The Discovery of the Kaons

Emily ConoverUniversity of Chicago

PH 3645/7/08

The state of particle physics in mid 1940’s-

Yukawa’s pion has been found

Dirac’s positron has been found

Idea of neutrinos was mostly accepted

The muon was confusing, but otherwise, it looked like things were falling together in an orderly fashion that was fairly well understood.

Rochester and Butler

Biographical Memoirs of Fellows of the Royal Society Vol. 47, (Nov., 2001)

Sir Clifford Charles Butler

1945 - Butler joins the physics department at Manchester University

1946- Butler begins work with George Rochester

They used a setup with a cloud chamber and a magnetic field to investigate particles in cosmic ray showers at ground level. Sets of counters provided a triggering system.

1947 - photos of V0 and V+ events are published

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V0 a+ + b- V+ c+ + neutral particle(s)Probably pions where c+ is a pion or muon

V+

V eventsRochester and Butler, Nature 160 (1947), 855

Masses of V particles were estimated at 500 ±200 MeV - unlike anything seen before

These eventually became known as the decays 0 and + + .

Cecil Powell and the Bristol Group

-We’ve already met him (in Anton’s talk)-His group used emulsions at a research station at Jungfraujoch (Switzerland)

In 1949 they publish a photo in Nature of a particle that comes to be known as the +.

http://www.ifjungo.ch/

3580 meters above sea level“The top of Europe”

http://nobelprize.org/nobel_prizes/physics/laureates/1950/powell-bio.html

+ + + -

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+

-

+

+

- captured by nucleus

Brown et al., Nature 163, 82 (1949)

Another heavy meson event

Early 1950’s developments:

More events were discovered - +, ’+, +, 0, +

Many papers written/photos published relating to the subject.

July 1953 - International Cosmic Ray Conference- Bagneres-de-Bigorre, France. The conference focused on the new particles.

During the conference a “Committee on Nomenclature” was formed to come up with a naming scheme: The generic name K meson was adopted to describe the new particles, and a symbol (mass) where Q=charge, n = number of decay products, and x specifies decay products.

Richard Dalitz (Bristol group - later at Chicago) - “We were all warned by the senior physicists at the conference [...] not to make any simplifying assumptions about the relationships between the particles observed. We should use only these ‘neutral, unbiased’ names [...] until we had firm evidence of any such relationship, beyond any doubt, since it appeared that we were facing a complicated situation.”

~1955 - Work shifted from cosmic ray studies to work with synchrotrons. At the Bevatron (Berkeley) and the Cosmotron (Brookhaven) data about the new particles was accumulated much more rapidly.

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KnxQ

The - puzzle

The two particles were similar - their masses agreed within several MeV, and their lifetimes were roughly equal.Thus they appeared to be related (the same particle?)

However, the decay products of the + have a parity of (-1)(-1)J, while those of the + have a parity of (-1)J, where J is the spin of the original K meson.

Thus, either +and + have opposite parity (and thus are different particles) or parity is not conserved in this decay.

Richard Dalitz - “I found myself unable to withstand the local pressures against any hypothesis of parity nonconservation. The argument against it was that parity violation was simply inconceivable and it was nonsensical even to mention this possibility.”

-Lee and Yang (1956) suggest that the particles are the same and parity is not conserved.-Wu et. al. (1957) - Parity nonconservation in beta decay of cobalt nuclei.Not inconceivable anymore!

+ and + are then accepted as the same particle, called K+.

+ + + -

+ + 0

It was then determined that the many particles could be described by four K mesons- K±, K0, and , where K- is the anti particle of K+, and the second neutral kaon is the antiparticle of the first.

They make up two isospin doublets, with K+ and K0 forming one doublet, and K- and forming a second. The existence of was required to complete the second isospin doublet.

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K0

What about the other particles?

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K0

In addition, other, heavier particles that created similar decays had been found which did not fit this scheme, e.g. the decay now known as p+ + - was discovered by Anderson at CalTech in 1950.

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K0

StrangenessThe particles were produced with copiously, but had long lifetimes.Had lifetimes ~10-10s, where a lifetime of ~10-23s would have been expected from the production rate.

Produced in a different manner than they decayed. Must be produced in pairs- Abraham Pais, 1952

1953 - Murray Gell-Mann and Kazuhiko Nishijima propose a strangeness scheme:

S = 2(Q - I3 - B/2)

For any interaction in which S ≠0, one of the above quantities (charge, baryon number, isospin) must not be conserved. The conservation of charge and baryon number was more fundamental and thus the reactions were taken to violate conservation of I3.

Thus the strange particles are produced in pairs in interactions which conserve strangeness (and isospin), explaining why they are produced frequently, but they decay through weaker interactions that do not conserve strangeness or isospin, which occur much more slowly .

Abraham Paishttp://scitation.aip.org/journals/doc/PHTOAD-ft/vol_54/iss_5/79_2.shtml

Gell-Mann on a Guinean Stamp!

http://www.mlahanas.de/Physics/Bios/MurrayGellMann.html

K0, both can decay to + - or 0 0

Fermi asks - How do we tell them apart?

Leads Gell-Mann and Pais to the idea that the K0 , and states mix,as K0

can become through K0 + - .

They create CP eigenstates. CP|KL> = -|KL> , CP|Ks> = |Ks>, one of which has a long lifetime, the other of which has a shorter lifetime.

Ks can decay to two pions (which have CP=+1), but due to CP conservation, KL can only decay to more complex modes (e.g. three pions). This leads to a longer lifetime for KL.

Of their 1955 article in the Physical Review, Cronin says (in his autobiography)- “You get shivers up and down your spine”

1956 - KL is discovered at Brookhaven.

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Ks =K0 + K0

2

KL =K0 − K0

2

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K0

Mixing of Neutral Kaons-CP Invariance

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K0

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K0

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K0

CP violation -

1964 Cronin and Fitch report that they have observed KL + - in a small fraction of decays - a process forbidden by CP conservation!

Cronin continues his studies of CP violation after coming to Chicago in 1970, wins Nobel prize in 1980.

James W. Cronin Val Fitch

http://nobelprize.org/nobel_prizes/physics/laureates/1980/

“The greatest pleasure a scientist can experience is to encounter an unexpected discovery” - Cronin in his Nobel lecture.

Conclusions -

Discovery of Kaons led to

-the idea of strangeness

-discovery of parity nonconservation

-discovery of CP violation