Fisica Sperimentale Nucleare e Subnucleare Lezione 4 Marina Cobal, UniUD e INFN Trieste
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�Fisica Sperimentale Nucleare e Subnucleare�Lezione 4
Marina Cobal,
UniUD e INFN Trieste
1) Introduction
2) Cosmic Rays
3) Basis 1) Quantum mechanics, Radioactivity
2) Scattering theory
4) Leptons
5) Hadrons1) Strangeness, quark model
2) Symmetries, Isospin
6) Standard Model1) Gauge invariance, QED: EM interaction
2) Parity, neutrinos: Weak interaction
3) QCD: Strong interaction
7) e+e- and DIS
8) Higgs and CKM
Plan
1900-1940
1945-1965
1965-1975
1975-2000
2000-2013
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 interactionstudies. Pion/Muon separation
Discovery of first elementary particles
What are leptons?
• Leptons are elementary particles which do not feel the strong force
• So named, to indicate a particle with a small mass
• At the time (1948) the known leptons had a mass muchlighter than the the mass of the proton
4
µ−
νµ
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
Leptons as we know them today
µ+
νµ
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
and anti-leptons
How did we get to know them?
e−
νe
⎛
⎝⎜⎜
⎞
⎠⎟⎟
τ −
ντ
⎛
⎝⎜⎜
⎞
⎠⎟⎟
electron
el. neutrino
muon
mu neutrino
tau
tau neutrino
e+
νe
⎛
⎝⎜⎜
⎞
⎠⎟⎟
τ +
ντ
⎛
⎝⎜⎜
⎞
⎠⎟⎟
positron
el. anti-nu
muon
mu anti-nu
tau
tau anti-nu
5
Electron
• 1896, J. J. Thomson, J. S. Townsend and H. A. Wilson: cathode rays are a bunch of particles
• Mass: ~1/1000 of the mass of the hydrogen ion (the leastmassive ion known)
• charge-to-mass ratio, e/m, independent of cathode material
6
• 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 t) For anti-leptons La = -1
• Lepton numbers are conserved in any reaction
÷÷ø
öççè
æ÷÷ø
öççè
æ÷÷ø
öççè
æ +++
tµ nt
nµ
n e
e
101101011011
µnµn e
enumbermuonnumberelectronnumberleptonLepton
----
Nonep
YesnpNoeNoepnYesepn
e
e
+®+
+®++®+®++®+
+
+
--
-
-
µ
µ
n
µngµ
nn
Consequence of the lepton nr conservation: some processes are not allowed
Lederman, Schwarts, Steinberger
Neutrinos
• Neutrinos cannot be registered by detectors, there are only indirectindications
• First indication of neutrino existence came from b-decays of a nucleus N
eeAZNAZN n+++® -),1(),(
• Electron is a stable particle, while muon and tauon have a finite lifetime:
tµ = 2.2 x 10-6 s and tt = 2.9 x 10-13 s
Muon decay in a purely leptonic mode:
The tau has a mass sufficient to produce even hadrons, but hasleptonic decays as well:
• Fraction of a particular decay mode with respect to all possibledecays is called branching ratio (BR)
BR of (a) is 17.84% and of (b) is 17.36%
µnnµ ++® --ee
tµ
t
nnµt
nnt
++®
++®--
--
)(
)(
bea e
1) Weak interactions of leptons are identical like electromagneticones (interaction universality)
2) One can neglect final state lepton masses for many basiccalculations
The decay rate for a muon is given by:
Where GF is the Fermi constant
Substituting mµ with mt one obtains same decay rates of tauonleptonic decays, for (a) and (b). It explains why BR of (a) and (b)have very close values
3
52
195)(
pnnµ µµ
mGe F
e =++®G --
Leptons Universality
Using the decay rate, the lifetime of a lepton is:
Here l stands for µ and t. Since muons have basically one decaymode, B= 1 in their case. Using experimental values of B and formula for G, one obtaines the ratio of µ and t lifetimes:
In very good agreement with independent experimentalmeasurements
• Universality of lepton interaction proved to big extent.Basically no difference between lepton generations, apart from the mass
)()(le
lel el
elBnnnnt --
--
®G®
=
75
103.1178.0 -×»÷÷ø
öççè
æ×»
t
µ
µ
t
tt
mm
The tau search
1960: A. Zichichi proposal at 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 charges
νHL
HL"
#$%
&'
The tau discovery
The tau was then searched for by Zichichi in 1967 in the e+ e–®t+ t–reaction at the ADONE ring in Frascati which did not have enoughenergy to produce a pair of the new lepton. The maximum ADONE energy was √s=3 GeV, below the threshold for t+t– production √s=3.554 GeV (20% 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. t 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 collinearsBackground: non-identified hadrons
e
µ
1977. PLUTO and DASP @ DESY confirm discovery1976. HL is called t from ”triton”, the third (P. Rapidis)
Neutrinos: the crisis around 1930
• Matter is made of: – Particles: g, e-, p – Atoms: Small nucleus of
protons surrounded by a cloud of electrons
before Pauli:Unique electron
energy?
Experimentalelectronenergy
® electron energy
®ev
ents
Observations:Nuclear b-decay:
3H →3He+e-
Energy conservationviolated?
Pauli: Variable electron energy!
Pauli's letter of the 4th of 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 Li6 nuclei and the continuous beta spectrum, I have hit upon a deseperate 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 1/ 2 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... … Unfortunately, I cannot appear in Tubingen personally since I am indispensable here in Zurich because of a ball on the night of 6/ 7 December. With my best regards to you, and also to Mr Back. Your humble servant . W. Pauli
Pauli�s hypothesis
Fermi theory of b decay
• What is a b-decay ? It is a neutron decay:
• Necessity of neutrino existence comes from the apparent energyand angular momentum non-conservation in observed reactions
• For the sake of lepton number conservation, electron must be accompanied by an anti-neutrino and not a neutrino!
• Mass limit for can be estimated from precise measurementsof the b-decay:
• Best results are obtained from tritium decay
it gives (~ zero mass)
eepn n++® -
en
emMEm Nee n
-D££
eeHeH n++® -33
2/2 ceVme£n
• The most powerful available sources of neutrinos, before the construction of protosinchrotrons (‘60) were the nuclear reactors.
• By the processes of fission ne are produced with a spectrum of energies of a few MeV. A few tens of meters from the core of a reactor of 1 GW, the flow is enormous F ~ 1017 m-2s-1
• Electronic neutrinos and antineutrinos can be revealed through the electronic "inverse beta decay", but the cross section is microscopic
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: En= 1MeV W1=Fs » 10–30 s–1
• So, for a total rate of: W = 10–3 Hz Þ Np = 1027
• If the target is made of H2O (10 p), in a mole (18 g) thereare NA 10/18 = 3.3 1023 protons
• So, one needs about 3000 moles Þ 50 kg• Detection efficiency, fiducial volume/total volume. Lets’ put »
1/4 Þ 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 1995with Perl (for discovery of t-lepton)
• Intense neutrino flux from nuclear reactor
• Inverse b decay
gg
n
+®+
+®+
-+
+
ee
enpeby followed
Power plant 0.7 GW(Savannah river plant USA)Producing ne
Scheme of the Reines and Cowan experiment2m
2m
• Target = 200 l of H2O • e+ immediately annihilates in
two g’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 g’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 Þ small• Cross section » expected value
“Neutrino” detected, finally
15-Oct-18
Science 124 (1956) 104
We now know it was electron antineutrino
Muon Neutrino detection
1959. B. Pontecorvo (in Russia) and M. Schwartz (in US) proposedindependently 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 extractedfrom the AGS at BNL is sentagainst a target. Hadronsand µ are filtered by13.5 mof Fe and neutrons withparaffin
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
15-Oct-18
• 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 nm
Muon Neutrino detection
How do electrons look like
Exposure of the chambers at the 400 MeV electron beam at Cosmotron
Muon Neutrino detection
28
),,( duup=
),,( ddun=
+- ee ,+- µµ ,
),(,),( uddu == -+ pp
g
n
Leptons (heavier copies of the electron)
The photon
The neutrino, postulated to explainbeta 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
n
n
n
ne + 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: Pontecorvohypothesis of neutrino oscillation
Nobel prize 2002
(Davis, Koshiba and Giacconi)
Flavour neutrino�s
• Neutrino�s from p →µ+n identified as nµ– �Two neutrino� hypothesis correct: ne and nµ– Lederman, Schwartz, Steinberger (Nobel Prize 1988)
“For the neutrino beam method and the demonstration of thedoublet structure of the leptons through the discovery of the muon neutrino”
Discovery of t-neutrino (2000)
DONUT collaborationProduction and detection of t-neutrino�s
t
nt
nt
t
ct
nTDs
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
• We observed three couples of leptons (tre “families”, “generations”)
• One lepton is charged (e–, µ–, t– ), the other is “its” neutrino (ne, nµ, nt)
• e–, µ– e t– have all the same characteristics, except for the mass• Charged leptons makes gravitational, electromagnetic and
weak interactions.• Neutrinos makes gravitational and weak interactions.
Flavour Mass Lifetime
e 0.5 MeV ∞
µ 106 MeV 2.2 µs
t 1777 MeV 0.29 ps
Determination of the Z0
line-shape: Reveals the number of ‘light neutrinos’Fantastic precision on Z0
parametersCorrections for phase of moon, water level in Lac du Geneve, passing trains,…
LEP (1989-2000): the 3 neutrino families
Nn 2.984�0.0017
MZ0 91.1852±0.0030 GeV
GZ0 2.4948 ±0.0041 GeV
Existence of only 3 neutrinosUnless the undiscovered neutrinos have mass mn>MZ/2
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