Production of atmospheric neutrinos

1
Atmospheric neutrinos
Takaaki Kajita (ICRR, U.of Tokyo)

• Production of atmospheric neutrinos
• Some early history (Discovery of atmospheric
  neutrinos, Atmospheric neutrino anomaly)
• Discovery of neutrino oscillations
• Studies of atmospheric neutrino oscillations
• Sub-dominant oscillations present and future-

2
Studies of atmospheric neutrino oscillations
3
Introduction
We know that neutrinos have mass
Future experiments
ne nm nt
ne nm nt
q23458
q12343
q13 lt 11
n3
n3
Atmospheric LBL
n2
n2
Solar KamLAND
n1
n1
Small q13 and Dm122 ltlt Dm232 ? OK to interpret
the present data with 2 flavor oscillation
framework P(na ? nb)1-sin22qijsin2(1.27Dmij2L/
E)
4
Event statistics in atmospheric neutrino
experiments
More than 20,000 now.
TK and Y.Totsuka, RMP73, 85 (2001)
5
Super-Kamiokande history and plan
today
1996 97 98 99 2000 01 02 03 04 05 06 07 08 09 2010 11
SK-I
SK-II
SK-III
accident
SK full reconstruction
T2K
K2K
6
(Dm2, sin22q)
7
SK-III atmospheric neutrino data
CC ne
SK-I hep-ex/0501064 SK-II 804 days
CC nm
No osc.
Osc.
8
Estimating the oscillation parameters
Transition point (as a function of
energy) ? Dm2
Accurate measurement possible due to small syst.
in up/down (2 or less)
Confirmation of non-oscillated flux
9
nm?nt 2-flavor oscillation analysis
(SK-I SK-II combined analysis)
CC ne
CC nm
Plep
UP through showering
FC 1ring e-like
FC mring e-like
FC 1ring m-like
FC mring m-like
PC stop
PC thru
UP through non-showering
Multi-GeV
UP stop
38 event type and momentum bins x 10 zenith
bins ? 380 bins
Sub-GeV
Each box has 10 zenith-angle bins
Since various detector related systematic errors
are different, SK-I and SK-II data bins are not
combined.
380 bins for SK-I 380 bins for SK-II ? 760
bins in total
10
Definition of c2
Number of data bins
Number of syst error terms
Poisson with systematic errors
Nobs observed number of events Nexp
expectation from MC ei systematic error
term si sigma of systematic error
c2 minimization at each parameter point (Dm2,
sin22q, ). Method (c2
version) G.L.Fogli et al., PRD 66, 053010 (2002).
11
70 systematic error terms
? (Free parameter) flux absolute normalization ?
Flux (nu_mu anti-nu_mu) / (nu_e anti-nu_e)
ratio ( E_nu lt 5GeV ) ? Flux (nu_mu
anti-nu_mu) / (nu_e anti-nu_e) ratio ( E_nu gt
5GeV ) ? Flux anti-nu_e / nu_e ratio ( E_nu lt
10GeV ) ? Flux anti-nu_e / nu_e ratio ( E_nu gt
10GeV ) ? Flux anti-nu_mu / nu_mu ratio ( E_nu lt
10GeV ) ? Flux anti-nu_mu / nu_mu ratio ( E_nu gt
10GeV ) ? Flux up/down ratio ? Flux
horizontal/vertical ratio ? Flux K/pi ratio ?
Flux flight length of neutrinos ? Flux spectral
index of primary cosmic ray above 100GeV ? Flux
sample-by-sample relative normalization ( FC
Multi-GeV ) ? Flux sample-by-sample relative
normalization ( PC Up-stop mu ) ? Solar
activity during SK1 ? Solar activity during
SK-II ? MA in QE and single-p ? QE models
(Fermi-gas vs. Oset's) ? QE cross-section ?
Single-meson cross-section ? DIS models (GRV vs.
Bodek's model) ? DIS cross-section ? Coherent-p
cross-section ? NC/CC ratio ? nuclear effect in
16O ? pion spectrum ? CC nt cross-section
Detector, reduction and reconstruction
(212) (SK-ISK-II, independent)
Flux (16)
? Reduction for FC ? Reduction for PC ? Reduction
for upward-going muon ? FC/PC separation ? Hadron
simulation (contamination of NC in 1-ring
m-like) ? Non-n BG ( flasher for e-like ) ? Non-n
BG ( cosmic ray muon for mu-like ) ? Upward
stopping/through-going mu separation ? Ring
separation ? Particle identification for 1-ring
samples ? Particle identification for multi-ring
samples ? Energy calibration ? Energy cut for
upward stopping muon ? Up/down symmetry of energy
calibration ? BG subtraction of up through m ?
BG subtraction of up stop m ? Non-ne
contamination for multi-GeV 1-ring electron ?
Non-ne contamination for multi-GeV multi-ring
electron ? Normalization of multi-GeV multi-ring
electron ? PC stop/through separation
n interaction (12)
12
nm ? nt 2 flavor analysis
1489 days (SK-1) 804 days (SK-II)
Dc2 distributions
Best Fit Dm2 2.5 x 10-3 eV2 sin2 2q 1.00 c2
839.7 / 755 dof (18)
Preliminary
1.9 x 10-3 eV2 lt Dm2 lt 3.1 x 10-3 eV2 sin2 2q gt
0.93 at 90 CL
13
L/E analysis
14
L/E analysis
SK collab. hep-ex/0404034
oscillation
decoherence
decay
Should observe this dip!

• Further evidence for oscillations
• Strong constraint on oscillation
• parameters, especially Dm2

15
L/E plot in 1998 SK evidence paper
Due to the bad L/E resolution, the dip was
completely washed out. (Or neutrinos decay.)
Something must be improved.
16
Selection criteria
Select events with high L/E resolution (D(L/E) lt
70)
Events are not used, if ?horizontally going
events
?low energy events

Similar cut for FC multi-ring m-like,
OD stopping PC, and
OD through-going PC
17
L/E distribution
SK-III, FCPC
MC (no osc.)
MC (osc.)
Mostly down-going
Mostly up-going
Osc.

• The oscillation dip is observed.

18
Allowed oscillation parameters from the SK-III
L/E analysis
SK-III
(preliminary)
Slightly unphysical region (Dc20.5)
2.0 x 10-3 eV2 lt Dm2 lt 2.8 x 10-3 eV2 sin2 2q gt
0.93 at 90 CL
Consistent with the zenith-angle analysis
19
SK-III L/E analysis and non-oscillation models
c2(osc)83.9/83dof c2(decay)107.1/83dof c2(decohe
rence)112.5/83dof
Oscillation gives the best fit to the data.
Decay and decoherence models disfavored by 4.8
and 5.3 s, resp.
20
Oscillation to nt or nsterile ?
21
Oscillation to nt or nsterile ?
m-like data show zenith-angle and energy
dependent deficit of events, while e-like data
show no such effect.
nm?nsterile
nm?nt
or
Difference in P(nm?nt) and P(nm?nsterile) due to
matter effect
Propagation
Z
Neutral current interaction
Interaction
22
Testing nm?nt vs. nm?nsterile
Neutral current
Matter effect
High E PC events (Evisgt5GeV)
Multi-ring e-like, with Evis gt400MeV
Up through muons
nt
nm?nsterile
nsterile
nm?nsterile
nm?nt
nm?nt
(PRL85,3999 (2000))
Pure nm?nsterile excluded
23
Limit on oscillations to nsterile
nm?(sinxnsterilecosxnt) If pure nm?nt,
sin2x0 If pure nm?nsterile, sin2x1
SK-1 data
Consistent with pure nm?nt
SK collab. draft in preparation
24
Seach for CC nt events
25
Search for CC nt events (SK-I)
CC nt MC
CC nt events
nt
hadrons
t
nt
hadrons
? Many hadrons ....
(But no big difference with other (NC)
events.) BAD t- likelihood
analysis ? Upward going only GOOD
Zenith angle
Only 1.0 CC nt FC events/ktonyr (BG (other
n events) 130 ev./ktonyr)


26
Selection of nt events
Pre-cuts E(visible) gt1,33GeV, most-energetic
ring e-like
Max. distance between primary vertex and the
decay-electron vertex
E(visible)
Number of ring candidates
Sphericity in the lab frame
nt MC Atm.n MC data
Sphericity in the CM frame
27
Likelihood / neural-net distributions
Down-going (no nt)
Likelihood
Neural-net
28
Zenith angle dist. and fit results
Hep-ex/0607059
Likelihood analysis
NN analysis
Data
scaled t-MC
Number of events
nm, ne, NC background
cosqzenith
cosqzenith
Fitted of t events Expected of t events
13848(stat) 15 / -32(syst) 13448(stat) 16 / -27(syst)
7826(syst) 7827 (syst)
Zero tau neutrino interaction is disfavored at
2.4s.
29
Sub-dominant oscillations - present and future -
Super-K
INO
MEMPHYS
Hyper-K
UNO
30
Present and future osc. experiments
Present Study of dominant oscillation channels
Future Study of sub-dominant oscillations
Known
Unknown
q12, Dm122
q23, Dm232
q13
Sign of Dm232
ne nm nt
n3
or
n2
n1
If q23 ?p/4, is it gtp/4 or ltp/4 ?
Solar, KamLAND
Atmospheric Long baseline
(CP)
? Future atmospheric exps
31
q13
32
Search for non-zero q13 in atmospheric neutrino
experiments
(Dm1220 and vacuum oscillation assumed)
Since ne is involved, the matter effect must be
taken into account.
33
Search for non-zero q13 in atmospheric neutrino
experiments
(Dm1220 and vacuum oscillation assumed)
MC, SK 20yrs
1multi-ring, e-like, 2.5 - 5 GeV
Assuming n3 is the heaviest
Electron appearance
s2130.05 s2130.00 null oscillation
cosQ
Electron appearance in the multi-GeV upward going
events.
34
SK-I multi-GeV e-like data
Multi-GeV, single-ring e-like
Multi-GeV, multi-ring e-like (special)
No evidence for excess of upward-going e-like
events ? No evidence for non-zero q13
35
q13 analysis from Super-K-I
Hep-ex/0604011
Normal
Inverted
36
c2 distributions
SK-1
CHOOZ limit
If the shape of c2 continues to be like this,
(factor 2) more data might constrain q13 at
90CL.
37
Future sensitivity to non-zero q13
s22q120.825 s2q230.40 0.60
s2q130.000.04 dcp45o Dm2128.3e-5
Dm2232.5e-3
Approximate CHOOZ limit
20yrs SK
sin2q230.60
0.55
0.50
3s
0.45
0.40
3s for 80yrs SK 4yrs HK
But probably after T2K/Nova
Positive signal for nonzero q13 can be seen if
q13 is near the CHOOZ limit and sin2q23 gt 0.5
38
Sign of Dm2
39
Can we discriminate positive and negative Dm2 ?
s(total) and ds/dy are different between n
and anti-n.
? If Dm232 is positive, resonance for n
? If Dm232 is negative, resonance for anti-n

n
ds/dy
n
y(En-Em)/En
SK atm. n MC
En(GeV)
40
Electron appearance for positive and negative Dm2
Single-ring e-like
Multi-ring e-like
Relatively high anti-ne fraction
Lower anti-ne fraction.
Positive Dm2 Negative Dm2 null oscillation
cosQ
cosQ
Small (Large) effect for Dm2 lt0 (gt0).
41
c2 difference (true wrong hierarchy)
Dm2 fixed, q23 free, q13 free Exposure
1.8Mtonyr 80yr SK 3.3yr HK

True
True
Similar sensitivity (sensitive if sin22q13gt0.04)
reported by INO (PRD 71, 013001 (2005).
42
Octant of q23
43
Solar oscillation effect in atmospheric neutrinos
ne nm nt
However, Diameter of the Earth (L)
12,800km, Typical atmospheric neutrino energy (E)
1GeV ? (L/E)-1 810-5 (km/GeV)-1
n3
Dm232
n2
Dm122
n1
So far, Dm122 has been neglected, because Dm122
(8.010-5) ltlt Dm232 (2.510-3)
Solar oscillation terms cannot be neglected !
?matter effect must be taken into account ?q13
0 assumed.
44
Solar term effect to atmospheric n
Peres Smirnov NPB 680 (2004) 479
Atmospheric neutrinos oscillation by (q12, Dm122).
s22q120.825 Dm2128.310-5 Dm2232.510-3 sin2
q130
w/o matter effect
with matter effect
45
Solar term effect to atmospheric n
However, due to the cancellation between nm?ne
and ne?nx, the change in the ne flux is small.
P(ne ? ne) 1 P2 P(ne ? nm)
P(nm ? ne) cos2q23 P2
P2 2n transition prob. ne ? nx by Dm122
ne flux (osc) f(ne0)(1-P2)f(nm0)cos2q23P2
Oscillation probability is different between
s2q230.4 and 0.6 ?
discrimination between q23 gtp/4 and ltp/4 might be
possible by studying low energy atmospheric ne
and nm events.
46
Effect of the solar terms to the sub-GeV m/e
ratio (zenith angle dependence)
Dm212 8.3 x 10-5 eV2 Dm223 2.5 x 10-3
eV2 sin2 2q12 0.82 sin2q130
Below 1.3GeV
Pm , e lt 400 MeV
Pm , e gt 400 MeV
sin2q23 0.6
2 flavor (sin22q23.96)
sin2q23 0.5
sin2q23 0.4
It could be possible to discriminate the octant
of q23, if sin2q23 is significantly away from
0.5.
47
Constraint on sin2q23 with and without the solar
terms
Solar terms off best-fit sin2 q23
0.50 Solar terms on best-fit sin2 q23
0.52 (sin2 2q23 0.9984)
w/o solar terms w/ solar terms
(preliminary)
Still (almost) maximum mixing is most favored.
48
Future q23 octant determination with the (12) and
(13) terms
s2q230.40 0.60 s2q130.000.04 dcp45o
1.8Mtonyr SK 80 yrs 3.3 HK yrs
90CL
90CL
sin22q230.96
sin22q230.99
Fit result
Test point
sin2q13
sin2q23
sin2q23
Discrimination between q23gtp/4 and ltp/4 is
possible for all q13.
Discrimination between q23gtp/4 and ltp/4 is
marginally possible only for sin2q13 gt0.04.
49
q23 octant determination and syst. errors
S.Nakayama, RCCN Int. Workshop on sub-dom. Atm.
Osc. 2004
Dm212 8.3 x 10-5 eV2 Dm223 2.5 x 10-3
eV2 sin2 2q12 0.82 sin2q130
0.8 Mtonyr SK 20yr HK 0.8yr
true
Pm , e lt 400 MeV
sin2q23 0.6
2 flavor (sin22q23.96)
sin2q23 0.5
sin2q23 0.4
50
Summary of atmospheric neutrino-2

• Present atmospheric neutrino data are nicely
  explained by nm ? nt oscillations.
• L/E analysis has shown evidence for oscillatory
  signature.
• The data are consistent with tau neutrino
  appearance.
• So far, no evidence for sub-dominant
  oscillations. But future atmospheric neutrino
  experiments are likely to give unique
  contribution to this field (especially solar
  term effect).

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End