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We present the first results from a recently concluded study of GRBs at$z\gtrsim5$ with the Karl G. Jansky Very Large Array (VLA). Spanning $1$ to$85.5$ GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A arethe most detailed joint radio and millimeter observations of a GRB afterglow at$z\gtrsim5$ to date. In conjunction with optical/near-IR and X-ray data, theobservations can be understood in the framework of radiation from a singleblast wave shock with energy $E_{\rm K,iso}\approx8.5\times10^{53}$ ergexpanding into a constant density environment with density, $n_0\approx8\,{\rmcm}^{-3}$. The X-ray and radio observations require a jet break at $t_{\rmjet}\approx0.6$ d, yielding an opening angle of $\theta_{\rmjet}\approx4^{\circ}$ and a beaming-corrected blast wave kinetic energy of$E_{\rm K}\approx2.2\times10^{50}$ erg. The results from our radio follow-upand multi-wavelength modeling lend credence to the hypothesis that detectedhigh-redshift GRBs may be more tightly beamed than events at lower redshift. Wedo not find compelling evidence for reverse shock emission, which may berelated to fast cooling driven by the moderately high circumburst density.

A VLA Study of High-redshift GRBs I - Multi-wavelength Observations and Modeling of GRB 140311A