Effective Field Theories for Physics Beyond the Standard Model

The wealth of data gathered by the LHC has tested the Standard Model to an unprecedented level, with searches for physics beyond the Standard Model placing strong constraints on new physics. As these constraints become more stringent, it is increasingly likely that physics beyond the Standard Model will be heavy. An effective field theory (EFT) provides a powerful framework for capturing the indirect effects of heavy new physics on low-energy observables. This thesis will explore the use of EFTs in the search for new physics beyond the Standard Model, taking three different perspectives. Firstly, we will perform a global analysis of the dimension-6 Standard Model Effective Field Theory (SMEFT) using data from the Higgs, top, diboson and electroweak sectors, combining these four data sectors in a global fit for the first time. We will assess the interplay between these sectors, shedding light on the interplay between the Higgs and top sectors in particular. By comparing with previous fits, we will highlight the increase in sensitivity to the SMEFT provided by new data from Run II of the LHC. We will assess the impact of our fit on UV completions of the SMEFT by considering simple single-field extensions of the Standard Model, matched to the SMEFT. Finally, we will scan through all 2-, 3-, 4- and 5-parameter combinations of SMEFT Wilson coefficients, calculating the pull from the Standard Model of each combination. Doing so produces a broad and model-independent search for signs of new physics, complementing our global fit. Secondly, we will produce a simultaneous determination of the parton distribution functions (PDFs) and the SMEFT using data from the high-mass tails of Drell-Yan distributions. These observables provide crucial constraints on energy-growing four- fermion EFT operators, while also constraining light quark PDFs in the large-x region. By neglecting this overlap and fitting PDFs using Standard Model assumptions, we may be reabsorbing new physics effects into the PDFs. By performing a simultaneous fit we will assess the interplay between the PDF and EFT effects. Doing so using both LHC data as well as projections for the High Luminosity LHC will allow us to quantify the impact of a consistent joint determination on the EFT constraints, both now and in the high-luminosity regime. Finally, we will turn to future hadron colliders in the hope that these may be able to directly detect physics beyond the Standard Model. The weak effective field theory has been used to parametrise hints at indirect evidence for new physics from measurements of rare B meson decays at experiments such as LHCb. The resulting shifts of the Wilson coefficients of the weak effective theory can be reproduced by extending the SM by a scalar leptoquark with flavour non-universal couplings to quarks and leptons. We will create projections for such a leptoquark, addressing the question: if a scalar leptoquark exists and can provide a solution to the neutral current B anomalies, could it be detected at future hadron colliders? By creating projections for the discovery potential of the High-Luminosity LHC and Future Circular Collider, we hope to strengthen the case for these future colliders.George and Lillian Schiff Studentshi

On the coupling of axion-like particles to the top quark

In this paper we explore the coupling of a light axion-like particle (ALP) to top quarks. We use high-energy LHC probes, and examine both the direct probe to this coupling in associated production of a top-pair with an ALP, and the indirect probe through loop-induced gluon fusion to an ALP leading to top pairs. Using the latest LHC Run II data, we provide the best limit on this coupling. We also compare these limits with those obtained from loop-induced couplings in diboson final states, finding that the $t\bar t$+MET channel is the best current handle on this coupling.Comment: 36 pages, 16 figure

Sensitivity of future hadron colliders to leptoquark pair production in the di-muon di-jets channel

Abstract: We estimate the future sensitivity of the high luminosity (HL-) and high energy (HE-) modes of the Large Hadron Collider (LHC) and of a 100 TeV future circular collider (FCC-hh) to leptoquark (LQ) pair production in the muon-plus-jet decay mode of each LQ. Such LQs are motivated by the fact that they provide an explanation for the neutral current B-anomalies. For each future collider, Standard Model (SM) backgrounds and detector effects are simulated. From these, sensitivities of each collider are found. Our measures of sensitivity are based upon a Run II ATLAS search, which we also use for validation. We illustrate with a narrow scalar (‘S3’) LQ and find that, in our channel, the HL-LHC has exclusion sensitivity to LQ masses up to 1.8 TeV, the HE-LHC up to 4.8 TeV and the FCC-hh up to 13.5 TeV

Sensitivity of future hadron colliders to leptoquark pair production in the di-muon di-jets channel

Abstract: We estimate the future sensitivity of the high luminosity (HL-) and high energy (HE-) modes of the Large Hadron Collider (LHC) and of a 100 TeV future circular collider (FCC-hh) to leptoquark (LQ) pair production in the muon-plus-jet decay mode of each LQ. Such LQs are motivated by the fact that they provide an explanation for the neutral current B-anomalies. For each future collider, Standard Model (SM) backgrounds and detector effects are simulated. From these, sensitivities of each collider are found. Our measures of sensitivity are based upon a Run II ATLAS search, which we also use for validation. We illustrate with a narrow scalar (‘S3’) LQ and find that, in our channel, the HL-LHC has exclusion sensitivity to LQ masses up to 1.8 TeV, the HE-LHC up to 4.8 TeV and the FCC-hh up to 13.5 TeV

Top, Higgs, diboson and electroweak fit to the Standard Model effective field theory

Abstract: The search for effective field theory deformations of the Standard Model (SM) is a major goal of particle physics that can benefit from a global approach in the framework of the Standard Model Effective Field Theory (SMEFT). For the first time, we include LHC data on top production and differential distributions together with Higgs production and decay rates and Simplified Template Cross-Section (STXS) measurements in a global fit, as well as precision electroweak and diboson measurements from LEP and the LHC, in a global analysis with SMEFT operators of dimension 6 included linearly. We present the constraints on the coefficients of these operators, both individually and when marginalised, in flavour-universal and top-specific scenarios, studying the interplay of these datasets and the correlations they induce in the SMEFT. We then explore the constraints that our linear SMEFT analysis imposes on specific ultra-violet completions of the Standard Model, including those with single additional fields and low-mass stop squarks. We also present a model-independent search for deformations of the SM that contribute to between two and five SMEFT operator coefficients. In no case do we find any significant evidence for physics beyond the SM. Our underlying Fitmaker public code provides a framework for future generalisations of our analysis, including a quadratic treatment of dimension-6 operators

Hide and seek: how PDFs can conceal New Physics

The interpretation of LHC data, and the assessment of possible hints of new physics, require the precise knowledge of the proton structure in terms of parton distribution functions (PDFs). We present a systematic methodology designed to determine whether and how global PDF fits might inadvertently 'fit away' signs of new physics in the high-energy tails of the distributions. We showcase a scenario for the High-Luminosity LHC, in which the PDFs may completely absorb such signs of new physics, thus biasing theoretical predictions and interpretations. We discuss strategies to single out the effects in this scenario, and disentangle the inconsistencies that stem from them. Our study brings to light the synergy between the high luminosity programme at the LHC and future low-energy non-LHC measurements of large-$x$ sea quark distributions. The analysis code used in this work is made public so that any users can test the robustness of the signal associated to a given BSM model against absorption by the PDFs.Comment: 48 pages, 23 figure

Parton distributions in the SMEFT from high-energy Drell-Yan tails

Abstract: The high-energy tails of charged- and neutral-current Drell-Yan processes provide important constraints on the light quark and anti-quark parton distribution functions (PDFs) in the large-x region. At the same time, short-distance new physics effects such as those encoded by the Standard Model Effective Field Theory (SMEFT) would induce smooth distortions to the same high-energy Drell-Yan tails. In this work, we assess for the first time the interplay between PDFs and EFT effects for high-mass Drell-Yan processes at the LHC and quantify the impact that the consistent joint determination of PDFs and Wilson coefficients has on the bounds derived for the latter. We consider two well-motivated new physics scenarios: 1) electroweak oblique corrections (Ŵ, Ŷ) and 2) four-fermion interactions potentially related to the LHCb anomalies in R(K(*)). We account for available Drell-Yan data, both from unfolded cross sections and from searches, and carry out dedicated projections for the High-Luminosity LHC. Our main finding is that, while the interplay between PDFs and EFT effects remains moderate for the current dataset, it will become a significant challenge for EFT analyses at the HL-LHC