Vetoing cosmogenic muons in a large liquid scintillator

Abstract

At upcoming medium baseline reactor neutrino experiments the spallation 9 Li background will be somewhat larger than the inverse β decay reactor neutrino signal. We use new FLUKA simulations of spallation backgrounds to optimize a class of veto strategies and find that surprisingly the optimal veto for the mass hierarchy determination has a rejection efficiency below 90%. The unrejected background has only a modest effect on the physics goals. For example ∆χ 2 for the hierarchy determination falls by 1.4 to 3 points depending on the muon tracking ability. The optimal veto strategy is essentially insensitive to the tracking ability, consisting of 2 meter radius, 1.1 second cylindrical vetoes of well tracked muons with showering energies above 3 to 4 GeV and 0.7 second full detector vetoes for poorly tracked muons above 15 to 18 GeV. On the other hand, as the uncertainty in θ 12 will be dominated by the uncertainty in the reactor neutrino spectrum and not statistical fluctuations, the optimal rejection efficiency for the measurement of θ 12 is 93% in the case of perfect tracking.

Status of the Jiangmen Underground Neutrino Observatory

“…Featuring a life time of 256 ms and Expected signal and background rates per day with various selection cuts. From [2] Section IBD Geo-s Accidental 9 172 ms, respectively, 99% of the 9 Li and 8 He isotopes produce IBD-like signatures at a 3 m distance to the muon track [3,4]. Therefore, the strategy for the muon veto includes a partial volume veto of the area of the liquid scintillator contained in a cylinder with a radius of 3 m around the muon track for a time of 1.2 s [5].…”

Measuring θ 12 despite an uncertain reactor neutrino spectrum

“…In Ref. [11] we demonstrate that, as a consequence, a very high spallation background rejection efficiency is optimal for the θ 12 measurement, higher than that for the mass hierarchy. This result is quite robust.…”

“…Our motivation for writing this paper now, when the covariance matrix for the uncertainties is not yet available, is as follows. In a companion paper [11] we consider the tracking requirements for cosmogenic muons for such experiments. For this, we need to know not the absolute value of the uncertainty in θ 12 , but rather its expected dependence on the background rejection efficiency.…”

Muon reconstruction with a geometrical model in JUNO

“…They have a long lifetime of 256 ms and 172 ms [3], respectively and their (β − + n) decay-channel mimics the correlated decay of IBD events. A detailed study of the production characteristics [4,5] showed that 99% of the cosmogenics are within 3 m distance to the muon track following approximately an exponential distribution. Therefore, it is possible to veto the cosmogenic background with a cylindrical volume around the muon track.…”