Measurement of the and B− meson lifetimes
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 . 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 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 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 (D0ℓ−) and D★+-lepton (D★+ℓ−) event samples consist mostly of B− and decays, respectively (charge conjugate modes are implied throughout this paper).
Previous measurements of the 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 rφ 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★+ℓ− and D0ℓ− 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★+ℓ− and D0ℓ− samples contain a mixture of and B− decays and the B−/ 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★+ℓ− and 2856 D0ℓ− candidates yields the following results for the and B− lifetimes and their ratio: 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)
- et al.
Gluonic enhancements in non-spectator beauty decays: an inclusive mirage though an exclusive possibility
Phys. Lett. B
(1992)et al.Aspects of heavy quark theory
Annu. Rev. Nucl. Part. Sci.
(1997)et al.Spectator effects in inclusive decays of beauty hadrons
Nucl. Phys. B
(1997) Measurement of the B− and meson lifetimes using semileptonic decays
Phys. Rev. D
(1998)Measurement of B hadron lifetimes using J/ψ final states at CDF
Phys. Rev. D
(1998)A measurement of B+ and B0 lifetimes using events
Z. Phys. C
(1995)Lifetimes of charged and neutral B hadrons using event topology
Z. Phys. C
(1995)A precise measurement of the Bd0 meson lifetime using a new technique
Z. Phys. C
(1997)L3 collaboration, Measurement of the B0d meson lifetime using the decay B0d→D★+Xlν, L3 Note 2142, June 30, 1998; paper...Improved measurement of the B0 and B+ meson lifetimes
Z. Phys. C
(1995)Measurement of the B+ and B0 lifetimes and search for CP(T) violation using reconstructed secondary vertices
Measurement of the B+ and B0 lifetimes using topological reconstruction of inclusive and semileptonic decays
Phys. Rev. Lett.
(1997)Measurement of the B+ and B0 lifetimes using topological vertexing at SLD
Improved measurement of the and B− meson lifetimes
Z. Phys. C
(1996)ALEPH: a detector for electron–positron annihilations at LEP
Nucl. Instrum. Methods A
(1990)The design, construction and performance of the ALEPH silicon vertex detector
Nucl. Instrum. Methods A
(1996)Performance of the ALEPH detector at LEP
Nucl. Instrum. Methods A
(1995)Heavy quark tagging with leptons in the ALEPH detector
Nucl. Instrum. Methods A
(1994)
Cited by (12)
Two-particle correlations in pp, p̄p̄ and K<inf>S</inf><sup>0</sup> K<inf>S</inf><sup>0</sup> pairs from hadronic Z decays
2005, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy PhysicsSearch for pentaquark states in Z decays
2004, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy PhysicsB physics at the Z<sup>0</sup> resonance
2001, Journal of Physics G: Nuclear and Particle PhysicsLifetimes of b-hadrons and mixing of neutral B-mesons: theoretical and experimental status
2024, European Physical Journal: Special TopicsDisintegration of beauty: a precision study
2023, Journal of High Energy Physics