Experimental Physics Department, University of Basel, Switzerland Measuring the pion-pion scattering length from atoms (pionium) with the DIRAC spectrometer at CERN Measuring the pion-pion scattering length from (  +  - ) atoms (pionium) with the DIRAC spectrometer at CERN Christian Schütz on behalf of DIRAC collaboration Schleching February 2004

83 Physicists from 19 Institutes 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. Lifetime Measurement of  +  - atoms to test low energy QCD predictions. DImeson Relativistic Atomic ComplexesDIRAC

Scattering length and the pionium lifetime How do we determine the pionium lifetime with DIRAC Results Systematic error sources Roadmap

DIRAC intends to determine the difference of the s-wave  scattering length |a 2 -a 0 | by measuring the lifetime of the pionium atom (  +  - atom). Scattering length and the pionium lifetime Lifetime due to decay of     atom: strong          (99.6%) el. magn.       (0.4%) Isospin conservation forces the lifetime to be linked to |a 2 -a 0 | Annihilation from higher l-states is strongly suppressed or, in practice, absent Method is independent of QCD models and constraints Goal of DIRAC: Measure  with accuracy of 0.3 fs

The     atom Atom migrates more than 10  m in the target and encounters at least 50’000 target atoms Produced in proton-nucleus collisions Atomic bound state (E b ≈ 1.86 keV) Strong interaction  I = 0,2 Annihilation (         ),      s (Colangelo et al.)

Excitation and break-up In collisions atom becomes excited Pions from break-up have very similar momenta and very small opening angle (small Q) and/or breaks up At the target exit this feature is smeared by multiple scattering

A 2π production and related processes The pionium atoms are produced by the Coulomb interaction of a pion pair in proton-target collisions In addition Coulomb pairs are created in the proton target interaction. The atoms production is proportional to the Coulomb pair production (N A =K  N CC ) Some atoms ( N A ) break up into atomic pairs (n A ) Breakup probability P br =n A / N A Coulomb pairs DIRAC measures  +  - pairs !

The break-up probability (P br ) has a one-to-one dependence on lifetime. Breakup Probability The errors do not scale proportionally There is an optimal target material for a given lifetime

Background Coulomb pairs. They are produced in one proton nucleus collision from fragmentation or short lived resonances and exhibit Coulomb interaction in the final state Non-Coulomb pairs. They are produced in one proton nucleus collision. One pion originates from a long lived resonance. They do not exhibit Coulomb interaction in the final state Accidental pairs. They are produced in two independent proton nucleus collision. They do not exhibit Coulomb interaction in the final state

Signal and background summary Pion pairs from atoms have very low Q CC background is Coulomb enhanced at very low Q Multiple scattering in the target smears the signals… …as well as the normalization (K) Intrinsic difficulty: mastering of multiple scattering in MC measuring 2 tracks with opening angle of 0.3 mrad

DIRAC Spectrometer 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 =0.5 MeV/c  QL =0.5 MeV/c Upstream detectors: MSGCs, SciFi, IH. Downstream detectors: DCs, VH, HH, C, PSh, Mu. Proton beam Angle to proton beam 5.7° Bending magnet

Trigger performance

Calibration Time difference spectrum at VH with e + e - T1 trigger. Mass distribution of p  - pairs from  decay.   =0.54 MeV/c 2

Atomic pair detection The time spectrum at VH provides us the criterion to select prompt (time correlated) and accidental (non correlated) pairs.

Fit Monte Carlo CC and NC background above the signal in Q/Ql Background distributions in Ql are very different. Not so in Q. Q is much more affected by multiple scattering than Ql. You expect different fit results in Q / Ql

Signal Ni 2001

Normalization and break-up probability Number of detected atomic pairs n A from measurement number of detected pairs n A _______________________________________________________________________ number of produced atoms N A Break-up probability P Br = Number of produced atoms N A from Coulomb pairs measurement using normalization N A = K N CC and… …taking into account efficiencies (using Monte Carlo)

Preliminary Lifetime Ni 2001

Breakup probability dependence At Q=2 MeV/c we are sensible ! At Q=4 MeV/c we are not sensible to the atomic pair shape Slight mismatch between residual and MC shape caused by…

Systematics Multiple scattering –Affect background, signal shape as well as the normalization Atomic excitations –Signal shape and normalization

Multiple Scattering Theoretical multiple scattering description is known with a precision of 5% for our conditions. DIRAC performed a dedicated multiple scattering measurement. Results are pending. In the mean time… ….we can study the effect of the MS using Monte Carlo Increase/decrease the multiple scattering angle in MC by 5%

The influence of the multiple scattering is only relevant when cutting into the signal ! Multiple scattering dP br = 6*10 -4 per 1% MS

Atomic pair shape at breakup The breakup of the atoms (E B =-1.86 keV) is modeled by computing the atomic transitions until the atom annihilates or breaks up. The transition cross sections are calculated accounting for multi-photon, magnetic, relativistic and other higher order effects. If the atom breaks up, the corresponding spectrum (  MeV/c) is considered to generate the relative momentum of the atomic pairs. The full simulation considers all transitions and spectra up to principal quantum number 7. Some approximations for higher states are also included. How much is the uncertainty introduced by the atomic pair shape ? 1s state has widest Q spectra Q=0 MeV/c is narrowest spectra

Atomic pair shape The influence of the atomic pair shape is only relevant when cutting into the signal ! dP br (extreme) = 0.008

Conclusion DIRAC is a very difficult experiment The 2001 results are consistent with a statistical accuracy on the lifetime of 0.35 fs Systematics are currently studied and dedicated measurements are evaluated. Expect Systematics not to exceed statistical accuracy Statistics including 2002/2003 will allow for a 0.25 fs statistical accuracy of t