Search for exclusive or semi-exclusive γγ production and observation of exclusive and semi-exclusive e+e− production in pp collisions at $ \sqrt{s}=7 $ TeV

Abstract

A search for exclusive or semi-exclusive γγ production, pp → p ( * ) + γγ + p ( * ) (where p * stands for a diffractively-dissociated proton), and the observation of exclusive and semi-exclusive e + e − production, pp → p ( * ) + e + e − + p ( * ) , in proton-proton collisions at √ s = 7 TeV, are presented. The analysis is based on a data sample corresponding to an integrated luminosity of 36 pb −1 recorded by the CMS experiment at the LHC at low instantaneous luminosities. Candidate γγ or e + e − events are selected by requiring the presence of two photons or a positron and an electron, each with transverse energy E T > 5.5 GeV and pseudorapidity |η| < 2.5, and no other particles in the region |η| < 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 → p ( * ) + γγ + 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.

Searching for Axionlike Particles with Ultraperipheral Heavy-Ion Collisions

“…In the remaining, background-free region we set a limit on the total signal rate. We also show the analogous limit from the p-p analysis performed by CMS [25], although we find it is not competitive with other LHC limits. For the BB operator, the expected limits from heavy-ion collisions are competitive with the other collider limits, whereas for the FF operator they are significantly stronger.…”

“…[24]. Given the absence of pile-up at Pb-Pb collisions, we do not expect such a veto to significantly degrade the signal efficiency (see also [25]). …”

“…Event selection: Our proposed analysis closely mimics the CMS search for exclusive γγ production in p-p collisions [25], as well as the analogous search for lightby-light scattering proposed in Ref. [26].…”

“…For p-p collisions, this process has been calculated [31][32][33] and constrained experimentally at 7 TeV with CMS [25]. To the best of our knowledge, no prediction is currently available for the analogous process in heavy-ion collisions.…”

“…We hereby require the e + e − to either have high rapidity |η| > 2.5 or Λ aFF in heavy-ion UPCs at the LHC (green solid and dashed curves, for a Pb-Pb luminosity of 1 nb −1 and 10 nb −1 , respectively). Shown for comparison is the limit from 36 pb −1 of exclusive p-p collisions [25] (red dot-dash). New and updated exclusion limits from LEPII (OPAL 2γ, 3γ) [38] and from the LHC (ATLAS 2γ, 3γ) [39,40] are indicated by the various shaded regions (see text).…”