The main problem is that XeTeX is not applying math italic correction:
- XITS has italic correction and needs it for proper spacing.
- In case of Latin Modern the font does bot provide italic correction at all.
- Asana math lacks italic correction as well, but the glyphs are spaced in a way that alleviates the need of italic correction.
- Euler, though have italic correction, its upright design makes lack of italic correction not very visible (though your example is wrong, you have to pass
math-style=upright
option for Euler alphabets to be used).
Why XeTeX stopped applying italic correction is unclear to me, that part of the code have not been touched for ages, but the code was flawed anyway so my guess is that it worked accidentally and some of the recent math cleanup broke.
However, there is a workaround: open the XITS font in a font editor (preferably FontForge) and set the width of space glyph to 0, this will cause the engine to apply italic correction again.
For the curious, the application of italic correction or not depends on whether the space factor of the font is zero, even in the OpenType math branch of the code, and though this is true for TFM math fonts, it is not essentially the case for OpenType font.
Also, OpenType math spec diverges from TeX algorithm on when italic correction should be applied, but it is very vague, and MS implementation seems to differ from what is actually documented, so it is not very well supported by XeTeX and LuaTeX yet.
Update: XeTeX's master branch handles this better now, until a more robust handling of italic correction is devised.
Best Answer
Seeing your template, in my humble opinion, it seems to use the
fourier
package.Here you can find several fonts with LaTeX math support: https://tug.dk/FontCatalogue/mathfonts.html
Addendum
This is only a bad copy for your template :-) You can change the style of the chapter with Sonny, Lenny, etc.: Fncychap: titoli fantasiosi di capitoli in classe book LaTeX