High Energy Physics Seminars This Term

Monte Carlo Event Generators are essential for the analysis and interpretation of Collider data. Event generation starts at energies of several hundred GeV with the hard interaction between quarks and gluons and ends at the scale of several hundred MeV with hadrons, leptons and photons as the final products. This energy gap is bridged by parton shower algorithms, which evolve the products of the hard interaction down to the hadronization scale. In this talk I will discuss recent improvements on the formal accuracy of parton showers and the performance of phase-space integrators, both crucial ingredients for precision physics at the HL-LHC.

The constituents of dark matter are still unknown, and the viable possibilities span a very large mass range. Specific scenarios for a thermal origin of dark matter sharpen this mass range to within about an MeV to 100 TeV. Most of the stable constituents of known matter have masses in the MeV to GeV range, and a thermal origin for dark matter works in a simple and predictive manner in this mass range as well, yet it remains largely unexplored. The Heavy Photon Search (HPS) at Jefferson Lab is a fixed target experiment that uses an electron beam to probe models of thermal dark matter involving sub-GeV dark photons. HPS searches for visibly decaying dark photons through two distinct methods - a resonance search in the e+e- invariant mass distribution and a displaced vertex search for long-lived dark photons. This seminar will give an overview of the theoretical motivations, the main experimental challenges and how they are addressed, the results for the 2016 Engineering Run, and future data and upgrades. In addition, an introduction to the Light Dark Matter eXperiment (LDMX), a planned next generation experiment at SLAC that will search for invisibly decaying dark photons through a missing-momentum experiment, will be presented.

The CMS experiment at CERN will be significantly upgraded during Long Shutdown 3 of the LHC (2026-2028) to operate with a 10-fold increase in luminosity and the associated event pileup of 140-200 proton-proton interactions per bunch crossing of the High-Luminosity LHC (HL-LHC).  The endcap calorimeter region, covering 1.5 < |η| < 3.0, will be exposed to very high radiation levels and to mitigate this, a new calorimeter, the High Granularity Calorimeter (CE), will replace the existing endcap calorimeter.  It will have higher transverse and longitudinal segmentation for both electromagnetic (CE-E) and hadronic (CE-H) sections to facilitate particle-flow reconstruction.  The fine structure of showers can be measured and used to enhance particle identification, whilst still achieving good energy resolution.  The CE-E, and a large fraction of CE-H, will use hexagonal silicon sensors produced from 8-inch wafers as active material, each with several hundreds of individual cells of 0.5-1 sq. cm cell size.  The remainder of the CE-H will use highly-segmented scintillators read out with SiPMs as active material.  An overview of the CE project, including motivation, design, timeline, and expected performance, will be presented in this talk together with a deeper look at the silicon sensor design and performance status on the eve of pre-production.

Higgs bosons produced at high momentum are rare, but measurable. The tails of their kinetic spectrum can provide a unique insight on whether anomalous interactions exist at the TeV scale. The production of Higgs boson pairs is even rarer - about 1000 times less frequent- but it can be enhanced in some new physics models, particularly when the pairs are produced at high momentum. This talk reviews how final states with jets have enabled the exploration of these elusive and possibly anomalous couplings, even in a difficult collider environment that is full of quarks and gluons like the Large Hadron Collider. We examine advances on particle jet identification that have drastically improved our ability to identify boosted Higgs and increased our physics reach. Finally, I will talk about how calorimetry detectors need more spatial precision, greater radiation tolerance, and smarter readout electronics to ensure that this and other search programs can continue at the upcoming High-Luminosity LHC run.

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