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Progress on Rare K and π Decays: BNL E949 Findings and Future Directions

This report presents updates from the E949 collaboration at the April APS meeting (2005), focusing on the rare decays K+ → π+νν and π0 → νν. These decays are critical for defining the Unitarity triangle and testing the Standard Model of particle physics. The presentation highlights the innovative experimental techniques employed, including the usage of a slowed-down K+ beam and advanced detection mechanisms. Improved background suppression strategies and future work in PNN2 analysis are also discussed, with implications for new physics and precision measurements of CKM matrix elements.

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Progress on Rare K and π Decays: BNL E949 Findings and Future Directions

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  1. Progress on rare K and π decays from BNL E949 • K+ π+νν with pπ<195Mev/c (PNN2) • πo νν For the E949 collaboration: Ilektra A. Christidi SUNY at Stony Brook April APS meeting 2005 Apr 19 2005

  2. K+ π+νν

  3. Goal: Independently define the Unitarity triangle/ CKM matrix (A, λ, η, ρ  Wolfenstein parameters) Vud Vub* + Vcd Vcb* + Vtd Vtb* = 0 Theory-motivation A better determination of Vtd from K+π+νν will provide a sensitive test of the SM by comparing the results from the K and B sector and probe new physics Rare K & π decays from E949 Ilektra A. Christidi

  4. Neutral Flavor Changing Process  all diagrams 2nd order • Main contribution of t in the loop • Very theoretically “clean” calculation (precision < 5%, uncertainties mainly from c sector) The SM K+π+νν BR  ellipse in ρ-η plane Rare K & π decays from E949 Ilektra A. Christidi

  5. “PNN1” “PNN2” Current situation Rare K & π decays from E949 Ilektra A. Christidi

  6. Incoming 700MeV/c beam K slowed down by BeO and AD • K+ stops & decays at rest in scintillating fiber target – measure delay (2ns) • Outgoing π+ momentum measured in UTC, energy & range in RS and target The measurement w/ E949 detector • π+stops & decays in RS – detect π+μ+e+ chain • Photons vetoed hermetically in BV-BVL, RS, EC, CO, USPV, DSPV Rare K & π decays from E949 Ilektra A. Christidi

  7. Incoming 700MeV/c beam K slowed down by BeO and AD • K+ stops & decays at rest in scintillating fiber target – measure delay (2ns) • Outgoing π+ momentum measured in UTC, energy & range in RS and target • π+stops & decays in RS – detect π+μ+e+ chain • Photons vetoed hermetically in BV-BVL, RS, EC, CO, USPV, DSPV • New/upgraded elements The measurement w/ E949 detector Rare K & π decays from E949 Ilektra A. Christidi

  8. “PNN1” PNN2 “PNN2” • 3-body decay w/ 2 missing particles: 0  pπ+ 227 MeV/c  • Signal: π+ + nothing , backgrounds (bg) vetoed ~ 10-11 ! • Need • particle identification (PID) • all other charged particles vetoed < 10-3 • redundant precise kinematic measurements Backgrounds Rare K & π decays from E949 Ilektra A. Christidi

  9. Main bg mechanism: K+π+ποwith π+scatter in target  • Simultaneous shift in range AND momentum • Photons head near beam direction, the weakest PV region of the detector • Goal: sensitivity equal to PNN1, s/b = 1 • 2  acceptance and 5  rejection • Improved PV: new detectors at central region (BVL) and small angles • Improved algorithms to identify π+scatters in target PNN2 analysis Rare K & π decays from E949 Ilektra A. Christidi

  10. In the Kπ2 kinematic box: • ~ 2  better rejection at nominal PNN1 acceptance (80%) or • ~ 5% more acceptance with E787 rejection ! • Good news for PNN2 as well… Extra tools for PNN2: • Active degrader (PV detector near beam) • Account for “veto blindness” (i.e. when an early accidental masks the γ pulse, by using double-pulse information and early-time loose veto. E787, E949 Photon Veto improvement Rare K & π decays from E949 Ilektra A. Christidi

  11. πo  νν analysis by K.Mizouchi, Kyoto University, Japan

  12. Theoretical motivation • Decay prohibited by helicity for massless ν • For massive ν, • therefore, for the max mντ= 18.2 MeV/c, BR < 2.110-9 • From cosmology: cooling through the pion pole γγπονν gives BR < 2.910-13 , but it’s model dependent • Existing limits: BR < 8.310-7 (90% CL) from E787 • Inclusive measurement: πο anything invisible  BSM Rare K & π decays from E949 Ilektra A. Christidi

  13. Tag ποfrom Kπ2(K+→π+πo)decay  pπo =pπ+= 205MeV/c • Signal: π+ + nothing , but with fixed 2-body kinematics • Two steps in selection/bg calculation: • Kπ2tag, non-Kπ2bg calculated from data w/ blind analysis: 3.07 events • Apply tightest PV & look in the box. Conservative limit: all events due to missed photons from detector inefficiencies. Poisson 90% CL Correction factors Background & measurement # of Kaons Kπ2acceptance Rare K & π decays from E949 Ilektra A. Christidi

  14. PV cuts tuned on 1/3 of the sample, randomly selected, with optimization procedure. • 1/3 sample discarded, to maintain blindness, and measurement is done with the remaining 2/3. • Open the 2/3 sample box: 99 events  99 - 3.07 = 95.93 good Kπ2tags  N90 = 110 • NK = 3.019109 • AKπ2 = 0.1173 • CdisCacc = 1.136 • So, Analysis & result Rare K & π decays from E949 Ilektra A. Christidi

  15. E787 upgrade into E949 worked as expected: x2 better PV rejection at PNN1 acceptance, x10 at low acceptances • PNN2 analysis under way, showing signs of improved bg suppression • Updated pi0->nunu BR limit, x3 better than before: 2.73x10-7 from 8.3 Conclusions Rare K & π decays from E949 Ilektra A. Christidi

  16. Extras

  17. Rare K & π decays from E949 Ilektra A. Christidi Candidate E787A Candidate E787C Previous (E787) results (2)

  18. Rare K & π decays from E949 Ilektra A. Christidi stuff

  19. Rare K & π decays from E949 Ilektra A. Christidi Delayed coincidence

  20. Rare K & π decays from E949 Ilektra A. Christidi Eμ = 4.1 MeV, Rμ ~ 1 mm, τπ =26 ns Ee < 53 MeV, τμ = 2.2 μs π+ μ+ e+ identification

  21. Rare K & π decays from E949 Ilektra A. Christidi BR dependence on s/b of cell where event is found: Toy MC for Junk code

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