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1. Introduc,on to neutrinos Experimental Astropar,cle Physics 1.1 Postula,on of the neutrino • Known constituents of the atom in 1930: protons and electrons • Neutron discovered by Chadwick in 1932 • Radioactivity discovered by Rutherford/Villard around 1900 • All these reactions are two-body decays α, β, γ‘s should be monoenergetic! Par,cle physics in the 1930s 1.1 Postula,on of the neutrinos Energy conserva,on in βdecays 1.1 Postula,on of the neutrinos • Continuous spectra observed in β-decay experiments • Violation of energy conservation?
Transcript of 74',+5-#289/.+/#.%#&8 ......A(%/-'?4,(%#(:#4%>*54'#3(3-%&*3 #!"!#7(/&*54,(%#(:#&8-#%-*&'.%(/#...
1. Introduc,on to neutrinos
Experimental Astropar,cle Physics
1.1 Postula,on of the neutrino
• Known constituents of the atom in 1930: protons and electrons
• Neutron discovered by Chadwick in 1932
• Radioactivity discovered by Rutherford/Villard around 1900
• All these reactions are two-body decays ��� α, β, γ‘s should be monoenergetic!
Par,cle physics in the 1930s 1.1 Postula,on of the neutrinos
Energy conserva,on in β-‐decays 1.1 Postula,on of the neutrinos
• Continuous spectra observed in β-decay experiments
• Violation of energy conservation?
Conserva,on of angular momentum 1.1 Postula,on of the neutrinos
• Nucleus consists of A spin-1/2 particles
• Mother and daughter nucleus have the same number of nuclides.
Both should have integral or half-integral spin.
But: The positron carries away spin 1/2!
Pauli‘s leCer 1.1 Postula,on of the neutrinos
Pauli‘s leCer Liebe Radioak+ve Damen und Herren,
Wie der Überbringer dieser Zeilen, den ich huldvollst an-‐zuhören bi@e, Ihnen des näheren auseinandersetzen wird, bin ich angesichts der "falschen" Sta+s+k der N-‐ und Li-‐6 Kerne, sowie des kon+nuierlichen beta-‐Spektrums auf ei-‐nen verzweifelten Ausweg verfallen um den "Wechselsatz“ der Sta+s+k und den Energiesatz zu re@en. Nämlich die Möglichkeit, es könnten elektrisch neutrale Teilchen, die ich Neutronen nennen will, in den Kernen exis+eren, welche den Spin 1/2 haben und das Ausschliessungsprinzip befol-‐gen und sich von Lichtquanten außerdem noch dadurch unterscheiden, dass sie nicht mit Lichtgeschwindigkeit lau-‐fen. Die Masse der Neutronen könnte von der gleichen Grössenordnung wie die Elektronenmasse sein und jeden-‐falls nicht grösser als 0,01 Protonenmassen. Das kon+nuier-‐liche beta-‐Spektrum wäre dann verständlich unter der An-‐nahme, dass beim beta-‐Zerfall mit dem Elektron jeweils noch ein Neutron emi[ert wird, derart, dass die Summe der Energien von Neutron und Elektron konstant ist. ...
1.1 Postula,on of the neutrinos
• Third particle emitted in beta decays:
Proper,es of the new par,cle 1.1 Postula,on of the neutrinos
• Four-fermion contact interaction
• Coupling described by Fermi Constant GF
• Very low cross-section:
Fermi‘s theory of weak interac,on 1.1 Postula,on of the neutrinos
AZ! A(Z-1)!
e+!
!e!
t!
GF!
e-!
e-!
e-!
e-!
"em! "em!
#!t!
n!
p!
u d d!
u d u! e-! !e!_!
W-!t!
1.2 Discovery of the neutrino
Clyde Cowan
Fred Reines
What neutrino source to use? 1.2 Discovery of the neutrino
• Their first idea: ���Neutrinos from a ���nuclear explosion!
Neutrinos from a nuclear reactor 1.2 Discovery of the neutrino
Savannah River reactor complex
Basic fission reac,on
n + 235U → A(Z) + 233−A(92 − Z) + 3n + 200 MeV
1.2 Discovery of the neutrino
Nuclear fission yield 1. Neutrino proper,es
n + 235U → A(Z) + 233−A(92 − Z) + 3n + 200 MeV
Nuclide chart 1. Neutrino proper,es
Produc,on of neutron-‐rich isotopes 1. Neutrino proper,es
Neutrinos from n-‐rich isotopes 1.2 Discovery of the neutrino
• Possible decay scheme:
Neutrinos flux from a nuclear reactor 1.2 Discovery of the neutrino
• Estimating the neutrino emission rate:
Reactors are a strong antineutrino source!
Hanford Experiment 1.2 Discovery of the neutrino
• ~1 m3 of organic liquid scintillator
• scintillation light read out���by photomultiplier tubes
• neutrino detection by���inverse beta decay:
Coincidence signature���to suppress single-event background
• no conclusive result,���too much background
Hanford Team 1.2 Discovery of the neutrino
Savannah River Experiment 1.2 Discovery of the neutrino
• Neutrino target:���two tanks (A,B) filled���with Cd-doped water
• Free protons for inverse beta decay
• Prompt positron annihilates into���two 511 keV gammas
• Cd has large neutron-���capture cross-section���& releases gammas
Savannah River Experiment 1.2 Discovery of the neutrino
Coincidence in adjacent scintillator detectors to ���discriminate all kinds of single-event backgrounds!
Savannah River – Shielding 1.2 Discovery of the neutrino
Shielding from cosmic ray particles and reactor backgrounds
Telegramme to Pauli (1954) 1.2 Discovery of the neutrino
Discovery was made based on a clear IBD event ���excess in case the reactor was running!
1.3 Neutrinos in the Standard Model
Par,cle Inventory of the SM 1.3 Neutrinos in the Standard Model
• Fermions make up matter
• Bosons are exchange���particles of the forces
• Fermions: spin 1/2���Bosons: integer spin
• Three families or flavors ���different in mass
Par,cle Masses 1.3 Neutrinos in the Standard Model
Forces ac,ng between the par,cles 1.3 Neutrinos in the Standard Model
Electromagne,c force 1.3 Neutrinos in the Standard Model
AZ! A(Z-1)!
e+!
!e!
t!
GF!
e-!
e-!
e-!
e-!
"em! "em!
#!t!
n!
p!
u d d!
u d u! e-! !e!_!
W-!t!
Strong nuclear force 1.3 Neutrinos in the Standard Model
Weak nuclear force 1.3 Neutrinos in the Standard Model
Weak currents 1.3 Neutrinos in the Standard Model
Chirality and Helicity 1.3 Neutrinos in the Standard Model
V-‐A theory of weak interac,on 1.3 Neutrinos in the Standard Model
W, Z couple only to left-handed (LH) particles and��� right-handed (RH) antiparticles!
Weak interaction violates parity P and���charge conjugaction C symmetries, but conserves CP!
!µ" e-!
e-!
t!
!µ"
Z0!p!
S!RH:! p!
S!LH:!
p!
S!LH !e
!
p!
S!LH !e
!_!
p!
S!RH !e
!
p!
S!RH !e
!_!
!" !"
#"
#"
Doublets in the weak force 1.3 Neutrinos in the Standard Model
• Left-handed (LH) particles and right-handed (RH) antiparticles form doublets with respect to the weak force.
Weak force in meson decays 1.3 Neutrinos in the Standard Model
For quarks, weak eigenstates are not identical to mass eigenstates!
W+!
u!
d!
W+!
!e!
e-!
etc., but not! W+!
!µ"
e-!
_!
W+!
u! d!
!µ"µ+"
#+"
W+!
u! s!
!µ"µ+"
K+"_!
but also!
Quark mixing: CKM matrix 1.3 Neutrinos in the Standard Model
mass eigenstates of down-like quarks
weak eigenstates coupling to up-like quarks
• Mixing in the quark sector is small
• In the SM, no equivalent in lepton sector:���no neutrino mass eigenstates no mixing!
1.4 Neutrino flavors and lepton number
How many neutrino flavors? 1.4 Neutrino flavors and lepton number
• How do we know there are three different neutrino flavors?
• Neutrino flavors are defined by weak charged current reactions
• Experimental test: produce neutrinos by pion decay: π+ µ+νµ
Detect νµ in CC interaction: should produce only muons!���
AGS @ Brookhaven Na,onal Lab 1.4 Neutrino flavors and lepton number
Alternate Gradient Synchrotron 1.4 Neutrino flavors and lepton number
• Protons are accelerated to���15 GeV in a ring accelerator
• Protons are grouped into bunches running���separately along���the ring.
• Upon reaching���the aimed-for energy,���protons are deflected���from the ring onto a ���fixed Be target.
Sketch of the AGS neutrino beamline
Be-‐target
• In fixed target, protons produce mostly π+ and K+ µ+νµ • Particles are boosted in direction of the proton beam • Charged particles stopped in beam dump only νµ‘s reach detector • Cherenkov counter for µ‘s to get time information
1.4 Neutrino flavors and lepton number
AGS Neutrino Spectrum
• Initial proton���energy: 15 GeV
1.4 Neutrino flavors and lepton number
AGS Spark Chambers
• Vertically stacked ���almunium plates
• Spaces in between ���filled with He/Ne gas
• CC interaction in Al: νµ + n p + µ- (e-?)
• µ/e‘s passing through���the interspaces will���ionize the gas
This causes sparks ���in the gas which were���photographed for analysis.
1.4 Neutrino flavors and lepton number
Event discrimina,on in spark chambers electron-like: electromagnetic showers muon-like: minimum-ionizing tracks
1.4 Neutrino flavors and lepton number
AGS Result electron-like: electromagnetic showers muon-like: minimum-ionizing tracks
• In coincidence with the beam spills,���only muon tracks were found in the spark chambers!
No electrons were created out of νµ. Muon and electron neutrino are different particles!
1.4 Neutrino flavors and lepton number
Melvin Schwartz – Nobel Prize 1988 1.4 Neutrino flavors and lepton number
DONUT Experimental Setup 1.4 Neutrino flavors and lepton number
• τ lepton known since the 70‘s, ντ discovered in DONUT in 2000. • setup very similar to AGS experiment, but higher energies
DONUT Detector 1.4 Neutrino flavors and lepton number
DONUT ντ event signature 1.4 Neutrino flavors and lepton number
• Neutrino target:���Sandwich of steel plates (ν target, 1mm) and emulsion sheets (0.1mm)
• CC interaction:���
• Fine spatial resolution needed to observe the τ decay kink, e.g.
Total number of ac,ve flavors 1.4 Neutrino flavors and lepton number
• All active neutrinos couple to the Z0 boson by weak neutral current
• Z0‘s were produced in large numbers in e+e- collisions at the DELPHI experiment at LEP (maximum CMS energy: 209 GeV)
• Mass, cross-sections, decay width and branchings were determined
• Z0 decays into fermion-���antifermion pairs: qq, ll, νν
• only particles with m < MZ/2 ≈ 45 GeV ���are produced in this decay: 5 quarks (not t), e, µ, τ, Nν neutrinos
e-! e+!
q,l,!"
t!
q,l,!"
Z0!
!"!"!"
_ _ _
Z0 decay width and amplitude 1.4 Neutrino flavors and lepton number
• Width of the peak:���total decay width of all possible final states (including neutrinos)
• Height of the peak:���decay amplitude of all channels visible in the detector (excluding ν‘s)
• From comparison to the theoretical prediction of width and height:
Nν = 3.00 ± 0.05
Neutrinos vs. An,neutrinos 1.4 Neutrino flavors and lepton number
• How do we know neutrinos and antineutrinos interact differently?
• First experiment by Ray Davis at Savannah River reactor in 1955.
• Neutrino source: Reactor emits only νe.
• Neutrino target: Chlorine tank (6 tons of C2Cl4).
• Detection reaction:
only possible for νe, not νe!
• 37Ar is extracted from tank and detected by re-decay to 37Cl.
No excess over background observed.
νe are different from νe.
_
_
Lepton and lepton family number 1.4 Neutrino flavors and lepton number
• The number of leptons in a reaction is conserved.
• The number of leptons of a given flavor is conserved.
• This is represented by conserved quantities:
