# 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 $\tilde t^*$?

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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.
The asterisk still isn't typographically ideal, because it runs into the other convention of using an asterisk to denote off-shell particles. To try to minimize confusion, I was using ${\tilde t}^\dagger$ in the draft of one paper about stops but my coauthors didn't like it so we reverted to asterisk in the end.... – Matt Reece Apr 24 '13 at 13:29