778 research outputs found
Evidence for Factorization Breaking in Diffractive Low-Q^2 Dijet Production
We calculate diffractive dijet production in deep-inelastic scattering at
next-to-leading order of perturbative QCD, including contributions from direct
and resolved photons, and compare our predictions to preliminary data from the
H1 collaboration at HERA. In contrast to recent experimental claims, evidence
for factorization breaking is found only for resolved, and not direct, photon
contributions. No evidence is found for large normalization uncertainties in
diffractive parton densities. The results confirm theoretical expectations for
the (non-)cancellation of soft singularities in diffractive scattering as well
as previous results for (almost) real photoproduction.Comment: 4 pages, 3 figure
Calculating two- and three-body decays with FeynArts and FormCalc
The Feynman diagram generator FeynArts and the computer algebra program
FormCalc allow for an automatic computation of 2->2 and 2->3 scattering
processes in High Energy Physics. We have extended this package by four new
kinematical routines and adapted one existing routine in order to accomodate
also two- and three-body decays of massive particles. This makes it possible to
compute automatically two- and three-body particle decay widths and decay
energy distributions as well as resonant particle production within the
Standard Model and the Minimal Supersymmetric Standard Model at the tree- and
loop-level. The use of the program is illustrated with three standard examples:
h->b\bar{b}, \mu->e\bar{\nu}_e\nu_\mu, and Z->\nu_e\bar{\nu}_e.Comment: 8 pages, 1 figur
Suppression factors in diffractive photoproduction of dijets
After new publications of H1 data for the diffractive photoproduction of
dijets, which overlap with the earlier published H1 data and the recently
published data of the ZEUS collaboration, have appeared, we have recalculated
the cross sections for this process in next-to-leading order (NLO) of
perturbative QCD to see whether they can be interpreted consistently. The
results of these calculations are compared to the data of both collaborations.
We find that the NLO cross sections disagree with the data, showing that
factorization breaking occurs at that order. If direct and resolved
contributions are both suppressed by the same amount, the global suppression
factor depends on the transverse-energy cut. However, by suppressing only the
resolved contribution, also reasonably good agreement with all the data is
found with a suppression factor independent of the transverse-energy cut.Comment: 28 pages, 11 figures, 3 table
Can new heavy gauge bosons be observed in ultra-high energy cosmic neutrino events?
A wide range of models beyond the Standard Model predict charged and neutral
resonances, generically called - and -bosons, respectively. In this
paper we study the impact of such resonances on the deep inelastic scattering
of ultra-high energy neutrinos as well as on the resonant charged current
scattering (Glashow resonance). We find that the effects of
such resonances can not be observed with the Pierre Auger Observatory or any
foreseeable upgrade of it.Comment: 5 pages, 2 figure
How robust is a thermal photon interpretation of the ALICE low-p_T data?
We present a rigorous theoretical analysis of the ALICE measurement of
low-p_T direct-photon production in central lead-lead collisions at the LHC
with a centre-of-mass energy of \sqrt{s_{NN}}=2.76 TeV. Using NLO QCD, we
compute the relative contributions to prompt-photon production from different
initial and final states and the theoretical uncertainties coming from
independent variations of the renormalisation and factorisation scales, the
nuclear parton densities and the fragmentation functions. Based on different
fits to the unsubtracted and prompt-photon subtracted ALICE data, we
consistently find T = 304 \pm 58 MeV and 309 \pm 64 MeV for the effective
temperature of the quark-gluon plasma (or hot medium) at p_T \in [0.8;2.2] GeV
and p_T \in [1.5;3.5] GeV as well as a power-law (p_T^{-4}) behavior for p_T >
4 GeV as predicted by QCD hard scattering.Comment: 18 pages, 7 figures, 1 tabl
Electroweak top-quark pair production at the LHC with bosons to NLO QCD in POWHEG
We present the calculation of the NLO QCD corrections to the electroweak
production of top-antitop pairs at the CERN LHC in the presence of a new
neutral gauge boson. The corrections are implemented in the parton shower Monte
Carlo program POWHEG. Standard Model (SM) and new physics interference effects
are properly taken into account. QED singularities, first appearing at this
order, are consistently subtracted. Numerical results are presented for SM and
total cross sections and distributions in invariant mass, transverse
momentum, azimuthal angle and rapidity of the top-quark pair. The remaining
theoretical uncertainty from scale and PDF variations is estimated, and the
potential of the charge asymmetry to distinguish between new physics models is
investigated for the Sequential SM and a leptophobic topcolor model.Comment: 33 pages, 15 figures, 1 table. Discussion added on uncertainties from
parton showers and NLL contribution
Dijet photoproduction of massless charm jets at next-to-leading order of QCD
We compute the charm dijet photoproduction cross section at next-to-leading
order of QCD in the zero-mass variable flavour number scheme, i.e. with active
charm quarks in the proton and photon. The results are compared to recent
measurements from the ZEUS experiment at HERA. The predictions for various
distributions agree well with the data, in particular for large momentum
fractions of the the partons in the photon, where direct photon processes
dominate. At low momentum fractions, the predictions are quite sensitive to the
charm content in the photon. The experimental data are shown to favour
parameterizations with a substantial charm quark density such as the one
proposed by Cornet et al.Comment: 18 pages, 11 figure
NLO+NLL limits on W' and Z' gauge boson masses in general extensions of the Standard Model
QCD resummation predictions for the production of charged (W') and neutral
(Z') heavy gauge bosons decaying leptonically are presented. The results of our
resummation code at next-to-leading order and next-to-leading logarithmic
(NLO+NLL) accuracy are compared to Monte Carlo predictions obtained with PYTHIA
at leading order (LO) supplemented with parton showers (PS) and FEWZ at NLO and
next-to-next-to-leading order (NNLO) for the -differential and total cross
sections in the Sequential Standard Model (SSM) and general SU(2)xSU(2)xU(1)
models. The LO+PS Monte Carlo and NNLO fixed-order predictions are shown to
agree approximately with those at NLO+NLL at small and intermediate ,
respectively, and the importance of resummation for total cross sections is
shown to increase with the gauge boson mass. The theoretical uncertainties are
estimated by variations of the renormalisation/factorisation scales and of the
parton densities, the former being significantly reduced by the resummation
procedure. New limits at NLO+NLL on W' and Z' boson masses are obtained by
reinterpreting the latest ATLAS and CMS results in general extensions of the
Standard Model.Comment: 26 pages, 10 figures, 6 table
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