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The Pesky Neutrino

• A vexing conundrum• A desperate remedy• Some ingenious experiments• Disappearance and oscillations• Current and future quests

David E. JaffePhysics Department

426th Brookhaven Lecture

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Some early history3/1896 Becquerel discovers radioactivity10/1896 Zeeman discovers the electron

(e-)1898 Rutherford identifies alpha-rays()

and beta-rays() from Uranium1899 -rays are shown to be electrons (e-)1900 Villard observes gamma () rays1914 Chadwick observes continuous -ray

energy spectrumRef: A.Pais, Rev.Mod.Phys. 49 (1977) 925.

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The continuous -ray spectrum

Kovarik & McKeehan, Phys.Rev 8 (1916) 574.

Energy of electron

Observed reaction: nucleus (A) decaying to another nucleus (A’) and an electron

A->A’ + e- Expect E(e-) = E(A) – E(A’)

Apparent violation of conservation of energy, the 1st law of thermodynamicsestablished in 1800’s for macroscopic systems. A vexing conundrum indeed!

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“Neutrino” hypothesis1930 Pauli proposes “a desperate

remedy to save…the law of conservation of energy”: a light (mass < m(e-)), neutral, highly-penetrating particle with spin ½. Pauli dubbed it the “neutron”.

1932 Chadwick discovers the neutron (n) with mass ~same as the proton (p)

1932-3 Fermi dubs Pauli’s particle the “neutrino” (). Develops theory of -decay and concludes m()<<m(e-).

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Neutrino () properties

-decay written as A(Z) -> A(Z+1) + e- + or

n->p + e- + : Explains continuous e- spectrum

How “highly penetrating” are neutrinos?Bethe & Peierls calculated the

interaction cross section using Fermi’s theory

Process Cross-section Interaction length

p->e+n 7x10-43 cm2 4x1019 cm(~40 light years)

Pb 10x10-24 cm2 3 cm

Need a massive target and copious source to be able to observe neutrinos

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Sources of neutrinosFusion and fission produce neutrinos

• Nuclear explosion– On/off possible, but destructive to detector

• Nuclear reactor– On/off possible

• The sun – Always on

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Observing neutrinos npv

eeprompt, 2 x 0.511 MeV)

CdCdnCd *

(delayed ~10s, ~6MeV)

Reines et al.,Phys.Rev.117 (1960) 1.

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First observation of neutrinos

Locate detector 12 meters underground to shield detector from cosmic rays and 11 meters from core of nuclear reactor. Accumulate data with reactor on and off.

Reactor onReactor off

Reactor onReactor off

Reines et al.,Phys.Rev.117 (1960) 1.

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The pesky neutrino

Postulated that the recently observed penetrating component of cosmic rays was a “heavy electron” (mass ~200x electron mass) and had a neutrino partner distinct from the partner of the electron.

This “heavy electron” is now called the muon

New puzzle: Are there two kinds of neutrinos?If so, how can we tell?

,e

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Sources of muon neutrinos

• Cosmic rays?– Same source as but always on

• Particle accelerator?– On/off possibleUse proton beam to create pions (direct

thepions towards a massive detector and allow

themto decay

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protons

13.5m iron shielding

Observing muon neutrinos

Target

Ten ton detector composed ofAluminum planes interleaved withspark chambers

A penetrating track appears,probably due to n->-p

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Phys.Rev.Lett. 9 (1962) 36.

Remove 4’ iron shielding,if rate due to neutrons,expect x100 increaseRemove 4’ iron shielding,replace by 4’ lead shielding close totarget to intercept pions before they decayand ‘turn off’ neutrinobeam

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A penetrating track appears,probably due to n->-p

For reactionne-p, an electromagneticshower is expected. The response of two modulesof the detector to a 400 MeV electron beam isshown at right and is distinct from the responseto a muon from n->-p shown below.

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Bruno Pontecorvo, Sov. Phys. JETP 6 (1958) 429

Some of the e from the sun are missing.

The pesky neutrinoExpectedrate of e

from the sun

Average measured rate

Possibility of oscillations in vacuum

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Some 30 years later…

…during which time many experiments clearly establishand quantify neutrino oscillations…

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Present knowledge of neutrinos

It is now clear that– There are 3 lepton doublets:1. electron: (e-,e)

2. muon: (-,)

3. tau:( -,)

– Neutrinos have mass that is much less than the electron mass

– Neutrinos can oscillate or mix; that is, can change into (for example)

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Neutrino oscillations

For the simplified case of 2 neutrinos( , e) the probability that a of energy E in GeV will turn into a e after traveling distance L in km is where in eV2 is the difference in the squares of the 1,2

mass states.

)/27.1(sin2sin)( 222 ELmP e 22

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2 mmm

Neutrinos are produced and detected as weak states…

…but neutrinos propagateas mass states.

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Neutrino oscillationsThe probability that a of energy E in GeV will turn into a e after traveling distance L in km is

where is in eV2

)/27.1(sin2sin)( 222 ELmP e 22

21

2 mmm

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The big picture• One explanation of the small mass is the “see-saw”

mechanism which proposes that every has a very heavy partner

• A consequence of the see-saw mechanism is that neutrinos can be their own antiparticles and violate conservation of lepton number.

• If in the early universe, the decay of is out of equilibrium and violates CP symmetry and lepton number, then a net excess of leptons is generated. This excess of leptons is partially converted to an excess of baryons as the universe cools.

• “CP symmetry” means that the interactions of matter and the interactions of antimatter are identical.

• Example of leptonic CP violation: • CP violation for quarks has been observed, but it is

too small to account for the matter-antimatter asymmetry of the universe. Can leptons do the job?

)()( ee PP

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Neutrino oscillationsNeed a 3x3 matrix to describe oscillations of 3 kinds of neutrinos:

All three angles () and phase (CP) must be non-zero to enable CP violation

ii

i

i

i

eeee

e

e

e

e

U

UUU

UUU

UUU

U

CP

CP

2/

2/1212

1212

1313

1313

2323

2323

3

321

321

321

00

00

001

100

0cossin

0sincos

cos0sin

010

sin0cos

cossin0

sincos0

001

7.06.04.0

7.06.04.0

5.08.0

05.0sin

06.050.0sin

03.031.0sin

10)3.02.2(||

10)3.09.7(

132

232

122

23231

25221

eVm

eVm

)/27.1(sin2sin1)( 231

213

2 ELmP ee

)/27.1(sinsin2sin)( 231

223

213

2 ELmP e

Current knowledgeOf neutrino parameters:

“Electron neutrino appearance”

“Electron neutrino disappearance”

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• High power beam produced by 120 GeV protons from the Main Injector at FNAL

• Two functionally identical detectors:– Near detectorNear detector (ND) at Fermilab

to measure the beam composition and energy spectrum

– Far DetectorFar Detector (FD), 735km away, in the Soudan Mine, Minnesota to search for evidence of oscillations

L=735 km

MMain IInjector NNeutrino OOscillation SSearch

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MINOSMINOS Neutrino production

Nea

r D

etec

tor

~1 km~1 km

Not to scale

21 m

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Neutrino detection

UV UV UV UVSteel

Scintillator

Orthogonal strips

Veto Shield

Coil

2.54 cm thick magnetized (1.2T) steel plates4.1x1cm scintillator strips grouped into orthogonal U,V planes

FAR DETECTOR

Far Det Near DetDanby et al.

Mass(t) 5400 1000 10

Size(m3

)8x8x30

3.8x4.8x16

1.1x2.2x1.5

Planes 484/484steel/scint

282/152 Steel/scint

50/45Al/spark

Danby et al.

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UZ

VZ

3.5m 1.8m 2.3m

Xn Xn Xene

Identifying neutrino interactions in MINOS

Charged current (CC)

Neutral current (NC)

CC e

Simulated MINOS events

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Measure the energy spectrum of charged current (CC) interactions in the far detector and compare with expectation based on the near detector

)/27.1(sinsin1)( 222 ELmP

98.0ionNormalizat

syst) (stat 00.12sin

eV10syst) (stat 74.2m

13.0232

2344.026.0

232

MINOS results are consistent with the“disappearance” of muonneutrinos between the near and far detectors

D.G.Michael et al., Phys. Rev. Lett. 97 (2006) 191801.

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→ e appearance search

1. NC events (primary background) 0 final states in hadronic system produce EM showers

2. Intrinsic beam e are identical to signal

3. High-y CC Hadronic shower dominates; muon track is very short or buried

4. FD: Oscillated generally shower-like; decays to e- ~20% of the time

Signal e candidate identification is basedon characteristic shower shape

1.NC

2.e

beam

3. CC

4. CC

Totalbkgd

signale

osc

9.75 2.2 1.4 1.2 14.5 7.3

Predicted rates based onsimulated results with• Oscillation parameters:

sin2(213) = 0.1|m32|2 = 2.710-3eV2

sin2(223) = 1• POT = 4x1020

e CC Event (MC)

)/27.1(sinsin2sin)( 231

223

213

2 ELmP e

ee ;Source of

beam e

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Determining 13 via disappearance

E

Lm

E

LmP ee 4

sin2sincos4

sin2sin1)(2

21212

213

42

31213

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Use same principles as Reines et al. for the 1st observation of the neutrino.To observe oscillations, we also need 1. Multiple detectors at different distances (L) from the reactors2. To measure the anti-neutrino energy (E)

Daya Bay NPP

• 4 reactor complex at Daya Bay, China currently producing ~23 x1020 e/s • 2 more reactors online in ~2010, for a total of ~35x1020 e/s • Adjacent to mountain, easy to construct tunnels to reach underground labs with sufficient overburden to suppress cosmic rays

Ling Ao II NPP

Ling Ao NPP

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Where To Place The Detectors ?

P(e e ) 1 sin2 213 sin2 m312 L

4E

cos4 13 sin2 212 sin2 m21

2 L

4E

• Place near detector(s) close to reactor(s) to measure raw flux and spectrum of e, reducing reactor-related systematic

• Position a far detector near the first oscillation maximum to get the highest sensitivity, and also be less affected by 12

• Since reactor e are low-energy, it is a disappearance experiment:

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0.1 1 10 100

Nos

c/Nn

o_os

c

Baseline (km)

Large-amplitudeoscillation due to 12

Small-amplitude oscillation due to 13

integrated over E

neardetector

fardetector

Sin2 = 0.1m2

31 = 2.5 x 10-3 eV2

Sin2 = 0.825m2

21 = 8.2 x 10-5 eV2

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Detect Inverse Decay in Gadolinium-loaded liquid

scintillator(LS)

• Time- and energy-tagged signal is a good tool to suppress background events.

• Energy of e is given by:

E Te+ + Tn + (mn - mp) + m e+ Te+ + 1.8

MeV 10-40 keV

The reaction is the inverse -decay in 0.1% Gd-doped liquid scintillator:

Arb

itra

ry

Flux Cross

Sectio

n

Observable Spectrum

From Bemporad, Gratta and Vogel

+ Gd Gd*

Gd + ’s(8 MeV)

(~30μs)

nepe

Reines et al. used neutron capture on Cadmium in water instead of Gadolinium in LS

ee

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Antineutrino detectors: then and now

20 t

Gd-LS

LSoil

1cm thick transparentacrylic vessels

5m

Reines et al.

Daya Bay near

1.3x1013 7x1010 Flux (e/sec/cm2)

11 363Distance to reactor(m)

1.2x1028 1.5x1030 Target protons

15+-2% 98% Positron efficiency

17+-6% 78% Neutron efficiency

36 1100 Events/target/day

The Daya Bay experiment will have8 antineutrino detectors with 20 tons oftarget mass.The inner surface of each detector willbe lined with 192 photomultiplier tubes to detect the energy from the productsof the neutrino inverse beta decay reaction

Roughly to scale

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Chooz = reactor expt with lowest limit on sin2213

Fast = 1 year’s Daya Bay data

Daya Bay = 3 year’s data

MINOS = approximate expectedlimit from appearance search

All limits at 90% confidence level

Sensitivity to sin2213

(1 year)

MIN

OS

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The pesky neutrino• The exotic properties of the neutrino have had

a key role in our understanding of the interaction and properties of matter for nearly 100 years.

• Current and future experiments will expand our knowledge of this elusive particle.

• “…in atomic theory, notwithstanding all the recent progress, we must still be prepared for new surprises.”

- Neils Bohr, Faraday Lecture, 1932

Thanks to Milind Diwan, Dick Hahn, Lauren Hsu, Vladimir Issakov, Steve Kettell,Laurence Littenberg and Minfang Yeh for their help with this presentation.

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Neutrinos in BNL Lectures23rd Brookhaven LectureJanuary 9, 1963Neutrino PhysicsLeon M. Lederman, Physics

73rd Brookhaven LectureMarch 20, 1968The Search for Solar NeutrinosRaymond Davis, Chemistry

132nd Brookhaven LectureJanuary 23, 1976The Brookhaven Solar Neutrino Experiments: Past, Present and FutureRay Davis, Chemistry

136th Brookhaven LectureMay 1976Neutrinos: Charm and SexRobert Palmer, Physics

189th Brookhaven LectureMarch 17, 1982Searching for Neutrino OscillationsMichael Murtagh, Physics

266th Brookhaven LectureJanuary 16, 1991Hunting for Elusive Solar NeutrinosRichard Hahn, Chemistry

419th Brookhaven LectureWednesday, November 15, 2006The Past 20 Years in Neutrino Science: Where Have We Been? Where Do We Go From Here?Richard Hahn, Chemistry Department