1. University of Santiago de Compostela, Spain Pionium Lifetime Measurement by DIRAC Pionium Lifetime Measurement by DIRAC Antonio Romero on behalf of DIRAC collaboration XXXV INTERNATIONAL MEETING ON FUNDAMENTAL PHYSICS 28 May - 1 June 2007, Santiago de Compostela, Spain

2. 90 Physicists from 18 Institutes Basel Univ., Bern Univ., Bucharest IAP, CERN, Dubna JINR, Frascati LNF- INFN, Ioannina Univ., Kyoto-Sangyo Univ., Kyushu Univ. Fukuoka, Moscow NPI, Paris VI Univ., Prague TU, Prague FZU-IP ASCR, Protvino IHEP, Santiago de Compostela Univ., Tokyo Metropolitan Univ., Trieste Univ./INFN, Tsukuba KEK. Lifetime Measurement of  +  - atoms to test low energy QCD predictions www.cern.ch/DIRAC DImeson Relativistic Atomic Complexes DIRAC

3. Pionium lifetime The lifetime of  +   atoms is dominated by charge exchange process into  0  0 : a 0 and a 2 are the  S-wave scattering lengths for isospin I=0 and I=2. Pionium is a hydrogen-like atom consisting of  + and  - mesons E B =-1.86 keV, r B =387 fm, p B ≈0.5 MeV π+π+ ππ π0π0 π0π0 at lowest order :

4. Pionium lifetime in QCD   = (5.8 ± 1.2) × 10 -2 significant correction At next-to-leading order in α and (m d – m u ) 2 : Measurement of  (10 %)  |a 0 -a 2 | (5%) J. Gasser et al, Phys. Rev. D62 (2001) 016008

5. Pionium lifetime in QCD The  scattering lengths have been calculated in the framework of Chiral Perturbation Theory (ChPT): G. Colangelo, J. Gasser and H. Leutwyler, Nucl. Phys. B603 (2001) 125.

6. Experimental results K + →  +  - e + e (K e4 ) decay New measurement at BNL (E865) S.Pislak et al., Phys.Rev. D 67 (2003) 072004 a 0 =0.216±0.013 ±0.003(syst) a 2 =-0.0454±0.0031 ±0.0013(syst) L. Rosselet et al., Phys. Rev. D 15 (1977) 574a 0 =0.26±0.05  N→  N near threshold M. Kermani et al., Phys. Rev. C 58 (1998) 3431a 0 =0.204±0.014 ±0.008(syst) C.D. Froggatt, J.L. Petersen, Nucl. Phys. B 129 (1977) 89a 0 =0.26±0.05 N.Cabibbo, Phys. Rev. Lett. 93, 121801 (2004) N.Cabibbo, G.Isidori, hep-ph/0502130 |a 0 -a 2 |= 0.261 ± 0.006 ( stat.) ±0.003 ( syst.) ±0.0013 (ext) ±0.013 (theor) K + →  +  0  0 and K L →3  0 NA48/2

7. The pionium is a Coulomb bound state:  + and  - originating from short lived sources ( , K*, ,...) and resonance decays may form a pionium atom. The differential cross section is: Lorentz Center of Mass to Laboratory factor. Wave function at origin (accounts for Coulomb interaction). Pion pair production from short lived sources. Production of pionium

8. Method of pionium detection Pionium is created in nS states then it interacts with target material: L.Nemenov, Sov.J.Nucl.Phys. 41 (1985) 629 Annihilation: A 2  →  0  0 Excitation: transitions between atomic levels Break-up(ionisation): characteristic “atomic” pairs n A Q cms <3MeV/c → in laboratory system E + ≈E -, small opening angle θ<3mrad Coulomb and atomic pairs are detected simultaneously:

9. Production of pionium Atoms are Coulomb bound state of two pions produced in one proton-nucleus collision Background processes: Coulomb pairs. They are produced in one proton nucleus collision from fragmentation or short lived resonances ( , K*, ,...) and exhibit Coulomb interaction in the final state: Non-Coulomb pairs. They are produced in one proton nucleus collision. At least one pion originates from a long lived resonance. No Coulomb interaction in the final state Accidental pairs. They are produced in two independent proton nucleus collision. They do not exhibit Coulomb interaction in the final state

10. DIRAC Spectrometer Upstream detectors: MSGCs, SciFi, IH. Downstream detectors: DCs, VH, HH, C, PSh, Mu.

11. Tracking principles Precison time-of-flight to reduce accidental and proton background Strong e+e- rejection by Čerenkov counters Unambiguous transverse momentum Q T by upstream tracking (MSGC+SFD) Longitudinal momentum Q L measured by fast drift chambers and upstream tracks DIRAC Spectrometer High high irradiation

12. Downstream detectors: Drift chambers Cherenkov Time-of-Flight Upstream detectors: MSGC/GEM SFD Ionisation Hodoscope Pre-Shower and Muon Counters unseen Nucl. Inst. Meth. A515 (2003) 467. DIRAC Spectrometer

13. Trigger performance

14. Calibrations Time difference spectrum at VH with e + e - T1 trigger. Mass distribution of p  - pairs from  decay.   =0.39 MeV/c 2 Positive arm mass spectrum, obtained by TOF difference, under  - hypothesis in the negative arm.

15. Accidental pairs, different proton interactions in the target Coulomb pairs. From short lived sources. r < 3 fm, < R( A 2   Non Coulomb pairs. From long lived sources. r ~ 1000 fm.  +  - pairs Time-of-Flight spectrum

16. Analysis based on MC Atoms are generated in nS states using measured momentum distribution for short-lived sources. The atomic pairs are generated according to the evolution of the atom while propagating through the target Background processes: Coulomb pairs are generated according to A C (Q)Q 2 using measured momentum distribution for short-lived sources. Non-Coulomb pairs are generated according to Q 2 using measured momentum distribution for long-lived sources. Monte Carlo simulation is restricted to detector response only, without relying on specific asumptions from proton-nucleus collision models

17. 2D  2 FIT TO (Q T, Q L ) SPECTRUM  3 (accidentals fraction) measured from TOF  1 and  free parameters in 10 independent 600 MeV/c  +  - momentum bins Atom signal defined as difference between prompt data and Monte Carlo with  = 0

18. Pionium signal in Qt vs Ql

19. PIONIUM BREAK-UP SIGNAL IN  +  - SPECTRUM · cos 

20. Pionium signal in Q =√ Q L 2 + Q T 2

21. PIONIUM TRANSVERSE SIGNAL Q L > 2 MeV/c Q L < 2 MeV/c

22. Pionium Longitudinal Signal

23. DETERMINATION OF BREAK-UP PROBABILITY Different extrapolation domains: QM analytical factor: Coulomb-pair background N CC determined from fit  1 parameter: Acceptance factors  i determined by Monte Carlo simulation Standard choice is Q L C = 2 MeV/c and Q T C = 5 MeV/c

24. Break-up probability as function of pionium momentum

25. P Br as function of Q L upper cut

26. Fraction of Coulomb pairs as function of momentum

27. Long-lived pairs (  2 ) /(0.6 GeV/c) Spectrum consistent with  +  - bound state formation Softer spectrum ( , ..) expected from Monte Carlo Atom Pairs Coulomb Pairs x 1/40 Coulomb Pairs

28. Breakup probability Breakup probability Summary of systematic uncertainties: Source  Q L trigger acceptance MSGC+SFD background Double Ionization cut Double-track resolution Target Impurity KK contamination ± 0.004 ± 0.006 ± 0.003 Total ± 0.008 P Br =0.435±0.016 (stat) ± 0.008 (syst) = 0.435 ± 0.018

29. Results from DIRAC DIRAC collaboration has built up a double arm spectrometer which provides a pair relative momentum (Q) resolution of 0.4 MeV/c for Q<30MeV/c More than 6000 of  +  - pairs from pionium break-up were observed The analysis of Ni 2001 data provides a lifetime measurement which translates into an S-wave amplitude measurement at rest : (ChPT)