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

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

 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

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 -  5 - 12 (+) 3.7 - 5 (+J/, ) 1.8 - 3.7 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

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

Radiative Return at Particle Factories pQCD Experiment KLOE: Energy region < 1 GeV, dominated by 2p- channel (r-resonance) s [mbarn] Experiment BaBar: Energy region 1 ... 3 GeV, dominated by higher multi- plicities (esp. 4p), so far data with 20 ... 50% 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

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

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

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

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

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 0.9% Luminosity ( LA Bhabhas ) Vacuum polarization FSR corrections Radiator function 0.5% Total theoretical Error 0.9% TOTAL ERROR 1.3% KLOE 2001 Data 140pb-1 Achim Denig The Radiative Return: A Review of Experiments

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 - 1 CMD-2 EPS (‘05) preliminary [0.37-0.93 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 agree @ 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

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 0.9% Luminosity * <<0.6% Vacuum Polarization 0.2% FSR Corrections 0.3% Radiator Function 0.5% Total theoretical error 0.9% *Goal 2002 data: Error < 1% Achim Denig The Radiative Return: A Review of Experiments

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

{ 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(900-1000 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

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

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

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

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

3 Hadrons: e+e-  p+p-p0 PRD 70 (2004) 072004 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: 1400 - 1450 () = 4507070 MeV 180 - 250 M() = 1660102 MeV/c2 1670  30 () = 2303020 MeV 315  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

Comparison with world data 4 Hadrons: e+e-  p+p-p+p- PRD 71 (2005) 052001 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

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

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

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

e+e- 3(p+p-), 2(p+p-)p0p0 , 2(p+p-)K+K- hep-ex/062006 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

2 Protons: e+e-  p p c2ppg Mpp pp PRD 73 (2006) 012005 Cross section parametrized by magnetic and electric formfactors GM and GE Experimental challenge: fight KK, pp, mm bkg. with 10-100 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

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

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

Conclusions KLOE Feasibility of the Radiative Return for high-precision measurement (Pion Formfactor @ 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

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 1 - 3 GeV more and more important for muon anomaly with increased precision < 1 GeV! Achim Denig The Radiative Return: A Review of Experiments