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University of Santiago de Compostela, Spain
Pionium Lifetime Pionium Lifetime Measurement by DIRACMeasurement by DIRAC
Antonio Romero on behalf of DIRAC collaboration
XXXV INTERNATIONAL MEETING ON FUNDAMENTAL PHYSICS
28 May - 1 June 2007, Santiago de Compostela, Spain
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90 Physicists from 18 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.
Lifetime Measurement of +- atoms to test low energy QCD predictionswww.cern.ch/DIRAC
DImeson Relativistic Atomic ComplexesDIRACDIRAC
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Pionium lifetimePionium lifetime
The lifetime of + atoms is dominated by charge exchange process into 00:
3
2
222 104
1
0
0
a0 and a2 are the S-wave scattering lengths for isospin I=0 and I=2.
Pionium is a hydrogen-like atom consisting of + and - mesons
EB=-1.86 keV, rB=387 fm, pB≈0.5 MeV
π+
ππ0
π0
220
3π2 )a(apα
9
2Γ
0at lowest order :
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Pionium lifetime in QCDPionium lifetime in QCD
2π
220
3
s1s1 Mδ)(1|aa|pα
9
2
τ
1Γ 2
π22
0π2π Mα
4
1MMp
= (5.8 ± 1.2) × 10-2 significant correction
At next-to-leading order in α and (md – mu )2 :
Measurement of (10%) |a0-a2| (5%)
J. Gasser et al , Phys. Rev. D62 (2001) 016008
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Pionium lifetime in QCDPionium lifetime in QCD
The scattering lengths have been calculated in the framework of Chiral Perturbation Theory (ChPT):
004.0265.0
0010.00444.0005.0220.0
20
20
aa
aa
fss 1.09.21
G. Colangelo, J. Gasser and H. Leutwyler, Nucl. Phys. B603 (2001) 125.
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Experimental resultsExperimental resultsK+→+-e+e (Ke4) decay
New measurement at BNL (E865)
S.Pislak et al., Phys.Rev. D 67 (2003) 072004
a0=0.216±0.013 ±0.003(syst)
a2=-0.0454±0.0031
±0.0013(syst)
L. Rosselet et al., Phys. Rev. D 15 (1977) 574a0=0.26±0.05
N→N near threshold
M. Kermani et al., Phys. Rev. C 58 (1998) 3431a0=0.204±0.014
±0.008(syst)
C.D. Froggatt, J.L. Petersen, Nucl. Phys. B 129 (1977) 89a0=0.26±0.05
N.Cabibbo, Phys. Rev. Lett. 93, 121801 (2004)
N.Cabibbo, G.Isidori, hep-ph/0502130|a0-a2|= 0.261
± 0.006 (stat.) ±0.003 (syst.)±0.0013 (ext) ±0.013 (theor)
K+→+00 and KL→30 NA48/2
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The pionium is a Coulomb bound state:
MeV1fm387)(
0keV858.1
2
B
PCB
PAR
JE
+ and - originating from short lived sources (, K*, ,...) and resonance decays may form a pionium atom. The differential cross section is:
Lorentz Center of Mass to Laboratory factor.
Wave function at origin (accounts for Coulomb interaction).
Pion pair production from short lived sources.
Production of pioniumProduction of pionium
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Method of pionium detectionMethod of pionium detection
Pionium is created in nS states then it interacts with target material:
decay 15 for 17c m
L.Nemenov, Sov.J.Nucl.Phys. 41 (1985) 629
Annihilation: A2→00
Excitation: transitions between atomic levels
Break-up(ionisation): characteristic “atomic” pairs nA
• Qcms<3MeV/c
• → in laboratory system E+≈E-, small opening angle θ<3mrad
Coulomb and atomic pairs are detected simultaneously:
Niforμmλ S 201int
C
Ath
A
ABr N
n
KN
nP
1
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Production of pioniumProduction of pionium
Atoms are Coulomb bound state of two pions produced in one proton-nucleus collision
Background processes:Coulomb pairs. They are produced in one proton nucleus collision from fragmentation or short lived resonances (, K*, ,...) and exhibit Coulomb interaction in the final state:
Non-Coulomb pairs. They are produced in one proton nucleus collision. At least one pion originates from a long lived resonance. No Coulomb interaction in the final state
Accidental pairs. They are produced in two independent proton nucleus collision. They do not exhibit Coulomb interaction in the final state
α/q)Mπ2exp(1
α/qMπ2(q)A
dpdp
σd(q)A
dpdp
σd
π
πC
0S
2
CC
2
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DIRAC SpectrometerDIRAC Spectrometer
Upstream detectors: MSGCs, SciFi, IH.
Downstream detectors: DCs, VH, HH, C, PSh, Mu.
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Tracking principles Tracking principles •Precison time-of-flight to reduce accidental and proton background
•Strong e+e- rejection by Čerenkov counters
•Unambiguous transverse momentum QT by upstream tracking (MSGC+SFD)
•Longitudinal momentum QL measured by fast drift chambers and upstream tracks
DIRAC Spectrometer DIRAC Spectrometer High high irradiation
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Downstream detectors:Drift chambersCherenkovTime-of-Flight
Upstream detectors:MSGC/GEMSFDIonisation Hodoscope
Pre-Shower and Muon Counters unseen
Nucl. Inst. Meth. A515 (2003) 467.
DIRAC Spectrometer DIRAC Spectrometer
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Trigger performanceTrigger performance
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CalibrationsCalibrations
Time difference spectrum at VH with e+e- T1 trigger.
Mass distribution of p- pairs from decay. =0.39 MeV/c2
Positive arm mass spectrum, obtained by TOF difference, under - hypothesis in the negative arm.
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Accidental pairs, different proton interactions in the target
Coulomb pairs. From short lived sources.r < 3 fm, < R(A2
Non Coulomb pairs. From long lived sources.r ~ 1000 fm.
t= 174 ps +- pairs
Time-of-Flight spectrumTime-of-Flight spectrum
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Analysis based on MCAnalysis based on MCAtoms are generated in nS states using measured momentum distribution for short-lived sources. The atomic pairs are generated according to the evolution of the atom while propagating through the target
Background processes:
Coulomb pairs are generated according to AC(Q)Q2 using measured momentum distribution for short-lived sources.
Non-Coulomb pairs are generated according to Q2 using measured momentum distribution for long-lived sources.
Monte Carlo simulation is restricted to detector response only, withoutrelying on specific asumptions from proton-nucleus collision models
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Qd
dn
Qd
dn
Qd
dn
Qd
dn
Qd
dnATACNCCCp
22322212
Qd
dn
NQd
dn i
i
i22
1 LT dQdQQd 2
2D 2 FIT TO (QT, QL) SPECTRUM
3 (accidentals fraction) measured from TOF
1 and free parameters in 10 independent 600 MeV/c +- momentum bins
Atom signal defined as difference between prompt data and Monte Carlo with = 0
1321
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Pionium signal in Qt vs Ql
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PIONIUM BREAK-UP SIGNAL IN +- SPECTRUM
· cos
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Pionium signal in Q =√ QL2+ QT
2
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PIONIUM TRANSVERSE SIGNAL
QL > 2 MeV/c
QL < 2 MeV/c
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Pionium Longitudinal Signal
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)(K
1
)(N
)(NP
expCC
ATBr
Qd(Q)A
/n1
π
)παM(2)(K
2C
33πth
)(K)(ε
)(εK th
AT
CCexp
)Q(0,)Q(0,Ω CL
CT
DETERMINATION OF BREAK-UP PROBABILITY
Different extrapolation domains:
QM analytical factor:
Coulomb-pair background NCC determined from fit 1 parameter:
Acceptance factors i determined by Monte Carlo simulation
Standard choice is QLC = 2 MeV/c and QT
C = 5 MeV/c
C
Ath
A
ABr N
n
KN
nP
1
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Break-up probability as function of pionium momentum
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PBr as function of QL upper cut
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Fraction of Coulomb pairs as function of momentum
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Long-lived pairs (2)
/(0.6 GeV/c)
Spectrum consistent with+- bound stateformation
Softer spectrum (, ..)expected from Monte Carlo
Atom Pairs
Coulomb Pairs x 1/40
Coulomb Pairs
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Breakup probabilityBreakup probability
Summary of systematic uncertainties:
Source
QL trigger acceptanceMSGC+SFD background Double Ionization cut Double-track resolution Target ImpurityKK contamination
± 0.004± 0.006± 0.003± 0.003± 0.003± 0.003
Total ± 0.008
PBr=0.435±0.016 (stat) ± 0.008 (syst) = 0.435 ± 0.018
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fssyststatS117.0111.0
266.0255.01 63.2
Results from DIRACResults from DIRAC• DIRAC collaboration has built up a double arm spectrometer which
provides a pair relative momentum (Q) resolution of 0.4 MeV/c for Q<30MeV/c
• More than 6000 of + - pairs from pionium break-up were observed• The analysis of Ni 2001 data provides a lifetime measurement which
translates into an S-wave amplitude measurement at rest :
fsS290.0278.01 63.2
110153.00146.002 015.0277.0277.0
MMaa
004.0265.020 aa (ChPT)