Results from the DIRAC Experiment at CERN DI meson R elativistic A tomic C omplex
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Transcript of Results from the DIRAC Experiment at CERN DI meson R elativistic A tomic C omplex
10.6.2004 L. Tauscher, Basel 1
Results from the DIRAC Experiment at CERN
DImeson Relativistic Atomic Complex
L. Tauscher, for the DIRAC collaborationFrascati, June 10 , 2004
16 InstitutesSpokesman: Leonid Nemenov, Dubna
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The scattering length
DIRAC measures the lifetime of the atom (of order 10-15 s)
Lifetime due to decay of atom: strong (99.6%)el. magn. (0.4%)
Method is independent of QCD models and constraints
The aim of DIRAC is to measure with an accuracy of 0.3 fsPhysics motivation: elementary quantity in soft QCD (similar to m)
Lifetime linked to s-wave scattering lengths (a2-a0)
Annihilation from higher l-states forbidden / strongly suppressed, in practice, absent
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The atom
Relativistic atom (≈17) migrates more than 10 m in the target and encounters around 100’000 atoms
Produced in proton-nucleus collisions (Coulomb final state interaction)
Atomic bound state (Eb ≈ 2.9 KeV)
Strong interaction I = 0,2
Annihilation, +(a0-a2),
≈ s
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Excitation and break-upIn collisions atom becomes excited
Pions from break-up have very similar momenta and very small opening angle (small Q)
and/or breaks up
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 decayBreak-up probability Pbr is linked to the lifetime (theory of relativistic atomic collisions)
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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BackgroundPairs of pions produced in high energy proton nucleus collisions are coherent,
• if they originate directly from hadronisation or• involve short lived intermediate resonances.
They are incoherent, • if one of them originates from long lived intermediate resonances, e.g. ’s (non-correlated)• because they originate from different proton collisions (accidentals)
Coherent pairs undergo Coulomb final state interaction and become “Coulomb-correlated”
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Coulomb correlated (C) backgroundCoherentpairs 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 atomsnumber 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)
Coulomb enhancement function (Sommerfeld, Gamov, Sacharov, etc):
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Signal and background summaryPion pairs from atoms have very low QC background is Coulomb enhanced at very low QMultiple scattering in the target smears the signals and the cutsDIRAC uses the C background as normalization for produced atoms
Intrinsic difficulty: multiple scattering in MCmeasuring 2 tracks with opening angle of 0.3 mrad
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DIRAC spectrometer
target vacuum vacuum
magnet
T2
negative
absorber
1 m
positive
T1
magnet: 1.65 T, 2.2 Tm
target vacuum
MSGC
vacuum
magnet
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
magnet: 1.65 T, 2.2 Tm
target vacuum
SFDMSGC
vacuum
magnet
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
magnet: 1.65 T, 2.2 Tm
target vacuum
SFDMSGC IH
vacuum
magnet
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
IH: scintillation dE/dx counters
magnet: 1.65 T, 2.2 Tm
target vacuum
SFDMSGC IH
vacuum
magnet
DC
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
IH: scintillation dE/dx counters
magnet: 1.65 T, 2.2 Tm
DC: high resolution drift chambers
target vacuum
SFDMSGC IH
vacuum
magnet
DC
HHVH
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
IH: scintillation dE/dx counters
magnet: 1.65 T, 2.2 Tm
DC: high resolution drift chambers
VH, HH: scintillation hodoscopes
target vacuum
SFDMSGC IH
vacuum
magnet
DC
HHVH
CT2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
IH: scintillation dE/dx counters
magnet: 1.65 T, 2.2 Tm
DC: high resolution drift chambers
VH, HH: scintillation hodoscopes
C: Cherenkov counters
target vacuum
SFDMSGC IH
vacuum
magnet
DC
HHVH
CPSh
Mu
T2
negative
absorber
1 m
positive
T1
MSGC: multi strip gas chambers
SFD: scintillation fiber detector
IH: scintillation dE/dx counters
magnet: 1.65 T, 2.2 Tm
DC: high resolution drift chambers
VH, HH: scintillation hodoscopes
C: Cherenkov counters
PSh: preshower detector
Mu: muon counters
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Timing
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Monte-CarloGenerators: tailored to the experiment1. Accidental background according to Nacc/Q Q2 with momentum distributions as
measured with accidentals2. 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
3. C background according to NC/Q fCC(Q)*Q2 with momentum distribution as measured but corrected for long-lived incoherent pion pairs (Fritjof)
4. 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 < 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.
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Experimental Q and Ql distributions (Ni2001)
Fit MonteCarlo C and nC background• outside the A signal region (Q > 4 MeV, QL > 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:
= nA(Qrec ≤ Qcut)/nA (from MonteCarlo)
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 atomsNA = 0.615kNC (Qrec ≤ Qcut)
nA(Qrec ≤ Qcut)_______________________________________________________________________
0.615kNC (Qrec ≤ Qcut)
PBr =
Number of atomic pairs:
nA = nA(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
Result:nA = 6560 ± 295 (4.5%)NC = 374282 ± 3561 (1.0%)NC (Q<4 MeV/c) = 106114
0.615k = 0.1383PBr = 0.447 ± 0.023stat (5.1%)
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Lifetime
PBr = 0.447 ± 0.023stat
stat
= 2.85 [fs] - stat
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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
= 2.85 [fs]
-
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Dual target technique
Both targets have the same properties in terms of:1. production of secondary particles by the beam,2. correlated and uncorrelated backgrounds (Nback),3. produced atoms (NA),4. integral multiple scattering,5. Measuring conditions
But: break up Pbrm is smaller than Pbr
s because of enhanced annihilation.
Two targets: 1. standard single layer Ni-target 2. 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.
Method to get rid of uncertainties fromnormalization, multiple scattering and shape
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Normalization-free determination of
from (independent of normalization)
Signal shape: Ns(Q) - Nm(Q) = (Pbrs - Pbr
m)NA SA(Q) Background shape: Ns(Q) - Nm(Q) = (1-) Nback(Q) = (1-) B(Q)
= Pbrs / Pbr
m B(Q) = BC(Q) + (1-) BnC(Q)
B: Monte Carlo shape function for the background, normalized to the no. of background-events
Nm(Q) = Pbrm NA SA(Q) + Nback(Q) Ns(Q) = Pbr
s NA SA(Q) + Nback(Q)SA(Q) : normalized Monte Carlo shape function for the atomic break up signal
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Rate combination for signal (2002) (preliminary)
Ns(Q) - Nm(Q) = (Pbrs-Pbr
m)NA = 825 ± 140 eventssignal shape well reproduced
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Rate combination for background (2002) (preliminary)
= 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.)Pt1999
24 GeV
Ni2000
24 GeV
Ti2000
24 GeV
Ti2001
24 GeV
Ni2001
24 GeV
Ni2002
20 GeV
Ni2002
24 GeV
Ni2003
20 GeVSum
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
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 (= 0.3 fs)
Using a subset of data
DIRAC has achieved a lifetime measurement with 16% accuracy