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


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


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


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 using background)


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”


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):


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


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


Timing


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


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.


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


Residuals in Q and Ql

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


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) /


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

Systematics from:• Normalization (C vs. nC determination)• Cut (Qcut) for PBr determination

• Multiple scattering simulation• Signal shape simulation• Many others


Lifetime with systematics

= 2.85 [fs]

-


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


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


Rate combination for signal (2002) (preliminary)

Ns(Q) - Nm(Q) = (Pbrs-Pbr

m)NA = 825 ± 140 eventssignal shape well reproduced


Rate combination for background (2002) (preliminary)

= 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

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

results from the dirac experiment at cern di meson r elativistic a tomic c omplex

Documents

atom momentum distribution

decay of pp atom

pp atom of order

strong pp pp

target collisions

breakup of produced

decayp p pairs

dynamics of atom