The Neutrino Story

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The Neutrino Story. What We Think It Tells Us and Why We May Be Wrong C.P. Burgess. Outline. The Most Feebly-Interacting Observed Particles Why are they observed at all? Basic Neutrino Properties How many types are there? How massive are they? Do they decay? - PowerPoint PPT Presentation

Text of The Neutrino Story

  • The Neutrino Story What We Think It Tells Us and Why We May Be Wrong

    C.P. Burgess

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • You Live in a Neutrino BathAround 1010 neutrinos per cm2 per sec pass through the Earths surface, coming to us from the Solar core.By comparison around 1 muon per cm2 per sec arrives at the Earths surface, caused by cosmic rays hitting the upper atmosphere.In 1987 (for about 10 secs) about the same neutrino flux reached the Earth from an exploding star about 150,000 light years away.

    Neutrinos interact very feebly with matter!

    Neutrinos

  • How Do We Know Neutrinos Exist?In a two-body decay, energy and momentum conservation uniquely fix the energy of the outgoing particles.n p + e-EN

    Neutrinos

  • How Do We Know Neutrinos Exist?Electrons produced by beta decay do not all have the same energy.Pauli proposed the existence of an unseen neutral particle to explain the observed electron spectrum.n p + e- + nn

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • Neutrinos in the LabIf neutrinos interact so weakly, how can they be produced for experiments in the lab?Pions are copiously produced by the strong interactions, but decay into neutrinos close to 100% of the time. Neutrino beams can be produced from pion beams, or from intensely radioactive sources like nuclear reactors.p+ m+ + nmpn

    Neutrinos

  • Two Kinds of ReactionsNeutrino beams cause two types of reactions when they hit targets. Charged current interactions always involve an electrically charged lepton (ie e, m or t).Neutral current interactions always involve missing energy and momentum, indicating the presence of an unseen neutrino among the final-state particles.mpXmnmpXnnCharged CurrentNeutral Current

    Neutrinos

  • More Than One Neutrino SpeciesNeutrinos produced with muons always* produce muons in charged-current interactions: n = nmNeutrinos produced with electrons always* produce electrons in charged-current interactions: n = neNeutrinos produced with tau leptons always* produce tau leptons in charged-current interactions: n = ntmpXmnenXen

    Neutrinos

  • Lepton Number ConservationThe results of charged-current experiments (until recently) were consistent with the existence of three different neutrino species with the separate conservation of Le, Lm and Lt.

    Neutrinos

  • How Massive Are They?Muon neutrino mass is inferred from the final muon energy in the two-body decay of the pion.nm: m < 0.19 MeV (90% cl)Tau neutrino mass is inferred from the spectrum of pion energies in the decay t 5p + nnt: m < 18.2 MeV (95% cl)ENmp /2

    Neutrinos

  • How Massive Are They?Muon neutrino mass is inferred from the final muon energy in the two-body decay of the pion.nm: m < 0.19 MeV (90% cl)Tau neutrino mass is inferred from the spectrum of pion energies in the decay t 5p + nnt: m < 18.2 MeV (95% cl)ENmp /2mn 0

    Neutrinos

  • The Electron Neutrino MassElectron neutrino masses are inferred from the shape of the electron spectrum in tritium beta decay.ne: m < 3 eV (95% cl)Kurie Plot

    Neutrinos

  • Do Neutrinos Decay?Limits are obtained by observing neutrino induced reactions downstream from a known source.

    Laboratory limits:ne: t > 300 sec (m/eV)nm: t > 0.11 sec (m/eV)

    Supernova 1987a: ne: t > 3 105 sec (m/eV)SNUs150,000 ly

    Neutrinos

  • Kinematics of Massless NeutrinosMassless particle states may be labelled by their momentum, p, and helicity, h.h = (p . s)/|p| =

    Observed ns have: h= +. CPT symmetry requires the existence of an antiparticle whose helicity is oppositen(p,h)n(p,-h)CPT

    Neutrinos

  • Kinematics of Massive NeutrinosIf neutrinos are massive, then the sign of their helicity can be changed by changing the frame of reference.

    h = (p . s)/|p|

    If massive, neutrinos having both signs of helicity must exist.n(p,h)n(p,-h)CPTn(p,h)n(p,-h)CPTBoost, if m 0Boost, if m 0

    Neutrinos

  • Majorana vs Dirac NeutrinosSince neutrinos are electrically neutral, they could be their own antiparticle (like the photon).If neutrinos carry a conserved charge (like Lepton number) then this can be used to distinguish particle from antiparticle.Neutrinos which are their own antiparticles are known as Majorana neutrinos.n(p,h)n(p,-h)CPTn(p,h)n(p,-h)CPTBoost, if m 0Boost, if m 0

    Neutrinos

  • Double Beta DecaySome unstable nuclei cannot decay by single beta emission, but can decay by the much more rare process of double-beta emission. eg for 76Ge: t = (1.5 0.19) 1021 ynn-

    Neutrinos

  • Neutrinoless Double Beta DecayIf neutrinos were Majorana and are massive then two neutrinos having the same helicity could mutually annihilate, leading to neutrinoless double beta decay.n-

    Neutrinos

  • Neutrinoless Double Beta Decayfor 76Ge: t > 1.6 1025 y implies < mn > < 0.3 eV (90% c.l.)2n bb decay0n bb decay

    Neutrinos

  • OutlineThe Most Feebly-Interacting Observed ParticlesWhy are they observed at all?Basic Neutrino PropertiesHow many types are there?How massive are they?Do they decay?Are they distinct from their antiparticles?TheoryThe Standard Model synthesisNeutrino Oscillations

    Neutrinos

  • The Fermi TheoryA purely phenomenological description

    Neutrinos

  • The Fermi TheoryA purely phenomenological descriptionNeutrinosonly appearin Weak Interactions

    Neutrinos

  • The Fermi TheoryA purely phenomenological descriptionChargedCurrent Interactions

    Neutrinos

  • The Fermi TheoryA purely phenomenological descriptionNeutralCurrent Interactions

    Neutrinos

  • The Fermi TheoryA purely phenomenological descriptionDestroys a neutrino and creates an electron

    Neutrinos

  • The Fermi TheoryA purely phenomenological descriptionDestroys andrecreates a neutrino withdifferent momentum

    Neutrinos

  • The Fermi TheoryA purely phenomenological description

    Neutrinos

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  • Theoretical FeaturesThe Fermi Theory is designed to include the following experimentally-observed features:All neutrino masses are zero;All three Lepton numbers are conserved;Neutrinos appear only in CC and NC interactions;All couplings are Universal (ie all charged current interactions are described by the single constant, G);Only left-handed (h = +1) neutrinos appear;CC and NC interaction strengths have same strength (ie r = 1);All neutrino interactions break C and P but preserve CP.

    Neutrinos

  • Descriptive, Not ExplanatoryThe Fermi Theory gives no understanding of why neutrinos have these features:Why are Lepton numbers and CP conserved? Why are there no right-handed neutrinos?Why are there only two types of interactions?Why are the neutrino couplings universal?Why is r = 1?Why are neutrinos massless?

    Neutrinos

  • Some Interactions NOT Included

    Neutrinos

  • Some Interactions NOT IncludedDescribes neutron decay into neutrino plus photon, which would violate conservation of lepton number.

    Neutrinos

  • Some Interactions NOT IncludedDescribes L=+1 right-handed neutrinos (or L=-1 left-handed anti-neutrinos).

    Neutrinos

  • Some Interactions NOT IncludedA Lepton-number conserving Dirac mass term.

    Neutrinos

  • Some Interactions NOT IncludedLepton-number violating Majorana mass terms.

    Neutrinos

  • The Standard ModelWeinberg (1967) and Salam (1968) unified the weak and electromagnetic interactions, using a symmetry proposed earlier by Glashow.Weak interactions are described by the exchange of either a massive, charged W boson (charged current) or a massive, neutral Z boson (neutral current).The interactions of spin-one particles like the W and Z bosons are strongly constrained by the consistency of special relativity and quantum mechani