# Four-divergence and Legendre transformation

As a study case, consider the following Lagrangian for a left-handed Weyl field $\chi \in \mathbb{C}^{2}$:

$$\mathcal{L} = \chi^{\dagger} \mathrm{i} \overline{\sigma}^{\rho} \partial_{\rho} \chi$$

where $\overline{\sigma}^{\rho} \equiv (1_{2},-\sigma^{i})$, with $\sigma^{i}$ the standard Pauli matrices. The corresponding Hamiltonian (density) is obtained, as customarily, by a Legendre transformation [with $a$ running, of course, over the two components of $\chi$]:

$$\mathcal{H} = \mathcal{L}\frac{\overleftarrow{\partial }}{\partial \dot{\chi^{a}}}\dot{\chi^{a}}-\mathcal{L} = \mathrm{i} \chi_{a} ^{\dagger }\dot{\chi}^{a}-\mathcal{L},$$

correctly yielding a Hamiltonian containing no time-derivatives: $\mathcal{H} = -\chi^{\dagger} \mathrm{i} \overline{\sigma}^{i} \partial_{i} \chi.$ Now, consider the above Lagrangian augmented by the four-divergence $-\frac{\mathrm{i}}{2}\partial _{\rho }\left( \chi ^{\dagger } \overline{\sigma }^{\rho }\chi \right)$ to bring it on the following explicitly hermitian form:

$$\mathcal{L}'=\frac{\mathrm{i}}{2}\left[ \chi ^{\dagger }\overline{\sigma } ^{\rho }\partial _{\rho }\chi -\left( \partial _{\rho }\chi \right) ^{\dagger }\overline{\sigma }^{\rho }\chi \right];$$

and consider the following ansatz for the associated Hamiltonian:

$$\mathcal{H}' = \mathcal{L}'\frac{\overleftarrow{\partial }}{\partial \dot{\chi }^{a}}\dot{\chi}^{a}+\mathcal{L}'\frac{\overleftarrow{\partial }}{\partial \left( \dot{\chi}^{a}\right) ^{\ast }}\left( \dot{\chi}^{a}\right) ^{\ast }- \mathcal{L}' = \frac{\mathrm{i}}{2}\chi ^{\dagger }\dot{\chi}^{a}-\frac{\mathrm{i}}{2} \chi _{a}\left( \dot{\chi}^{a}\right) ^{\ast }-\mathcal{L}'.$$

Note that the minus sign of the second term comes from passing the derivative through from the right, using anticommutativity. Unlike the former case, here $\mathcal{H}'$ will contain no time-derivatives only if for the second term the relation $\frac{\mathrm{i}}{2} \chi _{a}\left( \dot{\chi}^{a}\right) ^{\ast } = -\frac{\mathrm{i}}{2} \left( \dot{\chi}^{a}\right) ^{\ast } \chi _{a}$ is applied.

At the classical level where the components of $\chi$ are Grassmann-valued, $\chi$ being a spinor field, such a relation is of course fully valid, but I would be more comfortable if the time-derivatives would readily drop out. Is that possible? And if affirmative, then how should the above ansatz for the Legrendre transformation be modified?

• The case of Dirac spinors is sketched in 198054/84967. It might be useful to give it a read. Mar 11, 2017 at 9:39
• One should stress that the Hamiltonian is not guessed: there is no need to postulate an ansatz. There is a perfectly well-defined method to find the Hamiltonian of constrained systems (irrespective of whether the variables are even or odd). Google "Dirac-Bergmann algorithm". Mar 11, 2017 at 9:58
• Happy to hear that. My use of "ansatz" above was also meant in the rhetoric sense, as it was certainly my hope, and in fact expectation, that there exists some "well-defined method to find the Hamiltonian", as you put it. I have very little experience on these matters of quantization, and certainly so when dealing with constrained systems. Quite frustratingly, I generally feel lost, having never been properly trained on these matters during my education. There are all these different concepts as well as methods floating around, and I often feel I need some simple to follow 'recipes'. Mar 11, 2017 at 13:19

1. The main point is that since e.g. the Lagrangian density ${\cal L}$ should be real, the complex Grassmann-odd Weyl spinors $\chi$ and $\chi^{\dagger}$ are not independent variables. See also this Phys.SE post.
2. This leads to constraints. When the singular Legendre transformation is performed correctly, the Hamiltonian density ${\cal H}$ does not depend on velocity variables. See also this, this this, this and this related Phys.SE posts.