Physique au-delà du modèle standard

La physique au-delà du modèle standard se rapporte aux développements théoriques de la physique des particules nécessaires pour expliquer les défaillances du modèle standard, telles que l'origine de la masse, le problème de la violation CP de l'interaction forte, les oscillations des neutrinos, l'asymétrie matière-antimatière, et la nature de la matière noire et de l'énergie noire. Un autre problème réside dans le cadre mathématique du modèle standard lui-même : son incompatibilité avec celui de la relativité générale, au point que l'une ou l'autre ou les deux théories deviennent inopérantes face à certaines conditions, par exemple les singularités gravitationnelles connues de l'espace-temps comme le Big Bang ou l'horizon des événements des trous noirs. Les théories au-delà du modèle standard comprennent de nombreuses extensions du modèle standard jusqu'à la supersymétrie, telles que le modèle standard supersymétrique minimal (en abrégé MSSM pour Minimal Supersymmetric Standard Model)

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

Exploring beyond the Standard Model: the Heavy and the Dark

Kin Shanho Eliaou Mimouni

Physics beyond the Standard Model can appear as new particles too heavy to be produced in experiments (the energy frontier) or interacting too weakly to be seen in our measurements (the intensity frontier). In the first part of this thesis, we study two dark matter models of electroweak WIMP coming from two limiting cases in the MSSM: the Higgsino and the Wino model. The dark matter candidate is a particle too heavy to be directly seen in colliders but it could be seen indirectly by astrophysical observations. Dark matter particles in the galaxy can annihilate to Standard Model particles and be detected by satellites. We consider the charged neutral mass splitting as a free parameter in the theory and investigate its effect on the Sommerfeld enhancement which gives an important boost to the annihilation cross-section, the main observable of indirect detection. In the second part of this thesis, we present an idea of a fixed target experiment so search for dark sector models, a class of model with very weakly interacting particles. This experiment uses the experimental setup needed for a future muon collider but is independent from the muon collider program. We study the reach of our experiment for three benchmark models, the dark photon, the dark Higgs, the heavy neutral lepton and we compare its expected performances to current and future experiments. In the third part of this thesis, we consider a more model-independent approach to the search for new physics. If new states are heavy, they can be integrated out and their leading effects are encoded in effective operators made of Standard Model fields. Assuming baryon and lepton number conservation, one can classify the effective operators of dimension-6 which give the leading contribution and constrain the coefficient of these operators by looking at precision observables. We compile results from LEP-I and LEP-II experiments as well as neutrino scattering and other low-energy observables. We allow all operators to be present with an arbitrary flavour structure. Our result can then be used to translate these constaints to specific models of new physics.

EPFL2019

Measurement of time-dependent CP violation in B0->Dpi decays and optimisation of flavour tagging algorithms at LHCb

Vincenzo Battista

This thesis presents the results of a time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays using

3~\rm fb^{-1}

of proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider during Run 1 with a centre-of-mass energy of

7

(2011) and

8

(2012) TeV. The LHCb experiment is dedicated to the study of the properties of

b

-flavoured hadrons, in particular

CP

violation in the

B

meson system. The Standard Model of Particle Physics describes very precisely the mechanism and the amount of

CP

violation expected in the Universe. However, the observed matter-antimatter asymmetry is larger by several order of magnitude compared to the predictions. This could be explained by the existence of a new source of

CP

violation, originating in New Physics beyond the Standard Model. The time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays provides constraints on the angle

\gamma

of the Unitarity Triangle, one of the fundamental parameters of the Standard Model related to

CP

violation. Since no sizeable high-order Standard Model processes are expected to contribute, any deviation from the predictions would be an unambiguous signature of New Physics. The current experimental precision on

\gamma

is significantly lower than that of theoretical predictions. This motivates the effort for new experimental determinations of

\gamma

in order to reduce its uncertainty. The analysis of

\Bz\to\Dmp\pipm

decays, although not as sensitive as that obtained from decays of charged

B

mesons into

D^{(*)0}K^{(*)+}

final states, represents an independent and uncorrelated estimation of

\gamma

that contributes to the global combination of all

\gamma

measurements. The result obtained in this thesis is more precise than previous determinations from other experiments (BaBar, Belle) using

B^0\to D^{\mp}\pi^{\pm}

decays. Although based on a very large sample of about half a million signal events, it is still dominated by statistical uncertainties, indicating good prospects for future improvements in precision with more data from Run 2 and beyond. In addition to the

B^0\to D^{\mp}\pi^{\pm}

analysis, this thesis also summarizes the studies to improve the performances of the flavour tagging algorithms used by the LHCb collaboration to infer the flavour of neutral

B

mesons in time-dependent analyses. The performance of these algorithms, being correlated with the kinematics of the reconstructed particles as well as the complexity of the event (number of tracks and primary vertices), showed a significant decrease on Run 2 data (2015--2018), which were collected at a centre-of-mass energy of

13~\rm TeV

. Thanks to new implementations, these algorithms now have a performance similar to that obtained with Run 1 data.

EPFL2018

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

Measurement of time-dependent CP violation in B0->Dpi decays and optimisation of flavour tagging algorithms at LHCb

Vincenzo Battista

This thesis presents the results of a time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays using

3~\rm fb^{-1}

of proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider during Run 1 with a centre-of-mass energy of

7

(2011) and

8

(2012) TeV. The LHCb experiment is dedicated to the study of the properties of

b

-flavoured hadrons, in particular

CP

violation in the

B

meson system. The Standard Model of Particle Physics describes very precisely the mechanism and the amount of

CP

CP

violation, originating in New Physics beyond the Standard Model. The time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays provides constraints on the angle

\gamma

of the Unitarity Triangle, one of the fundamental parameters of the Standard Model related to

CP

\gamma

is significantly lower than that of theoretical predictions. This motivates the effort for new experimental determinations of

\gamma

in order to reduce its uncertainty. The analysis of

\Bz\to\Dmp\pipm

decays, although not as sensitive as that obtained from decays of charged

B

mesons into

D^{(*)0}K^{(*)+}

final states, represents an independent and uncorrelated estimation of

\gamma

that contributes to the global combination of all

\gamma

measurements. The result obtained in this thesis is more precise than previous determinations from other experiments (BaBar, Belle) using

B^0\to D^{\mp}\pi^{\pm}

B^0\to D^{\mp}\pi^{\pm}

analysis, this thesis also summarizes the studies to improve the performances of the flavour tagging algorithms used by the LHCb collaboration to infer the flavour of neutral

B

13~\rm TeV

. Thanks to new implementations, these algorithms now have a performance similar to that obtained with Run 1 data.

EPFL2018