Little Higgs

Another class of models with additional potentially light spin-0 fields is Little Higgs  [1,2,3]. In these models the SM Higgs doublet serves as a pseudo Goldstone boson of multiple approximate global symmetries. Explicit breaking of this set of symmetries is collective, namely, apparent only in the presence of at least two terms in the Lagrangian. This ensures that quadratically divergent diagrams contributing to the Higgs mass parameter require two loops, thereby allowing to push the cutoff scale to Λ ∼ (4π)2v ∼ 10 TeV instead of the usual 4πv ∼ 1 TeV.

In order to implement collective symmetry breaking, the electroweak gauge group is extended to a larger global symmetry, which is partially gauged. The partial gauging introduces the explicit breaking, which is crucial for having a nonzero Higgs mass as well as Yukawa couplings. In most Little Higgs models, all the spontaneously broken global generators are explicitly broken by the partial gauging, thereby giving mass to the associated Goldstone bosons. However, in some models, not all global generators are explicitly broken at leading order, either because they are collectively broken like the ones related to the Higgs doublet, or because that would interfere with collective symmetry breaking [4,5].A consequence of this is the presence of light (pseudo-)scalars a with direct couplings to the SM Higgs, which potentially leads to exotic Higgs decays [6,7].
If one imposes Minimal Flavor Violation (MFV)[8,9,10,11] in order to avoid large flavor changing neutral currents, the couplings of a to SM fermions are proportional to the SM Yukawas, and thus the coupling to the b quark is typically enhanced.

However, an enhanced decay rate of a to gluons is possible in some cases, as well as an enhanced rate to charm quarks – which arises for models with enhanced up-Yukawa couplings compared to down-Yukawa. The former possibility results in a “buried Higgs” [12,13] scenario, with the Higgs decaying to four gluon-originated jets, while the latter implies h → 4c decays, also known as “charming Higgs”  [14] (see also [15] for a more recent jet substructure study), where may decay to cc even if ma > 2mb.  Although the original version of the charming Higgs is excluded by the observed Higgs mass, other versions may exist (and in any case the same final state arises in other models, such as the type IV 2HDM+Scalar models mentioned in 2HDM+Scalar).

As a final comment, note that in models with multiple light pseudo-scalars, cascade decays among these particles, and more complex final states, such as haa′→(aaa)(aaa), could result.


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