Minimum Bias Au Research Articles

Qualification of DIRICH readout chain

The DIRICH front-end board (FEB) is a 32+1 channel FPGA based TDC readout module for Multianode Photomultipliers (MAPMT), and also Multianode MCPs. Due to its low cost and excellent timing precision in single photon measurement applications it will be used in the RICH detector of the Compressed Baryonic Matter (CBM) experiment at the future FAIR facility at GSI Darmstadt, Germany.In the regions of highest photon density at the CBM RICH one expects an average hit rate of ≃300 kHz per MAPMT pixel (6 mm × 6 mm) and ∼15% occupancy for minimum bias Au+Au collisions at 35 A GeV at an interaction rate of 10 MHz. In order to validate the high-rate capability of the DIRICH readout, a dedicated lab setup simulating realistic detector signals by employing a pulsed picosecond laser light source in combination with a constant current driven LED was commissioned. In this paper the setup is introduced and first results are discussed. We show that individual readout channels can withstand photon rates up to 2.2 MHz per pixel, limited only by maximum data rate capability and buffer size on the front-end board. Using the same setup, also effects of high photon occupancy on the MAPMTs are investigated, which might cause additional signals due to capacitive cross-talk within the MAPMT or readout chain. Occupancies of up to 55% (simultaneous photon hits on more than half of the MAPMT pixels) are investigated, indicating that in the expected occupancy range of 10%–15% the readout works flawlessly with very low cross-talk.

Measurement of D0 -meson + hadron two-dimensional angular correlations in Au+Au collisions at sNN=200 GeV

Open heavy flavor hadrons provide unique probes of the medium produced in ultra-relativistic heavy-ion collisions. Due to their increased mass relative to light-flavor hadrons, long lifetime, and early production in hard-scattering interactions, they provide access to the full evolution of the partonic medium formed in heavy-ion collisions. This paper reports two-dimensional (2D) angular correlations between neutral $D$-mesons and unidentified charged particles produced in minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. $D^0$ and $\bar{D}^0$ mesons are reconstructed via their weak decay to $K^{\mp} \pi^{\pm}$ using the Heavy Flavor Tracker (HFT) in the Solenoidal Tracker at RHIC (STAR) experiment. Correlations on relative pseudorapidity and azimuth $(\Delta\eta,\Delta\phi)$ are presented for peripheral, mid-central and central collisions with $D^0$ transverse momentum from 2 to 10 GeV/$c$. Attention is focused on the 2D peaked correlation structure near the triggered $D^0$-meson, the {\em near-side} (NS) peak, which serves as a proxy for a charm-quark containing jet. The correlated NS yield of charged particles per $D^0$-meson and the 2D widths of the NS peak increase significantly from peripheral to central collisions. These results are compared with similar correlations using unidentified charged particles, consisting primarily of light-flavor hadrons, at similar trigger particle momenta. Similar per-trigger yields and widths of the NS correlation peak are observed. The present results provide additional evidence that $D^0$-mesons undergo significant interactions with the medium formed in heavy-ion collision and show, for the first time, significant centrality evolution of the NS 2D peak in the correlations of particles associated with a heavy-flavor hadron produced in these collisions.

Charm and beauty isolation from heavy flavor decay electrons in Au+Au collisions at [formula omitted] = 200 GeV at RHIC

We present a study of charm and beauty isolation based on a data-driven method with recent measurements on heavy flavor hadrons and their decay electrons in Au+Au collisions at sNN = 200 GeV at RHIC. The individual electron pT spectra, RAA and v2 distributions from charmed and beauty hadron decays are obtained. We find that the electron RAA from beauty hadron decays (RAAb→e) is suppressed in minimum bias Au+Au collisions but less suppressed compared with that from charmed hadron decays at pT > 3.5 GeV/c, which indicates that beauty quark interacts with the hot-dense medium with depositing its energy and is consistent with the mass-dependent energy loss scenario. For the first time, the non-zero electron v2 from beauty hadron decays (v2b→e) at pT > 3.0 GeV/c is observed and shows smaller elliptic flow compared with that from charmed hadron decays at pT < 4.0 GeV/c. At 2.5 GeV/c <pT < 4.5 GeV/c, v2b→e is smaller than a number-of-constituent-quark (NCQ) scaling hypothesis. This suggests that beauty quark is unlikely thermalized and too heavy to be moved in a partonic collectivity in heavy-ion collisions at the RHIC energy.

Two-particle correlations on transverse momentum: an untapped resource for studying relativistic heavy-ion collision dynamics

Two-particle correlation projections onto two-dimensional transverse momentum coordinates (pt1,pt2) allow access to properties of the relativistic heavy-ion collision system which are complementary to that studied using angular correlations. Examples include the degree of thermal equilibration and the variance of dynamical fluctuations in hard-scattering processes. Results for minimum-bias Au + Au collisions at √sNN= 200 are presented, with the structures described by two phenomenological models. The correlations structures and extracted physical quantities are then compared to theoretical predictions. Conclusions from these comparisons regarding global equilibration, fluctuations in soft and semi-hard QCD processes, and the effects of the hot, dense collision medium are presented.

Phenomenological models of two-particle correlation distributions on transverse momentum in relativistic heavy-ion collisions

Two-particle, pair-number correlation distributions on two-dimensional transverse momentum ($p_{t1},p_{t2}$) constructed from the particle production in relativistic heavy-ion collisions allow access to dynamical processes in these systems beyond what can be studied with angular correlations alone. Only a few measurements of this type have been reported in the literature and phenomenological models, which facilitate physical interpretation of the correlation structures, are non-existent. On-going effort at the Relativistic Heavy-Ion Collider (RHIC) will provide a significant volume of these correlation measurements in the future. In anticipation of these new data two phenomenological models are developed which describe two-dimensional 2D correlation distributions on transverse momentum. One model is based on a collision event-by-event fluctuating blast wave. The other is based on event-by-event fluctuations in fragmenting color-flux tubes and in jets. Both models are shown to be capable of accurately describing the measured single-particle $p_t$ distributions for minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$~GeV. Both models are then applied to preliminary, charged-particle correlation measurements on 2D transverse momentum. The capabilities of the two models for describing the overall structure of these correlations, the stability of the fitting results with respect to collision centrality, and the resulting trends of the dynamical fluctuations are evaluated. In general, both phenomenological models are capable of qualitatively describing the major correlation structures on transverse momentum and can be used to establish the required magnitudes and centrality trends of the fluctuations. Both models will be useful for interpreting the forthcoming correlation data from the RHIC.

Nuclear modification factor and flow of charm and bottom quarks in Au+Au collisions at [formula omitted] by the PHENIX Experiment

Experimental results at RHIC and at the LHC show a similar strong suppression for light and heavy quark probes at high pT, and a possible quark mass dependence of suppression at low pT. More high precision measurements of separated charm and bottom are needed to quantify the dependence of medium effects on the quark mass. The PHENIX experiment measured separated electrons from bottom and charm decays using displaced vertex distributions at mid-rapidity |y|<0.35. Azimuthal anisotropy of electrons from separated charm and bottom decays are obtained from high statistics Au+Au data taken in 2014. PHENIX also measures muons from heavy quark decays in d+Au collisions at forward rapidity to study collective effects on heavy flavor production in small systems. These proceedings report the azimuthal anisotropy of charm and bottom at mid-rapidity in minimum bias Au+Au collisions and the anisotropy of single muons from heavy quark decays in d+Au collisions.

Discrepancy in low transverse momentum dileptons from relativistic heavy-ion collisions

The dilepton transverse momentum spectra and invariant mass spectra for low $p_T <0.15$~GeV/c in Au+Au collisions of different centralities at $\sqrt{s_{NN}}$ = 200 GeV are studied within the parton-hadron-string dynamics (PHSD) transport approach. The PHSD describes the whole evolution of the system on a microscopic basis, incorporates hadronic and partonic degrees-of-freedom, the dynamical hadronization of partons and hadronic rescattering. For dilepton production in p+p, p+A and A+A reactions the PHSD incorporates the leading hadronic and partonic channels (also for heavy flavors) and includes in-medium effects such as a broadening of the vector meson spectral functions in hadronic matter and a modification of initial heavy-flavor correlations by interactions with the partonic and hadronic medium. The transport calculations reproduce well the momentum integrated invariant mass spectra from the STAR Collaboration for minimum bias Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, while the description of the STAR data - when gating on low $p_T < 0.15$ GeV/c - is getting worse when going from central to peripheral collisions. An analysis of the transverse momentum spectra shows that the data for peripheral (60-80\%) collisions are well reproduced for $p_T>0.2$ GeV/c while the strong peak at low $p_T < 0.15$ GeV/c, that shows up in the experimental data for the mass bins ($0.4 < M < 0.7$ GeV and $1.2 < M < 2.6$ GeV), is fully missed by the PHSD and cannot be explained by the standard in-medium effects. This provides a new puzzle for microscopic descriptions of low $p_T$ dilepton data from the STAR Collaboration.

PHENIX measurements of single electrons from charm and bottom decays at midrapidity in Au+Au collisions

Heavy quarks (charm and bottom) are a sensitive probe of the Quark-Gluon Plasma (QGP) created in high-energy nuclear collisions. The modification of their phase-space distribution in the QGP reflects the physical property of the QGP because they are generated in the early stage of collisions and subsequently propagate through the QGP. PHENIX has published the first measurement of the nuclear modification factors RAA of separated charm and bottom hadron decay electrons in minimum bias Au+Au collisions at sNN=200GeV using the 2011 data set. In these proceedings, we present RAA(c→e) and RAA(b→e) in most central Au+Au collisions at sNN=200GeV using 1/8 of our large 2014–2016 data set.

Direct photon production from the low-mass e+e− measurements at STAR

We report the direct photon invariant yields in the transverse momentum ranges 1<pT<3 GeV/c and 5<pT<10 GeV/c at mid-rapidity derived from the low-mass e+e− invariant mass continuum in 0–80% minimum-bias Au+Au collisions at sNN=200 GeV. Centrality dependence of direct photon yields is presented. A clear excess of the invariant yield relative to the yield in p+p scaled by nuclear overlap function TAA is observed in the pT range 1<pT<3 GeV/c. The production follows TAA scaling at pT>6 GeV/c.

Measurement of D0 Meson Production and Azimuthal Anisotropy in Au+Au Collisions at [formula omitted

Due to the large masses, heavy-flavor quarks are dominantly produced in initial hard scattering processes and experience the whole evolution of the medium produced in heavy-ion collisions at RHIC energies. They are also expected to thermalize slower than light-flavor quarks. Thus the measurement of heavy quark production and azimuthal anisotropy can provide important insights into the medium properties through their interactions with the medium.In these proceedings, we report measurements of D0 production and elliptic flow (v2 via topological reconstruction using STAR's recently installed Heavy Flavor Tracker (HFT). The new measurement of the nuclear modification factor (RAA) of D0 mesons in central Au+Au collisions at sNN=200 GeV confirms the strong suppression at high transverse momenta (pT) reported in the previous publication with much improved precision. We also report the measurement of elliptic flow for D0 mesons in a wide transverse momentum range in 0–80% minimum-bias Au+Au collisions. The D0 elliptic flow is finite for pT>2 GeV/c and is systematically below that of light hadrons in the same centrality interval. Furthermore, several theoretical calculations are compared to both RAA and v2 measurements, and the charm quark diffusion coefficient is inferred to be between 2 and ∼12.

Production of e+e− from U+U collisions at [formula omitted] and Au+Au collisions at [formula omitted], and 200 GeV as measured by STAR

We present STAR's measurement of the e+e− continuum as a function of centrality, invariant mass, and transverse momentum for U+U collisions at sNN=193 GeV. Also reported are the acceptance-corrected e+e− invariant mass spectra for minimum-bias Au+Au collisions at sNN=27,39, and 62.4 GeV and U+U collisions at sNN=193 GeV. The connection between the integrated e+e− excess yields normalized by charge particle multiplicity (dNch/dy) at mid-rapidity and the lifetime of the fireball is discussed.

Direct virtual photon production in Au+Au collisions at [formula omitted

We report the direct virtual photon invariant yields in the transverse momentum ranges 1<pT<3 GeV/c and 5<pT<10 GeV/c at mid-rapidity derived from the dielectron invariant mass continuum region 0.10<Mee<0.28 GeV/c2 for 0–80% minimum-bias Au+Au collisions at sNN=200 GeV. A clear excess in the invariant yield compared to the nuclear overlap function TAA scaled p+p reference is observed in the pT range 1<pT<3 GeV/c. For pT>6 GeV/c the production follows TAA scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.

Measurements of azimuthal anisotropy for high pT charged hadrons at in Au+Au at RHIC-PHENIX

The azimuthal anisotropy of particle emission is a good observable to study the property of the quark gluon plasma (QGP), from the point of view of not only its collective dynamics but also the energy loss of hard scattered partons in it. We measured charged hadron v2 as a function of pT in 0.5 < pT < 10 GeV/c for minimum bias and several centrality Au+Au collisions at using the RHIC Year-2014 data, which significantly improved the statistical precision compared to the previous results. It was found that the v2’s for charged hadrons are consistently above zero up to pT = 10 GeV/c over the centralities. It was also found that there is a difference in v2 between charged hadrons and neutral pions for pT <7 GeV/c.

Insight from elliptic flow of open charm mesons using quark coalescence model at RHIC and LHC energies

A study of elliptic flow of open charm mesons, D0 and using quark coalescence as the mechanism of hadronization of heavy quarks will be presented. The coalescing partons are taken from a multi-phase transport model. The transverse momentum dependence of the elliptic flow parameter at mid-rapidity (|y| < 1.0) for minimum bias Au+Au collisions at > = 200 GeV (RHIC) and Pb+Pb collisions = 2.76 TeV (LHC) for different values of partonic interaction cross-section and QCD coupling constant will be discussed. We have compared our calculations with the experimentally measured data at the LHC energy. We will also present the effect of specific viscosity on elliptic flow of open charm mesons within the transport model approach. Our study indicates that the elliptic flow of open charmed mesons is more sensitive to viscous properties of QGP medium compared to light hadrons.

Single electrons from semi-leptonic charm and bottom hadron decays in Au+Au collisions at PHENIX

Heavy quarks are clean probes to explore the nature of strongly coupled quark gluon plasma created in high energy heavy ion collisions. The strong suppression of single electrons from semi-leptonic decays of heavy flavor hadrons was observed. To further understand the heavy quark suppressions, PHENIX installed the silicon vertex detector (VTX) which allows us to measure the bottom and charm productions separately from measurement of displaced tracks. For the first time, we observed the electrons from bottom hadron decays are less suppressed than those from charms for 3 < pT < 4 GeV/c and are similarly strongly suppressed for higher pT in minimum bias Au+Au collisions at √sNN = 200 GeV. We present the results of separated bottom and charm productions using the 2011 dataset with the VTX.

Single electron yields from semileptonic charm and bottom hadron decays inAu+Aucollisions atsNN=200GeV

The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy-flavor production in minimum bias Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV via the yields of electrons from semileptonic decays of charm and bottom hadrons. Previous heavy-flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks due to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au$+$Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV and find the fractions to be similar within the large uncertainties on both measurements for $p_T>4$ GeV/$c$. We use the bottom electron fractions in Au$+$Au and $p$$+$$p$ along with the previously measured heavy flavor electron $R_{AA}$ to calculate the $R_{AA}$ for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region $3<p_T<4$ GeV/$c$.

Spectra and elliptic flow for Λ, Ξ, and Ω in 200 A GeV Au+Au collisions

Using VISHNU hybrid model, we calculate the pT-spectra and elliptic flow of Λ, Ξ, and Ω in 200 A GeV Au+Au collisions. Comparisons with the STAR measurements show that the model generally describes these soft hadron data. We also briefly study and discuss the mass ordering of elliptic flow among π, K, p, Λ, Ξ, and Ω in minimum bias Au+Au collisions.

Energy dependence of acceptance-corrected dielectron excess mass spectrum at mid-rapidity in [formula omitted] collisions at [formula omitted] and 200 GeV

The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity |yee|<1 in minimum-bias Au+Au collisions at sNN=19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for Mee<1.1 GeV/c2. The integrated dielectron excess yield at sNN=19.6 GeV for 0.4<Mee<0.75 GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at sNN=17.3 GeV. For sNN=200 GeV, the normalized excess yield in central collisions is higher than that at sNN=17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at sNN=200 GeV is longer than those in peripheral collisions and at lower energies.

Dielectron production in Au + Au collisions from STAR

We present the systematic study of dielectron production from minimum bias Au + Au collisions at s NN = 19.6 , 39 , and 62.4 GeV along with the production from minimum bias Au + Au collisions at 200 GeV for reference. The spectra are compared to simulated hadronic contributions that do not contain ρ -meson contributions. In each comparison, there is an excess in the dielectron yield that is consistent with a medium modified ρ -meson and shows no dependence on s NN . To further understand the excess, the spectra are compared to the simulated hadronic contributions plus model calculations that include contributions from the medium and ρ -mesons with a broadened spectral function. These comparisons are consistent within uncertainties.

Beam energy dependence of dielectron production in Au+Au collisions from STAR at RHIC

We present the energy-dependent study of dielectron production in 0-80% minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ energies of 19.6, 27, 39, and 62.4 GeV in STAR. Invariant mass ($M_{ee}$) and transverse momentum ($p_T$) differential measurements of dielectron yields are compared to cocktail simulations of known hadronic sources and semi-leptonic charmed decays. The enhancement (excess yield) prominent in the Low-Mass Region (LMR) over the cocktail at all energies, is further compared to calculations of $\rho$ in-medium modifications. Within statistical and systematic uncertainties, we find that the model consistently describes this enhancement from SPS up to top RHIC energies in its $M_{ee}$- as well as $p_T$-dependence. Dielectron measurements drive the statistics for the future BES Phase-II program, which promises to improve our understanding of the LMR enhancement's trend with total baryon densities.