Experimental and Phenomenological Investigations of the MiniBooNE Anomaly
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Abstract and Figures
This thesis covers a range of experimental and theoretical efforts to elucidate the origin of the MiniBooNE low energy excess (LEE). We begin with the follow-up MicroBooNE experiment, which took data along the BNB from 2016 to 2021. This thesis specifically presents MicroBooNE's search for charged-current quasi-elastic (CCQE) interactions consistent with two-body scattering. The two-body CCQE analysis uses a novel reconstruction process, including a number of deep-learning-based algorithms, to isolate a sample of CCQE interaction candidates with purity. The analysis rules out an entirely -based explanation of the MiniBooNE excess at the confidence level. We next perform a combined fit of MicroBooNE and MiniBooNE data to the popular 3+1 model; even after the MicroBooNE results, allowed regions in - parameter space exist at the confidence level. This thesis also demonstrates that the MicroBooNE data are consistent with a -based explanation of the MiniBooNE LEE at the confidence level. Next, we investigate a phenomenological explanation of the MiniBooNE excess combining the 3+1 model with a dipole-coupled heavy neutral lepton (HNL). It is shown that a 500 MeV HNL can accommodate the energy and angular distributions of the LEE at the confidence level while avoiding stringent constraints derived from MINERA elastic scattering data. Finally, we discuss the Coherent CAPTAIN-Mills experiment--a 10-ton light-based liquid argon detector at Los Alamos National Laboratory. The background rejection achieved from a novel Cherenkov-based reconstruction algorithm will enable world-leading sensitivity to a number of beyond-the-Standard Model physics scenarios, including dipole-coupled HNLs.
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A bstract
This article reports global fits of short-baseline neutrino data to oscillation models involving light sterile neutrinos. In the commonly-used 3+1 plane wave model, there is a well-known 4.9 σ tension between data sets sensitive to appearance versus disappearance of neutrinos. We find that models that damp the oscillation prediction for the reactor data sets, especially at low energy, substantially improve the fits and reduce the tension. We consider two such scenarios. The first scenario introduces the quantum mechanical wavepacket effect that accounts for the source size in reactor experiments into the 3+1 model. We find that inclusion of the wavepacket effect greatly improves the overall fit compared to a three-neutrino model by ∆ χ ² / dof = 61 . 1 / 4 (7 . 1 σ improvement) with best-fit ∆ m ² = 1 . 4 eV ² and wavepacket length of 67 fm. The internal tension is reduced to 3.4 σ . If reactor-data only is fit, then the wavepacket preferred length is 91 fm ( > 20 fm at 99% CL). The second model introduces oscillations involving sterile flavor and allows the decay of the heaviest, mostly sterile mass state, ν 4 . This model introduces a damping term similar to the wavepacket effect, but across all experiments. Compared to a three-neutrino fit, this has a ∆ χ ² / dof = 60 . 6 / 4 (7 σ improvement) with preferred ∆ m ² = 1 . 4 eV ² and decay Γ = 0 . 35 eV. The internal tension is reduced to 3.7 σ .
For many years, the reactor event rates have been observed to have structure that deviates from prediction. Community discussion has focused on an excess compared to prediction observed at 5 MeV; however, other deviations are apparent. This structure has L dependence that is well-fit by the damped models. Before assuming this points to new physics, we urge closer examination of systematic effects that could lead to this L dependence.
We show results from the Coherent CAPTAIN Mills (CCM) 2019 engineering run which begin to constrain regions of parameter space for axionlike particles (ALPs) produced in electromagnetic particle showers in an 800 MeV proton beam dump, and further investigate the sensitivity of ongoing data-taking campaigns for the CCM200 upgraded detector. Based on beam-on background estimates from the engineering run, we make realistic extrapolations for background reduction based on expected shielding improvements, reduced beam width, and analysis-based techniques for background rejection. We obtain reach projections for two classes of signatures; ALPs coupled primarily to photons can be produced in the tungsten target via the Primakoff process, and then produce a gamma-ray signal in the liquid argon CCM detector either via inverse Primakoff scattering or decay to a photon pair. ALPs with significant electron couplings have several additional production mechanisms (Compton scattering, e+e− annihilation, ALP-bremsstrahlung) and detection modes (inverse Compton scattering, external e+e− pair conversion, and decay to e+e−). In some regions, the constraint is marginally better than both astrophysical and terrestrial constraints. With the beginning of a three year run, CCM will be more sensitive to this parameter space by up to an order of magnitude for both ALP-photon and ALP-electron couplings. The CCM experiment will also have sensitivity to well-motivated parameter space of QCD axion models. It is only a recent realization that accelerator-based large volume liquid argon detectors designed for low-energy coherent neutrino and dark matter scattering searches are also ideal for probing ALPs in the unexplored ∼MeV mass scale.
We consider the sterile neutrino, which is also known as heavy neutral lepton, interacting with the Standard Model (SM) active neutrino and photon via a transition magnetic moment, the so-called dipole portal, which can be induced from the more general dipole couplings which respect the full gauge symmetries of the SM. Depending on the interactions with SU(2)L and U(1)Y field strength tensors Wμνa and Bμν, we consider four typical scenarios and probe the constraints on the couplings with photon dγ at LEP using the analyses to search monophoton signature and the measurement of Z decay. We find that in the considered scenarios assuming the coupling with Z boson dZ≠0, the measurement of Z decaying into photon plus invisible particles can provide stricter constraints than the monophoton searches at the LEP1. The complementary constraints to existing experiments can be provided by the LEP. We also investigate the sensitivity on the dipole portal coupling dγ from the monophoton searches at future electron colliders, such as CEPC, and find that CEPC can explore the previously unconstrained parameter space by current experiments.
A bstract
The MiniBooNE excess persists as a significant puzzle in particle physics. Given that the MiniBooNE detector cannot discriminate between electron-like signals and backgrounds due to photons, the goal of this work is to study photon backgrounds in MiniBooNE in depth. We first consider a novel single-photon background arising from multi-nucleon scattering with coherently enhanced initial or final state radiation. This class of processes, which we dub “2p2h γ ” (two-particle–two-hole + photon) can explain ~40 of the ~560 excess events observed by MiniBooNE in neutrino mode. Second, we consider the background from neutral-current single- π ⁰ production, where two photons from π ⁰ → γγ decay are mis-identified as an electron-like shower. We construct a phenomenological likelihood that reproduces MiniBooNE’s π ⁰ → γγ background faithfully. Even with data-driven background estimation techniques, we find there is a residual dependence on the Monte Carlo generator used. Our results motivate a reduction in the significance of the MiniBooNE excess by 0 . 4 σ .
The MicroBooNE experiment searched for an excess of electron-neutrinos in the Booster Neutrino Beam (BNB), providing direct constraints on νe-interpretations of the MiniBooNE low-energy excess (LEE). In this article, we show that if the MiniBooNE LEE is caused instead by an excess of ν¯e, then liquid argon detectors, such as MicroBooNE, SBND, and ICARUS, would have poor sensitivity to it. This is due to a strong suppression of ν¯e–Ar40 cross sections in the low-energy region of the excess. The MicroBooNE results are consistent at the 2σ CL with a scenario in which the MiniBooNE excess is sourced entirely by ν¯e interactions. The opportune location of ANNIE, a Gd-loaded water Cherenkov detector, allows for a direct search for a ν¯e flux excess in the BNB using inverse beta-decay events.
We scrutinize the hypothesis that gauge singlet fermions—sterile neutrinos—interact with Standard Model particles through the transition magnetic moment portal. These interactions lead to the production of sterile neutrinos in supernovae followed by their decay into photons and active neutrinos which can be detected at γ-ray telescopes and neutrino detectors, respectively. We find that the nonobservation of active neutrinos and photons from sterile-neutrino decay associated to SN1987A yields the strongest constraints to date on magnetic-moment-coupled sterile neutrinos if their masses are inside a 0.1–100 MeV window. Assuming a near-future galactic supernova explosion, we estimate the sensitivity of several present and near-future experiments, including Fermi-LAT, e-ASTROGAM, DUNE, and Hyper-Kamiokande, to magnetic-moment-coupled sterile neutrinos. We also study the diffuse photon and neutrino fluxes produced in the decay of magnetic-moment coupled sterile neutrinos produced in all past supernova explosions and find that the absence of these decay daughters yields the strongest constraints to date for sterile neutrino masses inside a 1–100 keV window.
We estimate the sensitivity of the DUNE experiment to new physics particles interacting with neutrinos, considering the dipole portal to heavy neutral leptons and a neutrinophilic scalar with lepton-number 2 as examples. We demonstrate that neutrinos from the high-energy tail of the DUNE flux, with energies Eν≳5–10 GeV, may significantly improve the sensitivity to these models, allowing to search for particles as heavy as ≃10 GeV. We also study the impact of the so-called tau-optimized neutrino beam configuration, which slightly improves sensitivity to the new physics models considered here. For both models, we consider new production channels (such as deep-inelastic scattering) and provide a detailed comparison of different signatures in the detector.
We revisit models of heavy neutral leptons (neutrissimos) with transition magnetic moments as explanations of the 4.8σ excess of electronlike events at MiniBooNE. We first reexamine the preferred regions in the model parameter space to explain MiniBooNE, considering also potential contributions from oscillations due to an eV-scale sterile neutrino. We then derive constraints on the model using neutrino-electron elastic scattering data from MINERvA. To carry out these analyses, we have developed a detailed Monte Carlo simulation of neutrissimo interactions within the MiniBooNE and MINERvA detectors using the LeptonInjector framework. This simulation allows for a significantly more robust evaluation of the neutrissimo model compared to previous studies in the literature—a necessary step in order to begin making definitive statements about beyond the Standard Model explanations of the MiniBooNE excess. We find that MINERvA rules out a large region of parameter space, but allowed solutions exist at the 2σ confidence level. A dedicated MINERvA analysis would likely be able to probe the entire region of preference of MiniBooNE in this model.
A bstract
A variety of new physics scenarios allows for neutrinos to up-scatter into a heavy neutral lepton state. For a range of couplings and neutrino energies, the heavy neutrino may travel some distance before decaying to visible final states. When both the up-scattering and decay occur within the detector volume, these “double bang” events produce distinctive phenomenology with very low background. In this work, we first consider the current sensitivity at Super-Kamiokande via the atmospheric neutrino flux, and find current data may already provide new constraints. We then examine projected future sensitivity at DUNE and Hyper-Kamiokande, including both atmospheric and beam flux contributions to double-bang signals.