Muon beams are generated with large transverse and longitudinal emittances. In order to achieve the low emittances required by a muon collider, within the short lifetime of the muons, ionization cooling is required. Cooling schemes have been developed to reduce the muon beam 6D emittances to $\ensuremath{\approx}300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}\text{\ensuremath{-}}\mathrm{rad}$ in transverse and $\ensuremath{\approx}1--1.5\text{ }\text{ }\mathrm{mm}$ in longitudinal dimensions. The transverse emittance has to be further reduced to $\ensuremath{\approx}50--25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}\text{\ensuremath{-}}\mathrm{rad}$ with an upper limit on the longitudinal emittance of $\ensuremath{\approx}76\text{ }\text{ }\mathrm{mm}$ in order to meet the high-energy muon collider luminosity requirements. Earlier studies of the transverse cooling of low energy muon beams in high field magnets showed a promising performance, but did not include transverse or longitudinal matching between the stages. In this study we present the first complete design of the high field-low energy ionization cooling channel with transverse and longitudinal matching. The channel design was based on strong focusing solenoids with fields of 25--30 T and low momentum muon beam starting at $135\text{ }\mathrm{MeV}/\mathrm{c}$ and gradually decreasing. The cooling channel design presented here is the first to reach $\ensuremath{\approx}50$ micron scale emittance beam. We present the channel's optimized design parameters including the focusing solenoid fields, absorber parameters and the transverse and longitudinal matching.

http://inspirehep.net/record/1391727/files/openaccess_PhysRevSTAB_18_091001.pdf

High Field – Low Energy Muon Ionization Cooling Channel

Le travail presente dans cette these porte d’une part sur le developpement et la caracterisation d’une nouvelle generation de detecteurs Micromegas (MICROMEsh GAseous Structure) pour l’experience COMPASS au Cern, et d’autre part sur l’estimation de la contribution de canaux diffractifs a la production de pions et de kaons, dans le cadre de l’etude des fonctions de fragmentation de quarks en hadrons a COMPASS. De nouveaux detecteurs Micromegas d’une surface active de 40 x 40 cm² sont en cours de developpement pour le futur de l’experience COMPASS, a partir de 2015. Ces detecteurs devront fonctionner dans des flux de muons et hadrons approchant 8 MHz/cm². Pour cela, une zone centrale d’environ 5 cm de diametre, traversee par le faisceau, sera constituee de 1280 pixels, et des technologies permettant de fortement reduire l’impact des decharges seront adoptees. Quatre prototypes de geometrie finale utilisant deux types de technologies de reduction des decharges differentes ont ete produits au Cern et testes en conditions reelles a COMPASS entre 2010 et 2012. Trois d’entre eux sont des detecteurs hybrides utilisant une feuille GEM (Gas Electron Multiplier) en tant qu’etage de preamplification pour reduire la probabilite de decharge. Le dernier detecteur est equipe d’une structure resistive a « resistances enterrees », permettant la reduction de l’amplitude des decharges. Leurs performances sont presentees dans cette these. L’impact de ces resultats sur la production et la mise en œuvre de la serie finale de detecteurs est egalement discute. Les fonctions de fragmentation de quarks en hadrons l’hadronisation d’un quark de saveur q en un hadron h. Ces fonctions universelles interviennent dans l’expression de la section efficace de nombreux processus. Elles sont accessibles a COMPASS via la reaction de diffusion profondement inelastique semi-inclusive de muons sur des nucleons. Les observables permettant leur extraction dans ce cadre sont les multiplicites de hadrons, soit le nombre moyen de hadrons produits par evenement de diffusion profondement inelastique. Les mesons vecteurs issus de processus diffractifs produisent egalement des pions et kaons par leur desintegration. Ces processus n’impliquant pas l’hadronisation d’un quark, il semble justifie de ne pas les comptabiliser dans la mesure des multiplicites. Ce travail propose une etude Monte-Carlo des contributions de la production diffractive de mesons vecteurs rho et phi a la production de hadrons leger et aux evenements inclusifs. Des facteurs de correction des multiplicites sont finalement etablis. Enfin, l’effet de cette correction sur l’extraction des fonctions de fragmentation en pions est presente.

Développement de détecteurs Micromegas pixellisés pour les hauts flux de particules et évaluation de la contribution diffractive à la leptoproduction de hadrons à COMPASS

Polarized Drell–Yan measurements require a strong homogeneous magnetic field. A 2.5 T longitudinal field is generated with a superconducting solenoid carrying a nominal current of 650 A, while high homogeneity is obtained by using additional shim—and compensation coils. Since the target needs to be transversely polarized, a second main coil is required. A superconducting dipole coil encloses the solenoid and together they are integrated in a common cryostat. Field rotation, from longitudinal to transverse and vice versa, is possible by simultaneously powering both coils. During the complete procedure, the absolute field strength is never allowed to drop below 0.48 T to guarantee a high polarization percentage. In the meantime, the forces between the coils need to be controlled, and therefore the coils cannot both be carrying the nominal operational current, since this will destroy the system. The magnetic field rotation procedure is therefore uniquely developed for this system. In this paper, the magnet system is described and a summary of the operational experience gained during data taking is provided.

Operational Experience With the Combined Solenoid/Dipole Magnet System of the COMPASS Experiment at CERN

The aim of this Thesis is to contribute to the understanding of the nucleon structure through the study of two observables accessible in Semi-Inclusive Deep Inelastic Scattering (SIDIS) of high energy leptons off unpolarized protons: the transverse-momentum distributions and the amplitudes of the modulations in the azimuthal angle of the final state hadrons, the latter referred to as "azimuthal asymmetries". They give relevant information on the transverse momentum of the partons inside the nucleon and on the Boer-Mulders TMD PDF. This Thesis summarizes the work done in this direction during my PhD, which consisted in a complete analysis of part of the data collected in 2016 in COMPASS, a fixed target experiment at the CERN SPS using 160 GeV/$c$ $\mu^+$ and $\mu^-$ beams and a liquid hydrogen target. The data quality and the detector stability have been investigated, as well as the stability and the consistency of the reconstructed data. A significant effort has been put in the validation of the Monte Carlo simulations, necessary for the evaluation of the acceptance of the spectrometer and to estimate the contamination to the SIDIS sample by the hadrons produced in the decay of diffractively produced vector mesons (particularly $\rho^0$ and $\phi$). This diffractive process had also to be studied in detail and implemented in dedicated Monte Carlo simulations, reducing the systematic uncertainties affecting previous measurements. Other systematic effects have also been investigated, and the corresponding systematic uncertainties evaluated. A deep inspection of the various kinematic dependences has been performed. A phenomenological analysis of the new results is also presented, with a comparison of the current COMPASS findings with the previous ones obtained on a deuteron target.

Study of the Transverse-Momentum-Dependent structure of the nucleon in Semi-Inclusive DIS

The COMPASS-II experiment is a fixed target experiment, located at the CERN Prevessin area at the end of the M2-beamline of the Super Proton Synchrotron. The scattering of high energy leptons or hadrons of a nucleon target allows studying the spin structure of the nucleon and performing hadron spectroscopy.

The COMPASS-II Experiment

A 2.5 m long liquid hydrogen target has been developed for the COMPASS experiment at CERN to investigate the nucleon spin structure via the Deeply Virtual Compton Scattering (DVCS) process. To recognize exclusive DVCS events, produced photons and slow protons need to be detected. In order to do so, the material budget around the target has to be minimal. A 0.125 mm thick Kapton ® target cell and a 1 mm thick carbon fiber vacuum chamber with a Mylar ® window have been constructed and tested. Finally, the target system was successfully employed during the DVCS pilot run in COMPASS at the end of 2012. The objective of this paper is to give a detailed description of this newly developed liquid hydrogen target apparatus.

A 2.5 m long liquid hydrogen target for COMPASS

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A 2.5 m long liquid hydrogen target for COMPASS