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Neutrinos: No Mass, No Charge? No Problem! Kevin McFarland University of Rochester

Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

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Page 1: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Neutrinos:No Mass, No Charge?

No Problem!

Kevin McFarlandUniversity of Rochester

Page 2: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

The Mysterious Neutrino

Like most people, we physicists enjoy a good mysteryWhen you start investigating a mystery, you rarely know where it is going

if you knew who would be left standing at the end of the slasher flick, what fun would that be?

The story of the neutrino has been and continues to be a good mystery

and I will keep the telling of it simple,appropriate for the hour of the day…

Page 3: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

The Birth of the Neutrino

Wolfgang Pauli

Page 4: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

4th December 1930Dear 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 6Li 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 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...From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge. Unfortunately I will not be able to appear in Tübingen personally, because I am indispensable here due to a ball which will take place in Zürich during the night from December 6 to 7….Your humble servant,W. Pauli

4th December 1930Dear 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 6Li 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 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...From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge.

Your humble servant,W. Pauli

4th December 1930Dear 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 6Li 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 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...From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge.

Your humble servant,W. Pauli

Translation, Please?

Page 5: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Translation, Please?To save the law of conservation of energy?

If the above picture is complete, conservation of energy says β has one energy

but we observe this insteadPauli suggests “neutron” takes away energy!

β-decay

The Energy of the “β”

Page 6: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Who Cares About β-Decay?

To answer that, we have to knowabout the four fundamental forces

Gravityattractive force betweenparticles with mass or energylong range but very weakholds planets, galaxies, etc.together

Page 7: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Who Cares About β-Decay?

Electromagnetismattractive or replusive forcebetween particles with chargelong rangeholds atoms togetherkeeps matter from collapsing under the force of gravity

shockingly important!

Page 8: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Who Cares About β-Decay?

Strong Nuclear Forcethe nucleus of an atom containslots of protons that all repeleach other electromagneticallythe strong force binds themit’s a force that is short-rangebecause it is so strong!

Is it just me, or is it bizarre that matter can ever form with all these competing forces?

Page 9: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Who Cares About β-Decay?Weak Nuclear Force

its exciting role is to, well, make β-decaysthat sounds awfully anticlimactic… who cares?

actually,you do. A lot.

Fusion in the sun requires that a protonturn into a neutron. Inverse of β-decay!Without β-decay, we are stuck where the sun don’t shine…

Page 10: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Wow! Could β-decay beany more important?

actually, yes.to understand why, look atthe particle “periodic table”it has up and downquarks which makeprotons and neutronswhich bind with electrons to make atomsand neutrinos, of course!so what’s all the stuff to the right?

Page 11: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Yeah! What is that Stuff?

there just appear to be threecopies of all the matter thatreally matters…all that distinguishes the“generations” is their mass

-- I.I. Rabi

Page 12: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

A Brief History of the UniverseIn the beginning, the Universe wasvery small and very hot

Why small? Well, if we look at other galaxies, we see they are ALL moving away from us?

It is somethingwe did? No.

How do we know? Doppler.

Page 13: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

A Brief History of the UniverseIn the beginning, very small and very hot

Why hot?When you let a gas expand, it cools…

Now remember mass is energy (E=mc2)And heat is energy too.

Very early in the Universe, it was so hot that the masses of the different generations didn’t matterThen as the universe cools, suddenly generational mass differences were a big deal, and the massive generations needed to shed their extra mass (energy)

Physicists call this sort of thing symmetry breaking

Page 14: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

β-Decay and the UniverseExtra generations must have shed mass by decaying to light generations

Why? Well, we don’tsee the heavy onestoday in the Universe!

And the only way for that to happen is…β-Decay!!Just as neutrons could decay to protons by β-decay, so heavy generations decay to light.

Page 15: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

The Story so FarNeutrinos are essential for β-Decay to occur (Pauli’s idea)β-Decay:

makes the sun shineallows the cold Universe to be made of what we see today

So although we are not made of neutrinos,we wouldn’t be here without them!

Wow… maybe someone should study neutrinos…

Page 16: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

How to Hunt a Neutrino

How do we see any fundamental particle?Electromagneticinteractions kickelectrons awayfrom atomsThis is why radiation is ahealth hazard…But neutrinos don’t have electric charge. They only interact weakly.

Page 17: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

How Weak is Weak?

Weak is, in fact, way weak.A 3 MeV neutrino producedin fusion from the sun will travel

through water, on average, before interacting.The 3 MeV positron (anti-matter electron) produced in the same fusion process will travel 3 cm, on average.

Moral: to find neutrinos, you need a lot of neutrinos and a lot of detector!

Page 18: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Discovery of the Neutrino

Reines and Cowan (1955)Nobel Prize 19951 ton detectorNeutrinos from a nuclearreactor

Page 19: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Is there an easier way?

Why, yes! Leave it to Star Trek to point the way!Apparently, according to severalepisodes, Lt. Jordy LaForge’s VISORcan actually detect “neutrino fieldemissions”

and what do we do in science exceptemulate Star Trek?

So, let’s go “neutrino field emission” hunting!

Page 20: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Where are Neutrinos Found?Anywhere there are weak interactions!

The early UniverseDecays of heavy generationsleft a waste trail of 100/cm3 ofeach neutrino speciesThey are (now) very cold andslow and hard to detectBut if they have even a very small mass, theymake up much of the weight of the Universe

Page 21: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Where are Neutrinos Found?In the sun

If the sun shinesby fusion, energy reaching earth in light and in neutrinos is similar100 billion neutrinos per cm2 per second rain on us

Supernova 1987A (150000 light years away) exploded, releasing 100 times the neutrinos the sun will emit in its whole lifetime

we observed 11 neutrinos in detectors on earth!

Page 22: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Where are Neutrinos Found?Bananas?

We each contain about 20mg of 40K which is unstable and undergoes β decaySo each of us emits 0.3 billion neutrinos/sec

For the same reason, the radioactivityof the earth results in 10 millionneutrinos per cm2 per second here

Page 23: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Where are Neutrinos Found?Cosmic Rays

Cosmic rays from galaxyEach particle (mostly protons)has many GeV of energyCollisions in upper atmospherecreate particles which decay(weakly) to neutrinos

Can use same technique to produceneutrinos at accelerators

Page 24: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Is there no escape from Neutrinos?

Cosmic GallNeutrinos, they are very small.They have no charge and have no massAnd do not interact at all.The earth is just a silly ballTo them, through which they simply pass,Like dustmaids down a drafty hallOr photons through a sheet of glass.They snub the most exquisite gas,Ignore the most substantial wall,Cold-shoulder steel and sounding brass,Insult the stallion in his stall,

And, scorning barriers of class,Infiltrate you and me! Like tallAnd painless guillotines, they fallDown through our heads into the grass.At night, they enter at NepalAnd pierce the lover and his lassFrom underneath the bed - you callIt wonderful; I call it crass.

– John Updike

Page 25: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

νs… what are they good for?Neutrinos only feel the weak force

a great way to study the weak force!or applications of weak forces (i.e., the sun)

Is there just one weak interaction?one weak interaction (β decay, n p+e-+ν)connects electrons and neutrinos

but wait… there’s more. Another weak force discovered with νs!

ν

Gargamelle, event from neutral weak force

Page 26: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

What about this other weak force?

It turns out that this weak force was the “prediction” of a theory that unified the electromagnetic and weak forces

(Glashow, Salam, Weinberg, Nobel 1979)We still don’t know how to add the strong force

and gravity to this picture“unification” still drives muchof particle physics

Page 27: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

A confusing aside…(made in Rochester)

The basics of this neutral force are as expectedhowever…

… concluded the neutral weak force isa tiny bit too weak

NuTeVExperiment

(Profs. Bodek &McFarland atRochester)

Studied 1.7M neutrino and 0.35M anti-

neutrino interactions

Page 28: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Modern Neutrino HuntingRadiochemical Detector

Ray Davis (Nobel prize, 2002)ν+n p+e- (stimulated β-decay)Use this to produce an unstable isotope,ν+37Cl 37Ar+e- , which has 35 day half-life

Put 615 tons ofPerchloroethylenein a mine

expect one 37Ar atomevery 17 hours.

Page 29: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Modern Neutrino HuntingRan from 1969-1998Confirmed that sun shines from fusionBut found 1/3 of ν !

Page 30: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Modern Neutrino HuntingSuper-Kamiokande(Masatoshi Koshiba,UR PhD 1955, Nobel 2002)

Page 31: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Modern Neutrino HuntingThe Sun, imaged in neutrinos, bySuper-Kamiokande

The Sun, optical imageExistence of the sun confirmed by neutrinos!

Page 32: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Neutrino FlavorRemember that neutrinos were discovered by

the appearance of the positron is noaccident! it turns there are threeneutrinos, eachassociated with aparticular flavor

OK… so here’s a question…

p e nν +→

eeμ

νν μν ττ

↔↔↔

Page 33: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Would the real neutrino please stand up?

Are these neutrinos “of definite flavor”the “real neutrinos”

i.e., is a neutrino flavor eigenstate inan eigenstate of the neutrino mass matrix

Or are we looking at neutrino puree?

And of course, “why does anyone care?”

flavor flavor,mass eigenstates,

i iiUν ν= ∑

eeμ

νν μν ττ

↔↔↔

Page 34: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Neutrino FlavorMixing

What if neutrinosmixed?

“normal modes”not a or bbut a mix

We havelearned thisphenomenology!This is called

“neutrino flavor oscillation”a→b→a

Page 35: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

The Role of Neutrino Mass

There is an important condition for oscillation…

… the masses of the different mass eigenstates must be distinct!

Page 36: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Summary of Neutrino Oscillations

If neutrinos mass states mixto form flavors

and the masses are different…This would explain the disappearing solar νs!

since only electron flavor neutrinos make the detection reaction, ν+n→p+e-, occur

ννντ

flavor,mass eige

fn

lavstates,

or

i iiU νν = ∑

Page 37: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

More Neutrino Flavor ChangesPions decay to make amuon flavored neutrinoMuons decay to makeone muon and one electronflavored each

A very robust prediction

Page 38: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

What does a neutrino from the atmosphere look like?

Muons or electronsproduced in inverseβ-decay are goingnear cThis exceeds speedof light in water, soget Cerenkov lightCones of light (thinka boat wake in 3-D)intersect wall ofdetector and give rings

Page 39: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Atmospheric Neutrino Oscillations

Muon like neutrinos going through earth “disappear”

Takes time to happen

Page 40: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Future Neutrino HuntingNew Ideas afootProduce neutrinos at accelerators, send them long distances to massive detectorsGoal: study differencesbetween neutrinos andanti-neutrinos

Page 41: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Why Neutrinos and Anti-Neutrinos?Every fundamental particle has an anti-matter partner

When the meet, they annihilate into pure energy. Alternatively, energy can become matter plus anti-matter

Page 42: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

So you might ask…The early Universe had a lot of energy. Where is the anti-matter in the Universe?Good question… how do we know it isn’t around today?

look for annihilations.As far away as we can tell, today there aren’t big matter and anti-matter collisions

Page 43: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Our New Goal

Prove or disprove the hypothesis:neutrinos cause the matter anti-matter

asymmetry in the Universe!We are using accelerators to make neutrinos to study whether or not neutrino anti-neutrino differences seeded this as the Universe cooled…

Page 44: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

What does it take?Megawatts of acceleratedprotons to produce neutrinos

e.g., T2K beam: 0.8-4.0 MW100-1000kTon detectors,hundreds of km from source

1MTon is a cube of water,100 meters on a side

Experiments with 107 neutrinosseen to precisely measure howthey interact

MINERvA at FNAL, led by Rochester

UNO neutrino detector concept

~2010

~2020

~2008

Page 45: Kevin McFarland University of Rochesterpavone/particle-www/lectures... · The continuous beta spectrum would then become understandable by the ... Very early in the Universe, it was

Conclusions

Neutrinos exist! They are everywhere, so we’d better learn to live with them!Neutrino interferometry is established and now is a tool for studying neutrinos

long-term goal is to demonstrate matter andanti-matter differences can this seed the same asymmetry in the Universe?

The mystery continues…