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1 Apr
A Direct Cp Violation

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From: Peter Langston <psl>
Date: Thu,  1 Apr 99 12:30:27 -0800
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Subject: A Direct Cp Violation

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The American Institute of Physics Bulletin of Physics News
Number 420 March 29, 1999  by Phillip F. Schewe and Ben Stein

DIRECT CP VIOLATION has been observed at Fermilab by the KTeV collaboration.
An important way of apprehending the basic nature of time and space (in the
finest tradition of Greek philosophy) is to ask "what if" questions. For
example, will a collision between particles be altered if we view the whole
thing in a mirror?  Or what if we turn all the particles into antiparticles?
These propositions, called respectively parity (P) and charge conjugation
(C) conservation,  are upheld by all the forces of nature except the weak
nuclear force. And even the weak force usually conserves the compound
proposition of CP. In only one small corner of physics---the decay of K
mesons---has CP violation been observed, although physicists suspect that
CP violation must somehow operate on a large scale since it undoubtedly
helped bring about the present-day preponderance of matter over antimatter.
    K mesons (kaons) are unstable and do not exist outside the interiors of
neutron stars and particle accelerators, where they are artificially spawned
in K-antiK pairs amidst high energy collisions.  K's might be born courtesy
of the strong nuclear force, but the rest of their short lives are under
control of the weak force, which compels a sort of split personality:
neither the K nor anti-K leads a life of its own.  Instead each transforms
repeatedly into the other. A more practical way of viewing the matter is to
suppose that the K and anti-K are each a combination of two other particles,
a short lived entity called K1 which usually decays to two pions (giving K1
a CP value of +1) and a longer-lived entity, K2, which decays into three
pions (giving K2 a CP value of -1). This bit of bookkeeping enshrined the
idea then current that CP is conserved.
   All of this was overthrown when in 1964 the experiment of Jim Cronin and
Val Fitch showed that a small fraction of the time (about one case in every
500, a fraction called epsilon) the K2 turns into a K1, which subsequently
decays into two pions.  This form of CP violation is said to be indirect
since the violation occurs in the way that K's mix with each other and not
in the way that K's decay. One theoretical response was to say that this
lone CP indiscretion was not the work of the weak force but of some other
novel "superweak" force.  Most theorists came to believe, however, that the
weak force was responsible and, moreover, that CP violation should manifest
itself directly in the decay of K2 into two pions.  The strength of this
direct CP violation, characterized by the parameter epsilon prime, would be
far weaker than the indirect version.  For twenty years detecting a nonzero
value of epsilon prime has been the object of large-scale experiments at
Fermilab and for nearly as long at CERN.  In each case, beams of K's are
sent down long pipes in which the K-decay pions could be culled in sensitive
    At the APS Centennial meeting in Atlanta last week, both groups
discussed their work. The KTeV group at Fermilab reported a definite result:
a ratio of epsilon prime to epsilon equal to 28 (+/- 4) x 10^-4, larger than
the theoretical expectation. As for the NA48 group at CERN, Lydia
Iconomidou-Fayard ( said that data analysis was still
proceeding and no definite measurement could be reported at this time. The
principal conclusion was stated by KTeV co-spokesman Bruce Winstein
(, 773-702-7594): Before the new experiments direct
CP violation had not been established, owing to the large uncertainty in
the early measurements of epsilon prime; the new experiment, by contrast,
does succeed in establishing a nonzero value for epsilon prime, thus
providing a new way to probe (a parameter that can be measured in the lab)
this cosmologically-important and most mysterious feature of particle
physics.  (See figure at; background article:
Physics Today, October 1988.)

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