1,654 research outputs found
Novel hard semiexclusive processes and color singlet clusters in hadrons
Hard scattering to a three cluster final state is suggested as a method to
probe configurations in hadrons containing small size color singlet cluster and
a residual quark-gluon system of a finite mass. Examples of such processes
include
where could be a pion(kaon) or other state of finite mass which
does not increase with momentum transfer (). We argue that different
models of the nucleon may lead to very different qualitative predictions for
the spectrum of states . We find that in the pion model of nonperturbative
sea in a nucleon the cross section of these reactions is comparable
to the cross section of the corresponding two-body reaction. Studies of these
reactions are feasible using both fixed target detectors (EVA at BNL, HERMES at
DESY) and collider detectors with a good acceptance in the forward direction.Comment: 9 pages, 2 figures, to be published in the proceedings of the
Workshop: Exclusive Processes at High Momentum Transfer, Newport News,
Virginia, 15-18 May 200
Tracking fast small color dipoles through strong gluon fields at the LHC
We argue that the process gamma +A \to J/psi + gap + X at large momentum
transfer provides a quick and effective way to test onset of a novel
perturbative QCD regime of strong absorption for the interaction of small
dipoles at the collider energies. We find that already the first heavy ion run
at the LHC will allow to study this reaction with sufficient statistics via
ultraperipheral collisions hence probing the interaction of q\bar q dipoles of
sizes ~ 0.2 fm with nuclear media down to x ~ 10^{-5}.Comment: 4 pages, final version published in PR
Longitudinal Momentum Fraction X_L for Two High P_t Protons in pp->ppX Reaction
We present an analysis of new data from Experiment E850 at BNL. We have
characterized the inclusive cross section near the endpoint for pp exclusive
scattering in Hydrogen and in Carbon with incident beam energy of 6 GeV. We
select events with a pair of back-to-back hadrons at large transverse momentum.
These cross sections are parameterized with a form
, where is the ratio of the longitudinal momentum
of the observed pair to the total incident beam momentum. Small value of
may suggest that the number of partons participating in the reaction is large
and reaction has a strong dependence on the center-of-mass energy. We also
discuss nuclear effects observed in our kinematic region.Comment: 4 pages, 2 figures, to be published in Proceedings of CIPANP2000,
Quebec, May 22-28, 2000, requires aipproc.sty(included
The Physics of Ultraperipheral Collisions at the LHC
We discuss the physics of large impact parameter interactions at the LHC:
ultraperipheral collisions (UPCs). The dominant processes in UPCs are
photon-nucleon (nucleus) interactions. The current LHC detector configurations
can explore small hard phenomena with nuclei and nucleons at photon-nucleon
center-of-mass energies above 1 TeV, extending the range of HERA by a
factor of ten. In particular, it will be possible to probe diffractive and
inclusive parton densities in nuclei using several processes. The interaction
of small dipoles with protons and nuclei can be investigated in elastic and
quasi-elastic and production as well as in high
production accompanied by a rapidity gap. Several of these phenomena
provide clean signatures of the onset of the new high gluon density QCD regime.
The LHC is in the kinematic range where nonlinear effects are several times
larger than at HERA. Two-photon processes in UPCs are also studied. In
addition, while UPCs play a role in limiting the maximum beam luminosity, they
can also be used a luminosity monitor by measuring mutual electromagnetic
dissociation of the beam nuclei. We also review similar studies at HERA and
RHIC as well as describe the potential use of the LHC detectors for UPC
measurements.Comment: 229 Pages, 121 figure
Probing coherent charmonium photoproduction off light nuclei at medium energies
We demonstrate how the elementary amplitudes , the
amplitude of the nondiagonal transition, and
the total and cross sections can be determined from
measurements of the coherent and photoproduction off light
nuclei at moderate energies. For this purpose we provide a detailed numerical
analysis of the coherent charmonium photoproduction off silicon within the
generalized vector dominance model (GVDM) adjusted to account for the physics
of charmonium models and color transparency phenomenon.Comment: 8 pages, 5 figures (color
Energy Dependence of Nuclear Transparency in C(p,2p) Scattering
The transparency of carbon for (p,2p) quasi-elastic events was measured at
beam energies ranging from 6 to 14.5 GeV at 90 degrees c.m. The four momentum
transfer squared q*q ranged from 4.8 to 16.9 (GeV/c)**2. We present the
observed energy dependence of the ratio of the carbon to hydrogen cross
sections. We also apply a model for the nuclear momentum distribution of carbon
to normalize this transparency ratio. We find a sharp rise in transparency as
the beam energy is increased to 9 GeV and a reduction to approximately the
Glauber level at higher energies.Comment: 4 pages, 2figures, submitted to PR
Multiplicity dependence of jet-like two-particle correlations in p-Pb collisions at = 5.02 TeV
Two-particle angular correlations between unidentified charged trigger and
associated particles are measured by the ALICE detector in p-Pb collisions at a
nucleon-nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum
range 0.7 5.0 GeV/ is examined,
to include correlations induced by jets originating from low
momen\-tum-transfer scatterings (minijets). The correlations expressed as
associated yield per trigger particle are obtained in the pseudorapidity range
. The near-side long-range pseudorapidity correlations observed in
high-multiplicity p-Pb collisions are subtracted from both near-side
short-range and away-side correlations in order to remove the non-jet-like
components. The yields in the jet-like peaks are found to be invariant with
event multiplicity with the exception of events with low multiplicity. This
invariance is consistent with the particles being produced via the incoherent
fragmentation of multiple parton--parton scatterings, while the yield related
to the previously observed ridge structures is not jet-related. The number of
uncorrelated sources of particle production is found to increase linearly with
multiplicity, suggesting no saturation of the number of multi-parton
interactions even in the highest multiplicity p-Pb collisions. Further, the
number scales in the intermediate multiplicity region with the number of binary
nucleon-nucleon collisions estimated with a Glauber Monte-Carlo simulation.Comment: 23 pages, 6 captioned figures, 1 table, authors from page 17,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/161
Challenges in QCD matter physics - The Compressed Baryonic Matter experiment at FAIR
Substantial experimental and theoretical efforts worldwide are devoted to
explore the phase diagram of strongly interacting matter. At LHC and top RHIC
energies, QCD matter is studied at very high temperatures and nearly vanishing
net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was
created at experiments at RHIC and LHC. The transition from the QGP back to the
hadron gas is found to be a smooth cross over. For larger net-baryon densities
and lower temperatures, it is expected that the QCD phase diagram exhibits a
rich structure, such as a first-order phase transition between hadronic and
partonic matter which terminates in a critical point, or exotic phases like
quarkyonic matter. The discovery of these landmarks would be a breakthrough in
our understanding of the strong interaction and is therefore in the focus of
various high-energy heavy-ion research programs. The Compressed Baryonic Matter
(CBM) experiment at FAIR will play a unique role in the exploration of the QCD
phase diagram in the region of high net-baryon densities, because it is
designed to run at unprecedented interaction rates. High-rate operation is the
key prerequisite for high-precision measurements of multi-differential
observables and of rare diagnostic probes which are sensitive to the dense
phase of the nuclear fireball. The goal of the CBM experiment at SIS100
(sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD
matter: the phase structure at large baryon-chemical potentials (mu_B > 500
MeV), effects of chiral symmetry, and the equation-of-state at high density as
it is expected to occur in the core of neutron stars. In this article, we
review the motivation for and the physics programme of CBM, including
activities before the start of data taking in 2022, in the context of the
worldwide efforts to explore high-density QCD matter.Comment: 15 pages, 11 figures. Published in European Physical Journal
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