NEW RESULTS FROM
T2K EXPERIMENT
Atsuko K. Ichikawa for T2K collaboration
2011/June/15, 京大談話会

T2K 東海 TO 神岡
長基線ニュートリノ振動実験
水チェレンコフ検出器

大強度陽子加速器
comissioning have been started

Purebeam

Note: 1 GeV のmean free path = O(1a.u.)
280m

T2K COLLABORATIONInternational collaboration(~500 members, 59 institutes, 12 countries）

目次
1. ニュートリノ振動の現状
2. Introduction of T2K experiment
3. T2K history
4. Document 3/11
2. Search for νe appearancewith data upto 11th March(1.43 x 1020 protons on target )3. Conclusion and Prospect

Mass eigenstate

ニュートリノ振動とは？

12

2
1flavor eigenstate=|1>cos+|2>sine

m12
i L
2E

|1> em22
i L2Ecos+|2> esinP(     )     2L)
 sin 2 sin (m4E
222

太陽ニュートリノと大気ニュートリノ

太陽ニュートリノ
Super-K, SNO,
KamLAND(原子炉),
Bolexino,……
大気ニュートリノ
Super-K, K2K(加速器),MINOS(加速器)2010 PDGより

THREE FLAVOR MIXING IN LEPTON SECTOR
ニュートリノには、3世代あるので、3世代で混合を考えるべき
mass eigenstates
m1Weak eigenstates
e


0
1
U PMNS   0  c23
0  s23 e 
 1   CP    U MNS VM  2 
    3
0   c13
 s23  0
 c23   s13e i

0  s13e100

 i

 c13m3  c12  s12
 0

(cij  cos  ij , sij  sin  ij )大気ニュートリノm2

 s12
 c120太陽ニュートリノ

0

0
1 

12, 23, 13+ (+2 Majorana phase)m12, m23, m13

わかっていること
12  34  1
 23  45  8 (90%CL)
13  12 (90%CL)

 0.8 0.55  0.21

U MNS    0.4 0.60.7 
 0.4

0.
60.
7

=?

 0.97 0.23 0.004 U CKM   0.23 0.97 0.04  0.008 0.041oO

何故、クオークとレプトンでこんなRにも混合の様子が異なるのか？
13と23がキーポイント
CP(δ)は？
物質優勢宇宙を説明するには、レプトンセクターでもCPが破れていることが望ましい。

→e APPEARANCE

Leading term at around atm.
osciilation maximum

θ13
CPC
CPVSolarMatter effect(small in T2K)
a → -a, δ → -δ for P(νμ → νe)
L=295km, <Eν> ~0.6GeV

CP violating term introduced byinterference btw. 13 and 12

DESIGN PRINCIPLE OF T2Koff-axis (2.5°)

30GeV protons
from J-PARC MRπ→μν118 m

High Intensity  beam from J-PARC
Super-Kamiokande(SK) as a far neutrino detector• 22.5kt fiducial volume mass
• Excellent performance for single particle event• νe + n → e + p (T2K νe signal)

Less high energy tail by off-axis beam method• First application at long baseline experiment

FAR DETECTOR (SUPER-K)
Stable operation since April 1996
Water Cherenkov detector w/ fiducial
volume 22.5kton
Dead-time less DAQ system (2008~)
Detector performance is well-matched atsub GeV••

Excellent performance for single particle
event
Good e-like(shower ring) / μ-like separation(next page)~11000 x 20inch PMTs (inner detector, ID)

ELECTRON-LIKE AND
MUON-LIKE EVENT AT SK
e-likeParticle identification using
ring shape & opening angle

atmospheric ν dataMCμ-like
Probability that μ is mis-identified
as electron is ~1%

T2K SIGNAL & BACKGROUND
FOR e APPEARANCESignal = single electron event
oscillated νeCharged Current Quasi-elastic interaction :CCQE : νe + n → e + p(dominant process at T2K beam energy)

Background• intrinsic νe in the beam (from μ, K decays)<0.5% at energy peak

• π0 from Neutral Current interaction

π0E (GeV)

OFF-AXIS BEAM : INTENSE & NARROWBAND BEAM
0°

12°
3°

0022.5°58Pπ(GeV)

Pseud monochromatic beam utilizing
pion decay kinematics
T2K off-axis angle is 2.5°
peak energy at oscillation max.
(~0.6GeV at L=295km)
less high energy tail↓maximize physics sensitivity

OFF-AXIS NEAR DETECTOR(ND280)

νμ CC events rate measurementin present analysis
0.2 T UA1 magnet
Fine Grained Detector (FGD)• scintillator bars target (water target in FGD2)• 1.6ton fiducial mass for analysis

Time Projection Chambers (TPC)• better than 10% dE/dx resolution

Event display (data)• 10% momentum resolution at 1GeV/c
TPC2

TPC1

TPC3

νμdE/dx (TPC: data)

FGD1

FGD2

HISTORY• 1990年代東大原子核研究所大型ハドロン計画（ＪＨＦ）• 1999 原研の中性子科学研究計画との統合計画（J-PARC）• 2001.4 J-PARC本予算スタート６年で（νは２期）• 2003.8 文科省ニュートリノ予算を財務省へ• 2003.10 総合科学技術会議ニュートリノをＣランクに格付け• 2003.11 プロジェクト再検討• 2003.12 財務省ニュートリノプロジェクト認める-- 建設 -- 建設 -- 建設 -- 建設 -- 建設 -- 建設 -- 建設 -- 建設 -- 建設 -• 2009.4 ニュートリノ施設のコミッショニング開始(数ショット)• 2009.11 J-PARCで最初のニュートリノ観測• 2010.1 物理データ取得開始 (<20kW)-- 強度化 -- 強度化 -- 強度化 -- 強度化 -- 強度化 -- 強度化 -- 強度化 -• 2010.2 Super-Kで最初のJ-PARCニュートリノを観測 (~30kW)• 2011.3.10 ~145kW

Run 1

50kW

145kW

Run 2

# of bunch : 6 → 8
rep. rate : 3.64s→ 3.2s → 3.04s

Run 1 (Jan. ’10 - June ’10)

Run 2 (Nov. ’10 - Mar. ’11)

- 3.23 x 1019 p.o.t. for analysis- 50kW stable beam operation- 11.08 x 1019 p.o.t. for analysis- ~145kW beam operation

Total # of protons used for this analysis is 1.43 x 1020 pot
2% of T2K’s final goal and ~5 times exposure of the previous report

proton per pulse

accumulated # of protons

TOTAL # OF PROTONS USED FOR
ANALYSIS

 BEAM STABILITY
Stability of ν beam direction (INGRID)

Stability of ν interaction rate normalizedby # of protons (INGRID)INGRID ν int. rate stabilityRun 1+2 / Run 1 < 1%

ν beam dir. stability < 1mrad
Stability of beam direction (Muon monitor)

Beam dir. stability < 1mrad

DOCUMENT 3/11
加速器メンテナンスのためビームは停止中
15:00より Run1の結果をKEKにて発表予定
京都では、物二教室発表会開催中
14:46 最初の地震 → 最大震度 6強
東海にいたT2K関係者の多くが、(TV会議で)セミナーに出席しようと敷地内を移動中
京大の高エネグループ院生 5名を含む
地震直後に携帯で京都と連絡
人の被害はない模様
その後、数日間は連絡を取れなくなるDOCUMENT 3/11
東海地区は、電気、ガス、水が止まる。余震がひどい
交通手段も麻痺 (高速道路、鉄道ともに止まる。一般道も信号がつかない。ガソリンが手に入らない）
院生は、コンビニエンスストアで、食糧を調達

3/14 KEKがバスを手配し、ユーザーをすべて東海から引き上げる
院生は無事、帰京

研究所はしばらく閉鎖
その後、空間放射線レベルが上がる。東海での上昇は、最大3~4μSv/h。
すぐに下がる。
１週間後から徐々にInspection start(3/24 市川、車で帰宅。ガソリン、納豆が手に入らない）
4月に入ってから、高速道路、鉄道も復旧。徐々にユーザーも来訪可に。
損害は大きいが、修復可能！

Search for νe appearance

ANALYSIS OVERVIEW
1.

Apply νe selection criteria to the events at far detector (SK)

2.

Compare the observed number of events and the expected
number of events (for sin22θ13=0)→ search for νe appearance

Contents in this section✤ νe selection criteria✤The expected number of events at Far detector
using Hadron (pion) production measurement&ND event rate measurement✤ Systematic uncertainty✤ Observation at Far detector & Results✤ νe selection criteria✤ The expected number of events at Far detector✤ Systematic uncertainty✤ Observation at Far detector & Results

e SELECTION AT FAR DETECTOR (SK)The selection criteria were optimized for initial running condition
The selection criteria were fixed before data taking started to avoid bias

7 selection cuts
1. T2K beam timing & Fully contained (FC)(synchronized with the beam timing,no activities in the OD)

ODFVID

2. In fiducial volume (FV)
(distance btw recon. vertex and wall > 200 cm)* Events too close to the wall are difficult toaccurately reconstruct vertex* Reject events which are originated outside the ID* Define FV 22.5kton

3. Single electron(# of ring is one & e-like)atmospheric ν FC events

MCData30MeV < visible energy < 1330MeV

4. Visible energy > 100 MeV(visible energy =electron energy deposited in ID)

5. No decay electron observed(no delayed electron signal)* Reject low energy events, such as
NC background and decay
electrons produced byinvisible muons* Reject events with muons or pions
which are invisible or
mis-identified as electron(νμ events orCC non-QE events)

T2K MC

T2K MCsin22θ13 = 0.1

sin22θ13 = 0.1

rejects 14% of NC,
30% of νμ CC bkg.
98% signal
efficiency

rejects
85% of νμ CC bkg.
90% signal
efficiency

6. Reconstructedinvariant mass (Minv) < 105 MeV/c2* Suppress NC π0 background
Find 2nd e-like ring by forcing to fit light patternunder the 2 e-like rings assumption, and thenreconstruct invariant mass of these 2 e-like rings

T2K MC

demonstrate to reconstruct invariant
mass using atmospheric ν data
sin22θ13 = 0.1

rejects 71% of NC
background
91% signalefficiency

SK atm. ν data and MC(single ring e-like event withT2K νe selection)

Invariant mass (MeV/c2)

7. Reconstructed energy (Erec) < 1250 MeV* Reject intrinsic beam νe backgrounds at high energy* Signal (νμ→νe) has a sharp peak at E~600MeV
reconstruct energy assuming CCQE(with correcting nuclear potential)

T2K MCsin22θ13 = 0.1

rejects 36%of beam νe bkg.
98% signalefficiency

After all the selection criteria
background rejection :77 % for beam νe,99 % for NC
signal efficiency : 66 %for the number of events in FV✤ νe selection criteria✤ The expected number of events at Fardetector with 1.43 x 1020 p.o.t.

✤ Systematic uncertainty✤ Observation at Far detector & Results

EXPECTED # OF EVENTS AT FAR
DETECTOR
Normalization by Near detector
both for signal and background

ND νμ event rate
Measurement of the number of inclusive νμ charged-current events inND per p.o.t. using data collected in Run 1 (2.88 x 1019 p.o.t.)
Stability of the beam event rate is confirmed by INGRID measurement
INGRID ν int. rate stability Run 1+2 / Run 1 < 1%

F/N ratio for νe signal event ← systematic error cancelation(flux ) x (osc. prob.) x (x-section) x (efficiency) x (det. mass)

NEUTRINO FLUX PREDICTION
T2K Neutrino beam simulation basedon Hadron production measurements

actual beam profile &position(beam monitors meas.)graphitetargetπ, Khorn focusing,
decay is simulatedby GEANT3μνμNDproton beamHadron production in 30GeV proton + C• Use CERN NA61/SHINE pion measurement(large acceptance: >95% coverage of ν parent pions)
5~10% systematic error• Kaon, pion outside NA61 acceptance, other interactionin the target were based on FLUKA simulation• Secondary interaction x-sections outside the target were based on
experimental data

SK

Predicted neutrino flux

Predicted neutrino flux
106

SK1012

ND

104

SK

μ decay is dominated at
low energy

NA61 pion measurement
predicts the beam νe from pionorigin

μ INTERACTION RATES AT NEARmuon momentum in inclusive νμ CC events
DETECTOR (ν+μ + N → μ + X)
p.o.t. normalized

data is consistent with
MC based on the NA61 andν interaction simulation (w/o tuning)

Results

THE EXPECTED NUMBER OF
EVENTS FOR SIN22θ13=0
The expected number of events with 1.43 x 1020 p.o.t.

NexpSK tot. = 1.5 events
NCBeam νebackground backgroundThe expected #of events at SK

0.8

0.6

Oscillated
νμ→νeTotal(solar term)

0.11.5✤ νe selection criteria✤The expected number of events at Far detector✤ Systematic uncertainty✤ Observation at Far detector & Results

SYSTEMATIC UNCERTAINTY ON

NEXPSK

int. cross section

NEUTRINO FLUX UNCERTAINTY
Uncertainties in hadronproduction and interaction are
dominant sources
Error source
Pion production• NA61 systematic uncertainty in each pion’s (p,θ) bin
Kaon production• Used model (FLUKA) is compared with the data(Eichten et. al.) ineach kaon’s (p,θ) bin
Secondary nucleon production• Used model (FLUKA) is compared withthe experimental data
Secondary interaction cross sectionn,pπ, K

protongraphite target• Used model (FLUKA and GCALOR) is compared withthe experimental data of interaction x-section (π, K and nucleon)

Summary of  flux uncertainties on NexpSK for sin22θ13=0Hadronproduction& interaction

The uncertainty on NexpSK due to the beam flux uncertainty is 8.5%
Error cancellation works for some beam uncertainties INT. CROSS SECTIONUNCERTAINTY

Evaluate uncertainty on F/N ratio by varying the crosssection within its uncertainty
Main ν interaction in each event category
NC background : NC1π0Beam νe background : νe CCQESignal : νe CCQE
ND CC event : CCQE(50%)
CC1π(23%)
Cross section uncertainty
relative to the CCQE total x-section

Cross section uncertainties are
estimated by Data/MC comparison,model comparison and parametervariation
MiniBooNE data
MC (NEUT)

Uncertainty of σ(νe)/σ(νμ) = ±6%Pπ0 (GeV/c) INT. CROSS SECTIONUNCERTAINTY
ON NEXPSK FOR SIN22θ13=0
Error source

Uncertainty in pion’s
final state interaction
is dominant

The uncertainty on NexpSK due to the ν x-section uncertainty is 14%(sin22θ13=0)

FAR DETECTOR UNCERTAINTY
Uncertainty due to the SK detector uncertainty
Evaluation using control sample

One of big error sources:

detection efficiency of NC 1π0 backgroundcontrol sample with one data electron + one simulated γ
control sample dataMC

apply T2K νe selection and compare
the cut efficiency between control sample dataand its MC
invariant mass Minv [MeV] → difference is assigned as sys. error

Summary of Far detector systematic uncertainty
Error source

π0 rejection-3.6%

Ring counting

3.9%

8.3%

Electron PID

3.8%

8.0%

Invariant mass cut5.1%

8.7%

Fiducial volume cut etc.

1.4%

1.4%

Energy scale

0.4%

1.1%

Decay electron finding

0.1%

0.3%

-1.0%

7.6%

15%

Muon PID

Total

Evaluated byatmosphericνe enriched data

→ The total uncertainty on NMCSK tot. is 14.7 % (sin22θ13=0)(uncertainty on the background + solar term oscillated νe)

TOTAL SYSTEMATIC
UNCERTAINTIESSummary of systematic uncertainties on NexpSK total. for sin22θ13=0 and 0.1
cf.
sin22θ13=0:#sig = 0.1 #bkg = 1.4

int. cross section

sin22θ13=0.1:#sig = 4.1 #bkg = 1.3(due to small Far det.
uncertainty for signal)

NexpSK tot. = 1.5 ± 0.3 events for sin22θ13=0(w/ 1.43 x 1020 p.o.t.)✤ νe selection criteria✤The expected number of events at Far detector✤ Systematic uncertainty✤ Observation at Far detector & Results

SK EVENTS IN BEAM TIMING
Events in the T2K beam timing synchronized by GPS
relative event timing to the spill timing

Clear beam structure !

LE: low energy events
OD: hits at Outer Detector
FC: No hit at Outer Detector∆T0 = TGPS@SK - TGPS@J-PARC - TOF(~985μsec)

Run 1
Run 2

NUMBER OF T2K EVENTS AT FAR
DETECTOR

non-beambackground

The accidental contamination from atmospheric ν backgroundis estimated using the sideband events to be 0.023

APPLY e EVENT SELECTION
Fiducial volume cut

defined before the data collection
6 selection cuts in addition FC cut(distance between recon. vertex and wall > 200cm)

Single electron cut (# of ring is one & e-like)(MC w/sin22θ13 = 0.1)

(MC w/sin22θ13 = 0.1)

e-likeμ-likeVisible energy > 100 MeV(MC w/sin22θ13 = 0.1)

No decay electron(MC w/sin22θ13 = 0.1)

Invariant mass cut (Minv < 105 MeV/c2)(MC w/sin22θ13 = 0.1)

Reconstructed ν energy cut (Erec < 1250 MeV) :Final cut

(MC w/sin22θ13 = 0.1)

6 candidate events
remain after all cuts !!
(Nexp = 1.5 ± 0.3 at sin22θ13=0)A e CANDIDATE EVENT

visible energy : 1049 MeV# of decay-e : 02γ Inv. mass : 0.04 MeV/c2
recon. energy : 1120.9 MeV

FURTHER CHECKCheck several distribution of νe candidate events(MC w/sin22θ13 = 0.1)

Vertex distribution of νe candidate events
beam direction

These events are clustered at large R→ Perform several checks. for example* Check distribution of events outside FV → no indication of BG contamination* Check distribution of OD events → no indication of BG contamination* K.S. test on the R2 distribution yields a p-value of 0.03

RESULTS FOR e APPEARANCESEARCHWITH 1.43 X 1020 P.O.T.
The observed number of events is 6
The expected number of events is 1.5 ± 0.3for sin22θ13=0

Under the θ13=0 hypothesis, the probability toobserve six or more candidate events is 0.007(equivalent to 2.5σ significance)ALLOWED REGION OF SIN22θ13
AS A FUNCTION OF ΔM223(assuming sin22θ23=1, δCP=0)

Feldman-Cousins method was used

ALLOWED REGION OF
SIN22θ13
AS A FUNCTION OFδCP
2(assuming ∆m 23=2.4 x 10-3 eV2, sin22θ23=1)

90% C.L. interval & Best fit point (assuming ∆m223=2.4 x 10-3 eV2, sin22θ23=1, δCP=0)0.03 < sin22θ13 < 0.28

0.04 < sin22θ13 < 0.34

sin22θ13 = 0.11

sin22θ13 = 0.14

T2K NEXT STEPS
Aim for firmly establishing νe appearance andbetter determining the angle θ13

CONCLUSION

We reported new results on νμ → νe oscillation analysis
based on 1.43 x 1020 p.o.t. (2% exposure of T2K’s goal)• The expected number of events is 1.5 ± 0.3 (sin22θ13 = 0)
• 6 candidate events are observed• Under θ13=0 hypothesis, the probability to observe 6 or more candidate events is0.007 (equivalent to 2.5σ significance)• 0.03 (0.04) < sin22θ13 < 0.28 (0.34) at 90% C.L. for normal (inverted) hierarchy(assuming ∆m223=2.4 x 10-3 eV2, sin22θ23=1, δCP=0)

Indication of νe appearance
submitted to PRL

Resume experiment as soon as possible and improveanalysis method to conclude νe appearance phenomenon
νμ disappearance result with 1.43 x 1020 p.o.t. data will bereported this summer