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Neutrino Physics

Pedro OchoaMay 15th 2006

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I. Historical BackgroundJames Chadwick

Radioactive beta decay as understood in the twenties:

epn

Do you see any problems with this picture?

Energy conservation !

(also) Recoil of proton not always opposite to electron

(also) Spin seemed non-conserved

YES !

Observed electron (positron) spectrum

like for example in eNiCo 6060

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I agree that my remedy could seem incredible because one should have seen those neutrons very earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honored predecessor, Mr Debye, who told me recently in Bruxelles: "Oh, It's well better not to think to this at all, like new taxes". From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge. Unfortunately, I cannot appear in Tubingen personally since I am indispensable here in Zurich because of a ball on the night of 6/7 December. With my best regards to you, and also to Mr Back.

Your humble servant. W. Pauli

Wolfgang Pauli

Dear Radioactive Ladies and Gentlemen,

As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, how because of the "wrong" statistics of the N and Li6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and the electron is constant...

Note: In 1933 Pauli recognized the possibility of neutrinos having zero mass.

Do you know why they were not named neutrons after all?

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In 1934, Hans Bethe and Rudolf Peierls showed that the cross-section (related to the interaction probability) between neutrinos and matter should be extremely small…. BILLIONS of time smaller than that of an electron.

Most people thought this “neutrino” was never to be observed…

Never say never !

In 1953-56, Frederick Reines and Clyde Cowan made the first observation of electron antineutrinos.

How?

Because of tiny cross-section, need very abundant flux of neutrinos and/or large detector:

2 choices; go near a:-Nuclear bomb

-Nuclear plant

They chose the nuclear plant of Hanford, Washington (and later on Savannah river, SC)

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2 things happen after a neutrino interacts in the detector:

enpe

CdCdCdn 109109108 *

F. Reines got the Nobel Prize in 1995 for his contributions to neutrino physics.

ee

The detection of a gamma after 5µs of the detection of the initial gamma pair provided a unique signature for antineutrino events.

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A question remained: Are the neutrinos associated with the electron (i.e. from beta decay) different than the ones associated with the muon (i.e. pion decay)?

In modern terms: ? eEarlier failed attempts to observe the reaction suggested that even if the weak coupling appeared to be universal, the two neutrino species were different.

e

L. Lederman, M. Schwartz and J. Steinberger (Nobel Prize 1988), along with other collaborators answered this question, by showing that

pn goes, butpen

does not go!

Schematic of the experimental apparatus used at the Alternating Gradient

Synchrotron at BNL

muons leave nice tracksBeam made mostly of

In 34/40 interactions, they got a muon !

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It wasn’t until 2000 that the DONUT collaboration reported the observation of the tau neutrino:

Schematic of the DONUT beam at Fermilab

Observed

This concept for making a neutrino beam is very similar to NuMI, the beam aimed at MINOS.

ud

in their detector(5 interactions!)

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But not everything added up !

Since 1969 a physicist named Ray Davis tried to catch a few electron neutrinos from the sun every year through the reaction (Argon is a radioactive noble gas with half life ~35 days)

600 tons of chlorine

expectation based on solar model

AreCle3737

Only ~1/2 of the expected neutrinos were found !!! Later, GALLEX, SAGE and KAMIOKANDE reported similar results.

Either the solar model was wrong or…. (see next slide)

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II. Neutrino oscillations

Underlying principle: weak eigenstates mass eigenstates

The oscillation probability is given by:

where E[GeV], L[km], [ ], and2ijm

2eV

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Let’s see what this gives for the 2 flavor model (see board & next slide).

Do you understand this “mixing” concept?

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We have:

We obtain:

Do these oscillations happen for real? We’ll try to answer this question…

0

222

sin)2(sin)(L

Lt

0

222sin)2(sin1)(

L

Lte

20

4

m

pL

21

21

cossin

sincos

e

21 sincos)( 21 tiEtiE eet

e )0(If

then

where

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But before answering let’s have a word on cosmic rays…

cosmic rays (protons mostly)strike earth from

all directions

Note that:

Neutrinos produced by:

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Movie time !

1TeV proton shower on Chicago

http://astro.uchicago.edu/cosmus/projects/aires/

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The Super-Kamiokande Experiment

So cosmic rays give us a practically isotropic flux of muon neutrinos at the earth’s surface ! The Super-K experiment uses those neutrinos to study neutrino oscillations:

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Muon like event Electron like event

Two examples of events at SK:

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expectedobservedbest fit

What they observed (1998):

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The interpretation: Observation of oscillations!

E

Lmt

4sin)2(sin)(

2222

92.0)2(sin

104.3105.12

23223

eVmeV

Such that:

at 90% confidence level.

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The SNO Experiment

In 2001 the SNO collaboration announced that they observed:

1) ~1/3 of the electron neutrinos expected according to the solar model 2) ~exact flux of all types of neutrinos expected according to the model.

The electron neutrinos are also changing flavor !

1kton of heavy water

Charged current interaction (through

W)

Sensitive to

Neutral current interaction (through Z)

Sensitive to

e

x

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735 km

Far detectorNUMI beam & Near detector

Fermilab, IL Soudan, MN

250

200

150

100

50

0

# o

f C

C e

ven

ts

NUMI beam

Near detector

120 GeV protons from the Main Injector

Measures the unoscillated energy spectrum

Far detectorMeasures the oscillated energy spectrum

22 002.0 eVm 9.0)2(sin2

)(GeVE0 10 20

The MINOS Experiment

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How do you make a beam of neutrinos?

Focus positively charged particles

Hadrons decay into neutrinos (and

other stuff)

non-neutrino stuff gets absorbed

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Coil

Veto Shield

Far Detector Near Detector

The two detectors:

5.4 kton mass, 8x8x30m484 steel/scintillator planes

1 kton mass 3.8x4.8x15m282 steel and 153 scintillator planes

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What MINOS has seen (2006):

E

Lmt

4sin)2(sin1)(

222

2

completely consistent with:

E (GeV)

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The future for MINOS

MINOS confirmed the hypothesis of oscillations and will make a 10% measurement of :

2m

2006 results