Search

A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2002–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160 GeV=c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e- annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of pi+pi- pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x > 0:032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of pi+pi- measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions. A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised $^6$LiD (deuteron) and NH$_3$ (proton) targets is presented. All available data collected in the years 2002-2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160 GeV/$c$ at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from $e^+e^-$ annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of $\pi^{+}\pi^{-}$ pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range $x> 0.032$, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of $\pi^{+}\pi^{-}$ measured on the $^6$LiD target can be interpreted as indication for a cancellation of $u$ and $d$-quark transversity distributions.

Photodetection session

In the context of a project to upgrade the gas photon detectors of COMPASS RICH-1, we have performed an R&D programme aimed to develop photon detectors based on multi-layer arrangements of thick GEM electron multipliers coupled to a CsI photoconverter. For this purpose, thick GEMs have been characterised in detail including the gain performance, its dependance on the geometrical parameters and its time-evolution, a feature exhibited by the gas detectors with open insulator surfaces. The variation due to this evolution drammatically depends on the parameters themselves. In the present article we summarise the outcomes of the studies dedicated to the thick GEM gain and its evolution versus time. We also include a qualitative model which accounts for the peculiar details of the observed thick GEM gain time-evolution.