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Neutrinos:No Mass, No Charge?
No Problem!
Kevin McFarlandUniversity of Rochester
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…
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?
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”
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
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!
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?
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…
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?
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
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.
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
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.
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…
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.
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!
Discovery of the Neutrino
Reines and Cowan (1955)Nobel Prize 19951 ton detectorNeutrinos from a nuclearreactor
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!
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
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!
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
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
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
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
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
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
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.
Modern Neutrino HuntingRan from 1969-1998Confirmed that sun shines from fusionBut found 1/3 of ν !
Modern Neutrino HuntingSuper-Kamiokande(Masatoshi Koshiba,UR PhD 1955, Nobel 2002)
Modern Neutrino HuntingThe Sun, imaged in neutrinos, bySuper-Kamiokande
The Sun, optical imageExistence of the sun confirmed by neutrinos!
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
↔↔↔
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
↔↔↔
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
The Role of Neutrino Mass
There is an important condition for oscillation…
… the masses of the different mass eigenstates must be distinct!
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
eμ
ννντ
flavor,mass eige
fn
lavstates,
or
i iiU νν = ∑
More Neutrino Flavor ChangesPions decay to make amuon flavored neutrinoMuons decay to makeone muon and one electronflavored each
A very robust prediction
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
Atmospheric Neutrino Oscillations
Muon like neutrinos going through earth “disappear”
Takes time to happen
Future Neutrino HuntingNew Ideas afootProduce neutrinos at accelerators, send them long distances to massive detectorsGoal: study differencesbetween neutrinos andanti-neutrinos
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
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
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…
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
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…