Constraints on flavor-diagonal non-standard neutrino interactions from Borexino Phase-II 
The Borexino Collaboration
Journal of High Energy Physics 2020, 38 – Published 5 February 2020

[doi:10.1007/JHEP02(2020)038] (preprint on arXiv)


Data availability

The datasets generated during the current study are freely available: see below (scroll down). Additional information is available from the Borexino Collaboration spokesperson (spokesperson-borex@lngs.infn.it) upon reasonable request.


Abstract

The Borexino detector measures solar neutrino fluxes via neutrino-electron elastic scattering. Observed spectra are determined by the solar-νe survival probability Pee(E), and the chiral couplings of the neutrino and electron. Some theories of physics beyond the Standard Model postulate the existence of Non-Standard Interactions (NSI’s) which modify the chiral couplings and Pee(E). In this paper, we search for such NSI’s, in particular, flavor-diagonal neutral current interactions that modify the νee and ντe couplings using Borexino Phase II data. Standard Solar Model predictions of the solar neutrino fluxes for both high- and low-metallicity assumptions are considered. No indication of new physics is found at the level of sensitivity of the detector and constraints on the parameters of the NSI’s are placed. In addition, with the same dataset the value of sin2 θW is obtained with a precision comparable to that achieved in reactor antineutrino experiments


Open Data

In February 2020, the Borexino Collaboration in collaboration with Tatsu Takeuchi, Sanjib Kumar Agarwalla and Chen Sun published [Journal of High Energy Physics 2 (2020) 038] the analysis of flavor-diagonal non-standard neutrino interactions (NSI) based on the Phase-II data.

Within the last few decades, solar neutrino physics has evolved into a field of relevance not only for probing our understanding of the Sun but also for investigating and determining intrinsic neutrino properties. Solar neutrinos can be utilized as a probe for new physics beyond the Standard Model that affect neutrino interactions with the charged leptons and quarks.

Monochromatic nature of the ^7Be solar neutrinos results in an electron recoil spectrum whose shape is more sensitive to the \nu_e couplings than that from a continuous neutrino energy spectrum. It gives an advantage in using the Sun as a neutrino source for the study of NSI’s.

In this paper, the neutrino-flavor-diagonal NSI’s that affect \nu_e-e and \nu_{\tau}-e interactions were investigated since Borexino is particularly sensitive to this set of parameters. The obtained bounds showed remarkable improvement, regardless of the choice of metallicity in the Standard Solar Model (SSM). The bounds are comparable to the global ones. In particular, the best constraint to-date on \varepsilon ^L_e was obtained.

The same dataset, but without any NSI’s assumed, was used to measure \sin^2\theta_W, resulting in the value of 0.229 \pm 0.026 (stat+syst). The precision is comparable to that measured by reactor antineutrino experiments.

 

This first sequence of plots (left panels of Figure 5 in our paper) shows the  Log-likelihood profiles for the NSI parameters \varepsilon^R_e (red line) and \varepsilon^L_e (blue line) assuming HZ and LZ SSM’s.

The correspondent \chi^2 profiles are available here: \varepsilon^R_e-HZ profile, \varepsilon^L_e-HZ profile, \varepsilon^R_e-LZ profile, \varepsilon^L_e-LZ profile.

This sequence of plots (right panels of Figure 5 in our paper) shows the  Log-likelihood profiles for the NSI parameters \varepsilon^R_{\tau} (red line) and \varepsilon^L_{\tau} (blue line) assuming HZ and LZ SSM’s.

The correspondent \chi^2 profiles are available here: \varepsilon^R_{\tau}-HZ profile, \varepsilon^L_{\tau}-HZ profile, \varepsilon^R_{\tau}-LZ profile, \varepsilon^L_{\tau}-LZ profile.

The one-dimensional \chi^2 profiles were obtained considering one NSI parameter at-a-time, while remaining NSI parameters were fixed to zero.

In this plot (Figure 6 of the paper),  we report the allowed region for NSI parameters in \varepsilon^{R/L}_e plane. The bounds from LSND and TEXONO are provided for comparison. The contour obtained from the global analysis of solar neutrino experiments is presented by a dashed black line. Each contribution can be downloaded at the following links: TEXONO_top contour and TEXONO_bottom contour; LSND contour; Solar-KamLand contour; \varepsilon_e-HZ_Borexino contour; \varepsilon_e-LZ_Borexino contour.

The plot above (Figure 7 of our paper) shows the allowed region for NSI parameters in \varepsilon^{R/L}_{\tau} plane obtained in the present work. The contour from LEP is provided for comparison. Each contribution can be downloaded at the following links: \varepsilon_{\tau}-HZ_Borexino contour, \varepsilon_{\tau}-LZ_Borexino_right contour, \varepsilon_{\tau}-LZ_Borexino_left contourLEP contour.

Constraints on flavor-diagonal non-standard neutrino interactions from Borexino Phase-II

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