Low Q2 Research Articles

Charged current weak production of Δ(1232) induced by electrons and positrons

The charged current weak production of Δ(1232) from the free proton target induced by the electron/positron in the intermediate energy range corresponding to the beam energy available at JLab and Mainz, has been studied. The results for the differential scattering cross section dσdQ2, the angular distribution dσdΩΔ, and the total scattering cross section σ(Ee) for both the electron and positron induced processes are presented, for the various energies in the range of 0.5–4 GeV. The cross section σ(Ee) is found to be of the order of 10−39 cm2 for the electron/positron energies in the few GeV range. The availability of electron/positron beams having well-defined energy and direction with very high luminosity of the order of 1038–1039 cm−2 sec−1, makes it possible to observe the weak charged current production of Δ(1232) and determine the axial vector form factors CiA(Q2);(i=3–5). The sensitivity of the differential cross section dσdQ2 to the subdominant form factors C3A(Q2) and C4A(Q2) is found to be strong enough, especially in the low Q2 region, which can be used to determine them phenomenologically and to test the various theoretical models proposed to calculate them. Published by the American Physical Society 2024

The proton charge radius from dimuon photoproduction off the proton

We investigate the feasibility of measuring the proton charge radius through dimuon photoproduction off a proton target. Our findings indicate that the Bethe-Heitler mechanism, which dominates at small momentum transfers, allows for an extraction of the proton electromagnetic form factors in the extremely low Q2 region below 10−3 GeV2 in the spacelike region, when the incident photon beam energy exceeds several hundred MeV. The optimal kinematical region and a sensitivity study of the proton charge radius from dimuon photoproduction are presented. Such a measurement is expected to provide an alternative to the elastic muon-proton scattering measurements such as MUSE at PSI and AMBER at CERN.

Global fit of electron and neutrino elastic scattering data to determine the strange quark contribution to the vector and axial form factors of the nucleon

We present a global fit of neutral-current elastic (NCE) neutrino-scattering data and parity-violating electron-scattering (PVES) data with the goal of determining the strange quark contribution to the vector and axial form factors of the proton. Previous fits of this form included data from a variety of PVES experiments (PVA4, HAPPEx, G0, SAMPLE) and the NCE neutrino and anti-neutrino data from BNL E734. These fits did not constrain the strangeness contribution to the axial form factor GAs(Q2) at low Q2 very well because there was no NCE data for Q2<0.45 GeV2. Our new fit includes for the first time MiniBooNE NCE data from both neutrino and antineutrino scattering; this experiment used a hydrocarbon target and so a model of the neutrino interaction with the carbon nucleus was required. Three different nuclear models have been employed: a relativistic Fermi gas model, the superscaling approximation model, and a spectral function model. We find a tremendous improvement in the constraint of GAs(Q2) at low Q2 compared to previous work, although more data is needed from NCE measurements that focus on exclusive single-proton final states, for example from MicroBooNE. Published by the American Physical Society 2024

Nonperturbative photon qq¯ light front wave functions from a contact interaction model

We propose a method to calculate the qq¯ light front wave functions (LFWFs) of photons at low virtuality, i.e., the light front amplitude of γ*→qq¯ at low Q2, based on a light front projection approach. We exemplify this method using a contact interaction model within Dyson-Schwinger equations formalism and obtain the nonperturbative photon qq¯ LFWFs. In this case, we find the nonperturbative effects are encoded in the enhanced quark mass and a dressing function of covariant quark-photon vertex, as compared to the leading order quantum electrodynamics photon qq¯ LFWFs. We then use nonperturbative-effect modified photon qq¯ LFWFs to study the inclusive deep inelastic scattering HERA data in the framework of the color dipole model. The results demonstrate that the theoretical description of data at low Q2 can be significantly improved once the nonperturbative corrections are included in the photon LFWFs. Published by the American Physical Society 2024

Bjorken sum rule with analytic coupling at low Q2 values

Bjorken sum rule with analytic coupling at low Q2 values

Self-consistent description of HERA data at low Q2 and soft hadron production at LHC

Using the analytical expression for transverse momentum dependent (TMD) gluon density in a proton, a self-consistent simultaneous description of low Q2 data on proton structure function F2(x,Q2), reduced cross section for the electron-proton deep inelastic scattering at HERA and soft hadron production in pp collisions at the LHC is achieved in the framework of color dipole approach and modified quark-gluon string model.

Evaluation of Bjorken polarised sum rule with a renormalon-motivated approach

We use the known renormalon structure of Bjorken polarised sum rule (BSR) Γ‾1p−n(Q2) to evaluate the leading-twist part of that quantity. In addition, we include D=2 and D=4 Operator Product Expansion (OPE) terms and fit this expression to available experimental data for inelastic BSR. Since we use perturbative QCD (pQCD) coupling, which fails at low squared spacelike momenta Q2≲1GeV2 due to Landau singularities, the fit is performed for Q2≥Qmin2 where Qmin2≈(1.7±0.3)GeV2. Due to large BSR experimental uncertainties, the extracted value of the pQCD coupling has very large uncertainties, especially when Qmin2 is varied. However, when we fix the pQCD coupling to the known world average values, the D=2 and D=4 residue parameters can be determined within large but reasonable uncertainties.

Association of social vulnerability with receipt of hernia repair in Texas

BackgroundThe effect of social health determinants on hernia surgery receipt is unclear. We aimed to assess the association of the social vulnerability index with the likelihood of undergoing elective and emergency hernia repair in Texas. MethodsThis is a retrospective cohort analysis of the Texas Hospital Inpatient Discharge Public Use Data File and Texas Outpatient Surgical and Radiological Procedure Public Use Data File from 2016 to 2019. Patients ≥18 years old with inguinal or umbilical hernia were included. Social vulnerability index and urban/rural status were merged with the database at the county level. Patients were stratified based on social vulnerability index quartiles, with the lowest quartile (Q1) designated as low vulnerability, Q2 and Q3 as average, and Q4 as high vulnerability. Wilcoxon rank sum, t test, and χ2 analysis were used, as appropriate. The relative risk of undergoing surgery was calculated with subgroup sensitivity analysis. ResultsOf 234,843 patients assessed, 148,139 (63.1%) underwent surgery. Compared to patients with an average social vulnerability index, the low social vulnerability index group was 36% more likely to receive surgery (relative risk: 1.36, 95% CI 1.34–1.37), whereas the high social vulnerability index group was 14% less likely to receive surgery (relative risk: 0.86, 95% CI 0.85–0.86). This remained significant after stratifying for age, sex, insurance status, ethnicity, and urban/rural status (P < .05). For emergency admissions, there was no difference in receipt of surgery by social vulnerability index. ConclusionVulnerable patients are less likely to undergo elective surgical hernia repair, even after adjusting for demographics, insurance, and urbanicity. The social vulnerability index may be a useful indicator of social determinants of health barriers to hernia repair.

New determination of |Vub/Vcb| from Bs0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {B}_s^0 $$\\end{document}→ {K−, Ds−\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {D}_s^{-} $$\\end{document}}μ+ν

We update the full set of B¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\overline{B} $$\\end{document}s → K form factors using light-cone sum rules with an on-shell kaon. Our approach determines the relevant sum rule parameters — the duality thresholds — from a Bayesian fit for the first time. Using a modified version of the Boyd-Grinstein-Lebed parametrisation, we combine our sum rule results at low momentum transfer q2 with more precise lattice QCD results at large q2. We obtain a consistent description of the form factors in the full q2 range. Applying these results to a recent LHCb measurement of branching ratios for the decays Bs0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {B}_s^0 $$\\end{document} → {K−, Ds−\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {D}_s^{-} $$\\end{document}}μ+νμ, we determine the ratio of Cabibbo-Kobayashi-Maskawa elementsVubVcbq2<7GeV2=0.0681±0.0040andVubVcbq2>7GeV2=0.0801±0.0047,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\left|\\frac{V_{ub}}{V_{cb}}\\right|}_{q^2<7\\;{\ extrm{GeV}}^2}=0.0681\\pm 0.0040\\kern0.5em \ extrm{and}\\kern0.5em {\\left|\\frac{V_{ub}}{V_{cb}}\\right|}_{q^2>7\\;{\ extrm{GeV}}^2}=0.0801\\pm 0.0047, $$\\end{document}which are mutually compatible at the 1.9σ level. We further comment on the sensitivity to Beyond the Standard Model effects through measurements of the shape of Bs0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {B}_s^0 $$\\end{document} → K−μ+νμ decays, in light of recent limits on such effects from other exclusive b → uℓν processes.

Insights on the current semi-leptonic B-decay discrepancies — and how Bs→ μ+μ−γ can help

Bs → μ+μ−γ, measured at high q2 as a partially reconstructed decay, can probe the origin of the existing discrepancies in semi-leptonic b → s and b → c decays. We perform a complete study of this possibility. We start by reassessing the alleged discrepancies, with a focus on a unified EFT description. Using the SMEFT, we find that the tauonic Wilson coefficient required by R(D(*)) implies a universal muonic Wilson coefficient of precisely the size required by semi-muonic BR data and, separately, by semi-muonic angular analyses. We thus identify reference scenarios. Importantly, Bs → μ+μ−γ offers a strategy to access them without being affected by the long-distance issues that hamper the prediction of semi-leptonic B decays at low q2. After quantifying to the best of our knowledge the Bs → μ+μ−γ experimental over the long haul, we infer the Bs → μ+μ−γ sensitivity to the couplings relevant to the anomalies. In the example of the real-δC9,10 scenario, we find significances below 3σ. Such figure is to be compared with other single-observable sensitivities that one can expect from e.g. BR and angular data, whether at low or high q2, and not affected by long-distance issues such as narrow resonances or intermediate charmed di-meson rescattering.

Measurement of the double-differential muon-neutrino charged-current inclusive cross section in the NOvA near detector

We report cross-section measurements of the final-state muon kinematics for νμ charged-current interactions in the NOvA near detector using an accumulated 8.09×1020 protons on target in the NuMI beam. We present the results as a double-differential cross section in the observed outgoing muon energy and angle, as well as single-differential cross sections in the derived neutrino energy, Eν, and square of the four-momentum transfer, Q2. We compare the results to inclusive cross-section predictions from various neutrino event generators via χ2 calculations using a covariance matrix that accounts for bin-to-bin correlations of systematic uncertainties. These comparisons show a clear discrepancy between the data and each of the tested predictions at forward muon angle and low Q2, indicating a missing suppression of the cross section in current neutrino-nucleus scattering models.3 MoreReceived 24 September 2021Accepted 14 February 2023DOI:https://doi.org/10.1103/PhysRevD.107.052011Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNeutrino interactionsNucleus-neutrino interactionsParticles & Fields

On the impact of meson mixing on Bs→ ϕee angular observables at low q2

Decays based on the b → sγ transition are expected to yield left-handed photons in the Standard Model, but could be particularly sensitive to New Physics contributions modifying the short-distance Wilson coefficients mathcal{C} 7 and mathcal{C} 7′ defined in the low-energy Effective Field Theory. These coefficients can be determined by combining observables of several modes, among which Bs → ϕee at low q2, in a kinematic range where the photon-pole is dominant. We investigate the impact of Bs − overline{B} s mixing on the angular observables available for this mode, which induces a time-dependent modulation governed by interference terms between mixing and decay that are sensitive to the moduli and phases of mathcal{C} 7 and mathcal{C} 7′. These interference terms can be extracted through a time-dependent analysis but they also affect time-integrated observables. In particular, we show that the asymmetries {A}_T^{(2)} and {A}_T^{left(operatorname{Im}right); CP} receive terms of potentially similar size from mixing-independent and mixing-induced terms, and we discuss how the constraints coming from the angular analysis of Bs→ ϕee at low q2 should be interpreted in the presence of mixing.

A multidimensional study of the structure function ratio [formula omitted] from hard exclusive π+ electro-production off protons in the GPD regime

A multidimensional extraction of the structure function ratio σLT′/σ0 from the hard exclusive e→p→e′nπ+ reaction above the resonance region has been performed. The study was done based on beam-spin asymmetry measurements using a 10.6 GeV incident electron beam on a liquid-hydrogen target and the CLAS12 spectrometer at Jefferson Lab. The measurements focus on the very forward regime (t/Q2 ≪ 1) with a wide kinematic range of xB in the valence regime (0.17 <xB < 0.55), and virtualities Q2 ranging from 1.5 GeV2 up to 6 GeV2. The results and their comparison to theoretical models based on Generalized Parton Distributions demonstrate the sensitivity to chiral-odd GPDs and the directly related tensor charge of the nucleon. In addition, the data is compared to an extension of a Regge formalism at high photon virtualities. It was found that the Regge model provides a better description at low Q2, while the GPD model is more appropriate at high Q2.

Lepton-flavor-violating ALPs at the Electron-Ion Collider: a golden opportunity

Axion-like particles (ALPs) arise in a variety of theoretical contexts and can, in general, mediate flavor violating interactions and parity non-conservation. We consider lepton flavor violating ALPs with GeV scale or larger masses which may, for example, arise in composite dark sector models. We show that a future Electron-Ion Collider (EIC) can uncover or constrain such ALPs via processes of the type e AZ→ τ AZa, where AZ is a nucleus of charge Z and a is an ALP in the range mτ≤ ma ≲ 20 GeV. The production of the ALP can have a large Z2 enhancement from low Q2 electromagnetic scattering of the electron from a heavy ion. Using the gold nucleus (Z = 79) as an example, we show that the EIC can explore e − τ flavor violation, mediated by GeV-scale ALPs, well beyond current limits. Importantly, the EIC reach for this interaction is not sensitive to the lepton-flavor conserving ALP couplings, whose possible smallness can render searches using τ decays ineffective. We also discuss how the EIC electron beam polarization can provide a powerful tool for investigating parity violating ALPs.

Longitudinal structure function FL at low Q2 and low x with model for higher twist: An update

A reanalysis of the model for the longitudinal structure function FL(x,Q2) at low x and low Q2 was undertaken, in view of the advent of the EIC. The model is based on the photon-gluon fusion mechanism suitably extrapolated to the region of low Q2. It includes the kinematic constraint FL∼Q4 as Q2→ 0 and higher twist contribution which vanishes as Q2→∞. Revised model was critically updated and compared to the presently available data.

Experimental study of the behavior of the Bjorken sum at very low Q2

We present new data on the Bjorken sum Γ‾1p−n(Q2) at 4-momentum transfer 0.021≤Q2≤0.496 GeV2. The data were obtained in two experiments performed at Jefferson Lab: EG4 on polarized protons and deuterons, and E97110 on polarized 3He from which neutron data were extracted. The data cover the domain where chiral effective field theory (χEFT), the leading effective theory of the Strong Force at large distances, is expected to be applicable. We find that our data and the predictions from χEFT are only in marginal agreement. This is somewhat surprising as the contribution from the Δ(1232) resonance is suppressed in this observable, which should make it more reliably predicted by χEFT than quantities in which the Δ contribution is important. The data are also compared to a number of phenomenological models with various degrees of agreement.

On the effective lifetime of Bs\u2192 \u03bc\u03bc\u03b3

We consider the {B}_s^0 → μ+μ−γ effective lifetime, and the related CP-phase sensitive quantity {A}_{{Delta Gamma}_s}^{mu mu gamma} , as a way to obtain qualitatively new insights on the current B-decay discrepancies. Through a fit comparing pre- to post-Moriond-2021 data we identify a few theory benchmark scenarios addressing these discrepancies, and featuring large CP violation in addition. We then explore the possibility of telling apart these scenarios with {A}_{{Delta Gamma}_s}^{mu mu gamma} , once resonance-modeling and form-factor uncertainties are taken into account. We do so in both regions of low and high invariant di-lepton mass-squared q2. For low q2, we show how to shape the integration range in order to reduce the impact of the ϕ-resonance modelling on the {A}_{{Delta Gamma}_s}^{mu mu gamma} prediction. For high q2, we find that the corresponding pollution from broad-charmonium resonances has a surprisingly small effect on {A}_{{Delta Gamma}_s}^{mu mu gamma} . This is due to a number of cancellations, that can be traced back to the complete dominance of semi-leptonic operator contributions for high q2 — at variance with low q2 — and to {A}_{{Delta Gamma}_s}^{mu mu gamma} behaving like a ratio-of-amplitudes observable. Our study suggests that {A}_{{Delta Gamma}_s}^{mu mu gamma} is — especially at high q2 — a potentially valuable probe of short-distance CP-violating effects in the very same Wilson coefficients that are associated to current b → s discrepancies. Its discriminating power, however, relies on progress in form-factor uncertainties. Interestingly, high q2 is the region where {B}_s^0 → μ+μ−γ is already being accessed experimentally, and the region where form factors are more accessible through non-perturbative QCD methods.

Disentangling QCD and new physics in D \u2192 \u03c0\u2113+\u2113\u2212

In this paper we consider the decay D+ → π+ℓ+ℓ−, addressing in particular the resonance contributions as well as the relatively large contributions from the weak annihilation diagrams. For the weak annihilation diagrams we include known results from QCD factorisation at low q2 and at high q2, adapting the existing calculation for B decays in the Operator Product Expansion. The hadronic resonance contributions are obtained through a dispersion relation, modelling the spectral functions as towers of Regge-like resonances in each channel, as suggested by Shifman, imposing the partonic behaviour in the deep Euclidean. The parameters of the model are extracted using e+e− → (hadrons) and τ → (hadrons) + ντ data as well as the branching ratios for the resonant decays D+ → π+R(R → ℓ+ℓ−), with R = ρ, ω, and ϕ. We perform a thorough error analysis, and present our results for the Standard Model differential branching ratio as a function of q2. Focusing then on the observables FH and AFB, we consider the sensitivity of this channel to effects of physics beyond the Standard Model, both in a model independent way and for the case of leptoquarks.

Two-Photon Exchange in Elastic Electron Scattering on Hadronic Systems

In the present review, we discuss different aspects of the two-photon exchange (TPE) physics in elastic ep scattering at high Q2, as well as at low Q2. The imaginary part of the TPE amplitude gives rise to beam and target single-spin asymmetries. Different theoretical approaches to the calculation of these observables are considered. The real part of the TPE amplitude influences the unpolarized cross-section and double-spin observables and is, most likely, responsible for the discrepancy between two methods of measurements of the proton form factors. We review different methods of calculations of the TPE amplitudes in the framework of the “hadron” and “quark-gluon” approaches. We discuss the dispersion approach suitable for low and intermediate Q2, which includes elastic and inelastic intermediate hadronic states, as well as the connection of TPE with the proton radius puzzle. The present situation with direct experimental searches for the TPE amplitude in the e+p/e−p charge asymmetry is also discussed, as well as attempts to extract the TPE amplitudes from existing experimental data obtained by the Rosenbluth and double polarization techniques. The TPE physics in other processes such as elastic мp, e-nucleus, and еп scattering is also reviewed.

Exploring time like tranistions in pp, πp and AA reactions with HADES

Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon converting to dielectron pair (Dalitz decays) provides important information about baryon-photon coupling at low q2 in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at q2 near vector mesons ρ/ω poles has been predicted by various calculations reflecting strong baryon-vector meson couplings. The understanding of these couplings is also of primary importance for the interpretation of the emissivity of QCD matter studied in heavy ion collisions via dilepton emission. Dedicated measurements of baryon Dalitz decays in proton-proton and pion-proton scattering with HADES detector at GSI/FAIR are presented and discussed. The relevance of these studies for the interpretation of results obtained from heavy ion reactions is elucidated on the example of the HADES results.