Lifetime measurement of atoms consisting of     mesons

1. Lifetime measurement of atoms consisting of  mesons International Workshop onQuantum Chromodynamics       Conversano (Bari, Italy)  June 14-18  2003 Valeri Iazkov (SINP Moscow) on behalf of DIRAC collaboration

2. DIRAC DImeson Relativistic Atomic Complexes Lifetime Measurement of p+p- atoms to test low energy QCD predictions. 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, Waseda Univ. 83 Physicists from 19 Institutes

3. The Goal The goal of the DIRAC Experiment is to measure the p+ p- atom lifetime of order 3·10-15s with 10% precision. This measurement will provide in a model independent way the difference between Swave  scattering lengths |a2a0 | with 5 % precision. Low energy QCD - chiral perturbation theory - predicts nowadays scattering lengths with very high accuracy ~ 2 % . Therefore, such a measurement will be a sensitive check of the understanding of chiral symmetry breaking of QCD by giving an indication about the value of the quark condensate, an order parameter of QCD.

4. - = ( 2 ) L a a 0 . 20 0 2 - = ± ( 4 ) L a a 0 . 25 0 . 01 0 2 ( 6 ) L GChPT - = ± < ( 6 ) L a a 0 . 258 ( 3 %) 0 2 - = ± ( 6 ) L a a 0 . 265 0 . 004 ( 1 . 5 0 2 Theoretical Status In ChPT the effective Lagrangian which describes the pp interaction is an expansion in (even) terms: 1966Weinberg (tree): 1984Gasser-Leutwyler (1-loop): 1995Knecht et al. (2-loop): 1996Bijnens et al. (2-loop): 2001Colangelo et al. (& Roy): %) And the theoretical results for the scattering lengths up to 2-loops are:

5. = ± a 0 . 26 0 . 05 0 = ± ± a 0 . 216 0 . 013 0 . 004 ( syst) 0 ± 0 . 005 (theor) • G. Colangelo, J. Gasser, H. Leutwyler Phys.Rev.Lett. • 86 (2001) 5008-5010; ChPT analysis of BNL E865 data = ± a 0 . 221 0 . 026 ( 95 % CL ) 0 - = ± 1 0 . 26 0 . 05 a m C.D. Froggatt, J.L. Petersen, Nucl. Phys. B 129 (1977) 89 p 0 = ± ± M. Kermani et al., Phys. Rev. C 58 (1998) 3431 a 0 . 204 0 . 014 0 . 008 ( syst ) 0 Experimental data on pp scattering lengths can be obtained via the following indirect processes: Experimental Status = ± L. Rosselet et al., Phys. Rev. D 15 (1977) 574 a 0 . 28 0 . 05 0 M.Nagels et al., Nucl.Phys. B147 (1979) 189 Combined analysis of Ke4, Roy equation and peripheral reaction data BNL E865: S.Pislak et al., Phys.Rev.Lett. 87 (2001) 221801 a0-a2 from ChPT have been used S. Descotes et al. Eur.Phys.J. C24 (2002) 469-483; ChPT independent analysis of BNL E865 data …

6. Theoretical Motivation p+p atoms (A2p) in the ground state decay by strong interaction mainly into p0p0. Chiral Perturbation Theory (ChPT), which describes the strong interaction at low energies provides, at NLO in isospin symmetry breaking, a precise relation between G2p and the pp scattering lengths: J.Uretsky, J.Palfrey; S.Bilenky et al.) (Gall et al.; Gashi et al.; Jalloli et al; Sazdjian et al.;Ivanov et al) a0 and a2 are the strong pp S-wave scattering lengths for isospin I=0 and I=2. Gall et al.

7. A2π production Wave function at origin (accounts for Coulomb interaction). • The pionic atoms are produced by the Coulomb interaction of a pion pair in proton-target collisions (Nemenov): • The pionic atoms evolve in the target and some of them (nA) are broken. The broken atomic pairs are emitted with small C.M.S. relative momentum (Q < 3 MeV/c) and opening angle <0.3 mrad. • Also free Coulomb pairs are created in the proton-target collisions. • The atom production is proportional to the low relative momentum Coulomb pairs production (NA=KNC). • DIRAC aims to detect and identify Coulomb and atomic pairs samples to calculate the break-up probabilityPbr=nA/NA. Lorentz Center of Mass to Laboratory factor. Pion pair production from short lived sources.

8. Lifetime and breakup probability The Pbr value depends on the lifetime value, t. To obtain the precise Pbr(t) curve a large differential equation system must be solved: where s is the position in the target, pnlm is the population of a definite hydrogen-like state of pionium. The anlmn´l´m´ coefficients are given by: , if nlmn´l´m´, snlmn´l´m´beingthe electro-magnetic pionium-target atom cross section, N0 the Avogadro Number, r the material density and A its atomic weight. The detailed knowledge of the cross sections (Afanasyev&Tarasov; Trautmann et al) (Born and Glauber approach) together with the accurate solution of the differential equation system permits us to know the curves within 1%. dt=10% dPbr =4%

9. DIRAC Spectrometer Downstream detectors: DCs, VH, HH, C, PSh, Mu. Upstream detectors: MSGCs, SciFi, IH. Setup features: angle to proton beam =5.7 channel aperture =1.2·10–3 sr magnet 2.3 T·m momentum range 1.2p7 GeV/c resolution on relative momentum QX= QY=0.4 MeV/c  QL=0.6 MeV/c

10. Atoms detection The time spectrum at VH provides us the criterion to select real (time correlated) and accidental (non correlated) pairs. Coulomb pairs: Accidentals: Non-Coulomb pairs: N and f are obtained from a fit to the pion pairs Q spectrum in the range without atomic pairs Q > 3 MeV/c

11. Atoms detection Ni 2001 data |QL|<22 MeV/c |QX|<4 MeV/c |QY|<4 MeV/c

12. Atomic Pairs Pt 1999 Ni 2000 Ni 2002 Ti 2000+2001 Ni 2001

13. Atomic pairs

14. Conclusions and results • DIRAC collaboration has built up the double arm spectrometer which achieves 1 MeV/c resolution at low relative momentum (Q<30MeV/c) of particle pairs and has successfully demonstrated its capability to detect atoms after 2 years of running time. • In order to decrease systematic errors, the dedicated measurements with a multi-layer nickel target and measurements of multiple scattering all detectors and setup elements are performing in the end 2002 run and during present run of 2003. • Preliminary results have been achieved by analyzing data collected in 2001. The statistical accuracy in the lifetime determination reaches 25% and the systematic one is 30%. Analysis of data collected in 2000 and 2002 years together with the systematic error reduction allows us to improve accuracy up to the level of 14%.