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The DIRAC Experiment Status DI meson R elativistic A tomic C omplex

The DIRAC Experiment Status DI meson R elativistic A tomic C omplex. L. Tauscher, for the DIRAC collaboration CERN, April 27 , 2004. 16 Institutes Spokesman: Leonid Nemenov, Dubna. Lifetime linked to s-wave p  p  scattering lengths (a 2 -a 0 )

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The DIRAC Experiment Status DI meson R elativistic A tomic C omplex

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  1. The DIRAC Experiment StatusDImeson Relativistic Atomic Complex L. Tauscher, for the DIRAC collaboration CERN, April 27 , 2004 16 Institutes Spokesman: Leonid Nemenov, Dubna L. Tauscher, Basel

  2. Lifetime linked to s-wave ppscattering lengths (a2-a0) Annihilation from higher l-states forebidden / strongly suppressed or, in practice, absent The pp scattering length DIRAC intends to measure lifetime of the pp atom (of order 10-15 s) Lifetime due to decay of ppatom: strong pp pp(99.6%) el. magn. pp (0.4%) Method is independent of QCD models and constraints The aim of DIRAC is to measure t with an accuracy of 0.3 fs Physics motivation: see L. Nemenov, next talk L. Tauscher, Basel

  3. Strong interaction  I = 0,2 • Annihilation, p+p-p0p0(a0-a2), t≈ 310-15 s The p+p- atom Produced in proton-nucleus collisions (Coulomb final state interaction) Atomic bound state (Eb≈ 2.9 KeV) Relativistic atom (g ≈17) migrates more than 10 mm in the target and encounters at least 50’000 atoms L. Tauscher, Basel

  4. Excitation and break-up In collisions atom becomes excited and/or breaks up Pions from break-up have very similar momenta and very small opening angle (small Q) At target exit this feature is smeared by multiple scattering, especially in QT L. Tauscher, Basel

  5. Break-up probability Pbr is linked to the lifetime (theory of relativistic atomic collisions) Principle of measuring the lifetime Excitation and break-up of produced atoms (NA) are competing with decay p+p-pairs from break-up provide measurable signal nA Pbr is linked to nA : Pbr = nA/NA The number of produced atoms NA is not directly measurable  to be obtained otherwise (normalization) L. Tauscher, Basel

  6. Background • Pairs of pions produced in high energy proton nucleus collisions are coherent, • if they originate directly from hadronisation or • involve short lived intermediate resonances. • They areincoherent, • if one of them originates from long lived intermediate resonances, e.g. h’s (non-correlated) • because they originate from different proton collisions (accidentals) L. Tauscher, Basel

  7. Coulomb enhancement function (Sommerfeld, Gamov, Sacharov, etc): Coulomb correlated (CC) background Coherentp+p-pairs undergo Coulomb final state interaction  enhancement of low-Q event rate with respect to non-correlated events. • The same mechanism leads also to the formation of atoms • number of produced atoms NA and the Coulomb correlated background (NC) are in a calculable and fixed relation (normalization): NA = 0.615 * NC (Q ≤ 2 MeV/c) L. Tauscher, Basel

  8. Pion pairs from atoms have very low Q C background is Coulomb enhanced at very low Q Multiple scattering in the target smears the signals and the cuts DIRAC uses the C background as normalization for produced atoms Signal and background summary Intrinsic difficulty: mastering of multiple scattering in MC measuring 2 tracks with opening angle of 0.3 mrad L. Tauscher, Basel

  9. DIRAC spectrometer L. Tauscher, Basel

  10. Track reconstruction • Track reconstruction starts from the downstream part of the spectrometer, iterative • Search for at least one track in either of the two arms (VH, HH, hits in DC’s) • Extrapolate track through magnet onto target (beam-target X-section)  0’th approximation for momentum p • Search for hits in upstream detectors in vicinity of 0’th track (window given by multiple scattering) • Launch Kalman filter tracking for selected hits • Verify consistency of chosen hit-track assignment (time, 2 etc.) • Repeat Kalman filter tracking for selected hits 1st approximation for momentum p • Require track extrapolation to be close to beam-target X-section • Repeat Kalman filter tracking if necessary (track-hit assignment) • Alternative to Kalman filtering straight line fit • Require or not common vertex L. Tauscher, Basel

  11. Timing L. Tauscher, Basel

  12. Monte-Carlo Generators:tailored to the experiment Accidental background according to Nacc/Q  Q2 with momentum distributions as measured with accidentals Incoherent background according to NnC/Q  Q2 with momentum distribution as measured for one pion and Fritjof momentum distribution for long-lived resonances for second pion C background according to NC/Q  fCC(Q)*Q2 with momentum distribution as measured but corrected for long-lived incoherent pion pairs (Fritjof) Atomic pairs according to dynamics of atom-target collisions and atom momentum distribution for C background Geant4:full spectrometer simulation Detector simulation:full simulation of response, read-out, digitalization and noise Trigger simulation:full simulation of trigger processors Reconstruction:as for real data L. Tauscher, Basel

  13. Data from Ni taken in 2001 Best coherent data taking with full trigger and set-up • Typical cuts: • QT < 4 MeV/c • QL < 22 MeV/c • No of reconstructed events in prompt window : 570‘000 • For analysis subtract the accidental background from the prompt by proper scaling L. Tauscher, Basel

  14. Experimental Qtot and Ql distributions (Ni2001) • Fit MonteCarlo C and nC background • outside the A2p signal region (Qtot > 4 MeV, Ql > 2 MeV) • simultaneously to Qtot and Ql L. Tauscher, Basel

  15. Residuals in Qtot and Ql • Comments: • Qtot and Ql provide same number of events  background consistent • Signal shapes well reproduced L. Tauscher, Basel

  16. nA(Qrec ≤ Qcut) _______________________________________________________________________ e0.615kNC (Qrec ≤ Qcut) PBr = Normalization PBr = nA / NA Number of detected atomic pairs with Qrec ≤ Qcut: nA(Qrec ≤ Qcut)/nA = e (from MonteCarlo) Number of atomic pairs: nA = nA(Qrec ≤ Qcut) /e C background with Qinit ≤ 2 MeV NC(Qinit ≤ 2 MeV )= kNC (Qrec ≤ Qcut), k from MCarlo Number of produced atoms NA = 0.615kNC (Qrec ≤ Qcut) L. Tauscher, Basel

  17. Result: nA = 6560 ± 295 (4.5%) NC = 374282 ± 3561 (1.0%) NC (Q<4 MeV/c) = 106114 e0.615k = 0.1383 PBr = 0.447 ± 0.023stat(5.1%) Break-up from Ni2001 • Strategy: • Use MC shapes for background from Monte Carlo • Use MC shapes for the atomic signal • Fit in Q and QL simultaneously • Require that background composition in Q and QL are the same L. Tauscher, Basel

  18. PBr = 0.447 ± 0.023stat • + 0.42stat t = 2.85 [fs] - 0.33stat Lifetime L. Tauscher, Basel

  19. Systematics due to line shape PBr should be independent of the cut in Q/QL • Line shape alone is not sufficient • Integrating over the full signal leads to identical PBr L. Tauscher, Basel

  20. Systematics due to multiple scattering Multiple scattering measured to 5% in momentum range of DIRAC • Mult. scatt. alone not sufficient • Integrating over the full signal leads to identical PBr L. Tauscher, Basel

  21. Lifetime with systematics • + 0.48 t = 2.85 [fs] - 0.41 L. Tauscher, Basel

  22. Dual target technique Two targets: standard single layer Ni-target multi-layer Ni-target with the same thickness as the standard target, but segmented into 13 equally thick layers at distances of 1.0 mm. • This target has the same properties as the single layer one in terms of: • production of secondary particles by the beam, • correlated and uncorrelated backgrounds (Nback), • produced atoms (NA), • integral multiple scattering, • Measuring conditions But: break up Pbrm is smaller than Pbrs because of enhanced annihilation. L. Tauscher, Basel

  23. Signal shape: Background shape: Normalization-free determination oft t from r (independent of normalization) L. Tauscher, Basel

  24. Preliminary results of single/multilayer 2002 Ns - Nm = (Pbrs-Pbrm)NA = 825 ± 140 events signal shape well reproduced L. Tauscher, Basel

  25. Preliminary results of single/multilayer 2002 r = 1.86 ± 0.20stat Background shape from MonteCarlo is consistent also at low Q L. Tauscher, Basel

  26. Lifetime single/multilayer 2002 (preliminary) Result:  = 2.5 + 1.1- 0.9 [fs] L. Tauscher, Basel

  27. outlook Full statistic probably sufficient to reach the goal of 10% accuracy L. Tauscher, Basel

  28. Conclusion • DIRAC is a very difficult experiment • The apparatus worked fine • Systematic errors have been studied • Multiple scattering • Shape uncertainties • Many others • Systematic errors are smaller than the statistical errors • Full statistics sufficient to reach 10% accuracy • More analysis and systematics studies needed (2 more years) Request: 2 weeks of test with the DIRAC setup L. Tauscher, Basel

  29. Micro Drift Chamber High rate High double track resolution Low material budget Fast Stand-alone tracking 18 layers in special housing Overpressure 2 atm 26 ns max drift time L. Tauscher, Basel

  30. Micro Drift Chamber • Sucessfully operated in 2003 (3 weeks) • Rate >1.5x1011 pps • Break-down of 4 layers in second week • Remaining 16 layers ok Read-out and cabling improved Sensitivity 5 times better Lower HV possible 2 weeks of test in 2004 L. Tauscher, Basel

  31. Place all scattering objects between drift chamber 3 and 4 Instrumental resulution Measured scattering angles Multiple scattering measurement L. Tauscher, Basel

  32. Multiple scattering result • Conclusions: • Central part of mult. Scattering well reproduced by GEANT4 • More analysis work needed L. Tauscher, Basel

  33. Experimental difficulties Low atomic formation rate (10-9) High instantaneous particle flux (10MHz) High physics background High uncorrelated background Solution High resolution double arm spectrometer Precision tracking Particle identification Time resolution • Multilevel trigger: • First stage: 50 ns • Second stage: 200 ns (digital NN) • Third stage: >1.5 ms L. Tauscher, Basel

  34. Incoherent pairs follow the phase space: Non - correlated (NC) background Incoherent pairs from long-lived intermediate resonances have slightly different single particle momentum distributions than coherent pairs (Fritjof) Accidental pairs from two different interactions have similar single particle momentum distributions as coherent pairs L. Tauscher, Basel

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