Undulator-based production of polarized positrons Alexander, G.; Barley, J.; Batygin, Y. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2009, Volume:
610, Issue:
2
Journal Article
Peer reviewed
Open access
Full exploitation of the physics potential of a future International Linear Collider will require the use of polarized electron and positron beams. Experiment E166 at the Stanford Linear Accelerator ...Center (SLAC) has demonstrated a scheme in which an electron beam passes through a helical undulator to generate photons (whose first-harmonic spectrum extended to 7.9
MeV) with circular polarization, which are then converted in a thin target to generate longitudinally polarized positrons and electrons. The experiment was carried out with a 1-m-long, 400-period, pulsed helical undulator in the Final Focus Test Beam (FFTB) operated at 46.6
GeV. Measurements of the positron polarization have been performed at five positron energies from 4.5 to 7.5
MeV. In addition, the electron polarization has been determined at 6.7
MeV, and the effect of operating the undulator with a ferrofluid was also investigated. To compare the measurements with expectations, detailed simulations were made with an upgraded version of G
eant4
that includes the dominant polarization-dependent interactions of electrons, positrons, and photons with matter. The measurements agree with calculations, corresponding to 80% polarization for positrons near 6
MeV and 90% for electrons near 7
MeV.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The ...asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK0S→pπ+π−. A peak is observed in the pK0S 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 σ=4.3–6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ+(uudds̄) pentaquark baryon. No signal for an hypothetical Θ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that an isotensor Θ is excluded and an isovector Θ is unlikely.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
To investigate the relation of residual worst lead ST segment elevation (WL-STE) after ST segment myocardial infarction (STEMI) with infarct size and microvascular injury assessed by cardiovascular ...magnetic resonance (CMR) imaging.
WL-STE in patients with acute reperfused STEMI has been shown to identify high risk patients for major adverse cardiovascular events (MACE). However, the relation of WL-STE with myocardial damage is unknown.
In this multicentre study we analysed ECG data 90 min after primary percutaneous coronary intervention (PCI) in 763 STEMI patients. WL-STE was defined as the absolute magnitude of STE in the most affected lead on the post-PCI ECG. Patients were categorised into three groups (<1 mm, 1-2 mm, and ≥2 mm). CMR was performed within 1 week after infarction for comprehensive assessment of myocardial damage using a standardised protocol. The primary clinical endpoint was MACE defined as death, reinfarction, and new congestive heart failure within 12 months after infarction.
WL-STE <1 mm, 1-2 mm, and ≥2 mm was present in 155 (20%), 328 (43%), and 280 (37%) patients, respectively. Myocardial damage determined by CMR demonstrated a graded relationship of infarct size (median (IQR) 13.3 (6.2-20.3)%LV vs 13.7 (7.6-21.3)%LV vs 22.5 (15.6-31.2)%LV, p<0.001), the myocardial salvage index (60.8 (37.0-84.5) vs 55.0 (36.6-73.9) vs 42.7 (26.2-58.2), p<0.001), and microvascular obstruction (0.0 (0.0-0.9)%LV vs 0.0 (0-1.0)%LV vs 1.2 (0.0-3.6)%LV, p<0.001) across the three groups. WL-STE ≥2 mm was strongly associated with MACE 12 month after infarction (HR 1.93, 95% CI 1.11 to 3.37; p=0.02).
This largest CMR study to date correlating post-PCI WL-STE with markers of myocardial damage demonstrates that WL-STE is significantly associated with infarct size, myocardial salvage, microvascular obstruction, and MACE in a high risk STEMI population.
NCT00712101.
The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By ...comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
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.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A measurement of the virtual-photon asymmetry A2(x,Q2) and of the spin-structure function g2(x,Q2) of the proton are presented for the kinematic range 0.004<x<0.9 and 0.18 GeV2<Q2<20 GeV2. The data ...were collected by the HERMES experiment at the HERA storage ring at DESY while studying inclusive deep-inelastic scattering of 27.6 GeV longitudinally polarized leptons off a transversely polarized hydrogen gas target. The results are consistent with previous experimental data from CERN and SLAC. For the x-range covered, the measured integral of g2(x) converges to the null result of the Burkhardt–Cottingham sum rule. The x2 moment of the twist-3 contribution to g2(x) is found to be compatible with zero.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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