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July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
Tests of Chiral Perturbation Theory in KS rare decays at NA48
Tests of Chiral Perturbation Theory in KS rare decays at NA48
Riccardo Fantechi
INFN - Sezione di Pisa
On behalf of the NA48 collaboration
EPS HEP 2003 – Aachen July 19th, 2003
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
OutlineOutline
• Motivations for KS rare decays study
• KS
• KS 0
• A little step backwards: study of KL
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
The NA48 detectorThe NA48 detector
LKr Calorimeter:
(E)/E (E)/E 3.2%/√E 3.2%/√E 100MeV/E 100MeV/E 0.42%0.42%
Spectrometer:
pT kick ~250 MeV/c(P)/P (P)/P 0.48% 0.48% 0.009 P[GeV/c]% 0.009 P[GeV/c]%
M(00) ~ M(+-) ~ 2.5 MeV
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
Rare kaon decays with NA48 Rare kaon decays with NA48
• The NA48 detector has capability for rare kaon decays studies:– Good resolutions in charged and neutral sectors
– Flexible trigger for neutral decays allows to select decays in any number of
– Flexible trigger for charged decays allows to select events with more than 2 charged tracks
– Enough data acquisition bandwidth to accept a fraction of not ’/ triggers, to accumulate data from the intense KL
beam many results published
• Is it possible to have an intense KS beam ?
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
The NA48 beamsThe NA48 beams
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
First step towards NA48/1First step towards NA48/1
• A single KS beam was tested during a special run in 1999– No KL beam
– 6*109 ppp on KS target ( x200 wrt ’/)
– 2.3*108 KS decays for 60 < EK < 190 GeV
– SES ~ 4*10-8 with 10% acceptance– 40 hours 3-4 years of’operation
• The test was successful– Physics results published in the following years
– Define a proposal for a HIKs experiment NA48/1
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
NA48/1: High Intensity KS
Physics motivationsNA48/1: High Intensity KS
Physics motivations
• KS 0 e+ e-, KS 0 + -
– Bound indirect CP Violation in the corresponding KL decays
• Search for CP Violation in KS decays– KS 30, KS 0 + -
• Test of Chiral Perturbation Theory this talk– KS , KS 0 , KS 0 0
• Semileptonic and radiative neutral hyperon decays– Improve NA48 results on e-, -, ,
– Put an upper limit to BR (p) (double beta decay)
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
NA48/1 data takingNA48/1 data taking
• 2000 run – No drift chambers in the spectrometer– 400 GeV/c proton momentum– Modified production angle, same proton intensity– Modified duty cycle (3.2s/14.4s)– Far target run (KL) with magnetic field on – Near target run (HIKS) without magnetic field– 1010 KS decays collected in ~40 days
• 2002 run – Spectrometer operational– Improved beam line and duty cycle– New drift chamber readout – 4.4*1010 KS decays collected in ~60 days
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
Chiral perturbation theoryChiral perturbation theory
• PT is an effective field theory of Standard Model at low energies where QCD is non-perturbative
• Processes are described in perturbative expansion of momenta and masses:
p2 / ( 4 F)2, m2 / ( 4 F)2 where ( 4 F)2 1.2 GeV
• Higher order boson loops are divergent, they are compensated by counter-terms with effective couplings determined from experiments.
• Ideal to describe kaon decays ( mK 0.5 GeV )
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS KS
• KS is interesting because it is calculable in PT with no counter-terms and it is sensitive to loops
– Theoretical prediction: BR(KS = (2.1 ± 0.2)*10-6
– Result from NA31: BR(KS = (2.4 ± 0.9)*10-6
• Published result from 1999 HIKs run with dedicated trigger
– BR (KS = (2.58 ± 0.36st ± 0.22sy )*10-6
– Phys. Lett. B493 (2000) 29
– Systematic error limited by the error on PDG BR(KL
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – BackgroundKS – Background
– KS00 minimized using a 5 m fiducial region after the KS collimator.
• Decays with two lost have a maximum invariant mass of 458 MeV and the reconstructed vertex cannot be less than 9 m.
• Reduction to 5 m is due to the effect of overlapping showers.
– Hadronic interactions in the collimator evaluatedfrom high COG tails and suppressed by Ehac and shower width cut
– Accidental pairs eliminated with a cut on cluster time
– Dalitz decays K, 0 ee mainly due to closed e+e- pairs subtracted using MC
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – BackgroundKS – Background
– KL irreducible 1.5 times the KS in the decay volume.
• Use KL 30 to evaluate KL flux
• Use 2000 far target run to measure (KL)/(KL 30)
• Compute directly N KL using the above ratio
• Ratio of acceptances entering the calculation is close to 1 with less than 1% uncertainty
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
(KL )/(KL 3)(KL )/(KL 3)
• Measurement of (KL)/(KL 30)– 2000 far target run: similar detector conditions as in the
near target onedetector systematics cancel.– Choose the same decay volume as for KS
acceptance almost cancels– Background handling similar to KS – Contribution to syst. error from background evaluation and
acceptance calculation– Result:
(KL)/(KL 30) = (2.81 0.01st0.02sy)*10-3
PDG value = (2.77 ± 0.08 )*10-3
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS - ResultKS - Result
Global systematic correction
–1.8 1.4 %
Total number of Kx 19916
Number of KS 7461172
Normalise to KS
Statistical error 2.0 %
Statistical error on MC 0.6 %Published result [Phys. Lett. B551 (2003) 7]
BR(Ks ) = (2.780.06st 0.02MCst 0.06sy)*10-6
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – ResultKS – Result
•The NA48 result is compatible with the previous measurements
•It shows 30% difference wrt O(p4) PT predictions
•There is an indication for a large O(p6) contribution
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS KS
• Up to now not observed• NA48 from 1999 data:
BR(KS )zq>0.2 < 3.3*10-7 90% CL
• Predictions from PT:BR(KS )zq>0.2 = 3.8*10-8
Cut on Zq=m2/mK
2 to avoid the 20 poleWeak vertex momentum dependence
• In principle it is possible to study the structure of the weak vertex from the zq distribution
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – Preliminary resultKS – Preliminary result
•Data from “near target” run in 2000
•Normalized to KS
•Main background
– Beam activity time, anti counters
– KS , KS D
kinematical cuts
– KL non-reducible
– 0 n energy asymmetry
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – Preliminary resultKS – Preliminary result
• First observation – preliminary result:
BR(KS )Zq>0.2 = (4.9 1.6stat0.8 syst*10-8
Prediction from PT : BR(KS )Zq>0.2 = 3.8*10-8
The probability that 31 events were consistent with the bckg is 9*10-4
Events in the signal region 31.0 5.6
Beam activity 7.4 2.4
Background KS D
2.4 1.2
Background KL 3.8 0.2
Acceptance 0.7
Background estimation 13.6 2.8
Remaining events 17.4 6.2
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KS – Preliminary resultKS – Preliminary result
• Insufficient statistics to verify the chiral structure of the vertex
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
A little step backwards: KL A little step backwards: KL
• Motivation– At one loop PT (O(p4)) the decay rate is finite, but only
gives 1/3 of the measured rate– Calculations of O(p6) including vector meson exchange
reproduce the measured rate and allows a tail at low M– VMD contribution parametrized by av, to be measured,
which determines the CPC amplitude to KL e+e-
PT prediction of O(p6)
• BR(KL )
• av = -0.7
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KL - AnalysisKL - Analysis
• Data from ’/ runs in 1998 and 1999– Background from 30 with missing or overlapping
photons are rejected with combinatorial cuts• Background gives wrong K vertex with a good 0 vertex
– Additional cuts on the shower width of the cluster
– Good separation using a variable zmax set up as the maximum of the reconstructed z of various combinations of photons
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KL - AnalysisKL - Analysis
0<m34<0.135 0.135<m34<0.24 0.24<m34<0.26
~2500 candidates in the signal region (132 < m12 < 138 MeV)
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
KL - ResultsKL - Results
• Fitting av one obtains:
av= -0.46 ± 0.03st ± 0.03sy ± 0.02th
• Using the fitted av the branching ratio is:
BR (KL ) = (1.36 ± 0.03st ± 0.03sy ± 0.03norm)*10-6
• The systematics of both results are limited by background evaluation and acceptance calculation
• The value of av implies a negligible CP conserving contribution to KL e+e-
BR (KL e+e-)CPC = (4.7± 2.2)*10-13
Phys. Lett. B536 (2002) 229
KTeV
BR (KL ) = (1.68± 0.10)
av= -0.72 ± 0.08
July 19th, 2003 EPS HEP 2003 - Aachen R. Fantechi
ConclusionConclusion
• NA48/1 has completed data taking with a KS high intensity beam– First phase (neutral only) in 2000, completed in 2002 with
charged decays – Many results available, see other talks here at EPS HEP 2003
• Precise measurement of BR(KS )– Improve on NA48 first measurement, both on statistics and
systematics error– Show 30% discrepance from O(p4) PT prediction – Indication of a large O(p6) contribution
• Precise measurement of (KL )/ (KL 3)– Used in the above analysis, better result up to now
• First observation of the decay KS • A little step backwards: study of KL
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