CMS posted a new preprint on a search for several types of exotic particles on Monday using 19.7 fb-1 of integrated luminosity at 8 TeV center-of-mass energy. The paper focuses on the theoretical models of WIMP dark matter, extra dimensions, and unparticles.
WIMP, or weakly interacting massive particle, dark matter is the most popular model of particle dark matter. It postulates that dark matter is composed of heavy particles that don’t interact much with regular matter, similar to what one might term a superheavy neutrino. Theories of extra dimensions postulate that there are additional spatial dimensions to the familiar three. If these dimensions are finite in size (“compactified:” like a tiny loop or torus, to give examples in 1D and 2D) and are small enough, we could see particles that seem equivalent to Standard Model particles but are much heavier. Unparticle physics is something that I don’t know very about, but it apparently proposes the existence of scale-invariant fields: where things like the mass and momentum all scale in the same way. This is very different from particles, where the mass is always constant regardless of the momentum.
The CMS paper looks for a particular event topology and uses this to set exclusion limits on models of these exotic particles. The channel that they look at is monojets with large missing transverse energy. The idea is that when quarks in the protons in the beams collide to create WIMPs, Kaluza-Klein particles (related to extra dimensions), or unparticles, they can emit a gluon prior to the interaction or can emit one as part of the interaction (if there is an interaction between gluons, the new particles and another particle mediating the interaction). A high energy gluon will fragment into a number of quarks, which themselves organize into hadrons that are measured by the detector. Thus, high energy gluons lead to a cone-shaped spray of hadrons called a “jet.” The interactions CMS searches for are “monojets” because they have a high energy jet and nothing else. If the jet points away from the beam direction, there will be a large imbalance if the momenta of all the particles in the transverse directions are added. Conservation of momentum requires that the total transverse momentum be basically zero, so this is a sign that an effectively noninteracting particle (or unparticle) carried away the excess momentum. Some models will predict cross sections for this process, so the experimental result can be compared to theoretical models.
Backgrounds for these interactions, according to the paper, mostly involve intermediate vector boson (W and Z) decays with associated jet production in the same interaction. The Z in particular can decay to neutrinos, which are invisible, so any jets in the event could appear like a monojet with large missing transverse energy. The results in all cases of the analysis are consistent with the Standard Model. So, CMS can set exclusion limits on the properties of these theoretical models. As in many such cases, the analysis contains a number of caveats, so the limits are for the specific models tested not for the most generic versions of these theories.