Recent results by the NA48 collaboration on neutral kaons rare decays into
the pi+ pi- e+ e- final state are presented. A large CP-violating asymmetry
A(K_L)=(13.9+-2.7+-2.0)% has been observed in the K_L to pi+ pi- e+ e- decay.
The K_S to pi+ pi- e+ e- decay has been observed for the first time, showing no
such asymmetry.Comment: 6 pages, 5 figures, LaTeX. To be published in the Proceedings of the
  30th International Symposium on Multiparticle Dynamics (ISMD 2000), Tihany,
  Lake Balaton, Hungary, 9-15 Oct 200

Bizzeti, Andrea

English

arXiv

Recent results by the NA48 collaboration on neutral kaons rare decays into the $\pi^{+}\pi^{-}e^{+}e^{-}$ final state are presented. A large CP-violating asymmetry $A(K_{L})=(13.9 \pm 2.7 \pm 2.0)$% has been observed in the $K_{L} \to \pi^{+}\pi^{-} e^{+}e^{-}$ decay. The $K_{S} \to \pi^{+}\pi^{-}e^{+}e^{-}$ decay has been observed for the first time, showing no such asymmetry.Recent results by the NA48 collaboration on neutral kaons rare decays into the pi+ pi- e+ e- final state are presented. A large CP-violating asymmetry A(K_L)=(13.9+-2.7+-2.0)% has been observed in the K_L to pi+ pi- e+ e- decay. The K_S to pi+ pi- e+ e- decay has been observed for the first time, showing no such asymmetry

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arXiv:hep-ex/0102035v1  15 Feb 2001NEW RESULTS FROM NA48 EXPERIMENT ON NEUTRALKAON RARE DECAYSA. BIZZETIUniversita` di Modena e Reggio Emilia, Dipartimento di Fisica,via G. Campi, 213/A, I-41100 Modena, ItalyINFN - Sezione di Firenze, largo E. Fermi 2, I-50125 Firenze, Italyrepresenting the NA48 Collaboration:Cagliari, Cambridge, CERN, Dubna, Edinburgh, Ferrara, Firenze,Mainz, Orsay, Perugia, Pisa, Saclay, Siegen, Torino, Vienna, WarsawRecent results by the NA48 collaboration on neutral kaons rare decays into thepi+pi−e+e− final state are presented. A large CP-violating asymmetry A(KL) =(13.9 ± 2.7 ± 2.0)% has been observed in the KL → pi+pi−e+e− decay. TheKS → pi+pi−e+e− decay has been observed for the first time, showing no suchasymmetry.1 IntroductionThe NA48 experiment at CERN SPS, described in detail elsewhere1,2, hasbeen designed to measure the direct CP violation parameter ℜ(ǫ′/ǫ) in two-pion decays of neutral kaons. However, the quality of its simultaneous KLand KS beams3, high resolution detectors, fast trigger and performing dataacquisition system allow to investigate at the same time rare decays of neutralkaons and neutral hyperons. Rare decay data have been collected during 1997-1999 ℜ(ǫ′/ǫ) runs, as well as during dedicated runs with a high intensity KSand hyperon beam in 1999 and 2000.In this talk I will report recent results from the NA48 experiment on KLand KS decays into the π+π−e+e− final state, which provide a new opportu-nity to probe CP violation in the neutral kaon sector.2 KL → π+π−e+e−The e+e− pair in π+π−e+e− decays of neutral kaons is expected to orig-inate from a virtual photon5,6: KL,S → π+π−γ∗ → π+π−e+e−. TheKL → π+π−e+e− decay amplitude has two main components, one from CP-odd magnetic dipole (M1) direct photon emission, the other from the decayto the CP-even state π+π− with inner bremsstrahlung. Interference betweenthese amplitudes causes a large CP-violating asymmetry in the distributionof the angle ϕ between the normals to the π+π− and e+e− planes in the kaonbizzeti: submitted to World Scientific on November 21, 2013 1c.m. system:dΓdϕ= Γ1 cos2(ϕ) + Γ2 sin2(ϕ) + Γ3 sin(ϕ) cos(ϕ) . (1)A non-zero value of Γ3, i.e. a dependence of dΓ/dϕ on the sign of the CP-oddand T-odd variable sin(ϕ)cos(ϕ), constitutes a clear signature of CP violationwhich can be seen in the dΓ/dϕ distribution. The CP-violating asymmetryA =Nsin(ϕ) cos(ϕ)>0 −Nsin(ϕ) cos(ϕ)<0Nsin(ϕ) cos(ϕ)>0 +Nsin(ϕ) cos(ϕ)<0(2)is predicted by theory5,6 to be |A(KL)| = 15%·sin(φ+−+δ0(M2K)−δ¯1) ≈ 14%.The observation of such a large asymmetry has been reported recently7 andfound to be in agreement with theoretical predictions.During 1998 and 1999 run periods more than 1300 goodKL → π+π−e+e−events have been selected using a dedicated four-track trigger8. The eventidentification relies on the precise tracking by the magnetic spectrometer andon the good energy resolution of the electromagnetic calorimeter for e/π sep-aration. Background from KL → π+π−π0D → π+π−e+e−γ decays is stronglysuppressed using a kinematic cut4, while π+π−e+e− events from two overlaidKe3 decays are rejected by time constraints. Electrons from photon conver-sions in the material before the spectrometer are eliminated by requiring a2 cm minimum separation between the e+ and e− tracks in the first chamber.DataMCMC KL→p+p-p0DDouble Ke3 evts1337 candidatesinsignal regionS/B = 38KL→p+p-e+e-Figure 1. Invariant mass distribution of KL → pi+pi−e+e− candidate events.bizzeti: submitted to World Scientific on November 21, 2013 2Figure 1 shows the invariant mass distribution of the KL → π+π−e+e−candidate events passing all analysis cuts, together with the expected back-ground contributions. For the determination of the KL → π+π−e+e− branch-ing fraction fully reconstructed KL → π+π−π0D → π+π−e+e−γ decays areused as normalization channel. The correction for the detector acceptance isdetermined using a model by Heiliger and Sehgal6 with the inclusion of a formfactor in the M1 direct emission amplitude:FM1 = g˜M1[1 +a1/a2(M2ρ −M2K)c2 + 2MK(Ee+ + Ee−)](3)Using the values g˜M1 = 1.35+0.20−0.17 and a1/a2 = (−0.720 ± 0.029) (GeV/c)2measured by KTeV7 we obtain the preliminary result B(KL → π+π−e+e−) =(3.1± 0.1± 0.2) · 10−7, in fair agreement with the KTeV preliminary result9of (3.63± 0.11± 0.14) · 10−7.Figure 2 shows the sin(ϕ)cos(ϕ) distribution of KL → π+π−e+e− eventsafter acceptance correction. Our preliminary result on the observed asym-metry is A(KL) = (13.9 ± 2.7 ± 2.0)%, in good agreement with theoreticalpredictions6 and the recently published KTeV value7.KL→p+p-e+e-Asymmetry after acceptancecorrection = (13.9 ± 2.7(stat))%DataMCFigure 2. Acceptance corrected sin(ϕ)cos(ϕ) distribution of KL → pi+pi−e+e− candidateevents.bizzeti: submitted to World Scientific on November 21, 2013 33 KS → π+π−e+e−The KS → π+π−e+e− decay amplitude is largely dominated by the CP-eveninner bremsstrahlung process, so no CP-violating asymmetry is expected inthis case.This decay mode has been observed for the first time with a cleansample of 56 events selected from 1998 data8. The decay events comingfrom the KS beam have been selected using the very good vertex resolu-tion of the magnetic spectrometer and the precise timing of the taggingdetectors10. The invariant mass distribution of KS → π+π−e+e− candi-date events is shown in figure 3. Low invariant mass events in the plot aredue to KL → π+π−π0D → π+π−e+e−γ decays of KL mesons originating fromthe KS target. Using 105 fully reconstructed KL → π+π−π0D events fromthe KS target as normalization channel from 1998 data a branching fractionB(KS → π+π−e+e−) = (4.5± 0.7± 0.4) · 10−5.In 1999 two days of run were dedicated to the investigation of rare decaysof KS and neutral hyperons, with the proton beam intensity on the KS targetincreased by about a factor 200, the KL beam switched off and the AKSconverter at the beginning of the fiducial volume removed. The large amountof rare decay events collected during this short test run, equivalent to severalyears of operation with the standard KL + KS beam setup, allowed a moreprecise determination of the branching fraction and a measurement of theCP-violating asymmetry A(KS) at a few percent level. The invariant massdistribution of KS → π+π−e+e− candidate events from the full 1998+1999KS→p+p-e+e-KL→p+p-p0|→e+e-( g )56 eventsinsignal region↓ ↓Figure 3. Invariant mass distribution of KS → pi+pi−e+e− candidate events (1998 data).bizzeti: submitted to World Scientific on November 21, 2013 4data sample is shown in figure 4. The sin(ϕ)cos(ϕ) distribution of these eventsis plotted in figure 5: the resulting asymmetry A(KS) = (−0.2± 3.4± 1.4)%KS→p+p-e+e-KL→p+p-p0|→e+e-( g )921 eventsinsignal region↓Figure 4. Invariant mass distribution of KS → pi+pi−e+e− candidate events (1998+1999data).DataMCFigure 5. Acceptance corrected sin(ϕ)cos(ϕ) distribution of KS → pi+pi−e+e− candidateevents (1998+1999 data).bizzeti: submitted to World Scientific on November 21, 2013 5is consistent with zero. Our preliminary result on the branching fractionis B(KS → π+π−e+e−) = (4.3 ± 0.2 ± 0.3) · 10−5. Using this number toevaluate the inner bremsstrahlung contribution to the KL decay we obtainB(KIBL → π+π−e+e−) = (1.3±0.1)·10−7, in good agreement with theoreticalexpectations5,6.References1. V. Fanti et al., Phys. Lett. B 465, 335 (1999).2. T. Gershon, these proceedings.3. C. Biino et al., Proc. 6th European Particle Accelerator Conference,Stockholm, Sweden, 22-26 June 1998; IOP, Bristol, 1999.4. D. Leurs et al., Phys. Rev. 133B, 1276 (1964); A. Carrol et al., Phys.Rev. Lett. 44, 525 (1980); J. Adams et al., Phys. Rev. D 80, 4123(1998).5. L. M. Sehgal and M. Wanninger, Phys. Rev. D 46, 1035 (1992); ibid.46, 5209(E) (1992)6. P. Heiliger and L. M. Sehgal, Phys. Rev. D 48, 4146 (1993); ibid. 60,079902(E) (1999).7. A. Alavi-Harati et al., Phys. Rev. Lett. 84, 408 (2000).8. A. Lai et al., to be published in Phys. Lett. B.9. K. Senyo, Proc. Int. Europhysics Conference on High Energy Physics,Tampere, Finland, 15-21 July 1999; IOP, Bristol, 2000.10. P. Grafstro¨m et al., Nucl. Instrum. Methods A344, 487 (1994);H. Bergauer et al., Nucl. Instrum. Methods A419, 623 (1998).bizzeti: submitted to World Scientific on November 21, 2013 6

New results from NA48 experiment on neutral kaon rare decays

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