109 research outputs found
Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction
No full textEvidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK(S)(0) → ppi(+)pi(-). A peak is observed in the pK(S)(0) invariant mass spectrum at 1528 +/- 2.6(stat) +/- 2.1(syst) MeV. Depending on the background model, the naive statistical significance of the peak is 4-6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma = 4.3-6.2 MeV. This state may be interpreted as the predicted S = +1 exotic Theta(+)(uudd (s) over bar) pentaquark baryon. No signal for an hypothetical Theta(++) baryon was observed in the pK(+) invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely
Search for an exotic S =-2, Q =-2 baryon resonance at a mass near 1862 MeV in quasireal photoproduction
No full textA search for an exotic baryon resonance with has been performed in quasi-real photoproduction on a deuterium target through the decay channel . No evidence for a previously reported resonance is found in the invariant mass spectrum. An upper limit for the photoproduction cross section of 2.1 nb is found at the 90% confidence level. The photoproduction cross section for the is found to be between 9 and 24 nb
Spin density matrix elements in exclusive ω electroproduction on 1H and 2H targets at 27.5 GeV beam energy
No full textExclusive electroproduction of ω mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q2<1.0 GeV2, 3.0 GeV <W< 6.3 GeV, and −t′<0.2 GeV2. 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 Q2 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
→ωT and γ
∗
T
→ωT, 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
→ωT transition dominates over the two natural-parity-exchange amplitudes describing the γ
∗
L
→ωL and γ
∗
T
→ωT 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
Exclusive electroproduction on transversely polarized protons
No full textThe exclusive electroproduction of ρ0 mesons was studied with the hermes spectrometer at the Desy laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. Spin density matrix elements for this process were determined from the measured production- and decay-angle distributions of the produced ρ0 mesons. These matrix elements embody information on helicity transfer and the validity of s-channel helicity conservation in the case of a transversely polarized target. From the spin density matrix elements, the leading-twist term in the single-spin asymmetry was calculated separately for longitudinally and transversely polarized ρ0 mesons. Neglecting s-channel helicity changing matrix elements, results for the former can be compared to calculations based on generalized parton distributions, which are sensitive to the contribution of the total angular momentum of the quarks to the proton spin
Spin Filtering Studies at COSY
No full textUnderstanding the interplay of the nuclear interaction with polarized protons and the electromagnetic interaction with polarized electrons in polarized atoms is crucial to progress towards the PAX goal to eventually produce stored polarized
antiproton beams at FAIR. Presently, there exist two competing theoretical scenarios: one with substantial filtering of (anti)protons by atomic electrons, while the second one suggests a self-cancellation of the electron contribution to filtering. The issue can be clarified by studying the energy dependence of the polarization buildup in a proton beam at COSY at energies in the range from 20 to about 800 MeV. This Letter-of-Intent summarizes the physics case and possible experimental approaches to these studies at COSY
Measurement of the Spin-Dependence of the pbar-p Interaction at the AD-Ring
No full textWe propose to use an internal polarized hydrogen storage cell gas target in the AD ring to determine for the first time the two total spin-dependent pbar-p cross sections sigma_1 and sigma_2 at antiproton beam energies in the range from 50 to 450 MeV. The data obtained are of interest by themselves for the general theory of pbar-p interactions since they will provide a first experimental constraint of the spin-spin dependence of the nucleon-antinucleon potential in the energy range of interest. In addition, measurements of the polarization buildup of stored antiprotons are required to define the optimum parameters of a future, dedicated Antiproton Polarizer Ring (APR), intended to feed a double-polarized asymmetric pbar-p collider with polarized antiprotons. Such a machine has recently been proposed by the PAX collaboration for the new Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt, Germany. The availability of an intense stored beam of polarized antiprotons will provide access to a wealth of single- and double-spin observables, thereby opening a new window on QCD spin physics
Status Report and Beam-Time Request for COSY experiment #199 Spin–Filtering Studies at COSY
No full textWe report on the progress of the PAX experimental programme since the last PAC
meeting. During summer shutdown 2009 four new quadrupole magnets and a modified
vacuum system have been installed into the COSY ring to form a section with low β
functions. With the successful commissioning of this low-β insertion in January 2010 it has
also been discovered that intra-beam scattering effects are likely limiting the beam lifetime.
In order to get these effects under control and to improve the lifetime, to commission a
new detection system, and to finally perform a first series of spin–filtering measurements
with transverse polarisation, we request ten weeks of beam time.
Content
Pentaquark Θ+ search at HERMES
No full textThe earlier search at HERMES for narrow baryon states excited in quasireal photoproduction, decaying through the channel pK0S→pπ+π−, has been extended with improved decay-particle reconstruction, more advanced particle identification, and increased event samples. The structure observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical significance drops to about 2σ for data taken with a deuterium target. The number of events above background is 68+98−31(stat)±13(sys). No such structure is observed in the hydrogen data set
Spin-Filtering Studies at COSY
No full textWe propose to use an internal polarised target in the COSY ring to determine the polarisation
build–up in a proton beam. Spin–filtering experiments at COSY would provide
the necessary data to test our present understanding of spin–filtering processes in storage
rings.
Measurements of the polarisation build–up of stored protons are crucial to progress
towards the PAX goal to eventually produce stored polarised antiproton beams. The
availability of intense stored beams of polarised antiprotons will provide access to a wealth
of single– and double–spin observables, opening a new window on QCD spin physics. It
is planned to realise this experimental programme at the new Facility for Antiproton and
Ion Research (FAIR) at GSI in Darmstadt, Germany.
A recent experiment at COSY revealed that e~p spin–flip cross sections are too small to
cause a detectable depolarisation of a stored proton beam. This measurement rules out a
proposal to use polarised electrons to polarise a proton beam by ~ep spin–flip interactions.
Thus, our approach to provide a beam of polarised protons is based on spin–filtering using
an internal polarised gas target.
In total 22 weeks of beam time are needed to complete the experimental program at
COSY. We now ask for two weeks of beam time for commissioning of the low–β section
and measuring the machine acceptance
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