Transcript Document
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+()
• e+e+()
discussion
M.Davier BaBar pi pi
Tau08 22-25/9/2008
1
Hadronic Vacuum Polarization and Muon (g –2)
Contributions to the Standard
Model (SM) Prediction:
g 2
a
2
aQED
Born: (0) (s ) (s ) ( / (s ) )
0 [e+ehadrons()]
12 Im (s)
R(s)
pt
] |
Im[
ahad
3 2
2
ds
4 m2
K (s )
s
hadrons |2
+
ahad
+
aweak
2
Dominant uncertainty from
lowest order hadronic piece.
Cannot be calculated from
QCD (“first principles”) – but:
we can use experiment
h
a
d
R (s )
”Dispersion relation“
had
...
M.Davier BaBar pi pi
Tau08 22-25/9/2008
2
Goals of the Analysis
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 (MZ2) (EW tests, ILC)
Present systematic precision of e+e 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
M.Davier BaBar pi pi
Tau08 22-25/9/2008
3
Situation at ICHEP-Tau06
ahad [ee] = (690.9 ± 4.4) 10 –10
a [ee] = (11 659 180.5 ± 4.4had ± 3.5LBL ± 0.2QED+EW) 10 –10
including:
Hadronic HO
– ( 9.8 ± 0.1) 10 –10
Hadronic LBL
+ (12.0 ± 3.5) 10 –10
Electroweak
QED
.0
(15.4 ± 0.2) 10 –10
(11 658 471.9 ± 0.1) 10 –10
Knecht-Nyffeler, Phys.Rev.Lett. 88 (2002) 071802
Melnikov-Vainshtein, hep-ph/0312226
Davier-Marciano, Ann. Rev. Nucl. Part. Sc. (2004)
Kinoshita-Nio (2006)
BNL E821 (2004):
aexp = (11 659 208.0 6.3) 10 10
Observed Difference with BNL using e+e:
a [exp] – a [SM] =
(27.5 ± 8.4) 10 –10
3.3 „standard deviations“
M.Davier BaBar pi pi
Tau08 22-25/9/2008
4
The Relevant Processes
ISR
FSR
ISR + add. ISR
M.Davier BaBar pi pi
Tau08 22-25/9/2008
ISR + add. FSR
5
The Measurement
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
otherwise 3-4% syst error
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)
0
[()](s)
[()](s)
R(s)
Rexp(s)
[()](s) (1+FSR
) 0 [()](s) (1+FSR
) (1+addFSR
)
M.Davier BaBar pi pi
Tau08 22-25/9/2008
6
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
M.Davier BaBar pi pi
Tau08 22-25/9/2008
7
Analysis Steps
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 spectra
Consistency checks for (QED test, ISR luminosity) and
Unblinding R
Results on cross section and calculation of dispersion integral
M.Davier BaBar pi pi
Tau08 22-25/9/2008
8
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 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
M.Davier BaBar pi pi
Tau08 22-25/9/2008
9
Particle-related Efficiency Measurements
tag particle (track, ID)
γISR
candidate (p, , φ)
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 /K/ 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)
M.Davier BaBar pi pi
Tau08 22-25/9/2008
10
Data/MC Tracking Correction to , cross sections
single track efficiency
correlated loss probability f0
probability to produce more than 2 tracks f3
Ctrack
data
track
MC
track
2
(1 f 0 f 3 ) data
MC
(
1
f
f
)
0
3
1 Ctrack
1 Ctrack
m
M.Davier BaBar pi pi
and similarly for
m
Tau08 22-25/9/2008
11
Particle Identification
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 tagging
in efficiency measurements and to
further reject background in low cross
section regions.
* isolated muons M > 2.5 GeV
efficiency maps (p,v1,v2)
impurity (1.10.1) 103
* correlated efficiencies/close tracks
maps (dv1,dv2)
Barrel
ZIFR
XIFR
Forward Endcap
M.Davier BaBar pi pi
Tau08 22-25/9/2008
fIFR
YIFR
Backward Endcap
12
Data/MC PID corrections to and cross sections
Two running
periods with
different IFR
performance
M.Davier BaBar pi pi
Tau08 22-25/9/2008
13
PID separation and Global Test
All ‘xx’ solve for all xx(0) and compare
with no-ID spectrum and estimated syst. error
N(o)ii
hist: predicted from PID
dots: measured (no ID)
M.Davier BaBar pi pi
m (GeV)
Tau08 22-25/9/2008
14
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
M.Davier BaBar pi pi
Tau08 22-25/9/2008
15
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 f (0)
global test in background-rich region near cut boundary
BG fractions in 10-3 at m values
Fitted BG/predicted = 0.9680.037
multi-hadrons
m (GeV)
M.Davier BaBar pi pi
Tau08 22-25/9/2008
16
Additional ISR
Angular distribution
of add. ISR /beams!
Energy cut-off for
add. ISR in AfkQed
M.Davier BaBar pi pi
Tau08 22-25/9/2008
17
Additional FSR
FSR
Angle between add
and closest track
ISR
Large-angle add.ISR
in data AfkQed
Evidence for FSR
data AfkQed
M.Davier BaBar pi pi
Tau08 22-25/9/2008
18
2 cut Efficiency Correction
loose 2
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 2
additional loss for because of interactions
studied with sample of interacting events
agreement data/MC
M.Davier BaBar pi pi
Tau08 22-25/9/2008
1.00 0.16 loose 2
1.07 0.11 tight 2
syst error 0.5-1.5 10-3
19
Checking Known Distributions
Cos* in XX CM /
flat at threshold
1+cos2* b1
sin2*
b
P>1 GeV track requirement loss at cos*1
M.Davier BaBar pi pi
Tau08 22-25/9/2008
20
QED Test with sample
absolute comparison of mass spectra
in data and in simulation
J/y excluded
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
(0.2 – 5.0 GeV)
ISR efficiency 5.2 syst.
trig/track/PID 4.0
M.Davier BaBar pi pi
Tau08 22-25/9/2008
BaBar ee luminosity
21
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)
M.Davier BaBar pi pi
Tau08 22-25/9/2008
22
Systematic Uncertainties
ISR lumi
fitted w.r.t.
LO formula
increased to 20. for preliminary results, pending investigations
M.Davier BaBar pi pi
Tau08 22-25/9/2008
23
BaBar results
M.Davier BaBar pi pi
Tau08 22-25/9/2008
24
BaBar results in region
M.Davier BaBar pi pi
Tau08 22-25/9/2008
25
BaBar vs. other experiments at large mass
M.Davier BaBar pi pi
Tau08 22-25/9/2008
26
BaBar vs.other ee data (0.5-1.0 GeV)
CMD-2
direct relative comparison of cross
sections in the corresponding 2-MeV
BaBar bins (interpolation with 2 bins)
deviation from 1 of ratio w.r.t. BaBar
stat + syst errors included
SND
M.Davier BaBar pi pi
KLOE
Tau08 22-25/9/2008
27
BaBar vs.other ee data ( interference region)
mass calibration of BaBar checked with ISR-produced J/y
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
CMD-2
M.Davier BaBar pi pi
SND
Tau08 22-25/9/2008
28
BaBar vs. IB-corrected data (0.5-1.0 GeV)
relative comparison w.r.t. BaBar of
isospin-breaking corrected spectral
functions
BaBar data averaged in wider bins
and corrected for - interference
M.Davier BaBar pi pi
Tau08 22-25/9/2008
29
Computing a
a (1010)
ALEPH-CLEO-OPAL
(DEHZ 2006)
(DEHZ 2003)
(2008)
FSR correction was missing in Belle, new value 523.5 3.0 2.5
Direct comparison 0.630-0.958 GeV
M.Davier BaBar pi pi
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
Tau08 22-25/9/2008
30
Conclusions
BaBar analysis of and ISR 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 Belle
Contribution to a from BaBar agrees better with results
Deviation between BNL measurement and theory prediction
significantly reduced using BaBar data
a [exp ] – a [SM ]=(27.5 ± 8.4) 10 –10 (14.0 8.4) 1010
Wait for final results and contributions of multi-hadronic modes
M.Davier BaBar pi pi
Tau08 22-25/9/2008
31
Backup Slides
M.Davier BaBar pi pi
Tau08 22-25/9/2008
32
PID correction to cross section
Two running
periods with
different IFR
performance
M.Davier BaBar pi pi
Tau08 22-25/9/2008
33
Measurement of -ID efficiencies
• ‘’ 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
M.Davier BaBar pi pi
Tau08 22-25/9/2008
34
PID correction to cross section
M.Davier BaBar pi pi
Tau08 22-25/9/2008
35
2 cut Efficiency Correction: Interactions
loose 2
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)
Beam pipe
agreement data/MC
1.00 0.16 loose 2
1.07 0.11 tight 2
syst error 0.5-1.5 10-3
tight 2
M.Davier BaBar pi pi
Tau08 22-25/9/2008
36
Mass Calibration
Effect of a 1 MeV mass scale shift
(calibration with J/y : (0.16 0.16) MeV)
m (GeV)
M.Davier BaBar pi pi
Tau08 22-25/9/2008
37
Present BaBar Measurements
only statistical errors
syst. 5-10%
to obtain R in the energy range 1-2 GeV the processes
+30, +40, K+K-, KSKL, KSKL, KSK+ 0
remain to be measured
M.Davier BaBar pi pi
Tau08 22-25/9/2008
38
BaBar Impact on Multi-Hadronic Modes
•
•
•
Using ISR method the cross sections of e+e +0, 2+2, 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
Davier-EidelmanHoecker-Zhang 2003
total 696.37.2
More modes to come
M.Davier BaBar pi pi
Tau08 22-25/9/2008
39