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

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

Detailed Comparison of Next-to-Leading Order Predictions for Jet Photoproduction at HERA

The precision of new HERA data on jet photoproduction opens up the possibility to discriminate between different models of the photon structure. This requires equally precise theoretical predictions from perturbative QCD calculations. In the past years, next-to-leading order calculations for the photoproduction of jets at HERA have become available. Using the kinematic cuts of recent ZEUS analyses, we compare the predictions of three calculations for different dijet and three-jet distributions. We find that in general all three calculations agree within the statistical accuracy of the Monte Carlo integration yielding reliable theoretical predictions. In certain restricted regions of phase space, the calculations differ by up to 5%.Comment: 10 pages, 7 eps-figures. To appear in the Proceedings of the Workshop on ``Monte Carlo Generators for HERA Physics'', Hamburg 1998/9

Factorization scheme and scale dependence in diffractive dijet production at low Q^2

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. We study how the cross section depends on the factorization scheme and scale M_\gamma at the virtual photon vertex for the occurrence of factorization breaking. The strong M_\gamma-dependence, which is present when only the resolved cross section is suppressed, is tamed by introducing the suppression also into the initial-state NLO correction of the direct part.Comment: 14 pages, 6 figure

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 $2\cdot10^{-5}$ to $1\cdot10^{-5}$ 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, $1.1\cdot 10^{-5}$ 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.

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

SUSY-QCD corrections to stop annihilation into electroweak final states including Coulomb enhancement effects

We present the full $\mathcal{O}(\alpha_s)$ supersymmetric QCD corrections for stop-anti-stop annihilation into electroweak final states within the Minimal Supersymmetric Standard Model (MSSM). We also incorporate Coulomb corrections due to gluon exchange between the incoming stops. Numerical results for the annihilation cross sections and the predicted neutralino relic density are presented. We show that the impact of the radiative corrections on the cosmologically preferred region of the parameter space can become larger than the current experimental uncertainty, shifting the relic bands within the considered regions of the parameter space by up to a few tens of GeV.Comment: 20 pages, 13 figures, updated to version published in Phys. Rev.

Higgsino and gaugino pair production at the LHC with aNNLO+NNLL precision

We present a calculation of higgsino and gaugino pair production at the LHC at next-to-next-to-leading logarithmic (NNLL) accuracy, matched to approximate next-to-next-to-leading order (aNNLO) QCD corrections. We briefly review the formalism for the resummation of large threshold logarithms and highlight the analytical results required at aNNLO+NNLO accuracy. Our numerical results are found to depend on the mass and nature of the produced charginos and neutralinos. The differential and total cross sections for light higgsinos, which like sleptons are produced mostly at small x and in the s-channel, are found to be again moderately increased with respect to our previous results. The differential and total cross sections for gauginos are, however, not increased any more due to the fact that gauginos, like squarks, are now constrained by ATLAS and CMS to be heavier than about 1 TeV, so that also t- and u-channels play an important role. The valence quarks probed at large x then also induce substantially different cross sections for positively and negatively charged gauginos. The higgsino and gaugino cross sections are both further stabilized at aNNLO+NNLL with respect to the variation of renormalization and factorization scales. We also now take mixing in the squark sector into account and study the dependence of the total cross sections on the squark and gluino masses as well as the trilinear coupling controlling the mixing in particular in the sbottom sector.Comment: 29 pages, 16 figures. arXiv admin note: text overlap with arXiv:1911.0241