Observation of Exclusive Gamma Gamma Production in p pbar Collisions at sqrt{s}=1.96 TeV

TL;DR

The authors have observed exclusive γγ production in proton-antiproton collisions at √s=1.96 TeV, using data from 1.11±0.07 fb(-1) integrated luminosity taken by the Run II Collider Detector at Fermilab.

Abstract

We have observed exclusive γγproduction in proton-antiproton collisions at \sqrt{s}=1.96 TeV, using data from 1.11 \pm 0.07 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 GeV and pseudorapidity |η| < 1.0, with no other particles detected in -7.4 < η< +7.4. The two showers have similar E_T and azimuthal angle separation Δφ\sim π; 34 events have two charged particle tracks, consistent with the QED process p \bar{p} to p + e^+e^- + \bar{p} by two-photon exchange, while 43 events have no charged tracks. The number of these events that are exclusive π^0π^0 is consistent with zero and is < 15 at 95% C.L. The cross section for p\bar{p} to p+γγ+\bar{p} with |η(γ)| < 1.0 and E_T(γ) > 2.5$ GeV is 2.48^{+0.40}_{-0.35}(stat)^{+0.40}_{-0.51}(syst) pb.

Figures (4)

• Figure 1: Leading order diagrams for central exclusive production in $p(\bar{p})-p$ collisions: a) exclusive $\gamma\gamma$ production in $\bar{p}-p$ collisions; b) exclusive Higgs boson production in $p-p$ 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), $|\pi-\Delta\phi|$ 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 lpair.
• 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 $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV with theoretical predictions harland.