The DIRAC Experiment Status DI meson R elativistic A tomic C omplex
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Transcript of The DIRAC Experiment Status DI meson R elativistic A tomic C omplex
27.4.2004 L. Tauscher, Basel 1
The DIRAC Experiment Status
DImeson Relativistic Atomic Complex
L. Tauscher, for the DIRAC collaborationCERN, April 27 , 2004
16 InstitutesSpokesman: Leonid Nemenov, Dubna
27.4.2004 L. Tauscher, Basel 2
The scattering length
DIRAC intends to measure 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: see L. Nemenov, next talk
Lifetime linked to s-wave scattering lengths (a2-a0)
Annihilation from higher l-states forebidden / strongly suppressed or, in practice, absent
27.4.2004 L. Tauscher, Basel 3
The atom
Relativistic atom (≈17) migrates more than 10 m in the target and encounters at least 50’000 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
27.4.2004 L. Tauscher, Basel 4
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
27.4.2004 L. Tauscher, Basel 5
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)
27.4.2004 L. Tauscher, Basel 6
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)
27.4.2004 L. Tauscher, Basel 7
Coulomb correlated (CC) 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):
27.4.2004 L. Tauscher, Basel 8
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: mastering of multiple scattering in MCmeasuring 2 tracks with opening angle of 0.3 mrad
27.4.2004 L. Tauscher, Basel 9
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
27.4.2004 L. Tauscher, Basel 10
Track reconstruction
Track reconstruction starts from the downstream part of the spectrometer, iterative
•Search for at least one track in either of the two arms (VH, HH, hits in DC’s)•Extrapolate track through magnet onto target (beam-target X-section) 0’th approximation for momentum p•Search for hits in upstream detectors in vicinity of 0’th track (window given by multiple scattering)•Launch Kalman filter tracking for selected hits
•Verify consistency of chosen hit-track assignment (time, 2 etc.)•Repeat Kalman filter tracking for selected hits 1st approximation for momentum p•Require track extrapolation to be close to beam-target X-section•Repeat Kalman filter tracking if necessary (track-hit assignment) •Alternative to Kalman filtering straight line fit •Require or not common vertex
27.4.2004 L. Tauscher, Basel 11
Timing
27.4.2004 L. Tauscher, Basel 12
Monte-Carlo
Generators: tailored to the experiment1. Accidental background according to Nacc/Q Q2 with momentum distributions as
measured with accidentals2. Incoherent 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
27.4.2004 L. Tauscher, Basel 13
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 subtract the accidental background from the prompt by proper scaling
27.4.2004 L. Tauscher, Basel 14
Experimental Qtot and Ql distributions (Ni2001)
Fit MonteCarlo C and nC background• outside the A signal region (Qtot > 4 MeV, Ql > 2 MeV) • simultaneously to Qtot and Ql
27.4.2004 L. Tauscher, Basel 15
Residuals in Qtot and Ql
Comments:• Qtot and Ql provide same number of events background consistent• Signal shapes well reproduced
27.4.2004 L. Tauscher, Basel 16
Normalization
PBr = nA / NA
Number of detected atomic pairs with Qrec ≤ Qcut:
nA(Qrec ≤ Qcut)/nA = (from MonteCarlo)
C background with Qinit ≤ 2 MeV
NC(Qinit ≤ 2 MeV ) = kNC (Qrec ≤ Qcut), k from MCarlo
Number of produced atomsNA = 0.615kNC (Qrec ≤ Qcut)
nA(Qrec ≤ Qcut)_______________________________________________________________________
0.615kNC (Qrec ≤ Qcut)
PBr =
Number of atomic pairs:
nA = nA(Qrec ≤ Qcut) /
27.4.2004 L. Tauscher, Basel 17
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%)
27.4.2004 L. Tauscher, Basel 18
Lifetime
PBr = 0.447 ± 0.023stat
stat
= 2.85 [fs] - stat
27.4.2004 L. Tauscher, Basel 19
Systematics due to line shape
Line shape alone is not sufficientIntegrating over the full signal leads to identical PBr
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0 0.5 1 1.5 2
Ql (MeV/c)2 2.5 3 3.5 4
Q (MeV/c)
standard
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0 0.5 1 1.5 2
Ql (MeV/c)2 2.5 3 3.5 4
Q (MeV/c)
zero Qstandard
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0 0.5 1 1.5 2
Ql (MeV/c)2 2.5 3 3.5 4
Q (MeV/c)
zero Qstandard1s only
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0 0.5 1 1.5 2
Ql (MeV/c)2 2.5 3 3.5 4
Q (MeV/c)
zero Qstandard1s only+10% ms
PBr should be independent of the cut in Q/QL
27.4.2004 L. Tauscher, Basel 20
Systematics due to multiple scattering
Mult. scatt. alone not sufficientIntegrating over the full signal leads to identical PBr
Multiple scattering measured to 5% in momentum range of DIRAC
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
0 0.5 1 1.5 2Ql (MeV/c)
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
2 2.5 3 3.5 4Q (MeV/c)
standard MS
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
0 0.5 1 1.5 2Ql (MeV/c)
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
2 2.5 3 3.5 4Q (MeV/c)
MS - 5%standard MS
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
0 0.5 1 1.5 2Ql (MeV/c)
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
0.52
2 2.5 3 3.5 4Q (MeV/c)
MS - 5%standard MSMS + 5%
27.4.2004 L. Tauscher, Basel 21
Lifetime with systematics
= 2.85 [fs]
-
27.4.2004 L. Tauscher, Basel 22
Dual target technique
This target has the same properties as the single layer one 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: standard single layer Ni-target multi-layer Ni-target with the same thickness as the standard target, but segmented into 13 equally thick layers at distances of 1.0 mm.
27.4.2004 L. Tauscher, Basel 23
Normalization-free determination of
from (independent of normalization)
Signal shape:
Background shape:
27.4.2004 L. Tauscher, Basel 24
Preliminary results of single/multilayer 2002
Ns - Nm = (Pbrs-Pbr
m)NA = 825 ± 140 eventssignal shape well reproduced
27.4.2004 L. Tauscher, Basel 25
Preliminary results of single/multilayer 2002
= 1.86 ± 0.20stat
Background shape from MonteCarlo is consistent also at low Q
27.4.2004 L. Tauscher, Basel 26
Lifetime single/multilayer 2002 (preliminary)
Result: = 2.5 + 1.1- 0.9 [fs]
27.4.2004 L. Tauscher, Basel 27
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
27.4.2004 L. Tauscher, Basel 28
Conclusion
DIRAC is a very difficult experiment
The apparatus worked fine
Systematic errors have been studied• Multiple scattering• Shape uncertainties• Many others
Systematic errors are smaller than the statistical errors
Full statistics sufficient to reach 10% accuracy
More analysis and systematics studies needed (2 more years)
Request: 2 weeks of test with the DIRAC setup
27.4.2004 L. Tauscher, Basel 29
Micro Drift Chamber
Track 1 Track 2
Anode wire Potential strip
CathodesI
II
4 mm
80.0 mm
I
II
2.5 mm
High rateHigh double track resolutionLow material budgetFastStand-alone tracking
18 layers in special housingOverpressure 2 atm26 ns max drift time
27.4.2004 L. Tauscher, Basel 30
Micro Drift Chamber
1. Sucessfully operated in 2003 (3 weeks)
2. Rate >1.5x1011 pps3. Break-down of 4
layers in second week
4. Remaining 16 layers ok
1. Read-out and cabling improved
2. Sensitivity 5 times better
3. Lower HV possible
4. 2 weeks of test in 2004
27.4.2004 L. Tauscher, Basel 31
Multiple scattering measurementIDEntriesMeanRMS
341 563322-0.3059E-05 0.5618E-03
0
5000
10000
15000
20000
25000
-0.005 -0.004 -0.003 -0.002 -0.001 0 0.001 0.002 0.003 0.004 0.005
scatt. angle
Instrumental resulution
XY-Initial YMS
IDEntriesMeanRMS
342 573944-0.2936E-05 0.7462E-03
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
-0.005 -0.004 -0.003 -0.002 -0.001 0 0.001 0.002 0.003 0.004 0.005
scatt. angle
Measured scattering angles
Place all scattering objects between
drift chamber 3 and 4
27.4.2004 L. Tauscher, Basel 32
Multiple scattering result
reconstructed scattering distribution
scatt. angle
reconstructed scattering distribution
MonteCarlo (Moliere) ±5%
scatt. angle
Conclusions:1. Central part of mult. Scattering
well reproduced by GEANT4 2. More analysis work needed
27.4.2004 L. Tauscher, Basel 33
Experimental difficulties
Low atomic formation rate (10-9)
High instantaneous particle flux (10MHz)
High physics background
High uncorrelated background
1. High resolution double arm spectrometer
2. Precision tracking3. Particle identification4. Time resolution
Multilevel trigger:1. First stage: 50 ns2. Second stage: 200
ns(digital NN)
3. Third stage: >1.5 s
Solution
27.4.2004 L. Tauscher, Basel 34
Non - correlated (NC) background
Incoherent pairs follow the phase space:
Incoherent pairs from long-lived intermediate resonances have slightly different single particle momentum distributions than coherent pairs (Fritjof)
Accidental pairs from two different interactions have similar single particle momentum distributions as coherent pairs