Oscillating Neutrinos and T2K

UVic Physics/Astronomy Colloquium

Dean Karlen / University of Victoria & TRIUMF

Oscillating Neutrinos and T2K

Brief history of neutrinos

September 14, 2011 Oscillating Neutrinos and T2K Dean Karlen / University of Victoria and TRIUMF 2

• Neutrinos were postulated by W. Pauli in 1930 to explain the apparent energy non-conservation in nuclear beta decay • suggested that neutrinos only rarely interact with matter

• able to carry away energy undetected

• In the 1950s, Reines and Cowan (at Los Alamos) worked to develop an experiment that could detect neutrinos: • first idea: detonate a 20 kton fission bomb!

• approved, but not attempted

• next idea: use a nuclear reactor as a steady high intensity source

Discovery of neutrinos


• Reines and Cowan succeeded in detecting neutrinos at the Savannah River nuclear reactor in 1956 • reaction: inverse beta decay: prompt + delayed coincidence

• two 200 L tanks with water and cadmium salt to reduce n capture time to < 10 µs

• sandwiched by three scintillator tanks to detect the gammas

• (cf. T2K near detector tracker!)

p n

Cd

νe e+

e−

<10 µs

Reines and Cowan experiment


• Results: • signal/background: 3:1

• predicted cross section (1956):

(6.3 ± 1.5) × 10-44 cm2

• observed cross section:

6 × 10-44 cm2 (uncertainty ~ 5%)

• Agreement with theory too good? • 1957 discovery of parity non-conservation in weak interactions:

• revised IBD cross section: (10.0 ± 1.7) × 10-44 cm2

• subsequent reanalysis of the original data: 12−4+7 × 10−44 cm2

More than one kind of neutrino


• In 1962, the existence of a second kind of neutrino, νµ, was established by an experiment that directed high energy protons from the BNL AGS onto a target • neutrinos produced by the decay

of secondary hadrons tended to produce penetrating muons in the downstream detectors:

• 34 muon like, at most 5 electron like

Neutrino interactions


• Neutrinos only interact via the “weak” force • weak because interaction involves a heavy mediator (W or Z)

• eg: neutrino from beta decay

• eg: neutrino interactions in matter νe e−

n p

W

νµ µ−

n p

W

νe

e−

n p

W

Neutrinos change their identity


• In the 1980s and 90s, experiments were performed to measure the rate of νe and νµ interactions from natural neutrino sources: • neutrinos produced in the nuclear reactions in sun

• neutrinos produced in cosmic ray interactions with the atmosphere

⇒ The results did not agree with “Standard Model” predictions

• The prevailing theory of neutrino properties had to be modified to explain the results… • when produced, a neutrino has a definite type (flavour)

• electron, muon, or tau (depending on the charged lepton involved)

• when the neutrino interacts at some later time, it may produce a different type of charged lepton as if it changed its identity

• lepton flavour not conserved: the neutrino “oscillated”

PMNS explanation


• In the 1960s, Pontecorvo, Maki, Nakagawa, and Sakata postulated that neutrinos might behave that way…

• states of definite flavour = linear combination of states of definite mass

• if the masses differ, they get out of phase, leading to neutrino oscillation

• Solar and cosmic experiments: • 𝜃12 = 34 ± 2° , 𝜃23 = 45 ± 7°

• Δ𝑚212 = 8.0 ± 0.5 × 10−5 eV2 , Δ𝑚32

2 = 2.4 ± 0.1 × 10−3 eV2

• Reactor experiments: 𝜃13 < 11° @ 95% CL : Is it zero?

|𝜈𝛼⟩ = �𝑈𝛼𝛼∗ |𝜈𝛼⟩3

𝛼=1

𝛼 = 𝑒, 𝜇, 𝜏 𝑖 = 1,2,3

𝑈 =1 0 00 𝑐23 𝑠230 −𝑠23 𝑐23

×𝑐13 0 𝑠13𝑒−𝛼𝛿0 1 0

−𝑠13𝑒−𝛼𝛿 0 𝑐13×

𝑐12 𝑠12 0−𝑠12 𝑐12 0

0 0 1

𝑠23 = sin𝜃23 𝑐23 = cos𝜃23

θ13 oscillation experiments


• Experiments that measure neutrino oscillation involving νe over a suitable baseline can be sensitive to θ13 • Nuclear reactors produce a high flux of 𝜈𝑒 with E ~ 1 MeV

• Accelerators can produce high flux of νµ with E ~ 1 GeV

• baselines of several 100 km required

1 − 𝑃 𝜈𝑒� → 𝜈𝑒� ≈ sin2 2𝜃13 sin2Δ𝑚31

2 𝐿4𝐸𝜈

+ cos4 𝜃13 sin2 2𝜃12 sin2Δ𝑚21

2 𝐿4𝐸𝜈

Δ𝑚312 𝐿

4𝐸𝜈= 1.27

Δ𝑚312

eV2𝐿

kmGeV𝐸𝜈

∼𝜋2

⇒ 𝐿 ∼ 0.5 km 𝐸𝜈

MeV

Reactor measurements


• Three new experiments are just getting underway: • Double Chooz (France), Reno (Korea), Daya Bay (China)

• larger detectors: more events + more shielding: less background

• include an identical near detector to reduce sensitivity to modelling the neutrino flux and cross section and the detector properties

1 − 𝑃 𝜈𝑒� → 𝜈𝑒� ≈ sin2 2𝜃13 sin2Δ𝑚31

2 𝐿4𝐸𝜈

+ cos4 𝜃13 sin2 2𝜃12 sin2Δ𝑚21

2 𝐿4𝐸𝜈

sin2 2𝜃13 = 0.1

near far

Reno (Korea)


Daya Bay (China)


• Located near Hong Kong

• Multiple identical detectors at three sites: • 4 detectors at far site, 2 detectors at the 2 near locations

• compare detectors at same site to confirm systematic errors

• tunnels between detector locations allow them to be exchanged if the systematic errors dictate

• Daya Bay near detector started data taking August 15, 2011

• Ling Ao near detector to start later this year

• Far detector to start summer 2012

Daya Bay detectors


• “standard” 4 concentric volumes • outermost volume: water pool

• detectors performing well

Comparison of reactor experiments


• Specifications:

• Start of physics data collection:

Experiment Power (GW)

L near/far (m)

Depth n/f (mwe)

Target mass (tons)

sin22θ13 sensitivity*

Double Chooz 8.7 400/1050 110/300 8.3/8.3 0.03 RENO 16.4 290/1380 120/450 16/16 0.02 Daya Bay 17.7 360-1980 270/910 40,40/80 0.01

* 90% CL upper limit if θ13 = 0 (after 3 years running)

Experiment Near detector(s) Far detector Double Chooz end/2012 4/2011 RENO 8/2011 8/2011 Daya Bay 8/2011 + fall/2011 summer/2012

Significant competition between similar detectors over the coming years

Accelerator experiments


• An accelerator neutrino experiment can estimate θ13 by measuring the νe appearance probability: • start with a nearly pure beam of νµ

• several 100 km away, detect and distinguish νe and νµ

𝑃 𝜈𝜇 → 𝜈𝑒 ≈ sin2 𝜃23 sin2 2𝜃13 sin2Δ𝑚23

2 𝐿4𝐸𝜈

T2K


• The T2K project, approved in December 2003, arose from a fortunate coincidence of two major facilities needed for neutrino oscillation experiment separated by an appropriate distance: • The Japan Proton Accelerator Research Center (JPARC): the world’s

highest intensity proton beam

• Super Kamiokande: the world’s largest water Cherenkov detector

JPARC and neutrino beamline Super Kamiokande

T2K – an off axis experiment


• The beam axis is directed 2.5° away from SK to: • increase the flux of ~ 0.6 GeV neutrinos at SK – those most sensitive

to oscillation parameters for a 295 km baseline

• decrease the flux of higher energy neutrinos at SK – which can be mis-reconstructed as lower energy neutrinos (background)

Inte

ract

ion

rate

(arb

uni

ts)

𝐸𝜈 (GeV)

relation between Eν and pπ at fixed angles observed Eν spectrum at SK

Neutrino detection


• Charged Current interactions identify the flavour of the neutrino • For neutrinos with E ~ 0.6 GeV, the dominant cross section is

CC quasi- elastic (CCQE)

• The neutrino energy is estimated by measuring the momentum of the charged lepton and using momentum conservation

νl l−

n p

W

T2K - overview


• Several detector systems in place to monitor beam properties and stability: • beamline monitors: proton beam position and direction at target

• muon monitor: direction of µ from hadron decays

• on-axis near detector: ν direction and rate

• off-axis near detector: ν spectrum, rate, purity, ν interaction studies

30 GeV proton beam from J-PARC MR

π+,K+

ν

120m 0m 280m 295 km

off-axis μ-mon

ν

Near detectors SK

T2K - beamline


beam dump

muon monitor

horn

target

superconducting magnets

near detectors

To SuperK

T2K – near detectors


• On-Axis: INGRID • 14 identical modules

• iron/scintillator sandwich

• 7 tons each

INGRID

ND280

0°

2.5°

T2K – near detectors


• Off-Axis: ND280 • a multipurpose magnetic spectrometer (former UA1 magnet)

• pi-zero detector (P0D)

• tracker: scintillator (+ water) modules (FGDs) sandwiched by 3 TPCs

• electromagnetic calorimeters

• side muon range detectors

• UVic / TRIUMF / UBC groups led the tracker project • design, construction, installation,

operation

T2K – near detector tracker


• Neutrinos interact in one of the 2 fine-grain detectors and the reaction products are detected in the Time Projection Chambers

2.5 m

UVic connections


• The electronics connections for the fine-grained detector were designed and constructed at UVic (thanks to electronics shop)

UVic connections


• Our group led the TPC project from inception

• Still at UVic: TPC prototype built at TRIUMF/UVic in 2005 • proof of concept – full funding for the 3 TPCs followed

UVic connections


• 3 TPCs installed in Japan in 2009

TPCs distinguish electrons from muons with dE/dx

T2K – far detector (Super K)


• Operated since April 1996

• Fiducial volume: 22.5 kton

39 m

42 m

T2K – SK particle identification


SK atmospheric ν data compared with simulation:

T2K - analysis


• Evaluate this by counting the number of νe events at SK:

• 𝑁𝜈𝑒 background: νe contamination of T2K beam, π0 production

• 𝑁𝜈𝜇 no−oscillation: number of νµ events expected with no oscillation

• A complete simulation of the beamline and detectors, with internal and external data, is used to calculate these quantities such as:

• hadron production from target: NA61 experiment and others

• Nνµ interactions in the off-axis near detector

𝑃 𝜈𝜇 → 𝜈𝑒 ≈ sin2 𝜃23 sin2 2𝜃13 sin2Δ𝑚23

2 𝐿4𝐸𝜈

𝑃 𝜈𝜇 → 𝜈𝑒 = 𝑁𝜈𝑒 candidates − 𝑁𝜈𝑒 background𝜀 𝜈𝑒 selection 𝜎 𝜈𝑒

𝑁𝜈𝜇 no−oscillation𝜎 𝜈𝜇

�

T2K dataset


• Physics data collection from March 2010 – March 2011

• Proton intensity reached 145 kW

• Collected a few percent of the total expected dataset …

shutdown

March 11, 2011


• The first results from T2K were to be announced at a special seminar scheduled for 3:00 PM at the Japanese laboratory, KEK

• At 2:46 PM – magnitude 9 earthquake struck, followed by devastating tsunami waves

• No injuries or tsunami damage at the JPARC lab

• No damage to SK

JPARC

Structural damage


• Severe road damage on site

• Little building damage

• Accelerator not severely damaged • Returning to operation by end 2011

T2K On-Axis ND (INGRID)


• νµ interaction rate per POT stable

• direction stable and correctly centred

T2K Off-Axis ND (ND280): example events


T2K Off-Axis ND νµ CCQE selection


• Data compared to simulation (not tuned)

𝑅𝑁𝑁𝜇, Data 𝑅𝑁𝑁

𝜇, MC� = 1.036 ± 0.028 (stat) −0.037+0.044 (det. sys) ± 0.038 (phys. model)

T2K-SK event selection


• Fully contained events (start and stop inside fiducial volume) • in time with neutrino beam from J-PARC



• SK was well understood and well calibrated prior to T2K • All νe candidate selection criteria set prior to data collection:

• in time, fully contained events consistent with a single ring, plus:



• A total of 6 candidates selected • expected number if θ13 = 0 is 1.5 ± 0.3

• probability to see 6 or more events if θ13 = 0 is 0.007 (p-value) → 2.5σ

T2K-SK candidate event #1


• View from inside water tank • grey dots: unhit PMTs

• color: arrival time

• size: light collected

T2K-SK event distributions


• expect a uniform distribution in R2: K-S test p-value = 0.03 • do not see excess events outside fiducial volume

fiducial volume

T2K confidence intervals


• Additional factors that affect the oscillation probability • CP violation parameter δ

• mass hierarchy (normal or inverted)

• Chooz upper limit shown (in red) • independent of δ and mass

hierarchy

• T2K publication: • Phys. Rev. Lett. 107, 041801 (2011)

(released June 15, 2011)

sin2 2𝜃23 = 1 Δ𝑚23

2 = 2.4 × 10−3eV2

Global neutrino analysis


• Fogli et al, recently updated their analysis of all oscillation data • Find the significance for θ13 > 0 is more than 3σ

• Incorporating new models for reactor flux increases significance

• the near detectors at the new reactor experiments will check flux models

G.L. Fogli, E. Lisi, A. Marrone, A. Palazzo, and A.M. Rotunno arXiv:1106.6028v1

Summary


• Significant advances in measuring θ13 have come about in the past few years • precision reactor experiments are coming on-line

• T2K has given first strong indication that 𝜃13 ≠ 0

• In the coming years – watch to see how the different oscillation measurements compare with each other, and whether the PMNS description continues to hold

• Provided θ13 is large enough, new projects may move ahead to definitively measure the amount of CP violation in the lepton sector • important input to understanding the origin of the matter/anti-matter

asymmetry in the Universe

Acknowledgements


• T2K – International: • Canada, France, Germany, Italy, Japan, Korea, Poland, Russia,

Spain, Switzerland, UK, USA

• T2K – Canada: • UVic, UBC, TRIUMF, U Alberta, U Regina, York U, U Toronto

• T2K – UVic: • For TPC/FGD construction:

• P. Birney, C. Bojechko, N. Braam, K. Fransham, A. Gaudin, C. Hansen, R. Hasanen, N. Honkanen, DK, R. Langstaff, M. Lenckowski, J. Myslik, M. Pfleger, P. Poffenberger, M. Roney, V. Tvaskis

• Continuing with T2K Operation and Physics Analyses:

• C. Bojechko, A. Gaudin, A. Hillairet, DK, J. Myslik

MINOS result


• 62 events selected

• Fit energy and PID discriminant (LEM):

MINOS - T2K comparison


L. Whitehead BNL / MINOS

Prediction with T2K’s best fit point: Overlay of MINOS and T2K contours:

NOvA


• The next generation long baseline experiment at Fermilab is expected to start taking data in 2014

• When combined with T2K data, sensitive to δ and mass ordering

G. Feldman

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Oscillating Neutrinos and T2K

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