226 research outputs found
Coherent QED, Giant Resonances and Pairs in High Energy Nucleus-Nucleus Collisions
We show that the coherent oscillations of the e.m. field induced by the
collective quantum fluctuations of the nuclear matter field associated with the
giant resonances, with frequencies MeV, give
rise to a significant pair production in high energy Heavy Ion
collisions. The approximate parameterless calculation of such yield is in good
agreement with recent experimental observations.Comment: 27 pages, 13 figure
Finite temperature pion vector form factors in Chiral Perturbation Theory
We discuss the thermal behaviour of the pion vector form factors and
calculate them in one-loop Chiral Perturbation Theory. The perturbative result
is used to analyze the -dependent electromagnetic pion charge radius,
obtaining a rough estimate of the deconfinement critical temperature. Imposing
thermal unitarity, we generate the resonance pole for the form factor in
the center of mass frame. The peak height in the modulus of the form
factor decreases for increasing temperature, while its width increases and the
peak position is slightly shifted downwards for 150 MeV. These
results point in the direction suggested by many analysis of dilepton
production data in relativistic heavy ion collisions.Comment: 15 pages, 4 figures, final version to appear in Phys.Lett.B, added
references and comments, abstract change
Quark Dispersion Relation and Dilepton Production in the Quark-Gluon Plasma
Under very general assumptions we show that the quark dispersion relation in
the quark-gluon plasma is given by two collective branches, of which one has a
minimum at a non-vanishing momentum. This general feature of the quark
dispersion relation leads to structures (van Hove singularities, gaps) in the
low mass dilepton production rate, which might provide a unique signature for
the quark-gluon plasma formation in relativistic heavy ion collisions.Comment: 6 pages, Revtex, 2 PostScript figures, revised version to be
published in Phys. Rev. Let
Recent results from Pb-Au collisions at 158 GeV/c per nucleon obtained with the CERES spectrometer
During the 1996 lead run time, CERES has accumulated 42 million events,
corresponding to a factor of 5 more statistics than in 1995 and 2.5 million
events of a special photon-run. We report on the results of the low-mass
ee-pair analysis. Since the most critical item is the poor
signal-to-background ratio we also discuss the understanding of this
background, in absolute terms, with the help of a detailed Monte Carlo
simulation. We show preliminary results of the photon analysis and summarize
the results of the hadron analysis preliminarily reported on already at QM'97Comment: 10 pages, 9 figures, Proceedings of the XIV Int. Conf. on
Nucleus-Nucleus Collisions,Quark Matter 99, Torino, Italy, May 10 - 15, 199
Quark-Gluon Plasma: Status of Heavy Ion Physics
Lattice quantum chromodynamics (QCD), defined on a discrete space time
lattice, leads to a spectacular non-perturbative prediction of a new state of
matter, called quark-gluon plasma (QGP), at sufficiently high temperatures or
equivalently large energy densities. The experimental programs of CERN, Geneva
and BNL, New York of relativistic heavy ion collisions are expected to produce
such energy densities, thereby providing us a chance to test the above
prediction. After a brief introduction of the necessary theoretical concepts, I
will present a critical review of the experimental results already obtained by
the various experiments in order to examine whether QGP has already been
observed by them.Comment: 11 Pages, LaTeX, Plenary talk given at 6th Workshop in High Energy
Physics Phenomenology (WHEPP 6), Chennai (Madras), India, 3-15 Jan 2000 To
appear in Pramana, Journal of Physic
Medium Modifications of the Rho Meson at CERN/SPS Energies
Rho meson propagation in hot hadronic matter is studied in a model with
coupling to states. Medium modifications are induced by a change of
the pion dispersion relation through collisions with nucleons and in
the fireball. Maintaining gauge invariance dilepton production is calculated
and compared to the recent data of the CERES collaboration in central S+Au
collisions at 200 GeV/u. The observed enhancement of the rate below the rho
meson mass can be largely accounted for.Comment: 10 pages RevTeX and 2 figures (uuencoded .ps-files
Origin of the low-mass electron pair excess in light nucleus-nucleus collisions
We report measurements of electron pair production in elementary p+p and d+p
reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the
electron pairs were reconstructed for n+p reactions by detecting the proton
spectator from the deuteron breakup. We find that the yield of electron pairs
with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger
in n+p reactions as compared to p+p. A comparison to model calculations
demonstrates that the production mechanism is not sufficiently described yet.
The electron pair spectra measured in C+C reactions are compatible with a
superposition of elementary n+p and p+p collisions, leaving little room for
additional electron pair sources in such light collision systems.Comment: 11 pages, 2 figures, \usepackage{epsfig
Relativistic Heavy-Ion Physics: Experimental Overview
The field of relativistic heavy-ion physics is reviewed with emphasis on new
results and highlights from the first run of the Relativistic Heavy-Ion
Collider at BNL and the 15 year research programme at the SPS at CERN and the
AGS at BNL.Comment: 15 pages, 9 figures. Invited Talk at the Fourth International
Conference on Physics and Astrophysics of Quark Gluon Plasma, (ICPAQGP-2001)
Jaipur, India, November 26-30, 200
Probing nuclear expansion dynamics with -spectra
We study the dynamics of charged pions in the nuclear medium via the ratio of
differential - and -spectra in a coupled-channel BUU (CBUU)
approach. The relative energy shift of the charged pions is found to correlate
with the pion freeze-out time in nucleus-nucleus collisions as well as with the
impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb
force provides a 'clock' for the expansion of the hot nuclear system. Detailed
comparisons with experimental data for at 1 GeV/A and at
2.0 GeV/A are presented.Comment: 21 pages, latex, figures include
Event-by-event fluctuations of the mean transverse momentum in 40, 80, and 158 A GeV/c Pb-Au collisions
Measurements of event-by-event fluctuations of the mean transverse momentum
in Pb-Au collisions at 40, 80, and 158 A GeV/c are presented. A significant
excess of mean p_T fluctuations at mid-rapidity is observed over the
expectation from statistically independent particle emission. The results are
somewhat smaller than recent measurements at RHIC. A possible non-monotonic
behaviour of the mean p_T fluctuations as function of collision energy, which
may have indicated that the system has passed the critical point of the QCD
phase diagram in the range of mu_B under investigation, has not been observed.
The centrality dependence of mean p_T fluctuations in Pb-Au is consistent with
an extrapolation from pp collisions assuming that the non-statistical
fluctuations scale with multiplicity. The results are compared to calculations
by the RQMD and UrQMD event generators.Comment: 28 pages, 10 figure
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