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

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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

D. Leone

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

D. Leone

The BES detector

R measurements at e+e- colliders

D. Leone

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