Higgs Measurements at a Muon Collider

TL;DR

This study evaluates a μ+μ− collider operating in the Higgs s-channel at $\\sqrt{s}=M_H$ for a SM Higgs with $M_H=126.0$ GeV and $\\Gamma_H=4.21~\mathrm{MeV}$, using generator-level simulations to model Higgs and SM backgrounds with a beam width of $4.2~\mathrm{MeV}$. It leverages the enhanced $H^0\ell\ell$ coupling in the muon sector and the machine’s tight energy resolution to perform a direct scan of the narrow Higgs resonance, extracting $M_H$, $\\Gamma_H$, and branching ratios primarily from the $H^0\rightarrow b\bar{b}$ and $H^0\rightarrow WW^*$ channels, with gamma-gamma and tau-tau offering supplementary information. The results indicate that with $1~\mathrm{fb}^{-1}$, $M_H$ can be measured to ~0.25–0.3 MeV and $\\Gamma_H$ to ~0.45–0.9 MeV by combining channels, while the Higgs peak can be located with a few ×10^2 pb$^{-1}$ of luminosity under realistic beam conditions; a combined-channel approach significantly reduces luminosity demands and improves precision. The work underscores the muon collider’s potential for direct, high-precision Higgs measurements thanks to large cross sections and exceptional beam resolution, while acknowledging the need to address machine-induced backgrounds and to refine channel-specific analysis strategies.

Abstract

In light of the recent discovery of an approximately 126 GeV Higgs boson at the LHC, the particle physics community is beginning to explore the possibilities for a next-generation Higgs factory particle accelerator. In this report we study the s-channel resonant Higgs boson production and Standard Model backgrounds at a proposed μ+μ- collider Higgs factory operating at center-of-mass energy sqrt(s) = M_H with a beam width of 4.2 MeV. We study PYTHIA-generated Standard Model Higgs and background events at the generator level to identify and evaluate important channels for discovery and measurement of the Higgs mass, width, and branching ratios. We find that the H^0 -> bb and H^0 -> WW^* channels are the most useful for locating the Higgs peak. With an integrated luminosity of 1 fb^-1 we can measure a 126 GeV Standard Model Higgs mass accurately to within 0.25 MeV and its total width to within 0.45 MeV. Our results demonstrate the value of the high Higgs cross section and narrow beam resolution potentially achievable at a muon collider.

Figures (27)

• Figure 1: Conceptual rendering of MCDRCal01 detector. The silicon tracker (orange) outer radius is $122$ cm. The electromagnetic and hadronic calorimeters (gray) are made of Bismuth Germanium Oxide, or BGO. The EM calorimeter has an inner radius of 125 cm and has ten $2 cm$ thick layers, each segmented by a $1 cm \times 1 cm$ grid. The Hadronic calorimeter has an inner radius of $146 cm$ and has thirty$5 cm$ thick layers segmented by a $2 cm \times 2 cm$ grid. The muon calorimeter is made of steel-235, has an inner radius of 300 cm and has 22 $10 cm$ thick layers segmented by a $10 cm \times 10 cm$ grid. The detector has a 5 Tesla magnetic field. All calorimeters are fully sensitive. The cones are made of Tungsten surrounding the beamline and form an angle of 15 degrees. See Appendix \ref{['sec:evt-display']} for Higgs events simulated using this detector model.
• Figure 2: Simulated event counts for a scan across a 126.0 GeV Higgs peak with a 4.2 MeV wide Gaussian beam spread, counting all events except for $Z^0\rightarrow \nu_{\ell}\bar{\nu_{\ell}}$ decays. Data is taken in a 60 MeV range centered on the Higgs mass in bins separated by the beam width of 4.2 MeV. Total luminosity is $1~fb^{-1}$. Event counts are calculated as Poisson-distributed random variables and the data is fit to a Breit-Wigner convoluted with a Gaussian peak plus linear background. Fitted values of the free parameters are in Table \ref{['table:m-g-meas']}.
• Figure 3: Z boson masses in 10,000 PYTHIA-simulated $\mu^+\mu^-\rightarrow Z$ events at $\sqrt{s}=125.0GeV$. The low-mass region is dominated by the Drell-Yan process. There is a peak around the Z mass where intial-state Bremsstrahlung radiation allows the creation of an on-shell Z. The third region of interest is the peak at $125GeV$, the center of mass energy. This represents a process with no initial state radiation where the off-shell Z's produced are indistinguishable from the Higgs.
• Figure 4: Standard Model backgrounds at a $\mu^+\mu^-$ collider operating at $\sqrt{s}=126\ GeV$
• Figure 5: Simulated event counts for a scan across a 126.0 GeV Higgs peak with a 4.2 MeV wide Gaussian beam spread, counting all events with a total energy of at least 98.0 GeV visible to the detector. Data is taken in a 60 MeV range centered on the Higgs mass in bins separated by the beam width of 4.2 MeV. Event counts are calculated as Poisson-distributed random variables and the data is fit to a Gaussian peak plus linear background. The fit width is $5.16\pm0.24$ MeV and the error in the mass measurement is $0.26\pm0.19$ MeV.