AI summary · arXiv: 2611.04596v1 · hep-ex
A New Precise Measurement of the $e^+e^-\toπ^+π^-(γ)$ Cross Section with the Initial-State Radiation Method at BaBar
The essentials
BABAR measures the two-pion cross section with 460 fb⁻¹ of ISR data, finding a two-pion HVP contribution of 525.9 ± 3.5(stat) ± 3.8(syst) × 10⁻¹⁰ from 0.3 to 1.4 GeV.
Question
Can a new, complementary BABAR measurement of e⁺e⁻ → π⁺π⁻(γ) improve the experimental input to the two-pion hadronic vacuum polarization contribution to the muon magnetic anomaly? The analysis also assesses agreement with the earlier BABAR result and other measurements.
Method
The analysis uses 460 fb⁻¹ of BABAR data and measures the cross section from threshold to 1.4 GeV with initial-state radiation. It statistically separates pion and muon events using their angular distributions, unfolds the mass spectra, and obtains the pion cross section from the π⁺π⁻/μ⁺μ⁻ ratio, which reduces common systematic uncertainties. The fitted sample contains 7,617,066 events, including an estimated 135,249 background events; the muon spectrum is validated against QED with Rμμ = 0.9920 and total precision 0.69%.
Findings
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Result
The measured two-pion HVP contribution from 0.3 to 1.4 GeV is 525.9 ± 3.5(stat) ± 3.8(syst) × 10⁻¹⁰.
See the passage in the paper
The leading-order HVP contribution of the two-pion channel in the energy range of 0.3–1.4$\mathrm{\,Ge\kern-1.00006ptV}$ is $a_{\mu}^{\pi\pi}=525.9\pm 3.5_{\mathrm{stat}}\pm 3.8_{\mathrm{syst}}$
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Result
The new cross section is about 2% higher than the 2009 BABAR result in the ρ-resonance region, with differences typically below two standard deviations in the central region.
See the passage in the paper
The new cross section is about 2% higher than the 2009 BABAR result in the $\rho$-resonance region, with larger fluctuations in the threshold and high-mass regions where the angular separation has lower statistical power and suffers from the overwhelming dominance of the muon contribution.
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Result
The combined 2009 and 2026 BABAR result for the two-pion HVP contribution is 517.8 ± 2.4(stat) ± 3.3(syst) × 10⁻¹⁰.
See the passage in the paper
When combined with the BABAR 2009 measurement in the same energy range, the corresponding result is $517.8(2.4)(3.3)\times 10^{-10}$.
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Result
In the 0.6–0.8832 GeV comparison region, the new result is consistent with most prior results but differs from KLOE and BESIII by −4.2σ and −3.0σ, respectively.
See the passage in the paper
This 2026 result is within two standard deviations of most previous results, including the 2009 BABAR ($-1.4\sigma$) and the most recent CMD-3 ($+0.9\sigma)$ results, but is inconsistent with KLOE ($-4.2\sigma$) and BESIII ($-3.0\sigma$).
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Authors’ interpretation
The measurement provides a complementary BABAR input to the data-driven HVP determination by separating pions and muons statistically rather than relying on particle identification.
See the passage in the paper
This new measurement is a complementary BABAR input to the data-driven HVP determination.
Limitations
- The measurement is not competitive with the 2009 result in the low- and high-energy ρ-tail regions, where the pion signal is overwhelmed by the muon cross section.
See the passage in the paper
However, in the $\rho$ tail regions below 0.5$\mathrm{\,Ge\kern-1.00006ptV}$ and above 1$\mathrm{\,Ge\kern-1.00006ptV}$, where the $\pi\pi$ signal is overwhelmed by the large $\mu\mu$ cross section, the new measurement is not competitive with the 2009 result.
Why it matters
The authors present the result as a complementary and essentially independent BABAR measurement that adds a new input to data-driven HVP evaluations. Its ratio-based approach reduces common uncertainties, while the comparison highlights unresolved differences among measurements of the two-pion contribution.
Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.
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