The NA48/2 result, based on the data collected during the 2003 run, on direct
CP violation in $K^{\pm}\to\pi^{\pm}\pi^0\pi^0$ decay is presented. The main
goal of the experiment is to reach a sensitivity at level of $10^{-4}$ in the
measurement of the charge asymmetry parameter $A_g=(g^+-g^-)/(g^++g^-)$, where
$g$ is the \emph{linear slope} of the Dalitz plot in the $K\to 3\pi$ decay.
Thanks to the simultaneous collection of the two kaon charges and to the high
resolution of the main sub-detectors, the systematics uncertainties are kept
under the statistical error level. The experimental procedure, the analysis
technique and the main systematics are discussed to present the final result $$
A_g=(1.8\pm2.6)\times 10^{-4} $$ This result based on more than $45\cdot10^6$
events, correspondig to one half of the whole two year data taking, is about an
order of magnitude more precise with respect to the previous measurement.Comment: Talk given to "Rencontres de Moriond 2006 EW" on behalf of NA48/2
  collaboratio

Lamanna, G.

English

arXiv

This result based on more than $45\cdot10^6$ events, correspondig to one half of the whole two year data taking, is about an order of magnitude more precise with respect to the previous measurement.The NA48/2 result, based on the data collected during the 2003 run, on direct CP violation in $K^{\pm}\to\pi^{\pm}\pi^0\pi^0$ decay is presented. The main goal of the experiment is to reach a sensitivity at level of $10^{-4}$ in the measurement of the charge asymmetry parameter $A_g=(g^+-g^-)/(g^++g^-)$, where $g$ is the \emph{linear slope} of the Dalitz plot in the $K\to 3\pi$ decay. Thanks to the simultaneous collection of the two kaon charges and to the high resolution of the main sub-detectors, the systematics uncertainties are kept under the statistical error level. The experimental procedure, the analysis technique and the main systematics are discussed to present the final result $$ A_g=(1.8\pm2.6)\times 10^{-4} $$ This result based on more than $45\cdot10^6$ events, correspondig to one half of the whole two year data taking, is about an order of magnitude more precise with respect to the previous measurement

CERN Document Server

NEW RESULTS ON DIRECT CP VIOLATION IN CHARGED KAON DECAYS BY NA48/2 G. LAMANNAa Department of Physics €1 INFN Pisa, 3 Largo Pontecorvo, 56127 Pisa, Italy The NA48/2 result, based on the data collected during the 2003 run, on direct CP vio­lation in K± ---+ 7r±7ro7ro decay is presented. The main goal of the experiment is to reach a sensitivity at level of 10-4 in the measurement of the charge asymmetry parame­ter A9 = (g+ - g-)/(g+ + g-),  where g is the linear slope of the Dalitz plot in the K ---+ 37r decay. Thanks to the simultaneous collection of the two kaon charges and to the high reso­lution of the main sub-detectors, the systematics uncertainties are kept under the statistical error level. The experimental procedure, the analysis technique and the main systematics are discussed to present the final result A9 = (1.8 ± 2.6) x 10-• This result based on more than 45 · 106 events, correspondig to one half of the whole two year data taking, is about an order of magnitude more precise with respect to the previous measurement. 1 Introduction In spite of the fact that more than 40 years have passed since the discovery of the CP violation 1 ,  the full understanding of this phenomenon is still far to b e  reached. After the discovery of direct CP violation in the neutral kaon decay into two pions in the first part of the 90s, by the NA31 experiment 2 , and the clear'confirmation after few years , by NA483 and KTEV 4 , the only other environment in which this effect was detected is the neutral B system 5 . On the other hand, the study of the tiny effects due to the violation of this symmetry in all the systems where it's possible, represents an important window on the contribution of new physics beyond the aon behalf of NA48/2 Collaboration: Cambridge , CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze , Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna. 102 Standard Model (SM). In fact new effects could appear in a relevant way in the heavy quark loops which are at the core of the mechanism allowing CP violation in the decay. In the kaon sector the most promising places, besides t:1/t:, where this kind of contributions could play some role are the rates of GIM suppressed rare decays 6 and the charge asymmetry between charged kaons. In particular in K --+ 37!' this asymmetry could give a strong qualitative indication of the validity of the CKM description of the direct CP violation or the existence of possible sources outside of this paradigm. Quantitative tests or predictions are, at the moment, very difficult from a theoretical point of view due to the complexity of the calculation involving non-perturbative hadronic effects. As soon as the intense theoretical efforts to improve the understanding of this sector of the particle interactions will produce tools to make reliable predictions, the experimental measurements could also be used as strong quantitative test. From an experimental point of view, the easiest way to look for any possible differences between K+ and K- decay into three pions is to compare the shape of the Dalitz plot distribution instead of the decay rates. Exploiting the small Q value of the K --+ 37!' decay it's possible to use a polynomial expansion of the matrix element b: IM(u, v) l2 ,...., 1 + gu + hu2 + kv2 + . . .  (1)  where g,h,k are called linear and quadratic slope parameters and where the Lorentz invariant Dalitz plot parameters 8 u and v are related to the energy (Et) of "odd" and "even" pion (i.e. the unpaired pion and the same sign two pions, respectively) in the center of mass, through the definition: _ (sa - so) _ 2 mK/3 - E3 _ (s2 - s1) _ 2 Ei - E2 U - 2 - ffiK 2 , V - 2 - ffiK 2 m,. m,. m,. m,. (2) where s; = (pK - p;)2, being PK and p; the kaon and the pions (with i = 1, 2, 3 and i = 3 for the "odd" pion) four-momenta, so = (s1 + s2 + sa)/3, and mK, m,. the kaon and charged pion mass, respectively. The present value 8 for the linear slope c appearing in (1) are, for the two possible decay mode: (3) and (!hi , !k l  « lg! ) .  A difference between g+ and g-, the linear slopes in the K+ and K- decay, represents a signal of direct CP violation. This can be formalized defining the CP violating parameter Ag as: A = g+ - g- "" /j.g g g+ + g- 2g (4) where fj.g = g+ -g- and g is the average between g+ and g- . In the SM framework the prediction of the Ag value is very difficult due to hadronic effects contribution, but all the calculations 9 are in the range between 10-s and few 10-5 , both in the neutral (the K± --+ 7!'±7l'o7ro) and in the charged (the K± --+ 7!'±7!'+7!'-) decay mode. Calculations involving models beyond the SM allow for an enhancement up to the level of few 10-4. Several experiments in the past have searched for the asymmetry in "neutral" and "charged" mode as summarized in tab.I . The sensitivity reached so far is ,!l.t level of few 10-3 , dominated by the systematic uncertainties. The main goal of the NA48/2 experiment is to reach the sensitivity of 10-4 both in "neutral" and in "charged" mode, covering the gap existing between the experimental results and the "beyond the SM" theory predictions. bDue to the strong interaction process 7r+7T- -. 1T0tr0 this simple parametrization isn't fully adequate to describe the matrix element 7• Nevertheless this expansion is adopted here for the CP violation measurement. cThe linear slope for the v variable is forbidden for symmetry reasons. 1 03 Asymmetry # of events Au = (19 ± 125) · 10 ·4 115K A� = (2 ± 19) · 10-4 620K A� = ( -70 ± 53) · 10 4 3.2M A� = (22 ± 15 ± 37) . 10-4 54M Experiment CERN PS(1975) 1 1  Protvino IHEP (2005) 12 BN1 ACS (1970Y� HyperCP (2000) prelim.14 Table 1: Summary of the experimental situation both in "neutral" (A�) and in "charged" (A�) mode, before the NA48/2 results The NA48/2 experiment (basically an upgrade 15 of the NA48 experiment, devoted to the direct CP violation measurement in the K0 system) will be briefly described, the analysis of the charged asymmetry in K± ---> 1T±1To1To will be discussed and the final result, based on about a half of the whole NA48/2 data sample, will be presented. 2 NA48/2: beam line and experimental apparatus A novel high intensity beam line was designed in order to allow for the simultaneous collections of K+ and K- decays. The possibility to collect at the same time, with the same detector, the K ---> 31T decay of both charges, represents a fundamental point in the reduction of the systematic error. The beam transport line is schematically sketched in fig.3. The hadron beam, of both charges, is produced by the SPS 400 Ge V / c protons ("' 7 · 1011 protons per pulse) impinging on a 40 cm long and 2 mm diameter beryllium target with an angle of zero degrees. A magnetic device, called first achromat, selects the momentum of the beam in the range (60 ± 3) GeV /c, splitting the two charges. After being recombined the beams are focused by a quadruplet of quadrupoles. The serond achromat system houses a spectrometer for the beam, called KABES 16 , to measure the particles' momentum with a resolution of 1 %, useful in particular for rare decays studies. The beam, recombined again along the beam axis, contains "' 6.4 · 107 particles per 4.8 s spill (about 12 times more pions than kaons) with kaon charge ratio K+ /K- "' 1.8,  which is irrelevant for the measurement . In the 114 m long decay region the two beams, both ,...,5 mm wide (RMS) are superimposed at level of 1 mm. The central detector is based on the old NA48 detector described elsewhere 17 . The spectrometer magnet was operated in order to give PJ..kick = l20MeV/c and the resolution in momentum (GeV /c) is O'p/P = 1 .0% EB 0.044% · p. The 1Kr electromagnetic calorimeter is employed to detect the photons from 1To decay with an energy resolution (GeV) of O'E/E = 3.2%/VE EP 9%/E EP 0.42%. Thanks to these performances the resolution on the reconstructed kaon mass is good enough (0.9MeV/c2 in the "neutral" mode, l.7MeV/c2 in the "charged" mode) for a precise calibration of the detector instabilities. A two level trigger is employed to reduce the rate of data collected. A hardware level trigger (11) using informations from the fast hodoscope counter and from a dedicated 1Kr read out system is followed by a 12 section based on processors for fast reconstruction of kinematical quantities. In particular for the K± ---> 1T±1To1To online selection, the missing mass of the charged pion is required to be far from the 1To mass, in order to reduce the contribution of the more frequent 1T±1To decay. The final trigger rate is 10 KHz. 3 Strategy of the measurement The method to extract A� (A9 for the "neutral" mode) is based on the comparison between the u projections of the Dalitz plot distribution. The ratio between the two distributions is well described by: 104 R(u) = NK+ ex 1 + (g + �g)u + hu2 N K- 1 + gu + hu2 (5) SIMULTA."'1EOUS K'" AND K- BJ<;AMS Figure 1: The NA48/2 beam line and detector. Not to scale. The presence of magnetic fields both in the beam sector (achromat) and in the detector (spec­trometer magnet) introduces an intrinsic charge dependent acceptance of the apparatus. In order to equalize this asymmetry the main magnetic field are frequently reversed during the data taking (the spectrometer magnet on a daily basis, while the achromat system on a weekly basis) .  For each achromat polarity, it's possible to define two ratios in which the same side (the two sides are called Jura (J) and Saleve (S)) of the spectrometer is involved: NB+ Rs(u) = Nlfi� K- (6) In each of these ratios the acceptance asymmetry due to the spectrometer is cancelled. The full cancellation of the detector asymmetry is obtained exploiting the quadruple ratio: R( ) = R R R R "' fl( 1 + (g + �g )u + hu2 )4 u us UJ DS DJ l + gu + hu2 (7) where U and D stands for the path followed by the K+ in the achromat (up and down) and fl is an inessential normalization constant. This method is independent from the relative size of the fotir samples collected with different fields configuration and from the K+ and K- flux difference. The method allows for the cancellation of: • local detector bias (left-right asymmetry), thanks to the fact that each single ratio in (7), is defined in the same side of the detector; • beam local biases, because in each single ratio the path of the particle through the achromat is the same; • global time variation biases, because the decays from both charges are collected at the same time. The result remains sensitive only to the time variation of the detector with a characteristic time smaller than the inversion period of the magnetic field. In fact each of the ratios employed in the (7) is constructed Ufling data of subsequent days. Other systematic biases induced by effects not cancelled by th� alternation of the magnetic field (for instance the Earth's magnetic field and any misalignment of the spectrometer) have to be carefully corrected. The intrinsic cancellations of the acceptance asymmetries in the quadruple-ratio allow to avoid the use of simulation input for the measurement. Nevertheless a GEANT3 18 based MonteCarlo, including the time variation of beam and detectors, is used for systematic studies and as a cross-check of the result. 1 05 4 Data Analysis in K± --+ 7r±7ro7ro mode The reconstruction of the event in the "neutral" mode exploits the LKr and the spectrometer. In the preselection phase the tracks and the photons are selected according to quality criteria. The fiducial region of the detectors is chosen to avoid edge effects or miscalibrated regions. In particular the spectrometer's inner radial cut is chosen according to the actual beam position, whose displacement during the run is measured by three-track events. The decay vertex is recostructed from the / impact positions on the LKr, for each 7ro, by using the relation: Z;J = ml V E;EJ<i'fJ "o (8) Among all the possible / pairings, the two pairs with minimum vertex difference are selected and the final decay vertex is obtained as the arithmetic average of the two vertices. The kaon invariant mass is used to select the charged pion and to identify a good K± --+ 7r±7ro7ro, by requiring IMK± - MK± I < 6MeV/c2. The measurement of the charged pion momentum, PDG slightly biased by variable DCH misalignment, is corrected by exploiting the condition MK+ = MK- in the reconstruction of the K± --+ 11"±11"+11"- mode. The same decay mode is used to follow the variation of the absolute value of the spectrometer magnetic field after each periodical inversion, monitored online only at level of ,...., 10-3 . The position of the photons impact point is corrected to take into account the projective geometry of the calorimeter. These effects gives, after the correction, a negligible contribution to the systematics error. A potential source of systematic bias is the trigger. This was studied directly from the data with downscaled sample of control triggers, uncorrelated with the main trigger. In the "neutral" mode the Ll trigger level is essentially composed by a coincidence of a signal in the scintillating hodoscope (Ql) and a pattern compatible with 4 photons on the LKr (NTPEAK). The Ql efficiency is obtained by using all the events with one track in the charged hodoscope, re-weighted according to the distribution of the 3n events. The inefficiency (,...., 0.253) is constant during the run and the upper limit of the systematic error on D.g given by this part of the trigger is 0. 1 · 10-4. The largest systematic uncertainty (1.3 · 10-4) comes from the neutral part of the Ll .  The NTPEAK inefficiency is different at the beginning ("' 0. 73) and at the end (,..,, 3 3) of the data taking. The estimation of this systematics is limited by the statistics of the control sample. The L2 inefficiency (,...., 5.73 on average during the run) is mostly ("' 703) due to the DCH wires inefficiencies and was studied by using MonteCarlo simulation, including the detailed time variation of the spectrometer's behaviour. The systematic error assigned to D.g is 0.4 · 10-4, including also other effects like data buffer overflows, algorithm inefficiency and synchronization effects in the trigger chain. All the sources of systematics are summarized in table 4 . During the 2003 data taking (one half of the whole statistics) 40.3 · 106 K+ and 16.9 · 106 K­decays in the "neutral" 37r mode have been selected for the asymmetry measurement presented in this paper. The final result in terms of A9 (related to D.g through (4) ) , obtained as average of three independent analyses that agree within few 10-5, is: A� = (1.8 ± 2.2stat. i l .Ostat.(trig) ± 0 .8syst. ± 0.2ext. ) · 10-4 = (1 .8 ± 2.6) · 10-4 (9) ( where the external error depends on the uncertainties on the value of g(7r±7r07r0) ("-' 3.13) 8 .  This result, fully compatible with the SM predictions, is almost one order of magnitude better than the previous measurement by other experiments 11 , 12 . The ongoing analysis of the 2004 sample will improve the statistical error of a factor of "' 1 .4, with a similar reduction in the systematic error. 106 I Systematic effect U calculation & fitting LKr non linearity Shower overlapping Pion decay Spectrometer Alignment & Momentum scale Beam Geometry Accidentals Ll Trigger: Ql Ll Trigger: NTPEAK L2 Trigger Total systematic uncertainty Trigger statistical uncertainty 5 Result in charged mode I Effect on D..9 · 10 4 I ±0.4 ±0.1 ±0.5 ±0.5 < ±0.1 ±0.3 ±0.2 ±0.1 ±l.3(stat.) ±0.4 ±1 .0 ±1.3 An analogous analysis in the "charged" mode using 1 .6 · 109 K± ---> 11'±11'+11'- decays, from the 2003 sample, gives the result 19: A� = (1.7 ± 2 .lstat ± L4trig ± l.4syst) · 10-4 = (1.7 ± 2.9) · 10-4 (10) The statistical precision is similar with respect to the result given in (9) because, even though the "neutral" mode is statistically disfavored due to lower branching ratio and acceptance, the population density of the Dalitz plot is more favourable and 19(11'±11'011'0) 1  ,..., 3 · 19(11'±11'+11'-)I . A preliminary study on the combined 2003 and 2004 data (3.11 · 1011 events) leads to a compatible result 20: A� = (-1 .3 ± l.5stat ± 0.9trig ± l.4syst) · 10-4 = (-1.3 ± 2.3) · 10-4 (11) References 1. J. H. Christenson et al. Phys. Elev. Lett. 13, 138 (1964) 2. G. Barr et al. (NA31), Phys. Lett. B 317, 233 (1993) 3. A. Lai et al (NA48) ,  Eur, Phys. J. C 22, 231 (2001) J. R. Batley et al. (NA48), Phys. Lett. B 544, 97 (2002) 4. A. Alavi-Harati et al. (KTeV) Phys. Rev. D 67, 012005 (2003), Erratum: Phys. Rev. D 70, 079904 (2004) 5. K. Abe et al. (Belle), Phys. Rev. Lett. 93, 021601 (2004) B. Aubert et al. (Bahar) , Phys. Rev. Lett. 93, 131801 (2004) 6. G. D'Ambrosio and G. Isidori, Phys. Lett. B 530 (2002) 108 [arXiv:hep-ph/0112135] . 7. J. R. Batley et al. [NA48/2 Collaboration] , Phys. Lett. B 633 (2006) 173 [arXiv:hep­ex/0511056). 8 .  S. Eidelman et al. (PDG) Phys. Lett. B 592,1 (2004) 9. L. Maiani and N. Paver,"The Second DA<PNE Physics Handbook" ,INFN, LNF, Vol 1 . (1995) 51 � A. A. Belkov, A. V. �anyov, G. Bohm, A. Schaale and D. Ebert, Nucl. Phys. Proc. Suppl. 16 (1990) 367. A. A. Belkov, G. Bohm, A. V. Lanyov and A. Schaale, arXiv:hep-ph/9309329. G. Faldt and E. Shabalin, arXiv:hep-ph/0503241.  E. Gamiz, J. Prades and I .  Scimemi, arXiv:hep-ph/0410150. 1 07 10. G. D'Ambrosia, G. Isidori and G. Martinelli, Phys. Lett. B 480 (2000) 164 [arXiv:hep­ph/9911522] . E. Shabalin, ITEP-8-98 11 .  K. M. Smith et al. , Nucl. Phys. B 60 (1973) 411. 12. G.  A. Akopdzhanov et al. , Eur. Phys. J. C 40 (2005) 343 [arXiv:hep-ex/040600-S]. 13. W. T. Ford, P. A. Piroue, R. S .  Remmel, A. J. S.  Smith and P. A .  Souder, Phys. Rev. Lett. 25 (1970) 1370. 14. W. S. Choong, Ph.D. thesis, Berkeley (2000) LBNL-47014 15. R. Batley et al., CERN-SPSC-2000-003 16. B. Peyaud, Nucl. lnstrum. Meth. A 535 (2004) 247. 17. V. Fanti et al. (NA48) Phys. Lett. B 465, 335 (1999) 18. GEANT Description and Simulation Tool, CERN Program Library Long Writeup W5013, ( 1994) 19. J. R. Batley [NA48/2 Collaboration] , Phys. Lett. B 634 (2006) 474 [arXiv:hep­ex/0602014] . 20. E. Goudzovski arXiv:hep-ex/0604002 108 

New results on direct CP violation in charged kaon decays by NA48/2

The NA48/2 result, based on the data collected during the 2003 run, on direct CP violation in K ±  → π ± π 0 π 0 decay is presented. The main goal of the experiment is to reach a sensitivity at level of 10 −4 in the measurement of the charge asymmetry parameter Ag = (g + − g −)/(g + + g −), where g is the linear slope of the Dalitz plot in the K → 3π decay. Thanks to the simultaneous collection of the two kaon charges and to the high resolution of the main sub-detectors, the systematics uncertainties are kept under the statistical error level. The experimental procedure, the analysis technique and the main systematics are discussed to present the final result Ag = (1.8 ± 2.6)  × 10 −4 This result based on more than 45 · 10 6 events, correspondig to one half of the whole two year data taking, is about an order of magnitude more precise with respect to the previous measurement. 

G. Lamanna A

CiteSeerX

NEW RESULTS ON DIRECT CP VIOLATION IN CHARGED KAON DECAYS BY NA48/2

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.65.1661

New results on direct CP violation in charged kaon decays by NA48/2

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CiteSeerX