Laboratory of Nuclear Problems,

Joint Institute for Nuclear Research, Dubna

Measurement of atom lifetime

at DIRAC.

ICHEP02

Amsterdam, 25–31 July, 2002.

Leonid Afanasyev on behalf of DIRAC collaboration

83 Physicists from 19 Institutes

Basel Univ., Bern Univ., Bucharest IAP, CERN, Dubna JINR, Frascati LNF-INFN, Ioannina Univ., Kyoto-Sangyo Univ., Kyushu Univ. Fukuoka,

Moscow NPI, Paris VI Univ., Prague TU, Prague FZU-IP ASCR, Protvino IHEP, Santiago de Compostela Univ., Tokyo Metropolitan Univ., Trieste

Univ./INFN, Tsukuba KEK, Waseda Univ.

Lifetime Measurement of +- atoms to test low energy QCD predictions.

DImeson Relativistic Atomic Complexes

DIRACDIRAC

The goal of the DIRAC Experiment is to measure the +- atom

lifetime of order 3·10-15 s with 10% precision. This measurement will

provide in a model independent way the difference between

Swave scattering lengths |a2a0 | with 5 % precision. Low energy

QCD - chiral perturbation theory - predicts nowadays scattering

lengths with very high accuracy ~ 2 % . Therefore, such a

measurement will be a sensitive check of the understanding of

chiral symmetry breaking of QCD by giving an indication about the

value of the quark condensate, an order parameter of QCD.

The GoalThe Goal

Theoretical MotivationTheoretical Motivation+ atoms (A2 in the ground state decay by strong interaction mainly into 00.

3

2

222 104

1

0

0

Chiral Perturbation Theory (ChPt), which describes the strong interaction at low energies provides, at NLO in isospin symmetry breaking, a precise relation between 2 and the scattering lengths:

)1(|| 000 22

2202

LONLOLO

aaC

(Deser et al) (Gall et al., Jalloli et al, Ivanov et al)

a0 and a2 are the strong S-wave scattering lengths for isospin I=0 and I=2.

s1510)1.09.2()%2.18.5(

%5)/()(%10/ 2020 aaaa

Theoretical StatusTheoretical StatusIn ChPT the effective Lagrangian which describes the interaction is an expansion in (even) terms:

)4()4()2( LLLLeff

%)5.1(004.0265.0%)3(258.0

gChPT01.025.0

20.0

20)6(

20)6(

)6(20

)4(20

)2(

aaLaaL

LaaLaaL1966 Weinberg (tree):

1984 Gasser-Leutwyler (1-loop):1995 Knecht et al. (2-loop):1996 Bijnens et al. (2-loop):2001 Colangelo et al. (& Roy):

Tree

(Weinberg)

1-loop

(Gass.&Leut.)

2-loop

(Bijnens et al.)

2loop+Roy

(Colangelo et al.)

a0 0.16 0.203 0.219 0.220

a2 -0.045 -0.043 -0.042 -0.044

And the theoretical results for the scattering lengths up to 2-loops are:

Experimental StatusExperimental StatusExperimental data on scattering lengths can be obtained via the following indirect processes:

shold near thre :dependent Model NN

)( t,independen Model 4ee KeK

05.028.00 a L. Rosselet et al., Phys. Rev. D 15 (1977) 574

(theor)005.0syst)(004.0013.0216.00

a New measurement at BNL (E865)

S.Pislak et al., Phys.Rev.Lett. 87 (2001) 221801

C.D. Froggatt, J.L. Petersen, Nucl. Phys. B 129 (1977) 89

M. Kermani et al., Phys. Rev. C 58 (1998) 3431

10 05.026.0 ma

)syst(008.0014.0204.00 a

05.026.00 a M.Nagels et al., Nucl.Phys. B 147 (1979) 189Combined analysis of Ke4, Roy equation and peripheral reaction

NN data

…

AA22ππ production production•The pionic atoms are produced by the Coulomb interaction of a pion pair in proton-target collisions (Nemenov):

•Also free Coulomb pairs are created in the proton-target collisions.

•The atom production is proportional to the low relative momentum Coulomb pairs production (NA=KNC).

•The pionic atoms evolve in the target and some of them (nA) are broken. The broken atomic pairs are emitted with small C.M.S. relative momentum (Q < 3 MeV/c) and opening angle <0.3 mrad.

•DIRAC aims to detect and identify Coulomb and atomic pairs samples to calculate the break-up probability

Pbr=nA/NA.

qp

sCnlm

Anlm

qdpd

d

M

E

Pd

d

0

2)(3 )0()2(

Lorentz Center of Mass to Laboratory factor.

Wave function at origin (accounts for Coulomb interaction).

Pion pair production from short lived sources.

Lifetime and breakup probabilityLifetime and breakup probability

mln

mln

mln

nlm

nlm spads

sdp)(

)(

AN

amln

nlmmln

nlm

0

.00

.0/20

l

lPcMA

Na n

totnlmnlm

nlm

The Pbr value depends on the lifetime value, . To obtain the precise Pbr() curve a large differential equation system must be solved:

where s is the position in the target, pnlm is the population of a definite hydrogen-like state of pionium. The anlm

n´l´m´ coefficients are given by:

nlmn´l´m´ being the electro-magnetic

pionium-target atom cross section, N0 the Avogadro Number, the material density and A its atomic weight.

, if nlm n´l´m´,

The detailed knowledge of the cross sections (Afanasyev&Tarasov; Trautmann et al) (Born and Glauber approach) together with the accurate solution of the differential equation system permits us to know the curves within 1%.

=10% Pbr =4%

DIRAC SpectrometerDIRAC Spectrometer

Upstream detectors: MSGCs, SciFi, IH.

Downstream detectors: DCs, VH, HH, C, PSh, Mu.

Setup features:angel to proton beam =5.7 channel aperture =1.2·10–3 srmagnet 1.85 T·mmomentum range 1.2p7 GeV/cresolution on relative momentum QX= QY=0.4 MeV/c QL=0.6 MeV/c

CalibrationCalibration

Time difference spectrum at VH with e+e- T1 trigger.

Mass distribution of p- pairs from decay. =0.43 MeV/c2 <0.49 MeV/c2 (Hartouni et al.).

Positive arm mass spectrum, obtained by time difference at VHs, under - hypothesis in the negative arm.

These results show that our set-up fulfils the needs in time and momentum resolution.

Atoms detectionAtoms detection

Accidentals:

dQdN

acc

Non-Coulomb pairs:

dQdN

dQdN

acc

NC

corr

Coulomb pairs:

dQ

dNQA

dQ

dN accC

Ccorr )(

fQAN

dQdN

dQdN

dQdN

dQdNdQ

dN

QR Cacc

Ccorr

NCcorr

acc

corr

)()(

N and f are obtained from a fit to the pion pairs Q spectrum in the range without atomic pairs Q > 3 MeV/c

The time spectrum at VH provides us the criterion to select real (time correlated) and accidental (non correlated) pairs.

Atoms detectionAtoms detectionNi 2001 data

Corr.FunctQ

R(Q

)

0123456789

0 2.5 5 7.5 10 12.5 15 17.5 20

0

10000

20000

30000

0 2.5 5 7.5 10 12.5 15 17.5 20

010000

200003000040000

0 2.5 5 7.5 10 12.5 15 17.5 20

Real+Accidental

Entries 712418

Q

N

Accidental

Entries 1220603

Q

N

Real

Entries 646538

Q

N

0

10000

20000

30000

0 2.5 5 7.5 10 12.5 15 17.5 20

Atomic PairsAtomic Pairs

Atomic pairsQ

N

-200

0

200

400

600

800

1000

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5

Pt 1999

Ti 2000+2001

Ni 2000

Ni 2001

Ni 2002

Target Z Thick (m)

Pt 78 28

Ni 2000

Ni 2001

28 94

98

Ti 22 251

Atomic pairsQ

N

-20

0

20

40

60

80

100

120

140

0 2.5 5 7.5 10 12.5 15 17.5 20

Atomic pairsQ

N

-200

0

200

400

600

800

0 2.5 5 7.5 10 12.5 15 17.5 20

Atomic pairsQ

N

-100

0

100

200

300

0 2.5 5 7.5 10 12.5 15 17.5 20

Atomic pairsQ

N

-250

0

250

500

750

1000

1250

1500

1750

2000

0 2.5 5 7.5 10 12.5 15 17.5 20

Atomic pairsAtomic pairs

SampleTriggers 106

na at Q<2MeV/c

na at

Q<3MeV/c

Pt 1999 55.713043

(3.)

20777

(2.7)

Ni 2000 896920170 (5.4)

1335300

(4)

Ti 2000+01

9101170190 (6.2)

1495340 (4.4)

Ni 2001 6472686310 (8.7)

3500510 (6.9)

Ni 2002

(15 day)77

400100

(4)

545170

(3.2)+- relative c.m. momentum MeV/c

Ent

ries

/0.5

MeV

/c

Atomic Pairs

Monte Carlo

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Conclusions and resultsConclusions and results

Ni +Ti

Ti 2000/1

Ni 2000

Statistical error

Lifetime 10-15 s

Sample

1.1

8.08.2

5.1

3.14.5

9.0

7.06.3

%34

%26

%22

DIRAC collaboration has built up the double arm spectrometer which achieves 1 MeV/c resolution at low relative momentum (Q<30MeV/c) of particle pairs and has successfully demonstrated its capability to detect atoms after 2 years of running time.

Some preliminary lifetime results have been achieved with a 5000 atoms sample reaching an statistical accuracy of 22% in lifetime determination.

The accurate determination of systematical errors requires a measurement of the background.

A 10% accuracy on the atom lifetime value (goal of the experiment) will require running beyond 2002.