Constraining the Higgs width in Higgs production associated with a top quark pair

Abstract

In this thesis we study the width of the Higgs boson in the process pp → t ̄t 4l at LO using MadGraph5_aMC@NLO to generate the events. The contributions including the
Higgs signal, continuum background and interference were considered in order to calculate the expected number of events in a broad range of four-lepton invariant masses. Due to strongly enhanced off-shell contributions an upper bound on the Higgs width could be derived. By assuming that the coupling constants scale by a multiplicative constant, allowing the on-shell cross section to be compatible with the Standard Model for different Higgs widths, we calculate the expected number of events in the off-peak region using an integrated luminosity of 3000 fb−1. Considering the statistical uncertainty of the expected number of events for the process with the SM width we find an upper bound on the Higgs width Γ_H ≤ 3.54 Γ_H^SM at the 95% confidence level. This result translates into an upper limit on the branching ratio to invisible final states BRinv ≤ 0.47. We believe that the result can be improved by a more careful selection of the four-lepton invariant mass range. The major assumptions we use are that the top quarks can be perfectly reconstructed from their decay products and that we have an ideal detector. The cross section for this process is very small and therefore it is not particularly competitive to the limit on the Higgs width that can be derived from pp → H → 4l not including the top quarks. However, it still provides additional information which improves the understanding of the Higgs width.

Popular Abstract

The Standard Model of particle physics is the most promising candidate we have for a theory of everything; a theory that would be able to predict everything in our universe. There are four fundamental forces in nature: the electromagnetic force responsible for electric and magnetic fields and chemical processes, the weak force responsible for the beta decay, the strong force which holds the nucleus together inside atoms and the gravitational force. The Standard Model describes the first three fundamental forces mentioned and how the smallest constituents of matter interact via these forces. Gravity is not included for the simple reason that we do not yet know how to incorporate gravity into the mathematical framework of the Standard Model.

There are two types of particles in our universe: matter particles that make up you, me and everything around us, such as electrons, protons and neutrons, and then there are the force carrier particles which mediate the fundamental forces, such as the photon responsible for the electromagnetic force. All of these particles except the photon and the gluon (carrier of the strong force) have mass. What puzzled scientists in the early 60s was the fact that the carriers of the weak force (the $Z$ and $W$ bosons) were experimentally observed to have mass which was not consistent with the current theory. This was a mystery until 1964, when Peter Higgs, Francois Englert and Robert Brout provided a theory that explained how particles acquire mass through the so-called Higgs mechanism. This mechanism explains how particles interact with the Higgs field and by doing so, gain mass. The mediator of the Higgs field is the Higgs boson. Finding that particle was crucial in order to justify the Higgs mechanism. Finally, in 2012, 50 years later, the Higgs boson was discovered at CERN in Geneva. This was a remarkable discovery that earned Peter Higgs and Francois Englert the Nobel Prize in 2013.

As most particles in the Standard Model, the Higgs boson is an unstable particle; it will exist only a fleeting moment before disintegrating into other lighter particle species. There is a direct connection between a particles lifetime and its uncertainty in mass; a property called the decay width of the particle. The width tells us how probable it is for a particle to decay to other lighter particles or equivalently, how short its lifetime is. My thesis aims to investigate the width of the Higgs boson in a specific process which has not been studied before, in order to set a boundary on how large the Higgs width can be. This will be done by simulating a proton-proton collision where the Higgs boson is produced together with a top quark pair, which is the heaviest particle in the Standard Model. This will be done with a computer program that is able to perform simulations of collisions similar to those produced at CERN. Such analyses have been done with real data from particle accelerators at CERN for other processes to gain more insight into the properties of the Higgs boson. The goal is to establish that the measured Higgs boson is the same as the one predicted from the Standard Model. If the decay width proves to be larger than the prediction it would indicate that there is physics beyond the Standard Model. This would potentially lead to new physics with the opportunity to unravel some of the mysteries of our universe.

• author: Dahlén, Benjamin
• supervisor: Rikkert Frederix
• organization: Department of Physics; Theoretical Particle Physics - Undergoing reorganization
• course: FYTK03 20231
• year: 2023
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Higgs, width, cross section, on-shell, off-shell, invariant mass, Standard model, upper bound
• language: English
• id: 9123559
• date added to LUP: 2023-06-21 17:16:42
• date last changed: 2023-08-30 15:04:15