Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session DM: Hadrons: Spin Asymmetries |
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Chair: Nick Markov, Jefferson Lab |
Friday, October 30, 2020 8:30AM - 8:42AM |
DM.00001: Longitudinal Double-Spin Asymmetry for Inclusive and Di-Jet Production in Polarized Proton-Proton Collisions at $\sqrt{s}=200$ GeV Nicholas Lukow The contribution of the gluon helicity to the spin of the proton is being studied utilizing the unique capability of the Relativistic Heavy Ion Collider (RHIC) to collide polarized protons at varying energies. The kinematic coverage of the Solenoidal Tracker At RHIC (STAR) allows access to gluons through gluon-gluon and quark-gluon scattering processes which dominate jet production at low and intermediate transverse momenta. The polarized gluon distribution function, $\Delta g(x)$, can be constrained through a global analysis including measurements of the longitudinal double-spin asymmetries ($A_{LL}$) of inclusive jet and di-jet production. Inclusive jet $A_{LL}$ results published by STAR at mid-rapidity ($|\eta| < 1$ ) at $\sqrt{s}=200\,$GeV have been used in global analyses and show a non-zero truncated first moment of $\Delta g(x)$ for momentum fraction, $x$, greater than 0.05. An additional data sample of $43\,$pb$^{-1}$ has been collected in 2015 at the same collision energy. This new data sample is over twice as large as the previous sample and will improve the precision of $\Delta g(x)$ for $x>0.05$. The latest results from the analysis of the 2015 data will be presented. [Preview Abstract] |
Friday, October 30, 2020 8:42AM - 8:54AM |
DM.00002: Transverse Spin Dependent Azimuthal Correlations of Charged Pion Pairs in $p^{\uparrow}+p$ Collisions at $\sqrt s = 200$ GeV Babu Pokhrel The transversity distribution function, $h^{q}_{1}(x)$, a leading twist parton density which describes the distributions of transversely polarized quarks inside transversely polarized hadrons, is a fundamental component of the spin structure of the nucleon, and is loosely constrained by global fits. Being chiral odd, $h^{q}_{1}$ can be accessed only when it is coupled with another chiral-odd partner, such as transverse spin-dependent fragmentation function. This gives rise to the azimuthal correlation between the polarization of the struck quarks and the final state scalar mesons, called transverse single-spin asymmetry (TSSA), that directly measures quark transversity distribution. The STAR experiment at RHIC has previously measured TSSA using polarized proton-proton collision data from 2006 at $\sqrt s = 200$ GeV and 2011 at $\sqrt s = 500$ GeV. Both measurements reported nonzero asymmetries which are sensitive to $h^{q}_{1}$. We will present an update on the recent TSSA analysis using data from 2015 polarized proton-proton collisions at $\sqrt s = 200$ GeV, $\sim 30$ times larger sample than that from 2006, in the mid pseudorapidity region ($| \eta |< 1$) for exclusive charged pion pairs ($\pi^{+}\pi^{-}$) in the final state. [Preview Abstract] |
Friday, October 30, 2020 8:54AM - 9:06AM |
DM.00003: Lensing Mechanism Meets Small-$x$ Physics: Single Transverse Spin Asymmetry in $p^{\uparrow}+p$ and $p^{\uparrow}+A$ Collisions Melvin Santiago, Yuri Kovchegov We calculate the single transverse spin asymmetry in polarized proton-proton and polarized proton-nucleus collisions ($A_N$) generated by a partonic lensing mechanism. The polarized proton is considered in the quark-diquark model while its interaction with the unpolarized target is calculated using the small-$x$/saturation approach. The phase required for the asymmetry is caused by a final-state gluon exchange between the quark and diquark, as is standard in the lensing mechanism of Brodsky, Hwang and Schmidt. The expression we obtain for the asymmetry $A_N$ of the produced quarks has the following properties:(i) The asymmetry is generated by the dominant elastic scattering contribution and $1/N_c^2$ suppressed inelastic contribution;(ii) The asymmetry grows or oscillates with the produced quark's transverse momentum $p_T$ until the momentum reaches the saturation scale $Q_s$, and then only falls off as $1/p_T$ for larger momenta;(iii) The asymmetry decreases with increasing atomic number $A$ of the target for $p_T$ below or near $Q_s$, but is independent of $A$ for $p_T$ significantly above $Q_s$. We discuss how these properties may be qualitatively consistent with data published by the PHENIX collaboration and with preliminary data reported reported by the STAR collaboration. [Preview Abstract] |
Friday, October 30, 2020 9:06AM - 9:18AM |
DM.00004: Measurement of transverse single-spin asymmetries for dijet production in polarized $p$+$p$ collisions at $\sqrt{s}$ = 200 GeV at STAR Huanzhao Liu We report a new measurement of transverse single-spin asymmetries for pair-production of jets in collisions of transversely polarized protons at $\sqrt{s}$ = 200 GeV with data taken in 2012 and 2015 at STAR. The correlation between the transverse momentum of a parton ($\vec k_T$) and the transverse spin ($\vec S$) of its proton, moving in the longitudinal ($\vec p$) direction, is probed at a high $Q^2$ scale ($\sim$160 GeV$^2$). The corresponding Sivers $\langle k_T \rangle$ is calculated based on a simple kinematic model. By employing charge-tagging to separately enhance $u$- and $d$-quark contributions, we see non-zero Sivers effects for the first time in dijet production with transversely polarized proton collisions. The individual parton contributions ($u$, $d$, gluon+sea) to the measured $\langle k_T \rangle$ are extracted through bin-by-bin matrix inversion of the charge-sorted $\langle k_T \rangle$ results. [Preview Abstract] |
Friday, October 30, 2020 9:18AM - 9:30AM |
DM.00005: Longitudinal Double-Spin Asymmetries for Intermediate Rapidity Inclusive $\pi^0$ Production from Polarized $pp$ Collisions at $\sqrt{s}$ = 200 GeV at STAR William Solyst We present an update of the analysis towards a new measurement of the longitudinal double-spin asymmetry $A_{LL}$ for neutral pions ($\pi^0$'s) reconstructed in the STAR Endcap Electromagnetic Calorimeter (EEMC) from polarized proton-proton collisions at $\sqrt{s}=200$ GeV with data taken in 2009. The EEMC is optimized for measurement of $\pi^0$'s, with full azimuthal coverage for $1.086 < \eta < 2.0$ and a fine granularity, scintillator-based shower maximum detector to identify $\pi^0$'s signature di-photon decay. Neutral pion $A_{LL}$ is sensitive to the polarized gluon distribution function $\Delta g(x)$, as quark-gluon scattering is the dominant subprocess for $\pi^0$ production in the EEMC region. In the more forward pseudorapidity region of the EEMC ($\eta > 1.3$), where inclusive jet measurements are compromised due to increased charged-particle tracking inefficiencies, $\pi^0$'s can be reconstructed accurately, allowing us to probe lower values of partonic momentum fraction $x$, where $\Delta g(x)$ is poorly constrained. [Preview Abstract] |
Friday, October 30, 2020 9:30AM - 9:42AM |
DM.00006: Measurement of azimuthal decorrelation angle between the leading jet and the scattered lepton in deep inelastic scattering at HERA Amilkar Quintero We present an update of the measurements of the azimuthal decorrelation angle between the leading jet and scattered lepton in deep inelastic scattering, with the ZEUS detector at HERA. The studied data was collected in the HERA II data-taking period and corresponds to an integrated luminosity of $~$ 330 $pb^{-1}$. Azimuthal angular decorrelation has been proposed to study the $Q^2$ dependence of the evolution of the transverse momentum distributions (TMDs). Previous decorrelation measurements of two jets have been performed in proton-proton collisions at very high transverse momentum; these measurements are well described by perturbative QCD at next-to-leading order. The HERA kinematic region provides unique insight of nucleon structure to understand the small-x region. Also, electron-proton collisions measurements allow clean access to the quark TMD PDF, in particular to final state effects. The azimuthal decorrelation angle obtained in these studies shows good agreement with predictions from QCD calculations within uncertainties; however, there are parts of the phase space for which deviations of up to 20\% are observed. Dedicated theoretical predictions are to be tested in the future. [Preview Abstract] |
Friday, October 30, 2020 9:42AM - 9:54AM |
DM.00007: Helicity at Small x: Oscillations, Cross-Checks and LLA Corrections Yossathorn Tawabutr, Yuri Kovchegov A numerical solution is constructed for the recently-derived large-$N_c \& N_f$ small-$x$ helicity evolution equations with the aim to establish the small-$x$ asymptotics of the quark helicity distribution. (Here $N_c$ and $N_f$ are the numbers of quark colors and flavors.) We find that adding quarks to the evolution makes quark helicity distribution oscillate as a function of $\ln(1/x)$. The typical oscillation period depends on $N_f$ and spans many units of rapidity. This result may relate to the sign variation with $x$ seen in the strange quark data. In addition, we perform a cross check on this recently-derived helicity evolution equations by analytically solving the equations with a substantially different initial condition, obtaining the same asymptotics at large $N_c$. The fact that two large-$N_c$ evolution equations resulting from two different initial conditions give the same small-$x$ asymptotics provides a validity cross-check of the calculation. Finally, we derive the single-logarithmic corrections to the double-logarithmic equations derived previously. The more complete equations, once solved, will provide a more precise estimate of the quark helicity distribution at small $x$, contributing to the resolution of the proton spin puzzle. [Preview Abstract] |
Friday, October 30, 2020 9:54AM - 10:06AM |
DM.00008: Cross section measurements of kinematically reconstructed weak bosons in unpolarized $p+p$ collisions at STAR Salvatore Fazio We present cross sections for the weak bosons measured by the STAR experiment at RHIC in unpolarized proton-proton collisions at $\sqrt{s}$ = 500(510) GeV. The results combine data from 2011, 2012, 2013, corresponding to an integrated luminosity of 360~pb$^{-1}$. An update including the 2017 data ($\sim$ 340~pb$^{-1}$) will be also discussed. The differential $Z^{0}$ cross section, measured as a function of the boson's $p_{T}$, provides important constraints on the energy dependence of transverse momentum distributions of partons inside the proton. The $W^{+}/W^{-}$ cross-section ratio as a function of the boson's rapidity, is sensitive to the non-pertubative $\bar{d}/\bar{u}$ distribution. The probed $x$ range ($0.1 < x < 0.3$) covered by our data naturally complements the phase space accessed at the LHC, providing critical inputs to global fits. [Preview Abstract] |
Friday, October 30, 2020 10:06AM - 10:18AM |
DM.00009: Longitudinal Double-Spin Asymmetries for Dijet Production at Intermediate Pseudorapidity in Polarized Proton-Proton Collisions at $\sqrt{s}$ = 510 GeV Joseph Kwasizur The analysis of dijets produced in polarized $pp$ collisions at the Relativistic Heavy Ion Collider (RHIC) provides information on the gluon helicity contribution to the spin of the proton. Because quark-gluon and gluon-gluon scattering processes dominate jet production in $pp$ collisions at RHIC energies of $\sqrt{s}$ = 200 GeV and $\sqrt{s}$ = 510 GeV, the longitudinal double-spin asymmetry $A_{LL}$ for dijet production is sensitive to the polarized gluon distribution function $\Delta g(x)$. Dijet measurements at larger pseudorapidity and higher center-of-mass energy probe lower values of partonic momentum fraction $x$, a region where $\Delta g(x)$ is still poorly constrained. Previous measurements of dijet $A_{LL}$ at the Solenoidal Tracker at RHIC (STAR) have been carried out at either mid-rapidity or $\sqrt{s}$ = 200 GeV, or both. We present the status of the first analysis of $A_{LL}$ for dijets with at least one jet reconstructed at intermediate pseudorapidity (0.8 $< \eta <$ 2.0) in polarized $pp$ collisions at $\sqrt{s}$ = 510 GeV, based on data taken during the 2012 and 2013 RHIC running periods. [Preview Abstract] |
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