THE CERN Experiment P326 for Rare Kaon Decays Massimo Lenti INFN Sezione di Firenze

Outline of the presentation Physics motivations for K+→p+nn The beam The main background channels The apparatus The signal acceptance and S/B Time schedule and Conclusions P326 M. Lenti

Physics motivations for K+→p+nn CP Standard Model predictions BR(K+p+nn)  (1.6×10-5)|Vcb|4[sh2+(rc-r)2]  (8.0 ± 1.1)×10-11 BR(KLp0nn)  (7.6×10-5)|Vcb|4h2  (3.0 ± 0.6)×10-11 P326 M. Lenti

Physics motivations, II Theoretically very clean Sensitive to Vtd Very sensitive to New Physics Present (E787/949): BR(K+p+nn) = 1.47 ×10-10 with 3 events +1.30 -0.89 Need a 10% measurement (100 events): P326 P326 M. Lenti

] How many K+ decays? BR(K+→p+nn) ~ 8×10-11 Look for ~100 signal events Signal acceptance ~ 10% ~1013 K+ decays NA48/P326 Use the existing CERN accelerators NA48 experimental hall P326 M. Lenti

The Beam Primary Beam: 400 GeV/c protons 3×1012 protons/pulse (3×NA48/2) 4.8/16.8 s duty cycle Secondary Beam: 75 GeV/c momentum (Dp/p~1%) Beam acc.: 15 mstr (30×NA48/2) Total rate: 800 MHz K+ ~ 6% 4.8×1012 K+decays/y P326 M. Lenti

The Beam purity Only 6% K+ but: protons and electrons don’t decay... pions and muons decays cannot mimic K+ decays but beam-gas interactions !! Keep vacuum at 10-6 mbar: use existing NA48 decay tank (already directly measured) Tag the K+ in the beam: use a CEDAR P326 M. Lenti

The CEDAR The CEDAR is a Cherenkov counter Used at CERN since long time Vary gas pressure and diaphragm aperture to select K+ November 2006: test beam with a CEDAR 100 GeV/c beam Filled with Nitrogen Adapt to P326 needs: Fill with Hydrogen Change Phototubes and electronics Pions Protons Kaons P326 M. Lenti

Kinematics 92% K+ decays 8% K+ decays PK : beam spectrometer qKp K+ n n m2miss=(PK-Pp)2 92% K+ decays 8% K+ decays PK : beam spectrometer Pp : straw chambers spectrometer P326 M. Lenti

Beam Spectrometer (I) Achromat CEDAR Achromat p Gigatracker 3 Silicon Pixels stations across the 2nd Achromat: 36(X) × 48(Y) mm per station Beam rate: 800 MHz (“Gigatracker”), 50 MHz/cm2 P326 M. Lenti

Beam Spectrometer (II) s(PK)/PK ~ 0.4% s(qK) ~ 16 mrad 300×300 mm pixels 200 mm Si sensor + 100 mm chip 0.13 mm CMOS technology Low X/X0 s(t) ~ 200 ps/station Important for beam pile-up Strong R&D ongoning... P326 M. Lenti

Straw Chambers Spectrometer from T0 5.4 m 7.2 m Gas CF4-CO2-isoC4H10 3 coordinates 4 coordinates 2 coordinates 1 coordinate 10 cm 6 chambers with 4 double layers Ø 9.6 mm straw tubes in vacuum 0.1% X0 per view 130 mm hit resolution per view 2 magnets (270 and 360 MeV/c pt kick) holes follow beam path P326 M. Lenti

Kinematics Reco. Missing mass measurement: dominated by angle between qpK qp PK qK Pp Missing mass measurement: dominated by angle between Kaon and pion Double spectrometer: almost independent momentum measurement P326 M. Lenti

Background 1: K+→m+n (Km2) Largest BR: 63.4% Need ~10-12 rejection factor Kinematics: 10-5 Muon Veto: 10-5 Particle ID: 5×10-3 MAMUD RICH P326 M. Lenti

Muon Veto: MAMUD MAgnetized MUon Detector Sampling calorimeter + Magnet for beam deflection Em/hadronic clusters separation Sensitivity to MIP 10-5 m detection inefficiency P326 M. Lenti

RICH p-m 3s separation up to 35 GeV/c 18 m long Neon at 1 atm (p thr.: 12 GeV/c) 2000 PMT 18 mm granularity 100 ps resolution (to disentangle pileup in the Gigatracker) PMTs tested in 2006 CEDAR test Prototype test beam in 2007 P326 M. Lenti

Background 2: K+→p+p0 (Kp2) 2nd Largest BR: 20.9% Need ~10-12 rejection factor Kinematics: 5×10-3 Photon Veto: 10-5 per photon Large angle: 13 ANTIs (10 < acceptance < 50 mrad) Medium angle: NA48 LKr (1 < acceptance < 10 mrad) Small angle: IRC1,2 SAC (acceptance < 1 mrad) P326 M. Lenti

Large Angle Veto 13 ring em calorimeters in vacuum 10-4 ineff. 0.05<Eg<1 GeV 10-5 ineff. Eg>1 GeV Two options under test: lead-scint. tiles read by WLS fibers lead+scint.fibers (KLOE like) Prototypes under constructions Tests at Frascati tagged photon beam P326 M. Lenti

Small Angle Veto shaslyk calorimeter on the beam axis 10-5 ineff. High energy g Tested in October 2006 In the NA48 tagged photon beam (see later) P326 M. Lenti

Liquid Kripton Calorimeter Use the existing NA48 LKr calorimeter 10-5 ineff. Eg>5 GeV 10-4 ineff. 1<Eg<5 GeV Ineff. for Eg>10 GeV tested on data collected by NA48/2 (K+→p+p0) p g g P326 M. Lenti

Liquid Kripton Calorimeter (II) Kevlar window Magnet Calorimeter October 2006 test: Tagged photon beam Using the existing NA48 setup vacuum e- g Electron beam (25 GeV/c) Bremsstrahlung Drift chambers Energy deposition in LKr electron 2×108 electrons collected 10-5 ineff.sensitivity below 10 GeV Energy GeV g P326 M. Lenti X LKr cm

The P326 Layout 11 MHz 800 MHz 50 MHz P326 M. Lenti

Signal Acceptance Region I: 0<mmiss2<0.01 GeV2/c4 Region II: Remind: Km2 mmiss2 < 0 Kp2 mmiss2 = 0.0182 GeV2/c4 Momentum range: 15 <pp< 35 GeV/c Fiducial decay region: 60 m Acceptance: 4% (Region I), 13% (Region II): 17% (I+II) 10% goal feasible (after analysis cuts, etc.) P326 M. Lenti

Signal/Background S/B ~ 8 (Region I ~5, Region II ~9) Events/year Total Region I Region II Signal (acc=17%) 65 16 49 K+p+p0 2.7 1.7 1.0 K+m+n 1.2 1.1 <0.1 K+e+p+p-n ~2 negligible Other 3 – track decays ~1 K+p+p0g 1.3 K+m+ng 0.5 0.2 K+e+(m+) p0n,others - Total bckg. 9 3.0 6 S/B ~ 8 (Region I ~5, Region II ~9) P326 M. Lenti

Trigger Levels 11 MHz L0 trigger input 1track × m! × g! → 1 MHz L1 trigger input → PC farm Software trigger reduction ~ 40 P326 M. Lenti

Conclusions 2006-2007: R&D, test beam 2008-2010: Construction 2011: start data-taking Full approval and funding expected end of 2007 Still need to strengthen the collaboration Clear Physics case many other physics channels P326 M. Lenti