pif 8 leptoni - uniudcobal/site/pif_8_leptoni.pdf · m(µ−)=105.7 mev / c2 14 pion and muon decay...
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Leptons
Known particles at the end of the 30’s • Electron • Proton • Photon • Neutron • Positron • Muon
• Pion
Neutrino: a particle whose existence was hypotesized without a discovery!
Faraday, Goldstein, Crookes, J. J Thomson (1896)
Avogadro, Prout (1815)
Einstein (1905), Compton (1915)
Chadwick (1932)
Conventional birth date of Nuclear Physics
Anderson (1932)
Cosmic rays interaction studies. Pion/Muon separation
Discovery of first elementary particles
Muon decay (1930)
Decayeletrontrack
The mesotron puzzle..
Conversi, Pancini, Piccioni (1947) experiment
Conversi, Pancini, Piccioni (1947) experiment
Conversi, Pancini, Piccioni (1947) experiment
Conversi, Pancini, Piccioni (1947) experiment
Conversi, Pancini, Piccioni (1947) experiment
Conversi, Pancini, Piccioni (1947) experiment
Pion discovery
Pion discovery
2/7.105)( cMeVm =−µ
14
Pion and Muon decay sequence: a cascade of decays
Pion discovery (1947, Lattes, Powell Occhialini)
Muon decay
Nuclear Emulsion
)106.2( 8 s−−− ×=→ τνµπ µ
m(π − ) = 139.6 MeV / c2
)102.2( 6 se e−−− ×=→ τννµ µ
In all these decays, neutrinos are emitted !
Muon decay scheme
15
Pion – Muon
The pion in term of quarks
Experimental strategy: Exposure of Emusions to Cosmic Rays
e→→ µπ
Pion – Muon
Leptons • Leptons are s = ½ fermions, not subject to strong interactions
me < mµ < mτ
• Electron e-, muon µ- and tauon τ- have corresponding neutrinos:
νe, νµ and νt • Electron, muon and tauon have electric charge of e-. Neutrinos are neutral • Neutrinos have very small masses
• For neutrinos only weak interactions have been observed so far
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• Anti-leptons are positron e+, positive muons/tauons and anti-neutrinos • Neutrinos and anti-neutrinos differ by the lepton number. For leptons La = 1 (a = e,µ or τ) For anti-leptons La = -1 • Lepton numbers are conserved in any reaction
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛ +++
τµ ν
τν
µ
ν e
e
101101011011
µνµν e
enumbermuonnumberelectronnumberleptonLepton
−−−−
Nonep
YesnpNoeNoepnYesepn
e
e
+→+
+→+
+→
+→+
+→+
+
+
−−
−
−
µ
µ
ν
µν
γµ
ν
ν
Consequence of the lepton nr conservation: some processes are not allowed
Lederman, Schwarts, Steinberger
Neutrinos • Neutrinos cannot be registered by detectors, there are only indirect indications
• First indication of neutrino existence came from β-decays of a nucleus N
eeAZNAZN ν+++→ −),1(),(
• Electron is a stable particle, while muon and tauon have a finite lifetime:
τµ = 2.2 x 10-6 s and ττ = 2.9 x 10-13 s Muon decay in a purely leptonic mode: Tauon has a mass sufficient to produce even hadrons, but has leptonic decays as well: • Fraction of a particular decay mode with respect to all possible decays is called branching ratio (BR)
BR of (a) is 17.84% and of (b) is 17.36%
µννµ ++→ −−ee
τµ
τ
ννµτ
νντ
++→
++→−−
−−
)(
)(
bea e
assumptions
:
1) 1)
Weak
interactions of leptons are interactions identical like
of
leptons electromagnetic are identical ones like( interaction electromagnetic universality ones)
(2) interactionOne can universalityneglect ) final
can neglect masses for final many state lepton basic masses calculations for The manydecay rate for a muon basic given
calculations
The decay rate for a muon
is given by: Where GF is the Fermi constant Substituting mµ with mτ one ,
for (a) and (b). It obtainsexplains why
BR of (a) and (b) havesame very closedecay values
3
52
195)(
πννµ µµ
mGe F
e =++→Γ −−
Using the decay rate, the
Using the
decay lifetime of a lepton
is Here l stands : for µ and τ. Since have basically one decayHere l mode, B= 1 in stands for µ and τ. case. Using experimental
Since
good agreement with valuesindependent experimental of B and formula for Γ, measurements one
• Universality obtaines of lepton the ratio of µ and τ lifetimes: In no
very
)()(le
lel el
elBνννν
τ −−
−−
→Γ→
=
75
103.1178.0 −⋅≈⎟⎟⎠
⎞⎜⎜⎝
⎛⋅≈
τ
µ
µ
τ
ττ
mm
The tau search CERN PS
CERN PS
of the PAPLEP (Proton-AntiProton into LEpton Pairs)
It starts the search for the 3rd sequential lepton family, a replica of the first two.The “Heavy Lepton and its neutrino”
Searching for acoplanar lepton pairs of opposite chargesIt starts the search for the 3rd sequential lepton family, a replica of the
νHL
HL"
#$%
&'
The tau discovery The tau was then
was
reaction searched for by Zichichi in 1967 in the e+ e–→τ+ τ– reaction at the ADONE ring in Frascati which did not have enough of the new lepton
energy
The maximum ADONE energy was √s=3
to produce a
GeV, below the threshold for pair– production √s=3.554 of the new lepton20% less
.
!!) A lower limit for the heavy lepton (HL) mass was obtained
Simplfied from
Nuovo Cimento 17A (1973) 383
HL is here
The tau discovery 1971. M. Pearl e co. same idea at SPEAR (e+e- with E= 8 GeV) 1975. τ discovery with the Mark I experiment
Common processes
e+ + e− → e+ + e− 2 e (showers) opposite sign, collinearse+ + e− → µ+ + µ− 2 µ (penetrating) opposite sign, collinearse+ + e− → π + +π − 2 π (hadrons) opposite sign, collinearse+ + e− → π + +π − +π ˚ 2 π (hadrons) opposite sign, non collinears
Signal e+ + e− → τ + +τ − τ + → e+ +ν 's / τ − → µ− +ν 's e+ + e− → τ + +τ − τ + → µ+ +ν 's / τ − → e− +ν 's
Topology: eµ pair of opposite sign, non collinears Background: non-identified hadrons
e
µ
1977. PLUTO and DASP @ DESY confirm discovery 1976. HL is called τ from τριτον, the third (P. Rapidis)
Neutrinos: the crisis around 1930 • Matter is made of:
– Particles: γ, e-, p – Atoms: Small nucleus of
protons surrounded by a cloud of electrons
before Pauli: Unique electron
energy?
Experimental electron energy
→ electron energy
→ e
vent
s
Observations: Nuclear β-decay:
3H →3He+e-
Energy conservation violated?
Pauli: Variable electron energy!
Pauli's letterofthe4 thofD ecember1930
DearRadioactiveLadiesandGentlemen,
Asthebeareroftheselines,towhomIgraciouslyaskyoutolisten,willexplaintoyouinmoredetail,howbecauseofthe"wrong"statisticsoftheNandLi6nucleiandthecontinuousbetaspectrum,Ihavehituponadeseperateremedytosavethe"exchangetheorem"ofstatisticsandthelawofconservationofenergy. Namely,thepossibility thattherecouldexist in thenucleielectr ically neutralparticles,thatI w ishtocallneutrons,w hichhavesp in1/2 andobey theexclusionprinciple andwhichfurtherdifferfromlightquantainthattheydonottravelwiththevelocityoflight.Themassoftheneutronsshouldbeofthesameorderofmagnitudeastheelectronmassandinanyeventnotlargerthan0.01protonmasses.T hecontinuousbetaspectrumw ould thenbecomeunderstandablebytheassumptionthat in betadecayaneutronisemitted in additiontotheelectron suchthat thesumoftheenergiesoftheneutronandtheelectron isconstant... … Unfortunately, I cannotappear in T ubingenpersonally sinceI am indispensablehereinZ urichbecauseofaballonthen ightof6/7 D ecember. Withmybestregardstoyou,andalsotoMrBack.Yourhumbleservant.W.Pauli
Pauli’s hypothesis
Fermi theory of β decay
• Whatisaβ-decay?Itisaneutrondecay:
• Necessityofneutrinoexistencecomesfromtheapparentenergyandangularmomentumnon-conserva=oninobservedreac=ons• Forthesakeofleptonnumberconserva=on,electronmustbeaccompaniedbyanan=-neutrinoandnotaneutrino!• Masslimitforcanbees=matedfromtheprecisemeasurementsoftheβ-decay:
• Bestresultsareobtainedfromtri=umdecay
itgives(~zeromass)
eepn ν++→ −
eν
emMEm Nee ν
−Δ≤≤
eeHeH ν++→ −33
2/2 ceVme≤ν
• Themostpowerfulavailablesourcesofneutrinos,beforetheconstruc=onofprotosinchrotrons(60)werethenuclearreactors.
• BytheprocessesoffissionνeareproducedwithaspectrumofenergiesofafewMeV.Afewtensofmetersfromthecoreofareactorof1GW,theflowisenormous Φ ≈1017m-2s-1
• Electronicneutrinosandan=neutrinoscanberevealedthroughtheelectronic"inversebetadecay",butthecrosssec=onismicroscopic
Electron Neutrino detection
σ νe + p→ e+ + n( ) ≈10–47 Eν / MeV( )2 m2
Electron Neutrino detection σ νe + p→ e+ + n( ) ≈10–47 Eν / MeV( )2 m2
• Rate for p target Eν= 1MeV W1=Φσ ≈ 10–30 s–1 • So, for a total rate of: W = 10–3 Hz ⇒ Np = 1027 • If the target is made of H2O (10 p), in a mole (18 g) there are NA 10/18 = 3.3 1023 protons • So, one needs about 3000 moles ⇒ 50 kg • Detection efficiency, fiducial volume/total volume. Lets’ put ≈ 1/4 ⇒ Total mass needed ≈ 200 kg
The main problem is not the needed mass (albeit this was remarkable in 1958), but the control of the ”backgrounds” : • n from the reactor • background induced by cosmic rays • natural radioactivity
Electron Neutrino detection (1956) • Cowan & Raines
– Cowan nobel prize 1995 with Perl (for discovery of τ-lepton)
• Intense neutrino flux from nuclear reactor
• Inverse β decay
γγ
ν
+→+
+→+
−+
+
ee
enpeby followed
Power plant 0.7 GW (Savannah river plant USA) Producing νe
Scheme of the Reines and Cowan experiment 2m
2m
Target = 200 l of H2O e+ immediately annihilates in two γ’s at 180 ˚ between them, which go in two different containers of liquid scintillator adjacent. Compton electrons produce a flash of light. H2O is a good moderator and in a few tens of µs a neutron is thermalized. H2O doped with 40 kg of Cd which has a large cross section for capture of thermal n. The retarded γ’s are revealed in the scintillator. Detector at 10 m below a building (cosmic) + lot of care in shielding
• Observed: 3±0.2 events/h • Background ⇒ small • Cross section ≈ expected value
“Neutrino” detected, finally
6-Nov-17
Science 124 (1956) 104
We now know it was electron antineutrino
Muon Neutrino detection 1959. B. Pontecorvo (in Russia) and M. Schwartz (in US) proposed independently the use of neutrino beams produced in accelerator (they show that intensities should be enough). Which neutrinos:
π + → µ+ +ν? π − → µ− +ν?
1960. Lee e Yang. Should be different from the electron neutrino otherwise:
µ± → e± + γ1962. Schwartz, Lederman, Steinberger experiment. The proton beam extracted from the AGS at BNL is sent against a target. Hadrons and µ are filtered by13.5 m of Fe and neutrons with paraffin
Muon Neutrino detection
172 6 Historical track detectors
Between two discharges the produced ions are removed from the detec-tor volume by means of a clearing field . If the time delay between thepassage of the particle and the high-voltage signal is less than the mem-ory time of about 100 µs, the efficiency of the spark chamber is close to100%. A clearing field, of course, removes also the primary ionisation fromthe detector volume. For this reason the time delay between the passageof the particle and the application of the high-voltage signal has to bechosen as short as possible to reach full efficiency. Also the rise time ofthe high-voltage pulse must be short because otherwise the leading edgeacts as a clearing field before the critical field strength for spark formationis reached.
Figure 6.12 shows the track of a cosmic-ray muon in a multiplate sparkchamber [5, 44].
If several particles penetrate the chamber simultaneously, the proba-bility that all particles will form a spark trail decreases drastically withincreasing number of particles. This is caused by the fact that the firstspark discharges the charging capacitor to a large extent so that less volt-age or energy, respectively, is available for the formation of further sparks.This problem can be solved by limiting the current drawn by a spark.In current-limited spark chambers partially conducting glass plates aremounted in front of the metallic electrodes which prevent a high-currentspark discharge. In such glass spark chambers a high multitrack efficiencycan be obtained [45, 46].
Fig. 6.12. Track of a cosmic-ray muon in a multiplate spark chamber [44].
6.5 Spark chambers 171
Fig. 6.10. Single muon track in a stack of polypropylene-extruded plastic tubes.Such extruded plastic tubes are very cheap since they are normally used aspacking material. Because they have not been made for particle tracking, theirstructure is somewhat irregular, which can clearly be seen [37].
sparkgap
RL
scintillatorphotomultiplier
photomultiplier scintillator
particle trajectory
coincidence
20 kV
C
R
discriminators
Fig. 6.11. Principle of operation of a multiplate spark chamber.
In a spark chamber a number of parallel plates are mounted in a gas-filled volume. Typically, a mixture of helium and neon is used as countinggas. Alternatingly, the plates are either grounded or connected to a high-voltage supply (Fig. 6.11). The high-voltage pulse is normally triggeredto every second electrode by a coincidence between two scintillation coun-ters placed above and below the spark chamber. The gas amplification ischosen in such a way that a spark discharge occurs at the point of thepassage of the particle. This is obtained for gas amplifications between108 and 109. For lower gas amplifications sparks will not develop, whilefor larger gas amplifications sparking at unwanted positions (e.g. at spac-ers which separate the plates) can occur. The discharge channel followsthe electric field. Up to an angle of 30◦ the conducting plasma chan-nel can, however, follow the particle trajectory [8] as in the track sparkchamber.
• A number of parallel plates are mounted in a gas filled volume (typically, a mixture of He and Ne)
• Plates are alternatively connected to ground and to a high voltage supply • The high-voltage pulse is triggered by a coincidence between two
scintillation counters placed above and below the spark chamber • Gas amplification between 108 and 109 results in a spark discharge along
the trajectory of the particle.
a muon track
6-Nov-17
• 34 “single muon” events observed
• Additional 8 events compatible with background
• No electron observed
• Conclusion: the neutrino that is born together with a µ in the π decay when interacts produce a µ, not e.
• Two different conserved quantities exist, lepton flavours: ne and nµ
Muon Neutrino detection
How do electrons look like
Exposure of the chambers at the 400 MeV electron beam at Cosmotron
Muon Neutrino detection
39
),,( duup=
),,( ddun=
+− ee ,+− µµ ,
),(,),( uddu == −+ ππ
γ
ν
Leptons (heavier copies of the electron)
The photon
The neutrino, postulated to explain beta decay and observed in inverse beta decay, is always associated to a charged lepton.
The hadrons, particles made up of quarks and obeying mainly to strong nuclear interaction
Classification of elementary particles
Anti-neutrino’s vs neutrino’s
• Davis & Harmer – If the neutrino is same
particle as anti-neutrino then close to power plant:
Ar Cl
3718
3717 +→+
+→+
+→+
−
−
++
e
pen
nep
e
e
e
ν
ν
ν
νe + 37Cl → e- + 37Ar
-615 tons kitchen cleaning liquid -Typically one 37Cl → 37Ar/day -Chemically isolate 37Ar -Count radio-active 37Ar decay
• Reaction not observed: – Neutrino-anti neutrino not the
same particle – Little bit of 37Ar observed:
neutrino’s from cosmic origin (sun?)
– Rumor spread in Dubna that reaction did occur: Pontecorvo hypothesis of neutrino oscillation
Nobel prize 2002
(Davis, Koshiba and Giacconi)
Flavour neutrino’s
• Neutrino’s from π→µ+ν identified as νµ
– ‘Two neutrino’ hypothesis correct: νe and νµ
– Lederman, Schwartz, Steinberger (nobel prize 1988)
“For the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino”
Discovery of τ-neutrino (2000) DONUT collaboration Production and detection of τ-neutrino’s
τ
ντ
ντ
τ
cτ
νΤΔs
Neutrino flavours
• Neutrinos cannot be directly detected
• The charged lepton produced by the neutrino interaction in the detector identifies the neutrino flavour
Neutrino flavour CHANGES In the last 15 years we learnt that neutrino change flavour, provided time (flight distance) is given them to do so
• Oscillations and flavour conversion in matter, prove that neutrinos, contrary to the Standard model have non-zero mass
• Flavour states are superposition (mixing) of mass eigenstates
Flavour Mass Lifetime
e 0.5 MeV ∞
µ 106 MeV 2.2 µs
τ 1777 MeV 0.29 ps
We observed three couples of leptons (tre “families”, “generations”) One lepton is charged (e–, µ–, τ– ), the other is “its” neutrino (νe, νm, νt) e–, µ– e τ– have all the same characteristics, except for the mass Charged leptons makes gravitational, electromagnetic and weak interactions. Neutrinos makes gravitational and weak interactions.
Determination of the Z0 line-shape:
Reveals the number of ‘light neutrinos’ Fantastic precision on Z0 parameters
Corrections for phase of moon, water level in Lac du Geneve, passing trains,…
LEP (1989-2000): the 3 neutrino families
Nν 2.984±0.0017
MZ0 91.1852±0.0030 GeV
ΓZ0 2.4948 ±0.0041 GeV
Existence of only 3 neutrinos Unless the undiscovered neutrinos have mass mν>MZ/2