Saveur (physique) | EPFL Graph Search

La saveur, en physique des particules, est une caractéristique permettant de distinguer différents types de leptons et de quarks, deux sous-familles des fermions. Les leptons se déclinent en trois saveurs et les quarks en six saveurs. Les saveurs permettent de distinguer certaines classes de particules dont les autres propriétés (charge électrique, interactivité) sont similaires. Les dénominations des saveurs ont été introduites par Murray Gell-Mann, baptisant le quark étrange lors de la détection du kaon en 1964. Saveurs leptoniques Lepton On connaît trois saveurs de lepton : l'électron, le muon et le tauon. Chaque saveur est représentée par une paire de particules appelée un doublet faible. L'une des deux est une particule massive et chargée qui porte le même nom que sa saveur (comme l'électron) ; l'autre est une particule sans charge et de masse très faible appelée un neutrino (comme le neutrino-électron). Lorsque des particules interagissent, le nombre de leptons de m

Numerical investigations of 3D CFTs using conformal bootstrap methods

Marten Jan Reehorst

Conformal field theories (CFTs) play a very significant role in modern physics, appearing in such diverse fields as particle physics, condensed matter and statistical physics and in quantum gravity both as the string worldsheet theory and through the AdS/CFT correspondence. In recent years major breakthroughs have been made in solving these CFTs through a method called numerical conformal bootstrap. This method uses consistency conditions on the CFT data in order to find and constrain conformal field theories and obtain precise measurements of physical observables. In this thesis we apply the conformal bootstrap to study among others the O(2)- and the ARP^3- models in 3D. In the first chapter we extend the conventional scalar numerical conformal bootstrap to a mixed system of correlators involving a scalar field charged under a global U(1) symmetry and the associated conserved spin-1 current J. The inclusion of a conserved spinning operator is an important advance in the numerical bootstrap program. Using numerical bootstrap techniques we obtain bounds on new observables not accessible in the usual scalar bootstrap. Concentrating on the O(2) model we extract rigorous bounds on the three-point function coefficient of two currents and the unique relevant scalar singlet, as well as those of two currents and the stress tensor. Using these results, and comparing with a quantum Monte Carlo simulation of the O(2) model conductivity, we give estimates of the thermal one-point function of the relevant singlet and the stress tensor. We also obtain new bounds on operators in various sectors. In the second chapter we investigate the existence of a second-order phase transition in the ARP^3 model. This model has a global O(4) symmetry and a discrete Z_2 gauge symmetry. It was shown by a perturbative renormalization group analysis that its Landau-Ginzburg-Wilson effective description does not have any stable fixed point, thus disallowing a second-order phase transition. However, it was also shown that lattice simulations contradict this, finding strong evidence for the existence of a second-order phase transition. In this chapter we apply conformal bootstrap methods to the correlator of four scalars t transforming in the traceless symmetric representation of O(4) in order to investigate the existence of this second order phase transition. We find various features that stand out in the region predicted by the lattice data. Moreover, under reasonable assumptions a candidate island can be isolated. We also apply a mixed t-s bootstrap setup in which this island persists. In addition we study the kink-landscape for all representations appearing in the t times t OPE for general N. Among others, we find a new family of kinks in the upper-bound on the dimension of the first scalar operator in the "Box" and "Hook" representations.

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}

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

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}

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

Numerical investigations of 3D CFTs using conformal bootstrap methods

Marten Jan Reehorst

EPFL2020