r measurements at e + e - colliders
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R measurements at e+e- colliders
Debora Leone (IEKP – Universität Karlsruhe)for the KLOE collaboration
IFAETorino, 14th-16th April
D. Leone
R: very important quantity in particle physics: counts directly the number of quark flavours and their colours
R -
e+
e-
+
h a d ro n s-e
e+
q-
q
f la v o rc o lo r
= Qf2
lowestorder
To measure R: R scan: systematic variation of c.m. energy of the machine and measurement tot(e+e- hadrons)– Inclusive: directly measure tot
– Exclusive: sum up all channels of tot= I
Radiative return: trough ISR:
Definition
)μμeσ(e
hadrons)eσ(eR(s)
R measurements at e+e- colliders
Mhadr
d(e+ e- hadrons + )dM2
hadrons
(e+ e- hadrons) H(M2hadr )
D. Leone
Hadronic contribution to (g-2)
scm
eg
2Magnetic moment: with )1(2 ag
ahad includes contribution from not evaluable in pQCD,
but it can be provided by (e+ e- hadrons) by means of dispersion relations:
aSM = a
QED + ahad +
aEW
Due to quantum corrections:
ahad (e+e-) K(s) K(s) ~ 1/s~ 1/s
+ +q
q
B field
hadronsdecays
provide complementary information:CVC + SUiso(2)
hadrons
hadronsντ
dM
dN ahad ()
R measurements at e+e- colliders
D. Leone
Status of (g-2): theory vs. experiment
Comparison Experimental Value with Theory - Prediction
TH
EO
RY
’02/‘
03
e+ e- - Data: 2.7 s - Deviation
– Data: 1.4 s - Deviation
Exp
eri
men
t’0
2/‘
04 Experiment BNL-E821
Values for +(2002) and -(2004)in agreement with each other.Precision: 0.5 ppm
Theoretical values taken fromM. Davier, S. Eidelman, A. Höcker,
Z. Zhanghep-ex/0308213
R measurements at e+e- colliders
New cross section data have recently lowered theory error: a) CMD-2 (Novosibirsk/VEPP-2M) + - channel with 0.6% precision < 1 GeVb) -Data from ALEPH /OPAL/CLEO
D. Leone
12 - 5 - 12 (+)3.7 - 5 (+J/, )1.8 - 3.743 (+,)2 > 4 (+KK)
< 1.8 GeV
ahad
2[ahad]
<1.8 GeV
<1.8 GeV
1%
Calculations based on hep-ph/0308213Davier, Eidelman, Höcker, Zhang
e+e- data only!
2 contrib. ahad
8% [2m - 0.5 GeV] 54% [0.6 - 1.0 GeV] 10% [Rest<1.8 GeV]
2 contrib. dahad
8% [2m - 0.5 GeV] 34% [0.6 - 1.0 GeV] 31% [Rest< 1.8 GeV]1GeV
Hadron contribution to a
R measurements at e+e- colliders
D. Leone
The (e+e- +-) analysis @ CMD-2
R.R. Akhmetshin et al., Phys.Lett.B578:285-289,2004 Energy scan analysis with 0.6< s < 1.0 GeV
selection of collinear e+e e+e, e+e +, e+e in the fiducial volume separation of the three sample, by energy deposition rejection of cosmic events by means of the spatial distributionof the vertex
Sources of systematic errors:
– Event separation 0.2%– Radiative correction 0.4%– Detection efficiency 0.2%– Fiducial volume 0.2%– Correction for losses 0.2%– Beam energy determination 0.1%
TOTAL 0.6%
e
R measurements at e+e- colliders
D. Leone
|F|
2 L= 317.3 nb-1
2 E (MeV)
- interference
Pion form factor
3DππDN
R.C.ππ
μμR.C.μμee
R.C.ee
μμee
ππ2π Δ
ε)Δ)(1Δ(1σ
εσεσ
NN
N|F|
due to pion loss:– by decays in flight– by nuclear interaction
correction for misidentification
of
β1
βBW)BWM
sδ(1BW
(s)F
GSρ'ω2
ω
GSρ
π
Fit function according to
Gounari – Sakurai model:
3 terms for , , ’ ……..and (e+e- +-) |F(s)|2
R measurements at e+e- colliders
D. Leone
ahad @ VEPP-2M
ahad(total) = 70.2 10-9 (60.21 ppm)
ahad(VEPP-2M) = 60.16 10-9 (51.61 ppm)
ee
L= 119.6 nb-1
ChannelAccuracy
%Error in a
ppm
+ 0.6 0.26
+ 0 1.5 0.06
K+K 5.2 0.09
KLKS 1.9 0.02
+ 00 7 0.05
+ + 7 0.04
0,
6 0.02
Total 0.29
contributions to a above VEPP-
2M: 8.6 0.6 ppm
R measurements at e+e- colliders
D. Leone
R scan @ BES
R scan between 2 and 5 GeV:
March-May, 1998: 6 energy points at 2.6, 3.2, 3.4, 3.55, 4.6, 5.0 GeV
Feb.- June, 1999: 85 energy points at 2.0-4.8 GeV
δ)(1εL)μμeσ(e
NNNNR l γγllbkg
obshad
Nll = lepton pairsN = pairscorrection to to ISR
– kinematic cuts to select the event– use vertex-fitting procedure to remove residual background– luminosity determined using large angle Bhabha
Source of syst. Error
%
Luminosity 2-3Detection efficiency 2-3Trigger efficiency 0.5Radiative Correction 1-2Statistics 3-7
TOTAL 5-8
37704160
4040
4415
average error ~6.6%
R measurements at e+e- colliders
D. Leone
Radiative Return
Standard method for cross section measurement is the energy scan, i.e. the syst. variation of the the c.m. energy of the accelerator.
Particle factories (fixed c.m. energy) have the opportunity to measure the cross section (e+ e- hadrons ) as a function of the hadronic c.m. energy M 2
hadrons
by using the radiative return.
hadr
d(e+ e- hadrons + )d
hadrons
(e+ e- hadrons) H(hadr )
This method is a complementary approach to the energy scan
Precise knowledge of ISR – process: Radiator function )M,,H(Q 22
φθ
MC generator: Phokhara
“Radiative Return” to () resonance e+e- () +-
(H. Czyz, A. Grzelinska, J.H. Kühn, G. Rodrigo,
hep-ph/0308312)
R measurements at e+e- colliders
D. Leone
Diff. Cross Section d(+-)/dM
2
Obs. Spectrum dN/dM2
141 pb-1 of 2001 data
Diff. Cross Section d(+-)/dM
2
Ana
lysi
s F
low
Divide by H function
Event analysis
Divide by luminosity
VP, FSR correctionsVP, FSR corrections
50o<<130o
< 150> 1650
Drift ChamberEM Calorimeter
High statistics for ISR photons Low relative contribution from FSR Low relative background
50o < < 130o, < 15o and > 165o
No photon tagging
Analysis overview
R measurements at e+e- colliders
D. Leone
Luminosity measurement
KLOE uses large angle Bhabha events for the luminosity evaluation:
)1( BkgMC
NdtL
effective cross section, given by theoretical generator
interfaced with our simulation detector program
Precision determined byknowledge of radiative corrections
precision in collecting large angle Bhabha
Systematics on LuminosityTheory 0.5%AcceptanceBackgroundTrigger+Tracks+ClusteringKnowledge of s run-by-run
0.2%0.1%0.2%0.1%
TOTAL 0.5 % theory 0.3% exp = 0.6 %
2 independent generators used for radiative corrections:
BABAYAGA (Pavia group): eff = (428.80.3stat) nb
BHAGENF (Berends modified): eff = (428.50.3stat) nb
claimed by generator’s authors
Experimental systematic error are determinedby comparing data and MC angular andmomentum distribution
Polar Angle [°]
55o 125o
R measurements at e+e- colliders
D. Leone
e+e- spectrum
Statistics: 141pb-1 of 2001-Data 1.5 Million Events
interference
Efficiencies:- Trigger & Cosmic veto- Tracking, Vertex- - e- separation- Reconstruction filter- Trackmass-cut- Unfolding resolution- Acceptance
Background: - e+ eee- e+ e- -
Luminosity: 0.6%
1.0 %
0.5 %
Errors
R measurements at e+e- colliders
F=1, Phokhara
D. Leone
Final spectrum
To get the cross section for e+e- - division by radiator function H ISR-process calculated at NLO-level- radiative corrections: i) bare cross section subtraction of vacuum polarization contribution
ii) FSR corrections
Radiator function
)(Mdσ
)(Mdσ)(MFσ
2ππ1Fππγ,
2ππππγ22
πππππ
π
)(Mdσ
)(Mdσ)(MFσ
2ππ1Fππγ,
2ππππγ22
πππππ
π
R measurements at e+e- colliders
nb
e-
e-
e+
F(Q2)
e+
D. Leone
line = approach 1 + = approach 2
M GeV2 2( )
Ratio = approach 1 / approach 2
Pion Formfactor after FSR corrections
Both methods in excellent agreement!For the error on the model dependence ofFSR (scalar QED) we take 0.5%
R measurements at e+e- colliders
2 approaches for FSR corrections: • Approach 1: “exclusive NLO-FSR“ - Correcting measured for FSR - Use MC with pure ISR in analysis - Add FSR-contrib. to (ca. 0.8%) by hand
• Approach 2: “inclusive NLO-FSR“ - Correct for „unshifting“, i.e. s‘ Mpp
- Use MC with ISR + FSR in analysis
D. Leone
Summary on systematic error
Experiment
TOTAL 1.2%
The systematic error hascontributions from:
Theory– Radiator Function H 0.5%– Vacuum Polarization 0.2%– Luminosity 0.6%– FSR resummation 0.5%
TOTAL 1.0%
Calculating the dispersion integral:
ahad-(0.35< M
< 0.95 GeV2) = (389.2 0.8stat 4.7syst 3.9theo) 10-
10
ahad-(0.37< M
2< 0.93GeV2) =
(376.5 0.8stat 5.9syst+theo) 10-10 KLOE
(378.6 2.7stat 2.3syst+theo) 10-10 CMD-2
both measurement are in agreement within the errors e+e- - discrepancy for a is confirmed
R measurements at e+e- colliders
D. Leone
ISR method @ BABAR
e- (9 GeV)e+ (3 GeV)
ISR
f = hadrons or
)dL(s
1
)δ(1ε
dN)(sσ
dN
)δ(1ε
)δ(1ε
dN)(sσ
'fμμf
fγ'μμee
μμγ
radμμμμ
fμμf
fγ'f
s’ is the effective energy
– e+e- f many channels: , KK, pp, 0 , 4, 5, 6, , KK, KK, 2K2K, KK–R will be reconstructed from the sum of exclusive channels up to 2.5 GeV– inclusive approach also studied, limited by photon energy resolution for lower recoil masses
up to now very few data with large errors for s 1.4 GeV: R ~ 10-15%
data from different experiments and different systematics
R measurements at e+e- colliders
D. LeoneR measurements at e+e- colliders
e+e- → + @ BABARevents
/0.0
25
GeV
4 invariant mass distribution
data
ISR bkgd
non-ISR bkgd
ISR photon + 4 charged hadrons
1C fit in 4 hypothesis
perspectives on R measurements by BABAR:• comparable accuracy w.r.t. CMD-2 and KLOE for s < 1
GeV • few % accuracy for 1 < s < 3 GeV
BABARPRELIMINARY
D. LeoneR measurements at e+e- colliders
CONCLUSION
CMD2 and SND experiments at VEPP-2M and KLOE at DANE have improved the accuracy on R respectively in the regions E < 1.4 GeV and E < 1 GeV, the first one using the energy scan and the last one the ISR method. Their results are in agreement
BESII at BEPC has improved quite a lot the accuracy on R in the region 2<E<5 GeV
Other improvements are expected by BABAR in the region 1-3.5 GeV, using ISR
D. Leone
Riserva
R measurements at e+e- colliders
D. Leone
The VEPP-2M collider
Data collected by CMD-2 :
• 2107 decays
• 5106 + - events
• 3106 decays
• 2107 multi-hadrons
VEPP-2M parameters:
Beam energy 2(180-700) MeV
Peak luminosity 31030 cm-2 s-1
Interaction regions 2
Years of activity 1974-2000
R measurements at e+e- colliders
D. Leone
The CMD-2 detector
Drift chamber:R-m z=5 mm = 1.5 10-2 = 7 10-3
dE/dx = 0.2 Ep/p=(90 p2[GeV] +7)1/2 %
Calorimeter:Barrel:E/E = 8% E 100700 MeV=0.02 radiant
Endacap:E/E = 4 8 % E 100700 MeV=0.02 0.03 radiant Solid angle covered by cal:= 0.92 4 steradiant
R measurements at e+e- colliders
D. Leone
Energy resolution:
Angular resolution:
4
E
E(GeV)
4.2%
E
σ
00
φ 0.63E(GeV)
0.82σ
SND detector
1-vacuum chamber; 2 – drift chambers; 3 – internal scintillating counter; 6-NaI crystals; 7-vacuum phototriodes; 8-absorber; 9-streamer tubes; 11- scintillator plates;
R measurements at e+e- colliders
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The BEPC complex
e+e- machine in 2-5 GeV
the only e+e machine in2-5 GeV since 1989 L ~ 51030 cm-2 s-1 at theJ/peak
R measurements at e+e- colliders
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The BES detector
R measurements at e+e- colliders
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PEP II & BABAR
Design Performance
Peak Lumi 31033cm-2s-1 4.51033cm-2s-1
Lumi/day 135 pb-1 303 pb-1
Tracking: (pt)/pt = 0.13% pt 0.45% pt in GeV
EMC: E/E = 2.30 % E-1/4 1.35 %
R measurements at e+e- colliders
D. Leone
FSR treatment 1/2
Two complementary procedures to correct for FSR
FSR – exclusive approach FSR – inclusive approach
N(e+e- ISRFSR)Subtract FSR contribution Add back FSR contribution
Event analysis – Phokhara3 ISR Event analysis – Phokhara3 ISR+FSR
Division by Luminosity
(e+e- ISR (e+e- ISRFSR)
Division by radiator function H
(e+e-
(e+e- FSR)
Add back FSR by hand(sQED, Schwinger 1990)
Map M2(FSR) to M2
using MonteCarlo
R measurements at e+e- colliders
DANE & KLOE
e+e- collider @ S = M = 1019.4 MeV
achieved peak Luminosity: 8 1031 cm-2 s-1
2000-2002 data set: 500 pb-1
KLOE detector designed for CP violation studies good time resolution t= 57 ps /E(GeV) 50 psin calorimeter andhigh resolution drift chamber (p/p is 0.4 % for > 45°), ideal for the measurement of M.
pb
-
1
R measurements at e+e- colliders D. Leone
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