arXiv subjects

The STAR Collaboration

Publications and source records attributed to The STAR Collaboration.

54 records · Page 3Linked to original sources

arXiv2010

Strange and Multi-strange Particle Production in Au+Au Collisions at $\sqrt{s_{NN}}$ = 62.4 GeV

We present results on strange and multi-strange particle production in Au+Au collisions at $\sqrt{s_{NN}}=62.4$ GeV as measured with the STAR detector at RHIC. Mid-rapidity transverse momentum spectra and integrated yields of $K^{0}_{S}$, $Λ$, $Ξ$, $Ω$ and their anti-particles are presented for different centrality classes. The particle yields and ratios follow a smooth energy dependence. Chemical freeze-out parameters, temperature, baryon chemical potential and strangeness saturation factor obtained from the particle yields are presented. Intermediate transverse momentum ($p_T$) phenomena are discussed based on the ratio of the measured baryon-to-meson spectra and nuclear modification factor. The centrality dependence of various measurements presented show a similar behavior as seen in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

nucl-ex↗

arXiv2010

Measurement of the parity-violating longitudinal single-spin asymmetry for $W^{\pm}$ boson production in polarized proton-proton collisions at $\sqrt{s} = 500 $GeV

We report the first measurement of the parity violating single-spin asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized proton-proton collisions at $\sqrt{s}=500 $GeV by the STAR experiment at RHIC. The measured asymmetries, $A^{W^+}_{L}=-0.27\pm 0.10\;({\rm stat.})\pm 0.02\;({\rm syst.}) \pm 0.03\;({\rm norm.})$ and $A^{W^-}_{L}=0.14\pm 0.19\;({\rm stat.})\pm 0.02 \;({\rm syst.})\pm 0.01\;({\rm norm.})$, are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized DIS measurements.

hep-ex↗

arXiv2010

Inclusive pi^0, eta, and direct photon production at high transverse momentum in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV

We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.

hep-ex↗

arXiv2010

Balance Functions from Au+Au, d+Au, and p+p Collisions at $\sqrt{s_{NN}}$ = 200 GeV

Balance functions have been measured for charged particle pairs, identified charged pion pairs, and identified charged kaon pairs in Au+Au, d+Au, and p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. These balance functions are presented in terms of relative pseudorapidity, $Δη$, relative rapidity, $Δy$, relative azimuthal angle, $Δϕ$, and invariant relative momentum, $q_{\rm inv}$. In addition, balance functions are shown in terms of the three components of $q_{\rm inv}$: $q_{\rm long}$, $q_{\rm out}$, and $q_{\rm side}$. For charged particle pairs, the width of the balance function in terms of $Δη$ scales smoothly with the number of participating nucleons, while HIJING and UrQMD model calculations show no dependence on centrality or system size. For charged particle and charged pion pairs, the balance functions widths in terms of $Δη$ and $Δy$ are narrower in central Au+Au collisions than in peripheral collisions. The width for central collisions is consistent with thermal blast-wave models where the balancing charges are highly correlated in coordinate space at breakup. This strong correlation might be explained either by delayed hadronization or by limited diffusion during the reaction. Furthermore, the narrowing trend is consistent with the lower kinetic temperatures inherent to more central collisions. In contrast, the width of the balance function for charged kaon pairs in terms of $Δy$ shows little centrality dependence, which may signal a different production mechanism for kaons. The widths of the balance functions for charged pions and kaons in terms of $q_{\rm inv}$ narrow in central collisions compared to peripheral collisions, which may be driven by the change in the kinetic temperature.

nucl-ex↗

arXiv2010

Charged and strange hadron elliptic flow in Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV

We present the results of an elliptic flow analysis of Cu+Cu collisions recorded with the STAR detector at 62.4 and 200GeV. Elliptic flow as a function of transverse momentum is reported for different collision centralities for charged hadrons and strangeness containing hadrons $K_{S}^{0}$, $Λ$, $Ξ$, $ϕ$ in the midrapidity region $|eta|<1.0$. Significant reduction in systematic uncertainty of the measurement due to non-flow effects has been achieved by correlating particles at midrapidity, $|η|<1.0$, with those at forward rapidity, $2.5<|η|<4.0$. We also present azimuthal correlations in p+p collisions at 200 GeV to help estimating non-flow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with previously published results from Au+Au collisions at 200 GeV. We observe that $v_{2}$($p_{T}$) of strange hadrons has similar scaling properties as were first observed in Au+Au collisions, i.e.: (i) at low transverse momenta, $p_T<2GeV/c$, $v_{2}$ scales with transverse kinetic energy, $m_{T}-m$, and (ii) at intermediate $p_T$, $2<p_T<4GeV/c$, it scales with the number of constituent quarks, $n_q$. We have found that ideal hydrodynamic calculations fail to reproduce the centrality dependence of $v_{2}$($p_{T}$) for $K_{S}^{0}$ and $Λ$. Eccentricity scaled $v_2$ values, $v_{2}/ε$, are larger in more central collisions, suggesting stronger collective flow develops in more central collisions. The comparison with Au+Au collisions which go further in density shows $v_{2}/ε$ depend on the system size, number of participants $N_{part}$. This indicates that the ideal hydrodynamic limit is not reached in Cu+Cu collisions, presumably because the assumption of thermalization is not attained.

nucl-ex↗

arXiv2010

Observation of an Antimatter Hypernucleus

Nuclear collisions recreate conditions in the universe microseconds after the Big Bang. Only a very small fraction of the emitted fragments are light nuclei, but these states are of fundamental interest. We report the observation of antihypertritons - composed of an antiproton, antineutron, and antilambda hyperon - produced by colliding gold nuclei at high energy. Our analysis yields 70 +- 17 antihypertritons and 157 +- 30 hypertritons. The measured yields of hypertriton (antihypertriton) and helium3 (antihelium3) are similar, suggesting an equilibrium in coordinate and momentum space populations of up, down, and strange quarks and antiquarks, unlike the pattern observed at lower collision energies. The production and properties of antinuclei, and nuclei containing strange quarks, have implications spanning nuclear/particle physics, astrophysics, and cosmology.

nucl-ex↗

arXiv2010

Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy ion collisions. These domains are predicted to lead to charge separation of quarks along the system's orbital momentum axis. We investigate a three particle azimuthal correlator which is a ¶even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 GeV using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation.

nucl-ex↗

arXiv2009

Studying Parton Energy Loss in Heavy-Ion Collisions via Direct-Photon and Charged-Particle Azimuthal Correlations

Charged-particle spectra associated with direct photon ($γ_{dir} $) and $π^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $γ_{dir}$ and $π^0$. Assuming no associated charged particles in the $γ_{dir}$ direction (near side) and small contribution from fragmentation photons ($γ_{frag}$), the associated charged-particle yields opposite to $γ_{dir}$ (away side) are extracted. At mid-rapidity ($|η|<0.9$) in central Au+Au collisions, charged-particle yields associated with $γ_{dir}$ and $π^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|η|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $γ_{dir}$ and $π^0$, and independent of the $γ_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.

nucl-ex↗

arXiv2009

Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV

We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0's in their parent jets was found to be around 0.7 for electromagnetically triggered events.

hep-ex↗

arXiv2009

Growth of Long Range Forward-Backward Multiplicity Correlations with Centrality in Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and $\textit{p+p}$ collisions at $\sqrt{s_{NN}}$ = 200 GeV. Strong short and long range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short range correlations are observed in peripheral Au+Au collisions. Both the Dual Parton Model (DPM) and the Color Glass Condensate (CGC) predict the existence of the long range correlations. In the DPM the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC longitudinal color flux tubes generate the LRC. The data is in qualitative agreement with the predictions from the DPM and indicates the presence of multiple parton interactions.

nucl-ex↗

arXiv2009

Longitudinal Spin Transfer to $Λ$ and $\barΛ$ Hyperons in Polarized Proton-Proton Collisions at $\sqrt{s}$ = 200 GeV

The longitudinal spin transfer, $D_{LL}$, from high energy polarized protons to $Λ$ and $\barΛ$ hyperons has been measured for the first time in proton-proton collisions at $\sqrt{s} = 200 \mathrm{GeV}$ with the STAR detector at RHIC. The measurements cover pseudorapidity, $η$, in the range $|η| < 1.2$ and transverse momenta, $p_\mathrm{T}$, up to $4 \mathrm{GeV}/c$. The longitudinal spin transfer is found to be $D_{LL}= -0.03\pm 0.13(\mathrm{stat}) \pm 0.04(\mathrm{syst})$ for inclusive $Λ$ and $D_{LL} = -0.12 \pm 0.08(\mathrm{stat}) \pm 0.03(\mathrm{syst})$ for inclusive $\barΛ$ hyperons with $<η> = 0.5$ and $ = 3.7 \mathrm{GeV}/c$. The dependence on $η$ and $p_\mathrm{T}$ is presented.

hep-ex↗

arXiv2009

Yields and elliptic flow of $d(\bar{d})$ and $^{3}He(\bar{^{3}He})$ in Au+Au collisions at $\sqrt{s_{_{NN}}} =$ 200 GeV

We present the $p_{T}$ spectra, elliptic flow ($v_2$) and coalescence parameters $B_{2}$ for $d$, $\bar{d}$ ($1<p_{T}<4$ GeV/c) and $B_{3}$ for $^{3}He$, $\bar{^{3}He}$ ($2<p_{T}<6$ GeV/c) produced at mid-rapidity in Au+Au col lisions at $\sqrt{s_{_{NN}}} = 200$ GeV. The results are measured in the STAR experiment at RHIC. The spectra of the light nuclei show softer $p_T$ distributions than calculations from a blast-wave model in which the parameters were fixed from pion, kaon and proton $v_2$($p_T$) and $p_T$ distributions. The coalescence volume is found to track with pion HBT re sults for different collision geometries. The $v_2$ measurement for $d(\overline{d})$ as a function of transverse momentum $p_T$ is found to follow an approximate atomic mass number ($A$) scaling while that of $^{3}He(\bar{^{3}He}) $ deviates slightly from the scaling. A negative $v_{2}$ has been observed for $\bar{d}$ at low $p_{T}$, consiste nt with large radial flow in Au+Au collisions.

nucl-ex↗

arXiv2009

Long range rapidity correlations and jet production in high energy nuclear collisions

The STAR Collaboration at RHIC presents a systematic study of high transverse momentum charged di-hadron correlations at small azimuthal pair separation \dphino, in d+Au and central Au+Au collisions at $\rts = 200$ GeV. Significant correlated yield for pairs with large longitudinal separation \deta is observed in central Au+Au, in contrast to d+Au collisions. The associated yield distribution in \detano$\times$\dphi can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d+Au collisions, and a component which is narrow in \dphi and \textcolor{black}{depends only weakly on} $\deta$, the "ridge". Using two systematically independent analyses, \textcolor{black}{finite ridge yield} is found to persist for trigger $\pt > 6$ \GeVc, indicating that it is correlated with jet production. The transverse momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range ($2 < \pt < 4 \GeVc$).

nucl-ex↗

arXiv2009

Pion Interferometry in Au+Au and Cu+Cu Collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 and 200 GeV

We present a systematic analysis of two-pion interferometry in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 GeV and Cu+Cu collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 and 200 GeV using the STAR detector at RHIC. The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

nucl-ex↗

arXiv2009

Measurements of $ϕ$ meson production in relativistic heavy-ion collisions at RHIC

We present results for the measurement of $ϕ$ meson production via its charged kaon decay channel $ϕ\to K^+K^-$ in Au+Au collisions at $\sqrt{s_{_{NN}}}=62.4$, 130, and 200 GeV, and in $p+p$ and $d$+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity ($|y|<0.5$) $ϕ$ meson transverse momentum ($p_{T}$) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the $p_{T}$ spectra from $p+p$, $d$+Au and peripheral Au+Au collisions show power-law tails at intermediate and high $p_{T}$ and are described better by Levy distributions. The constant $ϕ/K^-$ yield ratio vs beam species, collision centrality and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for $ϕ$ production at RHIC. The $Ω/ϕ$ yield ratio as a function of $p_{T}$ is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}\sim 4$ GeV/$c$, but disagrees at higher transverse momenta. The measured nuclear modification factor, $R_{dAu}$, for the $ϕ$ meson increases above unity at intermediate $p_{T}$, similar to that for pions and protons, while $R_{AA}$ is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both $R_{cp}$ and $v_{2}$ for the $ϕ$ meson with respect to other hadrons in Au+Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV at intermediate $p_{T}$ is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate $p_{T}$ region at RHIC.

nucl-ex↗

arXiv2009

Measurement of D* Mesons in Jets from p+p Collisions at sqrt{s} = 200 GeV

We report the measurement of charged $D^*$ mesons in inclusive jets produced in proton-proton collisions at a center of mass energy $\sqrt{s}$ = 200 GeV with the STAR experiment at RHIC. For $D^{*}$ mesons with fractional momenta $0.2 < z < 0.5$ in inclusive jets with 11.5 GeV mean transverse energy, the production rate is found to be $N(D^{*+}+D^{*-})/N(\mathrm{jet}) = 0.015 \pm 0.008 (\mathrm{stat}) \pm 0.007 (\mathrm{sys})$. This rate is consistent with perturbative QCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.

hep-ex↗

arXiv2004

Non Identical strange particle correlations in Au-Au collisions at $\sqrt{s_{NN}}=$ 200 GeV from the STAR experiment

Information about the space-time evolution of colliding nuclei can be extracted correlating particles emitted from nuclear collisions. The high density of particles produced in the STAR experiment allows the measurement of non-identical strange particle correlations. Due to the absence of Coulomb interaction, $p-Λ$ and $\bar{p}-Λ$ systems are more sensitive to the source size than $p-p$ pairs. Strong interaction potential has been studied using $p-Λ$, and for the first time, $\bar{p}-Λ$ pairs. The experimental correlation functions have been described in the frame of a model based on the $p-n$ interaction. The first preliminary measurement of $π$ - $Ξ$ correlations has been performed, allowing to extract information about the freeze-out time and the space-time asymmetries in particle emission closely related to the transverse radial expansion and decay of resonances.

hep-ex↗

arXiv2004

Anti-d and Anti-He Production in Au+Au Collisions at RHIC

Ultra-relativistic Au+Au collisions at RHIC ($\sqrt{s}$=130, 200 GeV) are used to study production of rare anti-nuclei. These clusters of anti-nucleons are formed by coalescence, i.e. overlapping wave functions of anti-nucleons. The coalescence coefficients $B_2$ for Anti-$d$ and $B_3$ for Anti-$^3He$ are determined, and used to derive the fireball radius.

nucl-ex↗