Threshold corrections is a term that appears when you discuss effective field theories (EFTs). An EFT is an approximation of a full theory which is valid at low energies, ie below some threshold.

Let $A_{\mbox{eff}}$ be any amplitude as calculated in the EFT and $A_{\mbox{full}}$ the amplitude for the same process calculated in the full theory.

The threshold correction is defined as
$$A_{\mbox{full}}-A_{\mbox{eff}}$$
and it describes all the information that has been "integrated out" in the EFT.

It needs to be calculated/approximated somehow when the accuracy of the result of the EFT is not sufficient.

The threshold (and the region of validity) of the EFT are not fixed a priori. There are two different ways to think about this:

i)We can either set the threshold/scale at a value we desire and then keep as many terms in the expansion as required (truncating the rest), so that the theory is valid to the desired accuracy below that scale.

or

ii) Decide to keep a certain number of terms and then calculate the threshold to which this calculation gives acceptably accurate results.

If you are thinking more along the lines of i), then it is often the case that the threshold you have set initially for yourself is no longer good enough. For example you might have upgraded your accelerator to higher energies. If that's the case, then you will have to include corrections (extra terms) with regards to the old threshold, that will make your theory valid up to the new desired threshold. An example would be a new particle whose rest mass is accessible to the upgraded accelerator but not the old one. You would then have to include some further operators in your lagrangian to take the new particle into account.

Hope this helps!

This post imported from StackExchange Physics at 2014-04-21 16:24 (UCT), posted by SE-user Heterotic