Searching for beyond-Standard-Model solar neutrino interactions using directional detectors

TL;DR

This work investigates the potential of directionally sensitive gas-based detectors to probe beyond-Standard-Model neutrino-electron interactions using solar neutrinos. By formulating the full energy–angle dependent rate with NSIs via a density-matrix approach and modeling detector performance for a 30 m^3 He:CF$_4$ TPC, the authors evaluate how NSI parameters and a light Z' mediator could be constrained. They perform a profile-likelihood analysis with Asimov data to project 90% CL limits under various background and directional-performance scenarios, showing that modest background reductions plus directionality can rival Borexino sensitivities. The results motivate the CYGNO/Cygnus program as a viable route to complement existing solar-neutrino tests and to help resolve degeneracies in NSI parameter space through event-by-event neutrino energy reconstruction. Overall, the study demonstrates the practical feasibility and scientific payoff of small, directional gas detectors for low-energy neutrino physics and NSI searches.

Abstract

Micro-pattern gaseous detectors (MPGDs) are a class of technologies that enable the full three-dimensional spatial reconstruction of ionisation tracks from nuclear and electron recoils in gas. Anticipating near-future 30 m$^3$-scale time projection chambers with MPGD-based readout, we forecast the sensitivity of such directionally-sensitive low-energy recoil detectors to neutrino interactions beyond the Standard Model. We work in the framework of neutrino non-standard interactions (NSIs), and calculate the combined recoil energy-angle distributions of the electron recoil signal generated by solar neutrinos in atmospheric-pressure He:CF$_4$ gas. We estimate the expected exclusion limits that such an experiment could place on various NSI parameters, as well as the mass and coupling of a new light mediator that interacts with electrons and neutrinos. We find that with an achievable background reduction of around a factor of ten from current estimates for a 30 m$^3$ optical-readout detector using this gas mixture, sensitivity to NSI parameters would already approach Borexino's sensitivity. Directionality also allows for event-by-event neutrino energy reconstruction, which would provide a means to resolve some parameter degeneracies present in the modified neutrino cross section in this formalism. Our results strongly motivate the development of small-scale directionally-sensitive gas detectors for neutrino physics.

Figures (7)

• Figure 1: Yearly expected number of electron recoil events with energies $E_r>10$ keV, in a 30 m$^3$ gas detector containing a 60:40 mixture of He and CF$_4$ at atmospheric pressure. Each line shows the total rate as a function of a different $\varepsilon_{\alpha\beta}^e$ where all other values of $\varepsilon_{\alpha\beta}^e$ are kept at zero. The case for the Standard Model is shown with a dotted grey line. The thick part of each line shows the range of $\varepsilon$ allowed by Borexino at 90% CL. The event numbers shown here correspond to the sum over the $pp$ and $^7$Be fluxes.
• Figure 2: Two illustrative cases of the double-differential event rates as a function of recoil energy, $E_r$, and recoil angle with respect to the Sun, $\cos\theta_\odot$. The central coloured panels display $\textrm{d}^2R/\textrm{d}E_r\textrm{d}\cos\theta_\odot$ for the NSI case. We assign a set of $\varepsilon^e_{\alpha \beta}$ as labelled for the heavy mediator example (left), and ($m_{Z^\prime},g_{Z^\prime})$ as labelled for the $B-L$ light mediator example (right). The smaller panels to the left and above the main panel in each case show the event rate integrated over angle and energy, respectively. In all panels, the solid lines correspond to the NSI case while the dashed lines show the SM ($\forall \varepsilon_{\alpha \beta}^e = 0$ or $g_{Z^\prime} = 0$ for heavy and light mediators, respectively). In all of these cases, the lines correspond to the rate after it has been normalised by the maximum value so as to highlight differences in shape (although the total rate also differs, as shown in Fig. \ref{['fig:NumberOfEvents']}). In the main panels, the solid and dashed contours also delineate equal values of the rate normalised by the maximum value---the four levels correspond to 0.75, 0.5, 0.25 and 0.1.
• Figure 3: Similar to Fig. \ref{['fig:RecoilDistribution']}, only now we show the relative difference in the energy-angle distributions of the recoils, which quantifies the differences compared to the SM for all six of the NSI parameters. More precisely, we are showing $(r_{\rm NSI} - r_{\rm SM})/r_{\rm SM}$ where $r = (1/R)\textrm{d}^2R/\textrm{d}E_r\textrm{d}\cos\theta_\odot$ is the double differential event rate normalised by the total rate. For each panel, we set all other $\varepsilon_{\alpha \beta}^e$ to zero apart from the component labelled, which we set equal to 1. This figure highlights that $\varepsilon^e_{ee}$, $\varepsilon^e_{e\mu}$, and $\varepsilon^e_{e\tau}$ have the largest relative impact on the angular distributions.
• Figure 4: Projected median 90% CL exclusion limits on each $\varepsilon_{\alpha\beta}^e$ parameter (assuming all other $\varepsilon_{\alpha\beta}^e = 0$). We assume a detector of 30 m$^3$ volume with a 60:40 He:CF$_4$ gas mixture at atmospheric pressure. The three panels from left to right are for different background rates, measured relative to the total rate of neutrino-induced electron recoils under the SM, $R_\nu$. The various projected limits shown in blue are for different levels of directional sensitivity: idealised performance, realistic performance, and no directionality, from lightest to darkest. We emphasise that the "non-directional" case has an identical event rate, energy threshold and energy resolution as the "directional" cases. For comparison, in black we show the existing constraints from Borexino, demonstrating a competitive level of sensitivity despite the small size of the proposed experiment.
• Figure 5: Projected median 90% CL exclusion limits when two $\varepsilon_{\alpha\beta}^e$ parameters are considered simultaneously (assuming all other $\varepsilon_{\alpha\beta}^e = 0$). We assume a detector of 30 m$^3$ volume with a 60:40 He:CF$_4$ gas mixture at atmospheric pressure. The three columns correspond to three different pairs of NSI parameters. The top set of panels is for a background rate that is fifty times the neutrino electron recoil rate under the SM, while the bottom set of panels is for five times the neutrino rate. The various projected limits shown in blue are for different levels of directional sensitivity: idealised performance, realistic performance, and no directionality, from lightest to darkest. We emphasise that the "non-directional" case has an identical event rate, energy threshold and energy resolution as the "directional" cases.