New opportunities in h → 4ℓ

Abstract

The Higgs decay h → 4 has played an important role in discovering the Higgs and measuring its mass thanks to low background and excellent resolution. Current cuts in this channel have been optimized for Higgs discovery via the dominant tree level ZZ contribution arising from electroweak symmetry breaking. Going forward, one of the primary objectives of this sensitive channel will be to probe other Higgs couplings and search for new physics on top of the tree level ZZ 'background'. Thanks to interference between these small couplings and the large tree level contribution to ZZ, the h → 4 decay is uniquely capable of probing the magnitude and CP phases of the Higgs couplings to γγ and Zγ as well as, to a lesser extent, ZZ couplings arising from higher dimensional operators. With this in mind we examine how much relaxing current cuts can enhance the sensitivity while also accounting for the dominant non-Higgs continuum qq → 4 background. We find the largest enhancement in sensitivity for the hZγ couplings ( 100%) followed by hγγ ( 40%) and less so for the higher dimensional hZZ couplings (a few percent). With these enhancements, we show that couplings of order Standard Model values for hγγ may optimistically be probed by end of Run-II at the LHC while for hZγ perhaps towards the end of a high luminosity LHC. Thus an appropriately optimized h → 4 analysis can complement direct decays of the Higgs to on-shell γγ and Zγ pairs giving a unique opportunity to directly access the CP properties of these couplings.

Pseudo-observables in Higgs decays

“…As noted in Ref. [21], such corrections are very small (below the 1 % level compared to the tree-level terms) and practically unobservable, except in a few notable kinematical points. In particular, the only case where such corrections are relevant is for on-shell hardphoton amplitudes (given that they vanish at the tree level within the SM) or almost on-shell photon-exchange contributions in neutral-current amplitudes.…”

Electroweak bounds on Higgs pseudo-observables and $$h\rightarrow 4\ell $$ h → 4 ℓ decays

“…As shown in Ref. [23], decreasing the cut on m from 12 to 4 GeV substantially improves the sensitivity to these couplings, even excluding the m region around the ϒ resonances. Moreover, as demonstrated in Ref.…”

“…[23], manifests itself in Eq. (12) as a ∼25 times larger interference term of γ γ with κ Z Z in h → 4e(4μ) vs. h → 2e2μ.…”

“…[23][24][25][26][27] for a recent discussion). In this work we focus only on the invariant-mass distributions, both because the effect of the contact terms in h → 4 is the one less studied in the previous literature and because, as shown above, these PO are the less constrained at the moment (at least in the general TGC case).…”

ConstrainingCP-violating Higgs sectors at the LHC using gluon fusion

“…Accordingly, CP-odd effects in h → ZZ Ã decays and WBF are suppressed by Oðα EW Þ, so that these methods effectively project out the CP-odd part of the Higgs (although see Ref. [21] for a study which incorporates loop effects and Ref. [22] for a discussion of h → VV decays in some specific BSM models).…”