Fast Photon Detection for Particle Identification with COMPASS RICH-1

Particle identification at high rates is an important challenge for many current and future high-energy physics experiments. The upgrade of the COMPASS RICH-1 detector requires a new technique for Cherenkov photon detection at count rates of several $10^6$ per channel in the central detector region, and a read-out system allowing for trigger rates of up to 100 kHz. To cope with these requirements, the photon detectors in the central region have been replaced with the detection system described in this paper. In the peripheral regions, the existing multi-wire proportional chambers with CsI photocathode are now read out via a new system employing APV pre-amplifiers and flash ADC chips. The new detection system consists of multi-anode photomultiplier tubes (MAPMT) and fast read-out electronics based on the MAD4 discriminator and the F1-TDC chip. The RICH-1 is in operation in its upgraded version for the 2006 CERN SPS run. We present the photon detection design, constructive aspects and the first Cherenkov light in the detector.Comment: Proceedings of the Imaging 2006 conference, Stockholm, Sweden, 27-30 June 2006, 5 pages, 6 figures, to appear in NIM A; corrected typo in caption of Fig.

Fast photon detection for the COMPASS RICH detector

The COMPASS experiment at the SPS accelerator at CERN uses a large scale Ring Imaging CHerenkov detector (RICH) to identify pions, kaons and protons in a wide momentum range. For the data taking in 2006, the COMPASS RICH has been upgraded in the central photon detection area (25% of the surface) with a new technology to detect Cherenkov photons at very high count rates of several 10^6 per second and channel and a new dead-time free read-out system, which allows trigger rates up to 100 kHz. The Cherenkov photons are detected by an array of 576 visible and ultra-violet sensitive multi-anode photomultipliers with 16 channels each. The upgraded detector showed an excellent performance during the 2006 data taking.Comment: Proceeding of the IPRD06 conference (Siena, Okt. 06

The Fast Read-out System for the MAPMTs of COMPASS RICH-1

A fast readout system for the upgrade of the COMPASS RICH detector has been developed and successfully used for data taking in 2006 and 2007. The new readout system for the multi-anode PMTs in the central part of the photon detector of the RICH is based on the high-sensitivity MAD4 preamplifier-discriminator and the dead-time free F1-TDC chip characterized by high-resolution. The readout electronics has been designed taking into account the high photon flux in the central part of the detector and the requirement to run at high trigger rates of up to 100 kHz with negligible dead-time. The system is designed as a very compact setup and is mounted directly behind the multi-anode photomultipliers. The data are digitized on the frontend boards and transferred via optical links to the readout system. The read-out electronics system is described in detail together with its measured performances.Comment: Proceeding of RICH2007 Conference, Trieste, Oct. 2007. v2: minor change

First Measurement of the Transverse Spin Asymmetries of the Deuteron in Semi-Inclusive Deep Inelastic Scattering

First measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarized 6-LiD target are presented. The data were taken in 2002 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. The Collins asymmetry turns out to be compatible with zero, as does the measured Sivers asymmetry within the present statistical errors.Comment: 6 pages, 2 figure

Longitudinal double spin asymmetries in single hadron quasi-real photoproduction at high pTp_T

We measured the longitudinal double spin asymmetries $A_{LL}$ for single hadron muo-production off protons and deuterons at photon virtuality $Q^2$ < 1(GeV/$\it c$)$^2$ for transverse hadron momenta $p_T$ in the range 0.7 GeV/$\it c$ to 4 GeV/$\it c$ . They were determined using COMPASS data taken with a polarised muon beam of 160 GeV/$\it c$ or 200 GeV/$\it c$ impinging on polarised $\mathrm{{}^6LiD}$ or $\mathrm{NH_3}$ targets. The experimental asymmetries are compared to next-to-leading order pQCD calculations, and are sensitive to the gluon polarisation $\Delta G$ inside the nucleon in the range of the nucleon momentum fraction carried by gluons $0.05 < x_g < 0.2$

Interplay among transversity induced asymmetries in hadron leptoproduction

In the fragmentation of a transversely polarized quark several left-right asymmetries are possible for the hadrons in the jet. When only one unpolarized hadron is selected, it exhibits an azimuthal modulation known as Collins effect. When a pair of oppositely charged hadrons is observed, three asymmetries can be considered, a di-hadron asymmetry and two single hadron asymmetries. In lepton deep inelastic scattering on transversely polarized nucleons all these asymmetries are coupled with the transversity distribution. From the high statistics COMPASS data on oppositely charged hadron-pair production we have investigated for the first time the dependence of these three asymmetries on the difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure

Interplay among transversity induced asymmetries in hadron leptoproduction

In the fragmentation of a transversely polarized quark several left-right asymmetries are possible for the hadrons in the jet. When only one unpolarized hadron is selected, it exhibits an azimuthal modulation known as Collins effect. When a pair of oppositely charged hadrons is observed, three asymmetries can be considered, a di-hadron asymmetry and two single hadron asymmetries. In lepton deep inelastic scattering on transversely polarized nucleons all these asymmetries are coupled with the transversity distribution. From the high statistics COMPASS data on oppositely charged hadron-pair production we have investigated for the first time the dependence of these three asymmetries on the difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure

The Polarised Valence Quark Distribution from semi-inclusive DIS

The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of opposite charge has been measured by the COMPASS experiment at CERN. The data were collected in the years 2002-2004 using a 160 GeV polarised muon beam scattered off a large polarised ^6LiD target and cover the range 0.006 < x < 0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an evaluation of the first moment of Delta u_v + Delta d_v which is found to be equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously measured on the same data, this result favours a non-symmetric polarisation of light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard deviations, in contrast to the often assumed symmetric scenario Delta u-bar = Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list update

Leading-order determination of the gluon polarisation from semi-inclusive deep inelastic scattering data

Using a novel analysis technique, the gluon polarisation in the nucleon is re-evaluated using the longitudinal double-spin asymmetry measured in the cross section of semi-inclusive single-hadron muoproduction with photon virtuality $Q^2>1~({\rm GeV}/c)^2$. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/$c$ polarised muon beam impinging on a polarised $^6$LiD target. By analysing the full range in hadron transverse momentum $p_{\rm T}$, the different $p_{\rm T}$-dependences of the underlying processes are separated using a neural-network approach. In the absence of pQCD calculations at next-to-leading order in the selected kinematic domain, the gluon polarisation $\Delta g/g$ is evaluated at leading order in pQCD at a hard scale of $\mu^2= \langle Q^2 \rangle = 3 ({\rm GeV}/c)^2$. It is determined in three intervals of the nucleon momentum fraction carried by gluons, $x_{\rm g}$, covering the range $0.04 \!<\! x_{ \rm g}\! <\! 0.28$~ and does not exhibit a significant dependence on $x_{\rm g}$. The average over the three intervals, $\langle \Delta g/g \rangle = 0.113 \pm 0.038_{\rm (stat.)}\pm 0.036_{\rm (syst.)}$ at $\langle x_{\rm g} \rangle \approx 0.10$, suggests that the gluon polarisation is positive in the measured $x_{\rm g}$ range.Comment: 14 pages, 6 figure

Resonance Production and ππ\pi\pi S-wave in π−+p→π−π−π++precoil\pi^- + p \rightarrow \pi^- \pi^- \pi^+ + p_{recoil} at 190 GeV/c

The COMPASS collaboration has collected the currently largest data set on diffractively produced $\pi^-\pi^-\pi^+$ final states using a negative pion beam of 190 GeV/c momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, $0.5 < m_{3\pi} < 2.5$ GeV/c$^2$ , and in 11 bins of the reduced four-momentum transfer squared, $0.1 < t < 1.0$ (GeV/c)$^2$ . This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant $3\pi$ production. In this paper, we present detailed studies on selected $3\pi$ partial waves with $J^{PC} = 0^{-+}$, $1^{++}$, $2^{-+}$, $2^{++}$, and $4^{++}$. In these waves, we observe the well-known ground-state mesons as well as a new narrow axial-vector meson $a_1(1420)$ decaying into $f_0(980) \pi$. In addition, we present the results of a novel method to extract the amplitude of the $\pi^-\pi^+$ subsystem with $I^{G}J^{PC} = 0^+ 0^{++}$ in various partial waves from the $\pi^-\pi^-\pi^+$ data. Evidence is found for correlation of the $f_0(980)$ and $f_0(1500)$ appearing as intermediate $\pi^- \pi^+$ isobars in the decay of the known $\pi(1800)$ and $\pi_2(1880)$.Comment: 96 page