Catalog · Kaon · K010

K010 $\mathcal{B}(K_S\to\pi^0 e^+e^-)$

Short-lived neutral-kaon rare semileptonic electron mode

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
$BR(K_S \to $ pi0 e+ e-, $m_{ee}$ > 0.165 GeV)	3.0e-9 branching fraction	2026	Particle Data Group 2026 pdgLive listing S012.10 / NA48	source ↑
$BR(K_S \to $ pi0 e+ e-), full kinematic region	5.8e-9 branching fraction	2026	Particle Data Group 2026 pdgLive listing S012.10 / NA48	source ↑
NA48/1 observed signal candidates	7 events	2003	NA48/1 2003 arXiv hep-ex/0309075	source ↑
NA48/1 expected background	0.15 events	2003	NA48/1 2003 arXiv hep-ex/0309075	source ↑

Why this constrains the RS scan

This mode probes $s\to d e^+e^-$ semileptonic neutral-current structure, not the $\Delta F=2$ kaon-mixing lane already implemented in the repository. In warped or anarchic-flavor models it is sensitive, through a hadronic long-distance amplitude, to flavor-changing $Z$, photon-penguin, and heavy neutral-gauge effects. Its most immediate catalog role is as an experimental normalization for the indirect-CP part of $K_L\to\pi^0 e^+e^-$, so it should be kept distinct from short-distance theory prefactors or NDA normalizations.

What's changed since the original paper

No post-2008 measurement was found that supersedes the NA48/1 value used by PDG. The important developments are interpretive: the rare-kaon Snowmass white paper emphasizes $K_{L,S}\to\pi^0\ell^+\ell^-$ branching ratios and $q^2$ distributions as new-physics probes, with correlations to $\varepsilon'/\varepsilon$, $\varepsilon_K$, and $\Delta M_K$. The modern literature also treats this $K_S$ input as part of the indirect-CP normalization for the $K_L$ modes rather than as a standalone clean Wilson coefficient bound (aebischer\_buras\_kumar\_2022\_arxiv.txt, isidori\_smith\_unterdorfer\_2004\_arxiv.txt).

Validity and model dependence

Class: long-distance-limited rare kaon decay. The measured partial branching fraction is robust, but the full-kinematic number is model-dependent because it assumes a vector matrix element and a unit form factor. A model constraint must specify whether it uses the directly measured $m_{ee}>0.165$ GeV region or the extrapolated total rate, and whether it predicts the chiral long-distance amplitude or only a short-distance $s\to d e^+e^-$ Wilson coefficient.

Code coverage in this repo

NO. Focused greps over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no $K_S\to\pi^0 e^+e^-$, $S012.10$, $a_S$, or $s\to d e^+e^-$ implementation. The nearest kaon coverage is $\Delta F=2$: quarkConstraints/deltaf2.py:209 starts the default neutral-meson-mixing inputs, quarkConstraints/deltaf2.py:755 evaluates $\Delta m_K$, and quarkConstraints/modern/phenomenology.py:23 lists only $\varepsilon_K$, $K$, $B_d$, $B_s$, and $D0$ policy systems.

Implementation difficulty

HIGH. Production integration would need a new $\Delta S=1$ semileptonic rare-kaon observable, Wilson normalization for $s\to d e^+e^-$, and a documented treatment of the $K_S$ form factor or a conservative experimental-input-only interpretation. The existing SLL/SLR/VLL/VRR/LR $\Delta F=2$ basis does not cover this decay.

Key references

Process-local source keys before bibliography consolidation: PDG2026:K010, NA482003:KSPi0EE, IsidoriSmithUnterdorfer2004:KPi0LL, AebischerBurasKumar2022:RareKaons, and CFW2008:RSFlavor.

value 3.0e-9 +1.5e-9 -1.2e-9 stat; +/-0.2e-9 syst

AMBIGUOUS

Match 1 of 10 snapshot line 33

L30:   event_count: 7
L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9

Match 2 of 10 snapshot line 34

L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9

Match 3 of 10 snapshot line 35

L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9

Match 4 of 10 snapshot line 36

L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10

Match 5 of 10 snapshot line 37

L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10
L40:   syst_error_negative: 2.0000000000000003E-10

Match 6 of 10 snapshot line 38

L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10
L40:   syst_error_negative: 2.0000000000000003E-10
L41:   used_in_average: true

Match 7 of 10 snapshot line 45

L42:   used_in_fit: true
L43: 
L44: PDG footnote:
L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.

Match 8 of 10 snapshot line 48

L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51:

Match 9 of 10 snapshot line 49

L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

Match 10 of 10 snapshot line 50

L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

value 5.8e-9 +2.9e-9 -2.4e-9

AMBIGUOUS

Match 1 of 10 snapshot line 33

L30:   event_count: 7
L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9

Match 2 of 10 snapshot line 34

L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9

Match 3 of 10 snapshot line 35

L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9

Match 4 of 10 snapshot line 36

L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10

Match 5 of 10 snapshot line 37

L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10
L40:   syst_error_negative: 2.0000000000000003E-10

Match 6 of 10 snapshot line 38

L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9
L39:   syst_error_positive: 2.0000000000000003E-10
L40:   syst_error_negative: 2.0000000000000003E-10
L41:   used_in_average: true

Match 7 of 10 snapshot line 45

L42:   used_in_fit: true
L43: 
L44: PDG footnote:
L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.

Match 8 of 10 snapshot line 48

L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51:

Match 9 of 10 snapshot line 49

L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

Match 10 of 10 snapshot line 50

L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

value 7

AMBIGUOUS

Match 1 of 4 snapshot line 3

L1: Source key: NA482003:KSPi0EE
L2: Source: arXiv abstract page
L3: URL: https://arxiv.org/abs/hep-ex/0309075
L4: Access date: 2026-05-16
L5: arXiv: hep-ex/0309075
L6: DOI: 10.1016/j.physletb.2003.10.001

Match 2 of 4 snapshot line 5

L2: Source: arXiv abstract page
L3: URL: https://arxiv.org/abs/hep-ex/0309075
L4: Access date: 2026-05-16
L5: arXiv: hep-ex/0309075
L6: DOI: 10.1016/j.physletb.2003.10.001
L7: Journal reference: Phys. Lett. B576, 43 (2003)
L8: Title: Observation of the rare decay Ks->pi0e+e-

Match 3 of 4 snapshot line 7

L4: Access date: 2026-05-16
L5: arXiv: hep-ex/0309075
L6: DOI: 10.1016/j.physletb.2003.10.001
L7: Journal reference: Phys. Lett. B576, 43 (2003)
L8: Title: Observation of the rare decay Ks->pi0e+e-
L9: Authors: NA48/1 Collaboration
L10: Submitted: 2003-09-26

Match 4 of 4 snapshot line 16

L13: 
L14: A search for the decay Ks->pi0e+e- has been made by the NA48/1 experiment at
L15: the CERN SPS accelerator. Using data collected during 89 days in 2002 with a
L16: high-intensity Ks beam, 7 events were found with a background of 0.15 events.
L17: The branching fraction BR(Ks->pi0e+e-, m(ee) > 0.165 GeV/c^2) =
L18: (3.0^{+1.5}_{-1.2}(stat) +/-0.2 (syst)) x 10^{-9} has been measured. Using a
L19: vector matrix element and a form factor equal to one, the measurement gives

value 0.15

RESOLVED

Match snapshot line 16

L13: 
L14: A search for the decay Ks->pi0e+e- has been made by the NA48/1 experiment at
L15: the CERN SPS accelerator. Using data collected during 89 days in 2002 with a
L16: high-intensity Ks beam, 7 events were found with a background of 0.15 events.
L17: The branching fraction BR(Ks->pi0e+e-, m(ee) > 0.165 GeV/c^2) =
L18: (3.0^{+1.5}_{-1.2}(stat) +/-0.2 (syst)) x 10^{-9} has been measured. Using a
L19: vector matrix element and a form factor equal to one, the measurement gives

value 3e-09 +1.5e-9 -1.2e-9 stat; +/-0.2e-9 syst

AMBIGUOUS

Match 1 of 6 snapshot line 33

L30:   event_count: 7
L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9

Match 2 of 6 snapshot line 34

L31:   technique: NA48
L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9

Match 3 of 6 snapshot line 35

L32:   comment: m(ee)>0.165 GeV
L33:   value_text: (3.0+1.5-1.2+-0.2) E-9
L34:   value: 3.0000000000000004E-9
L35:   error_positive: 1.5132745950421556E-9
L36:   error_negative: 1.216552506059644E-9
L37:   stat_error_positive: 1.5000000000000002E-9
L38:   stat_error_negative: 1.2E-9

Match 4 of 6 snapshot line 45

L42:   used_in_fit: true
L43: 
L44: PDG footnote:
L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.

Match 5 of 6 snapshot line 49

L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

Match 6 of 6 snapshot line 50

L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.
L49:   BARR 1993B: <1100E-9 at 90% CL, NA31.
L50:   GIBBONS 1988: <45000E-9 at 90% CL, E731.
L51: 
L52: License note: minimal text snapshot only; no PDG PDF tracked.

value 5.8e-09 +2.9e-9 -2.4e-9

RESOLVED

Match snapshot line 45

L42:   used_in_fit: true
L43: 
L44: PDG footnote:
L45:   BATLEY 2003 extrapolate also to the full kinematical region using a constant form factor and a vector matrix element. The resulting branching ratio is 5.8+2.9-2.4E-9.
L46: 
L47: Older limits listed by PDG, not used in the average:
L48:   LAI 2001: <140E-9 at 90% CL, NA48.