Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session FG: Hadron Physics II |
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Chair: Abhay Deshpande, Stony Brook University Room: Surf |
Thursday, October 25, 2012 4:00PM - 4:12PM |
FG.00001: W Production in Polarized p-p Collisions: A Probe for Sea Quark Spin Daniel Jumper The PHENIX experiment at the Relativistic Heavy Ion Collider aims to measure spin dependent momentum distributions for quarks and anti-quarks inside the proton. This is accomplished by measuring parity violating longitudinal single spin asymmetries of muon yields from W-boson decay in 500 GeV polarized p-p collisions. Through the parity violating weak interaction, W-bosons offer a flavor-sensitive, direct probe of the quark and anti-quark spin distributions in the proton. This measurement is made at forward rapidity with the use of a recent muon trigger upgrade consisting of newly installed Resistive Plate Chambers and new trigger electronics for the muon tracking chambers. The trigger allows the selection of high momentum muons from W decay. This talk will introduce the measurement, discuss first results from the 2011 run and the analysis status of the 2012 run data. [Preview Abstract] |
Thursday, October 25, 2012 4:12PM - 4:24PM |
FG.00002: Recent STAR results on W boson production in polarized $p+p$ collisions at $\sqrt{s}=500$~GeV Jan Balewski The production of $W^\pm$ bosons in longitudinally polarized p+p collisions is an ideal tool to study the spin-flavor structure of the proton, because the spin-dependent $W$ production cross section $\Delta\sigma=\sigma(\overrightarrow{p} p) - \sigma(\overleftarrow{p} p)$ is directly sensitive to the polarization of the quarks and anti-quarks in the proton. At leading order in the standard model, $W^{\pm}$ production proceeds through $u+\bar{d} \rightarrow W^+$ and $d+\bar{u} \rightarrow W^-$. The STAR Electromagnetic Calorimeter is used to trigger on electrons and positrons from the weak decay of the $W$ boson and to provide a measure of the lepton energy, while the STAR Time Projection Chamber allowes for reconstruction of the lepton track and its charge sign. Background events from QCD processes were suppressed by isolation cuts around a candidate lepton track as well as vetoing on transverse energy opposite in azimuth of the candidate. The STAR experiment has collected about 100 pb$^{-1}$ of $W$ boson events from collisions of protons at $\sqrt{s}= 500$~GeV, longitudinally polarized at 50\%, during 2009-2012 running periods. We will present progress on determination of the single-spin asymmetry, $A_L=\Delta\sigma/(\sigma(\overrightarrow{p} p) +$ [Preview Abstract] |
Thursday, October 25, 2012 4:24PM - 4:36PM |
FG.00003: Longitudinal Spin Transfer to $\Lambda$ and $\bar\Lambda$ Hyperons Produced in Polarized Proton-Proton Collisions at $\sqrt{s} = 200$ GeV Ramon Cendejas The longitudinal spin transfer, $D_{LL}$, of $\Lambda$ and $\bar\Lambda$ hyperons in longitudinally polarized proton-proton collisions is sensitive to strange quark and anti-quark polarization in the polarized proton as well as to polarized fragmentation. The STAR collaboration previously published $D_{LL}$ from a data sample obtained in 2005 that corresponds to an integrated luminosity of 2 $pb^{-1}$ with 50\% beam polarization. The $D_{LL}$ results from the data were compared to the $D_{LL}$ model calculations. A considerably larger data sample corresponding to 25 $pb^{-1}$ with beam polarization of 57\% was obtained in 2009 using an upgraded instrument. The analysis of these data is in progress and is anticipated to considerably improve the precision of the $D_{LL}$ measurements. The status of the analysis will be discussed. [Preview Abstract] |
Thursday, October 25, 2012 4:36PM - 4:48PM |
FG.00004: $W^{\pm}$ Boson Production Measurement at Mid-rapidity in the PHENIX Experiment Mikhail Stepanov The measurement of $W^{\pm}$ production provides an important probe of the flavor-separated quark and antiquark helicity distributions in the proton. At mid-rapidity, $\mid \eta \mid \le 0.35$, using the PHENIX central arm detectors we observe $W^{\pm}$ boson decays through $e^{\pm}$ channels in polarized $p+p$ collisions at RHIC. In 2011, in 500 GeV $p+p$ collisions, PHENIX recorded data with an improved beam polarization and increased integrated luminosity in comparison with the previous 2009 data set: the beam polarization was 48$\%$ and the integrated luminosity was 16.7 pb$^{-1}$. After the silicon VTX detector was installed in 2011, which led to an increased background from conversion in additional material, a supplementary analysis technique was developed to extract the signal. In 2012, PHENIX collected approximately 30 pb$^{-1}$ of polarized $p+p$ collisions at $\sqrt{s}$ = 510 GeV with the beam polarization of 52$\%$; the silicon VTX detector was fully operational in 2012 and is ready to be used in the data analysis. The progress on extracting $W$-decay $e^{\pm}$ single spin asymmetries using the 2011 and 2012 data sets will be presented. [Preview Abstract] |
Thursday, October 25, 2012 4:48PM - 5:00PM |
FG.00005: Reduction of Background in Observation of $W$ Decay Using FVTX Tracker in PHENIX Abraham Meles One of the highlights of the spin program at RHIC over the next few years is the observation of the parity-violating asymmetry $A_L$ in the production of $W$ bosons in $\vec{p}+\vec{p}$ collisions at $\sqrt{s}$ = 500 GeV. This asymmetry is sensitive to the polarization of light quarks and anti-quarks in the proton. Observing the lepton ($e$ or $\mu$) from the decay of the $W$ is challenging due to a great background of hadronic processes. In PHENIX, in the forward and backward hemispheres, the muon spectrometers have been recently upgraded in order to provide additional tracking information to suppress those backgrounds. One of those upgrades is the Forward Vertex (FVTX) detector, a silicon-strip tracker. The ability of the FVTX to improve $W$ observation will be reviewed, and compared to real data from the first use of the FVTX in the RHIC 2012 run. [Preview Abstract] |
Thursday, October 25, 2012 5:00PM - 5:12PM |
FG.00006: Photon-jet Coincidence Measurements in Polarized pp Collisions at 200 GeV at STAR Seema Dhamija Recent inclusive measurements with polarized proton-proton collisions at RHIC provide significant constraints on the polarized gluon distribution, $\Delta g(x)$, integrated over the gluon momentum range 0.05 $<$ x $<$ 0.2. Determining the gluon helicity distribution as a function of the parton momentum fraction x requires sensitivity to the initial-state parton kinematics, which can be achieved with correlated probes such as di-jet or photon-jet coincidence measurements. Compared to di-jets, the $\gamma$-jet channel is lower in yield, but is dominated by a single partonic subprocess (quark-gluon Compton scattering). Through localized measurement of the photon energy in the forward direction (along with the jet thrust axis of the recoiling jet), more precise reconstruction of the parton kinematics is achievable as is sensitivity to lower values of x. A detailed systematic study of detector effects, and evaluation of $\gamma$-jet signal and background contributions in the experimental data, will be presented from the 2006 GeV p+p run. [Preview Abstract] |
Thursday, October 25, 2012 5:12PM - 5:24PM |
FG.00007: Inclusive Neutral Pion Cross Section Measurement with the STAR Endcap Electromagnetic Calorimeter Stephen Gliske The STAR endcap electromagnetic calorimeter (EEMC) is designed to allow measurement of cross sections and spin observables in the forward direction, $1 < \eta < 2$. Using the EEMC to measure double longitudinal spin asymmetries in photonic channels---such as inclusive $\pi^0$, prompt photon, and prompt photon + jet---allowes access to $\Delta G$, but covers a lower $x$ region than current mid-rapidity measurements. However, the high rate of conversions from material in front of the EEMC makes these photonic channels particularly challenging. Measuring the cross section for each channel verifies that the signal of interest can be properly reconstructed and isolated. Cross section measurements can also quantify the applicability of pQCD. Particle reconstruction with the STAR EEMC will be discussed, and the status of the neutral pion cross section measurement will be presented. [Preview Abstract] |
Thursday, October 25, 2012 5:24PM - 5:36PM |
FG.00008: Exclusive Jets in Deep Inelastic Scattering Christopher Lee, Daekyoung Kang, Iain Stewart Near x=1, final states in deep inelastic scattering have a single collimated jet of hadrons. We consider events away from this limit, but with a restriction on the final state requiring that it be two-jet-like in the center-of-momentum frame, with one jet along the incident proton direction. This ``beam'' jet is likely to have been formed by initial state radiation (ISR). This environment provides a relatively clean way to study the nature of ISR, in contrast to pp collisions at LHC, and provides an additional sensitive probe of the parton distributions inside the proton. We use soft collinear effective theory to predict the jet mass distributions to NNLL (next-to-next-to-leading-logarithmic) accuracy, the highest achieved to date in DIS. [Preview Abstract] |
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