UC Davis Particle Physics Seminars

Jet substructure is typically studied using clustering algorithms, such as kT, which arrange the jets' constituents into trees. Instead of considering a single tree per jet, Tuhin Roy proposes that multiple trees should be considered, weighted by an appropriate metric. Then each jet in each event produces a distribution for an observable, rather than a single value. Advantages of this approach include: 1) observables have significantly increased statistical stability; and, 2) new observables, such as the variance of the distribution, provide new handles for signal and background discrimination. For example, we find that employing a set of trees substantially reduces the observed fluctuations in the pruned mass distribution, enhancing the likelihood of new particle discovery for a given integrated luminosity. Furthermore, we introduce a new quantity 'volatility', a cut on which reduces the luminosity needed for a given significance requirement by a factor of two.

Nonlinear realizations describe the low-energy degrees of freedom of strongly coupled theories with spontaneously broken symmetry. When combined with SUSY, the Goldstone bosons of these theories are complex fields with "Goldstone fermion" super-partners. These, in turn, offer novel weakly interacting massive particle (WIMP) dark matter candidates. Flip Tanedo discusses how these can be used in realistic models of dark matter.

Guido D'Amico describes a new model of inflation using branes unwrapping from extra dimensions.

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.

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.

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.

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.

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.

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.

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.