Direct Measurement of \DeltaG/G at Compass

The gluon polarization $\Delta{G}/G$ is the key to a further clarification of the spin structure of the nucleon. The COMPASS collaboration at CERN has set out to undertake the direct determination of this quantity. It accesses the gluon distribution via the photon-gluon fusion process (PGF) in scattering polarized muons off a polarized deuteron target. And it explores three different channels to tag the PGF: open charm production and high transverse momentum (high p_T), in either electroproduction (Q^2 > 1 GeV^2) or quasi-real photoproduction (Q^2 < 1 GeV^2). The high p_T quasi-real photoproduction channel yields the most precise measurement. The result indicates that the gluon polarization is small. I describe its experimental aspects and its theoretical framework, based on PYTHIA. And I report on the preliminary results obtained in the other two channels, and on the prospects for future analysis and data taking.Comment: 11 pages, 5 figures. Prepared for the XI Workshop on High Energy Spin Physics (SPIN05), JINR, Dubna, Russia, September 27 - October 1, 2005. To be published with the conference proceeding

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

Single Hadron Multiplicities in SIDIS @ COMPASS

International audienceSingle hadron multiplicities in Semi-Inclusive Deep Inelastic Scattering (SIDIS) provide an important input for the understanding of the hadronisation process and its description in terms of Fragmentation Functions (FFs). COMPASS collaboration has undertaken a programme of measurements of these observables. I first give an overview of the results. And I then focus on the $K^-/K^+$ multiplicity ratio at large value of the energy fraction carried by the kaons, which results challenge the conventional picture of a factorisation between FFs and Parton Distribution Functions (PDFs)

Longitudinal spin structure of the nucleon: COMPASS legacy

International audienceThe COMPASS collaboration has dedicated a large fraction of its already twenty year long existence to the study of the polarised structure of the nucleon. This presentation addresses two of the investigated aspects: inclusive measurements and direct measurements of the gluon polarised PDF, Δg. In the first case, a summary of what constitutes COMPASS legacy is presented. This covers the actual measurements of structure function g 1, at both high and low Q 2, and the information extracted from them in terms of polarised PDFs, axial charge and Bjorken sum rule. In particular, the quark spin contribution to the nucleon spin is determined to be 0.26 < Δ∑ < 0.36 and from COMPASS data alone the Bjorken sum rule is verified to 9% accuracy. In the second case, the latest developments in the analysis of our data on semi-inclusive single-hadron production at high-pT are reported

Longitudinal spin structure of the nucleon at COMPASS

International audienceFor a large fraction of its already twenty year long existence, the COMPASS collaboration hasstudied the polarised structure of the nucleon via muon-nucleon scattering. This paper summa-rizes the final results obtained in longitudinally polarised DIS, either inclusive or semi-inclusive.For inclusive DIS, these cover the measurements of structure function g1 , and the information ex-tracted from them in terms of polarised PDFs (pPDFs) and Bjorken sum rule. From g1 measure-ments alone, combining COMPASS with world data, the quark spin contribution to the nucleonspin is determined to be 0.26 < ∆Σ < 0.36. And from COMPASS data alone, the Bjorken sumrule is verified to 9% accuracy. In semi-inclusive DIS, more information can be gained on thepPDFs of sea quarks and gluons. A selection of our achievements along this line are reported

Aging effects in the COMPASS hybrid GEM-Micromegas pixelized detectors

International audienceLarge-size hybrid and pixelized GEM-Micromegas gaseous detectors (40x40 cm$^2$ active area) were developed and installed in 2014 and 2015 for the COMPASS2 physics program which started at the same time. That program involved in particular two full years of Drell-Yan studies using a high-intensity pion beam on a thick polarized target. Although the detectors were placed behind a thick absorber, they were exposed to an important flux of low energy neutrons and photons. The detectors were designed to drastically reduce the discharge rate, a major issue for non-resistive Micromegas in high hadron flux, by a factor of more than 100 compared to the former ones. A hybrid solution was chosen where a pre-amplifying GEM foil is placed 2 mm above the micromesh electrode. A pixelized readout was also added in the center of the detector, where the beam is going through, in order to track particles scattered at very low angles. The combination of the hybrid structure and the pixelized central readout allowed the detector to be operated in an environment with particle flux above 10 MHz/cm$^2$ with very good detection efficiencies and spatial resolution. The performance has remained stable since 2015 in terms of gain and resolution, showing the interest of hybrid structures associating a GEM foil to a Micromegas board to protect gaseous detectors against discharges and aging effect

REST-for-Physics, a ROOT-based framework for event oriented data analysis and combined Monte Carlo response

International audienceThe REST-for-Physics (Rare Event Searches Toolkit for Physics) framework is a ROOT-based solution providing the means to process and analyze experimental or Monte Carlo event data. Special care has been taken to the traceability of the code and the validation of the results produced within the framework, together with the connectivity between code and stored data, registered through specific version metadata members. The framework development was originally motivated to cover the needs of Rare Event Searches experiments (experiments looking for phenomena having extremely low occurrence probability, like dark matter or neutrino interactions or rare nuclear decays). The framework components naturally implement tools to address the challenges in these kinds of experiments. The integration of a detector physics response, the implementation of signal processing routines, or topological algorithms for physical event identification are some examples. Despite this specialization, the framework was conceived thinking in scalability. Other event-oriented applications could benefit from the data processing routines and/or metadata description implemented in REST, being the generic framework tools completely decoupled from dedicated libraries. REST-for-Physics is a consolidated piece of software already serving the needs of different physics experiments - using gaseous Time Projection Chambers (TPCs) as detection technology - for detector data analysis and characterization, as well as generic R&D. Even though REST has been exploited mainly with gaseous TPCs, the code could be easily applied or adapted to other detector technologies. We present in this work an overview of REST-for-Physics, providing a broad perspective to the infrastructure and organization of the project as a whole. The framework and its different components will be described in the text

PandaX-III: Searching for neutrinoless double beta decay with high pressure136^{136}Xe gas time projection chambers

International audienceSearching for the neutrinoless double beta decay (NLDBD) is now regarded as the topmost promising technique to explore the nature of neutrinos after the discovery of neutrino masses in oscillation experiments. PandaX-III (particle and astrophysical xenon experiment III) will search for the NLDBD of$^{136}$Xe at the China Jin Ping Underground Laboratory (CJPL). In the first phase of the experiment, a high pressure gas Time Projection Chamber (TPC) will contain 200 kg, 90%$^{136}$Xe enriched gas operated at 10 bar. Fine pitch micro-pattern gas detector (Microbulk Micromegas) will be used at both ends of the TPC for the charge readout with a cathode in the middle. Charge signals can be used to reconstruct the electron tracks of the NLDBD events and provide good energy and spatial resolution. The detector will be immersed in a large water tank to ensure ~5 m of water shielding in all directions. The second phase, a ton-scale experiment, will consist of five TPCs in the same water tank, with improved energy resolution and better control over backgrounds

Search for exclusive photoproduction of Z(c)(+/-) (3900) at COMPASS

A search for the exclusive production of the Z(c)(+/-)(3900) hadron by virtual photons has been performed in the channel Z(c)(+/-)(3900). J/Psi pi(+/-). The data cover the range from 7GeV to 19GeV in the centre-of- mass energy of the photon-nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio BR(Z(c)(+/-)(3900)-> J/Psi pi(+/-)) x sigma(gamma N) -> Z(c)(+/-)(3900) N/sigma gamma N -> J/Psi N 3.7 x10(-3) has been established at the confidence level of90%. (C) 2015 The Authors. Published by Elsevier B.V.CERN managemen