THE PARTICLE PHYSICS REVOLUTION: 19731977

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THE PARTICLE PHYSICS REVOLUTION 1973-1977 (how
two quarks and two leptons were discovered in
only 4 years)
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Why this talk

• Motivated by Ullas History of Particle Physics
  Thursday seminar
• .. which ended with the neutral current discovery
  (1973)
• The following years,1973-1977, were enormously
  exciting
• I was lucky to be where most of the action took
  place (SLAC) hence you will get my personal,
  somewhat biased, perspective.
• Will use Feynman graphs, but very few formulas -
  otherwise talk would be too long.
• I hope that the younger among you will learn
  something and gain some perspective of how
  physics is done.

• Some of my sources
• Stanford Lepton-Photon conference, 1975
• D. Perkins, Introduction to High energy Physics,
  2nd edition, 1982
• M. Riordan, The Hunting of the Quark a True
  History of Modern Physics, 1987

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The frontiers of particle physics in the early
1970s
(as seen by some more easily seen as such now,
of course!)

• WEAK INTERACTIONS THE DISCOVERY OF NEUTRAL
  CURRENTS
• The discovery of neutrino interactions of type
  ??e -gt ??e (and ??N -gt ??Nhadrons) in 1973 (at
  CERN) indicated the existence of processes of
  type

In which a NEUTRAL boson (the Zº!) would be
exchanged. In a bubble chamber, one sees an
electron or hadrons appearing out of
nothing not easy to observe.
Zº
These interactions have intensity comparable to
the already-known charged current interactions
??e -gt??e and ??N -gt ??Nhadrons in which a
CHARGED boson (W) would be exchanged.
?
W
?, or Nhad
The Weinberg-Salam model (1967-68) had postulated
such interactions, in the context of electroweak
unification. But NONE of these bosons had been
observed!
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The first neutral current event Gargamelle
heavy-liquid bubble chamber, CERN, 1973
??e interaction
Electron irradiates, gammas make ee- pairs
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2. DEEP INELASTIC SCATTERING -gt HADRON
PRODUCTION IN ee- REACTIONS
The observation of SCALING in deep inelastic
e-nucleon scattering (DIS) suggested the presence
of POINTLIKE partons within nucleon, AND that
parton interactions al large q2 would be weak. In
almost perfect coincidence with theoretical
discovery (1973) of asymptotic freedom of QCD!
The DIS graph, turned around, is closely related
to ee- annihilation into hadrons
If charged partons are the quarks of SU3(flavor)
q(u) 2/3, q(d) q(s) -1/3, the cross-section
of ee- ? hadrons can be simply calculated.
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HADRON PRODUCTION IN ee- REACTIONS
It is described in terms of the ratio R
?had(ECM)/???(ECM) where ??? 4??2/3E2CM
(87/E2CM) nb because
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R
2
? q2i 3 where the sum is over all
accessible quark flavors and 3 for the three
colors of SU(3).
In the early 70s a new ee- storage ring, SPEAR,
came into action at SLAC
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SPEAR maybe the most successful HEP facility
ever built. It was never authorized hence it had
no roof, and was built on a parking lot. It was
built as an experiment, not an accelerator
this bypassed the funding rules!
Electrons, positrons from 2 mile Linac
1996
The big, general-purpose detector was here
I did four short experiments at this IR
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The Mark I, the in-house detector of the
SLAC-LBL group.
Conceived by B. Richter, built by R.Schwitters
(in picture), later director of ill-fated SSC.
Inaugurated the standard general purpose
colliding beams detector solenoidal B, tracker,
shower counter, muon detector.
A brilliant concept whose enormous success shaped
most of the following collider detectors.
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THE CONFUSING SITUATION OF R, UNTIL 1974
Points by Adone spread over large interval 2 very
high points by CEA... No respectable theory fit
data. A few less-respectable models arose and
rapidly died. Mark 1 1972-73 points NOT SHOWN
in this (Schwitters) plot at 1975 L-Ph conference
did not disagree with previous experiments...
Only because of the huge systematic errors.
Then in the fall of 1974 all hell broke loose
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ALL OF A SUDDEN, IN NOVEMBER 1974, SLAC (MARK I)
ANNOUNCED AN ENORMOUS SIGNAL THEY CALLED IT ?
The peak cross section is 100x background!
The width (? 2 MeV) is given by beam energy
spread (see later)
Why did it take gt2years to discover it?
Because it is so narrow! Noone had thought of
scanning the CM energy in 2 MeV steps! Resonances
were expected but with width ? 100 MeV. Hence
scans in 100 MeV steps were made, in summer 1974
(I was running an experiment then...) and in one
of these scans the RADIATIVE TAIL of the ? was
hit. Different runs were inconsistent, results
did not repeat... Then someone thought that a
narrow resonance might have been missed, and a
fine scan found the ?...
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THE DISCOVERY OF THE J BY S.C.C. TINGS GROUP
AT BROOKHAVEN
A narrow resonance of unexpectedly high mass,
decaying into ee-pairs. Detected in a very
difficult experiment under extreme background
conditions
Ting had the signal since June 74, but kept it
secret - he was afraid of being wrong. When he
heard that SLAC had it, he told a young postdoc,
Sau Lan Wu, to call Frascatis director G
Bellettini... Frascati had to push Adone beyond
design energy, but found the resonance in a week.
Cabibbo thought it was the Zº, because of an
asymmetry of leptonic... decays (statistics!)
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WHAT IS THE PHYSICAL WIDTH OF THE ? ? WHY SO
NARROW ?
Q. How to find the physical width ??1/? of a
very narrow resonance? A. Integrating the
Breit-Wigner cross-section over the CM
energy ??(E)dE (6?2/M2)?ee (subtracting
rad. tail) and BR(ee) ?ee/ ?
?(? ) 70 keV !!!! Compare to ?(?)
150 MeV (u?u d ?d)/?2 ?? ?(?) 8
MeV (u?u d ?d)/?2 ?(?) 4 MeV
s?s .. A clear hint of a new
quantum number or conservation law .....
Within days of experimental discovery, theorists
from Harvard, Princeton, etc. had a model
explaining its width and predicting several more
psi-like particles... the CHARMONIUM picture. The
J/? was not unexpected! Only its small width was
surprising.
To understand it, we must take a few steps back
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THE THEORETICAL ORIGIN OF THE CHARMED QUARK
In the late 60s there was no symmetry between
leptons and the (hypothetical) quarks
2 lepton (weak) isodoublets (W-S), connected by
charged current
A (strong) SU3 triplet, with charged current
Cabibbo mixing
?
Neutral strangeness-changing couplings were
ALLOWED, but very SUPPRESSED KºL??? 710-9
?
d?
In 1970, Glashow, Iliopoulos Maiani proposed a
4th charmed quark with Cabibbo-like coupling to
s, d quarks.
This CANCELS the strangeness-changing couplings,
and agrees with experiment if quark is not too
heavy (mc ? 1.8 GeV)
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A 1.5 - 1.8 GeV quark is much heavier than the
other three. Its kynematics and dynamics are
different.
The c quark is much heavier than the QCD mass
scale.
Hence the cc? atom will be smaller, hence
more WEAKLY bound than light quark composites
it will resemble quantum-mechanical atoms!
The J/? was a perfect candidate for a c?c atom...
And note that it had no net charm. Then it could
be narrow because its natural decay to charmed
mesons might be energetically forbidden
FORBIDDEN m? lt 2mD
ALLOWED m? lt 2mK
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BUT THERE IS MORE TO THE SMALL WIDTH OF THE
J/? ! Consider again the ?
BUT
Has ??1 MeV, because it is a disconnected graph
only gluons can go from initial to final state
Has ? 3 MeV, because phase space is small
This was known as the Okubo-Zweig-Iizuka rule.
Not quantitative, but was seen as related to
asymptotic freedom...
Now consider J/? instead of ? m 3.1 GeV, not
1.02 GeV... and with ?S getting smaller with
increasing energy, ?(J/?) lt ?(?) !
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... And if there is a narrow state, why not two?
Three?
In another another fine energy scan, a second
narrow-width particle was found, in Dec. 1974.
It was called ?(3684), and it decays mostly to
???. A new spectroscopy had emerged.
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POSITRONIUM AND CHARMONIUM TWO VERY SIMILAR
PARTICLE-ANTIPARTICLE SYSTEMS
The J/ ? and the ? were discovered before the
end of 2004. By mid 1975, there was evidence for
several more charmonium states, radiatively
coupled to the states directly produced in
ee-collisions
The well-known example of positronium served as a
guide. NOTE THAT THIS IS THE FIRST VERY CLEAR
CASE OF ATOM-LIKE SPECTROSCOPY IN A
QUARK-ANTIQUARK SYSTEM.
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BUT WHAT ABOUT CHARMED MESONS?
?
j/?

• The 1975 Mark I data on R showed (besides the
  narrow ? states)
• A complicated structure, suggestive of threshold
  effects. CHARMED MESONS were found in this energy
  region, in 1976...
• A rise of R of gt2 units, hard to explain in terms
  of only one more q2/3 quark (2/3)2 3 4/3

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More new and unexpected facts in 1974-75, in the
Mark 1 data, M. Perl found events of type ee- ?
e ? nothing This was clear evidence of pair
production of new particles. It could be D ???,
D- ?e-? (not yet discovered then) OR a new
sequential lepton. like a heavier muon U
????, U- ??-?? Now the U is called the ? lepton
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AND THE FIRST EVIDENCE FOR JETS, THE
MANIFESTATION OF QUARK-ANTIQUARK PRODUCTION ....
SPEAR operated with 3 GeV lt Ecms lt 7.4 GeV At
these low energies, quark jets are not very
collimated. Need SHAPE variables (SPHERICITY was
the first of many) to see how shape of event
varies with energy. Still, the evidence from MARK
I data (Gail Hansen) persuaded most
people.... At higher energies, jets became very
evident.
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BRINGING IT ALL TOGETHER H. HARARIS TALK AT THE
1975 STANFORD CONFERENCE
In a remarkable talk, Harari elucidated several
aspects of the J/? data, of the (then emerging)
spectroscopy, and addressed the issue of the (too
large) step in R.
He took the e? events as evidence for a
sequential lepton, that would decay mostly to
hadrons. In modern language ????W, W???, ?e,
3ud ?hadrons This gave ? 1 unit of R, thereby
explaining most of the rise of R above 4 GeV.
Note the coincidence, a new quark and a new
lepton with similar masses! (the fact that muons
and pions have similar masses also was confusing)
Harari then postulated a new quark doublet (t,
b) in order to preserve quark-lepton symmetry!
(and in Moriond, a few months later, he showed
how 3 doublets and the K-M matrix would be
necessary to have CP violation)
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THE DISCOVERY OF THE ? BY L. LEDERMANS GROUP
AT FERMILAB
In summer 1977, history repeated itself with a
two-arm spectrometer similar to Tings at BNL,
but made to detect muon pairs, Ledermans group
found enhancements in the dimuon mass spectrum.
They were the ground state and first radial
excitation of the bottomonium atom
?
?
At that point, we at PEP and PETRA colliders (lt
30-45 GeV Ecms) thought we were going to find the
top quark.. We were wrong by a factor of ? 10 on
the top quark mass
m??
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TAKING STOCK OF THE DISCOVERY OF CHARM, BEAUTY
AND OF THE TAU LEPTON YEARS LATER
Charm, a concept originating from the weak
interactions allowed to extend the W-S model to
quarks, while re-establishing quark-lepton
symmetry.
The discovery of hidden charm in ee-
interactions made quarks (more) believable as a
physical entity.
The psion decay dynamics and the hadron
production dynamics added experimental proofs to
QCD predictions
The (totally unexpected) discovery of a third
generation of leptons brought quark-lepton
symmetry on the stage again, and motivated
experiments that concluded in 1989 with the LEP
measurement of the number of light neutrinos
(1989) and the t-quark discovery(1996)at FNAL
and are still ongoing at B factories.
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A FEW COMMENTS IN CLOSING
In those years, several models arose and died
now you only hear of the ones that survived.
A few clever (and lucky) theorists were (almost)
always right. Not necessarily those who were
right in previous years past performance is no
guarantee of future success
Persistence was rewarded Perl had looked for
heavy leptons for gt10 years Lederman had
narrowly missed he J/???? in BNL in the late
60s... In other cases, experimenters were very
slow finding R structures, charmed mesons....
Some of us learned that the way to do good
physics is with a powerful, general detector that
will stay on the IR.
As you saw, things happened extremely fast they
had been slow in the previous years. Maybe it
will happen again at the LHC? (but dont count on
so many discoveries in such a short time)