Particle Physics: Status and Perspectives Part 4: The Standard Model Manfred Jeitler SS 2015

Cabibbo angle Strange quark is in some ways similar to Down quark Λ0 = (uds) “heavy brother” of proton but Strange quarks decay into Up quarks they are not completely separate, but somehow coupled to “first generation” for the Weak interactions, the quarks are not quite the same as the “physical” quarks (“mass eigenstates”) they appear to be “rotated”: ϑc ~ 13º What does “mixing of states” mean? Quantum mechanical concept: a “particle” can be a bit of one, and a bit of the other, and only measurement decides what it really is (“the wave function collapses during observation”) analogy to double-slit experiment, where the electron seems to go through both slits (--> interference)

neutral currents experimenting with neutrino beams how would you make a neutrino beam? “charged-current events”: νμ N  μ- X, μ N  μ+ X “neutral-current events”: νμ N  νμ X, μ N  μ X draw the elementary Feynman graphs! Can interpret as decay of a “real” W or as transformation of particles, mediated by a “virtual” W

charged current events neutral-current events were first seen in bubble-chamber experiments at CERN in 1973 these observations were an important element in our understanding of the Standard Model

neutral current

the GIM mechanism it was noted that all neutral-current transitions had ΔS=0 Strange quark would transform into Up but not into Down no “flavor-changing neutral currents” … at tree level! Glashow, Iliopoulos and Maiani proposed a mechanism to explain this now called the GIM mechanism

the GIM mechanism the “Charm” quark was postulated to be some sort of “heavier brother” of the Up quark, just like the Strange quark was a “heavier brother” of the Down quark this way, the ΔS=1 transition amplitudes (transitions between Down and Strange quarks) cancel exactly and one can explain why these transitions are never observed This paradigm can be observed time and again in modern particle physics: to explain an effect, or also its absence, a new particle is “introduced” that explains this behavior. Another case of this reasoning was the postulate of the neutrino, to explain the electron’s energy distribution in nuclear beta decay. Similar reasoning has lead to the prediction of the Higgs particle, which was found only 50 years after having been originally predicted, or “Supersymmetric” particles, which have not yet been observed .

completing the second generation of quarks: charm and the J/ψ particle the GIM mechanism had proposed another quark in the 2nd generation but it seems this prediction remained unheeded by experimentalists it was by chance that at the proton fixed-target accelerator at Brookhaven, a resonance in the cross section was observed at 3.1 GeV “AGS” = Alternating Gradient Synchrotron, at “BNL” = Brookhaven National Laboratory before this had been sufficiently confirmed to allow for publication, it seems that rumors about the discovery spread to the Stanford SPEAR accelerator, an e+e- collider and allowed to look for the resonance in the appropriate place due to the much lower background at the electron-positron collider, the resonance could be quickly confirmed there

J/ψ discovery

completing the second generation of quarks: charm and the J/ψ particle the new particle was understood to be a bound state of a new quark and its antiquark due to the shared discovery, the particle got the unusual name “ J/ψ “ Sam Ting ( ) from Brookhaven and Burt Richter from Stanford shared the Nobel prize for this discovery the new quark was dubbed “charm”: c the J/ψ has the quark content cc L3 was supposed to be named “SAM”, for “Schopper approves me”

“tree level” read the small print penguin diagram

“tree level” read the small print penguin diagram

the Zweig rule probabilities are higher for decays where quark lines are connected “fewer things have to change” explanation by number of gluons to be exchanged

conserved quantities (quantum numbers) mass/energy absolutely conserved no perpetual motion machine (alas!) charge upper limit: 10-19 new quantities: baryon number (=1/3 for any quark) seems absolutely conserved - but there could be a small violation arguments against absolute conservation of baryon number: baryogenesis (development of matter/antimatter asymmetry from Big Bang symmetry gauge theories: there seems to be no field associated to baryon number

conserved quantities (quantum numbers) lepton numbers Le, Lμ, Lτ conserved? violation not yet seen for charged leptons neutrino “oscillations” seen, however! properties that distinguish the quark “generations”: Strangeness S: -1 for Strange, +1 for antiStrange Charm C +1 for Charm, -1 for antiCharm Beauty B -1 for bottom, +1 for antiBottom (Truth T) violated only by Weak interactions generations live in “separate worlds” for other interactions how would you look for L number violation in charged leptons?

conserved quantities (quantum numbers) isospin conserved only by Strong interactions violated by Electromagnetic and Weak interactions proton and neutron described as two isospin states of the same particle (the “nucleon”) discrete symmetries parity, charge parity, time reversal see later

the third generation of quarks: beauty the Kobayashi-Maskawa theory of CP-violation had predicted a third generation of quarks in 1977, the first bound state of the “bottom” or “beauty” quark was found: the “upsilon” ( Υ ) a bb state, similar to the cc state of the J/ψ 400-GeV protons on fixed target at Fermilab (Chicago)

beauty - hidden in the Υ the Υ appears as a resonance peak in the cross section for the production of μ+μ--peaks

the W± and Z0 bosons the W± and Z0 gauge bosons are much too short-lived to reach a detector they may, however, be produced as “real” particles and their decay products can be seen due to their high mass, a high-energy accelerator was needed to produce them SPS (“Super Proton Synchtron”) at CERN colliding protons with antiprotons indirectly, these particles are visible as “virtual” particles in processes mediated by them, for example beta-decay for the W neutral-current events for the Z mW = 80.403 ± 0.029 GeV/c2 ΓW = 2.141 ± 0.041 GeV τ ~ 3*10-25 sec; how far does light travel in this time? How far could a W get? how far would a “virtual” W get? Is the picture of a “point interaction” justified? (When?) mZ = 91.1876 ± 0.0021 GeV/c2 Γz = 2.4952 ± 0.0023 GeV

the W± and Z0 bosons the “signatures” for the decays of W and Z bosons were, among others, high-energy leptons (arrow!) it is not so obvious, however, to disentangle these tracks from the large amount of low-energy “junk” !

discovery of the W± and Z0 bosons: the UA1 detector for chasing the W and Z bosons, the CERN SPS accelerator was converted into a proton-antiproton collider, upon the initiative of Carlo Rubbia the necessary narrow particle beam was achieved by means of “stochastic cooling”, invented by Simon van der Meer the bosons were detected in 1983, which earned Rubbia and van der Meer the 1984 Nobel prize

last one to have been found: the top quark top quark was last to be found (1995) people had been wondering for quite some time why it could not be found discovered at the Fermilab (Chicago) proton-antiproton collider (the “Tevatron”, from the 1 TeV beam energy) much heavier than expected (178 GeV)

top quark forms no hadron because short-lived decays always into b-quark

top quark forms no hadron because short-lived decays always into b-quark