PROBING THE LOW-X GLUON HELICITY DISTRIBUTION WITH DIJET DOUBLE SPIN ASYMMETRIES IN POLARIZED PROTON COLLISIONS AT \u3cem\u3e√S\u3c/em\u3e = 510 GEV

The proton is a complex subatomic particle consisting of quarks and gluons, and one of the key questions in nuclear physics is how the spin of the proton is distributed amongst its constituents. Polarized deep inelastic scattering experiments with leptons and protons estimate that the quark spin contribution is approximately 30%. The limited kinematic reach of these experiments, combined with the fact that they are only indirectly sensitive to the electrically neutral gluon, means they can provide very little information about the gluon contribution to the spin of the proton. In contrast, hadronic probes, such as polarized proton collisions provide direct access to the gluon helicity distribution. The production of jets in polarized proton collisions at STAR is dominated by quark-gluon and gluon-gluon scattering processes. The dijet longitudinal double spin asymmetry (ALL) is sensitive to the polarized parton distributions and may be used to extract information about the gluon contribution to the spin of the proton. Previous STAR jet measurements at √s = 200 GeV show evidence of polarized gluons for gluon momentum fractions above 0.05. The measurement of dijet ALL at √s = 510 GeV will extend the current constraints on the gluon helicity distribution to low momentum fractions and allow for the reconstruction of the partonic kinematics. Information about the initial state momentum provides unique constraints on the functional form of the gluon helicity distribution, thus reducing the uncertainty on extrapolations to poorly constrained regions. This thesis will present the first measurement of the dijet ALL at √s = 510 GeV, from polarized proton data taken during the 2012 RHIC run

Charged-to-Neutral Correlation at Forward Rapidity in Au + Au collisions at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e=200 GeV

Event-by-event fluctuations of the multiplicities of inclusive charged particles and photons at forward rapidity in Au+Au collisions at √SNN=200 GeV have been studied. The dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidence of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as base lines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A nonzero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation owing to particle decay. Results are compared to the expectations based on the generic production mechanism of pions owing to isospin symmetry, for which no significant (\u3c1%) deviation is observed

Beam-Energy-Dependent Two-Pion Interferometry and the Freeze-Out Eccentricity of Pions Measured in Heavy Ion Collisions at the STAR Detector

We present results of analyses of two-pion interferometry in Au+Au collisions at √SNN=7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV measured in the STAR detector as part of the BNL Relativistic Heavy Ion Collider Beam Energy Scan program. The extracted correlation lengths (Hanbury-Brown–Twiss radii) are studied as a function of beam energy, azimuthal angle relative to the reaction plane, centrality, and transverse mass (mT) of the particles. The azimuthal analysis allows extraction of the eccentricity of the entire fireball at kinetic freeze-out. The energy dependence of this observable is expected to be sensitive to changes in the equation of state. A new global fit method is studied as an alternate method to directly measure the parameters in the azimuthal analysis. The eccentricity shows a monotonic decrease with beam energy that is qualitatively consistent with the trend from all model predictions and quantitatively consistent with a hadronic transport model

\u3cem\u3eJ\u3c/em\u3e/\u3cem\u3eψ\u3c/em\u3e Production at Low \u3cem\u3ep\u3csub\u3eT\u3c/sub\u3e\u3c/em\u3e in Au + Au and Cu + Cu Collisions at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3eNN\u3c/em\u3e = 200 GeV with the STAR Detector

The J/ψ pT spectrum and nuclear modification factor (RAA) are reported for pT \u3c 5 GeV/c and |y| \u3c 1 from 0% to 60% central Au + Au and Cu + Cu collisions at √SNN = 200 GeV at STAR. A significant suppression of pT-integrated J/ψ production is observed in central Au + Au events. The Cu + Cu data are consistent with no suppression, although the precision is limited by the available statistics. RAA in Au + Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with pT. The data are compared to high-pT STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low pT are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration

Jet-Like Correlations with Direct-Photon and Neutral-Pion Triggers at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 200 GeV

Azimuthal correlations of charged hadrons with direct-photon (γdir) and neutral-pion (π0) trigger particles are analyzed in central Au+Au and minimum-bias p+p collisions at in the STAR experiment. The charged-hadron per-trigger yields at mid-rapidity from central Au+Au collisions are compared with p+p collisions to quantify the suppression in Au+Au collisions. The suppression of the away-side associated-particle yields per γdir trigger is independent of the transverse momentum of the trigger particle (ptrigT), whereas the suppression is smaller at low transverse momentum of the associated charged hadrons (passocT). Within uncertainty, similar levels of suppression are observed for γdir and π0 triggers as a function of zT (passocT / ptrigT). The results are compared with energy-loss-inspired theoretical model predictions. Our studies support previous conclusions that the lost energy reappears predominantly at low transverse momentum, regardless of the trigger energy

Jet-Hadron Correlations in √\u3csup\u3es\u3c/sup\u3eNN=200 GeV \u3cem\u3ep\u3c/em\u3e+\u3cem\u3ep\u3c/em\u3e and Central Au+Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at √sNN=200  GeV in STAR are presented. The trigger jet population in Au+Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (passocT) and enhanced at low passocT in 0%–20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions

Dielectron Azimuthal Anisotropy at Mid-Rapidity in Au + Au Collisions at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e=200 GeV

We report on the first measurement of the azimuthal anisotropy (v2) of dielectrons (e+e− pairs) at mid-rapidity from √SNN=200 GeV Au + Au collisions with the STAR detector at the Relativistic Heavy Ion Collider (RHIC), presented as a function of transverse momentum (pT) for different invariant-mass regions. In the mass region Meec2 the dielectron v2 measurements are found to be consistent with expectations from π0, η, ω, and ϕ decay contributions. In the mass region 1.1\u3cMee\u3c2.9GeV/c2, the measured dielectron v2 is consistent, within experimental uncertainties, with that from the c¯C contributions

ΛΛ Correlation Function in Au+Au Collisions at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e=200 GeV

We present ΛΛ correlation measurements in heavy-ion collisions for Au+Au collisions at √SNN=200  GeV using the STAR experiment at the Relativistic Heavy-Ion Collider. The Lednický-Lyuboshitz analytical model has been used to fit the data to obtain a source size, a scattering length and an effective range. Implications of the measurement of the ΛΛ correlation function and interaction parameters for dihyperon searches are discussed

Observation of Charge Asymmetry Dependence of Pion Elliptic Flow and the Possible Chiral Magnetic Wave in Heavy-Ion Collisions

We present measurements of π− and π+ elliptic flow, v2, at midrapidity in Au+Au collisions at √SNN=200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, Ach, based on data from the STAR experiment at RHIC. We find that π− (π+) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at √SNN=27  GeV and higher. At √SNN=200  GeV, the slope of the difference of v2 between π− and π+ as a function of Ach exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies

Observation of Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs in \u3cem\u3ep\u3c/em\u3e\u3csup\u3e↑\u3c/sup\u3e+\u3cem\u3ep\u3c/em\u3e at √\u3cem\u3es\u3c/em\u3e=200 GeV

We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p↑+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η\u3e0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions