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Limits based on Mandelstam Variables
Since the Mandelstam variables are the scalar product of 4-momenta, which are invariants, they are invariants as well. The sum of these invariant variables must also be invariant as well. Find the sum of the 3 Mandelstam variables when the two particles have equal mass in the center of mass frame gives:
[math]s+t+u=(4(m^2+ p \ ^{*2}))+(-2 p \ ^{*2}(1-cos\ \theta))+(-2 p \ ^{*2}(1+cos\ \theta))[/math]
[math]s+t+u \equiv 4m^2[/math]
Since
[math]s \equiv 4(m^2+\vec p \ ^{*2})[/math]
This implies
[math]s \ge 4m^2[/math]
In turn, this implies
[math] t \le 0 \qquad u \le 0[/math]
At the condition both t and u are equal to zero, we find
[math] t = 0 \qquad u = 0[/math]
[math]-2 p \ ^{*2}(1-cos\ \theta) = 0 \qquad -2 p \ ^{*2}(1+cos\ \theta) = 0[/math]
[math](-2 p \ ^{*2}+2 p \ ^{*2}cos\ \theta) = 0 \qquad (-2 p \ ^{*2}-2 p \ ^{*2}cos\ \theta) = 0[/math]
[math]2 p \ ^{*2}cos\ \theta = 2 p \ ^{*2} \qquad -2 p \ ^{*2}cos\ \theta = 2 p \ ^{*2}[/math]
[math]\cos\ \theta = 1 \qquad \cos\ \theta = -1[/math]
[math]\Rightarrow \theta_{t=0} = \arccos \ 1=0^{\circ} \qquad \theta_{u=0} = \arccos \ -1=180^{\circ}[/math]
Holding u constant at zero we can find the maximum of t
[math]s+t \equiv 4m^2[/math]
[math]\Rightarrow t=4m^2-s[/math]
[math]t=4m^2-4m^2+ 4p \ ^{*2})[/math]
[math]t=4p \ ^{*2}[/math]
[math]-2 p \ ^{*2}(1-cos\ \theta)=4p \ ^{*2}[/math]
[math](1-cos\ \theta)=-2[/math]
[math]-cos\ \theta=-3[/math]
[math] \theta_{max} \equiv \arccos 3[/math]
The domain of the arccos function is from −1 to +1 inclusive and the range is from 0 to π radians inclusive (or from 0° to 180°). This implies for arccos 3, the range will include imaginary numbers. We can approximate this value using a Taylor series expansion
[math]\arccos{x} \approx \frac{π}{2} - x - \frac{x^3}{6 - \frac{3 x^5}{40} [/math]