Création de paires | EPFL Graph Search

Une création de paires est la création d’un couple particule-antiparticule à partir d’un photon (ou d’un autre boson de charge neutre) ou d’une particule chargée se déplaçant à une vitesse relativiste. Définition approfondie Création de paires à partir de photons La production fait référence à la création d’une particule élémentaire et de son antiparticule, le plus souvent à partir d’un photon (ou un autre boson neutre). Cela est permis dès lors qu’il y a suffisamment d’énergie disponible dans le centre de masse pour créer la paire — au moins l’énergie de masse au repos totale des deux particules — et que la situation permet la conservation de l’énergie et de la quantité de mouvement. La somme de tous les autres nombres quantiques (moment cinétique angulaire, charge électrique) des particules produites doit être nulle — ainsi les particules créées auront-elles des valeurs opposées l’une par rapport à l’autre (par exemple, si une particule a une étrangeté de +1 al

Search for flavoured leptonic decays of long-lived particles in the LHCb detector

Matthieu Philippe Luther Marinangeli

New massive long-lived particles (LLP) are predicted by multiple beyond the Standard Model theories. This thesis presents the search of such long-lived particles decaying leptonically in the

e \mu \nu

final state. The search is performed using 2.31 fb

^{-1}

of proton-proton collisions collected by the LHCb detector at a center-of-mass energy of

\sqrt{s} = 13

TeV, seeking for displaced vertices of electrons and muons of opposite charges. LLPs with masses between 7 to

50

GeV/c

^2

, and lifetime between 2 and 50 ps, are explored in this search. Three kinds of LLP production modes are considered: the direct pair production of LLPs from quark interactions, the pair production from a Standard Model like Higgs boson with a mass of

125

GeV/c

^2

, and charged current production from an on-shell

W

boson with an additional lepton. No evidence of these long-lived states has been observed, upper limits at

95 \%

CL on the production cross-section times branching ratio have been set on the different production mode.

EPFL2020

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

Particle creation from non-topological solitons

Stephen Clark

Non topological solitons, Q balls can arise in many particle theories with U(1) global symmetries. As was shown by Cohen et al. [2], if the corresponding scalar field couples to massless fermions, large Q-balls are unstable and evaporate, producing a fermion flux proportional to the Q ball's surface. In this work we analyse Q-ball instabilities as a function of Q-ball size and fermion mass. In particular, we construct an exact quantum-mechanical description of the evaporating Q-ball. This new construction provides an alternative method to compute Q-Ball's evaporation rates. We shall also find a new expression for the upper bound on evaporation as a function of the produced fermion mass and study the effects of the size of the Q ball on particle production. We also analyse what happens if external fermion is scattered on a Q ball and demonstrate that it can be converted into antiparticle with a probability of the order of one. This result has important implications for astrophysical applications of dark matter Q balls.

EPFL2006

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

EPFL2013

Search for flavoured leptonic decays of long-lived particles in the LHCb detector

Matthieu Philippe Luther Marinangeli

New massive long-lived particles (LLP) are predicted by multiple beyond the Standard Model theories. This thesis presents the search of such long-lived particles decaying leptonically in the

e \mu \nu

final state. The search is performed using 2.31 fb

^{-1}

of proton-proton collisions collected by the LHCb detector at a center-of-mass energy of

\sqrt{s} = 13

TeV, seeking for displaced vertices of electrons and muons of opposite charges. LLPs with masses between 7 to

50

GeV/c

^2

125

GeV/c

^2

, and charged current production from an on-shell

W

boson with an additional lepton. No evidence of these long-lived states has been observed, upper limits at

95 \%

CL on the production cross-section times branching ratio have been set on the different production mode.

EPFL2020