(As pointed out by responders the speed of a neutral pion in a lab setting depends on the nature of the reaction which produced it. So the remaining question is "how is the speed determined?".)
Alvager et al 1964 reported evidence against Ritz's emitter theory in an experiment that generated neutral pions ($\pi_0$'s) with a velocity of $v \approx 0.99.c$. How is the velocity of a neutral pion determined? Does the determination invoke special relativity?
Alvager et al (http://mysite.verizon.net/cephalobus_alienus/papers/Alvager_et_al_1964.pdf) directed a pulsing beam of high energy ($19.2 GeV$) protons at a beryllium crystal and measured arrival times of resulting emitted $\gamma$ ray photons (energy $>= 6GeV$) at downstream detectors. The photons were emitted from the decay of $\pi_0$'s produced from collisions of the protons with the beryllium nuclei. The $\pi_0$'s had speeds $v = \beta.c$. They state that $1/(1-\beta^2) >45$ and so $\beta >= 0.989$. Later they use the value $\beta=0.99975$.
Time intervals between the peaks of successive photon bursts (interval ~105 ms) detected at two separate detectors were interpreted as indicating that, if the photons were travelling at some speed other than $c$ and given by $v=c+k.\beta.c$, then the value of $k$ must be very small (-3+/- 13 * 10e-5). (I think that this formula for altered photon velocity harks back to Fizeau experiments investigating the effect of moving water on the velocity of light. Applying simple galilean relativity in the Alvager et al experiments would predict a range of photon velocities $v$ between $c -\beta.c$ and $c + \beta.c$).
In a contemporary paper Velocity of Gamma Rays from a Moving Source T. A. Filippas and J. G. Fox, Phys. Rev. 135, B1071, 1964 ) used $\pi_0$'s with velocity $0.2c$ which decayed to produce 68 MeV $\gamma$ ray photons. They declare that "independently of relativity, and indeed of nuclear theory, there can be no reasonable doubt about the velocity of the ($\pi_0$) source(s)" based on reported aberration angle and Doppler energy shift for $\pi_0$'s produced by the same reaction ($\pi- + p \rightarrow \pi_0$).
I pose the question because it seems that Alvager et al were trying to confirm special relativity (SR) by assuming pion velocities which were themselves determined assuming SR.
Regarding the interpretation of the Alvager experiment the following note is interesting: (http://worldnpa.org/pipermail/memberschat_worldnpa.org/attachments/20090115/db6f6bc5/attachment.pdf)(since deleted).
The answer by AnnaV is very helpful. I made a follow-up question in which I was led to obtain a formula for $v$ myself:- $v=\sqrt(2KE/m_0)$. The community declared this solution correct but then deemed the question a pointless homework question and deleted/hid it. As it happens the formula does not seem to be correct when I try to apply the Filippas & Fox 1964 data ($\pi_0$ rest mass $m_0=135MeV/c^2$, $KE=68 MeV$) it gives $v_{calc} = 1.004c$ instead of the authors' value $v=0.2c$. Applying the Alvager et al data to it ($KE= 6000 MeV$) it gives $v_{calc}=9.428c$. So I am still in the dark on the Special Relativity approach. (The aberration method mentioned by Fox & Filippas 1964 is relatively simple to understand for me.)
