Measurement of the very rare K+ → π + ν ν̄ decay
Michal Zamkovsky
Université Catholique de Louvain, Belgium

August 22, 2021

Michal Zamkovsky

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NA62 experiment at CERN

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NA62 Detector layout

SPS Beam:Secondary beam:
Decay Region
400 GeV/c protons
75 GeV/c momentum, 1% RMS
60 m long fiducial region
12100µraddivergence(RMS)∼ 3 MHz K + decay rate
1.9 × 10 p/spill
2Vacuum O(10−6 ) mbar
3.5 s spill
60 × 30 mm transverse size
∼ 1018 POT/year

K + (6%)/π + (70%)/p(24%)
450 MHz of particles at GTK3
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NA62 Detector layout

Upstream detectors (K+ )Downstream detectors (π + )KTAG: differential Cherenkov
STRAW: track momentum spectrometercounter for K+ ID
CHOD: scintillator hodoscopes
GTK: Si pixel beam tracker
LKr/MUV1/MUV2: Calorimeters
CHANTI: Anti-counter for inelastic
RICH: Cherenkov counter for π/µ/e ID
beam-GTK3 interactions
LAV/SAC/IRC: Photon veto detectors
MUV3: Muon detector
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Theoretical motivation - Standard Model
FCNC loop processs → d coupling and highest CKM suppression (BR∼ |Vts × Vtd |2 )Very clean theoretically
Short distance contribution
and no hadronic uncertainties
Hadronic matrix element extracted
from well-known decay K+ → π 0 e + ν
Theoretical error budget
dominated by CKM parameters
SM predictions[Buras et al., JHEP 1511 (2015) 033]

BR (K+ → π + ν ν̄ ) =(8.39 ± 0.30) · 10−11

BR (K0L → π 0 ν ν̄ ) =(3.36 ± 0.05) · 10−11|Vcb |2.8 

|Vub |γ0.74= (8.4 ± 1.0) · 10−1173.2◦

0.0407
2 0.00388

Michal Zamkovsky|Vcb |0.0407

2 sin γ
sin 73.2

Lomonosov 2021

2= (3.4 ± 0.6) · 10−115 / 17

Theoretical motivation - Beyond the Standard Model
Simplified Z, Z’ models [Buras, Buttazzo,Knegjens, JHEP 1511 (2015) 166]
Littlest Higgs with T-parity [Blanke, Buras, Recksiegel, EPJ C76 (2016) no.4 182]
Custodial Randall-Sundrum [Blanke, Buras, Duling, Gemmler, Gori, JHEP 0903 (2009) 108]MSSM non-MFV [Blazek, Matak Int.J.Mod.Phys.A29 (2014) 1450162; Isidori et al. JHEP 0608 (2006) 064]
LFU violation models [Isidori et. al., Eur. Phys. J. C (2017) 77]
Leptoquarks [S. Fajfer, N. Košnik, L. Vale Silva, arXiv:1802.00786v1 (2018)]
Constraints from existing measurements (correlations model dependent):
Kaon mixing and CPV, CKM fit, K,B rare meson decays, NP limits from direct searches

K → πν ν̄ can discriminate among different new physics scenarios
Z 0 (5TeV ) in ConstrainedMFV

Randall − SundrumMichal Zamkovsky

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LittlestHiggs

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K+ → π + ν ν̄: Experimental status
History of K+ → π + ν ν̄ measurement

E787+E949−11BR(K+ → π + ν ν̄) = (17.3+11.5−10.5 ) × 10

[PRD 77, 052003 (2008),
PRD D 79, 092004 (2009)]

NA62 2016+2017 BR(K+ → π + ν ν̄) < 1.78 × 10−10 @ 90%CL [PLB 791 (2019) 156-166,
JHEP 11 (2020) 042]
NA62 2018 - THIS TALK
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Analysis strategy for K+ → π + ν ν̄Kaon decays in flightSignal: Time and space K+ − π + matching2
Regions defined by: mmiss= (PK − Pπ )2
The analysis is mostly cut based
Blind analysis: Signal and background ctrl
regions are kept blind throughout the analysis

Main background sources
Decay mode
BR
Main rejection tools
K+ → µ + ν(γ)
63% µ−ID + kinematics
K+ → π + π 0 (γ)
21% γ-veto + kinematics
K+ → π + π + π −6%multi + kinematics
K+ → π + π 0 π 02%
γ-veto + kinematics
5%e−ID + γ-veto
K+ → π 0 e + ν eK+ → π 0 µ + ν µ
3%µ−ID + γ-vetoMichal Zamkovsky

Requirements
O(100) ps timing between
sub-detectors
O(104 ) background suppression
with kinematics
O(107 ) µ-suppression(K+ → µ + ν)
> (107 ) π 0 -suppression(K+ → π + π 0 ,π 0 → γγ)

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Event selection
Signal regions
Three different ways to calculate mmiss
to avoid mis-reconstruction:
2mmiss= (STRAW , GTK )2mmiss= (RICH, GTK )2mmiss= (STRAW , Beam)

Selection
Single track in final state topology
matched with upstream K+π + identification
Photon rejection
Multi-track rejection
105 < Zvertex < 165 m
15 < Pπ + < 35 GeV/c in R1
15 < Pπ + < 45 GeV/c in R2(best µ/π discrimination in RICH& to leave at least 30 GeV of Emiss )Performanceε(µ) = 1 · 10−8 (64% π + efficiency)ε(π 0 ) = (1.4 ± 0.1) · 10−8
σ (mmiss ) = 1 · 10−3 GeV2 /c 4
σ (t) ∼ O(100) ps

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Upstream background

Background source if:
a kaon decays upstream, and only a pion enters in the decay region
there is an in-time pileup beam particle (in KTAG and GTK)
the upstream generated pion enters in the decay region and is scattered in
the first STRAW chamber
In 2018 collimator was replaced to remove early decays mechanism
Data sample split in subset S1 (Old collimator) and S2 (New collimator)

Upstream background
Michal Zamkovsky

S1: 0.54+0.39−0.21
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S2: 2.76+0.90−0.70
10 / 17Background from kaon decays
Data in bkg region after πννselection:γ-rejection appliedexpNbkg(region) = N(bkg ) · f kin (region); bkg = π + π 0 ∨ µ + ν

Fraction of bkg in signal region
Expected bkg in signal
regions after πνν selection measured on control data

Backgroundµ +νπ +π 0π +π −e +ν
π +π +π −
π + γγπ 0 `+ ν
Upstream
Total

Subset S1
0.23 ± 0.02
0.19 ± 0.06
0.10 ± 0.03
0.05 ± 0.02
< 0.01< 0.001.39
0.54+0−0.21.40
1.11+0−0.22

Subset S2
0.52 ± 0.05
0.45 ± 0.06
0.41 ± 0.10
0.17 ± 0.08
< 0.01< 0.001.902.76+0−0.70.91
4.31+0−0.72

Control K+ → π + π 0 data used to study the tails of the mmiss 2 distribution
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Background summary
Background expectation validated using control regions
Signal Regions are blinded!
Observed (expected) events in control
regions

Validation: expected vs observed
background eventsin control regions in bins of π +Michal Zamkovsky
momentum

Backgroundµ +νπ +π 0π +π −e+νπ +π +π −π + γγ
π 0 `+ ν
Upstream

Subset S1
0.23 ± 0.02
0.19 ± 0.06
0.10 ± 0.03
0.05 ± 0.02
< 0.01< 0.001
0.54+0.39−0.21

Subset S2
0.52 ± 0.05
0.45 ± 0.06
0.41 ± 0.10
0.17 ± 0.08
< 0.01< 0.001
2.76+0.90
−0.70

Total

1.11+0.40−0.22

4.31+0.91−0.72

Background estimates summed overRegion 1 and Region 2
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Opening signal regions - 2018 data set
17 events observed in signal region

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BR(K+ → π + ν ν̄) at NA62: Result
SES × 1010
Aπνν × 102
Expected SM signalExpected background
Observed events

2016 data
3.15 ± 0.24
4 ± 0.4
0.27 ± 0.04
0.15 ± 0.090
12017 data
0.39 ± 0.02
3 ± 0.3
2.16 ± 0.13
1.46 ± 0.30
22018 S1 data
0.54 ± 0.04
4 ± 0.4
1.56 ± 0.10
1.11+0.40−0.22
22018 S2 data
0.14 ± 0.01
6.4 ± 0.6
6.02 ± 0.39
4.31+0.91−0.72
15−11 at 68% CL
BR(K+ → π + ν ν̄) = (10.6+4.0−3.4 |stat ± 0.9syst ) × 10

Michal Zamkovsky

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14 / 17Interpretation of result
Large deviations from SMBR(K+ → π + ν ν̄) are excluded→ high precision measurement needed

Grossman-Nir limit:
BR(K+ → π + ν ν̄) < 4.3 · BR(KL → π 0 ν ν̄)[Phys. Lett. B 398, 163 (1997)]

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Future prospects for K+ → π + ν ν̄Data taking between CERN LS2 and LS3:
Upstream background suppression: beam line re-arranging to swipe away
upstream π + , adding a fourth Gigatracker station (GTK-4), newveto-counter system to detect upstream decay products
Additional off-axis calorimeter (HASC-2) to further suppress K+ → π + π 0backgroundRun at higher beam intensity foreseen
Goal: BR(K+ → π + ν ν̄) measurement with O(10%) statistical precision
Michal Zamkovsky

Lomonosov 2021

16 / 17Conclusion
Complete result from Run 1 (2016+2017+2018):
Observed 20 events
Expected background: ∼ 7.0−11 at 68% CLBR(K+ → π + ν ν̄) = (10.6+4−3.4 |stat ± 0.9syst ) × 10
3.5σ significance
Most precise measurement ever performed for the K+ → π + ν ν̄golden channel
Large BR(K+ → π + ν ν̄) values with respect to SM expectation start
to be excluded: high precision measurement needed

Important hardware improvements ongoing for the next data-taking:strong suppression of the main background sources
Plan to achieve O(10%) precision with data collected
between CERN LS2 and LS3Michal Zamkovsky

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