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First evidence for the exclusive two-photon production of W pairs, and new limits on anomalous quartic couplings

One year and a half after the first (and fully performed at CP3) measurement of exclusive processes at the LHC, four CP3 members made first ever observation of W pair production via photon fusion. As a consequence, it allowed to push the best limits on anomalous quartic couplings by two orders of magnitude. Finally, this ensured excellent perspectives for CMS toward further studies of the electroweak sector of the Standard Model using the two-photon processes.
By Laurent Forthomme on February 19, 2013
Additional information:

In spite of very challenging conditions due to high event pileup during the 2011 data-taking (about 4 times higher than in 2010) the improved track-based exclusivity selection developed at CP3 (in use for the former exclusive measurement of dimuon pairs photo-production) allowed efficient measurements of the exclusive production of lepton pairs.
This led to observation of two candidates of W pairs produced via photon-photon fusion, and where one W decayed into electronic channel and the other into muonic one.
Using data collected in 2011 by the CMS detector at CERN's LHC for an integrated luminosity of 5.05 fb-1, an upper limit to the cross-section of the exclusive two-photon production of W pairs is set to 1.9 fb at 95% C.L. within acceptance cuts, thus providing a mean to set very stringent constraints on anomalous quartic couplings due to extensions of the electroweak sector of the Standard Model.

Exclusive WW photo-production Feynman diagram  Transverse momentum distribution for the outgoing leptons pair  Updated limits on the AQGC parameters (OPAL limits are provided in the inset)
Exclusive γγ→W+W- production, transverse momentum distribution for the pair of outgoing leptons, and comparison between the OPAL and the new limits on the two AQGC parameters, provided the updated limits on the production cross-section

Reference url: https://cds.cern.ch/record/1518733

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