B.Malaescu ISR e+e- /g-2 HEP 2010 1 ISR cross sections in e + e - and implications on muon g-2...

B.Malaescu ISR e+e- /g-2

HEP 2010 1

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

Bogdan MALAESCU( LAL – Orsay )

( Representing the BaBar Collaboration )


HEP 2010 2

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 3

Lepton Magnetic Anomaly: from Dirac to QED

Dirac (1928) ge=2 ae=0

anomaly discovered: Kusch-Foley (1948) ae= (1.19 0.05) 103

and explained by O() QED contribution: Schwinger (1948) ae = /2 = 1.16 103

first triumph of QED

ae sensitive to quantum fluctuations of fields


HEP 2010 4

More Quantum Fluctuations

typical contributions:

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

Hadrons vacuum polarization light-by-light (models)

+ ? a new physics ?

Electroweak new physics at high mass scale

a much more sensitive to high scales


HEP 2010 5

Hadronic Vacuum Polarization and Muon (g –2)

2

2had

24

( )

( )3

m

a R sK

ss

ds

Dispersion relation

had

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

Im[ ] | hadrons |2

0 [ hadrons ( )]12 Im ( ) ( )FSR

pt

e es R s

Dominant uncertainty from lowest-order HVP pieceCannot be calculated from QCD (low mass scale), but one can use experimental data on e+e hadrons cross section


HEP 2010 6

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 frequenciesresult updated with new value for /p (+0.9 1010) (see new review in RPP2009 (Hoecker-Marciano))

aexp = (11 659 208.9 5.4 3.3) 1010

( 6.3) (0.54 ppm)

precessionrotation

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


HEP 2010 7

Measure [e+ e + (FSR)] with high accuracy for vacuum polarization calculations, using the ISR method e+ e + ISR (add.)

channel contributes 73% of ahad

Dominant uncertainty also from Present systematic precision of ee 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)

Goals of the BaBar Analysis

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 8

ISR FSR

ISR + add. ISR ISR + add. FSR

The Relevant Processes

e+ e + ISR (add.) and + ISR (add.) measured simultaneously

LO FSR negligible for at s(10.6 GeV)2


HEP 2010 9

BaBar / PEP II

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

Integrated luminosity = 531 fb-1

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

BaBar IFR: resistive plate chambers

BaBar SVT and DCH• precision tracking

BaBar EMC: 6580 CsI(Tl) crystals,

resolution ~1-2 % high E.


HEP 2010 10

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 /KK/ 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 to cross sections to cancel ee luminosity additional ISR vacuum polarization ISR photon efficiency correct for LO FSR (|FSR|2) contribution in (QED, <1% below 1 GeV) additional FSR photons measured

otherwise ~2% syst error

The Measurement


HEP 2010 11

Used an integrated luminosity of 232 fb-1 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 spectraConsistency checks for (QED test, ISR luminosity) and Unblinding R partial (√s`>0.5 GeV) preliminary results (Tau08, Sept. 2008)Additional studies and checksFinal results on cross section and calculation of dispersion integral

(PRL 103, 231801 (2009))

Analysis Steps


HEP 2010 12

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

MC Generators


HEP 2010 13

tag particle (track, ID)

candidate (p, , φ)

γISR

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 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

Particle-related Efficiency Measurements


HEP 2010 14

Kinematic FittingFirst analysis to measure cross sectionwith 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 0ISR (qq), and and (0)


HEP 2010 15

QED Test with sample

ISR efficiency 3.4 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 incomplete NLO using Phokhara

strong test (ISR probability drops out for cross section)

(0.2 3 GeV)

BaBar


HEP 2010 16

Obtaining the (FSR) cross section

Unfolded spectrum

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

Acceptance from MC + data/MC corrections

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

Additional ISR almost cancels in the procedure (() / () ratio)Correction (2.5 1.0) 103 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 17

Systematic uncertainties

s` intervals (GeV) errors in 103

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


HEP 2010 18

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

e+ e + (FSR) bare (no VP) cross section diagonal errors (stat+syst)


HEP 2010 19

BaBar results in region2-MeV energy intervals


HEP 2010 20

BaBar vs. other experiments at larger mass


HEP 2010 21

VDM fit of the pion form factor

add. FSR Running (VP)

BaBar Preliminary Fit




HEP 2010 22

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

CMD-2

SND KLOE

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

BaBar

BaBar Preliminary Fit BaBar Preliminary Fit



HEP 2010 23

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

CLEO Belle




HEP 2010 24

Computing a

*

* arXiv:0906.5443 M. Davier et al.

0.281.8 (GeV)BABAR 514.1 3.8previous e +e combined 503.5 3.5 * combined 515.2 3.5 *

*


HEP 2010 25

Including BaBar in the e+e Combination

arXiv: 0908.4300Davier-Hoecker-BM-Yuan-Zhang

Improved procedure and software(HVPTools) for combining cross section data with arbitrary pointspacing/binning, with full covariance matrices and error propagation


HEP 2010 26

Modes Energy [GeV] e+e –

+ – 20 4m – 1.8 16.8 ± 1.3 ± 0.2rad 21.4 ± 1.3 ± 0.6SU(2)

2 + 2 – (+BaBar) 4m – 1.8 13.1 ± 0.4 ± 0.0rad 12.3 ± 1.0 ± 0.4SU(2)

(782) 0.3 – 0.81 38.0 ± 1.0 ± 0.3rad –

(1020) 1.0 – 1.055 35.7 ± 0.8 ± 0.2rad –

Other excl. (+BaBar) 2m – 1.8 24.3 ± 1.3 ± 0.2rad –

J /, (2S) 3.08 – 3.11 7.4 ± 0.4 ± 0.0rad –

R [QCD] 1.8 – 3.7 33.9 ± 0.5theo –

R [data] 3.7 – 5.0 7.2 ± 0.3 ± 0.0rad –

R [QCD] 5.0 – 9.9 ± 0.2theo –

Other hadronic contributionsfrom Davier-Eidelman-Hoecker-Zhang (2006)

another large long-standing discrepancy in the + 20 channel !


HEP 2010 27

The Problematic +-20 Contribution

e+e- +20 data (CMD2 discarded)

preliminary ISR BaBar data: A. Petzold, EPS-HEP (2007)

only statistical errors

old contribution 16.8 1.3update 17.6 1.7 probably still underestimated (BaBar) 21.4 1.4

Preliminary


HEP 2010 28

BaBar Multi-hadronic Published Results

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

Statistical + systematic errors


HEP 2010 29

Where are we?

• including BaBar 2 results in the e+e combination + estimate of hadronic LBL contribution (Prades-de Rafael-Vainhstein, 2009) yields aSM[e+e] = (11 659 183.4 4.1 2.6 0.2) 1010

HVP LBL EW (4.9)

• E-821 updated result* (11 659 208.9 6.3) 1010

• deviation (ee) (25.5 8.0) 1010

(3.2 )

• updated analysis +Belle +revisited IB corrections• deviation () (15.7 8.2) 1010

(1.9 )

*new proposal submitted to Fermilab to improve accuracy by a factor 4


HEP 2010 30

Conclusions

BaBar analysis of andISR processes completed Precision goal has been achieved: 0.5% in region (0.6-0.9 GeV) Absolute cross section agrees with NLO QED within 1.1% eecross 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 dataContribution to a from BaBar is (514.1 2.23.1)1010 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 +-20

Deviation between BNL measurement and theory prediction reduced using BaBar 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 31

Backup Slides


HEP 2010 32

Data on e+e hadrons

CMD-2 (2004) CMD-2 (2006)

SND (2006) KLOE (2009)


HEP 2010 33

The Role of Data through CVC – SU(2)

hadrons

W hadrons

e+

e –

CVC: I =1 & V W: I =1 & V,A : I =0,1 & V

Hadronic physics factorizes (spectral Functions)2

( 1) 04I e es

0

0

2

22

0

2

0 BR

1 / 1

1

/

BR e

dN

N d

m

ms me s s

branching fractions mass spectrum kinematic factor (PS)


HEP 2010 34

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 – m0 leads to phase space (cross sec.) and width (FF) corrections

- mixing (EM – + decay) corrected using FF model

m – m0 *** and – 0 ***

• Electromagnetic decays: ***, , , l+l –

• Quark mass difference mu md (negligible)

Cirigliano-Ecker-Neufeld’ 02Lopez Castro et al.’ 06

Marciano-Sirlin’ 88

Braaten-Li’ 90

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

Flores-Baez-Lopez Castro’ 08Davier et al.’09


HEP 2010 35

Situation at ICHEP’06 / 08a

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

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

3.3 „standard deviations“

Observed Difference with BNL using e+e:

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

.0

Davier-Marciano (2004)

Kinoshita-Nio (2006)

e+e BNL

Estimate using data consistent with E-821


HEP 2010 36

Revisited Analysis using Data: Belle + new IB

CMD-2, SND KLOE

Relative comparison of and ee spectral functions ( green band)

arXiv:0906.5443 M.Davier et al.

Global test of spectral functions: prediction of BR using ee data

larger disagreement with KLOE

slope…


HEP 2010 37

Revisited Analysis using Data: including Belle


HEP 2010 38

Revisited Analysis Data: new IB corrections


HEP 2010 39

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


HEP 2010 40

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)


HEP 2010 41

Data/MC PID corrections to and cross sections

Two runningperiods withdifferent IFRperformance


HEP 2010 42

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

BaBar

BaBar


HEP 2010 43

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 10-2 at m values

BaBar


HEP 2010 44

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 much better study now, 2 independent methods

loose

secondary interactions data/MC 1.51 0.03 syst error 0.3 - 0.9 10-3


HEP 2010 45

Additional ISR

Angular distributionof add. ISR /beams!

Energy cut-off foradd. ISR in AfkQed


HEP 2010 46

Additional FSR

Angle between add and closest track

Large-angle add.ISR in data AfkQed

Evidence for FSR data AfkQed

FSR

ISR

data/MC 0.960.06 1.210.05


HEP 2010 47

Checking Known Distributions

Cos* in XX CM /

flat at threshold1+cos2*

sin2*

P>1 GeV track requirement loss at cos*1


HEP 2010 48

Unfolding Mass Spectrum measured mass spectrum distorted by resolution effects and FSR (m 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 a

)

Mass (GeV)

Absolute differenceunfolded(1) raw dataunfolded(2) unfolded(1)Statistical errors (band)

Relative difference

BaBar

BaBar


HEP 2010 49

Obtaining the () cross section

ratio ISR lumiLO formulashould behave smoothly (HVP effects on resonances cancel)

Use measured lumi in 50-MeV binsaveraged in sliding 250-MeV binsfor smoothing

Unfolded spectrum

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

Acceptance from MC + data/MC corrections

Additional ISR almost cancels in the procedure (() / () ratio)Correction (2.5 1.0) 103 cross section does not rely on accurate description of NLO in the MC generator


HEP 2010 50

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 51

f2 Story


HEP 2010 52

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 determined through VDM mass fit mω

fit 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



HEP 2010 53

Obtaining the average cross section

• 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

relative weights

integrand (dispersion integral)

error

error scale factor


HEP 2010 54

Consistency of Experiments with Average


HEP 2010 55

g-2 Values for Various Mass Intervals

(1010)


HEP 2010 56

Combined Spectral Functions Comparison


HEP 2010 57

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 (24), but not sufficient to establish a contribution from new physics


HEP 2010 58

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 20

• 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

B.Malaescu ISR e+e- /g-2 HEP 2010 1 ISR cross sections in e + e - and implications on muon g-2...

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