Search for W Boson Decays to Three Charged Pions

Area of Science:

• Particle Physics
• High-Energy Physics
• Standard Model Physics

Background:

• The W boson is a fundamental particle mediating the weak nuclear force.
• Rare decays of the W boson provide sensitive probes of the Standard Model and potential new physics.
• Previous searches have not explored the W boson decay to three charged pions.

Purpose of the Study:

• To perform the first search for the rare decay of the W boson into three charged pions (W -> 3π).
• To set an upper limit on the branching fraction of this decay.
• To provide data for theoretical calculations and comparisons.

Main Methods:

• Analysis of proton-proton collision data collected by the CMS experiment.
• Utilized data corresponding to an integrated luminosity of 77.3 fb⁻¹ at a center-of-mass energy of 13 TeV.
• Searched for a significant excess of events consistent with the W -> 3π decay signature above expected background.

Main Results:

• No significant excess of W -> 3π events was observed.
• An upper limit on the branching fraction was set at 1.01×10⁻⁶ at 95% confidence level.
• The result is the most stringent limit to date for this specific decay channel.

Conclusions:

• The rare decay of the W boson to three charged pions is highly suppressed or does not occur at a detectable rate.
• The established upper limit provides crucial constraints for theoretical models of particle decays.
• This search motivates further theoretical investigations into the branching fraction of W -> 3π.