The OLYMPUS experiment Milner, R.; Hasell, D.K.; Kohl, M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2014, Letnik:
741
Journal Article
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The OLYMPUS experiment was designed to measure the ratio between the positron–proton and electron–proton elastic scattering cross-sections, with the goal of determining the contribution of two-photon ...exchange to the elastic cross-section. Two-photon exchange might resolve the discrepancy between measurements of the proton form factor ratio, μpGEp/GMp, made using polarization techniques and those made in unpolarized experiments. OLYMPUS operated on the DORIS storage ring at DESY, alternating between 2.01GeV electron and positron beams incident on an internal hydrogen gas target. The experiment used a toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight detectors to measure rates for elastic scattering over the polar angular range of approximately 25°–75°. Symmetric Møller/Bhabha calorimeters at 1.29° and telescopes of GEM and MWPC detectors at 12° served as luminosity monitors. A total luminosity of approximately 4.5fb−1 was collected over two running periods in 2012. This paper provides details on the accelerator, target, detectors, and operation of the experiment.
We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading ...effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha, and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.
A comprehensive collection of results on longitudinal double-spin asymmetries is presented for charged pions and kaons produced in semi-inclusive deep-inelastic scattering of electrons and positrons ...on the proton and deuteron, based on the full HERMES data set. The dependence of the asymmetries on hadron transverse momentum and azimuthal angle extends the sensitivity to the flavor structure of the nucleon beyond the distribution functions accessible in the collinear framework. No strong dependence on those variables is observed. In addition, the hadron charge-difference asymmetry is presented, which under certain model assumptions provides access to the helicity distributions of valence quarks.
The OLYMPUS experiment used a 0.3T toroidal magnetic spectrometer to measure the momenta of outgoing charged particles. In order to accurately determine particle trajectories, knowledge of the ...magnetic field was needed throughout the spectrometer volume. For that purpose, the magnetic field was measured at over 36,000 positions using a three-dimensional Hall probe actuated by a system of translation tables. We used these field data to fit a numerical magnetic field model, which could be employed to calculate the magnetic field at any point in the spectrometer volume. Calculations with this model were computationally intensive; for analysis applications where speed was crucial, we pre-computed the magnetic field and its derivatives on an evenly spaced grid so that the field could be interpolated between grid points. We developed a spline-based interpolation scheme suitable for SIMD implementations, with a memory layout chosen to minimize space and optimize the cache behavior to quickly calculate field values. This scheme requires only one-eighth of the memory needed to store necessary coefficients compared with a previous scheme (Lekien and Marsden, 2005 1). This method was accurate for the vast majority of the spectrometer volume, though special fits and representations were needed to improve the accuracy close to the magnet coils and along the toroidal axis.
Exclusive electroproduction of
ω
mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of
Q
2
>
1.0
GeV
2
, 3.0 GeV
<
W
<
6.3 GeV, and
-
t
′
<
0.2
GeV
2
. ...Results on the angular distribution of the
ω
meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996–2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of
Q
2
or
-
t
′
. Violation of
s
-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse
ω
production by longitudinal and transverse virtual photons,
γ
L
∗
→
ω
T
and
γ
T
∗
→
ω
T
, is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the
γ
T
∗
→
ω
T
transition dominates over the two natural-parity-exchange amplitudes describing the
γ
L
∗
→
ω
L
and
γ
T
∗
→
ω
T
transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Exclusive ρ0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in ...the kinematic region 1.0 GeV2 < Q2 < 7.0 GeV2, 3.0 GeV < W < 6.3 GeV, and –t' < 0.4 GeV2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing ρ0-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F01/201/2, which describes the production of a longitudinal ρ0-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F01/201/2. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the π - ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 ρ0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract Exclusive $$\rho ^0$$ ρ 0 -meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely ...polarized hydrogen target, in the kinematic region 1.0 GeV $$^2<Q^2<$$ 2 < Q 2 < 7.0 GeV $$^2$$ 2 , 3.0 GeV $$<W<$$ < W < 6.3 GeV, and $$-t^\prime <$$ - t ′ < 0.4 GeV $$^2$$ 2 . Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing $$\rho ^{0}$$ ρ 0 -meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude $$F_{0\frac{1}{2}0\frac{1}{2}}$$ F 0 1 2 0 1 2 , which describes the production of a longitudinal $$\rho ^{0}$$ ρ 0 -meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to $$F_{0\frac{1}{2}0\frac{1}{2}}$$ F 0 1 2 0 1 2 . Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the $$\pi -\rho $$ π - ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 $$\rho ^0$$ ρ 0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Exclusive ρ0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in ...the kinematic region 1.0 GeV2<Q2< 7.0 GeV2, 3.0 GeV <W< 6.3 GeV, and -t′< 0.4 GeV2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing ρ0-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F012012, which describes the production of a longitudinal ρ0-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F012012. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the π-ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 ρ0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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