A search for millicharged particles, a simple extension of the standard model, has been performed with the ArgoNeuT detector exposed to the Neutrinos at the Main Injector beam at Fermilab. The ...ArgoNeuT liquid argon time projection chamber detector enables a search for millicharged particles through the detection of visible electron recoils. We search for an event signature with two soft hits (MeV-scale energy depositions) aligned with the upstream target. For an exposure of the detector of 1.0×10^{20} protons on target, one candidate event has been observed, compatible with the expected background. This search is sensitive to millicharged particles with charges between 10^{-3}e and 10^{-1}e and with masses in the range from 0.1 to 3 GeV. This measurement provides leading constraints on millicharged particles in this large unexplored parameter space region.
Using truth-level Monte Carlo simulations of particle interactions in a large volume of liquid argon, we demonstrate physics capabilities enabled by reconstruction of topologically compact and ...isolated low-energy features, or "blips," in large liquid argon time projection chamber (LArTPC) events. These features are mostly produced by electron products of photon interactions depositing ionization energy. The blip identification capability of the LArTPC is enabled by its unique combination of size, position resolution precision, and low energy thresholds. We show that consideration of reconstructed blips in LArTPC physics analyses can result in substantial improvements in calorimetry for neutrino and new physics interactions and for final-state particles ranging in energy from the MeV to the GeV scale. Blip activity analysis is also shown to enable discrimination between interaction channels and final-state particle types. In addition to demonstrating these gains in calorimetry and discrimination, some limitations of blip reconstruction capabilities and physics outcomes are also discussed.
We have examined the impact of new Daya Bay, Double Chooz, and RENO measurements on global fits of reactor antineutrino flux data to a variety of hypotheses regarding the origin of the reactor ...antineutrino anomaly. In comparing RENO and Daya Bay measurements of inverse beta decay (IBD) yield versus Pu239 fission fraction, we find differing levels of precision in measurements of time-integrated yield and yield slope, but similar central values, leading to modestly enhanced isotopic IBD yield measurements in a joint fit of the two datasets. In the absence of sterile neutrino oscillations, global fits to all measurements now provide 3σ preference for incorrect modeling of specific fission isotopes over common mismodeling of all beta-converted isotopes. If sterile neutrino oscillations are considered, global IBD yield fits provide no substantial preference between oscillation-including and oscillation-excluding hypotheses: hybrid models containing both sterile neutrino oscillations and incorrect U235 or Pu239 flux predictions are favored at only 1σ–2σ with respect to models where U235, U238, and Pu239 are assumed to be incorrectly predicted.
A
bstract
We analyzed the evolution data of the Daya Bay reactor neutrino experiment in terms of short-baseline active-sterile neutrino oscillations taking into account the theoretical uncertainties ...of the reactor antineutrino fluxes. We found that oscillations are disfavored at 2.6
σ
with respect to a suppression of the
235
U reactor antineutrino flux and at 2.5
σ
with respect to variations of the
235
U and
239
Pu fluxes. On the other hand, the analysis of the rates of the short-baseline reactor neutrino experiments favor active-sterile neutrino oscillations and disfavor the suppression of the
235
U flux at 3.1
σ
and variations of the
235
U and
239
Pu fluxes at 2.8
σ
. We also found that both the Daya Bay evolution data and the global rate data are well-fitted with composite hypotheses including variations of the
235
U or
239
Pu fluxes in addition to active-sterile neutrino oscillations. A combined analysis of the Daya Bay evolution data and the global rate data shows a slight preference for oscillations with respect to variations of the
235
U and
239
Pu fluxes. However, the best fits of the combined data are given by the composite models, with a preference for the model with an enhancement of the
239
Pu flux and relatively large oscillations.
Predictions of antineutrino fluxes produced by fission isotopes in a nuclear reactor have recently received increased scrutiny due to observed differences in predicted and measured inverse beta decay ...(IBD) yields, referred to as the “reactor antineutrino flux anomaly.” In this paper, global fits are applied to existing IBD yield measurements to produce constraints on antineutrino production by individual plutonium and uranium fission isotopes. We find that fits including measurements from highly U235-enriched cores and fits including Daya Bay’s new fuel evolution result produce discrepant best-fit IBD yields for U235 and Pu239. This discrepancy can be alleviated in a global analysis of all data sets through simultaneous fitting of Pu239, U235, and U238 yields. The measured IBD yield of U238 in this analysis is (7.02±1.65)×10−43 cm2/fission, nearly two standard deviations below existing predictions. Future hypothetical IBD yield measurements by short-baseline reactor experiments are examined to determine their possible impact on the global understanding of isotopic IBD yields. It is found that future improved short-baseline IBD yield measurements at both high-enriched and low-enriched cores can significantly improve constraints for U235, U238, and Pu239, providing comparable or superior precision to existing conversion- and summation-based antineutrino flux predictions. Systematic and experimental requirements for these future measurements are also investigated.
We report the first electron neutrino cross section measurements on argon, based on data collected by the ArgoNeuT experiment running in the GeV-scale NuMI beamline at Fermilab. A flux-averaged νe + ...νe total and a lepton angle differential cross section are extracted using 13 νe and νe events identified with fully automated selection and reconstruction. We employ electromagnetic-induced shower characterization and analysis tools developed to identify νe / νe-like events among complex interaction topologies present in ArgoNeuT data ( ⟨ E νe ⟩ = 4.3 GeV and ⟨ E νe ⟩ = 10.5 GeV ). The techniques are widely applicable to searches for electron-flavor appearance at short and long baseline using liquid argon time projection chamber technology. Notably, the data-driven studies of GeV-scale νe / νe interactions presented here probe an energy regime relevant for future DUNE oscillation physics.
Recent measurements of reactor-produced antineutrino fluxes and energy spectra are inconsistent with models based on measured thermal fission beta spectra. In this paper, we examine the dependence of ...antineutrino production on fission neutron energy. In particular, the variation of fission product yields with neutron energy has been considered as a possible source of the discrepancies between antineutrino observations and models. In simulations of low-enriched and highly-enriched reactor core designs, we find a substantial fraction of fissions (from 5% to more than 40%) are caused by nonthermal neutrons. Using tabulated evaluations of nuclear fission and decay, we estimate the variation in antineutrino emission by the prominent fission parents U235, Pu239, and Pu241 versus neutron energy. The differences in fission neutron energy are found to produce less than 1% variation in detected antineutrino rate per fission of U235, Pu239, and Pu241. Corresponding variations in the antineutrino spectrum are found to be less than 10% below 7 MeV antineutrino energy, smaller than current model uncertainties. We conclude that insufficient modeling of fission neutron energy is unlikely to be the cause of the various reactor anomalies. Our results also suggest that comparisons of antineutrino measurements at low-enriched and highly-enriched reactors can safely neglect the differences in the distributions of their fission neutron energies.
Neutrinos are one of the most promising messengers for signals of new physics Beyond the Standard Model (BSM). On the theoretical side, their elusive nature, combined with their unknown mass ...mechanism, seems to indicate that the neutrino sector is indeed opening a window to new physics. On the experimental side, several long-standing anomalies have been reported in the past decades, providing a strong motivation to thoroughly test the standard three-neutrino oscillation paradigm. In this Snowmass21 white paper, we explore the potential of current and future neutrino experiments to explore BSM effects on neutrino flavor during the next decade.
MeV-scale energy depositions by low-energy photons produced in neutrino-argon interactions have been identified and reconstructed in ArgoNeuT liquid argon time projection chamber (LArTPC) data. ...ArgoNeuT data collected on the NuMI beam at Fermilab were analyzed to select isolated low-energy depositions in the TPC volume. The total number, reconstructed energies, and positions of these depositions have been compared to those from simulations of neutrino-argon interactions using the fluka Monte Carlo generator. Measured features are consistent with energy depositions from photons produced by deexcitation of the neutrino’s target nucleus and by inelastic scattering of primary neutrons produced by neutrino-argon interactions. This study represents a successful reconstruction of physics at the MeV scale in a LArTPC, a capability of crucial importance for detection and reconstruction of supernova and solar neutrino interactions in future large LArTPCs.
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