# Epsilon Tensor in FeynCalc

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.

• Would Mathematica be a better home for this question? – Qmechanic Apr 12 '14 at 18:59
• I believe its just defined that way. The momentum as an index is defined to be an index contraction of a momentum with the Levi-Civita. – JeffDror Apr 12 '14 at 21:29
• I agree with @JeffDror. This is just a weird notation. – Melquíades Apr 29 '14 at 20:57