Study of the decay asymmetry parameter and CP violation parameter in the decay
Introduction
Parity violation in the weak decay of a spin hyperon into a spin baryon and a pseudoscalar meson is well known. An example is [1], where the angular distribution of the proton in the Λ rest frame is given by where P is the polarization of Λ along the z direction, θ is the polar angle and is the weak decay-asymmetry parameter. The latter is defined as where and are the parity-even and parity-odd decay amplitudes. In a non-relativistic picture, they correspond to the and orbital angular momenta of the proton–π system, respectively.
Eq. (1) can be generalized for the case of a baryon double decay chain like with in which each baryon weak decay is a process. If the is produced unpolarized, Eq. (1) for the Λ decay becomes where is the weak-decay asymmetry parameter of the process and θ is the helicity angle of the proton in the Λ rest frame, i.e. it is the supplement of the angle (i.e., π minus the angle) in the Λ rest frame between the proton and the direction of the . The distribution of the helicity angle, θ, determines the product , and, since is known (in this analysis we assume [2]), one can extract .
If CP were conserved exactly, of the process would be the negative of in decay. Just as with the Λ decay, there is the possibility of CP violation in decay (a weak decay with possible final state interactions). In this Letter we (1) measure and separately, (2) use them for the first measurement of the CP asymmetry parameter
and (3) having established that the difference is negligible within our errors, combine the data for particle and antiparticle decays to obtain the best value of the asymmetry parameter. In this analysis the is assumed to be unpolarized and thus the Λ longitudinal polarization (polarization along the Λ momentum direction in the rest frame) is . As discussed later, if the polarization is the maximum allowed in our data, the effect on our measurement is negligible.
FOCUS is a charm photoproduction experiment, an upgraded version of E687 [3], which collected data during the 1996–1997 fixed target run at Fermilab. Electron and positron beams obtained from the Tevatron proton beam produce, by means of bremsstrahlung, a photon beam (with typically endpoint energy) which interacts with a segmented BeO target [4]. The mean photon energy for triggered events is ∼180 GeV. A system of three multicell threshold Čerenkov counters is used to perform the charged particle identification, separating kaons from pions up to of momentum. Two systems of silicon microvertex detectors are used to track particles: the first system consists of 4 planes of microstrips interleaved with the experimental target [3], [5] and the second system consists of 12 planes of microstrips located downstream of the target. These detectors provide high resolution in the transverse plane (approximately 9 μm on the track position), allowing the identification and separation of charm production and decay vertices. The charged particle momentum is determined by measuring the deflections in two magnets of opposite polarity through five stations of multiwire proportional chambers.
Section snippets
Analysis of the decay mode + charge conjugate
Unless explicitly stated otherwise, both the particle and its charge conjugate are implied. The selection of events begins with the identification of Λ candidates on the basis of vertexing and loose Čerenkov cuts on the pion and proton decay particles as described in detail elsewhere [6]. The tracks of these charged daughters are used to form the decay vertex of the Λ and to determine its flight direction and momentum. The momentum of the Λ is used together with the momentum of a (or
Fitting procedure and extraction of and
In order to extract and , the data sample is divided into particle and antiparticle subsamples and each subsample is further divided into four equal slices of , spanning the range from −1 to +1. The Λπ mass plots of the resulting eight subsamples are shown in Fig. 2, Fig. 3. The number of events above background in these plots is the basic data needed to measure the slope and thus the product which is called α in this discussion of the fit procedure. These numbers are
Systematic checks
Many checks have been performed in order to assess the systematic errors on and .
In this analysis we assume the is produced unpolarized. We think this is a safe assumption for the FOCUS experiment. In fact if the were polarized the Λ polarization would be larger than in one hemisphere about the polarization and smaller in the other hemisphere, but the polarization averaged over all decay angles remains . Even a large, asymmetric error in the acceptance correction and
Final results and comparison with previous results and with the theoretical predictions
After all systematic checks, bias correction and error inflation the final results of the measurements are and . Previous results obtained for are listed in Table 1. The present analysis result is a higher precision and it is in agreement with earlier results. No previous measurements for have been reported.
Theoretical predictions of the two-body baryon-pseudoscalar decays of charmed baryons have been made by a number of authors in the past. They
Conclusions
Using data from the FOCUS (E831) experiment at Fermilab, we have studied the decay mode and measured the asymmetry parameter (under the CP conservation hypothesis) and the CP parameter . Our result for is consistent with previous measurements and is more accurate. Our result for is the first measurement of this quantity and is consistent with zero.
Acknowledgements
We wish to acknowledge the assistance of the staffs of Fermi National Accelerator Laboratory, the INFN of Italy, and the physics departments of the collaborating institutions. This research was supported in part by the US National Science Foundation, the US Department of Energy, the Italian Istituto Nazionale di Fisica Nucleare and Ministero della Istruzione Università e Ricerca, the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico, CONACyT-México, and the Korea Research
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