Particle Physics: Status and PerspectivesPart 4: The Standard Model

Manfred Jeitler

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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

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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

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charged currentevents

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neutral current

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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

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the GIM mechanism

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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

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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

“tree level”read the small print

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penguin diagram

“tree level”read the small print

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penguin diagram

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probabilities are higher for decays where quark lines are connected

“fewer things have to change”

explanation by number of gluons to be exchanged

the Zweig rule

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conserved quantities (quantum numbers)

mass/energy absolutely conserved no perpetual motion machine (alas!)

charge absolutely conserved upper limit: 10-19

new quantities: baryon number (=1/3 for any quark)

seems absolutely conserved - but there could be a small violation

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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

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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

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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)

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beauty - hidden in the Υ

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

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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

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the W± and Z0 bosons

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discovery of the W± and Z0 bosons:

the UA1 detector

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last one to have been found: the top quark

top quark

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

top quark

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