Modèle standard de la physique des particules

vignette|upright=2.0|Modèle standard des particules élémentaires avec les trois générations de fermions (trois premières colonnes), les bosons de jauge (quatrième colonne) et le boson de Higgs (cinquième colonne). Le modèle standard de la physique des particules est une théorie qui concerne l'électromagnétisme, les interactions nucléaires faible et forte, et la classification de toutes les particules subatomiques connues. Elle a été développée pendant la deuxième moitié du , dans une initiative collaborative mondiale, sur les bases de la mécanique quantique. La formulation actuelle a été finalisée au milieu des années 1970 à la suite de la confirmation expérimentale des quarks. Depuis, les découvertes du quark top (1995), du neutrino tauique (2000) et du boson de Higgs (2012) ont donné encore plus de crédibilité au modèle standard. Toutes les particules du modèle standard ont désormais été observées expérimentalement. Par son succès à expliquer une large variété de résultats expériment

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

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

Measurement of lifetimes for heavy flavour mesons using semileptonic B decays at LHCb

Brice Emile Maurin

This thesis work presents lifetime measurements of heavy-flavour mesons made with semileptonic

B^0

and

B^0_s

decays based on

3~{\rm fb}^{-1}

of data collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. The study of meson lifetimes is important to constrain phenomenological models for hadronic interactions based on the Standard Model of particle physics. Better understanding of hadronic interactions is essential for making precise predictions, which can then be confronted to experimental data in order to look for signs of physics beyond the Standard Model. We measure the differences between the decay widths of the

B^0_s

and

B^0

mesons,

\Delta(B)

, and between that of the

D_s^-

and

D^-

mesons,

\Delta(D)

, by analysing approximately 410000

B^0_s \to D_{s}^{(*)-}\mu^{+}\nu_{\mu}

and 110000

B^0 \to D^{(*)-}\mu^{+}\nu_{\mu}

decays, which are partially reconstructed in the same

K^+K^-\pi^-\mu^+

final state. We measure

\Delta(B) = -0.0115 \pm 0.0053~{\rm(stat)} \pm 0.0041~{\rm(syst)}~{\rm ps}^{-1}

and

\Delta(D) = 1.0131 \pm 0.0117~{\rm(stat)} \pm 0.0065~{\rm(syst)}~{\rm ps}^{-1}.

Using the obtained values of

\Delta(D)

and

\Delta(B)

and the

B^0

and

D^-

lifetimes as external inputs, we obtain a measurement of the flavour-specific

B^0_s

lifetime,

\tau_s^{\rm fs} = 1.547 \pm 0.013~{\rm(stat)} \pm 0.010~{\rm (syst)} \pm 0.004~{\rm(\tau_{B^0})}~{\rm ps},

and of the

D_s^-

lifetime,

\tau_{D_s^-} = 0.5064 \pm 0.0030~{\rm(stat)} \pm 0.0017~{\rm (syst)} \pm 0.0017~{\rm(\tau_{D^-})}~{\rm ps},

where the last uncertainties originate from the limited knowledge of the

B^0

and

D^-

lifetimes, respectively. Both results are compatible with, and improve upon, previous determinations. A feasibility study of a

D^0

lifetime measurement is performed, by measuring the difference between the decay widths of the

D^0

and

D^-

mesons,

\Delta(D)'

. We reconstruct approximately

2.2\times 10^6

B^0\to D^{*-}(\to \bar{D}^0 (\to K^+\pi^-)\pi^-)\mu^+\nu_{\mu}

and

1.6\times 10^6

B^0\to D^{(*)-}(\to K^+\pi^-\pi^- (X))\mu^+\nu_{\mu}

decays. We measure

\Delta(D)' = 1.4644 \pm 0.0043~{\rm(stat)} \pm 0.0132~{\rm(syst)}~{\rm ps}^{-1}

and with the

D^-

lifetime as external input, we get an estimate of the

D^0

lifetime,

\tau_{D^0} = 0.4122 \pm 0.0007~{\rm(stat)} \pm 0.0022~{\rm (syst)} \pm 0.0011~{\rm(\tau_{D^-})}~{\rm ps}.

This result is compatible with, but less precise than, current precision and thus validates the method. We discuss possible improvements with larger simulation samples and data sets.

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 lifetimes for heavy flavour mesons using semileptonic B decays at LHCb

Brice Emile Maurin

This thesis work presents lifetime measurements of heavy-flavour mesons made with semileptonic

B^0

and

B^0_s

decays based on

3~{\rm fb}^{-1}

B^0_s

and

B^0

mesons,

\Delta(B)

, and between that of the

D_s^-

and

D^-

mesons,

\Delta(D)

, by analysing approximately 410000

B^0_s \to D_{s}^{(*)-}\mu^{+}\nu_{\mu}

and 110000

B^0 \to D^{(*)-}\mu^{+}\nu_{\mu}

decays, which are partially reconstructed in the same

K^+K^-\pi^-\mu^+

final state. We measure

\Delta(B) = -0.0115 \pm 0.0053~{\rm(stat)} \pm 0.0041~{\rm(syst)}~{\rm ps}^{-1}

and

\Delta(D) = 1.0131 \pm 0.0117~{\rm(stat)} \pm 0.0065~{\rm(syst)}~{\rm ps}^{-1}.

Using the obtained values of

\Delta(D)

and

\Delta(B)

and the

B^0

and

D^-

lifetimes as external inputs, we obtain a measurement of the flavour-specific

B^0_s

lifetime,

\tau_s^{\rm fs} = 1.547 \pm 0.013~{\rm(stat)} \pm 0.010~{\rm (syst)} \pm 0.004~{\rm(\tau_{B^0})}~{\rm ps},

and of the

D_s^-

lifetime,

\tau_{D_s^-} = 0.5064 \pm 0.0030~{\rm(stat)} \pm 0.0017~{\rm (syst)} \pm 0.0017~{\rm(\tau_{D^-})}~{\rm ps},

where the last uncertainties originate from the limited knowledge of the

B^0

and

D^-

lifetimes, respectively. Both results are compatible with, and improve upon, previous determinations. A feasibility study of a

D^0

lifetime measurement is performed, by measuring the difference between the decay widths of the

D^0

and

D^-

mesons,

\Delta(D)'

. We reconstruct approximately

2.2\times 10^6

B^0\to D^{*-}(\to \bar{D}^0 (\to K^+\pi^-)\pi^-)\mu^+\nu_{\mu}

and

1.6\times 10^6

B^0\to D^{(*)-}(\to K^+\pi^-\pi^- (X))\mu^+\nu_{\mu}

decays. We measure

\Delta(D)' = 1.4644 \pm 0.0043~{\rm(stat)} \pm 0.0132~{\rm(syst)}~{\rm ps}^{-1}

and with the

D^-

lifetime as external input, we get an estimate of the

D^0

lifetime,

\tau_{D^0} = 0.4122 \pm 0.0007~{\rm(stat)} \pm 0.0022~{\rm (syst)} \pm 0.0011~{\rm(\tau_{D^-})}~{\rm ps}.

This result is compatible with, but less precise than, current precision and thus validates the method. We discuss possible improvements with larger simulation samples and data sets.

EPFL2018

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