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HEP Seminars & Vivas

25 Feb 2024

UCL HEP Seminars 2013

: Lea Reichhart (UCL)

First results from the LUX Dark Matter Experiment

A vast number of astronomical observation point towards the existence of an unknown dark component dominating the matter content of our Universe. The most compelling candidates for dark matter are the Weakly Interacting Massive Particles (WIMPs), which have great potential to be detected in deep underground low background experiments, looking for direct interactions of WIMPs with dedicated target materials. Very recently, the Large Underground Xenon (LUX) experiment, operated in the Davis Cavern of the SURF laboratory, USA, has announced results from its first science run. From an exposure of 85 days, having found no evidence of signal above expected background, LUX has set constraints on scalar WIMP-nucleon interactions above 7.6x10-46 cm2 at 33 GeV/c2 WIMP mass (90% C.L.) - three times more sensitive than any competing experiment. This first result also seriously challenges the interpretation of hints of signal detected in other experiments as arising from low-mass WIMPs.

: Bryan Lynn (UCL/CERN)

Chiral Symmetry Restoration, Naturalness and the Absence of Higgs-Mass Fine-Tuning 1: Global Theories

The Standard Model (SM), and the scalar sector of its zero-gauge-coupling limit -- the chiral-symmetric limit of the Gell Mann-Levy Model (GML) -- have been shown not to suffer from a Higgs Fine-Tuning (FT) problem due to ultraviolet quadratic divergences (UVQD). In GML all UVQD are absorbed into the mass-squared of pseudo Nambu-Goldstone (pNGB) bosons. Since chiral SU(2)_{L-R} symmetry is restored as the pNGB mass-squared or as the Higgs vacuum expectation value (VEV) are taken to zero, small values of these quantities and of the Higgs mass are natural, and therefore not Fine-Tuned. Our results on the absence of FT also apply to a wide class of high-mass-scale (M_{Heavy}>>m_{Higgs}) extensions to a simplified SO(2) version of GML. We explicitly demonstrate naturalness and no-FT for two examples of heavy physics, both SO(2) singlets: a heavy (M_S >> m_{Higgs}) real scalar field (with or without a VEV); and a right-handed Type 1 See-Saw Majorana neutrino with M_R >> m_{Higgs}. We prove that for |q^2| << M_{Heavy}^2, the heavy degrees of freedom contribute only irrelevant and marginal operators. The crucial common property of such high-mass-scale extensions is that they respect chiral SO(2)_{L-R} symmetry. GML is therefore natural and not FT, not just as a stand-alone renormalizable field theory, but also as a low energy effective theory with certain high-mass-scale extensions. Phenomenological consequences include the renewed possibility of thermal lepto-genesis, and subsequent baryon-number asymmetry, in the neutrino-MSM. We conjecture that, since gravity couples democratically to particles, certain quantum gravitational theories that respect chiral symmetry will also retain low-energy naturalness, and avoid FT problems for GML (and maybe the SM). Absent a SM FT problem, there should be no expectation that LHC will discover physics beyond the SM which is unrelated to neutrino mixing, the only known experimental failure of the SM.

: Matt Lilley (Imperial)

Feeling the Fusion Burn

The age of fusion energy is almost upon us, creating and sustaining hot plasmas of 150 million degrees is now a routine operation performed all around the world. We are ready for next big challenge - the burning plasma - in which the fusion reactions are self sustaining. This is a highly non thermal system which is prone to instability. The nonlinear character of the instability determines the fate of the plasma, either ignition or a mere fizzle. In this presentation we will explore the physical processes behind these burning plasma instabilities and discus the challenges that lie ahead.


: Nikos Konstantinidis (UCL) — CANCELED!

The High Luminosity LHC programme

As the LHC machine and experiments are preparing frantically to start data taking at design energy and luminosity (and slightly above), an equally intense and exciting programme of R&D and physics studies is ongoing for the High Luminosity (HL-) LHC project, proposed to start in about 10 years, that would deliver 3000/fb to each general purpose detector by the mid-2030s. I will discuss the science case for HL-LHC, the challenges for the accelerator and the experiments, and the ongoing R&D, particularly on the tracking and triggering systems of the experiments




: Gabriel Facini (CERN)

H -> bb in ATLAS

Since the discovery of a Higgs-­like boson by the ATLAS and CMS experiments at the LHC, the emphasis has shifted towards measurements of its properties and the search in more challenging channels in order to determine whether the new particle is the Standard Model (SM) Higgs boson. Of particular importance is the direct observation of the coupling of the Higgs boson to fermions. A comprehensive review of the latest ATLAS result in the search for the Higgs boson decaying to a b-­quark pair int associated production channel will be given.




: Lauren Tompkins (University of Chicago)

FTK: A hardware-based track finder for the ATLAS trigger

The spectacular performance of the LHC machine challenged the ATLAS and CMS detectors to contend with an average of 25 proton-proton interactions per beam crossing in 2012. Projections for 14 TeV running in 2015 and beyond suggest that the detectors should prepare for up to 80 interactions per crossing. In these dense environments, identifying the physics objects of interest, such as isolated leptons, taus and b-jets is of paramount importance for a successful physics program. The ATLAS experiment is developing a hardware based track finder, FTK, which will perform full silicon detector tracking within 100 microseconds of a Level 1 trigger accept at luminosities of 3x10^34 cm^-2 s^-1, significantly improving the track-based isolation, secondary vertex tagging and track-based tau finding done at Level 2. I will discuss the FTK design and performance prospects, as well as report on successful prototype tests completed thus far.





: Bhupal Dev, University of Manchester

Heavy Neutrino Searches at the LHC

One of the simplest extensions of the Standard Model to explain the non-zero neutrino masses is to introduce heavy neutrinos. In this talk, we will review the existing experimental constraints on the masses and mixing of these heavy neutrinos. We will then discuss their ongoing searches at the LHC and some recent efforts to improve their sensitivity.





: 1st Year Student PhD Talks

B05 Lecture Theatre in the Chadwick Building.

: Group Day + drinks (E1)

B17 (Basement) 1-19 Torrington Place.

: 2nd Year Student PhD Talks

E3/E7 Physics and Astronomy Department.

: Prof. Jose Valle, (IFIC/CSIC - U. Valencia)

Neutrinos and Dark Matter

I will review the status of neutrino mass and mixing parameters, theoretical modeling and cosmological implications. In particular I discuss how neutrino mass and dark matter may be closely connected and indicate possible direct, indirect and collider detection prospects.

: Brian Rebel, Fermilab

Liquid Argon Detectors at Fermilab: From R&D to LBNE

Liquid argon time projection chambers (LArTPCs) are an exciting new technology for neutrino detectors. This technology provides excellent position resolution that rivals bubble chamber images, but in a digital format. The striking advantage of liquid argon time projection chambers for neutrino physics is the ability to distinguish between electrons, produced in charged current interactions, and gammas, produced by the decay of neutral pions created in neutral current interactions, with high efficiency. This talk will outline the Fermilab R&D program aimed toward development of the multi-kiloton LBNE detector for long baseline neutrino physics. Results from the various aspects of the program will be presented, as well as the status of LBNE.

: Alexander Grohsjean, DESY

A quark comes of age: latest highlights in top quark physics.

The discovery of the top quark in 1995 at the Fermilab Tevatron collider was a remarkable confirmation of the standard model of particle physics. Its short lifetime provides the possibility to probe the properties of a bare quark With increasingly large integrated luminosities, the characteristics of this particle, as well as its production and decay properties have been measured with ever greater precision. The analysis of top-quark events triggered the development of new analysis tools and offered an excellent starting point for searches of new phenomena. In this summary, following a short historic perspective, I present recent measurements from the D0 experiment, as well as new ATLAS results from the LHC at 7 and 8 TeV.

: Jonathan Hays, Queen Mary

LHC Higgs results

Last year the ATLAS and CMS experiments announced the discovery of a new particle with a mass of around 126 GeV and a strong candidate for being a Higgs boson. Measurements with the full 2011+2012 dataset have further confirmed this. The latest results in experimental Higgs physics are presented from both experiments, concentrating on the new particle. This includes the continuing search for signals in those modes yet to find evidence for the new particle, property measurements in the diboson modes, and a variety of global fits to data across different channels to investigate compatibility with the Standard Model. Additionally, the compatibility of the results across experiments will be briefly discussed along with some thoughts on the outlook for the Higgs programme at the LHC.


: Dr Will Thomas, Centre for the History of Science, Technology and Medicine, Imperial College

Problems in Particle Detection 1930-1950: New Ways to Talk about the History of Physics

Early developments in particle physics were based on the analysis of cosmic rays and radioactive materials before these sources were supplanted by high-energy accelerators circa 1950. Commonly used particle detection technologies included cloud chambers, arrays of coincidence counters, and, after 1945, nuclear emulsions. These technologies all yielded fairly imprecise information about the particles passing through them, necessitating experimenters to deploy strategies to arrive at what they viewed as legitimate interpretations of events. These strategies included using inference to establish what sorts of particles were being detected, the aggregation of evidence, and an increasingly intensive use of nuclear physics knowledge to narrow a range of possible interpretations. It will be suggested that articulating the nature of these strategies, and paying attention to how experimenters deployed them, allows for a good way of discussing historical experimenters’ skill, certainly over and above a simple cataloguing of their discoveries.


: Gino Isidori, INFN, Frascati National Laboratories

Standard Model and beyond after the the Higgs discovery

We discuss the implications of the recent Higgs discovery, and particularly of the Higgs mass measurement, for the stability of Higgs potential and, more generally, for the completion of the Standard Model at high energies.


: Dr. Bela Majorovits, Max-Planck-Institut für Physik

Understanding Neutrinos?GERDA and the Neutrinoless Double Beta-Decay

Observation of neutrinoless double beta (0vbb) -decay could answer the question whether Neutrinos are their own anti-particles or not and could yield information on the absolute mass scale of neutrinos. The most stringent half-life limit for 76Ge is T1/2>1.9 1025 years. This can be translated to the lowest present limit for the effective Majorana neutrino mass of < 0.3eV. Part of the Heidelberg-Moscow collaboration claims to have observed 0vbb-decay in 76Ge with T1/2=1.2 1025 years, however this result is controversial. A short motivation for 0vbb-decay searches will be given. The principle of 0vbb-searches utilizing High Purity Germanium enriched in the isotope 76Ge detectors will be introduced. The general design features of the GERDA experiment - designed to confirm or refute the claim within one year of measurement will be shown. Results from the GERDA commissioning runs and the status of GERDA data taking with enriched detectors will be discussed. Plans and status of preparations of the second phase of the GERDA experiment will be shown.


: Mat Charles, Oxford

Charm results from LHCb

Highlights from LHCb's charm physics programme are presented, including searches for the highly suppressed decays D+ -> pi+ mu- mu+, D+ -> pi- mu+ mu+, and D0 -> mu- mu+; a measurement of meson mixing in D0 -> K+ pi-; and a search for CP violation in two-body D0 decays.


: Prof. Chris Mabey, Middlesex Business School

Big Lessons from Big Science

The ATLAS collaboration comprises 3000 physicists from 140 Institutes in 37 countries collaborating on a ‘big science’ project based at CERN near Geneva. As a loosely-coupled, global network of knowledge activists working at the forefront of science, it is prototypical of many knowledge-intensive agencies and firms. What can be learnt from this unusual collaboration about the way tacit knowledge is surfaced and exchanged across professional, cultural and geographic boundaries? ATLAS is feted as a remarkably democratic and highly productive partnership. How does it achieve this and what are the lessons for the effective leadership of knowledge? Chris will share insights from his recent ESRC-funded project (2009-12).


: Prof. Robert Thorne, UCL

Parton Distribution Functions at the LHC

I discuss the current status of parton distributions, compare to LHC data and present the range of predictions for LHC processes. Some significant discrepancies are found between different PDF sets, particularly regarding predictions for Higgs boson cross sections and the asymmetry between W^+ and W^- production. I examine possible causes for this, concentrating on issues of parameterisation dependence and the treatment of heavy flavours in the fits.


: Dr. Bobby Acharya, Kings College London

Generic Predictions from string/M theory for Particle Physics and Dark Matter


: Dr. Ricardo Silva, Department of Statistical Science, UCL

The Structure of the Unobserved

Hidden variables are important components in many multivariate models, as they explain dependencies among recorded variables and may provide a compressed representation of the data. In this talk, I will provide some overview of my line of work on how latent structure can be exploited in machine learning and computational statistics applications. In particular, we will go through the following topics: 1. How to measurement error problems have a causal interpretation and what can potentially be done to identify probabilistic and causal relations among variables that cannot be recorded without error 2. How dependencies among interacting individuals in a network can be explained by hidden common causes and what their roles are in prediction problems 3. How measurements can be compressed into fewer items without losing relevant information from the data, as postulated from a latent variable model, with applications in social sciences


: Prof. Buzz Baum, LMCB, UCL

A noisy path to order: refinement of a developing tissue

In my talk I will discuss the process of tissue refinement, whereby an ordered epithelial is generated from an initially disordered state through noisy processes that cause cells to compete for space and fate.


: Jennifer Smillie, (University of Edinburgh)



: Dr. Maurizio Piai, Swansea

Holographic techni-dilaton

I review the status of theoretical and phenomenological studies on the holographic techni-dilaton, a light composite scalar present in the spectrum of a class of strongly-coupled models of electroweak symmetry breaking. The experimental signatures of such scalar are similar to those of the Higgs particle of the minimal version of the Standard Model, with important observable differences is some of the search channels. The 125-126 GeV scalar discovered by ATLAS and CMS could be such particle, and I will discuss how to test this hypothesis in future theoretical as well as experimental studies.


: Prof. Jon Butterworth, UCL

Standard Model physics

I'll review a selection of LHC measurements of jets, photons and weak bosons, show comparisons to Standard Model predictions, and discuss some lessons learned and future prospects.