[Tex/LaTex] Helvetica-Light in Beamer disrupts the math fonts

beamerfontsxetex

I am trying to make the font of a Beamer presentation match that of a co-author who is using Powerpoint. The Powerpoint font is Helvetica Light.

In XeLatex I used \usepackage{fontspec} and \setmainfont{...},
as well as a \usefonttheme{professionalfonts}.

This seems to work. EXCEPT THAT, the math fonts are now broken:
anything that was in \mathbf{} appears to be transformed to HelveticaLight also.
Whereas math that is not \mathbf looks "normal" (I think serif, CM font). Inconsistent and ugly!

How can I cause the \mathbf{} symbols to look as they would if I had not
requested the helvetica font?

Here is an example:

\documentclass{beamer}
\usepackage[scaled]{helvet}     % does not seem to work
\usetheme{Antibes}
\usecolortheme{dove}
\usefonttheme{professionalfonts}
\usepackage{fontspec}
\setmainfont[Ligatures=TeX]{HelveticaLight.ttf}
\usepackage{beamerprosper}

\begin{document}

\begin{slide} { Bad Fonts Mixture }
Here is the problem:
\[
   \sum_k (d_k - \mathbf{b}^T \mathbf{x})^2
\]
The $\mathbf{b}^T \mathbf{x}$ is in helvetica light and looks bad next to the $d_k$.
\end{slide}

\end{document}

Best Answer

With LuaLaTeX (or may be XeLaTeX in case you use that) comment \usepackage[scaled]{helvet} and use \setsansfont instead of \setmainfont; that works for me. However, if you want to ensure that math is not touched by fontspec you should load it with \usepackage[no-math]{fontspec}.

And, in case you use pdfLaTeX, don't use fontspec (nor \setmainfont) and it should work (although not light).

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