Interference of Higgs diphoton signal and background in production with a jet at the LHC

TL;DR

This work investigates how interference between Higgs diphoton signal and continuum background affects the Higgs mass peak when an extra jet is present in the final state, comparing it to the inclusive γγ case. Using a heavy-top effective theory for Hgg, full one-loop Higgs couplings to photons, and several parton-level channels (gg, Q Qbar, and Qg), it derives analytical structures and performs numerical evaluations with realistic jet and photon cuts. The key finding is that, with pT(j) cuts around 25 GeV or higher, the interference-induced diphoton mass shift is substantially reduced—typically below about 20 MeV—due to both diminished gg interference and cancellations with quark-initiated processes. This contrasts with the larger negative shifts (order −100 MeV) reported for the inclusive γγ channel without jets, and suggests that higher-order (NLO) analyses are needed to firmly establish precision Higgs mass extraction in jet-tagged configurations while the exclusive γγ channel may dominate the interference-induced mass shifts at the 100 MeV level. Overall, jet-associated Higgs production substantially mitigates the diphoton mass shift from interference, with implications for precision Higgs mass measurements at the LHC.

Abstract

The Higgs mass determination from diphoton events at the LHC can be affected by interference between the Higgs resonant and continuum background amplitudes with the same initial and final states. For the leading order gluon fusion process, this shift was previously found to exceed 100 MeV, with some dependence on the diphoton mass resolution and the methods used to extract and fit the peak from data. In this paper, I consider the mass shift for the process pp --> jγγ that includes an additional central jet in the final state. For cuts on the transverse momentum of the jet of 25 GeV or more, the diphoton Higgs peak mass shift due to interference is found to be very small, due in part to less interference for the gluon-gluon initiated subprocess, and in part to a cancellation between it and the quark-gluon initiated subprocess.

Figures (6)

• Figure 2.1: The diphoton invariant mass distribution, with and without interference included, before including any experimental resolution effects, computed for $pp \rightarrow \gamma\gamma$ at leading order as described in section \ref{['sec:H']} from the partonic process $gg \rightarrow \gamma\gamma$, for 8 TeV $pp$ collisions at the LHC, with $p_T > 40$ GeV and $|\eta| < 2.5$ for the photons. Here $M_{\gamma\gamma} = \sqrt{h}$ is the diphoton mass. The three panels show exactly the same results but with different scales on the axes.
• Figure 2.2: The diphoton invariant mass distribution for $pp\rightarrow \gamma\gamma$ at leading order, with and without interference included, as in Figure \ref{['fig:unsmearedLO']}, but now including the effects of a Gaussian mass resolution with $\sigma_{\rm MR} = 1.7$ GeV. The two panels show the same results with different scales on the axes.
• Figure 2.3: The shift in the invariant mass distribution due to the interference effect, $\Delta M_{\gamma\gamma} \equiv M_{\gamma\gamma}^{\rm peak} - M_H$, for $pp \rightarrow \gamma\gamma$ at leading order, computed by a least-squares fit of the lineshape to a Gaussian with the same width $\sigma_{\rm MR}$ used to model the mass resolution. The solid line includes cuts $p_T > 40$ GeV and $|\eta| < 2.5$ on the photons, and the dashed line is what would be obtained without these cuts.
• Figure 4.1:
• Figure 4.2: The quantity $M_H \Gamma_H I(M_H^2)/P(M_H^2)$, where $I(h)$ and $P(h)$ are the functions defined in eq. (\ref{['eq:genform']}), as a function of the cut on the transverse momentum of the jet, $p^j_{T,{\rm cut}}$, with other cuts as described in the text, for $gg \rightarrow g\gamma\gamma$, for $Qg \rightarrow Q\gamma\gamma$ plus $\overline Qg \rightarrow \overline Q\gamma\gamma$, and for the combined $pp \rightarrow j \gamma\gamma$, for $\sqrt{s} = 8$ TeV. The result for the leading order $pp \rightarrow H \rightarrow \gamma\gamma$ with no jet requirement is also shown as the dashed line. The results for $\sqrt{s} = 13$ TeV are similar.