RARE KAON DECAYS: IL BUONO, IL BRUTTO, IL CATTIVO. Page: 5 of 10
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2 Lepton flavor violation
The state-of-the-art in lepton flavor violation (LFV) is set by the BNL ex-
periments E871 and E865 which searched for the decays KL -+ eF and
K+ _ 7r+f+e respectively. The two decay modes are complementary in that
the first probes parity odd couplings and the second parity even. The E871
result on KL -+ }teT actually predates the time frame of this review, but it
is the best limit available: BR(KL -+ pteF) < 4.7 x 10-12 3). Limits on new
physics are model dependent; it is typical to derive a limit in a "generic" sense
for a heavy particle exchanged at tree level. For the same coupling strength
as the electroweak couplings of the quarks, the limit on KL -> frieT probes
energy scales as high as 150 TeV. 4) E865 has completed analysis of its 1998
data set for K+ -, ?++e-. 5) The dominant background comes from the
overlap of multiple K+ decays, which are estimated from the time sidebands
and extrapolated into the signal region from the region of high K+ momentum.
Eight events are observed in the signal region, consistent with background ex-
pectations; a likelihood analysis is used to obtain the branching ratio limit.
Combining this with the E865 results from the 1995 and 1996 runs as well as
with the result from the predecessor experiment BNL E777, yields the final
E865 limit: BR(K+ - 7r+ +e) < 1.2 x 10-11. New results from KTeV are
also available on KL -+ x0pe+, 6) and KL -> e Le*jT t. 7)
3 Quark mixing and CP violation: K -> 7vv
The decays K+ -+ 7r+v and KL --+ irxAv have attracted much attention for
their potential (together with the decay K+ --) woe+v) to completely determine
the Unitarity Triangle from kaon decays alone. An inconsistency between the
unitarity relation in kaon decays (s -+ d transitions) with that in B decays
(b -+ d transitions) would provide clues to the flavor structure of physics beyond
the SM. The clean theoretical nature of the K -> 7rv decay modes was
discussed at this conference by Sehgal; a detailed review can be found in 8).
First results from BNL E949 on the decay K+ -3 7r+v have been
published recently. 9) The signal region is analyzed with a signal-to-noise
likelihood ratio technique. An event is seen in the 2002 dataset near the
K+ -+r+zv,, kinematic endpoint, albeit with poorer signal-to-noise ratio com-
pared to the previous two candidate events seen by E787; accordingly the new
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REDLINGER,G. RARE KAON DECAYS: IL BUONO, IL BRUTTO, IL CATTIVO., article, June 7, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc1412096/m1/5/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.