Kuleshov, Sergey

Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A0 , in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37 × 1011 electrons on target collected during 2016–2022 runs NA64 probes for the first time the well-motivated region of parameter space of benchmark thermal scalar and fermionic dark matter models. No evidence for dark matter production has been found. This allows us to set the most sensitive limits on the A0 couplings to photons for masses mA0 ≲ 0.35 GeV, and to exclude scalar and Majorana dark matter with the χ − A0 coupling αD ≤ 0.1 for masses 0.001 ≲ mχ ≲ 0.1 GeV and 3mχ ≤ mA0 .

We report measurements of the photon beam asymmetry for the reactions γ p → pπ0 and γ p → pη from the GLUEX experiment using a 9 GeV linearly polarized, tagged photon beam incident on a liquid hydrogen target in Jefferson Lab’s Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous π0 measurements and are the first η measurements in this energy regime. The results are compared with theoretical predictions based on t-channel, quasiparticle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.

We report on a direct search for sub-GeV dark photons (A0 ), which might be produced in the reaction e−Z → e−ZA0 via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75 × 109 electrons on target. We set new limits on the γ − A0 mixing strength and exclude the invisible A0 with a mass ≲100 MeV as an explanation of the muon gμ − 2 anomaly.

The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry T has been measured in photoproduction from the decay ω → π+π−π0, using a transversely polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant nonzero values are observed for these asymmetries, reaching about 30–40% in the third-resonance region. New measurements for the photon-beam asymmetry are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional ω photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from s-channel resonance production were found in addition to t-channel exchange processes.

We report the measurement of near-threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range Q2 from 2 to 4.5 GeV2 and W from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles E0+ and S0+ in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors Gπ0p 1 (Q2) and Gπ0p 2 (Q2). The data are compared to these generalized form factors and the results for Gπ0p 1 (Q2) are found to be in good agreement with the LCSR predictions, but the level of agreement with Gπ0p 2 (Q2) is poor.

Unpolarized and beam-polarized fourfold cross sections ðd4σ=dQ2dxBdtdϕÞ for the ep → e0 p0 γ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 (Q2; xB; t) bins over the widest phase space ever explored in the valencequark region. Several models of generalized parton distributions (GPDs) describe the data well at most of our kinematics. This increases our confidence that we understand the GPD H, expected to be the dominant contributor to these observables. Through a leading-twist extraction of Compton form factors, these results support the model predictions of a larger nucleon size at lower quark-momentum fraction xB.

New results on the single-differential and fully integrated cross sections for the process γvp → p π+π− are presented. The experimental data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the kinematic region of the reaction invariant mass W from 1.3 to 1.825 GeV and the photon virtuality Q2 from 0.4 to 1.0 GeV2 . The cross sections were obtained in narrow Q2 bins (0.05 GeV2 ) with the smallest statistical uncertainties achieved in double-pion electroproduction experiments to date. The results were found to be in agreement with previously available data where they overlap. A preliminary interpretation of the extracted cross sections, which was based on a phenomenological meson-baryon reaction model, revealed substantial relative contributions from nucleon resonances. The data offer promising prospects to improve knowledge on the Q2 evolution of the electrocouplings of most resonances with masses up to ∼1.8 GeV.

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of eþe− pairs observed in the excited 8 Be nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e−Z → e−ZX by a 100 GeV e− beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a eþe− pair. With 5.4 × 1010 electrons on target, no evidence for such decays was found, allowing us to set first limits on the X − e− coupling in the range 1.3 × 10−4 ≲ ϵe ≲ 4.2 × 10−4 excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A0 ) from nonobservation of the decay A0 → eþe− of the bremsstrahlung A0 with a mass ≲23 MeV

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 elasticscattering 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 nonzero 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 ε at Q2 ≈ 0.85 and 1.45 GeV2 . There are indications of a slight increase in R2γ with Q2.

© 2019 American Physical Society. First measurements of double-polarization observables in ω photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry F has been measured using circularly polarized, tagged photons in the energy range 1200-2700 MeV, and the beam-target asymmetries H and P have been measured using linearly polarized, tagged photons in the energy range 1200-2000 MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon (N∗) resonances. In particular, contributions from new N∗ resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.