Kaon Experiments at CERN: Recent Results and Prospects Francesca Bucci, INFN Sezione di Firenze Korea Institute for Advanced Study (KIAS), October 29, 2013

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 29/10/2013F. Bucci2

NA48/n and NA62 Fixed target experiments at the CERN Super Proton Synchrotron (SPS) 29/10/2013F. Bucci3

NA48/n History 29/10/2013F. Bucci : 2013 : NA48 Main goal: Search for direct CPV Measurement of  ’/  Beams: K L +K S NA48/1 Main goal: Rare K S decays, hyperon decays, CPV tests Beams: K S NA48/2 Main goal: Search for direct CPV Charge asymmetry measurement Beams: K + + K - NA62 Main goal: Measurement of the K +  + decays Beams: K +

Outline Recent results: NA48/2 and NA62 (R K phase)  ChPT Tests  Lepton Flavor Universality Prospects: NA62  K +  +  Other rare and forbidden decays studies 29/10/2013F. Bucci5

NA48/2 Beam(s) 29/10/2013F. Bucci6  SPS protons at 400 GeV/c  Simultaneous, unseparated, focused positive and negative hadronic beams  Kaon momentum: 60  3 GeV/c (NA48/2), 74  1 GeV/c (NA62-R K )

NA48/2 Detector 29/10/2013F. Bucci7 LKr electromagnetic calorimeter:  E /E = (3.2/√E  9.0/E  0.42)% (E in GeV)  x =  y = (4.2/ √E + 0.6)mm (E in GeV) Hodoscope:  t  150ps   100 m long decay region in vacuum  Similar acceptance for K + and K - decays Magnetic spectrometer:  p /p = (1.0  p)% (p in GeV/c) NA48/2  p /p = (0.48  p)% (p in GeV/c) NA62-R K Momentum kick: 120 MeV/c NA48/2 265 MeV/c NA62-R K beam

Outline Recent results: NA48/2 and NA62 (R K phase)  ChPT Tests  Lepton Flavor Universality Prospects: NA62  K +  +  Other rare and forbidden decays studies 29/10/2013F. Bucci8

Chiral Perturbation Theory The 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 ,K 0,K 0,  ) 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. 29/10/2013F. Bucci9

NA48/2 and NA62-R K ChPT Tests Radiative kaon decays:  K      0  EPJC 68 (2010)  K      e + e - PLB 659 (2008)  K   e   (SD)  K       Non radiative kaon decays:  K    0 e , K    0    K    +  - e  EPJC 54 (2008) , EPJC 70 (2010) , PLB 715 (2012)  K    0  0 e   K      0  0 EPJ C64 (2009) 589  K      +  - PLB (2007)  K     e + e -, K      +  - PLB 677 (2009) , PLB 697 (2011)  K      0 e + e - 29/10/2013F. Bucci10 ChPT is the ideal framework to describe kaon decays

K     Decay Theory K     Decay Theory 29/10/2013F. Bucci11 In the ChPT framework the differential rate is: D’Ambrosio, Portolés, PLB 386 (1996)  The leading contribution is at O(p 4 )  Rate and spectrum in z=m 2  /m 2 K at O(p 4 ) depend on a single unknown parameter N and L are fundamental ChPT parameters weak chiral lagrangian QCD loops and counterterms

K     Decay Theory K     Decay Theory 29/10/2013F. Bucci12 Predictions at O(p 4 ) and O(p 6 ) differ: may be tested experimentally ! D’Ambrosio, Portolés, PLB 386 (1996) Gerard, Smith, Trine, NPB 730 (2005) 1 The dominant amplitude A is responsible for a cusp at m  = m 2  Ecker, Pich, De Rafael, NPB 303 (1988)

K     Fit K     Fit 29/10/ Fit the z distributions to extract the value of the ĉ parameter in the framework of the ChPT O(p 4 ) and ChPT O(p 6 ) F. Bucci NA48/2 NA48/2 NA62-R K NA62-R K Data support the ChPT prediction of a cusp at the di-pion threshold O(p 6 )

K     Fit Results K     Fit Results 29/10/ Compute the BR in the full kinematic range assuming the ChPT O(p 6 ) BR 6 (K     ) = (1.01  0.06)  F. Bucci ChPT O(p 4 ) vs O(p 6 ) models cannot be discriminated within the current exp. sensitivity BR(K     ) vs ĉ d  /dz vs z PRELIMINARY

 Scattering and K Decays  The S-wave  scattering lengths a I 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 29/10/2013F. Bucci15 NPB 603 (2001) 125, PRL 86 (2001) 5008

 Cusp  Effect in K     0  0 Decays 29/10/2013F. Bucci16 Change of slope observed in the m 2  0  0 distribution at the region near m 2  0  0 = (2m  + ) 2 was interpreted as due to the strong  rescattering in the final state K     +  - Cabibbo PRL 93,2004 (2m  + ) 2 m 2  0  0 (GeV/c 2 ) 2 (2m  + ) 2 zoom m 2  0  0 (GeV/c 2 ) 2 [a 0,a 2 values in units of 1/m  + ]

Phase Shift in K   +  - e  (K e4 ) Decays Very 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) ].  Roy equations allow to connet measured phase shifts to  scattering lengths [PLB 36 (1971) 353]. 29/10/2013F. Bucci17 [a 0,a 2 values in units of 1/m  + ] best ChPT prediction 68% CL contours

 Scattering: Theory and Experiments Combine both the cusp [EPJ C64 (2009) 589] and K e4 results [EPJC 70 (2010) ] from NA48/2 29/10/2013F. Bucci18 Impressive agreement with ChPT [a 0,a 2 values in units of 1/m  + ]

Outline Recent results: NA48/2 and NA62 (R K phase)  ChPT Tests  Lepton Flavor Universality Prospects: NA62  K +  +  Other rare and forbidden decays studies 29/10/2013F. Bucci19

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 (f K ) 29/10/2013F. Bucci20

R K =  (K → e )/  (K →  ) in the SM  In the ratio R K =  (K→e /K→  ) hadronic uncertainties cancel  The SM prediction of R K has reached <0.1% precision   R K is the correction due to the Inner Bremsstrahlung part of the radiative K →e  process 29/10/2013F. Bucci21 [V. Cirigliano and I. Rosell Phys. Rev. Lett. 99 (2007) ] helicity suppression factor

R K 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 R K  LFV couplings, present at one loop level, can contribute to R K SM at the % level  Recent measurements of BR(B s  +  - ) and BR(B u  ) significantly lower the SUSY contribution to R K  Sensitive to SM extensions with 4 th generation, sterile 22 A.Masiero, P.Paradisi, R.Petronzio Phys. Rev. D74 (2006) , J. Girrbach and U. Nierste, arXiv: slepton mixing Fonseca, Romão, Teixiera, arXiv: /10/2013F. Bucci Lackers, Menzel, JHEP 1007 (2012) 006

Measurement Strategy  K e2, K  2 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 29/10/2013F. Bucci23 # signal events # background events acceptance measured PID efficiency LKr trigger efficiency LKr readout efficiency downscaling factor of K  2

R K Final Result Fit over 40 R K measurements (4 data samples  10 momentum bins) including correlations:  2 /ndf=47/39 29/10/2013F. Bucci24 R K =(2.488 ± 0.010) x10 -5,  R K /R K = 0.4% PLB 719 (2013) 326

R K World Average 29/10/2013F. Bucci world average : R K = (2.488 ± 0.009)  (  R K /R K = 0.36%) PDG 2010 (KLOE result): R K = (2.493 ± 0.031)  (  R K /R K = 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

Outline Recent results: NA48/2 and NA62 (R K phase)  ChPT Tests  Lepton Flavor Universality Prospects: NA62  K +  +  Other rare and forbidden decays studies 29/10/2013F. Bucci26

K  in the SM Flavour Changing Neutral Current (FCNC) process forbidden at tree level in the SM 27  Highest CKM suppression  very sensitive to New Physics  High theoretical cleanness : Dominated by short distance dynamics In case of K + →  +  small effects of long distance contributions due to charm Hadronic matrix element extracted by K + →  0 e +  SM predictions  BR(K L  0 )=(2.43  0.39  0.06)   BR(K +  + )=(7.81  0.75  0.29)   BR proportional to |V * ts V td |  theoretically clean V td dependance Parametric error dominated by V cb,  Pure theoretical error mostly LD contribution

K +  + Previous Meausrements E949/E787 experiment (BNL): 7 candidates observed in the two allowed kinematics regions 29/10/2013F. Bucci28  Low energy separated K + beam  K decays at rest  Hermetic coverage Experimental uncertainty > 50% NA62 aims at measuring the BR with an accuracy of 10%

K  beyond the SM 29 Several SM extensions predict sizable deviations for the BR E949/787 experimental uncertainty NA62 expected precision RSc: Randall-Sundrum, LHT: Littlest Higgs with T-parity, SM4: SM with 4 th generation (hep-ph/ , hep-ph/ ,hep-ph/ ) 29/10/2013F. Bucci

LAV CHANTI GTK KTAG-CEDAR Target protons hadron beam 75 GeV/c Vacuum <10 -5 mbar STRAW Tracker RICH CHOD LKr MUV SAV Signal signature:  Incoming high momentum (75 GeV/c) K +  Outgoing low momentum (<35 GeV/c)  + in time with the incoming K + The NA62 Beam and Detector 50 MHz 800 MHz 10 MHz 29/10/ Unseparated positive hadron beam (K +  6%) F. Bucci

Experimental Technique  High K momentum  Low  momentum to allow enough  missing  energy (p  0  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 29/10/2013F. Bucci31

Backgrounds 92% of kaon decays separated from signal by kinematic cuts 29/10/2013F. Bucci32 DecayBR K +  % K +  +  % K +  +  +  - 5.6% K +  +  0  0 1.8% K +  +  0 splits the signal region in 2   (m 2 miss ) < GeV 2 /c 4   (p K )/p K  0.2%,  (p  )/p   1 %  keep multiple scattering as low as possible   t =100 ps on  +,  t =150 ps on K +

Backgrounds 29/10/2013F. Bucci33 8% of kaon decays not separated from signal by kinematic cuts DecayBR K +  0 e + 5.1% K +  0  + 3.4% K++K++ 5.5  K +  +  0  1.5  K +  +  - e + 4  K +  +  - e + 1    identification inefficiency <   veto inefficiency    -  separation at level

Expected Sensitivity 29/10/2013F. Bucci34 Simple cut and count estimation with no optimization Background to be evaluated on data to reach the 10% accuracy

Tracking Detectors: GTK Composed of 3 hybrid silicon pixel stations mounted around four achromat magnets Inside the vacuum and subjet to a high and non-uniform beam rate (750 GHz in total ) 29/10/2013F. Bucci35 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 mm 60 mm GTK1 Pixel 23 GTK2 Pixel 23   245 ps    2  175 ps

Tracking Detectors: STRAWS Consists of 4 chambers intercepted in the middle by a dipole magnet Minimize multiple scattering: ultra-light material, integration in the vacuum tank 29/10/2013F. Bucci36 Measurement of  coordinates (  < 130  m),  and momentum (  (p)/p  0.3%) of charged particles originating from the decay Each chamber equipped with  1800 straw tubes, positioned in 4 views (u-v,x-y )

PID Detectors: KTAG-CEDAR Differential Cherenkov counter filled with H 2 for positive K (50 MHz) identification Without kaon tagging vacuum should be better than 6  mbar 29/10/2013F. Bucci37 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)

PID Detectors: RICH Ring Imaging Cherenkov detector composed of a cylindrical vessel (17 m long) filled with N e slightly above atmospheric pressure F. Bucci38 Project validated by strong R&D 17 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)  MisID Probability Momentum (GeV) Time Resolution (ps) 29/10/2013

Photon Veto System To suppress the dominant decay K +  +  0 (BR  21%):   0 rejection inefficiency at level (   detection inefficiency at )  Hermetic  coverage up to 50 mrad 29/10/2013F. Bucci39 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

Large Angle Veto (LAV) 12 stations formed by 4 to 5 overlapping rings of OPAL lead glasses 29/10/2013F. Bucci40 The first eleven are part of the vacuum decay tube, the last one is located outside the vacuum tank Inefficiency < for 100 MeV < E  < 35 GeV

Muon Veto 3 muon veto (MUV) stations (partially re-use of the NA48 hadron calorimeter) 29/10/2013F. Bucci41 MUV 3 MUV 1 MUV 2 IRON MUV1+MUV2  24 (MUV1) and 22 (MUV2) iron/scintillator layers  reach a factor 10 6 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)

Detector Status Detectors installed: KTAG, LAV(8/12), LKr, SAC Under construction/installation: CHANTI, STRAWS, RICH, IRC, MUV Installation completed by October /10/2013F. Bucci42

Outline Recent results: NA48/2 and NA62 (R K phase)  ChPT Tests  Lepton Flavor Universality Prospects: NA62  K +  +  Other rare and forbidden decays studies 29/10/2013F. Bucci43

LFV/LNV modes High 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 44 ModeUL at 90% CLExperimentNA62 acceptance K +  +  + e  BNL 777/865  10% K +  +  - e  BNL 865  10% K +  -  + e  BNL 865  10% K +  - e + e  BNL 865  5% K +  -  +   NA48/2  20% K +  - e + e  Geneva Saclay  2% K +  e - +  + no data  10%  0  + e  KTeV  2%  0  - e  KTeV  2% Expected decays in Fiducial Volume in 2 years of data taking: 1.2  K + decays, 2.5   0 decays NA62 single-event sensitivities:  for K + decays,  for  0 decays

Others DecayPhysicsPresent resultNA62 K +  + h, h  Heavy neutrinoLimits up to m h = 350 MeV RKRK LU and NP (2.488  0.010)  >  2 better K +  +  ChPT<500 events10 5 events K +  0  0 e + ChPT66000 eventsO(10 6 ) K +  0  0  + ChPT-O(10 5 ) 29/10/2013F. Bucci45

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 Its unprecedented statistics and its powerful detector features will allow to push further our knowledge of rare (and forbidden) kaon decays 29/10/2013F. Bucci46

29/10/2013F. Bucci47 Thank you for your attention

Additional Slides

The ChPT weak chiral lagrangian  The basic  S=1 O(p 4 ) 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 29/10/2013F. Bucci49

Radiative K → e  Decays  In K → e  (K e2  ),  can be produced via internal bremsstrahlung (IB) or direct- emission, the latter being dependent on the hadronic structure (SD)  R K 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. 29/10/2013F. Bucci50 K±K± e±e± e  IB K±K± e±e± e  SD

K L 0 →  0 K L 0 →  0 The charm contribution can be fully neglected since it proceeds in the SM almost entirely through direct CP violation → determination of  Need a huge number of K L decays NA48 K L flux corresponding to 3  /year NA62 possible K L flux 5-10 times NA48 one After SPS upgrade 100 times more ExpMachineMeas. or UL 90% CLNotes KTeVTevatron <5.710 -7 ( 0 → ee) E391aKEK-PS <2.610 -8 KOTOJ-PARCAim at 2.7 SM evts/3y KOPIOOpportunity at Project X ? F. Bucci 29/10/201351