Abstract
The most general renormalizable interaction between the Higgs sector and a new gauge-singlet scalar S is governed by two interaction terms: cubic and quartic. The quartic term is only loosely constrained by invisible Higgs decays and given current experimental limits about 10% of all Higgs bosons at the LHC can be converted to new scalars with masses up to mHiggs/2. By including this production channel, one significantly extends the reach of the LHC-based Intensity Frontier experiments. We analyze the sensitivity of the FASER experiment to this model and discuss modest changes in the FASER 2 design that would allow exploring an order-of-magnitude wider part of the Higgs portal’s parameter space.
Highlights
The smallness of the discrepancy is related to the smallness of the ratio pT / proportional to the energy (pL) for the Higgs bosons
We compute the number of scalars traveling through the FASER 2 fiducial volume and estimate the number of decay events, using eq (2.1) with the decay probability Pdecay averaged over the energies of scalars flying in the direction of the experiment
A modest increase in the geometrical acceptance would allow probing the whole mass range few GeV mS mh/2, as demonstrated by the blue dashed line in figure 8. This increase can be achieved for example by increasing the radius of the FASER 2 from 1 meter to 1.5 meters, which is allowed by the size of the TI12 tunnel where the experiment will be located
Summary
The up to date experimental constraints in the mS-θ plane can be found in the scalar portal section of [61]. The strongest experimental constraints on the parameter α come from the invisible Higgs decay. In the Standard Model the decay h → ZZ → 4ν has the branching ratio O(10−3). Current limits on the Higgs to invisible are BRinv < 0.19 at 95% CL [34]. In what follows we will assume that the branching ratio BRinv is saturated by the h → SS decay. The obtained constraints, do not restrict the parameters relevant for the FASER 2 experiment as they search for prompt decays of the scalars, while in our model the cτS ∼ O(100) meters
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