<p>The MiniBooNE low-energy excess stands as an unexplained anomaly in short-baseline neutrino oscillation experiments. It has been shown that it can be explained in the context of dark sector models. Here, we provide an overview of the possible new-physics solutions based on electron, photon, and dilepton final states. We systematically discuss the various production mechanisms for dark particles in neutrino-nucleus scattering. Our main result is a comprehensive fit to the MiniBooNE energy spectrum in the parameter space of dark neutrino models, where short-lived heavy neutral leptons are produced in neutrino interactions and decay to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> pairs inside the detector. For the first time, other experiments will be able to directly confirm or rule out dark neutrino interpretations of the MiniBooNE low-energy excess.</p>

United States

Asli M. Abdullahi

Poland

Jaime Hoefken Zink

Matheus Hostert

Italy

Belgium

Daniele Massaro

Silvia Pascoli

Panorama of new-physics explanations to the MiniBooNE excess

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> signal peak. A concrete example of such light new physics is <math><msup><mi>μ</mi><mo>−</mo></msup></math>-nucleon annihilation into a light dark sector, which can produce an energetic <math><msup><mi>e</mi><mo>−</mo></msup></math> as well as <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> by-products. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars.

Patrick J. Fox

Tony Menzo

Maxim Pospelov

Jure Zupan

Muon-induced baryon number violation

Long-standing anomalous experimental results from short-baseline neutrino experiments have persisted for decades. These results, when interpreted with one or more light sterile neutrinos, are inconsistent with numerous null results experimentally. However, if the sterile neutrino decays en route to the detector, this can mimic <math><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>→</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> oscillation signals while avoiding many of these external constraints. We revisit this solution to the MiniBooNE and LSND puzzles in view of new data from the MicroBooNE experiment at Fermilab. Using MicroBooNE’s liquid-argon time-projection chamber search for an excess of <math><msub><mi>ν</mi><mi>e</mi></msub></math> in the Booster beam, we derive new limits in two models’ parameter spaces of interest; where the sterile neutrino decays (I) via mixing with the active neutrinos, or (II) via higher-dimensional operators. We also provide an updated, comprehensive fit to the MiniBooNE neutrino- and antineutrino-beam data, including appearance (<math><msub><mi>ν</mi><mi>e</mi></msub></math>) and disappearance (<math><msub><mi>ν</mi><mi>μ</mi></msub></math>) channels. Despite alleviating the tension with muon neutrino disappearance experiments, we find that the latest MicroBooNE analysis rules out the decaying sterile neutrino solution in a large portion of the parameter space at more than 99% CL.

Kevin J. Kelly

Tao Zhou

Decaying sterile neutrinos at short baselines

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ + → 3e + 2e  −  2ν (  <math> <mi>B</mi> </math>  $$ \mathcal{B} $$  = 4.0 × 10 −10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015 μ + decays, depending on the signal selection strategy. We then turn to a neutrinoless μ + → 3e + 2e  −  decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs h  d . Following its production in μ + → e + h  d  decays, the dark Higgs can undergo a decay cascade to two e + e  −  pairs through two dark photons, h  d  → γ  d  γ  d  → 2(e + e  − ). We show that a μ + → 3e + 2e  −  search at the Mu3e experiment, with potential sensitivity to the branching ratio at the   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10 −12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.

Canada

New physics in multi-electron muon decays

We derive constraints on the couplings of light vector particles to all first-generation Standard Model fermions using leptonic decays of the charged pion, <math><mrow><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub><msub><mrow><mi>X</mi></mrow><mrow><mi>μ</mi></mrow></msub></mrow></math>. In models where the net charge to which <math><msub><mi>X</mi><mi>μ</mi></msub></math> couples to is not conserved, no lepton helicity flip is required for the decay to happen, enhancing the decay rate by factors of <math><mrow><mi>O</mi><mo>(</mo><msubsup><mrow><mi>m</mi></mrow><mrow><mi>π</mi></mrow><mrow><mn>4</mn></mrow></msubsup><mo>/</mo><msubsup><mrow><mi>m</mi></mrow><mrow><mi>e</mi></mrow><mrow><mn>2</mn></mrow></msubsup><msubsup><mrow><mi>m</mi></mrow><mrow><mi>X</mi></mrow><mrow><mn>2</mn></mrow></msubsup><mo>)</mo></mrow></math>. A past search at the SINDRUM-I spectrometer severely constrains this possibility. In the context of the hypothesized 17 MeV particle proposed to explain anomalous <math><mrow><mmultiscripts><mrow><mi>Be</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts></mrow></math>, <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math>, and <math><mrow><mmultiscripts><mrow><mi>C</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>12</mn></mrow></mmultiscripts></mrow></math> nuclear transitions claimed by the ATOMKI experiment, this limit rules out vector-boson explanations and poses strong limits on axial-vector ones.

Pion decay constraints on exotic 17 MeV vector bosons

The existence of right-handed neutrinos, or heavy neutral leptons (HNLs), is strongly motivated by the observation of neutrino masses and mixing. The mass of these new particles could lie below the electroweak scale, making them accessible to low-energy laboratory experiments. Additional new physics at high energies can mediate new interactions between the Standard Model particles and HNLs, and is most conveniently parametrized by the neutrino Standard Model Effective Field Theory, or νSMEFT for short. In this work, we consider the dimension six νSMEFT operators involving one HNL field in the mass range of   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (1) MeV &lt; M  N  &lt;   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (100) GeV. By recasting existing experimental limits on the production and decay of new light particles, we constrain the Wilson coefficients and new physics scale of each operator as a function of the HNL mass.1

Spain

Enrique Fernández-Martínez

Manuel González-López

Hungary

Josu Hernández-García

Jacobo López-Pavón

Effective portals to heavy neutral leptons

In rich dark sector models, dark photons heavier than tens of MeV can behave as semivisible particles; their decays contain both visible and invisible final states. We present models containing multiple dark fermions which allow for such decays and inscribe them in the context of inelastic dark matter and heavy neutral leptons scenarios. Our models represent a generalization of the traditional inelastic dark matter model by means of a charge conjugation symmetry. We revisit constraints on dark photons from <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> colliders and fixed-target experiments, including the effect of analysis vetoes on semivisible decays, <math><msup><mi>A</mi><mo>′</mo></msup><mo>→</mo><msub><mi>ψ</mi><mi>i</mi></msub><mo>(</mo><msub><mi>ψ</mi><mi>j</mi></msub><mo>→</mo><msub><mi>ψ</mi><mi>k</mi></msub><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup><mo>)</mo></math>. We find that in some cases the BABAR and NA64 experiments no longer exclude large kinetic mixing, <math><mi>ε</mi><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup></math>, and, specifically, the related explanation of the discrepancy in the muon (<math><mrow><mi>g</mi><mo>−</mo><mn>2</mn></mrow></math>). This reopens an interesting window in parameter space for dark photons with exciting discovery prospects. We point out that a modified missing-energy search at NA64 can target short-lived <math><msup><mi>A</mi><mo>′</mo></msup></math> decays and directly probe the newly-open parameter space.

CERN

Semivisible dark photon phenomenology at the GeV scale

A vector portal between the Standard Model and the dark sector is a predictive and compelling framework for thermal dark matter. Through co-annihilations, models of inelastic dark matter (iDM) and inelastic Dirac dark matter (i2DM) can reproduce the observed relic density in the MeV to GeV mass range without violating cosmological limits. In these scenarios, the vector mediator behaves like a semi-visible particle, evading traditional bounds on visible or invisible resonances, and uncovering new parameter space to explain the muon  $$(g-2)$$  <math> <mrow> <mo>(</mo> <mi>g</mi> <mo>-</mo> <mn>2</mn> <mo>)</mo> </mrow> </math>   anomaly. By means of a more inclusive signal definition at the NA64 experiment, we place new constraints on iDM and i2DM using a missing energy technique. With a recast-based analysis, we contextualize the NA64 exclusion limits in parameter space and estimate the reach of the newly collected and expected future NA64 data. Our results motivate the development of an optimized search program for semi-visible particles, in which fixed-target experiments like NA64 provide a powerful probe in the sub-GeV mass range.

Switzerland

Martina Mongillo

United Kingdom

Asli Abdullahi

Benjamin Banto Oberhauser

Paolo Crivelli

Laura Molina Bueno

Constraining light thermal inelastic dark matter with NA64

We propose new searches for <math><msup><mi>n</mi><mo>′</mo></msup></math>, a dark baryon that can mix with the Standard Model neutron. We show that IsoDAR, a proposal to place an intense cyclotron near a large-volume neutrino detector deep underground, can look for <math><mi>n</mi><mo>→</mo><msup><mi>n</mi><mo>′</mo></msup><mo>→</mo><mi>n</mi></math> transitions with much lower backgrounds than surface experiments. This opportune neutron-shining-through-a-wall search would be possible without any modifications to the primary goals of the experiment and would provide the strongest laboratory constraints on the <math><mi>n</mi><mo>−</mo><msup><mi>n</mi><mo>′</mo></msup></math> mixing for a wide range of mass splitting. We also consider dark neutrons as dark matter and show that their nuclear absorption at deep-underground detectors such as SNO and Borexino places some of the strongest limits in parameter space. Finally, we describe other <math><msup><mi>n</mi><mo>′</mo></msup></math> signatures, such as neutrons shining through walls at spallation sources, reactors, and the disappearance of ultracold neutrons.

David McKeen

India

Nirmal Raj

Dark sectors in neutron-shining-through-a-wall and nuclear-absorption signals

We propose a method to ease the challenges of exploring multidimensional parameter spaces in beyond-the-Standard Model theories. We evaluate the model likelihood for any choice of parameters by sampling the theory parameters intelligently and building a kernel density estimator. By reducing the number of expensive Monte Carlo simulations, this method provides an efficient way to test complex theories. We illustrate our technique to set new limits on a short-lived heavy neutrino <math><mrow><mi>N</mi></mrow></math>, proposed as an explanation of anomalies in neutrino experiments. Using a search for lepton pairs in the T2K near detector, we find exclusion limits on the model parameters in a vast region of parameter space, fully exploiting the advantages of our new method. With a single Monte Carlo simulation, we obtain the differential event rate for arbitrary choices of model parameters, allowing us to cast limits on any slice of the model parameter space. We conclude that <math><mi>N</mi></math> particles with lifetimes greater than <math><mi>c</mi><msup><mi>τ</mi><mn>0</mn></msup><mo>≳</mo><mn>3</mn><mtext> </mtext><mtext> </mtext><mi>cm</mi></math> are excluded by T2K data. We also derive model-independent constraints in terms of the total rate, lifetime, and <math><mi>N</mi></math> mass and provide an approximated analytical formula. This method can be applied in other branches of physics to explore the landscape of theory parameters efficiently.

Carlos A. Argüelles

Nicolò Foppiani

Efficiently exploring multidimensional parameter spaces beyond the Standard Model