Particle Physics

Models with two supersymmetric large extra dimensions (SLED) may provide a way to approach both the cosmological constant problem and dark energy. After reviewing these ideas, I shall discuss warped brane world solutions in 6D supergravity, as a laboratory in which to explore SLED and codimension two branes in general. Solving the linearized perturbations for all the bosonic fields, and some of the fermions, we can observe how the corresponding physics compares with 5D models and standard Kaluza-Klein compactifications. These results should help to better our understanding of the cancellations in the 4D effective vacuum energy with SLED.

It is well known that new physics at the electroweak scale could solve important puzzles in cosmology, such as the nature of dark matter and the origin of the cosmic baryon asymmetry. In this talk, I discuss some of the simplest, non-supersymmetric possibilities, their collider signatures, and the prospects for their discovery and identification at the LHC.

A modified version of PQCD considered in previous works is further investigated in the case of a vanishing gluon condensate, by retaining only the quark one. In this case the Green functions generating functional is expressed in a simple form in which Dirac’s delta functions are now absent from the free propagators. The new expansion implements the dimensional transmutation effect through a single interaction vertex in addition to the standard ones in mass less QCD. The results of an ongoing two loop evaluation of the vacuum energy will be presented. The potential is parameterized as a function of the quark mass (defined by the pole of the first corrections to the quark propagator), the assumed finite zero momentum limit of the coupling constant g and the dimensional regularization parameter. The first condensate dependent corrections to the gluon and quark self-energies and propagators are evaluated. Assuming the possibility of fixing a minimum of the potential at the experimental value of the top quark mass =173 GeV, we evaluate the pole of the simplest correction to the propagator of the composite operator describing the quark condensate. Then, alter adopting the idea from the former top condensate models, in which the Higgs field corresponds to the quark condensate, the obtained pole gave a first rough estimate for the Higgs mass =168.2 GeV. Although being inside of the recently experimentally excluded region: 160-170 GeV, this mass value has the chance of being modified by a better approximation being currently considered for the gluon propagator entering its evaluation.

Particle physics, cosmology, and the study of a wide variety of theoretical models – most notably ones involving extra dimensions of space. Randall works on several of the competing models of string theory in the quest to explain the fabric of the universe.

s-channel resonances are predicted by many models of Physics Beyond the Standard Model and it is quite possible that such an object will be discovered in the early years of the LHC program. If this occurs, the task will be to understand its origins. A brief survey of models that predict s-channel resonances will be given, concentrating mainly on extra neutral gauge bosons (Z' 's) arising from extended gauge theories. This will be followed by a description of how to search for a Z' and the resulting Z' discovery reach of the LHC. I will describe various diagnostic measurements to study Z' 's and describe some new observables we have proposed that can distinguish between models that take advantage of the ability to tag 3rd generation fermions.

The vacuum landscape of string theory can solve the cosmological constant problem, explaining why the energy of empty space is observed to be at least 60 orders of magnitude smaller than several known contributions to it. It leads to a 'multiverse' in which every type of vacuum is produced infinitely many times, and of which we have observed but a tiny fraction. This conceptual revolution has raised tremendous challenges in particle physics and cosmology. To understand the low-energy physics we observe, and to test the theory, we will need novel statistical tools and effective theories. We must also solve a long-standing fundamental problem in cosmology: how to define probabilities in an infinite universe where every possible outcome, no matter how unlikely, will be realized infinitely many times. This 'measure problem' is inextricably tied to the quantitative prediction of the cosmological constant.

I will revisit the phenomenology of the radion graviscalar in warped extra dimensions. This particle could be the lightest 'new physics' state to be discovered at the LHC in this type of models. Its phenomenology is very similar to the Standard Model (SM) Higgs, another potentially light scalar particle with which it could actually mix. When SM fields are moved from the boundary to the bulk of the extra dimension, new interesting effects appear in the scalar sector of the model. With a little bit of Higgs-radion mixing, it is possible to enhance importantly some decay channels of the mostly-radion scalar. Moreover, both the Higgs and the radion can now typically mediate Flavor Changing Neutral Currents at tree level. These will impose bounds on the flavor structure of the model, but also allow for interesting probes in current and future collider experiments.

The ATLAS experiment at the Large Hadron Collider (LHC) at CERN is completing final preparations for first high energy collisions in 2009. This talk will cover: the physics motivation of the LHC, highlights of the ATLAS experiment, commissioning, and prospects for new physics discoveries ahead.

An electroweak model in which the masses of the W and Z bosons and the fermions are generated by quantum loop graphs through a symmetry breaking of the vacuum is investigated. The model is based on a regularized quantum field theory in which the quantum loop graphs are finite to all orders of perturbation theory and the massless theory is gauge invariant, Poincaré invariant, and unitary to all orders. The breaking of the electroweak symmetry SUL(2) × UY (1) is achieved without a Higgs particle. A fundamental energy scale ΛW (not to be confused with a naive cutoff) enters the theory through the regularization of the Feynman loop diagrams. The finite regularized theory with ΛW allows for a fitting of low energy electroweak data. ΛW ~ 542 GeV is determined at the Z pole by fitting it to the Z mass mZ, and anchoring the value of sin²θw to its experimental value at the Z pole yields a prediction for the W mass mW that is accurate to about 0.5% without radiative corrections. The scattering amplitudes for WLWL → WLWL and e+e− → W+L W−L processes do not violate unitarity at high energies due to the suppression of the amplitudes by the running of the coupling constants at vertices. There is no Higgs hierarchy fine-tuning problem in the model. The unitary tree level amplitudes for WLWL → WLWL scattering and e+e− → W+L W−L annihilation, predicted by the finite electroweak model are compared with the amplitudes obtained from the standard model with Higgs exchange. These predicted amplitudes can be used to distinguish at the LHC between the standard electroweak model and the Higgsless model.

Over the last two years, the Compact Muon Solenoid (CMS) detector has been installed in the tunnel of the Large Hadron Collider (LHC) at CERN and commissioned to its full functionality. The CMS detector successfully collected beam halo and beam dump data, while the beams were circulating in the LHC in September 2008. After the LHC incident, the commissioning of CMS continued with a one month campaign of continuous cosmic rays data taking at nominal magnetic field. This allowed further tuning of the detector, consolidation of its operation and characterization of its performances. In this talk, the status of the CMS detector and its performance, in view of LHC collisions in 2009, will be described.