Future CERN Collider Could Pin Down the Higgs Decay to Tau Leptons with Sub-Percent Precision

More than a decade after the Higgs boson was discovered, physicists are already designing the machine that will measure its properties with unprecedented fidelity. The Future Circular Collider, or FCC-ee, has been placed at the forefront of the European Strategy for Particle Physics as CERN’s flagship project for the coming decades, and a new study published in The European Physical Journal C now quantifies just how well this proposed electron-positron collider could measure one of the most telling of all Higgs decays: the transformation of the Higgs boson into a pair of tau leptons. The answer is striking. Where today’s Large Hadron Collider experiments measure this decay channel with uncertainties of tens of percent, the FCC-ee could reach sub-percent precision in the dominant production mode, an improvement of at least an order of magnitude.

The tau channel occupies a special place in Higgs physics because it probes the Yukawa coupling between the Higgs boson and charged leptons, the mechanism through which the Higgs field is thought to give elementary particles their mass. Any deviation from the pattern predicted by the Standard Model in this vertex could signal new particles or forces lurking at energy scales far beyond the reach of any collider. The decay is also experimentally attractive: it has a relatively large branching fraction, and unlike the decay to bottom quarks it enjoys a comparatively clean background environment. It was the combined ATLAS and CMS analysis of Run I data that first established the observation of the Higgs boson decaying to tau pairs, with CMS later achieving the observation from a single experiment at 13 teraelectronvolts.

Today, however, the measurements remain coarse. The CMS collaboration reports a signal strength of 1.89 with an uncertainty of roughly 0.6 for Higgs production in association with a Z boson followed by decay to taus, and ATLAS finds a comparable spread, translating into relative uncertainties near 60 percent in that channel and around 20 percent in vector boson fusion. Even the projected High-Luminosity LHC, the machine’s final upgrade, is expected to reach only about 14 percent in the associated production channel and 4 to 8 percent in vector boson fusion. In the kappa framework, where coupling modifiers express the measured strength of each Higgs interaction relative to the Standard Model, the current precision on the tau coupling stands at roughly 8 percent, with HL-LHC projections improving this to about 1.9 percent.

The FCC-ee would change the calculus entirely. The core of its program is a dedicated run at a center-of-mass energy of 240 gigaelectronvolts, where approximately two million Higgs bosons would be produced in association with Z bosons for an integrated luminosity of 10.8 inverse attobarns. A second run near the top-quark production threshold, at 365 gigaelectronvolts with 3 inverse attobarns, would additionally open up Higgs production through vector boson fusion, a process never before observed in electron-positron collisions. Because the initial state consists of two well-defined pointlike particles, this production mode would allow model-independent measurements of Higgs properties that are impossible at a hadron collider, where the partonic collision energy is unknown and the underlying event obscures the final state.

The new analysis, carried out by researchers at the Karlsruhe Institute of Technology, EPFL and CIEMAT, projects the achievable precision on the Higgs-to-tau cross-section in both production modes using a fast simulation of the IDEA detector concept, one of the leading designs proposed for the FCC-ee, built around a short-drift wire chamber and a dual-readout calorimeter. Signal and background processes were generated with Monte Carlo tools including Whizard and Pythia, with all relevant Higgs decay modes and Standard Model backgrounds such as ZZ, WW and Drell-Yan production included. Events were organized into nine categories according to how the Z boson and the taus decay, and jets were clustered with the exclusive Durham algorithm, a choice particularly well suited to lepton colliders where the absence of pileup and underlying event allows the jet count to be fixed directly to the known parton multiplicity.

A central challenge in any tau analysis is reconstructing the tau leptons themselves. The tau is the least precisely characterized of the charged leptons, largely because it decays in numerous modes, most of them involving hadrons, within a distance of less than a millimeter from the collision point. The study compares two reconstruction strategies. The first is ParticleNet, a machine-learning algorithm based on a graph neural network architecture that assigns each jet a classification score reflecting the probability that it originated from a tau rather than a quark or gluon. With a score threshold of 0.5, the algorithm achieves a tau identification efficiency of 99.954 percent in simulated Higgs events while misidentifying fewer than 4 percent of quark jets. The second is an explicit reconstruction method that scans jet constituents to identify the exact tau decay mode, checking charge and mass consistency and counting charged and neutral particles to classify the decay.

The two approaches perform comparably in efficiency, so the choice between them hinges on whether decay-mode information is needed. For the cross-section measurement the authors adopted ParticleNet, since decay-mode identification was not exploited. The study also examined different ways of recovering the Higgs boson mass from tau-pair events, a nontrivial task because each tau decay carries away neutrinos that escape the detector unseen. The visible mass of the tau decay products carries the least information, but the collinear approximation and, most powerfully, the recoil mass technique, which reconstructs the system recoiling against the tau pair from the precisely known Z boson decay products and energy-momentum conservation, place the overwhelming majority of signal events within a narrow window around the Higgs mass. The recoil mass method captures 83.74 percent of events between 120 and 132 gigaelectronvolts, producing a remarkably sharp peak.

To separate signal from background, the analysis employs boosted decision trees trained separately in each category and at each collision energy, using kinematic features such as angular separations between the taus, missing momentum, and the reconstructed Higgs and Z boson masses. At the higher energy, where vector boson fusion contributes significantly to the same final states, a multi-class classifier distinguishes the two production modes, exploiting differences in missing transverse momentum and missing energy between their topologies. The final cross-sections are extracted through a maximum likelihood fit. The projected relative uncertainty on the product of the ZH cross-section and the Higgs-to-tau branching fraction reaches the sub-percent level, while the vector boson fusion channel, limited by its smaller event yield and higher background, delivers a less dramatic but still substantial improvement over the LHC.

Translated into the kappa framework, and combining the projected FCC-ee precisions on the Z and W coupling modifiers and the Higgs boson width, the study finds an expected relative precision of 0.47 percent on the tau Yukawa coupling from associated production, with vector boson fusion contributing at the 4.26 percent level. Such precision would transform the tau coupling from one of the weaker links in the Higgs sector into one of the sharpest tests of the Standard Model. The implications extend beyond the Higgs program: the FCC-ee’s Z-pole run alone would produce more than a hundred billion tau pairs, rivaling dedicated flavor factories and enabling improvements of several orders of magnitude in measurements of tau observables, from the lepton’s mass to searches for violations of lepton universality.

The study also carries a methodological message for the broader future-collider community. The tau reconstruction techniques evaluated here, from graph neural network jet tagging to explicit decay-mode assignment, apply equally to Z decays to tau pairs and other processes, and the authors note that combining the two approaches could trade a modest loss in efficiency for gains in purity in future analyses. As Europe weighs its post-LHC future, projections of this kind translate the abstract promise of a hundred-kilometer ring into concrete numbers: a machine that could measure the Higgs boson’s whisper to the lightest of its charged constituents with a precision that today’s colliders cannot approach, turning one of particle physics’s most elusive decay channels into one of its most incisive probes of new physics.

Subject of Research: Projected precision of Higgs boson decay to tau lepton cross-section measurements at the proposed FCC-ee electron-positron collider

Article Title: Projections of H(\rightarrow \tau \tau ) cross-section at FCC-ee

Article References: Giappichini, S., Klute, M., Presilla, M., Zuo, X., & Cepeda, M. (2026). Projections of H$$\rightarrow \tau \tau $$ cross-section at FCC-ee. The European Physical Journal C, 86(8), Article 1016. https://doi.org/10.1140/epjc/s10052-026-16238-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16238-2

Keywords: FCC-ee, Higgs boson, tau leptons, CERN, particle physics, cross-section measurement, Yukawa coupling, vector boson fusion, ParticleNet, machine learning, European Strategy for Particle Physics, detector simulation