July 19th, 2003EPS HEP Aachen R. Fantechi Tests of Chiral Perturbation Theory in K S rare decays at NA48 Riccardo Fantechi INFN - Sezione di Pisa On behalf of the NA48 collaboration EPS HEP 2003 – Aachen July 19 th, 2003

July 19th, 2003EPS HEP Aachen R. Fantechi Outline Motivations for K S rare decays study K S    K S   0   A little step backwards: study of K L   

July 19th, 2003EPS HEP Aachen R. Fantechi The NA48 detector LKr Calorimeter:  (E)/E  3.2%/√E  100MeV/E  0.42% Spectrometer: p T kick ~250 MeV/c  (P)/P  0.48%  P[GeV/c]%  M (  0  0 ) ~  M (  +  - ) ~ 2.5 MeV

July 19th, 2003EPS HEP Aachen R. Fantechi 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 K L beam  many results published Is it possible to have an intense K S beam ?

July 19th, 2003EPS HEP Aachen R. Fantechi The NA48 beams

July 19th, 2003EPS HEP Aachen R. Fantechi First step towards NA48/1 A single K S beam was tested during a special run in 1999 –No K L beam –6*10 9 ppp on K S target ( x200 wrt  ’/  ) –2.3*10 8 K S decays for 60 < E K < 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 HIK s experiment  NA48/1

July 19th, 2003EPS HEP Aachen R. Fantechi NA48/1: High Intensity K S Physics motivations K S   0 e + e -, K S   0  +  - –Bound indirect CP Violation in the corresponding K L decays Search for CP Violation in K S decays –K S  3  0, K S   0  +  - Test of Chiral Perturbation Theory  this talk –K S   , K S   0  , K S   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, 2003EPS HEP Aachen R. Fantechi NA48/1 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 (K L ) with magnetic field on –Near target run (HIK S ) without magnetic field –10 10 K S decays collected in ~40 days 2002 run – Spectrometer operational –Improved beam line and duty cycle –New drift chamber readout –4.4*10 10 K S decays collected in ~60 days

July 19th, 2003EPS HEP Aachen R. Fantechi Chiral 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: p 2 / ( 4  F  ) 2, m 2 / ( 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 ( m K  0.5 GeV )

July 19th, 2003EPS HEP Aachen R. Fantechi KS KS  KS KS  K S  is interesting because it is calculable in  PT with no counter-terms and it is sensitive to loops –Theoretical prediction: BR(K S  = (2.1 ± 0.2)*10 -6 –Result from NA31: BR(K S  = (2.4 ± 0.9)*10 -6 Published result from 1999 HIKs run with dedicated trigger –BR (K S  = (2.58 ± 0.36 st ± 0.22 sy )*10 -6 –Phys. Lett. B493 (2000) 29 –Systematic error limited by the error on PDG BR(K L 

July 19th, 2003EPS HEP Aachen R. Fantechi K S  – Background –K S  0  0 minimized using a 5 m fiducial region after the K S 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 evaluated  from 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, 2003EPS HEP Aachen R. Fantechi K S  – Background –K L  irreducible  1.5 times the K S  in the decay volume. Use K L  3  0 to evaluate K L flux Use 2000 far target run to measure  (K L  )/  (K L  3  0 ) 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, 2003EPS HEP Aachen R. Fantechi  (K L  )/  (K L  3   ) Measurement of  (K L  )/  (K L  3  0 ) –2000 far target run: similar detector conditions as in the near target one  detector systematics cancel. –Choose the same decay volume as for K S  acceptance almost cancels –Background handling similar to K S  –Contribution to syst. error from background evaluation and acceptance calculation –Result:  (K L  )/  (K L  3  0 ) = (2.81  0.01 st  0.02 sy )*10 -3 PDG value = (2.77 ± 0.08 )*10 -3

July 19th, 2003EPS HEP Aachen R. Fantechi K S  - Result Global systematic correction –1.8  1.4 % Total number of K x  Number of K S  7461  172 Normalise to K S     Statistical error  2.0 % Statistical error on MC  0.6 % Published result [Phys. Lett. B551 (2003) 7] BR(Ks  ) = (2.78  0.06 st  0.02 MCst  0.06 sy )*10 -6

July 19th, 2003EPS HEP Aachen R. Fantechi K S  – Result The NA48 result is compatible with the previous measurements It shows 30% difference wrt O(p 4 )  PT predictions There is an indication for a large O(p 6 ) contribution

July 19th, 2003EPS HEP Aachen R. Fantechi K S    Up to now not observed NA48 from 1999 data: BR(K S    ) zq>0.2 < 3.3* % CL Predictions from  PT: BR(K S    ) zq>0.2 = 3.8*10 -8 Cut on Zq=m  2 /m K 2 to avoid the 2  0 pole Weak vertex momentum dependence In principle it is possible to study the structure of the weak vertex from the z q distribution

July 19th, 2003EPS HEP Aachen R. Fantechi K S    – Preliminary result Data from “near target” run in 2000 Normalized to K S     Main background –Beam activity  time, anti counters –K S  , K S     D   kinematical cuts –K L    non-reducible –  0    n      energy asymmetry

July 19th, 2003EPS HEP Aachen R. Fantechi K S    – Preliminary result First observation – preliminary result: BR(K S    ) Zq>0.2 = (4.9  1.6 stat  0.8 syst  *10 -8 Prediction from  PT : BR(K S    ) 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 region31.0  5.6 Beam activity 7.4  2.4 Background K S     D 2.4  1.2 Background K L    3.8  0.2 Acceptance  0.7 Background estimation13.6  2.8 Remaining events17.4  6.2

July 19th, 2003EPS HEP Aachen R. Fantechi K S    – Preliminary result Insufficient statistics to verify the chiral structure of the vertex

July 19th, 2003EPS HEP Aachen R. Fantechi A little step backwards: K L    Motivation –At one loop  PT (O(p 4 )) the decay rate is finite, but only gives 1/3 of the measured rate –Calculations of O(p 6 ) including vector meson exchange reproduce the measured rate and allows a tail at low M  –VMD contribution parametrized by a v, to be measured, which determines the CPC amplitude to K L    e + e -  PT prediction of O(p 6 ) BR(K L    )   a v = -0.7

July 19th, 2003EPS HEP Aachen R. Fantechi K L    - Analysis Data from  ’/  runs in 1998 and 1999 –Background from 3  0 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 z max set up as the maximum of the reconstructed z of various combinations of photons

July 19th, 2003EPS HEP Aachen R. Fantechi K L    - Analysis 0<m 34 < <m 34 < <m 34 <0.26 ~2500 candidates in the signal region (132 < m 12 < 138 MeV)

July 19th, 2003EPS HEP Aachen R. Fantechi K L    - Results Fitting a v one obtains: a v = ± 0.03 st ± 0.03 sy ± 0.02 th Using the fitted a v the branching ratio is: BR (K L    ) = (1.36 ± 0.03 st ± 0.03 sy ± 0.03 norm )*10 -6 The systematics of both results are limited by background evaluation and acceptance calculation The value of a v implies a negligible CP conserving contribution to K L    e + e - BR (K L    e + e - ) CPC = (4.7± 2.2)* Phys. Lett. B536 (2002) 229 KTeV BR (K L    ) = (1.68± 0.10) a v = ± 0.08

July 19th, 2003EPS HEP Aachen R. Fantechi Conclusion NA48/1 has completed data taking with a K S 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(K S  ) –Improve on NA48 first measurement, both on statistics and systematics error –Show 30% discrepance from O(p 4 )  PT prediction –Indication of a large O(p 6 ) contribution Precise measurement of  (K L  )/  (K L  3   ) –Used in the above analysis, better result up to now First observation of the decay K S    A little step backwards: study of K L   