Transcript No Slide Title

New results on rare kaon and
pion decays from BNL E949
Dmitry Vavilov
E949 is an high rate K+ decay at rest experiment with the
primary goal of measuring |Vtd| via a measurement of the
branching ratio of the ultra-rare charged kaon decay
K++ n`n. The report mostly concerns related limits
from the decays K+ +gg , K+ +g and 0  n`n.
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E949 Collaboration
US
BNL, FNAL, New Mexico
Canada
Triumf, UBC, Alberta
Japan
Fukui, KEK, Kyoto, Osaka
Russia
IHEP, INR
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E949 Detector in LESB II at BNL
IHEP contributions: MC calculation for micro-Collar, Downsteam PV, RS Monitoring
system, Slow Control, Data Analysis
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E787 / E949 Data Taking Summary
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K++ n`n

3d candidate event “E949A”
found
E787A E787C E949A
0.006

0.02
0.07
Updated branching
Br(K++ n`n) = (1.5 +1.3-0.9 ) x 10-10

SM predicted branching
Br(K++ n`n) = (0.78±0.12 ) x 10-10

PNN2 analysis is in progress
Goal is to reach S/N1 at
comparable to PNN1 sensitivity
2x Acc and 5x Rej improvements
needed
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K+ +gg Introduction
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K+ +gg Offline Analysis
Backgrounds:
K+ +0 (K2)
• + kinematics measured
•
•
Photon energies/angles mis-measured
Or photon overlaps charged track
K2 with K Decay In Flight (DIF)
• + Kinematics measured
•
Fakes K decay at rest
K+ m+ng , 0m+ng or K2 with  DIF
• m+ kinematics measured
•   m  e decay sequence fooled
Two beam particles
•
Дмитрий Вавилов
Suppressed with good segmentation and
timing in the K tracking system
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Backgrounds, Acceptance, Sensitivity
Backgrounds
Number of Events
K2
Overlapping Photon
Muon
K decay in flight
Two Beam
Total
0.017 ± 0.006
0.065 ± 0.065
0.090 ± 0.020
0.025 ± 0.014
<0.006 (90%CL)
0.197 ± 0.070
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K+ +gg, K+ +g results
Blind analyses
No events seen in signal boxes
Br(K+ +gg ) < 8.3 x 10-9 (90%CL)
p() > 213 MeV/c
Assuming spectrum from ChPT with unitarity
corrections (c = 1.8)
Factor of 8 sensitivity improvement over
E787; but, cannot confirm or rule of
unitarity corrections.
Same data set is used to search for K+ +g
which violates angular momentum
conservation and gauge invariance
Br(K+ +g ) < 2.3 x 10-9 (90%CL)
Factor of 160 improvement over E787
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0  n`n
• Forbidden by angular momentum conservation if W’s and neutrinos are
purely left handed. Can proceed if neutrinos have mass.
Br(0  n`n) < 1.1 x 10-9 for m(nt)<18.2 MeV/c2
• Best previous limit comes from E787
Br(0  n`n) < 8.3 x 10-7 (90%CL)
• Method:
Copious supply of clean 0 from K+ +0 tagged by monochromatic +.
Look for K2 events with no activity other than K+ and +.
Trigger sample is same as K+ + n`n. Select K2 events; apply tightest
photon veto.
Tune photon veto on 1/3 of data, use other 2/3 for search.
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0  n`n
0 Rejection
Maxed out at ~4x106
99 events survive all cuts
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0  n`n Results

99 events remain after all photon veto cuts

We presume these to be 0  gg with undetected photons
– The e949 photon veto inefficiency in not known and probably not
knowable at this level.
– A background subtraction is not a reasonable option.

The corresponding branching ratio upper limit is:
Br(0  n`n) < 2.7 x 10-7 (90%CL)
Factor of 3 improvement beyond E787
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Conclusions
 Present E949 results
Br(K++ n`n) = (1.5 +1.3-0.9 ) x 10-10 [3 events, E787(2) + E949(1)]
Br(K+ +gg ) < 8.3 x 10-9 (90%CL) p() > 213 MeV/c
Br(K+ +g ) < 2.3 x 10-9 (90%CL)
Br(0  n`n) < 2.7 x 10-7 (90%CL)
 Expected new E949 Results
Br(K++ n`n) from region below Kp2 peak
 Further Progress in ultra-rare Kaon decays
P326(Cern), Jparc(Japan)
References
K+ + n`n
Phys.Rev.Lett. 93 (2004) 031801, hep-ex/0403036
K+ +gg, K+ +g
T. Yoshioka, Univ. of Tokyo thesis, KEK K-decay Report 2005-2, Hep-ex/0505069, Phys.Lett.B 623 (2005) 192-199
0  n`n
K.Mizouchi, Kyoto Uinv. Thesis (in preparation), hep-ex/0506028, submitted to Phys.Rev.D. Rapid Communications
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K+ +gg in Chiral Perturbation Theory
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