Recent T2K results on CP violation in the lepton sector

presented by

Per Jonsson

Imperial College London

On Behalf of the T2K Collaboration

Miami 2016, December 14-20 2016, Fort Lauderdale, USA.

18/12/16 2Miami 2016

Outline● Neutrino Oscillations

● The T2K Experiment

● Beam

● Near detectors

● Far detector

● Results

● θ23

and Δm2

32

● Electron anti-neutrino appearance

● δCP

and mass hierarchy

● Future Plans

● Conclusions

18/12/16 3Miami 2016

Three flavour mixing in lepton sector

PMNS

Pontecorvo-Maki-Nakagawa-Sakata Mixing Matrix

The neutrino flavour eigenstates are mixtures of the mass eigenstates

18/12/16 4Miami 2016

Neutrino Oscillations● Neutrino oscillations discovered in 1998

● Neutrinos have non-zero mass and mixing angles

● Many open questions:

● What is the neutrino mass hierarchy?

– Is Δm2

31 positive or negative?

● Is there CP violation in the lepton sector?

– CP symmetry is violated if δCP

≠ 0,π

● Is the θ23

mixing angle maximal?

– If not, which quadrant?

● What are the precise values of the mixing angles θ

ij?

● Majorana or Dirac? Δm2

ij= m2

i-m2

j

Δm2

21 = (7.53 ±0.2)x10-5 eV2

Δm2

32 = (2.44 ±0.06)x10-3 eV2

sin2θ12

= 0.304 ±0.01 (θ12

~33°)

sin2θ23

= 0.514 ±0.06 (θ23

~45°)

sin22θ13

= 0.085 ±0.005 (θ13

~8°)

PDG2015

18/12/16 5Miami 2016

Neutrino oscillation probabilities at T2K

Muon neutrino survival probability:

Electron neutrino appearance probability (expansion by α = Δm2

21 / Δm2

31):

For anti-neutrinos, replace δ and x with -δ and -x

● Leading term depends on θ13

and θ23

,

● CP-violating phase δ => P(νμ → ν

e) ≠ P(ν

μ → ν

e),

● Matter effect gives sensitivity to mass hierarchy: sign of x.

E=0.6 GeVL=295 km

Sensitive to θ23

and Δm2

32 Comparing neutrino and anti-neutrino

disappearance: test of CPT symmetry

18/12/16 6Miami 2016

Effect of CP violation at T2K

● Asymmetric effect:

● δCP

= -π/2 → maximizes P(νμ → ν

e), minimizes P(ν

μ → ν

e)

● δCP

= +π/2 → minimizes P(νμ → ν

e), maximizes P(ν

μ → ν

e)

● Effect of δCP

and Mass Hierarchy on appearance probability is similar. Size of effect:

– δCP

: 0 to (±) 20% effect

– Mass hierarchy: ±10%

18/12/16 7Miami 2016

The T2K Collaboration

Include collaboration info here

The T2K collaboration includes about 500 members from 11 countries (Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, UK, USA).

18/12/16 8Miami 2016

The T2K ExperimentOff-axis ν

μbeam 2.5°

Neutrino flux peaks at 0.6 GeVLess than 1% ν

e under the peak

Two production modes:Neutrino and anti-neutrino

280 m

νμ

νμ /ν

e

SK: 50 kt water, 22.5 kt fiducial INGRIDND280

In Japan

18/12/16 9Miami 2016

J-PARC

17July. 2009Bird’s eye view of J-PARC

Linac

Rapid Cycling Synchrotron

Main Ring

NeutrinoTo Kamioka

Near Detector

18/12/16 10Miami 2016

T2K Beam Line

● 30 GeV proton beam from J-PARC main ring

● 90 cm graphite target

● Neutrinos produced from pion and kaon decays

● For anti-neutrinos: Invert focusing-horn polarity which selects the charged pions and kaons

● Dedicated hadron production measurements from NA61/SHINE crucial for T2K (anti)neutrino flux predictions

● Off-axis beam: centre of beam direction 2.5°off from direction to SK

● Narrow beam energy peak around oscillation maximum,

● Fewer high-energy neutrinos, so less problematic background

18/12/16 11Miami 2016

Near Detectors: INGRID● On-axis detector,

● Consists of 16 modules, 7 horizontal, 7 vertical and 2 off diagonal, each module is a cube of 1 m3,

● Each module is a sandwich of 11 scintillator and 10 iron layers, surrounded by 4 veto planes,

● Neutrino beam centre is obtained with accuracy of ~ 0.1 mrad from horizontal/vertical distribution of neutrino event rate,

10mrad10mrad

Horizontal VerticalProvides cross section measurements and constraining beam flux

18/12/16 12Miami 2016

Near Detectors: ND280● Off-axis detector

● 2 Fine-Grained Detectors (FGDs) of scintillator bars, one has water target,

● 3 gas-filled TPCs to reconstruct and identify charged particle tracks

● 0.2 T magnetic field used for charged particle momentum determination

● Information on neutrino flux and interactions from reconstructed tracks

P0D: π0 detectorECAL: Electromagnetic CALorimeterSMRD: Side Muon Range Detector

neutrinos

ND280 strongly reduces systematic uncertainties of T2K oscillation analysis

18/12/16 13Miami 2016

Far Detector: Super Kamiokande

● Situated 1000 m under ground in Kamioka mine, 295 km from J-PARC

● 50 kt water Cherenkov detector with a fiducial volume of 22.5 kt

● Inner Detector is instrumented with 11129 20-inch PMTs

● Outer Detector has 1885 8-inch PMTs

39.3 m

41.4 m

18/12/16 14Miami 2016

Particle ID and Event Selection

● Select 1 ring events for analysis

νe→e

νμ selection

● μ-like PID● p

μ > 200 MeV/c

● At most 1 Michel electron

νe

selection

● e-like PID● p

e> 100 MeV/c

● No Michel electron● E

rec < 1.25 GeV

● Pass π0 rejection

Sharp Cherenkov ring,only direct light from μ

Fuzzier Cherenkov ring,due to scatter/EM shower

νμ→μ

Excellent μ/e separationProbability to misidentify a muon as an electron < 1%

18/12/16 15Miami 2016

Data Sample

Results with full good quality data up to May 27:● ν-mode: 7.48x1020 POT● ν-mode: 7.47x1020 POT

● Continuous rise in beam power:~225 kW (2014) → ~420kW (2016)● Accumulated almost the same number of POT in ν- and ν-mode

POT=Protons On Target

18/12/16 16Miami 2016

Analysis Method

NND

~(σΦ)ND

εND

NSK

~(σΦ)SK

εSK

POSC

ΔNSK

/NSK

~ 5%

θ13

, θ23

, Δm2

31/32, δ

CP

(θ12

and Δm2

21 fixed

from other experiments)

18/12/16 17Miami 2016

Systematic Uncertainties● Expected events

at SK as a function of reconstructed neutrino energy

● ND280 reduces the systematic uncertainties on expected neutrino events at SK from 12-14% to 5-6%

νμ

νμ

νe

νe

18/12/16 18Miami 2016

Results

The results presented in this talk are from the analysis of the combined T2K neutrino and anti-neutrino data samples (ν

μ,ν

μ ,ν

e,ν

e) with four

parameters (δCP

, θ13

, θ23

, Δm2

32) fitted

simultaneously.

18/12/16 19Miami 2016

νμand ν

μdisappearance

Previous publications: PRL 112 181801 (2014) PRD 91 07210 (2015) PRL 116, 181801(2016)

Neutrino mode Anti-neutrino mode

Disappearance and distortion of

energy spectrum for both νμand ν

μ

νμ

νμ

νμand ν

μ results are consistent

Preliminary

No evidence of CPT violation

18/12/16 20Miami 2016

Constraints on θ23

and Δm2

32

18/12/16 21Miami 2016

νe and ν

e appearance

Neutrino mode Anti-neutrino mode

Previous publications: PRL 112, 061802(2014) PRD 91, 072010(2015)

Clear appearance signal for νe

More statistics needed for νe

νe

νe

18/12/16 22Miami 2016

Constraints on θ13

and δCP

Reactor experiment constraint from PDG 2015 (sin2 2θ

13= 0.085 ±0.005) shown

● θ13

in agreement with reactor

experiments

● T2K begins to probe δCP

● δCP

~ -π/2 and NH preferred

● T2K disfavors region of δCP

= +π/2

T2K-only data fit

Number of νe and ν

e candidates

compared with predictions :

● Number of observed events shows larger asymmetry than expected for δ

CP = -π/2 and NH

18/12/16 23Miami 2016

Constraints on δCP

90% CL constraints on δCP

from

Feldman-Cousins method

Reactor constraint: sin2 2θ

13= 0.085 ±0.005 (PDG 2015)

● Best fit gives δCP

= -1.791, Normal

Hierarchy

● The allowed 90% CL intervals are: -3.13 < dCP < -0.39 (NH) and -2.09 < dCP < -0.74 (IH)

● CP conserving values δCP

= 0 and p are excluded at 90% C.L.

CP conservation hypothesis excluded at 90% CL

Allowed 90% CL

18/12/16 24Miami 2016

Future Plans

18/12/16 25Miami 2016

T2K-II● T2K: ends when 7.8x1021 POT is reached (expected around 2021)

● T2K-II: proposed extension up to 2026 for 20x1021 POT

● Stage 1 status at this summer's J-PARC PAC (Program Advisory Committe)

● Plan to gradually increase the beam intensity (currently ~ 400 kW) up to 1 MW in 2021

● J-PARC accelerator and beam-line upgrade:

● Beam power up to 1.3 MW in ~2026

● Upgrade of near detectors to improve systematic uncertainties

● Possible increase of SK fiducial volume T2K-II targeted scenario

[arXiv: 1609.04111]

A step towards Hyper-Kamiokande

18/12/16 26Miami 2016

Physics Potential of T2K-II

Sensitivity for sin2θ23

=0.43 With full T2K-II statistics able to:

● Exclude CP conservation hypothesis at more than 3σ if δ

CP ~-π/2

● Measure θ23

with resolution ~1.7°

Unknown Mass Hierarchy Mass Hierarchy Known

18/12/16 27Miami 2016

Conclusions● Since 2010 T2K has accumulated ~ 1.5x1021 POT, 19% of expected total,

split equally in neutrino and anti-neutrino mode

● First search for CP violation in the lepton sector with analysis using both neutrino and anti-neutrino data (ν

μ/ν

μ disappearance, ν

e/ν

e appearance)

● Leading results for θ23

and Δm2

32

– Data prefer maximal (θ23

= 45°) νμ/ν

μ disappearance

● CP conservation hypothesis excluded at 90% CL

– δCP

= [-3.13,-0.39] NH, [-2.09,-0.74] IH

● Expect to double neutrino data by summer 2017 and to reach 7.8x1021 POT around 2021

● Proposed extension to T2K:

● Accumulate 20x1021 POT by 2026,

● Reach > 3σ sensitivity to δCP

~-π/2

18/12/16 28Miami 2016

BACKUP

18/12/16 29Miami 2016

Flux Prediction● Neutrino and anti-neutrino flux prediction tuned

with hadron spectra from NA61/SHINE

● Flux uncertainty reduced from ~30% to ~10% (thin target data)

● Less than 1% electron (anti)neutrino component at the peak,● <10% wrong-sign background,

● Using predictions of flux correlations between near/far, neutrino/anti-neutrino beam ,νμ /ν

e

18/12/16 30Miami 2016

NA61/SHINE experiment at CERN SPS● Large-acceptance detector with very good capabilities of charge and mass

measurements

● Located in the CERN North Area

● Cover almost full T2K {p,Θ} phase space

● Measure pion, proton and kaon production with 31 GeV/c proton beam on carbon target

● Thin 2cm target (Eur. Phys. J. C 76, 84 (2016))

● T2K replica target (published π± yields: Eur. Phys. J. C 76, 617 (2016))

18/12/16 31Miami 2016

Systematics

● Improvements given by measurements with ND280 data

18/12/16 32Miami 2016

Near detector neutrino sample for systematics constraint

Neutrino samples, best-fit distributions1 μ- candidates

18/12/16 33Miami 2016

Near detector anti-neutrino sample for systematics constraint

Anti-neutrino samples, best-fit distributions

1 μ+ candidates

18/12/16 34Miami 2016

SK νμ /ν

μselection

No magnetic field: same selection for neutrino and anti-neutrino beam

18/12/16 35Miami 2016

SK νe /ν

e selection

No magnetic field: same selection for neutrino and anti-neutrino beam

18/12/16 36Miami 2016

SK spectra

● Predicted spectra at SK compared to candidate events● Energy reconstructed assuming 2-body kinematics