SOX (experiment)

SOX (experiment)
Name	SOX
Project	Neutrino physics
Location	Gran Sasso National Laboratory
Start	2013
End	2016
Participants	Borexino Collaboration, Italian National Institute for Nuclear Physics, European Organization for Nuclear Research, INFN

SOX (experiment)

SOX was a proposed short-baseline neutrino oscillation experiment designed to test the existence of light sterile neutrinos using a radioactive source placed near the Borexino detector at the Gran Sasso National Laboratory. The project united teams from the Borexino Collaboration, the Italian National Institute for Nuclear Physics, and partner institutions including European Organization for Nuclear Research groups to probe anomalies reported by experiments such as LSND, MiniBooNE, and reactor antineutrino measurements. SOX aimed to exploit the ultra-low background environment of Borexino to perform a high-statistics, high-precision oscillation search over metre-scale baselines.

Overview

SOX was conceived in response to long-standing anomalies like the LSND excess, the Reactor antineutrino anomaly, and hints from the Gallium anomaly observed in calibrations at GALLEX and SAGE. The collaboration planned to deploy intense artificial sources—radioactive neutrino emitters such as Chromium-51 and Cerium-144/Praseodymium-144 mixtures—in a bore external to the Borexino inner vessel to search for disappearance or spectral distortions attributable to oscillations into sterile states motivated by models discussed in papers from groups at Massachusetts Institute of Technology, Stanford University, University of California, Berkeley, and University of Oxford. SOX combined expertise drawn from institutions including Max Planck Society, Centre National de la Recherche Scientifique, and ETH Zurich.

Experimental Design

The SOX strategy centered on deploying a compact, high-activity neutrino source in proximity to the Borexino scintillator to create a controlled, quasi-pointlike flux enabling baseline-dependent studies similar to those proposed for PROSPECT and STEREO. The design used principles established in GALLEX and SAGE calibrations and aimed to achieve sensitivity to squared-mass differences in the region favored by global fits from teams at Brookhaven National Laboratory, Los Alamos National Laboratory, and Lawrence Livermore National Laboratory. Source choices, deployment geometry, and shielding were optimized through simulations performed using toolkits developed at CERN and validated by groups from Princeton University and University of Tokyo.

Detector and Instrumentation

Borexino is a large-volume liquid scintillator detector previously used for precision solar neutrino measurements reported by collaborations including Borexino Collaboration and results compared with predictions from models by John Bahcall and the Standard Solar Model. The detector comprises an inner nylon vessel surrounded by buffer fluids and a water tank instrumented with photomultiplier tubes similar to instrumentation at Super-Kamiokande and SNO. SOX planned to use Borexino's existing instrumentation—PMT arrays, data acquisition systems, and calibration ports—while adding dedicated source-handling hardware engineered in partnership with teams from ENEA, CERN, and INFN. Electronics and reconstruction algorithms were to draw on techniques developed for KamLAND and JUNO.

Calibration and Sources

Two primary source options were studied: a monochromatic electron-capture source based on Chromium-51 and a beta-emitting antineutrino source based on the Cerium-144/Praseodymium-144 decay chain. Source production, radiochemical processing, and encapsulation were to be coordinated with facilities such as Mayak Production Association and national laboratories like Oak Ridge National Laboratory. Calibration campaigns referenced methods from GALLEX and SAGE and planned to deploy low-activity tracers and gamma standards used historically at LNGS and in experiments at Gran Sasso National Laboratory. Shielding designs took into account lessons from transport and handling protocols set by International Atomic Energy Agency guidance and safety frameworks at European Commission laboratories.

Data Analysis and Results

Analysis workflows were to implement spectral unfolding, event reconstruction, and oscillation fits using statistical frameworks common to collaborations at Fermilab and CERN, employing likelihood techniques analogous to those used by Daya Bay and Double Chooz. Expected observables included position-dependent count-rate modulations and energy-spectrum distortions compared against Monte Carlo predictions from toolchains maintained at Lawrence Berkeley National Laboratory and IN2P3. Although the SOX deployment was ultimately cancelled due to difficulties in source procurement and radiochemical characterization—issues similar to challenges faced in specialized projects at Mayak Production Association—the collaboration produced detailed sensitivity studies and background characterizations informing null-result hypothesis testing strategies used by groups at Gran Sasso National Laboratory and elsewhere.

Systematic Uncertainties and Sensitivity

Key systematics identified included source activity and spectral uncertainties tied to nuclear data from evaluations by National Nuclear Data Center, source-detector distance uncertainties informed by surveys at Gran Sasso National Laboratory, detector response calibrated against campaigns inspired by GALLEX, and background contributions benchmarked against Borexino solar analyses. Sensitivity projections incorporated covariances for detector energy scale and resolution using methods developed by teams at Columbia University and University of Chicago, estimating reach into sterile-mixing parameter space competitive with short-baseline programs like MicroBooNE and ICARUS under optimistic production scenarios.

Scientific Impact and Legacy

Although the SOX source deployment did not occur, the initiative influenced neutrino source handling protocols, calibration methodologies, and sterile-neutrino search strategies across the field, informing designs for experiments such as PROSPECT, STEREO, and proposals at JUNO. SOX outputs—technical reports, simulation packages, and background studies—contributed to reviews by committees convened by International Committee for Future Accelerators, and its interdisciplinary collaborations strengthened ties among groups at INFN, CERN, Max Planck Society, and national laboratories across Europe and the United States. The project remains an instructive case in coordinating radiochemical production, underground deployment logistics, and precision low-background neutrino physics research.

Category:Neutrino experiments