Observation of Exclusive $\gamma \gamma$ Production in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We have observed exclusive $\gamma \gamma$ production in proton-antiproton collisions at $\sqrt{s}=1.96\mathrm{TeV}$, using data from $1.11\pm 0.07{\mathrm{fb}}^{-1}$ integrated luminosity taken by the Run II Collider Detector at Fermilab. We selected events with two electromagnetic showers, each with transverse energy $E_T>2.5\mathrm{GeV}$ and pseudorapidity $|\eta |<1.0$, with no other particles detected in $-7.4<\eta <+7.4$. The two showers have similar $E_T$ and azimuthal angle separation $\Delta \varphi \sim \pi$; 34 events have two charged particle tracks, consistent with the QED process $p\overline{p}\to p+e^{+}{e}^{-}+\overline{p}$ by two-photon exchange, while 43 events have no charged tracks. The number of these events that are exclusive ${\pi }^0{\pi }^0$ is consistent with zero and is $<15$ at 95% C.L. The cross section for $p\overline{p}\to p+\gamma \gamma +\overline{p}$ with $|\eta (\gamma )|<1.0$ and $E_T(\gamma )>2.5\mathrm{GeV}$ is ${2.48}_{-0.35}^{+0.40}(\mathrm{stat}{)}_{-0.51}^{+0.40}(\mathrm{syst})\mathrm{pb}$.

Figure 1

Leading order diagrams for central exclusive production in $p(\overline{p})\mathrm{\text{−}}p$ collisions: (a) exclusive $\gamma \gamma$ production in $\overline{p}\mathrm{\text{−}}p$ collisions; (b) exclusive Higgs boson production in $p\mathrm{\text{−}}p$ collisions. Note the screening gluon that cancels the color flow from the interacting gluons.

Figure 2

The $e^{+}{e}^{-}$ candidates: invariant mass distribution (a). The two-photon candidates: invariant mass distribution (b), $|\pi -\Delta \varphi |$ distribution (c), and $p_T$ distribution of the two photons (d). All error bars are statistical. The MC predictions for $\gamma \gamma$ are normalized to data. The QED prediction for $e^{+}{e}^{-}$ is normalized to the delivered luminosity and efficiencies. The MC samples for the QED process were generated with the LPAIR program [25].

Figure 3

Estimate of ${\pi }^0{\pi }^0$ background fraction in the candidate sample. Distribution of reconstructed CES showers per event for data compared to $\gamma \gamma$ and ${\pi }^0{\pi }^0$ Monte Carlo simulations (a). Background fraction estimate using Pearson’s ${\chi }^2$ test to fit the composition hypothesis to the data distribution (b).

Figure 4

Comparison of the measured cross section for the exclusive $\gamma \gamma$ production in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ with theoretical predictions [11].