Recent Belle results from \Upsilon(5S) sample

Recent Belle results from sample

(On behalf of the Belle collaboration)
Laboratoire de Physique des Hautes Énergies,
École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

The large data sample recorded with the Belle detector at the energy provides a unique opportunity to study the poorly-known meson. Two analyses, performed with a data sample representing an integrated luminosity of 121 , are presented: the measurement of the and branching fractions, and the observation of the decay which is the first observation of a baryonic decay. In addition, we present new results of a measurement of the CKM angle with tagged events.

29 October 2011

LPHE Note 2011-04


The Belle experiment [1], located at the interaction point of the KEKB asymmetric-energy collider, was designed for the study of mesons111The notation “” refers either to a or a . Moreover, charge-conjugated states are implied everywhere. produced in annihilation at a center-of-mass (CM) energy corresponding to the mass of the resonance (). However, a data sample of integrated luminosity has been recorded and analyzed at the energy of the resonance (), above the threshold.

Apart from the continuum events, the process can produce different kinds of final states involving a pair of non-strange mesons [2] (, , , , , , and ), a pair of mesons (, and ), or final states involving a light bottomonium resonance below the open-beauty threshold [3]. The and mesons always decay by emission of a photon. The total cross section at the energy was measured to be  pb [4] and the fraction of events to be % [5]. The dominant production mode, , represents of the events, as measured with events [6].

candidates are fully reconstructed from the final-state particles. From the reconstructed four-momentum in the center-of-mass, , two observables are used to extract the signal yield: the energy difference and the beam-constrained mass . The corresponding branching fraction is then computed using the total efficiency (including sub-decay branching fractions) determined with Monte-Carlo (MC) simulations, , and the number of mesons produced via the process, .

1 Study of

The decay is the counterpart if the already-observed decay. The study of baryonic decays is important as the latest observations [7] exhibit a baryon-antibaryon mass peak near the kinematic threshold and tend to have larger branching fractions than two-body decays.

We fully reconstruct the decay via and . After a fit of the two vertices, only candidates for which the () invariant mass lies within 100 (4 ) of the PDG value [5] are retained. The continuum is rejected with requirements on second-to-zeroth Fox-Wolfram moment ratio [8], , and the cosine of thrust angle, .

A two-dimensional binned fit on and leads to a first -significant (including systematic effects) observation of events (Fig. 1). This is the first observation of a baryonic decay. The measured branching fraction,

where the uncertainty due to the branching fraction is quoted separately, is compatible with that of [5].

Figure 1: (left) and (right) distributions of the candidates (histogram) together with the fit result (solid curve). The dotted curve shows its background component.

2 Study of

decays to eigenstates are important for -violation measurements [9]. The mode is especially interesting for the hadron-collider experiments because it can be reconstructed from charged tracks only.

The candidates are formed with oppositely-charged electron or muon pairs, while candidates are formed with pairs. A mass and vertex constrained fit is then applied to the candidates. If more than one candidate per event satisfies all the selection criteria, the one with the value the closest to the expected signal mean is selected. The main background is the continuum, which is reduced by requiring . The signal is fitted using the energy difference, , and the mass, , distributions. Two resonances, and , are included in the fit.

We obtain a 8.4 observation of events and the first evidence for with events [10]. We extract the branching fractions and , which are in agreement with other hadron-collider experiments [11].

3 Measurement of with tagging

Because the mass is above the threshold, a significant number of events are present in the data sample [2]. The sign of the pion indicates whether the event contains a () or a (). With decaying to a eigenstate, the asymmetry, , the CKM angle can be determined via the relation [12]: , where .

From a clean sample of fully reconstructed events, we simultaneously fit the missing masses of the and candidates by adding a charged pion. The fit involves three signal components for the , (+c.c.) and classes of events. A total signal of events is obtained together with the asymmetry (stat.). While this analysis clearly suffers from lack of statistics, it nevertheless demonstrates that can be measured by this alternative method.

Figure 2: (left) and (right) missing mass distributions for selected candidates (data points) together with the fit result (solid curve) and its background component (dashed curve).


We presented new results on decays obtained from 121 of data recorded by the Belle detector. While modes with large statistics can provide precise measurements of branching fractions and properties, first observations of several -eigenstate decays are a confirmation of the large potential of our 120 data sample and advocate an ambitious program at super- factories.


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