The discovery of the Higgs boson at the CERN Large Hadron Collider (LHC) has opened an unprecedented era in the field of precision measurements. In view of the upcoming data from the 13 TeV Run of the LHC, accurate predictions for processes that involve the production of the Higgs boson are fundamental, in order to further investigate the phenomenon of electroweak symmetry breaking. At the energies reached by the LHC, the dominant corrections come from the QCD sector, where the first perturbative order (NLO) typically gives a contribution of 20-30% to the cross section. Electroweak (EW) NLO corrections, instead, amount to 5-10%, comparable with the second QCD perturbative order. Even if their effect is rather mild on the integrated cross sections, NLO EW corrections are known to alter the shape of many kinematic quantities, like the high-energy tails of some transverse-momentum distributions. Moreover, they play an important role in the study of the nature of dark matter and in its indirect detection. A precise knowledge of the high-energy tails, including EW corrections, is then mandatory in order to test the validity of these models. The EW sector has many differences with respect to QCD. Since in QED and QCD the virtual and real corrections involve the exchange of massless particles, they are separately divergent, and only their sum is finite. In these theories, then, the inclusion of real corrections is mandatory. When considering weak corrections, instead, because of the exchange of massive bosons, all the contributions to the NLO cross section are finite. Moreover, the radiated particles would subsequently decay, giving final states that can be experimentally distinguished from the leading-order ones. There are no technical reasons, then, to include the real emission of W and Z bosons. The result of considering only virtual EW corrections is the appearance of logarithms that involve the ratio between the energy of the process and the EW scale. They would represent the divergent part of the virtual corrections if the exchanged bosons were massless. These corrections become relevant at the TeV scale, where they usually give sizable negative contributions in the high-energy tails of some transverse-momentum distributions. It has been proven that, in the limit in which all the kinematic invariants involved are of the same order and much greater than the EW scale, the structure of EW virtual corrections is universal: it can then be computed once for all and applied to the desired processes. These high-energy results can be used to obtain a next-to-leading logarithmic (NLL) approximation of the complete EW virtual corrections with a reduced computational cost. This thesis is focused on the description of two kinds of high-energy processes that involve the production of a Higgs boson, including both QCD and EW corrections. The first process is one of the main production modes of the Higgs boson at hadron colliders: the associated production with a vector boson, HV. This process is very important, since it allows to study the decay channel into a bottom-quark pair, that has the highest branching ratio in the Higgs boson mass region. The second relevant process analyzed is the associated production with a vector boson and a jet, HVj, that contributes to the background for supersymmetric events. At present, both the HV and Hvj processes are described at NLO+PS QCD accuracy. The NLO+PS accuracy in the EW sector is instead missing. The aim of this thesis is the description of the HV and HVj associated production processes at NLO+PS QCD+EW accuracy. We present numerical predictions for proton-proton collisions at a center-of-mass energy of 13 TeV, focusing on observables that are sensitive to EW corrections. For each process, we analyze the differences between the NLO EW corrections and their NLL approximation, and the impact of the EW sector on the QCD results.

La scoperta del bosone di Higgs a LHC al CERN ha dato il via a un'era senza precedenti nel campo delle misure di precisione. In vista delle imminenti misure dai Run a 13 TeV di LHC è fondamentale avere previsioni accurate per processi che coinvolgono la produzione di un bosone di Higgs, per investigare a fondo il fenomeno di rottura spontanea della simmetria elettrodebole. Alle energie raggiunte da LHC, le correzioni dominanti provengono dal settore di QCD, in cui il primo ordine perturbativo (NLO) tipicamente dà contributi del 20-30% alla sezione d'urto. Le correzioni NLO elettrodeboli (EW), invece, ammontano al 5-10%, comparabili col secondo ordine perturbativo di QCD. Anche se il loro effetto è moderato sulla sezione d'urto totale, le correzioni NLO EW modificano l'andamento di molte grandezze cinematiche, come le code ad alta energia di alcune distribuzioni di momento trasverso. Inoltre, svolgono un ruolo importante nello studio della materia oscura e nella sua rivelazione indiretta. Una conoscenza precisa di queste distribuzioni, considerando anche le correzioni EW, è quindi fondamentale per verificare la validità di questi modelli. Il settore EW presenta molte differenze rispetto alla QCD. Poiché in QED e QCD le correzioni virtuali e reali coinvolgono lo scambio di particelle massless, esse sono separatamente divergenti, e solo la loro somma è finita. In queste teorie, quindi, l'inclusione delle correzioni reali è obbligatoria. Considerando correzioni deboli, invece, per via dello scambio di particelle massive, tutti i contributi alla sezione d'urto NLO sono finiti. Inoltre, le particelle emesse decadono, dando stati finali che possono essere sperimentalmente distinti da quelli al leading order. Non ci sono ragioni tecniche, quindi, per includere l'emissione reale di bosoni W e Z. Considerando solo correzioni virtuali EW compaiono dei logaritmi del rapporto tra l'energia del processo e la scala EW, che rappresenterebbero la parte divergente delle correzioni virtuali se le particelle scambiate fossero massless. Queste correzioni diventano rilevanti a energie dell'ordine del TeV, dove danno solitamente notevoli contributi negativi alle code ad alta energia di alcune distribuzioni di momento trasverso. È stato dimostrato che, nel limite in cui tutti gli invarianti cinematici sono dello stesso ordine e molto maggiori della scala EW, la struttura delle correzioni EW virtuali è universale: può quindi essere calcolata una volta per tutte, e poi applicata ai processi desiderati. Questi risultati possono essere utilizzati per ottenere, con un costo computazionale ridotto, un'approssimazione next-to-leading logarithimic (NLL) delle correzioni EW virtuali complete. Questa tesi è incentrata sulla descrizione di due tipi di processi che coinvolgono la produzione di un bosone di Higgs, includendo correzioni sia di QCD sia EW. Il primo processo è uno dei principali modi di produrre il bosone di Higgs in collisori adronici: la produzione associata con un vettore bosone, HV. Questo processo è molto importante, perché permette di studiare il canale di decadimento in una coppia di quark bottom, che ha il più alto branching ratio nella regione di massa del bosone di Higgs. Il secondo processo rilevante è la produzione associata con un bosone vettore e un jet, HVj, che contribuisce al background per eventi supersimmetrici. Allo stato attuale, i processi HV e HVj sono descritti con un'accuratezza NLO+PS in QCD. L'accuratezza NLO+PS nel settore EW è tuttora mancante. Lo scopo di questa tesi è quindi la descrizione dei processi di produzione associata HV e HVj con accuratezza NLO+PS QCD+EW. Sono state effettuate previsioni teoriche per collisioni protone-protone a un'energia del centro di massa di 13 TeV, concentrandosi su osservabili sensibili alle correzioni EW. Per ogni processo vengono analizzate le differenze tra le correzioni NLO EW e la loro approssimazione NLL, e l'impatto del settore EW sui risultati di QCD.

(2017). Electroweak and strong next-to-leading-order corrections to HV and HVj production at hadron colliders. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2017).

Electroweak and strong next-to-leading-order corrections to HV and HVj production at hadron colliders

GRANATA, FEDERICO
2017

Abstract

The discovery of the Higgs boson at the CERN Large Hadron Collider (LHC) has opened an unprecedented era in the field of precision measurements. In view of the upcoming data from the 13 TeV Run of the LHC, accurate predictions for processes that involve the production of the Higgs boson are fundamental, in order to further investigate the phenomenon of electroweak symmetry breaking. At the energies reached by the LHC, the dominant corrections come from the QCD sector, where the first perturbative order (NLO) typically gives a contribution of 20-30% to the cross section. Electroweak (EW) NLO corrections, instead, amount to 5-10%, comparable with the second QCD perturbative order. Even if their effect is rather mild on the integrated cross sections, NLO EW corrections are known to alter the shape of many kinematic quantities, like the high-energy tails of some transverse-momentum distributions. Moreover, they play an important role in the study of the nature of dark matter and in its indirect detection. A precise knowledge of the high-energy tails, including EW corrections, is then mandatory in order to test the validity of these models. The EW sector has many differences with respect to QCD. Since in QED and QCD the virtual and real corrections involve the exchange of massless particles, they are separately divergent, and only their sum is finite. In these theories, then, the inclusion of real corrections is mandatory. When considering weak corrections, instead, because of the exchange of massive bosons, all the contributions to the NLO cross section are finite. Moreover, the radiated particles would subsequently decay, giving final states that can be experimentally distinguished from the leading-order ones. There are no technical reasons, then, to include the real emission of W and Z bosons. The result of considering only virtual EW corrections is the appearance of logarithms that involve the ratio between the energy of the process and the EW scale. They would represent the divergent part of the virtual corrections if the exchanged bosons were massless. These corrections become relevant at the TeV scale, where they usually give sizable negative contributions in the high-energy tails of some transverse-momentum distributions. It has been proven that, in the limit in which all the kinematic invariants involved are of the same order and much greater than the EW scale, the structure of EW virtual corrections is universal: it can then be computed once for all and applied to the desired processes. These high-energy results can be used to obtain a next-to-leading logarithmic (NLL) approximation of the complete EW virtual corrections with a reduced computational cost. This thesis is focused on the description of two kinds of high-energy processes that involve the production of a Higgs boson, including both QCD and EW corrections. The first process is one of the main production modes of the Higgs boson at hadron colliders: the associated production with a vector boson, HV. This process is very important, since it allows to study the decay channel into a bottom-quark pair, that has the highest branching ratio in the Higgs boson mass region. The second relevant process analyzed is the associated production with a vector boson and a jet, HVj, that contributes to the background for supersymmetric events. At present, both the HV and Hvj processes are described at NLO+PS QCD accuracy. The NLO+PS accuracy in the EW sector is instead missing. The aim of this thesis is the description of the HV and HVj associated production processes at NLO+PS QCD+EW accuracy. We present numerical predictions for proton-proton collisions at a center-of-mass energy of 13 TeV, focusing on observables that are sensitive to EW corrections. For each process, we analyze the differences between the NLO EW corrections and their NLL approximation, and the impact of the EW sector on the QCD results.
OLEARI, CARLO
High-energy,; phenomenology,; Higgs,; QCD,; electroweak
High-energy,; phenomenology,; Higgs,; QCD,; electroweak
FIS/01 - FISICA SPERIMENTALE
English
11-apr-2017
FISICA E ASTRONOMIA - 86R
29
2015/2016
open
(2017). Electroweak and strong next-to-leading-order corrections to HV and HVj production at hadron colliders. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2017).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/158128
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