Modified Higgs Sectors and NLO Associated Production

TL;DR

This work shows that beyond-SM Higgs sectors can induce substantial one-loop corrections to Higgs associated production, beyond what is inferred from h→γγ decays. By analyzing two concrete SM extensions—vector-like leptons and new electroweak scalars—the authors quantify BSM electroweak corrections to e+e- → Zh at a 250 GeV lepton collider and to pp → Zh at the LHC, finding potential deviations exceeding 1–3% at lepton colliders and up to ~10% at the LHC. Crucially, these corrections can be large even when Higgs diphoton rates are near SM values, making Zh production a powerful indirect probe of modified Higgs sectors. The results highlight the importance of including BSM precision calculations in Higgs physics programs and suggest that precision Zh measurements could reveal new electroweak states beyond direct reach.

Abstract

Many beyond the Standard Model (BSM) scenarios involve Higgs couplings to additional electroweak fields. It is well established that these new fields may modify Higgs gamma-gamma and gamma-Z decays at one-loop. However, one unexplored aspect of such scenarios is that by electroweak symmetry one should also expect modifications to the Higgs Z-Z coupling at one-loop and, more generally, modifications to Higgs production and decay channels beyond tree-level. In this paper we investigate the full BSM modified electroweak corrections to associated Higgs production at both the LHC and a future lepton collider in two simple SM extensions. From both inclusive and differential NLO associated production cross sections we find BSM-NLO corrections can be as large as O(>10%) when compared to the SM expectation, consistent with other precision electroweak measurements, even in scenarios where modifications to the Higgs diphoton rate are not significant. At the LHC such corrections are comparable to the involved QCD uncertainties. At a lepton collider the Higgs associated production cross section can be measured to high accuracy (O(1%) independent of uncertainties in total width and other couplings), and such a deviation could be easily observed even if the new states remain beyond kinematic reach. This should be compared to the expected accuracy for a model-independent determination of the Higgs diphoton coupling at a lepton collider, which is O(15%). This work demonstrates that precision measurements of the Higgs associated production cross section constitute a powerful probe of modified Higgs sectors and will be valuable for indirectly exploring BSM scenarios.

Figures (8)

• Figure 1: New electroweak-charged fields coupled to the Higgs contribute at one loop to (a) $h\rightarrow \gamma \gamma$ decays and (b) $h \rightarrow \gamma Z$ decays. The branching ratios to these final states are sensitive to the total Higgs width, which depends on Higgs couplings to other SM and BSM fields. The total rate in these channels also depends on the Higgs production cross section. Thus experimental determination of the $h\gamma\gamma$ and $h \gamma Z$ couplings, either at the LHC or a lepton collider, is subject to uncertainties in all of the Higgs couplings. For these reasons, at a $250$ GeV lepton collider with $250 \text{ fb}^{-1}$ integrated luminosity the $h \gamma \gamma$ vertex can be determined to an accuracy of $\mathcal{O} (15\%)$Klute:2013cx.
• Figure 2: Higgs associated production. New electroweak-charged fields coupled to the Higgs typically contribute to this amplitude at one loop, interfering with the tree-level amplitude. The $hZZ$ coupling can be measured to a high degree of accuracy ($\mathcal{O} (1\%)$Klute:2013cx) at a lepton collider by measuring $Z$-recoils in the inclusive associated production process, regardless of uncertainties regarding the total Higgs width. Hence a lepton collider gains better sensitivity to the $hZZ$ coupling than to the $h \gamma \gamma$ and $h \gamma Z$ couplings, and opens up the possibility to discover new physics through anomalous contributions to the $hZZ$ vertex.
• Figure 3: SM electroweak one-loop corrections to the associated production process at a lepton collider. (a) Total associated production cross section at a linear collider as a function of $\sqrt{s}$ and (b) differential cross section at $\sqrt{s} = 250$ GeV. In both panels the tree-level result is plotted in red (middle of a set of three), the result with only weak corrections included in blue (top of three), and the full one-loop result in black (bottom of three). The various $Z$-boson polarizations are plotted in continuous (unpolarized), dashed (longitudinal) and dotted (transverse) lines.
• Figure 4: Contours of the modified Higgs diphoton decay rate, $R_{h\gamma\gamma}$ (dashed red), and BSM corrections to the associated production cross section $\delta \sigma_{Zh}$ (solid black) as defined in Sec. \ref{['sec:BSM']}, for the vector-like fourth lepton generation model. Contours of the lightest charged fermion mass, $m_{E_1}$, are also shown (dotdashed purple) but not labelled. For our restricted parameter choice we have $m_{E_1} = |m_V-m_{Ch}|$ so the lightest fermion mass can be read off from the intersection of these lines with the $x$ or $y$ axes. (a) corresponds to the case where the Higgs contributions to charged and neutral fermion masses are equal, $\Delta_\nu = 0$ GeV, (see Eq. (\ref{['eq:VL4hcoup']})) and (b) to the case where the neutral fields couple more strongly to the Higgs than charged fermions, $\Delta_\nu = 50$ GeV. Regions where precision electroweak corrections exceed measured values by $1\sigma$ are shaded, with $\Delta S$ in gray and $\Delta T$ in blue. Parameter values where modifications to the diphoton rate are not significant ( e.g.$\mathcal{O} (\lesssim 2 \sigma)$ corresponding to the region to the left of the $R_{h\gamma\gamma} = 1.3$ contour) typically imply a reduction in the associated production which can comfortably exceed $1\%$ compared to the SM prediction. This would be observable at a $250$ GeV lepton collider which can measure this cross section to $\mathcal{O} (1\%)$ accuracy.
• Figure 5: Precision electroweak constraints on the electroweak scalar model. By far the strongest constraint comes from the $T$-parameter and consistency at $1 \sigma$ with precision electroweak measurements requires $|\Delta_\phi| < 84$ GeV. Inconsistency with the $S$-parameter only arises in small wedges of parameter space. We also require that the neutral scalar has mass greater than $m_{\phi_0}>110$ GeV to ensure consistency with LEP bounds.