September 27, Proposal to Measure the Rare Decay K     at the CERN SPS CERN, Dubna, Ferrara, Florence, Frascati, Mainz, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome, Saclay, San Luis Potosi, Sofia, Turin CERN-SPSC SPSC-P-326

September 27, Physics Motivation The Kobayashi-Maskawa mechanism appears to be the main (only?) source of CP-violation We must look for inconsistencies in the Standard Model using independent observables affected by small theoretical uncertainties and different sensitivity to new physics The rare process K     belongs to the theoretically cleanest decays in the field of K- and B-mesons It allows one to determine |V td | independently from B 0 -B 0 mixing (LHCb), thus providing a decisive test of the Standard Model

September 27, Setting the bar for the next generation of K + →  + experiments 100 events Mean=SM 100 events Mean=E787/949 Current constraint on  plane ? E787/E949: BR(K + →  + ) = × SM Prediction : 8.0 ± 1.1 

September 27, P-326 Detector Layout 800 MHz beam  /K/p K+K+ ++ ~11 MHz

September 27, Timeline 2006 –Tests in the present K12 beam –Construction, installation and tests of the new beam (2007) and new detectors ( ) –Data Taking

September 27, Status We build on the experience and infrastructure of NA48, NA48/1 and NA48/2 but it is really a new experiment with large capital investment needed The proposal was submitted in June 2005 The open presentation to the SPSC was given on September 27, 2005 Response from SPSC referees: “…The physics case was discussed and was, again, enthusiastically endorsed by the committee. On the experimental side, the committee got the message that every aspect of the proposed experiment is at the same time critical and challenging.”

September 27, Other Material

September 27, Other Physics Opportunities The situation is similar to NA48, which was designed to measure “only”  ’/  but produced many more measurements Accumulating ~100 times the flux of NA48/2 will allow us to address, for instance: 1.Cusp like effects (  scattering) –K       e  2.Lepton Flavour Violation K        e , K       e +, (K e2 /K  2 ) 3.Search for new low mass particles –K      X –K       P (pseudoscalar sGoldstino) 4.Study rare     decays 5.Improve greatly on rare radiative kaon decays 6.Compare K + and K - (alternating beam polarity) –K        (CPV interference) –T-odd Correlations in K l4 7.And possibly, given the quality of the detector, topics in hadron spectroscopy

September 27, Background rejection Guidance: S/B = 10 ~ rejection 1)Kinematical rejection based on the missing mass: 2) Veto and Particle ID , , charged particles  –  e separation

September 27, Backgrounds kinematically constrained DecayBR K +    K  2 )0.634 K +  +  K +  +  +  - K +    0  % of K + decays Allows us to define the signal region K   +  0 forces us to split it into two parts Region I: 0 < m 2 miss < 0.01 GeV 2 /c 4 Region II: < m 2 miss < GeV 2 /c 4

September 27, Backgrounds not kinematically constrained Decay BR K +  0 e + (K e3 ) K3K3K3K K2K2K2K2 5.5×10 -3 +0K++0+0K++0 1.5×10 -3 K e4 4×10 -5 K4K4K4K4 1× % of K + decays They span accross the signal regions Must rely on Particle ID and veto

September 27, Signal Acceptance Acceptance (5 m < Z vertex < 65 m) REGION I: 4% REGION II: 13% Total: 17% For safety, a 10% acceptance is quoted in the proposal

September 27, Signal & backgrounds from K decays / year TotalRegion IRegion II Signal K++0K++0 2.7±0.21.7±0.21.0±0.1 K2K2 1.2±0.31.1±0.3<0.1 K e4 2±2negligible2±2 K   +  +   and other 3-tracks bckg. 1±1negligible1±1 22 1.3±0.4negligible1.3±0.4 K2K2 0.4±0.10.2±0.1 K e3, K  3,others negligible  Total bkg9±33.0±0.26±3

September 27, Beam: Present K12 (NA48/2) New HI K + > 2006 Factor wrt 2004 SPS protons per pulse on T101 x x Duty cycle (s./s.)4.8 / Solid angle (  sterad)  0.40  Av. K + momentum (GeV/c)6075 K + ~ 1.5 Mom. band RMS: (  p/p in %)  4  1 ~0.25 Area at Gigatracker (cm 2 )  7.0  14  2.0 Total beam per pulse (x 10 7 ) per Effective spill length MHz MHz/cm 2 (gigatracker) ~45 (~27) ~24(~15) Eff. running time / yr (pulses)3 x K + decays per year1.0x x10 12  48 New high-intensity K + beam for P-326 Already Available

September 27, SPS Availability There are two approved competitors for beam: LHC and CNGS P-326 requires ~ SPS pulses/year with 4.8 s flat top For comparison, the Fixed Target request quoted in the Villars Report (SPSC-M-730) is pulses/year P-326 is completely compatible with the simultaneous running of COMPASS in the M2 beam line and with experiments and tests in the H2, H4, H6 and H8 beams

September 27, Possibly the Cleanest SM test In The phase  derives from Z 0 diagrams (  S=1) whereas in A(J/  K S ) originates in the box diagram (  B=2) Any non-minimal contribution to Z 0 diagrams would be signalled by a violation of the relation: A deviation from the predicted rates of SM would be a clear indication of new physics Complementary programme to the high energy frontier: –When new physics will appear at the LHC, the rare decays may help to understand the nature of it

September 27, Kaon Rare Decays and the SM Kaons provide quantitative tests of SM independent from B mesons… …and a large window of opportunity exists! |V td | G. Isidori Im t = A 2 5  Re t = A 2 5 

September 27, K + →  +  : State of the art BR(K + →  + ) = × Compatible with SM within errors hep-ex/ PRL93 (2004) Stopped K ~0.1 % acceptance AGS