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

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.

Comprehensive geoneutrino analysis with Borexino

 

The Borexino collaboration published a paper [Physical Review D 101, (2020) 012009] on an updated geoneutrino measurement in January 2020 using the data obtained from Dec 2007 to Apr 2019. The updated statistics and the improved analysis techniques, such as an increased fiducial volume, improved veto for cosmogenic backgrounds, extended energy and coincidence time windows, as well as a more efficient \alpha/\beta particle discrimination, led to more than a factor two increase in exposure and an improvement in the precision from 26.2% to 17.5%, when compared to the previous measurement in 2015.

The measured geoneutrino signal at Gran Sasso was 47.0^{+8.4}_{-7.7} (stat) ^{+2.4}_{-1.9} (sys) TNU.

The paper also provides the geological interpretations of the obtained results, namely, the estimation of the mantle signal by exploiting the relatively well-known lithospheric contribution, the calculation of the radiogenic heat, as well as the comparison of these results to the various predictions. The null-hypothesis of the mantle signal was rejected at 99% C.L. for the first time. Even though the results were compatible with all the Earth models, a 2.4\sigma tension was observed with those models that predict the lowest concentration of heat-producing elements inside the mantle. Additionally, the upper limits for a hypothetical georeactor that might be present at different locations inside the Earth were obtained. Particular attention was given to the details of the analysis techniques which might be useful for next generation liquid scintillator detectors.

Here below we provide some important figures and their respective data categorized into: theoretical spectra, spectral fit inputs, spectral fit results, and geological inputs.

Theoretical Spectra

 

    In this plot (Fig. 14a of our paper) we show the energy spectra for ^{238}U, ^{235}U, ^{232}Th and ^{40}K geoneutrinos normalized to one decay from the head element of the chain. Input data can be found here.

 

       This plot (Fig. 14b of our paper) shows geoneutrino fluxes from ^{238}U, ^{232}Th and ^{40}K and their sum at LNGS as a function of geoneutrino energies calculated adopting geophysical and geochemical inputs. Inputs are available here.

Figure 19 of our paper shows the energy spectra for reactor antineutrinos with and without excess at 5 MeV expected at Gran Sasso calculated using PRIS database and the correction factor from Daya Bay for the excess.  Data available here.

Spectral fit inputs

The final unbinned likelihood spectral fit is done using the charge of the 154 golden candidates and the Monte-Carlo PDFs of signal and backgrounds. The contribution of the three main backgrounds namely accidentals, cosmogenic ^9Li and (\alpha,\,n) are constrained. Geoneutrino and reactor antineutrino contributions are usually left free.

The fit is done in the following configurations:

  • Figure 48a: U/Th fixed to chondritic ratio (The fit is also done after constraining the atmoshperic neutrino background contribution and after constraining the reactor antineutrino contribution).

  • Figure 48b: U and Th contributions as free parameters.

  • Figure 52a: Extraction of mantle geoneutrino signal after constraining the contribution from bulk lithosphere.

  • Extraction of upper limits on georeactor placed at different positions after constraining the contribution of reactor antineutrinos.
Inputs:

Charge (available here) of the 154 golden candidates used for the unbinned likelihood spectral fit.

Monte-Carlo charge PDFs of geoneutrinos (U/Th fixed) , reactor antineutrinos , ^{238}U geoneutrinos , ^{232}Th geoneutrinos as in Figure 32 of our paper are available here.

Monte-Carlo charge PDFs of cosmogenic ^{9}Li, (\alpha,\,n) interactions, atmospheric neutrinos, and accidental background data as in Figure 33 and 37 of the paper are available here.  

Monte-Carlo charge PDFs of georeactor placed at different positions as in Figure 34 of the paper are available here.

Spectral fit results

Confidence contours:

Figure 48c: Reactor vs geoneutrino confidence contours for U/Th fixed to chondritic ratio in the fit.  Inputs can be found here.

 

Figure 48d: ^{238}U vs ^{232}Th geoneutrino confidence contours when their contributions are left free in the fit. Inputs can be found here.

Likelihood profiles

Figure 49a: Likelihood profile of geoneutrinos from the spectral fit after fixing U/Th to the chondritic ratio. Input can be found here.Figure 49b: Likelihood profile of reactor neutrinos from the spectral fit after fixing U/Th to the chondritic ratio. Input is available here.Figure 49c: Likelihood profile of ^{238}U geoneutrinos from the spectral fit after leaving U and Th as free parameters in the spectral fit. Input can be found here.Figure 49d: Likelihood profile of ^{232}Th geoneutrinos from the spectral fit after leaving U and Th as free parameters in the spectral fit. Input is available here.Figure 52b: Likelihood profile of mantle geoneutrinos after constraining the contribution from bulklithosphere. Input can be found here.Figure 57: Likelihood profile of georeactor at different positions after constraining the contribution from reactor-antineutrinos. Input is available here.

Geological interpretations

Figure 50: Comparison of the geoneutrino signal obtained from Borexino with different theoretical models. Inputs can be downloaded here.

 

Figure 54: Signal of mantle geoneutrinos vs Radiogenic heat for cosmochemical, geochemical, geodynamical, and fully radiogenic models along with Borexino results. Input can be found here.

 

Figure 55: Radiogenic heat obtained with Borexino results compared with different theoretical models along with the total surface heat flux. Data can be downloaded here.

 

Figure 56: Convective Urey Ratio obtained with Borexino results compared with different theoretical models. Input are available here.