359 research outputs found
Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches
We use several diverse parameterizations of diffractive parton distributions,
extracted in leading twist QCD analyses of the HERA diffractive deep inelastic
scattering (DIS) data, to make predictions for leading twist nuclear shadowing
of nuclear quark and gluon distributions in DIS on nuclei. We find that the
HERA diffractive data are sufficiently precise to allow us to predict large
nuclear shadowing for gluons and quarks, unambiguously. We performed detailed
studies of nuclear shadowing for up and charm sea quarks and gluons within
several scenarios of shadowing and diffractive slopes, as well as at central
impact parameters. We compare these leading twist results with those obtained
from the eikonal approach to nuclear shadowing (which is based on a very
different space-time picture) and observe sharply contrasting predictions for
the size and Q^2-dependence of nuclear shadowing. The most striking differences
arise for the interaction of small dipoles with nuclei, in particular for the
longitudinal structure function F_{L}^{A}.Comment: 43 pages, 16 figures, requires JHEP style fil
Determination of the longitudinal structure function at HERA
Recent results from the HERA experiment H1 on the longitudinal stucture
function of the proton are presented. They include proton structure
function analyses with particular emphasis on those kinematic regions which are
sensitive to . All results can be consistently described within the
framework of perturbative QCD.Comment: 16 pages, 11 figures (requires iopart, iopams and epsfig); Talk
presented in the Intern. Workshop on New Trends in HERA Physics 2001, 17-22
June 2001, Ringberg Castle, Tegernsee, Germany; To appear in the Proceeding
Deep Inelastic Scattering at HERA
Results from the H1 and ZEUS experiments at HERA on deep inelastic scattering
are reviewed. The data lead to a consistent picture of a steep rise in the F_2
structure function and in the gluon density within the proton. Important new
information on the partonic structure of diffraction is emerging from H1 and
ZEUS. The space-like region in which the weak and electromagnetic interactions
become of equal strength is being explored for the first time. A possible
excess of events at high x and Q^2 compared to the expectations of the Standard
Model has been observed in both experiments.Comment: 80 pages, 42 figure
Open Heavy Flavor Production in QCD -- Conceptual Framework and Implementation Issues
Heavy flavor production is an important QCD process both in its own right and
as a key component of precision global QCD analysis. Apparent disagreements
between fixed-flavor scheme calculations of b-production rate with experimental
measurements in hadro-, lepto-, and photo-production provide new impetus to a
thorough examination of the theory and phenomenology of this process. We review
existing methods of calculation, and place them in the context of the general
PQCD framework of Collins. A distinction is drawn between scheme dependence and
implementation issues related to quark mass effects near threshold. We point
out a so far overlooked kinematic constraint on the threshold behavior, which
greatly simplifies the variable flavor number scheme. It obviates the need for
the elaborate existing prescriptions, and leads to robust predictions. It can
facilitate the study of current issues on heavy flavor production as well as
precision global QCD analysis.Comment: 13 pages, 10 figures, Proceedings of Ringberg Workshop: New Trends in
HERA Physics 2001, Munich, German
A Parametrisation of the Inclusive Diffractive Cross Section at HERA
A recently proposed parametrization for the deep inelastic diffractive cross
section is used to describe the H1 94 data. We find two possible solutions, and
we discuss them is some detail.Comment: 13 pages, 7 eps figures, latex fil
Quaero@H1: An Interface to High-pT HERA Event Data
Distributions from high-pT HERA event data analyzed in a general search for
new physics at H1 have been incorporated into Quaero, an algorithm designed to
automate tests of specific hypotheses with high energy collider data. The use
of Quaero@H1 to search for leptoquarks, R-parity violating supersymmetry, and
excited quarks provides examples to develop intuition for the algorithm's
performance.Comment: Submitted to Eur. Phys. J.
Measuring R-parity-violating couplings in dilepton production at the LHC
We revisit the issue of probing R-violating couplings of supersymmetric
theories at hadronic colliders, particularly at the LHC. Concentrating on
dimuon production, an evaluation of the optimal sensitivity to the R-violating
coupling is performed through a maximum likelihood analysis. The measurement
uncertainties are evaluated through a study of fully generated events processed
through a fast simulation of the ATLAS detector. It is found that a host of
R-violating couplings can be measured to a statistical accuracy of better than
10%, over a significant part of the m_{tilde f} -- lambda parameter space still
allowed by low energy measurements. Since the bounds thus obtained do not
simply scale as the squark mass, one can do significantly better at the LHC
than at the Tevatron. The same analysis can also be extended to assess the
reach of the LHC to effects due to any non-SM structure of the four-fermion
amplitude, caused by exchanges of new particles with different spins such as
leptoquarks and gravitons that are suggested by various theoretical ideas.Comment: 28 pages, 14 figures (uses JHEP3.cls
The pion structure function and jet production in
Despite its theoretical and practical importance, the pion structure is still
badly constrained, particularly at low and in the sea-quark and gluon
sectors. Recently ZEUS have presented data on dijet photoproduction with a
leading neutron, which is dominated by slightly off-shell pion exchange and can
be used to constrain the pion densities down to . We
compare a recent NLO calculation to the ZEUS data and find that the lower gluon
densities of SMRS seem to be preferred by the data. Theoretical uncertainties,
in particular from the pion flux, are discussed in some detail.Comment: Talk presented at the Ringberg Workshop on ``New Trends in HERA
Physics 2001''. 12 pages, 10 postscript figure
Regge behaviour of distribution functions and t and x-evolutions of gluon distribution function at low-x
In this paper t and x-evolutions of gluon distribution function from
Dokshitzer-Gribov-Lipatov-Altarelli-Parisi(DGLAP) evolution equation in leading
order(LO) at low-x, assuming the Regge behaviour of quark and gluon at this
limit, are presented. We compare our results of gluon distribution function
with MRST 2001, MRST 2004 and GRV '98 parameterizations and show the
compatibility of Regge behaviour of quark and gluon distribution functions with
perturbative quantum chromodynamics(PQCD) at low-x. We also discuss the
limitations of Taylor series expansion method used earlier to solve DGLAP
evolution equations, in the Regge behaviour of distribution functions.Comment: 19 pages, 7 figure
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