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Measurement of the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays at Belle II

T0 review · 0 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read Belle II measures the D+ -> pi+ pi0 CP asymmetry at (-1.8 ± 0.9) percent.

desk verdict A careful, incremental charm CP measurement that does what it claims; the only real caveat is a 2.7-sigma tension with the older Belle result that the paper waves away a bit too quickly. read the letter →

arxiv 2506.07879 v2 pith:EX4CPTLH submitted 2025-06-09 hep-ex

Belle II Collaboration: I. Adachi , L. Aggarwal , H. Ahmed , H. Aihara , N. Akopov , S. Alghamdi , M. Alhakami , A. Aloisio
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K. Amos M. Angelsmark N. Anh Ky C. Antonioli D. M. Asner H. Atmacan V. Aushev M. Aversano R. Ayad V. Babu H. Bae N. K. Baghel P. Bambade Sw. Banerjee S. Bansal M. Barrett M. Bartl J. Baudot A. Baur A. Beaubien F. Becherer J. Becker J. V. Bennett F. U. Bernlochner V. Bertacchi M. Bertemes E. Bertholet M. Bessner S. Bettarini V. Bhardwaj B. Bhuyan F. Bianchi D. Biswas A. Bobrov D. Bodrov A. Bondar G. Bonvicini J. Borah A. Boschetti A. Bozek M. Bračko P. Branchini R. A. Briere T. E. Browder A. Budano S. Bussino Q. Campagna M. Campajola L. Cao G. Casarosa C. Cecchi M.-C. Chang P. Cheema L. Chen B. G. Cheon K. Chilikin J. Chin K. Chirapatpimol H.-E. Cho K. Cho S.-J. Cho S.-K. Choi S. Choudhury I. Consigny L. Corona J. X. Cui S. Das E. De La Cruz-Burelo S. A. De La Motte G. De Pietro R. de Sangro M. Destefanis A. Di Canto Z. Doležal I. Domínguez Jiménez T. V. Dong X. Dong M. Dorigo K. Dugic G. Dujany P. Ecker D. Epifanov J. Eppelt R. Farkas P. Feichtinger T. Ferber T. Fillinger C. Finck G. Finocchiaro F. Forti A. Frey B. G. Fulsom A. Gabrielli A. Gale E. Ganiev M. Garcia-Hernandez R. Garg G. Gaudino V. Gaur V. Gautam A. Gaz A. Gellrich D. Ghosh H. Ghumaryan G. Giakoustidis R. Giordano A. Giri P. Gironella Gironell B. Gobbo R. Godang O. Gogota P. Goldenzweig W. Gradl E. Graziani D. Greenwald Y. Guan K. Gudkova I. Haide H. Hayashii S. Hazra C. Hearty M. T. Hedges A. Heidelbach G. Heine I. Heredia de la Cruz M. Hernández Villanueva T. Higuchi M. Hoek M. Hohmann R. Hoppe P. Horak C.-L. Hsu T. Humair T. Iijima K. Inami G. Inguglia N. Ipsita A. Ishikawa R. Itoh M. Iwasaki D. Jacobi W. W. Jacobs D. E. Jaffe E.-J. Jang Q. P. Ji S. Jia Y. Jin A. Johnson K. K. Joo J. Kandra K. H. Kang G. Karyan T. Kawasaki F. Keil C. Ketter C. Kiesling C.-H. Kim D. Y. Kim J.-Y. Kim K.-H. Kim Y.-K. Kim H. Kindo K. Kinoshita P. Kodyš T. Koga S. Kohani K. Kojima A. Korobov S. Korpar E. Kovalenko R. Kowalewski P. Križan P. Krokovny Y. Kulii R. Kumar K. Kumara T. Kunigo A. Kuzmin Y.-J. Kwon K. Lalwani T. Lam L. Lanceri J. S. Lange T. S. Lau M. Laurenza R. Leboucher F. R. Le Diberder M. J. Lee C. Lemettais P. Leo P. M. Lewis C. Li H.-J. Li L. K. Li S. X. Li W. Z. Li Y. Li Y. B. Li Y. P. Liao J. Libby J. Lin S. Lin M. H. Liu Q. Y. Liu Y. Liu Z. Liu D. Liventsev S. Longo A. Lozar T. Lueck C. Lyu Y. Ma M. Maggiora S. P. Maharana R. Maiti G. Mancinelli R. Manfredi E. Manoni M. Mantovano D. Marcantonio S. Marcello C. Marinas C. Martellini A. Martens T. Martinov L. Massaccesi M. Masuda D. Matvienko S. K. Maurya M. Maushart J. A. McKenna Z. Mediankin Gruberová R. Mehta F. Meier D. Meleshko M. Merola C. Miller M. Mirra S. Mitra K. Miyabayashi G. B. Mohanty S. Mondal S. Moneta A. L. Moreira de Carvalho H.-G. Moser R. Mussa I. Nakamura M. Nakao Y. Nakazawa M. Naruki Z. Natkaniec A. Natochii M. Nayak M. Neu S. Nishida R. Okubo H. Ono Y. Onuki E. R. Oxford G. Pakhlova S. Pardi K. Parham H. Park J. Park S.-H. Park B. Paschen A. Passeri S. Patra S. Paul T. K. Pedlar I. Peruzzi R. Pestotnik L. E. Piilonen P. L. M. Podesta-Lerma T. Podobnik A. Prakash C. Praz S. Prell E. Prencipe M. T. Prim S. Privalov H. Purwar P. Rados G. Raeuber S. Raiz V. Raj K. Ravindran J. U. Rehman M. Reif S. Reiter M. Remnev L. Reuter D. Ricalde Herrmann I. Ripp-Baudot G. Rizzo S. H. Robertson J. M. Roney A. Rostomyan N. Rout D. A. Sanders S. Sandilya L. Santelj C. Santos V. Savinov B. Scavino C. Schmitt M. Schnepf K. Schoenning C. Schwanda A. J. Schwartz Y. Seino A. Selce K. Senyo J. Serrano M. E. Sevior C. Sfienti W. Shan G. Sharma X. D. Shi T. Shillington T. Shimasaki J.-G. Shiu D. Shtol B. Shwartz A. Sibidanov F. Simon J. Skorupa R. J. Sobie M. Sobotzik A. Soffer A. Sokolov E. Solovieva S. Spataro B. Spruck M. Starič P. Stavroulakis S. Stefkova L. Stoetzer R. Stroili M. Sumihama H. Svidras M. Takizawa S. S. Tang K. Tanida F. Tenchini F. Testa O. Tittel R. Tiwary D. Tonelli E. Torassa K. Trabelsi F. F. Trantou I. Tsaklidis M. Uchida I. Ueda T. Uglov K. Unger Y. Unno K. Uno S. Uno P. Urquijo Y. Ushiroda S. E. Vahsen R. van Tonder K. E. Varvell M. Veronesi V. S. Vismaya L. Vitale R. Volpe M. Wakai S. Wallner M.-Z. Wang X. L. Wang A. Warburton S. Watanuki C. Wessel E. Won B. D. Yabsley S. Yamada W. Yan S. B. Yang J. Yelton K. Yi J. H. Yin K. Yoshihara J. Yuan Y. Yusa L. Zani F. Zeng M. Zeyrek B. Zhang V. Zhilich J. S. Zhou Q. D. Zhou L. Zhu R. Žlebčík
This is my paper · ORCID
classification hep-ex
keywords CPviolationcharmphysicsDmesondecaysBelleIIasymmetrychargemeasurementneutralpionreconstructione+e-collisions
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper reports a measurement of the charge-parity asymmetry in $D^+ \to \pi^+\pi^0$ decays using $428\,\mathrm{fb}^{-1}$ of $e^+e^- \to c\bar{c}$ data collected by the Belle II experiment. The result is $A_{CP}(D^+ \to \pi^+\pi^0) = (-1.8 \pm 0.9 \pm 0.1)\%$, where the first uncertainty is statistical and the second systematic, making it the most precise determination of this quantity to date. The value agrees with the Standard Model expectation of approximate CP symmetry and with earlier CLEO, Belle, and LHCb measurements. The measurement matters because this decay is a clean new-physics probe: its $\pi^+\pi^0$ final state has isospin $I=2$ and is reached through a $\Delta I = 3/2$ transition, so any significant asymmetry would unambiguously signal physics beyond the Standard Model. The result is consistent with zero within about two standard deviations.

What carries the argument

The central mechanism is the subtraction identity $$A_{CP}(D^+ \to \pi^+\$pi^{0}$) = A_{\rm raw}^{\pi^+\$pi^{0}$} - A_{\rm raw}^{\pi^+ $K_S^{0}$} + A_{$K^{0}$},$$ where $A_{\rm raw}$ is the raw yield asymmetry from a mass fit and $A_{K^0}$ corrects for CP violation and detection effects in the neutral-kaon system. Since the $D^+ \to \pi^+ K_S^0$ control mode shares the same $D$-meson production asymmetry and the same charged-pion detection asymmetry as the signal, the subtraction cancels those instrumental asymmetries to first order. A kinematic weighting of the control sample reduces residual differences between the signal and control kinematic distributions, and the assigned systematic uncertainties of 0.096% and 0.053% cover what remains. The analysis is performed separately in tagged and null-tag categories and the results are then combined.

What would settle it

An independent analysis of the same $428\,\mathrm{fb}^{-1}$ data that removes the production and detection asymmetries with a different control mode, or with a differently modeled $A_{K^0}$ correction, would settle the point: if its central value shifted by more than the quoted $0.1\%$ systematic uncertainty, the cancellation assumption behind the subtraction would be incomplete.

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Extended reading notes

Core claim

Belle II reconstructs $D^+ \to \pi^+\pi^0$ decays, with the $\pi^0$ reconstructed in the $\gamma\gamma$ channel, and splits the sample into 'tagged' decays that come from a reconstructed $D^{*+} \to D^+\pi^0$ and 'null-tag' decays without such a tag. Fits to the $\pi^+\pi^0$ invariant-mass distribution in each charge give raw asymmetries of $(-2.9 \pm 1.8)\%$ and $(-0.4 \pm 1.0)\%$ for the two samples. The raw asymmetries of a $D^+ \to \pi^+ K_S^0$ control sample, $(0.54 \pm 0.53)\%$ and $(0.33 \pm 0.30)\%$, are subtracted to remove the production asymmetry of $D$ mesons and the detection asymmetry of charged pions, and a correction $A_{K^0} = (-0.422 \pm 0.007)\%$ (tagged) and $(-0.418 \pm 0.007)\%$ (null tag) accounts for neutral-kaon effects. The resulting asymmetries, $(-3.9 \pm 1.8)\%$ and $(-1.1 \pm 1.0)\%$, are combined to obtain $A_{CP}(D^+ \to \pi^+\pi^0) = (-1.8 \pm 0.9 \pm 0.1)\%$, which the paper identifies as the most precise measurement to date and as consistent with CP symmetry.

Load-bearing premise

The measurement assumes that after kinematic weighting, the control decay $D^+ \to \pi^+ K_S^0$ has exactly the same $D$-meson production asymmetry and charged-pion detection asymmetry as the signal decay $D^+ \to \pi^+\pi^0$; if that cancellation is incomplete beyond the assigned systematic uncertainties, the central value would shift.

Editorial extensions

If this is right

  • The result supersedes the previous most precise measurement of this asymmetry and becomes the reference value for $A_{CP}(D^+ \to \pi^+\pi^0)$.
  • The consistency with zero means no direct CP violation is observed in this mode at the current sensitivity, matching the Standard Model expectation.
  • The measurement sharpens the constraint on beyond-standard-model contributions: a future observation of a percent-level asymmetry in this decay would be a clean new-physics signal.
  • The tagged and null-tag samples give consistent values, supporting the reliability of the subtraction procedure.
  • The 30% precision improvement over Belle, on roughly half the luminosity, shows that better neutral-pion reconstruction and selection purity can be as important as integrated luminosity.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • As an editorial extension: with the full Belle II dataset, the same analysis should reach a total uncertainty near 0.3%, making this mode a sharper test of the Standard Model; even if the present central value persists, a deviation could become significant.
  • The control-sample cancellation strategy transfers naturally to other singly Cabibbo-suppressed charged charm decays with neutral final-state particles, where production and detection asymmetries must be removed in the same way.
  • A valuable cross-check would be an independent measurement of this asymmetry using a control mode that does not contain a $K_S^0$, which would test the size and modeling of the $A_{K^0}$ correction.
  • Combining this result with the existing LHCb measurement in a future average would reduce the total uncertainty and could reveal whether the current two-standard-deviation offset from zero is a statistical fluctuation or a trend.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 5 minor

Summary. The paper reports a measurement of the time-integrated CP asymmetry in D+ -> pi+ pi0 using 428 fb^-1 of Belle II data. Signal and control D+ -> pi+ K_S candidates are reconstructed in two mutually exclusive categories, tagged and null-tag, and the raw charge asymmetries are extracted from unbinned maximum-likelihood fits to the corresponding invariant-mass distributions. The production and detection asymmetries are removed by subtracting the control-mode raw asymmetry and adding the K0-K0bar mixing and regeneration correction A_K0, following Eq. (5). The tagged and null-tag results, (-3.9 +/- 1.8 +/- 0.2)% and (-1.1 +/- 1.0 +/- 0.1)%, are combined to give A_CP(D+ -> pi+ pi0) = (-1.8 +/- 0.9 +/- 0.1)%. The analysis is blinded and validated with pseudoexperiments and full simulation.

Significance. If correct, this is the most precise A_CP(D+ -> pi+ pi0) measurement to date, with a total uncertainty of about 0.9%, and it improves on both the LHCb and Belle results. The result is consistent with no direct CP violation at the 2-sigma level and with Standard Model expectations. The main assumption, that the production and charged-pion detection asymmetries cancel between signal and control modes after kinematic weighting, is addressed explicitly: the weighting is applied and the residual shifts are assigned as systematic uncertainties. The paper is strengthened by the blinding procedure, pseudoexperiment and full-simulation validation, and consistency checks across data-taking conditions and kinematic variables.

minor comments (5)
  1. [Abstract; Conclusion] The statement that the result agrees with previous measurements is potentially misleading because the central value differs from the Belle result (2.3 +/- 1.2 +/- 0.2)% by about 2.7 standard deviations; please quantify the compatibility with each previous measurement, for example with a chi2 or p-value, and rephrase the agreement claim accordingly.
  2. [Systematic uncertainties, kinematic weighting] The description of the kinematic weighting is brief; please specify the exact variables used for the weighting beyond the c.m.s. D+ polar angle and charged-pion kinematics, and state clearly whether the weighting is applied to the control sample only, since this is the key cancellation step in Eq. (5).
  3. [Signal selection] The D_s+ contamination of about 0.4% is stated to be neglected; please provide a quantitative bound on the resulting shift in A_CP, for example using the expected D_s+ CP asymmetry, rather than only stating that the effect is small.
  4. [Fit-model systematic uncertainties] For reproducibility, it would be helpful to list the individual shifts for each alternative fit model rather than only the quadrature sums, namely 0.119%/0.044% for the D+ -> pi+ pi0 fit and 0.122%/0.048% for the D+ -> pi+ K_S fit.
  5. [Combination of tagged and null-tag results] The combination assumes uncorrelated fit-model systematic uncertainties between the tagged and null-tag samples; since the same alternative models are used, a short justification or a check with a fully correlated treatment would be useful.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the CP asymmetry is extracted by subtracting raw asymmetries and an externally computed kaon correction; no target quantity enters the inputs.

full rationale

The measurement chain is self-contained. The raw asymmetry A^{pi+pi0} (Eq. 2) is a directly fitted yield asymmetry. Equation (5) subtracts the control-mode raw asymmetry A^{pi+KS}, so the production and detection asymmetries cancel by construction, not by fitting the target. The control mode is Cabibbo-favored and theoretically expected to have no direct CP asymmetry, which is an external physics assumption rather than an input derived from the result. The only nontrivial external correction, A_K0, is computed from the well-known K0-K0bar mixing and CP-violation parameters, interaction cross sections, and detector material following Ref. [63]; this is an independent LHCb result, not a self-citation, and it does not involve A_CP(D+ -> pi+ pi0). Kinematic differences between signal and control modes are addressed by explicit weighting, with the resulting small shifts assigned as systematic uncertainties. The analysis is blinded, validated with pseudoexperiments and full simulation, and no step in Eqs. (1)-(6) defines the target in terms of itself or fits the target from a closely related quantity.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

No ad hoc free parameters or invented entities. The result is a measured asymmetry; the floated fit parameters are standard nuisance parameters, and the correction factors come from established physics or external references.

assumptions (4)
  • domain assumption The raw asymmetry is approximated as a linear sum of the CP asymmetry, the production asymmetry A_P^D, and the charged-pion detection asymmetry A_epsilon^pi+ (Eq. 3).
    This small-asymmetry approximation underpins the extraction; it is standard in charm asymmetry measurements and validated with pseudoexperiments.
  • domain assumption The Cabibbo-favored control mode D+ -> pi+ Ks has no direct CP asymmetry, so its raw asymmetry is A_P^D + A_epsilon^pi+ + A_K0 (Eq. 4).
    This is the theoretical basis for the subtraction; direct CP violation is negligible for a single-weak-phase Cabibbo-favored decay.
  • domain assumption The neutral-kaon asymmetry A_K0 is computed following Ref. [63] using known K0-K0bar mixing parameters, CP-violation parameters, cross sections, and detector material density.
    This external correction shifts the result; its uncertainty (0.007%) is included in the systematic budget.
  • domain assumption Simulation reproduces the mass shapes and selection responses well enough that some signal PDF parameters can be fixed from simulation.
    Johnson SU parameters are fixed from MC; floated Gaussian parameters absorb data-MC differences, and alternative fit models are tested as systematics.

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Cite this review

Pith. "Pith review of Measurement of the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays at Belle II." pith.science (2026). https://pith.science/paper/EX4CPTLH

@misc{pith2026250607879,
  author       = {Pith},
  title        = {Pith review of: Measurement of the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays at Belle II},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EX4CPTLH}},
  note         = {Machine review of arXiv:2506.07879}
}
abstract

We measure the CP asymmetry in $D^+ \to \pi^+ \pi^0$ decays reconstructed in $e^+ e^-$ collisions at the Belle II experiment using a data set corresponding to an integrated luminosity of 428 fb$^{-1}$. A control sample of $D^+ \to \pi^+ K_{S}$ decays is used to correct for detection and production asymmetries. The result, $A_{CP}(D^+ \to \pi^+\pi^0) =(-1.8 \pm 0.9 \pm 0.1)\%$, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of CP symmetry from the standard model, and with results of previous measurements.

Figures

Figures reproduced from arXiv: 2506.07879 by the authors.

Figure 1
Figure 1. Distributions of m(π +π 0 ) for (left) tagged and (right) null-tag D + → π +π 0 candidates, with fit projections overlaid. The bottom panels show the asymmetry as a function of mass, with fit projections overlaid. distributions of the charged pion must agree to cancel the charged-pion detection asymmetry. The control samples are weighted to correct for observed small differences in these kinematic distributions. The… view at source ↗
Figure 2
Figure 2. Distributions of m(π +K0 S ) for (left) tagged and (right) null-tag D + → π +K0 S candidates, with fit projections overlaid. The bottom panels show the asymmetry as a function of mass, with fit projections overlaid. Starting Grant No. 947006 “InterLeptons”; Natural Sciences and Engineering Research Council of Canada, Compute Canada and CANARIE; National Key R&D Program of China under Contract No. 2024YFA1610503, and… view at source ↗

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Reviewed August 7, 2026 · model on record in the stance chip above.