Session MB: Heavy Ions and Heavy Flavor

System size scan of heavy flavor $R_{AA}$ and $v_n$ using PbPb, XeXe, ArAr, and OO collisions

Experimental measurements indicate no suppression (e.g. $R_{pPb} \sim 1$) but a surprisingly large D meson $v_2$ was measured in pPb collisions. In order to understand these results we use Trento+v-USPhydro+DAB-MOD to make predictions and propose a system size scan at the LHC involving $^{208}$PbPb, $^{129}$XeXe, $^{40}$ArAr, and $^{16}$OO collisions. Whatever the chosen transport model, we find that the nuclear modification factor approaches unity as the system size is decreased, but nonetheless, in the 0-10$\%$ most central collisions $v_2\{2\}$ is roughly equivalent regardless of system size. These results arise from a rather non-trivial interplay between the shrinking path length and the enhancement of eccentricities in smaller systems. Finally, we also find a surprising sensitivity of D mesons $v_2\{2\}$ in 0-10$\%$ centrality class at $p_T = 2-10 \,{\rm GeV}$ to the slight deformation of $^{129}$Xe recently found at LHC.   

Measurements of charmonia production in p+p collision at $\sqrt{s}$ = 510 and 500 GeV at the STAR experiment

Quarkonium states produced in heavy-ion collisions serve as essential probes in studying the Quark-Gluon Plasma. It requires a good understanding of quarkonium production mechanism when interpreting the observed quarkonium suppression in heavy-ion collisions. There are several popular models on the market for describing quarkonium production, such as the Color Singlet Model, the Color Evaporation Model, the nonrelativistic QCD (NRQCD) formalism including also the Color Octet Mechanism, and the Color Glass Condensate + NRQCD formalism. Precise measurements of quarkonium spectrum from low to high transverse momentum can provide important tests of the models and deepen our understanding of the quarkonium production mechanism in p+p collisions. STAR is one of the running heavy-ion experiments in the world and provides a large acceptance coverage to study quarkonium production at mid-rapidity. In this presentation, we will present the latest results and progress of measuring production cross sections of $J/\psi$ and $\psi$(2S) from the STAR experiment in $\sqrt{s}$ = 510 and 500 GeV p+p collisions.   

$b\bar{b}$ production at forward rapidity in $p$+$p$ collisions at $\sqrt{s}=510$ GeV

Heavy flavor quarks are an important probe of the initial state of the Quark Gluon Plasma formed in heavy-ion collisions. Bottom and charm quarks are produced early in the collision, primarily through hard interactions, and experience the full time evolution of the medium. Understanding bottom quark production in $p$+$p$ collisions gives a baseline reference for studying larger collision systems. The measurement of the $b\bar{b}$ cross section gives insight into $b$ quark production mechanisms which can directly test pQCD predictions. Utilizing the unique properties of neutral $B$ meson oscillation, the $b\bar{b}$ signal is extracted from like-sign dimuons with invariant mass of 5-10 GeV/$c^2$ at forward rapidity. Measuring like-sign dimuons within this mass range provides an enriched bottom signal with minimal amount of open charm background and without any contributions from quarkonia or Drell-Yan pairs. In this talk, we report the $b\bar{b}$ differential cross section and extrapolated total cross section. The azimuthal opening angle between muon pairs from $b\bar{b}$ decays and their $p_T$ distributions will also be presented. The total cross section at $\sqrt{s}=510$ GeV is compared to world data at different energies and to a perturbative quantum chromodynamics calculation.   

Beauty-hadron decay electron production in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV with ALICE

The LHC heavy-ion program aims at investigating the properties of the quark-gluon plasma (QGP), a state of matter in which quarks and gluons are deconfined. Heavy-flavor quarks (charm and beauty) are effective probes of the QGP, as they are created via initial hard partonic scatterings and subsequently experience the full evolution of the QGP medium. With the ALICE detector, we are able to study beauty quarks through measurements of beauty-hadron decay electrons at mid-rapidity. Since beauty hadrons have a longer lifetime than other electron sources, their decay electrons can be separated using the distance of closest approach of the track to the primary vertex. In this talk, the $R_{\rm{AA}}$ of beauty-hadron decay electrons in 0-10% and 30-50% Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be presented. The results will be compared with model predictions and other measurements to assess the mass dependence of in-medium energy loss.   

Electron-muon correlations in p$+$p and p$+$A collisions at $\sqrt{s}=200~$GeV using the PHENIX detector at RHIC

The measurement of electron-muon pairs, coming from heavy quark pair decays, suffers less from background as compared to $e^+e^-$ or $\mu^+\mu^-$ pairs since neither direct lepton production nor resonance decays produce this type of correlated signal. Heavy-flavor modification in relativistic p(d)$+$A collisions is sensitive to different kinds of strong-interaction physics ranging from modifications of the nuclear wave function to initial- and final-state energy loss. PHENIX has published $e$-$\mu$ correlations in p$+$p and d$+$Au collisions, where a suppression was seen in the pair yield per collision in d$+$Au collisions. In the years 2014 and 2015, PHENIX collected high luminosity datasets for p$+$p, p$+$Al, p$+$Au and $^3$He$+$Au collisions. A systematic study of $e$-$\mu$ correlations in these systems will help us to understand modifications to nuclear parton distribution function (nPDF) at different regions in \textbf{Bjorken x} space, and as a function of nuclear medium density. In this talk, we will present the current status of these studies in small collision systems.   

Heavy-flavour production in small systems with ALICE at the LHC

Heavy quarks are produced in the initial stages of hadronic collisions via hard scattering processes and experience the complete evolution of the hot and dense medium created in heavy-ion collision, known as quark-gluon plasma. Measurement of their production in pp collisions provides baseline for observations of hot-medium effects in heavy-ion collisions, as well as tests of perturbative QCD calculations. A deeper understanding of heavy-flavour production in Pb$-$Pb collisions requires a detailed study of Cold Nuclear Matter effects in order to disentangle the role of initial- and final-state effects on their production. Measurements in small systems at high multiplicity recently gained additional interest due to the observation of the signal typical of the heavy-ion phenomenology. To study in detail the (initial- and/or final-state) effects playing a role on these observations, measurement of heavy-flavour production as a function of multiplicity is performed. In this talk, the most recent results on production cross section, nuclear modification factor and multiplicity dependence studies of production of D mesons and heavy-flavour hadron decay electrons at mid-rapidity and of muons at forward rapidity will be presented. The results will be compared with model calculations.   

Measurements of quarkonium suppression in Au+Au collisions at sqrt{sNN} = 200 GeV with the STAR experiment

The quark-gluon plasma (QGP) is believed to have existed in the early universe, and can be created in laboratory through ultra-relativistic heavy-ion collisions. Quarkonia are expected to dissociate in the QGP due to the color screening of the quark-antiquark potential by the surrounding partons. Such a dissociation occurs when the quarkonium size exceeds the medium Debye radius, which is inversely proportional to the medium temperature. Consequently, quarkonium suppression in heavy-ion collisions was proposed as a strong evidence of the QGP formation. Furthermore, the three bottomonium states of different binding energies are expected to dissociate at different temperatures, which can be used to constrain the temperature of the QGP. In this talk, we will report the latest measurements of $J/\psi$ and $\Upsilon$ suppression in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV by the STAR experiment at RHIC. A strong suppression of high-$p_{\rm{T}}$ $J/\psi$ is observed in head-on Au+Au collisions. Furthermore, a sequential suppression pattern is observed for the $\Upsilon$ mesons, with the excited states more suppressed than the ground state. These measurements will be compared with model calculations and the physics implications will be discussed.   

$J/\psi$ production in Au+Au collisions at $\sqrt{s} = 54.4$ GeV

Heavy quarkonia are ideal probes of the Quark-Gluon Plasma. $J/\psi$ is the most abundantly produced quarkonium state accessible experimentally and its suppression due to the color screening effects in hot and dense medium has been suggested as a signature of the formation of the QGP. Besides the screening effects, there are other mechanisms, such as the cold nuclear effects and charm quark recombination, which could affect the $J/\psi$ yield in heavy-ion collisions. Measurements of $J/\psi$ production in different collision energies and systems will help to understand the interplay of these mechanisms for $J/\psi$ production. STAR has measured the suppression of $J/\psi$ production at mid-rapidity in Au+Au collisions at $\sqrt{s}$ = 39, 62.4 and 200 GeV. The nuclear modification factors show no significant collision energy dependence from SPS to RHIC top energy. However, uncertainties of the STAR measurements at 39 and 62.4 GeV are large. In 2017, STAR took a large sample of 54.4 GeV Au+Au collisions and the statistics is more than ten times of the 39 and 62.4 GeV Au+Au data. In this talk, we will present the measurement of inclusive $J/\psi$ production in Au+Au collisions at $\sqrt{s} = 54.4$ GeV by STAR experiment and discuss physics implications.   

Study of Heavy-Flavor Production at Forward Rapidity in Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV

At RHIC energies, heavy flavor quarks are produced predominantly via hard interaction, prior to the formation and evolution of the Quark-Gluon Plasma (QGP). Once produced, they are excellent probes of parton energy loss mechanism and transport coefficients in both cold nuclear matter and the QGP medium. We measure yields as a function of transverse momenta of muons from the decay of heavy flavor hadrons in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. The measurement will be carried out in forward rapidity region, and a previously published PHENIX measurement in $p+p$ collisions will be used as a baseline for determination of nuclear modification factor. In this talk, current status of the analysis and the techniques applied will be presented.   

Study of $J/\psi$ production with jet activity in $pp$ collisions at $\sqrt{s}$ = 200 GeV at the STAR experiment

The fragmentation function is believed to play a crucial role to differentiate different production mechanisms of $J/\psi$. In order to have a better understanding of charm quarks and gluons fragmenting into $J/\psi$, production of $J/\psi$ mesons in jets has been studied in $pp$ collisions at the LHC and RHIC. The measured $J/\psi$ production is found to be much less isolated than nonrelativistic QCD (NRQCD) prediction as implemented in PYTHIA. The dependence of $J/\psi$ production on jet activity (the number of jets per event) is another observable which could be easily compared with theoretical predictions to understand the relation between jet fragmentation and $J/\psi$ production. In this talk, we will present the progress of the study of $J/\psi$ production with jet activity at mid-rapidity in $pp$ collisions at $\sqrt{s}$ = 200 GeV at the STAR experiment.