\Upsilon
The Greek letter Upsilon (\Upsilon) is used in mathematics and science to represent various concepts, such as a type of particle in physics or a variable in equations.
Decay of an Upsilon particle into an electron-positron pair
\Upsilon \to e^+ e^-
Summation notation using Upsilon as the variable
\Upsilon = \sum_{i=1}^{n} x_i
Second partial derivative in tensor notation
\Upsilon_{ij} = \frac{\partial^2 \phi}{\partial x_i \partial x_j}