Search for exclusive or semi-exclusive photon pair production and observation of exclusive and semi-exclusive electron pair production in pp collisions at sqrt(s) = 7 TeV

TL;DR

In pp collisions at $\sqrt{s}=7$ TeV, CMS searches for exclusive and semi-exclusive central production of photon pairs and observes exclusive/semi-exclusive $e^+e^-$ production, using a low-pileup data sample of $36\ \text{pb}^{-1}$. The analysis employs dedicated Monte Carlo generators (ExHuME for $\gamma\gamma$ and lpair for $\gamma\gamma\to e^+e^-$) and a stringent exclusivity framework to suppress pileup and inelastic backgrounds, with detailed efficiency and background studies. No diphoton candidates are found, yielding an upper limit on $\sigma(pp\to p(*)\gamma\gamma p(*))$ of $<1.18$ pb at 95% CL, while 17 exclusive/semi-exclusive $e^+e^-$ events are observed in agreement with the predicted $16.3\pm1.3$ events. These results validate the exclusivity techniques and provide empirical constraints on diffractive photon-pair production and the related rapidity-gap survival probability, informing expectations for exclusive Higgs production and diffraction in hadron colliders.

Abstract

A search for exclusive or semi-exclusive photon pair production, pp to p(*) + photon pair + p(*) (where p(*) stands for a diffractively-dissociated proton), and the observation of exclusive and semi-exclusive electron pair production, pp to p(*) + ee + p(*), in proton-proton collisions at sqrt(s) = 7 TeV, are presented. The analysis is based on a data sample corresponding to an integrated luminosity of 36 inverse picobarns recorded by the CMS experiment at the LHC at low instantaneous luminosities. Candidate photon pair or electron pair events are selected by requiring the presence of two photons or a positron and an electron, each with transverse energy ET > 5.5 GeV and pseudorapidity abs(eta) < 2.5, and no other particles in the region abs(eta) < 5.2. No exclusive or semi-exclusive diphoton candidates are found in the data. An upper limit on the cross section for the reaction pp to p(*) + photon pair + p(*), within the above kinematic selections, is set at 1.18 pb at 95% confidence level. Seventeen exclusive or semi-exclusive dielectron candidates are observed, with an estimated background of 0.85 +/- 0.28 (stat.) events, in agreement with the QED-based prediction of 16.3 +/- 1.3 (syst.) events.

Figures (6)

• Figure 1: The dominant diagrams for (a) exclusive diphoton production and (b) exclusive Higgs boson production in pp collisions. Note the screening gluon that cancels the color flow from the interacting gluons and therefore allows the protons to stay intact. For exclusive $\gamma\gamma$ production, the contributions from ${q}\overline{{q}}\xspace\rightarrow \gamma\gamma$ and $\gamma\gamma\rightarrow \gamma\gamma$ are both theoretically estimated to be less than 1% of ${g}\xspace{g}\xspace\rightarrow\gamma\gamma$Khoze:2004ak.
• Figure 2: The Feynman diagrams for (a) exclusive ${e}^+{e}^-$ production and semi-exclusive ${e}^+{e}^-$ production with (b) either or (c) both protons dissociating in pp collisions.
• Figure 3: Exclusivity efficiency as a function of the bunch-by-bunch luminosity.
• Figure 4: Comparison of the upper limit (at 95% CL) derived with the present data and four theoretical predictions. The upper limit is on the sum of the exclusive and semi-exclusive $\gamma\gamma$ production cross sections (where it is required that no particles from the proton dissociation have $|\eta| < 5.2$), while the theoretical predictions are for exclusive $\gamma\gamma$ production only. If the contributions from semi-exclusive production are included, the predictions increase by a factor of $\sim$2 private.
• Figure 5: Distributions of (a) the invariant mass and (b) the transverse momentum of the ${e}^+{e}^-$ pairs, compared to the lpair predictions (histograms) for the three processes contributing to exclusive and semi-exclusive $\gamma \gamma \rightarrow {e}^+{e}^-\xspace$ production, passed through the full detector simulation and reconstruction. The simulation is normalized to the integrated luminosity of the data sample (36$\,\text{pb}^\text{$-$1}$), and does not include the estimated $0.85 \pm 0.28$ background events.