Events at Physics |
Events During the Week of May 7th through May 14th, 2023
Sunday, May 7th, 2023
- Academic Calendar
- Final exams
- Abstract: *Note: actual end time may vary.* URL:
Monday, May 8th, 2023
- Academic Calendar
- Final exams
- Abstract: *Note: actual end time may vary.* URL:
- Graduate Program Event
- PhD Final Defense
- Higgs production with a top quark pair in the diphoton channel, di-Higgs production in the bb-diphoton channel, and Inner Tracker upgrade with the ATLAS detector at the Large Hadron Collider
- Time: 2:00 pm - 3:30 pm
- Place: 4274 Chamberlin
- Speaker: Alex Zeng Wang, Physics Graduate Student
- Abstract: Since the discovery of the Higgs boson in 2012, the diphoton (γγ) decay channel of the Higgs boson has been one of the most potent channels due to the contrast between the smoothly falling background and sharply peaked signal in the diphoton invariant mass.
In 2018, with up to 80 fb−1 of data at √s = 13 TeV, ATLAS observed Higgs boson production in association with a pair of top quarks (ttH) with a significance of 5.8σ by combining measurements in the γγ, bb, ZZ, and multi-lepton Higgs decay channels. The ttH production cross-section was measured to be 670 ± 90 (stat) +110 -100 (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1σ.
With 140 fb−1, a search for non-resonant Higgs pair production in the bbγγ final state was performed. No significant signal was observed and upper limits at 95% confidence level were set. The observed limit on the SM cross-section was 130fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7].
Both the ttH (H → γγ) and HH → bbγγ analyses will benefit tremendously from the increased statistics expected from the High-Luminosity LHC (HL-LHC). To ensure the continued efficiency of the detector in the harsh HL-LHC environment, ATLAS will install a new Inner Tracker (ITk) consisting of silicon pixel sensors in its innermost layer. At SLAC National Accelerator Laboratory, a variety of electrical tests are performed for the construction of a prototype integrated pixel system, in order to provide early feedback and validation of the ITk design. - Host: Professor Sau Lan Wu
- Graduate Program Event
- PhD Final Defense
- Investigation of Higgs Boson Decaying to Di-muon and Dark Matter Produced in Association with a Higgs Boson decaying to Di-b-quark
- Time: 3:30 pm - 5:00 pm
- Place: 4274 Chamberlin
- Speaker: Jay Chan, Physics Graduate Student
- Abstract: The discovery of the Standard Model (SM) Higgs boson by ATLAS and CMS at the LHC in 2012 marked a major milestone in particle physics. However, many questions remain unanswered, which has led to an active research program to search for either rare SM phenomena or Beyond Standard Model (BSM) physics that involve the Higgs boson. In this dissertation, I present two example searches involving the Higgs boson, using proton-proton (pp) collision data collected by the ATLAS detector.
The first search tackles the problem of how the SM Higgs couples to the second-generation fermions. It searches for the dimuon decay of the SM Higgs boson (H →μμ) using data corresponding to an integrated luminosity of 139 fb−1 collected by the ATLAS detector in pp collisions at √s = 13 TeV at the LHC. To identify this rare decay, we train boosted decision trees to separate signal and background. We obtain an observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV of 2.0σ (1.7σ). The observed upper limit on the cross-section times branching ratio for pp →H →μμ is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a H → μμ signal assuming the absence (presence) of a SM signal is 1.1 (2.0). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is μ = 1.2 ±0.6.
In the second search, we look for Dark Matter produced in association with a Higgs boson decaying to b-quarks. This search uses the same dataset as the H → μμ search and targets events that contain large missing transverse momentum and either two b-tagged small-radius jets or a single large-radius jet associated with two b-tagged subjets. We split events into multiple categories that target different phase spaces of the Dark Matter signals. We do not observe a significant excess from the SM prediction. We interpret the results using two benchmark models with two Higgs doublets extended by either a heavy vector boson Z′ (Z′−2HDM) or a pseudoscalar singlet a (2HDM+a) that provide a Dark Matter candidate χ. For Z′−2HDM, the observed limits extend up to a Z′ mass of 3.1 TeV at 95% confidence level for a mass of 100 GeV for the Dark Matter candidate. For 2HDM+a, we exclude masses of a up to 520 GeV and 240 GeV for tan β = 1 and tan β = 10, respectively, and for a Dark Matter mass of 10 GeV. Additionally, we set limits on the visible cross sections, which range from 0.05 fb to 3.26 fb, depending on the regions of missing transverse momentum and b-quark jet multiplicity.
In addition to the two physics analyses, I present a new method to correct data for the detector effect, referred to as unfolding, which is a key procedure in the high energy experiments. This new unfolding method allows to unfold data without having any artificial binning and is also able to profile nuisance parameters simultaneously, which provides much higher flexibility and increases the reusability for different downstream tasks. It will benifit any future analyses including Higgs physics and Dark Matter searches. - Host: Sau Lan Wu
Tuesday, May 9th, 2023
- Academic Calendar
- Final exams
- Abstract: *Note: actual end time may vary.* URL:
- Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
- Neutron Star Weather Forecasting
- Time: 2:00 pm
- Place: Join Zoom Meeting Meeting ID: 912 3071 4547
- Speaker: Joonas Nättilä , Columbia University, Flatiron Institute
- Abstract: Neutron stars are extremely compact stellar remnants with a mass of ~1.5 Msun and a radius of ~12km. They have an upper layer consisting of a plasma atmosphere and a Coulomb-liquid ocean. Like any stratified fluid envelope on a rotating spherical body, the layers will develop complex lateral motions---weather & climate. I will use well-known results from geophysical fluid dynamics to predict what kind of "weather" we can expect on neutron stars. Understanding the spatiotemporal variability of atmospheres and oceans on neutron stars is important for interpreting observations of their X-ray emissions.
- Host: A. Baha Balantekin
- Graduate Program Event
- PhD Final Defense
- Mathematics of evolving ecologies in chemical reaction networks and the origins of biochemical life
- Time: 3:00 pm - 5:00 pm
- Place: 4274 Chamberlin
- Speaker: Praful Gagrani, Physics Graduate Student
- Abstract: Modern physics and biology satisfactorily explain the passage from the Big Bang to the formation of Earth and the first cells to present-day life, respectively. However, the origins of biochemical life still remain an open question. Any answer to the question must explain how an evolving ecology of polymers of ever-increasing length could come about on a planet that otherwise consisted only of small molecules. In this work, we use chemical reaction network theory to develop a formalism for defining a notion of ecology as well as different categories of evolution. We then construct abstract polymer models that can exhibit dynamics such that attractors in the polymer composition space with a higher average polymer length are also more probable. Furthermore, we demonstrate a polymerization model in two monomers that exhibits historical contingency. While this work provides a step forwards towards understanding the origins of biochemical life, we conclude with a discussion of the limitations of our formalism and directions where future work is needed.
- Host: Eric Smith
Wednesday, May 10th, 2023
- Academic Calendar
- Final exams
- Abstract: *Note: actual end time may vary.* URL:
Thursday, May 11th, 2023
- Academic Calendar
- Final exams
- Abstract: *Note: actual end time may vary.* URL:
- Graduate Program Event
- PhD Final Defense
- New Approaches to Transverse NMR Gyroscopes
- Time: 9:00 am - 11:00 am
- Place: 5310 Chamberlin or
- Speaker: Susan Sorensen, Physics Graduate Student
- Abstract: In this thesis, I describe three approaches to the continuous operation of a transverse spin-exchange optically pumped NMR gyroscope. I demonstrate the simultaneous, continuous excitation of $^{131}$Xe and $^{129}$Xe using first modulations of the optical pumping (polarization modulation, PM), then using modulations of the applied bias field (pulse density modulation, PDM), and then using a combined approach that includes both polarization and field modulations (hybrid operation). I describe the theory behind the operation of the device, and the apparatus used to perform the experiments presented in this thesis. I then present results for each of the three operation modes - PM, PDM, and hybrid operation. For each, I give several key performance metrics, including linewidth, signal-to-noise ratio, field suppression factor, cross talk, angle-random walk, and bias instability. The field suppression factor and cross talk are unique metrics which have generally been ignored in this field of study. This thesis includes the best bias instability ever measured with this device, at $\sim$420~nHz. I close with a comparison of the three methods and a discussion of proposed future works.
- Host: Thad Walker
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Gravitational Wave Science with Gamma Rays
- Time: 2:30 pm - 3:30 pm
- Place: CH4274/https://wipac-science.zoom.us/j/98113285101?pwd=RlNKY2I4Uzg5K2QxVWJmZTJ4eVpWQT09
- Speaker: Matthew Kerr , US Naval Research Laboratory
- Abstract: The Fermi Large Area Telescope (LAT) has now detected gamma-ray pulsations from over 100 millisecond pulsars, and its <300ns times tamping and nearly-unchanged experimental setup enable high-precision pulsar timing over its 15-year baseline. It is thus a "Gamma-ray Pulsar Timing Array" (PTA). Gamma rays are unaffected by propagation through the ionized interstellar medium, and the high-energy data thus enable the separation of propagation effects from the "spin noise" intrinsic to pulsars, perhaps arising from crustal superfluid turbulence. This capability in turns provides a clean measurement the correlated signals expected to be induced by nHz gravitational waves from merging supermassive black holes. I will compare this approach with radio PTAs and present recent results from the Gamma-ray PTA. In a second part, I will also present an overview and early results from Glowbug, a sensitive gamma-ray burst experiment built at NRL and recently launched to the International Space Station. Its large area and sensitive onboard burst detection algorithm will improve the odds of detecting short gamma-ray bursts associated with neutron star mergers, and its primary mission will completely overlap the O4 observing run of LIGO/Virgo/Kagra.
- Host: Ke Fang
Friday, May 12th, 2023
- Graduate Program Event
- PhD Final Defense
- Pressure-driven tearing and energy transport in finite beta Reversed Field Pinch computations
- Time: 10:00 am - 12:00 pm
- Place:
- Speaker: Urvashi Gupta, Department of Physics Graduate Student
- Abstract: Pressure-driven effects on magnetic relaxation dynamics and energy transport of an inductively-driven Reversed-Field Pinch device are investigated in a 3D cylindrical magnetohydrodynamics model. Pressure-driven dynamics in RFPs are often assumed to be small. However, unfavorable average curvature in RFPs means that pressure does influence tearing and consequently transport along stochastic field lines. In this work, nonlinear NIMROD computations are applied to model the RFP at experimentally relevant plasma-$\beta$ values. Self-consistent evolution of fluctuations from an Ohmic steady state that includes thermal conduction and heating results in tearing dominant relaxed states with sustained reversal. Linear computations are applied to profiles extracted from the relaxed nonlinear state to study the sources of free energy for the fluctuations. The parallel current drive and pressure-curvature drive for the obtained linear eigenfunctions are found to be comparable and only the sum of both the terms surpasses the stabilizing contribution to drive tearing modes. Energy transport via fluctuation-induced conduction and convection is computed for the nonlinear relaxed states and qualitative agreement with recent experimental results is observed. The heat flux densities are further decomposed to assess the significance of different orders of correlations. The implications of artificial particle diffusion on convective transport and the overall relaxed state are also discussed for this self-consistent MHD model.
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Probing The Impact of Cluster Weak Lensing Bias On DES Y1 Cluster Cosmological Results
- Time: 12:00 pm - 1:00 pm
- Place: Chamberlin 5280
- Speaker: Johnny Esteves, University of Michigan
- Abstract: Over the last two decades of large photometric surveys, optical cluster cosmology has presented a promising way to improve cosmological measurements and our current understanding of dark matter and dark energy. The Dark Energy Survey (DES) Y1 cluster's analysis has revealed a new systematic effect that affects weak lensing measurements, called weak lensing bias. The weak lensing bias arises from the correlation between the mass proxy (such as richness or λ) and the lensing profile for a given mass. The effect is mainly caused by the presence of projected red galaxies along the line of sight, which can bias the mass estimate. However, how this systematic effect impacted the Y1 cosmology is still unclear.
In this talk, I will present a novel forward modeling methodology that was developed specifically for Y3 and outline the technical computing challenges involved in solving multi-dimensional integrals. These challenges have been overcome by utilizing graphics processing units (GPUs), which provide exceptional computational speed and accuracy. In addition, I will present my work on the commissioning of the Rubin Observatory, which will provide invaluable data for advancing our cosmological understanding of the Universe. - Host: Keith Bechtol
- Graduation Event
- Graduate Celebration Event
- Time: 3:00 pm - 5:15 pm
- Place: 2241 Chamberlin Hall
- Speaker: Sharon Kahn and Evan Heintz
- Abstract: Come celebrate and congratulate our 2023 graduates from both the undergraduate and Ph.D. Programs! We will celebrate the Ph.D. graduates from 3:00 - 3:30, enjoy treats and ice cream from 3:30 - 4:00, and then celebrate the undergraduates from 4:00 - 5:15.
- Academic Calendar
- Graduate School Spring 2023: Master's degree deadline
- Time: 4:00 pm - 5:00 pm
- Abstract: Degree candidates must complete all steps: CONTACT: 262-2433, gsacserv@grad.wisc.edu
Saturday, May 13th, 2023
- No events scheduled
Sunday, May 14th, 2023
- Academic Calendar
- Graduate School Spring 2023: Doctoral degree deadline
- Time: 11:55 pm - 12:55 am
- Abstract: Degree candidates must complete all steps: CONTACT: 262-2433, gsacserv@grad.wisc.edu