Effects of bulk symmetry breaking on AdS/QCD predictions

We put forward a new bottom-up AdS/QCD holographic model bearing a distinct treatment of the pion fields. We argue that a standard approach to the pion description is neither transparent nor totally satisfactory. In the paper we provide a new one based on a broadened realization of some holographic principles. The reasoning and the effect of these modifications are explained in detail. The resulting model has a different set of parameters than the standard AdS/QCD case. We use them to calculate an extensive list of QCD quantities and find a rather good agreement with the experimental data

Algunes qüestions de renormalització a la cromodinàmica quàntica

[cat] Sembla obligat començar fent referència a les raons teòriques i experimentals que ens fan creure en la Cromodinàmica Quàntica com en la Teoria de Camps adient a la descripció del mon hadrònic. Per a creure en una teoria necessitem, seguint Wightman, dos requisits: ha d'estar lliure de contradiccions i ha de satisfer un rang més o menys ampli d'observacions experimentals. És en aquest sentit que hom creu en les equacions de Maxwelll, però no ho fa en l’antiga teoria quàntica anterior a la Mecànica Ondulatòria. En els dos aspectes la resposta ha d'ésser matisada.Pel que coneixem fins aquest moment QCD és una teoria lliure d'inconsistències internes, al menys a un nivell molt fonamental. És una teoria de gauge -i creiem en les teories de gauge per a l'explicació de totes les interaccions- convenientment definida (en un sentit ampli del terme), renormalitzable i unitària. A grans trets, el problema que ens impedeix donar una resposta conclusiva és la probable inexistència de la matriu S almenys en un sentit pertorbatiu

Searching for P- and CP-odd effects in HIC

In this talk we will summarize the main results from our recent work concerning the possibility that a new metastable phase occurs in some heavy ion collisions (HIC). This phase would be characterized by the breaking of two characteristic symmetries of strong interactions; namely P and CP. We investigate the experimental consequences of parity breaking in such a situation and propose suitable observables to elucidate the presence this phenomenon

Pulsar timing arrays and the cosmological constant

In this talk I review how a non-zero cosmological constant Λ effects the propagation of gravitational waves and their detection in pulsar timing arrays (PTA). If Λ ≠ 0 it turns out that waves are anharmonic in cosmological Friedmann-Robertson- Walker coordinates and although the amount of anharmonicity is very small it leads to potentially measurable effects. The timing residuals induced by gravitational waves in PTA would show a peculiar angular dependence with a marked enhancement around a particular value of the angle subtended by the source and the pulsars. This angle depends mainly on the actual value of the cosmological constant and the distance to the source. Preliminary estimates indicate that the enhancement can be rather notorious for supermassive black hole mergers and in fact it could facilitate the first direct detection of gravitational waves while at the same time representing a 'local' measurement of Λ

Unitarity and causality constraints in composite Higgs models

We study the scattering of longitudinally polarized W bosons in extensions of the Standard Model where anomalous Higgs couplings to gauge sector and higher-order O(p4) operators are considered. These new couplings with respect to the Standard Model should be thought of as the low-energy remnants of some new dynamics involving the electroweak symmetry breaking sector. By imposing unitarity and causality constraints on the WW scattering amplitudes, we find relevant restrictions on the possible values of the new couplings and the presence of new dynamical resonances above 300 GeV. We investigate the properties of these new resonances and their experimental detectability. Custodial symmetry is assumed to be exact throughout, and the calculation avoids using the equivalence theorem as much as possible

Oblique corrections in the Dine-Fischler-Srednicki axion model

In the Minimal Standard Model (MSM) there is no degree of freedom for dark matter. There are several extensions of the MSM introducing a new particle - an invisible axion, which can be regarded as a trustworthy candidate at least for a part of the dark matter component. However, as it is extremely weakly coupled, it cannot be directly measured at the LHC. We propose to explore the electroweak sector indirectly by considering a particular model that includes the axion and derive consequences that could be experimentally tested. We discuss the Dine-Fischler-Srednicki (DFS) model, which extends the two-Higgs doublet model with an additional Peccei-Quinn symmetry and leads to a physically acceptable axion. The non-linear parametrization of the DFS model is exploited in the generic case where all scalars except the lightest Higgs and the axion have masses at or beyond the TeV scale. We compute the oblique corrections and use their values from the electroweak experimental fits to put constraints on the mass spectrum of the DFS model

Longitudinal WW scattering in light of the 'Higgs boson' discovery

WW scattering is dominated at high energies by their longitudinal components, which are the most sensitive to the nature of the electroweak symmetry breaking. Prior to the discovery at the LHC of a Higgs-like particle, unitarization tools were extensively used to show that, in the absence of a light Higgs boson, new resonances resulting from the would-be strongly interacting electroweak sector would appear, and furthermore these techniques would approximately predict their masses, widths, and signal strengths. With the discovery of a Higgs-like particle now firmly established, we reinvestigate these techniques assuming this particle couples exactly as in the standard model (SM), but still being open to the possibility of an extended symmetry breaking sector. While the SM itself is free from problems with perturbative unitarity in the electroweak sector, "anomalous" self-couplings of the vector bosons low-energy remnants of such higher-energy symmetry breaking sectors are easily shown to reintroduce them. We demonstrate how new resonances should still appear in the scattering of electroweak vector bosons after imposing constraints from unitarity, and we discuss their ability to be probed with current and future LHC data

Chiral perturbation theory vs. linear sigma model in a chiral imbalance medium

We compare the chiral perturbation theory (ChPT) and the linear sigma model (LSM) as realizations of low energy quantum chromodynamics (QCD) for light mesons in a chirally-imbalanced medium. The relations between the low-energy constants of the chiral Lagrangian and the corresponding constants of the linear sigma model are established as well as the expressions for the decay constant of π -meson in the medium and for the mass of the a0. In the large Nc count taken from QCD the correspondence of ChPT and LSM is remarkably good and provides a solid ground for the search of chiral imbalance manifestations in pion physics. A possible experimental detection of chiral imbalance (and therefore a phase with local parity breaking) is outlined in the charged pion decays inside the fireball

Photons in a cold axion background and strong magnetic fields: Polarimetric consequences

In this work we analyze the propagation of photons in an environment where a strong magnetic field (perpendicular to the photon momenta) coexists with an oscillating cold axion background with the characteristics expected from dark matter in the galactic halo. Qualitatively, the main effect of the combined background is to produce a three-way mixing among the two photon polarizations and the axion. It is interesting to note that in spite of the extremely weak interaction of photons with the cold axion background, its effects compete with those coming from the magnetic field in some regions of the parameter space. We determine (with one plausible simplification) the proper frequencies and eigenvectors as well as the corresponding photon ellipticity and induced rotation of the polarization plane that depend both on the magnetic field and the local density of axions. We also comment on the possibility that some of the predicted effects could be measured in optical table-top experiments

Bose-Einstein graviton condensate in a Schwarzschild black hole

We analyze in detail a previous proposal by Dvali and Gómez that black holes could be treated as consisting of a Bose-Einstein condensate of gravitons. In order to do so we extend the Einstein-Hilbert action with a chemical potential-like term, thus placing ourselves in a grand-canonical ensemble. The form and characteristics of this chemical potential-like piece are discussed in some detail. We argue that the resulting equations of motion derived from the action could be interpreted as the Gross-Pitaevskii equation describing a graviton Bose-Einstein condensate trapped by the black hole gravitational field. After this, we proceed to expand the ensuring equations of motion up to second order around the classical Schwarzschild metric so that some non-linear terms in the metric fluctuation are kept. Next we search for solutions and, modulo some very plausible assumptions, we find out that the condensate vanishes outside the horizon but is non-zero in its interior. Inspired by a linearized approximation around the horizon we are able to find an exact solution for the mean-field wave function describing the graviton Bose-Einstein condensate in the black hole interior. After this, we can rederive some of the relations involving the number of gravitons N and the black hole characteristics along the lines suggested by Dvali and Gómez