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Aguilar, A. C., Binosi, D., Ibañez, D., & Papavassiliou, J. (2014). New method for determining the quark-gluon vertex. Phys. Rev. D, 90(6), 065027–26pp.
Abstract: We present a novel nonperturbative approach for calculating the form factors of the quark-gluon vertex in terms of an unknown three-point function, in the Landau gauge. The key ingredient of this method is the exact all-order relation connecting the conventional quark-gluon vertex with the corresponding vertex of the background field method, which is Abelian-like. When this latter relation is combined with the standard gauge technique, supplemented by a crucial set of transverse Ward identities, it allows the approximate determination of the nonperturbative behavior of all 12 form factors comprising the quark-gluon vertex, for arbitrary values of the momenta. The actual implementation of this procedure is carried out in the Landau gauge, in order to make contact with the results of lattice simulations performed in this particular gauge. The most demanding technical aspect involves the approximate calculation of the components of the aforementioned (fully dressed) three-point function, using lattice data as input for the gluon propagators appearing in its diagrammatic expansion. The numerical evaluation of the relevant form factors in three special kinematical configurations (soft-gluon and quark symmetric limit, zero quark momentum) is carried out in detail, finding qualitative agreement with the available lattice data. Most notably, a concrete mechanism is proposed for explaining the puzzling divergence of one of these form factors observed in lattice simulations.
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Barranco, L., Boubekeur, L., & Mena, O. (2014). Model-independent fit to Planck and BICEP2 data. Phys. Rev. D, 90(6), 063007–7pp.
Abstract: Inflation is the leading theory to describe elegantly the initial conditions that led to structure formation in our Universe. In this paper, we present a novel phenomenological fit to the Planck, WMAP polarization (WP) and the BICEP2 data sets using an alternative parametrization. Instead of starting from inflationary potentials and computing the inflationary observables, we use a phenomenological parametrization due to Mukhanov, describing inflation by an effective equation of state, in terms of the number of e-folds and two phenomenological parameters alpha and beta. Within such a parametrization, which captures the different inflationary models in a model-independent way, the values of the scalar spectral index n(s), its running and the tensor-to-scalar ratio r are predicted, given a set of parameters (alpha, beta). We perform a Markov Chain Monte Carlo analysis of these parameters, and we show that the combined analysis of Planck and WP data favors the Starobinsky and Higgs inflation scenarios. Assuming that the BICEP2 signal is not entirely due to foregrounds, the addition of this last data set prefers instead the phi(2) chaotic models. The constraint we get from Planck and WP data alone on the derived tensor-to-scalar ratio is r < 0.18 at 95% C.L., value which is consistent with the one quoted from the BICEP2 Collaboration analysis, r = 0.16(-0.05)(+0-06), after foreground subtraction. This is not necessarily at odds with the 2 sigma tension found between Planck and BICEP2 measurements when analyzing data in terms of the usual n(s) and r parameters, given that the parametrization used here, for the preferred value n(s) similar or equal to 0.96, allows only for a restricted parameter space in the usual (n(s), r) plane.
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Strege, C., Bertone, G., Besjes, G. J., Caron, S., Ruiz de Austri, R., Strubig, A., et al. (2014). Profile likelihood maps of a 15-dimensional MSSM. J. High Energy Phys., 09(9), 081–59pp.
Abstract: We present statistically convergent profile likelihood maps obtained via global fits of a phenomenological Minimal Supersymmetric Standard Model with 15 free parameters (the MSSM-15), based on over 250M points. We derive constraints on the model parameters from direct detection limits on dark matter, the Planck relic density measurement and data from accelerator searches. We provide a detailed analysis of the rich phenomenology of this model, and determine the SUSY mass spectrum and dark matter properties that are preferred by current experimental constraints. We evaluate the impact of the measurement of the anomalous magnetic moment of the muon (g – 2) on our results, and provide an analysis of scenarios in which the lightest neutralino is a subdominant component of the dark matter. The MSSM-15 parameters are relatively weakly constrained by current data sets, with the exception of the parameters related to dark matter phenomenology (M-1, M-2, mu), which are restricted to the sub-TeV regime, mainly due to the relic density constraint. The mass of the lightest neutralino is found to be < 1.5TeV at 99% C.L., but can extend up to 3 TeV when excluding the g – 2 constraint from the analysis. Low-mass bino-like neutralinos are strongly favoured, with spin-independent scattering cross-sections extending to very small values, similar to 10(-20) pb. ATLAS SUSY null searches strongly impact on this mass range, and thus rule out a region of parameter space that is outside the reach of any current or future direct detection experiment. The best-fit point obtained after inclusion of all data corresponds to a squark mass of 2.3 TeV, a gluino mass of 2.1 TeV and a 130 GeV neutralino with a spin-independent cross-section of 2.4 x 10(-10) pb, which is within the reach of future multi-ton scale direct detection experiments and of the upcoming LHC run at increased centre-of-mass energy.
Keywords: Supersymmetry Phenomenology
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n_TOF Collaboration(Mendoza, E. et al), Domingo-Pardo, C., & Tain, J. L. (2014). Measurement and analysis of the Am-243 neutron capture cross section at the n_TOF facility at CERN. Phys. Rev. C, 90(3), 034608–16pp.
Abstract: Background: The design of new nuclear reactors and transmutation devices requires to reduce the present neutron cross section uncertainties of minor actinides. Purpose: Improvement of the Am-243(n, gamma) cross section uncertainty. Method: The Am-243(n, gamma) cross section has been measured at the n_TOF facility at CERN with a BaF2 total absorption calorimeter, in the energy range between 0.7 eV and 2.5 keV. Results: The Am-243(n, gamma) cross section has been successfully measured in the mentioned energy range. The resolved resonance region has been extended from 250 eV up to 400 eV. In the unresolved resonance region our results are compatible with one of the two incompatible capture data sets available below 2.5 keV. The data available in EXFOR and in the literature have been used to perform a simple analysis above 2.5 keV. Conclusions: The results of this measurement contribute to reduce the Am-243(n, gamma) cross section uncertainty and suggest that this cross section is underestimated up to 25% in the neutron energy range between 50 eV and a few keV in the present evaluated data libraries.
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LHCb Collaboration(Aaij, R. et al), Martinez-Vidal, F., Oyanguren, A., Ruiz Valls, P., & Sanchez Mayordomo, C. (2014). Measurement of the Xi(-)(b) and Omega(-)(b) baryon lifetimes. Phys. Lett. B, 736, 154–162.
Abstract: Using a data sample of pp collisions corresponding to an integrated luminosity of 3 fb(-1), the Xi(-)(b) and Omega(-)(b) baryons are reconstructed in the Xi(-)(b) -> J/psi Xi(-) and Omega(-)(b) -> J/psi Omega(-) decay modes and their lifetimes measured to be tau(Xi(-)(b)) = 1.55(-0.09)(+0.10) (stat) +/- 0.03 (syst) ps, tau(Omega(-)(b)) = 1.54(-0.21)(+0.26) (stat) +/- 0.05 (syst) ps. These are the most precise determinations to date. Both measurements are in good agreement with previous experimental results and with theoretical predictions.
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