419 research outputs found
The spread of the gluon k_t-distribution and the determination of the saturation scale at hadron colliders in resummed NLL BFKL
The transverse momentum distribution of soft hadrons and jets that accompany
central hard-scattering production at hadron colliders is of great importance,
since it has a direct bearing on the ability to separate new physics signals
from Standard Model backgrounds. We compare the predictions for the gluonic
k_t-distribution using two different approaches: resummed NLL BFKL and DGLAP
evolution. We find that as long as the initial and final virtualities (k_t)
along the emission chain are not too close to each other, the NLL resummed BFKL
results do not differ significantly from those obtained using standard DGLAP
evolution. The saturation momentum Q_s(x), calculated within the resummed BFKL
approach, grows with 1/x even slower than in the leading-order DGLAP case.Comment: 24 pages, 8 figures, An improved, slightly more precise NLL
resummation is used and the figures are updated accordingly. The conclusions
are unchange
Exclusive photoproduction of hard dijets and magnetic susceptibility of QCD vacuum
We argue that coherent production of hard dijets by linearly polarized real
photons can provide direct evidence for chirality violation in hard processes,
the first measurement of the magnetic susceptibility of the quark condensate
and the photon distribution amplitude. It can also serve as a sensitive probe
of the generalized gluon parton distribution. Numerical calculations are
presented for HERA kinematics.Comment: 4 pages, 4 figure
Exotic hybrid mesons in hard electroproduction
We estimate the sizeable cross section for deep exclusive electroproduction
of an exotic hybrid meson in the Bjorken regime. The production
amplitude scales like the one for usual meson electroproduction, i.e. as
. This is due to the non-vanishing leading twist distribution amplitude
for the hybrid meson, which may be normalized thanks to its relation to the
energy momentum tensor and to the QCD sum rules technique. The hard amplitude
is considered up to next-to-leading order in and we explore the
consequences of fixing the renormalization scale ambiguity through the BLM
procedure. We study the particular case where the hybrid meson decays through a
meson pair. We discuss the generalized distribution
amplitude and then calculate the production amplitude for this process. We
propose a forward-backward asymmetry in the production of and
mesons as a signal for the hybrid meson production. We briefly comment on
hybrid electroproduction at very high energy, in the diffractive limit where a
QCD Odderon exchange mechanism should dominate. The conclusion of our study is
that hard electroproduction is a promissing way to study exotic hybrid mesons,
in particular at JLAB, HERA (HERMES) or CERN (Compass)
Factorization and high-energy effective action
I demonstrate that the amplitude for high-energy scattering can be factorized
as a convolution of the contributions due to fast and slow fields. The fast and
slow fields interact by means of Wilson-line operators -- infinite gauge
factors ordered along the straight line. The resulting factorization formula
gives a starting point for a new approach to the effective action for
high-energy scattering in QCD.Comment: 34 pages, Latex, 13 postscript figures, submitted to PR
Next-to-leading and resummed BFKL evolution with saturation boundary
We investigate the effects of the saturation boundary on small-x evolution at
the next-to-leading order accuracy and beyond. We demonstrate that the
instabilities of the next-to-leading order BFKL evolution are not cured by the
presence of the nonlinear saturation effects, and a resummation of the higher
order corrections is therefore needed for the nonlinear evolution. The
renormalization group improved resummed equation in the presence of the
saturation boundary is investigated, and the corresponding saturation scale is
extracted. A significant reduction of the saturation scale is found, and we
observe that the onset of the saturation corrections is delayed to higher
rapidities. This seems to be related to the characteristic feature of the
resummed splitting function which at moderately small values of x possesses a
minimum.Comment: 34 page
Multi-particle Production and Thermalization in High-Energy QCD
We argue that multi-particle production in high energy hadron and nuclear
collisions can be considered as proceeding through the production of gluons in
the background classical field. In this approach we derive the gluon spectrum
immediately after the collision and find that at high energies it is
parametrically enhanced by ln(1/x) with respect to the quasi-classical result
(x is the Bjorken variable). We show that the produced gluon spectrum becomes
thermal (in three dimensions) with an effective temperature determined by the
saturation momentum Qs, T= c Qs/2pi during the time ~1/T; we estimate
c=sqrt{2pi}/2 ~ 1.2. Although this result by itself does not imply that the
gluon spectrum will remain thermal at later times, it has an interesting
applications to heavy ion collisions. In particular, we discuss the possibility
of Bose-Einstein condensation of the produced gluon pairs and estimate the
viscosity of the produced gluon system.Comment: 25 pages, 4 figures; typos fixed; discussions expanded; we added a
new section IV in which we argue that at high energies the production
mechanism discussed in the paper is parametrically enhanced by ln(1/x) with
respect to the quasi-classical resul
Limiting fragmentation in hadron-hadron collisions at high energies
Limiting fragmentation in proton-proton, deuteron-nucleus and nucleus-nucleus
collisions is analyzed in the framework of the Balitsky-Kovchegov equation in
high energy QCD. Good agreement with experimental data is obtained for a wide
range of energies. Further detailed tests of limiting fragmentation at RHIC and
the LHC will provide insight into the evolution equations for high energy QCD.Comment: 28 pages, 10 figures (2 new figures, text slightly expanded, and some
additional references
Veneziano Ghost Versus Isospin Breaking
It is argued that an account for the Veneziano ghost pole, appearing in
resolving the U(1) problem, is necessary for understanding an isospin violation
in the system. By virtue of a perturbative expansion
around the ( ) symmetric Veneziano solution, we
find that the ghost considerably suppresses isospin breaking gluon and s-quark
matrix elements. We speculate further on a few cases where the proposed
mechanism can play an essential role. We discuss the isospin violation in
meson-nucleon couplings and its relevance to the problem of charge asymmetric
nuclear forces and possible breaking of the Bjorken sum rule. It is shown that
the ghost pole could yield the isospin violation of order 2 \% for the couplings and 20 \% for the
Bjorken sum rule.Comment: 16 pages , Preprint TAUP-2127-9
Light Cone Sum Rules for the pi0-gamma*-gamma Form Factor Revisited
We provide a theoretical update of the calculations of the pi0-gamma*-gamma
form factor in the LCSR framework, including up to six polynomials in the
conformal expansion of the pion distribution amplitude and taking into account
twist-six corrections related to the photon emission at large distances. The
results are compared with the calculations of the B-> pi l nu decay and pion
electromagnetic form factors in the same framework. Our conclusion is that the
recent BaBar measurements of the pi0-gamma*-gamma form factor at large momentum
transfers are consistent with QCD, although they do suggest that the pion DA
may have more structure than usually assumed.Comment: 20 pages, 14 figures, 5 table
Spin-dependent twist-4 matrix elements from the instanton vacuum: Flavor-singlet and nonsinglet
We estimate the twist-4 spin-1 nucleon matrix element f_2 in an
instanton-based description of the QCD vacuum. In addition to the
flavor-nonsinglet we compute also the flavor-singlet matrix element, which
appears in next-to-leading order of the (1/N_c)-expansion. The corresponding
twist-3 spin-2 matrix elements d_2 are suppressed in the packing fraction of
the instanton medium, (\bar \rho)/(\bar R) << 1. We use our results to estimate
the leading (1/Q^2) power corrections to the first moment of the proton and
neutron spin structure functions G_1, as well as the intrinsic charm
contribution to the nucleon spin.Comment: 17 pages, 4 eps figures include
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