2008/07/31 Tomonori Kusano Tohoku University

Search for active neutrino disappearance using neutral-

current interactions in the MINOS long-baseline

experiment

2008/07/31Tomonori Kusano Tohoku University

Disappearance of

Several experiment shows disappearance while propagating from the production point. reason: - →oscillation (Super-Kamiokande has reported appearance.) - →soscillation (s:sterile neutrino)

→soscillation could explain the disappearance.

Number of neutrino flavors indicated from Z boson decay width

3 flavors of neutrinocoupling with electro-weakcurrent is indicated.

This is a cross section for the e+e-→(), near the Z resonance.

But existence of sterile neutrino can’t be excluded.

Target Process

# of NC event would show us the fraction of →s.

→e

→s

NC:neutral current

and e can couple toZ boson. # of NC events would NOT changed.

s can’t couple to Z boson. NC events would be suppressed.

Assuming →oscillation (e,,s )

Overview of MINOS experiment

Neutrino beam is provided from 120 GeV protons.

Near Detector at Felmilab,and Far detector,734km away, at Soudan.

Compare the results of the two detectors.

734 km

Near Far

Neutrino Beam

120 GeV protons from the Main injector.neutrino beam components

92.9%anti- 5.8%e and anti-e 1.3%

→(99%)

→(63%)

Neutrino Beam configuration Beam energy spectrum can be chcanged by adjusting the target position.

Low energy configuration(3.3GeV) is selected for this analysis.

Near Detector

0.98kt mass, fiducial mass 27t, 282 steel,153 scintillator plane,(Hadronic shower generate scintillation light), 1.4T Magnetic field(Separation for )

4.8m

15m

Far Detector

5.4kt mass,3.8kt fiducial mass,484 steel / scintillator,1.5T magnetic field.

8.0m

30m

Pre-selection To reject poorly reconstructed events,Event must be separated 40ns separated 1m in the longitudinal direction within 120ns.

GPS time stamp to reject beam spill from noise, cosmic muons, poorly reconstructed events.

Event Reconstruction Sigbkg bkg

→miss　 identified as NC events

Selection for NC event Event topology Event Length < 60 planes Track extension < 5 planes (short event)

(at the Near Detector)

←

←

Energy distribution at the Near Detector

Good agreement between the Data and MC.

This is the reconstructed Energy of NC-like events at the near Detector.

Prediction of the energy spectrum at the Far detector Near Detector Data

→Correct Near Detector MC

→Far detector MC (Estimated from Near Detector)

Compare the Data at the Far Detector(BO⇆X)

Energy distribution at the far detector

Assuming → ,→e oscillations,

CC background are estimated.

This is a reconstructed energy spectrum for NC events at the Far detector.

Calculation of R

NData:measured event count at Far Detector

BCC:predicted(from Near Detector) CC BG from all flavor

SNC:predicted number of NC interactionsdisappearance of nm occurs for neutrino true energy<6GeV→data is separated to two region

R differs from 1 by 1.3

Results kept R= 0.780.03(stat)+0.05

-0.04(syst.)for 0<E<120(GeV)

→sfraction(1-R)/(1-R) = fs

rate = 0.030.39(stat)+0.27

-0.36(syst.) (1-R)/(1-R) <0.80 at90% C.L.

Single mass-squared splitting

Assuming single mass-squared splitting,

m : atmospheric mass squared splittingL : 735kmE : energy of the neutrinos: phenomenological parameters

(Energy independent)

Including e appearance

fsrate = 0.480.40(stat)+0.17

-0.25(syst.)

fs = 0.43+0.23-0.27(stat.+syst.)

fs <0.80 at90% C.L.

Summary