310 research outputs found
Cut Vertices and Semi-Inclusive Deep Inelastic Processes
Cut vertices, a generalization of matrix elements of local operators, are
revisited, and an expansion in terms of minimally subtracted cut vertices is
formulated. An extension of the formalism to deal with semi-inclusive deep
inelastic processes in the target fragmentation region is explicitly
constructed. The problem of factorization is discussed in detail.Comment: LaTex2e, 24 pages including 17 postscript figure
Evolution of Parton Fragmentation Functions at Finite Temperature
The first order correction to the parton fragmentation functions in a thermal
medium is derived in the leading logarithmic approximation in the framework of
thermal field theory. The medium-modified evolution equations of the parton
fragmentation functions are also derived. It is shown that all infrared
divergences, both linear and logarithmic, in the real processes are canceled
among themselves and by corresponding virtual corrections. The evolution of the
quark number and the energy loss (or gain) induced by the thermal medium are
investigated.Comment: 21 pages in RevTex, 10 figure
Wilson line correlator in the MV model: relating the glasma to deep inelastic scattering
In the color glass condensate framework the saturation scale measured in deep
inelastic scattering of high energy hadrons and nuclei can be determined from
the correlator of Wilson lines in the hadron wavefunction. These same Wilson
lines give the initial condition of the classical field computation of the
initial gluon multiplicity and energy density in a heavy ion collision. In this
paper the Wilson line correlator in both adjoint and fundamental
representations is computed using exactly the same numerical procedure that has
been used to calculate gluon production in a heavy ion collision. In particular
the discretization of the longitudinal coordinate has a large numerical effect
on the relation between the color charge density parameter g^2 mu and the
saturation scale Qs. Our result for this relation is Qs = 0.6 g^2 mu, which
results in the classical Yang-Mills value for the "gluon liberation
coefficient" c = 1.1.Comment: 8 pages, 10 figures, RevTEX4, V2: typo corrections, V3: small
clarifications, to be published in EPJ
Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions
A natural framework to understand the energy dependence of bulk observables
from lower energy experiments to the LHC is provided by the Color Glass
Condensate, which leads to a "geometrical scaling" in terms of an energy
dependent saturation scale Q_s. The measured charged multiplicity, however,
seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does
for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling.
We argue that this difference between pp and AA collisions can be understood
from the effect of DGLAP evolution on the value of the saturation scale, and is
consistent with gluon saturation observations at HERA.Comment: RevTeX, 8 pages, 4 figures. V2: modified discussion of fragmentation,
published in EPJ
Saturation and parton level Cronin effect: enhancement vs suppression of gluon production in p-A and A-A collisions
We note that the phenomenon of perturbative saturation leads to transverse
momentum broadening in the spectrum of partons produced in hadronic collisions.
This broadening has a simple interpretation as parton level Cronin effect for
systems in which saturation is generated by the "tree level" Glauber-Mueller
mechanism. For systems where the broadening results form the nonlinear QCD
evolution to high energy, the presence or absence of Cronin effect depends
crucially on the quantitative behavior of the gluon distribution functions at
transverse momenta kt outside the so called scaling window. We discuss the
relation of this phenomenon to the recent analysis by Kharzeev-Levin-McLerran
of the momentum and centrality dependence of particle production in
nucleus-nucleus collisions at RHIC.Comment: 22 pages LaTex, 7 eps-figures, discussion of evolved gluon
distribution revised significantl
Cronin Effect and High-p_T Suppression in pA Collisions
We review the predictions of the theory of Color Glass Condensate for gluon
production cross section in p(d)A collisions. We demonstrate that at moderate
energies, when the gluon production cross section can be calculated in the
framework of McLerran-Venugopalan model, it has only partonic level Cronin
effect in it. At higher energies/rapidities corresponding to smaller values of
Bjorken x quantum evolution becomes important. The effect of quantum evolution
at higher energies/rapidities is to introduce suppression of high-p_T gluons
slightly decreasing the Cronin enhancement. At still higher energies/rapidities
quantum evolution leads to suppression of produced gluons at all values of p_T.Comment: 32 pages, 8 figures, v2: extended and improved discussion, references
adde
The Heumann-Hotzel model for aging revisited
Since its proposition in 1995, the Heumann-Hotzel model has remained as an
obscure model of biological aging. The main arguments used against it were its
apparent inability to describe populations with many age intervals and its
failure to prevent a population extinction when only deleterious mutations are
present. We find that with a simple and minor change in the model these
difficulties can be surmounted. Our numerical simulations show a plethora of
interesting features: the catastrophic senescence, the Gompertz law and that
postponing the reproduction increases the survival probability, as has already
been experimentally confirmed for the Drosophila fly.Comment: 11 pages, 5 figures, to be published in Phys. Rev.
Deep inelastic scattering off a N=4 SYM plasma at strong coupling
By using the AdS/CFT correspondence we study the deep inelastic scattering of
an R-current off a N=4 supersymmetric Yang-Mills (SYM) plasma at finite
temperature and strong coupling. Within the supergravity approximation valid
when the number of colors is large, we compute the structure functions by
solving Maxwell equations in the space-time geometry of the AdS_5 black
three-brane. We find a rather sharp transition between a low energy regime
where the scattering is weak and quasi-elastic, and a high-energy regime where
the current is completely absorbed. The critical energy for this transition
determines the plasma saturation momentum in terms of its temperature T and the
Bjorken x variable: Q_s=T/x. These results suggest a partonic picture for the
plasma where all the partons have transverse momenta below the saturation
momentum and occupation numbers of order one.Comment: Version accepted for publication in JHEP: more references added; some
technical points were displaced from Sect. 4 to the new Appendix
Shadowing Effects on Vector Boson Production
We explore how nuclear modifications to the nucleon structure functions,
shadowing, affect massive gauge boson production in heavy ion collisions at
different impact parameters. We calculate the dependence of , and
production on rapidity and impact parameter to next-to-leading order in
Pb+Pb collisions at 5.5 TeV/nucleon to study quark shadowing at high . We
also compare our Pb+Pb results to the rapidity distributions at 14 TeV.Comment: 25 pages ReVTeX, 12 .eps figures, NLO included, version accepted for
publication in Physical Review
Generalized parton distributions and Deeply Virtual Compton Scattering in Color Glass Condensate model
Within the framework of the Color Glass Condensate model, we evaluate quark
and gluon Generalized Parton Distributions (GPDs) and the cross section of
Deeply Virtual Compton Scattering (DVCS) in the small- region. We
demonstrate that the DVCS cross section becomes independent of energy in the
limit of very small , which clearly indicates saturation of the DVCS
cross section. Our predictions for the GPDs and the DVCS cross section at
high-energies can be tested at the future Electron-Ion Collider and in
ultra-peripheral nucleus-nucleus collisions at the LHC.Comment: 20 pages, 8 Figure
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