1. First Anti-Neutrino Oscillation Results from the T2K Experiment
Alfons Weber University of Oxford
STFC/RAL

2. Measurements Overview Introduction The Experiments Summary Beam
Near Detectors
Super-Kamiokande
Measurements
Disappearing muon anti-neutrinos
Appearing electron anti-neutrinos
Other measurements
Summary
Sep 2015
Alfons Weber

3. Neutrino Mixing The PMNS Matrix
Pontecorvo-Maki- Nakagawa-Sakata
Assume that neutrinos do have mass:
mass eigenstates  weak interaction eigenstates
Analogue to CKM-Matrix in quark sector!
Normal Hierarchy
Inverted Hierarchy
weak “flavour eigenstates”
Mass eigenstates
m1, m2, m3
Unitary mixing matrix:
3 mixing angles
& complex phases
Sep 2015
Alfons Weber

4. Who is Who beamνμ disappearance Solar & reactor
ν–less double beta decay
νe appearance in νμ beam or
νe disappearance in reactor experiments
Sep 2015
Alfons Weber

5. Neutrino Oscillations
If mass and weak eigenstates are different:
Neutrino is produced in weak eigenstate
It travels a distance L as a mass eigenstate
It will be detected in a (possibly) different weak eigenstate
Sep 2015
Alfons Weber

6. Muon Neutrino Disappearance
Monte Carlo (MINOS Experiment)
(Input parameters: sin22θ = 1.0, Δm2 = 3.35x10-3 eV2 )
spectrum ratio
νμ spectrum
Unoscillated
Oscillated
Sep 2015
Alfons Weber

7. A Little Bit of Math
Sep 2015
Alfons Weber

8. Collaboration
Sep 2015
Alfons Weber

9. T2K Experiment
Sep 2015
Alfons Weber

10. T2K experimental overview
The oscillation probability for numu disappearnce, is sinusoidal, and dpeneds on L and E, the distance the neutrinos travel and their energy.
In order to maximinze this probability, this argument would need to be of order 1), and also to observe the lepton through charged current interactions, then I need to go hundreds of km.
Long baseline experiments are driven by intense neutrino sources, and are tuned to a beam energy relevant for oscillation physics.
J-PARC accelerator
Near detectors
Off-axis: ND On-axis: INGRID
Far detector
Sep 2015
Alfons Weber

11. Design Principle 0º
Sep 2015
Alfons Weber

12. Neutrino InteractionsW
Charged Current Quasi-Elastic (CCQE) ν p n
Neutrino energy from lepton momentum and angle:
e- or -
2 body kinematics and assumes the target nucleon is at rest
The beam travels essentially a fixed distance L, and we probe the oscillation dip around the relevant region in energy. T2K’s peak flux energy is ~0.6 GeV, so the dominant interaction is charged current quasi-elastic interactions (CCQE). I’ll talk later about why this is an oversimplification, but for now the neutrino interactons on a bound neutron, and produces a muon or an electron in the detector. The kinematics of the lepton are used to infer the neutirno energy, assuming simple 2 body inematics.
Some relevant background processes are those which mimic CCQE, such as single pion production. In this case there is a lepton and a pion in thef inal sate. When the pion is absorbed or not reconstructed, then this event appears “CCQE-like” but is actually due to a higher energy event. This smears out our determination of the oscillation probability.
Neutral current events can also be a background to the nue appearnce (NCpi0) or numu disappearance analysis (Ncpi+) W ν N N’ Δ π
CCπ
NCπ
Additional significant processes:
CCQE-like multinucleon interaction
Charged current single pion production (CCπ)
Neutral current single pion production (NCπ) Z ν N N’ Δ π
e- or  -
Sep 2015
Alfons Weber

13. Beamline MC Hadronic interactions
Pions/Kaons - use CERN NA61/SHINE pion measurement (large acceptance: >95% coverage of ν parent pions)
Pions outside NA61 acceptance, other interaction use FLUKA simulation
Secondary interactions outside the target based on experimentally measured cross-sections
GEANT3 transport simulation used downstream of target
Sep 2015
Alfons Weber

14. NA61/SHINE @ CERN ~13m ~10m Hadron(π, K) yield measurement
30 GeV proton
High-acceptance ToFs and spectrometers
Measurement program
2 cm thin target - 4%λI
T2K Replica target
~13m
~10m
Sep 2015
Alfons Weber

15. Flux Prediction Flux prediction external or in-situ measurements
p-C data
alignment and off-axis angle
π± , K± production from NA61/SHINE
The distribution of momentum and production angle for secondary pi+ that produce a neutrino (directly or through daughter particles) in the 250 kA flux.  The phase space of the NA (new) and (old) thin target data are also shown by the region outlined with black lines.  The reduced coverage at low momentum and high angle for 2009 are due to the fact tehat the preliminary 2009 data only use the TOF-dE/dx analysis.  In that region, the 2007 data is used for tuning.
NA61 has thick target data to be analyzed. Modified BMPT prameterization <10% tuning applied to all channel
Uncertainties are comparable for neutrino or antineutrino mode operation (10-15%)
Sep 2015
Alfons Weber

16. Flux Prediction II µ- µ+ π- π+ νµ νµ
Neutrino mode operation
Antineutrino mode operation
FLUKA/Geant3-based neutrino beam simulation (PRD 87, )
Significant neutrino component to antineutrino mode beam
Increases in event rate due to lower antineutrino cross section
“wrong sign” component
“Intrinsic” ~0.5% electron (anti)neutrino component
The beam travels essentially a fixed distance L, and we probe the oscillation dip around the relevant region in energy. T2K’s peak flux energy is ~0.6 GeV, so the dominant interaction is charged current quasi-elastic interactions (CCQE). I’ll talk later about why this is an oversimplification, but for now the neutrino interactons on a bound neutron, and produces a muon or an electron in the detector. The kinematics of the lepton are used to infer the neutirno energy, assuming simple 2 body inematics.
Some relevant background processes are those which mimic CCQE, such as single pion production. In this case there is a lepton and a pion in thef inal sate. When the pion is absorbed or not reconstructed, then this event appears “CCQE-like” but is actually due to a higher energy event. This smears out our determination of the oscillation probability.
Neutral current events can also be a background to the nue appearnce (NCpi0) or numu disappearance analysis (Ncpi+) µ- µ+ π- π+ νµ νµ
Sep 2015
Alfons Weber

17. Data Taking
Maximum beam power is 371kW!
Stable operation at ~345kW achieved!
11.0x1020 (total) = 7.0x1020 (ν) + 4.0x1020 (ν-bar)
Partial POT for ND280, so confirm beam stability
Full antineutrino up to June 2015
Protons on Target (POT) for the antineutrino analyses today:
Run 5c+6 datasets for far detector, Super-K: 4.0 x 1020 POT
Run 5c datasets for off-axis near detector, ND280: 4.3 x 1019 POT
Sep 2015
Alfons Weber

18. Near detectors Muon monitor INGRID (On-axis detector )
spill-by-spill monitoring
INGRID (On-axis detector )
measures beam intensity/direction
1 mrad precision in 1 day
ND280 (same off-axis angle as SK)
Detailed flux measurement
Exclusive cross-section measurements
ν beam
Sep 2015
Alfons Weber

19. Beam Stability neutrino beam profile measured with INGRID
Maximum beam power is 371kW!
Stable operation at ~345kW achieved!
10.3x1020 (total) = 7.0x1020 (ν) + 3.3x1020 (ν-bar)
Partial POT for ND280, how does the rest of the running look?
neutrino beam profile measured with INGRID
scintillator/iron tracking detectors (0-0.9 degrees off-axis)
POT normalized event rate stable to better than 1%
Beam direction is stable to within 1 mrad
corresponds to a 2% shift to peak in off-axis neutrino energy
Sep 2015
Alfons Weber

20. Analysis Strategy Flux Model ND280 Detector Model NA61/SHINE Data
ND280 Data
INGRID/Beam monitor Data
ND data reduces flux and
cross-section uncertainties
ND280 Fit
External Cross-section Data
Cross-section Model
Oscillation Fit
Super-K Data
Super-K Detector Model
Oscillation Parameters
Sep 2015
Alfons Weber

21. Reducing Uncertainties
Neutrino
flux
prediction
Neutrino
cross
section
model
Far Detector
selection,
efficiency
Neutrino
flux
prediction
Neutrino
cross
section
model
Near Detector
selection,
efficiency
N_{FD}\sim\Phi(E_\nu)\sigma(E_\nu)\epsilon_{FD}P(\nu_\mu\rightarrow\nu_e)
N_{ND}\sim\Phi(E_\nu)\sigma(E_\nu)\epsilon_{ND}
Sep 2015
Alfons Weber

22. νμ ND280 Detector dE/dx (TPC: data) 0.2 T magnet
Recycled from CERN
Recycled from CERN
Plastic scintillator detectors:
Fine Grained Detector (FGD)
1.6 ton fiducial mass for analysis
π0 detector (P0D)
ECals and SMRD
Time projection chambers (TPC)
better than 10% dE/dx resolution
10% p resolution at 1GeV/c
Micromegas from CERN
Example: neutrino candidate in antineutrino mode
Muon-like track
TPC
ECAL
FGD νμ
FGD2
TPC3
TPC2
TPC1
FGD1
dE/dx (TPC: data)
Sep 2015
Alfons Weber

23. neutrino selection, neutrino mode samples
ND Event Samples (I)
CC1π
CCOther
neutrino selection, neutrino mode samples
CC0π
Select CC νμ candidates based on interactions with μ-
Select highest momentum track with negative charge, and PID consistent with a muon
Event samples provide information on flux, cross section model
Separated based on presence of charged pion in final state (CC0π, CC1π, CC Other)
Pions identified using track multiplicity, dE/dx in TPCs, photons in ECALs
Dominant detector systematics are pion re-interactions, MC statistics and out of fiducial volume backgrounds
Example: neutrino candidate in antineutrino mode
Sep 2015
Alfons Weber

24. ND Event Samples (II)
Select CC νμ candidates based on interactions with μ+
highest momentum track, positive charge, and PID consistent with muon
Two sub-samples based on track multiplicity:
CC1-Track,
CC>1 Track
Complementary selection of neutrino candidates in antineutrino mode
CC1Track: antineutrino selection, antineutrino mode
CC inclusive:
neutrino selection, antineutrino mode
Dominant detector systematics are pion re-interactions, MC statistics and out of fiducial volume backgrounds
Sep 2015
Alfons Weber

25. Near Detector Fit (I)
Far detector expectations tuned with fit to ND samples (p,θ)
(Anti-)Neutrino fluxes highly correlated between ND and FD
Cross sections highly correlated
Significant reduction to overall uncertainties
CC0π: neutrino selection, neutrino mode
Dominant detector systematics are pion re-interactions, MC statistics and out of fiducial volume backgrounds
C1Track: Antineutrino selection, antineutrino mode
CC1π: neutrino selection, neutrino mode
Sep 2015
Alfons Weber

26. Near Detector Fit (II) Predicted flux at FD is generally increased
Some cross-section parameters are significantly different to prior values
In general error on parameters is decreased
PRELIMINARY
νμ-flux in νμ-mode beam
PRELIMINARY
Sep 2015
Alfons Weber

27. Super-K Detector Located in Mozumi mine Water Cherenkov detector
2700 m.w.e. overburden
Water Cherenkov detector
22.5 kt fiducial mass
Inner detector
inch PMTs
Outer veto
inch PMTs
New DAQ system
No deadtime
Excellent μ/e separation
Probability to reconstruct μ as e ~1%
Sep 2015
Alfons Weber

28. Particle Identification
μ-like MC
e-like MC
Sep 2015
Alfons Weber

29. PID (II)
Sep 2015
Alfons Weber

30. Previous T2K Measurements
90% CL constrain on δCP
First measurement νe appearance (7.3 σ)
Measurement of θ13 (w/ & w/o reactor constrain
sin2θ13=0.095±0.001)
Open Questions
Mass Hierarchy
CP Phase δCP
World-leading measurement of θ23
Significant measurement of Δm223
Abe, K. et al, Physical Review D 91.7 (2015):
Sep 2015
Alfons Weber

31. T2K Oscillation Analysis
Muon antineutrino disappearance
Fit for and
Use separate parameters for neutrino interactions
Other oscillation parameters fixed to T2K neutrino data and PDG2014
Test of NSI or CPT theorem
Electron antineutrino appearance
Search for presence of appearance with antineutrinos
Necessary step toward future CPV searches
\bar{\theta}_{23}
Sep 2015
Alfons Weber

32. T2K Search for Muon neutrinos
Sep 2015
Alfons Weber

33.  Selection (I) FD Event selection Muon-like PID
Fully contained in fiducial volume
1 ring
≤1 decay electron(s)
pμ > 200 MeV
Beam direction
Fiducial volume boundary
Events during run 5
Events during run 6
Out of fiducial volume events
Sep 2015
Alfons Weber

34.  Selection (II)
Sep 2015
Alfons Weber

35. Predicted Event Rates
Predominantly antineutrino interactions, but significant components from other channels
Expect 34.6 events w/ oscillation
Expect events w/o oscillation
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Add selection?
Sep 2015
Alfons Weber

36. Systematics
PRELIMINARY
The near detector significantly reduces the systematic uncertainty in the predicted event rate at far detector
Systematic
Without ND
With ND
Flux and Cross-section
Common to ND280/SK
9.2%
3.4%
Super-K Only
Multi-nucleon effect on oxygen
9.5%
All Super-K Only
10.0%
All
13.0%
10.1%
Final State Interaction/Secondary Interaction at Super-K
2.1%
Super-K Detector
3.8%
Total
14.4%
11.6%
Explain plot: “Prefit” = MC prediction without ND280 constraint, “Postfit” = MC prediction with ND280 constraint
Note this is all for numubar (nuebar similar magnitude)
ND Measurements on water not jet included
Sep 2015
Alfons Weber

37. Antineutrino Disappearance
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Add selection?
Likelihood based estimation of oscillation parameters
Binned in reconstructed neutrino energy
Other oscillation parameters fixed to T2K neutrino data and PDG2014
34 events observed
103.6 expected (w/o osc.)
Sep 2015
Alfons Weber

38. Comparison with MINOS Results compatible with MINOS combined beam+atm
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Results compatible with MINOS combined beam+atm
P. Adamson et al., Phys. Rev. Lett. 110 (2013) 25,
Sep 2015
Alfons Weber

39. Comparison with Neutrino Result
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Sep 2015
Alfons Weber

40. T2K Search for Electron neutrinos
Sep 2015
Alfons Weber

41. Identifying e
Sep 2015
Alfons Weber

42. Anti-neutrino e-like selection
e Events
FD Event Selection
Electron-like PID
In fiducial volume
1 ring
No decay electrons
pe > 100 MeV
Eν < 1250 MeV
Pass π0-rejection
Anti-neutrino e-like selection
3 events
Vertex X (cm)
Vertex Y (cm)
PRELIMINARY
Beam direction
Fiducial volume boundary
Events during run 5
Events during run 6
Out of fiducial volume events
Sep 2015
Alfons Weber

43. νe Selection
Sep 2015
Alfons Weber

44. Appearance Analysis β scales green Normal hierarchy Inverted hierarchy
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Test of no e appearance hypothesis:
 = 0 or 1
Sep 2015
Alfons Weber

45. Data does not favor or disfavor e appearance
Rate Measurement
Generate an ensemble of test experiments with β=0 (no e appearance)
p-value: fraction of test experiments that have as many or more candidate events as T2K data
Sensitivity: mean p-value for an ensemble of fake data experiments with β=1
Rate only
p-value
Likelihood ratio
3 evts
0.26
0.9
Rate only
p-value
Mean p-value
0.20
Likelihood ratio:
L(β=0)/L(β=1) is close to 1
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
Data does not favor or disfavor e appearance
Sep 2015
Alfons Weber

46. Rate & Shape 4646 Background only Signal only
νe uncertainty is largest—but
these events are rare
๏ Neutral current uncertainty
dominates at low energy
-2 \Delta \textrm{ln} \mathcal{L} = -2 \textrm{ln} \frac{\mathcal{L}_{marg}(\beta=0)}{\mathcal{L}_{marg}(\beta=1)}
Sep 2015
Alfons Weber
4646

47. Rate & Shape P-values from data Likelihood ratio No evidence for
-2Δln(Lp-θ) = -1.16
P-values from data
Likelihood ratio
No evidence for
β = 1 over β = 0
-2Δln(LEREC)= 0.16
Shape term
P-value
Erec
0.16
Lepton P-θ
0.34
PRELIMINARY
Shape term
B10
Erec
1.1
Lepton P-θ
0.6
Distribution of test statistic for β = 0 using Lepton P-θ shape information
Sep 2015
Alfons Weber

48. T2K Potential So far, 14% of T2K design POT taken
-mode: 6.9 x 1020 POT;
-mode: 4.0 x 1020 POT
Final sensitivity may be sufficient to find indication for
CPV and/or octant (combined with others)
Sep 2015
Alfons Weber

49. Future Systematics
Nuclear effects can enhanced “CCQE” cross section and change reconstructed energy
CCQE interaction simulated on single nucleon (1p1h)
But contribution from correlated pair of nucleons (2p2h) J. Nieves, I. Ruiz Simo, and M. J. Vicente Vacas, PRC (2011)
M. Martini, M. Ericson, G. Chanfray, and J. Marteau, PRC (2009)
Most oscillation experiments employ the use of a near detector to determine the iniital rate of charged current interactions, and T2K is no different. 280 m away from the neutrino production point, we use a set of offaxis tracking detectors called ND280 which sit within the UA1 magnet. Neutrinos interact on scintillator tracking detector, and then are reconstructed thrioguh surrounding TPCs. The muon momentum is determined from the curvatire in the field. Additional tracks and timing information are used to indicate if there is a pion in the final state, and thus separate the CC numu candidates into CC0pi (which is CCQE enhanced) and CC1pi, and all other CC interactions.
295km later, after oscillation has occurred, we observe numu and nue interactions at Super-Kamiokande, a 50kton water Cherenkov detector.
CCQE interactions produce just the lepton above cherenkov threshold, so we select single ring interactions and use the characteristics of the ring to determine if the particle was electron like or muon like and thus a muon neutrino or an electron neutrino. Interactions which aren’t CCQE are vetoed with the presence of one and only one ring, and decay electron tagging, there are also additional selection cuts to remove neutral current interactions which can mimic nue candites.
The electron or muon momentum and angle with respect to the beam direction is determined from the ring, and this is used to determine the energy of the neutrino.
T2K collab PRL 112, (2014)
Picture by M. Martini
Sep 2015
Alfons Weber

50. Cross Section Systematics
ND280 flux
Eν - EνQE smearing for Eν=0.8 GeV
SK oscillated flux
Eν - EνQE smearing for Eν=0.8 GeV
What is the effect of this n the oscilation analyssi? Well, if the entire cross section was just increased, clearly that would cancel in the event rate at the near and far detector. The issue is that a shift in the underlying true neutrino energy distribution is not really observable at the near detector. This plot shows the ND flux, with a enu -> ereco smearing. The low side tail of this is due to multinucleon events feeding down. The effect of this feed down at the far detector is different– in th eoscillated flux these events fill in the peak and so affect the measurement of th23.
Sep 2015
Alfons Weber

51. CC 0π Final State
Solid: Martini et al
Dashed: Nieves et al
Martini and Ericson, Phys.Rev. C90 (2014) 2,
measured cross-section as a function of muon momentum for the first muon costheta bin compared to Monte Carlo models. Black dots are results from data with shape (systematics and statistical) uncertainties; gray band is the normalization uncertainty due to flux; red solid line is the prediction from Martini model; red dotted line is the prediction from Nieves model.
New measurement muon kinematics for muon, muon + proton, both with no pion in final state from ND280 off-axis beam
Sep 2015
Alfons Weber

52. T2K Cross Sections Cross section measurements Target Reported in
Detector
 CC inclusive CH
PRD 87, (2013)
ND280, Tracker
 CCQE
Accepted by PRD
e CC inclusive
PRL 113, (2014)
 NC π0
CH/Water
Publication in progress
ND280, P0D
 NC elastic
Water
PRD 90, (2014) SK
CH/Fe
PRD 90, (2014)
INGRID
PRD 91, (2015)
 CC coherent
 CCπ+
 CC0π
What we did on T2K was to test how unsimulated multinucleon interactions could potentially affect the analysis. We generated fake data ynder flux, detector and cross section uncertainties. For each of these fake data sets, we took two cases, with and without a 2p2h multinucleon model present, and fit that for oscillation. We can see the effect of this unsimulated process on our determination of theta23. Theere is an increased uncertainty associated with this process at the level of 3%, and in the case of the larger Martini model, also a shift to the mean indicating a bias in the analysis assuming this model is the correct one. While these uncertainties are within our current systematic uncertainty, they would be considered significant relative to other uncertainties.
As we move toward precision measurements, this will become more and more important.
Sep 2015
Alfons Weber

53. Thank You! CERN contributions to T2K UA1/NOMAD magnet
And infrastructure
Production and testing of micromegas
TS/DEM group
NA61/SHINE experiment
Hadron production
CERN-KEK cooperation on superconducting magnets
For neutrino beam line
Test beams
For ND detector components
Sep 2015
Alfons Weber

54. Summary & Conclusion
T2K made its first measurement using an anti-neutrino beam
Clear measurement of muon anti-neutrino disappearance
No clear signal for electron neutrino appearance
Statistics very low
Anti-neutrino measurements are statistically limited
And will be for foreseeable future
More anti-neutrino running is planned
Much more data expected
May have first indication for CP-Violation
Sep 2015
Alfons Weber

55. Back-up