Measurement of the B0 and B meson lifetimes

https://doi.org/10.1016/S0370-2693(00)01093-5Get rights and content

Abstract

The lifetimes of the B0 and B mesons are measured using a sample of about four million hadronic Z decays collected from 1991 to 1995 with the Aleph detector at LEP. The data sample has been recently reprocessed, achieving a substantial improvement in the tracking performance. Semileptonic decays of B0 and B mesons are partially reconstructed by identifying events containing a lepton with an associated D★+ or D0 meson. The proper time of the B meson is estimated from the measured decay length and the momentum of the D-lepton system. A fit to the proper time of 1880 D★+ and 2856 D0 candidates yields the following results: τB0=1.518±0.053±0.034 ps, τB=1.648±0.049±0.035 ps, τBB0=1.085±0.059±0.018.

Introduction

Measurements of the individual b-hadron lifetimes represent an important test of the present knowledge of nonspectator effects in the b-hadron decay dynamics, such as Pauli interference, W exchange and weak annihilation. Based on the heavy-quark expansion formalism, the difference between the lifetimes of the b baryons and mesons is predicted to depend on 1/mb2 and higher-order terms, whereas meson–meson differences depend only on 1/mb3 and higher-order terms [1]. The predicted hierarchy is τΛbB0∼τBsB. Differences are expected to be at the level of a few percent, which sets the scale of the experimental precision required.

This paper reports an improved measurement of the B0 and B lifetimes with the Aleph detector at LEP, using approximately four million hadronic decays of the Z, collected in the period 1991–1995. This data sample was recently reprocessed, achieving higher efficiency and better resolution in the track reconstruction, which is highly beneficial for this analysis.

Semileptonic decays of B0 and B mesons are partially reconstructed by identifying events containing a lepton (electron or muon) with an associated D0 or D★+ meson. The resulting D0-lepton (D0) and D★+-lepton (D★+) event samples consist mostly of B and B0 decays, respectively (charge conjugate modes are implied throughout this paper).

Previous measurements of the B0 and B lifetimes are reported in [2], [3].

Section snippets

The ALEPH detector

A detailed description of the Aleph detector can be found elsewhere [4], [5]. A high-resolution vertex detector (VDET) consisting of two layers of silicon with double-sided readout provides measurements in the and z directions at average radii of 6.5 cm and 11.3 cm, with 12 μm precision at normal incidence. The VDET provides full azimuthal coverage, and polar angle coverage to |cosθ|<0.85 for the inner layer only and |cosθ|<0.69 for both layers. Outside the VDET, particles traverse the inner

Event selection

The D★+ and D0 event samples consist of an identified lepton (e or μ) associated with a fully reconstructed D★+ or D0 candidate. The selection of muons and electrons is described in detail in [7]. For this analysis, lepton candidates are required to have a momentum of at least 2.0 GeV/c for electrons and 2.5 GeV/c for muons.

The D★+ and D0 candidates are reconstructed from charged tracks and π0's that form an angle of less than 45° with the lepton candidate. These charged tracks are also

Lifetime measurement

An unbinned likelihood fit for the lifetimes is performed. For each event the probability of observing a proper time t given the lifetime τ is calculated: the probability density function F(t,σt,τ) is obtained by convoluting an exponential distribution with the properly normalised κ distribution and with a Gaussian function which takes into account the resolution on the decay length.

Both the D★+ and D0 samples contain a mixture of B0 and B decays and the B/B0 mixture in the samples

Conclusions

The lifetimes of the charged and neutral B mesons have been measured with the full statistics collected by the Aleph detector at and around the Z peak energy. The data sample was recently reprocessed, achieving improved tracking performance. A maximum likelihood fit to the proper time distributions of 1880 D★+ and 2856 D0 candidates yields the following results for the B0 and B lifetimes and their ratio: τB0=1.518±0.053±0.034 ps,τB=1.648±0.049±0.035 ps,τBB0=1.085±0.059±0.018, where the

Acknowledgements

We thank our colleagues in the accelerator divisions for the excellent performance of LEP. Thanks also to the many engineering and technical personnel at CERN and at the home institutes for their contributions to the performance of the Aleph detector. Those of us from non-member states thank CERN for its hospitality.

References (14)

There are more references available in the full text version of this article.

Cited by (12)

View all citing articles on Scopus
7

Supported by CICYT, Spain.

15

Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain.

View full text