As the experiment collaborations get ready for Run 2 at the LHC, the situation of the searches for new physics is rather different from what it was in 2009, when Run 1 began. Many models have been constrained and many limits have been set. Yet a fundamental question remains: why is the mass of the newly discovered Higgs boson so much below the Planck energy scale? This is the so-called hierarchy problem. Quantum corrections to the mass of the Higgs boson that involve known particles such as the top quark are divergent and tend to push the mass to a very high energy scale. To account for the relatively low mass of the Higgs boson requires fine-tuning, unless some new physics enters the picture to save the situation.
A variety of theories beyond the Standard Model attempt to address the hierarchy problem. Many of these predict new particles whose quantum-mechanical contributions to the mass of the Higgs boson precisely cancel the divergences. In particular, models featuring heavy partners of the top quark with vector-like properties are compelling, because the cancellations are then achieved in a natural way. These models, which often assume an extension of the Standard Model Higgs sector, include the two-Higgs doublet model (2HDM), the composite Higgs model, and the little Higgs model. In addition, theories based on the presence of extra dimensions of space often predict the existence of vector-like quarks.
The discovery of the Higgs boson was a clear and unambiguous target for Run 1. In contrast, there could be many potential discoveries of new particles or sets of particles to hope for in Run 2, but currently no model of new physics is favoured a priori above any other.
One striking feature common to many of these new models is that the couplings with third-generation quarks are enhanced. This results in final states containing b quarks, vector bosons, Higgs bosons and top quarks that can have significant Lorentz boosts, so that their individual decay products often overlap and merge. Such “boosted topologies” can be exploited thanks to dedicated reconstruction algorithms that were developed and became well established in the context of the analyses of Run-1 data.
Searches for top-quark partners performed by CMS on the data from Run 1 span a large variety of different strategies and selection criteria, to push the mass-sensitivity as high as possible. These searches have now been combined to reach the best exclusion limit from the Run-1 data: heavy top-quark partners with masses below 800 GeV are now excluded at the 95% confidence level. The figure shows a simulated event with a top-quark partner decaying into a top-quark plus a Higgs boson (T → tH) in a fully hadronic final state.
CMS plans to employ these techniques to analyse boosted topologies not only in the analysis framework, but for the very first time also in the trigger system of the experiment when the LHC starts up this year. The new triggers for boosted topologies are expected to open new regions of phase space, which would be out of reach otherwise. Some of these searches are expected to already be very sensitive within the first few months of data-taking in 2015. The higher centre-of-mass energy increases the probability for pair production of these new particles, as well as of single production. The CMS collaboration is now preparing to exploit the early data from Run 2 in the search for top-quark partners produced in 13 TeV proton collisions.
