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Jan/15/04 Su Dong Low Energy R Measurements with ISR 1
Low Energy R Measurements with ISR Su Dong
Stanford Linear Accelerator Center
CLEO-c/BES c//QCD workshop, Beijing, Jan/15/2004
Jan/15/04 Su Dong Low Energy R Measurements with ISR 2
Content
• The need for improved R measurements for muon g-2 and running QED.
• Initial State Radiation (ISR) basic characteristics and experimental issues.
• Current measurements at KLOE and BaBar.
• Radiative corrections.
• Prospects.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 3
R measurements
R = (e+e- Hadrons) / (e+e- +-)
A large number of measurements scattered at various energy
ranges, over the last 3 decades.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 4
R Measurements: the last progress
BES
Focus down runningQED
=> mHiggs from precision EW
CMD-2
Prediction for muong-2=> Test physics beyond SM
Foundation of the SM: prediction for precision EW tests relies on R
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The Need for R in Muon g-2
a= (g-2)/2
a x1011
QED 11658406 + 3 Hadronic (LO) ~7000 + ~60Hadronic (NLO) 101 + 6Hadronic (light-by-light) 80 + 40
Weak 152 + 4
The leading order hadronic correction cannot be calculated from perturbative QCD => Need experimental R(s) measurement.
Most contributions and uncertainty come from s’< 3 GeV2. Low energy measurements traditionally done by summing exclusive modes, dominated by mode contribution.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 6
The status of a
Muon g-2 measurement is still 2.7 from e+e- based prediction.e+e- and data agree better after CMD2 rad corr bug fix ( up ~3%, while quoted error was +0.6% !), but still has discrepancy at s = 0.7-0.9 GeV2.New e+e- R measurements would be very desirable !
Aug/03 review by: Davier, Eidelman, Hoecker, Zhang hep-ph/0308213
Jan/04 !
Jan/15/04 Su Dong Low Energy R Measurements with ISR 7
The running QED
s = (s)]Most useful at s=Mz
2 to confront the ensembleof precision electroweakmeasurements.
(Mz) x104
Leptonic 314.98 Top -0.70 + 0.05 Hadronic 276.1 + 3.6 * (* Burkhardt &Pietrzyk)The hadronic vacuum polarization
again mostly requires experimentalR measurements at low-medium energiesMost uncertainty from s1/2 ~ 1-5 GeV still.
Note: Point to point R measurements would be great, but the primary need is a precise INTEGRAL.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 8
R Measurements: The New approach with ISR
Operating at a fixed CM energy to simultaneously explore the whole lower energy range below with initial state radiation (ISR)
(don’t have to fight over when to operate on what energy. They are there all the timeparasitic to whatever else you want to do !)
Rapid rise in both theoretical and experimental interests.
(the possibility of R measurement with ISR actually first emerged from CLEO data in 1995 as a background to the b->s analyses…)
Becoming truly competitive with the luminosity of the B/-c/factories. BaBar and Dane already started working at s1/2=10.6 GeV and s1/2=~1 GeV respectively.
It’s still in the early days and there are more questions than answers.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 9
ISR cross section at Ecm=(4s)
ISR photon mostly along beamline. Only using ~5-10% events with photon in calorimeter fiducial (~|cos*|<0.8 for good containment),but still integrate to ~0.05nb of +had events below s1/2~7 GeV.(compare to the non-radiative ~1nb BB and 3.4nb udsc at s1/2=10.58GeV)
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The ISR Experimental Approaches
KLOE @ Ecm=1.02 GeV : untagged >165o OR <15o
and in 45o<<135o
BaBar @ Ecm=10.58 GeVTagged ISR |cos*|<0.85
Inclusive OR exclusive final states recon.
cos*= - 0.92 cos*= 0.72
cos*= 0.89
Jan/15/04 Su Dong Low Energy R Measurements with ISR 11
Features of Measurements with ISR • Simultaneous collection of data at all CM energy s½ below with no
point-to-point normalization (same luminosity at all s½), and really `sees’ all s½ integrated (c.f. scan points can miss peak)
• Tagged ISR photon eliminates many background sources, particularly beam-gas/beam-wall and cosmics which do bother experiments at low energies. Hopefully will allow more efficient hadronic selection with less bias.
• Selecting ISR photon well within fiducial also forces recoil hadronic system aiming at fiducial detector region (and events more collimated due to recoil boost) with no bias on the hadronic system itself. This should reduce hadronic event signal blur due to acceptance losses.
• The recoil boost to the hadronic system hardens particle momentum spectra to reduce loss due to detection cut off at low momentum (useful for tuning MC model).
Jan/15/04 Su Dong Low Energy R Measurements with ISR 12
ISR statistics compared to low energy e+e-
CMD2: published L ~ 200 pb-1 (has x5 more data now being analyzed)BaBar ISR: current L ~ 150 fb-1 + efficiency estimate, |cos*|<0.80 tag KLOE ISR: current L ~ 140 pb-1 , |cos*|>0.966 (no tag) actual analysis c- factory ISR: for L ~ 3 fb-1
, |cos*|<0.90
Ecm= 0.61—0.96 GeV:
Luminosity HadronsCMD2 180K Bhabha 114K (more data on peak)BaBar ISR 350K 1540K (more data at s½=0.8-1 GeV vs CMD2)KLOE ISR 20M Bhabha 1500K (untagged) c-factory 250K
Ecm= 2—5 GeV: HadronsBES-I ~85KBaBar ISR 2810K (=30 points at 100 MeV s½ step with 0.35% stat.) c-factory 193K in range s’1/2=2.0-2.7 GeV for s1/2 =3.77 GeV
Jan/15/04 Su Dong Low Energy R Measurements with ISR 13
Experimental Issues
Because the rather special topology of the ISR events and we are aiming at precision measurements, it is important to examine detector design, especially trigger and filter strategies to ensure the data preservation.
BaBar + KKMC ISR
Jan/15/04 Su Dong Low Energy R Measurements with ISR 14
Monte Carlo Simulation for ISRNo generic MC which can do everything yetJETSET: Can generate down to CM energy of 2 GeV but no resonances and only LO ISR. EVA/AFKQED: LO ISR + FSR + Leading Log structure function,
for [Binner et al.; Arbuzov et al.]; 3,4, and other modes without FSR [Czyz,Kuhn + BaBar extensions] PHOKHARA: ISR with NLO for +FSR with latest V3.0 [G.Rodrigo et al.]. BaBar has incorporated generic qq mode with JETSET. KKMC: LO ISR + structure function, empirical tabulation of resonances and exclusive final state (still a development version). [S.Jadah et al.] LUND AreaLawMC: Not an ISR MC, but low energy hadronic
decays match BES data. [Anderson/BES] Cooperation between B/c-/ factories is highly desirable !
Jan/15/04 Su Dong Low Energy R Measurements with ISR 15
BaBar: Exclusive R ratio measurementsFully reconstruct the final state and use e+e- -> events as a `luminosity’ normalizer to do a classical R ratio measurement:
Advantages:• Fully reconstructed final state give good resolution for s’. This is very
important for the a integral for g-2 with strong s’ dependence.• The ISR photon efficiency cancels out.• The initial state radiation corrections and ISR spectrum shape
uncertainties largely cancel out. • Some tracking efficiency systematic partially cancel out especially for
the 2 track modes.Disadvantage: • Need to determine efficiency separately.• Statistical precision dominated by the lower stat. sample.• Unlikely to be workable for high multiplicity modes.
)'()1(
)1()'( s
dN
dNs
eefradf
radff
Jan/15/04 Su Dong Low Energy R Measurements with ISR 16
BaBar Radiative : Luminosity
Topology: two charged tracks + hard photon + Muon ID
1 tag2 tag
Compare data cross section with MC simulation after efficiency and radiative corrections.
Calibrate muon efficiency from data itself: tag 1 and check efficiency of the other.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 17
BaBar Radiative : Resolution
Use J/ signal to monitor the resolution directly
Apply energy and momentum conservation using a 1C kinematic fit to improve final state reconstruction resolution.
After fit
16 MeV
8 MeV
Before fit
Jan/15/04 Su Dong Low Energy R Measurements with ISR 18
BaBar +- Pion Form Factor with Pion Form Factor with - - interference interference
BaBar data covering the full mass range. Systematic study in progress (tough goal at ~1%)
89 fb-1
BaBar preliminary
Interferencewith
Jan/15/04 Su Dong Low Energy R Measurements with ISR 19
BaBarBaBar Preliminary
Publication being reviewed within BaBar
Jan/15/04 Su Dong Low Energy R Measurements with ISR 20
BaBar
BaBar covers the full mass range. Publication (also include KK, KKKK) submission imminent.
BaBar Preliminary89 fb-1
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KLOE analysis scheme• Untagged ISR in beam pipe.• Use 2 track momentum and E,P
conservation for single ISR to define MTRK from
• MTRK cut: Each of ee0 background < 5%. Also suppress FSR effect.
• Correct for selection efficiency, background and radiative correction vs s’ (up to 7%
• Normalize to Bhabha Lumi for final spectrum.
• Issues remain: more FSR checks; Bhabha lumi generator discrepancy (2% vs BHWIDE). Try ?
ee
signalregion
M
(GeV2)
MTRK (MeV)
tail
0)( 22
21
222
222
1
qpp
MpMpM trktrk
Jan/15/04 Su Dong Low Energy R Measurements with ISR 22
KLOE preliminary resultS
elec
tion
ef
fici
ency
80%
60% 0(e+
e-
)
(nb
)
KLOE result: a (0.37-0.93) = 378.4 0.8stat 4.5syst 3.0theo 3.8FSR
CMD-2 revised: a (0.37-0.93) = 378.6 2.7stat 2.3syst+theo
M2GeV2 M2GeV2
Jan/15/04 Su Dong Low Energy R Measurements with ISR 23
BaBar: Inclusive R MeasurementsIt is necessary to approach the higher s’ final states with inclusive measurement, but partial final state does not determine reduced CM energy. What about using the ISR photon energy ? Resolution enough ? Does it matter ? The goal is < +5% (BES total err ~7%).
Assume this is OK, the procedure would be:• Make inclusive hadronic final state selection with rather forgiving
cuts for high efficiency (ISR tag defeat many background without biasing hadronic final state).
• Using either standard luminosity measurement and use theoretical ISR spectrum shape, or as normalization.
• Include in the final state selection and subtract out using theoretical cross sections.
• Final state efficiency from MC calibrated by special data samples (the high efficiency hopefully squeeze down room for systematic uncertainty.)
Can work up to s’1/2~ 5-6 GeV before photon background from Normal non-radiative hadronic events becoming significant.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 24
ISR photon resolution relevance
had integrand fortunately similar to ISR cross section at low s½
~just counting ISR photons and s½ resolution hardly matters ! but not so lucky with g-2 a…
Jan/15/04 Su Dong Low Energy R Measurements with ISR 25
ISR photon resolution effects
Low s’½ sensitive to photon energy resolution.
High E* range covers wide range of s’.
Use e+e- -> events (and rad Bhabha) as calibration source
Jan/15/04 Su Dong Low Energy R Measurements with ISR 26
ISR photon resolution test: Smeared spectrum
Using the R
spectrum in
KKMC
as a toy model.
Inject data
resolution.
Detector
fiducial
|cos|<0.8
Jan/15/04 Su Dong Low Energy R Measurements with ISR 27
ISR photon resolution test: integral
The absolute total effect of integral at 6 GeVcompared to perfect detector:
~7% All clusters
~3% Crystal center
~3% Rough edge corr.
~0.5% Gaussian
We can surely calibratefrom data to a small fraction of the 3% !
Jan/15/04 Su Dong Low Energy R Measurements with ISR 28
ISR inclusive analysis: background
BaBar MC test with KKMC for inclusive e+e- +hadron selection, after photon cluster shape cut and 0->veto, but before any final state event shape cuts. Background mainly from high momentum 0 in uds events.
S’ (GeV2)
ISR signal
backgr
Jan/15/04 Su Dong Low Energy R Measurements with ISR 29
NLO Radiative Corrections
G. Rodrigo et al. hep-ph/0106132: Higher order corrections to
radiative spectrum at low s’ is smaller and flatter if e+e- Ecm is higher.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 30
FSR effect magnitude in
S’1/2 (GeV) S’1/2 (GeV)
S1/2=10.58 GeV S1/2=4 GeV
Phokhara 3.0 generator: compare turning FSR on/off
Jan/15/04 Su Dong Low Energy R Measurements with ISR 31
Final State Radiation
Who is radiating ?
Hadron or the quark ?
(Collinear soft radiation are probably mostly from hadrons, but what about wide angle hard FSR ?)
ISR+FSR interference significant at Dane, but may be not for BaBar at low s’ ?
Not everyone agrees…
But there are various ways to test
this from data. We are looking into them…
Jan/15/04 Su Dong Low Energy R Measurements with ISR 32
Initial and Final State Radiation
Data MC
BaBar QED charge asymmetry due to initial state and final state interference
- N ()
- N ()
Pion polar angle Asymetry
• data • MC
-50%
50%
Showing 20% discrepancy with MC (phokhara 3.0)
KLOE
Learning from data
Jan/15/04 Su Dong Low Energy R Measurements with ISR 33
Prospects• Very promising prospects for R measurements with ISR events.
Statistics at BaBar, KLOE are significantly exceeding existing energy scan measurements. CLEO-c and BES-III ISR will also have more statistics than current scan data.
• The ISR measurements also have very different characteristic from the fixed CM energy measurement environment, and many clear systematic advantages, yet each approach also has some weakness.
• Even among ISR measurements, there are very different approaches for different experiments designed to suite the different needs of a and had.
• With all measurements statistically strong, it will all come down to systematics. Measurements from more varieties of environment and methods are essential to establish a convincing picture.
The fun has just begun in a new arena …
Jan/15/04 Su Dong Low Energy R Measurements with ISR 34
Backup Slides
Jan/15/04 Su Dong Low Energy R Measurements with ISR 35
The status of aAug/02
Jan/15/04 Su Dong Low Energy R Measurements with ISR 36
Uncertainty dominated by R measurementerrors in 1-7 GeV region still.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 37
Many different evaluations.
Red solid circles: Data driven – only use perturbative QCD at >10 GeV. Black open circles: Some additional assumptions to allow use of perturbative QCD at intermediate and low energy range.
The recent BES measurementsof R (at ~+7% precision) hadsignificant impact (more so for
pure data driven calculations)
had shift –0.00030 Electroweak fit from sin2w
mH + ~23 GeV
Current (data driven, inc. BES)
had) ~ +0.00036
Equivalent to sin2w +0.00011
Compare to current experimental
measurements: sin2w +0.00017Burkhardt & Pietrzyk PLB B513, 46 (2001)
Jan/15/04 Su Dong Low Energy R Measurements with ISR 38
The companion spectral function measurements
Conserved Vector Current (CVC)
(e+e- +-) v(- -0) Measurement of the decay -0spectral function at LEPand CLEO. Similar comparison also work for 4 modes.
Some isospin symmetry violating effects need to becorrected (difference in radiative corrections, charged vs. neutral masses, mixing, EM decays etc.), amount to -157+28 x10-11 on a.
( See Aug/02 review by DEHS:
Davier, Eidelman, Hoecker, Zhang, hep-ph/0208177 )
Jan/15/04 Su Dong Low Energy R Measurements with ISR 39
BaBar Radiative : Efficiency Obtain the efficiencies directly from the DATA
11
22
BINS in P,,
MUONIDID
MUONTAGTAGBARREL: Z, FORWARD ECAP: X,Y
Example: Radiative DimuonsExample: Radiative DimuonsCan be selected with very high purity ~98%
Other test samples:Other test samples:Pi: events 0 K: B events D0 K
Jan/15/04 Su Dong Low Energy R Measurements with ISR 40
ISR photon resolution test
Use e+e- -> data:CM energy and cluster directions => predict energy E0 and examinemeasured E/E0.
All cluster E/E0 tail dominated by crystaledge effects. Can check using clusters hitting center of crystal.
Jan/15/04 Su Dong Low Energy R Measurements with ISR 41
KLOE analysis details
BKG/TOT e e
M2 (GeV2)
5%BKG/TOT +
1
1.02
1.04
1.06
1.08
0.2 0.4 0.6 0.8 1.0
(s) from F. Jegerlehner
(s)
(corr
ecti
on
tos
))
M2(GeV2)
• without TrackMass cut• with TrackMass cut
(ISR+FSR) – ISR ISR
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