The Well-tempered kaon or Searching for very rare kaon decays For the Use and Profit of the Physical Youth Desirous of Learning as well as for the Pastime of those Already Skilled in this Study drawn up and written by Marco S. Sozzi of the University of Pisa and INFN Anno 2008 CKM08 - September 2008 M.Sozzi The Well-tempered kaon Program Praeludium Introduction Toccata LFV, K + + + Divertimento K L (K S ) 0 + Fuga K + + Ricercar K L 0 Coda Conclusions CKM08 - September 2008 M.Sozzi The Well-tempered kaon Introduction Kaons: lightest kind of flavoured matter A large and significant part of particle physics Is there still something to be learned by using kaons? Praeludium The minimal flavour laboratory: All the features of flavour physics are present Very simple (minimal) system Very nicely accessible experimentally (large rates, large BRs, small backgrounds, ) Usually pretty hard for theorists CKM08 - September 2008 M.Sozzi The Well-tempered kaon LF Violation? Already done Several dedicated efforts BNL AGS experiments (E865) set very high standards Flux and backgrounds are now limiting further progress Sensitivity to NP up to ~ 100 TeV Mode90% BR limitExp K+ ++eK+ ++e 1.3 10 11 BNL E865 K+ +e+K+ +e+ 5.2 10 10 BNL E865 K+ e+e+K+ e+e+ 6.4 10 10 BNL E865 K+ ++K+ ++ 3.0 10 9 BNL E865 K+ +e+K+ +e+ 5.0 10 10 BNL E865 KL 0eKL 0e 7.56 10 11 FNAL KTeV Toccata Impressive reach. No new LFV projects. Background limitations CKM08 - September 2008 M.Sozzi The Well-tempered kaon New KTeV LFV limit Byproduct of / experiment: KTeV exploited its large K L sample and PID capability to search for LFV as a s d e transition, on the full dataset: KTeV 4.21 exp, 5 found 0.66 exp, 0 found Toccata CKM08 - September 2008 M.Sozzi The Well-tempered kaon Loop-induced: K + + + Long-distance ( *) dominated (S,T contributions negligible) Model-dependent predictions for individual BR. BR ratio: in agreement with theory (>0.23). + e + e : BR at 3%, form factor slope at 6%. + + : BR at 17%, form factor slope at 50%. Toccata Limited fundamental physics to be learned from very precise measurement (unless way off !). CKM08 - September 2008 M.Sozzi The Well-tempered kaon New NA48/2 K + + e + e PDG average: BR = (2.880.13)10 7 Top sample: E865 (10K events) New measurement in flight by NA48/2: Kaon flux: 1.7x candidates (0.6% background) Normalization to K + + 0 D NA48/2 preliminary: Toccata FF fits do not discriminate between models 1.4 from E865 (10K evts): (2.94 0.05 0.13 0.05)10 7 CKM08 - September 2008 M.Sozzi The Well-tempered kaon K+++K+++ Poor agreement in PDG BR data (scale factor 2.7) PDG-average: BR = (8.11.4)10 8 World sample < 800 evts. Top sample: E865 (400 events) Severe background from K 3 (E865: 6.5% bkg) Most recent measurement by FNAL (4% K in secondary beam), 110 events in 20% of statistics Linear FF fit agrees with ee Toccata NA48/2 analysis ongoing (few thousand events, O(1%) bkg) NA48/2 CKM08 - September 2008 M.Sozzi The Well-tempered kaon CP violation in K decays So far the most straightforward manifestation of CP violation ( / ) was not seen. K + : width asymmetry O(10 5 ) in SM, can be larger with SUSY. Useful handle from m dependence. First measurements available, insufficient statistics: HyperCP (2002): Small SM effect, good NP probe. A (K ) = 0.02 0.11 0.04 A (K e e ) = Toccata New NA48/2 (2008) preliminary: CKM08 - September 2008 M.Sozzi The Well-tempered kaon The re-tuning issue Kaons have been very successful indeed in uncovering most of the qualitative features of the flavour sector in the SM BUT they are often ill-behaved: troubles with low-energy (m K ~O( )) quark processes cannot be evaded when dealing with light hadrons (until our theory colleagues will solve QCD). Can often relate observable kaon properties to one another ( PT), but usually lack solid (unfailing) predictive approaches. That is: RE-TUNING is often required for each piece CKM08 - September 2008 M.Sozzi The Well-tempered kaon The Well-Tempered kaon Some very rare FCNC decays are largely SD-dominated and can be computed to exceptionally high (few percent) parametric precision, unmatched by any other FCNC quark process This usually goes together with other SD-effects (very high mass-scale sensitivity, CP violation) Kaon decays thus switch to quantitative testing of the SM (behaving properly in any key!) K decays CKM08 - September 2008 M.Sozzi The Well-tempered kaon SD K decays KL 0e+eKL 0e+e 10 11 (CPV dir 3 ) < 2.8 10 10 (FNAL KTeV) CPC+CPV 3 ev. (2.05 bkg) KL 0+KL 0+ 10 11 (CPV dir 1 ) < 3.8 10 10 (FNAL KTeV) CPC+CPV 2 ev. (0.87 bkg) K + + 8.210 11 (at 5% par.) 1.05 10 10 (BNL E787+E949) Dedicated expts. 3+4 evts. ( bkg) K L 0 2.810 11 (at 2% par.) < 6.7 10 8 (KEK E391a) CPV dir Nothing to nothing Praeludium A single hadronic current with well-measured (K-) matrix element (isospin), strong suppression of LD effects and few effective operators: this gives theoretical HEAVEN (and experimental hell) CKM08 - September 2008 M.Sozzi The Well-tempered kaon K L 0 + - : not only and Divertimento Three contributions: (1) The computable SD CPV part (2) An indirect ( ) CPV part (known from if K S decays measured) (3) A LD ( * *) CP-conserving part (related to its on-shell counterpart K S 0 ) CKM08 - September 2008 M.Sozzi The Well-tempered kaon (2) CPV ind: K S 0 + KS 0e+eKS 0e+e KS 0+KS 0+ NA48/1 NA48/ events 6 events Divertimento Would still profit from improvements CKM08 - September 2008 M.Sozzi The Well-tempered kaon (3) CPC: K L 0 Interesting by itself in PT: no free parameters at O(p 4 ) New KTeV result on full dataset 1982 events Normalization to K L 0 0 30% background (K L 3 0 ) Supersedes previous KTeV result (half dataset) BR=(1.550.070.08)10 6 Agrees with NA result: BR=(1.270.040.01)10 6 Divertimento Relevant for CPC K L 0 e + e CKM08 - September 2008 M.Sozzi The Well-tempered kaon KL 0+-KL 0+- Isidori et al. EPJ C36 (2004) 57 (2) K S measurements: indirect CP- violating term dominates. (3) KL measurements: CP-allowed contribution small. Sensitivity of BR to CKM phase depends on (unmeasurable) relative sign of the two CP-violating terms. Theoretical predictions: constructive interference. Interesting correlations w. K Divertimento CONSTR. INT. DESTR. INT. CPC (th. 30% + K L 0 ) CPV ind (K S 0 + ) 50% CPV dir 10% CPV int 25% CPC (th. + K L 0 ) CPV ind (K S 0 e + e ) 50% CPV dir 8% CPV int 25% (NA48/1) CKM08 - September 2008 M.Sozzi The Well-tempered kaon KL 0+-KL 0+- With the ancillary measurements done these decays are now re-admitted into the SD exclusive club (paying membership) Divertimento Irreducible Greenlee (QED) background: BR(K L e + e ) 610 7 Dalitz plot analysis, time-dependent analysis, polarization: KTeV KL 0e+eKL 0e+e KL 0+KL 0+ require a further experimental step in intensity/rate capability Bkg limited, excellent EM calorime try Smaller SD part, larger CPC, smaller bkg CKM08 - September 2008 M.Sozzi The Well-tempered kaon K : the holy grail The best measurement of |V td |? Rather a highly-sensitive search for NP Comparison of the two can discriminate by itself the flavour structure of NP c,u contribution K + only (37%) Praeludium Theoretically clean also BSM ! CKM08 - September 2008 M.Sozzi The Well-tempered kaon Searching for the holy grail Normal science: when the processes which can be easily handled by theorists are experimental nightmares (and vice versa). (Apologies to T. Kuhn) Just look for a decay of a short-lived particle with 10 9 background, with a poor signature and no kinematic closure. High fluxes, high vacuum, high ermeticity, excellent vetoing, excellent resolutions An interesting challenge Praeludium The theorist The experimentalist CKM08 - September 2008 M.Sozzi The Well-tempered kaon To the faithful departed KAMI b. Batavia, d. Batavia, people from USA, Brasil, Greece, Russia >50M$ to measure 100 K L 0 and K L 0 + in flight b. Batavia, d. (twice) Washington, people from USA, Russia 100M$ to measure 100 K + + in flight (sep. beam) b. Brookhaven, d. Washington, people from USA, Russia, China, Japan, Switzerland 150M$ to measure 60 K L 0 in flight b. Brookhaven, d. Washington, 2002 in infancy 68 people from USA, Canada, Russia, Japan ~2M$ to measure ~10 K + + w. stopped K Praeludium CKM08 - September 2008 M.Sozzi The Well-tempered kaon K + + BR(SM) = (8.5 0.7)10 11 Non-parametric theoretical uncertainty ~ 5% (11% overall) Background from K decays: + (0.63), and + 0 (0.21), beam scattering, charge-exchange + K 3 and pion nuclear interactions. No kinematic constraints. VETO !!! Background suppression : redundancy is a must. Stopped K: separated beam, high effective decay rate, kinematics. BNL (3+4 events). In-flight K: high veto efficiency, high acceptance. First search: 1969 (10 -4 ) Observation: 1997 ( ) Fuga NA62 The long march CKM08 - September 2008 M.Sozzi The Well-tempered kaon K + + : BNL E787-E949 Study of region 2 below + 0 peak: higher background (nuclear int.): S/B = many times worse E787+E949: 4 events Bkg: 2.150.43 Region 1 between two 2-body peaks (smaller background): BR = ( ) Bkg: 0.300.03 (P=0.001) Fuga E787+E949: 3 events CKM08 - September 2008 M.Sozzi The Well-tempered kaon 100 events Mean=SM 100 events Mean=E787/949 Today ? ? Tomorrow (2012?) (4.4 events) BR(exp) = 2 x BR(SM) K + + science (or) fiction Fuga CKM08 - September 2008 M.Sozzi The Well-tempered kaon In Europe: CERN NA62 New approach: decay in flight 75 GeV/c unseparated K + beam (K 6.6%) Reuse parts of NA48 apparatus and existing SPS protons (3.310 12 ppp 1/20) Sensitivity not limited by beam flux Acceptance 14% NA62 Fuga Proposal 2005: CERN-SPSC , SPSC-P-326 Bern, Birmingham, CERN, Dubna, Ferrara, Fairfax, Firenze, Frascati, IHEP, INR, Louvain, Mainz, Merced, Napoli, Perugia, Pisa, Roma I+II, Saclay, San Luis Potosi, SLAC, Sofia, Triumf, Torino Expect O(100) events with S/B10 in 2 years (2012+) CKM08 - September 2008 M.Sozzi The Well-tempered kaon NA62 Background rejection: - vetoing: high-performance LKr calorimeter, lead-glass rings; low- E = low inefficiency - Kinematics: vacuum (straw) spectrometer, Si K + tracking in 1 GHz - PID(/ ): Ne RICH and veto/hadron calorimeter - Positive Kaon ID: beam RICH Fuga Several challenges: 200 ps 1 GHz Si beam tracker, 0.5% X 0 /station Vacuum spectrometer Vetoing Triggering and fully efficient DAQ CKM08 - September 2008 M.Sozzi The Well-tempered kaon Fuga Scientific approval from CERN and INFN R&D towards completion Strenghtening the collaboration NA62 24MCHF 92% kinematically constrained bkg. 8% not-constrained bkg. Acc. 3.5% Acc. 10.9% CKM08 - September 2008 M.Sozzi The Well-tempered kaon Ricercar E17 E391a K L 0 BR(SM) = (2.5 0.4)10 11 Non-parametric theoretical uncertainty ~ 1% (16% overall) No CPC part, indirect CPV tiny: pure SD interference CPV (dominant operator, if no LFV) Background from K decays: 0 0 (8.710 4, CPV), 0 0 0 (19.5%), + 0 (12.5%), halo neutron inelastic interactions. Unmeasurable parent particle. No kinematic constraints. VETO !!! High-energy approach: excellent vetoing, pencil beam for kinematics, high p T Low-energy approach: K L TOF for kinematics, large prod. angle, large beam emittance, tracking Grossman-Nir bound: BR(K L ) < 4.4 BR (K + ) from isospin CKM08 - September 2008 M.Sozzi The Well-tempered kaon Ricercar K L 0 Old KTeV limits (high-energy) BR < 1.5 10 6 1 event (3.71 bkg) n interactions BR < 5.9 10 7 0 events (0.120.05 bkg) n interactions, 3 0 D Dalitz approach cannot reach SM level Beam halo neutron interactions very challenging 0 (98.8%) 0 e + e (1.2%) m( ) constraint Better p T resolution CKM08 - September 2008 M.Sozzi The Well-tempered kaon At a new US machine? Most recent US proposals: a K L 0 and a stopped K + + experiment at the (improved) 8 GeV FNAL booster, in the perspective of a future MW proton driver (Project-X): FNAL Project-X Praeludium b. Batavia 2002, d. Batavia 2006, reborn Batavia 2007 with: Booster: 16kW, 22.510 12 ppp/1.4s (60% of BNL AGS): Project-X: 200 kW, x booster: (100% duty factor assumed) CKM08 - September 2008 M.Sozzi The Well-tempered kaon FNAL PAC June 2008 Kaons at FNAL? May P5: only possible in a HEP fund doubling scenario K L 0 : low-energy, small emittance beam, detector in vacuum, high(er) acceptance 30 booster, Project-X K + + : stopped, low-energy (500 MeV/c, higher stop probability) separated beam, finely-segmented compact detector w. high magnetic field 40 booster, Project-X Also discussed high-energy, in-flight, RF-separated beam exp. (200 ev/year) Unfortunately, unlikely CKM08 - September 2008 M.Sozzi The Well-tempered kaon In Japan: E391a (KEK) KEK 12 GeV/c PS (2 ppp ). Pilot project for J-PARC experiment Pencil beam (kinematics, vetoing): p K 2 GeV/c, n/K 60 Photon vetoes at 1 MeV level, high acceptance High vacuum, hadronic background MC Large p T (>120 MeV/c) and decay vertex cuts Double veto region (3 0 w. back ) Ricercar CKM08 - September 2008 M.Sozzi The Well-tempered kaon E391a new limit Run I partial analysis (10% of data): Run II: Solved problem of matter on beam and DAQ inefficiency, reduced n flux Control bkg: 1.90.2, obs. 3, 0.390.08, obs. 2 Background: exp. 0.410.11, obs. 0 (0.11 0 0, 0.30 halo n) Acceptance: 0.67%, 5.1109 k Single-event sensitivity: 2.8910 8 Still above Grossman-Nir bound Ricercar Comparable Run III CKM08 - September 2008 M.Sozzi The Well-tempered kaon 50 GeV, 3.310 14 ppp, 20-50% DC, 750 kW J-PARC J-PARC complex Hadron hall Beam to experiments in second half of 2008 Ricercar CKM08 - September 2008 M.Sozzi The Well-tempered kaon In Japan: J-PARC E14 Improved beamline (reduced halo, n/K 7, lower background, veto) Step 1: re-use E391a barrel Thicker and more fine-grained CsI calorimeter (KTeV) Waveform digitization Upgraded vetos (KOPIO-like beam hole veto) Goal: 2.7 events (S/N 1.6) in 3 years Construction 2009, first physics run 2011 ? Step 2: 100 SM events (S/N 4.8), new dedicated target ? Flux x10, Time x10, Performance x10 Ricercar CKM08 - September 2008 M.Sozzi The Well-tempered kaon More ideas/plans K + + J-PARC: E949-like stopped experiment K L 0 Protvino: U70: E391a-like, 10 GeV, tracking K L 0 CERN: at improved LHC proton injector (SPS+), high- energy, long hermetic vetoing decay region K L 0 a super-intense phi-factory: K L of known momentum and direction (some orders of magnitude away from any presently conceivable project) A high-intensity (MW) proton driver ( GeV/c) is the best source of K (synergy with program but high DC required). High yield: better experiments and access to more powerful analysis approaches CKM08 - September 2008 M.Sozzi The Well-tempered kaon Coda New Physics (if it exists) is small, and not too weird concerning flavour: If Nature is not MFV it is almost MFV (remember superweak?) Large suppressions (V ts V td ) make K attractive: Rare decay (small SM contribution) can see NP in large effect Well-controlled decay can see NP in a not-so-large effect Or maybe there is NO new physics at the TeV-scale (interesting in a way, provided the SM Higgs is not there): Experimentally K SES and high-energy scale reach are unrivaled F=1 processes are different from those constraining the CKM matrix So why kaons? CKM08 - September 2008 M.Sozzi The Well-tempered kaon Kaons, once again. Nature rewards perseverance. Now just go and measure it. There's nothing remarkable about it. All one has to do is hit the right keys at the right time and the instrument plays itself . J.S. Bach CKM08 - September 2008 M.Sozzi The Well-tempered kaon Spare slides CKM08 - September 2008 M.Sozzi The Well-tempered kaon Future machines? A high intensity p driver (>10 14 ppp) would be very valuable for ultimate K measurements. Sinergies with neutrino physics? With LHC injectors upgrade? Energy in the tens of GeV range, slow extraction (high DC) Intense K + beam: K CP asymmetries Intense K + RF-separated beam: K + + Tertiary K 0 beam: CPT tests at Planck scale (K phases) Intense K L beam: K L 0, K L 0 e + e -, K L 0 + - (with several handles), diverse program CKM08 - September 2008 M.Sozzi The Well-tempered kaon KL 0+-KL 0+- Isidori et al. EPJ C36 (2004) 57 (2) K S measurements: indirect CP- violating term dominates. (3) KL measurements: CP-allowed contribution small. Sensitivity of BR to CKM phase depends on (unmeasurable) relative sign of the two CP-violating terms. Theoretical predictions: constructive interference. BR(K L 0 e + e - ) CPV 17 (ind) 9 (interf) + 4 (dir) BR(K L 0 + - ) CPV 8 (ind) 3 (interf) + 2 (dir) Divertimento CONSTR. INT. DESTR. INT. Interesting correlations w. K CKM08 - September 2008 M.Sozzi The Well-tempered kaon Return to 2007-level funding (1) e conversion (2) Participation to super-B factory abroad No K physics in this scenario CKM08 - September 2008 M.Sozzi The Well-tempered kaon LFV with K + Impressive reach. No new LFV projects. Better background suppression would be required BNL E865 Toccata CKM08 - September 2008 M.Sozzi The Well-tempered kaon KTeV ( ) FNAL Main Injector (120 GeV) p Double K L beam ( =70 GeV/c) Regenerated K S Pure CsI calorimeter Data taking in 1997 and physicists, 12 USA/Japan institutions CKM08 - September 2008 M.Sozzi The Well-tempered kaon P K spectra, 60 3 GeV/c cm cm m He tank + spectrometer Momentum selection Focusing sweeping Cleaning Beam spectrometer (0.7%) ~1 ppp K+K+ KK Beams coincide within ~1mm all along 114m decay volume, always in vacuum focusing beams BM z magnet vacuum tank not to scale K+K+ KK 2 3 M K/spill ( /K ~ 12) decay products stay in pipe 0 prod. angle K decays per year collected CERN SPS (450 GeV) p, twin simultaneous K beams 130 physicists, 16 European institutions NA48/2 ( ) CKM08 - September 2008 M.Sozzi The Well-tempered kaon K + + e + e : Form Factor Different models for F(z): O(p 6 ) + unitarity, VMD z