1 / 26

Results from the DIRAC Experiment at CERN DI meson R elativistic A tomic C omplex

Results from the DIRAC Experiment at CERN DI meson R elativistic A tomic C omplex. L. Tauscher, for the DIRAC collaboration Frascati, June 10 , 2004. 16 Institutes Spokesman: Leonid Nemenov, Dubna. Lifetime linked to s-wave p  p  scattering lengths (a 2 -a 0 )

kevin-roberts
Download Presentation

Results from the DIRAC Experiment at CERN DI meson R elativistic A tomic C omplex

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Results from the DIRAC Experiment at CERNDImeson Relativistic Atomic Complex L. Tauscher, for the DIRAC collaboration Frascati, June 10 , 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 forbidden / strongly suppressed, in practice, absent The pp scattering length DIRAC measures the 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: elementary quantity in soft QCD (similar to mp) 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 around 100’000 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 t (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 using background) 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. • Coherent pairs undergo Coulomb final state interaction and become “Coulomb-correlated” • 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 (C) 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: multiple scattering in MC measuring 2 tracks with opening angle of 0.3 mrad L. Tauscher, Basel

  9. DIRAC spectrometer L. Tauscher, Basel

  10. Timing L. Tauscher, Basel

  11. Monte-Carlo Generators:tailored to the experiment Accidental background according to Nacc/Q  Q2 with momentum distributions as measured with accidentals Non-correlated 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

  12. 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 the accidental background in the prompt window is obtained by proper scaling and kept fixed. L. Tauscher, Basel

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

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

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

  16. 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

  17. PBr = 0.447 ± 0.023stat • + 0.44stat t = 2.85 [fs] - 0.38stat Lifetime L. Tauscher, Basel

  18. Systematics • Systematics from: • Normalization (C vs. nC determination) • Cut (Qcut) for PBr determination • Multiple scattering simulation • Signal shape simulation • Many others L. Tauscher, Basel

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

  20. Dual target technique Method to get rid of uncertainties from normalization, multiple scattering and shape • Two targets: • standard single layer Ni-target • multi-layer Ni-target with the same thickness as the standard target, but • segmented into 12 equally thick layers at distances of 1.0 mm. • Both targets have the same properties 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

  21. Normalization-free determination oft Nm(Q) = Pbrm NASA(Q) + Nback(Q)Ns(Q) = Pbrs NASA(Q) + Nback(Q) SA(Q) : normalized Monte Carlo shape function for the atomic break up signal Signal shape: Ns(Q) - Nm(Q) = (Pbrs - Pbrm)NASA(Q) Background shape: Ns(Q) - r Nm(Q) = (1-r) Nback(Q) = (1-r) B(Q) r = Pbrs / Pbrm B(Q) = w BC(Q) + (1-w) BnC(Q) B: Monte Carlo shape function for the background, normalized to the no. of background-events t from r (independent of normalization) L. Tauscher, Basel

  22. Rate combination for signal (2002) (preliminary) Ns(Q) - Nm(Q) = (Pbrs-Pbrm)NA = 825 ± 140 events signal shape well reproduced L. Tauscher, Basel

  23. Rate combination for background (2002) (preliminary) r = 1.86 ± 0.20stat Background shape from MonteCarlo is consistent also at low Q L. Tauscher, Basel

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

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

  26. Conclusion Using a subset of data DIRAC has achieved a lifetime measurement with 16% accuracy • Systematic errors have been studied • Normalization consistency in Q, QL • Cut uncertainties • Multiple scattering • Shape uncertainties • Many others • Systematic errors < statistical errors • Full statistics sufficient to reach 10% accuracy (st = 0.3 fs) L. Tauscher, Basel

More Related