Constraining the Higgs boson width with ZZ production at the LHC

Abstract: We point out that existing measurements of $pp \to ZZ$ cross-section at the LHC in a broad range of ZZ invariant masses allow one to derive a model-independent upper bound on the Higgs boson width, thanks to strongly enhanced off-shell Higgs contribution. Using CMS data and considering events in the interval of ZZ invariant masses from 100 to 800 GeV, we find $\Gamma_H \le 38.8 \times \Gamma_H^{SM} \approx 163$ MeV, at the $95\%$ confidence level. Restricting ZZ invariant masses to $M_{ZZ} \ge 300$ GeV range, we estimate that this bound can be improved to $\Gamma_H \le 21 \times \Gamma_H^{SM} \approx 88$ MeV. Under the assumption that all couplings of the Higgs boson to Standard Model particles scale in a universal way, our result can be translated into an upper limit on the branching fraction of the Higgs boson decay to invisible final states. We obtain $Br(H \to inv) < 0.84 (0.78)$, depending on the range of ZZ invariant masses that are used to constrain the width. We believe that an analysis along these lines should be performed by experimental collaborations in the near future and also in the run II of the LHC. We estimate that such analyses can, eventually, be sensitive to the Higgs boson width as small as $\Gamma_H \sim 10 \times \Gamma_H^{SM}$.

• The paper introduces a novel method for constraining the Higgs boson width using off-shell ZZ production at the LHC.
• It employs invariant mass distribution analysis of ZZ pairs from CMS data to establish 95% confidence level upper limits on the Higgs width relative to the Standard Model.
• The findings offer both theoretical insights and practical guidance for refining experimental strategies in future high-luminosity LHC operations.

Constraining the Higgs Boson Width at the LHC

The paper by Caola and Melnikov provides a quantitative exploration of the potential to constrain the total width of the Higgs boson ($\Gamma_H$) using precision measurements from the Large Hadron Collider (LHC), particularly focusing on $ZZ$ production in proton-proton collisions. This work is crucial for enhancing our understanding of Higgs boson properties beyond the minimal assumptions of the Standard Model (SM). The authors employ the invariant mass distribution of $ZZ$ pairs, resulting from Higgs-mediated processes, to inform model-independent upper bounds on $\Gamma_H$.

Key Findings

The analysis leverages data primarily from the Compact Muon Solenoid (CMS) experiment to establish constraints on the Higgs boson width by considering its off-shell contributions to $ZZ$ final states—a methodology benefitting from the significantly enhanced off-shell Higgs contributions. Utilizing event data in the $ZZ$ invariant mass range of $100$ to $800 \, \text{GeV}$, the authors assert a $95\%$ confidence level upper limit $$\Gamma_H \le 38.8 \; \Gamma_{H}^{\text{SM}} \approx 163 \, \text{MeV}$$. Further, restricting the $ZZ$ invariant masses to $M_{ZZ} \ge 300 \, \text{GeV}$ refines this bound to $$\Gamma_H \le 21 \; \Gamma_H^{\text{SM}} \approx 88 \, \text{MeV}$$.

Methodology Overview

The methodology hinges on the extended mass range beyond $2 m_Z$, where the Higgs can decay off-shell, leading to $ZZ$ production. The key insight is that within this extended mass range, the off-shell Higgs boson contributes significantly to the total cross-section, and this contribution scales linearly with $\Gamma_H$. The authors utilize theoretical estimates along with CMS data to compare expected and observed event rates, deriving upper limits on $\Gamma_H$. The analysis accounts for background corrections, considering both the signal and interference from Standard Model $gg \to ZZ$ processes.

Theoretical and Practical Implications

The implications of this work are twofold: theoretically, it sets a precedent for further studies on the Higgs boson width using off-shell production mechanisms which are less affected by the uncertainties in direct width measurements. Practically, it guides experimental strategies under current and future LHC operations, pointing to the viability of tighter constraints on $\Gamma_H$ by optimizing the invariant mass range and enhancing sensitivity through angular distributions of decay products, among others.

Furthermore, the constraint on $\Gamma_H$ can be translated into limits on the branching ratio for invisible Higgs decays, with this study suggesting Br($H \to \text{inv}) < 0.84$. Such branching ratio constraints are essential for new physics searches aiming to detect deviations from the SM caused by hypothesized invisible particles.

Future Directions

The authors suggest potential improvements in constraining $\Gamma_H$ through increased luminosity and systematic error reduction, advocating for dedicated analysis by experimental collaborations. The paper underscores the need for refining theoretical estimates and detector simulations, especially at invariant masses significantly above the Higgs mass.

In conclusion, this paper makes a significant contribution to the field of particle physics by demonstrating an innovative method to constrain the Higgs boson width through off-shell production analysis at the LHC. Such methods could provide complementary insights into coupling deviations and exotic decay modes, paving the way for novel investigations in high-energy physics.