M.Davier BaBar pi pi Tau08 22-25/9/2008 1

Precision Measurement of the e+e+() Cross Section with the

ISR Method Michel Davier (LAL – Orsay)

(on behalf of the BaBar Collaboration)

• physics goals • analysis steps• e+e+()• eediscussion

M.Davier BaBar pi pi Tau08 22-25/9/2008 2

Hadronic Vacuum Polarization and Muon (g –2)Contributions to the Standard Model (SM) Prediction: ha weakQED d2

2

ga aaa

2

2had

24

( )

( )3

m

a R sK

ss

ds

”Dispersion relation“

had

had

...

Dominant uncertainty from lowest order hadronic piece. Cannot be calculated from QCD (“first principles”) – but: we can use experiment

2(0)Born: ( ) ( ) / ( )s s s

Im[ ] | hadrons |2

)( ])( hadrons[ )(Im 120

sReespt

M.Davier BaBar pi pi Tau08 22-25/9/2008 3

Measure R = pt (also RKK) with high accuracy for vacuum polarization calculations, using the ISR method

channel contributes 73% of ahad

Dominant uncertainty also from Also important to increase precision on (MZ

2) (EW tests, ILC) Present systematic precision of ee experiments CMD-2 0.8% SND 1.5% in agreement KLOE (ISR from 1.02 GeV) 1.3% some deviation in shape Big advantage of ISR: all mass spectrum covered at once, from

threshold to 4-5 GeV, with same detector and analysis Interesting to compare to spectral functions from decays discrepancy /e+e evaluations (3.0 1.1)%

aim for a measurement with <1% accuracy

Goals of the Analysis

M.Davier BaBar pi pi Tau08 22-25/9/2008 4

Situation at ICHEP-Tau06a

had [ee ] = (690.9 ± 4.4) 10 –10

a [ee ] = (11 659 180.5 ± 4.4had ± 3.5LBL ± 0.2QED+EW) 10 –10

Hadronic HO – ( 9.8 ± 0.1) 10 –10

Hadronic LBL + (12.0 ± 3.5) 10 –10

Electroweak (15.4 ± 0.2) 10 –10

QED (11 658 471.9 ± 0.1) 10 –10

inclu-ding:

a [exp ] – a [SM ] = (27.5 ± 8.4) 10 –10

3.3 „standard deviations“

Observed Difference with BNL using e+e:

BNL E821 (2004):a

exp = (11 659 208.0 6.3) 10 10

Knecht-Nyffeler, Phys.Rev.Lett. 88 (2002) 071802

Melnikov-Vainshtein, hep-ph/0312226

.0

Davier-Marciano, Ann. Rev. Nucl. Part. Sc. (2004)

Kinoshita-Nio (2006)

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ISR FSR

ISR + add. ISR ISR + add. FSR

The Relevant Processes

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ISR photon at large angle in EMC 1 (for efficiency) or 2 (for physics) tracks of good quality identification of the charged particles separate /KK/ event samples kinematic fit (not using ISR photon energy) including 1 additional photon obtain all efficiencies (trigger, filter, tracking, ID, fit) from same data measure ratio of to cross sections to cancel ee luminosity additional ISR vacuum polarization ISR photon efficiency still need to correct for |FSR|2 contribution in and additional FSR, both calculated in QED, but also checked in data (ISR-FSR interference, additional detected photons)

)1( )1(

)( )]()([ )1(

)]()([ )]()([ )]()([ )(R 0

0

exp

addFSRFSRFSR

sRs

ssss

otherwise 3-4% syst error

The Measurement

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PEP-II is an asymmetric e+e collider operating at CM energy of (4S).

Integrated luminosity = 531 fb-1

BaBar EMC: 6580 CsI(Tl) crystals,

resolution ~1-2 % high E.

BaBar DIRC• particle ID up to 4-5 GeV/c

BaBar IFR: resistive plate chambers

BaBar / PEP II

BaBar SVT and DCH• precision tracking

M.Davier BaBar pi pi Tau08 22-25/9/2008 8

230.8 fb-1 4S) on-peak & off peak)

Geometrical acceptance Triggers (L1 hardware, L3 software), background-filter efficiencies Tracking efficiency Particle ID matrix (ID and mis-ID efficiencies) K Kinematic fitting reduce non 2-body backgrounds 2 cut efficiency additional radiation (ISR and FSR) secondary interactions

Unfolding of mass spectraConsistency checks for (QED test, ISR luminosity) and Unblinding R Results on cross section and calculation of dispersion integral

Analysis Steps

M.Davier BaBar pi pi Tau08 22-25/9/2008 9

Acceptance and efficiencies determined initially from simulation, with data/MC corrections applied Large simulated samples, typically 10 data, using AfkQed generator

AfkQed: lowest-order QED with additional radiation: ISR with structure function method, assumed collinear to the beams and with limited energy FSR using PHOTOS Phokhara 4.0: exact second-order QED matrix element, no more radiation

Studies comparing Phokhara and AfkQed at 4-vector level with fast simulation

MC Generators

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tag particle (track, ID)

candidate (p, , φ)

γISR

benefit from pair production for particle ID kinematically constrained events efficiency automatically averaged over running periods measurement in the same environment as for physics, in fact same events! applied to particle ID with samples, tracking, study of secondary interactions… assumes that efficiencies of the 2 particles are uncorrelated in practice not true this is where 95% of the work goes! study of 2-particle overlap in the detector (trigger,tracking, EMC, IFR) required a large effort to reach per mil accuracies (hence the duration of the analysis)

Particle-related Efficiency Measurements

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MC

data

MCtrack

datatrack

track ff

ffC

)1(

)1(

30

30

2

m

trackC1

trackC1

m

and similarly for

single track efficiency correlated loss probability f0

probability to produce more than 2 tracks f3

Data/MC Tracking Correction to cross sections

M.Davier BaBar pi pi Tau08 22-25/9/2008 12

Particle identification required

to separate XX final processes Define 5 ID classes using cuts and

PID selectors (complete and

orthogonal set) Electrons rejected at track definition

level (Ecal, dE/dx) All ID efficiencies measured

xI

a tighter ID (h) is used for taggingin efficiency measurements and to further reject background in low crosssection regions.

Particle Identification

Barrel

Backward EndcapForward Endcap

ZIFR

IFRXIFR

YIFR

* isolated muons M> 2.5 GeV efficiency maps (p,v1,v2) impurity (1.10.1) 103

* correlated efficiencies/close tracks maps (dv1,dv2)

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Data/MC PID corrections to and cross sections

Two runningperiods withdifferent IFRperformance

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PID separation and Global Test

All ‘xx’ solve for all xx(0) and compare with no-ID spectrum and estimated syst. error

hist: predicted from PIDdots: measured (no ID)

m(GeV)

N(o)ii

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Kinematic Fitting

kinematic fits to X X ISR add

ISR photon defined as highest energy

Add. ISR fit: add assumed along beams

Add. ‘FSR’ if add detected

Each event recorded on 2D plot

Typical regions defined

Loose 2 cut (outside BG region in plot) for and in central region

Tight 2 cut (ln(2+1)<3) for in tail region

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Backgrounds

background larger with loose 2 cut used in 0.5-1.0 GeV mass range q q and multi-hadronic ISR background from MC samples + normalization from data using signals from 0ISR (qq), and and (0) global test in background-rich region near cut boundary

m (GeV)

multi-hadrons

Fitted BG/predicted = 0.9680.037BG fractions in 10-3 at m values

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Additional ISR

Angular distributionof add. ISR /beams!

Energy cut-off foradd. ISR in AfkQed

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Additional FSR

Angle between add and closest track

Large-angle add.ISR in data AfkQed

Evidence for FSR data AfkQed

FSR

ISR

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2 cut Efficiency Correction

depends on simulation of ISR (FSR), resolution effects (mostly ISR direction) for and

2 cut efficiency can be well measured in data because of low background

main correction from lack of angular distribution for additional ISR in AfkQed common correction: 1% for loose 2, 7% for tight

additional loss for because of interactions studied with sample of interacting events agreement data/MC

loose

1.00 0.16 loose 2

1.07 0.11 tight 2

syst error 0.5-1.5 10-3

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Checking Known Distributions

Cos* in XX CM /

flat at threshold1+cos2*

sin2*

P>1 GeV track requirement loss at cos*1

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QED Test with sample

(0.2 – 5.0 GeV)

ISR efficiency 5.2 syst.trig/track/PID 4.0

BaBar ee luminosity

absolute comparison of mass spectra in data and in simulation

simulation corrected for data/MC efficiencies

AfkQed corrected for incorrect NLO using Phokhara

results for different running periods consistent: (7.9 7.5) 10-3

full statistics

J/excluded

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Unfolding Mass Spectrum

measured mass spectrum distorted by resolution effects and FSR (m vs. s’) unfolding uses mass-transfer matrix from simulation 2 MeV bins in 0.5-1.0 GeV mass range, 10 MeV bins outside most salient effect in - interference region (little effect on a

)

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Systematic Uncertainties

increased to 20. for preliminary results, pending investigations

ISR lumifitted w.r.t.LO formula

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BaBar results

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BaBar results in region

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BaBar vs. other experiments at large mass

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BaBar vs.other ee data (0.5-1.0 GeV)

CMD-2

SND KLOE

direct relative comparison of crosssections in the corresponding 2-MeVBaBar bins (interpolation with 2 bins)

deviation from 1 of ratio w.r.t. BaBar

stat + syst errors included

M.Davier BaBar pi pi Tau08 22-25/9/2008 28

BaBar vs.other ee data interference region)

CMD-2 SND

mass calibration of BaBar checked with ISR-produced J/expect (0.16 0.16) MeV at peak mass can be determined through mass distribution fit (in progress) Novosibirsk data precisely calibrated using resonant depolarization comparison BaBar/CMD-2/SND in - interference region shows no evidence for a mass shift

M.Davier BaBar pi pi Tau08 22-25/9/2008 29

BaBar vs. IB-corrected data (0.5-1.0 GeV)

relative comparison w.r.t. BaBar ofisospin-breaking corrected spectralfunctions

BaBar data averaged in wider binsand corrected for - interference

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Computing a

FSR correction was missing in Belle, new value 523.5 3.0 2.5

ALEPH-CLEO-OPAL (DEHZ 2006) (DEHZ 2003) (2008)

Direct comparison 0.630-0.958 GeV BaBar 369.3 0.8 2.2 CMD-2 94-95 362.1 2.4 2.2 CMD-2 98 361.5 1.7 2.9 SND 361.0 1.2 4.7

a(1010)

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Conclusions

BaBar analysis of andISR processes completed Precision goal has been achieved: 0.6% in region (0.6-0.9 GeV) Absolute cross section agrees with NLO QED within 1.2% Preliminary results available for in the range 0.5-3 GeV Structures observed in pion form factor at large masses

Comparison with results from earlier experiments discrepancy with CMD-2 and SND mostly below large disagreement with KLOE better agreement with results, especially BelleContribution to a from BaBar agrees better with resultsDeviation between BNL measurement and theory prediction significantly reduced using BaBar dataa [exp ] – a [SM ]=(27.5 ± 8.4) 10 –10 (14.0 8.4) 1010

Wait for final results and contributions of multi-hadronic modes

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Backup Slides

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PID correction to cross section

Two runningperiods withdifferent IFRperformance

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• ‘’ ID is a set of negative conditions

• use sample from ISR-produced with h tag: 0.6<m<0.9 GeV impurity = (3.70.5) 103

• ID and mis-ID efficiencies stored in 2D maps

• unlike muons, efficiency sample is not from isolated tracks

• biases from tagging and correlated loss studied with MC 10-3 level

Measurement of -ID efficiencies

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PID correction to cross section

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2 cut Efficiency Correction: Interactions

loose

tight

secondary interactions mostly from beam pipe (tight doca cut on tracks)

tag events with interactions using displaced vertex with a ‘bad’ track in transverse plane (Rxy)

agreement data/MC 1.00 0.16 loose 1.07 0.11 tight syst error 0.5-1.5 10-3

Beam pipe

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Mass Calibration

Effect of a 1 MeV mass scale shift (calibration with J/ : (0.16 0.16) MeV)

m (GeV)

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Present BaBar Measurements

to obtain R in the energy range 1-2 GeV the processes K+K-, KSKL, KSKL KSK+

remain to be measured

only statistical errors

syst. 5-10%

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BaBar Impact on Multi-Hadronic Modes

• Using ISR method the cross sections of e+e , , K+K, 2K+2K reactions have been measured from threshold to 4.5 GeV.

• These are the most precise measurements to date for c.m. energies greater than 1.4 GeV.

• Examples: contributions to ahad (1010) from 2+ 2 (0.56 – 1.8 GeV)

from all e+ e exp. 14.21 0.87exp 0.23rad from all data 12.35 0.96exp 0.40SU(2)

from BaBar 12.95 0.64exp 0.13rad

from + 0 (1.055 1.8 GeV)

from all e+ e exp. 2.45 0.26exp 0.03rad

from BaBar 3.31 0.13exp 0.03rad

• More modes to come

Davier-Eidelman-Hoecker-Zhang 2003

total 696.37.2