REVIEW
Untagged B -> X_(s+d) gamma CP asymmetry as a probe for new physics
High Energy Physics - Phenomenology · Submitted 18 Dec 2003 · 63 references
Pith's verdict: A careful, genuinely useful extension of the Gambino–Misiak machinery with a real two-loop MFV scan; the b→d reach bound is the one headline number that leans on the dipole-only assumption.
T0 · AI review
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Referee report
The paper
The paper claims
The untagged CP asymmetry in $B$ decays to a photon is essentially zero in the Standard Model, and the paper argues that how far it can move in supersymmetric and general new-physics scenarios separates classes of models.
The reading
What carries the argument
The load-bearing identity is the U-spin plus CKM unitarity cancellation: the unnormalized rate asymmetry summed over the strange and down channels vanishes in the limit of equal down and strange quark masses, because the Jarlskog invariant controls all Standard Model CP violation and the hadronic matrix elements are related by U-spin. Written out, the untagged asymmetry is $A_{CP}(B o X_{s+d}gamma) = (A_s + R_{ds} A_d)/(1+R_{ds})$ with $R_{ds} = SigmaGamma_d/SigmaGamma_s$, and the Standard Model cancellation appears as $mathrm{Im}(epsilon_s + |V_{td}/V_{ts}|^2 epsilon_d) = 0$. The second piece of machinery is the pair of model-independent next-to-leading-log formulae, Eqs. (42) and (43), which express the branching ratios and tagged asymmetries as explicit polynomials in the rescaled complex coefficients $R_7 = C_7^{tot}/C_7^{SM}$ and $R_8 = C_8^{tot}/C_8^{SM}$, their right-handed analogues, and the CKM ratios $epsilon_q$, with numerical coefficient tables given for two photon energy cuts and two charm-mass choices. These formulae are what let the authors vary the strange and down sectors independently, which no minimal-flavour model permits.
Extended reading notes
Core claim
The central claim is that the direct CP asymmetry of the untagged inclusive channel $B$ to $X_{s+d}$ plus a photon is a strict Standard Model null test whose size, if it is ever non-zero, sorts new physics into classes. In minimal flavour violation the magnetic and chromomagnetic Wilson coefficients are the same for the strange and down sectors to very high precision, so the ratio of the two rates stays at its Standard Model value and the untagged asymmetry is simply proportional to the tagged one, Eq. (68); after the electric dipole moment bounds are imposed on the flavour-blind phases, the surviving asymmetries stay below about 2%. In a model-independent framework where the strange and down dipole coefficients are independent, new physics in the strange sector alone can drive the untagged asymmetry to $pm 10%$ while still satisfying the measured branching ratio and the Belle CP bound, again in strict proportion to the tagged asymmetry. New physics in the down sector alone is much less visible: the untagged asymmetry does not exceed about 2% even for very large effects in the $B$ to $X_d$ plus photon rate or asymmetry. The practical corollary the authors draw is that a measurement of the untagged asymmetry makes a separate tagged measurement in the strange sector largely redundant, and that a few-percent measurement would distinguish minimal from general flavour models.
Load-bearing premise
The model-independent conclusion that new physics in the down sector cannot push the untagged asymmetry beyond about 2% rests on the assumption stated in Section 2 that new physics enters only through the dipole operators $O_7$ and $O_8$ and their right-handed partners, leaving the four-quark operators at their Standard Model values; the paper does not scan those coefficients or quantify how much room the omission leaves.
Editorial extensions
If this is right
- A fully inclusive untagged CP asymmetry measurement reaching the few-percent level becomes a sharper Standard Model test than the tagged $B$ to $X_s$ plus photon asymmetry, because the Standard Model prediction for it is a structural zero rather than a small number.
- If the untagged asymmetry is measured, the tagged strange-sector asymmetry adds little: in every scenario the authors study the two track each other, and the difference is bounded below 1.3% when only strange-sector dipoles are shifted.
- Minimal flavour violation with extra phases, constrained by the electric dipole moments, is expected to be indistinguishable from the Standard Model in this observable at current factories, so a positive signal would point to general flavour violation.
- The updated Standard Model reference values, $mathrm{Br}(B o X_s gamma) = (3.61^{+0.24}_{-0.40} pm 0.02_{CKM} pm 0.24_{param} pm 0.14_{scale}) imes 10^{-4}$ and $A_{CP} = (0.44^{+0.15}_{-0.10} pm 0.03 pm {}^{+0.19}_{-0.09})%$ at $E_gamma > 1.6$ GeV, set the baseline against which any new-physics interpretation of the untagged number must be judged.
- The ratio $mathrm{Br}(B o X_d gamma)/mathrm{Br}(B o X_s gamma) = (3.82^{+0.11}_{-0.18} pm 0.42_{CKM} pm 0.08 pm 0.15) imes 10^{-2}$ is now dominated by CKM uncertainties, so improving the CKM fit sharpens the down-sector side of the same analysis.
Reading between the lines
- The structural zero is more valuable than a small predicted number: a null observable with a symmetry proof behind it converts a modest experimental error into a large discovery reach, which suggests looking for other U-spin or isospin-protected rate asymmetries in radiative and semileptonic rare decays.
- The result that down-sector-only new physics is nearly invisible in the untagged asymmetry implies the observable is a strange-sector probe in disguise; a dedicated $B$ to $X_d$ plus photon asymmetry measurement, hard as it is, would be the only direct handle on down-sector CP phases.
- The proportionality between tagged and untagged asymmetries in every scenario examined is itself a testable pattern: a future measurement of both quantities whose ratio departs from one would signal a breakdown of the dipole-only assumption, effectively measuring the four-quark operator contributions the analysis leaves at their Standard Model values.
- The electric dipole moment bounds do most of the work in suppressing the minimal-flavour-with-phases region, so improved neutron and electron EDM limits would tighten the predicted untagged asymmetry further and make any observed few-percent signal correspondingly harder to accommodate in that framework.
Original abstract
The direct CP asymmetry in the untagged inclusive channel B -> X_(s+d) gamma provides a strict test of the standard model. It has been shown beyond the partonic level that this asymmetry is negligibly small thanks to U-spin relations and to the unitarity of the CKM matrix. In the present paper we investigate this relation beyond the SM; in particular, we analyse to which extent deviations from this prediction are possible in supersymmetric scenarios. We analyse the minimal flavour violation scenario, including tan(beta) enhanced terms and using the complete two-loop renormalization group running. Our analysis fully takes into account also the EDM constraints on the supersymmetric phases. We investigate possible correlations between the tagged and the untagged CP asymmetries and the indirect sensitivity of the latter to the B -> X_d gamma CP asymmetry. Furthermore, we derive general model-independent formulae for the branching ratios and CP asymmetries for the inclusive B -> X_d gamma and B -> X_s gamma modes, and update the corresponding SM predictions.
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.
The abstract, sentence by sentence
The direct CP asymmetry in the untagged inclusive channel B -> X_(s+d) gamma provides a strict test of the standard model. It has been shown beyond the partonic level that this asymmetry is negligibly small thanks to U-spin relations and to the unitarity of the CKM matrix. In the present paper we investigate this relation beyond the SM; in particular, we analyse to which extent deviations from this prediction are possible in supersymmetric scenarios. We analyse the minimal flavour violation scenario, including tan(beta) enhanced terms and using the complete two-loop renormalization group running. Our analysis fully takes into account also the EDM constraints on the supersymmetric phases. We investigate possible correlations between the tagged and the untagged CP asymmetries and the indirect sensitivity of the latter to the B -> X_d gamma CP asymmetry. Furthermore, we derive general model-independent formulae for the branching ratios and CP asymmetries for the inclusive B -> X_d gamma and B -> X_s gamma modes, and update the corresponding SM predictions.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The direct CP asymmetry in the untagged inclusive channel B -> X_(s+d) gamma provides a strict test of the standard model. | background 1.00 | 0.07 | 0.06 | asserted 1.00 |
| s1 | It has been shown beyond the partonic level that this asymmetry is negligibly small thanks to U-spin relations and to the unitarity of the CKM matrix. | background 1.00 | 0.06 | 0.07 | asserted 1.00 |
| s2 | In the present paper we investigate this relation beyond the SM; in particular, we analyse to which extent deviations from this prediction are possible in supersymmetric scenarios. | contribution 0.99 | 0.64 | 0.07 | asserted 0.58 |
| s3 | We analyse the minimal flavour violation scenario, including tan(beta) enhanced terms and using the complete two-loop renormalization group running. | method 0.82 | 0.88 | 0.04 | asserted 0.86 |
| s4 | Our analysis fully takes into account also the EDM constraints on the supersymmetric phases. | method 0.78 | 0.82 | 0.04 | asserted 0.99 |
| s5 | We investigate possible correlations between the tagged and the untagged CP asymmetries and the indirect sensitivity of the latter to the B -> X_d gamma CP asymmetry. | contribution 1.00 | 0.87 | 0.05 | hedged possibility 0.42 |
| s6 | Furthermore, we derive general model-independent formulae for the branching ratios and CP asymmetries for the inclusive B -> X_d gamma and B -> X_s gamma modes, and update the corresponding SM predictions. | contribution 0.94 | 0.95 | 0.10 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:hep-ph/0312260 by the authors.
References
How the literature uses it
44 arXiv papers cite this in 68 sentences
Papers citing it · 36 sentences without a confident label
Uses its method or tool 16 papers
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We use the NLL calculation of Ref. [ref] , which easily and analytically incorporates new physics from the Wilson coefficients.
Constraining the Charged Higgs Mass in the MSSM: A Low-Energy Approach · $b→sγ$ · 1.00
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As stated before we have implemented the calculation of $(b→s γ)$ as introduced in [ref] , and adopted in [ref] for the contribution of beyond SM effects.
Constraints on Supersymmetric Flavour Models from b->s gamma · $𝐛→𝐬 γ$ beyond minimal flavour violation · 0.99
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Formulas are given in or are adapted from Refs. [ref] .
Model-independent analysis of $b\to d$ processes · Rare $b →d$ decay observables · 0.93
Uses its data or values 6 papers
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We observe a very good agreement between the SM prediction in <ref> and the extrapolated experimental value from BaBar [ref] , [equation] Including NP effects from $C_7,8^(')$ using Ref. [ref] , we find the following parametrization [equation] with [equation] The central values of the factors $a^(B̅ X_d)_i$ are compiled in <ref>.
Model-independent analysis of $b\to d$ processes · b to d gamma · 0.89
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[ref] . $C_7,8^(0)tot$ and $C_7R,8R^(0) SUSY$ receive large contributions from diagrams mediated by gluino and down squarks exchange and diagrams with chargino and up squarks exchange.
Supersymmetric contributions to $\bar{B}_s \to \phi \pi^0$ and $\bar{B}_s \to \phi \rho^0$ decays in SCET · SUSY contributions to the branching ratios of $B̅_s→ϕπ^0$ and $B̅_s→ϕρ^0$ decays · 0.85
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The present NLL-prediction reads [ref] (for $E_γ> 1.6 GeV$): [equation] The large CKM-uncertainty is due to $V_td$.
News on Penguins · The inclusive decay $B̅ →X_s,d γ$ · 0.90
Compares with it 1 paper
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compared to an earlier estimate of the short-distance part [ref]
B-decay CP-asymmetries in SUSY with a U(2)^3 flavour symmetry · $A_CP(B→X_sγ)$ · 0.98
Cites it as background 3 papers
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The first studies of the magnitude of the untagged asymmetry in the context of physics beyond the SM were in Ref. [ref] , and the importance of this observable was emphasised in Ref. [ref] .
CP asymmetries of ${\overline B}\to X_s/X_d\gamma$ in models with three Higgs doublets · Direct CP asymmetries of $B→X_s γ$ and $B→X_s+d γ$ · 0.86
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These effects in $b→sγ$ have been studied in great detail over the years, see Refs. [ref] for the most recent and complete discussion.
Supersymmetric Flavor-Changing Sum Rules as a Tool for b -> s gamma · Application: $b→sγ$ · 0.99
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Supersymmetric examples of this kind were studied in [ref] , where new phases were discussed.
General Minimal Flavor Violation · Introduction · 0.89
Discussion
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