G. Pierazzini

First Evidence for Direct CP Violation

Since CP violation was first observed in the decay of the long-lived neutral kaon into two pions [1], it remains one of the enigmas in particle physics. Whilst CP violation is manifest in neutral kaon decays, the search for CP-violating effects has elsewhere been unsuccessful. In the phenomenology of CP violation in the neutral kaon system [2], the short- and long-lived mass eigenstates are usually defined in terms of the CP eigenstates K1 (CP = + 1) and K2 (CP = - 1) as Ks ≈ K1 + ?K2 and KL ≈ K2 + ?K1. The parameter ? describes CP violation induced by kaon state-mixing. Direct CP violation may also occur in the decay of K2 into two pions with a relative amplitude ?′, which is non-zero in the case of a phase difference between the amplitudes A0 and A2 for the decay into isospin O and 2 states of two pions. Before the present measurement, all experimental results were compatible with ? = 2.27 × 10-3 exp (i 43.70) and with the Superweak Model [3], in which state-mixing is the only source of CP violation and ?′ = 0. In the Standard Model with six weakly interacting quarks [4], direct CP violation as well as state-mixing is introduced by transitions via heavy-quark intermediate states. Based on this, a small, but non-zero, value of ?′ is predicted [5]. To a good approximation, ?′ /? is related to the double ratio R of the relative decay rates of the long- and short-lived neutral kaons into two neutral and two charged pions as Re (?′/?) = 1/6 × (1 - R).

A measurement of the phases of the CP-violating amplitudes in K0→2π decays and a test of CPT invariance

Abstract The phases of the CP-violating amplitudes in K0→π+π− and K0→2π0 decays, φ+−=46.9°±2.2° and φ00=47.1°±2.8°, have been measured in the same experiment, and a direct comparison gives the phase difference φ00−φ+−=0.2°±2.9°. This result leads to an upper limit on possible CPT violation in the K0 mass matrix, of |(m K 0 −m K 0 )/m K 0 | −18 at the 95% confidence level and is the most stringent test of the equality of particle and antiparticle masses.

Reaction π−p→ηn in the 15 to 40 GeV/c momentum range

A high statistics measurement of the reaction π−p → π0n has been performed at the Serpukhov accelerator for 15, 20, 25, 30 and 40 GeV/c incident pion momentum using the NICE set-up with its associated 648-channel hodoscope spectrometer for γ-ray detection. More than 3 million charge-exchange events have been recorded in total. It is found that the spin-flip and non-spin-flip amplitudes can be parametrized, for small |t|, as exponentials with the same slopes to within a few percent. Also the behaviour of the differential cross section for small and medium |t| agrees with the prediction of a geometrical s-channel model which describes binary reactions in terms of a complex pole b0(s). The imaginary part of this universal pole, Im b0(s), has been determined and found to be growing logarithmically with s.

The Beam and Detector for a High Precision Measurement of {CP} Violation in Neutral Kaon Decays

The K0 beam and detector used for a high-precision measurement of the CP-violation parameter ϵ′ at the CERN Super Proton Synchrotron (SPS) are described. The beam provides KL and KS alternately through a common decay region. The detection of the decays is based on wire chambers and calorimeters without employing a magnet. The trigger and readout system achieve a high selectively for the suppressed, CP-violating, two-pion decays of the KL by incorporation of hard-wired processors. The readout is based on Fastbus for maximum data rates.

Reaction π− p → η′n in the 15–40 GeV/c momentum range☆

Abstract Measurements were made of the cross section of the reactions π−p → ν′(958)n, η′ → 2γ at momenta at 15, 20, 25, 30 and 40 GeV/c. The experiment was carried out on the IHEP 70 GeV accelerator using the 648 channel hodoscope spectrometer NICE for γ-ray detection. A total of 6000 η′ mesons were recorded. A sharp drop is seen in the differential cross section for t → 0. The dependences of the differential cross sections for the π−p → η′n and π−p → η n on t are identical. On the basis of the ratio of the cross sections for these reactions at t = 0, i.e. R( η′ n ) t=0 = 0.55 ± 0.06 , the singlet-octet mixing angle for pseudoscalar mesons was determined to be β = −(18.2 ± 1.4)°.

Observation of the decay KL→π0γγ

Abstract The decay mode K L → π 0 γγ has been observed with a signal of 21 events and an expected background of 1.5±0.9 events. A branching ratio for decays with invariant γγ masses above 280 MeV of (2.1±0.6)×10 −6 is calculated. This result is compared with the values estimated from theoretical models and has implications for the CP conserving contribution to K L → π 0 e + e − decay.

Observation of the decay Ks → 2γ and measurement of the decay rates KL → 2γ and KS → 2γ

Abstract The decay rates of K L → 2γ and K S → 2γ have been measured at the CERN SPS. The results are Γ (K L →2 γ )/ χ (K L →2 π 0 )=0.632±0.004±0.008 and Γ (K S →2 γ )/ Γ (K L →2 γ )=2.3 ±1.0±0.4. This is the first observation of K S →2γ decays.

Observation of the decay K(L) ---> pi0 gamma gamma

Abstract The decay mode K L → π 0 γγ has been observed with a signal of 21 events and an expected background of 1.5±0.9 events. A branching ratio for decays with invariant γγ masses above 280 MeV of (2.1±0.6)×10 −6 is calculated. This result is compared with the values estimated from theoretical models and has implications for the CP conserving contribution to K L → π 0 e + e − decay.

Observation of a spin 4 neutral meson with 2 GeV mass decaying in π° π°

Abstract The invariant mass spectrum of neutral meson states from π − p interactions at 40 GeV/ c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a hodoscope spectrometer with a computer on-line was used. A clear structure on the mass spectrum of dipions produced in the reaction π − p→π°π°n is observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M = (2020±30)MeV and the estimate of the full width is (180±60) MeV.

Analysis of the reaction π−p → π0ηn at 40 GeV/c beam momentum

The reaction π−p→π0ηn↳2y↳2y has been analyzed using data of an experimental performed at the 70 GeV accelerator, with the NICE 648 channel hodoscope spectrometer for γ ray detection. Events with 4γ seen are used for the analysis. A method is applied, which allows the determination of the number of π0η events for each mass, cosθGJ and t bin. Mass spectra, t distributions and decay angular distributions for the π0η system are presented. The cross section for the production of A20 is found to be 2.7 ± 1.1 μb at 40 GeV/c beam momentum. No indication of a resonant 1− state in the π0η system is observed, in spite of the fact that this state is allowed for the π0η system on the same footing as the observed 0+ and 2+ resonances.