Journal Club

Seminar Room

Wednesday 27th of February, 2013

Hour: 10:00

Title: Exclusive diffractive photon bremsstrahlung at the LHC

Presented by: Manuel Masip Mellado

Ref: http://arxiv.org/abs/1302.4346v1

Abstract:

We calculate differential distributions for the $p p \to p p \gamma$ reaction at the LHC energy $\sqrt{s} = 14$ TeV. We consider diffractive classical bremsstrahlung mechanisms including effects of non point-like nature of protons. In addition, we take into account (vector meson)-pomeron, photon-pion as well as photon-pomeron exchange processes for the first time in the literature. Predictions for the total cross section and several observables related to these processes e.g. differential distributions in pseudorapidities and transverse momenta of photons or protons are shown and discussed. The integrated diffractive bremsstrahlung cross section ($E_{\gamma}>100$ GeV) is only of the order of $\mu$b. We try to identify regions of the phase space where one of the mechanisms dominates. The classical bremsstrahlung dominates at large forward/backward photon pseudorapidities, close to the pseudorapidities of scattered protons. In contrast, the photon-pomeron (pomeron-photon) mechanism dominates at midrapidities but the related cross section is rather small. In comparison the virtual-omega rescattering mechanism contributes at smaller angles of photons (larger photon rapidities). Photons in the forward/backward region can be measured by the Zero Degree Calorimeters (ZDCs) installed in experiments at the LHC while the midrapidity photons are difficult to measure (small cross section, small photon transverse momenta). Protons could be measured by ALFA detector (ATLAS) or TOTEM detector at CMS. The exclusivity could be checked with the help of main central detectors.

Hour: 10:00

Title: Fixing the EW scale in supersymmetric models after the Higgs discovery

Presented by: Manuel Pérez-Victoria Moreno de Barreda

Ref: http://arxiv.org/abs/1302.5262v1

Abstract:

TeV-scale supersymmetry was originally introduced to solve the hierarchy problem and therefore fix the electroweak (EW) scale in the presence of quantum corrections. Surprisingly, the numerical methods that evaluate the likelihood (or chi^2=-2*ln L) to fit the experimental data and test the SUSY models do not account for fixing the EW scale. When this constraint is implemented, the likelihood (or chi^2) receives a significant correction (delta_chi^2) that worsens the data fits of the models. We estimate this correction for the models: constrained MSSM (CMSSM), models with non-universal gaugino masses (NUGM) or higgs soft masses (NUHM1, NUHM2), the NMSSM and the general NMSSM (GNMSSM). Except the GNMSSM model and for a higgs mass m_h=126 GeV, one finds that in these models (delta_chi^2)/ndf >1.5, which violates the usual condition of a good fit already before fitting the observables, other than the EW scale itself (ndf=number of degrees of freedom). Given its significant (negative) implications for SUSY models, it is suggested that future data fits properly account for this effect, if one remains true to the original goal of SUSY.

Hour: 10:00

Title: Roles of a coherent scalar field on the evolution of cosmic structures

Presented by: Rafael Cerezo

Ref: http://arxiv.org/abs/astro-ph/9610042v2

Abstract:

A coherently oscillating scalar field, an axion as an example, is known to behave as a cold dark matter. The arguments were usually made in the Newtonian context. Ratra proved the case in relativistic context using the synchronous gauge. In this paper we present another proof based on a more suitable gauge choice, the uniform-curvature gauge, which fits the problem. By a proper time averaging the perturbed oscillating scalar field behaves as a cold dark matter on the relevant scales including the superhorizon scale.