Ⅱ-6. Particle Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Baryon spectrum of an SU(4) composite Higgs theory†
E. T. Neil∗1,∗2 [for the Tel Aviv-Colorado Collaboration]
The possibility that the Higgs boson is not a fundamental scalar, but rather a composite bound stateof some new strongly-coupled sector, provides an intriguing scenario for new physics beyond the Standard
Model. Current measurements of the properties of theHiggs boson provide only weak constraints on the possible scale of compositeness for the Higgs. If the Higgs
is indeed a composite bound state, then there will be
an associated spectrum of other new composite particles which may be within discovery reach of the Large
Hadron Collider (LHC) or other future colliders. The
overall pattern of the spectrum is directly related tothe underlying fundamental structure, just as the spectrum of QCD is predicted by the quark masses and thegauge symmetry group SU(3).
The full spectrum of a strongly-coupled gauge theory
cannot be predicted by traditional perturbative methods. Instead, lattice simulations can be used to obtain
numerical but systematically controlled results for thespectrum of a particular theory. In this work, we choseto study an SU(4) gauge theory proposed as a composite Higgs model by Ferretti.1) This model includes
two types of fermions, charged under the fundamental
4 and “sextet” two-index antisymmetric 6 representations of SU(4). We have developed a lattice code capable of dealing with fermions in multiple representations, with previous results on the “meson” spectrum2)
and thermodynamic properties of this theory.3)
Composite bound states can be enumerated byclassifying all SU(4)-singlet combinations of fermions.
Writing the 4 fermions as q and the 6 fermions as Q,
we can identify bound states such as qqqq which are
direct analogues of the familiar QCD baryons. However, there is an additional baryon-like bound statecomposed of qqQ fermions. This “chimera baryon” isespecially interesting in the context of the composite
Higgs model, in which it has the same quantum numbers as the fundamental top quark. Mixing between
the top quark and this top-partner chimera is responsible for the generation of the observed top mass.
Using standard lattice spectroscopy techniques, we
compute the mass of the chimera and the other baryonlike states for a range of input fermion masses. The
physical limit for the composite Higgs model requires
taking the massless limit for the sextet fermions m6 →0, but the other fermion mass m4 remains a free parameter of the model. Figure 1 shows our results forthe spectrum versus m4 with 1σ error bands. The units
are set in terms of F6 , the sextet NGB decay constant,†∗1∗2Fig. 1. Spectrum of bound states computed from lattice simulation for SU(4) gauge theory with two Diracfermions in the 4 and 6 (“sextet”) gauge representations. Bound-state masses (vertical axis) are shown versus the squared fundamental Nambu-Goldstone boson2(NGB) mass MP,4 , which is proportional to the input
fermion mass m4 , treated as a free parameter. In thecontext of the composite Higgs model, F6 is boundedby experiment to be F6 ≳ 1 TeV, so the plot can beread in TeV units. Most bound states are seen to beheavier than 5 TeV for the entire parameter space.

which is related to the Higgs vacuum expectation
value; experimental constraints require F6 ≳ 1 TeV,
so the plot units are equivalent to TeV scale.
We see that for the full parameter space, all boundstates are found to be quite heavy, with masses of order 5 TeV or larger, and thus likely out of the reach ofthe LHC. However, the fundamental pseudo-NambuGoldstone boson (NGB, purple curve) may be withinreach for at least part of the parameter space. Future
lattice studies will focus on non-perturbative contributions to the Higgs potential in this model, which should
greatly constrain the allowed parameters of the model.
References
1) G. Ferretti, JHEP 1406, 142 (2014).
2) V. Ayyar, T. DeGrand, D. C. Hackett, W. I. Jay,E. T. Neil, Y. Shamir, B. Svetitsky, Phy. Rev. D 97,
no. 7, 074505 (2018).
3) V. Ayyar, T. DeGrand, D. C. Hackett, W. I. Jay,
E. T. Neil, Y. Shamir, B. Svetitsky, Phy. Rev. D 97,no. 11, 114502 (2018).

Condensed from the article in Phy. Rev. D 97, no. 11, 114505(2018)RIKEN Nishina Center
Department of Physics, University of Colorado, Boulder- 97 -