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Decays of $B$ Mesons to Two Light Pseudoscalars

High Energy Physics - Phenomenology · Submitted 18 Apr 1994 · 28 references

Pith's verdict: A clean, honest SU(3) method paper: the new content is the Eq. (23) triangle and the rate-only γ extraction, and the authors are upfront that the whole thing rests on dropping E, A, PA.

T0 · AI review

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arXiv IDhep-ph/9404283 (version 1)
Pith IDPRSWDZHA, Pith's permanent record of this paper and its review
CitationDavid London, Jonathan L. Rosner, Michael Gronau, Oscar F. Hernandez. "Decays of $B$ Mesons to Two Light Pseudoscalars." arXiv:hep-ph/9404283 (1994). https://pith.science/paper/PRSWDZHA
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Referee report
The paper

The paper claims

The decays of B mesons into two light pseudoscalars obey triangle relations in flavour SU(3), and one of those triangles turns measured decay rates of charged B mesons into a determination of a CP-violating quark-mixing phase.

The reading

The paper asks a practical question: given that CP violation is supposed to come from phases in the quark-mixing matrix, which B-meson decays let you actually read those phases off, and with the fewest and easiest measurements? Its answer is that the charmless two-body decays $B to PP$ (a B meson into two pseudoscalar mesons such as pions and kaons) are tied together by relations that follow from treating the three light quarks as a symmetry triplet. Written as complex numbers, those relations are triangles: three amplitudes whose vector sum closes. The most useful one links the isospin-3/2 part of $B to pi K$ to the single amplitude $B^+ to pi^+ pi^0$, scaled by the small ratio $r_u = V_{us}/V_{ud} approx 0.23$. If the relations hold, a handful of measured rates for charged B decays — all of them self-tagging, so no flavour tagging or time-dependent analysis is needed — fix the weak phase $gamma$, and the fuller set of $pi pi$, $pi K$ and $K bar K$ rates fixes two weak phases together with several differences of strong (final-state) phase shifts.

What carries the argument

The machinery is a graphical decomposition of $B to PP$ amplitudes that is equivalent to an SU(3) reduced-matrix-element expansion: five independent combinations of six diagrams, so that twelve amplitudes per strangeness sector satisfy seven linear relations. The relations come in three kinds — pure isospin triangles within $B to pi pi$ and within $B to pi K$, triangles that mix strange and non-strange final states, and cross-sector relations that use $T'/T = C'/C = E'/E = A'/A = r_u$ for tree-type graphs and $P'/P = PA'/PA = r_t$ for penguins. Neglecting $E$, $A$ and $PA$ translates into two simple statements about reduced matrix elements, ${8_1} = -sqrt{5}{1}/4$ and ${8_3} = {27}/4$, which is what converts the triangles into rate equalities such as $Gamma(B_s to pi^+ K^-) = Gamma(B^0 to pi^+ pi^-)$ and $Gamma(B^0 to pi^- K^+) = Gamma(B_s to K^- K^+)$.

Extended reading notes

Core claim

The central claim is that SU(3) flavour symmetry, organised graphically into a small set of topological amplitudes (tree $T$, colour-suppressed $C$, penguin $P$, exchange $E$, annihilation $A$, penguin annihilation $PA$, and their strangeness-changing primed partners), produces relations among $B to PP$ amplitudes that are simple enough to be used as measurement tools. The load-bearing relation is Eq. (23): $$sqrt{2},A(B^+ to $pi^{0}$ K^+) + A(B^+ to pi^+ $K^{0}$) = r_u, sqrt{2},A(B^+ to pi^+ $pi^{0}$),$$ which says that the combination of $B to pi K$ amplitudes carrying isospin $3/2$ is a pure 27-plet and is therefore proportional, with the known factor $r_u$, to the $B^+ to pi^+ pi^0$ amplitude. Because the $B^+ to pi^+ K^0$ amplitude is dominated by the penguin with weak phase $pi$ while the $pi^+ pi^0$ amplitude carries the tree phase $gamma$, the two triangles formed by this relation and its charge conjugate have a relative orientation that measures $gamma$ — provided the annihilation contribution $A'$ can be dropped, in which case the rates for $B^+ to pi^+ K^0$ and $B^- to pi^- bar K^0$ should be equal. The paper then shows that if the exchange, annihilation and penguin-annihilation graphs are neglected altogether, the surviving relations collapse into ten statements — six rate equalities, three triangles and one quadrangle — from which $gamma$, $alpha$, and the strong-phase differences $delta_P - delta_2$ and $delta_{P'} - delta_{3/2}$ can all be extracted from rates alone.

Load-bearing premise

The extraction of $gamma$, and the ten relations that follow, rest on treating the exchange, annihilation and penguin-annihilation diagrams (and their primed versions) as negligible, a suppression the authors justify by $f_B/m_B$ and helicity arguments but explicitly qualify as valid only if rescattering effects are unimportant.

Editorial extensions

If this is right

  • Four measured rates — $B^+ to pi^0 K^+$, $B^- to pi^0 K^-$, $B^+ to pi^+ K^0$ and $B^+ to pi^+ pi^0$ — suffice to determine the weak phase $gamma$, with no time-dependent measurement and no tagging of the initial B flavour.
  • A CP-violating rate difference between $B^+ to pi^0 K^+$ and $B^- to pi^0 K^-$ requires both two distinct weak subprocesses and a nonzero strong-phase difference between the $I = 1/2$ and $I = 3/2$ channels, so observing no asymmetry does not by itself rule out the mechanism.
  • Rates for the full set of $pi pi$, $pi K$ and $K bar K$ charge states yield two CKM phases and several strong-phase differences, and the overlapping constructions provide independent cross-checks on SU(3) breaking.
  • Several channels are predicted to be strongly suppressed or absent if the annihilation-type graphs are negligible: $B^0 to K^+ K^-$, $B_s to pi^+ pi^-$ and $B_s to pi^0 pi^0$ should all be near zero, making searches at the $10^{-7}$ level a direct test of the assumption.
  • The same triangle logic applied to $B to pi bar D$ decays, where a single weak phase governs all channels, isolates final-state phase differences between $I = 1/2$ and $I = 3/2$ amplitudes; present data already bound the difference below about $35^circ$ at 90% confidence.

Reading between the lines

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

  • The strategy is a template that outlives the specific SU(3) relations: whenever a symmetry forces one combination of amplitudes to be proportional to a single well-measured mode, that mode becomes a calibration leg for a triangle, and the phase information moves from interference in time to interference in the complex plane.
  • Because the $gamma$ extraction is over-determined by the four rates, the residual of the triangle closure is itself a quantitative handle on SU(3) breaking — a use the paper notes only in passing when it suggests replacing $r_u$ by $(f_K/f_pi), r_u$ in Eq. (66).
  • The rate equalities involving $B_s$ decays were not testable with the data available in 1994; they become sharp predictions for any experiment that can produce and identify $B_s$ mesons, and a measured $B_s to pi^+ pi^-$ rate above background would directly contradict the neglected-rescattering premise rather than merely strain it.
  • The paper's insistence that strong-phase differences are needed for direct CP violation in charged-B decays suggests the natural follow-up is to ask whether the rescattering that would generate those phases can be computed, since that is what separates a rate-based phase extraction from a phase extraction that is robust to it.
Original abstract

The decays $B \to PP$, where $P$ denotes a pseudoscalar meson, are analyzed, with emphasis on charmless final states. Numerous triangle relations for amplitudes hold within SU(3) symmetry, relating (for example) the decays $B^+ \to \pi^+ \pi^0,~ \pi^0 K^+$, and $\pi^+ K^0$. Such relations can improve the possibilities for early detection of $CP$-violating asymmetries. Within the context of a graphical analysis of decays, relations are analyzed among SU(3) amplitudes which hold if some graphs are neglected. One application is that measurements of the rates for the above three $B^+$ decays and their charge-conjugates can be used to determine a weak CKM phase. With measurements of the remaining rates for $B$ decays to $\pi \pi,~\pi K$, and $K \bar K$, one can obtain two CKM phases and several differences of strong phase shifts.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,044 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.41 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.04 · compares with prior results 0.06 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

The decays $B \to PP$, where $P$ denotes a pseudoscalar meson, are analyzed, with emphasis on charmless final states. Numerous triangle relations for amplitudes hold within SU(3) symmetry, relating (for example) the decays $B^+ \to \pi^+ \pi^0,~ \pi^0 K^+$, and $\pi^+ K^0$. Such relations can improve the possibilities for early detection of $CP$-violating asymmetries. Within the context of a graphical analysis of decays, relations are analyzed among SU(3) amplitudes which hold if some graphs are neglected. One application is that measurements of the rates for the above three $B^+$ decays and their charge-conjugates can be used to determine a weak CKM phase. With measurements of the remaining rates for $B$ decays to $\pi \pi,~\pi K$, and $K \bar K$, one can obtain two CKM phases and several differences of strong phase shifts.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The decays $B \to PP$, where $P$ denotes a pseudoscalar meson, are analyzed, with emphasis on charmless final states. contribution 0.88 0.74 0.06 asserted 0.92
s1 Numerous triangle relations for amplitudes hold within SU(3) symmetry, relating (for example) the decays $B^+ \to \pi^+ \pi^0,~ \pi^0 K^+$, and $\pi^+ K^0$. result 0.58 0.33 0.13 asserted 1.00
s2 Such relations can improve the possibilities for early detection of $CP$-violating asymmetries. interpretation 0.99 0.11 0.14 hedged possibility 0.96
s3 Within the context of a graphical analysis of decays, relations are analyzed among SU(3) amplitudes which hold if some graphs are neglected. method 0.67 0.70 0.07 asserted 0.86
s4 One application is that measurements of the rates for the above three $B^+$ decays and their charge-conjugates can be used to determine a weak CKM phase. interpretation 0.63 0.32 0.20 asserted 0.50
s5 With measurements of the remaining rates for $B$ decays to $\pi \pi,~\pi K$, and $K \bar K$, one can obtain two CKM phases and several differences of strong phase shifts. result 0.40 0.23 0.26 hedged possibility 0.75

What kind of paper the classifier reports this is: theoretical result 0.96 · evidence from proof or derivation 0.56.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1. Diagrams describing decays of
Figure 2. The two triangles are fixed by measurements of all six decay r
Figure 6. FIG. 6. Isospin triangle for

Showing 3 of the paper's 6 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-ph/9404283 by the authors.

References
How later papers used it

68 later papers cite it

Evidence: some. 52 of the citing papers share no author with it, and 12 of those carry a confident label from Pith's citation classifier.

Pith read 9 of the 106 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 1 of 12 papers with this label
How the literature uses it

54 arXiv papers cite this in 91 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 62 sentences without a confident label

Uses its method or tool 10 papers
  • [ref] , which made use of the well established topological amplitude formalism [ref] and asymptotic relations [ref] in the large $m_b$ limit.

    Two-body Baryonic $B_{u,d,s}$ and $B_c$ to Charmless Final State Decays · Introduction · 0.98

  • [ref] , which was used to study two-body baryonic $B$ decays, $B→'$, making use of the well established topological amplitude formalism [ref] .

    Charmless Semileptonic Baryonic $B_{u,d,s}$ Decays · Introduction · 0.99

  • Within the diagrammatic approach of Refs. [ref] , $B$-decay amplitudes are expressed in terms of six diagrams[The annihilation, exchange and penguin-annihilation diagrams are neglected, as they are expected to be very small in the SM.]: the color-favored and color-suppressed tree amplitudes $T'$ and $C'$, the gluonic penguin amplitudes $P'_tc$ and $P'_uc$, and the color-favored and color-suppressed electroweak penguin amplitudes $$ and $$ (the primes on the amplitudes indicate $$ transitions).

    Uniting Low-energy Semileptonic and Hadronic Anomalies within SMEFT · $$ · 0.99

Compares with it 1 paper
Cites it as support 1 paper
Cites it as background 16 papers

Discussion

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