Leptons and Weak interactions

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1 PHY771, 8/28/2014 Tomasz Skwarnicki 1 Historical introduction to Elementary Particles: Leptons and Weak interactions Tomasz Skwarnicki Syracuse University Griffiths, 2 nd ed., ,1.10

2 PHY771, 8/28/2014 Tomasz Skwarnicki 2 Yukawa s prediction of a meson. Lepton, meson, baryon terminology. Protons and neutrons are held together in nuclei by a force much stronger than electrostatic repulsions of protons Yukawa in 1934: Strong interactions must be mediated by a particle Since strong forces drop quickly to zero outside the nucleus, the strong force carrier must have a large mass From the size of a nucleus he estimated the mass of the particle exchanged by nucleons to be between the electron and proton masses (~100 MeV/c 2 ) Hideki Yukawa Japan As middle-weight it got labeled meson, in between lightweight electron ( lepton ) and heavy-weight proton ( baryon ) Nobel Prize 1947 Lepton-meson-baryon terminology has survived, but its meaning has changed since then: Leptons are spin ½ particles which don t participate in strong interactions, which have no substructure (like electron) Baryons are half-integer spin ( 1 / 2, 3 / 2, ) particles which do interact strongly, made out of 3 quarks (like proton, neutron) Mesons are integer spin particles (0, 1, 2, ) which do interact strongly, made out of quark anti-quark pair (like Yukawa meson)

3 PHY771, 8/28/2014 Tomasz Skwarnicki 3 Muon discovery 1937 Anderson (positron discovery in 1932) and Nedermeyer discovered a mid-weight particle in cosmic rays in 1937 by having curvatures in magnetic field in between electron and proton Initially it was identified with Yukawa meson, thus called mu meson (later muon ) However, it was later shown (1946) that majority of cosmic mu mesons did not interact with nuclei they were very penetrating Therefore, they were not Yukawa mesons! In fact, muons are leptons; a heavier version of electrons. To this day we don t understand why nature needs muons. Muon discovery was the first time second generation particles were observed. However, people did not realize that right away because of the case of mistaken identity. To this day some old-timers call muon a meson. This is terribly wrong in today s terminology since muon is a lepton. Anderson and Nedermeyer at Caltech with magnetic cloud chamber Famous quote from Isidor Rabi

4 PHY771, 8/28/2014 Tomasz Skwarnicki 4 Pion discovery 1946 C.F.Powell developed a method of detecting cosmic rays in photographic emulsion and found one middle-weight particle decaying to another (1946): There were two middle-weight particles in cosmic rays! They were named pion (π) and muon (µ) muons are by far more common in cosmic rays at the ground level (it is the other way round in upper parts of the atmosphere) Pion is the particle Yukawa had predicted Yukawa received Nobel prize in 1947, Powell in 1950 e µ π p n Particle Why kinks? See next Mass MeV/c Cecil Frank Powell England e µ π

5 PHY771, 8/28/2014 Tomasz Skwarnicki 5 Neutrinos and weak interactions In 1930 electron energy spectrum in radioactive β decay was measured and indicated that something else was also produced which was not visible as rays Expected for 3-body decay Z P Z+1 D e - x Expected for 2-body decay Z P Z+1 D e - Z Z+1 The suggestion that an undetected neutral particle was present came from Pauli. In 1933 Enrico Fermi published theory of β decays and called the particle invented by Pauli neutrino (ν) i.e. little neutral one. x Since the endpoint of the observed electron spectrum comes close to the 2- body expectations, the missing particle must be very light m x ~0 In Fermi theory ( 4-fermion interactions ), β decays are mediated by a new type of force called weak. Neutrinos are mass-less leptons (spin ½), which have only weak charge. Participating only in weak interactions, neutrinos are super penetrating and escape detection. Underlying process in β decay is n p e - ν.

6 PHY771, 8/28/2014 Tomasz Skwarnicki 6 µ e ν ν e π,µ get stopped by emulation and decay at rest Back to Powell s observation (1946) µ ν ν e µ π p The observed e energy spectrum indicated 3-body decay Lifetime τ >5x10 26 y 2.2x10-6 s 2.6x10-8 s >2x10 29 y cτ 658 m 7.8 m Since neutrinos are present, these are both weak decays. In fact, these are dominant decays of charged pions and muons. Since these are weak decays, charged pion, muon and (free) neutron are relatively long lived travel many meters before decaying. ν π π µ ν 880 s The observed µ energy spectrum indicated 2-body decay n Average decay time for a particle at rest. 2.6x10 11 m Actual average decay paths are longer than cτ and depend on exact velocity because clock is ticking slower for relativistic particles. Proton and electron are absolutely stable and don t decay. Neutrons are often absolutely stable in nuclei if their decay would violate energy conservation in nuclear β decay.

7 PHY771, 8/28/2014 Tomasz Skwarnicki 7 Direct observation of neutrino 1956 Until 1950 neutrinos were observed only indirectly by their effects in decays of other particles some people remained skeptical about their existence Neutrinos are so penetrating that they can easily pass a shield as thick as thousand light years. The only chance to detect them is to have a very intense source of them. Neutrinos are copiously produced in nuclear reactions. Cowan and Reines set up apparatus near Savannah River reaction in South Carolina with neutron flux 5x10 13 ν/cm 2 /s and observed ν p n e + (inverse of neutron β decay) in This firmly established existence of neutrions. Fred Reines ( ) and Clyde Cowan ( ) Reines received Nobel Prize in 1995 Sun is a distant, but a huge nuclear reactor. It produces ~10 11 ν/cm 2 /s at Earth s surface. They constantly pass through you Nowadays there are experiments studying solar neutrions.

8 PHY771, 8/28/2014 Tomasz Skwarnicki 8 Antineutrinos In cosmic rays both negative and positive pions (and muons) were observed. Like electrons and positrons, these are particles and antiparticles to each other. Neutral particle can be its own antiparticle (e.g. photon), but not necessarily (antineutron is different from neutron) Is there an antineutrino? Is one of these antineutrino? (cannot happen for a free proton, but it does happen for protons in some nuclei!) β decay: n p e - ν β + decay: p n e + ν Crossing symmetry If AB CD happens, then cross-reactions A BCD AC BD CD AB can also happen (if kinematically allowed) π µ ν π + µ + ν Suppose n p e - ν i.e. reactors produce antineutrinos Cowan and Reines: ν p n e + This cross-reaction must exist: ν n p e But if neutrino is the same as antineutrino then reactor experiments must also show: ν n p e Not observed! (A. Davies in 1956) Antineutrinos exist!

9 PHY771, 8/28/2014 Tomasz Skwarnicki 9 Lepton number and its conservation Konopinski and Mahmoud 1953 defined a lepton number (L): L(lepton) = +1 L(antilepton)= -1 L(not lepton)= 0 and suggested that total lepton number is a conserved quantity (like electric charge conservation) : Σ i L i = const Reaction sought by Davies would violate this conservation law: ν n p e Σ i L i 1+1=2 1+1=2 ν n p e Σ i L i -1+1=0 1+1=2 Implications for other decays: π µ ν π + µ + ν Σ i L i 0 1+(-1)=0 µ e ν ν Σ i L i (-1)=1

10 PHY771, 8/28/2014 Tomasz Skwarnicki 10 Conservation of electron and muon numbers µ e γ Σ i L i 1 1+0=1 Allowed by lepton number conservation, but not observed (we are still looking for it ) This leads to a postulate that there are distinct electron neutrino and muon neutrinos, and that lepton numbers defined separately for electron and muon families are conserved µ e γ Σ i L µ i 1 0+0=0 Σ i L e i 0 1+0=1 µ e ν e ν µ Σ i L µ i =1 Σ i L e i 0 1+(-1)+0=0 OK

11 PHY771, 8/28/2014 Tomasz Skwarnicki 11 Confirmation of two-neutrino hypothesis 1962 Hypothesis that there are two distinct neutrinos: electron neutrino and muon neutrino (each with its own antiparticle) was confirmed experimentally by Lederman Schwartz Steinberger in 1962: An accelerator based experiment at Brookhaven (NY), in which muon anti-neutrino beam was produced via: π µ ν µ and directed into proton-reach target: ν µ p n µ + ν µ p n e + Observed Not observed

12 PHY771, 8/28/2014 Tomasz Skwarnicki 12 Tau lepton Discovered at SPEAR collider at SLAC with LBL detector e + e τ + τ τ µ ν µ ν τ τ + e + ν e ν τ Martin Perl 1927 US Nobel Prize 1995 Yet much heavier version of electron Much shorter lifetime Comes with its own neutrino. Tau lepton number is a conserved quantity. The same puzzle as with muon who ordered that?

13 PHY771, 8/28/2014 Tomasz Skwarnicki 13 Intermediate boson of weak interactions W ± In Fermi theory of weak interactions, four-fermions had contact interactions. This could be fixed by introduction of a heavy, spin-1 (i.e. vector boson ), charged weak-force carrier (W - ) It gave a good quantitative description of the data on weak interactions, but had a divergent behavior when extrapolated to reactions at very high energies Consistent formulation of quantum field theory for weak interactions proved theoretically more difficult than for electromagnetic (QED), but eventually was accomplished

14 PHY771, 8/28/2014 Tomasz Skwarnicki 14 Electroweak theory - W ±,Z 0,H 0 Consistent quantum field theory of weak interactions by Glashow-Weinberg-Salam (~ ) made a number of bold predictions: In addition to weak interactions mediated by W ± ( charged currents ) there should also be a heavy, neutral intermediate vector boson of weak interactions Z 0 ( neutral currents ) Weak and electromagnetic interactions are different aspects of electroweak interactions (one theory describes both). What makes W ±,Z 0 have large masses (~ 90 x proton mass), while γ has no mass, are interactions with a scalar (spin 0) Higgs field, which should also manifest itself in a form of a real scalar particle H 0 Given various experimental constraints on the parameters of this theory, W and Z 0 masses were precisely predicted. Predictions for H 0 mass were very uncertain.

15 PHY771, 8/28/2014 Tomasz Skwarnicki 15 Experimental confirmation of neutral weak currents Neutrino scattering on protons observed at CERN in Gargamelle bubble chamber in 1973 ν µ e - ν µ e - e - e - Glashow-Weinberg-Salam get Nobel Prize in 1979

16 PHY771, 8/28/2014 Tomasz Skwarnicki 16 Observation of W ±,Z Z 0 e + e - Observed at Super Proton Synchrotron (SPS) at CERN in 1983 with masses as predicted Carlo Rubbia 1934 Italy Nobel Prize 1984 Simon van der Meer Dutch W - e - ν e

17 PHY771, 8/28/2014 Tomasz Skwarnicki 17 Higgs discovery - H H 0 γγ Observed at Large Hadron Collider at CERN in 2012 with mass close to the lower end of the predicted range H 0 µ + µ µ + µ Peter Higgs 1929 UK (Edinburgh) Nobel Prize 2013 Francois Englert 1932 Belgium