The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS 190 GeV/c π^{-} beam and a transversely polarized ...ammonia target. Three azimuthal asymmetries giving access to different transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are extracted using dimuon events with invariant mass between 4.3 GeV/c^{2} and 8.5 GeV/c^{2}. Within the experimental uncertainties, the observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD PDFs extracted from DY have a sign opposite to the one extracted from semi-inclusive deep-inelastic scattering (SIDIS) data. We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs. A recent COMPASS SIDIS measurement was obtained at a hard scale comparable to that of these DY results. This opens the way for possible tests of fundamental QCD universality predictions.
We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160GeV/c polarised μ+ and μ− beams of the CERN SPS impinging on a liquid hydrogen target. We ...determine the dependence of the average of the measured μ+ and μ− cross sections for deeply virtual Compton scattering on the squared four-momentum transfer t from the initial to the final proton. The slope B of the t-dependence is fitted with a single exponential function, which yields B=(4.3±0.6stat|sys−0.3+0.1)(GeV/c)−2. This result can be converted into a transverse extension of partons in the proton, 〈r⊥2〉=(0.58±0.04stat|sys−0.02+0.01±0.04model)fm. For this measurement, the average virtuality of the photon mediating the interaction is 〈Q2〉=1.8(GeV/c)2 and the average value of the Bjorken variable is 〈xBj〉=0.056.
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
(
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
T
, the different
p
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
Δ
g
/
g
is evaluated at leading order in pQCD at a hard scale of
μ
2
=
⟨
Q
2
⟩
=
3
(
GeV
/
c
)
2
. It is determined in three intervals of the nucleon momentum fraction carried by gluons,
x
g
, covering the range
0.04
<
x
g
<
0.28
and does not exhibit a significant dependence on
x
g
. The average over the three intervals,
⟨
Δ
g
/
g
⟩
=
0.113
±
0
.
038
(
stat
.
)
±
0
.
036
(
syst
.
)
at
⟨
x
g
⟩
≈
0.10
, suggests that the gluon polarisation is positive in the measured
x
g
range.
Basic instrumentation for Hall A at Jefferson Lab Arrington, J.; Averett, T.; Beise, E.J. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2004, Volume:
522, Issue:
3
Journal Article
Peer reviewed
Open access
The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro- and photo-induced reactions at very high luminosity and good momentum and angular ...resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2×10
−4. A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of
10
38
cm
−2
s
−1
. The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium.
Discharge studies in micromegas detectors in a 150 GeV/c pion beam Procureur, S; Aune, S; Ball, J ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2011, Volume:
659, Issue:
1
Journal Article
Peer reviewed
A detailed study of several Micromegas detectors prototyped for the COMPASS and CLAS12 experiments is presented. Using a 150 GeV/c pion beam, the discharge probability was measured for several ...detector variants including bulk and non-bulk Micromegas. A detector equipped with an additional GEM foil as pre-amplification stage was also tested. A resistive coating of the readout strips was found to reduce the amplitude of the discharge by at least two orders of magnitude which was below the detection limit of the experimental setup. The effects of the micro-mesh type and material were investigated as well as the influence of the drift gap. Response in the presence of a 1.5 T transverse magnetic field was also studied. The measurements presented were performed during a RD51 beam test period.
Discharge studies in Micromegas detectors in low energy hadron beams Charles, G.; Anfreville, M.; Aune, S. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2011, Volume:
648, Issue:
1
Journal Article
Peer reviewed
We present measurements of discharge rates in various Micromegas detectors in low energy hadron beams for applications foreseen at the future experiments CLAS12 at JLab and MINOS at GSI. The tests ...were performed in the T11 beam line of the CERN/PS using hadron beams with momentum between 0.2 and 3GeV/c. The discharge probability decreases smoothly below 1GeV/c, except at some specific energies where narrow peaks are observed. The position of the peaks corresponds to the stop point of protons and heavier particles, predicted by the Geant4 simulation. Discharge rates were also measured using Micromegas prototypes equipped with a GEM foil as foreseen for the COMPASS experiment at CERN.
A major advance in accurate electron beam polarization measurement has been achieved at Jlab Hall A with a Compton polarimeter based on a Fabry–Perot cavity photon beam amplifier. At an electron ...energy of 4.6
GeV and a beam current of 40
μ
A, a total relative uncertainty of 1.5% is typically achieved within 40
min of data taking. Under the same conditions monitoring of the polarization is accurate at a level of 1%. These unprecedented results make Compton polarimetry an essential tool for modern parity-violation experiments, which require very accurate electron beam polarization measurements.
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.
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