899 research outputs found
Low x saturation at HERA ?
We compare the predictions of two distinct dipole models for inclusive and
exclusive diffractive processes. While only one of these dipole models contains
perturbative saturation dynamics, we show that the predictions of both models
are fully consistent with the available HERA data, indicating no compelling
evidence for saturation at present HERA energies.Comment: 8 pages, 4 figures, presented at the 26th
Montreal-Rochester-Syracuse-Toronto (MRST) conference held at Concordia
University, Montreal, Canada, 12th-14th May 2004. To appear in the
proceeding
Observing a Light CP-Violating Higgs Boson in Diffraction
Light CP-violating Higgs bosons with mass lower than 70 GeV might have
escaped detection in direct searches at the LEP collider. They may remain
undetected in conventional search channels at the Tevatron and LHC. In this
Letter we show that exclusive diffractive reactions may be able to probe for
the existence of these otherwise elusive Higgs particles. As a prototype
example, we calculate diffractive production cross-sections of the lightest
Higgs boson within the framework of the Minimal Supersymmetric Standard Model
with explicit CP violation. Our analysis shows that the challenging regions of
parameter space corresponding to a light CP-violating Higgs boson might be
accessible at the LHC provided suitable proton tagging detectors are installed.Comment: 9 pages, LaTeX, 2 figures, version as to appear in Phys. Rev.
Extracting the Distribution Amplitudes of the rho meson from the Color Glass Condensate
We extract the leading twist-2 and subleading twist-3 Distribution Amplitudes
(DAs) of the rho meson using the HERA data on diffractive rho photoproduction.
We do so using several Colour Glass Condensate (CGC) inspired and a Regge
inspired dipole models. We find that our extracted twist-2 DA is not much model
dependent and is consistent with QCD Sum Rules and lattice predictions. The
extracted twist-3 DA is more model dependent but is still consistent with the
Sum Rules prediction.Comment: 21 pages, 10 figures, 3 tables. Section 6 revised, figures 8 and 9
and table 3 updated. Conclusions essentially unchange
Diffractive production of high pt photons at HERA
We study the diffractive production of high pt photons at HERA. We have
implemented the process as a new hard sub-process in the HERWIG event generator
in order to prepare the ground for a future measurement.Comment: 4 pages, 4 figures. Contribution to the 1999 UK Phenomenology
Workshop on Collider Physics, Durham, U
Diffractive vector boson production at the Tevatron
The diffractive production of vector bosons at the Tevatron is studied. We
take a hard pomeron flux and use the H1 parton density functions extracted from
their measurement of F_2^{D(3)}. To this we add a reggeon exchange
contribution. We find that the ratio of diffractive to non-diffractive W boson
production is in good agreement with the CDF data. We note that the poorly
understood reggeon contribution might be as large as the pomeron contribution
in the kinematic range of the data. We also note that our pomeron exchange
contribution is much smaller than earlier predictions due to our use of a hard
pomeron flux. All our results have been obtained using the lowest order matrix
elements and we have compared to the results obtained using a modified version
of Herwig which is capable of generating a wide variety of diffractive
scattering processes. Gap survival is estimated using Pythia and is found to be
around 60%. We make predictions which could be compared to future measurements
of diffractive Drell-Yan production.Comment: 11 pages, 4 figure
On the breaking of collinear factorization in QCD
We investigate the breakdown of collinear factorization for non-inclusive
observables in hadron-hadron collisions. For pure QCD processes, factorization
is violated at the three-loop level and it has a structure identical to that
encountered previously in the case of super-leading logarithms. In particular,
it is driven by the non-commutation of Coulomb/Glauber gluon exchanges with
other soft exchanges. Beyond QCD, factorization may be violated at the two-loop
level provided that the hard subprocess contains matrix element contributions
with phase differences between different colour topologies.Comment: Version 2: minor improvements for journal publicatio
High--Energy Photon--Hadron Scattering in Holographic QCD
This article provides an in-depth look at hadron high energy scattering by
using gravity dual descriptions of strongly coupled gauge theories. Just like
deeply inelastic scattering (DIS) and deeply virtual Compton scattering (DVCS)
serve as clean experimental probes into non-perturbative internal structure of
hadrons, elastic scattering amplitude of a hadron and a (virtual) "photon" in
gravity dual can be exploited as a theoretical probe. Since the scattering
amplitude at sufficiently high energy (small Bjorken x) is dominated by parton
contributions (= Pomeron contributions) even in strong coupling regime, there
is a chance to learn a lesson for generalized parton distribution (GPD) by
using gravity dual models. We begin with refining derivation of
Brower-Polchinski-Strassler-Tan (BPST) Pomeron kernel in gravity dual, paying
particular attention to the role played by complex spin variable j. The BPST
Pomeron on warped spacetime consists of a Kaluza-Klein tower of 4D Pomerons
with non-linear trajectories, and we clarify the relation between Pomeron
couplings and Pomeron form factor. We emphasize that the saddle point value j^*
of the scattering amplitude in the complex j-plane representation is a very
important concept in understanding qualitative behavior of the scattering
amplitude. The total Pomeron contribution to the scattering is decomposed into
the saddle point contribution and at most a finite number of pole
contributions, and when the pole contributions are absent (which we call saddle
point phase), kinematical variable (q,x,t) dependence of ln (1/q) evolution and
ln(1/x) evolution parameters gamma_eff. and lambda_eff. in DIS and t-slope
parameter B of DVCS in HERA experiment are all reproduced qualitatively in
gravity dual
Central exclusive production of longlived gluinos at the LHC
We examine the possibility of producing gluino pairs at the LHC via the
exclusive reaction pp -> p+gluino+gluino+p in the case where the gluinos are
long lived. Such long lived gluinos are possible if the scalar super-partners
have large enough masses. We show that it may be possible to observe the
gluinos via their conversion to R-hadron jets and measure their mass to better
than 1% accuracy for masses below 350 GeV with 300/fb of data.Comment: 13 pages, 9 figures. Minor corrections to version
Diffractive photoproduction at large momentum transfer in coherent hadron - hadron interactions at CERN LHC
The vector meson production in coherent hadron-hadron interactions at LHC
energies is studied assuming that the color singlet -channel exchange
carries large momentum transfer. We consider the non-forward solution of the
BFKL equation at high energy and large momentum transfer and estimate the
rapidity distribution and total cross section for the process , where can be a proton or a nucleus. We predict large rates,
which implies that the experimental identification can be feasible at the LHC.Comment: 10 pages, 5 figures, 1 table. Version to be published in Physical
Review
Saturation and geometric scaling in DIS at small x
We present various aspects of the saturation model which provides good
description of inclusive and diffractive DIS at small x. The model uses parton
saturation ideas to take into account unitarity requirements. A new scaling
predicted by the model in the small x domain is successfully confronted with
the data.Comment: Presented at New Trends in HERA Physics 2001, Ringberg Castle,
Tegernsee, Germany, 17-22 June 2001, minor corrections, some references adde
- …