Past exam of the particle physics course of the University of Oxford / 2015 / 3 / c

TODO: the numerical resonance-area reading is unavailable from the retained source. Page 6 retains the upper data panels but loses the lower part of the scanned data image, including the numerical horizontal energy calibration. A vertical cross section scale and an apparent peak cannot supply $\int \sigma dE$ without that calibration. The decoded image contains 2,834,898 of 4,437,216 required RGB bytes; the archived bundle has the same truncation. No horizontal scale or natural decay width is inferred from the display width.

The recoverable numerical conversion is nevertheless definite. With $M\simeq 3.1\mathrm{GeV}$, $B_{ee}=0.060$, $ϵ=0.50$, and $1{\mathrm{GeV}}^{-2}=389400\mathrm{nb}$, b givesThis coefficient assumes the plotted exclusive electronic sample has not already been corrected for acceptance. A restored area, with continuum subtraction and acceptance convention checked, would complete the estimate. A tabulated modern J/psi meson width would be external information rather than a reading of these data.

The narrow scattering resonance indicates suppressed strong interaction decay. In the quark model, $c\bar{c}$ is below the open-charm quark $D\bar{D}$ threshold, so its constituents cannot simply emerge in separate charmed mesons. The OZI rule suppresses annihilation into light hadrons; for a $1^{--}$ color-singlet state the leading purely gluonic channel needs three gluons. The observed leptonic channels remain competitive. Its mass and $I=0$ fit charmonium; the small width alone would not uniquely identify the new quark flavour without the wider spectrum and threshold evidence.