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

2. 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)

3. 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

4. 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

5. neutral current

6. 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

7. 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 .

8. 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

9. J/ψ discovery

10. 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”

11. “tree level” read the small print
penguin diagram

12. “tree level” read the small print
penguin diagram

13. 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

14. conserved quantities (quantum numbers)
mass/energy
absolutely conserved
no perpetual motion machine (alas!)
charge
upper limit:
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

15. 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, for antiCharm
Beauty B 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?

16. 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

17. 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)

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

19. 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 = ± GeV/c2
ΓW = ± 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 = ± GeV/c2
Γz = ± GeV

20. 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” !

21. 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

22. 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)

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

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