A few days ago I started to use the Mathematica package FeynCalc and one thing confuses me: Assume we have a four-vector $p_\mu$ and we contract it with the epsilon tensor. FeynCalc produces $\varepsilon^{\mu\nu\rho\sigma} p_\sigma = \varepsilon^{\mu\nu\rho p}$...what does the momentum as an index mean?

Furthermore, the FeynCalc documentation says "Eps[a, b, c, d] is the head of the totally antisymmetric $\varepsilon$ (Levi-Civita) tensor. The a,b,... may have head LorentzIndex, Momentum or Integer." What is an epsilon tensor with a momentum index? I never saw this before.

This post imported from StackExchange Physics at 2014-04-13 11:29 (UCT), posted by SE-user P.I.