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We examine in detail the semi-leptonic decay $\Lambda_b \to \Lambda_c\tau{\bar \nu}_{\tau}$, which may confirm previous hints, from the analogous$B$ decay, of a new physics beyond the standard model. First of all, startingfrom rather general assumptions, we predict the partial width of the decay.Then we analyze the effects of five possible new physics interactions, adoptingin each case five different form factors. In particular, for each term beyondthe standard model, we find some constraints on the strength and phase of thecoupling, which we combine with those found by other authors in analyzing theanalogous semi-leptonic decays of $B$. Our analysis involves some dimensionlessquantities, substantially independent of the form factor, but which, owing tothe constraints, turn out to be strongly sensitive to the kind of non-standardinteraction. We also introduce a criterion thanks to which one can discriminateamong the various new physics terms: the left-handed current and thetwo-higgs-doublet model appear privileged, with a neat preference for theformer interaction. Lastly, we suggest a differential observable that could, inprinciple, help to distinguish between the two cases.

Detailed Study of the Decay Lambda_b --> Lambda_c tau {\bar \nu}_{\tau}