Rock muon rate at ND Chris Marshall Lawrence Berkeley National - - PowerPoint PPT Presentation

Rock muon rate at ND

Chris Marshall Lawrence Berkeley National Laboratory CERN ND workshop 22 January, 2017

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Rock muons

• 100s m of rock between decay pipe and detector hall
• Muons from neutrino interactions in rock will overlap

with interactions in detector

• High-energy tail will produce forward, high-energy muons

Earth E > 0.4 GeV/m θ < detector radius / distance

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Questions

• What is the rate of rock muons intersecting detector?
• What is the energy spectrum at the face of the detector?
• How does this depend on the air gap between the rock

and the front face of the ND?

Earth E > 0.4 GeV/m θ < detector radius / distance

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Ideal estimate

• Simulate neutrino flux, including beam divergence as a

function of distance

• Simulate neutrino interactions in some large volume or

rock

• Propagate muons through rock to detector
• Make plots

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Quick, simplified study

• Distribute CC events uniformly along a line
• Use GENIE muon kinematics as a function of neutrino

energy to estimate what fraction of interactions would produce a “rock muon”

• Do this for DUNE and MINERvA fluxes
• Calculate DUNE rock muon rate relative to MINERvA,

and peg to observed MINERvA rate

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Fluxes (FHC)

• Optimized 80GeV flux at ND
• MINERvA LE flux from latest flux paper

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Event rates

• FHC νμ CC only, integrated out to Eν = 40 GeV

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Determine probability of muon intersecting detector

• Assume perfectly-focussed beam, perpendicular to face
• f cylindrical detector
• Assume flat distribution of neutrino interactions in last

100m of rock, muons lose 0.4 GeV/m (ρ~2 g/cm3)

Earth

100m rock 100m rock air gap

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Determine probability of muon intersecting detector

• Use GENIE to form PDF of muon momentum and

angle in slices of neutrino energy

0.5 < Eν < 1 GeV 30 < Eν < 32 GeV log(1-cosθ) log(1-cosθ)

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Probability of intersecting detector

• Integrate over the p-θ distributions to form probability
• f intersecting detector as a function of neutrino energy

and distance from face (with 18m air gap) DUNE r = 1.75m MINERvA r = 1.08m

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• Prob. vs. neutrino energy
• Integrate out the

distance assuming flat distribution

• Probability that a CC

neutrino interaction produces a muon that intersects the detector as a function of neutrino energy

r = 1.75m r = 1.08m

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Weight event rate by that probability for DUNE, MINERvA

• Result is a relative rock muon rate between DUNE and

MINERvA, per POT

X

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Result

• Shown for 18m air

gap, and normalized to the detector surface area

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Result

• MINERvA LE FHC was 0.33 front-entering rock

muons per 1013 POT (1.08m radius)

• For DUNE 80GeV flux, DUNE ND with 1.75m radius

and 7.5E13 protons per spill ~ 4.4 per spill with an 18m air gap

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Air gap

• Rate rises quickly as you

push detector closer to the rock

• NuMI gap is ~18m to

MINERvA scintillator planes

• Plot is for 1.75m radius

and DUNE 80GeV flux, based on observed MINERvA rock rate

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Things neglected

• Focussing/beam spread differences between NuMI and

DUNE

• Higher-angle events from wider-angle neutrinos, which

could be lower in energy → more rock muons for DUNE

• Fluxes above 40 GeV→ fewer rock muons for DUNE
• Beam angle w.r.t. detector axis (assumed 0)
• Side-entering “rock” muons, because MINERvA can't

distinguish from outer detector interactions

• Other materials in detector hall – treated as empty vacuum

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Conclusions

• DUNE will see ~4 rock muons per spill
• This number is extremely sensitive to the flux,

especially the flux tail

• This number is sensitive to the air gap between the rock

and the detector because of acceptance effects

• We should think about this when designing the hall