Kaon System Research Articles

The NA48 event-building PC farm

The NA48 experiment at the CERN SPS aims to measure the parameter Re(/spl epsiv/'//spl epsiv/) of direct CP violation in the neutral kaon system with an accuracy of 2x10/sup -4/. Based on the requirements of: high event rates (up to 10 kHz) with negligible dead time; support for a variety of detectors with very wide variation in the number of readout channels; data rates of up to 150 MByte/s sustained over the beam burst; level-3 filtering and remote data logging in the CERN computer center; the collaboration has designed and built a modular pipelined data flow system with 40 MHz sampling rate. The architecture combines custom-designed components with commercially available hardware for cost effectiveness and flexibility. To increase the available data bandwidth and to add filtering and monitoring capabilities, the original custom-built event builder hardware has been replaced by a farm of 24 Intel PentiumII based PCs running the Linux operating system during the shutdown between the 1997 and 1998 data taking periods. During the data taking period 1998 the system has been successfully operated taking ca. 70 Terabyte of data.

Direct CP-violation as a test of quantum mechanics

Direct CP-violating effects in the neutral kaon system result in violations of certain Bell-like inequalities. The new experimental results on the determination of the phenomenological parameter $\varepsilon'$ allow to dismiss a large class of “hidden variable” alternatives to quantum mechanics.

Direct

Direct CP-violating effects in the neutral kaon system result in violations of certain Bell-like inequalities. The new experimental results on the determination of the phenomenological parameter epsilon' allow to dismiss a large class of ``hidden variable'' alternatives to quantum mechanics.

CONSTRAINTS ON A PARITY-CONSERVING/TIME-REVERSAL-NON-CONSERVING INTERACTION

Time-reversal-invariance non-conservation has for the first time been unequivocally demonstrated in a direct measurement at CPLEAR. One then can ask the question: What about tests of time-reversal-invariance in systems other than the kaon system? Tests of time-reversal-invariance can be distinguished as belonging to two classes: the first one deals with parity violating (P-odd)/time-reversal-invariance-odd (T-odd) interactions, while the second one deals with P-even/T-odd interactions (assuming CPT conservation this implies C-conjugation non-conservation). Limits on a P-odd/T-odd interaction follow from measurements of the electric dipole moment of the neutron (with a present upper limit of 6 × 10-26 e.cm [95% C.L.]). It provides a limit on a P-odd/T-odd pion-nucleon coupling constant which is less than 10-4 times the weak interaction strength. Experimental limits on a P-even/T-odd interaction are much less stringent. Following the standard approach of describing the nucleon-nucleon interaction in terms of meson exchanges it can be shown that only charged rho-meson exchange and A 1-meson exchange can lead to a P-even/T-odd interaction. The better constraints stem from measurements of the electric dipole moment of the neutron and from measurements of charge-symmetry breaking in neutron-proton elastic scattering. The latter experiments were executed at TRIUMF (497 and 347 MeV) and at IUCF (183 MeV). Weak decay experiments may provide limits which will possibly be comparable. All other experiments, like gamma decay experiments, detailed balance experiments, polarization–analyzing power difference determinations, and five-fold correlation experiments with polarized incident nucleons and aligned nuclear targets, have been shown to be at least an order of magnitude less sensitive. The question then emerges: is there room for further experimentation?

Lattice matrix elements confronting the experimental value of ε′/ε

A new lattice estimate of K -> 2 pi transitions claims, contrary to all other computations, that the hadronic matrix element for the gluon penguin operator Q_6 has opposite sign and, in addition, is much larger than the vacuum saturation approximation. We comment under what conditions (if any) it is possible to reconcile this lattice result with the experimental value of eps'/eps. The dramatic impact of new physics in the kaon system that seems to be required is not easily accommodated within our present theoretical understanding.

Neutral-kaon system in dense matter and heavy-ion collisions

Above a critical matter density the propagating modes of the neutral kaon system are essentially eigenstates of strangeness, but below it they are almost complete eigenstates of CP. We estimate the real and imaginary parts of the energies of these modes and their mixing at all densities up to a nuclear matter density of 2×10 14 g/cm 3. In a heavy-ion collision the strong interactions create eigenstates of strangeness, and these propagate adiabatically until the density has fallen to the critical value, whereupon the system undergoes a sudden transition to (near) eigenstates of CP. We estimate the critical density to be 20 g/cm 3, and that this density will be reached about 2×10 5 fm/c after the end of the collision.

The NA48 experiment at the CERN SPS

The NA48 experiment at the CERN SPS aims at measuring the direct CP violation parameter, R (ε′/ε), in the system of neutral kaons with a precision of ∼ 2 × 10 −4. The measurement is performed comparing K L and K S decay rates into pairs of charged and neutral pions. The status of the experiment will be presented: after a description of the apparatus, with special attention to some of its components, the detector performances and the achievements of the 1997 run will be presented together with the improvements of the 1998 run.

Contribution of CPLEAR to the physics of the neutral-kaon system

We report the results of the CPLEAR experiment on CP-, T-, and CPT-symmetries in the neutral kaon system. CP-violation parameters are given for the K 0, K 0 → 2π, 3π decay channels. For the first time, T-violation is measured by a direct method using semileptonic decays. The CPT symmetry is tested through the parameters Im(δ) with a precision of 10 −5 and Re(δ) with a precision of a few 10 −4. With these two measurements, the difference in mass and width of K 0 and K 0 are bound to within 10 −18 GeV. In the limit of CPT invariance in the neutral kaon decays, the K 0 and K 0 masses are equal within 10 −19 GeV.

DISSIPATIVE CONTRIBUTIONS TO ε′/ε

The time evolution and decay of the neutral kaon system can be described using quantum dynamical semigroups. Non-standard terms appear in the expression of relevant observables; they can be parametrized in terms of six new phenomenological constants. We discuss how the presence of these constants affects the determination of the parameter ε′/ε.

Chemical fluctuations in high-energy nuclear collisions

Fluctuations of the chemical composition of the hadronic system produced in nuclear collisions are discussed using the Φ-measure which has been earlier applied to study the transverse momentum fluctuations. The measure is expressed through the moments of the multiplicity distribution and then the properties of Φ are discussed within a few models of multiparticle production. A special attention is paid to the fluctuations in the equilibrium ideal quantum gas. The system of kaons and pions, which is particularly interesting from the experimental point of view, is discussed in detail.

Non-standard Neutral Kaon Dynamics from Infinite Statistics

The neutral kaon system can be effectively described by non-unitary, dissipative, completely positive dynamics that extend the usual treatment. In the framework of open quantum systems, we show how the origin of these non-standard time evolutions can be traced to the interaction of the kaon system with a large environment. We find that a heat bath of quanta obeying infnite statistics could constitute a realistic example of such an environment.

Contribution of CPLEAR to the physics of the neutral-kaon system

We report the results of the CPLEAR experiment on CP-, T- and CPT-symmetries in the neutral kaon system. CP-violation parameters are given for different decay channels. For the first time T-violation is measured by a direct method using semileptonic decays. The CPT symmetry in the kaons decays is tested through the parameters Im( δ ), from the Bell-Steinberger relation, with a precision of 10 −5 and Re( δ ) with a precision of a few 10 −4 (two order of magnitude better than the previous measurement). These two measurements allow us to bound the difference in mass and width of K 0 and ¯K 0 to be equal within 10 −18 GeV. In the limit of CPT invariance in the neutral kaon decays, the K 0 and ¯K 0 masses are equal within 10 −19 GeV.

Fully SupersymmetricCPViolation inKandBSystems

We analyze $\mathrm{CP}$ violation in supersymmetric extensions of the standard model with heavy scalar fermions of the first two generations. Neglecting intergenerational mixing in the sfermion mass matrices and thus considering only chargino, charged Higgs-boson, and $W$-boson diagrams we show that it is possible to fully account for $\mathrm{CP}$ violation in the kaon system even in the absence of the standard CKM phase. This opens new possibilities for large supersymmetric contributions to $\mathrm{CP}$ violation in the $B$ system.

Spontaneous CP Violation from High-Dimensional Operators

We investigate spontaneous CP violation that arises from high-dimensional operators in an effective Lagrangian with the symmetry group SU(3)×SU(2)×U(1)×Z7. We show that such a model does not necessarily require a very large scale Λ by simultaneously satisfying the constraints from the parameter in the neutral kaon system and the electric dipole moment (EDM) of the neutron. We find that the EDM of the electron in this model could be at the level of 10−27 ecm. However, the CP violating muon polarizations in η ,KL → μ+μ− decays are small.

Neutron electric dipole moment and spontaneous CP breaking

In a model where CP is spontaneously broken, the aspon model, it is shown by extending previous arguments about B decay and the value of Re( ϵ ′/ ϵ) in the kaon system, that the neutron electric dipole moment D n is bounded from below by D n ≥10 −29e.cm.

CP-violating phases in the CKM matrix in orbifold compactifications

The picture of CP-violation in orbifold compactifications in which the T-modulus is at a complex fixed point of the modular group is studied. CP-violation in the neutral kaon system and in the neutron electric dipole moment are both discussed. The situation where the T-modulus takes complex values on the unit circle which are not at a fixed point is also discussed.

Test of special relativity and equivalence principle fromKphysics

A violation of Local Lorentz Invariance (VLI) and hence the special theory of relativity or a violation of equivalence principle (VEP) in the Kaon system can, in principle, induce oscillations between $K^0$ and $\bar{K}^0$. We construct a general formulation in which simultaneous pairwise diagonalization of mass, momemtum, weak or gravitational eigenstates is not assumed. %and the maximum attainable %velocities of the velocity eigenstates are different. We discuss this problem in a general way and point out that, as expected, the VEP and VLI contributions are indistinguishable. We then insist on the fact that VEP or VLI can occur even when CPT is conserved. A possible CP violation of the superweak type induced by VEP or VLI is introduced and discussed. We show that the general VEP mechanism (or the VLI mechanism, but not both simultaneously), with or without conserved CPT, could be clearly tested experimentally through the energy dependence of the $K_L-K_S$ mass difference and of $\eta_{+-}$, $\eta_{00}$, $\delta$. Constraints imposed by present experiments are calculated.

Time performance of the liquid krypton calorimeter of the NA48 experiment

The NA48 experiment aims to measure direct CP violation in the neutral kaon system. The measurement of the event time is crucial for this experiment, particularly for the neutral modes, since this allows the identification of the kaon type (KL or KS). The time performance of the liquid krypton calorimeter measured in 1996 with 50GeV electrons is presented. A time resolution better than 260ps has been achieved and the tails of the time distribution are at the level of 10−5. This time performance of the calorimeter is in excellent agreement with expectation.

Test of special relativity from K physics

A breakdown of the Local Lorentz Invariance and hence the special theory of relativity in the Kaon system can, in principle, induce oscillations between K 0 and K ̄ 0 . We construct a general formulation in which simultaneous pairwise diagonalization of mass, velocity or weak eigenstates is not assumed and the maximum attainable velocities of K 0 and K 0 are different. This mechanism permits Local Lorentz Invariance violation in a manner that need not violate CPT conservation. We show that such a CPT-conserved violation of special relativity could be clearly tested experimentally through the energy dependence of the K L − K S mass difference and discuss constraints imposed by present experiments.

The CPLEAR-Experiment at CERN

The CPLEAR experiment uses tagged K 0 and K 0 produced in p p annihilation at rest to measure CP-, T- and CPT-violation parameters in the neutral kaon system. The results of these measurements and some implications are reported.