How can we measure particles that hardly ever interact with a detector material? For example, if particles don't interact with the electromagnetic or strong force, they don't ionize or excite atoms or collide with atomic nuclei, leaving no sign of their passage that we can collect in the CMS detector.
Such invisible particles are produced at LHC. While neutrinos are the only invisible particles in the Standard Model, many models of physics beyond the Standard Model predict the existence of other invisible particles and their productions at LHC. For instance, in supersymmetric models, if R-parity is conserved, the cascade decay of supersymmetric partners ends with the LSPs (lightest supersymmetric particles), which are often invisible particles, i.e., neutralinos. As another example, if WIMPs (weakly interacting massive particles) are dark matter and are produced at LHC, they will be stable and won't interact with the CMS detector; they will be invisible particles. Therefore, if we can somehow measure invisible particles, it will enhance the precision of our measurements and the sensitivity of our searches in many physics analyses at CMS.
Missing transverse momentum (MET) is what we use to infer the presence of invisible particles or even to estimate their transverse momentum. MET is the imbalance in the transverse momentum of all visible particles. It is a physics object that is demanding to reconstruct. The accurate reconstruction of MET requires that we use only and all visible particles produced in the collision and that we measure their transverse momenta accurately. Furthermore, it will be more and more challenging as the number of pile-up interactions increases.
This Hands-on Advanced Tutorials Session is an introduction to MET in CMS. It covers the basics of MET reconstruction and MET corrections in CMS, including how to access and use MET in CMSSW.