Di-Boson production

At the Large Hadron Collider (LHC) protons have been colliding for three years, at three different centre of mass energy: 900GeV, 7TeV and 8TeV. The produced data have being analysed to first understand all the known physics processes and subsequently to probe new physics phenomena. The production of di-bosons is predicted from the Standard Model of particle physics and its cross section is predicted to the next-to-leading order (NLO) accuracy.
In particular pairs of Z bosons are predicted to be produced at LHC. A schematic view using the Feynman diagram is shown in the figure below.
Among the possibile interactions which can lead to the production of pairs of Z bosons, some are not allowed by the Standard Model. In particular, neutral di-boson self-interactions via trilinear couplings (TGC) are forbidden. The process of simultaneous production of two Z bosons is sensitive to this coupling. Consequently its cross section and kinematic distributions measurements can be used to test SM predictions. Deviations can arise from production of new particles which decay in Z-diboson or from new interactions between neutral gauge bosons, not allowed in the SM, referred as anomalous triple gauge couplings (aTGCs). ZZ production is the only channel which provides direct access to ZZZ coupling. Therefore, this process is very important to test the electroweak sector of the SM and its gauge structure at high energies.

 

Enlarged view: di boson production
Schematic view of the di-boson production process

Z-diboson studies are of great importance in searches for new physics. Production of ZZ pairs and their decay products constitute an irreducible background for many Higgs and physics beyond SM (ex. SUper SYmmetry) searches. For example, ZZ boson analysis is important for search of a low-mass Higgs boson produced in association with a Z boson and decaying in two b quarks, while the weak boson decays into leptons (VH(bb) analysis).
Due to the small cross section of Z pairs production and the overwhelming background, ZZ events can be also used to confirm CMS experiment capability to discriminate small signals.

Enlarged view: z diboson CMS
Invariant mass of the two b-tagged jets
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