**Thesis Defense**

**Siwei Qiu**

**August 21, 2013**

2:30pm (206 JFB)

**Title: **** "Semileptonic B->D Decay at Nonzero Recoil with 2+1 Flavors of Improved Staggered Quarks "**

**Abstract:**

The purpose of this thesis is to carry out a precise test of the standard model of physics, i.e. the model that describes the strong and electroweak interactions between quarks, leptons and gauge bosons. Such a test may well lead to the discovery of new physics beyond the standard model. More specifically this thesis gives a precise determination of |V_cb |, one of the parameters of the standard model which controls the semileptonic decay process B® Dln. This parameter is one of several that make up the CKM matrix. If the standard model is correct that matrix must be unitary. A deviation from unitary would be a sign of new physics. Although this is mainly an electroweak process, it occurs in the environment of the strong interactions. The strong interaction effects are encoded in the form factor G for the vector current transition from the B meson to the D meson. In this thesis, we calculate the form factor. Experimental measurement provides the product G|V_cb |. So theory and experiment together give |V_cb |. We calculate the form factor in a numerical simulation, using lattice quantum chromodynamics. Ours is the first numerical calculation of the full form factor (nonzero recoil) that takes into account all the effects of the strong interactions (including sea quarks) and carries out an extrapolation to physical quark masses in the limit of zero lattice spacing. We give the theoretical background of the calculation, describe our data analysis, give a detailed analysis of all sources of error, and fit our result and the experimental data from the Babar collaboration to get our final result, namely, |V_cb |=0.0402(20).