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.
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.
Phys. Rev. C 107, 014907 (2023) The PHENIX experiment reports systematic measurements at the Relativistic
Heavy Ion Collider of $\phi$-meson production in asymmetric Cu$+$Au collisions
at ...$\sqrt{s_{_{NN}}}$=200 GeV and in U$+$U collisions at $\sqrt{s_{_{NN}}}$=193
GeV. Measurements were performed via the $\phi\rightarrow K^{+}K^{-}$ decay
channel at midrapidity $|\eta|<0.35$. Features of $\phi$-meson production
measured in Cu$+$Cu, Cu$+$Au, Au$+$Au, and U$+$U collisions were found to not
depend on the collision geometry, which was expected because the yields are
averaged over the azimuthal angle and follow the expected scaling with
nuclear-overlap size. The elliptic flow of the $\phi$ meson in Cu$+$Au,
Au$+$Au, and U$+$U collisions scales with second-order-participant eccentricity
and the length scale of the nuclear-overlap region (estimated with the number
of participating nucleons). At moderate $p_T$, $\phi$-meson production measured
in Cu$+$Au and U$+$U collisions is consistent with coalescence-model
predictions, whereas at high $p_T$ the production is in agreement with
expectations for in-medium energy loss of parent partons prior to their
fragmentation. The elliptic flow for $\phi$ mesons measured in Cu$+$Au and
U$+$U collisions is well described by a (2+1)D viscous-hydrodynamic model with
specific-shear viscosity $\eta/s=1/4\pi$.
Phys. Rev. C 106, 014908 (2022) Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects;
however, the collective behavior observed in these collisions shows a hint of
...hot-nuclear-matter effects. The identified-particle spectra, especially the
$\phi$ mesons which contain strange and antistrange quarks and have a
relatively small hadronic-interaction cross section, are a good tool to study
these effects. The PHENIX experiment has measured $\phi$ mesons in a specific
set of small collision systems $p$$+$Al, $p$$+$Au, and $^3$He$+$Au, as well as
$d$$+$Au Phys. Rev. C {\bf 83}, 024909 (2011), at $\sqrt{s_{_{NN}}}=200$ GeV.
The transverse-momentum spectra and nuclear-modification factors are presented
and compared to theoretical-model predictions. The comparisons with different
calculations suggest that quark-gluon plasma may be formed in these small
collision systems at $\sqrt{s_{_{NN}}}=200$ GeV. However, the volume and the
lifetime of the produced medium may be insufficient for observing
strangeness-enhancement and jet-quenching effects. Comparison with calculations
suggests that the main production mechanisms of $\phi$ mesons at midrapidity
may be different in $p$$+$Al versus $p/d/$$^3$He$+$Au collisions at
$\sqrt{s_{_{NN}}}=200$ GeV. While thermal quark recombination seems to dominate
in $p/d/$$^3$He$+$Au collisions, fragmentation seems to be the main production
mechanism in $p$$+$Al collisions.
Phys. Rev. C 109, 044912 (2024) The measurement of the direct-photon spectrum from Au$+$Au collisions at
$\sqrt{s_{_{NN}}}=200$ GeV is presented by the PHENIX collaboration using the
...external-photon-conversion technique for 0\%--93\% central collisions in a
transverse-momentum ($p_T$) range of 0.8--10 GeV/$c$. An excess of direct
photons, above prompt-photon production from hard-scattering processes, is
observed for $p_T<6$ GeV/$c$. Nonprompt direct photons are measured by
subtracting the prompt component, which is estimated as $N_{\rm coll}$-scaled
direct photons from $p$$+$$p$ collisions at 200 GeV, from the direct-photon
spectrum. Results are obtained for $0.8<p_T<6.0$ GeV/$c$ and suggest that the
spectrum has an increasing inverse slope from ${\approx}0.2$ to 0.4 GeV/$c$
with increasing $p_T$, which indicates a possible sensitivity of the
measurement to photons from earlier stages of the evolution of the collision.
In addition, like the direct-photon production, the $p_T$-integrated nonprompt
direct-photon yields also follow a power-law scaling behavior as a function of
collision-system size. The exponent, $\alpha$, for the nonprompt component is
found to be consistent with 1.1 with no apparent $p_T$ dependence.
Phys. Rev. C 109, 044907 (2024) The invariant yield of electrons from open-heavy-flavor decays for $1<p_T<8$
GeV/$c$ at midrapidity $|y|<0.35$ in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$ =
200 GeV ...has been measured by the PHENIX experiment at the Relativistic Heavy
Ion Collider. A displaced-vertex analysis with the PHENIX silicon-vertex
detector enables extraction of the fraction of charm and bottom hadron decays
and unfolding of the invariant yield of parent charm and bottom hadrons. The
nuclear-modification factors $R_{AA}$ for electrons from charm and bottom
hadron decays and heavy-flavor hadrons show both a centrality and a quark-mass
dependence, indicating suppression in the quark-gluon plasma produced in these
collisions that is medium sized and quark-mass dependent.
Phys. Rev. C 107, 024914 (2023) The measurement of direct photons from Au$+$Au collisions at
$\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$
Gev/$c$ is presented by ...the PHENIX collaboration at the Relativistic Heavy Ion
Collider. A significant direct-photon yield is observed in both collision
systems. A universal scaling is observed when the direct-photon $p_T$ spectra
for different center-of-mass energies and for different centrality selections
at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$
for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon
spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier
by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV
published by ALICE. The scaling power $\alpha$ seems to be independent of
$p_T$, center of mass energy, and collision centrality. The spectra from
different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The
spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of
$0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to
$0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of
low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV
suggests a common source of direct photons for the different collision energies
and event centrality selections, and suggests a comparable space-time evolution
of direct-photon emission.
We present measurements of the cross section and double-helicity asymmetry
$A_{LL}$ of direct-photon production in $\vec{p}+\vec{p}$ collisions at
$\sqrt{s}=510$ GeV. The measurements have been ...performed at midrapidity
($|\eta|<0.25$) with the PHENIX detector at the Relativistic Heavy Ion
Collider. At relativistic energies, direct photons are dominantly produced from
the initial quark-gluon hard scattering and do not interact via the strong
force at leading order. Therefore, at $\sqrt{s}=510$ GeV, where
leading-order-effects dominate, these measurements provide clean and direct
access to the gluon helicity in the polarized proton in the
gluon-momentum-fraction range $0.02<x<0.08$, with direct sensitivity to the
sign of the gluon contribution.
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