THE PARTICLE PHYSICS REVOLUTION: 1973-1977
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THE PARTICLE PHYSICS REVOLUTION: 1973-1977 (how two quarks and two leptons were discovered in only 4 years) PowerPoint PPT Presentation


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THE PARTICLE PHYSICS REVOLUTION: 1973-1977 (how two quarks and two leptons were discovered in only 4 years). Why this talk. Motivated by Ulla’s “History of Particle Physics” Thursday seminar .. which ended with the neutral current discovery (1973)

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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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June 28 2007

THE PARTICLE PHYSICS REVOLUTION: 1973-1977

(how two quarks and two leptons were discovered in only 4 years)

IFAE Thursday Meeting - M.C.-S.


June 28 2007

IFAE Thursday Meeting - M.C.-S.


Why this talk

Why this talk

  • Motivated by Ulla’s “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

IFAE Thursday Meeting - M.C.-S.


June 28 2007







e, or N

e

e

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 -> e (and N ->N+hadrons) 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.

These interactions have intensity comparable to the already-known “charged current interactions” e ->e and N -> N+hadrons in which a CHARGED boson (W±) would be exchanged.

, or N+had

The Weinberg-Salam model (1967-68) had postulated such interactions, in the context of electroweak unification. But NONE of these bosons had been observed!

IFAE Thursday Meeting - M.C.-S.


June 28 2007

The first neutral current event : Gargamelle heavy-liquid bubble chamber, CERN, 1973

e interaction

Electron irradiates, gammas make e+e- pairs

IFAE Thursday Meeting - M.C.-S.


June 28 2007

e-

e-

e+

hadrons

hadrons

e-

N

2. DEEP INELASTIC SCATTERING -> HADRON PRODUCTION IN e+e- REACTIONS

The observation of SCALING in deep inelastic e-nucleon scattering (DIS) suggested the presence of POINTLIKEpartons 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 e+e- 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 e+e- hadrons can be simply calculated.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

e+

hadrons

e-

e+

e-

HADRON PRODUCTION IN e+e- REACTIONS

It is described in terms of the ratio

R = had(ECM)/(ECM) where  = 42/3E2CM = (87/E2CM) nb

because

2

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 e+e- storage ring, SPEAR, came into action at SLAC

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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 >2years 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 ...

IFAE Thursday Meeting - M.C.-S.


June 28 2007

THE DISCOVERY OF THE J BY S.C.C. TING’S GROUP AT BROOKHAVEN

A narrow resonance of unexpectedly high mass, decaying into e+e-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 Frascati’s 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!)

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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

IFAE Thursday Meeting - M.C.-S.


June 28 2007

e



c

-sinc

e

cosc

u

cosc

d

sinc

s

IFAE Thursday Meeting - M.C.-S.


June 28 2007

e



c

-sinc

e

cosc

u

cosc

d

sinc

s

s

KºL

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 = 7·10-9

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)

IFAE Thursday Meeting - M.C.-S.


June 28 2007

1.5 GeV

c

s

QCD

150 MeV

u, d

1-5 MeV

K+

D+

s

u

c

d

J/

u

d

s

K-

c

D-

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 < 2mD

ALLOWED

m < 2mK

IFAE Thursday Meeting - M.C.-S.


June 28 2007

+

º

-

K+

s

u

u

s

K-

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/) < () !

IFAE Thursday Meeting - M.C.-S.


June 28 2007

... 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.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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 e+e-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.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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 >2 units, hard to explain in terms of only one more q=2/3 quark: (2/3)2 ·3 = 4/3

IFAE Thursday Meeting - M.C.-S.


June 28 2007

More new and unexpected facts: in 1974-75, in the Mark 1 data, M. Perl found events of type

e+e- 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

IFAE Thursday Meeting - M.C.-S.


June 28 2007

AND THE FIRST EVIDENCE FOR JETS, THE MANIFESTATION OF QUARK-ANTIQUARK PRODUCTION ....

SPEAR operated with 3 GeV < Ecms < 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.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

BRINGING IT ALL TOGETHER: H. HARARI’S 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, 3·ud 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)

IFAE Thursday Meeting - M.C.-S.


June 28 2007

THE DISCOVERY OF THE  BY L. LEDERMAN’S GROUP AT FERMILAB

In summer 1977, history repeated itself: with a two-arm spectrometer similar to Ting’s at BNL, but made to detect muon pairs, Lederman’s 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 (< 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

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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 e+e- 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.

IFAE Thursday Meeting - M.C.-S.


June 28 2007

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 >10 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 don’t count on so many discoveries in such a short time)

IFAE Thursday Meeting - M.C.-S.


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