In the framework of the FAIR project, the PAX collaboration has suggested new experiments using polarized antiprotons. The central physics issue is now to study the polarization build-up by spin ...filtering of antiprotons via multiple passages through an internal polarized gas target. The goals for spin-filtering experiments with protons at COSY are to test our understanding of the spin-filtering processes and to commission the setup for the AD experiments with antiprotons at the AD (CERN). Spin-filtering experiments with antiprotons at the AD will allow us to determine the total spin-dependent transversal and longitudinal cross sections. The low-beta section at COSY is composed of two superconducting quadrupole magnets on each side of the target, while at the AD, we will use three quadrupoles on each side. Technical problems and details of the superconducting quadrupoles with their respective cryogenics will be discussed. A flexible cryogenic system fitting at COSY and AD ring characterized by a low liquid helium consumption will be discussed. The status of the quadrupoles will be reported as well.
We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement ...of the spin-flip cross section in low-energy electron–proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.
The PHENIX experiment measured the centrality dependence of two-pion
Bose-Einstein correlation functions in $\sqrt{s_{_{NN}}}=200$~GeV Au$+$Au
collisions at the Relativistic Heavy Ion Collider at ...Brookhaven National
Laboratory. The data are well represented by L\'evy-stable source
distributions. The extracted source parameters are the correlation-strength
parameter $\lambda$, the L\'evy index of stability $\alpha$, and the
L\'evy-scale parameter $R$ as a function of transverse mass $m_T$ and
centrality. The $\lambda(m_T)$ parameter is constant at larger values of $m_T$,
but decreases as $m_T$ decreases. The L\'evy scale parameter $R(m_T)$ decreases
with $m_T$ and exhibits proportionality to the length scale of the nuclear
overlap region. The L\'evy exponent $\alpha(m_T)$ is independent of $m_T$
within uncertainties in each investigated centrality bin, but shows a clear
centrality dependence. At all centralities, the L\'evy exponent $\alpha$ is
significantly different from that of Gaussian ($\alpha=2$) or Cauchy
($\alpha=1$) source distributions. Comparisons to the predictions of
Monte-Carlo simulations of resonance-decay chains show that in all but the most
peripheral centrality class (50%-60%), the obtained results are inconsistent
with the measurements, unless a significant reduction of the in-medium mass of
the $\eta'$ meson is included. In each centrality class, the best value of the
in-medium $\eta'$ mass is compared to the mass of the $\eta$ meson, as well as
to several theoretical predictions that consider restoration of $U_A(1)$
symmetry in hot hadronic matter.
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 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.
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