We present a model for Monte Carlo simulation of the fragmentation of a polarized quark. The model is based on string dynamics and the P30 mechanism of quark pair creation at string breaking. The ...fragmentation is treated as a recursive process, where the splitting function of the subprocess q→h+q′ depends on the spin density matrix of the quark q. The P30 mechanism is parametrized by a complex mass parameter μ, the imaginary part of which is responsible for single spin asymmetries. The model has been implemented in a Monte Carlo program to simulate jets made of pseudoscalar mesons. Results for single hadron and hadron pair transverse-spin asymmetries are found to be in agreement with experimental data from SIDIS and e+e− annihilation. The model predictions on the jet-handedness are also discussed.
The investigation of the spin structure of the nucleon via semi-inclusive deep inelastic scattering on polarised nucleons is updated with the most recent results of the JLab, HERMES, and COMPASS ...experiments. A short description is given of these experiments, which are complementary in phase space and use sophisticated and different techniques to polarise the nucleon targets. The cases of target spin parallel or orthogonal to the direction of the incoming lepton require different experimental techniques, have a different phenomenology and need a different theoretical treatment. After reviewing these differences, the most recent transverse spin advances are presented, and evidence is given that the new data already allow for a rather precise extraction of the transversity and of the Sivers PDFs.
In recent years information on the transversity distribution h1 has been obtained by combining the Collins asymmetry results from semi-inclusive deep inelastic scattering (SIDIS) data on transversely ...polarized nucleon targets with the information on the fragmentation function of a transversely polarized quark from the asymmetries measured in e+e− annihilation into hadrons. An alternative method was proposed a long time ago, which does not require the e+e− data but allows one to get ratios of the u and d quark transversity distributions from the SIDIS data alone. The method utilizes the ratio of the difference of the Collins asymmetries of positively and negatively charged hadrons produced on transversely polarized proton and deuteron targets. We have applied this method to the COMPASS proton and deuteron data and extracted the ratio h1dv/h1uv. The results are compared to those obtained in a previous point-by-point extraction based both on SIDIS and e+e− data.
After pioneering gaseous detectors of single photon for RICH applications using CsI solid state photocathodes in MWPCs within the RD26 collaboration and by the constructions for the RICH detector of ...the COMPASS experiment at CERN SPS, in 2016 we have upgraded COMPASS RICH by novel gaseous photon detectors based on MPGD technology. Four novel photon detectors, covering a total active area of 1.5m2 have been installed in order to cope with the challenging efficiency and stability requirements of the COMPASS physics programme. They are the first application in an experiment of MPGD-based single photon detectors. All aspects of the upgrade are presented, including engineering, mass production, quality assessment and performance.
Perspectives for further developments in the field of gaseous single photon detectors are also presented.
The novel MPGD-based photon detectors of COMPASS RICH-1 consist of large-size hybrid MPGDs with multi-layer architecture including two layers of Thick-GEMs and a bulk resistive MicroMegas. The top ...surface of the first THGEM is coated with a CsI film which also acts as photo-cathode. These detectors have been successfully in operation at COMPASS since 2016. Concerning bias-voltage supply, the Thick-GEMs are segmented in order to reduce the energy released in case of occasional discharges, while the MicroMegas anode is segmented into pads individually biased with positive voltage while the micromesh is grounded. In total, there are about ten different electrode types and more than 20000 electrodes supplied by more than 100 HV channels, where appropriate correlations among the applied voltages are required for the correct operation of the detectors. Therefore, a robust control system is mandatory, implemented by a custom designed software package, while commercial power supply units are used. This sophisticated control system allows to protect the detectors against errors by the operator, to monitor and log voltages and currents at 1 Hz rate, and automatically react to detector misbehavior. In addition, a voltage compensation system has been developed to automatically adjust the biasing voltage according to environmental pressure and temperature variations, to achieve constant gain over time. This development answers to a more general need. In fact, voltage compensation is always a requirement for the stability of gaseous detectors and its need is enhanced in multi-layer ones.
In this paper, the HV system and its performance are described in details, as well as the stability of the novel MPGD-based photon detectors during the physics data taking at COMPASS.
We describe a Monte Carlo code for the fragmentation of polarized quarks into pseudoscalar mesons. The quark jet is generated by iteration of the splitting q → h + q′ where q and q′ indicate quarks ...and h a hadron. The splitting function describing the energy sharing between q′ and h is calculated on the basis of the Symmetric Lund Model where the quark spin is introduced through spin matrices as foreseen in the 3P0 mechanism. A complex mass parameter is introduced for the parametrisation of the Collins effect. The results for the Collins analysing power and the comparison with the Collins asymmetries measured by the COMPASS collaboration are presented. For the first time preliminary results on the simulated azimuthal asymmetry due to the Boer-Mulders function are also given.
We present nucleon–antinucleon scattering experiments performed at the Low Energy Antiproton Ring (LEAR) of CERN. The data are reviewed and the underlying physics is discussed, in particular by ...comparison with the predictions of current models based on meson exchange and short-range absorption. A detailed description is given of protonium, which gives information on the interaction at zero energy and is the initial state when annihilation occurs at rest.
In 2016 we have upgraded the COMPASS RICH by novel gaseous photon detectors based on MPGD technology. Four new photon detectors, covering a total active area of 1.5
m
2
, have been installed in order ...to cope with the challenging efficiency and stability requirements of the COMPASS physics programme. The new detector architecture consists in a hybrid MPGD combination: two layers of THGEMs, the first of which also acts as a reflective photocathode thanks to CsI coating, are coupled to a bulk Micromegas on a pad-segmented anode. These detectors are the first application in an experiment of MPGD-based single photon detectors. Presently, we are further developing the MPGD-based PDs to make them adequate for a setup at the future EIC collider. All aspects of the COMPASS RICH-1 Photon Detectors upgrade are presented: R&D, engineering, mass production, QA and performance; the on-going development for collider application is also presented.
We report about the development status of large area gaseous single photon detectors based on a novel hybrid concept for RICH applications.
The hybrid concept combines Thick Gaseous Electron ...Multipliers (THGEMs) coupled to CsI, working as a photon sensitive pre-amplification stage, and Micromegas, as a multiplication stage. The most recent achievements within the research and development programme consist in the assembly and study of 300×300mm2 hybrid photon detectors, the optimization of front-end electronics, and engineering towards large area detectors. Hybrid detectors with an active area of 300×300mm2 have been successfully operated in laboratory conditions and at a CERN PS T10 test beam, achieving effective gains in the order of 105 and good time resolution (σ=7ns); APV25 front-end chips have been coupled to the detector resulting in noise levels lower than 1000 electrons; the production and characterization of 300×600mm2 THGEMs is ongoing.
A set of hybrid detectors with 600×600mm2 active area is envisaged to upgrade COMPASS RICH-1 at CERN in 2016.
•The hybrid detector is a novel concept for large area gaseous photon detectors.•The hybrid concept combines Thick Gaseous Electron Multipliers and Micromegas.•The COMPASS RICH-1 upgrade will adopt hybrid detectors to replace the existing MWPCs
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