Abstract
A search is described for the production of a pair of bottom-type vector-like quarks (VLQs), each decaying into a b or $\mathrm{\bar{b}}$ quark and either a Higgs or a Z boson, with a mass greater than 1000 GeV. The analysis is based on data from proton-proton collisions at a 13 TeV center-of-mass energy recorded at the CERN LHC, corresponding to a total integrated luminosity of 137 fb$^{-1}$. As the predominant decay modes of the Higgs and Z bosons are to a pair of quarks, the analysis focuses on final states consisting of jets resulting from the six quarks produced in the events. Since the two jets produced in the decay of a highly Lorentz-boosted Higgs or Z boson can merge to form a single jet, nine independent analyses are performed, categorized by the number of observed jets and the reconstructed event mode. No signal in excess of the expected background is observed. Lower limits are set on the VLQ mass at 95% confidence level equal to 1570 GeV in the case where the VLQ decays exclusively to a b quark and a Higgs boson, 1390 GeV for when it decays exclusively to a b quark and a Z boson, and 1450 GeV for when it decays equally in these two modes. These limits represent significant improvements over the previously published VLQ limits.
Highlights
One of the biggest puzzles in elementary particle physics concerns the large difference between the electroweak scale and the Planck scale, and the related problem of the unexpectedly low value of the Higgs boson mass [1]
Supersymmetry provides an elegant solution to this problem [2,3], the lack of evidence for the production of supersymmetric particles at the CERN LHC indicates that, if supersymmetry is realized in nature, it is broken at an energy scale greater than a few TeV and, does not solve the fine tuning of the 125 GeV Higgs boson mass
Background from standard model (SM) processes is reduced by requiring that the jets are consistent with the production of a pair of bosons, that the reconstructed vectorlike quarks (VLQs) have equal masses, and that some of the jets are tagged as originating from b quarks
Summary
One of the biggest puzzles in elementary particle physics concerns the large difference between the electroweak scale and the Planck scale, and the related problem of the unexpectedly low value of the Higgs boson mass [1]. Several alternative theories have been proposed for solving this fine tuning problem These theories include composite Higgs models [4,5,6], in which the Higgs boson is not a fundamental particle, but rather contains constituents bound by a new type of gauge interaction, and little Higgs models [7,8], in which the Higgs boson is a pseudo-Nambu–Goldstone boson that arises from spontaneous breaking of a global symmetry at the TeV energy scale. Requiring VLQs to have renormalizable couplings to the SM quarks permits only four types of VLQs, defined by their charge q: q 1⁄4 −1=3 ðBÞ, q 1⁄4 þ2=3 ðTÞ, q 1⁄4 −4=3 ðXÞ, and q 1⁄4 þ5=3 ðYÞ [12] These are arranged into seven multiplets: two singlets (T and B), three doublets (TB, XT, and YB), and two triplets (XTB and TBY) [13]. The analysis presented here improves on these results by using the full 137 fb−1 dataset collected by CMS in 2016–2018, and by fully reconstructing the event kinematics, thereby allowing the mass of the B to be reconstructed
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