Compositeness and bounds on "preon" mass Since the discovery of Higgs particle, compositeness models are not so popular despite the fact they have interesting features.
Peskin, in the article "Compositeness of quarks and leptons" (pdf), gives arguments to be careful or pessimistic about compositeness. In fact, from the flavor changing neutral current process:
$$\Gamma (\mu \rightarrow e\gamma)= \dfrac{m_\mu^5}{M^4_X}$$
he guesses the bound $M_X>200\,\mathrm{TeV}$, as, he reasons, 
$$BR(\mu \rightarrow e\gamma)<2\cdot 10^{-10}.$$
What is the exact way to get $M_X>200\,\mathrm{TeV}$ from the above two equations? And how to get a general bound from measured branching ratio , a known width AND a theoretical lagrangian (operator) coefficient?
Remark: $BR(A\rightarrow X)=\Gamma(A\rightarrow X)/\Gamma$
 A: From one of the references of the above paper, I get the formula (natural units are used):
$$M_X=E_X\equiv \Lambda=\left(\dfrac{192\pi^3\alpha}{G_F^2BR(\mu\rightarrow e\gamma)}\right)^{1/4}$$
Inserting the values of $\alpha,\pi, G_F$ and the given BR I get the 200 TeV bound (despite the fact a missing 1/3 factor is lost in the Maiani's et al. paper, as I read 64 instead 192, but the bound is similar in any case). The presence of (mass)⁵ also remembers me the theory of proton decay, is the same principle operating somehow? I mean, how can the formula above be extended to quarks FCNC and proton decay? I believe that
$$\Gamma_{GUT}=g^2_{GUT}\dfrac{M_p^5}{M_X^4}$$
Note and additional question: Using PSI/MEG (2011) value, 2.4x10⁻¹² for the BR, we get that the preon scale is about 600TeV, beyond the reach of LHC upgrades and even beyond the reach of the Chinese Big Collider!!!!! We could require a PeV collider in order to reach sensitivities of compositeness but, are PeV cosmic ray better for this purpose?
