Neutrinos:No Mass, No Charge?

No Problem!

Prof. Kevin McFarlandExperimental HEP GroupUniversity of Rochester

The Mysterious Neutrino

Like most people, we physicists enjoy a good mystery When 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…

The Birth of the Neutrino

Wolfgang 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. 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 instead

Pauli suggests “neutron” takes away energy! “The exchange theorem of statistics”, by the way, refers to the fact that a spin½ neutron

can’t decay to an spin½ proton + spin½ electron

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 between

particles with mass or energy long range but very weakholds planets, galaxies, etc.

together

Who Cares About β-Decay?

Electromagnetismattractive or replusive force

between particles with charge long rangeholds atoms togetherkeeps matter from collapsing under the force

of gravity shockingly important!

Who Cares About β-Decay?

Strong Nuclear Force the nucleus of an atom contains

lots of protons that all repeleach other electromagnetically

the strong force binds them it’s a force that is short-range

because it is so strong!

Gravity, Electromagnetism and the Strong Force are responsible for the structure of matter!

Who Cares About β-Decay? Weak Nuclear Force

its exciting role is to, well, make β-decays that 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 at

the particle “periodic table” it has up and down

quarks which makeprotons and neutrons

which bind with electrons to make atoms and 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 Universe In the beginning, the Universe was

very small and very hotWhy 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? Redshift…

A Brief History of the Universe In 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 matter

Then as the universe cools, suddenly generational mass differences were a big deal, and the massive generations needed to shed their extra mass (energy)

Particle Physicists call this symmetry breaking

β-Decay and the Universe Extra generations must have shed mass by decaying to

light generationsWhy? Well, we don’t

see 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 Far Neutrinos 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? Electromagnetic

interactions kickelectrons awayfrom atoms

This 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 produced

in 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 nuclear

reactor p e n

Is there an easier way?

Why, yes! Leave it to Star Trek to point the way! Apparently, according to several

episodes, Lt. Jordy LaForge’s VISORcan actually detect “neutrino fieldemissions” and what do we do in science except

emulate Star Trek?

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

Where are Neutrinos Found? We should find neutrinos anywhere there are weak

interactions!

The early Universe Decays of heavy generations

left a waste trail of 100/cm3 ofeach neutrino species

They are (now) very cold andslow and hard to detect

But 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 similar

100 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!

woo-hoo!

Where are Neutrinos Found? Bananas?

We each contain about 20mg of 40K which is unstable and undergoes β decay

So 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 atmosphere

create particles which decay(weakly) to neutrinos

Can use the 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 mass

And do not interact at all.

The earth is just a silly ball

To them, through which they simply pass,

Like dustmaids down a drafty hall

Or 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 tall

And painless guillotines, they fall

Down through our heads into the grass.

At night, they enter at Nepal

And pierce the lover and his lass

From underneath the bed - you call

It wonderful; I call it crass.

– John Updike

s… 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 ( decay, np+e-+ν)

connects electrons and neutrinos

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

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 expected however…

… 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

Solar Neutrino Hunting Radiochemical Detector

Ray Davis (Nobel prize, 2002)ν+np+e- (stimulated β-decay)Use this to produce an unstable isotope,

ν+37Cl37Ar+e- , which has 35 day half-life Put 615 tons of

Perchloroethylenein a mine

expect one 37Ar atomevery 17 hours.

Modern Neutrino Hunting Ran from 1969-1998 Confirmed that sun

shines from fusion But found 1/3 of ν !

Modern Solar Neutrino Hunting Super-Kamiokande

(Masatoshi Koshiba, UR PhD 1955, Nobel Laureate 2002)

Modern Neutrino Hunting The Sun, imaged in neutrinos, by

Super-Kamiokande

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

Neutrino Flavor Remember 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 n

ee

Would the real neutrino please stand up? Are these neutrinos “of definite flavor”

the “real neutrinos” i.e., is a neutrino flavor eigenstate in

an 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 ii

U

ee

Neutrino FlavorMixing What if neutrinos

mixed? “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 s!

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

e

flavor,mass eige

fn

lavstates,

or

i ii

U

Schoedinger-ology…

So each neutrino wavefunctionhas a time-varying phase in its rest frame

Now, imagine you produce a neutrino of definite momentum but is a mixture of two masses, m1, m2

so pick up a phase difference in lab frame

/iEte

22 2 2 1

1 1 2

22 2 2 2

2 2 2

1

1

mE p m p

p

mE p m p

p

2 21 2 1 2( ) ( )

Lci E E i m m

p

Schoedinger-ology (cont’d) Phase difference

Phase difference leads to interference effect, just like with sound waves

Analog of “volume disappearing” in beats is original neutrino flavor disappearing

2 21 2 1 2( ) ( )

Lci E E i m m

E

More Neutrino Flavor Changes Pions decay to make a

muon flavored neutrino Muons decay to make

one muon and one electronflavored each

A very robust prediction

What does a neutrino from the atmosphere look like? Muons or electrons

produced in inverse-decay are goingnear c

This exceeds speedof light in water, soget Cerenkov light

Cones of light (thinka boat wake in 3-D)intersect wall ofdetector and give rings

Atmospheric Neutrino Oscillations

Muon like neutrinos going through earth “disappear” probably change to tau neutrinos

Future Neutrino Hunting New Ideas afoot Produce neutrinos at accelerators, send them

long distances to massive detectors Goal: study differences

between 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 accelerated

protons to produce neutrinos e.g., T2K beam: 0.8-4.0 MW

100-1000kTon detectors,hundreds of km from source 1MTon is a cube of water,

100 meters on a side Experiments with 107 neutrinos

seen 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 and

anti-matter differences can this seed the same asymmetry in the Universe?

The mystery continues…