Azimuthal asymmetries in exclusive electroproduction of a real photon from an unpolarized deuterium target are measured with respect to beam helicity and charge. They appear in the distribution of ...these photons in the azimuthal angle
ϕ around the virtual-photon direction, relative to the lepton scattering plane. The extracted asymmetries are attributed to either the deeply virtual Compton scattering process or its interference with the Bethe–Heitler process. They are compared with earlier results on the proton target. In the measured kinematic region, the beam-charge asymmetry amplitudes and the leading amplitudes of the beam-helicity asymmetries on an unpolarized deuteron target are compatible with the results from unpolarized protons.
The exclusive electroproduction of π+ mesons was studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off an internal hydrogen gas target. ...The virtual-photon cross sections were measured as a function of the Mandelstam variable t and the squared four momentum −Q2 of the exchanged virtual photon. A model calculation based on Generalized Parton Distributions is in fair agreement with the data at low values of |t| if power corrections are included. A model calculation based on the Regge formalism gives a good description of the magnitude and the t and Q2 dependences of the cross section.
Results of inclusive measurements of inelastic electron and positron scattering from unpolarized protons and deuterons at the HERMES experiment are presented. The structure functions
F
2
p
and
F
2
d
...are determined using a parameterization of existing data for the longitudinal-to-transverse virtual-photon absorption cross-section ratio. The HERMES results provide data in the ranges 0.006 ≤
x
≤ 0.9 and 0.1 GeV
2
≤
Q
2
≤ 20 GeV
2
, covering the transition region between the perturbative and the non-perturbative regimes of QCD in a so-far largely unexplored kinematic region. They are in agreement with existing world data in the region of overlap. The measured cross sections are used, in combination with data from other experiments, to perform fits to the photon-nucleon cross section using the functional form of the ALLM model. The deuteron-to-proton cross-section ratio is also determined.
The massive binary system Eta Carinae and the surrounding H ii complex, the Carina nebula, are potential particle acceleration sites from which very high energy (VHE; E≥ 100 GeV) γ-ray emission could ...be expected. This paper presents data collected during VHE γ-ray observations with the HESS telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-h data set no hint of significant γ-ray emission from Eta Carinae has been found and an upper limit on the γ-ray flux of
(99 per cent confidence level) is derived above the energy threshold of 470 GeV. Together with the detection of high energy (HE; 0.1 ≤E≤ 100 GeV) γ-ray emission by the Fermi Large Area Telescope up to 100 GeV, and assuming a continuation of the average HE spectral index into the VHE domain, these results imply a cut-off in the γ-ray spectrum between the HE and VHE γ-ray range. This could be caused either by a cut-off in the accelerated particle distribution or by severe γ-γ absorption losses in the wind collision region. Furthermore, the search for extended γ-ray emission from the Carina nebula resulted in an upper limit on the γ-ray flux of
(99 per cent confidence level). The derived upper limit of ∼23 on the cosmic ray enhancement factor is compared with results found for the old-age mixed-morphology supernova remnant W28.
The non-thermal nature of the X-ray emission from the shell-type supernova remnants (SNRs) G1.9+0.3 and G330.2+1.0 is an indication of intense particle acceleration in the shock fronts of both ...objects. This suggests that the SNRs are prime candidates for very-high-energy (VHE; E > 0.1 TeV) γ-ray observations. G1.9+0.3, recently established as the youngest known SNR in the Galaxy, also offers a unique opportunity to study the earliest stages of SNR evolution in the VHE domain. The purpose of this work is to probe the level of VHE γ-ray emission from both SNRs and use this to constrain their physical properties. Observations were conducted with the H.E.S.S. (High Energy Stereoscopic System) Cherenkov Telescope Array over a more than six-year period spanning 2004–2010. The obtained data have effective livetimes of 67 h for G1.9+0.3 and 16 h for G330.2+1.0. The data are analysed in the context of the multiwavelength observations currently available and in the framework of both leptonic and hadronic particle acceleration scenarios. No significant γ-ray signal from G1.9+0.3 or G330.2+1.0 was detected. Upper limits (99 per cent confidence level) to the TeV flux from G1.9+0.3 and G330.2+1.0 for the assumed spectral index Γ = 2.5 were set at 5.6 × 10−13 cm−2 s−1 above 0.26 TeV and 3.2 × 10−12 cm−2 s−1 above 0.38 TeV, respectively. In a one-zone leptonic scenario, these upper limits imply lower limits on the interior magnetic field to B
G1.9 ≳ 12 μG for G1.9+0.3 and to B
G330 ≳ 8 μG for G330.2+1.0. In a hadronic scenario, the low ambient densities and the large distances to the SNRs result in very low predicted fluxes, for which the H.E.S.S. upper limits are not constraining.
Multidimensional study of hadronization in nuclei Akopov, N.; Aschenauer, E. C.; Augustyniak, W. ...
The European physical journal. A, Hadrons and nuclei,
09/2011, Letnik:
47, Številka:
9
Journal Article
Recenzirano
Odprti dostop
.
Hadron multiplicities in semi-inclusive deep-inelastic scattering were measured on neon, krypton, and xenon targets relative to deuterium at an electron(positron)-beam energy of 27.6GeV at HERMES. ...These ratios were determined as a function of the virtual-photon energy
, its virtuality Q
2
, the fractional hadron energy
z
and the transverse hadron momentum with respect to the virtual-photon direction p
t
. Dependences were analysed separately for positively and negatively charged pions and kaons as well as protons and antiprotons in a two-dimensional representation. Compared to the one-dimensional dependences, some new features were observed. In particular, when
z
> 0.4 positive kaons do not show the strong monotonic rise of the multiplicity ratio with
as exhibited by pions and negative kaons. Protons were found to behave very differently from the other hadrons.
The transverse-target single-spin asymmetry for inclusive deep-inelastic scattering with effectively unpolarized electron and positron beams off a transversely polarized hydrogen target was measured, ...with the goal of searching for a two-photon exchange signal in the kinematic range 0.007<xB<0.9 and 0.25GeV2<Q2<20GeV2. In two separate regions Q2>1GeV2 and Q2<1GeV2, and for both electron and positron beams, the asymmetries are found to be consistent with zero within statistical and systematic uncertainties, which are of order 10−3 for the asymmetries integrated over xB.
Hyperon production in the threshold region was studied in the reaction pp→K+Λp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three ...different beam momenta of pbeam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of ε=204, 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9±0.8±2.0 μb, 28.4±1.3±2.2 μb and 35.0±1.3±3.0 μb. Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence of the N(1650)-resonance at pbeam=2.95 GeV/c, whereas for the higher momenta an increasing relative contribution of the N(1710)- and/or of the N(1720)-resonance was observed. In addition, the pΛ-final-state interaction turned out to have a significant influence on the Dalitz plot distribution.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image) Hard exclusive electroproduction of ... mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering ...27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive ... transition form factor.
The HERMES hydrogen and deuterium nuclear-polarized gas targets have been in use since 1996 with the polarized electron beam of HERA at DESY to study the spin structure of the nucleon. Polarized ...atoms from a Stern–Gerlach Atomic Beam Source are injected into a storage cell internal to the HERA electron ring. Atoms diffusing from the center of the storage cell into a side tube are analyzed to determine the atomic fraction and the atomic polarizations. The atoms have a nuclear polarization, the axis of which is defined by an external magnetic holding field. The holding field was longitudinal during 1996–2000, and was changed to transverse in 2001. The design of the target is described, the method for analyzing the target polarization is outlined, and the performance of the target in the various running periods is presented.
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