Phys. Rev. C 109, 054910 (2024) The PHENIX experiment has performed a systematic study of identified
charged-hadron ($\pi^\pm$, $K^\pm$, $p$, $\bar{p}$) production at midrapidity
in $p$$+$Al, ...$^3$He$+$Au, Cu$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV and
U$+$U collisions at $\sqrt{s_{_{NN}}}=193$ GeV. Identified charged-hadron
invariant transverse-momentum ($p_T$) and transverse-mass ($m_T$) spectra are
presented and interpreted in terms of radially expanding thermalized systems.
The particle ratios of $K/\pi$ and $p/\pi$ have been measured in different
centrality ranges of large (Cu$+$Au, U$+$U) and small ($p$$+$Al, $^3$He$+$Au)
collision systems. The values of $K/\pi$ ratios measured in all considered
collision systems were found to be consistent with those measured in $p$$+$$p$
collisions. However the values of $p/\pi$ ratios measured in large collision
systems reach the values of $\approx0.6$, which is $\approx2$ times larger than
in $p$$+$$p$ collisions. These results can be qualitatively understood in terms
of the baryon enhancement expected from hadronization by recombination.
Identified charged-hadron nuclear-modification factors ($R_{AB}$) are also
presented. Enhancement of proton $R_{AB}$ values over meson $R_{AB}$ values was
observed in central $^3$He$+$Au, Cu$+$Au, and U$+$U collisions. The proton
$R_{AB}$ values measured in $p$$+$Al collision system were found to be
consistent with $R_{AB}$ values of $\phi$, $\pi^\pm$, $K^\pm$, and $\pi^0$
mesons, which may indicate that the size of the system produced in $p$$+$Al
collisions is too small for recombination to cause a noticeable increase in
proton production.
Recently, the n\(^3\)He collaboration reported a measurement of the parity-violating (PV) proton directional asymmetry \(A_{\mathrm {PV}} = (1.55\pm 0.97~\mathrm {(st\ at)} \pm 0.24~\mathrm ...{(sys)})\times 10^{-8}\) in the capture reaction of \({}^3\)He\((\vec {n},{\mathrm p}){}^3\)H at meV incident neutron energies. The result increased the limited inventory of precisely measured and calculable PV observables in few-body systems required to further understand the structure of hadronic weak interaction. In this letter, we report the experimental and theoretical investigation of a parity conserving (PC) asymmetry \(A_{\mathrm {PC}}\) in the same reaction (the first ever measured PC observable at meV neutron energies). As a result of S- and P-wave mixing in the reaction, the \(A_{\mathrm {PC}}\) is inversely proportional to the neutron wavelength \(\lambda\). The experimental value is \((\lambda\times A_{\mathrm {PC}})\equiv\beta= (-1.97 \pm 0.28~\mathrm{(stat)}\pm 0.12~\mathrm{(sys)}) \times 10^{-6}\) Amstrongs. We present results for a theoretical analysis of this reaction by solving the four-body scattering problem within the hyperspherical harmonic method. We find that in the \({}^3\)He\((\vec {n},{\mathrm p}){}^3\)H reaction, \(A_{\mathrm {PC}}\) depends critically on the energy and width of the close \(0^-\) resonant state of \({}^4\)He, resulting in a large sensitivity to the spin-orbit components of the nucleon-nucleon force and even to the three-nucleon force. The analysis of the accurately measured \(A_{\mathrm {PC}}\) and \(A_{\mathrm {PV}}\) using the same few-body theoretical models gives essential information needed to interpret the PV asymmetry in the \({}^3\)He\((\vec {n}, {\mathrm p}){}^3\)H reaction.
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √s(NN)=200 GeV. The p(T) of the photon is an excellent approximation to the initial ...p(T) of the jet and the ratio z(T)=p(T)(h)/p(T)(γ) is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I(AA), the ratio of hadron yield opposite the photon in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z(T). The associated hadron yield at low z(T) is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
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The double helicity asymmetry in neutral pion production for p_T = 1 to 12 GeV/c has been measured with the PHENIX experiment in order to access the gluon spin contribution, Delta-G, to the proton ...spin. Measured asymmetries are consistent with zero, and at a theory scale of \mu^2 = 4 GeV^2 give Delta-G^0.02,0.3 = 0.1 to 0.2, with a constraint of -0.7 < Delta-G^0.02,0.3 < 0.5 at Delta-chi^2 = 9 (~3 sigma) for our sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for our measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta-G constraint on the choice of theoretical scale, a dominant uncertainty in these predictions.
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For Au + Au collisions at 200 GeV, we measure neutral pion production with good statistics for transverse momentum, pT, up to 20 GeV/c. A fivefold suppression is found, which is essentially constant ...for 5 < pT < 20 GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parametrization for the transport coefficient of the medium, e.g., q in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au + Au collisions.
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Neutral pion transverse momentum (p(T)) spectra at midrapidity (|y| less than or approximately 0.35) were measured in Cu+Cu collisions at sqrts(NN)=22.4, 62.4, and 200 GeV. Relative to pi_(0) yields ...in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N(coll) the pi_(0) yields for p(T) more than or approximately 2 GeV/c in central Cu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu+Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.
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Yields for J/psi production in Cu+Cu collisions at sqrt s_NN=200 GeV have been measured over the rapidity range |y|<2.2 and compared with results in p+p and Au+Au collisions at the same energy. The ...Cu+Cu data offer greatly improved precision over existing Au+Au data for J/psi production in collisions with small to intermediate numbers of participants, in the range where the quark-gluon plasma transition threshold is predicted to lie. Cold nuclear matter estimates based on ad hoc fits to d+Au data describe the Cu+Cu data up to N_part approximately 50, corresponding to a Bjorken energy density of at least 1.5 GeV/fm(3).
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The PHENIX experiment has measured open heavy-flavor production via semileptonic decay over the transverse momentum range 1 < p(T) < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in ...d+Au and p + p collisions at √sNN = 200 GeV. In central d+Au collisions, relative to the yield in p + p collisions scaled by the number of binary nucleon-nucleon collisions, a suppression is observed at forward rapidity (in the d-going direction) and an enhancement at backward rapidity (in the Au-going direction). Predictions using nuclear-modified-parton-distribution functions, even with additional nuclear-p(T) broadening, cannot simultaneously reproduce the data at both rapidity ranges, which implies that these models are incomplete and suggests the possible importance of final-state interactions in the asymmetric d + Au collision system. These results can be used to probe cold-nuclear-matter effects, which may significantly affect heavy-quark production, in addition to helping constrain the magnitude of charmonia-breakup effects in nuclear matter.
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Charged-pion-interferometry measurements were made with respect to the second- and third-order event plane for Au+Au collisions at sqrts_{NN}=200 GeV. A strong azimuthal-angle dependence of the ...extracted Gaussian-source radii was observed with respect to both the second- and third-order event planes. The results for the second-order dependence indicate that the initial eccentricity is reduced during the medium evolution, which is consistent with previous results. In contrast, the results for the third-order dependence indicate that the initial triangular shape is significantly reduced and potentially reversed by the end of the medium evolution, and that the third-order oscillations are largely dominated by the dynamical effects from triangular flow.
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High-momentum two-particle correlations are a useful tool for studying
jet-quenching effects in the quark-gluon plasma. Angular correlations between
neutral-pion triggers and charged hadrons with ...transverse momenta in the range
4--12~GeV/$c$ and 0.5--7~GeV/$c$, respectively, have been measured by the
PHENIX experiment in 2014 for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$~GeV.
Suppression is observed in the yield of high-momentum jet fragments opposite
the trigger particle, which indicates jet suppression stemming from in-medium
partonic energy loss, while enhancement is observed for low-momentum particles.
The ratio and differences between the yield in Au$+$Au collisions and $p$$+$$p$
collisions, $I_{AA}$ and $\Delta_{AA}$, as a function of the trigger-hadron
azimuthal separation, $\Delta\phi$, are measured for the first time at the
Relativistic Heavy Ion Collider. These results better quantify how the yield of
low-$p_T$ associated hadrons is enhanced at wide angle, which is crucial for
studying energy loss as well as medium-response effects.
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