Transverse single-spin asymmetries to probe the transverse-spin structure of the proton have been measured for neutral pions and nonidentified charged hadrons from polarized proton-proton collisions ...at midrapidity and square root(s) = 200 GeV. The data cover a transverse momentum (pT) range 1.0-5.0 GeV/c for neutral pions and 0.5-5.0 GeV/c for charged hadrons, at a Feynman-x value of approximately zero. The asymmetries seen in this previously unexplored kinematic region are consistent with zero within errors of a few percent. In addition, the inclusive charged hadron cross section at midrapidity from 0.5 < pT < 7.0 GeV/c is presented and compared to next-to-leading order perturbative QCD (pQCD) calculations. Successful description of the unpolarized cross section above approximately 2 GeV/c suggests that pQCD is applicable in the interpretation of the asymmetry results in the relevant kinematic range.
Flow coefficients ν(n) for n=2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at √s(NN)=200 GeV, are measured relative to event planes Ψ(n), determined at large rapidity. ...We report ν(n) as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The ν(n) are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint can serve to improve the precision of the extracted shear viscosity to entropy density ratio η/s.
Phys. Rev. D 105, 032003 (2022) In 2015, the PHENIX collaboration has measured single-spin asymmetries for
charged pions in transversely polarized proton-proton collisions at the center
of mass ...energy of $\sqrt{s}=200$ GeV. The pions were detected at central
rapidities of $|\eta|<0.35$. The single-spin asymmetries are consistent with
zero for each charge individually, as well as consistent with the previously
published neutral-pion asymmetries in the same rapidity range. However, they
show a slight indication of charge-dependent differences which may suggest a
flavor dependence in the underlying mechanisms that create these asymmetries.
Back-to-back hadron pair yields in d+Au and p+p collisions at √s(NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were ...detected with the trigger hadron at pseudorapidity |η|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<η<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case, the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with a low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p(T), and η points to cold nuclear matter effects arising at high parton densities.
Phys. Rev. D 105, 032004 (2022) In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider
recorded $p$$+$$p$, $p$$+$Al, and $p$$+$Au collision data at center of mass
energies of ...$\sqrt{s_{_{NN}}}=200$ GeV with the proton beam(s) transversely
polarized. At very forward rapidities $\eta>6.8$ relative to the polarized
proton beam, neutrons were detected either inclusively or in (anti)correlation
with detector activity related to hard collisions. The resulting single spin
asymmetries, that were previously reported, have now been extracted as a
function of the transverse momentum of the neutron as well as its longitudinal
momentum fraction $x_F$. The explicit kinematic dependence, combined with the
correlation information allows for a closer look at the interplay of different
mechanisms suggested to describe these asymmetries, such as hadronic
interactions or electromagnetic interactions in ultra-peripheral collisions,
UPC. Events that are correlated with a hard collision indeed display a mostly
negative asymmetry that increases in magnitude as a function of transverse
momentum with only little dependence on $x_F$. In contrast, events that are not
likely to have emerged from a hard collision display positive asymmetries for
the nuclear collisions with a kinematic dependence that resembles that of a UPC
based model. Because the UPC interaction depends strongly on the charge of the
nucleus, those effects are very small for $p$$+$$p$ collisions, moderate for
$p$$+$Al collisions, and large for $p$$+$Au collisions.
PHENIX has measured the centrality dependence of midrapidity pion, kaon, and proton transverse momentum distributions in d+Au and p+p collisions at root s(NN) = 200 GeV. The p+p data provide a ...reference for nuclear effects in d+Au and previously measured Au+Au collisions. Hadron production is enhanced in d+Au, relative to independent nucleon-nucleon scattering, as was observed in lower energy collisions. The nuclear modification factor for (anti)protons is larger than that for pions. The difference increases with centrality but is not sufficient to account for the abundance of baryon production observed in central Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The centrality dependence in d+Au shows that the nuclear modification factor increases gradually with the number of collisions encountered by each participant nucleon. We also present comparisons with lower energy data as well as with parton recombination and other theoretical models of nuclear effects on particle production.
The first measurement of direct photons in Au + Au collisions at (square root)S(NN) = 200 GeV is presented. The direct photon signal is extracted as a function of the Au + Au collision centrality and ...compared to next-to-leading order perturbative quantum chromodynamics calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.
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