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ISR cross sections in e + e - and implications on muon g-2. Bogdan MALAESCU ( LAL – Orsay ) ( Representing the BaBar Collaboration ). Outlook. The muon magnetic anomaly The BaBar ISR (Initial State Radiation) analysis Test of the method: e + e   +   ()

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isr cross sections in e e and implications on muon g 2

ISR cross sections in e+e- and implications on muon g-2

Bogdan MALAESCU

( LAL – Orsay )

( Representing the BaBar Collaboration )

HEP 2010

outlook
Outlook
  • The muon magnetic anomaly
  • The BaBar ISR (Initial State Radiation) analysis
  • Test of the method: e+e+()
  • Results on e+e+ () (PRL 103, 231801  (2009))
  • Combination of all e+e data
  • Discussion and conclusion

HEP 2010

slide3

Lepton Magnetic Anomaly: from Dirac to QED

Dirac (1928) ge=2 ae=0

anomaly discovered:

Kusch-Foley (1948) ae= (1.19  0.05) 103

and explained by O() QED contribution:

Schwinger (1948) ae = /2 = 1.16 103

first triumph of QED

 ae sensitive to quantum fluctuations of fields

HEP 2010

slide4

More Quantum Fluctuations

+ ? a new physics ?

typical contributions:

QED up to O(4), 5 in progress (Kinoshita et al.)

Hadronsvacuum polarizationlight-by-light (models)

Electroweak new physics at high mass scale

 a much more sensitive to high scales

HEP 2010

slide5

Hadronic Vacuum Polarization and Muon (g–2)

Dominant uncertainty from lowest-order HVP piece

Cannot be calculated from QCD (low mass scale), but one can use experimental

data on e+e hadrons cross section

Im[ ]  | hadrons |2

had

Dispersion relation

HEP 2010

slide6

The E-821 Direct a Measurement at BNL*

Storage ring technique pioneered at CERN (Farley-Picasso…, 1970s), with μ+ and μ- data

a obtained from a ratio of frequencies

result updated with new value for /p (+0.9 1010)

(see new review in RPP2009 (Hoecker-Marciano))

aexp = (11 659 208.9  5.4  3.3) 1010

( 6.3) (0.54 ppm)

precessionrotation

*Bennet et al., 2006, Phys. Rev. D 73, 072003

HEP 2010

slide7

Goals of the BaBar Analysis

  • Measure [e+ e +  (FSR)] with high accuracy for vacuum polarization calculations, using the ISR method e+ e +  ISR (add.)
  •  channel contributes 73% of ahad
  • Dominant uncertainty also from 
  • Present systematic precision of ee experiments

CMD-2 0.8% SND 1.5% in agreement

KLOE (ISR from 1.02 GeV) 2005 1.3% some deviation in shape

2008 0.9% better agreement

  • Big advantage of ISR: all mass spectrum covered at once (from threshold to 3 GeV in BABAR), with same detector and analysis
  • The ISR method needs a very large amount of data at an energy larger than the one at which we measure the cross section
  • Measure simultaneously +  ISR (add.) and +  ISR (add.)
  • Compare the measured +  ISR (add.) spectrum with QED prediction
  • Compare to spectral functions from previous e+ e data and  decays

 aim for a measurement with <1% accuracy (syst. errors at per mil level)

great interest to clarify the situation as magnitude of possible discrepancy

with SM is of the order of SUSY contributions with masses of a few 100 GeV

HEP 2010

slide8

The Relevant Processes

e+ e +  ISR (add.) and +  ISR (add.) measured simultaneously

ISR

FSR

LO FSR negligible for pp

at s(10.6 GeV)2

ISR + add. ISR

ISR + add. FSR

HEP 2010

slide9

BaBar / PEP II

BaBar EMC:

  • 6580 CsI(Tl) crystals, resolution ~1-2 % high E.
  • PEP-II is an asymmetric e+ecollider operating at CM energy of (4S).
  • Integrated luminosity = 531 fb-1

BaBar IFR:

  • resistive plate chambers
  • BaBar DIRC
  • particle ID up to 4-5 GeV/c
  • BaBar SVT and DCH
  • precision tracking

HEP 2010

slide10

The Measurement

  • ISR photon at large angle, detected in the EMC
  • detected tracks of good quality: 1 (for efficiency) or 2 (for physics)
  • identification of the charged particles (DIRC, DCH)
  • separate pp/KK/mm event samples
  • kinematic fit (not using ISR photon energy) including 1 additional photon (discussed

later)

  • obtain all efficiencies (trigger, filter, tracking, ID, fit) from same data
  • measure ratio of ppg(g) to mmg(g) cross sections to cancel

ee luminosity

additional ISR

vacuum polarization

ISR photon efficiency

  • correct for LO FSR (|FSR|2) contribution in mmg(g) (QED, <1% below 1 GeV)
  • additional FSR photons measured

otherwise ~2% syst error

HEP 2010

slide11

Analysis Steps

Used an integrated luminosity of 232 fb-1 (U(4S) on-peak & off peak)

  • Geometrical acceptance (using Monte Carlo simulation)
  • All efficiencies measured on data (data/MC corrections)
  • 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 spectra
  • Consistency checks for  (QED test, ISR luminosity) and 
  • Unblinding R  partial (√s`>0.5 GeV) preliminary results (Tau08, Sept. 2008)
  • Additional studies and checks
  • Final results on  cross section and calculation of dispersion integral

(PRL 103, 231801  (2009))

HEP 2010

slide12

MC Generators

  • Acceptance and efficiencies determined initially from simulation,

with data/MC corrections applied

  • Large simulated samples, typically 10  data, using AfkQed generator
  • AfkQed: lowest-order (LO) QED with additional radiation:

ISR with structure function method,  assumed collinear to the beams and

with limited energy

FSR using PHOTOS

  • Phokhara 4.0: (almost) exact second-order QED matrix element, limited to NLO
  • Studies comparing Phokhara and AfkQed at 4-vector level with fast simulation
  • QED test with    () cross section requires reliable NLO generator
  • (FSR) cross section obtained through () / () ratio, rather insensitive to

detailed description of radiation in MC

HEP 2010

slide13

Particle-related Efficiency Measurements

tag particle (track, ID)

γISR

candidate (p, , φ)

  • benefit from pair production for tracking and 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 p/K/m samples, tracking…
  • assumes that efficiencies of the 2 particles are uncorrelated
  • in practice not true  study of 2-particle overlap in the detector

(trigger,tracking, EMC, IFR) required a large effort to reach per mil accuracies

HEP 2010

slide14

Kinematic Fitting

  • First analysis to measure cross section
  • with additional photons (NLO)
  • Two 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

  • Loose 2 cut (outside BG region in plot)

for  and  in central  region

  • Tight 2 cut (ln(2+1)add.ISR<3)

for  in  tail region

  • q q and multi-hadronic ISR background

from MC samples + normalization from

data using signals from 0ISR (qq),

and  and  (0)

ppg(g)

HEP 2010

slide15

QED Test with mmg sample

  • absolute comparison of mm mass spectra in data and in simulation
  • simulation corrected for data/MC efficiencies
  • AfkQed corrected for incomplete NLO using Phokhara
  • strong test (ISR probability drops out for  cross section)

BaBar

(0.2  3 GeV)

BaBar ee luminosity

ISR  efficiency 3.4 syst.

trig/track/PID 4.0

HEP 2010

slide16

Obtaining the (FSR) cross section

Acceptance from MC + data/MC corrections

Unfolded spectrum

Effective ISR luminosity from () analysis (similar equation + QED)

 mass spectrum unfolded (B. M. arXiv:0907.3791) for detector response

Additional ISR almost cancels in the procedure (() / () ratio)

Correction (2.5 1.0) 103  cross section does not rely on accurate

description of NLO in the MC generator

ISR luminosity from () in 50-MeV energy intervals

(small compared to variation of efficiency corrections)

HEP 2010

slide17

Systematic uncertainties

s` intervals (GeV) errors in 103

Dominated by particle ID (-ID, correlated ‘’, -ID in ISR luminosity)

HEP 2010

slide18

BaBar results (arXiv:0908.3589, PRL 103, 231801  (2009))

e+ e  +  (FSR) bare (no VP) cross section diagonal errors (stat+syst)

HEP 2010

slide19

BaBar results in  region

2-MeV energy intervals

HEP 2010

slide21

VDM fit of the pion form factor

 Running (VP)

add. FSR

BaBar Preliminary Fit

BaBar Preliminary Fit

BaBar Preliminary Fit

HEP 2010

slide22

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

direct relative comparison of cross sections with BaBar fit (stat + syst errors included)

(green band)

BaBar

CMD-2

BaBar Preliminary Fit

BaBar Preliminary Fit

SND

KLOE

BaBar Preliminary Fit

BaBar Preliminary Fit

HEP 2010

slide23

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

  • relative comparison w.r.t. BaBar of
  • spectral functions corrected for

isospin-breaking(IB)

  • IB corrections: radiative corr.,  masses,
  • - interference,  masses/widths
  • each  data normalized to its own BR

ALEPH

BaBar Preliminary Fit

CLEO

Belle

BaBar Preliminary Fit

BaBar Preliminary Fit

HEP 2010

slide24

Computing ampp

0.281.8 (GeV)

BABAR 514.1  3.8

previous e +e combined 503.5  3.5 *

 combined 515.2  3.5 *

*

*

* arXiv:0906.5443 M. Davier et al.

HEP 2010

slide25

Including BaBar in the e+e Combination

arXiv: 0908.4300

Davier-Hoecker-BM-Yuan-Zhang

Improved procedure and software

(HVPTools) for combining cross

section data with arbitrary point

spacing/binning, with full covariance

matrices and error propagation

HEP 2010

slide26

Other hadronic contributions

from Davier-Eidelman-Hoecker-Zhang (2006)

 another large long-standing discrepancy in the +  20 channel !

HEP 2010

slide27

The Problematic +-20 Contribution

preliminary ISR BaBar data:

A. Petzold, EPS-HEP (2007)

e+e-  +20 data (CMD2 discarded)

only statistical errors

Preliminary

old contribution 16.8  1.3

update 17.6  1.7 probably still underestimated (BaBar)

 21.4  1.4

HEP 2010

slide28

BaBar Multi-hadronic Published Results

Statistical + systematic errors

Still more channels under analysis: K+K, KK with K0

HEP 2010

slide29

Where are we?

  • including BaBar 2 results in the e+e combination + estimate of hadronic
  • LBL contribution (Prades-de Rafael-Vainhstein, 2009) yields
  • aSM[e+e] = (11 659 183.4 4.1 2.6 0.2) 1010
  • HVP LBL EW (4.9)
  • E-821 updated result* (11 659 208.9 6.3) 1010
  • deviation (ee) (25.5  8.0) 1010
  • (3.2 )
  • updated  analysis
  • +Belle +revisited IB corrections
  • deviation () (15.7  8.2) 1010
  • (1.9 )

*new proposal submitted to Fermilab

to improve accuracy by a factor 4

HEP 2010

slide30

Conclusions

  • BaBar analysis of p+p-and m+m-ISR processes completed
  • Precision goal has been achieved: 0.5% in  region (0.6-0.9 GeV)
  • Absolute m+m- cross section agrees with NLO QED within 1.1%
  • eep+p-()cross section very insensitive to MC generator
  • full range of interest covered from 0.3 to 3 GeV
  • Comparison with data from earlier experiments
  • fair agreement with CMD-2 and SND, poor with KLOE
  • agreement with t data
  • Contribution to am from BaBar is (514.1 2.23.1)1010 in 0.28-1.8 GeV
  • BaBar result has an accuracy (0.7%) comparable to combined previous results
  • Contribution from multi-hadronic channels will continue to be updated with
  • more results forthcoming from BaBar, particularly +-20
  • Deviation between BNL measurement and theory prediction reduced using

BaBar p+p- data

a[exp] –a[SM] =(19.8 ±8.4)10–10+- from BaBar only

25.5  8.0 combined e+e- including BaBar

HEP 2010

slide32

Data on e+e  hadrons

CMD-2 (2006)

CMD-2 (2004)

SND (2006)

KLOE (2009)

HEP 2010

slide33

The Role of Data through CVC – SU(2)

W: I=1 &V,A

CVC: I=1 &V

: I=0,1 &V



e+

hadrons

W

e–

hadrons

Hadronic physics factorizes (spectral Functions)

branching fractionsmass spectrum kinematic factor (PS)

HEP 2010

slide34

SU(2) Breaking

  • Corrections for SU(2) breaking applied to  data for dominant  – + contrib.:
    • Electroweak radiative corrections:
      • dominant contribution from short distance correction SEW
      • subleading corrections (small)
      • long distance radiative correction GEM(s)
    • Charged/neutral mass splitting:
      • m–  m0leads to phase space (cross sec.) and width (FF) corrections
      •  - mixing (EM    – + decay) corrected using FF model
      • m–  m0 *** and –  0 ***
    • Electromagnetic decays:      ***,    ,    ,   l+l –
    • Quark mass difference mu  md (negligible)

Marciano-Sirlin’ 88

Braaten-Li’ 90

Cirigliano-Ecker-Neufeld’ 02

Lopez Castro et al.’ 06

Alemany-Davier-Höcker’ 97, Czyż-Kühn’ 01

Flores-Baez-Lopez Castro’ 08

Davier et al.’09

HEP 2010

slide35

Situation at ICHEP’06 / 08

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

e+e  BNL

.0

Knecht-Nyffeler (2002), Melnikov-Vainhstein (2003)

Davier-Marciano(2004)

Kinoshita-Nio (2006)

Observed Difference with BNL using e+e:

Estimate using  data consistent with E-821

HEP 2010

slide36

Revisited Analysis using  Data: Belle + new IB

Relative comparison of  and ee spectral functions

( green band)

arXiv:0906.5443 M.Davier et al.

slope…

KLOE

CMD-2, SND

Global test of spectral functions:

prediction of  BR using ee data

 larger disagreement with KLOE

HEP 2010

slide39

Data/MC Tracking Correction to ppg,mmg cross sections

  • single track efficiency
  • correlated loss probability f0
  • probability to produce more than 2 tracks f3

and similarly for pp

mmm

mpp

HEP 2010

slide40

Particle Identification

* isolated muons Mmm > 2.5 GeV

 efficiency maps (p,v1,v2)

impurity (1.10.1) 10-3

* correlated efficiencies/close tracks

 maps (dv1,dv2)

  • 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

xI

Barrel

ZIFR

fIFR

XIFR

  • a tighter  ID (h) is used for tagging

in efficiency measurements and to

further reject background in low cross

section regions.

YIFR

Forward Endcap

Backward Endcap

HEP 2010

slide41

Data/MC PID corrections to mm and pp cross sections

mmg

Two running

periods with

different IFR

performance

ppg

HEP 2010

slide42

PID separation and Global Test

All ‘xx’  solve for all xx(0) and compare

with no-ID spectrum and estimated syst. error

BaBar

BaBar

  • N(o)ii

hist: predicted from PID

dots: measured (no ID)

mpp(GeV)

HEP 2010

slide43

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 0ISR (qq), and  and  (0)

  • global test in background-rich region near cut boundary

BG fractions in 10-2 at mpp values

Fitted BG/predicted = 0.9680.037

BaBar

pp

multi-hadrons

mpp (GeV)

HEP 2010

slide44

2 cut Efficiency Correction

loose c2

  • depends on simulation

of ISR (FSR), resolution

effects (mostly ISR 

direction) for mm and pp

  • c2 cut efficiency can be

well measured in mm data

because of low background

  • main correction from lack of angular distribution for

additional ISR in AfkQed

  • common correction: 1% for loose c2, 7% for tight c2
  • additional loss for pp because of interactions

studied with sample of interacting events

much better study now, 2 independent methods

mmgg

secondary interactions

data/MC 1.51  0.03

syst error 0.3 - 0.9 10-3

HEP 2010

slide45

Additional ISR

mmgg

Angular distribution

of add. ISR /beams!

Energy cut-off for

add. ISR in AfkQed

ppgg

HEP 2010

slide46

Additional FSR

FSR

Angle between add 

and closest track

mmgg

ISR

Large-angle add.ISR

in data  AfkQed

Evidence for FSR

data  AfkQed

ppgg

data/MC

 0.960.06

 1.210.05

HEP 2010

slide47

Checking Known Distributions

Cosq* in XX CM /g

mm

flat at threshold

1+cos2q* bm1

pp

sin2q*  

P>1 GeV track requirement  loss at cos*1

HEP 2010

slide48

Unfolding  Mass Spectrum

  • measured mass spectrum distorted by resolution effects and FSR (mpp vs. s’)
  • iterative unfolding method (B. M. arXiv:0907-3791) : one step is sufficient
  • mass-transfer matrix from simulation with corrections from data
  • 2 MeV bins in 0.5-1.0 GeV mass range, 10 MeV bins outside
  • most salient effect in - interference region (little effect on ampp)

BaBar

Absolute difference

unfolded(1)  raw data

unfolded(2)  unfolded(1)

Statistical errors (band)

Mass (GeV)

BaBar

Relative difference

HEP 2010

slide49

Obtaining the () cross section

Acceptance from MC + data/MC corrections

Unfolded spectrum

Effective ISR luminosity from () analysis (similar equation + QED)

Additional ISR almost cancels in the procedure (() / () ratio)

Correction (2.5 1.0) 103   cross section does not rely on accurate

description of NLO in the MC generator

ratio mm ISR lumi / LO formula

should behave smoothly

(HVP effects on resonances cancel)

Use measured lumi in 50-MeV bins

averaged in sliding 250-MeV bins

for smoothing

HEP 2010

slide50

Changes since preliminary results at Tau08

  • preliminary results Sept. 2008: only 0.5-3 GeV (excess/expect. near threshold)
  • problem explored (Oct. 2008- Feb. 2009): trigger/BGFilter, ee background
  • `’ re-investigated  direct measurement achieved using ID probabilities

before: model for correlated loss, no precise direct check

 significant changes  efficiency for ISR lumi  +0.9%

 contamination in `’ sample 

  cross section  1.8% 0.525 GeV

1.0% 0.775 GeV

1.4% 0.975 GeV

  • other changes: - MC unfolding mass matrix corrected for data/MC differences

(small) - ISR lumi now used in 50-MeV sliding bins, instead of global fit

- cancellation of add ISR in / ratio studied/corrected

  • extensive review

HEP 2010

slide51

f2 Story

HEP 2010

slide52

BaBar vs.other ee data (r-w interference region)

  • mass calibration of BaBar checked with ISR-produced J/y mm
  • expect (0.16  0.16) MeV at  peak
  •  mass determined through VDM mass fit

mωfit mωPDG = (0.12  0.29) MeV

  • Novosibirsk data precisely calibrated using resonant depolarization
  • comparison BaBar/CMD-2/SND in - interference region shows

no evidence for a mass shift

SND

CMD-2

BaBar Preliminary Fit

BaBar Preliminary Fit

HEP 2010

slide53

Obtaining the average cross section

relative weights

  • local weighted average performed
  • full covariance matrices and error
  • propagation
  • local 2 used for error rescaling
  • average dominated by BaBar and
  • KLOE, BaBar covering full range

error scale factor

integrand (dispersion integral)

error

HEP 2010

slide57

Discussion

  • BaBar 2 data complete and the most accurate, but expected precision
  • improvement on the average not reached because of discrepancy with KLOE
  • however, previous /ee disagreement strongly reduced
  • 2.9 (2006)  2.4 ( update)  1.5 (including BaBar)
  • a range of values for the deviation from the SM can be obtained, depending
  • on the 2 data used:
  • BaBar 2.4
  • all ee 3.2
  • all ee BaBar 3.7
  • all ee KLOE 2.9
  •  1.9
  • all approaches yield a deviation, but SM test limited by systematic effects not
  • accounted for in the experimental analyses (ee) and/or the corrections to  data
  • at the moment some evidence for a deviation (24), but not sufficient to establish
  • a contribution from new physics

HEP 2010

slide58

Perspectives

  • first priority is a clarification of the BaBar/KLOE discrepancy:
  • - origin of the ‘slope’ (was very pronounced with the 2004 KLOE results,
  • reduced now with the 2008 results)
  • - normalization difference on  peak (most direct effect on a)
  • - Novosibirsk results in-between, closer to BaBar
  • - slope also seen in KLOE/ comparison; BaBar agrees with 
  • further checks of the KLOE results are possible: as method is based on MC
  • simulation for ISR and additional ISR/FSR probabilities  test with  analysis
  • contribution from multi-hadronic channels will continue to be updated with
  • more results forthcoming from BaBar, particularly 2 20
  • more ee data expected from VEPP-2000 in Novosibirsk
  • experimental error of E-821 direct a measurement is a limitation, already now
  •  new proposal submitted to Fermilab to improve accuracy by a factor 4
  •  project at JPARC

HEP 2010

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