Stefan E. Müller Laboratori Nazionali di Frascati

A precise KLOE measurement of (e+e-+-) and extraction of a 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 State

Radiation (FSR)• large integrated luminosity

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

ss

(e+e-+) with ISR:Particle factories have the opportunity to measure the cross section (e+ e- ) as a function of the pionic c.m. energy s = M2

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

s ·d(e+ e- + )

ds(e+ e- ) H(s,s)

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

2

Neglecting

FSR

DEAR,

,SIDDHARTA

e+e- - collider with =m1.0195 GeVs

DANE: A -Factory

Integrated Luminosity

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

Total KLOE int. Luminosity:

L dt ~ 2500 pb1 (2001 - 05)

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

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

3

KLOE DetectorDriftchamber

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

4

KLOE DetectorElectromagnetic Calorimeter

E/E = 5.7% / E(GeV)

T = 54 ps / E(GeV) 50 ps(Bunch length contribution subtracted from constant term)

Excellent timing resolution

E/E = 5.7% / E(GeV)

T = 54 ps / E(GeV) 50 ps(Bunch length contribution subtracted from constant term)

Excellent timing resolution

5

QuickTime™ and a decompressor

are needed to see this picture.

€

rp=

rpmiss=−(

rp+ +

rp−)

statistics: 242pb-1

3.5 Million Events

high statistics for ISR events low relative FSR contribution suppression of background

Event Selection2 pion tracks at large angles

50o< <130o

Photons at small angles < 15o or > 165o

photon momentum from kinematics:

6

Event selection

To further clean the samples from radiative Bhabha events, we use a particle ID estimator for each charged track based on Calorimeter Information and Time-of-Flight.

• Experimental challenge: control backgrounds from

– – ee ee – ee ,

removed using kinematical cuts in trackmass MTrk and Missing Mass Mmiss

defined by 4-momentum conservation under the hypothesis of eex

€

Mmiss= EX2 −pX

2

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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Mmiss:

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

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 (ss0

from F. Jegerlehner

ii) FSR Cross section must be incl. for FSR

for use in the dispersion integral of a

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

in the passage ss

FSR contr. (sQED):

Net effect of FSR is ca. 0.8%

€

s ⋅dσ ππγ

dsπ

= σ ππ (sπ ) × H(s,s)

ss

8

Vac. Pol. corr:

Radiator:

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

KLOE measures L with Bhabha scattering55° < < 125°

acollinearity < 9°

p 400 MeV

e

e

F. Ambrosino et al. (KLOE Coll.)

Eur.Phys.J.C47:589-596,2006

generator used forgenerator used for effeff

BABAYAGA (Pavia group):

C. M.C. Calame et al., C. M.C. Calame et al., NPB584 (2000) 459

C. M.C. Calame et al., C. M.C. Calame et al., NPB758 (2006) 22

new version (BABAYAGA@NLO) gives 0.7% decrease in cross section,

and better accuracy: 0.1%

Systematics on Luminosity

Theory 0.1 %

Experiment 0.3 %

TOTAL 0.1 % th 0.3% exp = 0.3%

Luminosity:9

New:

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

improved offline-event filter reduces systematic uncertainty to < 0.1%

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

improved machine conditions (luminosity and background) in the new data set

Reconstruction filter 0.6%

Background 0.3%

Mtrk cuts 0.2%

Particle ID 0.1%

Tracking 0.3%

Vertex 0.3%

Trigger 0.3%

Acceptance 0.3%

Unfolding 0.2%

Luminosity (0.5th 0.3exp)%

0.6%

improvements (vertexrequirement dropped) in the selection

error table on exp. systematics in the published work

€

ddsπ

= Nobs −Nbkg

Δsπ

⋅1

εsel

⋅1

L

€

ddsπ

= Nobs −Nbkg

Δsπ

⋅1

εsel

⋅1

L

PLB606(2005)12

Improvements 10

total,2005= 1.3%

New result from 2002 data:

Offline Filter negligible

Background 0.6%

Trackmass/Miss. Mass

0.2%

/e-ID 0.3%

Tracking 0.3%

Trigger 0.1%

Acceptance (Miss) 0.1%

Acceptance () negligible

Unfolding 0.2%

Software Trigger 0.1%

Luminosity(0.1th 0.3exp)%

0.3%

Radiator H(0.5th 0.2exp)%

0.5%

Vacuum polarization

0.1%

Systematic errors on a:

Total= 1.0%

€

(sπ ) =πα 2βπ

3

3sFπ (sπ )

2

experimental fractional error on a = 0.9 %

stat. error only

KLOE 2008

11

Evaluating a

2005 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 0.8stat 3.5sys3.5theo) · 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 0.8stat3.5sys3.0theo) · 10-10

2008:

a(0.35-0.95GeV2) = (389.4 0.6stat3.3sys 2.0theo) · 10-10a(0.35-0.95GeV2) = (389.4 0.6stat3.3sys 2.0theo) · 10-10

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Summary of the results:

a Summary13

KLOE result in agreement with CMD2 and SND

a comparisonComparison with a from CMD2 and SND in the range

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

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|F|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. error shown)

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

Large angle analysis:

Threshold region non-trivial due to irreducible FSR-effects, which have to be estimated from MC using phenomenological models (interference effects unknown)

ff

FSR f0

important! small!

2002 data 485000 eventsL = 240 pb-1

s [GeV2]

Nr.

of

ev

en

ts /

0.0

1 G

eV

2

Prelim

inar

y

independent complementary analysis threshold region (2m)2 accessible ISR photon detected (4-momentum constraints) lower signal statistics larger contribution from FSR events large background contamination irreducible background from decays (f0)

Pion tracks: 50o< <130o

At least 1 photon with 50o< <130o and EMeV photon detected

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Large angle analysis:

2002 Data L = 240 pb-1

s [GeV2]

Dedicated 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 contstraints

• 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

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 )

Difference between the MC models contributes to systematic uncertainty

17

•

Conclusions and Outlook:18

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

We are also measuring the pion form factor using the bin-by-bin ratio of pionover muon (instead of using absolute normalization with Bhabhas).

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

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

Backup:

Comparing 2008 with 2005 result:

stat. error only