Stefan E. MüllerLaboratori Nazionali di Frascati

A precise KLOE measurement of (e+e-+-)and extraction of aµ between 0.35 and 0.95 GeV2

14. International QCD Conference (QCD08)

Montpellier, 7-12 July 2008

(for the KLOE collaboration)

Requirements:• precise evaluation of radiator function• good suppression, or understanding, of Final StateRadiation (FSR)• large integrated luminosity

Advantages:• overall energy scale √ s is well known and applies toall values of s• syst. errors from luminosity, √ s, rad. corrections enteronly once, don’t have to be studied for each point of s• data comes as together with the standard physicsprogram of the experiment

EVA + PHOKHARA MC Generator(S.Binner, J.H.Kühn, K.Melnikov, PLB459,1999)(H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC27,2003)

1st KLOE publication (based on 140 pb-1)A. Aloisio et al., PLB606(2005)12

π+

π−ss

σ(e+e-→π+π−) with ISR:Particle factories have the opportunity to measure the crosssection σ(e+ e- → +) as a function of the pionic c.m. energys = M2

using initial state radiation (radiative return toenergies below the collider energy s).

s · dσ(e+ e- → + + γ )ds

= σ(e+ e- → +) H(s, s)

requires precise calculation of the radiator H(s, s)

1

Neglecting

FSR

DEAR,

,SIDDHARTA

e+e- - collider with =mΦ≈1.0195 GeVs

DAΦNE: A Φ-Factory

Integrated Luminosity

Peak Luminosity Lpeak= 1.4 • 1032cm-2s-1

Total KLOE int. Luminosity: ∫L dt ~ 2500 pb−1 (2001 - 05)

2006:• Energy scan with 4 points around mΦ-peak• 225pb-1 at = 1 GeV s

New result is based on 240pb-1 from 2002 data

2

KLOE DetectorDriftchamber

σp/p = 0.4% (for 900 tracks)σxy ≈ 150 µm, σz ≈ 2 mm

Excellent momentum resolution

3

KLOE DetectorElectromagnetic Calorimeter

σE/E = 5.7% / √E(GeV)σT = 54 ps / √E(GeV) ⊕ 50 ps

(Bunch length contribution subtracted from constant term)

Excellent timing resolution

4

!

r p " =

r p miss = #(

r p + +

r p # )

statistics: 242pb-1

3.5 Million Events

high statistics for ISR events low relative FSR contribution suppression of φ → +0 background

Event Selection2 pion tracks at large angles

50o< θπ <130o

Photons at small angles θγ < 15o or θγ > 165o

photon momentum from kinematics:

5

Event selection

To further clean the samples fromradiative Bhabha events, we use aparticle ID estimator for each chargedtrack based on Calorimeter Informationand Time-of-Flight.

• Experimental challenge: control backgrounds from

– φ→ π+π−π0

– e+e− → e+e− γ– e+e− → µ+µ− γ,

removed using kinematical cuts in trackmass MTrk and Missing Mass Mmiss

!

s " p1

2+ Mtrk

2" p

2

2+ Mtrk

2( )2

" ( p1

+ p2)2

= 0

defined by 4-momentum conservation under the hypothesis of e+e−→π+π−x

!

Mmiss = EX2"pX

2

6

Mmiss:

defined by 4-momentum conservation under the hypothesis of 2 tracks with equal mass and one γ

MTrk:

s

Radiative corrections

- ISR-Process calculated at NLO-level PHOKHARA generator (H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC27,2003)

Precision: 0.5%

Radiator-Function H(s,s) (ISR):

Radiative Corrections:i) Bare Cross Section divide by Vacuum Polarisation (s)=((s)/(0))2

from F. Jegerlehnerii) FSR Cross section σππ must be incl. for FSR

for use in the dispersion integral of aµ

FSR corrections have to be taken into account in the efficiency eval. (SA Acceptance, MTrk) and

in the passage sπ → sγ∗

FSR contr. (sQED):

Net effect of FSR is ca. 0.8%

!

s "d#$$%

ds$=#$$ (s$ ) & H(s,sπ)

s > s

7

Vac. Pol. corr:

Radiator:

(H.Czyż, A.Grzelińska, J.H.Kühn, G.Rodrigo, EPJC33,2004)

KLOE measures L with Bhabha scattering

55° < θ < 125°acollinearity < 9°

p ≥ 400 MeV

e−

e+

γ

F. Ambrosino et al. (KLOE Coll.)Eur.Phys.J.C47:589-596,2006

generator used forgenerator used for σeffeff

BABAYAGA (Pavia group): C. M.C. Calame et al., C. M.C. Calame et al., NPB584 (2000) 459 C. M.C. C. M.C. Calame et Calame et al., al., NPB758 (2006) 22

new version (BABAYAGA@NLO) gives0.7% decrease in cross section,

and better accuracy: 0.1%

TOTAL 0.1 % th ⊕ 0.3% exp = 0.3%

0.3 %Experiment

0.1 %TheorySystematics on Luminosity

Luminosity:8

New:

30% inefficiency (veto of cosmic rays)recovered by introducing 3rd level trigger

improved offline-event filter reducessystematic uncertainty to < 0.1%

new generator BABAYAGA@NLO,error on σBhabha from 0.5% to 0.1%

improved machine conditions (luminosity andbackground) in the new data set

0.2%Unfolding

0.3%Tracking

0.6%Luminosity (0.5th ⊕ 0.3exp)%

0.3%Acceptance0.3%Trigger0.3%Vertex

0.1%Particle ID0.2%Mtrk cuts0.3%Background0.6%Reconstruction filter

improvements (vertexrequirement dropped) in the selection

error table on exp.systematics in thepublished work

!

d"##$

ds# =

Nobs %Nbkg

&s# '1

(sel

'1

L

PLB606(2005)12

Improvements9

total,2005 = 1.3%

New result from 2002 data:

0.1%Acceptance (Miss)

0.1%Software Trigger0.3%Luminosity(0.1th ⊕ 0.3exp)%

negligibleAcceptance ()0.2%Unfolding

0.1%Trigger0.3%Tracking0.3%/e-ID0.2%Trackmass/Miss. Mass0.6%Background

negligibleOffline Filter

0.1%Vacuum polarization0.5%Radiator H(0.5th ⊕ 0.2exp)%

Systematic errors on aµ:

Total = 1.0%

!

"## (s# ) =#$ 2%#

3

3sF# (s# )

2

experimental fractional error on aµ = 0.9 %

stat. error only

KLOE 2008

10

Evaluating aµ

2005 published result (Phys. Lett. B606 (2005) 12):

Applying update for trigger eff. and change in Bhabha-cross section used forluminosity evaluation:

aµ(0.35-0.95GeV2) = (388.7 ± 0.8stat± ±3.5sys±3.5theo) · 10-10

!

aµ

"" =1/4" 3ds#(e+

e$ %" +"$

) K(s)

0.35GeV2

0.95GeV2

&

Dispersion integral for 2π-channel in energy interval 0.35 <Mππ2<0.95 GeV2

aµ(0.35-0.95GeV2) = (384.4 ± 0.8stat±3.5sys±3.0theo) · 10-10

2008:

aµ(0.35-0.95GeV2) = (389.4 ± 0.6stat±3.3sys ±2.0theo) · 10-10

11

Summary of the results:

aµ Summary:

12

KLOE result in agreement with CMD2 and SND

aµ comparison:

Comparison with aµ from CMD2 and SND in the range

0.630-0.958 GeV :Phys. Lett. B648 (2007) 28

13

|F|2 comparison:

only statistical errors are shown

Light grey band: KLOE stat. errorDark grey band: KLOE total error (stat. + syst.)

Data points: CMD and SND results compared to KLOE value extrapolated to the

corresponding s (only stat. errorshown)

14

|F|2 |F|2 CMD,SND / |F|2 KLOE - 1

Large angle analysis:

Threshold region non-trivialdue to irreducible FSR-effects,which have to be estimated fromMC using phenomenologicalmodels (interference effectsunknown)

φ, ρφ, ρ

ππ

ππγγ φφ

ff00

γγ

ππ

ππ

φφ

ρρ

ππ

ππ

γγ

& &

FSR f0 ρπ

important! small!

2002 data 485000 eventsL = 240 pb-1

s [GeV2]

Nr.

of e

vent

s / 0

.01

GeV

2

Prelim

inary independent complementary analysis threshold region (2mπ)2 accessible γISR photon detected (4-momentum constraints)

lower signal statistics larger contribution from FSR events large φ → π+π−π0 background contamination irreducible background from φ decays (φ → f0 γ → ππ γ)

Pion tracks: 50o< θπ <130o

At least 1 photon with 50o< θγ <130o and E > 50 MeV photon detected

15

Large angle analysis:

2002 Data L = 240 pb-1

sπ [GeV2]

Dedicated selection cuts:• Exploit kinematic closure of the event Cut on angle Ω btw. ISR-photon and missing momentum

• Kinematic fit in π+π−π0 hypothesis using 4-momentum and π0-mass as contstraints

• FSR contribution added back to cross section (estimated from PHOKHARA generator)

π+

π−

misspr

ãpr

Ω

γ

Error on LA dominatedby syst. uncertainty onf0 contr.

LA stat.+syst.error

• Reducible Background from π+π−π0 and µ+µ−γ well simulated by MC

Preliminary

Dominating uncertainty from model dependence of irreducible background f0 +.Using different models for f0-decay and using input from dedicated KLOE f0 analyses(with f0+ and f0 00)

Difference between the MC models contributes to systematic uncertainty

16

•

Conclusions and Outlook:17

• The result from new data agrees with the updated result from the published KLOE analysis• KLOE results also agree with recent results on aµ

from the CMD2 and SND experiments at VEPP-2M in Novosibirsk• better agreement in spectrum between different experiments than in the past

We have evaluated the contribution to aµ in the range between

0.35 - 0.95 GeV2 using cross section data obtained via ISR events with photon emission at small angles.

Independent analysis is in progress using detected photons emitted at large angle

• Syst. uncertainty dominated by model-dependence of scalar mesonsWe are also measuring the pion form factor using the bin-by-bin ratio of pionover muon yields (instead of using absolute normalization with Bhabhas).

Finally we are measuring pion form factor from data taken at √s= Mφ - 20 MeV (1000 MeV)

• suppression of background from -decays• determination of f0-parameters