Réactions nucléaires avec des ions lourds

Les réactions nucléaires avec des noyaux lourds (ou avec des ions lourds) sont des réactions provoquées par la collision de deux noyaux atomiques accélérés, soit naturellement comme les rayons cosmiques ou lors de la nucléosynthèse stellaire, soit artificiellement par des accélérateurs. On les distingue des réactions avec des particules légères (photons, protons, neutrons ou particule α) ; les noyaux du faisceau vont du plus léger comme le lithium (nombre de nucléons 6) aux plus lourds comme l'uranium ( 238). Le paramètre essentiel est l'énergie cinétique des noyaux accélérés, qui se mesure en MeV par nucléon (traditionnellement notée MeV/A). Désormais, la gamme d’énergie accessible est très étendue, elle va de quelques MeV/A à la quelques TeV/A (un facteur 10). Il en résulte une très grande variété de phénomènes. Par exemple : à basse énergie, création de deux noyaux en sortie, proches des noyaux initiaux ; à plus haute énergie, multifragmentation (noyaux légers et nucléons isolés

First steps towards interlocking modular microfluidic cooling substrates (i-MμCS) for future silicon tracking detectors in High Energy Physics (HEP)

Massimo Angeletti, Philippe Renaud

In future High Energy Physics detectors, the coverage of large surfaces with silicon pixel chip sensors poses a challenge for the sensors positioning, for their cooling, assembly, and interconnection. The use of a cooling substrate on which the sensors are glued is typically limited by the bulky and complicated hydraulic interconnection between adjacent substrates. In this research, a new type of cooling substrate is presented. Its design is based on microchannels, where additive manufacturing of plastic and ceramic materials has been considered an alternative to the current silicon etching process. A solution to the mechanical and hydraulic interconnection problem is achieved through a modular interlocking concept. Design optimisation was followed having identified three relevant parameters, plug-and-ply, interchangeability and sealing performance, which qualify the substrates interconnection and guaranty their correct positioning. This paper poses the bases to a new substrate category where modularity, re-workability and easy connectivity are the strong points. This concept could find applications also outside High Energy Physics experiments such as hardware cloud computing and medical detectors.

2022

Aspects of Gauge Theories Vacua

Adrien Florio

This thesis is concerned with gauge theories, their complicated vacuum and resulting effects. After an introduction to the subject, it is divided into four parts. Firstly, we treat the problem of chiral charge dynamics at finite temperature. Quantum field theory predicts a possibility for massless fermions to be transferred into electromagnetic fields with non-zero helicity and vice-versa. This phenomenon has applications ranging from cosmology to heavy-ions physics. We present a numerical investigation from first principles of the resulting complex dynamics and find a qualitative agreement with previous studies based on hydrodynamical approaches but measure rates that differ by up to an order of magnitude. We interpret this effect as contributions coming from small scales not previously taken into account. Secondly, we present a study of open-boundary conditions in lattice QCD at finite temperature. They were designed to ease up the problem of "topological freezing", which plagues numerical simulations close to the continuum limit. In particular, we determine the length of the "boundary zone" for two different temperatures. We also use the boundary effects to extract screening masses. Thirdly, we move on to present a compendium of lattice techniques, including some new algorithms, to perform real-time classical simulations of bosonic matter, Abelian and non-Abelian gauge fields in an expanding universe. We also briefly introduce CosmoLattice, a numerical software designed to perform such simulations, which are particularly interesting to study the reheating phase of our universe. Finally, we study yet another technique to probe non-perturbative sectors of field theories. Namely, we show that one can reconstruct the Schwinger pair production rate, which is the rate of production of particles due to the presence of a strong electric field, using only a few terms of the weak magnetic field expansion. This surprising result is obtained by using techniques coming from the field of resurgence and the analysis of asymptotic expansions. We conclude this work by presenting some general outlooks, sharing aspects of all these different yet related topics.

EPFL2020

Direct search for Higgs boson in LHCb and contribution to the development of the vertex detector

Laurent Locatelli

The LHCb experiment (Large Hadron Collider beauty) is one of the four experiments under construction at the LHC (Large Hadron Collider) at CERN near Geneva. It is planned to start in 2007 and its goal is the study of b-quark physics. The LHC is a circular accelerator in which collide protons-protons at a center-of-mass energy of √s = 14 TeV. This generates a large number of high energy bb pairs which are predominantly produced in the same forward cone. The LHCb detector is therefore a forward single arm spectrometer designed to exploit the large bb production cross section (σbb ~ 500 μb) and to perform precise measurements of CP violation in b-hadrons decays. One of the actual greatest challenges in High Energy Physics is the discovery of the Higgs boson which is responsible for the Model Standard particles mass generation through the Spontaneous Symmetry Breaking process. The Higgs mass is not known and cannot be predicted by the theory. However the recent results of LEP at CERN have shown that mH0 > 114 GeV/c2. Below ~ 150 GeV/c2 the Higgs decay into two b-quarks H0 → bb dominates. The two quarks emitted back-to-back in the H0 rest frame form a string which fragments, giving rise to hadronization in jets containing b-hadrons. The aim of this thesis is to assess the feasibility to discover a Higgs boson with intermediate mass at LHCb by using the detector sensibility to b-hadrons in order to reconstruct these jets using jets reconstruction algorithms. The study is focused on the mechanisms in which the Higgs boson is produced in association with a gauge boson decaying leptonically H0 + W± → bb + ℓνℓ and H0 + Z0 → bb + ℓ+ℓ- for Higgs masses in the range 100 - 130 GeV/c2. The gauge bosons decay produces hard leptons quite often isolated from the b-jets. Hence an isolated lepton with high transverse momentum is required in order to reject the large QCD background. Several important background channels which also provide two b-quarks and an isolated lepton – like tt → W+b W-b, Z0 + W± → bb + ℓνℓ, Z0 + Z0 → bb + ℓ+ℓ-, W± + b-jets, Z0 + b-jets and generic bb – are studied in parallel. The idea is to find observables which behave differently for backgrounds and Higgs signal and to exploit these differences in the framework of a neural network, precisely in order to discriminate background from signal. The LHCb experiment needs a high capability to identify b-hadrons despite their very short lifetime τB ~ 1.5 · 10-12 s. The Vertex Locator (VeLo) is a sub-detector placed around the p-p interaction point which has to provide accurate measurements of the b-hadrons production and decay points by reconstructing secondary vertices. The second part of this thesis is a technical contribution to the development of the VeLo analogue transmission line. It consists in testing several hardware and software methods to improve the VeLo analogue transmission between the on-detector part of the readout and the off-detector electronics. Because the ~ 60 m line introduces an important attenuation, several cables and line drivers configurations with frequency and gain compensation are studied in order to obtain the best results in terms of signal-to-noise ratio and channel crosstalk. The different contributions to the noise are also studied and an estimation of the contribution due to the Beetle pipeline non-uniformity is given in order to see if a specific correction is needed or if it can be suppressed by a standard common noise correction procedure.