REVIEW
Perturbative corrections to semileptonic b decay distributions
High Energy Physics - Phenomenology · Submitted 8 Mar 2005 · 32 references
Pith's verdict: A solid, well-cross-checked one-loop plus BLM calculation for semileptonic b decay; the tau-sector structure functions W4 and W5 are the one genuinely unverified piece, and the authors say so themselves.
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Referee report
The paper
The paper claims
The paper derives the complete one-loop QCD corrections to all five structure functions that govern the differential rate of semileptonic bottom-quark decay, with the soft singularity subtracted so the result can be integrated numerically,.
The reading
What carries the argument
The load-bearing device is the treatment of the soft singularity. The real-emission contribution is written as $D(q̂^2,û,λ̂^2)$, which in the massless-gluon limit becomes $D_0(q̂^2,û)/û$ and diverges logarithmically at $û = 0$; the authors add and subtract a term $A(q̂^2,λ)δ(û)$, where $A$ is the integral of the endpoint approximation $D_s$ over the allowed range, so that the bracketed remainder is regular and the divergence sits entirely in $A$. The singular piece of $A$, $A ∼ (2 - a^{-1}log((1+a)/(1-a)))log λ̂$ with $a = √{λ_b^0}/ω̂$, cancels exactly against the corresponding soft logarithm $V_0(q̂^2,λ)$ from the virtual diagrams. What survives is a plus distribution $(1/û)_+$ defined by $∫ dû (1/û)_+ f(û) = ∫ dû [f(û)-f(0)]/û$, which is what makes the result integrable with arbitrary cuts. The same finite-gluon-mass machinery then feeds the BLM resummation, since the $λ^2$ dependence of the one-loop coefficient is exactly what the large-$β_0$ weight in Eq. (3.5) requires.
Extended reading notes
Core claim
The paper's claim is that the full order-$α_s$ structure functions for $b → cℓν̄$ can be written compactly and analytically, in the form of Eq. (2.47): a tree-level term proportional to $δ(û)$, a virtual-plus-soft correction proportional to $δ(û)$ and built from the finite combination $S(q̂^2) = lim_{λ→0}[A(q̂^2,λ) - V_0(q̂^2,λ)]$, and a real-emission piece split into a plus-distribution term $T_i D_0 (1/û)_+$ plus regular functions $R_i$. The authors state that they agree with all the real-emission contributions of the earlier calculation of Ref. [17] but disagree with its virtual contributions to $W_3$, $W_4$ and $W_5$, while their $V_1, V_2, V_3$ are reproduced correctly; the discrepancy would therefore show up only in the decay rate into massive leptons. On top of the one-loop result, Section 3 gives a convergent integral representation, Eq. (3.15), for the BLM coefficients of the triple differential rate, obtained by integrating the finite-gluon-mass calculation against a weight in $λ^2$.
Load-bearing premise
The higher-order numbers in the tables are the large-$β_0$ part of the series only, and the paper never estimates the size of the terms it drops, so the third and fourth columns are indicative of the running-coupling ladder rather than a bound on the full perturbative correction.
Editorial extensions
If this is right
- Any moment of the semileptonic decay distribution — leptonic energy, hadronic invariant mass, hadronic energy — can be computed at order $α_s$ with arbitrary experimental cuts from one set of formulae, rather than case by case.
- Decays into $τ$ leptons become accessible at the same order, because all five structure functions are given, not just the three that survive for massless leptons.
- The BLM tower $O(α_s^n β_0^{n-1})$ can be evaluated for the differential rate and hence for truncated moments, extending earlier BLM results that existed only for the total width and the lepton energy spectrum.
- The tabulated numbers in Tables 1 to 4 give reference values for the first few leptonic and hadronic moments at tree level, one loop, and the second and third BLM orders, in the pole-mass scheme with $m_b = 4.6$ GeV and $m_c = 1.15$ GeV.
- Because the authors' virtual contributions to $W_3$, $W_4$ and $W_5$ differ from Ref. [17] while $V_1, V_2, V_3$ agree, the massive-lepton channel is the place where one of the two calculations must fail.
Reading between the lines
- The disagreement with Ref. [17] is a concrete, resolvable discrepancy rather than a matter of convention: an independent evaluation of the $b → cτν̄$ rate at order $α_s$ would decide it, and the paper supplies enough intermediate structure ($V_i$, $I_x$, $I_y$, $I_{xy}$) for that check to be made without redoing the loop integrals.
- The plus-distribution construction is generic: the same add-and-subtract of an endpoint integral could be applied to other heavy-quark spectra where a soft logarithm sits at a kinematic edge, such as $b → u$ or radiative decays, and the paper notes that the $b → u$ structure functions follow as the $m_c → 0$ limit.
- The higher-order columns of the tables inherit the usual status of BLM results: they track the piece of the series that runs with the number of light fermions, so their use in fitting $|V_{cb}|$ carries an unquantified remainder that only a full two-loop calculation at the relevant kinematic points could bound.
Original abstract
We compute O(alpha_s) and O(alpha_s^n beta_0^{n-1}) (BLM) corrections to the five structure functions relevant for b->q l nu decays and apply the results to the moments of a few distributions of phenomenological importance. We present compact analytic one-loop formulae for the structure functions, with proper subtraction of the soft divergence.
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
We compute O(alpha_s) and O(alpha_s^n beta_0^{n-1}) (BLM) corrections to the five structure functions relevant for b->q l nu decays and apply the results to the moments of a few distributions of phenomenological importance. We present compact analytic one-loop formulae for the structure functions, with proper subtraction of the soft divergence.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We compute O(alpha_s) and O(alpha_s^n beta_0^{n-1}) (BLM) corrections to the five structure functions relevant for b->q l nu decays and apply the results to the moments of a few distributions of phenomenological importance. | contribution 0.99 | 0.96 | 0.44 | asserted 1.00 |
| s1 | We present compact analytic one-loop formulae for the structure functions, with proper subtraction of the soft divergence. | contribution 0.52 | 0.95 | 0.18 | asserted 1.00 |
What was examined, and what was not
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References
How later papers used it
47 later papers cite it
1 paper by the same authors is listed below: 1 used its method or tool.
Uses its method or tool 1 of 4 papers with this label · 1 by the same authors
By the same authors
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Performed the calculation of BLM corrections using the massive gluon technique and integration over the gluon mass, a standard procedure described in this paper.
Inclusive semileptonic B decays and the determination of |V_ub| (2007) (same authors) · 0.90 · paraphrased: the licence does not allow quoting
How the literature uses it
44 arXiv papers cite this in 76 sentences
Papers citing it · 50 sentences without a confident label
Uses its method or tool 4 papers
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In the analysis of [ref] the theoretical prediction for the moments $M_HQE$ are calculated perturbatively in a Heavy-Quark Expansion (HQE) in the kinetic-mass scheme up to $𝒪(1/m_b^3)$ with perturbative contributions [ref] resulting in a dependence on six parameters: the running masses of the b- and c-quarks, $m_b(μ)$ and $m_c(μ)$, the parameters $$ and $$ at $𝒪(1/m_b^2)$ in the HQE, and, at $𝒪(1/m_b^3)$, the parameters $$ and $$.
Limit on a Right-Handed Admixture to the Weak $b \to c$ Current from Semileptonic Decays · Fit Setup · 0.82
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The $O(^2 β_0)$ corrections to the hadronic structure functions and therefore to the triple differential distribution have been computed in Ref. [ref] , which gives explicit numerical results for the leptonic and hadronic form factors, as well as a code implementing the calculation.
The $q^2$ moments in inclusive semileptonic $B$ decays · BLM corrections · 0.86
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The original code underwent a subsequent upgrade to include the full $O(β_0 ^2)$, computed numerically following [ref] and implemented through precise interpolation formulas, and in this form has been employed in recent HFAG analyses.
B semileptonic moments at NNLO · Hadronic mass moments at NNLO · 0.95
Uses its data or values 3 papers
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They read W^(1)_i=w_i^(0) { S_i δ(û) -2 (1 - E_0 I_1) [1/û]_+ +θ(û)/(ρ+ û) } + R_i θ(û) , where $S_i= S+Δ_i$ and S = 2E_0 (I_2,0-I_4,0) -1 -1-ρ-6q̂^2/4q̂^2 lnρ-(1 - ρ)^2 - 6 q̂^2 (1 + ρ) + 5 (q̂^2)^2 /4q̂^2 I_1,0 ; Δ_1 = -ρ/E_0 I_1,0; Δ_2= 1-ρ/4q̂^2 lnρ+((1-ρ)^2/4q̂^2-1+ρ/4)I_1,0; Δ_3=0, and the functions $R_i$ are given in Eqs. (2.32-2.34) of Ref. [ref] .[The variables $ω̂$, $λ_b$, and $τ$ of Ref. [ref] correspond to $-2 E_0$, $λ$ and $(1-t)/(1+t)$, respectively.] The integrals $I_1$, $I_1,0$, $I_2,0$, and $I_4,0$ are given in Eqs. (A.6-8) of [ref] and the plus distribution is defined by its action on a generic test function $f(û)$:[ Ref. [ref] uses $û_+$ as upper limit, and the two definitions can be easily related, see [ref] .] ∫f(û) [1/û]_+dû = ∫_0^1 f(û)-f(0)/û dû.
Perturbative corrections to power suppressed effects in $\bar B\to X_u\ell\nu$ · Notation and $O()$ corrections · 0.81
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They read W^(1)_i=w_i^(0) { S_i δ(û) -2 (1 - E_0 I_1) [1/û]_+ +θ(û)/(ρ+ û) } + R_i θ(û) , where $S_i= S+Δ_i$ and S = 2E_0 (I_2,0-I_4,0) -1 -1-ρ-6q̂^2/4q̂^2 lnρ-(1 - ρ)^2 - 6 q̂^2 (1 + ρ) + 5 (q̂^2)^2 /4q̂^2 I_1,0 ; Δ_1 = -ρ/E_0 I_1,0; Δ_2= 1-ρ/4q̂^2 lnρ+((1-ρ)^2/4q̂^2-1+ρ/4)I_1,0; Δ_3=0, and the functions $R_i$ are given in Eqs. (2.32-2.34) of Ref. [ref] .[The variables $ω̂$, $λ_b$, and $τ$ of Ref. [ref] correspond to $-2 E_0$, $λ$ and $(1-t)/(1+t)$, respectively.] The integrals $I_1$, $I_1,0$, $I_2,0$, and $I_4,0$ are given below in Eqs. ([ref],[ref],[ref]) and the plus distribution is defined by [1/û]_+ =lim_ε→0 [ lnε δ(û) + 1/û θ(û - ε) θ(1-û) ]
Kinetic operator effects in B-> X_c l nu at O(alpha_s) · Leading Order results · 0.83
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Gambino for providing the results of Refs. [ref] in electronic form.
Kolya: an open-source package for inclusive semileptonic $B$ decays · Acknowledgments · 0.91
Compares with it 5 papers
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Starting from the $O()$ contributions to the form factors $W_i$ given in [ref] , we find excellent numerical agreement with [ref] , namely [equation] where $𝒞_0^NLO,F$ is given in Eq. (50) of [ref] .
The $q^2$ moments in inclusive semileptonic $B$ decays · The calculation · 0.95
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Ref. [ref] uses $û_+$ as upper limit, and the two definitions are related by the simple expression [1/û]_+,[10] =[1/û]_+ - lnû_+ δ(û)- θ(û - û_+) θ(1-û)/û.
Kinetic operator effects in B-> X_c l nu at O(alpha_s) · Leading Order results · 0.84
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We compare the lattice results with the corresponding OPE prediction (red curve) including $O(1/m^3)$ [ref] and $O(α_s)$ [ref] terms with $α_s=0.27$.
Inclusive semi-leptonic decays from lattice QCD · Introduction · 0.81
Cites it as support 1 paper
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However, we point out that also in the $MS$ mass scheme, the $α_s$ corrections to the semileptonic decay rates have a bad convergence as well if one uses the normalization point $μ=m_Q$ [ref] .
The Heavy Quark Expansion for Inclusive Semileptonic Charm Decays Revisited · Charm-Quark Mass and QCD Corrections · 0.98
Cites it as background 12 papers
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The Wilson coefficient $C^Γ_Q̅Q$ has been calculated to the $𝒪(α_s^3)$ order [ref] , where the highest order calculation has been done recently [ref] .
Determination of $|V_{cb}|$ using N$^3$LO perturbative corrections to $\Gamma(B \to X_c \ell \nu)$ and 1S masses · Introduction · 0.85
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The leading terms in these double expansions are given by the free $b$ quark decays, while the $O(α_s,α_s^2 β_0)$ perturbative corrections [ref] and the $O (Λ^2_QCD/m_b^2, Λ^3_QCD/m_b^3)$ non-perturbative corrections [ref] have been known for a long time.
Perturbative corrections to power suppressed effects in semileptonic B decays · Introduction · 0.88
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In the free-quark approximation, corrections up to $O(α_s^2)$ are available [ref] together with the leading $β_0$ terms at higher orders [ref] , where $β_0$ is the one-loop coefficient of the QCD beta function.
Third order corrections to the semi-leptonic \boldmath{$b\to c$} and the muon decays · Introduction · 0.91
Discussion
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