Pion (particule) | EPFL Graph Search

Un pion ou méson pi est une des trois particules : π, π+ ou π−. Ce sont les particules les plus légères de la famille des mésons. Elles jouent un rôle important dans l'explication des propriétés à basse énergie de la force nucléaire forte ; notamment, la cohésion du noyau atomique est assurée par l'échange de pions entre les nucléons (protons et neutrons). Nom et notation Le substantif masculin pion (prononcé en français standard) est composé de pi, transcription de la lettre grecque π, et de -on, tiré de électron. Le symbole du pion est la lettre (« pi ») de l'alphabet grec, en référence à l'initiale de « pseudo(-)scalaire ». Caractéristiques Les pions ont un spin égal à 0. Comme leur nom l'indique, les pions , et ont une charge électrique respectivement égale à 1, −1 et 0. Les pions sont des mésons, c'est-à-dire des hadrons constitués d'une paire quark-antiquark. Ils sont tous trois composés de quarks de la première génération (quark ou antiquark up et

Average polarization of 12B in 12C(μ, ν) 12B(g.s.) reaction: Helicity of the π-decay muon and nature of the weak coupling

The helicity, h -, of μ - in π-decay has been determined as positive (h -≥+0.90) from the average polarization, P av≡〈J B·s μ〉, of 12B produced in the μ -+ 12C→ν μ+ 12B reaction. We obtain also dynamical information on μ-capture: (i) the weak magnetism form factor, μ=4.5±1.1, and (ii) the sum of the induced pseudoscalar (g p) and the 2nd class induced tensor (g T) couplings versus g A, ( g P+g T) g A=7.1±2.7. The latter result, adopting the "canonical" value of g P g A, leads to g T g A=+1±2.7 which is compatible with zero and in strong contradiction with the value {reversed tilde equals}-6 recently advocated by Kubodera, Delorme and Rho. © 1977.

1977

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

MeV-scale seesaw and leptogenesis

Valerie Fiona Domcke, Marco Drewes, Michele Lucente

We study the type-I seesaw model with three right-handed neutrinos and Majorana masses below the pion mass. In this mass range, the model parameter space is not only strongly constrained by the requirement to explain the light neutrino masses, but also by experimental searches and cosmological considerations. In the existing literature, three disjoint regions of potentially viable parameter space have been identified. In one of them, all heavy neutrinos decay shortly before big bang nucleosynthesis. In the other two regions, one of the heavy neutrinos either decays between BBN and the CMB decoupling or is quasi-stable. We show that previously unaccounted constraints from photodisintegration of nuclei practically rule out all relevant decays that happen between BBN and the CMB decoupling. Quite remarkably, if all heavy neutrinos decay before BBN, the baryon asymmetry of the universe can be quite generically explained by low-scale leptogenesis, i.e. without further tuning in addition to what is needed to avoid experimental and cosmological constraints. This motivates searches for heavy neutrinos in pion decay experiments.

SPRINGER2021

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