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Evidence for pK -atoms with DIRAC-II

Evidence for pK -atoms with DIRAC-II. R & D development Data taking Analysis and results. Yves Allkofer University of Zurich 20 th November 2008. y. sin(k’r). Incoming (outgoing ) large distance wave function. r. pK - atoms have 2 isospin contributions: 1/2 and 3/2

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Evidence for pK -atoms with DIRAC-II

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  1. Evidence for pK-atoms with DIRAC-II • R & D development • Data taking • Analysis and results Yves Allkofer University of Zurich 20th November 2008

  2. y sin(k’r) Incoming (outgoing ) large distance wave function r pK -atoms have 2 isospin contributions: 1/2 and 3/2 a01/2 and a03/2 sin(kr+d) -V0 Scattering length a0 Introduction to scattering length

  3. pK scattering lengths K+ K+ P.Estabrooks et al.,Nucl.Phys.B133(1978)490 p- p- experiment D++ p S-wave scattering length ChPT a03/2[mp-1] V.Bernard et al.,Nucl.Phys. B357 (1991) 2757 Dispersions relations theory P.Buettiker et al,Eur.Phys.J C33 (2004) 409 a01/2 [mp-1]

  4. pK-atoms The decay channel of pK-atoms: rB • rB (1S) = 1/(ma)= 250 fm • DE 2S-2P = -0.3eV (em inter.) -1.1±0.1eV (strong inter.) • E(1S) = ma2/2= -2.9 keV -9 eV (DE strong inter.) What we aim to measure • t1S = 3.7 ± 0.4 fs J. Schweizer, Phys. Lett. B 587 (2004) 33 • DIRAC measures the number of ionized pairs so-called atomic pairs (competing process to the decay). • The number of ionized pairs depends on t. DIRAC’s approach: • Alternatively, DIRAC measures the scattering length through the lifetime ofpK-atoms.

  5. The DIRAC-II spectrometer Pt AKp

  6. The Čerenkov detectors e || p K p e p K || p e p || K p Heavy gas (veto) Aerogel (coincidence) N2 (veto) e p K p One N2 module with n=1.0003 (1bar) One C4F10 module with n=1.0014 3 aerogel modules in left arm: 2 with n=1.015 and 1 with n=1.008

  7. Čerenkov radiation and aerogel For a kaon/proton separation one has to choose a radiator for which the v(kaon)>c/n and v(proton)<c/n • AEROGEL: • is identical to quartz with low density r = 30 - 300 mg/cm3 • has a refractive index between gas and solid radiator. n=1 + 0.21r • poor optical transmittance due to absorption and Rayleight scattering.

  8. DIRAC’s requirement for K/p separation GeV/c Kaon spectrum 8 n=1.008 L1 6 H2 H1 n=1.015 4 50 60 80 horizontal position (cm) 70 Simulation • Issues : • Distance between 2 PMs large • For the n=1.008 module the light production is small • kaons suppressed by a factor 6 compared to pions and protons

  9. The cosmic ray test setup LED n=1.05 Results are used to tune the Monte-Carlo

  10. GEANT4 Simulation n=1.015 n=1.008 • strong impact position dependence due to absorption in aerogel • low light yield due to a low refractive index • Need for new designs

  11. The pyramid design n=1.05 Npe Npe Simulation Vertical position (cm) The pyramid design cancels out the impact position dependence

  12. Wavelength shifter? Quantum efficiency (%) Absorption length Labs (m) 10 Simulation Createdphotons 28% 1 Photons reaching the PMs 0.1 Labs =0. 1 m @ 250 nm Labs = 1 m @ 310 nm 500 100 300 700 450 350 Wavelength (nm) 250 550 200 400 600 Wavelength (nm) Wavelength (nm) Shifting light from UV to blue improves the light collection efficiency

  13. Placement of aerogel The aerogel counter • Aerogel with n=1.008 • 14 liters (250 pieces) Budker/ Boreskov Institute Novosibirsk • Aerogel with n=1.015 • 24 liters (248 pieces) Matsushita (Panasonic) Electric Works

  14. First run in 2007: detector performance Kaon detection efficiency: for L1: 85 - 90% for H1, H2: 95% Proton rejection factor: for L1: 10-15 for H1,H2: 20 Kaons (K-) L (no beam in 2006 due to PS magnet breakdown) • using the heavy gas and N2 Čerenkovas veto only kaons and protons are remaining. • How to disentangle them? We need a pure proton and kaon beam!

  15. Data analysis: event types Background Signal Accidental pairs Nacc Non Coulomb pairs NnC Coulomb pairs, NC pK atoms NA(produced) Time correlated events decay ionization p0K0 pK detected pp- Atomic pairs DIRAC looks for an excess of pairs with a very low relative momentum Q

  16. Event selections • electrons/positrons rejection (N2Čerenkov counter, preshower detector) • muons rejection (muon counter, preshower detector) • pions rejection (heavy gas Čerenkov counter) • proton rejections for K+p- candidates (aerogel Čerenkov counter) • |QL|< 20 MeV/c • |QT|< 8 MeV/c • 4 < P(kaon) < 8 GeV/c • 1.2 < P(pion) < 2.1GeV/c

  17. Background description Data: accidental pairs Monte-Carlo: non-Coulomb pairs Monte-Carlo: Coulomb pairs • Non-Coulomb and accidental pairs have the same shape and can be extracted directly from data using timing • Coulomb pairs have an enhancement for low QL and are simulated

  18. Coulomb correlated p-K+-pairs |QL| distribution for time correlated events (Coulomb pairs, accidentals and non-Coulomb pairs) divided by accidental pairs. Prompt pairs (accidental Coulomb and non Coulomb pairs) Accidental pairs Existence of Coulomb correlated p-K+ pairs is demonstrated without the use of Monte-Carlo. NC=858±247 in signal region.

  19. Expected atomic-pairs NA = 198 ± 57 (produced atoms) Pion is the number atomic pairs (nA) divided by the number of produced atoms (NA): From theory nA = 104 ± 30 (atomic pairs) The number of Coulomb pairs (NC) and produced atoms (NA) are proportional: From the detection of 858 ± 247 Coulomb pairs one expect 104 ± 30 atomic pairs (without the use of MC).

  20. Direct observation of pK atomic pairs atoms Residuals Signal region Total background Non-Coulomb correlated pairs Coulomb correlated pairs 143±53 detected p-K+ atomic pairs (104 expected). A similar analysis leads to 29±15 detected p+K- atomic pairs (38 expected).

  21. p- K+ + p+ K- atomic pairs p+ K- p- K++ p+ K- atomic pairs: 173 ±54 atoms Atomic pairs Coulomb pairs 29±15 165 ±108 p- K+ + p+K- p- K+ Atomic pairs Coulomb pairs 143±52 972 ±233 Pion=(64±25)%

  22. Lifetime measurement • An atom while traveling through the target can either: • be (de-)exited (Pex): (ex: 2S-->2P) • be ionized (Pion): • decay (Pdecay): Pion=1-Pdecay-Pex t ≥1.5·10-15 s with 84% confidence level (3.7±0.4 has been predicted).

  23. Summary • Thanks to efficient particle identification from pioneering run 2007: • observation of Coulomb correlation in pK-pairs production. Atoms must also be produced. • first direct evidence for pK-atoms production (173±54 detected atomic pairs). • first experimental estimation of a lower limit of pK atoms lifetime (1.5 fs).

  24. DIRAC-II Collaboration] This work has been published in: Y.Allkofer et al., Nucl. Instr. Meth. A 582 (2007) 497, Y. Allkofer et al., Frascati Physics Series Vol. XLVI (2008), Y. Allkofer et al., Nucl. In str. Meth. A 585 (2008)84

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