kaon physics europe koval...kaon physics in europe 3rd j-parc symposium tsukuba, japan, 23 –26...

Kaon Physics in Europe

3rd J-PARC Symposium Tsukuba, Japan, 23 – 26 September 2019

Michal Koval Charles University in Prague

michal.koval@cern.ch

on behalf of the NA62 collaboration

Introduction

• Selection of recent experimental results from kaon physics in Europe will be presented in this talk• NA62, NA48/2, KLOE & KLOE-2, LHCb

• The latest results were presented at the International Conference on Kaon Physics 2019• https://indico.cern.ch/event/769729/

• See reference talks for analysis details

25. 9. 2019 J-PARC Symposium 2019 Kaon Physics in Europe michal.koval@cern.ch 1

NA62 @ CERN – SPS

• New result on 𝐾+ → 𝜋+𝜈 ҧ𝜈• New results on searches for:

• Heavy neutral lepton• Invisible dark photon• Lepton number violation

Reference talks at KAON 2019 conference:• G. Ruggiero• E. Goudzovski• T. Spadaro• V. Duk

NA62 Experiment at CERN – SPS

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

SPSNA62:center of the LHC

Jura mountains

Geneva airport

Primary goal: • Measurement of BR(𝐾+ → 𝜋+𝜈 ҧ𝜈)

Technique:• Kaon decay-in-flight• Kaon momentum: 75GeV/c

Requirements:• 1013 𝐾 decays• Signal acceptance 𝒪(10%)• 𝒪(1012) background rejection

NA62 Beam and Detector

Cherenkovkaon tagger t=70ps

Spectrometerstraw chambers

~5 MHz K+ Decays

t=70ps

[NA62 Detector Paper, 2017 JINST 12 P05025]

SPS Beam:• 400 GeV/c• 2⋅1012 protons / spill• 3.5s spill• Target: Beryllium

Secondary Positive Beam:• Unseparated hadrons:

K+(6%)/ +(70%)/p(24%) • Momentum: 75GeV/c (±1%)• Divergence < 100rad• Transverse size: (60 x 30) mm2

• 750MHz beam rate @ GTK

Decay Region:• 60m long fiducial region• ∼5MHz K+ decay rate• Vacuum ∼ 10-6 mbar

NA62 Runs

NA62 timeline:

• Dec 2008: NA62 approval• 2008 – 2014

Detector R&D and installation• 2014 Pilot run• 2015 Commissioning• 2016 – 2018 NA62 Run 1• 2021 – 2023 NA62 Run 2 (TBA)

NA62 luminosity:

Previous NA62 K+ → 𝜋+𝜈 ҧ𝜈 result

New preliminary results

𝐾+ → 𝜋+𝜈 ҧ𝜈

• FCNC process forbidden at tree level

• Highly CKM suppressed (BR ~ |Vts*Vtd|2)

Theoretically very clean:

• dominant short-distance contribution

• hadronic matrix element extracted from

precisely measured BR(K+ → π0e+ν)BR(𝐾+ → 𝜋+𝜈 ҧ𝜈) = (8.4 ± 1.0) × 10−11

SM Prediction, error CKM parametric [Buras et al., JHEP 1511 (2015) 033]:

Sensitivity to New Physics:

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

IntlJModPhys.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, et al. arXiv:1802.00786]

𝐾+ → 𝜋+𝜈 ҧ𝜈 Experimental Status

E787/E949: Kaon decay at rest technique

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)

NA62: Kaon decay in flight technique

BR(𝐾+ → 𝜋+𝜈 ҧ𝜈) < 14 × 10−10 @ 95% CLPhys. Lett. B 791, 156 (2019)

NA62 Data 2016

𝐾+ → 𝜋+𝜈 ҧ𝜈 Event Selection

Selection criteria:

• Single track topology

• 𝜋+ identification

• Photon rejection

• Multi-track rejection

Performances:

• 휀𝜇+ ∼ 10−8

• 64% 𝜋+ efficiency

• 휀𝜋0 ∼ 1.4 ⋅ 10−8

• 𝜎 𝑚𝑚𝑖𝑠𝑠2 = 1 ⋅ 10−3 GeV2/c4

• 𝜎𝑇 ∼ O(100ps)

𝐾+ → 𝜋+𝜋+𝜋−

𝐾+ → 𝜋+𝜋0

𝐾+ → 𝜇+𝜈

NA62 Control data

𝐾+decay events in the fiducial decay region

Signal and background control regions are kept blind throughout the analysis

𝑚𝑚𝑖𝑠𝑠2 = 𝑃𝐾 − 𝑃𝜋+

𝐾+ → 𝜋+𝜈 ҧ𝜈: Opening the Box

NA62 2017 DataPreliminary

𝐾+ → 𝜋+𝜈 ҧ𝜈: Box Opened

NA62 2017 DataPreliminary

2 events observed in signal region

𝐾+ → 𝜋+𝜈 ҧ𝜈: 2016+2017 Result• Counting experiment:

• Upper limits (CLs method):

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

Events observed 3

Single event sensitivity 0.346 ± 0.017 × 10−10

Expected background 1.65 ± 0.31

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%

𝐾+ → 𝜋+𝜈 ҧ𝜈: Historical Perspective

𝐾+ → 𝜋+𝜈 ҧ𝜈 and 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈New Grossman – Nir limit:

BR(𝐾𝐿 → 𝜋0𝜈 ҧ𝜈) < 8.14 × 10−10 @ 90% CL

Heavy Neutral Leptons

• Observation of neutrino oscillations → massive neutrinos need to be accommodated in SM

• Example of a SM extension: Neutrino Minimal SM (νMSM) Asaka et al., PLB 620 (2005) 17

• HNL represent the fermion portal to a possible hidden sector of particles

• GeV-scale HNLs can be observed via their production and decay

• NA62 can search for HNL production in kaon decays:

𝐾+ → 𝑙+𝑁 (𝑙 = 𝜇, 𝑒)

Conservative limits on couplings assuming HNL decay products are not observable

Global limits on 𝑈𝑒42vs 𝑚4

Global limits on 𝑈𝜇42vs 𝑚4

De Gouvêa and Kobach, PRD93 (2016) 033005

HNL Production Search @ NA62• Search for HNL production in 𝐾+ → 𝑙+𝑁 (𝑙 = 𝜇, 𝑒) decays in 2016+2017 data

• Squared missing mass: 𝑚𝑚𝑖𝑠𝑠2 = 𝑃𝐾 − 𝑃𝑙+

• HNL production signal: a spike above continuous missing mass spectrum

𝐾+ → 𝑒+𝜈, BR = 1.6 × 10−5 , 1.19M candidates

𝐾+ → 𝜇+𝜈 ,𝜇+ → 𝑒+𝜈𝜈

𝐾+ → 𝜇+𝜈 ,

𝜋+ → 𝑒+𝜈

𝐾+ → 𝜇+𝜈, BR = 64% , 920M candidates

HNL search region

HNL Production Limits from NA62• No statistically significant excess found

• Upper limits on 𝑈𝑙42 vs 𝑚𝐻𝑁𝐿 found

• New preliminary NA62 resultbased on ∼ 1/3 of the full data set

• Improvements over earlier production searches by up to two orders of magnitude in terms of 𝑈𝑙4

• Expected improvement with the full data set (2016 – 2018): less than a factor of 2

• For 𝑈𝑒42:

• Improvement on the PS191 HNL decay search limit

• Excluded range allowed by Big Bang Nucleosynthesis up to 300 MeV/𝑐2mass

Limits from HNL production searches

Search for Dark Photon @ NA62• A new light vector boson 𝐴′, representing the vector portal to a hidden sector

• The minimal 𝐴′ scenario: kinetic mixing with the SM photon (mixing parameter 휀)

• Possible production at NA62 in: 𝜋0 → 𝛾𝐴′

• If 𝑚𝐴′ > 2 𝑚𝜒, 𝜒 ∈ hidden sector: sizeable invisible width of 𝐴′

• Search for invisible dark photon production at NA62 (2016 data set):

• Select a large sample of 𝜋0s with known momentum using 𝐾+ → 𝜋+𝜋0 decays

• 𝐴′ signal selection: 𝜋0 tag in 𝐾2𝜋 + 1 detected photon + no additional activity

• Search for peaks in the 𝑚𝑚𝑖𝑠𝑠2 = 𝑃𝐾 − 𝑃𝜋+ − 𝑃𝛾

2spectrum in the 30, 130 MeV range

𝜋0 → 𝛾𝛾 background artificially enhanced

Dark Photon Limits from NA62

PRD 97 (2018)

JHEP 05 (2019) 182

𝜺𝟐

𝑴𝑨′[MeV/𝒄𝟐]

PRL 119 (2017)

• No statistically significant excess found in 2016 data

• Upper limits on 휀2 vs 𝑚𝐴′ set

• Published most stringent limits in the 𝑚𝐴′∈ 60, 110 MeV/𝑐2

Dark Photon Limits from NA62

• No statistically significant excess found in 2016 data

• Upper limits on 휀2 vs 𝑚𝐴′ set

• Excluded region now covered by NA64 (𝑒−dump + 𝐸𝑚𝑖𝑠𝑠)

• Expected yield improvement at NA62 using full 2016-2018 data set: O(100)

Lepton Number Violation

• LNV kaon decays 𝐾+ → 𝜋−𝑙+𝑙+ , Δ𝐿 = 2; ΔL𝑙 = 2, 𝑙 = 𝑒, 𝜇

• Can be mediated by a massive Majorana neutrino[JHEP 0905 (2009) 030, Phys. Lett. B491 (2000) 285]

• Experimental status:BR(𝐾+ → 𝜋−𝑒+𝑒+) < 6.4 × 10−10@ 90% CL BNL E865: [PRL 85 (2000) 2877]BR(𝐾+ → 𝜋−𝜇+𝜇+) < 8.6 × 10−11@ 90% CL NA48/2: [PL B769 (2017) 67]

• New search for 𝐾+ → 𝜋− 𝑙+ 𝑙+ at NA62 [Phys. Lett. B 797 (2019) 134794]:

• Subset of 2017 data: ∼3 months of data taking, blind analysis procedure

• Corresponding SM decay modes 𝐾+ → 𝜋+ 𝑙+ 𝑙− used for normalization

Search for 𝐾+ → 𝜋− 𝑒+ 𝑒+ @ NA62

SM selection: 𝐾+ → 𝜋+ 𝑒+ 𝑒− LNV selection: 𝐾+ → 𝜋− 𝑒+ 𝑒+

SM 𝐾+ → 𝜋+ 𝑒+ 𝑒− candidates observed: 𝑁 = 2484

Kaon decays in the fiducial volume: 𝑁𝐾 = 2.14 ± 0.07 × 1011

Expected background in the signal region: 𝑁 = 0.16 ± 0.03

Observed events: 𝑁 = 0, CLs upper limit: BR(𝐾+ → 𝜋− 𝑒+ 𝑒+) < 2.2 × 10−10 @90% CL

Signalregion

Search for 𝐾+ → 𝜋− 𝜇+ 𝜇+ @ NA62

SM selection: 𝐾+ → 𝜋+ 𝜇+ 𝜇− LNV selection: 𝐾+ → 𝜋− 𝜇+ 𝜇+

SM 𝐾+ → 𝜋+ 𝜇+ 𝜇−candidates observed: 𝑁 = 8357

Kaon decays in the fiducial volume: 𝑁𝐾 = 7.94 ± 0.23 × 1011

Expected background in the signal region: 𝑁 = 0.91 ± 0.41

Observed events: 𝑁 = 1, CLs upper limit: BR(𝐾+ → 𝜋− 𝜇+ 𝜇+) < 4.2 × 10−11 @90% CL

Signalregion

NA62 Future ProspectsK+ → 𝜋+𝜈 ҧ𝜈:

• 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

• Plans for ≥2021 data taking: • Suppression of the dominant upstream background

by re-arranging beam line set-up• Run at higher beam intensities foreseen

Many ongoing analyses @ NA62; stay tuned for new results!

• Searches for rare & forbidden decays

• Exotic particle searches

• Precision measurements

NA48/2 @ CERN – SPS • Predecessor of NA62, similar detector setup• Simultaneous 𝐾+& 𝐾− beam, 𝑝 = 60 ± 3 GeV/c • Data taking: 2003 – 2004

• First observation and measurement of 𝐾± → 𝜋±𝜋0𝑒+𝑒−

• Measurement of charged kaon semileptonic form factors

Reference talks at KAON 2019 conference:• B. Bloch-Devaux• G. Lamanna

𝐾± → 𝜋±𝜋0𝑒+𝑒− Observation at NA48/2• BR measurement of 𝐾± → 𝜋±𝜋0𝑒+𝑒−: test of ChPT predictions:

• NA48/2: 4919 signal candidates observed, background estimation: 241(20)

𝐵𝑅 = 4.237 ± 0.063𝑠𝑡𝑎𝑡 ± 0.033𝑠𝑦𝑠𝑡 ± 0.126𝑒𝑥𝑡 × 10−6 = 4.237 ± 0.145 × 10−6

• Published in PLB 788 (2019) 552

• Error dominated by external uncertainty on the 𝜋𝐷0 branching ratio (normalization channel)

• In agreement with ChPT calculations from EPJ C78 (2018) 265: 𝐵𝑅 =4.229 × 10−6

EPJ C20 (2001) 371; EPJ C72 (2012) 1872; EPJ C78 (2018) 265

Charged Kaon Semileptonic Form Factors• 𝐾± → 𝜋0𝑙±𝜈 (𝑙 = 𝑒, µ) used in precise determination of CKM matrix element |𝑉𝑢𝑠|

• Differential decay rate:

• NA48/2 measurementJHEP 1810 (2018) 150Analysis of 2004 minimum bias sampleSelected events: 4.4 × 106 𝐾𝑒3

2.3 × 106 𝐾𝜇3

Combined 𝐾𝑙3 result:1σ ellipse (39.4% CL)

Compatible with previous results

Improved precision

Form factors

KLOE & KLOE-2 @ DAφNE

• New measurement of BR(𝐾𝑆 → 𝜋±𝑙∓𝜈)• 𝐾𝑆 semileptonic charge asymmetry• Direct CPT tests in Kaon transitions

Reference talks at KAON 2019 conference:• E. Selce• E. De Lucia

KLOE & KLOE-2 @ DAφNE • DAφNE Frascati φ-factory:

an 𝑒+𝑒− collider @ 𝑠 = 1019.4 MeV/𝑐2= 𝑀Φ

• Total KLOE + KLOE-2 luminosity: ∼ 8 fb−1

∼ 2.4 × 1010Φ decays∼ 8 × 109 𝐾𝐿 − 𝐾𝑆 pairs

• 𝐾𝑆 production taggedby detecting 𝐾𝐿 interactionin the calorimeter

• Efficiency ∼ 30%

KLOE-2

𝐾𝑆 → 𝜋±𝑙∓𝜈 Measurement

• Kaon semileptonic decays important for determination of CKM matrix element 𝑉𝑢𝑠• BR(𝐾𝑆 → 𝜋±𝑒∓𝜐) = (7.046 ± 0.078stat ± 0.049syst) × 10−4 [PLB 636 (2006) 173]

• Measurement by KLOE with 0.4 fb−1

• Finalization of a new analysis of full KLOE data set ongoing

• BR 𝐾𝑆 → 𝜋±𝜇∓𝜐

• No existing measurement, expected value: 0.666 × BR(𝐾𝑆 → 𝜋±𝑒∓𝜐)• New preliminary result from KLOE:

BR(𝐾𝑆 → 𝜋±𝜇∓𝜐) = (4.57 ± 0.11stat ± 0.16syst) × 10−4

𝐾𝑆 → 𝜋±𝑒∓𝜐 before fit𝐾𝑆 → 𝜋±𝜇∓𝜐 before fit

Signal counts obtained by fitting MC shapes to data in the distribution of

𝑀𝑙2 = 𝐸𝐾𝑆,tag − 𝐸𝜋 − 𝑝mis

2− 𝑝𝑙

Charge Asymmetry in Semileptonic Kaon Decays

• 𝐴𝑆 = 1.5 ± 9.6stat ± 2.9syst × 10−3 , KLOE with 410 pb−1 [PLB 636 (2006) 173]

• 𝐴𝐿 = 3.332 ± 0.058stat ± 0.047syst × 10−3, KTeV Coll. [PRL 88 181601 (2002)]

• New KLOE result with 1.6 fb−1 [JHEP 09 (2018) 2]:

𝐴𝑆 = −4.9 ± 5.7stat ± 2.6syst × 10−3

• Combined KLOE result [JHEP 09 (2018) 2]:𝐴𝑆 = −3.8 ± 5.0stat ± 2.6syst × 10−3

• Almost x2 precision improvement, with KLOE-2 data: 𝛿𝐴𝑆 𝑠𝑡𝑎𝑡 → ∼ 3 × 10−3

Direct Test of CPT with Kaons• Direct and model independent tests of CPT symmetry through the comparison of

transition rates between flavor and CP eigenstates

DRCPT: cleanest CPT observable, ≠ 1 ⇒ CPT violationnever measured to date

preliminary

Using 𝐴𝐿 from KTeV and 𝐴𝑆:

1.7 fb−1

LHCb @ CERN – LHC

• Kaons at LHCb• New result on 𝐾𝑆 → 𝜇+𝜇−

Reference talks at KAON 2019 conference:• F. Dettori• M. Ramos Pernas

Kaons at LHCb• About 1 strange hadron per event produced at LHCb (Compared to: ∼ 10−3 𝐵𝑆

• Reconstruction and trigger however bring this number downAlves et al. arXiv:1808.03477

• Hardware trigger (L0): limiting factor for strange physics both in Run-I and Run-II

• Software improvements for strange physics in Run 2 [LHCb-PUB-2017-023]:

• Allowed to achieve new result on 𝐾𝑆 → 𝜇+𝜇− (see next slides)

• Future prospects (Run-III) – L0 hardware trigger is removed:

• Great improvements possible for many channels

• Kaon decay sensitivity studies are promising [arXiv:1808.03477], e.g.:

𝐾𝑆 → 𝜇+𝜇−

• Strongly suppressed Flavor-changing Neutral Current (FCNC) transition

• Dominated by long distance contributions through 𝐾0 → 𝛾𝛾

• SM prediction: BR(𝐾𝑆 → 𝜇+𝜇−) = 5.18 ± 1.50LD ± 0.02SD × 10−12

[JHEP 05 (2018) 024], [JHEP 01 (2004) 009], [NPB 366 (1991) 189]

• Some constraints already in-place for many BSM scenarios:

• Leptoquarks [JHEP 02 (2018) 101]

• SUSY [JHEP 04 (2018) 019] [JHEP 05 (2018) 024]

• SM extensions [PRL 119:201802]

𝐾𝑆 → 𝜇+𝜇− Experimental Status

Large production of 𝐾𝑆 and a good muon identification needed

First measurement from the 70s[PLB44 (1973) 217]

LHCb results:• First measurement with data from 2011

[JHEP 01 (2013) 090]• Combination with full Run-I in 2016

[EPJ-C (2017) 77:678]• Full Run-II data sample studied and

presented in 2019 [LHCB-CONF-2019-002]

𝐾𝑆 → 𝜇+𝜇− Result

• No significant signal observed so far

• Obtained limits (combined with Run-I):

BR(𝐾𝑆 → 𝜇+𝜇−) < 2.1 × 10−10 at 90% CLBR(𝐾𝑆 → 𝜇+𝜇−) < 2.6 × 10−10 at 95% CL

• Excellent prospects for the LHCb upgrade!

KLEVER: 𝐾𝐿 → 𝜋0𝜈 ҧ𝜈 at CERN – SPS

M. Moulson @ KAON 2019

KLEVER: Status and Timeline

M. Moulson @ KAON 2019

Summary• NA62 @ CERN – SPS:

• New preliminary limit: BR(𝐾+ → 𝜋+𝜈 ҧ𝜈) < 1.85 × 10−10 at 90% CL• Combined result from 2016+2017 data, analysis of 2018 data ongoing

• Plans to continue data taking starting in 2021 with improved conditions

• New limits on HNL, dark photon, LNV kaon decay modes

• NA48/2 @ CERN – SPS: • First measurement of BR(𝐾± → 𝜋±𝜋0𝑒+𝑒−) = 4.237 ± 0.145 × 10−6

• New measurement of charged kaon semileptonic form factors

• KLOE @ DAφNE • New preliminary measurement BR(𝐾𝑆 → 𝜋±𝜇∓𝜐) = (4.57 ± 0.11stat ± 0.16syst) × 10−4

• New results from semileptonic charge asymmetry & direct CPT tests

• LHCb @ CERN – LHC:• New limit on BR(𝐾𝑆 → 𝜇+𝜇−) < 2.1 × 10−10 at 90% CL

• Kaon physics experiments continue to produce important & interesting results

• Ongoing analyses → more new results are expected in the future!

25. 9. 2019 J-PARC Symposium 2019 Kaon Physics in Europe michal.koval@cern.ch 40Thank you for your attention.

kaon physics europe koval...kaon physics in europe 3rd j-parc symposium tsukuba, japan, 23 –26...

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