Bulletin of the American Physical Society
2019 Fall Meeting of the APS Division of Nuclear Physics
Volume 64, Number 12
Monday–Thursday, October 14–17, 2019; Crystal City, Virginia
Session GG: Nucleon Structure at Colliders |
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Chair: Sanghwa Park Room: Salon A |
Tuesday, October 15, 2019 2:00PM - 2:12PM |
GG.00001: Investigating Nucleon Structure and Hadronization with Hadrons in Jets at STAR James Drachenberg The STAR collaboration at RHIC provides insight into the spin structure of the nucleon through collisions of longitudinally and transversely polarized beams of protons. Spin-dependent azimuthal distributions of hadrons within jets from transversely polarized proton collisions provide access to the transversity distribution function at a range of $x$ complementary to semi-inclusive deep inelastic scattering (SIDIS) experiments but at a much higher range of $Q^2$. Asymmetries from STAR data collected in 2011 at $\sqrt{s}=500$ GeV and in 2012 at $200$ GeV give the first experimental hints that the universality of this ``Collins mechanism'' may extend to proton-proton collisions, as it does in SIDIS and $e^{+}e^{-}$. The STAR data also provide unique insight to the in-jet transverse momentum dependence of the Collins asymmetry, crucial for a deeper understanding of the Collins fragmentation function. The final 2011 and preliminary 2012 STAR hadron-in-jet data will be presented and discussed in context with recent global transversity analyses and model calculations. [Preview Abstract] |
Tuesday, October 15, 2019 2:12PM - 2:24PM |
GG.00002: Measurement of transverse single-spin asymmetries for di-jet production in polarized $p$+$p$ collisions at $\sqrt{s}$ = 200 GeV at STAR Huanzhao Liu Our understanding of the origin of the nucleon spin remains incomplete. Determining partonic orbital angular momentum (OAM) contributions is particularly challenging experimentally. One possible handle on partonic OAM would be detection of a non-zero Sivers effect, which characterizes 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. Experimentally, the Sivers function can be accessed by searching for a spin-dependent preference in di-jet transverse opening angle, which reverses direction when the beam polarization direction is flipped. A previous effort by STAR using 1~pb$^{-1}$ of $p$+$p$ data taken in 2006 at $\sqrt{s}$ = 200 GeV did not find a significant effect due to limited statistics. In 2012 and 2015, STAR accumulated much larger datasets ($\sim$33 times the di-jets after event selection) at $\sqrt{s}$ = 200 GeV. Moreover, a new technique has been implemented to tag the flavor of the fragmenting partons, to avoid cancellation between $u$ and $d$ quark effects, which are expected to have opposite signs. The current status of the measurement of Sivers asymmetries using STAR 2012 and 2015 $p$+$p$ data at $\sqrt{s}$ = 200 GeV will be reported. [Preview Abstract] |
Tuesday, October 15, 2019 2:24PM - 2:36PM |
GG.00003: Measurement of the longitudinal spin asymmetries for weak boson production in polarized proton-proton collisions at $\sqrt{s}=510$ GeV at RHIC Bernd Surrow The STAR experiment at RHIC has provided significant contributions to our understanding of the spin structure and dynamics of the proton. The production of $W^\pm$ bosons in longitudinally polarized p-p collisions at $\sqrt{s}=510$ GeV provides a direct probe of the spin-flavor structure of the proton through the measurement of the parity-violating single-spin asymmetry, $A_{L}$. $W^{-(+)}$ 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 respective charged lepton is measured. The STAR experiment is well equipped to measure $W^\pm \rightarrow e^\pm + \nu$. The main STAR detector sub-systems used in this measurement are the Time Projection Chamber and Electromagnetic Calorimeters. The published 2011 / 2012 STAR $A_L$ results based on $86\,$pb$^{-1}$ of data provided significant impact in constraining the helicity distributions of $\bar{u}$ and $\bar{d}$ quarks. In 2013, STAR collected an additional, larger data sample of $250\,$pb$^{-1}$. Final published results from the 2013 dataset for the measurement of $W^{\pm}$ $A_{L}$ and $A_{LL}$, and $Z$ $A_{L}$ will be presented including a discussion of the impact on the sea-quark helicity distribution functions. [Preview Abstract] |
Tuesday, October 15, 2019 2:36PM - 2:48PM |
GG.00004: Azimuthal Transverse Single-Spin Asymmetries of Charged Pions Within Jets from Polarized $pp$ Collisions at $\sqrt{s}$ = 200 GeV Ting Lin Understanding the internal spin structure of the nucleon remains an open question in strong interaction physics. Transversity, which describes the transverse spin structure of quarks in a transversely polarized proton, is still quite unconstrained in global analyses. It can be accessed through channels that couple to another chiral-odd distribution like the Collins fragmentation function or the interference fragmentation function. STAR reported the first measurements of Collins asymmetries from jet + $\pi^{\pm}$ production in polarized proton+proton collisions at $\sqrt{s}$ = 500 GeV and 200 GeV based on the data taken during the years 2011 and 2012. These results probe higher momentum scales ($Q^{2}$ $\sim$ 960 $\mathrm{GeV^{2}}$ for 500 GeV and $\sim$ 170 $\mathrm{GeV^{2}}$ for 200 GeV) than the measurements from semi-inclusive deep inelastic scattering (SIDIS, $Q^{2}$ $<$ 20 $\mathrm{GeV^{2}}$) and enable the test of the evolution, universality and factorization breaking in the transverse momentum dependent (TMD) formalisms. Status of the measurement of Collins asymmetry from 2015 proton+proton collisions at $\sqrt{s}$ = 200 GeV with much higher statistics will be presented. [Preview Abstract] |
Tuesday, October 15, 2019 2:48PM - 3:00PM |
GG.00005: (CEU) Monte Carlo simulation studies for unfolding hadron-in-jet multiplicity measurements at STAR Roy Salinas Hadronic jets are among the most striking phenomena seen in high energy physics, but the conversion of scattered quarks and gluons into hadrons remains mysterious. The multiplicities of hadrons in jets have been proposed as a way to gain a deeper insight into fragmentation functions, in particular for the gluon. The STAR experiment at Brookhaven National Lab has long used spin asymmetries in jet production to constrain such things as gluon helicity. Recent studies have made use of spin asymmetries of pions within jets to investigate transversity and the Collins fragmentation function. Unpolarized hadron-in-jet measurements can lead to new information such as better constraints on the gluon fragmentation function and a clearer picture of the transverse momentum distribution of hadrons within jets. Producing accurate multiplicity measurements requires a multi-dimensional unfolding to correct for effects such as bin migrations for which Monte Carlo simulations have proven to be an effective tool. The status of a Monte Carlo analysis will be shown, along with a discussion on possible unfolding techniques. [Preview Abstract] |
Tuesday, October 15, 2019 3:00PM - 3:12PM |
GG.00006: Charm Production in Charged Current Deep Inelastic Scattering at HERA Jae Nam Charm production in charged current deep inelastic scattering has been measured for the first time in $e^{\pm}p$ collisions, using data collected with the ZEUS detector at HERA, corresponding to an integrated luminosity of $358 \textrm{ pb}^{-1}$. Results are presented separately for $e^+p$ and $e^-p$ scattering at a center-of-mass energy of $\sqrt{s} = 318 \textrm{ GeV}$ within a kinematic phase-space region of $200 \textrm{ GeV}^2 < Q^2 < 60000 \textrm{ GeV}^2$ and $y < 0.9$, where $Q^2$ is the squared four-momentum transfer and $y$ is the inelasticity of deep inelastic scattering. The measured cross sections of electroweak charm production, although not statistically significant, are consistent with expectations. [Preview Abstract] |
Tuesday, October 15, 2019 3:12PM - 3:24PM |
GG.00007: Measurement of azimuthal decorrelation angle between the leading jet and the scattered lepton in deep inelastic scattering at HERA Amilkar Quintero The azimuthal decorrelation angle between the leading jet and scattered lepton in deep inelastic scattering is being studied in the ZEUS detector at HERA. The data was taken 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) and understand the small-x region, providing unique insight to nucleon structure. 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 azimuthal decorrelation angle obtained in these studies shows good agreement with predictions from QCD calculations. [Preview Abstract] |
Tuesday, October 15, 2019 3:24PM - 3:36PM |
GG.00008: Heavy quark pair production and polarized gluon distributions at JLab and EIC Gary Goldstein Heavy quark pairs can be produced in e$+$p collisions at JLab and prolifically at an EIC. At intermediate values of momentum fraction x for each gluon in g$+$g to quark$+$antiquark, the spin dependences of gluon distributions leave imprints on the momentum and spin correlations of the quark pairs. These correlations are distinguishable from the quark distribution mechanism. Decays of such spin entangled heavy quark pairs produce a variety of correlations among pairs of the 3-momenta of the decay products. The different angular correlations will be presented and related to measurable distributions of decay products. Some models for spin dependent gluon transverse momentum distributions and generalized transverse momentum distributions will be used to simulate the spin correlations, illustrating how to measure the gluon polarizations in electroproduction. [Preview Abstract] |
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