Why can the Euler beta function be interpreted as a scattering amplitude? The Wikipedia article on the Veneziano Amplitude claims that the Euler beta function can be interpretted as a scattering amplitude. Why is this?
In another word, when the  Euler beta function is interpreted as a scattering amplitude, what features does it have that make it able to explain strong force of mesons?
What properties of (or why) the Euler beta function (when interpreted as scattering amplitude) has string behavior?
 A: A function can be interpretable as a scattering amplitude if that function satisfies the axioms of relativistic S-matrix theory [1]:


*

*Lorentz invariance

*Unitarity (Not realized by the beta function, but may be dropped if the function is interpreted as a Born approximation to the exact amplitude)

*T, C, P invariance (only for strong nucl. interactions)

*Analyticity: singularities in invariant energy complex plane correspond to particle poles or thresholds, in a way that doesn't violate causality.

*Crossing symmetry.

*Power boundedness

*Well-behaved particle poles (stable particle masses are positive, and their residues should be negative)

*Analyticity of the second kind: analytic in the complex angular momentum plane.


Then the function should also match some of the empirical facts gathered by experimentalists.  When the beta function was proposed the following list would be conjured:


*

*All poles in the complex angular momentum plane move to the right linearly with increasing energy at a universal rate.

*Diffractive scattering (not realized by the Euler beta function, but Virasoro's amplitude does realize this)

*Inclusive high energy reaction exhibit scaling (also not realized by Euler beta)

*Particle spectrum (determined by the poles) in agreement with the quark model (not realized either, but Chan and Paton's method get close)

*Reproduces data from weak and electromagnetic processes (not realized.)


References:
John Schwarz "Dual Resonance Theory" Phys Rep 8, no. 4, (1973) 269-335
A: I certainly don't have a complete answer, but what I do know is the following. The Veneziano amplitude is the following:
$$
\mathcal{A}^{(4)} \propto \lambda\left(B(-\kappa s -1, -\kappa t -1) + B(-\kappa s -1, -\kappa u -1) + B(-\kappa t -1, -\kappa u -1)\right),
$$
where $s, t, u$ are the Mandelstem variables and $B(x,y)$ the Euler-Beta function. Note that the above formula is symmetric under exchange of $s,t,u$. I think I read somewhere that physicists expected the strong interaction to be symmetric under such momentum exchanges, but I do not know precisely why. Note that the above formula is what one obtains in string theory for the scattering of open string tachyons, with $\lambda = g_c$ (closed string coupling constant) and $\kappa = \alpha'$.
