The Higgs Width in Higgs production in association with a W+ boson

Abstract

The width of the Higgs boson is a quantity that cannot be measured directly from the on-shell Higgs peak; it would have to be measured indirectly. In 2013, Caola and Melnikov developed a method to determine the Higgs width by using off-shell and on-shell cross-sections and applying the narrow-width approximation. In this thesis, we will use the indirect method of finding the Higgs width for the production of the Higgs boson in association with a W+ boson. The specific decay mode of the scalar boson will be its decay to two Z bosons. In theory, this specific decay mode of the scalar boson must result in an off-shell and on-shell region. We simulated proton-proton collisions that produce the Higgs and W+ boson using the MadGraph5_MC@NLO software and, using the generated data, we were able to plot a histogram for when the Higgs width equals the Standard Model Higgs boson width calculated at LO accuracy. The same was done when the width is two times the Standard Model Higgs boson width and when it is five times the Standard Model Higgs boson width. From the analysis of the histograms and finding the total cross-section of the on-shell and off-shell region in each of the histograms, we found that the results agree with the relationship between the on-shell cross-section and the Higgs width when the narrow-width approximation is applied. The same can be said for the off-shell cross-section.

Popular Abstract

Mass is an intrinsic property of all matter; this is what we've been taught in school. However, particle physicists do not think this is the case. Not all particles have mass; an example of a massless particle is the photon, the particle of light. All particles get their mass from the Higgs field, whose particle manifestation is the Higgs boson discovered at CERN in 2012. The discovery of the Higgs boson provided evidence of the existence of the Higgs field and this confirmed the validity of the Higgs mechanism. This theory explains how particles get mass from the Higgs field. But this doesn't mean that our work with the Higgs boson is over. In fact, it was just the beginning of a new chapter in particle physics! There's more to investigate about the Higgs boson such as its width. A measurement of this property of the boson will provide clues to the existence of new particles.

How particles gain mass from the Higgs field and what the Higgs boson is can be explained easily by using an analogy created by physicist David Miller of University College London. Imagine the Higgs field as a room full of physicists at a cocktail party. When the tax collector enters the room, no one interacts with him because who likes to pay taxes? The tax collector isn't held back by people so he moves freely. One could think of the tax collector as a massless particle such as the photon. When a famous person such as Peter Higgs, the physicist who theorized the existence of the Higgs boson, enters the room, the physicists are thrilled to see him and everyone tries to talk to him. Peter Higgs interacted with the crowd much more than the tax collector and he represents a heavy particle. When the tax collector and Peter Higgs are out of the room there is no difference between their masses but when they enter the room Peter Higgs has much more mass than the tax collector.

The Higgs boson is a particle just like the photon and electron. It has no electric charge and has a mass of about 125 GeV (which is 125 times heavier than the proton). It is the particle of the Higgs field just as the photon is the particle of the electromagnetic field. Unlike protons and electrons, the Higgs boson doesn't live very long, its lifetime is only 1.6 x 10^-22 s. Nevertheless, we’ve still detected it albeit indirectly. It is the short lifetime of the Higgs boson which physicists find very useful.

Short-lived particles decay into more stable particles and this is the case with the Higgs boson. Thus in the detector what we detect is not the Higgs boson but its final decay products which are photons. Due to mass-energy conservation, the total energy of the photons will be equal to the total mass of the Higgs boson. If we plot a graph of the total number of particles detected over the total energy of the detected particles we get a hump in the graph which has a considerable width. This is known as the width of the Higgs boson and the hump represents the Higgs particle. All short-lived particles have a width and this width is related to their lifetime. The longer the lifetime the narrower the width.

There are different ways of producing Higgs bosons. So far not all of the production modes have been investigated by experimental particle physicists. The production mode that we will be looking at is one of the modes that hasn’t been investigated yet. By generating events in a particle collision simulator we can get to find a value for the Higgs width obtained by the production mode we are interested in. If the width is different from the value given by the Standard Model, then this indicates that there may be new physics such as the existence of dark matter particles.

There’s still more to uncover about the Higgs boson. Even though it was discovered more than 10 years ago it still is relevant to the particle physics community. The Higgs boson may hold the answer to the existence of dark matter particles and we’ll never know unless we work hard and do our research.

• author: Fernando, Wijeratne Mohandiramge Umashi Navisha
• 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 boson, Higgs width, off-shell, on-shell, narrow-width approximation, MadGraph5_MC@NLO, W boson, supersymmetry, dark matter
• language: English
• id: 9127278
• date added to LUP: 2023-06-21 17:25:10
• date last changed: 2023-08-30 15:02:13