GPD study programme of COMPASS at CERN Kouznetsov, O.
Nuclear and particle physics proceedings,
January-March 2016, 2016-01-00, Letnik:
270-272
Journal Article
Recenzirano
COMPASS is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. High energy polarized muon beams available at CERN and the ...option of using either positive or negative ones, give an excellent opportunity for studying GPD, through the DVCS process and the HEMP reaction. The first result of COMPASS GPD programme came from transverse target-spin asymmetries measured in hard exclusive ρ0 production using a transversely polarized proton target. Recent results on transverse target-spin asymmetries for the exclusive ω production are presented as well. Projections of the achievable accuracies and preliminary results of 2012 DVCS pilot run measurements are discussed. Beginning of that run was devoted to commissioning of the long (2.5 m) liquid hydrogen target and new detectors, the large recoil proton detector and large angle electromagnetic calorimeter. These detectors will be a backbone for the future measurements of exclusive processes during the dedicated data taking in 2016–2017.
Spin Physics Experiments at NICA-SPD Kouznetsov, O.; Savin, I.
Nuclear and particle physics proceedings,
January-March 2017, 2017-01-00, Letnik:
282-284
Journal Article
Recenzirano
Nuclotron based Ion Collider fAcility (NICA) is a flagship project of the Joint Institute for Nuclear Research which is expected to be operational by 2021. Main tasks of “NICA Facility” are study of ...hot and dense baryonic matter, investigation the polarisation phenomena and the nucleon spin structure. The material presented here based on the Letter of Intent (LoI) dedicated to nucleon spin structure studies at NICA. Measurements of asymmetries in the lepton pair (Drell-Yan) production in collisions of non-polarised, longitudinally and transversely polarised proton and deuteron beams to be performed using the specialized Spin Physics Detector (SPD). These measurements can provide an access to all leading twist collinear and Transverse Momentum Dependent Parton Distribution Functions (TMD PDFs) in nucleons. The measurements of asymmetries in production of J/ψ and direct photons, which supply complimentary information on the nucleon structure, will be performed simultaneously. The set of these measurements permits to tests the quark-parton model of nucleons at the QCD twist-2 level with minimal systematic errors.
Eight proton transverse-spin-dependent azimuthal asymmetries are extracted in four regions of the photon virtuality Q2 from the COMPASS 2010 semi-inclusive hadron measurements in deep inelastic ...muon–nucleon scattering. These Q2 regions correspond to the four regions of the di-muon mass Q2 used in the ongoing analyses of the COMPASS Drell–Yan measurements, which allows for a future direct comparison of the nucleon transverse-momentum-dependent parton distribution functions extracted from these two alternative measurements. In addition, for the azimuthal asymmetries induced by the Sivers transverse-momentum-dependent parton distribution function various two-dimensional kinematic dependences are presented. The integrated Sivers asymmetries are found to be positive with an accuracy that appears to be sufficient to test the sign change of the Sivers function predicted by Quantum Chromodynamics.
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.
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive
ω
meson muoproduction on the proton at COMPASS using 160 GeV/
c
polarised
μ
+
and
μ
-
beams impinging on a liquid ...hydrogen target. The measurement covers the range 5.0 GeV/
c
2
<
W
<
17.0 GeV/
c
2
, with the average kinematics
⟨
Q
2
⟩
=
2.1 (GeV/
c
)
2
,
⟨
W
⟩
=
7.6
GeV/
c
2
, and
⟨
p
T
2
⟩
=
0.16
(GeV/
c
)
2
. Here,
Q
2
denotes the virtuality of the exchanged photon,
W
the mass of the final hadronic system and
p
T
the transverse momentum of the
ω
meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (
γ
T
∗
→
V
L
) indicate a violation of
s
-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing
W
. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive
ω
production.
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.
Single hadron azimuthal asymmetries of positive and negative hadrons produced in muon semi-inclusive deep inelastic scattering off longitudinally polarised deuterons are determined using the 2006 ...COMPASS data and also combined all deuteron COMPASS data. For each hadron charge, the dependence of the azimuthal asymmetry on the hadron azimuthal angle
ϕ
is obtained by means of a five-parameter fitting function that besides a
ϕ
-independent term includes four modulations predicted by theory:
sin
ϕ
,
sin
2
ϕ
,
sin
3
ϕ
and
cos
ϕ
. The amplitudes of the five terms have been extracted, first, for the hadrons in the whole available kinematic region. In further fits, performed for hadrons from a restricted kinematic region, the
ϕ
-dependence is determined as a function of one of three variables (Bjorken-
x
, fractional energy of virtual photon taken by the outgoing hadron and hadron transverse momentum), while disregarding the others. Except the
ϕ
-independent term, all the modulation amplitudes are very small, and no clear kinematic dependence could be observed within experimental uncertainties.
PDF
