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HEP Seminars & Vivas

20 May 2024

UCL HEP Seminars 2023

: Mieczyslaw Witold Krasny (LPNHE, Sorbonne University Paris & CERN) -- G08, Roberts Building

Gamma Factory -- a tool-driven revolution?

In this talk, I shall present the Gamma Factory project, being developed at CERN over the last 7 years. Its goal is to broaden significantly the present CERN research programme by including a new component which can be open by the novel-type light source. The proposed, partially-stripped-ion-beam-driven light source is the backbone of the Gamma Factory project. It could be realized at CERN by re-using the infrastructure of the already existing accelerators and by profiting from the recent progress in the laser technology. It could extend the scientific life of the LHC storage rings beyond its HL-LHC phase. Gamma Factory could push the intensity limits of the presently operating light-sources by at least 7 orders of magnitude, reaching the flux of up to 10^18 photons/s, in the particularly interesting gamma-ray energy domain of 0.1 — 400 MeV, which is presently nor accessible to the FEL photon sources. The partially stripped ion beams, the unprecedented-intensity energy-tuned gamma beams, together with the gamma-beam-driven secondary beams of polarized positrons, polarized muons, neutrinos, neutrons, and radioactive ions constitute the basic research tools of the Gamma Factory. A broad spectrum of new research opportunities, in a vast domain of uncharted fundamental and applied physics territories, could be opened by the Gamma Factory. Examples of new research opportunities and the status of the project development will be presented in this talk. 

Matthieu Hentz PhD Viva (90 High Holborn)

Examiners: Kris Thielemans (UCL), Mike Taylor (Manchester)

: Julia Harz (Mainz) -- G08 Roberts Building

Confronting Dark Matter Freeze-In during Reheating with Constraints from Inflation

I will discuss the production of Dark Matter (DM) in a minimal freeze-in model, specifically considering a non-instantaneous reheating phase after inflation. I will demonstrate that for low reheating temperatures, bosonic or fermionic reheating from monomial potentials can lead to a different evolution in the DM production and hence to distinct predictions for the parent particle lifetime and mass, constrained by long-lived particle (LLP) searches. I will demonstrate that the extent to which the standard DM freeze-in production can be modified crucially depends on the underlying inflationary model. Our findings underscore the impact of the specific dynamics of inflation on DM freeze-in production and highlight their importance for the interpretation of collider signatures.

: Robert Svoboda (University of California, Davis) -- Physics A1/3 (NOTE UNUSUAL DATE + TIME)

A Hybrid Optical Detector for Neutrino Physics

Many of our breakthroughs in neutrino physics over the last three decades came as a result of new technology that allowed kiloton scale detectors with good particle tracking and energy resolution to be placed deep underground. In this talk, I will present the physics case for Theia, a large hybrid optical detector being proposed for the new Sanford Underground Research Facility (SURF) now under construction. Theia would use novel target materials, fast timing, excellent energy resolution, and photon spectral sorting to enable a broad program in neutrino physics using both the beam from the new Long Baseline Neutrino Facility and from natural neutrino sources. I will review the physics of Theia and give a summary of the several large demonstrators now under construction.

: Graham Van Goffrier (UCL) -- B20, Drayton House

Chiral EFT for 0𝜈𝛽𝛽 Decay

Neutrinoless double-beta (0𝜈𝛽𝛽) decay is an as-yet unobserved nuclear process, which stands to provide crucial insights for model-building beyond the Standard Model of particle physics. Its detection would simultaneously confirm the hypothesis that neutrinos are Majorana fermions, thus violating lepton-number conservation, and provide the first measurement of the absolute neutrino mass scale. In this talk, I will explore the formulation of 0𝜈𝛽𝛽-decay within chiral effective field theory. I will review the justifications for a renormalisation-enhanced "contact term", a short-range two-nucleon effect which is unaccounted for in traditional nuclear approaches to the process, and highlight the most precise computation of its size to date, whose precision is limited by a truncation to elastic intermediate hadronic states. I will then show how an extension of this analysis to a subleading class of inelastic intermediate states can reduce the uncertainty on the contact coefficient by half, delivering an updated value for this important quantity. Such ab initio nucleon-level calculations represent inputs for nuclear many-body methods, and show promise for the resolution of large disparities between 0𝜈𝛽𝛽 nuclear matrix-element estimates from different many-body methods.

: Rebeca Beltran Lloria (Instituto de Física Corpuscular) -- Drayton House B20

Exploring heavy neutral leptons at the LHC: phenomenology beyond the minimal scenario

Searches for heavy neutral leptons (HNLs) at the LHC have received increasing attention recently. In the minimal scenario, HNLs are produced and decay through mixing with active neutrinos. However, in non-minimal extensions, HNL interactions are often described within the context of effective field theories (EFTs). In this seminar, we will explore the signatures of GeV scale HNLs at the LHC and the sensitivity of the future far detectors to different HNL scenarios. We investigate first the production of HNLs directly in proton-proton collisions and second, the scenario where they are sufficiently light to be produced in meson decays, all following an EFT approach, where HNLs are typically long-lived owing to the suppression of the effective interactions by a very high new physics energy scale.

: Brian Foster (Oxford) -- Chris Ingold Ramsay G21

A Hybrid, Asymmetric, Linear Higgs Factory based on Plasma Acceleration - the HALHF concept

Plasma-wakefield acceleration promises orders of magnitude higher gradients than can be achieved via conventional radio-frequency cavities. It is now starting to be used in real user facilities. However, its application to particle-physics colliders has always been complicated by the difficulty in accelerating positrons. I will introduce the basics of plasma wakefield acceleration and explain the difficulty with accelerating positrons. HALHF avoids this by using a conventional linac to accelerate positrons, resulting in an asymmetric-energy, hybrid linear facility which is much smaller, greener and cheaper than any conventional alternative Higgs-factory proposal. I will outline the HALHF layout and principles, and the R&D path to making it a reality.

Saad Shaikh PhD Viva (D17, Physics)

Examiners: Ryan Nichol (UCL), Tony Price (Birmingham)

: Maxwell Chertok (University of California, Davis)

The High Luminosity LHC CMS Outer Tracker Upgrade

The venerable CMS silicon strip tracker has detected charged particles emanating from high energy LHC proton-proton collisions at increasingly higher energies since its debut in 2008. Data from this detector played a major role in the Higgs boson discovery in 2012, and is centrally used in virtually every data analysis. Radiation damaged and with a data readout too slow for inclusion in the Level-1 trigger, the tracker will be replaced for the upcoming High Luminosity LHC (HL-LHC) era. In this talk, I present the HL-LHC Outer Tracker upgrade, a complete replacement for the strip tracker using state-of-the-art components, materials, and electronics. I will focus on OT detector mechanics and the status of the detector production.

: Suchita Kulkarni (University of Graz)

Exploring strongly interacting dark matter

Understanding the laws governing the dark matter dynamics in the Universe is undoubtedly one of the pressing questions inastro-particle physics. While investigations have focused on elementary particles as potential dark matter candidates, an equally interesting possibility arises if dark matter is a composite particle instead. I will take an overview of construction of such dark matter theories by extending the Standard Model with new non-Abelian sectors. I will further illustrate the benefits of connecting these constructions with lattice calculations. I will sketch the avenues of progress and highlight some of the open questions in this direction. Finally, I will exemplify the experimental signatures and impact of dark matter phenomenology.

: Thomas Teubner (Liverpool)

The puzzles of g-2: a status update

In this talk we will review the status of g-2 of the muon. The main emphasis will be on the Standard Model prediction and in particular the determination of the hadronic contributions which are the limiting factor. Recent developments, the current state of the discrepancy between experiment and theory as well as future prospects to resolve the puzzles will be discussed.

: Tomas Gonzalo (KIT)

Global studies of beyond the Standard Model theories: dark matter and supersymmetry

Beyond the Standard Model (BSM) theories aim to explain some of the missing ingredients of the Standard Model, yet no clear evidence for any of them has been found so far. Furthermore, most of these models introduce many new parameters, and make strong predictions for a multitude of experimental searches in particle physics, astrophysics and cosmology. Therefore, contrasting these theories with experimental data requires smart strategies to sample complicated parameter spaces and perform rigorous combinations of experimental results from many sources. In this talk I will introduce GAMBIT, a tool developed to make statistical studies of BSM theories easy and fast, and allow the study of models with many parameters and multiple experimen- tal searches. I will also show the results of various global studies of dark matter and supersymmetric models, where I will focus on their survival status in light of recent data and also on highlighting potential strategies to explore the remaining unconstrained parameter regions.

: Peter Bradshaw (UCL)

Uncovering the Secrets of Symmetries

Symmetries form a foundational part of the modern particle physics programme, from the interactions between particles to the very structure of the space-time they inhabit. We traditionally construct physical models out of representations of a given set of symmetries, associating collections of these representations with primitive physical objects such as the fields of elementary particles. The formalism of representation theory is well-developed and understood mathematically, and physicists have no trouble using them to extract phenomenological predictions. However, the physical interpretation of a representation is often not straightforward and sometimes requires additional notions outside the domain of the symmetry; this is also true when considering the similarities and differences between the physical properties of two different representations. In this talk, we will begin to overcome these difficulties and reveal the hidden physical structures within a representation. First, we will consider the usual approach to representation theory and motivate a new approach in terms of associative algebras. We will then consider an example of such methods applied to the symmetry governing non-relativistic spin and naturally entail the identification of physical properties beyond spin eigenstates. Finally, we will explore the potential that further studies of this kind may have for the myriad symmetries of particle physics

: Asli Abdullahi (Fermilab)

Exotic $e^+e^-$ production at MicroBooNE

We present a phenomenological study of MicroBooNE’s ability to investigate $e^+e^-$ final states produced in dark photon mediated neutrino-upscattering—a beyond-Standard Model process proposed as a solution to the MiniBooNE anomaly. Utilising the similarities shared between the observable signatures of neutrino-induced $e^+e^-$ and single photons originating in the radiative decays of the $\Delta(1232)$ resonance, we consider MicroBooNE’s recent public data from its search for neutral current $\Delta(1232)$ radiative decays to test benchmark models of $e^+e^-$ production, and investigate MicroBooNE’s sensitivity to heavy neutrino models in which the neutrino sector is strongly coupled to a dark sector.

: Sinead Farrington (Edinburgh)

Precision at the LHC

Precision at the LHC will continue to challenge theoretical calculations and tools. In this seminar I will discuss some of the LHC’s precision measurements in recent years, trends in them, and will shamelessly seek audience input to a workshop being organised later this year on the topic of legacy precision at the HL-LHC; what to work towards and how to achieve it.

: Sukanya Sinha (Manchester)

Not a jet all the way - an exploration of strongly interacting dark sector in ATLAS and beyond

As classic WIMP-based signatures for dark matter at the LHC have found no compelling evidence, several phenomenological studies have raised the possibility of accessing a strongly-interacting dark sector through new collider-event topologies. If dark hadrons exist, their evolution and hadronization procedure are currently little constrained. They could decay promptly and result in QCD-like jet structures, even though the original decaying particles are dark sector ones; they could behave as semi-visible jets (SVJ); or they could behave as completely detector-stable hadrons. ATLAS and CMS have recently come up with the first results, but this is only the beginning. In this talk, I will discuss the public results of the first t-channel ATLAS search for semi-visible jets, and the multiple pioneering studies we have performed on SVJ, covering three published or soon to be published papers. Finally, although not related to SVJ, I will cover our published work about an innovative search for dark photons in events with a top quark and a lepton jet

: David Yallup (Cambridge)

Evidence is all you need: Nested Sampling for particle physics

The Bayesian vs. Frequentist debate has raged for many years in particle physics, with the battle lines largely drawn and a set of established techniques falling firmly in favour of frequentist thinking. The same is largely true on the cosmology side of the fence, where the dominant paradigm is decidedly Bayesian. This is a curious fact that is worth considering, how have two adjacent fields that are so similar in ultimate goals and theoretical basis ended up at loggerheads when it comes to inference? To try and unravel some of this mystery, this talk will discuss the ‘Rosetta Stone’ of inference tasks in particle physics – searching for bumps in smoothly falling spectra at colliders. I will aim to extract the signal (the higgs) from the noise (the diphoton background), however in doing so I will really be attempting to extract the signal (why we use Nested Sampling so much in Cosmology) from the noise (the Bayesian vs. Frequentist debate). An “evidence based” approach (powered by Nested Sampling) affords unique perspectives on some long standing inference problems for High Energy Physics.

: Seb Jones (UCL)

The QTNM collaboration: a project for absolute neutrino mass measurement

The observation of neutrino oscillations provides proof of non-zero neutrino masses, something which was not predicted in the minimal Standard Model. However, these same neutrino oscillation experiments do not provide information on the absolute scale of the neutrino masses, which remain unknown. The neutrino masses are most directly accessed through those experiments which measure the shape of the beta-decay energy spectrum. In particular, a technique known as Cyclotron Radiation Emission Spectroscopy (CRES) offers the opportunity to measure neutrino masses lower than the current upper mass limit of 0.8 eV/c^2 achieved by the KATRIN collaboration. The Quantum Technologies for Neutrino Mass (QTNM) collaboration aims to utilise CRES, along with recent breakthroughs in quantum technologies, to build a demonstrator apparatus for measuring the neutrino mass. It is hoped that this demonstrator will make significant contributions towards an experiment with a neutrino mass sensitivity of O(10 meV). I will present an overview of the principles of neutrino mass measurement as well as the QTNM collaboration.

: Dieter Roehrich (Bergen)

Proton-CT - imaging with protons in particle therapy

A novel imaging modality using protons promises to overcome some limitations of particle therapy. Being able to position the Bragg peak accurately inside the tumor is a major advantage of charged particles, but incomplete knowledge about a crucial tissue property, the stopping power, limits its precision. A proton/helium-CT scanner provides direct information about the stopping power and has the potential to reduce range uncertainties significantly. A (clinical) prototype of a proton-CT has been designed and is being constructed in Bergen. The latest developments in Monolithic Active Pixel Sensors (MAPS) technology allow the fabrication of an extremely high granularity, large area silicon/absorber sandwich calorimeter with 41 sensitive layers of MAPS. A complete CT reconstruction of a simulated anthropomorphic paediatric head phantom shows that the concept of a single-sided detector setup and realistic pencil beam parameters gives a spatial uncertainty and a stopping power resolution sufficient for proton therapy treatment planning. The expected performance based on simulations, first beam test results and the status of the construction will be presented.