Scalar top quark (stop) pair production A rather simple question:
Starting from an electrically neutral state, pairs of top quarks are produced as top and anti-top, and denoted as $t\bar t$. 
Now the production of pairs of scalar top quarks, the supersymmetric partners of the top quarks, seems to be commonly denoted as $\tilde t \tilde t^*$ (e.g. 1, 2), rather than $\tilde t \bar{\tilde t}$. Why this notational difference? What is $\tilde t^*$?
 A: There are two aspects. One is sort of trivial and comprehensible; the other is a bit technical.
The trivial reason is that $\tilde t \bar{\tilde t}$ has two "accents" on top of each other and the symbol therefore occupies too much vertical space which is undesirable because we may get overlapping characters and/or non-uniform spacing between lines. The asterisk in $\tilde t \tilde t^*$ is horizontally shifted so the vertical space is saved. 
However, there also exists a more technical reason. The bar $\bar t$ is not "just" the complex conjugation or a symbol of antiparticles. This bar is a symbol for the Dirac conjugate spinor, $\bar t = t^\dagger \gamma_0$. And indeed, the top quark is described by the Dirac spinor $t$ and $\bar t$ is the most natural form of its complex (plus other operations) conjugation which makes the construction of Lorentz-covariant express more intuitive.
On the other hand, the top squark $\tilde t$ isn't a Dirac spinor; it is a scalar field. It's more usual to denote complex conjugate scalar fields by the asterisk.
