1. The Radiative Return: Review of Experiments
Thank the organizers for the invitation and the possiblity to introduce myself and to present the work which I have been doing over the last years.
I would like to discuss a new method in Hadron Physics – the so called Rad. Return – for precision measurements at particle factories.
Achim Denig
Universität Karlsruhe
BINP Novosibirsk
February 27, 2006
Achim Denig The Radiative Return: A Review of Experiments

2. Outline: The Radiative Return: Why and How?
Gesellschaft für
Schwerionenforschung
Darmstadt
Outline:
The Radiative Return: Why and How?
Results from KLOE at the phi-factory DAFNE
Results from BaBar at the B-factory PEP-II
Conclusions and Outlook
I start with a brief introduction into the SM of particle physics with special emphasis on strong interaction and the consequences of QCD
Then I present the new method Rad.Return which allows cross section measurements at particle factories
We will see that xsection measurements are closely linked to the muon anomaly, which allows a high precision test of the SM
We have measured in our group in Karlsruhe the pion form factor with the KLOE detector via Rad.Return. This measurement and
the consequence on the muon anomaly is presented next.
Finally we will see that the Rad.Return technique is also very well suited for hadron spectrocopy measurements, which we are now
performing at BaBar.
Finally I close with the conclusions
Achim Denig The Radiative Return: A Review of Experiments

3.  The Radiative Return: Why and How? gISR Fp
Let‘s start with a reminder of the basic concepts of the SM of particle physics
Achim Denig The Radiative Return: A Review of Experiments

4. into dispersion integral
Why?
Cross section measurements are of utmost importance for hadronic contribution
to muon anomaly amhadr and for knowledge of fine structure constant at s=mZ2!
12 - 
(+)
(+J/, ) 4
3 (+,) 2
> 4 (+KK)
ahad
<1.8 GeV
Experimental input
into dispersion integral
2p needed with ca.
1% precision!
Davier, Eidelman, Höcker, Zhang
Precision measurements above f-peak allow to study the spectrum of
vector resonances (J PC = 1 - -), which today is poorly known, also:
 Production of resonance pairs with limited choice of quantum numbers
 Consecutive decays allow to study resonances with all quantum numbers
 see BaBar discovery of Y(4260) resonance (m=42608 MeV, =8823 MeV )
Talk J. Izen
Achim Denig The Radiative Return: A Review of Experiments

5. Radiative Return r0 gISR
Modern particle factories, as DAFNE or PEP-II are designed for a fixed
center-of-mass energy: s = mf = 1.02 GeV in the case of DAFNE,
(4s) in case of B-factories: Energy-scan not possible!
New and completely complementary ansatz:
Consider events with Initial State Radiation (ISR)
S. Binner, J.H. Kühn, K. Melnikov, Phys.Lett. B459 (1999) 279
gISR
‘Radiative Return’ to r(w)-resonance:
e+ e- r(w) + g  p+ p-+ g r0
MC- Generator PHOKHARA = NLO
J. Kühn, H. Czyż, G. Rodrigo
Radiator-Function H(s)
ISR
H(s)
dshadr + g
ds(e+ e- hadr+gISR ) =
dMhadr2
dMhadr2
for (2mp)2 < Mhadr2 < s
Achim Denig The Radiative Return: A Review of Experiments

6. Radiative Return at Particle Factories
pQCD
Experiment
KLOE:
Energy region
< 1 GeV,
dominated by 2p-
channel
(r-resonance)
s [mbarn]
Experiment
BaBar:
Energy region
GeV,
dominated by
higher multi-
plicities (esp. 4p),
so far data with
% errors!
(4s)
DAFNE
s=1.02 GeV
PEP-II
s=10.6 GeV
Measurments are badly needed for a better understanding of the anomalous magnetic moment of the muon 1 10
s [GeV]
Using the method of the Radiative Return one can
study the entire energy region below ca. 4 GeV!
Achim Denig The Radiative Return: A Review of Experiments

7. at the phi-factory DAFNE:
Results from KLOE
at the phi-factory DAFNE:
The Pion Formfactor
High Precision needed!
Achim Denig The Radiative Return: A Review of Experiments

8. Selection p+p-gISR Pion tracks at large angles 50o < qp < 130o
The KLOE Detector
a) Photons at small angles
qg < 15o and qg > 165o
 No photon tagging
• High statistics for ISR photons
Negligible contribution of FSR
• Reduced background
b) Photons at large angles
50o < qg < 130o
 Photon tagging possible
• Measurement of threashold region
Increased contribution of FSR
Contribution f  f0(980) g  p +p - g
Achim Denig The Radiative Return: A Review of Experiments

9. Small Angle Analysis p+p-g g p+p-p0 S i g n a l R e g i o n m+m-g
Experimental challenge: fight hughe
background from f  p+ p- p0
m+ m- g and e+e- g, reduced by
means of kinematic cuts (trackmass),
and likelihood function (e-p-separation)
MTRK (MeV)
220
200
180
160
140
120
100
p+p-g g
p+p-p0
Normalization to integrated luminosity,
which is obtained from large-angle-
Bhabha events (clean exp. selection)
S i g n a l
R e g i o n mp
Background from LO-FSR negligible
(reduced by acceptance cuts: small Qg),
NLO-FSR not reduced and not a
background, efficiency has to be known
m+m-g
e+e-g
0.4
0.5
0.6
0.7
0.8
0.9 1.
0.3
mr2
Mpp2 (GeV2)
Achim Denig The Radiative Return: A Review of Experiments

10. Background p+p-g Muon Bkg.: Trackmass p+p-p0 Bkg.: Missing Mass
Mmiss[MeV]
0.6 < Mpp2<0.64 GeV2
Data MC
p+p-p0
p+p-g
0.3 < Mpp2<0.34 GeV2
p+p-p0 Bkg.: Missing Mass
0.62 < Mpp2<0.67 GeV2
0.32 < Mpp2<0.37 GeV2
m+m-g
MTrk[MeV]
Muon Bkg.: Trackmass
Achim Denig The Radiative Return: A Review of Experiments

11. Result Small Angles s ( e+e-  p+p- ) (nb) TOTAL ERROR 1.3% KLOE
Phys. Lett. B606 (2005) 12
s ( e+e-  p+p- ) (nb)
0.4
0.5
0.6
0.7
0.8
0.9
0.3
200
400
600
800
1000
1200
1400
Mpp2 (GeV2)
Acceptance
0.3%
Trigger
Tracking
Vertex
Reconstruction filter
0.6%
Particle ID
0.1%
Trackmass cut
0.2%
Background
Unfolding effects
Total experimental Error %
Luminosity ( LA Bhabhas )
Vacuum polarization
FSR corrections
Radiator function
0.5%
Total theoretical Error %
TOTAL ERROR 1.3%
KLOE
2001 Data
140pb-1
Achim Denig The Radiative Return: A Review of Experiments

12. KLOE data points from 0.35 to 0.95 GeV2
Comparison with recent e+e- Data
Comparison of e+e- experiments
2p contribution to amhadr
PLB 578 (2004) 285
CMD-2 (‘04)
JETP, Vol. 101, No 6 (2005) 1053
s / sKLOE
CMD-2 EPS (‘05)
preliminary
[ GeV2]
SND (‘05), c2 = 1
interpolation of 60
KLOE data points from 0.35 to 0.95 GeV2
KLOE (‘05)
ampp [10-10 ]
Mpp2 (GeV2)
CMD-2 (‘04) and KLOE high Mpp
some disagreement btw. KLOE and
CMD-2/SND on the r peak
fair agreement btw all 4 data sets
CMD-2 and KLOE agree within 0.5 s
disagreement btw KLOE and SND ca.1.5
Achim Denig The Radiative Return: A Review of Experiments

13. Analysis 2002 Data: Small Angles
The goals for the analysis of 2002 data:
 considerable reduction of errors
Acceptance
0.3%
Trigger *
<0.3%
Tracking
0.3%
Changes of online and
offline reconstruction
Vertex
<0.3%
F. Nguyen
C.M. Calame
Reconstruction Filter *
<< 0.6%
New version of
BABAYAGA
with full NLO
Particle ID
0.1%
Trackmass Cut
0.2%
Background
0.3%
p+p-g
m+m-g
dN/dM2 (events/GeV2)
2002
Data
Normalisation to Muons
M2 (GeV2)
Unfolding Effects
0.2%
Total experimental error %
Luminosity *
<<0.6%
Vacuum Polarization
0.2%
FSR Corrections
0.3%
Radiator Function
0.5%
Total theoretical error %
*Goal 2002 data: Error < 1%
Achim Denig The Radiative Return: A Review of Experiments

14. Analysis 2002 Data: Large Angles
Talk D. Leone
Analysis 2002 Data: Large Angles
the threshold region is accessible
photon tagging is possible
(4-momentum constraints)
lower signal statistics
large FSR contributions
large   p+p-p0 background
irreducible bkg. from f decays
PRO & CONTRA
p+p-p0 MC
p+p-g MC<
By means of dedicated selection
cuts it is possible to fight the
dominant   p+p-p0 background
Analysis in very advanced state
preliminary
50o<qp<130o
50o<qg<130o
L = 240 pb-1
Mpp2 (GeV2)
Threshold region non-trivial
due to irreducible FSR-effects,
which have to be cut from MC
using phenomenol. Models
(interference effects unknown) r p g f f0
&
FSR rp
Achim Denig The Radiative Return: A Review of Experiments

15. { DAFNE Off-Resonance-Run Physics Case:
s ( MeV ) F
s = 1023
s = 1030
s = 1018
s = 1010
s = 1000
Run-Nr.
Physics Case:
background-free Radiative Return
200pb-1 allow to be statistically com-
petitive with VEPP-2000 at threshold
a f-scan allows to study the model-
dependence in desciption of f0(980)
background-free gg - program
spp( MeV), dependence on s
KaonLoop fit
On Dec. 6, 2005 DAFNE has started
going off-resonance
Scientific Committee of LNF recommends
to run off-resonance up to end March 2006
in order to acquire an integegrated
luminosity of 200pb-1 at s = 1.00 GeV
In addition it was recommended to
perform a f-scan of 4 points, 10pb-1 each
No-Structure fit {
on-peak
Data 2002
Achim Denig The Radiative Return: A Review of Experiments

16. First look into off-peak-data
DAFNE Off-Resonance-Run
Run at s = 1.00 GeV started on Dec. 16
 Good machine performance off-peak:
Status off-peak running Feb. 24, 2006:
167pb-1 written on tape!
Dec. 16
Feb. 24
Jan. 6
167pb-1
Off-Peak-
Performance:
> 5pb-1/day
Intgerated Luminosity / pb-1
Feb. 1
XMas
Run-Nr.
We hope to continue up to end of March
and to acquire > 200pb-1
Trackmass [MeV]
Number of Events / MeV
10pb-1
ONPEAK
OFFPEAK
p+p-p0
p+p-g
First look into off-peak-data
very promising !
Large photon angle acceptance cuts
PID: both tracks identified as pions
Achim Denig The Radiative Return: A Review of Experiments

17. at the B-factory PEP-II
Results from BaBar
at the B-factory PEP-II
Higher Multiplicities at higher Energies (so far)
I have chosen some highlights
Also BELLE at KEK-B started analyes
Achim Denig The Radiative Return: A Review of Experiments

18. Radiative Return at BaBar
Center-of-mass energy of machine PEP-II (s=m(4s)=10.6 GeV) far from
mass range of interest (ca. < 4 GeV )
 requires high energy photon Eg=(3-5.3)GeV
 requires high integrated luminosity of PEP-II
(Run1-4: ca. 240 fb-1)
Hard ISR-photon back-to-back to hadrons
 only acceptance for large angle photons
 photon tagging!
e+e- ® m+m-g
1 muon ID
Normalizations to integrated luminosity,
cross checked with radiative muon pairs,
which are selected with high precision
 allows to measure R-Ratio directly
2 muon IDs
Achim Denig The Radiative Return: A Review of Experiments

19. Radiative Return at BaBar
Mass resolution of hadronic system largely improved by means of a
kinematic fit; constraints: energy, momentum (evt. p0 mass)
c2-distribution of kinematic fit is the
main tool for background subtraction
 long tail due radiative corrections (NLO)
 remaining background obtained from
MC (for qq events) or from data with
sideband technique (for ISR events)
Systematics of c2-cut together with
background and tracking efficiency
(in neutral cases p0 efficiency) main
parameters to be studied MC
Data +
Background from (4s) and from B-decays is very small
 taken out by requirement of an ISR-photon with Eg > 3 GeV
 it remains background from other ISR-events and continuum e+e-qq
Achim Denig The Radiative Return: A Review of Experiments

20. 3 Hadrons: e+e-  p+p-p0
PRD 70 (2004)
DM2
BaBar
SND
Spectrum dominated
by w, f, J/ resonances
BR(J/3) = (2.18±0.19)%
PDG: (1.50 ±0.20)%
BES: (2.10±0.12)%
N=920±34
Coverage of wide mass region in one experiment:
systematic error ca. 5% < 2.5 GeV
Inconsistency found with DM2 results > 1.4 GeV
Fit of the mass spectrum up to 1.8 GeV
M() = 13502020 MeV/c2 PDG:
() = 4507070 MeV
M() = 1660102 MeV/c  30
() = 2303020 MeV  35
Mw,f, Gw,f and phases between w and f, w’, w’’ fixed in the fit
Achim Denig The Radiative Return: A Review of Experiments

21. Comparison with world data
4 Hadrons: e+e-  p+p-p+p-
PRD 71 (2005)
Average BG fractions
Systematic errors:
12% for m4 < 1 GeV,
5% for 1 < m4 < 3 GeV, 16% above
 best measurement above 1.4 GeV
Events/0.025 GeV
data
ISR Background
Comparison with world data
Non-ISR Background MC
Coverage of wide region in one experiment
 no point-to-point normalization
problems when evaluating (g-2)mhadr
Achim Denig The Radiative Return: A Review of Experiments

22. Substructures in e+e-  p+p-p+p-
EPJ C18(2000)497
Comparison with Monte-Carlo (Czyż, Kühn):
assumes a1(1260) p contribution dominant
assumes additional f0(1370) r contribution
 confirmed by experiment!
J/y®p+p-p+p-
y(2S)®J/y(m+m-)p+p-
No J/ in MC
a1(1260)
Extract J/ , (2S) Branching Ratios:
J/
BR (J/ +-+-)=(3.610.26 0.26)10-3
PDG: (4.0  1.0) 10-3
r(770)
f0(1370)?
BR ((2S)  +-J/ )=( 36.11.5 3.7) %
PDG: (31.0  2.8) %
 Channel p+p- p0p0 in preparation!
Achim Denig The Radiative Return: A Review of Experiments

23. e+e- K+K-p+p-, K+K-K+K-
The K+K-p+p- final state
The K+K- K+K- final state
First measurement ever, which
benefits from good PID-capabilities
Systematic error: ca. 25%, few backgr.
Large improvement in precision and
mass range, in agreement with DM-1
Systematic error: 15%, domin. Kaon-ID
Substructures: - dominant K*(892)Kp
- substantial fp+p- and rK+K-
- f f0(980) under study w. more statistics
BR (J/ K+K-+-) =(6.090.50 0.53)10-3
PDG: (7.2  2.3) 10-3
BR (J/ K+K-K+K-) = ( 6.71.0 3.1 )10-4
PDG: - no entry -
Achim Denig The Radiative Return: A Review of Experiments

24. 6 Hadrons: e+e- 3(p+p-), 2(p+p-)p0p0
The 3(p+p-) final state
The 2(p+p-) p0p0 final state
Large improvement in precision
Systematic error: 20%, bkg. dominated
very little substructure: ~1 r/event
Dip near 1950 MeV, already seen by
FOCUS in diffractive photoproduction
Hughe improvement in precision and
mass range, differences to world data
Systematic error: 20%, bkg. dominated
Very rich substructure:
- w, w3p, 3p seen
- r, r0, f0(980), .... present
Dip near 1950 MeV also in this mode
Also first measurement of 2(p+p-) K+K-
Achim Denig The Radiative Return: A Review of Experiments

25. e+e- 3(p+p-), 2(p+p-)p0p0 , 2(p+p-)K+K-
hep-ex/ accepted by PRD
M3(p+p-)
Combined fit of continuum and resonance
 good agreement between charged and
partly neutral mode
 width significantly larger than FOCUS
Continuum
m6p=1.88±0.03 GeV
G6p=130±30 MeV
f6p=21±40 deg
J/ 3(+-) = (4.400.29 0.29) (4.0  2.0)
J/ 2(+-)00 = (16.51.0 1.8) no entry -
J/ 2(+-) K+K-=(5.090.42 0.35) (3.1  1.3)
y(2S)®2(p+p-)p0p0=(5.3±1.6±0.6) no entry -
y(2S)®2(p+p-)K+K-=(2.1±1.0±0.2) no entry -
Measured BR‘s (in units 10-3):
PDG
2(p+p-)K+K-
2(p+p-)p0p0
J/
(2S)
Resonance
Continuum
M2(p+p-)p0p0
m4p2p0=1.86±0.02 GeV
G4p2p0=160±20 MeV
f4p2p0=-3±15 deg
Resonance
Achim Denig The Radiative Return: A Review of Experiments

26. 2 Protons: e+e-  p p c2ppg Mpp pp
PRD 73 (2006)
Cross section parametrized by magnetic
and electric formfactors GM and GE
Experimental challenge: fight KK, pp, mm bkg. with
higher cross section
Require 2 good proton tracks (dE/dx, DIRC)
Perform 3C kinematic fit to beam 4-momentum
c2ppg
Mpp pp
KKg
K+K-
ppg
mmg
Can measure |GE/GM| by fitting the
distribution of the cosine of helicity angle
See rise, then fall of |GE/GM|
Not in agreement with LEAR data (PS170), except at threshold
 GE and GM equal at high masses
Achim Denig The Radiative Return: A Review of Experiments

27. e+e-  p p Peak at threshold Sharp dips at 2.25 and 3.0 GeV?!
Defining the effective proton form factor:
Peak at threshold
Complicated structure is observed:
Sharp dips at 2.25
and 3.0 GeV?!
No explanation ...
Good agreement
with pQCD-
prediction at
high Q2
NN-session
Achim Denig The Radiative Return: A Review of Experiments

28.  Conclusions and Outlook
Achim Denig The Radiative Return: A Review of Experiments

29. Conclusions KLOE
Feasibility of the Radiative Return for high-precision
measurement (Pion 1%) proven
Gradually improve precision for untagged analyis
 Goal < 1%, Normalisation to muon pairs
Tagged photon analysis provides independent cross check:
 Issues 1: FSR (model dependence), background from f0(980)
Dedicated DAFNE-run off-resonance (s=1.00 GeV)
with integrated luminosity >200pb-1 will substantially
improve precision for tagged and untagged analyses
Clarify confusing situation of Pion Formfactor < 1GeV;
needed for interpretation of E821-Measurement of am !
Achim Denig The Radiative Return: A Review of Experiments

30. Conclusions BaBar Radiative-Return-Program
started in 2000 as work- around
(energy scan impossible) !
Today fully complementary
and competitive!
Conclusions BaBar
Extensive Program for ISR Analyses at BaBar
Many exclusive ISR channels already measured more in progress (pp, p+p-p0p0, KK, D(*)D(*)…), also more data!
 Continuous coverage from threshold to Ecm~4.5 GeV
Precisions (  5% ) > 1.4 GeV considerably improved
Some channels measured for the first time ever
 Many J/ψ BR’s better than world average
Great potential for hadron spectroscopy
 see Y(4260) and structure at 1950 MeV
 more to come, coupled channel analyses
Energy region GeV more and more important for
muon anomaly with increased precision < 1 GeV!
Achim Denig The Radiative Return: A Review of Experiments