Non-photonic Electron Spectra Research Articles

Recent PHENIX results on open heavy flavor

Throughout the history of the RHIC physics program, questions concerning the dynamics of heavy quarks have generated much experimental and theoretical investigation. A major focus of the PHENIX experiment is the measurement of these quarks through their semi-leptonic decay channels at mid and forward rapidity. Heavy quark measurements in p+p collisions give information on the production of heavy flavor, without complications from medium effects. New measurements in d +Au and Cu+Cu indicate surprising cold nuclear matter effects on these quarks at midrapidity and provide a new baseline for interpretation of the observed suppression in Au+Au collisions. When considered all together, these measurements present a detailed study of nuclear matter across a wide range of system size and temperature. Here, we present preliminary PHENIX measurements of non-photonic electron spectra and their centrality dependence in d+Au and Cu+Cu and discuss their implications on the current understanding of parton energy loss in the nuclear medium.

Heavy-quark Langevin dynamics and single-electron spectra in nucleus-nucleus collisions

The stochastic dynamics of heavy quarks in the fireball produced in heavy-ion collisions is followed through numerical simulations based on the Langevin equation. The modification of the final pT spectra (RAA) of c and b quarks, hadrons and single-electrons with respect to pp collisions is studied. The transport coefficients are evaluated treating separately the contribution of soft and hard collisions. The initial heavy-quark spectra are generated according to NLO-pQCD, accounting for nuclear effects through recent nPDFs. The evolution of the medium is obtained from the output of two hydro-codes (ideal and viscous). The heavy-quark fragmentation into hadrons and their final semileptonic decays are implemented according to up-to-date experimental data. A comparison with RHIC data for non-photonic electron spectra is given.

Overview of open heavy flavor measurements in STAR

We present an overview of measurements related to open heavy flavor production. Results from indirect reconstruction of heavy flavor via semi-leptonic decays in p + p and minimum bias Au+Au collisions at = 200 GeV, and results from D meson measurements via their hadronic decay channel in minimum bias Cu+Cu and Au+Au collisions are discussed. For the semi-leptonic decays the focus is on partonic energy loss, RAA, and the use of e-h and e-D0 azimuthal correlation measurements to extract the B meson contribution to the non-photonic electron spectrum. For the hadronic decay the focus is on open charm yields in Cu+Cu collisions and first measurements of D meson in heavy ion collisions using a secondary vertexing technique.

Theory and phenomenology of heavy flavor at the RHIC

We review the problem of heavy-quark diffusion in the quark–gluon plasma and its ramifications for heavy-quark spectra in heavy-ion collisions at the RHIC. In particular, we attempt to reconcile underlying mechanisms of several seemingly different approaches that have been put forward to explain the large suppression and elliptic flow of non-photonic electron spectra. We also emphasize the importance of a quantitative description of the bulk medium evolution to extract reliable values for the heavy-quark diffusion coefficient.

Heavy flavor in STAR

The strong suppression of high pT hadrons in central Au+Au collisions observed at RHIC is generally interpreted as being a consequence of the energy loss of energetic partons in the hot and dense matter before fragmenting. The study of heavy quark production is testing our understanding of this scenario.The recent results on heavy flavor measurements from the STAR experiment at RHIC are presented. The total charm production cross section, extracted from d+Au, Au+Au and Cu+Cu collisions at s√S N N = 200 GeV, scales with the number of binary collisions. The high pT non-photonic electron spectra show a strong suppression in central Au+Au collisions. Current theoretical models do not explain this observation satisfactorily. The relative contribution of electrons from bottom decays in the non-photonic electron spectrum predicted in FONLL calculations is consistent with the value determined from azimuthal electron-hadron correlations.The measurement of the T cross section in p+p collision is also presented and is found to be consistent with NLO calculations.

Heavy Flavor Physics in STAR

The STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC) measured heavy flavor production via two independent channels: hadronic and semileptonic decays. The nonphotonic electron spectrum was measured over a wide range of p T (up to 10 GeV/ c). The low- p T region was constrained by muon measurements. The extracted charm cross section, in p+p, d+Au, and Au+Au collisions, exhibits binary scaling suggestive of charm origins in the early phase of the collision. The nuclear modification factor shows a consistent pattern of suppression at high p T in central heavy-ion collisions not explained by current theoretical models.

Identifying Bottom Contribution in Non-photonic Electron Spectra and v2 from Au+Au Collisions at RHIC

We present a study on the spectra and elliptic flow v 2 of electrons from heavy flavor (charm and bottom) decays with the relative contributions of charm and bottom hadrons given by the PYTHIA Monte Carlos simulations. We made a simultaneous fit to both measured non-photonic electron spectra and v 2 distributions. The results suggest that the bottom contribution is not dominant for electron p T < 5 GeV / c in the 200 GeV Au+Au collisions.

Open charm measurements at STAR

We report on the measurements of D0 meson production via direct reconstruction through the hadronic decay channel D0→Kπ in minimum bias d+Au and Au+Au collisions at \(\sqrt{s_{NN}}=200\mathrm{GeV}\) with pT up to ∼3 GeV/c. We derive the charm production total cross-section per nucleon–nucleon collision from a combination of three measurements: the D0 meson spectra, the non-photonic electron spectra from charm semi-leptonic decays obtained in p+p, d+Au, and Au+Au collisions, and the charm-decayed single muon (prompt muon) spectra at low pT in Au+Au collisions. The cross-section is found to follow binary scaling, which is a signature of charm production exclusively at the initial impact. The implications of charm quark energy-loss and thermalization in the strongly interacting matter are discussed.

Heavy flavour production at STAR

We present measurements on D0 meson production via direct reconstruction of its hadronic decay channel D0 → Kπ in minimum bias d + Au and Au+Au collisions at with pT up to ∼3 GeV/c. Non-photonic electron spectra from the charm semi-leptonic decays are analysed from the same data set as well as in p + p collision at using the STAR time-of-flight (TOF) and Barrel EMC (BEMC) detectors, respectively. Results of the charm-decayed single muon (prompt muon) spectra are also presented at low pT in Au+Au collisions measured by the TOF detector. The charm production total cross-section per nucleon–nucleon collision is measured to be 1.26 ± 0.09 (stat.) ± 0.23 (sys.) mb in minimum bias Au+Au collisions, which is consistent with the Nbin scaling compared to 1.4 ± 0.2 ± 0.4 mb in minimum bias d + Au collisions, and supports the idea that charm quarks should be produced mostly via parton fusion at an early stage in relativistic heavy-ion collisions. A blast-wave model fit to the low pT (<2 GeV/c) non-photonic electrons, prompt muons and D0 spectra shows that charm hadrons may kinetically freeze out earlier than light hadrons with a smaller collective velocity. The nuclear modification factors (RAA) of the non-photonic electrons in central Au+Au collisions are significantly below unity at pT > ∼2 GeV/c, which indicates a significant amount of energy loss for heavy quarks in Au+Au collisions. The charm transverse momentum distribution must have been modified by the hot and dense matter created in central Au+Au collisions at the Relativistic Heavy-Ion Collider.

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high pT hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high pT is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high pT provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<pT (GeV/c)<10. The corresponding nuclear modification factors (RAA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models.

Dependence of the non-photonic electron spectrum on the charmed baryon-to-meson ratio

In this talk I argue that a substantial fraction of the non-photonic electron suppression in Au+Au collisions could arise as a result of an enhanced Λc/D ratio rather than purely from jet-quenching. At intermediate transverse momentum (2<pT<6 GeV/c), the baryon-to-meson ratio in Au+Au collisions is enhanced compared to p+p collisions. Since charm-baryon decays produce electrons less frequently than charm-meson decays, the non-photonic electron spectrum is sensitive to the Λc/D ratio. I show the dependence of the non-photonic electron spectrum on the baryon-to-meson ratio for charm hadrons. As an example, I assume that the Λc/D ratio is the same as the Λ/K0 S ratio. I show that even if the total charm quark yield in Au+Au collisions scales with the number of binary nucleon-nucleon collisions (Nbin), the electron spectrum at 2<pT<5 GeV/c will be suppressed relative to Nbin scaled p+p collisions by as much as 20%.

Suppression of nonphotonic electrons from enhancement of charm baryons in heavy ion collisions

At intermediate transverse momentum (2 < p_T < 6 GeV/c), baryon production in Au+Au collisions is enhanced compared to p+p collisions. Since charm baryon decays produce electrons less frequently than charm meson decays, the non-photonic electron spectrum is sensitive to the Lambda_c/D ratio. In this report we study the dependence of the non-photonic electron spectrum on the baryon-to-meson ratio for charm hadrons. As an example, we take the Lambda_c/D ratio to have the same form as the Lambda/K^0_S ratio. In this case, even if the total charm quark yield in Au+Au collisions scales with the number of binary nucleon-nucleon collisions (N_bin), the electron spectrum at 2 < p_T < 5 GeV/c is suppressed relative to N_bin scaled p+p collisions by as much as 20%.

Open Charm Production at RHIC: Recent Results from STAR

We report recent measurements on the open charm production in d+Au and p+p collisions at \(\sqrt {s_{NN} } = 200\) GeV from the STAR detector at RHIC. The two independent measurements — direct open charm hadron (D0, D* etc.) reconstruction and non-photonic single electron spectrum — provide consistent results. The mid-rapidity charm differential cross section per nucleon-nucleon collision from d+Au collisions at RHIC is \(d\sigma _{c\bar c}^{NN} /dy = 0.30 \pm 0.04(stat.) \pm 0.09(syst.)\) mb, which is higher than predictions from most of the NLO pQCD calculations. Implications for charmonium production in Au+Au collisions will also be discussed.

Nuclear Modification of Electron Spectra and Implications for Heavy Quark Energy Loss inAu+AuCollisions atsNN=200 GeV

The PHENIX experiment has measured midrapidity ([FORMULA: SEE TEXT]) transverse momentum spectra ([FORMULA: SEE TEXT]) of electrons as a function of centrality in Au+Au collisions at [FORMULA: SEE TEXT]. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of pi0 and eta mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in p+p collisions. A significant suppression of electrons at high pT is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.

Centrality Dependence of Charm Production from a Measurement of Single Electrons inAu+AuCollisions atsNN=200 GeV

The PHENIX experiment has measured midrapidity transverse momentum spectra (0.4<p(T)<4.0 GeV/c) of single electrons as a function of centrality in Au+Au collisions at sqrt[s(NN)]=200 GeV. Contributions from photon conversions and Dalitz decays of light neutral mesons are measured by introducing a thin (1.7% X0) converter into the PHENIX acceptance and are statistically removed. The subtracted nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons containing heavy quarks, mainly charm at lower p(T). For all centralities, the charm production cross section is found to scale with the nuclear overlap function, T(AA). For minimum-bias collisions the charm cross section per binary collision is N(cc )/T(AA)=622+/-57(stat)+/-160(syst) microb.