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Neutrino Physics 2Pedro OchoaMay 22nd 2006

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What about solar neutrinos and the solar neutrino problem?

KamLAND usesthe entire Japanese

nuclear powerindustry as a

long baseline source

Kashiwazaki

Takahama

Ohi

Kamland is an experiment which studies the disappearance of reactor neutrinos

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In a fission reactor, there is a flux of associated with 235U, 239Pu, 241Pu and 238U that you can predict to a good accuracy.

ev

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You can detect these antineutrinos via inverse beta decay: enpve

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How the detector looks from the inside

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So if there are oscillations, this spectrum will be distorted:

ELmvvP ee 4

sin2sin1cos)(2122

122

134

At Kamland’s average L of about 180 km, the disappearance probability in the three neutrino model is, to a very good approximation:

Notice similarity with 2 flavor approx.

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Oscillations were observed indeed (2002)!! Kamland was actually the first experiment to observe the disappearance of “earthly” electron antineutrinos

Other experiments hand’t seen anything (they were too close)

Conclusive evidence of reactor disappearance:ev

)(047.0)(044.0658.0 syststatN

NN

oscno

bkgdobs

%998.99( C.L.) 45.0tan

109.7

122

25212

eVmBest fit values:

The solar neutrino problem was finally solved !

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III. Open QuestionsWhat are some of the unsolved problems in Neutrino Physics?

Everything fits the model extremely well !

Things make sense in the light that neutrino mass eigenstates mix with the weak eigenstates creating the oscillation phenomenon measured in many experiments. We think that:

45.0tan

108

122

25212

eVm9.0)2(sin

103

232

23223

eVm

Well, almost…. there is always a black sheep !!

We know that neutrinos have mass !!!!!

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Baseline ~30 m

Neutrino Energy 20-55 MeV,

Let’s first discuss “The LSND Anomaly” (1995)

nepe

detect prompt e track, 20<Ee<60 MeV

(+ scintillation)

ee

eOscillations?

• Search for throughe

• neutron capture:

dnp 2.2 MeV scintillation signal, 186µs later

The Liquid Scintillator Neutrino Detector Experiment:

Stop at Cu target

• Beam of protons on water produces π+ mainly

e

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The interior of the LSND detector:

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0.64.229.87

• Through they observed:

• excess of:

• oscillation probability:

evnepve

)%045.0067.0264.0(

Do you see a problem with this picture?

What the LSND experiment saw:

Yes !! Only two independent m2 if three neutrinos !!

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How to explain the LSND anomaly?

1) There are more neutrinos :

But LEP showed that there are three active (i.e. that interact with the Z) neutrino flavors only… the extra neutrinos would have to be sterile !

2) CPT Violation (in other words, ): vv mm

3) Some weird combination (CPT + 1 sterile neutrino, sterile neutrino decay…)

Before it could all nicely fit in a spectrum like

But now it would have to be something like, which is unlikely.

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Other experiments have ruled out parts of the LSND allowed region:

green=unexplored

The MiniBoone experiment at Fermilab will be able to put this issue to rest

D=12m

800 tons of mineral oil

If MiniBoone finds a signal new exciting physics !!

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Let’s change topics now. Earlier I said that:45.0tan

108

122

25212

eVm9.0)2(sin

103

232

23223

eVm

What parameter am I leaving out? 13 !!! Is it zero or just very small?Nobody knows…

U

is directly related to whether or not there is CP violation in the neutrino sector!

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Best limit on θ13 comes from a reactor experiment called CHOOZ:

m2 = 0.0025 eV2

ELmvvP ee 4

sin2sin1)(2232

132

At their baseline (~1km):

MINOS will actually expand that limit (or discover θ13):

At MINOS baseline (~735 km):

ELmvvP e 4

sinsin2sin)(2232

232

132

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NOVA (NuMI Off-Axis Experiment) will be able to assess this much better:

But remember:

ELmvvP e 4

sinsin2sin)(2232

232

132

By going off-axis we can get more neutrinos in the energy region where we’re more interested.

At 14mrad the spectrum peaks just above the first oscillation maximum.

Use the same NuMI beam that used for MINOS !!

Given the current limit of θ13 set by CHOOZ, the oscillation probability cannot be larger than 5%. This is why to study these oscillations we need a monster detector.

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What will be achieved:

NOVA may also address one of the biggest puzzles in neutrino physics:

What is the right hierarchy?(note: we know m2

21 > 0 from solar neutrinos)

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This would be achieved through something called Matter Effect:

The basic concept is that electron neutrinos, besides oscillating in the usual way, can interact with the electrons in rock while they propagate:

evev ee e

eev

evW t

But this will not happen for ,,, vvve

This creates a small difference in the probability of seeing vs. The direction of this effect depends on the mass hierarchy.

ev ev

This effect must be disentangled from possible CP violation, which also implies that )()( ee vvPvvP

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Besides the hierarchy, there’s something about neutrino masses we still don’t know:

??

We don’t know the absolute scale of the neutrino masses !!

There is actually a way of searching for this directly…

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Endpoint energyE=18.57keV

Neutrino mass should affect the spectrum of tritium decay:

An experiment called KATRIN (Karlsruhe Tritium Neutrino Experiment) in Germany will look for this effect.

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KATRIN will be able to measure the neutrino mass down to 0.2eV (90% CL).

Observing this effect is a major technological challenge. The way they’ll do it is with a “MAC-E-Filter”:

The beta electrons are transformed into a broad beam of electrons flying almost parallel to the magnetic field lines.

Because of the electrostatic potential, all electrons with enough energy to pass the barrier will make it to the detector.

Varying the E-field allows to measure the beta spectrum in a integrating mode.

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The other way is through Neutrinoless Double Beta Decay (denoted 0vββ):

2 neutrino Double Beta Decay is actually known to exist and allowed by the Standard Model:

But 0vββ has not been seen (convincingly at least):

What is the condition for this to happen? That neutrinos are their own antiparticle !! (i.e. they are Majorana Particles)

If 0vββ is observed then we’d know for sure that vv

No piece of cake. First calculated in 1935 by M. Goeppert-Mayer, and first observed in 1987 (any ideas why so hard to observe?)

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The 2vββ decay is a major background for the 0vββ search.

It is a very hard measurement !! T1/2 (2vββ) ~ 1020 yearsT1/2 (0vββ) ~ 1025-27 years.

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If 0vββ is observed, would that tell us something about the neutrino mass?

Yes !! The effective neutrino mass (due to mixing) can be disentangled:

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2/1

1mMG

Tv

F

Remember: electron neutrino has no definite mass

Calculating the matrix elements is no picnic, and many authors disagree among themselves:

Called the Matrix Element

Several 0vββ candidates. Each experiment uses a different one (and different technique)

Rodin et al, nucl-th/0503063

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We cannot go through all the proposed experiments that are going to try to measure 0vββ.

However, you should know that somebody claims to have observed it already.

Most sensitive experiments to date are based on germanium 76. This is one of them.

Note that only a subset of the Heidelberg-Moscow collaboration claims the observation of 0vββ. The collaboration actually split over this.

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Would you buy this?

Other experiments will look in the same region to confirm/disprove it.

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SUMMARY & CONCLUSIONS

Neutrino physics is a field full of surprises, and with plenty of room for discovery !!