Kaon Experiments at CERN: Recent Results and Prospects

Francesca Bucci, INFN Sezione di Firenze Korea Institute for Advanced Study (KIAS), October 29, 2013

F. Bucci 2

Introduction

Kaon decays have played a key role in the shaping of the Standard Model (SM)

Parity violation GIM mechanism CP violation

Kaon decays continue to have an important impact on flavor dynamics

ChPT Tests Constraining physics beyond the SM

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NA48/n and NA62Fixed target experiments at the CERN Super Proton Synchrotron (SPS)

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NA48/n History

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199719981999200020012002200320042005

:2013

::

NA48Main goal: Search for direct CPVMeasurement of ’/Beams: KL+KSNA48/1

Main goal: Rare KS decays, hyperon decays, CPV testsBeams: KS

NA48/2Main goal: Search for direct CPV Charge asymmetry measurementBeams: K+ + K-NA62

Main goal: Measurement of the K+ + decaysBeams: K+

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Outline

Recent results: NA48/2 and NA62 (RK phase)

ChPT Tests Lepton Flavor Universality

Prospects: NA62

K++ Other rare and forbidden decays studies

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NA48/2 Beam(s)

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SPS protons at 400 GeV/c Simultaneous, unseparated, focused positive and negative hadronic beams Kaon momentum: 603 GeV/c (NA48/2), 741 GeV/c (NA62-RK)

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NA48/2 Detector

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LKr electromagnetic calorimeter: sE/E = (3.2/√E 9.0/E 0.42)% (E in GeV)

sx =sy = (4.2/ √E + 0.6)mm (E in GeV)

Hodoscope: st 150ps

100 m long decay region in vacuum Similar acceptance for K+ and K- decays

Magnetic spectrometer:sp/p = (1.0 0.044 p)% (p in GeV/c) NA48/2sp/p = (0.48 0.009 p)% (p in GeV/c) NA62-RK

Momentum kick: 120 MeV/c NA48/2 265 MeV/c NA62-RK

beam

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Outline

Recent results: NA48/2 and NA62 (RK phase)

ChPT Tests Lepton Flavor Universality

Prospects: NA62

K++ Other rare and forbidden decays studies

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Chiral Perturbation TheoryThe Chiral Perturbation Theory (ChPT) is the effective field theory of quantum

chromodynamics (QCD) at low energies

In the ChPT the eight lightest hadrons (,0,K,K0,K0,) are the Goldstone bosons (GB) due to the spontaneous breakdown of the chiral symmetry

The chiral Lagrangian can be organized in terms of the increasing number of GB fields derivatives or, equivalently, in terms of the increasing powers of their momentum (chiral powers)

In the chiral expansion, the intrinsic hadronic uncertainties are parametrized by low energy constants (LECs) whose value must be determined by phenomenology.

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4eff

2eff

0effeff LLLL

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NA48/2 and NA62-RK ChPT Tests

Radiative kaon decays:

K 0 EPJC 68 (2010) 75-87 K e+ e- PLB 659 (2008) 493-499 K e (SD) K

Non radiative kaon decays:

K 0 e , K 0 K + - e EPJC 54 (2008) 411-423 , EPJC 70 (2010) 635-657, PLB 715 (2012) 105-115 K 0 0 e K 0 0 EPJ C64 (2009) 589

K + - PLB 6495-6 (2007) 349-358 K e+ e-, K + - PLB 677 (2009) 246-254, PLB 697 (2011) 107-115 K 0 e+ e-

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ChPT is the ideal framework to describe kaon decays

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K Decay Theory

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In the ChPT framework the differential rate is:

22

2222223

2

)(,,14

1,,ˆ

8zDzBrzyzCzByzcAz

m

zyK

D’Ambrosio, Portolés, PLB 386 (1996) 403-412

The leading contribution is at O(p4) Rate and spectrum in z=m2

/m2K at O(p4) depend on a single unknown

parameter

N and L are fundamental ChPT parameters

181514109

2

233

128ˆ NNNLLc

weak chiral lagrangian

QCD loops and counterterms

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K Decay Theory

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Predictions at O(p4) and O(p6) differ: may be tested experimentally !D’Ambrosio, Portolés, PLB 386 (1996) 403-412Gerard, Smith, Trine, NPB 730 (2005) 1

The dominant amplitude A is responsible for a cusp at m = m2Ecker, Pich, De Rafael, NPB 303 (1988) 665-702

13

K Fit

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Fit the z distributions to extract the value of the ĉ parameter in the framework of the ChPT O(p4) and ChPT O(p6)

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NA48/2 NA62-RK

Data support the ChPT prediction of a cusp at the di-pion threshold

O(p6) O(p6)

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K Fit Results

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Compute the BR in the full kinematic range assuming the ChPT O(p6) BR6 (K) = (1.010.06)10-6

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ChPT O(p4) vs O(p6) models cannot be discriminated within the current exp. sensitivity

BR(K) vs ĉ d/dz vs z

PRELIMINARY

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Scattering and K Decays

The S-wave scattering lengths aI in the isospin I=0 and I=2 states are precisely predicted by ChPT

The DIRAC collaboration at CERN produced +- atoms to measure its lifetime and obtained

The scattering lengths can also be determined by precise measurements of the kaon decays with pions in the final state

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syst.stat.2533.0 0078.00073.0

0080.00078.020

aa

NPB 603 (2001) 125, PRL 86 (2001) 5008

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Cusp Effect in K00 Decays

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Change of slope observed in the m2p0p0 distribution at the region near m2

p0p0= (2mp+)2

was interpreted as due to the strong pp rescattering in the final state K+-

Cabibbo PRL 93,2004

(2mp+)2

m2p0p0 (GeV/c2)2

(2mp+)2

zoom

m2p0p0 (GeV/c2)2

0149.00096.00129.00241.0 0088.00029.00048.02571.0 220 aaa[a0,a2 values in units of 1/m+]

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Phase Shift in K+-e(Ke4) DecaysVery clean hadronic environment since there are only two pions in the final state

The shift () between the phases of the S-wave, I=0 and P-wave, I=1 form factors is measured as a function of the +- invariant mass [EPJC 70 (2010) 635-657].

Roy equations allow to connet measured phase shifts to scattering lengths [PLB 36 (1971) 353] .

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best ChPT prediction

68% CL contours

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Scattering: Theory and Experiments

Combine both the cusp [EPJ C64 (2009) 589] and Ke4 results [EPJC 70 (2010) 635-657] from NA48/2

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Impressive agreement with ChPT

[a0,a2 values in units of 1/m+]

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Outline

Recent results: NA48/2 and NA62 (RK phase)

ChPT Tests Lepton Flavor Universality

Prospects: NA62

K++ Other rare and forbidden decays studies

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K+l+ Sensitivity to New Physics Leptonic kaon decays within the SM are mediated by a charged current at

tree level

The natural size of the non-standard contributions depends on the particular BSM scenario

In Models with 2 Higgs Doublet (2HDM-II including SUSY) sizable charged Higgs (H) exchange contributions

obstructed by hadronic uncertainties (fK )

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222

222

||18

)lK( usKK

llKF VfM

MMMG

40 tanβGeV, 50010 3

HSM

M

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RK=G(K→en)/G(K→mn) in the SM

In the ratio RK =G(K→en/K→mn) hadronic uncertainties cancel The SM prediction of RK has reached <0.1% precision

dRK is the correction due to the Inner Bremsstrahlung part of the radiative K →eng process

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5corr. rad.K

2

2μ

2K

2e

2K

2

μ

eSMK 10001.0477.21R

R

mm

mm

m

m

[V. Cirigliano and I. Rosell Phys. Rev. Lett. 99 (2007) 231801]

helicity suppression factor

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RK beyond the SM Charged Higgs bosons (H) appearing in any model with two Higgs doublets

(including SUSY case) can contribute at tree level

Such contribution does not affect the ratio RK

LFV couplings, present at one loop level, can contribute to RKSM at the % level

Recent measurements of BR(B s+-) and BR(Bu ) significantly lower the SUSY contribution to RK

Sensitive to SM extensions with 4th generation, sterile

2

2

22

SM

βtan1lK

lK

H

K

su

s

m

m

mm

m

βtan1RR 62

13

2

e

τ

4

H

KSMK

LFVK m

m

m

m

A.Masiero, P.Paradisi, R.Petronzio Phys. Rev. D74 (2006) 0011701, J. Girrbach and U. Nierste, arXiv:1202.4906

slepton mixing

Fonseca, Romão, Teixiera, arXiv:1250.1411

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Lackers, Menzel, JHEP 1007 (2012) 006

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Measurement Strategy Ke2, Km2 collected simultaneously

MC simulations used to a limited extent

PID, trigger, read out efficiencies and muon halo bkg are measured directly from data

Analysis performed in bins of the reconstructed lepton momentum

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LKReeeB

eBeK fKKAf

KKAf

KNKN

KNKN

DR

11

22

22

22

22

# signal events

# background eventsacceptance

measured PID efficiency

LKr trigger efficiency

LKr readout efficiency

downscaling factor of Km2

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RK Final ResultFit over 40 RK measurements (4 data samples 10 momentum bins)

including correlations: c2/ndf=47/39

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RK=(2.488 ± 0.010) x10-5 , RK/RK = 0.4%PLB 719 (2013) 326

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RK World Average

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2013 world average : RK= (2.488 ± 0.009) 10-5 (RK/RK= 0.36%)

PDG 2010 (KLOE result): RK= (2.493 ± 0.031) 10-5 (RK/RK= 1.3%)

Consistency with older measurements and with the Standard Model

Experimental accuracy still one order of magnitude away from the SM prediction Motivation for improved precision measurement

2013 average

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Outline

Recent results: NA48/2 and NA62 (RK phase)

ChPT Tests Lepton Flavor Universality

Prospects: NA62

K++ Other rare and forbidden decays studies

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K in the SMFlavour Changing Neutral Current (FCNC) process forbidden at tree level in the SM

Highest CKM suppression very sensitive to New Physics High theoretical cleanness :

• Dominated by short distance dynamics• In case of K+→p+ nn small effects of long distance contributions due to charm• Hadronic matrix element extracted by K+→p0e+n

SM predictions BR(KL0)=(2.43 0.39 0.06)10-11

BR(K++)=(7.81 0.75 0.29)10-11

BR proportional to |V*tsVtd| theoretically clean Vtd dependance

Parametric error dominated by Vcb ,

Pure theoretical error mostly LD contribution

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K++ Previous MeausrementsE949/E787 experiment (BNL):

7 candidates observed in the two allowed kinematics regions

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Low energy separated K+ beam K decays at rest Hermetic coverage

1015.105.1 1073.1 :'08] [E787/E949

KBR

Experimental uncertainty > 50%

NA62 aims at measuring the BR with an accuracy of 10%

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K beyond the SMSeveral SM extensions predict sizable deviations for the BR

E949/787experimental uncertainty

NA62expected precision

RSc: Randall-Sundrum, LHT: Littlest Higgs with T-parity, SM4: SM with 4th generation

(hep-ph/0906.5454, hep-ph/0812.3803,hep-ph/0604074)

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LAV

CHANTI

GTKKTAG-CEDAR

Target

protonshadron beam

75 GeV/c

Vacuum <10-5 mbar

STRAWTracker

RICH

CHOD

LKr

MUV

SAV

Signal signature: Incoming high momentum (75 GeV/c) K+

Outgoing low momentum (<35 GeV/c) p+ in time with the incoming K+

The NA62 Beam and Detector

50 MHz 800 MHz

10 MHz

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Unseparated positive hadron beam (K+ 6%)

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Experimental Technique

High K momentum Low momentum to allow enough missing energy (p0 40 GeV/c) to be

detected by hermetic veto detectors (LAV,IRC,SAC,LKr)

Particle identification to separate and (RICH, MUV)

Kinematical rejection with lightweight spectrometers (GTK, STRAW)

Beam particle identification and inelastic event suppression (KTAG, CHANTI) Fast timing

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Backgrounds 92% of kaon decays separated from signal by kinematic cuts

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Decay BR

K++ 63.5%

K++0 20.7%

K+++- 5.6%

K++00 1.8%

2πK2miss PPm

K++0 splits the signal region in 2

(m2miss ) < 10-3 GeV2/c4

(pK)/pK 0.2%, (p)/p 1 %

keep multiple scattering as low as possible

t=100 ps on + , t=150 ps on K+

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Backgrounds

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8% of kaon decays not separated from signal by kinematic cuts

Decay BR

K+0e+ 5.1%

K+0+ 3.4%

K++ 5.510-3

K++0 1.510-3

K++-e+ 410-5

K++-e+ 110-5

identification inefficiency < 10-5

veto inefficiency 10-5

- separation at 10-3 level

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Expected Sensitivity

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Simple cut and count estimation with no optimizationBackground to be evaluated on data to reach the 10% accuracy

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Tracking Detectors: GTKComposed of 3 hybrid silicon pixel stations mounted around four achromat magnetsInside the vacuum and subjet to a high and non-uniform beam rate (750 GHz in total)

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1 sensor, 10 bump-bonded chips

Provide precise momentum ((p)/p 0.2%), time (t 175 ps at 300V bias) and angular measurements (() 16 rad)

30 m

m

60 mm

GTK1 Pixel 23GTK2 Pixel 23

245 ps 2 175 ps

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Tracking Detectors: STRAWSConsists of 4 chambers intercepted in the middle by a dipole magnet

Minimize multiple scattering: ultra-light material, integration in the vacuum tank

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Measurement of coordinates ( < 130 m), and momentum ((p)/p 0.3%)

of charged particles originating fromthe decay

Each chamber equipped with 1800 straw tubes, positioned in 4 views (u-v,x-y)

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PID Detectors: KTAG-CEDARDifferential Cherenkov counter filled with H2 for positive K (50 MHz) identification

Without kaon tagging vacuum should be better than 610-8 mbar

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Upgraded form of the CEDAR built for the SPS secondary beam

New PMTs and electronics, modified mechanics /optics

Excellent time resolution required (t =100 ps)

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PID Detectors: RICHRing Imaging Cherenkov detector composed of a cylindrical vessel (17 m long)

filled with Ne slightly above atmospheric pressure

Project validated by strong R&D17 m long radiator, 1 mirror, 400 PMTs

Requirements: / separation in 15< p <35 GeV/c with

mis-id probability <10-2

Measure crossing time with t =100 ps Level 0 trigger for charged tracks

Momentum (GeV)

m M

isID

Pro

babi

lity

Momentum (GeV)

Tim

e Re

solu

tion

(ps)

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Photon Veto SystemTo suppress the dominant decay K++0 (BR 21%): 0 rejection inefficiency at 10-8 level ( detection inefficiency at 10-4) Hermetic coverage up to 50 mrad

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3 different detectors to cover 3 different angular regions Large Angle Vetoes (LAV): 8.5 – 50 mrad The NA48 Liquid krypton calorimeter (LKr): 1 – 8.5 mrad Small Angle Vetoes: 1 mrad

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Large Angle Veto (LAV)12 stations formed by 4 to 5 overlapping rings of OPAL lead glasses

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The first eleven are part of the vacuum decay tube, the last one is located outside the vacuum tank

Inefficiency < 10-4 for 100 MeV < E < 35 GeV

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Muon Veto3 muon veto (MUV) stations (partially re-use of the NA48 hadron calorimeter)

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MUV 3

MUV 1MUV 2IRON

MUV1+MUV2 24 (MUV1) and 22 (MUV2)

iron/scintillator layers reach a factor 106 in muon rejection

(combined with the RICH)

MUV3 After 80 cm iron, scintillator tiles

with direct photon detection Fast muon trigger (L0), 1 ns

resolution (10 MHz muon rate)

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Detector Status

Detectors installed: KTAG, LAV(8/12), LKr, SACUnder construction/installation: CHANTI, STRAWS, RICH, IRC, MUV

Installation completed by October 201429/10/2013

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Outline

Recent results: NA48/2 and NA62 (RK phase)

ChPT Tests Lepton Flavor Universality

Prospects: NA62

K++ Other rare and forbidden decays studies

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LFV/LNV modesHigh fluxes and PID/veto capabilities of NA62 are well suited to look for Lepton Flavor/ Lepton Number Violation mode both in kaon and pion decays

Mode UL at 90% CL Experiment NA62 acceptanceK+++e- 1.310-11 BNL 777/865 10%K++-e+ 5.210-10 BNL 865 10%K+-+e+ 5.010-10 BNL 865 10%K+-e+e+ 6.410-10 BNL 865 5%K+-++ 1.110-9 NA48/2 20%K+-e+e+ 2.010-8 Geneva Saclay 2%K+e-++ no data 10%0+e- 3.810-10 KTeV 2%0-e+ 3.410-9 KTeV 2%

Expected decays in Fiducial Volume in 2 years of data taking: 1.21013 K+ decays, 2.5 1012 0 decays

NA62 single-event sensitivities: 10-12 for K+ decays, 10-11 for 0 decays

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Others

Decay Physics Present result NA62

K++h,h Heavy neutrino Limits up to mh= 350 MeV

RK LU and NP (2.4880.010)10-5 > 2 better

K++ ChPT <500 events 105 events

K+00e+ ChPT 66000 events O(106)

K+00+ ChPT - O(105)

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Conclusions

• More than 60 years after their discovery, kaons provide a unique playground for testing our ideas on fundamental physics

• NA48 and NA62 collaborations are analyzing data taken in past years and producing very precise results for leptonic, non-leptonic and semileptonic kaon decays

• The new generation NA62 apparatus will start its data taking in fall 2014. Its unprecedented statistics and its powerful detector features will allow to push further our knowledge of rare (and forbidden) kaon decays

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Thank you for your attention

Additional Slides

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The ChPT weak chiral lagrangian The basic S=1 O(p4) chiral lagrangian can be written as:

37 poorly known Ni coefficients and Wi operators Combination of these couplings are accessible by measuring kaon decays

branching ratio and form factors

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L

iiSSS WNFGUDUDFGLLL

286

48

41

211

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Radiative K→eng Decays In K→eng (Ke2g), g can be produced via internal bremsstrahlung (IB) or

direct-emission, the latter being dependent on the hadronic structure (SD)

RK is defined to be inclusive of the IB, ignoring however the SD contributions To compare data with SM prediction the SD contribution must be carefully

estimated and subtracted.

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K±

e±

ne

gIB

K±

e±

ne

gSD

KL0→p0nn

The charm contribution can be fully neglected since it proceeds in theSM almost entirely through direct CP violation → determination of h

Need a huge number of KL decaysNA48 KL flux corresponding to 31010/yearNA62 possible KL flux 5-10 times NA48 oneAfter SPS upgrade 100 times more

Exp Machine Meas. or UL 90% CL Notes

KTeV Tevatron <5.710-7(0→eeg)

E391a KEK-PS <2.610-8

KOTO J-PARC Aim at 2.7 SM evts/3y

KOPIO Opportunity at Project X ?

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