Flavour Changing Neutral Current decays at LHCb

Christoph Langenbruch (for the LHCb collaboration)

arxiv: 2605.18084 · v1 · pith:TUKUWZAGnew · submitted 2026-05-18 · ✦ hep-ex

Flavour Changing Neutral Current decays at LHCb

Christoph Langenbruch (for the LHCb collaboration) This is my paper

Pith reviewed 2026-05-20 00:33 UTC · model grok-4.3

classification ✦ hep-ex

keywords flavour changing neutral currentsb to s mu mu decaysLHCb experimentangular observablesbranching fractionslepton flavour universalityrare B meson decaysStandard Model tensions

0 comments

The pith

LHCb measurements of b to s muon pair decays show persistent tensions with Standard Model predictions.

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

The paper presents the latest LHCb results on flavour changing neutral current decays, which proceed only through quantum loops in the Standard Model and are therefore rare. It focuses on semileptonic b to s lepton pair transitions and radiative modes, using observables such as branching fractions, angular distributions, CP asymmetries and lepton flavour universality tests to search for new physics effects. The central results include a legacy analysis of the decay B0 to K*0 mu+ mu- based on the full 8.4 inverse femtobarn dataset from LHC Run 1 and Run 2. Consistent deviations from Standard Model expectations appear across branching fraction and angular measurements in b to s mu+ mu- channels. The strength of these deviations hinges on how theoretical calculations treat non-perturbative hadronic contributions.

Core claim

Long-standing tensions between experimental data and Standard Model predictions are observed in b to s mu+ mu- decays, appearing consistently in both branching fraction and angular observable measurements, with the key legacy result coming from the decay B0 to K*0 mu+ mu- using the complete Run 1 and Run 2 dataset of 8.4 fb^{-1}.

What carries the argument

The effective operators in the b to s ell ell transition Hamiltonian, accessed through measurements of branching fractions, angular observables, CP asymmetries and lepton flavour universality ratios in semileptonic and radiative decays.

If this is right

Improved precision on angular observables can further constrain the possible new physics operator structures contributing to the effective Hamiltonian.
Measurements in additional channels such as b to s e e or radiative b to s gamma decays can test the consistency of any new physics explanation.
Lepton flavour universality ratios provide an independent probe that is less affected by some hadronic uncertainties.
Future data taking can reduce experimental uncertainties and sharpen the comparison with theory.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Better lattice QCD calculations of hadronic effects would directly test whether the tensions persist or are artifacts of current theory approximations.
These results motivate targeted searches for related anomalies in other flavour changing processes at the same experiment.
If new physics is responsible, the pattern of deviations could guide model building for extensions beyond the Standard Model.

Load-bearing premise

The significance of the observed tensions depends on assumptions made about hadronic uncertainties in the Standard Model theoretical predictions.

What would settle it

A new theoretical computation of the hadronic form factors or matrix elements that brings the Standard Model prediction for the B0 to K*0 mu+ mu- angular observables and branching fraction into agreement with the measured central values within uncertainties would remove the tension.

read the original abstract

Flavour Changing Neutral Current (FCNC) decays are forbidden at lowest perturbative order in the Standard Model (SM) and only allowed via quantum loops. These transitions are therefore heavily suppressed in the SM, and New Physics (NP) can give significant contributions through virtual corrections. Of particular interest are semileptonic $b\to s(d)\ell^+\ell^-$ and radiative $b\to s(d)\gamma$ decays that allow to not only to search for the presence of NP, but also probe its potential operator structure through a multitude of observables. These observables include measurements of branching fractions, angular observables, CP-asymmetries and tests of lepton flavour universality. Long-standing tensions of data with SM predictions have been observed in $b\to s\mu^+\mu^-$ decays, consistently in measurements of branching fractions and angular analyses. However, the significance of these tensions depends on assumptions on hadronic uncertainties in the SM prediction. These proceedings summarise the most recent results on FCNC decays from the LHCb experiment, including the legacy measurement of the key decay $B^0\to K^{*0}\mu^+\mu^-$, using $8.4\mathrm{fb}^{-1}$ of data from the LHC Run 1 and 2.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

LHCb's proceedings update the legacy B0 to K*0 mu mu measurement with full Run 1+2 data but add no new handling of the dominant hadronic uncertainties.

read the letter

This note from LHCb pulls together recent results on flavour-changing neutral currents, with the main new piece being the legacy analysis of B0 to K*0 mu+ mu- using the complete 8.4 fb-1 from Runs 1 and 2. The summary reports the usual pattern of lower-than-expected branching fractions and angular observables in b to s mu mu transitions, consistent with earlier LHCb publications. That update is the concrete addition here, and the collaboration presents it clearly as an experimental measurement compared against external SM predictions. The text also covers other channels like radiative decays and lepton-flavour-universality tests, giving a compact overview of where the data stand. Experimental results from a large collaboration like LHCb rest on validated reconstruction and selection chains, so the raw measurements themselves look reliable on the face of it. The abstract is upfront that any claimed tension with the Standard Model depends on how the hadronic form factors are treated, which is the right caveat to include. The soft spot is that the note does not supply its own sensitivity study or envelope of predictions under alternate lattice or LCSR inputs. Different form-factor choices can shift the SM expectations by amounts comparable to the observed deviations, and readers still need to consult the full analysis papers or separate theory work to see how much the pulls change. That limitation is typical for a proceedings summary rather than a flaw in the underlying data. This piece is useful for experimentalists or model builders who want a quick status report on LHCb's FCNC program. It does not introduce new techniques or resolve open questions, so I would not cite it directly; the detailed publications are the ones to reference. The experimental content is solid enough that a serious editor should send it to peer review rather than desk-reject, even if the journal version would need more detail on systematics and theory comparisons.

Referee Report

1 major / 2 minor

Summary. The paper is a proceedings summary from the LHCb collaboration on flavour-changing neutral current decays. It emphasizes semileptonic b→s(d)ℓ⁺ℓ⁻ and radiative b→s(d)γ transitions, reports long-standing tensions between data and SM predictions in b→sμ⁺μ⁻ branching fractions and angular observables, and presents the legacy measurement of B⁰→K^{*0}μ⁺μ⁻ using the full 8.4 fb^{-1} Run 1+2 dataset.

Significance. If the measurements are robust, the work supplies updated experimental constraints on potential new physics in rare B decays and strengthens inputs to global Wilson-coefficient fits. Credit is given for the use of the complete LHC Run 1 and 2 luminosity in the key legacy analysis, which is a standard strength of LHCb publications.

major comments (1)

[Discussion of tensions in b→sμ⁺μ⁻ decays] The manuscript states that the significance of the tensions depends on assumptions on hadronic uncertainties in the SM prediction. However, it supplies no explicit sensitivity study or envelope of predictions under alternate B→K* form-factor parametrizations (lattice QCD versus LCSR). Since such variations can shift observables by amounts comparable to the reported deviations, this limits the strength of the claim of consistent tensions across branching fractions and angular analyses.

minor comments (2)

[Abstract] The abstract would benefit from one or two concrete numerical highlights or key observables from the legacy B⁰→K^{*0}μ⁺μ⁻ measurement to give readers immediate context.
Add explicit references to the primary LHCb papers that detail the data selection, fit procedures, and systematic uncertainty evaluation for the reported results.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the positive recommendation of minor revision. We address the single major comment below.

read point-by-point responses

Referee: [Discussion of tensions in b→sμ⁺μ⁻ decays] The manuscript states that the significance of the tensions depends on assumptions on hadronic uncertainties in the SM prediction. However, it supplies no explicit sensitivity study or envelope of predictions under alternate B→K* form-factor parametrizations (lattice QCD versus LCSR). Since such variations can shift observables by amounts comparable to the reported deviations, this limits the strength of the claim of consistent tensions across branching fractions and angular analyses.

Authors: We agree that an explicit illustration of the impact of form-factor choices would strengthen the discussion. As this is a concise proceedings summary of recent LHCb results, the detailed sensitivity studies comparing lattice QCD and LCSR parametrizations (including the resulting envelopes on branching fractions and angular observables) are presented in the full legacy analysis paper on B⁰→K*⁰μ⁺μ⁻ that is referenced in the manuscript. We will add a short clarifying sentence in the revised version noting that the quoted tension significances already incorporate the dominant hadronic uncertainties from the form-factor inputs adopted in that analysis, together with a pointer to the specific sections of the full paper where the lattice-versus-LCSR variations are quantified. revision: yes

Circularity Check

0 steps flagged

No significant circularity in experimental measurements summary

full rationale

The paper is a proceedings summary of direct LHCb experimental measurements of FCNC decays, including the legacy B^0 to K*0 mu+ mu- analysis with 8.4 fb^-1 of Run 1+2 data. Central claims concern observed tensions with external SM predictions, but the text explicitly flags that significance depends on hadronic uncertainties in those predictions. No derivation chain, ansatz, or fitted parameter is presented that reduces by construction to the paper's own inputs; results are reported as independent data compared against external benchmarks whose limitations are acknowledged.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is an experimental summary paper; the central claims rest on Standard Model loop calculations and hadronic form-factor estimates taken from prior theory literature rather than new postulates introduced here.

axioms (1)

domain assumption Hadronic uncertainties in SM predictions for b→sℓℓ decays limit the significance of observed tensions.
Explicitly stated in the abstract as the factor controlling interpretation of the data.

pith-pipeline@v0.9.0 · 5747 in / 1210 out tokens · 44106 ms · 2026-05-20T00:33:27.317103+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/AbsoluteFloorClosure.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

the significance of these tensions depends on assumptions on hadronic uncertainties in the SM prediction
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

SM predictions for these processes are affected by hadronic uncertainties from local form factors and non-local charm-loop contributions

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

32 extracted references · 32 canonical work pages · 9 internal anchors

[1]

W. G. Parrott et al.Phys. Rev. D, 107(1):014511, 2023. [arXiv:2207.13371] [Erratum: Phys.Rev.D 107, 119903 (2023)]

work page arXiv 2023
[2]

Roel Aaij et al.Phys. Rev. Lett., 127(15):151801, 2021. [arXiv:2105.14007]

work page arXiv 2021
[3]

$B\to V\ell^+\ell^-$ in the Standard Model from Light-Cone Sum Rules

Aoife Bharucha et al.JHEP, 08:098, 2016. [arXiv:1503.05534]

work page internal anchor Pith review Pith/arXiv arXiv 2016
[4]

R. R. Horgan et al.PoS, LATTICE2014:372, 2015. [arXiv:1501.00367]

work page internal anchor Pith review Pith/arXiv arXiv 2015
[5]

Differential branching fractions and isospin asymmetries of $B \to K^{(*)}\mu^{+}\mu^{-}$ decays

R. Aaij et al.JHEP, 06:133, 2014. [arXiv:1403.8044]

work page internal anchor Pith review Pith/arXiv arXiv 2014
[6]

Differential branching fraction and angular analysis of $\Lambda^{0}_{b} \rightarrow \Lambda \mu^+\mu^-$ decays

Roel Aaij et al.JHEP, 06:115, 2015. [arXiv:1503.07138] [Erratum: JHEP 09, 145 (2018)]

work page internal anchor Pith review Pith/arXiv arXiv 2015
[7]

Aaij et al.JHEP, 01:159, 2026

R. Aaij et al.JHEP, 01:159, 2026. [arXiv:2509.12805]

work page arXiv 2026
[8]

William Detmold et al.Phys. Rev. D, 87(7):074502, 2013. [arXiv:1212.4827]

work page internal anchor Pith review Pith/arXiv arXiv 2013
[9]

William Detmold and Stefan Meinel.Phys. Rev. D, 93(7):074501, 2016. [arXiv:1602.01399]

work page arXiv 2016
[10]

Vardan Khachatryan et al.Phys. Lett. B, 753:424–448, 2016. [arXiv:1507.08126]

work page internal anchor Pith review Pith/arXiv arXiv 2016
[11]

Technical report, CERN, 2026

CMS Collaboration. Technical report, CERN, 2026. [CMS-PAS-BPH-23-003]

work page 2026
[12]

Roel Aaij et al. 2026. [arXiv:2601.06878]

work page arXiv 2026
[13]

Aaij et al.JHEP, 12:151, 2025

R. Aaij et al.JHEP, 12:151, 2025. [arXiv:2507.14401]

work page arXiv 2025
[14]

Roel Aaij et al. 2026. [arXiv:2601.03083]

work page internal anchor Pith review Pith/arXiv arXiv 2026
[15]

Roel Aaij et al. 2025. [arXiv:2512.18053]

work page arXiv 2025
[16]

Optimizing the basis of B->K*ll observables in the full kinematic range

Sebastien Descotes-Genon et al.JHEP, 05:137, 2013. [arXiv:1303.5794]

work page internal anchor Pith review Pith/arXiv arXiv 2013
[17]

Roel Aaij et al.Phys. Rev. Lett., 125(1):011802, 2020. [arXiv:2003.04831]

work page arXiv 2020
[18]

Aram Hayrapetyan et al.Phys. Lett. B, 864:139406, 2025. [arXiv:2411.11820]

work page arXiv 2025
[19]

Danny van Dyk et al.Eur. Phys. J. C, 82(6):569, 2022. [arXiv:2111.15428]

work page arXiv 2022
[20]

Gubernari, M

Nico Gubernari et al.JHEP, 09:133, 2022. [arXiv:2206.03797]

work page arXiv 2022
[21]

Marcel Alguer´ o et al.Eur. Phys. J. C, 83(7):648, 2023. [arXiv:2304.07330]

work page arXiv 2023
[22]

David M. Straub. 2018. [arXiv:1810.08132]

work page internal anchor Pith review Pith/arXiv arXiv 2018
[23]

Roel Aaij et al. 2026. [arXiv:2603.13223]

work page arXiv 2026
[24]

LHCb-PAPER-2025-066, in preparation

Roel Aaij et al. LHCb-PAPER-2025-066, in preparation

work page 2025
[25]

Aaij et al

R. Aaij et al.Phys. Rev. Lett., 131(5):051803, 2023. [arXiv:2212.09152]

work page arXiv 2023
[26]

Aaij et al., Measurement of lepton universality parameters in B+ → K+ℓ+ℓ− and B0 → K ∗0ℓ+ℓ− decays, Phys

R. Aaij et al.Phys. Rev. D, 108(3):032002, 2023. [arXiv:2212.09153]

work page arXiv 2023
[27]

[arXiv:2505.03483]

Roel Aaij et al.JHEP, 07:198, 2025. [arXiv:2505.03483]

work page arXiv 2025
[28]

Roel Aaij et al.Phys. Rev. Lett., 134(18):181803, 2025. [arXiv:2412.11645]

work page arXiv 2025
[29]

Roel Aaij et al.Phys. Rev. Lett., 134(12):121803, 2025. [arXiv:2410.13748]

work page arXiv 2025
[30]

Roel Aaij et al.Phys. Rev. Lett., 128(19):191802, 2022. [arXiv:2110.09501]

work page arXiv 2022
[31]

Aaij et al

Roel Aaij et al.JHEP, 05:040, 2020. [arXiv:1912.08139]

work page arXiv 2020
[32]

Roel Aaij et al. 2025. [arXiv:2511.16564]

work page arXiv 2025

[1] [1]

W. G. Parrott et al.Phys. Rev. D, 107(1):014511, 2023. [arXiv:2207.13371] [Erratum: Phys.Rev.D 107, 119903 (2023)]

work page arXiv 2023

[2] [2]

Roel Aaij et al.Phys. Rev. Lett., 127(15):151801, 2021. [arXiv:2105.14007]

work page arXiv 2021

[3] [3]

$B\to V\ell^+\ell^-$ in the Standard Model from Light-Cone Sum Rules

Aoife Bharucha et al.JHEP, 08:098, 2016. [arXiv:1503.05534]

work page internal anchor Pith review Pith/arXiv arXiv 2016

[4] [4]

R. R. Horgan et al.PoS, LATTICE2014:372, 2015. [arXiv:1501.00367]

work page internal anchor Pith review Pith/arXiv arXiv 2015

[5] [5]

Differential branching fractions and isospin asymmetries of $B \to K^{(*)}\mu^{+}\mu^{-}$ decays

R. Aaij et al.JHEP, 06:133, 2014. [arXiv:1403.8044]

work page internal anchor Pith review Pith/arXiv arXiv 2014

[6] [6]

Differential branching fraction and angular analysis of $\Lambda^{0}_{b} \rightarrow \Lambda \mu^+\mu^-$ decays

Roel Aaij et al.JHEP, 06:115, 2015. [arXiv:1503.07138] [Erratum: JHEP 09, 145 (2018)]

work page internal anchor Pith review Pith/arXiv arXiv 2015

[7] [7]

Aaij et al.JHEP, 01:159, 2026

R. Aaij et al.JHEP, 01:159, 2026. [arXiv:2509.12805]

work page arXiv 2026

[8] [8]

William Detmold et al.Phys. Rev. D, 87(7):074502, 2013. [arXiv:1212.4827]

work page internal anchor Pith review Pith/arXiv arXiv 2013

[9] [9]

William Detmold and Stefan Meinel.Phys. Rev. D, 93(7):074501, 2016. [arXiv:1602.01399]

work page arXiv 2016

[10] [10]

Vardan Khachatryan et al.Phys. Lett. B, 753:424–448, 2016. [arXiv:1507.08126]

work page internal anchor Pith review Pith/arXiv arXiv 2016

[11] [11]

Technical report, CERN, 2026

CMS Collaboration. Technical report, CERN, 2026. [CMS-PAS-BPH-23-003]

work page 2026

[12] [12]

Roel Aaij et al. 2026. [arXiv:2601.06878]

work page arXiv 2026

[13] [13]

Aaij et al.JHEP, 12:151, 2025

R. Aaij et al.JHEP, 12:151, 2025. [arXiv:2507.14401]

work page arXiv 2025

[14] [14]

Roel Aaij et al. 2026. [arXiv:2601.03083]

work page internal anchor Pith review Pith/arXiv arXiv 2026

[15] [15]

Roel Aaij et al. 2025. [arXiv:2512.18053]

work page arXiv 2025

[16] [16]

Optimizing the basis of B->K*ll observables in the full kinematic range

Sebastien Descotes-Genon et al.JHEP, 05:137, 2013. [arXiv:1303.5794]

work page internal anchor Pith review Pith/arXiv arXiv 2013

[17] [17]

Roel Aaij et al.Phys. Rev. Lett., 125(1):011802, 2020. [arXiv:2003.04831]

work page arXiv 2020

[18] [18]

Aram Hayrapetyan et al.Phys. Lett. B, 864:139406, 2025. [arXiv:2411.11820]

work page arXiv 2025

[19] [19]

Danny van Dyk et al.Eur. Phys. J. C, 82(6):569, 2022. [arXiv:2111.15428]

work page arXiv 2022

[20] [20]

Gubernari, M

Nico Gubernari et al.JHEP, 09:133, 2022. [arXiv:2206.03797]

work page arXiv 2022

[21] [21]

Marcel Alguer´ o et al.Eur. Phys. J. C, 83(7):648, 2023. [arXiv:2304.07330]

work page arXiv 2023

[22] [22]

David M. Straub. 2018. [arXiv:1810.08132]

work page internal anchor Pith review Pith/arXiv arXiv 2018

[23] [23]

Roel Aaij et al. 2026. [arXiv:2603.13223]

work page arXiv 2026

[24] [24]

LHCb-PAPER-2025-066, in preparation

Roel Aaij et al. LHCb-PAPER-2025-066, in preparation

work page 2025

[25] [25]

Aaij et al

R. Aaij et al.Phys. Rev. Lett., 131(5):051803, 2023. [arXiv:2212.09152]

work page arXiv 2023

[26] [26]

Aaij et al., Measurement of lepton universality parameters in B+ → K+ℓ+ℓ− and B0 → K ∗0ℓ+ℓ− decays, Phys

R. Aaij et al.Phys. Rev. D, 108(3):032002, 2023. [arXiv:2212.09153]

work page arXiv 2023

[27] [27]

[arXiv:2505.03483]

Roel Aaij et al.JHEP, 07:198, 2025. [arXiv:2505.03483]

work page arXiv 2025

[28] [28]

Roel Aaij et al.Phys. Rev. Lett., 134(18):181803, 2025. [arXiv:2412.11645]

work page arXiv 2025

[29] [29]

Roel Aaij et al.Phys. Rev. Lett., 134(12):121803, 2025. [arXiv:2410.13748]

work page arXiv 2025

[30] [30]

Roel Aaij et al.Phys. Rev. Lett., 128(19):191802, 2022. [arXiv:2110.09501]

work page arXiv 2022

[31] [31]

Aaij et al

Roel Aaij et al.JHEP, 05:040, 2020. [arXiv:1912.08139]

work page arXiv 2020

[32] [32]

Roel Aaij et al. 2025. [arXiv:2511.16564]

work page arXiv 2025