The KEK Experiment E246 has provided the best upper limit of the transverse muon polarization in Kμ3 decays to date. The E246 detector is now being upgraded by the TREK collaboration and upcoming ...experiments with this detector at the J-PARC facility will allow new high-precision measurements in search for physics beyond the Standard Model. The first round of planned experiments include the search for lepton universality violation in a measurement of the ratio of the Ke2 and Kμ2 decay widths, as well as the search for heavy sterile neutrinos and possible massive gauge boson A' in the 10 to 100 MeV mass range. The search for T violation in kaon decays requires a higher intensity hadron beam and will be carried out later.
The branching ratio of the structure dependent (SD) radiative K+→e+νeγ decay relative to that of the K+→e+νe(γ) decay including the internal bremsstrahlung (IB) process (Ke2(γ)) has been measured in ...the J-PARC E36 experiment using plastic scintillator/lead sandwich detectors, in contrast to the previous E36 measurement, which used a CsI(Tl) calorimeter. In the analysis, the effect of IB was also taken into account in the SD radiative decay as Ke2γ(γ)SD. By combining the new data with the previous E36 result after revision for the IB correction for Ke2γ(γ)SD, a new value Br(Ke2γ(γ)SD)/Br(Ke2(γ))=1.20±0.07 has been determined. This is consistent with a recent lattice QCD calculation, but larger than the expectation of Chiral Perturbation Theory (ChPT) at order O(p4) and the previous KLOE value. Using the method to relate form factor and branching ratio described in the KLOE paper, the present result is also consistent with the form factor prediction based on a gauged nonlocal chiral quark model, but larger than that from ChPT at order O(p6).
The structure dependent radiative K+→e+νγ (Ke2γSD+) decay was investigated with stopped positive kaons. The e+ momentum spectra containing 574±30Ke2γSD+ events with a K+→μ+ν (Kμ2) background of 28±19 ...events were measured with and without a photon in coincidence and analyzed with Monte Carlo simulations for acceptance and detector response to extract the ratio of the branching ratio of the Ke2γSD+ decay and the K+→e+ν decay including the internal bremsstrahlung process (Ke2(γ)). A value of Br(Ke2γSD+)/Br(Ke2(γ))=1.12±0.07stat±0.04syst was obtained. This indicates a partial branching ratio, Br(Ke2γSD+,pe>200MeV/c,Eγ>10MeV)/Br(Kμ2)=(1.85±0.11stat±0.07syst)×10−5, which is 25% (∼2.5σ) higher than the previous experimental result.
The J-PARC E36 experiment will search for a violation of lepton universality by a precise measurement of the ratio of the kaon decay widths RK=Γ(K+→e+ν)/Γ(K+→μ+ν). Charged particles will be ...identified by the combination of three independent systems: a lead-glass Cherenkov counter, an aerogel Cherenkov counter, and a time-of-flight measurement. The performance of the lead-glass Cherenkov counter was investigated with e+, μ+, and π+ beams in the relevant momentum region from the K+ decays. By using a polyethylene degrader to slow down the beam momentum in front of the lead-glass block, we succeeded in reducing the muon mis-identification probability down to 5% while maintaining a high e+ detection efficiency of 98%.
We present the results of our final analysis of the full data set of g(1)(p) (Q(2)), the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in 2000-2001. Polarized ...electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV were scattered from proton targets ((NH3)-N-15 dynamically polarized along the beam direction) and detected with CLAS. From the measured double spin asymmetries, we extracted virtual photon asymmetries A(1)(p) and A(2)(p) and spin structure functions g(1)(p) and g(2)(p) over a wide kinematic range (0.05 GeV2 < Q(2) < 5 GeV2 and 1.08 GeV< W < 3 GeV) and calculated moments of g(1)(p). We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large x, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion.
Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four ...momentum transfer (Q2). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elastic- scattering cross section, at Q2 ≈ 0:85 and 1.45 GeV2 over a large range of virtual photon polarization ε. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a non-zero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q2 ≈ 2 GeV2. These data are consistent with an increase in R2γ with decreasing " at Q2 ≈ 0:85 and 1.45 GeV2. There are indications of a slight increase in R2γ with Q2.
This paper reports new exclusive cross sections for $e p \to e' \pi^+ \pi^- p'$ using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities ...2.0 GeV2 < Q2 < 5.0 GeV2 in the center-of-mass energy range 1.4 GeV < W < 2.0 GeV, which covers a large part of the nucleon resonance region. Using a model developed for the phenomenological analysis of electroproduction data, we see strong indications that the relative contributions from the resonant cross sections at W < 1.74 GeV increase with $Q^2$. These data considerably extend the kinematic reach of previous measurements. Exclusive $e p \to e' \pi^+ \pi^- p'$ cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV.
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