Proton decay

Summary

In particle physics, proton decay is a hypothetical form of particle decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron. The proton decay hypothesis was first formulated by Andrei Sakharov in 1967. Despite significant experimental effort, proton decay has never been observed. If it does decay via a positron, the proton's half-life is constrained to be at least 1.67e34 years. According to the Standard Model, the proton, a type of baryon, is stable because baryon number (quark number) is conserved (under normal circumstances; see chiral anomaly for an exception). Therefore, protons will not decay into other particles on their own, because they are the lightest (and therefore least energetic) baryon. Positron emission and electron capture – forms of radioactive decay which see a proton become a neutron – are not proton decay, since the proton interacts with other particles within the atom. Some beyond-the-Standard Model grand unif

Official source

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Hadron beams - approximately 200 MeV protons and 400 MeV/u fully stripped carbon ions - are better suited to treat deep-seated tumours than X ray beams - produced by electrons accelerated by linear accelerators (linacs) to 5-25 MeV - because they leave the maximum energy density at the end of their range in matter, in the so-called Bragg peak. This physical property allows dose depositions that are more conformal to the tumour target and spare much better the surrounding healthy tissues, so that hadron therapy treatment rooms could substitute X ray therapy rooms (about 20 000 worldwide) if the needed accelerators could be made of similar dimensions and costs. Cyclotrons and synchrotrons are at the heart of today proton and carbon ion therapy centres, respectively. At the end of 2015, more than 130 000 patients have been treated with proton beams and almost 20 000 with carbon ions [Particle Therapy Co-Operative Group data, \url{https://www.ptcog.ch/index.php/patient-statistics}]. High frequency hadron therapy linacs have been studied in the last 30 years. Their main advantage is represented by their active beam energy modulation, which permits quick treatments with superior beam quality and novel dose delivery techniques. This thesis is the last of many research works on the development of linear accelerators for hadron therapy. The preliminary design of two linear accelerator facilities, for proton therapy (TULIP) and carbon ion therapy (CABOTO), was completed. The introductory Chapter will review the rationale of hadron therapy as a treatment methodology, together with a discussion of the advantages of linacs in hadron therapy compared to state-of-art technologies. Finally, the most important past research activities based on which this thesis started will be presented. The second Chapter describes the RF design of accelerating structures performed for the proton and carbon ion therapy facilities later on presented. The third Chapter is dedicated to the proton linac design, called TULIP: TUrning LInac for Protontherapy. The forth Chapter describes the carbon ion linac design, called CABOTO: CArbon BOoster for Therapy in Oncology. The fifth Chapter presents the experimental activity performed on the backward travelling wave prototype built. Goal of the test is to study the high-gradient break-down limitation of S-Band cavities. The thesis is completed by an Appendix where the beam dynamics codes developed to design the linacs are discussed.

EPFL2018

Phenomenology of 10(32) dark sectors

Michele Redi

We postulate an exact permutation symmetry acting on 10(32) standard model copies as the largest possible symmetry extension of the standard model. This setup automatically lowers the fundamental gravity cutoff down to TeV, and thus, accounts for the quantum stability of the weak scale. We study the phenomenology of this framework and show that below TeV energies the copies are well hidden, obeying all the existing observational bounds. Nevertheless, we identify a potential low energy window into the hidden world, the oscillation of the neutron into its dark copies. At the same time, proton decay can be suppressed by gauging the diagonal baryon number of the different copies. This framework offers an alternative approach to several particle physics questions. For example, we suggest a novel mechanism for generating naturally small neutrino masses that are suppressed by the number of neutrino species. The mirror copies of the standard model naturally house dark matter candidates. The general experimentally observable prediction of this scenario is an emergence of strong gravitational effects at the LHC. The low energy permutation symmetry powerfully constrains the form of this new gravitational physics and allows to make observational predictions, such as, production of micro black holes with very peculiar properties.

2009

The possibility to explain the baryon asymmetry in the Universe through the leptogenesis mechanism in the context of Adjoint SU(5) is investigated. In this model the neutrino masses are generated through the Type I and Type III seesaw mechanisms, and the field responsible for the Type III seesaw, called rho_3, generates the B-L asymmetry needed to satisfy the observed value of the baryon asymmetry in the Universe. We find that the CP asymmetry originates only from the vertex correction, since the self-energy contribution is not present. When neutrino masses have a normal hierarchy, successful leptogenesis is possible for 10^{11} GeV < M_{\rho_3}^{NH} < 4 10^{14} GeV. When the neutrino hierarchy is inverted, the allowed mass range changes to 2 10^{11} GeV < M_{\rho_3}^{IH} < 5 10^{11} GeV. These constraints make possible to rule out a large part of the parameter space in the theory which was allowed by the unification of gauge interactions and the constraints coming from proton decay.

2008

EPFL2018