The workshop “Shedding light on X17” brings together scientists looking for the existence of a possible new light particle, often referred to as X17. This hypothetical particle can explain the resonant structure observed at  $$\sim $$  <math> <mo>∼</mo> </math>   17 MeV in the invariant mass of electron-positron pairs, produced after excitation of nuclei such as  $$^8\hbox {Be}$$  <math> <mrow> <msup> <mrow></mrow> <mn>8</mn> </msup> <mtext>Be</mtext> </mrow> </math>   and  $$^4\hbox {He}$$  <math> <mrow> <msup> <mrow></mrow> <mn>4</mn> </msup> <mtext>He</mtext> </mrow> </math>   by means of proton beams at the Atomki Laboratory in Debrecen. The purpose of the workshop is to discuss implications of this anomaly, in particular theoretical interpretations as well as present and future experiments aiming at confirming the result and/or at providing experimental evidence for its interpretation.

United States

Daniele Alves

Italy

Daniele Barducci

Gianluca Cavoto

France

Luc Darmé

Spain

Luigi Delle Rose

Germany

Luca Doria

Jonathan Feng

André Frankenthal

Ashot Gasparian

United Kingdom

Evgueni Goudzovski

Carlo Gustavino

Egypt

Shaaban Khalil

Bulgaria

Venelin Kozhuharov

Hungary

Attila Krasznahorkay

Tommaso Marchi

Manuel Meucci

Gerald Miller

Stefano Moretti

Marco Nardecchia

Enrico Nardi

Czechia

Hugo Natal da Luz

Giovanni Organtini

Switzerland

Angela Papa

Ann-Kathrin Perrevoort

Vlasios Petousis

Gabriele Piperno

Mauro Raggi

Francesco Renga

Patrick Schwendimann

Rudolf Sýkora

Claudio Toni

Paolo Valente

Cecilia Voena

Cheuk-Yin Wong

Xilin Zhang

Shedding light on X17: community report

We combine Newton's variational method with ideas from eigenvector continuation to construct a fast &amp; accurate emulator for two-body scattering observables. The emulator will facilitate the application of rigorous statistical methods for interactions that depend smoothly on a set of free parameters. Our approach begins with a trial K or T matrix constructed from a small number of exact solutions to the Lippmann–Schwinger equation. Subsequent emulation only requires operations on small matrices. We provide several applications to short-range potentials with and without the Coulomb interaction and partial-wave coupling. It is shown that the emulator can accurately extrapolate far from the support of the training data. When used to emulate the neutron-proton cross section with a modern chiral interaction as a function of 26 free parameters, it reproduces the exact calculation with negligible error and provides an over 300x improvement in CPU time.

J.A. Melendez

C. Drischler

A.J. Garcia

R.J. Furnstahl

Fast &amp; accurate emulation of two-body scattering observables without wave functions

In 2016, the ATOMKI collaboration announced [Phys. Rev. Lett. 116, 042501 (2016)] observing an unexpected enhancement of the <math><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup></math>-<math><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math>pair production signal in one of the <math><mmultiscripts><mrow><mi>Be</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts></math> nuclear transitions induced by an incident proton beam on a $^{7}$Li target. Many beyond-standard-model physics explanations have subsequently been proposed. One popular theory is that the anomaly is caused by the creation of a protophobic vector boson (X) with a mass around 17 MeV [e.g. Phys. Rev. Lett. 117, 071803 (2016)] in the nuclear transition. We study this hypothesis by deriving an isospin relation between photon and X couplings to nucleons. This allows us to find simple relations between protophobic X-production cross sections and those for measured photon production. The net result is that X production is dominated by direct transitions induced by <math><mi>E</mi><msup><mrow><mn>1</mn></mrow><mrow><mi>X</mi></mrow></msup></math> and <math><mi>L</mi><msup><mrow><mn>1</mn></mrow><mrow><mi>X</mi></mrow></msup></math> (transverse and longitudinal electric dipoles) and <math><mi>C</mi><msup><mrow><mn>1</mn></mrow><mrow><mi>X</mi></mrow></msup></math> (charge dipole) without going through any nuclear resonance (i.e. Bremsstrahlung radiation) with a smooth energy dependence that occurs for all proton beam energies above threshold. This contradicts the experimental observations and invalidates the protophobic vector boson explanation.

Gerald A. Miller

Can a protophobic vector boson explain the ATOMKI anomaly?

Eigenvector continuation (EC) has been shown to accurately and efficiently reproduce ground states for targeted sets of Hamiltonian parameters. It uses as variational basis vectors the corresponding ground-state eigensolutions from selected other sets of parameters. Here we extend the EC approach to scattering using the Kohn variational principle. We first test it using a model for S-wave nucleon-nucleon scattering and then demonstrate that it also works to give accurate predictions for non-local potentials, charged-particle scattering, complex optical potentials, and higher partial waves. These proofs-of-principle validate EC as an accurate emulator for applying Bayesian inference to parameter estimation constrained by scattering observables. The efficiency of such emulators is because the accuracy is achieved with a small number of variational basis elements and the central computations are just linear algebra calculations in the space spanned by this basis.

P.J. Millican

Efficient emulators for scattering using eigenvector continuation

We present a systematic short time expansion for the generating function of the one point height probability distribution for the KPZ equation with droplet initial condition, which goes much beyond previous studies. The expansion is checked against a numerical evaluation of the known exact Fredholm determinant expression. We also obtain the next order term for the Brownian initial condition. Although initially devised for short time, a resummation of the series allows to obtain also the long time large deviation function, found to agree with previous works using completely different techniques. Unexpected similarities with stationary large deviations of TASEP with periodic and open boundaries are discussed. Two additional applications are given. (i) Our method is generalized to study the linear statistics of the Airy point process, i.e. of the GUE edge eigenvalues. We obtain the generating function of the cumulants of the empirical measure to a high order. The second cumulant is found to match the result in the bulk obtained from the Gaussian free field by Borodin and Ferrari [1,2], but we obtain systematic corrections to the Gaussian free field (higher cumulants, expansion towards the edge). This also extends a result of Basor and Widom [3] to a much higher order. We obtain large deviation functions for the Airy point process for a variety of linear statistics test functions. (ii) We obtain results for the counting statistics of trapped fermions at the edge of the Fermi gas in both the high and the low temperature limits.

Alexandre Krajenbrink

Pierre Le Doussal

Sylvain Prolhac

Systematic time expansion for the Kardar–Parisi–Zhang equation, linear statistics of the GUE at the edge and trapped fermions

Recently the experimentalists in Krasznahorkay (2016) [1] announced observing an unexpected enhancement of the e+ – e− pair production signal in one of the 8 Be nuclear transitions. The subsequent studies have been focused on possible explanations based on introducing new types of particle. In this work, we improve the nuclear physics modeling of the reaction by studying the pair emission anisotropy and the interferences between different multipoles in an effective field theory inspired framework, and examine their possible relevance to the anomaly. The connection between the previously measured on-shell photon production and the pair production in the same nuclear transitions is established. These improvements, absent in the original experimental analysis, should be included in extracting new particle's properties from the experiment of this type. However, the improvements can not explain the anomaly. We then explore the nuclear transition form factor as a possible origin of the anomaly, and find the required form factor to be unrealistic for the 8 Be nucleus. The reduction of the anomaly's significance by simply rescaling our predicted event count is also investigated.

Can nuclear physics explain the anomaly observed in the internal pair production in the Beryllium-8 nucleus?

We improve the Monte-Carlo based QCD sum rules by introducing the rigorous Hölder-inequality-determined sum rule window and a Breit-Wigner type parametrization for the phenomenological spectral function. In this improved sum rule analysis methodology, the sum rule analysis window can be determined without any assumptions on OPE convergence or the QCD continuum. Therefore, an unbiased prediction can be obtained for the phenomenological parameters (the hadronic mass and width etc.). We test the new approach in the ρ meson channel with re-examination and inclusion of αs corrections to dimension-4 condensates in the OPE. We obtain results highly consistent with experimental values. We also discuss the possible extension of this method to some other channels.

China

Qi-Nan Wang

Canada

Zhu-Feng Zhang

T.G. Steele

Hong-Ying Jin

Zhuo-Ran Huang

A comprehensive revisit of the ρ meson with improved Monte-Carlo based QCD sum rulesSupported by NSFC (11175153, 11205093, 11347020), Open Foundation of the Most Important Subjects of Zhejiang Province, and K. C. Wong Magna Fund in Ningbo University, TGS is Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Z. F. Zhang and Z. R. Huang are Grateful to the University of Saskatchewan for its Warm Hospitality

We consider pion production in parity-violating electron scattering (PVES) in the presence of nucleon strangeness in the framework of partial wave analysis with unitarity. Using the experimental bounds on the strange form factors obtained in elastic PVES, we study the sensitivity of the parity-violating asymmetry to strange nucleon form factors. For forward kinematics and electron energies above 1 GeV, we observe that this sensitivity may reach about 20% in the threshold region. With parity-violating asymmetries being as large as tens p.p.m., this study suggests that threshold pion production in PVES can be used as a promising way to better constrain strangeness contributions. Using this model for the neutral current pion production, we update the estimate for the dispersive Î³Z -box correction to the weak charge of the proton. In the kinematics of the Qweak experiment, our new prediction reads Reâ¡Î³ZV(E=1.165Â GeV)=(5.58Â±1.41)Ã10â3 , an improvement over the previous uncertainty estimate of Â±2.0Ã10â3 . Our new prediction in the kinematics of the upcoming MESA/P2 experiment reads Reâ¡Î³ZV(E=0.155Â GeV)=(1.1Â±0.2)Ã10â3 .

Mikhail Gorchtein

South Africa

Hubert Spiesberger

How strange is pion electroproduction?

We report an improved low-energy extrapolation of the cross section for the process Be7(p,Î³)B8 , which determines the 8 B neutrino flux from the Sun. Our extrapolant is derived from Halo Effective Field Theory (EFT) at next-to-leading order. We apply Bayesian methods to determine the EFT parameters and the low-energy S -factor, using measured cross sections and scattering lengths as inputs. Asymptotic normalization coefficients of 8 B are tightly constrained by existing radiative capture data, and contributions to the cross section beyond external direct capture are detected in the data at E&lt;0.5Â MeV . Most importantly, the S -factor at zero energy is constrained to be S(0)=21.3Â±0.7Â eVb , which is an uncertainty smaller by a factor of two than previously recommended. That recommendation was based on the full range for S(0) obtained among a discrete set of models judged to be reasonable. In contrast, Halo EFT subsumes all models into a controlled low-energy approximant, where they are characterized by nine parameters at next-to-leading order. These are fit to data, and marginalized over via Monte Carlo integration to produce the improved prediction for S(E) .

Kenneth M. Nollett

D.R. Phillips

Halo effective field theory constrains the solar 7 Be + p → 8 B + γ rate

We generalize forward real Compton amplitude to the case of the interference of the electromagnetic and weak neutral current, formulate a low-energy theorem, relate the new amplitudes to the interference structure functions and obtain a new set of sum rules. We address a possible new sum rule that relates the product of the axial charge and magnetic moment of the nucleon to the 0th moment of the structure function <math><msub><mrow><mi>g</mi></mrow><mrow><mn>5</mn></mrow></msub><mo>(</mo><mi>ν</mi><mo>,</mo><mn>0</mn><mo>)</mo></math>. For the dispersive γZ-box correction to the proton's weak charge, the application of the GDH sum rule allows us to reduce the uncertainty due to resonance contributions by a factor of two. The finite energy sum rule helps addressing the uncertainty in that calculation due to possible duality violations.

Forward Compton scattering with weak neutral current: Constraints from sum rules