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Particle Physics: Particle Discoveries Jun.Prof . Dr. Schott. The following QN are conserved in weak interactions Electric charge-conservation: Define LQ which is +1 for positive charge particles and -1 for negative charged particles
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Particle Physics: Particle Discoveries Jun.Prof. Dr. Schott
The following QN are conserved in weak interactions • Electric charge-conservation: Define LQ which is +1 for positive charge particles and -1 for negative charged particles • Lepton-Flavour-Number: Define Lepton-Flavour Number (Le,Lμ,Lτ), where each lepton is +1 and each anti-lepton −1 • Baryon-Number: For each incoming quark at a given vertex, one also has to have one outgoing quark • Lets look at the following reaction • Has to be weak interaction (Neutrinos are not charged) • No charge transport => Neutral Current Discovery ofthe Neutral Current (1/4)
Why do we want Anti-Neutrinos? • If we would use neutrinos, then we would also have an alternative reaction νμ + e− → μ− + νe, which is caused by a charged current and could fake the neutral current • Basic idea • The idea is therefore to create a beam of ν ̄μ and let it interact with matter (i.e. electrons). • Also possible to measure the relative strength of the charged and neutral current, by looking into the reactions • Counting number of NC and CC events allows to measure cross-section and hence their respective strength Discovery ofthe Neutral Current (2/4)
Discovery ofthe Neutral Current (4/4) • Result • Strength of NC and CC is not the same!
Two independent experiments • Standford (B. Richter et. al.) and Brookhaven (S. Ting et. al.). • Both groups found the bound state of charmonium with a mass of mJ/Ψ ≈ 3.1GeV and a width of Γ = 93keV • Much smaller than other known resonances, e.g. Γ(ρ(776)) = 150M eV • Its width if far too small to explain it with another resonance only based on u-, d- and s-quarks. • The new charm-quantum number explains the long life-time of the resonance: OZI-rule Discovery oftheCharm Quark (1/2)
Problem in the 1960s: • Why do weneedmuons? Are theyspecial? • Martin Perl: maybe there is also another charged lepton • Perl started to look into the production of new heavy leptons in e+e− collisions at SPEAR (SLAC), √s = 8 GeV: • e+ + e− →L++L− →e±+μ∓+ ETMiss • Many attractive features: • Analogous to muon decay • Will disappear below a certain treshold • Unique signature (since e and μ in the event) Discovery ofthe Tau-Lepton (1/2)
Use of the MARK-1 Detectors, which, was also used for the c-quark discovery • One of the first general purpose detectors: • measurement of time-of-flight via triggers-counters, allows the determination of velocity • wire-chambers in the magnetic field allow the determination of momentum • electrons were identified via a shower in the calorimeter • hadrons are stopped with the iron shielding • muonwere identified at the outer wire-chambers Discovery ofthe Tau-Lepton (2/2)
proton-anti-proton collider (SPS) at CERN with a center of mass energy √s = 600GeV • SPS Collider was using stochastic cooling to ensure a dense proton/anti-proton beam. • The detection of the W-Boson was based on the following reaction: • Question: Calculate the collision energy of the collider that is needed to produce W’s with a mass of 80 GeV Discovery ofthe W/Z Bosons (1/5)
UA1 and UA2 detectors are already full 4π general purpose detectors, including • Inner Detector • Electromagnetic Calorimeter • Hadronic Calorimeter • Muon Chambers Discovery ofthe W/Z Bosons (2/5)
Discovery of a three new Bosons • W+ • W- • Z • The mass of W+/W- bosons (≈80 GeV), which are responsible for the charged current, is different than the Z Boson mass of (≈90 GeV) • The interaction strength of CC and NC is different! • => The Z Boson cannot be the neutral W3 Boson which we had in our SU(2)L theory of the weak interaction • => Need Electroweak Unification! Summary
