HadAtom03 13-17 October 2003, Trento, Italy

1. Observation of  atoms at DIRAC and its lifetime estimation. HadAtom03 13-17 October 2003, Trento, Italy Valery Brekhovskikh on behalf of DIRAC collaboration Experimental Physics Department, Institute for High Energy Physics (Protvino)

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. 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. Experimental Status Experimental data on pp scattering lengths can be obtained via the following indirect processes: L. Rosselet et al., Phys. Rev. D 15 (1977) 574 M.Nagels et al., Nucl.Phys. B 147 (1979) 189 Combined analysis of Ke4, Roy equation and peripheral reaction data New measurement at BNL (E865) S.Pislak et al., Phys.Rev. D 67 (2003) 072004 C.D. Froggatt, J.L. Petersen, Nucl. Phys. B 129 (1977) 89 … M. Kermani et al., Phys. Rev. C 58 (1998) 3431

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, give a precise relation between G2p and the pp scattering lengths: (Deser et al) (Gall et al., Jalloli et al, Ivanov et al) a0 and a2 are the strong pp S-wave scattering lengths for isospin I=0 and I=2.

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): • 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). • 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. • 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´ being the 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: angel 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. Calibration These results show that our set-up fulfils the needs in time and momentum resolution. Positive arm mass spectrum, obtained by time difference at VHs, under p- hypothesis in the negative arm. Time difference spectrum at VH with e+e- T1 trigger. Mass distribution of pp- pairs from L decay. sL=0.43 MeV/c2 <0.49 MeV/c2 (Hartouni et al.).

11. 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

12. Single and multilayer measurements

13. Atoms pairs Ni 2001 data

14. Atomic pairs

15. Atomic pairs (F) Ni 2000 Ni 2001 Ni 2002 (20) Ni 2003 (M) Ni 2002 (S) Ni 2003 (S)

16. Atomic pairs (QL) Ni 2000 Ni 2001 Ni 2002 (20) Ni 2002 (S) Ni 2003 (S) Ni 2003 (M)

17. Atoms pairs (noUP) Ni 2000 Ni 2001 Ti 2001 Ni 2000 noUp Ti 2001 noUP Ni 2001 noUP

18. Atoms pairs (S-M) In the end of 2002 run there was introduced a segmented Ni target consisting of 12 planes with 1 mm gap between each. The combined thickness of all planes is approximately equal to that of the single layer 98 micron Ni target. Single and multilayer target event distributions are identical in all respects but one: the multilayer target yields a lower number of dissociated pairs due to the annihilations in the interlayer gaps. One can at once obtain the signal from the difference between the single and multilayer distributions

19. Atomic pairs (Table)

20. Q and F Q<2MeV/c Q<1MeV/c F<1 F<2.3

21. 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 at the end 2002 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 26%. Analysis of data collected in 2000 and 2003 together with the systematic error reduction allows us to improve accuracy up to the level of 14%.