Search for new quarks addresses unnaturalness

The Standard Model (SM) is a triumph of modern physics, with unprecedented success in explaining the subatomic world. The Higgs boson, discovered in 2012, was the capstone of this amazing theory, yet this newly known particle raises many questions.  For example, interactions between the Higgs boson and the top quark should lead to huge quantum corrections to the Higgs boson mass, possibly as large as the Planck mass (>10¹⁸ GeV). Why, then, is the observed mass only 125 GeV? Finding a solution to this “hierarchy problem” is one of the top motivations of many new theories of particle physics.

A common feature in several of these theories is the existence of vector-like quarks – in particular, a vector-like top quark (T) that could naturally cancel the large quantum corrections caused by the SM top quark. Like other quarks, vector-like quarks are spin-½ particles that interact via the strong force and, like all spin-½ particles, they have left-handed and right-handed versions. The unique feature of vector-like quarks is their ambidexterity: while the weak force only interacts with left-handed SM particles, it would interact the same way with both the right- and left-handed versions of vector-like quarks. This also gives vector-like quarks more options in how they can decay. Unlike the Standard Model top quark, which almost always decays to a bottom quark and W boson (t→Wb), a vector-like top quark could decay three different ways: T→Wb, T→Zt, or T→Ht.

The search for vector-like quarks in ATLAS spans a wide range of dedicated analyses, each focusing on a particular experimental signature (possibly involving leptons, boosted objects or large missing transverse energy). The breadth of the programme allows ATLAS to be sensitive to most relevant decays of vector-like top quarks, and also those of vector-like bottom quarks, thus increasing the chances of discovery. The creation of particle–antiparticle pairs is the most probable production mechanism for vector-like quarks with mass around or below 1 TeV. For higher masses, single production of vector-like quarks may have a larger rate.

ATLAS recently performed a statistical combination of all the individual searches that looked for pair-production of vector-like quarks. While the individual analyses were designed to be sensitive to particular sets of decays, the combined results provide increased sensitivity to all considered decays of vector-like top quarks with masses up to 1.3 TeV. No vector-like top or bottom quarks were found. The combination allowed ATLAS to set the most stringent exclusion bounds on the mass of a vector-like top quark for arbitrary sets of branching ratios to the three decay modes (figure, left).

As the limits on vector-like quarks reach higher masses, the importance of searching for their single production rises. Such searches are also interesting from a theoretical perspective, since they allow one to constrain parameters of the production model (figure, right).

Given these new strong limits on vector-like quarks and the lack of evidence for supersymmetry, the theoretical case for a naturally light Higgs boson is not looking good! But nature probably still has a few tricks up her sleeve to get out of this conundrum.