july 19th, 2003eps hep 2003 - aachen r. fantechi tests of chiral perturbation theory in k s rare...

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


OutlineOutline

• Motivations for KS rare decays study

• KS

• KS 0

• A little step backwards: study of KL


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


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 ?


The NA48 beamsThe NA48 beams


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


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)


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


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 )


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


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


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


(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


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


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


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


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



• 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



• Insufficient statistics to verify the chiral structure of the vertex


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


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


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)


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


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

july 19th, 2003eps hep 2003 - aachen r. fantechi tests of chiral perturbation theory in k s rare...

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