Graziano Venanzoni Laboratori Nazionali di Frascati
KLOE results on Hadronic cross section
Topical Workshop on (g-2)
Glasgow, 25-26 Oct 2007
(for the KLOE collaboration)
Outlook
• Motivation of (e+e- ) at low energy
• Reminder: KLOE published results on |F and awith 2001 data (normalization to Bhabha)
• Update of |F and a with 2002 data, using two selection schemes (large and small angle)
• Work in progress:– Extract |F via bin-by-bin normalisation to
– Use data collected at 1 GeV
Motivation: High Precision Test of the Standard Model
Anomalous magnetic moment of the muon
Fine structure constant at Z0-mass QED (MZ)
Hadronic Cross Section & Muon-Anomaly
Muon-Anomaly a= (g-2)/2 =
aaaaatheo QED had weak New physics
Dispersion-Relation Channel = (e+ e gives >70% contribution to a
had!
• K(s) = analytic kernel-function, • above typically 2…5 GeV, use pQCD
Alternative: Spectral function from decay ( Hadrons) taking intoaccount isospin breaking corrections
ahad had (s)
Hadronic Vacuum Polarization
2nd largest contribution, pQCD not applicableError of hadronic contribution dominates total error of a!
µ+
q
q
µ+
M. Davier
had and the muon anomaly
(e+e-+) with ISR:Since DANE runs at fixed energy (1020 MeV), we measure the cross section (e+ e- hadrons ) as a function of the hadronic c.m. energy M hadr by using ISR (”radiative return”):
This method is a complementary approach to the standard energy scan.
• Luminosity, acceptance (and other normalizations) enter only once !• Requires precise calculations of the radiator H
PHOKHARA MC NLO Generator
(H. Czyż, A. Grzelińska, J.H. Kühn, G. Rodrigo, Eur. Phys. J. C 27, 2003)
Neglecting FSR effects:4m
2 s = M m
2
S. Binner, J.H. Kühn, K. Melnikov, Phys. Lett. B 459, 1999
)(2
2 sdM
dM
H(s)
Rho - Resonanz
dds
0.1 0.3 0.5 0.7 0.9M
[GeV2]
a.u
.
Radiatorfunction H(s)
0.1 0.3 0.5 0.7 0.9M
[GeV2]
H. Czyz
DEAR,
e+e- - collider with =m1.0195 GeVs
DANE: A -Factory
Integrated Luminosity
Peak Luminosity Lpeak= 1.5 • 1032cm-2s-1
Total KLOE int. Luminosity:
L dt ~ 2500 pb1 (2001 - 05)
2006:• Energy scan with 4 points around m-peak• 220 pb-1 at = 1000 MeV s
This talk is based on 240pb-1 from 2002 data!
KLOE DetectorDrift chamber
p/p = 0.4% (for 900 tracks)
xy 150 m, z 2 mm
Excellent momentum resolution
p/p = 0.4% (for 900 tracks)
xy 150 m, z 2 mm
Excellent momentum resolution
KLOE DetectorElectromagnetic Calorimeter
E/E = 5.7% / E(GeV)
T = 54 ps / E(GeV) 100 ps(Bunch length contribution subtracted from constant term)
Excellent timing resolution
E/E = 5.7% / E(GeV)
T = 54 ps / E(GeV) 100 ps(Bunch length contribution subtracted from constant term)
Excellent timing resolution
a) Via absolute Normalisation to VLAB Luminosity (as in 2001 analysis):
€
dσππ +γ (γ )
obs
dMππ
2=
ΔNObs − ΔNBkg
ΔMππ
2⋅
1
εSel⋅
1
Ldt∫
€
s( ) ≈ Mππ
2dσ
ππ +γ (γ )
obs
dMππ
2⋅
1
H(s)
Relation between |F|2 and the cross section ee
Obtain from (ISR) - radiative cross section ddM2 via theoretical radiator function H(s):
ddM2 is obtained by subtracting background from observed event spectrum, divide by selection efficiencies, and int. luminosity:
Extracting |F|2 from eve:
1)
2)
3)
€
Fπ2 =
3s
πα 2βπ3
σ ππ s( )
b) Via bin-by-bin Normalisation to rad. Muon events (see later…)
Event Selection
Pion tracks at large angles 50o< <130o
a) Photons at small angles < 15o or > 165o
€
rp=
rpmiss=−(
rp+ +
rp−)
• High statistics for ISR photons• Very small contribution from FSR• Reduced background contamination
Photon momentum from kinematics:
Event Selection
b) Photons at large angles 50o < < 130o
Photon is explicitly measured in the detector!
• Threshold region accessible• Increased contribution from FSR• Contribution from f0(980)
Pion tracks at large angles 50o< <130o
a) Photons at small angles < 15o or > 165o
€
rp=
rpmiss=−(
rp+ +
rp−)
Photon momentum from kinematics:
• High statistics for ISR photons• Very small contribution from FSR• Reduced background contamination
Acceptance 0.3%
Trigger 0.3%
Tracking 0.3%
Vertex 0.3%
Reconstruction filter 0.6%
Particle ID 0.1%
Trackmass cut 0.2%
Background 0.3%
Unfolding effects 0.2%
Total experimental Error 0.9%
Acceptance 0.3%
Trigger 0.3%
Tracking 0.3%
Vertex 0.3%
Reconstruction filter 0.6%
Particle ID 0.1%
Trackmass cut 0.2%
Background 0.3%
Unfolding effects 0.2%
Total experimental Error 0.9%Luminosity ( LA Bhabhas ) 0.6%
Vacuum polarization 0.2%
FSR corrections 0.3%
Radiator function 0.5%
Total theoretical Error 0.9%
TOTAL ERROR 1.3%Statistical error negligible (1.5 Million events) Phys. Lett. B606 (2005) 12
Published result, 2001 data
a(0.35-0.95 GeV2) = (388.7 0.8stat4.9syst) · 10-
10
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(e+e- ) nbKLOE
2001 Data140pb-1
Published KLOE Result: Phys. Lett. B606 (2005) 12
• additional online software trigger level introduced to recover cosmic veto inefficiency in 2002 (that gave an inefficiency of up to 30% in 2001)
• Improved offline-event filter reduces its systematic uncertainty to <0.1%, was the largest contribution (0.6%!) to the error in published result
• New event generator BABAYAGA@NLO - theoretical error of Bhabha effective cross section decreases from 0.5% to 0.1% - Bhabha cross section lowered by 0.7%; therefore pion form factor decreases by 0.7%
• Larger data set allows for more refined evaluation of systematic errors associated with selection efficiencies; evaluate all contributions again
• 2002 data less effected by pile-up events from machine background
• Trigger issue in 2001 data resolved (see below)
C. M. Calame et al., Nucl. Phys. B758 (2006) 227
2002 data: improvements wrt 2001
Trigger 2001 update
Impact of update on trigger correction on 2001 cross section:
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KLOE 2001 TRG updatedKLOE 2001 published
KLOE 2001 TRG updatedKLOE 2001 published
Changes published value on a by 0.4%
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Small angle analysis 2002
€
dσ ππγ
dMππ2 =
Nobs−Nbkg
ΔMππ2 ×
1εSelect.
× 1 L
€
dσ ππγ
dMππ2 =
Nobs−Nbkg
ΔMππ2 ×
1εSelect.
× 1 L
Interference
M2
(GeV2)
Statistics: 242pb-1 of 2002-data 3.4 Million Events
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Miss<150 or Miss>1650
Preliminary
M2
(GeV2)
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Small Angle Analysis• Experimental challenge: Fight background from ee , , reduced by means of kinematical cuts in (trackmass), and likelihood function
• Background from LO-FSR negligible (reduced by acceptance cuts: small ), NLO-FSR not reduced and not a background, efficiency has to be known
• Normalization to integrated luminosity, which is obtained from large-angle- Bhabha events (clean exp. selection)
LO-FSR NLO-FSR
m
m
( ) 0)( 2221
222
222
1 ==+−+−+− γφ qppMpMpM trktrk
rrrr
m
• Surviving background evaluated by MC and subtracted
signal region
Background:Main backgrounds estimated from MC shapes fitted to data distribution in MMain backgrounds estimated from MC shapes fitted to data distribution in MTrkTrk
((eeee
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0.42< M2 < 0.44 GeV2
DataMCee
MTrk [MeV]
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DataMCee
0.68< M2 < 0.7 GeV2
• Bhabha contamination small • events only fitted below 0.6 GeV2 • -peak is cut by event streaming and elliptical Cut in MTrk
• Excellent agreement on TRK mass spectrum Between data and MC
MTrk [MeV]
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Luminosity:At KLOE, Luminosity is measured using „Large angle Bhabha“ events (55o<e<125o)
KLOE is its own Luminosity Monitor!
Generator used for Bhabha cross section:
–BABAYAGA (Pavia group):
eff = (428.00.3stat) nbC. M. Calame et al., C. M. Calame et al., Nucl. Phys. B758 (2006) 227
The luminosity is given by the number of Bhabha events divided for an effective cross
section obtained by folding the theory with the detector simulation.
Systematics on Luminosity
Theory 0.10 %
Acceptance 0.25 %
Background () 0.08 %
Tracking+Clustering
Energy Calibration
0.13 %
0.10 %
Knowledge of s run-by-run 0.10 %
TOTAL 0.10 % theo 0.32% exp = 0.34 %
Quoted theor. accuracy:0.1%
New version of generator (BABAYAGA@NLO) gives 0.7% decrease in cross section
compared to previous version
F. Ambrosino et al. (the KLOE Coll.) Eur.Phys.J.C47:589-596,2006
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F=1
(
) w
ith
F=
1 [n
b]
Radiative corrections
Radiator-Function H(s) (ISR):
i) Bare Cross Section divide by Vacuum Polarisation
from F. Jegerlehner: http://www-com.physik.hu-berlin.de/~fjeger/
ii) FSR - Corrections Cross section must be incl. for FSR
FSR corrections have to be taken into account in the efficiency eval. (small angle acceptance,
MTrk) and in the passage M2
FS
R c
on
tr. (
sQE
D)
Net effect of FSR is ca. 0.8%:
- ISR-Process calculated at NLO-level PHOKHARA generator (Czyż, Kühn et.al) Precision: 0.5%
× )(2
2 sdM
dM ππ
ππ
ππγππ σ
σH(s)
Radiative Corrections:
Correcting for FSR energy:
Go from M2 M2
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[GeV2]
[
GeV
2 ]MonteCarlo
Use special version of PHOKHARA which Use special version of PHOKHARA which allows to determine whether photon comes allows to determine whether photon comes from initial or final state from initial or final state build matrix which build matrix which relates relates to .
The presence of FSR results in a shift of the measured quantity M2
towards lower values:
M2
M2
ISR only: M2
FSR photon present: M2
(FSR)
M2
Small angle result from 2002 data:
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Offline Filter negligible
Background 0.3%
Trackmass/Miss. Mass 0.2% (prelim)
/e-ID 0.3%
Vertex 0.5%
Tracking 0.4%
Trigger 0.2%
Acceptance () negligible
M2 M (FSR corr.) 0.3% (prelim)
Software Trigger 0.1 %
Luminosity 0.3%
Acceptance (Miss) 0.1%
Radiator H 0.5%
Vacuum polarization negligible
Systematic errors on a:
Total= 1.1%
Preliminary
Comparison 2001-2002:
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(updated)
Evaluating a with small angle
2001 published result (Phys. Lett. B606 (2005) 12):
Applying update for trigger eff. and change in Bhabha-cross section used for luminosity evaluation:
a(0.35-0.95GeV2) = (388.7 0.8stat4.9syst) · 10-10
€
aμππ =1/4π 3 ds σ (e+e− → π +π −) K(s)
0.35GeV2
0.95GeV2
∫
Dispersion integral for 2-channel in energy interval 0.35 <M2<0.95 GeV2
a(0.35-0.95GeV2) = (384.4 0.8stat4.9syst) ·
10-10
2002 preliminary:
a(0.35-0.95GeV2) = (386.3 0.6stat3.9syst) · 10-10a
(0.35-0.95GeV2) = (386.3 0.6stat3.9syst) · 10-10
Large angle analysis:
Threshold region non-trivial due to irreducible FSR-effects, to be computed from MC using phenomenological models (interference effects unknown)
ff
FSR f0
important! small!
important cross check with small angle analysisthreshold region is accessible photon is explicitly required (allows 4-momentum constraints)
lower statistics for signal FSR not negligible large background irreducible bkg. from decays
240 pb-1
2002 Data
M2 [GeV2]
Nr.
of
even
ts /
0.01
GeV
2
Preliminary
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Large angle analysis (cont‘d):
2002 Data L = 240 pb-1
M2 [GeV2]
Apply specific selection cuts:
• Exploit kinematic closure of the event
Cut on angle btw. ISR-photon and missing momentum
• Kinematic fit in hypothesis using 4-momentum and 0-mass as constraints
• FSR contribution added back to cross section (estimated from PHOKHARA generator)
misspr
ãpr
Error on LA dominated by syst. uncertainty on f0 contr.
LA stat.+syst. error
• Reducible background from and µ+µwell simulated by MC
Preliminary
• Model dependence of irreducible background from f0 is the dominating uncertainty. Computed using different models for f0-decay and input from dedicated KLOE f0 analyses (with f0 decaying to charged and neutral pions).
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Range of comparison
Comparison SA-LA 2002
Small angle:
a(0.50-0.85GeV2) =
(255.4 0.4stat 2.5syst) · 10-10
Large angle:
a(0.50-0.85GeV2) =
(252.5 0.6stat 5.1syst) · 10-10
Range of comparison
LA stat.+syst. error
LASASA
a between 0.5 - 0.85 GeV2:
Preliminary
(60% (=3.1×10-10) of systematical error due to f0-uncertainty)
Asymmetry:
Binner, Kühn, Melnikov, Phys. Lett. B 459, 1999
check the validity of the FSR model (Phokhara uses sQED, i.e. pointlike pions)
Czyz, Grzelinska, Kühn, hep-ph/0412239
radiative decays of the into scalar mesons decaying to + -contribute to the charge asymmetry
Possibility to study the properties of scalar mesons with charge asymmetry
€
A =N(θ+ > 90o) − N(θ+ < 90o)
N(θ+ > 90o) + N(θ+ < 90o)
€
A =N(θ+ > 90o) − N(θ+ < 90o)
N(θ+ > 90o) + N(θ+ < 90o)
In case of non-vanishing FSR contribution, the interference term
between ISR and FSR is odd under exchange + - .This gives riseto a non-vanishing forward-backward asymmetry:
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KLOE DataMC [ISR + FSR]MC* [ISR+FSR+f0(980)]
*G. Pancheri et. al., arXiv:0706.3027
a KLOE summary
Jegerlehner (hep-ph/0703125):
Using the new KLOE result increases difference from 3.2to 3.4
€
Δaμ = aμexp − aμ
the = (28.7 ± 9.1) ⋅10−10
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Preliminary
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a Summary
Comparison with a from CMD2 and SND in the range
0.630-0.958 GeV :Phys. Lett. B648 (2007) 28 Preliminary
Conclusions:
KLOE has extracted a in the range between 0.35 - 0.95 GeV2
using cross section data obtained via the radiative return with photon emission at small angles.
• The preliminary result from 2002 data agrees with the updated result from the published KLOE analysis based on 2001 data
•Data from an independent and complementary KLOE measurement (Large angle analysis) of the cross section yelds a
in the range between 0.5 - 0.85 GeV2
• All three KLOE results are in good agreement
•KLOE results also agree with recent a results from the
CMD2 and SND experiments at VEPP-2M in Novosibirsk
Outlook:
• Refine the small angle analysis by unfolding for detector resolution, to release the cross section table
• Continue evaluation of resonance contributions in the large angle analysis to decrease model dependence from f0(980) contribution
• Measure the pion form factor via bin-by-bin ratio of pions over muons (Normalization to radiative muon pairs)
• Obtain pion form factor from data taken at s = 1000 MeV (off-peak data)
See next slides…
Extracting |F|2 from norm. to
)(/
/)( s
sdd
sdds Born
obs
obsBorn ′
′′
≈′
(exact only in the absence of FSR)
In presence of FSR:
Corr. due to FSR „Observed“ cross sections
€
Fπ s'( )2⋅ 1+η (s')( ) =
4 1+ 2mμ2 s'( )βμ
βπ3
⋅dσ ππγ ds'( )
ISR+FSR
dσ μμγ ds'( )ISR
Luminosity ( LA Bhabhas ) 0.6%
Vacuum polarization 0.2%
FSR corrections 0.3%
Radiator function 0.5%
Total theoretical Error 0.9%
Some Effects cancel out in the ratio:
x0.3%
needs to select µµ events with similar precision as
separation:separation is achieved in the analysis by a rigid cut in MTrk:
Data
N
o m
an‘s
lan
d
MTrk[MeV]
Additional tool used in evaluation of
efficiencies: ID by means of a Neural Network (NN):
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MTrk< 115 MeV
MTrk> 130 MeV
NN >0.7 for NN <0.2 for
Data-- MC
The spectra of selected events for the small angle analysis from 242.62
pb-1 of data taken in 2002:
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0.87 Mio. events3.42 Mio events
Spectra after SMA selection:
Pions Muons
DANE Off-peak data:
Run-Nr
s (
MeV
)
s = 1023s = 1023
s = 1030s = 1030
s = 1018s = 1018
s = 1010s = 1010
s = 1000s = 1000
• It allows to extract |F|2 down to thresh. in a background-free enviroment• 220 pb-1, competitive with VEPP-2M at threshold• Study model-dependence of f0(980) (in connection with on-peak data)
• Run at √s=1000 MeV between Dec. 2005 and March 2006 220 pb-1 on tape
• Energy scan around -peak
4 scan points, 10 pb-1 each
MC Phokhara
Data 2006 off-peakTagged photons
Preliminary
Forward-Backward-Asymmetry
MM22 (GeV (GeV22))
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http://www.lnf.infn.it/conference/phipsi08/
Backup slides
Reminder: Comparison with e+e- and - Data
s [GeVs [GeV22]]
Relative difference btw. publishedKLOE (2001) and CMD-2, SND
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/
KL
OE -
1
interpolation of 60KLOE data points from
0.35 to 0.95 GeV2Plot from I. Logashenko using non-published data from CMD-2 and SND!
KLOE published
SND hep-ex/0605013CMD-2 published
CMD-2 prel. ALEPH -data
••••••
Relative difference btw. e+e Data and -spectral function from ALEPH
s [GeV]s [GeV]
KLOE confirmed the CMD-2 discrepancy with tau data and “...invalidates the use of -data until a better understanding of the discrepancies is achieved” (A. Höcker ICHEP-04)Some disagreement btw. KLOE and SND/CMD-2 seen at low and high masses aµ
had from all recent e+e- experiments agree now within 0.5 Waiting for new results from KLOE (this talk), Babar, and data (Belle)