COHERENT enlightenment of the neutrino dark side

Abstract

In the presence of non-standard neutrino interactions (NSI), oscillation data are affected by a degeneracy which allows the solar mixing angle to be in the second octant (aka the dark side) and implies a sign flip of the atmospheric mass-squared difference. This leads to an ambiguity in the determination of the ordering of neutrino masses, one of the main goals of the current and future experimental neutrino program. We show that the recent observation of coherent neutrino-nucleus scattering by the COHERENT experiment, in combination with global oscillation data, excludes the NSI degeneracy at the 3.1σ (3.6σ) CL for NSI with up (down) quarks.The standard three-flavour oscillation scenario is supported by a large amount of data from neutrino oscillation experiments. The determination of oscillation parameters (see, e.g., Ref.[1]) is very robust, and for a broad range of new physics scenarios only small perturbations of the standard oscillation picture are allowed by data. There is, however, an exception to this statement: in the presence of non-standard neutrino interactions (NSI) [2-4] a degeneracy exists in oscillation data, leading to a qualitative change of the lepton mixing pattern. This was first observed in the context of solar neutrinos, where for suitable NSI the data can be explained by a mixing angle θ 12 in the second octant, the so-called LMA-Dark (LMA-D) [5] solution. This is in sharp contrast to the established standard MSW solution [2,6], which requires a mixing angle θ 12 in the first octant.The origin of the LMA-D solution is a degeneracy in oscillation probabilities due to a symmetry of the Hamiltonian describing neutrino evolution in the presence of NSI [7][8][9][10]. This degeneracy involves not only the octant of θ 12 but also a change in sign of the larger neutrino mass-squared difference, ∆m 2 31 , which is used to parameterize the type of neutrino mass ordering (normal versus inverted). Hence, the LMA-D degeneracy makes it impossible to determine the neutrino mass ordering by oscillation experiments [10], and therefore jeopardizes one of the main goals of the upcoming neutrino oscillation program. As discussed in Refs. [5,[10][11][12], non-oscillation data (such as that from neutrino scattering experiments) is needed to break this degeneracy.Recently, coherent neutrino-nucleus scattering has been observed for the first time by the COHERENT experiment [13], using neutrinos produced at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory. The observed interaction rate is in good agreement with the Standard Model (SM) prediction and can be used to constrain NSI. In this Letter we show that this result excludes the LMA-D solution at 3.1σ (3.6σ) CL for NSI with up (down) quarks when combined with oscillation data.NSI formalism and the LMA-D degeneracy. We consider the presence of neutral-current (NC) NSI in the form of dimension-six four-fermion operators, following the notation of Ref. [8]. Since we are interested in the contribution of the NSI to the effective potential of ne...

Improved global fit to Non-Standard neutrino Interactions using COHERENT energy and timing data

“…In our previous work [28] we performed a fit to COHERENT data using the available information at that time, which included only the total event rates observed. Last year the collaboration released publicly both the energy and timing information of the events [35].…”

“…As discussed in Refs. [22,26,[28][29][30], non-oscillation data (such as those collected by neutrino scattering experiments) is needed to break this degeneracy. However, for light enough mediators (M med O(10) GeV) bounds from deep inelastic neutrino scattering experiments, such as CHARM [31] and NuTeV [32], can be successfully avoided [12,26,29].…”

“…To this end, in Ref. [28] we combined the first results from the COHERENT experiment [34] with previous bounds on NSI from a global fit to oscillation data [24]. The outcome of such analysis showed that it was already possible to reject the LMA-D degeneracy beyond 3σ in models where the NSI operators involved only a single quark flavor (either up or down).…”

Accelerator and reactor complementarity in coherent neutrino-nucleus scattering

“…Using neutrinos from a stopped pion beam and a 14.6 kg CsI[Na] target, Coherent measured a best-fit count of 134 AE 22 CEνNS events, well in excess of the expected backgrounds. The measured rate was found to be 77 AE 16% of the standard model (SM) prediction, and constrains NSI for some terms more strongly than previous deep inelastic scattering and oscillation experiments [1][2][3]. Properties of the neutron distribution may also be extracted from the measurement [4], and within certain frameworks, models for light dark matter can be probed [5].…”

“…We further take an uncertainty of 5% on the experimental background reported by Coherent over the relevant recoil energy range. We note that previous studies of NSI using the Coherent experiment [2,3,16,23] have instead assumed that the per-flavor flux can be identified through a combination of timing and spectral signatures, which would appear to be a reasonable future expectation.…”