The Discovery of the Kaons

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The Discovery of the Kaons
Emily Conover University of Chicago PH 364 5/7/08
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The state of particle physics in mid 1940s-
Yukawas pion has been found Diracs 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.
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Rochester and 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
Sir Clifford Charles Butler
Biographical Memoirs of Fellows of the Royal
Society Vol. 47, (Nov., 2001)
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V events
Rochester and Butler, Nature 160 (1947), 855
V0 ? a b-
V ? c neutral particle(s)
Probably pions
where c is a pion or muon
Masses of V particles were estimated at 500 200
MeV - unlike anything seen before
These eventually became known as the decays ?0 ?
?? and ? ?? ??.
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Cecil Powell and the Bristol Group

• -Weve already met him (in Antons talk)
• His group used emulsions at a
• research station at Jungfraujoch (Switzerland)

http//nobelprize.org/nobel_prizes/physics/ laurea
tes/1950/powell-bio.html
3580 meters above sea level The top of Europe
http//www.ifjungo.ch/
In 1949 they publish a photo in Nature of a
particle that comes to be known as the ?.
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Another heavy meson event
?? ? ? ? -
Brown et al., Nature 163, 82 (1949)
? - captured by nucleus
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Early 1950s 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
Qcharge, 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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?? ? ? ? - ? ? ? ?0
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.
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What about the other particles?
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.
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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Strangeness
The 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 Pais
http//scitation.aip.org/journals/doc/PHTOAD-ft/vo
l_54/iss_5/79_2.shtml
Gell-Mann on a Guinean Stamp!
http//www.mlahanas.de/Physics/Bios/MurrayGellMann
.html
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Mixing of Neutral Kaons- CP Invariance
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. CPKLgt
-KLgt , CPKsgt Ksgt, one of which has a long
lifetime, the other of which has a shorter
lifetime. Ks can decay to two pions (which have
CP1), 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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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
The greatest pleasure a scientist can experience
is to encounter an unexpected discovery - Cronin
in his Nobel lecture.
http//nobelprize.org/nobel_prizes/physics/laureat
es/1980/
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Conclusions -
Discovery of Kaons led to -the idea of
strangeness -discovery of parity
nonconservation -discovery of CP violation