic and inelastic scatterings are included in our calculations; and heavy and light flavor partons are treated on the same footing. Within this LBT model, we provide good descriptions of heavy and light hadron suppression and anisotropic flow in heavy-ion collisions. Angular correlation functions between heavy and light flavor hadrons are studied for the first time and shown able to quantify not only the amount of heavy quark energy loss, but also how the parton energy is re-distributed in parton showers.

D meson nuclear modification factor and vn harmonics in PbPb collisions at 5.02 TeV with CMS

The measurement of heavy flavor production is a powerful tool to study the properties of the high-density QCD medium created in heavy-ion collisions as heavy quarks are sensitive to the transport properties of the medium and may interact with the QCD matter differently from light quarks. In particular, the comparison between the nuclear modification factors (RAA) of light- and heavy-flavor particles provides insights into the expected flavor dependence of in-medium parton energy loss. Furthermore, azimuthal anisotropy coefficients (vn) of heavy-flavor particles provide insights into the degree of the thermalization of the bulk medium at low pT, and unique information about the path length dependence of heavy quark energy loss at high pT. Using the large pp and PbPb samples collected at 5.02 TeV during the 2015 LHC run, high precision open charm measurements are performed with the CMS detector in a wide transverse momentum range. This allows us to set an important milestone in our understanding of the interactions between charm quarks and the medium. In this talk, the most recent results of the RAA, v2 and v3 of prompt D0 mesons in PbPb collisions at 5.02 TeV are presented and compared to the same results for charged particles (dominated by light flavor hadrons) at the same energy.

Event-by-event vn correlations of soft hadrons and heavy mesons in heavy ion collisions

Combining event-by-event hydrodynamics with heavy quark energy loss we compute correlations between the heavy and soft sectors for elliptic and triangular flow harmonics v2 and v3 of D0 mesons in PbPb collisions at 2.76 TeV and 5.02 TeV. Our results indicate that v3 is strongly influenced by the fragmentation temperature and that it builds up later than v2 during the evolution of the system.

PHENIX Measurement of B → J/ψ in Cu+Au collisions at [formula omitted] and in p+p collisions at 200 GeV and 510 GeV

This study is part of the RHIC program to probe properties of the Quark-Gluon Plasma medium properties using heavy quark energy loss. Part of the control of these measurements is to understand initial production of heavy quarks and how initial conditions in nucleus collisions can alter their yields. This manuscript reports the effort made by the PHENIX collaboration to measure total B-mesons yields, by looking at the fraction of muons from non-prompt J/ψ decays at displaced vertices in p+p and Cu+Au collisions. The total cross-sections of b-quarks in 200 GeV and 510 GeV p+p collisions follow the increasing trend from fixed target experiments to high energy results from the Tevatron and the LHC. Integrated pT and centrality B-meson yields in Cu+Au integrated over transverse momentum and centrality are consistent with no nuclear modification or some enhancement, in contrast to prompt J/ψ which shows a strong suppression.

The different energy loss mechanisms of inclusive and b-tagged reconstructed jets within ultra-relativistic heavy-ion collisions

The phenomenon of jet quenching provides essential information about the properties of hot and dense matter created in ultra-relativistic heavy-ion collisions. Recent results from experiments at the Large Hadron Collider (LHC) show evidence for an unexpectedly similar suppression of both light and heavy flavor jets. Furthermore, the role of radiative energy loss of heavy quarks is still under active discussion within the theoretical community. By employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), which numerically solves the 3+1 D Boltzmann equation both for light and heavy flavor partons, we calculate the nuclear modification factor of inclusive and b-tagged reconstructed jets in 0–10% central sLHC=2.76 A TeV Pb + Pb collisions. Based on perturbative QCD cross sections we find a suppression of both light and heavy flavor jets. While the inclusive jets are slightly too strong suppressed within Bamps in comparison with data, both elastic + radiative and only elastic interactions lead to a realistic b-tagged jet suppression. To further investigate light and heavy flavor energy loss we predict the R dependence of inclusive and b-tagged jet suppression. Furthermore, we propose the medium modification of b-tagged jet shapes as an observable for discriminating between different heavy quark energy loss scenarios.

High pT spectra and anisotropy of light and heavy hadrons

Data driven studies of heavy-ion results has played a big role in highlighting interesting features of these complex systems. In this proceeding, a simple QGP-brick interpretation of the RAA and v2 at high pT (pT≈10 GeV/c) is presented. This interpretation draws attention to two fundamental questions: is there an effect of the asymmetric QGP expansion on the v2 at high pT? is there an energy loss difference between quarks and gluons? Finally, it is discussed how these studies can be extended using Event-Shape Engineering and how they can be applied to compare the energy loss of light and heavy quarks.

Prompt D0 meson vn harmonics in PbPb collisions at 5.02 TeV

Due to large mass, heavy quarks are produced primarily at early stages of heavy-ion collisions, and therefore experience the full evolution of the system and carry information about the extent of thermalization of the QGP. Azimuthal anisotropy parameters (vn) of charm and bottom hadrons provide unique information about the path length dependent interactions between heavy quarks and the medium. To what extent heavy quarks at low pT flow with the medium is a good measure of the interaction strength. At high pT, v2 and v3 from path length dependent energy loss provide a powerful tool to study heavy quark energy loss mechanisms. With the large statistics PbPb samples at 5.02 TeV collected by the CMS detector during the 2015 LHC run, azimuthal anisotropy v2 and v3 of D0 meson is measured over a wide pT range and at different centralities. In this paper, the most recent results of prompt D0 meson vn parameters are presented, and compared to the charged hadron vn at the same energy and the latest theoretical calculations.

Heavy meson flow harmonics in event-by-event viscous relativistic hydrodynamics

Event-by-event viscous hydrodynamics is combined with heavy quark energy loss models to compute heavy flavor flow cumulants v2{2}, v3{2}, and v2{4} as well as the nuclear modification factors of D0 and B0 mesons in PbPb collisions at 2.76 TeV. Our results indicate that bottom quarks can flow as much as charm quarks in the pT range 8–30 GeV.

Bottomonium continuous production from unequilibrium bottom quarks in ultrarelativistic heavy ion collisions

We employ the Langevin equation and Wigner function to describe the bottom quark dynamical evolutions and their formation into a bound state in the expanding Quark Gluon Plasma (QGP). The additional suppressions from parton inelastic scatterings are supplemented in the regenerated bottomonium. Hot medium modifications on ϒ(1S) properties are studied consistently by taking the bottomonium potential to be the color-screened potential from Lattice results, which affects both ϒ(1S) regeneration and dissociation rates. Finally, we calculated the ϒ(1S) nuclear modification factor RAArege from bottom quark combination with different diffusion coefficients in Langevin equation, representing different thermalization of bottom quarks. In the central Pb–Pb collisions (b=0) at sNN=5.02 TeV, we find a non-negligible ϒ(1S) regeneration, and it is small in the minimum bias centrality. The connections between bottomonium regeneration and bottom quark energy loss in the heavy ion collisions are also discussed.

Energy loss of heavy quarks and B and D meson spectra in PbPb collisions at LHC energies

We study the production and evolution of charm and bottom quarks in hot partonic medium produced in heavy ion collisions. The heavy quarks loose energy in the medium which is reflected in the transverse momentum spectra of heavy mesons. The collisional energy loss of heavy quarks has been calculated using QCD calculations. The radiative energy loss is obtained using two models namely reaction operator formalism and generalized dead cone approach. The nuclear modification factors, RAA as a function of transverse momentum by including shadowing and energy loss are calculated for D0 and B+ mesons in PbPb collisions at sNN=5.02 TeV and for D0 mesons at sNN=2.76 TeV and are compared with the recent measurements. The radiative energy loss from generalized dead cone approach alone is sufficient to produce measured D0 meson RAA at both the LHC energies. The radiative energy loss from reaction operator formalism plus collisional energy loss gives good description of D0 meson RAA. For the case of B+ meson, the radiative energy loss from generalized dead cone approach plus collisional energy loss gives good description of the CMS data. The radiative process is dominant for charm quarks while for the bottom, both the radiative process and the elastic collisions are important.