Exclusive hadronic W decay: W → πγ and W → π+π+π−

Tom Melia

doi:10.1016/j.nuclphysbps.2015.09.341

Abstract

In this talk I will present results which assess the possibility of the LHC observing the decays W→πγ and W→π+π+π−. No exclusive hadronic decay of a fundamental standard model boson has ever been observed before and, at the LHC, the huge QCD background and trigger challenges present serious difficulties in making such measurements. The latter problem can be addressed by studying Ws in a tt‾ environment, exploiting the triggering opportunities this brings, and I discuss the use of an isolation technique – requiring single particle jets – which can help to separate signal from the QCD background. The conclusions drawn here are that while the W→πγ decay seems unlikely to be measured at the LHC, W→π+π+π− could be observed by the end of the high luminosity run.

Highlights

There will be of order 1011 W bosons produced at the LHC, which motivates a push for a new level of precision in the known decay modes and parameters of this particle, including branching ratios to exclusive hadronic final states
A more detailed study that takes into account realistic detector conditions needs to be performed
Tracking information will be very useful for dealing with this, since the single pion tracks should be required to point to the interaction vertex; whether this can control the amount of contamination remains to be seen after a detailed study

Summary

Introduction

There will be of order 1011 W bosons produced at the LHC, which motivates a push for a new level of precision in the known decay modes and parameters of this particle (summarised in the Particle Data Group review, [8]), including branching ratios to exclusive hadronic final states. Because of the huge QCD background at the LHC, exclusive hadronic decay modes of these W bosons are hard to subsequently observe. The remainder of this talk is restricted to reviewing the study of the decays W → πγ and W → π+π+π− in this ttenvironment, as presented in [1]. The technique studied is that of requiring single particle jets – jets which consist of a single pion (or purely photons for a ‘single photon’ jet) This technique exploits the fact that QCD evolution generically only very rarely produces a well isolated pion or photon. From the perturbative picture it is expected that this decay mode would be at least a factor of αEM larger than the W+ → π+γ decay discussed above, and a branching ratio could be as high as 10−5 and be in no conflict with experiment

Monte Carlo Study

Discussion