Antimatière | EPFL Graph Search

En physique des particules, l'antimatière est l'ensemble des antiparticules qui ont la même masse (la masse d'une antiparticule n'a cependant jamais pu être mesurée en 2018) et le même spin, mais des charges, nombres baryoniques et nombres leptoniques opposés aux particules ordinaires. Il est supposé que l'antimatière n'existe qu'en quantités infimes dans l'Univers local, soit dans les rayons cosmiques, soit produite en laboratoire. Les travaux sur l'antimatière consistent en grande partie à expliquer la rareté de l'antimatière par rapport à la matière. Selon la théorie du Big Bang, la matière et l'antimatière devraient avoir été présentes en quantités égales. La totale absence, du moins en apparence, d'antimatière dans notre univers, reste l'un des mystères majeurs du modèle standard. Principe La différence entre matière et antimatière se fait au niveau des charges (dont la charge électrique) : les particules composant l'antimatière ont d

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

Baryogenesis, Dark Matter and Neutrino Oscillations from Right-Handed Neutrinos

Laurent Canetti

Our knowledge of our universe is deeply related to our understanding of physics at the sub- atomic scale. Indeed, at its early stage, our universe was so hot and so dense that the only relevant interactions were those between fundamental particles. Therefore, to improve our comprehension of the phenomena that take place over very large scales, we should improve our understanding of the physics at extremely small ones. The so-called Standard Model of particle physics (SM) provides us with a description of these fundamental particles and their interactions. Despite its enormous success, this model fails to explain several experimental evidences. In this thesis, we focus on the following three questions that are not answered by the SM. What is the so-called dark matter that is present in every galaxy? Why is our present universe made of matter with almost no trace of anti-matter? Finally, why can a neutrino of one family transform into a neutrino of another family? In this thesis, we show that the simple addition of three new neutrinos to the Standard Model allows us to answer the three questions above. The model that realizes this scenario is known as Neutrino Minimal Standard Model (νMSM). In this work, we give a comprehensive summary of all known constraints in the νMSM. We present the first complete quantitative study of the parameter space of the model where no physics beyond the νMSM is needed to simultaneously explain the three phenomena cited above. Moreover, these new particles can be looked for using current day experimental techniques and our results provide a guideline for future experimental searches.

EPFL2013

Matter and antimatter in the universe

Laurent Canetti, Marco Drewes

We review observational evidence for a matter-antimatter asymmetry in the early universe, which leads to the remnant matter density we observe today. We also discuss bounds on the presence of antimatter in the present-day universe, including the possibility of a large lepton asymmetry in the cosmic neutrino background. We briefly review the theoretical framework within which baryogenesis, the dynamical generation of a matter-antimatter asymmetry, can occur. As an example, we discuss a testable minimal particle physics model that simultaneously explains the baryon asymmetry of the universe, neutrino oscillations and dark matter.

Iop Publishing Ltd2012