Alina Kleimenova(Université catholique de Louvain)on behalf of NA62 Collaboration

Searching for long-lived particles at the LHC: Fifth workshop of the LHC LLP Community

CERN 2019, 29th May

Search for exotics at NA62

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Outline● The NA62 experiment at the CERN SPS ● NA62 exotics searches:

– Heavy Neutral Leptons (HNL)– Dark Photon (DP)– Future prospects

● Summary

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The NA62 experiment

~30 institutes, ~200 participants from: Birmingham, Bratislava, Bristol, Bucharest, CERN, Dubna, GMU-Fairfax, Ferrara, Firenze, Frascati, Glasgow, Lancaster, Liverpool, Louvain, Mainz, Moscow, Napoli, Perugia, Pisa, Prague, Protvino, Roma I, Roma II, San Luis Potosi, Sofia, Torino, TRIUMF, Vancouver UBC

NA62 – fixed target kaon experiment at CERN SPSMain goal: measurement of the BR(K+→π+νν)̅ with 10% precision using novel kaon-in-flight technique.Theoretical prediction:Br[Buras et al., JHEP11(2015)033]Latest experimental result:BR[Phys. Rev D 79, 092004 (2009)]Broader physics program:● Rare/forbidden kaon decays● Searches for exotic processesTime scale:● 2014 – Pilot run● 2015 – Commissioning run: ~1% of design intensity, no beam tracker● 2016 - Commissioning run + Physics run (30 days)● 2017 – Physics run (161 days)● 2018 – Physics run (217 days)● 2019-2020 – LS2

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Detector overview Performances:

● GTK-KTAG-RICH time resolution: 𝛰(100 ps) ● 𝛰(104) background suppression from kinematics ● 𝛰(107) muon rejection for 15 <p(π+)< 35 GeV● 𝛰(108) π0 rejection of for E(π0)> 40 GeV [NA62 Detector Paper, JINST 12 (2017), P05025]

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NA62

Beam

RICHKTAG

Spectrometer

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HNL searches in kaon decaysAnalysis principle:● Heavy neutral lepton produced in K+ → μ+N/e+N due to mixing with SM neutrinos● BR● N long lived and escapes the detectorSearch strategy:● Peak searches in the missing mass spectrum: m2miss = (PK – Pl)2● Count nsig in sliding mass window across m2miss Data: ● Collected over 5 days of data taking in 2015● Minimum bias trigger at 1% of nominal intensity

Kinematic factor

[R. Shrock PLB

96 (1980) 159]

RK ≈ 2.5 ⨯10-5Mixing strength

NK ~3⨯108K+ → e+ν1.7k candidates NK ~1⨯108

K+ → μ+ν24M candidatesSignal region Signal region

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HNL – 2015 result● No HNL signal was observed ● Improvement on UL on |Ue4|2 in 170-448 MeV/c2● Improvement on UL on |Uμ4|2 in 250-373 MeV/c2 ● O(10) improvement foreseen with current NA62 data (2016-2018)

NA62 collaboration, Phys.Lett.B778 (2018) 137

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Dark photon searches in π0 decaysAnalysis principle:● Dark photon produced in π0 decay via kinetic mixing ϵ with SM photon K±→π±π0, π0 → A’γ:γ:● Br● A’γ:→ invisible or A’γ: long livedSearch strategy:● Peak searches in the missing mass spectrum: m2miss = (PK – Pπ– Pγ)2● Count nsig in sliding mass window across m2miss ● Convert nsig to limit on ϵ:Data: ● 5% of 2016 data (1% of full available statistics) 0.00075 0.01765Signal region

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Dark Photon – 2016 result● No DP signal was observed ● Improved limits on ϵ2 in 60-110 MeV/c2 range ● Sets new upper limits on: B(

arxiv:1903.08767

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Beam dump mode of NA62Normal data taking BD mode

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Future prospects: NA62++● The NA62++ experiment uses the same NA62 apparatus in a “beam dump” mode

– NA62++ can collect O(1018) protons on target (POT) in O(3) months – Possibility to run in this mode in 2023

● The physics prospects of NA62++ have been studied as part of the ‘Physics Beyond Colliders – Beyond the Standard Model’γ: working group [arxiv:1901.09966]● The following slide shows NA62++ prospects assuming O(1018) POT and negligible background● The limits are set at 90% CL, and are compared to other results expected on a 5-year timescaleNote:● The results of studies based on O(1016) POT show negligible background level● A detailed, G4-based MC simulation developed within the PBC reproduces the distribution of single-track background events● Improvements to the NA62++ setup (upstream veto, beamline modifications, higher intensity) are not included here, but initial studies are promising

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HNL decay searches● HNL decay searches to visible final states with at least two charged tracks, where HNL produced in leptonic decays of B and D mesons● Expected sensitivity in the plane HNL coupling to one SM generation vs HNL mass

– account for geometrical acceptance– zero-background assumption

[PBC-BSM report, arXiv:1901.09966] [arXiv:1801.04207]

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Dark photon decay searches● Decay searches for A' → ll● Expected sensitivity in the plane A' mixing with SM photons vs A' mass :

– account for geometrical acceptance– zero-background assumption

● Sensitivity expected to be higher than shown:– including direct QCD production of A'– Including A' production in the collimator (here, only target)

[PBC-BSM report, arXiv:1901.09966]

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ALP decay searches● Decay searches for a → γγ, where a produced via elastic scattering of beam proton dumped onto collimator material ● Analysis of 2017-2018 data is ongoing (2·1016 POT)● Expected sensitivity in the plane ALP coupling to photons vs ALP mass

– account for geometrical acceptance– zero-background assumption[arXiv:1904.02091]

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Summary● The NA62 experiment is a powerful laboratory to make searches for exotic decays:

– Results of recent searches for the decays K+→l+N and π0→γA' were presented. Worlds-best limits have been set.– Further results and useful studies will be obtained with the NA62 full data set analysis (2016-2018)

● The prospects for the NA62++ “dump mode” experiment were presented:– Searches for exotic decays at NA62++ can improve limits in a variety of models, including Heavy neutral leptons (HNL), dark photons (A′), and axion-like particles (ALP)

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[Backup slides]

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Future prospects 10-15 years scaleMixing with SM photon Mixing with SM HiggsCoupling to SM photon

a → γγ A' → ll S → llElectron coupling dominant Muon coupling dominant Tau coupling dominant

All HNL decays with at least two charged tracks are considered, assuming zero background[PBC-BSM report, arXiv:1901.09966]

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ALP decay searches● Decay searches for a → γγ, where a produced via elastic scattering of beam proton dumped onto collimator material ● Analysis of 2017-2018 data is ongoing (2·1016 POT)● Expected sensitivity in the plane ALP coupling to photons vs ALP mass

– account for geometrical acceptance– zero-background assumption

[PBC-BSM report, arXiv:1901.09966]

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HNL details● Mass scan with a step 1 MeV/c2 in mass window |m-mN|<1.5σN → background is proportional to mass resolution● Expected background (and stat.error) estimated from fits to the sidebands● Local signal significance never exceeds 2.2σ

K+ → e+N K+ → μ+N

Expected UL Observed UL

Relaxed selection

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ResultsDecay in flight technique proved to be working: ● 1 event observed

– Nπνν(SM)=(0.267±0.001stat±0.020sys±0.032ext)– Nbkg= 0.152+0.093-0.033stat±0.013sys

● Nk = ● Considering 1 observed event:BR(K+ → π+νν)̅<14⨯10-10 at 95% CLResults published in Phys. Lett. B 791 (2019) 156-166