Let $k$ be a field.


$G/k$ be a simply connected semisimple algebraic group. 


Let $X/k$ be a smooth affine $k$-scheme. 


**Question**: Is every principal $G$ bundle on $X\times {\mathbb A}^1$ a pull back from $X$?



Lindel's theorem for semisimple simply connected G
In full generality, the answer is no. There are examples of Parimala of non-extended torsors for special orthogonal groups over $\mathbb{A}^2\_{\mathbb{R}}$, see e.g. Amer J. Math. 100 (1978), 913-924, (admittedly the group is not simply-connected in this case). These examples are also discussed in Lam's book "Serre's problem on projective modules". Further examples of a dramatic failure of Serre's problem for anisotropic groups were given by Raghunathan, see Principal bundles on affine space and bundles on the projective line, Math. Ann. 285 (1989), 309-332. 


The answer is yes, if you restrict to $k$ being an infinite field, $G$ being an isotropic group (i.e. $G$ should contain a split torus of rank $\geq 1$) and consider only rationally trivial torsors. This can be found as Theorem 3.1 in my paper: Rationally trivial torsors in $\mathbb{A}^1$-homotopy theory, J. K-Theory 7 (2011), 541-572 (essentially all the relevant ingredients were in the literature before, but quite scattered; the result is a combination of a general local-global principle for torsors, the results of Raghunathan for torsors on affine spaces, and the extension of Lindel's method by Colliot-Thélène and Ojanguren)


