# Theory Seminars Archive

Thursday, September 28, 2017

Speaker: Ariel Edery

Time: 3:30PM -- 4:30PM

Location: HP4351

Title:  Where does the scale originate from in going from pure R^gravity to the equivalent Einstein gravity?

Abstract: Higher-derivative gravity theories have the well-known problem of not being unitary. However, there is one exception: pure R^2 gravity (R^2 gravity alone). Not only is it unitary but it has recently been shown by a few groups that this theory is equivalent to Einstein gravity with a non-zero cosmological constant minimally coupled to a massless scalar field. The original theory is scale-invariant but the equivalent Einstein theory is not. A crucial question is where does the scale originate from?" We point out that the conformal transformation that is performed in going from R^2 to Einstein gravity naturally selects a background spacetime with non-zero Ricci scalar R. This vacuum state does not preserve the symmetry of the action and the scale symmetry is spontaneously broken. We identify the massless scalar field as the Nambu-Goldstone boson associated with the broken symmetry.

Thursday, September 14, 2017
Speaker: Miki Gintner

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: The direct and indirect limits for the strong BSM SU(2)_L+R vector resonance phenomenology

Abstract: First, a simple effective model for the early LHC signs ofstrongly-interacting new physics including a new SU(2)_L+R vectorresonance triplet will be introduced. Then, the current limits for the model's free parameters based on the latest LHC measurements will be shown. Besides, the low-energy electroweak precision data restrictionsalong with the unitarity limits on the validity of the model will be discussed.

Thursday, July 27, 2017
Speaker: Jérôme Claude

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Search for a doubly charged Higgs boson through vector boson scattering with the ATLAS detector

Abstract: The hierarchy problem suggests that there must be physics beyond the Standard Model (SM) in the electroweak sector. A deviation from SM-predicted values in the Higgs coupling to vector bosons would allow these new physics in the form of an extension to the Higgs sector. The Georgi-Machacek model introduces scalars in higher isospin multiplets to the SM, including a doubly-charged Higgs, while preserving custodial symmetry. This new scalar is potentially accessible as a resonance through charged vector boson scattering (W+ W+ → W+ W+) in the 30 fb-1 of √s = 13 TeV data collected by the ATLAS detector at the Large Hadron Collider (LHC).

This search focuses on the final state of this resonance which includes a pair of same-sign leptons accompanied by a pair of forward jets and missing transverse momentum (MET). Same-sign leptons in the final state are a rare feature of the standard model, and so the background is dominated by charge misidentification. Techniques to reduce these types of background are presented, and the potential for discovery or exclusion is discussed.

Thursday, July 13, 2017

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Flavour change study in the Higgs bosons production at the LHeC

Abstract: TBD

Tuesday, July 4, 2017

Speaker: Jim Cline

Time:  11AM

Location: HP4351

Title: B decay anomalies from nonabelian horizontal symmetry

Abstract:

I consider a theory of flavor in which matter transforms under a local SU(3)_L x SU(3)_R flavor symmetry group, acting in the same way on quarks and leptons. It breaks to a U(1) subgroup whose associated Z' gauge boson can explain the observed anomalies in B->K l l decays. To satisfy ATLAS dilepton and perturbativity constraints, the Z' mass should be not far above 5 TeV, hence close to discovery at LHC. New contributions to B-\bar B and K-\bar K mixing are not much below current limits, and possibly also lepton-flavor violating decays mu -> 3e, tau -> 3l and violation of CKM unitarity.

Thursday, April 13, 2017

Speaker: Josh Berger

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Hydrogen Axion Star: Metallic Hydrogen Bound to a QCD Axion BEC

Abstract: As a cold dark matter candidate, the QCD axion may form Bose-Einstein condensates, called axion stars, with masses around 10−11M⊙. In this paper, we point out that a brand new astrophysical object, a Hydrogen Axion Star (HAS), may well be formed by ordinary baryonic matter becoming gravitationally bound to an axion star. We study the properties of the HAS and find that the hydrogen cloud has a high pressure and temperature in the center and is likely in the liquid metallic hydrogen state. Because of the high particle number densities for both the axion star and the hydrogen cloud, the feeble interaction between axion and hydrogen can still generate enough internal power, around 10^13 W×(ma/5 meV)^4, to make these objects luminous point sources. High resolution ultraviolet, optical and infrared telescopes can discover HAS via black-body radiation.

Thursday, April 6, 2017

Speaker: Masha Baryakhtar

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Searching for Ultralight Particles with Black Holes and Gravitational Waves

Abstract:

The LIGO detection of gravitational waves has opened a new window on the universe. I will discuss how the process of superradiance, combined with gravitational wave measurements, makes black holes into nature's laboratories to search for new light bosons, from axions to dark photons. When a bosonic particle's Compton wavelength is comparable to the horizon size of a black hole, superradiance of these bosons into bound "Bohr levels" extracts energy and angular momentum from the black hole. The occupation number of the levels grows exponentially and the black hole spins down. One candidate for such an ultralight boson is the QCD axion with decay constant above the GUT scale. Current black hole spin measurements disfavor a factor of 30 (>100) in axion (vector) mass; future measurements can provide evidence of a new boson. Particles transitioning between levels and annihilating to gravitons may produce thousands of monochromatic gravitational wave signals, and turn LIGO into a particle detector.

Thursday, March 23, 2017

Speaker: Sean Tulin

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Dark matter self-interactions and small scale structure

Abstract: Astrophysical observations of the structure of galaxies and clusters are no longer simply proving the existence of dark matter (DM), but have sharpened into a tool probing the particle physics of DM. I review small scale structure anomalies for cold DM and their possible implications for DM physics, such as the existence of forces in the dark sector. New results on cluster scales provide a new important handle for constraining DM's interactions, its mass, and the mass of dark mediator particles. I discuss how self-interactions arise in particle physics models and are complementarity with other types of dark matter searches.

Thursday, March 16, 2017

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Searching for compressed spectra

Abstract: I will present two scenarios where standard searches for SUSY at the LHCare not very efficient because the mass splitting between the LSP and the NLSPis small and therefore there is not a lot of MET in the events. I will then use differentobservables to be able to improve the reach of the LHC in this kind of situations.
Thursday, March 9, 2017

Speaker: Yuhsin Tsai

Time: 3:30PM -- 4:30PM

Location: HP4351

Title: Probing dark showers at the LHCb

Abstract: Dark shower is a generic feature of the Hidden Valley (HV)
model, which produces bound states with a high multiplicity, low
masses, and long lifetimes. The showering process can arise, for
example, in Neutral Naturalness models, or in dark matter scenarios
that explain the possible signal of galactic center gamma-ray. A
collider search of such signals requires good invariant-mass and
vertex resolution, as well as a good particle id to veto the
background. I will explain why the LHCb experiment has a great
potential in seeing these dark shower events and compare the estimated
sensitivity to that of the future ATLAS/CMS searches.