UC Davis Particle Physics Seminars

Latest Episodes:

Baryogenesis and Superparter Oscillations in R-symmetric SUSY Oct 15, 2012

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Weak scale SUSY with R-symmetry is a plausible solution to the SUSY flavor problem. In this talk, Yuhsin Tsai discusses two interesting aspects of R-symmetric SUSY models: electroweak baryogenesis and superpartner oscillations. Regarding baryogenesis, the new superpotential couplings between the adjoints and the Higgs can increase the strength of the electroweak phase transition, and large CP-violating phases consistent with electric dipole moments can generate large baryon asymmetry. R-symmetric models can also give spectacular signals at the LHC. If a tiny Majorana mass splits the degenerate mass eigenstates of gauginos and higgsinos, states produced at the LHC can oscillate between the spraticles and their Dirac mass partners. With displaced decays, the oscillation can be visible in the distribution of displaced vertex lengths at the LHC.

Quark/Gluon Tagger in Theory and Data Oct 02, 2012

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Jason Gallicchio explains how distinguishing light-quark jets from gluon jets on an event-by-event basis could significantly enhance the reach for many new physics searches at the Large Hadron Collider. Through an exhaustive search of existing and novel jet substructure observables, we find that a combination of two simple variables, the charge track multiplicity and the p_T-weighted linear radial moment (width), can filter out over 85-95% of the gluon jets while keeping more than half of the light-quark jets. I will discuss applications, address theoretical issues in the definitions of quark and gluon jets, and show progress that ATLAS has made in measuring these observables.

A Complete Model of Low-Scale Gauge Mediation Oct 01, 2012

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Recent signs of a Standard Model-like Higgs at 125 GeV point towards large A-terms in the MSSM. This presents special challenges for gauge mediation, which by itself predicts vanishing A-terms at the messenger scale. In this paper, we review the general problems that arise when extending gauge mediation to achieve large A-terms, and the mechanisms that exist to overcome them. Using these mechanisms, we construct weakly-coupled models of low-scale gauge mediation with extended Higgs-messenger couplings that generate large A-terms at the messenger scale and viable mu/Bmu-terms. Our models are simple, economical, and complete realizations of supersymmetry at the weak scale.

Precision measurement of the W boson mass at CDF Jun 04, 2012

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The W boson mass is a crucial parameter in the standard model of particle physics, providing constraints on the mass of the Higgs boson as well as on new physics models via quantum loop corrections. Bo Jayatilaka describes the measurement of the W boson mass using data corresponding to 2.2/fb of integrated luminosity collected with the CDF II detector. The measurement, performed using electron and muon decays of W bosons, yields a mass of 80387±19 MeV. This represents the single most precise measurement of the W boson mass ever performed, significantly surpassing the precision of all prior measurements combined. An updated world average of the W boson mass, including all recent Tevatron measurements, as well as the resulting standard model implications, is presented.

The ATLAS experiment a the LHC May 22, 2012

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Both the ATLAS and CMS detectors have operated with high efficiency in the first two years of LHC operation and produced an impressive number of physics results. In this talk I will discuss some of the differences between the detectors and describe some unique features of the ATLAS detector and how these features have been exploited in our physics analyses. I will describe some selected recent results and conclude with an overview of the planned, near term ATLAS detector upgrades.

Observing signals of the bulk matter RS model May 21, 2012

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The bulk matter Randall-Sundrum (RS) model is a setup where Standard Model (SM) matter and gauge fields reside in the bulk of 5D warped spacetime while the Higgs field is confined on the IR brane. The wavefunctions of the 1st and 2nd generation matter particles are localized towards the UV brane and those of the 3rd generation towards the IR brane, so that the hierarchical structure of the Yukawa couplings arises geometrically without hierarchy in fundamental parameters. This paper discusses an experimental test of this model in the case where the Kaluza-Klein scale is far above the collider scale, but the model is combined with 5D Minimal SUSY Standard Model (MSSM) and SUSY particles are in the reach of collider experiments. A general SUSY breaking mass spectrum consistent with the bulk matter RS model is considered: SUSY breaking sector locates on the IR brane and its effects are mediated to 5D MSSM through a hybrid of gravity mediation, gaugino mediation and gauge mediation.

A natural 125 GeV Higgs in the NMSSM May 14, 2012

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I look for a golden region in the parameter space of the NMSSM with a low cut-off where a SM-like Higgs with a mass m_h ~ 125 GeV can be obtained in a minimal bottom-up approach of field content.

Like-sign dilepton searches as a probe of new physics with the ATLAS experiment May 08, 2012

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The experimental signature of two leptons with the same electric charge is a great way to search for new physics as the backgrounds from Standard Model processes are quite low and a large variety of new physics models can produce this signature, e.g models with extended Higgs sectors, Supersymmetry, additional quark generations, and many more. Beate Heinemann reviews the ATLAS analyses in this signature based on the 2011 LHC data.

Seiberg duality versus hidden local symmetry May 07, 2012

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It is widely believed that the emergent magnetic gauge symmetry of SQCD is analogous to a hidden local symmetry (HLS). James Barnard explores this idea in detail, deriving the entire (spontaneously broken) magnetic theory by applying the HLS formalism to spontaneously broken SU(N) SQCD. We deduce the Kahler potential in the HLS description, and show that gauge and flavour symmetry are smoothly restored along certain scaling directions in moduli space. We propose that it is these symmetry restoring directions, associated with the R-symmetry of the theory, that allow full Seiberg duality. Reconsidering the origin of the magnetic gauge bosons as the rho-mesons of the electric theory, colour-flavour locking allows a simple determination of the parameter "a". Its value continuously interpolates between a=2 on the baryonic branch of moduli space - corresponding to "vector meson dominance" - and a=1 on the mesonic branch.

Composite Weak Bosons at the LHC May 01, 2012

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Harald Fritzsch discusses a model of composite weak bosons.