Kuleshov, Sergey

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 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.

We have measured the nuclear transparency of the incoherent diffractive A(e, e ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced ρ0’s on a nucleus relative to deuterium, which is sensitive to ρ A interaction, was studied as function of the coherence length (lc ), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q 2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q 2 dependence was measured, which is consistent with calculations that included the color transparency effects.

We report on the measurement of the 𝛾⁢𝑝→𝐽/𝜓⁢𝑝 cross section from 𝐸𝛾=11.8  GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find that the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section 𝑑⁢𝜎/𝑑⁢𝑡 has an exponential slope of 1.67±0.39  GeV−2 at 10.7 GeV average energy. The LHCb pentaquark candidates 𝑃+ 𝑐 can be produced in the 𝑠 channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions ℬ⁡(𝑃+ 𝑐→𝐽/𝜓⁢𝑝) and cross sections 𝜎⁡(𝛾⁢𝑝→𝑃+ 𝑐)×ℬ⁡(𝑃+ 𝑐→𝐽/𝜓⁢𝑝).

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.

Exclusive 0 electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in Q2, xB, t, and , in the Q2 range from 1.0 to 4:6 GeV2, t up to 2 GeV2, and xB from 0.1 to 0.58. Structure functions T þ L, TT, and LT were extracted as functions of t for each of 17 combinations of Q2 and xB. The data were compared directly with two handbag-based calculations including both longitudinal and transversity generalized parton distributions (GPDs). Inclusion of only longitudinal GPDs very strongly underestimates T þ L and fails to account for TT and LT, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity-flip and helicity nonflip processes. The results confirm that exclusive 0 electroproduction offers direct experimental access to the transversity GPDs.

The CLAS detector was used to obtain the first ever measurement of the electromagnetic decay of the þð1385Þ from the reaction p ! K0 þð1385Þ. A real photon beam with a maximum energy of 3.8 GeV was incident on a liquid-hydrogen target, resulting in the photoproduction of the kaon and hyperon. Kinematic fitting was used to separate the reaction channel from the background processes. The fitting algorithm exploited a new method to kinematically fit neutrons in the CLAS detector, leading to the measured decay widths ratio þð1385Þ ! þ =þð1385Þ ! þ0 ¼ 11:95 2:21ðstatÞ þ0:53 1:21ðsysÞ% and a deduced partial width of 250:0 56:9ðstatÞ þ34:3 41:2ðsysÞ keV. A U-spin symmetry test using the SU(3) flavor-multiplet representation yields predictions for the þð1385Þ ! þ and 0ð1385Þ ! partial widths that agree with the experimental measurements

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 .

AbstractWe report the results of a search for a new vector boson ($$ A'$$ A ′ ) decaying into two dark matter particles $$\chi _1 \chi _2$$ χ 1 χ 2 of different mass. The heavier $$\chi _2$$ χ 2 particle subsequently decays to $$\chi _1$$ χ 1 and an off-shell Dark Photon $$ A'^* \rightarrow e^+e^-$$ A ′ ∗ → e + e - . For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay $$A'\rightarrow \chi \overline{\chi }$$ A ′ → χ χ ¯ and axion-like or pseudo-scalar particles $$a \rightarrow \gamma \gamma $$ a → γ γ . With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for $$A'$$ A ′ masses from 2$$m_e$$ m e up to 390 MeV and mixing parameter $$\varepsilon $$ ε between $$3\times 10^{-5}$$ 3 × 10 - 5 and $$2\times 10^{-2}$$ 2 × 10 - 2 .

We have measured cross sections for the 3He ! pd reaction at photon energies of 0.4–1.4 GeV and a center-of-mass angle of 90 . We observe dimensional scaling above 0.7 GeV at this center-of-mass angle. This is the first observation of dimensional scaling in the photodisintegration of a nucleus heavier than the deuteron