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Christopher G. Lester

Publications and source records attributed to Christopher G. Lester.

At least 19 recordsLinked to original sources
arXiv2024

On the spatial distribution of the Large-Scale structure: An Unsupervised search for Parity Violation

We use machine learning methods to search for parity violations in the Large-Scale Structure (LSS) of the Universe, motivated by recent claims of chirality detection using the 4-Point Correlation Function (4PCF), which would suggest new physics during the epoch of inflation. This work seeks to reproduce these claims using methods originating from high energy collider analyses. Our machine learning methods optimise some underlying parity odd function of the data, and use it to evaluate the parity odd fraction. We demonstrate the effectiveness and suitability of these methods and then apply them to the Baryon Oscillation Spectroscopic Survey (BOSS) catalogue. No parity violation is detected at any significance.

astro-ph.CO↗
arXiv2024

Statistical divergences in high-dimensional hypothesis testing and a modern technique for estimating them

Hypothesis testing in high dimensional data is a notoriously difficult problem without direct access to competing models' likelihood functions. This paper argues that statistical divergences can be used to quantify the difference between the population distributions of observed data and competing models, justifying their use as the basis of a hypothesis test. We go on to point out how modern techniques for functional optimization let us estimate many divergences, without the need for population likelihood functions, using samples from two distributions alone. We use a physics-based example to show how the proposed two-sample test can be implemented in practice, and discuss the necessary steps required to mature the ideas presented into an experimental framework. The code used has been made available for others to use.

physics.data-an↗
arXiv2022

Using unsupervised learning to detect broken symmetries, with relevance to searches for parity violation in nature. (Previously: "Stressed GANs snag desserts")

Testing whether data breaks symmetries of interest can be important to many fields. This paper describes a simple way that machine learning algorithms (whose outputs have been appropriately symmetrised) can be used to detect symmetry breaking. The original motivation for the paper was an important question in Particle Physics: "Is parity violated at the LHC in some way that no-one has anticipated?" and so we illustrate the main idea with an example strongly related to that question. However, in order that the key ideas be accessible to readers who are not particle physicists but who are interesting in symmetry breaking, we choose to illustrate the method/approach with a 'toy' example which places a simple discrete source of symmetry breaking (the handedness of human handwriting) within a idealised particle-physics-like context. Readers interested in seeing extensions to continuous symmetries, non-ideal environments or more realistic particle-physics contexts are provided with links to separate papers which delve into such details.

hep-ph↗
arXiv2022

A method to challenge symmetries in data with self-supervised learning

Symmetries are key properties of physical models and of experimental designs, but any proposed symmetry may or may not be realized in nature. In this paper, we introduce a practical and general method to test such suspected symmetries in data, with minimal external input. Self-supervision, which derives learning objectives from data without external labelling, is used to train models to predict 'which is real?' between real data and symmetrically transformed alternatives. If these models make successful predictions in independent tests, then they challenge the targeted symmetries. Crucially, our method handles filtered data, which often arise from inefficiencies or deliberate selections, and which could give the illusion of asymmetry if mistreated. We use examples to demonstrate how the method works and how the models' predictions can be interpreted. Code and data are available at https://zenodo.org/record/6861702.

hep-ph↗
arXiv2022

Hunting for vampires and other unlikely forms of parity violation at the Large Hadron Collider

Non-Standard-Model parity violation may be occurring in LHC collisions. Any such violation would go unseen, however, as searches are for it are not currently performed. One barrier to searches for parity violation is the lack of model-independent methods sensitive to all of its forms. We remove this barrier by demonstrating an effective and model-independent way to search for parity-violating physics at the LHC. The method is data-driven and makes no reference to any particular parity-violating model. Instead, it inspects data to construct sensitive parity-odd event variables (using machine learning tools), and uses these variables to test for parity asymmetry in independent data. We demonstrate the efficacy of this method by testing it on data simulated from the Standard Model and from a non-standard parity-violating model. This result enables the possibility of investigating a variety of previously unexplored forms of parity violation in particle physics. Data and software are shared at https://zenodo.org/record/6827724

hep-ph↗
arXiv2022

Lorentz and permutation invariants of particles III: constraining non-standard sources of parity violation

Comparisons of the positive and negative halves of the distributions of parity-odd event variables in particle-physics experimental data can provide sensitivity to sources of non-standard parity violation. Such techniques benefit from lacking first-order dependence on simulations or theoretical models, but have hitherto lacked systematic means of enumerating all discoverable signals. To address that issue this paper seeks to construct sets of parity-odd event variables which may be proved to be able to reveal the existence of any Lorentz-invariant source of non-standard parity violation which could be visible in data consisting of groups of real non space-like four-momenta exhibiting certain permutation symmetries.

hep-ph↗
arXiv2021

Chiral Measurements

Searches for parity violation at particle physics collider experiments without polarised initial states or final-state polarimeters lack a formal framework within which some of their methods and results can be efficiently described. This document defines nomenclature which is intended to support future works in this area, however it has equal relevance to searches concerned with more conventional (and already well supported) pre-existing tests of parity-violation since it can be viewed as a formal axiomatisation of process of gaining evidence about parity violation regardless of its cause.

hep-ph↗
arXiv2020

Lorentz and permutation invariants of particles I

A theorem of Weyl tells us that the Lorentz (and parity) invariant polynomials in the momenta of $n$ particles are generated by the dot products. We extend this result to include the action of an arbitrary permutation group $P \subset S_n$ on the particles, to take account of the quantum-field-theoretic fact that particles can be indistinguishable. Doing so provides a convenient set of variables for describing scattering processes involving identical particles, such as $pp \to jjj$, for which we provide an explicit set of Lorentz and permutation invariant generators.

hep-ph↗
arXiv2019

Search for Non-Standard Sources of Parity Violation in Jets at $\sqrt s$=8 TeV with CMS Open Data

The Standard Model violates parity, but only by mechanisms which are invisible to Large Hadron Collider (LHC) experiments (on account of the lack of initial state polarisation or spin-sensitivity in the detectors). Nonetheless, new physical processes could potentially violate parity in ways which are detectable by those same experiments. If those sources of new physics occur only at LHC energies, they are untested by direct searches. We probe the feasibility of such measurements using approximately 0.2 inverse femtobarns of data which was recorded in 2012 by the CMS collaboration and made public within the CMS Open Data initiative. In particular, we test an inclusive three-jet event selection which is primarily sensitive to non-standard parity violating effects in quark-gluon interactions. Within our measurements, no significant deviation from the Standard Model is seen and no obvious experimental limitations have been found. We discuss other ways that searches for non-standard parity violation could be performed, noting that these would be sensitive to very different sorts of models to those which our measurements constrain. We hope that our initial studies provide a valuable starting point for rigorous future analyses using the full LHC datasets at 13 TeV with a careful and less conservative estimate of experimental uncertainties.

hep-ex↗
arXiv2019

Biased bootstrap sampling for efficient two-sample testing

The so-called 'energy test' is a frequentist technique used in experimental particle physics to decide whether two samples are drawn from the same distribution. Its usage requires a good understanding of the distribution of the test statistic, T, under the null hypothesis. We propose a technique which allows the extreme tails of the T-distribution to be determined more efficiently than possible with present methods. This allows quick evaluation of (for example) 5-sigma confidence intervals that otherwise would have required prohibitively costly computation times or approximations to have been made. Furthermore, we comment on other ways that T computations could be sped up using established results from the statistics community. Beyond two-sample testing, the proposed biased bootstrap method may provide benefit anywhere extreme values are currently obtained with bootstrap sampling.

physics.data-an↗
arXiv2014

Improving estimates of the number of fake leptons and other mis-reconstructed objects in hadron collider events: BoB's your UNCLE. (Previously "The Matrix Method Reloaded")

We consider current and alternative approaches to setting limits on new physics signals having backgrounds from misidentified objects; for example jets misidentified as leptons, b-jets or photons. Many ATLAS and CMS analyses have used a heuristic matrix method for estimating the background contribution from such sources. We demonstrate that the matrix method suffers from statistical shortcomings that can adversely affect its ability to set robust limits. A rigorous alternative method is discussed, and is seen to produce fake rate estimates and limits with better qualities, but is found to be too costly to use. Having investigated the nature of the approximations used to derive the matrix method, we propose a third strategy that is seen to marry the speed of the matrix method to the performance and physicality of the more rigorous approach.

hep-ph↗
arXiv2013

Properties of MT2 in the massless limit

Although numerical methods are required to evaluate the stransverse mass, MT2, for general input momenta, non-numerical methods have been proposed for some special clases of input momenta. One special case, considered in this note, is the so-called `massless limit' in which all four daughter objects (comprising one invisible particle and one visible system from each `side' of the event) have zero mass. This note establishes that it is possible to construct a stable and accurate implementation for evaluating MT2 based on an analytic expression valid in that massless limit. Although this implementation is found to have no significant speed improvements over existing evaluation strategies, it leads to an unexpected by-product: namely a secondary variable, that is found to be very similar to MT2 for much of its input-space and yet is much faster to calculate. This is potentially of interest for hardware applications that require very fast estimation of a mass scale (or QCD background discriminant) based on a hypothesis of pair production -- as might be required by a high luminosity trigger for a search for pair production of new massive states undergoing few subsequent decays (eg di-squark or di-slepton production). This is an application to which the contransverse mass MCT has previously been well suited due to its simplicity and ease of evaluation. Though the new variable requires a quadratic root to be found, it (like MCT) does not require iteration to compute, and is found to perform better then MCT in circumstances in which the information from the missing transverse momentum (which the former retains and the latter discards) is both reliable and useful.

hep-ph↗
arXiv2013

A search for direct heffalon production using the ATLAS and CMS experiments at the Large Hadron Collider

The first search is reported for direct heffalon production, using 23.3/fb per experiment of delivered integrated luminosity of proton-proton collisions at rootS = 8TeV from the Large Hadron Collider. The data were recorded with the ATLAS and the CMS detectors. Each exotic composite is assumed to be stable on the detector lifetime (tau >> ns). A particularly striking signature is expected. No signal events are observed after event selection. The cross section for heffalon production is found to be less than 64ab at the 95% confidence level.

hep-ph↗
arXiv2013

Significance Variables

Many particle physics analyses which need to discriminate some background process from a signal ignore event-by-event resolutions of kinematic variables. Adding this information, as is done for missing momentum significance, can only improve the power of existing techniques. We therefore propose the use of significance variables which combine kinematic information with event-by-event resolutions. We begin by giving some explicit examples of constructing optimal significance variables. Then, we consider three applications: new heavy gauge bosons, Higgs to $ττ$, and direct stop squark pair production. We find that significance variables can provide additional discriminating power over the original kinematic variables: $\sim$ 20% improvement over $m_T$ in the case of $H\rightarrowττ$ case, and $\sim$ 30% impovement over $m_{T2}$ in the case of the direct stop search.

hep-ph↗
arXiv2012

Finding Higgs bosons heavier than 2 m_W in dileptonic W-boson decays

We reconsider observables for discovering a heavy Higgs boson (with m_h > 2m_W) via its di-leptonic decays h -> WW -> l nu l nu. We show that observables generalizing the transverse mass that take into account the fact that both of the intermediate W bosons are likely to be on-shell give a significant improvement over the variables used in existing searches. We also comment on the application of these observables to other decays which proceed via narrow-width intermediates.

hep-ph↗
arXiv2012

Re-weighing the evidence for a Higgs boson in dileptonic W-boson decays

We reconsider observables for discovering and measuring the mass of a Higgs boson via its di-leptonic decays: H --> WW* --> l nu l nu. We define an observable generalizing the transverse mass that takes into account the fact that one of the intermediate W-bosons is likely to be on-shell. We compare this new variable with existing ones and argue that it gives a significant improvement for discovery in the region m_h < 2 m_W.

hep-ph↗
arXiv2011

The stransverse mass, MT2, in special cases

This document describes some special cases in which the stransverse mass, MT2, may be calculated by non-iterative algorithms. The most notable special case is that in which the visible particles and the hypothesised invisible particles are massless -- a situation relevant to its current usage in the Large Hadron Collider as a discovery variable, and a situation for which no analytic answer was previously known. We also derive an expression for MT2 in another set of new (though arguably less interesting) special cases in which the missing transverse momentum must point parallel or anti parallel to the visible momentum sum. In addition, we find new derivations for already known MT2 solutions in a manner that maintains manifest contralinear boost invariance throughout, providing new insights into old results. Along the way, we stumble across some unexpected results and make conjectures relating to geometric forms of M_eff and H_T and their relationship to MT2.

hep-ph↗