Higgs CP studies and other Higgs properties at ATLAS and CMS

This paper presents recent measurements of Higgs boson properties, including its mass, width, and CP nature in various couplings, derived from proton-proton collision data at 13 TeV and 13.6 TeV collected by the ATLAS and CMS experiments.

The Gist

Imagine the universe is a giant, complex machine, and the Higgs boson is a tiny, invisible gear that helps everything else get its weight. Scientists at the Large Hadron Collider (LHC) are like mechanics trying to understand exactly how this gear works. They use two massive, high-tech cameras (called ATLAS and CMS) to take snapshots of particle collisions happening at nearly the speed of light.

This paper is a report card from these two cameras, summarizing what they learned about the Higgs gear using data from recent "runs" of the machine. Here is what they found, broken down into simple ideas:

1. Weighing the Gear (Mass)

The first thing the scientists wanted to know was: How heavy is this gear?

• The Challenge: The Higgs boson is very unstable; it breaks apart almost instantly. To find its weight, the scientists looked at the pieces it leaves behind, specifically when it turns into two flashes of light (photons).
• The Result: It's like weighing a ghost by measuring the shadow it casts. By using incredibly precise calibrations (like using a known weight to check a scale), they measured the Higgs mass to be 125.14 GeV.
• The Takeaway: When they combined this new measurement with older data, the result was 125.07 GeV. This matches perfectly with what the "Standard Model" (the rulebook of physics) predicts. The ATLAS and CMS cameras agree with each other, confirming the weight is correct.

2. How Fast Does It Disappear? (Width)

In physics, "width" isn't about how wide an object is; it's a measure of how quickly a particle decays (falls apart).

• The Challenge: The rulebook says the Higgs should disappear in a tiny fraction of a second (about 4.1 MeV wide). But the cameras are a bit "blurry" (their resolution is about 1 GeV), making it hard to see such a tiny width directly.
• The Trick: The scientists looked at "off-shell" events. Imagine a car driving slightly faster or slower than the speed limit. By comparing Higgs bosons that behave "normally" (on-shell) with those that act a bit "strange" (off-shell), they could estimate the width.
• The Result: They found the width is roughly 3.9 MeV, which fits the rulebook. They also set a "speed limit" saying the width is definitely less than 92 MeV.
• The Takeaway: The Higgs is disappearing at the exact rate the rulebook predicted. No hidden, extra-heavy particles are messing with the timing.

3. Is the Gear Symmetric? (CP Properties)

This is the most detective-like part of the paper. Scientists are looking for a specific type of symmetry called CP (Charge-Parity).

• The Analogy: Imagine looking at the Higgs boson in a mirror. If the mirror image looks exactly the same as the real thing, it's "CP-even." If the mirror image is different (like a left hand looking like a right hand), it's "CP-odd" or "CP-violating."
• Why it matters: The rulebook says the Higgs should be perfectly symmetric (CP-even). But the universe has a mystery: there is more matter than antimatter. To explain this, physicists need to find a "broken mirror" somewhere.
• The Investigation:
  • ATLAS looked at how the Higgs interacts with force-carrying particles (like W and Z bosons) in many different ways. They checked if the interactions looked different in a mirror.
  • CMS looked at how the Higgs interacts with tau leptons (heavy cousins of electrons). They analyzed the angles at which the tau particles fly apart, like checking the spin of a top.
• The Result:
  • ATLAS found no evidence of a broken mirror. The interactions look symmetric.
  • CMS initially saw a tiny hint of asymmetry in their new data (Run 3), but when they combined it with older data (Run 2), the result smoothed out. The final measurement shows the Higgs is 99% likely to be symmetric, just as the rulebook says.
  • The "mirror" is still intact. No new sources of asymmetry were found.

The Bottom Line

The scientists at ATLAS and CMS have taken a very close look at the Higgs boson's weight, its lifespan, and its symmetry.

• The Weight: Confirmed.
• The Lifespan: Confirmed.
• The Symmetry: Confirmed.

Everything they found so far fits perfectly with the current "Standard Model" of physics. While they haven't found the "new physics" (like the broken mirror needed to explain the universe's matter imbalance) yet, they have tightened the rules significantly. They are essentially saying, "The Higgs is behaving exactly as we thought it would, and we are now watching it even more closely with better tools."

Technical Summary

Technical Summary: Higgs CP Studies and Other Higgs Properties at ATLAS and CMS

Problem and Motivation
The Standard Model (SM) does not predict the mass of the Higgs boson ($m_H$), necessitating precise experimental determination to validate electroweak vacuum stability and test the consistency of the electroweak sector. While the SM predicts a Higgs width ($\Gamma_H$) of approximately 4.1 MeV, direct measurement is hindered by detector resolutions on the order of 1 GeV; deviations from this value would signal Beyond the Standard Model (BSM) physics. Furthermore, while the SM Higgs is predicted to be CP-even, the observed matter-antimatter asymmetry of the universe requires additional sources of Charge-Parity (CP) violation, which could manifest in Higgs couplings within BSM scenarios. This paper addresses these open questions by presenting recent measurements of Higgs properties using proton-proton collision data from the LHC.

Methodology and Data Sets
The analysis utilizes data recorded by the ATLAS and CMS detectors during LHC Run 2 ($\sqrt{s} = 13$ TeV, 138–140 fb$^{-1}$) and Run 3 ($\sqrt{s} = 13.6$ TeV, 62–164 fb$^{-1}$). The methodologies vary by observable:

• Mass Measurement (CMS): The $H \to \gamma\gamma$ channel is used despite its low branching ratio (0.23%) due to its clean final state. Photon energy scales are calibrated using $Z \to ee$ and $Z \to \mu\mu\gamma$ decays. Events are categorized via a boosted decision tree, followed by a fit to the diphoton invariant mass.
• Width Measurement (CMS): Two approaches are employed. First, the ratio of off-shell to on-shell $H \to WW^* \to e\nu\mu\nu$ decays is analyzed, separating production mechanisms using a deep neural network. Second, an on-shell $H \to \gamma\gamma$ analysis exploits the interference between the signal and non-resonant $gg \to \gamma\gamma$ background to constrain the width.
• CP Properties (ATLAS & CMS):
  • Vector Boson Couplings: ATLAS performs a combined measurement across $H \to \gamma\gamma$, $H \to \tau\tau$, $H \to ZZ^*$, $H \to WW^*$, and WH/$H \to b\bar{b}$ channels. Observables asymmetric around zero are used to constrain CP-odd components within the Standard Model Effective Field Theory (SMEFT) formalism (Warsaw basis). Fits are performed for linear and quadratic terms of dimension-six operators ($c_{H\tilde{W}}$, $c_{H\tilde{B}}$, $c_{H\tilde{WB}}$).
  • Tau Yukawa Coupling (CMS): Using Run 3 data, the CP structure of the Higgs-tau coupling is probed via angular correlations in $H \to \tau\tau$ decays. The acoplanarity angle ($\phi_{CP}$) between tau decay planes in the Higgs rest frame is used to constrain the effective CP mixing angle ($\alpha_{H\tau\tau}$).

Key Contributions and Results

• Higgs Mass: The latest CMS measurement yields $m_H = 125.14 \pm 0.11 \text{ (syst.)} \pm 0.10 \text{ (stat.)}$ GeV. Combined with Run 1 data, this results in $m_H = 125.07 \pm 0.13$ GeV. This value is consistent with ATLAS measurements and previous CMS results in the $H \to ZZ^* \to 4\ell$ channel.
• Higgs Width:
  • From off-shell/on-shell $H \to WW^*$ ratios, CMS derives $\Gamma_H = 3.9^{+2.7}_{-2.2}$ MeV, consistent with the SM.
  • From on-shell $H \to \gamma\gamma$ interference, an upper limit of $\Gamma_H < 92$ MeV (95% CL) is set, representing a significant improvement over previous on-shell constraints, though still weaker than off-shell comparisons.
• CP-Violating Vector Boson Couplings:
  • ATLAS (Run 2): A combined analysis constrains the CP-violating parameter $c_{H\tilde{W}}$. The combined 95% CL interval is $[-0.14, 0.49]$, improving limits by over 40% compared to the most sensitive individual channel ($H \to \tau\tau$). No evidence of CP violation is observed.
  • ATLAS (Run 3): Using optimal-observable techniques, the Run 3 result yields $c_{H\tilde{W}} = 0.24$ with a 95% CL of $[-0.35, 0.88]$. The combination of Run 2 and Run 3 results gives $c_{H\tilde{W}} = 0.25$ ($[-0.23, 0.75]$).
  • CMS (Run 2/3): A simultaneous measurement of eight Higgs-vector boson couplings in $H \to ZZ^*$ finds a fractional CP-violating contribution $f_{a3} = 0.16$ ($95\% \text{ CL } [0.01, 0.50]$). This deviation is attributed to data asymmetry in a kinematic observable and remains statistically consistent with the SM given the large parameter space.
• CP Structure of Tau Yukawa Coupling:
  • CMS (Run 3): The mixing angle is measured as \alpha_{H\tau\tau} = 36^{+33}_{-30}^\circ, showing a slight preference for CP violation compared to the SM expectation of $0^\circ$.
  • Combined (Run 2 + Run 3): The combined result yields $\alpha_{H\tau\tau} = 7 \pm 16^\circ$ (expected $0 \pm 14^\circ$). The data are more compatible with the CP-even distribution. This represents the most precise measurement of $\alpha_{H\tau\tau}$ by CMS to date.

Significance and Claims
The paper claims that these measurements provide a comprehensive overview of Higgs boson properties, confirming the consistency of the Higgs mass and width with SM predictions. The authors emphasize that no significant deviations from SM expectations are observed across the mass, width, and CP property measurements. The significance of the work lies in the improved precision of these constraints, particularly the combination of Run 2 and Run 3 data which enhances sensitivity to CP-violating effects. The authors note that many results remain limited by statistical uncertainties and that further improvements are anticipated with the accumulation of additional Run 3 data.