Narrow Results

Modern experiments in hadronic physics require excellent detection and tracking capabilities in very high flux environment. Probing the quark and gluon content of the nucleon indeed necessitates semi-inclusive or exclusive reactions with very small cross-sections. Micromegas detectors have therefore been rapidly identified as natural candidates to equip the hottest region of the COMPASS spectrometer at CERN. Benefitting from several years of smooth operation at COMPASS, the Micromegas have been proposed to equip the future central tracker of the CLAS12 experiment at the Jefferson Laboratory. This project triggered an important R&D on the bulk and resistive technologies and inspired several projects involving cylindrical trackers. This paper focuses on the design and performance of the COMPASS and CLAS12 Micromegas, and reviews the R&D made on the discharge reduction in large hadron fluxes.

Diffractive dissociation reactions at COMPASS provide clean access to mesons with masses below ≈ 2.5 GeV/c². This paper presents a partial wave analysis of about 420 000 π^-Pb → π^-π^-π⁺Pb events at 190 GeV/c beam momentum and with four-momentum transfer t' ∈ [0.1,1] GeV²/c². The well-known a₁(1260), a₂(1320) and π₂(1670) mesons are resolved with high quality. Also the less established states π(1800) and a₄(2040) are seen. In addition, a resonance in the spin-exotic J^PC = 1^-+ wave is observed. A mass-dependent fit results in a mass and width of and , respectively, which is consistent with the disputed hybrid candidate π₁(1600).

This paper presents an analysis of π^- Pb → X^- Pb → π^-π^-π⁺ Pb events at 190 GeV/c beam momentum and low four-momentum transfer t′ < 0.01 (GeV/c)². Coherent scattering off the lead nucleus as a whole dominates with contributions from Reggeon, Pomeron and photon exchange. Photoproduction becomes apparent at lowest t′ and can be extracted statistically as well as by a partial-wave analysis, indicating also the overlap of diffractive and photo-production of the 3π events.

In 2016 and 2017, the COMPASS Collaboration at CERN collected a large sample of DIS events with a longitudinally polarized 160 GeV/$c$ muon beam scattering off a liquid hydrogen target. Part of the collected data has been analyzed to extract preliminary results for the amplitudes of the modulations in the azimuthal angle of the charged hadrons and for their transverse momentum distributions. These observables give relevant information for the study of the transverse momentum and spin structure of the nucleon. The results, presented here for the first time, have been obtained from part of the data collected in 2016. In the analysis, a new method for the evaluation of the contribution of exclusive diffractive processes has been implemented. The azimuthal asymmetries exhibit strong kinematic dependencies, similar to those observed in COMPASS deuteron measurements. The transverse momentum distributions show smooth exponential trends and are compatible with the previous measurement on deuteron.

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The valence transversity distributions of the u- and the d-quarks have been extracted point-by-point from single-hadron production and dihadron production data measured in semi-inclusive deep inelastic scattering and in e⁺e annihilation. The extraction is based on some simple assumptions and does not require any parametrization. The transversity distributions are found to be compatible with each other and with previous analyses.

COMPASS is a high-energy physics experiment operating at the SPS at CERN. Wide physics program of the experiment comprises study of hadron structure and spectroscopy with high energy muon and hadrons beams. As for the muon-program, one of the important objectives of the COMPASS experiment is the exploration of the transverse spin structure of the nucleon via spin (in)dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and transversely polarized ${}^{6}LiD$ (in 2002, 2003 and 2004) and $N{H}_3$ (in 2007 and 2010) targets. The experimental results obtained by COMPASS for unpolarized target azimuthal asymmetries, Sivers and Collins effects and other azimuthal observables play an important role in the general understanding of the three-dimensional nature of the nucleon. Giving access to the entire twsit-2 set of transverse momentum dependent parton distribution functions and fragmentation functions COMPASS data triggers constant theoretical interest and is being widely used in phenomenological analyses and global data fits. In this review main focus is given to the very recent results obtained by the COMPASS collaboration from first ever multi-dimensional extraction of transverse spin asymmetries.

The COMPASS measurements of the azimuthal asymmetries in the hadron pairs production in DIS on transversely polarised targets are here presented. Results on oppositely charged pion and kaon pairs produced on both deuteron and proton targets are shown. They have been used to update the calculation of the Transversity function which was previously extracted from the asymmetries measured from un-identified hadron pairs. Also a new method for the direct measurement of the integral of Transversity using both COMPASS and BELLE results is presented.

The valence transversity distributions of the u- and the d-quarks have been extracted point-by-point from single-hadron production and dihadron production data measured in semi-inclusive deep inelastic scattering and in $e^{+}{e}^{}$ annihilation. The transversity distributions are found to be compatible with each other and with previous analyses.

In the fragmentation of a transversely polarized quark a left-right asymmetry, the Collins asymmetry, is expected for each hadron produced in the process $\mu N\to {\mu }^{\prime }{h}^{+}{h}^{-}X$. Similarly, an asymmetry is also expected for the hadron pair, the dihadron asymmetry. Both asymmetries have been measured to be different from zero on transversely polarised proton targets and have allowed for first extractions of the transversity distributions. From the high statistics COMPASS data we have further investigated these asymmetries getting strong indications that the two mechanisms are driven by a common physical process.

Successful realization of polarized Drell-Yan physics program is one of the main goals of the second stage of the COMPASS experiment. Drell-Yan measurements with high energy (190 GeV/c) pion beam and transversely polarized NH3 target have been initiated by a pilot-run in the October 2014 and will be followed by 140 days of data taking in 2015. In the past twelve years COMPASS experiment performed series of SIDIS measurements with high energy muon beam and transversely polarized deuteron and proton targets. Results obtained for Sivers effect and other target transverse spin dependent and unpolarized azimuthal asymmetries in SIDIS serve as an important input for general understanding of spin-structure of the nucleon and are being used in numerous theoretical and phenomenological studies being carried out in the field of transvers-spin physics. Measurement of the Sivers and all other azimuthal effects in polarized Drell-Yan at COMPASS will reveal another side of the spin-puzzle providing a link between SIDIS and Drell-Yan branches. This will be a unique possibility to test universality and key-features of transverse momentum dependent distribution functions (TMD PDFs) using essentially same experimental setup and exploring same kinematic domain. In this review main physics aspects of future COMPASS polarized Drell-Yan measurement of azimuthal transverse spin asymmetries will be presented, giving a particular emphasis on the link with very recent COMPASS results obtained for SIDIS transverse spin asymmetries from four ”Drell-Yan” $Q^2$-ranges.

Deeply Virtual Compton Scattering (DVCS) is considered as the “golden channel” for accessing Generalised Parton Distributions, which contain information on the 3D imaging of the nucleon, the composition of its spin and pressure distributions within it. We present a summary of the recent, ongoing and future DVCS measurements in the valence quark region accessible at Jefferson Lab and in the sea-quark region reached by COMPASS, and their implication for the study of nucleon structure.

COMPASS is a multi-purpose fixed-target experiment at CERN aimed at studying the structure and spectrum of hadrons. It has collected the so far world’s largest data set on diffractive production of the K⁻π⁻π⁺ decay, which in principle gives access to all kaon states. We performed an elaborate partial-wave analysis, using model-selection techniques to select the wave set based on a large systematically constructed pool of allowed partial waves. The partial- wave decomposition reveals signals in the mass region of well-known states, such as K₁(1270) and K₁(1400). In addition, we observe potential signals from excited states, such as K₁(1650).

Utilizing the 160 GeV/c polarized muon beams provided by the CERN SPS, COMPASS studies the Generalized Parton Distributions (GPDs) via Deeply Virtual Compton Scattering (DVCS) and Hard Exclusive Meson Production (HEMP). In this article, the GPD measurement program at COMPASS is introduced and some of the recent results are given.

A fast photon-detection system for the detector RICH-1 of the COMPASS Experiment at CERN SPS is in operation since the 2006 run. It is based on the use of Multi-Anode Photomultipliers (MAPMTs) coupled to individual fused silica lens telescopes and fast read-out electronics. It has been designed taking into account the high photon flux in the central region of the detector and the high rate requirements of the COMPASS Experiment. We present the photon-detection design and construction, together with its characterization and measured performances based on the data collected in 2006.

For its physics program with a high-intensity hadron beam of 2 · 10⁷particles/s, the COMPASS experiment at CERN requires tracking of charged particles scattered by very small angles with respect to the incident beam direction. While good resolution in time and space is mandatory, the challenge is imposed by the high beam intensity, requiring radiation-hard detectors which add very little material to the beam path in order to minimise secondary interactions.

To this end, a set of triple-GEM detectors with pixel readout in the beam region and 2-D strip readout in the periphery is currently being built. The pixel size has been chosen to be 1×1 mm², which constitutes a compromise between the spatial resolution achievable and the number of readout channels. Surrounding the pixel area, a 2-D strip readout with a pitch of 400 μm has been realised on the same printed circuit foil. In total an active area of 10 × 10 cm² is covered using 2048 readout channels. Analogue readout by the APV25 ASIC has been chosen in order to profit from amplitude measurements which help to improve the spatial resolution by clustering neighbouring hit strips or pixels. A detector prototype has been tested successfully in the 5 · 10⁷particles/s COMPASS muon beam, as well as in a focused hadron beam. The design of the detector and first results concerning its performance as a beam tracker will be presented.

The measuring of transverse single spin asymmetries (SSA) is part of the physics program at COMPASS, a fixed target experiment at CERN SPS. In 2002-04 the COMPASS experiment has collected data with a transversely polarised ⁶LiD target using a 160 GeV/c polarised µ⁺ beam. By measuring transverse SSA one has access to the transversity distribution function Δ_Tq(x). This is one of the three quark distribution functions, which are needed to fully describe the spin structure of the nucleon at leading twist. At COMPASS three different quark polarimeters have been used to access transversity: the Collins effect, which produces an azimuthal asymmetry in the single hadron distribution, the azimuthal target spin asymmetries of charged hadron pairs and the transverse polarisation of Λ hyperons.

In addition the SSA arising from the correlation between the transverse nucleon spin and the quark intrinsic transverse momentum (Sivers effect), was measured, together with six more transverse target spin asymmetries.

All the asymmetries measured on the deuteron target are small and compatible with zero.

COMPASS is a running fixed-target experiment at the CERN SPS with a rich physics program focused on nucleon spin structure and on hadron spectroscopy. One of the main goals of the spin program is the measurement of the transverse spin effects in semi-inclusive DIS off transversely polarised nucleons. In the years 2002, 2003 and 2004 data have been taken using a 160 GeV/c naturally polarised µ⁺ beam and a deuterium target (⁶LiD) transversely polarised respect to the beam direction. In 2007 the run year has been devoted to collect data with a proton (NH₃) target. The preliminary results for the Collins and Sivers asymmetries, extracted from the 2007 data with transverse target polarisation, are presented here. Results are also compared with existing model predictions.

Generalized parton distributions (GPDs) provide a new and powerful framework for a complete description of the nucleon structure. They can provide a three-dimensional picture of how the quarks and gluons form a nucleon. GPDs can be probed experimentally in hard exclusive meson production or deeply virtual Compton scattering (DVCS). The COMPASS experiment at CERN is a unique place to study these reactions. At COMPASS, a high energy polarized positive or negative muon beam is scattered off a polarized or unpolarized fixed target. First results for exclusive ρ⁰ meson production are shown. The transverse target spin asymmetry for exclusively produced ρ⁰ on a transversely polarized deuteron target has been measured. Prospects for future measurements of DVCS and exclusive meson production at COMPASS will be shown. The experiment will use the existing COMPASS spectrometer with a new target, a new recoil detector and extended calorimetry. Simulations for different models and a test of the recoil detector have been performed.

The COMPASS experiment is investigating possible physics aspects connect with the use of the polarized target (PT) together with the secondary (CERN–SPS) hadron beams. The main purpose of this project is to study the Drell–Yan reactions as a clear tool to measure the parton distribution functions, in particular the Boer-Mulders, the Sivers and the transversity functions.