Neutrinos: No Mass, No Charge No Problem

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NeutrinosNo Mass, No Charge?No Problem!

• Prof. Kevin McFarland
• Experimental HEP Group
• University of Rochester

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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

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The Birth of the Neutrino

• Wolfgang Pauli

4
Translation, Please?
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 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 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 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 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 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
5
Translation, Please?

• To save the law of conservation of energy?
• If the above picture is complete, conservation of
  energy says ß 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 cant
  decay to an spin½ proton spin½ electron

ß-decay
The Energy of the ß
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Who Cares About ß-Decay?

• To answer that, we have to knowabout the four
  fundamental forces
• Gravity
• attractive force betweenparticles with mass or
  energy
• long range but very weak
• holds planets, galaxies, etc.together

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Who Cares About ß-Decay?

• Electromagnetism
• attractive or replusive forcebetween particles
  with charge
• long range
• holds atoms together
• keeps matter from collapsing under the force of
  gravity
• shockingly important!

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Who Cares About ß-Decay?

• Strong Nuclear Force
• the nucleus of an atom containslots of protons
  that all repeleach other electromagnetically
• the strong force binds them
• its a force that is short-rangebecause it is so
  strong!
• Gravity, Electromagnetism and the Strong Force
  are responsible for the structure of matter!

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Who Cares About ß-Decay?

• Weak Nuclear Force
• its exciting role is to, well, make ß-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 ß-decay!
• Without ß-decay, we are stuck where the sun dont
  shine

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Wow! Could ß-decay beany more important?

• actually, yes.
• to understand why, look atthe particle periodic
  table
• it has up and downquarks which makeprotons and
  neutrons
• which bind with electrons to make atoms
• and neutrinos, of course!
• so whats all the stuff to the right?

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Yeah! What is that Stuff?

• there just appear to be threecopies of all the
  matter thatreally matters
• all that distinguishes thegenerations is their
  mass

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A Brief History of the Universe

• In 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? Redshift

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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 (Emc2)
• And heat is energy too.
• Very early in the Universe, it was so hot that
  the masses of the different generations didnt
  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

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ß-Decay and the Universe

• Extra generations must have shed mass by decaying
  to light generations
• Why? Well, we dontsee the heavy onestoday in
  the Universe!
• And the only way for that to happen is
• ß-Decay!!
• Just as neutrons could decay to protons
  byß-decay, so heavy generations decay to light.

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The Story so Far

• Neutrinos are essential for ß-Decay to occur
  (Paulis idea)
• ß-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 wouldnt be here without them!
• Wow maybe someone should study neutrinos

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How to Hunt a Neutrino

• How do we see any fundamental particle?
• Electromagneticinteractions kickelectrons
  awayfrom atoms
• This is why radiation is ahealth hazard
• But neutrinos dont have electric charge. They
  only interact weakly.

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How Weak is Weak?

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

53 light-years
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Discovery of the Neutrino

• Reines and Cowan (1955)
• Nobel Prize 1995
• 1 ton detector
• Neutrinos from a nuclearreactor

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Is there an easier way?

• Why, yes! Leave it to Star Trek to point the
  way!
• Apparently, according to severalepisodes, Lt.
  Jordy LaForges VISORcan actually detect
  neutrino fieldemissions
• and what do we do in science exceptemulate Star
  Trek?
• So, lets go neutrino field emission hunting!

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Where are Neutrinos Found?

• We should find neutrinos anywhere there are weak
  interactions!
• The early Universe
• Decays of heavy generationsleft 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

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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!

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Where are Neutrinos Found?

• Bananas?
• We each contain about 20mg of 40K which is
  unstable and undergoes ß 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

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Where are Neutrinos Found?

• Cosmic Rays
• Cosmic rays from galaxy
• Each particle (mostly protons)has many GeV of
  energy
• Collisions in upper atmospherecreate particles
  which decay(weakly) to neutrinos
• Can use the same technique to produceneutrinos
  at accelerators

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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

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ns 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 (b decay, n?pe-?)connects
  electrons and neutrinos
• but wait theres more. Another weak force
  discovered with ns!

Gargamelle, event from neutral weak force
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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 dont know how to add the strong force
  and gravity to this picture
• unification still drives muchof particle
  physics

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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

NuTeV Experiment (Profs. Bodek McFarland
at Rochester) Studied 1.7M neutrino and 0.35M
anti-neutrino interactions
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Solar Neutrino Hunting

• Radiochemical Detector Ray Davis (Nobel prize,
  2002)
• ?n?pe- (stimulated ß-decay)
• Use this to produce an unstable isotope,
  ?37Cl?37Are- , which has 35 day half-life
• Put 615 tons ofPerchloroethylenein a mine
• expect one 37Ar atomevery 17 hours.

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Modern Neutrino Hunting

• Ran from 1969-1998
• Confirmed that sun shines from fusion
• But found 1/3 of ? !

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Modern Solar Neutrino Hunting

• Super-Kamiokande(Masatoshi Koshiba, UR PhD 1955,
  Nobel Laureate 2002)

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Modern Neutrino Hunting

• The Sun, imaged in neutrinos, bySuper-Kamiokande

The Sun, optical image
Existence of the sun confirmed by neutrinos!
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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 heres a question

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Would the real neutrino please stand up?

• Are these neutrinos of definite flavorthe
  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?

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Neutrino FlavorMixing

• What if neutrinosmixed?
• normal modesnot a or bbut a mix
• We havelearned thisphenomenology!

This is called neutrino flavor
oscillationa?b?a
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The Role of Neutrino Mass

• There is an important condition for
  oscillation the masses of the different
  mass eigenstates must be distinct!

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Summary of Neutrino Oscillations

• If neutrinos mass states mixto form flavors
• and the masses are different
• This would explain the disappearing solar ns!
• since only electron flavor neutrinos make the
  detection reaction, ?n?pe-, occur

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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

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Schoedinger-ology (contd)

• Phase difference
• Phase difference leads to interference effect,
  just like with sound waves
• Analog of volume disappearing in beats is
  original neutrino flavor disappearing

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More Neutrino Flavor Changes

• Pions decay to make amuon flavored neutrino
• Muons decay to makeone muon and one
  electronflavored each

• A very robust prediction

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What does a neutrino from the atmosphere look
like?

• Muons or electronsproduced in inverseb-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

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Atmospheric Neutrino Oscillations

• Muon like neutrinos going through earth
  disappear
• probably change to tau neutrinos

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Future Neutrino Hunting

• New Ideas afoot
• Produce neutrinos at accelerators, send them long
  distances to massive detectors
• Goal study differencesbetween neutrinos
  andanti-neutrinos

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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

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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 isnt around
  today?
• look for annihilations.
• As far away as we can tell, today there arent
  big matter and anti-matter collisions

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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

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What does it take?

• Megawatts of acceleratedprotons 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 neutrinosseen to precisely
  measure howthey interact
• MINERvA at FNAL, led by Rochester

2010
UNO neutrino detector concept
2020
2008
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Conclusions

• Neutrinos exist! They are everywhere, so wed
  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