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Corfu Summer Institute on Elementary Physics, 1998

PROCEEDINGS

TIME REVERSAL, CP AND CPT VIOLATION STUDIES IN THE CPLEAR EXPERIMENT AT CERN

Christos A. Eleftheriadis∗ Nuclear Physics and Elementary Division, Aristotle University, 54006 Thessaloniki, Greece E-mail: Christos.Eleftheriadis@.ch

Abstract: The CPLEAR experiment at CERN studies Time Reversal, CP and CPT symmetries in the neutral system, which is so far the only system where CP violation has been observed. In the so-called ‘golden channels’, - produces a neutral kaon, a charged kaon and a charged . The of the neutral kaon is tagged with the charge of the accompanying kaon. Decay rate between K0 and K0 are measured. Symmetry violating parameters connected to Time Reversal, CP and CPT symmetries are extracted from decays to various final states.

∗For the CPLEAR Collaboration: A. Apostolakis1, E. Aslanides11,G.Backenstoss2, P. Bargassa13, O. Behnke17, A. Benelli2,V.Bertin11,F.Blanc7,13, P. Bloch4 , P. Carlson15, M. Carroll9,E.Cawley9, M.B. Chertok3, M. Danielsson15,M.Dejardin14, J. Derre14, A. Ealet11, C. Eleftheriadis16,W.Fetscher17, M. Fidecaro4, A. Filipˇciˇc10,D.Francis3,J.Fry9, E. Gabathuler9, R. Gamet9, H.-J. Gerber17,A.Go4,A.Haselden9,P.J.Hayman9, F. Henry-Couannier11, R.W. Hollander6, K. Jon-And15, P.-R. Kettle13, P. Kokkas4,R.Kreuger6,R.LeGac11,F.Leimgruber2, I. Mandi´c10, N. Manthos8, G. Marel14, M. Mikuˇz10, J. Miller3,F.Montanet11 , A. Muller14, T. Nakada13 , B. Pagels17, I. Papadopoulos16 , P. Pavlopoulos2, G. Polivka2,R.Rickenbach2,B.L.Roberts3,T.Ruf4, L. Sakeliou1,M.Sch¨afer17, L.A. Schaller7, T. Schietinger2,A.Schopper4,L.Tauscher2, C. Thibault12, F. Touchard11,C.Touramanis9, C.W.E. Van Eijk6, S. Vlachos2, P. Weber17,O.Wigger13,M.Wolter17,D.Zavrtanik10,D.Zimmerman3.

1University of Athens, Greece, 2University of Basle, Switzerland, 3Boston University, USA, 4CERN, Geneva, Switzerland, 5LIP and University of Coimbra, Portugal, 6Delft University of Technology, Netherlands, 7University of Fribourg, Switzerland, 8University of Ioannina, Greece, 9University of Liverpool, UK, 10J. Stefan Inst. and Phys. Dep., University of Ljubljana, Slovenia, 11CPPM, IN2P3-CNRS et Universit´e d’Aix-Marseille II, France, 12CSNSM, IN2P3-CNRS, Orsay, France, 13Paul Scherrer Institut (PSI), Villigen, Switzerland, 14CEA, DSM/DAPNIA, CE-Saclay, France, 15Royal Institute of Technology, Stockholm, Sweden, 16University of Thessaloniki, Greece, 17ETH-IPP Z¨urich, Switzerland.

1. Introduction variance. Given CPT invariance and CP viola- tion, Time Reversal has to be violated. Neutral are the lightest strange mesons The CPLEAR experiment [1] used a new ex- and the only system where violation of - perimental method to study T, CP and CPT symmetry has been seen. This vi- symmetries in the decay of neutral kaons by mea- olation is connected to a CP-violating term in suring the rate asymmetries between K0 and K0 0 0 + the K - K mixing matrix. On the other hand, decays to various final states, namely π π−, πeν, + 0 0 0 0 0 0 strong theoretical arguments support CPT in- π π−π , π π and π π π . The extraction of Corfu Summer Institute on Elementary Particle Physics, 1998 Christos A. Eleftheriadis∗

+ 0 + the physics parameters is obtained through the R (t)=R(Kt=0 π−e ν) → measurements of the time-dependent decay rate 0 + R−(t)=R(Kt=0 π e−ν) → asymmetries : Assuming that the ∆S =∆Q rule holds, the first 0 () 0 () RK f t RK f t Af (t)= → − → . and the fourth process are the ones which are di- 0 ()+ 0 ( ) RK f t RK f t → → rectly allowed, whereas the second and the third

For instance the CP-violation parameter η+ is − may take place only after a ∆S = 2 transition. + obtained from the decay rates to π π− and the The four decay rates may thus be written as: CPT-violation parameter Re(δ) is measured from + 0 0 R (t)=R[K t=0 K (t)] the semileptonic decay rates. Time Reversal sym- → 0 0 metry is directly probed for the first time by the R−(t)=R[K t=0 K (t)] → 0 0 + 0 0 difference between the rates of K K and R (t)=R[Kt=0 K (t)] → → 0 0 K K , as measured in two of the semilep- 0 0 → R−(t)=R[Kt=0 K (t)] tonic decay channels. CPLEAR results are sum- → marized in the present paper. The We recall that from the Low En- + R(t) R−(t) AT (t)= − ergy Antiproton Ring (LEAR) were stopped in a + R(t)+R−(t) target at high pressure and annihilated measures the difference between the probability with . The neutral kaons were produced of an initial K0 being a K0 at time t and the prob- through the so-called ‘golden channels’ ability of an initial K0 beingaK0 again at time + 0 K−π K t, hence the amount of T violation (fig. 1) (for a pp + 0 → K π−K full discussion of the related phenomenology see

3 [3], [4]). with a branching ratio of 2 10− for each ≈ × channel. Since strong interactions conserve strange- ness, it is clear that at production time the neu- tral kaon and the charged one have opposite stran- geness, thus permitting the tagging of the neu- tral kaon. Detailed information on the CPLEAR detector can be found in reference [2].

2. Semileptonic decays exp Figure 1: The AT (t) asymmetry (τS is the KS There are four possible semileptonic decays of the lifetime). neutral kaons, two of them being allowed by the ∆S =∆Q rule and the two being forbidden. The The data were corrected with respect to de- decay rates are: tection efficiencies and regeneration effects. The decay time resolution was folded to the fitting + 0 + R (t)=R(K t=0 π−e ν) → function. The main background source is the 0 + R−(t)=R(K t=0 π e−ν) two-pion decays of neutral kaons, limited at early →

2 Corfu Summer Institute on Elementary Particle Physics, 1998 Christos A. Eleftheriadis∗ decay times. At late decay times there are contri- butions from three-pion decays and from semilep- tonic decays with reversed pion and as- signment. The background contribution is nearly negligible in the total error. The experimental result for the asymmetry is

exp 3 A =(6.6 1.3stat 1.0syst) 10− T ± ± × exp Figure 2: The Aδ (t) asymmetry. and it is the first direct measurement of Time

Reversal violation [3]. The asymmetry From the semileptonic decays, other parame- + + R−(t)+R (t) R (t) R−(t) ters may also be measured through the asymme- A (t)= − − ∆m + + exp R−(t)+R (t)+R (t)+R−(t) try Aδ (t)(fig.2),suchasRe(δ), Im(δ) (which is a function of ∆m and Re(x). From this asym- are CPT violating) and Re(x )andIm(x+) (which − are connected to the ∆S =∆Q rule, with x also metry (fig. 3), assuming no CPT-violating decay − CPT-violating) [5]. The values obtained, free of amplitudes, we obtain [9]: assumptions, are: 10 1 ∆m =(0.5295 0.0020stat 0.0003syst) 10 hs¯ − ± ± × 4 3 Re(δ)=(3.0 3.3stat 0.6syst) 10− , Re(x)=( 1.8 4.1stat 4.5syst) 10− ± ± × − ± ± × 2 Im(δ)=( 1.5 2.3stat 0.3syst) 10− , − ± ± × 2 Re(x )=(0.2 1.3stat 0.3syst) 10− , − ± ± × 2 Im(x+)=(1.2 2.2stat 0.3syst) 10− , ± ± × with correlation coefficients as given in [5]. Assuming that the ∆S =∆Q rule holds, then

Re(x )=Im(x+) = 0 and the analysis yields −

4 Re(δ)=(2.9 2.6stat 0.6syst) 10− , ± ± × 3 Im(δ)=( 0.9 2.9stat 1.0syst) 10− − ± ± ×

These values improve the limits obtained by a re- Figure 3: The A∆m(t) asymmetry for πeν decays. analysis of two previous experiments [6] by two and one orders of magnitude respectively. + 3. π π− decays The final results of the CPLEAR analysis, assuming only unitarity, through the Bell–Stein- The differences in the decay rates of the CP con- berger relation, and using recent CPLEAR and jugates K0 and K0 to two charged is a sign PDG values [7] improve further the limits for the of CP violation. In fig 4, these decay rates, cor- parameters mentioned above [8]. rected for the acceptance, are shown separately,

3 Corfu Summer Institute on Elementary Particle Physics, 1998 Christos A. Eleftheriadis∗ for K0 and K0 initially tagged. The difference in the time dependence of the decay rates demon- strates the CP violation effect.

+ Figure 5: Neutral kaon decay to π π−: the decay

rate asymmetry A+ after background subtraction. −

performed during the last year of operation of the CPLEAR detector [11]. Figure 4: Neutral kaon decay to π+π : the decay − With rates of K0 ( )andK0 ( ) are shown separately. • ◦ 7 1 ∆m = (530.1 1.4) 10 hs¯ − ± × By forming the time-dependent asymmetry and

τS =(89.32 0.08)psec R 0 + (t) αR 0 + (t) ± K π π− K π π− A+ (t)= → − → − R 0 + (t)+αR + (t) [7] the fit gives [10]: K π π− K0 π π− → 1 → (ΓS ΓL)t η+ e 2 − cos(∆mt φ+ ) = 2| −| − − η+ =(2.264 0.023stat 0.026syst 2 (ΓS ΓL)t − 1+ η+ e − | −| ± ± | −| 3 0.007τS ) 10− , the acceptances common to K0 and K0 cancel ± × 0 0 0 0 φ+ =43.19 0.53stat 0.28syst 0.42 . and the systematic uncertainties reduce accord- − ± ± ± ∆m ingly. The normalization factor α includes the 4. Summary difference in the tagging efficiencies of K0and K0, which is a reflection of the the different detection The first direct observation of T violation and + + 0 0 efficiencies of the pairs K π− and K−π used tests of CPT invariance in K K mixing are re- − to tag K0 and K0, see [10] . The asymmetry is ported. Measurements of other symmetry violat- shown in fig. 5 and it is fitted for η+ , φ+ ing parameters are also reported, such as η+ , | −| − | −| and α. The data are corrected, event by event, φ+ and the ones related to the ∆S =∆Q rule, − for regeneration effects. Since there were no data as well as a measurement of ∆m. In all cases the available for the difference between the forward previous experimental values were improved by scattering amplitudes of K0 and K0 in the hun- CPLEAR. dreds of MeVs region, dedicated runs were

4 Corfu Summer Institute on Elementary Particle Physics, 1998 Christos A. Eleftheriadis∗

References

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