307 research outputs found
Determination of Strange Sea Quark Distributions from Fixed-target and Collider Data
We present an improved determination of the strange sea distribution in the
nucleon with constraints coming from the recent charm production data in
neutrino-nucleon deep-inelastic scattering by the NOMAD and CHORUS experiments
and from charged current inclusive deep-inelastic scattering at HERA. We
demonstrate that the results are consistent with the data from the ATLAS and
the CMS experiments on the associated production of -bosons with
-quarks. We also discuss issues related to the recent strange sea
determination by the ATLAS experiment using LHC collider data.Comment: 23 pages, 14 figure
Error Estimates on Parton Density Distributions
Error estimates on parton density distributions are presently based on the
traditional method of least squares minimisation and linear error propagation
in global QCD fits. We review the underlying assumptions and the various
mathematical representations of the method and address some technical issues
encountered in such a global analysis. Parton distribution sets which contain
error information are described.Comment: Latex, 12 pages, 5 figures. Needs iopart.cls and iopart12.clo.
Presented at New Trends in HERA Physics 2001, Ringberg Castle, Tegernsee,
Germany, June 17-22, 200
Parton distributions from deep-inelastic-scattering data
We perform the analysis of existing light-targets deep-inelastic-scattering
(DIS) data in the leading-order (LO), next-to-leading-order (NLO), and
next-to-next-to-leading-order (NNLO) QCD approximations and extract PDFs
simultaneously with the value of the strong coupling constant and
the high-twist contribution to the structure functions. The main theoretical
uncertainties and experimental uncertainties due to all sources of experimental
errors in data are estimated, the latter generally dominate for the obtained
PDFs. The uncertainty in Higgs boson production cross section due to errors in
PDFs is % for the LHC and varies from 2% to 10% for the Fermilab
collider under variation of the Higgs boson mass from to . For the -boson production cross section the uncertainty is % for the both colliders. The value of is obtained, while the high-twist terms do
not vanish up to the NNLO as required by comparison to data
Modeling power corrections to the Bjorken sum rule for the neutrino structure function F_1
Direct measurements of the the structure functions F_1^{nu p} and F_1^{nu n}
at a neutrino factory would allow for an accurate extraction of alpha_s from
the Q^2-dependence of the Bjorken sum rule, complementing that based on the
Gross-Llewellyn-Smith sum rule for F_3. We estimate the power (1/Q^2-)
corrections to the Bjorken sum rule in the instanton vacuum model. For the
reduced matrix element of the flavor-nonsinglet twist-4 operator
ubar_g_Gdual_gamma_gamma5_u - (u -> d) we obtain a value of 0.18 GeV^2, in good
agreement with the QCD sum rule calculations of Braun and Kolesnichenko. Our
result allows to reduce the theoretical error in the determination of alpha_s.Comment: 3 pages, 1 figure, uses iopart.cls. Proceedings of the 4th NuFact'02
Workshop "Neutrino Factories based on Muon Storage Rings", Imperial College,
London, July 1-6, 200
New parton distributions in fixed flavour factorization scheme from recent deep-inelastic-scattering data
We present our QCD analysis of the proton structure function
to determine the parton distributions at the next-to-leading order (NLO). The
heavy quark contributions to , with = , have been
included in the framework of the `fixed flavour number scheme' (FFNS). The
results obtained in the FFNS are compared with available results such as the
general-mass variable-flavour-number scheme (GM-VFNS) and other prescriptions
used in global fits of PDFs. In the present QCD analysis, we use a wide range
of the inclusive neutral-current deep-inelastic-scattering (NC DIS) data,
including the most recent data for charm , bottom , longitudinal
structure functions and also the reduced DIS cross sections
from HERA experiments. The most recent HERMES data for
proton and deuteron structure functions are also added. We take into account
ZEUS neutral current DIS inclusive jet cross section data from HERA
together with the recent Tevatron Run-II inclusive jet cross section data from
CDF and D{\O}. The impact of these recent DIS data on the PDFs extracted from
the global fits are studied. We present two families of PDFs, {\tt KKT12} and
{\tt KKT12C}, without and with HERA `combined' data sets on DIS. We
find these are in good agreement with the available theoretical models.Comment: 23 pages, 26 figures and 4 tables. V3: Only few comments and
references added in the replaced version, results unchanged. Code can be
found at http://particles.ipm.ir/links/QCD.ht
A Critical Appraisal and Evaluation of Modern PDFs
We review the present status of the determination of parton distribution
functions (PDFs) in the light of the precision requirements for the LHC in Run
2 and other future hadron colliders. We provide brief reviews of all currently
available PDF sets and use them to compute cross sections for a number of
benchmark processes, including Higgs boson production in gluon-gluon fusion at
the LHC. We show that the differences in the predictions obtained with the
various PDFs are due to particular theory assumptions made in the fits of those
PDFs. We discuss PDF uncertainties in the kinematic region covered by the LHC
and on averaging procedures for PDFs, such as advocated by the PDF4LHC15 sets,
and provide recommendations for the usage of PDF sets for theory predictions at
the LHC.Comment: 70 pages pdflatex, 19 figures, 17 tables; final versio
What did HERA teach us about the structure of the proton?
Starting in 2008 the H1 and ZEUS experiments have been combining their data
in order to provide the most complete and accurate set of deep-inelastic data
as the legacy of HERA. The present review presents these combinations, both
published and preliminary, and explores how they have been used to give
information on the structure of the proton. The HERAPDF parton distribution
functions (PDFs) are presented and compared with other current PDFs and with
data from the Tevatron and LHC colliders.Comment: 49 pages, 49 figures, to be published in J.Phys.
Meaningful characterisation of perturbative theoretical uncertainties
We consider the problem of assigning a meaningful degree of belief to
uncertainty estimates of perturbative series. We analyse the assumptions which
are implicit in the conventional estimates made using renormalisation scale
variations. We then formulate a Bayesian model that, given equivalent initial
hypotheses, allows one to characterise a perturbative theoretical uncertainty
in a rigorous way in terms of a credibility interval for the remainder of the
series. We compare its outcome to the conventional uncertainty estimates in the
simple case of the calculation of QCD corrections to the e+e- -> hadrons
process. We find comparable results, but with important conceptual differences.
This work represents a first step in the direction of a more comprehensive and
rigorous handling of theoretical uncertainties in perturbative calculations
used in high energy phenomenology.Comment: 28 pages, 5 figures. Language modified in order to make it more
'bayesian'. No change in results. Version published in JHE
PDF and scale uncertainties of various DY distributions in ADD and RS models at hadron colliders
In the extra dimension models of ADD and RS we study the dependence of the
various parton distribution functions on observable of Drell-Yan process to NLO
in QCD at LHC and Tevatron energies. Uncertainties at LHC due to factorisation
scales in going from leading to next-to-leading order in QCD for the various
distributions get reduced by about 2.75 times for a range . Further uncertainties arising from the error on experimental
data are estimated using the MRST parton distribution functions.Comment: 27 pages, 11 figures, the version to appear in European Physical
Journal
High precision fundamental constants at the TeV scale
This report summarizes the proceedings of the 2014 Mainz Institute for
Theoretical Physics (MITP) scientific program on "High precision fundamental
constants at the TeV scale". The two outstanding parameters in the Standard
Model dealt with during the MITP scientific program are the strong coupling
constant and the top-quark mass . Lacking knowledge on the
value of those fundamental constants is often the limiting factor in the
accuracy of theoretical predictions. The current status on and
has been reviewed and directions for future research have been identified.Comment: 57 pages, 24 figures, pdflate
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