A48 NNA62 s d ν ν

+ + K → π νν – NA62 First Result

Bob Velghe* , on behalf of the NA62 collaboration

CIPANP 2018 Palm Springs, CA, May 29, 2018

*TRIUMF, Vancouver, BC Outline of the Talk

10−4 Camerini Experimental upper limit

10−5

Klems 10−6 Cable ) ¯ ν −7 Asano ν 10

+ E787 π

+ + → → π νν + − Decay K 8 K ( 10 B

−9 NA62 Experiment 10 E949

10−10

10−11 1970 1980 1990 2000 2010 2020 Publication year

In blue: Dahl 1969, Gaillard 1974, Ellis 1988, Buchalla 1996, Mescia 2007, Brod 2011 and Buras 2015 + + K → π νν – In the

s d W u, c, t s d ds u, c, t u, c, t u, c, t WW 0 W W Z0 Z l ν ν νν ν ν Flavour Changing Neutral Current: GIM suppression, involved CKM matrix elements are small (|Vts| ≈ 0.039, |Vtd| ≈ 0.008) + 0 + Hadronic matrix element related to K → π e νe decay F. Mescia and C. Smith [arXiv:0705.2025]

In terms of the CKM parameters:

.  2 8 h i0.74 + +  −11 |Vcb| γ B K → π νν = (8.39 ± 0.30) × 10 40.7 × 10−3 73.2° − = (8.4 ± 1.0) × 10 11

A. J. Buras et al [arXiv:1503.02693]

+ + +1.15 −10 B( → π νν¯) = (1.73 ) × 10 E787/949 Collaboration [arXiv:0808.2459] K −1.05 1 / 22 + + K → π νν – Beyond the Standard Model + + K → π νν¯ has been studied in many BSM scenarios. To name a few: 0 I Z models, A. Buras et al [arXiv:1211.1896],[arXiv:1507.08672] I Randall and Sandrum models, M. Blanke et al [arXiv:0812.3803] I Littlest Higgs models, M. Blanke et al [arXiv:1507.06316] I Supersymmetry, M. Tanimoto, K. Yamamoto [arXiv:1603.07960], T. Blažek, P. Maták [arXiv:1410.0055] I Lepton Flavour Violation models. M. Bordone et al [arXiv:1705.10729]

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A. J. Buras et al [arXiv:1507.08672] M. Bordone et al [arXiv:1705.10729] 2 / 22 Setup & Measurement Principle NA62 / CERN SPS

NA62 is a decay in flight experiment. The main objective is to + + measure B(K → π νν¯) with a relative uncertainty around 10 %.

(Also, heavy neutrinos, lepton flavour universality, exotic physics, etc.) CMS

I 2005 Proposal, North Area ALICE LHC LHC-b I 2009 Approved, TI8 SPS TT10 TI2 ATLAS AWAKE I 2010 Technical design, West Area AD TT60

I 2012 Technical run, TT2 BOOSTER ISOLDE East Area I 2014–15 Pilot runs, LINAC 2 PS n-TOF CTF3 I 2016–18 Physics runs. LINAC 3

14 countries, 31 institutes, 214 authors.

3 / 22 + + K → π νν – Measurement Principle

2 2 The missing mass squared, mmiss = (pk − pπ) , gives an handle on 92 % of the background channels → Core aspect of the experiment.

1

Γ 2 miss +→π+π0 γ d K ( ) dm

tot -1 1 10 Γ (γ) + + ν µ K →π+ µ νν × 10 + → ( 10 ) 10-2 K K+→π+π+π- +→π+π0 π0 10-3 K + +π0ν K → e e

-4 + π0 ν µ 10 + →µ K

10-5

+ ν -6 - e e 10 π+ π + → K

10-7 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 2 2 4 mmiss [GeV /c ]

I Identification of K and π, I Measurements of K and π momentum, I Vetoes for γ and µ, ps I O (100 ) timing capabilities for K - π matching.

+ + + + 0 Main kaon backgrounds: K → µ νµ (γ), K → π π (γ), + + + − + + − + K → π π π , K → π π e νe. 4 / 22 Kaon Collected

This talk: 2016 data, 4 weeks of data taking, 35 − 40% of the nominal intensity. 2017 data, about 23 weeks of data taking, 60 − 65% of the nominal intensity, higher data quality → about 10× more data.

11 + ≈ 1 × 10 good decays. 12 + K > 3 × 10 good K decays.

5 / 22 NA62 / CERN SPS – Layout

Beam: 75 GeV/c ± 1%, K, π and p (6:70:23), 750 MHz.

L = 53 m L = 24 m L = 38 m Ø = 1.92 m Ø = 2.4 m Ø = 2.8 m RICH CHOD LKr

Bend. KTAG GTK Magnet SAC

CHANTI

LAV 1-11 MNP33 IRC MUVs Magnet STRAWs LAV 12

T + 70 m T + 265 m

NA62 Collaboration [arXiv:1703.08501]

Kaon & pion tracking, PID, calorimeters, hermetic photon vetos, veto, hodoscope and inelastic interactions veto → redundancy.

Minimize beam – gas interactions: vacuum 10−6 mbar.

Signal selection sketch: K - π association, 15 < Pπ < 35 GeV/c, decay vertex in fiducial volume, no photon / muon / upstream activity.

6 / 22 Signal Selection Kaon Identification & Tracking

Average beam intensity: 2016 → 35 − 40%, 2017–2018 → 60 − 65%.

KTag – Diff. Cherenkov counter,

I N2 (H2) gas radiator, I Kaon time resolution ≈ 70 ps, I > 98% K ID efficiency.

GigaTracker – Silicon pixel tracker, I Sensor surface is 60 by 27 mm – Match beam size,

I Thickness ≤ 0.5 % X/X0 – Minimize beam induced background, + + I Temporal resolution < 150 ps – K – π matching.

+ KTag and GTK : 75% K reconstruction and identification efficiency.

7 / 22 Pion Tracking

Pion spectrometer – STRAW

Four STRAW chambers, I 4 views / chamber, 448 straws / view, I 1.3 m long dipole (0.9 Tm), I straws are 2.1 m long, 9.8 mm in diam., 36 µm walls,

I X/X0 < 1.8%,

I 70% Ar, 30% CO2.

> 95% reconstruction efficiency.

8 / 22 Particle Identification

Pion / muon separation – RICH & Calorimeters

Ring-imaging Cherenkov detector, I Ne gas radiator, I Ring time resolution ≈ 80 ps, 2 I µ/π separation > 10 (15 < P < 35 GeV/c).

Likelihood PID discriminant. Efficiency 2.5 × 10−3 / 0.75 for µ+/π+.

Calorimeters, I Electromagnetic calo. (LKr), I Hadronic calo. (MUV1,2), I Scintillator pads behind 80 cm Fe wall (MUV3).

MUV3 and BDT classifier. Efficiency 0.6 × 10−5 / 0.77 for µ+/π+. 9 / 22 Photon VETOs

Main cuts: I No in-time signals in LAVs and SAV, + I No in-time signals in hodoscope and LKr (if not associated with π ), I Segment rejection in Straw.

Typical timing coincidence: ±3/ ± 5 ns, energy dependent for LKr.

+ + − Fraction of K → π π0 passing the cuts: 2.5 × 10 8.

10 / 22 Signal Selection – Result

Signal and control regions blinded, selection developed on about 10% of the data set.

11 / 22 Single Event Sensitivity Single Event Sensitivity (SES) + + K → π π0 (from control data) used as normalization channel.

+ + − K → π νν acceptance (MC) (4.0 ± 0.1) × 10 2 Random veto efficiency 0.76 ± 0.04 Trigger efficiency 0.87 ± 0.2

−10 SES = (3.15 ± 0.01stat. ± 0.24syst.) × 10

−  Source δ SES 10 10 Random veto ±0.17 Definition of π+π0 region ±0.10 Simulation of π+ interactions ±0.09 NK ±0.05 Trigger efficiency ±0.04 Extra activity ±0.02 GTK pileup simulation ±0.02 Momentum spectrum ±0.01 Total ±0.24 12 / 22 Backgrounds Evaluation + + K → π π0 (γ) Assume that π0 rejection cuts and kinematic cuts are independent, + kinematic rejection measured on π π0 with tagged π0 (γγ in LKr).

I Radiative tail in R2 estimated from MC, I Single-γ veto efficiency measured on data, 0 I π γ rejection on the radiative tail estimated from data.

Radiative tail ×6 bigger but π0γ rejection ×30.

exp. exp. Region Nππ Region Nππγ R1 0.022 ± 0.004 ± 0.002 R1 0 R2 0.037 ± 0.006 ± 0.003 R2 0.005 ± 0.005syst.

13 / 22 + + − + K → π π e ν

Estimated using MC, ≈ 4 × 108 events generated.

2 2 4 0.026 < mmiss < 0.072 GeV /c region used for validation, free from other background processes.

Example: single π− events, full πνν selection, STRAW multiplicity cuts inverted.

exp . = . +0.024 ± . Nππeν 0 018−0.017 0 009

14 / 22 + + + − K → π π π Kinematic rejection in R2 ≤ 10−4, corrected for selection bias using the MC.

exp. Nπππ = 0.002 ± 0.001 ± 0.002 15 / 22 + + K → µ νµ (γ) + + Same approach as K → π π0 (γ), assume that PID rejection cuts and kinematic cuts are independent. Kinematic rejection measured on + µ νµ sample, applying the γ rejection.

exp. Region Nµν(γ) R1 0.019 ± 0.003 ± 0.003 R2 0.0012 ± 0.0002 ± 0.0006 16 / 22 Upstream Backgrounds Estimation based on a “bifurcation” analysis. GTK3 Last dipole π

GTK2

Final collimator

exp. +0.090 upstream = 0.050 N −0.030 17 / 22 Upstream Backgrounds Estimation based on a “bifurcation” analysis. Last dipole GTK3 π

GTK2

Final collimator

exp. +0.090 upstream = 0.050 N −0.030 17 / 22 Upstream Backgrounds Estimation based on a “bifurcation” analysis. Last dipole GTK3 π Ks GTK2

Final collimator

exp. +0.090 upstream = 0.050 N −0.030 17 / 22 Backgrounds Summary

Process Expected events R1 R2 R1+R2 + + 0 K → π π (γ) 0.022 0.037 0.064 ± 0.007 ± 0.006 . +0.090 Upstream backgrounds – – 0 050−0.030 + → π+π− +ν . . +0.024 ± . K e 0 0 018 0 018−0.017 0 009 + + + − K → π π π 0 0.0020 0.002 ± 0.001 ± 0.002 + + K → µ ν (γ) 0.019 0.0012 0.020 ± 0.003 ± 0.003 Total backgrounds – – 0.15 ± 0.09 ± 0.01 + + K → π νν (SM) 0.069 0.198 0.267 ± 0.001 ± 0.020 ± 0.032

18 / 22 Preliminary Results Preliminary Results

19 / 22 The Candidate in the RICH

Run 6646, burst 953, event 543854.

20 / 22 2016 Data Set – Summary

Cut based analysis of about 4 weeks worth of data.

+ +  −10 B K → π νν < 14 × 10 95% C.L.

Candidate 1 11 NK (1.21 ± 0.02) × 10 SES (3.15 ± 0.01 ± 0.24) × 10−10 + + Expected SM K → π νν 0.267 ± 0.001 ± 0.020 ± 0.032ext. Expected background 0.15 ± 0.09 ± 0.01

Decay-in-flight technique works!

21 / 22 Prospects

More decays collected in 2017/2018: I Data quality greatly improved in 2017/2018, I Higher beam intensity (40–45% → 60–65% of nominal), I 161 days in 2018, 217 days scheduled for 2018, I More sophisticated data analysis (cut base → multi-variate). Already > 20 × more data on tape.

+ + About 20 K → π νν SM events expected before LS2 (end of 2018).

22 / 22

+ + B (K → π νν) – Theoretical Error Budget

The branching ratio, summing over the three neutrino flavours reads [arXiv:hep-ph/0405132]:

" 2 + +  Im λt B → π νν = κ+ (1 + ∆ ) ( ) K EM λ5 Xt xt

 2# Re λc Re λt + [ + δ , ] + ( ) , (1) λ Pc Pc u λ5 Xt xt

λ = ∗ = 2/ 2 ∆ ≈ − . % where i VisVid, xt mt MW. The parameter EM 0 3 encodes the QED long distance radiative corrections [arXiv:0705.2025v2].

 8 −10 |Vus| κ+ = (0.5173 ± 0.0025) × 10 , (2) 0.225 summarises the long-distance contributions extracted from the + 0 + K → π e νe decay [arXiv:0705.2025v2]. + + B (K → π νν) – Theoretical Error Budget

Table: Error budget of the parameters entering in the K → πνν branching ratio computation [arXiv:1503.02693].

Quantity Error budget (%) Comment

|Vcb| 9.9 – γ 6.7 – Pc 1.8 Charm quark contribution δPc,u 2.9 Long distance charm-quark contribution Xt 0.9 Top-quark contribution Other 0.5 – Bifurcation Analysis

Estimate the number of background event in the signal region (A) using 0 0 0 control regions B ,C and D :

A: signal region

0 0 00 0 00 A : control region, B , B , C , C and 0 B' B'' D' D : control samples. If the two cuts are independent:

0 0 → = B C A' A 0 C'' D Box TRIM5 cut @ A C' 00 00 0 B C → A = 0 K - π matching D