Searching for doubly charged Higgs bosons using same-sign hadronic tau final states - A focused study on the charge-flip background of hadronic taus

Abstract

The search for doubly charged Higgs bosons (H±±) aims at resolving the mass generation mechanism of neutrinos in the Standard Model. The current search by the ATLAS Same-Sign Dilepton group uses the decay product of H±± to a pair of same-sign hadronic τ-leptons, which is an extension of their previous search using same-sign dielectrons and dimuons. This thesis focuses on the study of one of the backgrounds, known as the charge-flip background, as a result of the misidentification of the charge of hadronic τ-leptons by the ATLAS detector. To
estimate this background, three methods are used with the data-driven and tag-and-probe method applied to MC simulated samples and the template fit method applied to detector data collected from 2015 to 2017 by the ATLAS detector at √s = 13 TeV with a total integrated luminosity of 79.8 fb−1. The charge-flip rate of hadronic τ-leptons rises from around 0.3% to 2.2% with the
increase of pseudorapidity from 0 to 2.5, while the charge-flip rate for detector ranges from 0.4% to 3.8%. This leads to a successful calculation of the scale factor equalling to 1.52±0.42.

Popular Abstract

At the border of France and Switzerland, near the city of Geneva, there is a big circular tunnel with circumference 27 km buried 100 m underground. Inside the tunnel, the Large Hadron Collider, which is a super-conducting accelerator and collider, is installed. The Large Hadron Collider accelerates charged particles like protons to almost the speed of light (same as saying extremely high energy) and collides at four locations, where each location corresponds to one of four LHC experiments: ALICE, ATLAS, CMS, and LHCb. According to the famous mass-energy equivalence equation E = mc2 deduced by Albert Einstein, energy and mass can be converted into each other. Therefore, an explosion of various types of particles is created when highly energetic protons collide with each other. Particles, the fundamental building blocks of the Universe, can be classified based on the spin into two types: fermions (half-integer spin e.g. 1/2 and 3/2) and bosons (integer spin e.g. 0 and 1). Particles interact with each other through four fundamental forces: electromagnetic, weak, strong and gravitational force, where the last one is usually negligible in particle physics since its strength is much lower than other three forces in microscopic scale. Although huge amount of particles are created, they are not elementary particles—the smallest constituents and most basic constituents of matter, but the combinations of elementary particles (such as hadrons, which consist of quarks). Particle physicists are most interested in elementary particles. The best current theoretical description of the interplay of the electromagnetic, weak and strong force with elementary particles is called the Standard Model. The current known elementary particles in the Standard Model can be classified into four types: quarks, leptons, gauge bosons (a.k.a. vector bosons) and Higgs bosons (a.k.a. scalar bosons), where the first two types are both fermions and the third and fourth type are bosons. Leptons, which consist of three charged leptons (electrons, muons and taus) and three types of noncharged leptons called neutrinos (electron neutrinos, muon neutrinos and tau neutrinos), do not participate in the strong interaction. Neutrinos are the most abundant particles in the Universe but also one of the least understood ones because they interact very weakly with matter, however, they are incredibly difficult to detect. Quarks, which form neutrons, protons etc., carry a charge which is a fraction of that for the electron. Gauge bosons refer to force carrier particles (photons for electromagnetic force, Z0 and W± bosons for weak force and gluons for strong force). Lastly, the Standard Model Higgs bosons generate the mass of other Standard Model particles (except neutrinos) through the Higgs mechanism. Although the Standard Model is an extremely successful theory, it fails to explain many phenomena. One of them, which is also the goal of our research group within the ATLAS collaboration, is to explore the mystery of the generation mechanism of the mass of the neutrinos, because the Standard Model Higgs bosons cannot explain why neutrinos have non-zero mass. One way to explain the mass of neutrinos is by introducing one more type of Higgs bosons known as the doubly charged Higgs boson. As suggested by its name, this extra Higgs boson carries two times the charge of an electron. If doubly charged Higgs bosons really exist, it is expected that they can be created through the collision of protons in the Large hadron Collider. Our group perform an analysis focusing on observing the particular signature caused by each doubly charged Higgs boson decaying into a pair of same-charge leptons. The data of doubly charged Higgs bosons decaying into same-charge electron and muon pairs have been studied. Currently, the heaviest type of lepton, tau, is added to the analysis. In order to be able to distinguish the decay signals of doubly charged Higgs boson from the sea of particles created by the collision, it is crucial to understand the Standard Model background. This master thesis focuses on estimating one of the backgrounds, known as the charge-flip background, as a result of the misidentification of the charge of one of the tau lepton coming from an opposite-charge pair so that the opposite-charge pair is misidentified as a same-charge pair. With a good understanding of the charge-flip background, the search for doubly charged Higgs bosons can be facilitated.

• author: Qiu, Shi
• supervisor: Else Lytken
• organization: Particle and nuclear physics; Department of Physics
• course: FYSM60 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: doubly charged Higgs bosons, neutrino, ATLAS, charge-flip, charge misidentification
• language: English
• id: 8991933
• date added to LUP: 2019-08-20 09:28:45
• date last changed: 2019-08-20 09:28:45