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Measurements of Higgs boson production cross section in the four-lepton final state in proton-proton collisions at $\sqrt{s}$ = 13.6 TeV

Using 2022 proton-proton collision data at $\sqrt{s}$ = 13.6 TeV corresponding to an integrated luminosity of 34.7 fb $^{-1}$ , the CMS Collaboration measured the inclusive and differential Higgs boson production cross sections in the four-lepton final state, finding results consistent with Standard Model predictions.

Original authors: CMS Collaboration

Published 2026-07-28

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CC BY 4.0

Original authors: CMS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic Lego set. For decades, scientists have been trying to figure out exactly how the pieces snap together to build everything we see, from the tiniest atoms to the biggest stars. They have a "rulebook" for this, called the Standard Model, which predicts how particles should behave. But for a long time, there was one missing piece in the rulebook: a special particle called the Higgs boson. Think of the Higgs boson as the "glue" that gives other particles their weight. Without it, the universe would be a chaotic mess of massless particles zipping around at the speed of light, unable to form atoms, planets, or people.

Scientists found this "glue" particle back in 2012, but finding it was just the beginning. Now, they need to check if the glue works exactly the way the rulebook says it should. They do this by smashing protons together at incredible speeds, creating a tiny, super-hot explosion that briefly recreates the conditions of the early universe. When these protons crash, they sometimes produce a Higgs boson, which instantly falls apart into other particles. By studying how often this happens and how the pieces fly apart, scientists can test if our understanding of the universe is correct or if there's a new, hidden rule waiting to be discovered.

This paper is a fresh report from the CMS experiment at CERN, the massive particle collider in Switzerland. The team took a huge snapshot of data collected in 2022, when the collider was running at a record-breaking energy level of 13.6 TeV (that's a lot of energy packed into a tiny space). They were looking for a very specific, rare event: a Higgs boson that decays into four "leptons" (which are like electrons and muons, the lightweight cousins of heavy particles). This is like looking for a specific four-leaf clover in a field of three-leaf ones; it's rare, but when you find it, it's very clean and easy to spot.

The researchers measured exactly how often this four-lepton event happened. They found that for every 34.7 "inverse femtobarns" of data (a unit that basically means "how much data we collected"), the probability of this event occurring, known as the production cross-section, was 2.89 femtobarns, with a small range of uncertainty (between 2.40 and 3.42). When they compared this to what the Standard Model predicted, the theory said it should be 3.09 femtobarns (with a range of 2.85 to 3.36).

The result is a perfect match. The number they measured sits right inside the range the theory predicted. This means the "glue" is behaving exactly as the rulebook says it should, even at these super-high energies. The scientists also looked at how the Higgs boson moves—how fast it goes sideways and how far it travels before stopping—and found that these details also line up perfectly with the predictions.

So, what does this mean? It doesn't mean we found a new particle or a new law of physics. Instead, it's a victory lap for the existing rulebook. It confirms that our current understanding of how the universe works is incredibly robust. The Higgs boson is doing its job, and the Standard Model is holding up strong under the most extreme conditions we can create. While this might sound like "nothing new," in the world of particle physics, proving that the universe is exactly as we thought it was is a huge deal. It tells us that if there are any secrets left to find, they are hiding in places we haven't looked yet, not in the basic behavior of the Higgs boson itself.

Technical Summary: Measurements of the Higgs boson production cross section in the four-lepton final state in proton-proton collisions at $\sqrt{s} = 13.6$ TeV

Problem and Context
The discovery of the Higgs boson (H) in 2012 by the ATLAS and CMS Collaborations completed the Standard Model (SM) of particle physics. Since then, characterizing the Higgs boson's properties—such as its mass, spin, parity, width, and production cross sections—has been a primary objective to validate SM predictions. The four-lepton decay channel ( $H \to ZZ \to 4\ell$ , where $\ell = e, \mu$ ) is a "golden channel" for these studies due to its clean signature, high signal-to-background ratio, and the ability to fully reconstruct final-state kinematics, despite its low branching fraction ( $1.3 \times 10^{-4}$ ).

While previous measurements have utilized data from Run 1 ( $\sqrt{s} = 7, 8$ TeV) and Run 2 ( $\sqrt{s} = 13$ TeV), this paper presents the first measurements of the Higgs boson production cross section in the $H \to ZZ \to 4\ell$ channel using data collected at the higher center-of-mass energy of $\sqrt{s} = 13.6$ TeV. The analysis aims to measure inclusive and differential fiducial cross sections to test the SM predictions at this new energy scale.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at the CERN LHC in 2022, corresponding to an integrated luminosity of $34.7 \text{ fb}^{-1}$ .

Event Selection: Candidate events are required to contain at least four well-identified, isolated leptons (electrons or muons) consistent with $H \to ZZ \to 4\ell$ production. The selection targets three mutually exclusive channels: $4e$ , $4\mu$ , and $2e2\mu$ . Leptons must satisfy specific transverse momentum ( $p_T$ ) and pseudorapidity ( $\eta$ ) requirements (e.g., leading lepton $p_T > 20$ GeV, subleading $p_T > 10$ GeV). Two same-flavor, opposite-sign lepton pairs are reconstructed as $Z$ boson candidates ( $Z_1$ and $Z_2$ ) with invariant masses in the ranges $40 < m_{Z1} < 120$ GeV and $12 < m_{Z2} < 120$ GeV. The signal region is defined by the four-lepton invariant mass $105 < m_{4\ell} < 160$ GeV.
Background Estimation:
- Irreducible Background: The dominant background arises from $ZZ$ production via quark-antiquark annihilation ( $q\bar{q} \to ZZ$ ) and gluon fusion ( $gg \to ZZ$ ). These are estimated using simulated events normalized to theoretical calculations at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO).
- Reducible Background: Contributions from processes where jets or hadrons are misidentified as leptons (denoted as "ZX") are estimated using data-driven control samples based on lepton misidentification rates.
Signal Modeling and Statistical Analysis: The signal is modeled using a double-sided Crystal Ball (DCB) function centered at the Higgs mass ( $m_H = 125.38$ GeV). The analysis employs an unbinned maximum likelihood fit to the observed $m_{4\ell}$ distribution. The fiducial cross section is extracted directly from the fit, correcting for detector effects via a likelihood-based unfolding procedure using a response matrix derived from simulation.
Systematic Uncertainties: Major sources of uncertainty include the integrated luminosity (1.4%), lepton reconstruction and identification efficiencies (ranging from 0.8% to 11% depending on the channel), lepton momentum scale and resolution, and theoretical uncertainties in the background modeling (K-factors, PDFs, and renormalization/factorization scales).

Key Contributions and Results
The paper reports the first inclusive and differential fiducial cross section measurements for the $H \to ZZ \to 4\ell$ process at $\sqrt{s} = 13.6$ TeV.

Inclusive Fiducial Cross Section: The measured inclusive fiducial cross section is:
$\sigma_{\text{fid}} = 2.89^{+0.53}_{-0.49} (\text{stat})^{+0.29}_{-0.21} (\text{syst}) \text{ fb}$
This result is in agreement with the Standard Model expectation of $3.09^{+0.27}_{-0.24}$ fb. The dominant systematic uncertainty arises from the electron selection efficiency.
Differential Cross Sections: The analysis also provides differential cross sections as a function of the Higgs boson transverse momentum ( $p_T^H$ ) and the absolute value of its rapidity ( $|y^H|$ ). The distributions show good agreement with SM predictions derived from POWHEG and NNLOPS generators.
Channel Breakdown: The results are presented separately for the $4e$ , $4\mu$ , and $2e2\mu$ final states, with the combined inclusive result providing the most precise measurement.

Significance
The paper claims that these measurements constitute an important test of the Standard Model at the new center-of-mass energy of 13.6 TeV. By measuring the cross sections in a fiducial phase space that closely matches the experimental acceptance, the results minimize model dependence. The consistency of the measured inclusive and differential cross sections with SM expectations reinforces the validity of the Standard Model description of Higgs boson production and decay in the four-lepton channel at higher energies. The results are consistent with previous measurements from Run 1 and Run 2, extending the precision of Higgs property characterization into the Run 3 era.

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