Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session X11: Mini-Symposium: Recent Results from RHIC II |
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Sponsoring Units: DNP Chair: Matthias Grosse Perdekamp, University of Illinois at Urbana-Champaign Room: Maryland C |
Tuesday, February 16, 2010 10:45AM - 10:57AM |
X11.00001: PHENIX measurements of sqrt(s) dependence of pi0 production in pp collisions at mid-rapidity Alexander Bazilevsky Neutral pion measurements in proton-proton (pp) collisions serve to explore the structure of the proton, to constrain fragmentation mechanisms and to establish the baseline to study medium effects in heavy ion collisions. Perturbative Quantum Chromodynamics (pQCD) is a crucial tool in interpreting measurements which involve high pT particle production. PHENIX has already reported the differential cross section for inclusive pi0 production in pp collisions at mid-rapidity at $\sqrt{s}$=62.4 and 200 GeV. In 2009 RHIC Run PHENIX collected data at $\sqrt{s}$=500 GeV. Here we report on status of the pi0 cross section analysis and compare the measurements at different $\sqrt{s}$ to NLO pQCD calculations. xT scaling will be also examined. [Preview Abstract] |
Tuesday, February 16, 2010 10:57AM - 11:09AM |
X11.00002: Dihadron fragmentation functions within reconstructed jets in $p+p$ collisions at $sqrt(s)$=200 GeV in STAR Muhammad Elnimr Dihadron azimuthal correlations between two high transverse momentum hadrons are commonly used to study the medium modification in heavy-ion collisions at RHIC. However, near-side jet-like correlation show little modification relative to that measured in $p+p$ and $d+Au$ collisions whereas the away-side is significantly suppressed in central $Au+Au$ collisions at $\sqrt{s_{NN}} = $200 GeV[1]. Dihadron correlations within the same jet were measured in DIS experiments such as HERMES [2]. These showed only minimal variation with the choice of nuclear target , even though the single inclusive production of leading hadrons is highly suppressed for heavier targets. Measurements of di-hadron fragmentation functions shall provide a better basis for the interpretation of near-side correlations already measured at RHIC [3]. We present measurements of the dihadron fragmentation function $D(z_{1}, z_{2})$ of charged hadrons within fully reconstructed jets in $p+ p$ collisions at $\sqrt{s_{NN}} = $200 GeV in the STAR experiment. We also calculate the ratio $D(z_{1},z_{2})/D(z_{1})$ and compare it to the rescaled fragmentation function $D(z_{2}/(1-z_{1}))$ in line with [3]. We investigate the prospect of using such comparison to evaluate the quark/gluon flavor of jets as function of the jet energy at RHIC. [Preview Abstract] |
Tuesday, February 16, 2010 11:09AM - 11:21AM |
X11.00003: Status of Neutral Dijet Analysis on Data from 200GeV Proton Proton Collisions Using the STAR Detector at RHIC B.S. Page In the past, STAR has probed the gluon distribution $\Delta g(x)$ of the proton using inclusive measurements, which integrate over a broad range of Bjorken $x$. To date, these measurements have been able to place a strong constraint on the partial integral of $\Delta g(x)$ in the $x$ range 0.03 to 0.3, however, they have little sensitivity to the shape of $\Delta g(x)$ as a function of $x$. Dijet measurements on the other hand provide direct access to parton-level kinematics at leading order and thus allow for the investigation of the shape of $\Delta g(x)$. The study of dijets at forward rapidity in STAR is complicated by the falling charged particle tracking efficiency of the Time Projection Chamber for $|\eta| > 1$. Therefore, to access the lower $x$ region found at forward rapidity, it will be necessary to study jets detected only by their neutral component which can be observed by the STAR Endcap Electromagnetic Calorimeter for $1<\eta<2$. Investigations into the impact of lost tracking information on the reconstruction of partonic kinematics, as well as analysis of 2009 data are under way. The current status of these studies will be presented. [Preview Abstract] |
Tuesday, February 16, 2010 11:21AM - 11:33AM |
X11.00004: Measurement of gluon polarization contribution to proton spin through Charged Pion probes at PHENIX Sook Hyun Lee The RHIC spin program is designed to study the nucleon spin structure. A major research is to determine the gluon contribution to the total proton spin with the use of channels in longitudinally polarized proton-proton collisions. When taken along with other channels (e.g. associated with neutral $\pi^0$ mesons, direct photons and $\eta$ particles) the single inclusive charged pion channel can constrain the contribution of gluon spin, $\Delta G$, additionally it can give the sign information on $\Delta G$. The goal of this analysis is to measure the asymmetry $A_{LL}$ as a function of transverse momentum($p_T$) of charged pion with central arm tracking in PHENIX and eventually to use it along with other data to extract $\Delta G$ using next to leading order pQCD calculation. For this purpose, the kinematic region of interest is $p_T >$ 5GeV/c where quark-gluon and gluon-gluon hard processes dominate charged pion production at midrapidity. The status of the charged pion $A_{LL}$ analysis on the 2009 Run data at center of mass energies of 200GeV with $\sim$55\% polarization and integrated luminosity $\sim$15~pb$^{-1}$ will be presented. [Preview Abstract] |
Tuesday, February 16, 2010 11:33AM - 11:45AM |
X11.00005: PHENIX Measurements of the Double Longitudinal Helicity Asymmetry in Neutral Pion Production in Polarized p+p Collisions at $\sqrt{s} =$ 200 and 500 GeV Andrew Manion Measurement of the gluon spin contribution to the proton spin, $\Delta G$, is an important component of the RHIC spin program. One particular focus is measuring the $\pi^{0}$ double longitudinal helicity asymmetry, $A_{LL}$, which is proportional to $\Delta G$. The large $p+p \rightarrow \pi^{0}$ cross section coupled with the high resolution of the PHENIX EM-Calorimeter make this an attractive avenue for constraining $\Delta G$. Significant constraints have already come from 2005 and 2006 PHENIX measurements of $\sqrt{s} =$ 200 GeV polarized proton collisions. In 2009, $\sqrt{s} =$ 200 GeV running saw a more than 60\% increase in the figure of merit ($P^4 L$) due a large increase in luminosity ($L$) with $\sim$55\% polarization ($P$). Also, the first data at $\sqrt{s} =$ 500 GeV were recorded and will push the constraints to lower momentum fraction x. After a brief review of past results, the status of the 2009 analysis will be presented. [Preview Abstract] |
Tuesday, February 16, 2010 11:45AM - 11:57AM |
X11.00006: Non-photonic electron double longitudinal spin asymmetry measurements in polarized $\vec{p}+\vec{p}$ collisions at STAR Priscilla Kurnadi The production of heavy flavor quarks by gluon-gluon fusion in polarized p+p collisions is sensitive to the polarized gluon distribution function, delta g. Sensitivity to heavy quark production can be attained by the detection of the non-photonic electrons resulting from the semi-leptonic decays of heavy mesons containing $b$ and $c$ quarks. The challenge in obtaining the non-photonic signal stems from the need to separate the signal from a background of photonic electrons, which are produced from other processes such as conversions of photons from $\pi^0$s converting on material in the detector. I will discuss the status of an analysis to extract the double longitudinal spin asymmetry, $A_{LL}$, of non-photonic electrons from the $\sqrt{s} = 200 GeV$ polarized $pp$ collision data collected by STAR during the 2005 and 2006 RHIC runs. [Preview Abstract] |
Tuesday, February 16, 2010 11:57AM - 12:09PM |
X11.00007: Measurement of transverse single spin asymmetries of very forward $\eta$ mesons in $\vec{p}+p$ collisions in PHENIX David Kleinjan The measurement of transverse single spin asymmetries (SSA or $A_N$) provide insight into the structure of the nucleon. Several mechanisms have been proposed that attempt to explain $A_N$ based on QCD, and additional measurements of $A_N$ for different processes further constrain these models. Using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC), we study transversely polarized p+p collisions. Results from PHENIX and other RHIC experiments show significant asymmetries in the very forward region, which could be due to contributions from both the Sivers and the Collins effects. Studying the species as well as the kinematic dependencies of these transverse SSAs will help to disentangle the origin of the observed asymmetries. Therefore, measurements of $A_N$ with inclusive $\eta$ mesons at very forward rapidities is an important tool for the understanding of these asymmetries. In 2008, the PHENIX experiment collected 5.2 pb$^{-1}$ integrated luminosity in $\vec{p}+p$ collisions at $\sqrt{s}$=200 GeV. The status of $\eta$ meson asymmetry analysis at very forward rapidity will be shown. [Preview Abstract] |
Tuesday, February 16, 2010 12:09PM - 12:21PM |
X11.00008: Status and prospects of the STAR W boson spin physics program at RHIC Bernd Surrow The STAR Collaboration has recently completed the first data taking period in 2009 (Run 9) of polarized p+p collisions at $\sqrt{s}=500\,$GeV. This opens a new era in the study of the spin-flavor structure of the proton based on the production of $W^{-(+)}$ bosons. $W^{-(+)}$ bosons are produced at leading order in $\bar{u}+d\,(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{\nu}_{e}\,(e^{+}+\nu_{e})$, where only the respective charged lepton is measured. The discrimination of $\bar{u}+d (\bar{d}+u)$ quark combinations requires distinguishing between high $p_{T}$ $e^{-(+)}$ through their opposite charge sign, which in turn requires precise tracking information. At mid rapidity, STAR will rely at first on the existing Time Projection Chamber augmented in the future by high precision silicon detectors close to the interaction region. At forward rapidity, new tracking capabilities will be provided by the Forward GEM Tracker, consisting of six triple-GEM detectors currently under construction. A brief overview will be provided on recent results on the production of W bosons during the first data taking period in 2009 (Run 9) of polarized p+p collisions at $\sqrt{s}=500\,$GeV. The main emphasis will be placed on future projections of the STAR W spin program at mid-rapidity and forward rapidity including a discussion of the STAR Forward GEM Tracker project. [Preview Abstract] |
Tuesday, February 16, 2010 12:21PM - 12:33PM |
X11.00009: Measurement of the Cross Section for $W$ Boson Production at $\sqrt{s}=500$ GeV at STAR Justin Stevens The production of $W^{-(+)}$ bosons in longitudinally polarized p+p collisions at RHIC provides a new means of studying the spin-flavor asymmetries of the proton sea quark distributions. $W^{-(+)}$ bosons are produced in $\bar{u}+d\,(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{\nu}_{e}\,(e^{+}+\nu_{e})$, where only the charged lepton is detected. Precise tracking information, provided by the STAR Time Projection Chamber (TPC) at mid-rapidity, allows for a determination of the charge sign of the high $p_T$ $e^{-(+)}$. The large acceptance of the TPC and Electromagnectic Calorimeters is well suited to place isolation requirements on the $e^{-(+)}$ and to veto on the away side energy, which reduces the large QCD background by several orders of magnitude yielding a clean $W$ signal. The status of the $W$ production cross section analysis from the STAR Collaboration's 2009 data at $\sqrt{s}=500$ GeV will be presented. [Preview Abstract] |
Tuesday, February 16, 2010 12:33PM - 12:45PM |
X11.00010: The parity-violating spin asymmetry $A_L$ for $W^{\pm}$ bosons with STAR at RHIC Jan Balewski The STAR experiment has acquired its first set of $W$-boson events from collisions of longitudinally polarized protons at $\sqrt{s}= 500$~GeV. The STAR Electromagnetic Calorimeter triggered on electrons/positrons from the weak decay of the $W$ and provided the energy of the lepton, while the STAR Time Projection Chamber allowed reconstruction of the lepton track and its charge sign. The QCD physics background was suppressed by isolation cuts around a candidate lepton track as well as vetoing on transverse energy opposite in azimuth. In the standard model leading-order $W^{\pm}$ production is through $u+\bar{d} \rightarrow W^+$ and $d+\bar{u} \rightarrow W^-$. These interactions are ideal tools to study the spin-flavor structure of the proton, because the spin-dependent $W$ production cross section $\Delta\sigma=\sigma(\overrightarrow{p} p) - \sigma(\overleftarrow{p} p)$ depends strongly on the polarization of the quark and anti-quark in the proton, with $\overrightarrow{p} (\overleftarrow{p})$ representing a proton with its spin aligned with (against) its momentum. We will present the status of the STAR measurement of $A_L=\Delta\sigma/(\sigma(\overrightarrow{p} p) + \sigma(\overleftarrow{p} p))$ for mid-rapidity charge separated $W^+$ and $W^-$. [Preview Abstract] |
Tuesday, February 16, 2010 12:45PM - 12:57PM |
X11.00011: Forward polarization studies at STAR Xuan Li As part of our program to understand the internal structure of the proton, we intend to measure $\gamma$-jet asymmetries and high x Lambda polarization in $\sqrt s \ge 200$ GeV polarized proton-proton collisions. To facilitate these measurements, we are testing a Forward Hadron Calorimeter (FHC), which is proposed to be installed behind the existing Forward Meson Spectrometer (FMS) at STAR with cosmic ray muons. We will present the status of cosmic ray tests of existing FHC modules. Simulation studies of Lambda reconstruction at forward rapidities at STAR using the FMS and FHC will also be presented. [Preview Abstract] |
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