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

3. Introduction to scattering length Incoming (outgoing ) large distance wave function  atoms  have 2 isospin contributions: 1/2 and 3/2 a 0 1/2 and a 0 3/2 Scattering length a 0 -V 0 r sin(kr+  ) sin(k’r) 

4. a 0 3/2 [m  -1 ] theory a 0 1/2 [m  -1 ] S-wave scattering length ChPT Dispersions relations P.Buettiker et al,Eur.Phys.J C33 (2004) 409 V.Bernard et al.,Nucl.Phys. B357 (1991) 2757  K scattering lengths P.Estabrooks et al.,Nucl.Phys.B133(1978)490 experiment p      

5.  K-atoms  E 2S-2P = -0.3eV (em inter.) -1.1±0.1eV (strong inter.) The decay channel of  K- atoms : J. Schweizer, Phys. Lett. B 587 (2004) 33  1S = 3.7 ± 0.4 fs rBrB r B (1S) = 1/(  )= 250 fm E (1S) =    = -2.9 keV -9 eV (  E strong inter.) What we aim to measure Alternatively, DIRAC measures the scattering length through the lifetime of  K- atoms. DIRAC’s approach:  DIRAC measures the number of ionized pairs so- called atomic pairs (competing process to the decay). The number of ionized pairs depends on .

6. The DIRAC-II spectrometer A  Pt

7. One C 4 F 10 module with n=1.0014 3 aerogel modules in left arm: 2 with n=1.015 and 1 with n=1.008 The Č erenkov detectors e  Aerogel (coincidence) Heavy gas (veto) N 2 (veto) e  K || p e ||  K p e  K p K p One N 2 module with n=1.0003 (1bar)

8. Č erenkov radiation and aerogel AEROGEL: is identical to quartz with low density  = 30 - 300 mg/cm 3 has a refractive index between gas and solid radiator. n=1 + 0.21  poor optical transmittance due to absorption and Rayleight scattering. For a kaon/proton separation one has to choose a radiator for which the v(kaon)>c/n and v(proton)<c/n

9. DIRAC’s requirement for K/p separation n=1.015 n=1.008 6 4 8 GeV/c Kaon spectrum 50 60 70 horizontal position (cm) 80 H1 H2 L1 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 Simulation

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

11. n=1.015 GEANT4 Simulation 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

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

13. L abs =0. 1 m @ 250 nm L abs = 1 m @ 310 nm 0.1 1 10 Wavelength (nm) 250 350 450 550 Absorption length L abs (m) Wavelength shifter? Simulation Wavelength (nm) 200 600 Created photons Photons reaching the PMs 400 100 500 300700 Wavelength (nm) Quantum efficiency (%) 28% Shifting light from UV to blue improves the light collection efficiency

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

15. First run in 2007: detector performance (no beam in 2006 due to PS magnet breakdown) using the heavy gas and N 2 Č erenkov as veto only kaons and protons are remaining. How to disentangle them? We need a pure proton and kaon beam!  Kaons (K - ) Kaon detection efficiency: for L1: 85 - 90% for H1, H2: 95% Proton rejection factor: for L1: 10-15 for H1,H2: 20

16.  K atoms N A (produced) Accidental pairs N acc Non Coulomb pairs N nC Coulomb pairs, N C Signal Background Data analysis: event types p-p- Time correlated events  0 K 0  K decayionization detected Atomic pairs DIRAC looks for an excess of pairs with a very low relative momentum Q

17. electrons/positrons rejection (N 2 Č erenkov counter, preshower detector) muons rejection (muon counter, preshower detector) pions rejection (heavy gas Č erenkov counter) proton rejections for K +  - candidates (aerogel Č erenkov counter) |Q L |< 20 MeV/c |Q T |< 8 MeV/c 4 < P(kaon) < 8 GeV/c 1.2 < P(pion) < 2.1GeV/c Event selections

18. Background description Data: accidental pairs Monte-Carlo: Coulomb pairs Monte-Carlo: non-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 Q L and are simulated

19. Coulomb correlated  - K + -pairs Accidental pairs Prompt pairs (accidental Coulomb and non Coulomb pairs) |Q L | distribution for time correlated events (Coulomb pairs, accidentals and non- Coulomb pairs) divided by accidental pairs. Existence of Coulomb correlated   K + pairs is demonstrated without the use of Monte-Carlo. N C =858±247 in signal region.

20. Expected atomic-pairs The number of Coulomb pairs (N C ) and produced atoms (N A ) are proportional: N A = 198 ± 57 (produced atoms) P ion is the number atomic pairs (n A ) divided by the number of produced atoms (N A ):  From theory n A = 104 ± 30 (atomic pairs) From the detection of 858 ± 247 Coulomb pairs one expect 104 ± 30 atomic pairs (without the use of MC).

21. Coulomb correlated pairs Non-Coulomb correlated pairs Total background 143±53 detected   K + atomic pairs (104 expected). Direct observation of  K atomic pairs A similar analysis leads to 29±15 detected   K - atomic pairs (38 expected). Residualsatoms Signal region

22.  - K +  + K -  - K +  + K - atomic pairs: 173 ±54  - K + Atomic pairs Coulomb pairs 143±52 972 ±233 Atomic pairs Coulomb pairs 29±15 165 ±108  + K -  - K + +  + K - atomic pairs P ion =(64±25)% atoms

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

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

25. DIRAC-II Collaboration] CERN Geneva, Switzerland Santiago de Compostela University Santiago de Compostela, Spain Czech Technical University Prague, Czech Republic IFIN-HH Bucharest, Romania Institute of Physics ASCR Prague, Czech Republic JINR Dubna, Russia Nuclear Physics Institute ASCR Rez, Czech Republic IHEP Protvino, Russia University of Messina Messina, Italy SINP of Moscow State University Moscow, Russia INFN-Laboratori Nazionali di Frascati Frascati, Italy Basel University Basel, Switzerland Trieste University and INFN-Trieste Trieste, Italy Bern University Bern, Switzerland Kyoto Sangyou University Kyoto, Japan Zurich University Zurich, Switzerland (Y.Allkofer, c.Amsler, S.Horikawa, C.Regenfus, J.Rochet) KEK Tsukuba, Japan Tokyo Metropolitan University Tokyo, Japan 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