Search for the Phi(1860) Pentaquark at COMPASS

Narrow Xi-pi+- and Xi-bar+pi+- resonances produced by quasi-real photons have been searched for by the COMPASS experiment at CERN. The study was stimulated by the recent observation of an exotic baryonic state decaying into Xi-pi-, at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states Xi(1530)^0 and Xi-bar(1530)^0 are clearly seen, no exotic baryon is observed in the data taken in 2002 and 2003.Comment: 10 pages, 5 figure

Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons

Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise. © 2015

Performance of Multiplexed XY Resistive Micromegas detectors in a high intensity beam

We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 × 10 5e−/(s-cm2). So far, all studies with multiplexed Micromegas have only been reported for tests with radioactive sources and cosmic rays. The use of multiplexed modules in high intensity environments was not explored due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. For the specific mapping and beam intensities analyzed in this work with a multiplexing factor of five, more than 50% level of ambiguity is introduced due to particle pile-up as well as fake clusters due to the mapping feature. Our results prove that by using the additional information of cluster size and integrated charge from the signal clusters induced on the XY strips, the ambiguities can be reduced to a level below 2%. The tested detectors are used in the CERN NA64 experiment for tracking the incoming particles bending in a magnetic field in order to reconstruct their momentum. The average hit detection efficiency of each module was found to be 96% at the highest beam intensities. By using four modules a tracking resolution of 1.1% was obtained with 85% combined tracking efficiency.ISSN:0168-9002ISSN:1872-957

Review on the development of cryogenic silicon detectors

In this paper, we report on the performance of heavily irradiated silicon detectors operated at cryogenic temperatures. The results discussed here show that cryogenic operation indeed represents a reliable method to increase the radiation tolerance of standard silicon detectors by more than one order of magnitude. In particular, a 400 mum thick "double- p" silicon detector irradiated up to 1 x 10(15) n/cm(2) delivers a mip signal of about 27 000 electrons when operated at 130 K and 500 V bias. The position resolution of an irradiated microstrip detector, and "in situ" irradiation of a pad detector during operation in the cold are also discussed

Silicon detectors irradiated "in situ" at cryogenic temperatures

Though several studies have proved the radiation tolerance of silicon detectors at cryogenic temperatures, following room temperature irradiation, no previous investigation has studied the behaviour of detectors irradiated "in situ" at low temperatures. In this work, effects of irradiation of 450 GeV protons at 83 K will be presented, showing that after a dose of 1.2 x 10(15) p cm(-2) a charge collection efficiency (CCE) of 55% is reached at 200 V before the annealing. The same results were found at the end of the irradiation. after the sample has spent more then one year at room temperature, This shows that the CCE recovery by low temperature operation is not affected by the temperature of irradiation and by the reverse annealing

Observation of a J**PC = 1-+ exotic resonance in diffractive dissociation of 190-GeV/c pi- into pi- pi- pi+

Alekseev M, Alexakhin VY, Alexandrov Y, et al. Observation of a J**PC = 1-+ exotic resonance in diffractive dissociation of 190-GeV/c pi- into pi- pi- pi+. Phys.Rev.Lett. 2010;104(24): 241803.The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the pi- pi- pi+ final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1 GeV^2/c^2. The well-known resonances a1(1260), a2(1320), and pi2(1670) are clearly observed. In addition, the data show a significant natural parity exchange production of a resonance with spin-exotic quantum numbers J^PC = 1-+ at 1.66 GeV/c^2 decaying to rho pi. The resonant nature of this wave is evident from the mass-dependent phase differences to the J^PC = 2-+ and 1++ waves. From a mass-dependent fit a resonance mass of 1660 +- 10+0-64 MeV/c^2 and a width of 269+-21+42-64 MeV/c^2 is deduced