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.
Evidence for a flavor asymmetry between the $\bar{u}$ and $\bar{d}$ quark distributions in the proton has been found in deep-inelastic scattering and DrellYan experiments. The pronounced dependence ...of this flavor asymmetry on x (fraction of nucleon momentum carried by partons) observed in the Fermilab E866 Drell-Yan experiment suggested a drop of the $\bar{d}$(x)/$\bar{u}$(x) ratio in the x > 0.15 region. We report results from the SeaQuest Fermilab E906 experiment with improved statistical precision for $\bar{d}$(x)/$\bar{u}$(x) in the large x region up to x = 0.45 using the 120 GeV proton beam. Two different methods for extracting the Drell-Yan cross section ratios, σpd/2σpp, from the SeaQuest data give consistent results. The $\bar{d}$(x)/$\bar{u}$(x) ratios and the $\bar{d}$(x)- $\bar{u}$(x) differences are deduced from these cross section ratios for 0.13 < x < 0.45. The SeaQuest and E866/NuSea $\bar{d}$(x)/$\bar{u}$(x) ratios are in good agreement for the x≲0.25 region. The new SeaQuest data, however, show that $\bar{d}$(x) continues to be greater than $\bar{u}$(x) up to the highest x value (x = 0.45). The new results on $\bar{d}$(x)/$\bar{u}$(x) and $\bar{d}$(x)-$\bar{u}$(x) are compared with various parton distribution functions and theoretical calculations.
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive
ρ
0
meson muoproduction at COMPASS using 160 GeV/
c
polarised
μ
+
and
μ
-
beams impinging on a liquid hydrogen ...target. The measurement covers the kinematic range 5.0 GeV/
c
2
<
W
<
17.0 GeV/
c
2
, 1.0 (GeV/
c
)
2
<
Q
2
<
10.0 (GeV/
c
)
2
and 0.01 (GeV/
c
)
2
<
p
T
2
<
0.5 (GeV/
c
)
2
. Here,
W
denotes the mass of the final hadronic system,
Q
2
the virtuality of the exchanged photon, and
p
T
the transverse momentum of the
ρ
0
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 dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive
ρ
0
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.
A
bstract
Beam-helicity and beam-charge asymmetries in the hard exclusive leptoproduction of real photons from an unpolarised hydrogen target by a 27.6 GeV lepton beam are extracted from the H
ermes
...data set of 2006-2007 using a missing-mass event selection technique. The asymmetry amplitudes extracted from this data set are more precise than those extracted from the earlier data set of 1996-2005 previously analysed in the same manner by H
ermes
. The results from the two data sets are compatible with each other. Results from these combined data sets are extracted and constitute the most precise asymmetry amplitude measurements made in the H
ermes
kinematic region using a missing-mass event selection technique.
A
bstract
The beam-helicity asymmetry in exclusive electroproduction of real photons by the longitudinally polarized Hera positron beam scattering off an unpolarized hydrogen target is measured at ...Hermes. The asymmetry arises from deeply virtual Compton scattering and its interference with the Bethe-Heitler process. Azimuthal amplitudes of the beam-helicity asymmetry are extracted from a data sample consisting of
ep
→
epγ
events with detection of all particles in the final state including the recoiling proton. The installation of a recoil detector, while reducing the acceptance of the experiment, allows the elimination of background from
ep
→
eN πγ
events, which was estimated to contribute an average of about 12% to the signal in previous Hermes publications. The removal of this background from the present data sample is shown to increase the magnitude of the leading asymmetry amplitude by 0
.
054 ± 0
.
016 to −0
.
328 ± 0
.
027 (stat
.
) ± 0
.
045 (syst
.
).
Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton ...scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon:
with respect to the target polarisation and
with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin φ and cos(0φ) amplitudes observed respectively for the
and
asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2φ) modulation in the
asymmetry with a magnitude similar to that of the sin φ modulation is observed.
PDF
