Measurement of the Hadronic Cross Section via Radiative Return at DA F NE
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Transcript of Measurement of the Hadronic Cross Section via Radiative Return at DA F NE
Achim Denig Radiative Return @ DANE MENU 2004
Measurement of the Hadronic Cross Section
via Radiative Returnat DANE
Achim Denig for the KLOE Collaboration
MENU 2004September 3rd
Beijing, IHEP
Universität KarlsruheInstitut für Experimentelle Kernphysik
Achim Denig Radiative Return @ DANE MENU 2004
hep-ex/0407048, sent to Phys. Lett. B
Outline:
Muon Anomaly Radiative Return at KLOE
Results & Outlook
Further KLOE-Results: St. Müller (Session B2)
Achim Denig Radiative Return @ DANE MENU 2004
...2/2/)2g(a
newweakhadQEDtheor aaaaa
Motivation: Determination of Hadronic Vacuum Polarization = High Precision Test of the Standard Model
• Anomalous magnetic moment of the muon a = (g2)• Running fine structure constant at Z0-mass QED (MZ)
Dirac-Theory: (g 2 ) = 0Quantum corrections: (g 2 ) 0 due to corrections of: - electromagnetic interaction - weak interaction - strong interaction (and maybe NEW PHYSICS ???)
Hadronic Vacuum Polarization
hadrons
+ +
B field
q
q
2nd largest contrib., cannot be calculated in pQCDError of hadronic contribution is dominating total error !
Muon - Anomaly
Achim Denig Radiative Return @ DANE MENU 2004
Hadronic contribution to a can be estimated by means of a dispersion integral:
- K(s) = analytic kernel-function, almost flat in s- above 2…5 GeV, use pQCD
Hadronic Cross Section
Input Dispersion Integral:
a) hadronic electron-positron cross section data, e.g. e+e + b) hadronic - decays (CVC-theorem, taking into account isospin breaking corrections), e.g. 0
2m4
2
2had
s
)s(K̂)s(Rds
3
ma
)*ee(
)hadronsqq*ee()s(R
tot
tot
H
1 / s2 makes lowenergy contributionsespecially important:
eein the range < 1 GeVcontributes to 70% !
Achim Denig Radiative Return @ DANE MENU 2004
Muon-Anomaly: Theory vs. Experiment
a 11 659 000 ∙ 1010
TH
EO
RY
’20/
‘03
Exp
erim
ent
’20/
‘04 Experiment BNL-E821
Values for +(2002) and -(2004)in agreement with each other.Precision: 0.5ppm
Dispersion integral for hadronic contribution to aevaluated for:
a) e+ e-Data ONLY CMD-2 (VEPP-2M) measured channel with 0.6% precision < 1 GeV 2.7 – Deviation
b) -Data ONLY (if available) ALEPH /OPAL/CLEO 1.4 – Deviation
Theoretical values taken fromDavier, Eidelman, Höcker,
Zhang: hep-ex/0308213
a 11 659 000 ∙ 1010
e+e data and data differ for + channel in a specific energy window
above 0.6 GeV2 (above the -peak)!
Status u
p
to July ‘04
Achim Denig Radiative Return @ DANE MENU 2004
Radiative Return• Standard method for cross section measurement is the energy scan, i.e. the systematic variation of the c.m.s.-energy of the accelerator
• DANE is a - factory and therefore designed for a fixed c.m.s.-energy: s = m = 1 019 MeV; a variation of the energy is not foreseen in near future
Complementary approach:Take events with Initial State Radiation (ISR)
Cross section as a function of the 2-Pion invariant mass s=M
“Radiative Return” to -resonance:
e+ e
d(e+ e )dM
ISR
MC- Generator PHOKHARA = NLO
J. Kühn, H. Czyż, G. RodrigoRadiator-Function H(s)
)(2
2 sdM
dM
H(s)
s
Achim Denig Radiative Return @ DANE MENU 2004
L
1
1
M
NN
dM
d
..2
bkgobs
2
AcceptSelect
Background Signal
Selections-Efficiency
Luminosity
Acceptance
Analysis
• Final state e+e- relatively easy signature, however cross section measurement on percent level is a challenging task (normali- zation, efficiencies, background)
• KLOE Detector designed for CP – violation, we are having a high resolution tracking chamber ideal for the measurement of M!
Analysis-Items:
Achim Denig Radiative Return @ DANE MENU 2004
Selection
500< < 1300
< 150 > 1650
Pion tracks at large angles 50° < < 130°
• High statistics for ISR events• Reduced background contamination• Low relative contribution of FSR
Photons at small angles < 15° and > 165°
are shadowed byquadrupoles near the I.P.
Drift ChamberEM Calorimeter
)pp(pp miss NO PHOTON TAGGING
Achim Denig Radiative Return @ DANE MENU 2004
Background
ee
signalregion
MTRK (MeV)
m
tail
1) Pion-Electron-Separation Rad. Bhabhas e+ee+eare separated by means of a Likelihood- Method (Signature of EmC-Clusters and TOF of particle tracks)
m
M GeV2 2( )m
0)( 2221
222
222
1 qppMpMpM trktrk
2) Kinematic Separation e+e
using „Trackmass“-variable
M – dependent MTRK-Cut
3) Residual BackgroundFit Trackmass-Spectra for signal and background with free normalization parameters (shape from MC)
Achim Denig Radiative Return @ DANE MENU 2004
Analysis (e+e-
Background: - e+ e e+ e - e+ e -
Efficiencies:- Trigger & Cosmic veto- Tracking, Vertex- - e- separation- Reconstruction filter- Trackmass-cut- Unfolding resolution- Acceptance
Luminosity:Bhabhas at large angles > 55°, eff = 430 nb,
0.9%
0.3%
0.3%exp
0.5%theo
Statistics: 141pb-1 of 2001-Data 1.5 Million Events
Interference
High Statistics!
High Resolution!
M GeV2 2( )
Errors:
Achim Denig Radiative Return @ DANE MENU 2004
Extraction (e+e- )Radiator-Function (ISR):
- ISR-Process calculated at NLO-level Generator PHOKHARA (Kühn et.al)- Comparison with KKMC (Jadach et.al.) Precision: 0.5%
Radiative Corrections:i) Bare Cross Section divide by Vacuum Polarisation
ii) FSR - Corrections Cross section must be incl. for FSR
Vacuum Polarization Cross Section
Radiative Return requires ISR photon be inclusive for ISR-FSR-events e+ e ISRFSR
F=1
(
) m
it F
=1
s (GeV2)
Achim Denig Radiative Return @ DANE MENU 2004
Aim: Cross Section e+e-
TOTAL ERROR 1.3%
To be compared with 0.9%CMD-2 Error: exp.+syst.+stat.
2232
2
|)(|3
)(
MFM
ee
Cross Section (e+e-
• Result published now• Considerable improvement in near future
Achim Denig Radiative Return @ DANE MENU 2004
2 Contribution to ahadr
a= (388.7 0.8stat 3.5syst
3.5theo) 10-10
• Dispersions Integral for the 2-Pion-Channel in Energy Range 0.35 <M
2<0.95 GeV2
• Comparison with CMD-2 in the Energy Range 0.37 <M
2<0.93 GeV2
(375.6 0.8stat 4.9syst+theo) 10-10
(378.6 2.7stat 2.3syst+theo) 10-10
KLOE
CMD2
2
2
GeV95.0
GeV35.0
3 )()ππ(π4/1 sKeedsa
• KLOE data points are not in excellent but in a fair agreement with CMD-2: KLOE higher at low s and lower at large s
1.3% Error0.9% Error
Pion Formfactor
CMD-2KLOE
0.4 0.5 0.6 0.7 0.8 0.9s [GeV]
45
40
35
30
25
20
15
10
5
45
0
Achim Denig Radiative Return @ DANE MENU 2004
Comparison with Data
At large values of s (>m) KLOE & CMD-2 deviate from -Data > 10%
unsufficient understanding of isospin breaking corrections in -Data ?!
Rel. difference of e+e-data with respect to -data (avg.)
Achim Denig Radiative Return @ DANE MENU 2004
Conclusions
M. Davier, A. Höcker @ ICHEP04 Including:
• New KLOE data• New 4th order QED contribution (Kinoshita) hep-ph/0402206• New Light-by-light contribution (Melnikov, Vainshtein) hep-ph/0312226
– data excluded!
Theory (SM) - Experiment a
exp - atheo = ( 25.2 ± 9.2 ) ·10-10
2.7 “standard deviations”
Including KLOE
• Status of the anomalous magnetic moment of the muon today (August ’04):
• KLOE has proven feasibility of the Radiative Return for high-precision hadronic cross section measurements:
New
hep-ex/0407048, sent to Phys. Lett. B
Achim Denig Radiative Return @ DANE MENU 2004
Outlook: KLOE• Measure () in the region close to threshold, M < 600 MeV, responsible for 20% of a
- This region excluded by angular selection in small angle photon approach
- Complementary analysis at large photon angles
• At large photon angles the amount of FSR is large! test model of scalar QED (i.e. poinlike pions) Measure Charge Asymmetry and compare data with MC
Charge asymmetry is due to different C-Parity of ISR- and FSR-amplitudes
50 70 90 110 130
-20
-10
0
20
10
Asymmetry [%]
Polar Angle [°]
• Data• MC
K L O EP R E L I M I N A R Y
))
)))
(θN(θN
(θN(θNA(θ
ππ
ππ
Achim Denig Radiative Return @ DANE MENU 2004
Outlook: Muon AnomalyTheory:
• BaBar fills gap between DANE, VEPP-2M ( < 1.4 GeV ) and BES ( > 2 GeV) and will measure all relevant exclusive channels (see e.g. hep-ex/0408078)
• KLOE/CMD-2 threshold region < 0.4 GeV2
• Future measurements at BES, VEPP-2000, CLEO, BELLE, DANE-2?
• “Real Theory“: Rad. corrections, Light-by-light-scattering, Isospin breaking ...
Experiment:
• New beam time for BNL-E821 (still limited by statistics)!?
• Ambitious new project for 5th generation (g-2)experiment at J-PARC
a(theor.) 3…4 · 10-10
a(exp.) < 2…3 · 10-10
Actual Difference: 25 ± 9 · 10-10 Fu
ture