Êtes-vous un étudiant de l'EPFL à la recherche d'un projet de semestre?

Travaillez avec nous sur des projets en science des données et en visualisation, et déployez votre projet sous forme d'application sur GraphSearch.

Découvrez-en plus sur les applications Graph.

Graph
Search

fr|en

Se Connecter

Concept

Radioactivité β

Résumé

La radioactivité β, radioactivité bêta ou émission bêta (symbole β) est, à l'origine, un type de désintégration radioactive dans laquelle une particule bêta (un électron ou un positon) est émise. On parle de désintégration bêta moins (β) ou bêta plus (β) selon qu'il s'agit de l'émission d'un électron (particule chargée négativement) ou d'un positon (particule chargée positivement). L'émission β est notamment ce qui permet la conversion d'un neutron en proton, par exemple dans les cas de transmutation comme du tritium (T) qui se transforme en hélium 3 (He) : : ⟶ + e + . Aujourd'hui, la désintégration β se généralise à toutes les réactions nucléaires impliquant les neutrinos ou anti-neutrinos se résumant par la relation suivante : : + e + ν. Dans cette dernière relation, un électron ou un neutrino se transforme en son antiparticule par son passage de l'autre côté de la double flèche. Toutes ces réactions sont régies par la force nucléaire faible et sont possibles si

Source officielle

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées

Chargement

Personnes associées

Chargement

Unités associées

Chargement

Concepts associés

Chargement

Cours associés

Chargement

Séances de cours associées

Chargement

In a future beta-beam facility radioactive ions (6He and 18Ne) are produced, accelerated and then stored in a large Decay Ring (DR), where they eventually produce anti-neutrino and neutrino beams through β± decay. CERN is one of the candidate sites for the beta-beam facility, as existing machines like the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) could be used for the acceleration. The DR is composed of two long straight sections and two arcs. The straight sections host the collimators and two bumps. The beam is injected into the ring at nearly 92 GeV per nucleon. Beam losses occur in different sections of the machine: relevant losses include collimation losses in the collimation section and the bumps, and decay losses in the magnets in the arcs. This work focuses on two radiation protection aspects related to the operation of the DR, namely the induced radioactivity and the air activation generated by collimation and decay losses. All the calculations are performed with the Monte Carlo transport code FLUKA and are based on a continuous three-month operation. The induced radioactivity in the machine components and the expected residual dose rates inside the tunnel during maintenance are calculated for three different waiting times. Airborne activity is evaluated through the convolution of predicted particle spectra in the tunnel with isotope-production cross-sections. Using activity-to-dose coefficients, previously calculated for the ISOLDE facility at CERN, and a laminar flow model for the air diffusion, the airborne activity is converted into effective dose to the reference population group. The results show that residual dose rates during maintenance decrease significantly in a week after the shutdown of the machine, reaching values that correspond, according to CERN area classification, to a limited stay area. The effective dose given to the reference population in one year of operation is below the reference value for CERN emissions into the environment.

2011

Chargement

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

Chargement

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

Chargement

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