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


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


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


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


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)


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)


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):


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


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


target vacuum

SFDMSGC

vacuum

magnet

T2

negative

absorber

1 m

positive

T1


SFD: scintillation fiber detector


target vacuum

SFDMSGC IH

vacuum

magnet

T2

negative

absorber

1 m

positive

T1



IH: scintillation dE/dx counters


target vacuum

SFDMSGC IH

vacuum

magnet

DC

T2

negative

absorber

1 m

positive

T1





DC: high resolution drift chambers

target vacuum

SFDMSGC IH

vacuum

magnet

DC

HHVH

T2

negative

absorber

1 m

positive

T1






VH, HH: scintillation hodoscopes

target vacuum

SFDMSGC IH

vacuum

magnet

DC

HHVH

CT2

negative

absorber

1 m

positive

T1







C: Cherenkov counters

target vacuum

SFDMSGC IH

vacuum

magnet

DC

HHVH

CPSh

Mu

T2

negative

absorber

1 m

positive

T1







C: Cherenkov counters

PSh: preshower detector

Mu: muon counters


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


Timing


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


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


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


Residuals in Qtot and Ql

Comments:• Qtot and Ql provide same number of events background consistent• Signal shapes well reproduced


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) /


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%)


Lifetime

PBr = 0.447 ± 0.023stat

stat

= 2.85 [fs] - stat


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


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%


Lifetime with systematics

= 2.85 [fs]

-


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.


Normalization-free determination of

from (independent of normalization)

Signal shape:

Background shape:


Preliminary results of single/multilayer 2002

Ns - Nm = (Pbrs-Pbr

m)NA = 825 ± 140 eventssignal shape well reproduced


Preliminary results of single/multilayer 2002

= 1.86 ± 0.20stat

Background shape from MonteCarlo is consistent also at low Q


Lifetime single/multilayer 2002 (preliminary)

Result: = 2.5 + 1.1- 0.9 [fs]


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


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


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


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


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


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


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


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

Documents

The DIRAC Experiment Status DI meson R elativistic A tomic C omplex