Dresden 2013 – scientific programme
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T: Fachverband Teilchenphysik
T 19: Flavourphysik (Theorie) 2
T 19.8: Talk
Monday, March 4, 2013, 18:35–18:50, WIL-B122
Form factor for semi-leptonic B→πℓν decays — •Felix Bahr1, Fabio Bernardoni1, John Bulava2, Rainer Sommer1, Hubert Simma1, and Alberto Ramos1 — 1NIC, DESY Zeuthen — 2CERN, Geneva
The CKM-Matrix element Vub is currently insufficiently well known; there exist differences between its value obtained from different methods of determination, inclusive B→ Xuℓ ν, exclusive B→ πℓν and exclusive B→ τ ν decays. To investigate these, as well as to match improved experimental data, high precision lattice QCD calculations are required, keeping systematic errors under control. To this end, we simulate semi-leptonic B→π ℓ ν decays of pseudoscalar B mesons made up of a b and a light u or d quark. The hadronic matrix element
| ⟨ ⟨ | π(pπ) | ⎪ ⎪ | ū γµb | ⎪ ⎪ | B(pπ+q) | ⟩ ⟩ | = | ⎛ ⎝ | pB + pπ | ⎞ ⎠ | f+(q2) + | ⎛ ⎝ | pB − pπ | ⎞ ⎠ | f−(q2), (1) |
q2=(pB −pπ)2, can be parametrised by the form factors f+, f−. We work on an L3× 2L hypercubic lattice with finite lattice spacing a and Nf=2 seaquark flavours. To keep finite size effects under control, we require pion masses to fulfill Lmπ 4, which for lattice spacings of a≈0.05 … 0.08 fm and sizes L=32,48,64 corresponds to mπ≈190…450 MeV. Having simulated for various a and mπ, we will eventually extrapolate to the physical point, a→ 0, mπ→ mπphys. The b quark is treated in Heavy Quark Effective Theory, where in leading order the b quark is static and one then performs an expansion in 1/mb. We use a model independent parametrisation for the q2 dependence of f+(q2) to perform a combined fit of experimental and theory data to obtain Vub.