Inclusive Measurements of |V(ub)| From BaBar
The Cabibbo-Kobayashi-Maskawa (CKM) matrix element V{sub ub} is a fundamental parameter of the Standard Model, representing the coupling of the b quark to the u quark. It is one of the smallest and least known elements of the CKM matrix. With the increasingly precise measurements of decay-time-dependent CP asymmetries in B-meson decays, in particular the angle {beta} [1, 2], improved measurements of the magnitude of V{sub ub} will allow for stringent experimental tests of the Standard Model mechanism for CP violation [3]. The extraction of |V{sub ub}| is a challenge, both theoretically and experimentally. Theoretically, the weak decay rate for b {yields} uev can be calculated at the parton level. It is proportional to |V{sub ub}|{sup 2} and m{sub b}{sup 5}, where m{sub b} is the b-quark mass. To relate the B-meson decay rate to |V{sub ub}|, the parton-level calculations have to be corrected for perturbative and non-perturbative QCD effects. These corrections can be calculated using various techniques: heavy quark expansions (HQE) [4] and QCD factorization [5]. They make use of specific assumptions and are affected by different uncertainties. It is therefore important to make redundant measurements by using several experimental techniques, and different theoretical frameworks. Experimentally, the principal challenge is to separate the signal B {yields} X{sub u}ev decays from the 50 times larger B {yields} X{sub c}ev background. This can be achieved by selecting regions of phase space in which this background is highly suppressed. In addition, exploiting the available kinematic variables which discriminate between rare charmless semileptonic decays and the much more abundant decays involving charmed mesons, gives different sensitivities to the underlying theoretical calculations and assumptions. In inclusive measurements, the most common kinematic variables discussed in the literature, each having its own advantages, are the lepton energy (E{sub {ell}}), the hadronic invariant mass (M{sub X}), and the leptonic invariant mass squared (q{sup 2}). In this letter we present three different determinations of |V{sub ub}|, based on the analysis of semileptonic B decays collected with the BABAR detector at the PEP-II e{sup +}e{sup -} storage ring. Charmless semileptonic B decays are selected: (a) in the electron momentum interval of 2.0-2.6 GeV/c [6]; (b) using measurements of the electron energy and the invariant mass squared of the electron-neutrino pair [7]; (c) in kinematic regions where the dominant background from semileptonic B decays to charm is reduced by requirements on the hadronic mass M{sub X} and the momentum transfer q{sup 2} (M{sub X} < 1.7 GeV/c{sup 2} and q{sup 2} > 8 GeV{sup 2}/c{sup 4}) [8].
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 881549
- Report Number(s):
- SLAC-PUB-11822; TRN: US0603170
- Resource Relation:
- Conference: Prepared for EPS International Europhysics Conference on High Energy Physics (HEP-EPS 2005), Lisbon, Portugal, 21-27 Jul 2005
- Country of Publication:
- United States
- Language:
- English
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