40 research outputs found
Color fluctuation approximation for multiple interactions in leading twist theory of nuclear shadowing
The leading twist theory of nuclear shadowing predicts the shadowing
correction to nuclear parton distributions at small by connecting it to the
leading twist hard diffraction in electron-nucleon scattering. The
uncertainties of the predictions are related to the shadowing effects resulting
from the interaction of the hard probe with nucleons. We argue that
the pattern of hard diffraction observed at HERA allows one to reduce these
uncertainties. We develop a new approach to the treatment of these multiple
interactions, which is based on the concept of the color fluctuations and
accounts for the presence of both point-like and hadron-like configurations in
the virtual photon wave function. Using the developed framework, we update our
predictions for the leading twist nuclear shadowing in nuclear parton
distributions of heavy nuclei at small .Comment: 16 pages, 4 figures. Revised to address the Referee's comments.
Matches the published version, PLB 687 (2010) 167
Dijet production as a centrality trigger for p-p collisions at CERN LHC
We demonstrate that a trigger on hard dijet production at small rapidities
allows to establish a quantitative distinction between central and peripheral
collisions in pbar-p and p-p collisions at Tevatron and LHC energies. Such a
trigger strongly reduces the effective impact parameters as compared to minimum
bias events. This happens because the transverse spatial distribution of hard
partons (x >~ 10^{-2}) in the proton is considerably narrower than that of soft
partons, whose collisions dominate the total cross section. In the central
collisions selected by the trigger, most of the partons with x >~ 10^{-2}
interact with a gluon field whose strength rapidly increases with energy. At
LHC (and to some extent already at Tevatron) energies the strength of this
interaction approaches the unitarity ('black-body') limit. This leads to
specific modifications of the final state, such as a higher probability of
multijet events at small rapidities, a strong increase of the transverse
momenta and depletion of the longitudinal momenta at large rapidities, and the
appearance of long-range correlations in rapidity between the forward/backward
fragmentation regions. The same pattern is expected for events with production
of new heavy particles (Higgs, SUSY). Studies of these phenomena would be
feasible with the CMS-TOTEM detector setup, and would have considerable impact
on the exploration of the physics of strong gluon fields in QCD, as well as the
search for new particles at LHC.Comment: 17 pages, Revtex 4, 14 EPS figures. Expanded discussion of some
points, added 3 new figures and new references. Included comment on
connection with cosmic ray physics near the GZK cutoff. To appear in Phys Rev
Asymmetric Colliding Nuclear Matter Approach in Heavy Ion Collisions
The early stage of a heavy ion collision is governed by local non-equilibrium
momentum distributions which have been approximated by colliding nuclear matter
configurations, i.e. by two Lorentz elongated Fermi ellipsoids. This approach
has been extended from the previous assumption of symmetric systems to
asymmetric 2-Fermi sphere configurations, i.e. to different densities. This
provides a smoother transition from the limiting situation of two
interpenetrating currents to an equilibrated system. The model is applied to
the dynamical situations of heavy ion collisions at intermediate energies
within the framework of relativistic transport (RBUU) calculations. We find
that the extended colliding nuclear matter approach is more appropriate to
describe collective reaction dynamics in terms of flow observables, in
particular, for the elliptic flow at low energies.Comment: 21 pages, 8 figures, accepted for publication in Nuclear Physics
On the shape of a rapid hadron in QCD
We visualize the fundamental property of pQCD: the smaller size of the
colorless quark-gluon configurations leads to a more rapid increase of its
interaction with energy. Within the frame of dipole model we use the
factorization theorem to generalize the DGLAP approximation and/or leading
approximation and evaluate the interaction of quark dipole with a
target. In the limit of fixed and we found the increase with
energy of transverse momenta of quark(antiquark) within q pair produced
by strongly virtual photon. The average is evaluated analytically
within the double logarithmic approximation. We demonstrate that the invariant
mass of the q pair increases with the energy as
, where for transverse
photons, and as
for longitudinal photons, where at the energies of the
order GeV (). The magnitude of the effect
depends strongly on the small behavior of the gluon distribution. Similar
pattern of the energy dependence of is found in the LO DGLAP
approximation generalized to account for factorization. We discuss the
impact of the found phenomenon on the dependence of the coherence length on the
initial energy and demonstrate that the shape of final hadron state in DIS has
biconcave form instead of pancake. Some implications of the found phenomena for
the hard processes in pp collisions are discussed.Comment: 14 pages, 2 figures, uses epstopdf.sty typos correcte
Energy losses in the black disc regime and correlation effects in the STAR forward pion production in dAu collisions
We argue that in the small x processes, in the black disc QCD regime (BDR) a
very forward parton propagating through the nuclear matter should loose a
significant and increasing with energy and atomic number fraction of its
initial energy as a result of dominance of inelastic interactions, causality
and energy-momentum conservation. We evaluate these energy losses and find them
to lead to the significant suppression of the forward jet production in the
central NA collisions at collider energies with a moderate suppression of
recoiling jet at central rapidities. We confront our expectations with the
recent RHIC data of the STAR collaboration on the probability, , for
emission of at least one fast hadron at a central rapidity in association with
production of a very forward high neutral pion in and
collisions. We calculate the A-dependence of , and find that the data imply
a strong suppression of leading pion production at central impact parameters.
We also conclude that production of recoil jets in the hard subprocess is not
suppressed providing further evidence for the dominance of peripheral
collisions. Both features of the data are consistent with the onset of BDR. We
suggest new phenomena and new observables to investigate BDR at RHIC and LHC.Comment: 18 pages,final text to be published in Phys.Lett.B, evaluation of the
fractional energy energy losses in the black disk regime is ellaborate
Triple-Pomeron Matrix Model for Dispersive Corrections to Nucleon-Nucleus Total Cross Section
Dispersive corrections to the total cross section for high-energy scattering
from a heavy nucleus are calculated using a matrix model, based on the
triple-Pomeron behavior of diffractive scattering from a single nucleon, for
the cross section operator connecting different states of the projectile
nucleon . Energy-dependent effects due to the decrease in longitudinal momentum
transfers and the opening of more channels with increasing energy are included.
The three leading terms in an expansion in the number of inelastic transitions
are evaluated and compared to exact results for the model in the uniform
nuclear density approximation for the the scattering of nucleons from Pb^{208}
for laboratory momenta ranging from 50 to 200 GeV/c.Comment: 16 pages, 2 figures, RevTex
Effects of momentum-dependent symmetry potential on heavy-ion collisions induced by neutron-rich nuclei
Using an isospin- and momentum-dependent transport model we study effects of
the momentum-dependent symmetry potential on heavy-ion collisions induced by
neutron-rich nuclei. It is found that symmetry potentials with and without the
momentum-dependence but corresponding to the same density-dependent symmetry
energy lead to significantly different predictions on several
-sensitive experimental observables especially for energetic
nucleons. The momentum- and density-dependence of the symmetry potential have
to be determined simultaneously in order to extract the
accurately. The isospin asymmetry of midrapidity nucleons at high transverse
momenta is particularly sensitive to the momentum-dependence of the symmetry
potential. It is thus very useful for investigating accurately the equation of
state of dense neutron-rich matter.Comment: The version to appear in Nucl. Phys. A. A paragraph and a figure on
neutron and proton effective masses in neutron-rich matter are adde
Higher twists and maxima for DIS on nuclei in high density QCD region
We show that the ratio of different structure functions have a maximum which
depends on and . We argue that these maxima are proportional to the
saturation scale. The analysis of leading and higher twist contributions for
different observables is given with the aim of determining the kinematic region
where high parton density effects could be seen experimentally.Comment: 16 pages of Latex file,8 figures in eps file
Fragment Formation in Central Heavy Ion Collisions at Relativistic Energies
We perform a systematic study of the fragmentation path of excited nuclear
matter in central heavy ion collisions at the intermediate energy of . The theoretical calculations are based on a Relativistic
Boltzmann-Uehling-Uhlenbeck () transport equation including stochastic
effects. A Relativistic Mean Field () approach is used, based on a
non-linear Lagrangian, with coupling constants tuned to reproduce the high
density results of calculations with correlations.
At variance with the case at Fermi energies, a new fast clusterization
mechanism is revealed in the early compression stage of the reaction dynamics.
Fragments appear directly produced from phase-space fluctuations due to
two-body correlations. In-medium effects of the elastic nucleon-nucleon cross
sections on the fragmentation dynamics are particularly discussed. The
subsequent evolution of the primordial clusters is treated using a simple
phenomenological phase space coalescence algorithm.
The reliability of the approach, formation and recognition, is investigated
in detail by comparing fragment momentum space distributions {\it and
simultaneously} their yields with recent experimental data of the
collaboration by varying the system size of the colliding system, i.e. its
compressional energy (pressure, radial flow). We find an excellent agreement
between theory and experiment in almost all the cases and, on the other hand,
some limitations of the simple coalescence model. Furthermore, the temporal
evolution of the fragment structure is explored with a clear evidence of an
earlier formation of the heavier clusters, that will appear as interesting
of the high density phase of the nuclear Equation of State ().Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7,
two refs. added, Nucl.Phys.A, accepte
The Impact of QCD and Light-Cone Quantum Mechanics on Nuclear Physics
We discuss a number of novel applications of Quantum Chromodynamics to
nuclear structure and dynamics, such as the reduced amplitude formalism for
exclusive nuclear amplitudes. We particularly emphasize the importance of
light-cone Hamiltonian and Fock State methods as a tool for describing the
wavefunctions of composite relativistic many-body systems and their
interactions. We also show that the use of covariant kinematics leads to
nontrivial corrections to the standard formulae for the axial, magnetic, and
quadrupole moments of nucleons and nuclei.Comment: 25 pages, uuencoded postscript file---To obtain a hard copy of this
paper, send e-mail to [email protected] and ask fo