78 research outputs found
Energy Loss versus Shadowing in the Drell-Yan Reaction on Nuclei
We present a new analysis of the E772 and E866 experiments on the nuclear
dependence of Drell-Yan (DY) lepton pair production resulting from the
bombardment of , Be, C, Ca, Fe, and W targets by 800 GeV/c protons at
Fermilab. We employ a light-cone formulation of the DY reaction in the rest
frame of the nucleus, where the dimuons detected at small values of Bjorken x_2
<< 1 may be considered to originate from the decay of a heavy photon radiated
from an incident quark in a bremsstrahlung process. We infer the energy loss of
the quark by examining the suppression of the nuclear-dependent DY ratios seen
as a function of projectile momentum fraction x_1 and dimuon mass M. Shadowing,
which also leads to nuclear suppression of dimuons, is calculated within the
same approach employing the results of phenomenological fits to deep inelastic
scattering data from HERA. The analysis yields -dE/dz =2.73 +/- 0.37 +/- 0.5
GeV/fm for the rate of quark energy loss per unit path length, a value
consistent with theoretical expectations including the effects of the inelastic
interaction of the incident proton at the surface of the nucleus. This is the
first observation of a nonzero energy loss effect in such experiments.Comment: 43 pages including 17 figure
Time Evolution of Jets and Perturbative Color Neutralization
In-medium production of leading hadrons in hard reactions, carrying the main
fraction of the jet momentum, involves two stages: (i) the parton originated
from the hard process propagates through the medium radiating gluons due to the
initial hard collision, as well as to multiple interactions in the medium; (ii)
perturbative color neutralization, e.g. picking up an anti-colored parton
produced perturbatively, followed by evolution and attenuation of the
(pre)hadron in the medium. The color neutralization (or production) length for
leading hadrons is controlled by coherence, energy conservation and Sudakov
suppression. The pT-broadening is a sensitive and model independent probe for
the production length. The color neutralization time is expected to shrink with
rising hard scale. In particular, we found a very fast energy dissipation by a
highly virtual parton: half of the jet energy is radiated during the first
Fermi. Energy conservation makes the production of leading hadrons at longer
times difficult.Comment: Based on talk given by B.K. at the Fifth International Conference on
Perspectives in Hadronic Physics, Trieste, May 200
The role of IL-6 in skin fibrosis and cutaneous wound healing
The timely resolution of wound healing is critical for restoring the skin as a protective barrier. The switch from a proinflammatory to a reparative microenvironment must be tightly regulated. Interleukin (IL)-6 is a key modulator of the inflammatory and reparative process: it is involved in the differentiation, activation, and proliferation of leukocytes, endothelial cells, keratinocytes, and fibroblasts. This review examines the role of IL-6 in the healing of cutaneous wounds, and how dysregulation of IL-6 signaling can lead to either fibrosis or a failure to heal. The role of an IL-6/TGF-β feedback loop is discussed in the context of fibrogenesis, while IL-6 expression and responses in advanced age, diabetes, and obesity is outlined regarding the development of chronic wounds. Current research on therapies that modulate IL-6 is explored. Here, we consider IL-6′s diverse impact on cutaneous wound healing
Nuclear effects in the Drell-Yan process at very high energies
We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and
in nucleus-nucleus collisions within the light-cone color dipole formalism.
This approach is especially suitable for predicting nuclear effects in the DY
cross section for heavy ion collisions, as it provides the impact parameter
dependence of nuclear shadowing and transverse momentum broadening, quantities
that are not available from the standard parton model. For p(D)+A collisions we
calculate nuclear shadowing and investigate nuclear modification of the DY
transverse momentum distribution at RHIC and LHC for kinematics corresponding
to coherence length much longer than the nuclear size. Calculations are
performed separately for transversely and longitudinally polarized DY photons,
and predictions are presented for the dilepton angular distribution.
Furthermore, we calculate nuclear broadening of the mean transverse momentum
squared of DY dileptons as function of the nuclear mass number and energy. We
also predict nuclear effects for the cross section of the DY process in heavy
ion collisions. We found a substantial nuclear shadowing for valence quarks,
stronger than for the sea.Comment: 46 pages, 18 figures, title changed and some discussion added,
accepted for publication in PR
Glue drops inside hadrons
We present experimental evidences for the existence of a semi-hard scale in
light hadrons. This includes the suppression of gluon radiation that is seen in
high mass hadron diffraction; the weak energy dependence of hadronic total
cross sections; the small value of the Pomeron trajectory slope measured in
photoproduction of J/Psi; the weakness of gluon shadowing in nuclei; shortage
of gluons in the proton revealed by an unusual behavior of the proton structure
function in the soft limit, and the enhanced intrinsic transverse momentum of
quarks and gluons, which considerably exceeds the inverse hadronic size. All
these observations suggest that gluons in hadrons are located within spots of a
small size relative to the confinement radius.Comment: Based on talks given by B.P at the Fifth International Conference on
Perspectives In Hadronic Physics, Trieste, May 2006, and by B.K. at the
Workshop on Future Prospects in QCD at High Energies, Brookhaven, Jly 200
Puzzles of J/Psi production off nuclei
Nuclear effects for J/Psi production in pA collisions are controlled by the
coherence and color transparency effects. Color transparency onsets when the
time of formation of the charmonium wave function becomes longer than the
inter-nucleon spacing. In this energy regime the effective break-up cross
section for a c-cbar dipole depends on energy and nuclear path length, and
agrees well with data from fixed target experiments, both in magnitude and
energy dependence. At higher energies of RHIC and LHC coherence in c-cbar pair
production leads to charm quark shadowing which is a complement to the high
twist break up cross section. These two effects explain well with no adjusted
parameters the magnitude and rapidity dependence of nuclear suppression of
J/Psi observed at RHIC in dAu collisions, while the contribution of leading
twist gluon shadowing is found to be vanishingly small. A novel mechanism of
double color filtering for c-cbar dipoles makes nuclei significantly more
transparent in AA compared to pA collisions. This is one of the mechanisms
which make impossible a model independent "data driven" extrapolation from pA
to AA. This effect also explains the enhancement of nuclear suppression
observed at forward rapidities in AA collisions at RHIC, what hardly can be
related to the produced dense medium. J/Psi is found to be a clean and
sensitive tool measuring the transport coefficient characterizing the dense
matter created in AA collisions. RHIC data for pT dependence of J/Psi
production in nuclear collisions are well explained with the low value of the
transport coefficient q_0-hat<0.5 GeV^2/fm.Comment: 15 pages, 8 figures, Invited talk at the Workshop "Saturation, the
Color Glass Condensate and Glasma: What Have we Learned from RHIC?", BNL, May
10-12, 201
Parton model versus color dipole formulation of the Drell-Yan process
In the kinematical region where the center of mass energy is much larger than
all other scales, the Drell-Yan process can be formulated in the target rest
frame in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. Since the mechanisms for heavy dilepton production appear
very different in the dipole approach and in the conventional parton model, one
may wonder whether these two formulations really represent the same physics. We
perform a comparison of numerical calculations in the color dipole approach
with calculations in the next-to-leading order parton model. For proton-proton
scattering, the results are very similar at low x_2 from fixed target to RHIC
energies, confirming the close connection between these two very different
approaches. We also compare the transverse momentum distributions of Drell-Yan
dileptons predicted in both formulations. The range of applicability of the
dipole formulation and the impact of future Drell-Yan data from RHIC for
determining the color dipole cross section are discussed. A detailed derivation
of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure
Nuclear Hadronization: Within or Without?
Nuclei are unique analyzers for the early stage of the space-time development
of hadronization. DIS at medium energies is especially suitable for this task
being sensitive to hadronization dynamics, since the production length is
comparable with the nuclear size. This was the driving motivation to propose
measurements at HERMES using nuclear targets, and to provide predictions based
on a pQCD model of hadronization [1]. Now when the first results of the
experiment are released [2,3], one can compare the predictions with the data.
The model successfully describes with no adjustment the nuclear effects for
various energies, zh, pT, and Q2, for different flavors and different nuclei.
It turns out that the main source of nuclear suppression of the hadron
production rate is attenuation of colorless pre-hadrons in the medium. An
alternative model [4] is based upon an ad hoc assumption that the colorless
pre-hadron is produced outside the nucleus. This model has apparent problems
attempting to explain certain features of the results from HERMES. A good
understanding of the hadronization dynamics is important for proper
interpretation of the strong suppression of high-pT hadrons observed in heavy
ion collisions at RHIC. We demonstrate that the production length is even
shorter in this case and keeps contracting with rising pT.Comment: Latex 34 p. Based on talks given by B.Z.K. at the Fourth
International Conference on Perspectives in Hadronic Physics, Trieste, Italy,
May 12-16, 2003; and at the EuroConference on Hadron Structure Viewed with
Electromagnetic Probes, Santorini, Greece, October 7-12, 200
Systemic long-term metabolic effects of acute non-severe paediatric burn injury
A growing body of evidence supports the concept of a systemic response to non-severe thermal trauma. This provokes an immunosuppressed state that predisposes paediatric patients to poor recovery and increased risk of secondary morbidity. In this study, to understand the long-term systemic effects of non-severe burns in children, targeted mass spectrometry assays for biogenic amines and tryptophan metabolites were performed on plasma collected from child burn patients at least three years post injury and compared to age and sex matched non-burn (healthy) controls. A panel of 12 metabolites, including urea cycle intermediates, aromatic amino acids and quinolinic acid were present in significantly higher concentrations in children with previous burn injury. Correlation analysis of metabolite levels to previously measured cytokine levels indicated the presence of multiple cytokine-metabolite associations in the burn injury participants that were absent from the healthy controls. These data suggest that there is a sustained immunometabolic imprint of non-severe burn trauma, potentially linked to long-term immune changes that may contribute to the poor long-term health outcomes observed in children after burn injury
Space-time evolution of hadronization
Beside its intrinsic interest for the insights it can give into color
confinement, knowledge of the space-time evolution of hadronization is very
important for correctly interpreting jet-quenching data in heavy ion collisions
and extracting the properties of the produced medium. On the experimental side,
the cleanest environment to study the space-time evolution of hadronization is
semi-inclusive Deeply Inelastic Scattering on nuclear targets. On the
theoretical side, 2 frameworks are presently competing to explain the observed
attenuation of hadron production: quark energy loss (with hadron formation
outside the nucleus) and nuclear absorption (with hadronization starting inside
the nucleus). I discuss recent observables and ideas which will help to
distinguish these 2 mechanisms and to measure the time scales of the
hadronization process.Comment: 6 pages, 4 figures. Based on talks given at "Hot Quarks 2006",
Villasimius, Italy, May 15-20, 2006, and at the "XLIV internataional winter
meeting on nuclear physics", Bormio, Italy, Jan 29 - Feb 5, 2006. To appear
in Eur.Phys.J.
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