Sigma coupling to photons:: Hidden scalar in γγ→π0π0

There has long been speculation about the nature of the sigma resonance. For three decades Jaffe has argued for a tetraquark composition, while others have claimed it is largely a glueball. A key pointer to its nature is its coupling to two photons. Consequently, there have been recent proposals to observe this important scalar hiding in gamma gamma ->pi(0)pi(0). We show here that the sigma is already crouching in this cross section exactly as measured 20 years ago. What is new is that precise knowledge of the position of the sigma pole, provided by the analysis of the Roy equations, now allows its two photon coupling to be accurately fixed. Its two photon width is found to be (4.1 +/- 0.3) keV, a value far too large for a wholly gluonic, or even a tetraquark, state.