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Concept# Flavour (particle physics)

Summary

In particle physics, flavour or flavor refers to the species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles. They can also be described by some of the family symmetries proposed for the quark-lepton generations.
Quantum numbers
In classical mechanics, a force acting on a point-like particle can only alter the particle's dynamical state, i.e., its momentum, angular momentum, etc. Quantum field theory, however, allows interactions that can alter other facets of a particle's nature described by non dynamical, discrete quantum numbers. In particular, the action of the weak force is such that it allows the conversion of quantum numbers describing mass and electric charge of both quarks and leptons from one discrete type to another. This is known as a flavour change, or flavour transmutation. Due to their quan

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A quark (kwɔːrk,_kwɑːrk) is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and

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Related courses (12)

PHYS-416: Particle physics II

Presentation of the electroweak and strong interaction theories that constitute the Standard Model of particle physics. The course also discusses the new theories proposed to solve the problems of the Standard Model.

PHYS-415: Particle physics I

Presentation of particle properties, their symmetries and interactions.
Introduction to quantum electrodynamics and to the Feynman rules.

PHYS-206: Physics IV

Wave physics, Introduction to quantum mechanics.

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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.

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.

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.

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