-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
G. K. Mallot,
A. Mann,
C. Marchand,
A. Martin,
J. Marzec,
T. Matsuda,
G. Meshcheryakov,
W. Meyer,
T. Michigami,
Yu. V. Mikhailov
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
M. Wilfert,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The gluon polarisation in the nucleon was measured using open charm
production by scattering 160 GeV/c polarised muons off longitudinally polarised
protons or deuterons. The data were taken by the COMPASS collaboration between
2002 and 2007. A detailed account is given of the analysis method that includes
the application of neural networks. Several decay channels of D^0 mesons are
investigated. Longitudinal spin asymmetries of the D meson production
cross-sections are extracted in bins of D^0 transverse momentum and energy. At
leading order QCD accuracy the average gluon polarisation is determined as
(Delta g/G)^LO=-0.06 +/- 0.21 (stat.) +/- 0.08 (syst.) at the scale <mu^2> ~13
(GeV/c)^2 and an average gluon momentum fraction ~ 0.11. For the first time,
the average gluon polarisation is also obtained at next-to-leading order QCD
accuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at the
scale <mu^2> ~ 13 (\GeV/c)^2 and ~ 0.20.
11/2012;
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
M. Wilfert,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The production of D* and D mesons in inelastic scattering of 160 GeV/c muons
off a ^6LiD target has been investigated with the COMPASS spectrometer at CERN
for 0.003 (GeV/c)^2 < Q^2 < 10 (GeV/c)^2 and 3x10^-5< x_Bj < 0.1. The study is
based on 8100 events where a D^0 or anti D^0 is detected subsequently to a D*+
or D*- decay, and on 34000 events, where only a D^0 or anti D^0 is detected.
Kinematic distributions of D*, D and K*_2 are given as a function of their
energy E, transverse momentum p_T, energy fraction z, and of the virtual photon
variables nu, Q^2 and x_Bj. Semi-inclusive differential D* production
cross-sections are compared with theoretical predictions for D* production via
photon-gluon fusion into open charm. The total observed production
cross-section for D*+/- mesons with laboratory energies between 22 and 86 GeV
is 1.9 nb. Significant cross-section asymmetries are observed between D*+ and
D*- production for nu<40 GeV and z>0.6.
European Physical Journal C 11/2012; 72(12). · 3.63 Impact Factor
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badełek,
F. Balestra, [......],
M. Wilfert,
R. Windmolders,
W. Wiślicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The transverse target spin azimuthal asymmetry A_UT in hard exclusive
production of rho^0 mesons was measured at COMPASS by scattering 160 GeV/c
muons off transversely polarised protons and deuterons. The measured asymmetry
is sensitive to the nucleon helicity-flip generalised parton distributions E^q,
which are related to the orbital angular momentum of quarks in the nucleon. The
Q^2, x_B and p_t^2 dependence of A_UT is presented in a wide kinematic range.
Results for deuterons are obtained for the first time. The measured asymmetry
is small in the whole kinematic range for both protons and deuterons, which is
consistent with the theoretical interpretation that contributions from GPDs E^u
and E^d approximately cancel.
07/2012;
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badełek,
F. Balestra, [......],
L. Wang,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The cross section for production of charged hadrons with high transverse
momenta in scattering of 160 GeV/c muons off nucleons at low photon
virtualities has been measured at the COMPASS experiment at CERN. The results,
which cover transverse momenta from 1.1 to 3.6 GeV/c, are compared to a
next-to-leading order perturbative Quantum Chromodynamics (NLO pQCD)
calculation in order to evaluate the applicability of pQCD to this process in
the kinematic domain of the experiment. The shape of the calculated
differential cross section as a function of transverse momentum is found to be
in good agreement with the experimental data, but the normalization is
underestimated by NLO pQCD. This discrepancy may point towards the relevance of
terms beyond NLO in the pQCD framework. The dependence of the cross section on
the pseudo-rapidity and on the charge of the hadrons is also discussed.
07/2012;
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
M. Wilfert,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The COMPASS Collaboration at CERN has measured the transverse spin azimuthal
asymmetry of charged hadrons produced in semi-inclusive deep inelastic
scattering using a 160 GeV positive muon beam and a transversely polarised NH_3
target. The Collins asymmetry of the proton was extracted in the Bjorken x
range 0.003<x<0.7. These new measurements confirm with higher accuracy previous
measurements from the COMPASS and HERMES collaborations, which exhibit a
definite effect in the valence quark region. The asymmetries for negative and
positive hadrons are similar in magnitude and opposite in sign. They are
compatible with model calculations in which the u-quark transversity is
opposite in sign and somewhat larger than the d-quark transversity distribution
function. The asymmetry is extracted as a function of Bjorken $x$, the relative
hadron energy $z$ and the hadron transverse momentum p_T^h. The high statistics
and quality of the data also allow for more detailed investigations of the
dependence on the kinematic variables. These studies confirm the leading-twist
nature of the Collins asymmetry.
05/2012;
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
M. Wilfert,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The COMPASS Collaboration at CERN has measured the transverse spin azimuthal
asymmetry of charged hadrons produced in semi-inclusive deep inelastic
scattering using a 160 GeV positive muon beam and a transversely polarised NH_3
target. The Sivers asymmetry of the proton has been extracted in the Bjorken x
range 0.003<x<0.7. The new measurements have small statistical and systematic
uncertainties of a few percent and confirm with considerably better accuracy
the previous COMPASS measurement. The Sivers asymmetry is found to be
compatible with zero for negative hadrons and positive for positive hadrons, a
clear indication of a spin-orbit coupling of quarks in a transversely polarised
proton. As compared to measurements at lower energy, a smaller Sivers asymmetry
for positive hadrons is found in the region x > 0.03. The asymmetry is
different from zero and positive also in the low x region, where sea-quarks
dominate. The kinematic dependence of the asymmetry has also been investigated
and results are given for various intervals of hadron and virtual photon
fractional energy. In contrast to the case of the Collins asymmetry, the
results on the Sivers asymmetry suggest a strong dependence on the
four-momentum transfer to the nucleon, in agreement with the most recent
calculations.
05/2012;
-
H. Fonvieille,
G. Laveissiere,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa,
B. D. Anderson, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W. -M. Zhang,
J. Zhao,
Z. -L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson
Lab using the exclusive photon electroproduction reaction (e p --> e p gamma).
This paper gives a detailed account of the analysis which has led to the
determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the
electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and
beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76
GeV^2. These data, together with the results of VCS experiments at lower
momenta, help building a coherent picture of the electric and magnetic GPs of
the proton over the full measured Q^2-range, and point to their non-trivial
behavior.
05/2012;
-
GDH and Collaborations,
J. Ahrens,
S. Altieri,
J. R. M. Annand,
H. -J. Arends,
R. Beck,
M. A. Blackston,
C. Bradtke,
A. Braghieri, N. d'Hose, [......],
M. Rost,
T. Rostomyan,
D. Ryckbosch,
M. Schumacher,
B. Seitz,
G. Tamas,
A. Thomas,
R. Van de Vyver,
H. R. Weller,
F. Zapadtka
[show abstract]
[hide abstract]
ABSTRACT: The helicity dependence of the total cross-section and the invariant-mass distributions in the (pπ±) and (π+π-) final states for the γp → pπ+π- reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed
with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this
channel is found to be predominantly excited by the intermediate production of a Δπ state, both the measured unpolarized and
the helicity-dependent observables are generally not well described by the existing theoretical models.
European Physical Journal A 04/2012; 34(1):11-21. · 2.19 Impact Factor
-
D Neyret,
P Abbon,
M Anfreville,
Y Bedfer,
E Burtin,
C Coquelet, N d'Hose,
D Desforge,
A Giganon,
D Jourde,
F Kunne,
A Magnon,
N Makke,
C Marchand,
B Paul,
S Platchkov,
F Thibaud,
M Usseglio,
M Vandenbroucke
[show abstract]
[hide abstract]
ABSTRACT: New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm2, 10 times larger than for the present Micromegas detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Two solutions for a reduction of the impact of discharges have been studied, with Micromegas detectors using resistive layers and using an additional GEM foil. The performance of such detectors has been measured during beam test periods. A large size prototype with nominal active area and pixelized read-out has been produced and installed in the COMPASS spectrometer in 2010. In 2011 prototypes featuring an additional GEM foil, as well as a resistive prototype, were tested in similar conditions and preliminary results from those detectors are very promising. We present here the project and report on its status, in particular the performance of large size prototypes with an additional GEM foil.
Journal of Instrumentation 03/2012; 7(03):C03006. · 1.87 Impact Factor
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badellek,
F. Balestra, [......],
L. Wang,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlevd,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: First measurements of azimuthal asymmetries in hadron-pair production in
deep-inelastic scattering of muons on transversely polarised ^6LiD (deuteron)
and NH_3 (proton) targets are presented. The data were taken in the years
2002-2004 and 2007 with the COMPASS spectrometer using a muon beam of 160 GeV/c
at the CERN SPS. The asymmetries provide access to the transversity
distribution functions, without involving the Collins effect as in single
hadron production. The sizeable asymmetries measured on the NH_ target indicate
non-vanishing u-quark transversity and two-hadron interference fragmentation
functions. The small asymmetries measured on the ^6LiD target can be
interpreted as indication for a cancellation of u- and d-quark transversities.
02/2012;
-
C. Adolph,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. A. Antonov,
A. Austregesilo,
B. Badelek,
F. Balestra, [......],
L. Wang,
R. Windmolders,
W. Wislicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: We present a determination of the gluon polarisation Delta g/g in the
nucleon, based on the longitudinal double-spin asymmetry of DIS events with a
pair of large transverse-momentum hadrons in the final state. The data were
obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon
beam scattering off a polarised ^6LiD target. The gluon polarisation is
evaluated by a Neural Network approach for three intervals of the gluon
momentum fraction x_g covering the range 0.04 < x_g < 0.27. The values obtained
at leading order in QCD do not show any significant dependence on x_g. Their
average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (syst.) at x_g=0.09
and a scale of mu^2 = 3 (GeV/c)^2.
02/2012;
-
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. Austregesilo,
B. Badelek,
F. Balestra,
J. Barth,
G. Baum, [......],
N. V. Vlassov,
R. Windmolders,
W. Wiślicki,
H. Wollny,
K. Zaremba,
M. Zavertyaev,
E. Zemlyanichkina,
M. Ziembicki,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The COMPASS collaboration at CERN has investigated the \pi^- \gamma -> \pi^-
\pi^- \pi^+ reaction at center-of-momentum energy below five pion masses,
sqrt(s) < 5 m(\pi), embedded in the Primakoff reaction of 190 GeV pions
impinging on a lead target. Exchange of quasi-real photons is selected by
isolating the sharp Coulomb peak observed at smallest momentum transfers, t' <
0.001 (GeV/c)^2. Using partial-wave analysis techniques, the scattering
intensity of Coulomb production described in terms of chiral dynamics and its
dependence on the 3\pi-invariant mass m(3\pi) = sqrt(s) were extracted. The
absolute cross section was determined in seven bins of $\sqrt{s}$ with an
overall precision of 20%. At leading order, the result is found to be in good
agreement with the prediction of chiral perturbation theory over the whole
energy range investigated.
11/2011;
-
M G Alekseev,
V Yu Alexakhin,
Yu Alexandrov,
G D Alexeev,
A Amoroso,
A Austregesilo,
B Badełek,
F Balestra,
J Ball,
J Barth, [......],
R Windmolders,
W Wiślicki,
H Wollny,
K Zaremba,
M Zavertyaev,
E Zemlyanichkina,
M Ziembicki,
J Zhao,
N Zhuravlev,
A Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π- π- π+ final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420,000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1 GeV2/c2. The well-known resonances a1(1260), a2(1320), and π2(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J(PC)=1-+ at 1.66 GeV/c2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC)=2-+ and 1++ waves. From a mass-dependent fit a resonance mass of (1660±10(-64)(+0)) MeV/c2 and a width of (269±21(-64)(+42)) MeV/c2 are deduced, with an intensity of (1.7±0.2)% of the total intensity.
Physical Review Letters 06/2010; 104(24):241803. · 7.37 Impact Factor
-
D Neyret,
M Anfreville,
Y Bedfer,
E Burtin, N d'Hose,
A Giganon,
B Ketzer,
I Konorov,
F Kunne,
A Magnon,
C Marchand,
B Paul,
S Platchkov,
M Vandenbroucke
[show abstract]
[hide abstract]
ABSTRACT: New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm2, 10 times larger than for the present detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Studies were done with the present detectors moved in the beam, and two first pixelized prototypes are being tested with muon and hadron beams in real conditions at COMPASS. We present here this new project and report on two series of tests, with old detectors moved into the beam and with pixelized prototypes operated in real data taking condition with both muon and hadron beams.
Journal of Instrumentation 12/2009; 4(12):P12004. · 1.87 Impact Factor
-
G. Laveissière,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa,
B. D. Anderson,
K. A. Aniol, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W.-M. Zhang,
J. Zhao,
Z.-L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: We have made the first measurements of the virtual Compton scattering (VCS) process via the H(e, e'p)γ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the W-dependence at fixed Q2=1 GeV2 and for the Q2 dependence at fixed W near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed Q2 dependence is smooth. The measured ratio of H(e, e'p)γ to H(e, e'p)π0 cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to real Compton scattering (RCS) at high energy and large angles, our VCS data at the highest W (1.8-1.9 GeV) show a striking Q2 independence, which may suggest a transition to a perturbative scattering mechanism at the quark level.
Phys. Rev. C. 01/2009; 79(1).
-
The MAMI-A1 Collaboration: P. Janssens,
L. Doria,
P. Achenbach,
C. Ayerbe Gayoso,
D. Baumann,
J. C. Bernauer,
I. K. Bensafa,
R Böhm,
D. Bosnar,
E Burtin, [......],
S. Sanchez Majos,
B. S. Schlimme,
M. Seimetz,
S. Širca,
G. Tamas,
R. Van de Vyver,
L. Van Hoorebeke,
A. Van Overloop,
Th. Walcher,
M. Weinriefer
[show abstract]
[hide abstract]
ABSTRACT: The cross section of the $ep \to e' p' \gamma$ reaction has been measured at $Q^2 = 0.33$ (GeV/c)$^2$. The experiment was performed using the electron beam of the MAMI accelerator and the standard detector setup of the A1 Collaboration. The cross section is analyzed using the low-energy theorem for virtual Compton scattering, yielding a new determination of the two structure functions $P_LL}-P_{TT}/epsilon$ and $P_{LT}$ which are linear combinations of the generalized polarizabilities of the proton. We find somewhat larger values than in the previous investigation at the same $Q^2$. This difference, however, is purely due to our more refined analysis of the data. The results tend to confirm the non-trivial $Q^2$-evolution of the generalized polarizabilities and call for more measurements in the low-$Q^2$ region ($\le$ 1 (GeV/c)$^2$).
04/2008;
-
The COMPASS Collaboration,
M Alekseev,
V Yu Alexakhin,
Yu Alexandrov,
G D Alexeev,
A Amoroso,
A. Arbuzov,
B Badełek,
F Balestra,
J Ball, [......],
R Windmolders,
S Wirth,
W Wiślicki,
H Wollny,
K Zaremba,
M. Zavertyaev,
E Zemlyanichkina,
J Zhao,
R Ziegler,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The measurements of the Collins and Sivers asymmetries of identified hadrons
produced in deep-inelastic scattering of 160 GeV/c muons on a transversely
polarised 6LiD target at COMPASS are presented. The results for charged pions
and charged and neutral kaons correspond to all data available, which were
collected from 2002 to 2004. For all final state particles both the Collins and
Sivers asymmetries turn out to be small, compatible with zero within the
statistical errors, in line with the previously published results for not
identified charged hadrons, and with the expected cancellation between the u-
and d-quark contributions.
02/2008;
-
E S Ageev,
V Yu Alexakhin,
Y. Alexandrov,
G D Alexeev,
A Amoroso,
B Badelek,
F Balestra,
J Ball,
G Baum,
Y Bedfer, [......],
U Wiedner,
M Wiesmann,
R Windmolders,
S Wirth,
W Wilicki,
A M Zanetti,
K Zaremba,
J Zhao,
R Ziegler,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: We present a precise measurement of the deuteron longitudinal spin asymmetry $A_1^d$ and of the deuteron spin-dependent structure function $g_1^d$ at $Q^2 < $ 1~(GeV/$c$)$^2$ and $4\cdot$10$^{-5} < x < $~2.5$\cdot$10$^{-2}$ based on the data collected by the COMPASS experiment at CERN during the years 2002 and 2003. The statistical precision is tenfold better than that of the previous measurement in this region. The measured $A_1^d$ and $g_1^d$ are found to be consistent with zero in the whole range of $x$.
Physics Letters B 04/2007; 647(5-6):330-340. · 3.95 Impact Factor
-
I. K. Bensafa,
P. Achenbach,
M. Ases Antelo,
C. Ayerbe,
D. Baumann,
R. Böhm,
D. Bosnar,
E. Burtin,
X. Defaÿ, N. D'Hose, [......],
G. Rosner,
S. Sánchez Majos,
M. Seimetz,
S. Širca,
T. Spitzenberg,
G. Tamas,
R. Van de Vyver,
L. Van Hoorebeke,
Th. Walcher,
M. Weis
[show abstract]
[hide abstract]
ABSTRACT: The beam-helicity asymmetry has been measured simultaneously for the reactions
e® \;\stackrel{{\rightarrow}}{{e}}\;
p→epγ and
e® \;\stackrel{{\rightarrow}}{{e}}\;
p→epπ
0 in the Δ(1232)-resonance region at Q
2 = 0.35(GeV/c)2. The experiment was performed at MAMI with a longitudinally polarized beam and an out-of-plane detection of the proton. The
results are compared with calculations based on dispersion relations for virtual Compton scattering and with the MAID model
for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining
discrepancies may be ascribed to an imperfect parametrization of some γ
(*)
N→πN multipoles, mainly contributing to the non-resonant background. The beam-helicity asymmetry in both channels (γ and π
0) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization.
European Physical Journal A 03/2007; 32(1):69-75. · 2.19 Impact Factor
