Pion Exchange Currents Research Articles

First Results on the Scalar WIMP-Pion Coupling, Using the XENON1T Experiment.

We present first results on the scalar coupling of weakly interacting massive particles (WIMPs) to pions from 1 t yr of exposure with the XENON1T experiment. This interaction is generated when the WIMP couples to a virtual pion exchanged between the nucleons in a nucleus. In contrast to most nonrelativistic operators, these pion-exchange currents can be coherently enhanced by the total number of nucleons and therefore may dominate in scenarios where spin-independent WIMP-nucleon interactions are suppressed. Moreover, for natural values of the couplings, they dominate over the spin-dependent channel due to their coherence in the nucleus. Using the signal model of this new WIMP-pion channel, no significant excess is found, leading to an upper limit cross section of 6.4×10^{-46} cm^{2} (90%confidence level) at 30 GeV/c^{2} WIMP mass.

Improved Limits for Higgs-Portal Dark Matter from LHC Searches.

Searches for invisible Higgs decays at the Large Hadron Collider constrain dark matter Higgs-portal models, where dark matter interacts with the standard model fields via the Higgs boson. While these searches complement dark matter direct-detection experiments, a comparison of the two limits depends on the coupling of the Higgs boson to the nucleons forming the direct-detection nuclear target, typically parametrized in a single quantity f_{N}. We evaluate f_{N} using recent phenomenological and lattice-QCD calculations, and include for the first time the coupling of the Higgs boson to two nucleons via pion-exchange currents. We observe a partial cancellation for Higgs-portal models that makes the two-nucleon contribution anomalously small. Our results, summarized as f_{N}=0.308(18), show that the uncertainty of the Higgs-nucleon coupling has been vastly overestimated in the past. The improved limits highlight that state-of-the-art nuclear physics input is key to fully exploiting experimental searches.

Charged-pion electroproduction above the resonance region

[Background] Above the nucleon resonance region, the $N(e,e'\pi^\pm)N'$ data cannot be explained by conventional hadronic models. For example, the observed magnitude of the transverse cross section is significantly underestimated in a framework with Reggeized background amplitudes. [Purpose] Develop a phenomenological framework for the $N(e,e'\pi^\pm)N'$ reaction at high invariant mass $W$ and deep photon virtuality $Q^2$. [Method] Building on the work of Kaskulov and Mosel, a gauged pion-exchange current is introduced with a running cutoff energy for the proton electromagnetic transition form factor. A new transition form factor is proposed. It respects the correct on-shell limit, has a simple physical interpretation and reduces the number of free parameters by one. [Results] A study of the $W$ dependence of the $N(e,e'\pi^\pm)N'$ lends support for the newly proposed transition form factor. In addition, an improved description of the separated and unseparated cross sections at $-t \lesssim 0.5 \;\text{GeV}^2$ is obtained. The predictions overshoot the measured unseparated cross sections for $-t > 0.5 \;\text{GeV}^2$. Introducing a strong hadronic form factor in the Reggeized background amplitudes brings the calculations considerably closer to the high $-t$ data. [Conclusions] Hadronic models corrected for resonance/parton duality describe the separated pion electroproduction cross sections above the resonance region reasonably well at low $-t$. In order to validate the applicability of these models at high $-t$, separated cross sections are needed. These are expected to provide a more profound insight into the relevant reaction mechanisms.

Dissecting deuteron Compton scattering I: The observables with polarised initial states

A complete set of linearly independent observables in Compton scattering with arbitrarily polarised real photons off an arbitrarily polarised spin-1 target is introduced, for the case that the final-state polarisations are not measured. Adopted from the one widely used, e.g., in deuteron photo-dissociation, it consists of 18 terms: the unpolarised cross section, the beam asymmetry, 4 target asymmetries and 12 asymmetries in which both beam and target are polarised. They are expressed by the helicity amplitudes and —where available— related to observables discussed by other authors. As application to deuteron Compton scattering, their dependence on the (isoscalar) scalar and spin dipole polarisabilities of the nucleon is explored in Chiral Effective Field Theory with dynamical Δ(1232) degrees of freedom at order e 2 δ 3. Some asymmetries are sensitive to only one or two dipole polarisabilities, making them particularly attractive for experimental studies. At a photon energy of 100 MeV, a set of 5 observables is identified from which one may be able to extract the spin polarisabilities of the nucleon. These are experimentally realistic but challenging and mostly involve tensor-polarised deuterons. Relative to Compton scattering from a nucleon, sensitivity to the “mixed” spin polarisabilities γ E1M2 and γ M1E2 is increased because of the interference with the D wave component of the deuteron and with its pion-exchange current. An interactive Mathematica 9.0 notebook with results for all observables at photon energies up to 120 MeV is available from hgrie@gwu.edu.

Relativistic calculation of deuteron threshold electrodisintegration at backward angles

The threshold electrodisintegration of the deuteron at backward angles is studied in instant form Hamiltonian dynamics, including a relativistic one-pion-exchange potential (OPEP) with off-shell terms as predicted by pseudovector coupling of pions to nucleons. The bound and scattering states are obtained in the center-of-mass frame, and then boosted from it to the Breit frame, where the evaluation of the relevant matrix elements of the electromagnetic current operator is carried out. The latter includes, in addition to one-body, also two-body terms due to pion exchange, as obtained, consistently with the OPEP, in pseudovector pion-nucleon coupling theory. In order to estimate the magnitude of the relativistic effects we perform, for comparison, the calculation with a nonrelativistic phase-equivalent Hamiltonian and consistent one-body and two-body pion-exchange currents. Our results for the electrodisintegration cross section show that, in the calculations using one-body currents, relativistic corrections become significant (i.e., larger than 10%) only at high momentum transfer $Q$ (${Q}^{2}\ensuremath{\simeq}40$ fm${}^{\ensuremath{-}2}$ and beyond). However, the inclusion of two-body currents makes the relativistic predictions considerably smaller than the corresponding nonrelativistic results in the ${Q}^{2}$ region (18--40) fm${}^{\ensuremath{-}2}$. The calculations based on the relativistic model also confirm the inadequacy, already established in a nonrelativistic context, of the present electromagnetic current model to reproduce accurately the experimental data at intermediate values of momentum transfers.

The role of the pion pair term in the theory of the weak axial meson exchange currents

The structure of the weak axial pion exchange current is discussed in various models. It is shown how the interplay of the chiral invariance and the double counting problem restricts uniquely the form of the pion potential term, in the case when the nuclear dynamics is described by the Schroedinger equation with the static nucleon-nucleon potential.

Parity-violating interaction effects in thenpsystem

We investigate parity-violating observables in the $np$ system, including the longitudinal asymmetry and neutron-spin rotation in $\stackrel{P\vec}{n}p$ elastic scattering, the photon asymmetry in $\stackrel{P\vec}{n}p$ radiative capture, and the asymmetries in deuteron photodisintegration $d(\stackrel{P\vec}{\ensuremath{\gamma}},n)p$ in the threshold region and electrodisintegration $d(\stackrel{P\vec}{e},{e}^{\ensuremath{'}})np$ in quasielastic kinematics. To have an estimate of the model dependence for the various predictions, a number of different, latest-generation strong-interaction potentials---Argonne ${v}_{18}$, Bonn 2000, and Nijmegen I---are used in combination with a weak-interaction potential consisting of $\ensuremath{\pi}$-, $\ensuremath{\rho}$-, and $\ensuremath{\omega}$-meson exchanges---the Desplanques-Donoghue-Holstein (DDH) model. The complete bound and scattering problems in the presence of parity-conserving, including electromagnetic, and parity-violating potentials are solved in both configuration and momentum space. The issue of electromagnetic current conservation is examined carefully. We find large cancellations between the asymmetries induced by the parity-violating interactions and those arising from the associated pion-exchange currents. In the $\stackrel{P\vec}{n}p$ capture, the model dependence is nevertheless quite small, because of constraints arising through the Siegert evaluation of the relevant ${E}_{1}$ matrix elements. In quasielastic electron scattering these processes are found to be insignificant compared to the asymmetry produced by $\ensuremath{\gamma}\text{\ensuremath{-}}Z$ interference on individual nucleons. These two experiments, then, provide clean probes of different aspects of weak-interaction physics associated with parity violation in the $np$ system. Finally, we find that the neutron-spin rotation in $\stackrel{P\vec}{n}p$ elastic scattering and asymmetry in deuteron disintegration by circularly polarized photons exhibit significant sensitivity both to the values used for the weak vector-meson couplings in the DDH model and to the input strong-interaction potential adopted in the calculation. This reinforces the conclusion that these short-ranged meson couplings are not in themselves physical observables; rather, the parity-violating mixings are the physically relevant parameters.

A Six-Quark Dressed-Bag Description of np→dγ Radiative Capture

The radiative capture process np→dγ is considered within the framework of a recently developed six-quark dressed-bag model for the nucleon-nucleon interaction. The calculations presented here include both the nucleon current and the meson-exchange current contributions. The latter uses short-range hadronic form factors for the pion exchange currents consistent with the soft cut-off parameter ΛπNN from the NN-potential. Contributions of the pion exchange current and Δ-isobar current to the total cross-section still cannot explain the discrepancy between the theoretical and experimental cross-sections. Possibilities for new types of meson exchange currents associated with chiral fields inside multi-quark dressed-bag states in nuclei are discussed.

Parity-violating interactions and currents in the deuteron

We investigate parity-violating asymmetries in polarized n p radiative capture, and deuteron electro-disintegration in quasi-elastic kinematics, using the DDH model for the parity-violating nucleon-nucleon interaction. We find dramatic cancellations between the asymmetries induced by the parity-violating interaction and those arising from the associated parity-violating pion-exchange currents. In np capture, the model-dependence of the result is nevertheless quite small because of constraints arising through the Siegert evaluation of the relevant E1 matrix element. In quasi-elastic electron scattering these processes are found to be insignificant comared to the asymmetry produced by the gamma-Z interference on individual nucleons. These two experiments, then, provide clean probes of different aspects of weak-interaction physics associated with parity violation in the np system.

Dynamics of the 16O(e, e'p) reaction at high missing energies.

We measured the cross section and response functions for the quasielastic 16O(e,e'p) reaction for missing energies 25< or =E(m)< or =120 MeV at missing momenta P(m)< or =340 MeV/c. For 25<E(m)<50 MeV and P(m) approximately 60 MeV/c, the reaction is dominated by a single 1s(1/2) proton knockout. At larger P(m), the single-particle aspects are increasingly masked by more complicated processes. Calculations which include pion exchange currents, isobar currents, and short-range correlations account for the shape and the transversity, but for only half of the magnitude of the measured cross section.

Comment on “Octet baryon magnetic moments in the chiral quark model with configuration mixing”

A recent paper by Linde, Ohlsson, and Snellman comes to the conclusion that a general ``sum rule for magnetic moments is always fulfilled in the chiral quark model, independently of SU(3) symmetry breaking.'' This conclusion is shown to be wrong because it does not take into account pion exchange currents, which must arise in any calculation of pion emission that is consistent with isotopic spin conservation.

Quantum Monte Carlo studies of relativistic effects in light nuclei

Relativistic Hamiltonians are defined as the sum of relativistic one-body kinetic energy, two- and three-body potentials and their boost corrections. In this work we use the variational Monte Carlo method to study two kinds of relativistic effects in the binding energy of 3H and 4He. The first is due to the nonlocalities in the relativistic kinetic energy and relativistic one-pion exchange potential (OPEP), and the second is from boost interaction. The OPEP contribution is reduced by about 15% by the relativistic nonlocality, which may also have significant effects on pion exchange currents. However, almost all of this reduction is canceled by changes in the kinetic energy and other interaction terms, and the total effect of the nonlocalities on the binding energy is very small. The boost interactions, on the other hand, give repulsive contributions of 0.4 (1.9) MeV in 3H (4He) and account for 37% of the phenomenological part of the three-nucleon interaction needed in the nonrelativistic Hamiltonians.

Chiral symmetry restoration and parity mixing

We derive the expressions of the vector and axial current from a chiral Lagrangian restricted to nucleons and pions. They display mixing terms between the axial and vector currents. We study the modifications in the nuclear medium of the coupling constants of the axial current, namely the pion decay constant and the nucleonic axial one due to the requirements of chiral symmetry. We express the renormalizations in terms of the local scalar pion density. The latter also governs the quark condensate evolution and we discuss the link between this evolution and the renormalizations. In the case of the nucleon axial coupling constant this renormalization corresponds to a new type of pion exchange currents, with two exchanged pions. We give an estimate for the resulting quenching. Although moderate it helps explaining the quenching experimentally observed.

Exchange Currents in Baryons

This paper reviews calculations of the electromagnetic properties of baryons using the constituent quark model. We start with a short discussion of spontaneous chiral symmetry breaking, which is essential in understanding the transition from QCD to the constituent quark model. We then discuss a chiral version of the constituent quark model, which simulates the symmetries and dynamical content of the underlying field theory in terms of gluon, pion and sigma exchange between constituent quarks. We show that the electromagnetic current charge and current operators, usually approximated by one-body operators (impulse approximation), must be supplemented by appropriate two-body terms (exchange currents). The latter represent the gluon and pion exchange degrees of freedom in the electromagnetic current operator. These exchange currents must be included for reasons of completeness and consistency. Most importantly, however, they are needed in order for the electromagnetic current to be conserved. We also study the effect of scalar exchange currents connected with the confinement and sigma exchange potentials. By including these twobody exchange currents we go beyond the single-quark impulse approximation, which has mainly been used up to now. The inclusion of gluon- pion-, and scalar-exchange currents in the quark potential model is the new point of the present work. We show that for some observables, such as the magnetic moments, charge, and magnetic radii of the proton and charged Δ (1232) states, exchange currents contribute at the level of some 10%. The same holds true for the magnetic moments of the entire baryon octet, with the exception of the Ξ- magnetic moment. On the other hand, the neutron charge radius, the quadrupole moments of the Δ, and the N → Δ transition quadrupole moment, are dominated by pion and gluon exchange contributions to the charge density operator. The inclusion of the pion and gluon exchange currents leads to a neutron charge radius of the correct size and sign. Based on the gluon and pion exchange current diagrams, we derive parameter-free relations between the neutron charge radius, the quadrupole moment of the Δ, and the N → Δ transition quadrupole moment. Neglecting configuration mixing, we find that the neutron charge radius and the N → Δ transition quadrupole moment are simply related as QN→Δ = r2 n√2. The implications of Siegert's theorem for the calculation of the E2 form factor in the N → Δ transition are studied. Finally, we discuss the axial coupling constant of the nucleon. We show that the inclusion of axial pair exchange currents does not significantly alter the NRQM prediction.

Photocouplings of nucleon resonances with gluon and pion exchange currents

Photocouplings for the nucleon resonances most relevant to total photoabsorption are calculated in a constituent quark model including gluon and pion exchange. The sensitivity of the results to the different ingredients of the model is discussed.

Radiative proton capture to discrete states in 12C at 98 and 176 MeV

A high-resolution magnetic pair spectrometer for medium energy photons has been used for studies of the exclusive (p, γ) reaction on 11B at 98 and 176 MeV. We present angular distributions of the differential cross section for transitions to the ground and first three excited states of 12C. The distributions from the three lowest lying states are compared with predictions from a microscopic continuum shell-model calculation using a realistic finite-ranged effective interaction with tensor components. Exchange current effects are examined in detail as both Δ(1232) isobar and pion exchange currents are rigorously included. In general, a good theoretical description of the data is obtained. The predominant reaction mechanism involves photon emission by the nucleonic one-body current.

Isovector meson-exchange currents in the light-front dynamics

In the light-front dynamics, there is no pair term that plays the role of the dominant isovector pion exchange current. This current gives rise to the large and experimentally observed contribution to the deuteron electrodisintegration cross-section near threshold for pseudo-scalar πNN coupling. We show analytically that in leading 1m order the amplitude in the light-front dynamics coincides, however, with the one given by the pair term. At high Q2, it consists of two equal parts. One comes from extra components of the deuteron and final state relativistic wave functions. The other results from the contact NNπγ interaction which appears in the light-front dynamics. This provides a transparent link between relativistic and non-relativistic approaches.

Two-nucleon knock-out investigated with the semi-exclusive 12C( e, e′ p) reaction

The 12C( e, e′ p) reaction has been studied in the dip region, at an energy transfer ω = 212 MeV and a three-momentum transfer | q| = 270 MeV/c. The data cover a missing energy range 0–180 MeV and include missing momenta up to 750 MeV/c. The cross sections, measured at various angles between the detected proton and q, show a clear signature for two-nucleon knock-out. At large angles, the measured strength can mainly be attributed to the coupling of the virtual photon to pion exchange currents and intermediate Δ-excitation. The data at small angles cannot be explained by these mechanisms alone. It is suggested that additional strength originates from the emission of a correlated nucleon pair.

Electromagnetic reactions on the deuteron at intermediate energies

Disintegration of the deuteron and pion production from the deuteron in electromagnetic reactions at intermediate energies are described. Consistency with the corresponding hadronic reactions is required. The relation between the mechanism for pion production and the traditional pion exchange current is important for the consistency.

Two-nucleon knockout contributions to the 12C( e, e′ p) reaction in the dip and Δ (1232) regions

The contributions from 12C( e, e′ pn) and 12C( e, e′ pp) to the semi-exclusive 12C( e, e′ p) cross section have been calculated in an unfactorized model for two-nucleon emission. We assume direct two-nucleon knockout after virtual photon coupling with the two-body pion-exchange currents in the target nucleus. Results are presented at several kinematical conditions in the dip and Δ (1232) regions. The calculated two-nucleon knockout strength is observed to account for a large fraction of the measured ( e, e′ p) strength ahe two-nucleon emission threshold.