Results from the DIRAC Experiment at CERN DImeson Relativistic Atomic Complex L. Tauscher, for the DIRAC collaboration Frascati, June 10 , 2004 16 Institutes Spokesman: Leonid Nemenov, Dubna 10.6.2004 L. Tauscher, Basel

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%) Lifetime linked to s-wave pp scattering lengths (a2-a0) Annihilation from higher l-states forbidden / strongly suppressed, in practice, absent 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) 10.6.2004 L. Tauscher, Basel

The p+p- atom Produced in proton-nucleus collisions (Coulomb final state interaction) Atomic bound state (Eb ≈ 2.9 KeV) Strong interaction  I = 0,2 Annihilation, p+p-p0p0 (a0-a2), t≈ 310-15 s Relativistic atom (g ≈17) migrates more than 10 mm in the target and encounters around 100’000 atoms 10.6.2004 L. Tauscher, Basel

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 10.6.2004 L. Tauscher, Basel

Principle of measuring the lifetime Excitation and break-up of produced atoms (NA) are competing with decay Break-up probability Pbr is linked to the lifetime t (theory of relativistic atomic collisions) 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) 10.6.2004 L. Tauscher, Basel

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 are incoherent, if one of them originates from long lived intermediate resonances, e.g. h’s (non-correlated) because they originate from different proton collisions (accidentals) 10.6.2004 L. Tauscher, Basel

Coulomb correlated (C) background Coherent p+p- pairs undergo Coulomb final state interaction  enhancement of low-Q event rate with respect to non-correlated events. Coulomb enhancement function (Sommerfeld, Gamov, Sacharov, etc): 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) 10.6.2004 L. Tauscher, Basel

Signal and background summary 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 Intrinsic difficulty: multiple scattering in MC measuring 2 tracks with opening angle of 0.3 mrad 10.6.2004 L. Tauscher, Basel

DIRAC spectrometer 10.6.2004 L. Tauscher, Basel

Timing 10.6.2004 L. Tauscher, Basel

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 10.6.2004 L. Tauscher, Basel

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. 10.6.2004 L. Tauscher, Basel

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 10.6.2004 L. Tauscher, Basel

Residuals in Q and Ql Comments: Q and QL provide same number of events  background consistent Signal shapes well reproduced 10.6.2004 L. Tauscher, Basel

Normalization e0.615kNC (Qrec ≤ Qcut) 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) nA(Qrec ≤ Qcut) _______________________________________________________________________ e0.615kNC (Qrec ≤ Qcut) PBr = 10.6.2004 L. Tauscher, Basel

Break-up from Ni2001 e0.615k = 0.1383 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 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%) 10.6.2004 L. Tauscher, Basel

Lifetime t = 2.85 [fs] PBr = 0.447 ± 0.023stat + 0.44stat - 0.38stat 10.6.2004 L. Tauscher, Basel

Systematics Systematics from: Normalization (C vs. nC determination) Cut (Qcut) for PBr determination Multiple scattering simulation Signal shape simulation Many others 10.6.2004 L. Tauscher, Basel

Lifetime with systematics + 0.48 t = 2.85 [fs] - 0.41 10.6.2004 L. Tauscher, Basel

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. 10.6.2004 L. Tauscher, Basel

Normalization-free determination of t Nm(Q) = Pbrm NA SA(Q) + Nback(Q) Ns(Q) = Pbrs NA SA(Q) + Nback(Q) SA(Q) : normalized Monte Carlo shape function for the atomic break up signal Signal shape: Ns(Q) - Nm(Q) = (Pbrs - Pbrm)NA SA(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) 10.6.2004 L. Tauscher, Basel

Rate combination for signal (2002) (preliminary) Ns(Q) - Nm(Q) = (Pbrs-Pbrm)NA = 825 ± 140 events signal shape well reproduced 10.6.2004 L. Tauscher, Basel

Rate combination for background (2002) (preliminary) r = 1.86 ± 0.20stat Background shape from MonteCarlo is consistent also at low Q 10.6.2004 L. Tauscher, Basel

Lifetime single/multilayer (2002) (preliminary) Result:  = 2.5 + 1.1- 0.9 [fs] 10.6.2004 L. Tauscher, Basel

Number of Atomic pairs (approx.) outlook Number of Atomic pairs (approx.) Pt1999 24 GeV Ni2000 Ti2000 Ti2001 Ni2001 Ni2002 20 GeV Ni2003 Sum Sharp selection 280 1300 900 1500 6500 2000 2600 16600 Loose selection (high background) 27000 Full statistic probably sufficient to reach the goal of 10% accuracy 10.6.2004 L. Tauscher, Basel

DIRAC has achieved a lifetime measurement with 16% accuracy 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) 10.6.2004 L. Tauscher, Basel