-
S. Ceci,
M. Döring,
E. Epple,
C. Fernández-Ramírez,
A. Fix,
M. Fritsch,
R. W. Gothe,
B. Grube,
H. Haberzettl,
C. Hanhart, [......], M. Ostrick,
K. Peters,
W. Przygoda,
J. J. Sanz-Cillero,
V. Shklyar,
I. I. Strakovsky,
A. Svarc,
A. P. Szczepaniak,
L. Tiator,
Y. Wunderlich
[show abstract]
[hide abstract]
ABSTRACT: The mini-proceedings of the Workshop on PWA tools in Hadronic Spectroscopy
held in Mainz from February 18th to 20th, 2013.
04/2013;
-
M. Oberle,
B. Krusche,
J. Ahrens,
J. R. M. Annand,
H. J. Arends,
K. Bantawa,
P. A. Bartolome,
R. Beck,
V. Bekrenev,
H. Berghaeuser, [......],
D. I. Sober,
A. Starostin,
I. Supek,
M. Thiel,
A. Thomas,
M. Unverzagt,
D. P. Watts,
D. Werthmueller,
L. Witthauer,
F. Zehr
[show abstract]
[hide abstract]
ABSTRACT: Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz
for the photoproduction of neutral pion pairs in the reactions
$\vec{\gamma}p\rightarrow p\pi^0\pi^0$ and $\vec{\gamma}d\rightarrow
(n)p\pi^0\pi^0$, $\vec{\gamma}d\rightarrow (p)n\pi^0\pi^0$ off free protons and
off quasi-free nucleons bound in the deuteron for incident photon energies up
to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of
longitudinally polarized electrons and tagged with the Glasgow magnetic
spectrometer. Decay photons from the $\pi^0$ mesons, recoil protons, and recoil
neutrons were detected in the 4$\pi$ covering electromagnetic calorimeter
composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction
of the final state, excellent agreement was found between the results for free
and quasi-free protons. This demonstrates that the free-nucleon behavior of
such observables can be extracted from measurements with quasi-free nucleons,
which is the only possibility for the neutron. Contrary to expectations, the
measured asymmetries are very similar for reactions off protons and neutrons.
The results are compared to the predictions from the Two-Pion-MAID reaction
model and (for the proton) also to the Bonn-Gatchina coupled channel analysis.
04/2013;
-
Y. Maghrbi,
R. Gregor,
S. Lugert,
J. Ahrens,
J. R. M. Annand,
H. J. Arends,
R. Beck,
V. Bekrenev,
B. Boillat,
A. Braghieri, [......],
S. Schadmand,
S. Schumann,
D. Sober,
S. Starostin,
I. Supek,
A. Thomas,
M. Unverzagt,
D. P. Watts,
D. Werthmueller,
F. Zehr
[show abstract]
[hide abstract]
ABSTRACT: Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei
has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca,
and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The
experiments were performed at the MAMI accelerator in Mainz, using the Glasgow
photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting
of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass
spectra for heavy nuclei to small invariant masses has been observed for
$\pi^0$ pairs but also for the mixed-charge pairs. The precise results allow
for the first time a model-independent analysis of the influence of pion
final-state interactions. The corresponding effects are found to be large and
must be carefully considered in the search for possible in-medium modifications
of the $\sigma$-meson. Results from a transport model calculation reproduce the
shape of the invariant-mass distributions for the mixed-charge pairs better
than for the neutral pairs, but also for the latter differences between model
results and experiment are not large, leaving not much room for
$\sigma$-in-medium modification.
04/2013;
-
Y. Maghrbi,
B. Krusche,
J. Ahrens,
J. R. M. Annand,
H. J. Arends,
R. Beck,
V. Bekrenev,
B. Boillat,
A. Braghieri,
D. Branford, [......],
S. Schumann,
D. Sober,
A. Starostin,
I. Supek,
C. M. Tarbert,
A. Thomas,
M. Unverzagt,
D. P. Watts,
D. Werthmueller,
F. Zehr
[show abstract]
[hide abstract]
ABSTRACT: Coherent photoproduction of $\pi^0$-mesons from threshold ($E_{th} \approx$
136 MeV) throughout the $\Delta$-resonance region and of $\eta$-mesons close to
the production threshold ($E_{th} \approx$ 570 MeV for $\eta$) has been
measured for $^7$Li nuclei. The experiment was performed using the
tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS
detectors combined to give an almost 4$\pi$ solid-angle electromagnetic
calorimeter. The reactions were identified by a combined invariant-mass and
missing-energy analysis. A comparison of the pion data to plane-wave impulse
modelling tests the nuclear mass form factor. So far coherent $\eta$-production
had been only identified for the lightest nuclear systems ($^2$H and $^3$He).
For $^3$He a large enhancement of the cross section above plane-wave
approximations had been reported, indicating the formation of a quasi-bound
state. The present Li-data for $\eta$-production agree with a plane-wave
approximation. Contrary to $^3$He, neither a threshold enhancement of the total
cross section nor a deviation of the angular distributions from the expected
form-factor dependence were observed.
03/2013;
-
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;
-
D. Hornidge,
P. Aguar Bartolome,
J. R. M. Annand,
H. J. Arends,
R. Beck,
V. Bekrenev,
H. Berghaeuser,
A. M. Bernstein,
A. Braghieri,
W. J. Briscoe, [......],
M. H. Sikora,
A. Starostin,
I. Supek,
M. Thiel,
A. Thomas,
L. Tiator,
M. Unverzagt,
D. P. Watts,
D. Werthmueller,
L. Witthauer
[show abstract]
[hide abstract]
ABSTRACT: A precision measurement of the photon asymmetry $\Sigma$ and differential
cross sections $d\sigma/d\Omega$ for the $\gamma p \rightarrow \pi^0p$ reaction
in the near-threshold region has been performed with a tagged photon beam and
almost $4\pi$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging
facility along with the Crystal Ball/TAPS multi-photon detector system and a
cryogenic liquid hydrogen target were used. These data allowed for a precise
determination of the energy dependence of the real parts of the $S$- and all
three $P$-wave amplitudes for the first time and provide the most stringent
test to date of the predictions of Chiral Perturbation Theory and its energy
region of convergence. The upper limit of agreement is between 165 and 175 MeV
incident photon lab energy, $\simeq25$ MeV above threshold.
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;
-
F. Zehr,
B. Krusche,
P. Aguar,
J. Ahrens,
J. R. M. Annand,
H. J. Arends,
R. Beck,
V. Bekrenev,
B. Boillat,
A. Braghieri, [......],
T. Rostomyan,
S. Schumann,
D. Sober,
A. Starostin,
I. Supek,
C. M. Tarbert,
A. Thomas,
M. Unverzagt,
Th. Walcher,
D. P. Watts
[show abstract]
[hide abstract]
ABSTRACT: Precise total cross-sections and invariant-mass distributions have been
measured for photoproduction of pion pairs off the proton producing
$p\pi^0\pi^0$ and $n\pi^+\pi^0$ final states from the threshold region up to
800 MeV incident photon energy. Additionally, beam helicity asymmetries have
been measured in the second resonance region (550 MeV - 820 MeV). The
experiment was performed at the tagged photon beam of the Mainz MAMI
accelerator with the Crystal Ball and TAPS detectors combined to give an almost
4$\pi$ solid-angle electromagnetic calorimeter. The results are much more
precise than any previous measurements and confirm the chiral perturbation
theory predictions for the threshold behavior of these reactions. In the second
resonance region, the invariant-mass distributions of meson-meson and
meson-nucleon pairs are in reasonable agreement with model predictions, but
none of the models reproduce the asymmetries for the mixed-charge channel.
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;
-
M. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. Austregesilo,
B. Badełek,
F. Balestra,
J. Ball,
J. Barth, [......],
Q. Weitzel,
R. Windmolders,
W. Wiślicki,
H. Wollny,
K. Zaremba,
E. Zemlyanichkina,
M. Ziembicki,
J. Zhao,
N. Zhuravlev,
A. Zvyagin
[show abstract]
[hide abstract]
ABSTRACT: The longitudinal polarisation transfer from muons to Λand
[`\varLambda]\bar{\varLambda}
hyperons,
DLL\varLambda([`\varLambda])D_{LL}^{\varLambda(\bar{\varLambda})}
, has been studied in deep-inelastic scattering off an unpolarised isoscalar target at the COMPASS experiment at CERN. The
spin transfers to Λand
[`\varLambda]\bar{\varLambda}
produced in the current fragmentation region exhibit different behaviours as a function of x and x
F
. The measured x and x
F
dependences of D
LL
Λ
are compatible with zero, while
DLL[`\varLambda]D_{LL}^{\bar{\varLambda}}
tends to increase with x
F
, reaching values of 0.4–0.5. The resulting average values are D
LL
Λ
= −0.012±0.047±0.024 and
DLL[`\varLambda]D_{LL}^{\bar{\varLambda}}
= 0.249±0.056±0.049. These results are discussed in the frame of recent model calculations.
European Physical Journal C 04/2012; 64(2):171-179. · 3.63 Impact Factor
-
F. W. Wieland,
J. Barth,
K. -H. Glander,
J. Hannappel,
N. Jöpen,
F. Klein,
E. Klempt,
R. Lawall,
D. Menze, M. Ostrick,
E. Paul,
I. Schulday,
W. J. Schwille
[show abstract]
[hide abstract]
ABSTRACT: The reaction γp → K
+
Λ(1520) was measured in the energy range from threshold to 2.65 GeV with the SAPHIR detector at the electron stretcher facility
ELSA in Bonn. The Λ(1520) production cross-section was analyzed in the decay modes pK
−, $
n\bar K^0
$
n\bar K^0
, Σ
±
π
∓, and Λπ
+
π
− as a function of the photon energy and the squared four-momentum transfer t . While the cross-sections for the inclusive reactions rise steadily with energy, the cross-section of the process γp → K
+
Λ(1520) peaks at a photon energy of about 2.0 GeV, falls off exponentially with t , and shows a slope flattening with increasing photon energy. The angular distributions in the t-channel helicity system indicate neither a K nor a K
* exchange dominance. Rather, the rapid change of the angular distribution with energy suggests that, in addition to t -channel K and K* exchange, amplitudes for formation of nucleon resonances in the s-channel play an important role.
European Physical Journal A 04/2012; 47(4):1-11. · 2.19 Impact Factor
-
The COMPASS Collaboration,
M. G. Alekseev,
V. Yu. Alexakhin,
Yu. Alexandrov,
G. D. Alexeev,
A. Amoroso,
A. Austregesilo,
B. Badełek,
F. Balestra,
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: Azimuthal asymmetries in semi-inclusive production of positive (h
+) and negative hadrons (h
−) have been measured by scattering 160 GeV muons off longitudinally polarised deuterons at CERN. The asymmetries were decomposed
in several terms according to their expected modulation in the azimuthal angle φ of the outgoing hadron. Each term receives contributions from one or several spin and transverse-momentum-dependent parton
distribution and fragmentation functions. The amplitudes of all φ-modulation terms of the hadron asymmetries integrated over the kinematic variables are found to be consistent with zero within
statistical errors, while the constant terms are nonzero and equal for h
+ and h
− within the statistical errors. The dependencies of the φ-modulated terms versus the Bjorken momentum fraction x, the hadron fractional momentum z, and the hadron transverse momentum phTp_{h}^{T} were studied. The x dependence of the constant terms for both positive and negative hadrons is in agreement with the longitudinal double-spin
hadron asymmetries, measured in semi-inclusive deep-inelastic scattering. The x dependence of the sin φ-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the φ-modulation amplitudes are consistent with zero within the statistical errors.
European Physical Journal C 04/2012; 70(1):39-49. · 3.63 Impact Factor
-
V. L. Kashevarov,
A. Fix,
S. Prakhov,
P. Aguar-Bartolomé,
J. R. M. Annand,
H. J. Arends,
K. Bantawa,
R. Beck,
V. Bekrenev,
H. Berghäuser, [......],
I. I. Strakovsky,
I. M. Suarez,
I. Supek,
C. M. Tarbert,
M. Thiel,
A. Thomas,
M. Unverzagt,
D. P. Watts,
D. Werthmüller,
F. Zehr
[show abstract]
[hide abstract]
ABSTRACT: The gamma p --> pi0 pi0 p reaction has been measured from threshold to 1.4
GeV using the Crystal Ball and TAPS photon spectrometers together with the
photon tagging facility at the Mainz Microtron. The experimental results
include total and differential cross sections as well as specific angular
distributions, which were used to extract partial-wave amplitudes. In
particular, the energy region below the D13(1520) resonance was studied.
04/2012;
-
The CBELSA/TAPS Collaboration,
I. Jaegle,
T. Mertens,
A. Fix,
F. Huang,
K. Nakayama,
L. Tiator,
A. V. Anisovich,
J. C. S. Bacelar,
B. Bantes, [......],
S. V. Shende,
V. Sokhoyan,
A. Süle,
V. V. Sumachev,
T. Szczepanek,
U. Thoma,
D. Trnka,
R. Varma,
D. Walther,
C. Wendel
[show abstract]
[hide abstract]
ABSTRACT: Quasi-free photoproduction of h¢ \eta{^\prime} -mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment
was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production
threshold up to 2.5GeV. The h¢ \eta{^\prime} -mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good
agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with
recoil neutrons provides the first data for the g \gamma
n
® \rightarrow
n
h¢ \eta{^\prime} reaction. In addition, also first estimates for coherent h¢ \eta{^\prime} -production off the deuteron have been obtained. In agreement with model predictions, the total cross-section for this channel
is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account
contributions from nucleon resonances and t -channel exchanges.
European Physical Journal A 04/2012; 47(1):1-18. · 2.19 Impact Factor
-
The CBELSA/TAPS Collaboration,
I. Jaegle,
B. Krusche,
A. V. Anisovich,
J. C. S. Bacelar,
B. Bantes,
O. Bartholomy,
D. E. Bayadilov,
R. Beck,
Y. A. Beloglazov, [......],
A. Süle,
V. V. Sumachev,
T. Szczepanek,
U. Thoma,
D. Trnka,
R. Varma,
D. Walther,
C. Wendel,
D. Werthmüller,
L. Witthauer
[show abstract]
[hide abstract]
ABSTRACT: Precise data for quasi-free photoproduction of η-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for
incident photon energies up to 2.5GeV. The η-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and
re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to η-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution
of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of
the η-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free η-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around
1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass
spectrum of the η-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W ≈ 1665 MeV and FWHM Γ ≈ 25 MeV. The data are compared to the results of different models.
European Physical Journal A 04/2012; 47(8):1-25. · 2.19 Impact Factor
-
The Crystal Ball at MAMI, TAPS, and Collaborations,
V. L. Kashevarov,
A. Fix,
P. Aguar-Bartolomé,
L. K. Akasoy,
J. R. M. Annand,
H. J. Arends,
K. Bantawa,
R. Beck,
V. Bekrenev, [......],
A. Starostin,
I. M. Suarez,
I. Supek,
C. M. Tarbert,
M. Thiel,
A. Thomas,
M. Unverzagt,
D. P. Watts,
I. Zamboni,
F. Zehr
[show abstract]
[hide abstract]
ABSTRACT: Total and differential cross-sections for the reaction gp®p0hp\ensuremath \gamma p\rightarrow\pi^0\eta p have been measured with the Crystal Ball/TAPS detector using the tagged photon facility at the MAMI C accelerator in Mainz.
In the energy range Eg = 0.95-1.4\ensuremath E_{\gamma} = 0.95\mbox{--}1.4 GeV the reaction is dominated by the excitation and sequential decay of the D(1700)D33\ensuremath \Delta(1700)D_{33} -resonance. Angular distributions measured with high statistics allow us to determine the ratio of hadronic decay widths
GhD/GpS11\ensuremath \Gamma_{\eta\Delta}/\Gamma_{\pi S_{11}} and the ratio of the helicity amplitudes A3/2/A1/2\ensuremath A_{3/2}/A_{1/2} for this resonance.
European Physical Journal A 04/2012; 42(2):141-149. · 2.19 Impact Factor
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The CBELSA/TAPS Collaboration,
M. Nanova,
J. C. S. Bacelar,
B. Bantes,
O. Bartholomy,
D. Bayadilov,
R. Beck,
Y. A. Beloglazov,
R. Castelijns,
V. Crede, [......],
V. Sokhoyan,
A. Süle,
V. V. Sumachev,
T. Szczepanek,
U. Thoma,
D. Trnka,
R. Varma,
D. Walther,
Ch. Weinheimer,
Ch. Wendel
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ABSTRACT: The exclusive reactions γp → K
*0Σ+(1189) and γp → K
0π0Σ+(1189) , leading to the p 4π0 final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5GeV. The experiment
has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were
combined to a photon detector system of almost 4π geometrical acceptance. Differential and total cross-sections are reported.
At energies close to the threshold, a flat angular distribution has been observed for the reaction γp → K
0π0Σ+ suggesting dominant s -channel production. Σ*(1385) and higher-lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions
of the reaction γp → K
*0Σ+ indicates a t -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical
predictions.
European Physical Journal A 04/2012; 35(3):333-342. · 2.19 Impact Factor
