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Register a new userRecent results from STAR experiment in Au+Au collisions at $sqrt{s_{NN}}$ = 9.2 GeV
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Debasish Das
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Theoretical models suggest that the Quantum Chromo-Dynamics (QCD) phase diagram has a critical point demarcating the order of transition between the two phases: the hadron gas, in which the quarks are confined and the Quark-Gluon Plasma (QGP). The central goal of the experiments with relativistic heavy-ion collisions is to create and study such form of matter called the QGP and understand the QCD phase diagram. The STAR (Solenoidal Tracker At RHIC) detector is pertinent for the RHIC (Relativistic Heavy Ion Collider) energy scan program where we plan to explore this exciting physics possibility using heavy-ion collisions at various center of mass energies. A first test run with Au+Au collisions at $sqrt{s_{NN}}$ = 9.2 GeV took place in early 2008. We present the recent STAR results from this run of the identified particles (pion, kaon and proton) transverse momentum spectra and ratios. Also we shall present and discuss the results of the azimuthal anisotropy parameters ($v_{1}$, $v_{2}$) along with the pion interferometry measurements. These recent results from Au+Au collisions at $sqrt{s_{NN}}$ = 9.2 GeV are compared with other SPS and RHIC measurements.
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We present the first measurements of identified hadron production, azimuthal anisotropy, and pion interferometry from Au+Au collisions below the nominal injection energy at the Relativistic Heavy-Ion Collider (RHIC) facility. The data were collected using the large acceptance STAR detector at $sqrt{s_{NN}}$ = 9.2 GeV from a test run of the collider in the year 2008. Midrapidity results on multiplicity density (dN/dy) in rapidity (y), average transverse momentum (<pT>), particle ratios, elliptic flow, and HBT radii are consistent with the corresponding results at similar $sqrt{s_{NN}}$ from fixed target experiments. Directed flow measurements are presented for both midrapidity and forward rapidity regions. Furthermore the collision centrality dependence of identified particle dN/dy, <pT>, and particle ratios are discussed. These results also demonstrate the readiness of the STAR detector to undertake the proposed QCD critical point search and the exploration of the QCD phase diagram at RHIC.
We present measurements of $e^+e^-$ production at midrapidity in Au$+$Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass ($m_{ee} <$ 5 GeV/$c^2$) and pair transverse momentum ($p_T$ $<$ 5 GeV/$c$), for minimum bias and for five centrality classes. The ee yield is compared to the expectations from known sources. In the low-mass region ($m_{ee}=0.30$--0.76 GeV/$c^2$) there is an enhancement that increases with centrality and is distributed over the entire pair pt range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to $2.3pm0.4({rm stat})pm0.4({rm syst})pm0.2^{rm model}$ or to $1.7pm0.3({rm stat})pm0.3({rm syst})pm0.2^{rm model}$ for minimum bias collisions when the open-heavy-flavor contribution is calculated with {sc pythia} or {sc mc@nlo}, respectively. The inclusive mass and $p_T$ distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the $rho$ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region ($m_{ee}$ = 1.2--2.8 GeV/$c^2$), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
At RHIC, enhancements in the baryon-to-meson ratio for light hadrons and hadrons containing strange quarks have been observed in central heavy-ion collisions compared to those in p+p and peripheral heavy-ion collisions in the intermediate transverse momentum ($p_T$) range (2 $<$ $p_T$ $<$ 6 GeV/$c$). This can be explained by the hadronization mechanism involving multi-parton coalescence. $Lambda_{c}$ is the lightest charmed baryon with mass close to that of $D^0$ meson, and has an extremely short life time (c$tau$$sim$60 $mu$m). Different models predict different magnitudes of enhancement in the $Lambda_{c}$/$D^0$ ratio depending on the degree to which charm quarks are thermalized in the medium and how the coalescence mechanism is implemented. In these proceedings, we report the first measurement of $Lambda_{c}$ production in heavy-ion collisions using the Heavy Flavor Tracker at STAR. The invariant yield of $Lambda_{c}$ for 3 $<$ $p_T$ $<$ 6 GeV/$c$ is measured in 10-60% central Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV. The $Lambda_{c}$/$D^0$ ratio is compared to different model calculations, and the physics implications are discussed.
Yields, correlation shapes, and mean transverse momenta pt{} of charged particles associated with intermediate to high-pt{} trigger particles ($2.5 < pt < 10$ GeVc) in d+Au and Au+Au collisions at $snn=200$ GeV are presented. For associated particles at higher $pt gtrsim 2.5$ GeVc, narrow correlation peaks are seen in d+Au and Au+Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $pt < 2$ GeVc, a large enhancement of the near- ($dphi sim 0$) and away-side ($dphi sim pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au+Au collisions compared to d+Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $dphi sim pi$ in central Au+Au collisions.
We report preliminary results of hypertriton observation in heavy-ion collisions at RHIC. We have identified 157 +- 30 candidates in the current sample containing ~10^8 Au+Au events at sqrt{s_{NN}} = 200 GeV. The production rate of hypertriton is close to that of helium 3. No extra penalty factor is observed for hypertriton, in contrast to results observed at the AGS.
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