Neutrinos: No Mass, No Charge? No Problem! Prof. Kevin McFarland Experimental HEP Group University 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 1930 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 6 Li 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 1930 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 6 Li 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 1930 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 6 Li 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?

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 know about the four fundamental forces Gravity  attractive force between particles with mass or energy  long range but very weak  holds planets, galaxies, etc. together

Who Cares About β-Decay? Electromagnetism  attractive or replusive force between particles with charge  long range  holds atoms together  keeps 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 repel each 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 proton turn into a neutron. Inverse of β-decay!  Without β-decay, we are stuck where the sun don’t shine…

Wow! Could β-decay be any more important? actually, yes. to understand why, look at the particle “periodic table” it has up and down quarks which make protons 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 three copies of all the matter that really 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 hot  Why small? Well, if we look at other galaxies, we see they are ALL moving away from us?  It is something we 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=mc 2 ) 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 generations  Why? Well, we don’t see the heavy ones today 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 shine  allows 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 kick electrons away from atoms This is why radiation is a health 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 1995  1 ton detector  Neutrinos from a nuclear reactor

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 VISOR can actually detect “neutrino field emissions”  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/cm 3 of each neutrino species  They are (now) very cold and slow and hard to detect  But if they have even a very small mass, they make up much of the weight of the Universe

Where are Neutrinos Found? In the sun  If the sun shines by fusion, energy reaching earth in light and in neutrinos is similar  100 billion neutrinos per cm 2 per second rain on us Supernova 1987A ( 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 40 K which is unstable and undergoes β decay  So each of us emits 0.3 billion neutrinos/sec For the same reason, the radioactivity of the earth results in 10 million neutrinos per cm 2 per second here

Where are Neutrinos Found? Cosmic Rays  Cosmic rays from galaxy  Each particle (mostly protons) has many GeV of energy  Collisions in upper atmosphere create particles which decay (weakly) to neutrinos Can use the same technique to produce neutrinos at accelerators

Is there no escape from Neutrinos? Cosmic Gall Neutrinos, 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, n  p+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 much of particle physics

A confusing aside… (made in Rochester) The basics of this neutral force are as expected  however…  … concluded the neutral weak force is a tiny bit too weak NuTeV Experiment (Profs. Bodek & McFarland at Rochester) Studied 1.7M neutrino and 0.35M anti- neutrino interactions

Solar Neutrino Hunting Radiochemical Detector Ray Davis (Nobel prize, 2002)  ν+n  p+e - (stimulated β-decay)  Use this to produce an unstable isotope, ν +37 Cl  37 Ar+e -, which has 35 day half-life  Put 615 tons of Perchloroethylene in a mine expect one 37 Ar atom every 17 hours.

Modern Neutrino Hunting Ran from 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 no accident!  it turns there are three neutrinos, each associated with a particular flavor OK… so here’s a question…

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?”

Neutrino Flavor Mixing What if neutrinos mixed?  “normal modes” not a or b but a mix We have learned this phenomenology! 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 mix to 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

Schoedinger-ology… So each neutrino wavefunction has a time-varying phase in its rest frame Now, imagine you produce a neutrino of definite momentum but is a mixture of two masses, m 1, m 2 so pick up a phase difference in lab frame

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

More Neutrino Flavor Changes Pions decay to make a muon flavored neutrino Muons decay to make one muon and one electron flavored each A very robust prediction

What does a neutrino from the atmosphere look like? Muons or electrons produced in inverse  -decay are going near c This exceeds speed of light in water, so get Cerenkov light Cones of light (think a boat wake in 3-D) intersect wall of detector 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 and anti-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: MW kTon detectors, hundreds of km from source  1MTon is a cube of water, 100 meters on a side Experiments with 10 7 neutrinos seen to precisely measure how they 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…