Results from the DIRAC Experiment at CERN DImeson Relativistic Atomic Complex

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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
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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)
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The pp- 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 100000
atoms
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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
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Principle of measuring the lifetime
Excitation and break-up of produced atoms (NA)
are competing with decay
pp- 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)
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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. hs
  (non-correlated)
• because they originate from different proton
  collisions (accidentals)

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Coulomb correlated (C) background
Coherent pp- 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)

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Signal and background summary
Intrinsic difficulty multiple scattering in
MC measuring 2 tracks with opening angle of
0.3 mrad
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DIRAC spectrometer
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Timing
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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
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Data from Ni taken in 2001
Best coherent data taking with full trigger and
set-up

• Typical cuts
• QT lt 4 MeV/c
• QL lt 22 MeV/c
• No of reconstructed events in prompt window
  570000
• For analysis the accidental background in the
  prompt window is obtained by proper scaling and
  kept fixed.

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Experimental Q and Ql distributions (Ni2001)

• Fit MonteCarlo C and nC background
• outside the A2p signal region (Q gt 4 MeV, QL gt 2
  MeV)
• simultaneously to Q and QL

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Residuals in Q and Ql

• Comments
• Q and QL provide same number of events ?
  background consistent
• Signal shapes well reproduced

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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)
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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

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Lifetime
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Systematics

• Systematics from
• Normalization (C vs. nC determination)
• Cut (Qcut) for PBr determination
• Multiple scattering simulation
• Signal shape simulation
• Many others

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Lifetime with systematics

• 0.48
• t 2.85 fs
• - 0.41

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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.

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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)
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Rate combination for signal (2002) (preliminary)
Ns(Q) - Nm(Q) (Pbrs-Pbrm)NA 825 140
events signal shape well reproduced
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Rate combination for background (2002)
(preliminary)
r 1.86 0.20stat Background shape from
MonteCarlo is consistent also at low Q
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Lifetime single/multilayer (2002) (preliminary)
Result ? 2.5 1.1- 0.9 fs
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outlook
Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.) Number of Atomic pairs (approx.)
Pt1999 24 GeV Ni2000 24 GeV Ti2000 24 GeV Ti2001 24 GeV Ni2001 24 GeV Ni2002 20 GeV Ni2002 24 GeV Ni2003 20 GeV Sum
Sharp selection 280 1300 900 1500 6500 2000 2600 1500 16600
Loose selection (high background) 27000
Full statistic probably sufficient to reach the
goal of 10 accuracy
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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 lt statistical errors
• Full statistics sufficient to reach 10 accuracy
  (st 0.3 fs)