1,193 research outputs found
Recent high pT measurements in STAR
After five years of data taking, the STAR experiment at the Relativistic
Heavy Ion Collider (RHIC) at Brookhaven National Laboratory provides precise
measurements of particle production at high transverse momentum in p-p, d-Au,
and Au-Au collisions at sqrt(s) = 200 GeV. We review recent results on the
flavor dependence of high pT particle suppression and hadron particle spectra
at sqrt(s) = 62.4 GeV. New results on two-particle angular correlations for
identified trigger particles and for low momentum associated charged hadrons in
p-p and Au-Au as well as near-side correlations will be presented
and discussed.Comment: 5th International Conference on Physics and Astrophysics of Quark
Gluon Plasma, Calcutta. 8 pages, 10 figures, submitted to J. Phys. G: Nucl.
Part. Phy
Recent results from the STAR spin program at RHIC
The STAR experiment uses polarized p+p collisions at RHIC to determine the
contributions to the spin of the proton from gluon spin and from orbital
angular momentum of the quarks and gluons. Selective STAR measurements of the
longitudinal double spin asymmetry for inclusive jet and inclusive hadron
production are presented here. In addition, we report measurements of the
transverse spin asymmetry for di-jet production at mid-rapidity and the
transverse single-spin asymmetry for forward pi0 productionComment: 4 pages, 5 figures, presented at GHP06 conferenc
Beam-Energy Dependence of Charge Balance Functions from Au+Au Collisions at RHIC
Balance functions have been measured in terms of relative pseudorapidity
() for charged particle pairs at the Relativistic Heavy-Ion
Collider (RHIC) from Au+Au collisions at = 7.7 GeV to 200
GeV using the STAR detector. These results are compared with balance functions
measured at the Large Hadron Collider (LHC) from Pb+Pb collisions at
= 2.76 TeV by the ALICE Collaboration. The width of the
balance function decreases as the collisions become more central and as the
beam energy is increased. In contrast, the widths of the balance functions
calculated using shuffled events show little dependence on centrality or beam
energy and are larger than the observed widths. Balance function widths
calculated using events generated by UrQMD are wider than the measured widths
in central collisions and show little centrality dependence. The measured
widths of the balance functions in central collisions are consistent with the
delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing
of the balance function in central collisions at = 7.7 GeV
implies that a QGP is still being created at this relatively low energy.Comment: 8 pages, 5 figures, submitted to Physical Review
Observation of excess J/ yield at very low transverse momenta in Au+Au collisions at 200 GeV and U+U collisions at 193 GeV
We report on the first measurements of J/ production at very low
transverse momentum ( 0.2 GeV/c) in hadronic Au+Au collisions at
200 GeV and U+U collisions at 193
GeV. Remarkably, the inferred nuclear modification factor of J/ at
mid-rapidity in Au+Au (U+U) collisions reaches about 24 (52) for 0.05
GeV/c in the 60-80 collision centrality class. This noteworthy enhancement
cannot be explained by hadronic production accompanied by cold and hot medium
effects. In addition, the distribution of J/ for the very low
range is presented for the first time. The distribution is consistent
with that expected from the Au nucleus and shows a hint of interference.
Comparison of the measurements to theoretical calculations of coherent
production shows that the excess yield can be described reasonably well and
reveals a partial disruption of coherent production in semi-central collisions,
perhaps due to the violent hadronic interactions. Incorporating theoretical
calculations, the results strongly suggest that the dramatic enhancement of
J/ yield observed at extremely low originates from coherent
photon-nucleus interactions. In particular, coherently produced J/'s in
violent hadronic collisions may provide a novel probe of the
quark-gluon-plasma
Measurement of interaction between antiprotons
One of the primary goals of nuclear physics is to understand the force
between nucleons, which is a necessary step for understanding the structure of
nuclei and how nuclei interact with each other. Rutherford discovered the
atomic nucleus in 1911, and the large body of knowledge about the nuclear force
since acquired was derived from studies made on nucleons or nuclei. Although
antinuclei up to antihelium-4 have been discovered and their masses measured,
we have no direct knowledge of the nuclear force between antinucleons. Here, we
study antiproton pair correlations among data taken by the STAR experiment at
the Relativistic Heavy Ion Collider and show that the force between two
antiprotons is attractive. In addition, we report two key parameters that
characterize the corresponding strong interaction: namely, the scattering
length (f0) and effective range (d0). As direct information on the interaction
between two antiprotons, one of the simplest systems of antinucleons, our
result provides a fundamental ingredient for understanding the structure of
more complex antinuclei and their properties.Comment: 25 pages, 4 figures. Submitted to Nature. Under media embarg
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