arXiv subjects

Lijuan Ruan

Publications and source records attributed to Lijuan Ruan.

27 records · Page 2Linked to original sources

arXiv2010

Production and energy loss of strange and heavy quarks

Data taken over the last several years have demonstrated that RHIC has created a hot, dense medium with partonic degrees of freedom. Identified particle spectra at high transverse momentum ($p_T$) and heavy flavor are thought to be well calibrated probes thus serve as ideal tools to study the properties of the medium. We present $p_T$ distributions of particle ratios in p+p collisions from STAR experiment to understand the particle production mechanisms. These measurements will also constrain fragmentation functions in hadron-hardon collisions. In heavy ion collsions, we highlight: 1) recent measurements of strange hadrons and heavy flavor decay electrons up to high $p_T$ to study jet interaction with the medium and explore partonic energy loss mechanisms; and 2) $Υ$ and high $p_T$ $J/ψ$ measurements to study the effect of color screening and other possible production mechanisms.

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arXiv2009

Improving the dE/dx calibration of the STAR TPC for the high-pT hadron identification

We derive a method to improve particle identification (PID) at high transverse momentum ($p_T$) using the relativistic rise of the ionization energy loss ($rdE/dx$) when charged particles traverse the Time Projection Chamber (TPC) at STAR. Electrons triggered and identified by the Barrel Electro-Magnetic Calorimeter (BEMC), pure protons and pions from $Λ\to p+π^{-}$ ($\barΛ\to \bar{p}+π^{+}$), and $K^{0}_{S}\toπ^{+}+π^{-}$ decays are used to obtain the $dE/dx$ value and its width at given $βγ=p/m$. We found that the deviation of the $dE/dx$ from the Bichsel function can be up to $0.4σ$ ($\sim3%$) in p+p collisions at $\sqrt{s_{NN}}=200$ GeV taken and subsequently calibrated in year 2005. The deviation is approximately a function of $βγ$ independent of particle species and can be described with a function of $f(x) = A+\frac{B}{C+x^{2}}$. The deviations obtained with this method are used to re-calibrate the data sample from p+p collision for physics analysis of identified hadron spectra and their correlations up to transverse momentum of 15 GeV/$c$. The ratio of $e^{-}/e^{+}$ (dominantly from $γ$-conversion) is also used to correct the residual asymmetry in the negative and positive charged hadrons due to momentun distortion in the STAR TPC.

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arXiv2009

Spectra and radial flow at RHIC with Tsallis statistics in a Blast-Wave description

We have implemented the Tsallis statistics in a Blast-Wave model and applied it to mid-rapidity transverse-momentum spectra of identified particles measured at RHIC. This new Tsallis Blast-Wave function fits the RHIC data very well for $p_T<$3 GeV/$c$. We observed that the collective flow velocity starts from zero in p+p and peripheral Au+Au collisions growing to 0.470 $\pm$ 0.009($c$) in central Au+Au collisions. The $(q-1)$ parameter, which characterizes the degree of non-equilibrium in a system, changes from $0.100\pm0.003$ in p+p to $0.015\pm0.005$ in central Au+Au collisions, indicating an evolution from a highly non-equilibrated system in p+p collisions toward an almost thermalized system in central Au+Au collisions. The temperature and collective velocity are well described by a quadratic dependence on $(q-1)$. Two sets of parameters in our Tsallis Blast-Wave model are required to describe the meson and baryon groups separately in p+p collisions while one set of parameters appears to fit all spectra in central Au+Au collisions.

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arXiv2008

Prototype Performance of Novel Muon Telescope Detector at STAR

Research on a large-area, cost-effective Muon Telescope Detector has been carried out for RHIC and for next generation detectors at future QCD Lab. We utilize state-of-the-art multi-gap resistive plate chambers with large modules and long readout strips in detector design \cite{MTDLDRD}. The results from cosmic ray and beam test will be presented to address intrinsic timing and spatial resolution for a Long-MRPC. The prototype performance of a novel muon telescope detector at STAR will be reported, including muon identification capability, timing and spatial resolution.

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arXiv2007

Physics with Identified Particles at STAR

New physics results with identified particles at STAR are presented. Measurements at low $p_T$ address bulk properties of the collision, while those at high $p_T$ address jet energy loss in the bulk matter produced. Between these extremes, measurements at intermediate $p_T$ address the interplay between jets and the bulk. We highlight: measurements of $v_2$ fluctuations as a new, sensitive probe of the initial conditions and the equation of state; correlations involving multi-strange particles, along with ratios of identified particles to test coalescence as a mechanism of particle production at intermediate $p_T$; three particle azimuthal correlation to search for conical emission; and the energy and particle-type dependence of hadron production at high $p_T$ to study quark and gluon jet energy loss.

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arXiv2005

Measurement of $Δ$ and $K^{*}$ Production in $d$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

The measurements of the transverse momentum spectra and the invariant mass distributions of $Δ(1232)$ $\to πp$, $K^{*}(892)\to πK$ resonances in $d$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using the STAR Time Projection Chamber (TPC) at RHIC are presented. The in-medium modification of the $Δ$ and $K^{*}$ mass and width has been studied as a function of transverse momentum ($p_T$). The particle ratios $K^{*}/K$, $Δ/p$ and the average transverse momentum ($ $) as a function of different collision centrality has been reported. The nuclear modification factors ($R_{CP}$ and $R_{dAu}$) of $Δ$ and $K^{*}$ are discussed.

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arXiv2005

Thesis: Pion, Kaon, Proton and Antiproton Spectra in d+Au and p+p Collisions at $\sqrt{s_{NN}}$= 200GeV at the Relativistic Heavy Ion Collider

Identified mid-rapidity particle spectra of $π^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ from 200 GeV p+p and d+Au collisions are reported. This data were taken in 2003 run from the Solenoidal Tracker at RHIC (STAR) experiment. A time-of-flight detector based on multi-gap resistive plate chamber (MRPC) technology is used for particle identification. The intrinsic timing resolution 85 ps was achieved after the calibration. We observe that the spectra of $π^{\pm}$, $K^{\pm}$, $p$ and $\bar{p}$ are considerably harder in d+Au than those in p+p collisions. In $\sqrt{s_{_{NN}}} = 200$ GeV d+Au collisions, the nuclear modification factor $R_{dAu}$ of protons rise faster than those of pions and kaons. Besides, the particle-species dependence of the Cronin effect is observed to be significantly smaller than that at lower energies. The ratio of the nuclear modification factor ($R_{dAu}$) between $(p+\bar{p})$ and charged hadrons ($h$) in the transverse momentum range $1.2<p_{T}<3.0$ GeV/c is measured to be $1.19\pm0.05$(stat)$\pm0.03$(syst) in minimum-bias collisions and shows little centrality dependence. The yield ratio of $(p+\bar{p})/h$ in minimum-bias d+Au collisions is found to be a factor of 2 lower than that in Au+Au collisions, indicating that the relative baryon enhancement observed in heavy ion collisions at RHIC is due to the final state effects in Au+Au collisions. The mechanism for Cronin effect is also discussed in this thesis by comparison with the recombination model and with the initial multiple parton scattering model. From the comparisons, we conclude that the Cronin effect in 200 GeV d+Au collisions is not initial state effect only, and that final state effect plays an important role.

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arXiv2005

$π, K, p$ and $\bar{p}$ production from Au+Au collisions at $\sqrt{s_{_{NN}}} = 62.4$ GeV

The preliminary results of $π^{\pm}, K^{\pm}, p$ and $\bar{p}$ spectra are reported from Au+Au collisions at $\sqrt{s_{_{NN}}} = 62.4$ GeV. Particle identification is from the Time Projection Chamber and Time-of-Flight system at STAR. The nuclear modification factor $R_{CP}$ for mesons ($π^{\pm}, K^{\pm}$) and baryons ($p, \bar{p}$) will also be discussed.

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arXiv2004

Open Charm Yields in 200 GeV p+p and d+Au Collisions at RHIC

Open charm spectra at mid-rapidity from direct reconstruction of $D^{0}\to Kπ$ and indirect electron/positron measurements via charm semileptonic decay in p+p and d+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV are reported. The combined spectra cover open charm transverse momentum range $0.1{}^{<}_{\sim}p_{T}{}^{<}_{\sim}6$ GeV/$c$. The electron spectra show approximate binary scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon interaction at mid-rapidity for open charm production at RHIC is $dσ^{NN}_{c\bar{c}}/dy$=0.31$\pm$0.04(stat.)$\pm$0.08(syst.) mb. The next-to-leading-order pQCD calculation underpredicts the charm yields.

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