0. Neutral Current Interactions in MINOS
Alexandre Sousa, University of Oxford
for the MINOS Collaboration
Neutrino Events in MINOS
Neutrino interactions occur in two types, charged current (CC) and neutral current (NC)
In the MINOS detectors, a nm CC event appears as a long muon track with hadronic activity at the vertex
The energy of the muon is determined from range if it stops in the detector, or by its curvature in the magnetic field
NC events appear as short diffuse hadronic showers
Hadronic shower energy is determined from the amount of light produced in the scintillator
Charged Current Interaction
Neutral Current Interaction
Near Detector
Far Detector
3 m CC
1.5 m NC
In the Near Detector (ND), ~8 events are observed per beam spill (~107 fiducial events collected to date)
ND events are separated using timing and spatial cuts. Mis-assignment of portions of an event may occur
Problem not present in Far Detector (FD), due to low event rate
En = Eshower + pm
55%/E/GeV
6% range, 10% curvature
Near Detector Data Cleaning
Neutral Current Event Selection
Two approaches are employed to tackle pathological events
High Event Multiplicity (HEM) cleaning, consisting of several cuts
- Time separation between events Dt > 40ns
- Longitudinal spatial separation Dz > 1m, if 40ns < Dt < 120ns
- (#strips/plane)/#planes < 1.0
- Total number of event strips < 4
- If event energy < 5 GeV and #shower planes > #track planes, #strips in partially instrumented region < 4
Low Event Multiplicity (LEM) cleaning, consisting of accepting only beam spills with no more than two events reconstructed
After applying cleaning cuts, NC event selection is achieved by performing the cuts:
Event Length < 40 planes
Event has no reconstructed track
Event has a track and the track is no more than 10 planes longer than the shower
Energy distributions of the selected NC events, with each of the two ND cleaning methods applied, are shown
Monte Carlo
HEM Cleaning Applied
Monte Carlo
NC-like Energy Spectrum
NC-like Energy Spectrum
Courtesy of T. Raufer
Courtesy of T. Raufer
Monte Carlo
Split events reconstructed from single neutrino interaction
Event with vertex erroneously reconstructed inside fiducial volume
HEM Cleaning Applied
LEM Cleaning Applied
Sterile Neutrino Search with MINOS
Δm2 =
sin22θ =
fs =
2.1210-3 eV
ND νμ CC spectrum
ND NC spectrum
Cross sections
Neutrino flux
FD predicted NC spectrum
Neutrino oscillations in MINOS are dominated by nm to nt mode, driven by Dm2atm
NC interactions are not affected by the dominant oscillation mode
If additional neutrinos are necessary to explain observational data (cf. Maltoni and Schwetz, hep-ph: ), they cannot couple to the Z0 and must therefore be sterile.
Consider a 4th neutrino with mass n4, and flavor ns:
Oscillations may now occur between nm and nx where
and fs is the fraction of events converted to sterile neutrinos
MINOS can search for the extra flavor(s) by looking for neutrinos that do not have NC interactions, resulting in depletion of the number of NC events observed at the Far Detector by:
Neutrino mass spectrum when a 4th sterile neutrino is added
Courtesy of P. Litchfield
In order to measure the sterile fraction at the FD, the ND data are employed to:
Measure the neutrino flux using the CC spectrum
Constrain cross-sections using NC selected spectrum
Toy MC showing FD prediction with and without sterile oscillations
Toy MC showing sterile oscillation signature in neutrino and visible energy
Toy Simulation
Fitting the observed FD NC spectrum with the FD prediction, MINOS will make a measurement of the sterile fraction. The plot below shows the expected sensitivity for an exposure of 7.4×1020 protons-on-target and the oscillation parameters:
90% C.L.
99% C.L.
68% C.L.
MINOS sensitivity to sterile fraction
Expect first results later this year for an exposure of ~3.5×1020 protons-on-target!