New Result on 𝐊 →𝛑 · Perugia, 10/09/2019 On behalf of the NA62 Collaboration. ... Si pixel beam tracker + Straw tracker LKr Calorimeter from NA48 Cerenkov counters for K

New Result on 𝐊+ → 𝛑+𝛎ത𝛎from the NA62 Experiment

Giuseppe Ruggiero (Lancaster University)

KAON 2019

Perugia, 10/09/2019

On behalf of the NA62 Collaboration

Outline

K+ → π+νതν @ NA62 (2017 Data)

• Physics case

• The NA62 experiment

• Data selection

• 𝜋0 rejection and the 𝜋0 → invisible decay

• Determination of the single event sensitivity

• Background studies

• Result

210/09/2019 Giuseppe Ruggiero - Kaon 2019

𝐊 → 𝛑𝛎ത𝛎 decays: a theoretically clean environment

• FCNC loop processes: sd coupling and highest CKM suppression

• Very clean theoretically: Short distance contribution. No hadronic uncertainties.

• SM predictions [Buras et al. JHEP 11 (2015) 33]

BR 𝐾+ → 𝜋+𝜈 ҧ𝜈 = 8.39 ± 0.30 ∙ 10−11𝑉𝑐𝑏

0.0407

2.8𝛾

73.2°

0.74

= 0.84 ± 0.10 ∙ 10−10

BR 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 = 3.36 ± 0.05 ∙ 10−11𝑉𝑢𝑏

0.00388

2𝑉𝑐𝑏

0.0407

2sin 𝛾

sin 73.2°

2

= 0.34 ± 0.06 ∙ 10−10


𝐊 → 𝛑𝛎ത𝛎 NP Sensitivity

• 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 [Tanimoto, Yamamoto, PTEP 2016 (2016) no.12, 123B02; Blazek, Matak, IntlJModPhys.A

29 (2014), 1450162; Isidori et al. JHEP 0608 (2006) 064]

• LFU violation models [Isidori et. al., Eur. Phys. J. C (2017) 77]

• Leptoquarks [S.Fajfer, et al. arXiv:1802.00786]

• Constraints from existing measurements (correlations model dependent)

Randall - SundrumZ’ (5 TeV) in constrained MFV LFU violation


𝐊+ → 𝛑+𝛎ത𝛎 Experimental State of the Art

𝐾+ decay in flight technique NA62𝐾+ decay at rest technique E787/E949

Data 2016

NA62

𝐵𝑅 𝐾+ → 𝜋+𝜈 ҧ𝜈 = 1.73−1.05+1.15 × 10−10

Phys. Rev. D 77, 052003 (2008)

Phys. Rev. D 79, 092004 (2009)

Phys. Lett. B 791, 156 (2019)


NA62 @ CERN - SPS

LHC

NA62

SPS

Primary goal:

Measurement of BR(K+ → π+νതν)

Technique:

K Decay – in – flight

Broader Physics program

~200 participants: Birmingham, Bratislava, Bristol, Bucharest, CERN, Dubna (JINR), Fairfax (GMU), Ferrara,Firenze, Frascati, Glasgow, Lancaster, Liverpool, Louvain-la-Neuve, Mainz, Moscow (INR), Naples, Perugia,Pisa, Prague, Protvino (IHEP) , Rome I, Rome II, San Luis Potosi, Torino, TRIUMF, Vancouver (UBC)


NA62 Layout

Secondary positive beamDecay region and Detectors

Momentum 75 GeV/c, 1% rms

Divergence (RMS) 100 μrad

Transverse Size 60 × 30mm2

Composition ΤK+ 6% Τπ+ 70% p 24%

2017 typical Intensity 19 × 1011 ppp (450 MHz @ GTK3)

Fiducial region 60 m

K+ decay rate ~ 3 MHz

Vacuum 𝒪 10−6 mbar

Si pixel beam tracker + Straw tracker

LKr Calorimeter from NA48

Cerenkov counters for K id, RICH for p/m id

[NA62 Detector Paper, 2017 JINST 12 P05025]

MNP33400 GeV/c

SPS protons


NA62 Runs

810/09/2019 Giuseppe Ruggiero - Kaon 2019* Including periods of beam off

𝐏𝐫𝐨𝐭𝐨𝐧𝐬𝐨𝐧𝐭𝐚𝐫𝐠𝐞𝐭

2014-15 (16) Pilot run, Commissioning runs

2016 Physics run (45 days*)



𝐃𝐚𝐭𝐞

NA62 Runs: Data Analyzed


~19 × 1011 protons per spill (3 s)

~2 × 1012 K+ decays

This talk

2014-15 (16) Pilot run, Commissioning runs




* Including periods of beam off

𝐏𝐫𝐨𝐭𝐨𝐧𝐬𝐨𝐧𝐭𝐚𝐫𝐠𝐞𝐭

𝐃𝐚𝐭𝐞

𝐊+ → 𝛑+𝛎ത𝛎 Decay-in-flight

mmiss2 = PK+ − Pπ+

2 Process Branching ratio

K+ → π+π0 γ 0.2067

K+ → μ+𝜈 γ 0.6356

K+ → π+π+π− 0.0558

K+ → π+π−e+ν 4.25 ∙ 10−5




2 Process Branching ratio

K+ → π+π0 γ 0.2067

K+ → μ+𝜈 γ 0.6356

K+ → π+π+π− 0.0558

K+ → π+π−e+ν 4.25 ∙ 10−5

Kinematic signal identificationR

eg

ion

1

Region 2




2

𝟏𝟓 < 𝐏𝛑+ < 𝟑𝟓 GeV/c

Particle ID (Cherenkov detectors)

Particle ID (Calorimeters, 𝜇 - veto)

Photon veto

Process Branching ratio

K+ → π+π0 γ 0.2067

K+ → μ+𝜈 γ 0.6356

K+ → π+π+π− 0.0558

K+ → π+π−e+ν 4.25 ∙ 10−5

Kinematic signal identificationR

eg

ion

1

Region 2


NA62 Cornerstones

𝒪 100 ps Timing between sub-detectors

𝒪 103 ÷ 104 Background suppression from kinematics

> 107 Muon suppression

> 107 π0 (from K+ → π+π0) suppression


𝐊+ → 𝛑+𝛎ത𝛎 Analysis

• Trigger streams (hardware L0 + software L1): • «PNN»: kaon and charged particle identification, photon and muon veto

• «Control»: minimum bias

• Offline Analysis:

• Data samples: PNN; Control: K+ → π+π0, K+ → μ+ν, K+ → π+π+π−

• Blind analysis procedure: signal /control regions masked during the analysis

1) Selection

2) Determination of the single event sensitivity

3) Estimation and validation of the background

4) Opening of signal regions and results


𝐊+ → 𝛑+𝛎ത𝛎 Selection

Selection

• K+- π+ matching

• K+ decays in the decay volume

• π+ identification (PID)

• photon rejection

• Multi-track rejection


Measured performance

GTK-KTAG-RICH timing: 𝒪 100 ps

σ mmiss2 ~ 10−3 ΤGeV2 c4

π+ ID: εμ~10−8, επ+~64%

𝜋0 rej: 𝜀𝜋0~1.4 ∙ 10−8, 𝑝𝜋+𝜖 15,35 GeV/c

𝑚𝑚𝑖𝑠𝑠2 : π+/K+ 3-momentum from Straw/Gigatracker, 𝜋+ mass hypothesis

Control data

π+π+π− Region

π+π0 Region

μ+ν

Region

Signal Region 2

Signal

Region 1

𝐾+decays selected before PID

and 𝛾/multi-track rejection

𝝅+ 𝐦𝐨𝐦𝐞𝐧𝐭𝐮𝐦 [𝐆𝐞𝐕/𝐜]

𝒎𝒎𝒊𝒔𝒔

𝟐Τ

𝑮𝒆𝑽𝟐𝒄𝟒

𝛑𝟎 rejection and search for 𝛑𝟎 → invisibleA-priori evaluation of π0 rejection in 𝐾+ → 𝜋+𝜋0 (0.015 < 𝑚𝑚𝑖𝑠𝑠

2 < 0.021 ΤGeV2 c4)

• Same selection, and trigger stream as 𝐾+ → 𝜋+𝜈 ҧ𝜈, about Τ1 3 of the data used for 𝜋𝜈𝜈

• Single-𝛾 detection efficiency from data minimum-bias 𝐾+ → 𝜋+𝜋0 (Tag & Probe)

• π0 rejection evaluated from convolution with MC 𝐾+ → 𝜋+𝜋0 𝛾

• Validation: side-bands with expected rejection 𝒪 10−7 where 𝜋0 → invisible excluded

• π0 rejection expected: 2.8−2.1+5.0 × 10−9 (𝜋+ momentum 25-40 GeV/c)


[E949, PRD72 (2005)]

Result• BR(π0 → invisible) normalized to 𝜋0 → 𝛾𝛾

• Background expected: 10−8+22 (𝐾+ → 𝜋+𝜋0)

• Events observed: 12

BR π0 → invisible < 4.4 × 10−9 @ 90% CLUL 60 times stronger than past measurement

𝝅𝝂𝝂: Data After Selection

Region 2

Region 1

π+π0

Control Regions

Upstream background control region

μ+νControl Region

High - P

control region

Control and signal regions MASKED


NA62 Preliminary

π+π+π−

Control Region

𝝅𝝂𝝂 Single Event Sensitivity (S.E.S.)

S. E. S. =Br πνν

Nπνν𝑒𝑥𝑝

• Ratio of πνν and π+π0 acceptances allows cancellation of systematic effects

• Computation in bins of π+ momentum and instantaneous beam intensity

Nπνν𝑒𝑥𝑝

≈ Nππ εRV εtriggerAπννAππ

Br πνν

Br ππ

Nπνν𝑒𝑥𝑝

Expected number of πνν events

Br πνν SM πνν branching ratio

Nππ K+ → π+π0 from control πνν-like selected without γ/multiplicity rejection

εRV πνν loss due to γ/multi-track rejection because of random activity

εtrigger PNN trigger efficiency

Aπνν,ππ Monte Carlo acceptances for πνν (~3.0%*) and π+π0 (~8.5%)

Br ππ PDG K+ → π+π0 branching ratio

1810/09/2019 Giuseppe Ruggiero - Kaon 2019* Vector form factors

𝝅𝝂𝝂 Single Event Sensitivity

𝛆𝐑𝐕 measured on K+ → μ+ν data 𝛆𝒕𝒓𝒊𝒈𝒈𝒆𝒓 measured on data


• Intensity measured event-by-event using Gigatracker time sidebands

NA62 Preliminary𝜀𝑅𝑉 2017 = 0.638 ± 0.014

𝝅𝝂𝝂 S.E.S: Results

• Integrated over beam intensity and 𝜋+ momentum

• Error budget (S.E.S.)

• External error on 𝑁𝜋𝜈𝜈𝑒𝑥𝑝

from 𝐵𝑟(𝜋𝜈𝜈) = 0.84 ± 0.10 × 10−10


𝑆. 𝐸. 𝑆. = 0.389 ± 0.021 × 10−10 𝑁𝜋𝜈𝜈𝑒𝑥𝑝

= 2.16 ± 0.12 ± 0.26𝑒𝑥𝑡

𝝅𝝂𝝂: Data After Selection

π+π0

Control Regions

Upstream background control region

μ+νControl Region

High - P

control region

Control and signal regions MASKED


NA62 Preliminary

π+π+π−

Control Region

Region 2

Region 1

𝐊+ → 𝛑+𝛑𝟎 Backgrounds

Fraction of π+π0

in signal region measured on control data

Data in π+π0 regionafter πνν selection:γ −rejection applied

Minimum bias K+ → π+π0 data used

to study the tails of the mmiss2

Expected K+ → π+π0

in signal regions afterπνν selection


Background Validation

K+ → π+π0 background expectedand observed in control regions(CR1+CR2) after πνν selection

K+ → μ+ν background expected andobserved in control region (CR) afterπνν selection


Expected Background from 𝑲+

decays


Normal 𝐊+Decay

Achromat

𝐊+(𝟕𝟓 𝐆𝐞𝐕)

Collimator

1st Straw Chamber

Decay region


𝜸, 𝝂

𝜸, 𝝂

Decay vertex

Gigatracker

z

y


Normal 𝐊+Decay

Achromat


Collimator

1st Straw Chamber

Decay region


𝜸, 𝝂

𝜸, 𝝂

Decay vertex

z

y

𝐾+ → 𝜋+𝜈 ҧ𝜈 projected at collimator (MC)


NA62 Preliminary

Upstream Decay / Interaction

Achromat


Collimator

1st Straw Chamber

Decay region

𝜸𝜸

• K+ decays/interacts in the achromat

• Secondary 𝜋+ downstream

• Beam elements block additional particles


z

y

Upstream Background

Achromat


Collimator

1st Straw Chamber

Decay regionIn-time pileupbeam particle

𝜸𝜸




• Pileup beam particle tagged as K+

Fake decay vertex


z

y

Upstream Background

Achromat


Collimator

1st Straw Chamber


𝜸𝜸

Fake decay vertex




• 𝜋+ scattering in straw chamber 1



z

y

NA62 Preliminary

Upstream Background

Achromat


Collimator

1st Straw Chamber


𝜸𝜸

Fake decay vertex







z

y

NA62 PreliminaryRegion removed offline

Upstream Background

Achromat


Collimator

1st Straw Chamber


𝜸𝜸






Count events on data withinverted 𝐾 − 𝜋 matching

Estimate the probability tooccur from data/simulation


z

y

Fake decay vertex

Total Expected Background


Final Background ValidationSignal and background evaluated in the 35-40 GeV/c signal-like regions

𝐛𝐚𝐜𝐤𝐠𝐫𝐨𝐮𝐧𝐝 = 𝟎. 𝟔𝟐 ± 𝟎. 𝟏𝟑

𝐬𝐢𝐠𝐧𝐚𝐥 = 𝟎. 𝟏𝟑 ± 𝟎. 𝟎𝟏


NA62 Preliminary

Final Background Validation1 event observed in 35-40 GeV/c region


NA62 Preliminary

Opening the Box


NA62 Preliminary


NA62 Preliminary

Box Opened


NA62 Preliminary

Box Opened2 events observed in signal region

2017 Result• Counting experiment

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 = 0.20−0.20+0.69 × 10−10• Two-sided 68% band:

• Upper limits (CLs method)


Observed Expected (background only) CL

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.76 × 10−10 𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.41 × 10−10 90%

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 2.11 × 10−10 𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.76 × 10−10 95%

Events observed 2

Single event sensitivity 0.389 ± 0.021 × 10−10

Expected background 1.50 ± 0.30

𝒎𝒎𝒊𝒔𝒔𝟐 Signal and Background 2017


NA62 Preliminary

2016+2017 Result

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 = 0.47−0.47+0.72 × 10−10• Two-sided 68% band:

• Upper limits (CLs method)


Observed Expected (background only) CL

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.85 × 10−10 𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.32 × 10−10 90%

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 2.44 × 10−10 𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.62 × 10−10 95%

Events observed 3

Single event sensitivity 0.346 ± 0.017 × 10−10

Expected background 1.65 ± 0.31

• 2016 and 2017 data uncorrelated

𝐊+ → 𝛑+𝛎ത𝛎 Experimental Status


PRD 10 (1974) 897

NPB 304 (1988) 205

PRD 54 (1996) 6782

PRD 76 (2007) 034017

PRD 83 (2011) 034030

JHEP 11 (2015) 33

NA62 Preliminary

𝐊+ → 𝛑+𝛎ത𝛎 Experimental Status


NA62 Preliminary

𝑲+ → 𝝅+𝝂ത𝝂 and 𝑲𝑳 → 𝝅𝟎𝝂ത𝝂


• New Grossman – Nir limit: 𝐵𝑟 𝐾𝐿 → 𝜋0𝜈𝜈 < 8.14 × 10−10 @ 90% 𝐶𝐿

Conclusions

• Two events in signal region observed in 2017 data• S. E. S. = 0.389 ± 0.021 × 10−10

• Expected background: 1.50 ± 0.30 events


• Constraints on the largest enhancements allowed by NP models

• 𝐾+ → 𝜋+𝑋, 𝑋 →invisible at the 𝜋0 mass:

• 2016+2017 NA62 result:

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 < 1.85 × 10−10 @ 90% 𝐶𝐿

𝐵𝑟 𝐾+ → 𝜋+𝜈 ҧ𝜈 = 0.47−0.47+0.72 × 10−10

𝐵𝑟 π0 → invisible < 4.4 × 10−9 @ 90% 𝐶𝐿

Plans

• 2018 data analysis in progress• 2 × 2017 data sample

• On-going studies to increase signal efficiency

• On-going studies to reduce the random veto

• Shape analysis


• >2021 data taking: • Plans to strongly suppress the upstream background

• Run at higher beam intensity foreseen

10/09/2019 Giuseppe Ruggiero - Kaon 2019 46

SPARES

Particle Identification

Calorimeters RICH


NA62 Preliminary

Upstream Background Estimate

• Probability to tag a pileup beam particle as a K+ depends on• Closest distance of approach (CDA) between kaon and pion track

• Timing between KTAG – Gigatracker – RICH


• Probability estimation («mis-tag probability»)• CDA shape of upstream events from simulation validated with data

• Computed as a function of KTAG – Gigatracker and KTAG – RICH time difference

• Background estimation• Number of upstream events from data selected with inverted K+/π+ matching

• Upstream events weigthed with the mis-tag probability as a function of the KTAG – Gigatracker and KTAG – RICH time difference

• Estimation validated on data inverting cuts against upstream events

Upstream Background Validation


• Sample: upstream-enriched data sets selected

in mmiss2 signal regions

• Expectation: result of the upstream estimation applied to a given sample

NA62 Preliminary

𝐊+ → 𝛑+𝛑−𝐞+𝛎 (𝑲𝒆𝟒) Background:

Validation


• Sample: Ke4-enriched data

sets selected in mmiss2 signal

regions

• Expectation: MC normalized to 𝐾+ → 𝜋+𝜋0 like 𝜋𝜈𝜈

NA62 Preliminary

Documents

New Result on 𝐊 →𝛑 · Perugia, 10/09/2019 On behalf of the NA62 Collaboration. ... Si pixel beam tracker + Straw tracker LKr Calorimeter from NA48 Cerenkov counters for K