Measuring the Higgs Branching Fraction into two Photons at Future Linear \ee Colliders

TL;DR

This study evaluates the precision with which BF($H \rightarrow \gamma\gamma$) can be measured for a SM-like Higgs with $M_H=120$ GeV at a future linear $e^+e^-$ collider with $L=1~\mathrm{ab}^{-1}$ at $\sqrt{s}=350$ and $500$ GeV, using Higgsstrahlung and WW fusion production. It models signal and irreducible backgrounds, including $Z\gamma\gamma$ and $\nu\bar{\nu}\gamma\gamma$, via CompHEP with ISR/beamstrahlung and simulates detector response with SIMDET; a multivariate Higgs-likeness discriminant is employed to optimize signal extraction. For unpolarized beams, the BF measurement achieves about $18\%$ precision at $350$ GeV and $16\%$ at $500$ GeV, with polarization potentially reducing the uncertainty to roughly $10$–$12\%$ on $\sigma\cdot BF$ and yielding a diphoton partial width precision around $11$–$13\%$ once the total width is constrained. The results underscore the value of beam polarization and multi-channel analyses for precision Higgs coupling tests at future linear colliders and their potential to reveal or constrain new physics through the $H \rightarrow \gamma\gamma$ channel.

Abstract

We examine the prospects for measuring the \gaga branching fraction of a Standard Model-like Higgs boson with a mass of 120 GeV at the future TESLA linear \ee collider, assuming an integrated luminosity of 1 ab$^{-1}$ and center-of-mass energies of 350 GeV and 500 GeV. The Higgs boson is produced in association with a fermion pair via the Higgsstrahlung process \ee $\to ZH$, with $Z \to$ \qq or \nn, or the WW fusion reaction $e^+e^- \to ν_e \bar{ν_e} H$. A relative uncertainty on BF(\hgg) of~16% can be achieved in unpolarized \ee collisions at $\sqrt{s}$=~500 GeV, while for $\sqrt{s}$=~350 GeV the expected precision is slightly poorer. With appropriate initial state polarizations $Δ$BF(\hgg)/BF(\hgg) can be improved to 10%. If this measurement is combined with the expected error for the total Higgs width, a precision of 10% on the \gaga Higgs boson partial width appears feasible.

Figures (5)

• Figure 1: Background diagrams for the reaction $e^+e^- \rightarrow d \bar{d}\gamma \gamma$.
• Figure 2: Background diagrams for the reaction $e^+e^-\rightarrow\nu\bar{\nu}\gamma \gamma$.
• Figure 3: "Higgs-likeness" probability for $e^+e^-\:$$\rightarrow HZ \rightarrow q \bar{q} \gamma \gamma$ events (shaded) and the background considered.
• Figure 4: $M_{\gamma \gamma}$ invariant mass distributions for 350 GeV: a) $q \bar{q} \gamma \gamma$ and b) $\nu \bar{\nu} \gamma \gamma$ events. The background in the histograms has been averaged to avoid accidental fluctuations.
• Figure 5: $M_{\gamma \gamma}$ invariant mass distributions for 500 GeV: a) $q \bar{q} \gamma \gamma$ and b) $\nu \bar{\nu} \gamma \gamma$ events. The background in the histograms has been averaged to avoid accidental fluctuations.