960 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
Non-Diagonal and Mixed Squark Production at Hadron Colliders
We calculate squared helicity amplitudes for non-diagonal and mixed squark
pair production at hadron colliders, taking into account not only loop-induced
QCD diagrams, but also previously unconsidered electroweak channels, which turn
out to be dominant. Mixing effects are included for both top and bottom
squarks. Numerical results are presented for several SUSY benchmark scenarios
at both the CERN LHC and the Fermilab Tevatron, including the possibilities of
light stops or sbottoms. The latter should be easily observed at the Tevatron
in associated production of stops and sbottoms for a large range of stop masses
and almost independently of the stop mixing angle. Asymmetry measurements for
light stops at the polarized BNL RHIC collider are also briefly discussed.Comment: 22 pages, 11 figure
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
New Results for Light Gravitinos at Hadron Colliders - Tevatron Limits and LHC Perspectives
We derive Feynman rules for the interactions of a single gravitino with
(s)quarks and gluons/gluinos from an effective supergravity Lagrangian in
non-derivative form and use them to calculate the hadroproduction cross
sections and decay widths of single gravitinos. We confirm the results obtained
previously with a derivative Lagrangian as well as those obtained with the
non-derivative Lagrangian in the high-energy limit and elaborate on the
connection between gauge independence and the presence of quartic vertices. We
perform extensive numerical studies of branching ratios, total cross sections,
and transverse-momentum spectra at the Tevatron and the LHC. From the latest
CDF monojet cross section limit, we derive a new and robust exclusion contour
in the gravitino-squark/gluino mass plane, implying that gravitinos with masses
below to eV are excluded for
squark/gluino-masses below 200 and 500 GeV, respectively. These limits are
complementary to the one obtained by the CDF collaboration,
eV, under the assumption of infinitely heavy squarks and gluinos. For the LHC,
we conclude that SUSY scenarios with light gravitinos will lead to a striking
monojet signal very quickly after its startup.Comment: 30 pages, 12 figures. Tevatron limit improved and unitarity limit
included. Version to be published in Phys. Rev.
Transverse-Momentum Resummation for Slepton-Pair Production at the LHC
We perform a first precision calculation of the transverse-momentum (q_T)
distribution of slepton pair and slepton-sneutrino associated production at the
CERN Large Hadron Collider (LHC). We implement soft-gluon resummation at the
next-to-leading logarithmic (NLL) level and consistently match the obtained
result to the pure fixed-order perturbative result at leading order (LO) in the
QCD coupling constant, i.e. O(alpha_s). We give numerical predictions for
stau_1 stau_1^* and stau_1 sneutrino_tau^* + stau_1^* sneutrino_tau production,
also implementing recent parameterizations of non-perturbative effects. The
results show a relevant contribution of resummation both in the small and
intermediate q_T-regions and little dependence on unphysical scales and
non-perturbative contributions.Comment: 4 pages, 2 figure
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
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
Factorization Breaking in Dijet Photoproduction with a Leading Neutron
The production of dijets with a leading neutron in ep-interactions at HERA is
calculated in leading order and next-to-leading order of perturbative QCD using
a pion-exchange model. Differential cross sections for deep-inelastic
scattering (DIS) and photoproduction are presented as a function of several
kinematic variables. By comparing the theoretical predictions for DIS dijets to
recent H1 data, the pion flux factor together with the parton distribution
functions of the pion is determined. The dijet cross sections in
photoproduction show factorization breaking if compared to the H1
photoproduction data. The suppression factor is S = 0.48 (0.64) for resolved
(global) suppression.Comment: 16 pages, 5 figure
A singlet doublet dark matter model with radiative neutrino masses
We present a detailed study of a combined singlet-doublet scalar and
singlet-doublet fermion model for dark matter. These models have only been
studied separately in the past. We show that their combination allows for the
radiative generation of neutrino masses, but that it also implies the existence
of lepton-flavour violating (LFV) processes. We first analyse the dark matter,
neutrino mass and LFV aspects separately. We then perform two random scans for
scalar dark matter imposing Higgs mass, relic density and neutrino mass
constraints, one over the full parameter space, the other over regions where
scalar-fermion coannihilations become important. In the first case, a large
part of the new parameter space is excluded by LFV, and the remaining models
will be probed by XENONnT. In the second case, direct detection cross sections
are generally too small, but a substantial part of the viable models will be
tested by future LFV experiments. Possible constraints from the LHC are also
discussed.Comment: 27 pages, 18 figures, 2 table
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