[Paper Review] The use of a high intensity neutrino beam from the ESS proton linac for measurement of neutrino CP violation and mass hierarchy

E. Baussan, M. Dracos+4 more|arXiv (Cornell University)|Dec 20, 2012

Neutrino Physics Research8 references19 citations

TL;DR

This paper proposes using the European Spallation Source (ESS) proton linac—already under construction in Lund, Sweden—to generate a high-intensity 5 MW neutrino beam in parallel with neutron production. By deploying a megaton-scale water Cherenkov detector at a 400 km baseline, the study shows that CP violation could be discovered at 5σ in 48% of the δ phase space with 8 years of antineutrino and 2 years of neutrino running, while mass hierarchy could be determined at 3σ over most of the δ range.

ABSTRACT

It is proposed to complement the ESS proton linac with equipment that would enable the production, concurrently with the production of the planned ESS beam used for neutron production, of a 5 MW beam of 10$^{23}$ 2.5 GeV protons per year in microsecond short pulses to produce a neutrino Super Beam, and to install a megaton underground water Cherenkov detector in a mine to detect $ν_e$ appearance in the produced $ν_μ$ beam. Results are presented of preliminary calculations of the sensitivity to neutrino CP violation and the mass hierarchy as a function of the neutrino baseline. The results indicate that, with 8 years of data taking with an antineutrino beam and 2 years with a neutrino beam and a baseline distance of around 400 km, CP violation could be discovered at 5 $σ$ (3 $σ$) confidence level in 48% (73%) of the total CP violation angular range. With the same baseline, the neutrino mass hierarchy could be determined at 3 $σ$ level over most of the total CP violation angular range. There are several underground mines with a depth of more than 1000 m, which could be used for the creation of the underground site for the neutrino detector and which are situated within or near the optimal baseline range.

Motivation & Objective

• To explore the feasibility of using the ESS proton linac to produce a high-intensity neutrino beam for precision measurements of CP violation and neutrino mass hierarchy.
• To determine the optimal baseline distance for maximizing sensitivity to CP violation in the neutrino sector.
• To assess the potential of a megaton-scale water Cherenkov detector in deep underground mines for detecting νₑ appearance in a νₘu beam.
• To evaluate the cost-effectiveness and technical viability of upgrading the ESS facility to concurrently produce neutrinos and neutrons.
• To identify suitable underground mine sites in Europe with sufficient depth (>1000 m) and proximity to the ESS for hosting a large neutrino detector.

Proposed method

• Utilize the existing 2.5 GeV ESS proton linac, which operates at 5 MW for neutron production, to simultaneously accelerate H⁻ ions to produce a 5 MW proton beam for neutrino production.
• Inject H⁻ ion pulses into the linac during 70 ms gaps between proton bunches, strip electrons in an accumulator ring, and compress the beam into microsecond pulses for injection into a neutrino target.
• Employ a neutrino horn to focus pions produced in the target into a forward beam, which decay into muon neutrinos in a 20 m decay tunnel.
• Simulate νₘu → νₑ oscillation probabilities using Monte Carlo methods, accounting for beam energy, baseline distance, and detector response.
• Design a megaton-scale water Cherenkov detector located at least 1000 m underground (3000 m w.e.) to suppress cosmic ray backgrounds.
• Evaluate sensitivity to CP violation and mass hierarchy using statistical significance (5σ and 3σ) across the full range of the CP-violating phase δ.

Experimental results

Research questions

• RQ1 What is the optimal baseline distance between the ESS neutrino source and a far detector for maximizing sensitivity to CP violation in the neutrino sector?
• RQ2 Can the ESS proton linac be upgraded to produce a 5 MW neutrino beam concurrently with its primary neutron production mission?
• RQ3 What fraction of the CP-violating phase δ range can be probed with 5σ significance using 8 years of antineutrino and 2 years of neutrino running at a 400 km baseline?
• RQ4 To what extent can the neutrino mass hierarchy be determined at 3σ confidence level using this setup across the full δ range?
• RQ5 Which existing or reactivatable underground mines in Europe are suitable for hosting a megaton-scale neutrino detector with sufficient depth and proximity to the ESS?

Key findings

• With a 400 km baseline, CP violation could be discovered at 5σ confidence level in 48% of the total δ phase space, and at 3σ in 73% of the range, using 8 years of antineutrino and 2 years of neutrino running.
• The neutrino mass hierarchy could be determined at 3σ confidence level over most of the δ phase space without additional optimization, using the same 400 km baseline and data-taking strategy.
• The optimal baseline range for CP violation sensitivity lies between 200 km and 550 km, with Oskarshamn, Zinkgruvan, and Garpenberg mines identified as prime candidates.
• The proposed upgrade to the ESS linac—adding an H⁻ source, 5 MW RF power, accumulator ring, and neutrino target—would enable concurrent neutrino and neutron beam production at modest additional cost.
• Several deep mines in Sweden and Finland (e.g., Zinkgruvan at 1100 m depth, 365 km from ESS; Pyhsälmi at 1440 m, 1140 km from ESS) are viable for hosting the detector due to depth and proximity.
• The detector volume of ~650,000 m³ would require ~100 MEUR for excavation and lining, assuming existing infrastructure such as transport shafts is available.