Observation of pi^+pi^-pi^+pi^- Photoproduction in Ultra-Peripheral Heavy Ion Collisions at STAR

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 81 (2010) 044901, 2010.
Inspire Record 838875 DOI 10.17182/hepdata.98963

We present a measurement of pi^+pi^-pi^+pi^- photonuclear production in ultra-peripheral Au-Au collisions at sqrt(s_{NN}) = 200 GeV from the STAR experiment. The pi^+pi^-pi^+pi^- final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The pi^+pi^-pi^+pi^- invariant mass spectrum of the coherent events exhibits a broad peak around 1540 pm 40 MeV/c^2 with a width of 570 pm 60 MeV/c^2, in agreement with the photoproduction data for the rho^0(1700). We do not observe a corresponding peak in the pi^+pi^- final state and measure an upper limit for the ratio of the branching fractions of the rho^0(1700) to pi^+pi^- and pi^+pi^-pi^+pi^- of 2.5 % at 90 % confidence level. The ratio of rho^0(1700) and rho^0(770) coherent production cross sections is measured to be 13.4 pm 0.8 (stat.) pm 4.4 (syst.) %.

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Hadronic resonance production in $d$+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV at RHIC

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 78 (2008) 044906, 2008.
Inspire Record 776722 DOI 10.17182/hepdata.97116

We present the first measurements of the $\rho(770)^0$, $K^*$(892), $\Delta$(1232)$^{++}$, $\Sigma$(1385), and $\Lambda$(1520) resonances in $d$+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, reconstructed via their hadronic decay channels using the STAR detector at RHIC. The masses and widths of these resonances are studied as a function of transverse momentum ($p_T$). We observe that the resonance spectra follow a generalized scaling law with the transverse mass ($m_T$). The $<p_T>$ of resonances in minimum bias collisions is compared to the $<p_T>$ of $\pi$, $K$, and $\bar{p}$. The $\rho^0/\pi^-$, $K^*/K^-$, $\Delta^{++}/p$, $\Sigma(1385)/\Lambda$, and $\Lambda(1520)/\Lambda$ ratios in $d$+Au collisions are compared to the measurements in minimum bias $p+p$ interactions, where we observe that both measurements are comparable. The nuclear modification factors ($R_{dAu}$) of the $\rho^0$, $K^*$, and $\Sigma^*$ scale with the number of binary collisions ($N_{bin}$) for $p_T >$ 1.2 GeV/$c$.

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Longitudinal scaling property of the charge balance function in Au + Au collisions at 200 GeV

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Lett.B 690 (2010) 239-244, 2010.
Inspire Record 844983 DOI 10.17182/hepdata.97118

We present measurements of the charge balance function, from the charged particles, for diverse pseudorapidity and transverse momentum ranges in Au + Au collisions at 200 GeV using the STAR detector at RHIC. We observe that the balance function is boost-invariant within the pseudorapidity coverage [-1.3, 1.3]. The balance function properly scaled by the width of the observed pseudorapidity window does not depend on the position or size of the pseudorapidity window. This scaling property also holds for particles in different transverse momentum ranges. In addition, we find that the width of the balance function decreases monotonically with increasing transverse momentum for all centrality classes.

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Three-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.Lett. 105 (2010) 022301, 2010.
Inspire Record 840812 DOI 10.17182/hepdata.102404

We report the first three-particle coincidence measurement in pseudorapidity ($\Delta\eta$) between a high transverse momentum ($p_{\perp}$) trigger particle and two lower $p_{\perp}$ associated particles within azimuth $\mid$$\Delta\phi$$\mid$$&lt;$0.7 in $\sqrt{{\it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $\Delta\eta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $\Delta\eta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.

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

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 82 (2010) 034909, 2010.
Inspire Record 839470 DOI 10.17182/hepdata.101353

Charged-particle spectra associated with direct photon ($\gamma_{dir} $) and $\pi^0$ 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 $\gamma_{dir}$ and $\pi^0$. Assuming no associated charged particles in the $\gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($\gamma_{frag}$), the associated charged-particle yields opposite to $\gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|\eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $\gamma_{dir}$ and $\pi^0$ 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 $|\eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $\gamma_{dir}$ and $\pi^0$, and independent of the $\gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.

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Energy and system size dependence of \phi meson production in Cu+Cu and Au+Au collisions

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Lett.B 673 (2009) 183-191, 2009.
Inspire Record 800796 DOI 10.17182/hepdata.101351

We study the beam-energy and system-size dependence of \phi meson production (using the hadronic decay mode \phi -- K+K-) by comparing the new results from Cu+Cu collisions and previously reported Au+Au collisions at \sqrt{s_NN} = 62.4 and 200 GeV measured in the STAR experiment at RHIC. Data presented are from mid-rapidity (|y|&lt;0.5) for 0.4 &lt; pT &lt; 5 GeV/c. At a given beam energy, the transverse momentum distributions for \phi mesons are observed to be similar in yield and shape for Cu+Cu and Au+Au colliding systems with similar average numbers of participating nucleons. The \phi meson yields in nucleus-nucleus collisions, normalised by the average number of participating nucleons, are found to be enhanced relative to those from p+p collisions with a different trend compared to strange baryons. The enhancement for \phi mesons is observed to be higher at \sqrt{s_NN} = 200 GeV compared to 62.4 GeV. These observations for the produced \phi(s\bar{s}) mesons clearly suggest that, at these collision energies, the source of enhancement of strange hadrons is related to the formation of a dense partonic medium in high energy nucleus-nucleus collisions and cannot be alone due to canonical suppression of their production in smaller systems.

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Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at \sqrt{s}=200 GeV

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.Lett. 101 (2008) 222001, 2008.
Inspire Record 777766 DOI 10.17182/hepdata.97117

We report precision measurements of the Feynman-x dependence, and first measurements of the transverse momentum dependence, of transverse single spin asymmetries for the production of \pi^0 mesons from polarized proton collisions at \sqrt{s}=200 GeV. The x_F dependence of the results are in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with pQCD-based calculations.

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Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 81 (2010) 054908, 2010.
Inspire Record 830676 DOI 10.17182/hepdata.98577

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 orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a \P-even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 and 62~GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation.

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Enhanced strange baryon production in Au+Au collisions compared to p+p at sqrts = 200 GeV

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 77 (2008) 044908, 2008.
Inspire Record 750816 DOI 10.17182/hepdata.105866

We report on the observed differences in production rates of strange and multi-strange baryons in Au+Au collisions at sqrts = 200 GeV compared to pp interactions at the same energy. The strange baryon yields in Au+Au collisions, then scaled down by the number of participating nucleons, are enhanced relative to those measured in pp reactions. The enhancement observed increases with the strangeness content of the baryon, and increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at lower collision energy sqrts =17.3 GeV. The previous observations are for the bulk production, while at intermediate pT, 1 < pT< 4 GeV/c, the strange baryons even exceed binary scaling from pp yields.

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Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR

The STAR collaboration Abelev, B.I. ; Aggarwal, M.M. ; Ahammed, Z. ; et al.
Phys.Rev.C 79 (2009) 034909, 2009.
Inspire Record 793126 DOI 10.17182/hepdata.104931

Identified charged particle spectra of $\pi^{\pm}$, $K^{\pm}$, $p$ and $\pbar$ at mid-rapidity ($|y|<0.1$) measured by the $\dedx$ method in the STAR-TPC are reported for $pp$ and d+Au collisions at $\snn = 200$ GeV and for Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. ... [Shortened for arXiv list. Full abstract in manuscript.]

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