Jan/15/04 Su DongLow Energy R Measurements with ISR1 Su Dong Stanford Linear Accelerator Center CLEO-c/BES c/  /QCD workshop, Beijing, Jan/15/2004

Jan/15/04 Su DongLow Energy R Measurements with ISR2 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 DongLow Energy R Measurements with ISR3 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 DongLow Energy R Measurements with ISR4 R Measurements: the last progress BES Focus down running  QED => m Higgs from precision EW CMD-2 Prediction for muon  g-2 => Test physics beyond SM Foundation of the SM: prediction for precision EW tests relies on R

Jan/15/04 Su DongLow Energy R Measurements with ISR5 The Need for R in Muon g-2 a  = (g-2)  /2 a  x10 11 QED Hadronic (LO) ~ ~60 Hadronic (NLO) Hadronic ( light-by-light) Weak The leading order hadronic correction cannot be calculated from perturbative QCD => Need experimental R(s) measurement. Most contributions and uncertainty come from s’< 3 GeV 2. Low energy measurements traditionally done by summing exclusive modes, dominated by     mode contribution.

Jan/15/04 Su DongLow Energy R Measurements with ISR6 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 = GeV 2. New e + e - R measurements would be very desirable ! Aug/03 review by: Davier, Eidelman, Hoecker, Zhang hep-ph/ Jan/04 !

Jan/15/04 Su DongLow Energy R Measurements with ISR7 The running  QED  s  =  (s)] Most useful at s=M z 2 to confront the ensemble of precision electroweak measurements.  (M z ) x 10 4 Leptonic Top Hadronic * (* Burkhardt &Pietrzyk) The hadronic vacuum polarization again mostly requires experimental R measurements at low-medium energies Most uncertainty from s 1/2 ~ 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 DongLow Energy R Measurements with ISR8 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 time parasitic 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 Da  ne already started working at s 1/2 =10.6 GeV and s 1/2 =~1 GeV respectively. It’s still in the early days and there are more questions than answers.

Jan/15/04 Su DongLow Energy R Measurements with ISR9 ISR cross section at E cm =  (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 s 1/2 ~7 GeV. (compare to the non-radiative ~1nb BB and 3.4nb udsc at s 1/2 =10.58GeV)

Jan/15/04 Su DongLow Energy R Measurements with ISR10 The ISR Experimental Approaches    E cm =1.02 GeV     : untagged   >165 o OR  <15 o and  in 45 o <  <135 o Ecm=10.58 GeV Tagged ISR  |cos  *|<0.85 Inclusive OR exclusive final states recon. cos  *= cos  *= 0.72 cos  *= 0.89

Jan/15/04 Su DongLow Energy R Measurements with ISR11 Features of Measurements with ISR Simultaneous collection of data at all CM energy s ½ below with no point-to-point normalization (same luminosity at all s ½ ), and really `sees’ all s ½ 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 DongLow Energy R Measurements with ISR12 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 E cm = 0.61—0.96 GeV: Luminosity Hadrons CMD2 180K Bhabha 114K (more data on  peak) BaBar ISR 350K  1540K (more data at s ½ =0.8-1 GeV vs CMD2) KLOE ISR 20M Bhabha 1500K (  untagged) c-  factory 250K E cm = 2—5 GeV: Hadrons BES-I ~85K BaBar ISR 2810K (=30 points at 100 MeV s ½ step with 0.35% stat.) c-  factory 193K in range s’ 1/2 = GeV for s 1/2 =3.77 GeV

Jan/15/04 Su DongLow Energy R Measurements with ISR13 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 DongLow Energy R Measurements with ISR14 Monte Carlo Simulation for ISR No generic MC which can do everything yet JETSET: 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 DongLow Energy R Measurements with ISR15 BaBar: Exclusive R ratio measurements Fully 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.

Jan/15/04 Su DongLow Energy R Measurements with ISR16 BaBar Radiative  : Luminosity Topology: two charged tracks + hard photon + Muon ID 1  tag 2  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 DongLow Energy R Measurements with ISR17 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 DongLow Energy R Measurements with ISR18 BaBar  +  -  Pion Form Factor with  -  interference BaBar data covering the full mass range. Systematic study in progress (tough goal at ~1%) 89 fb -1 BaBar preliminary Interference with 

Jan/15/04 Su DongLow Energy R Measurements with ISR19 BaBar        BaBar Preliminary Publication being reviewed within BaBar

Jan/15/04 Su DongLow Energy R Measurements with ISR20 BaBar          BaBar covers the full mass range. Publication (also include KK , KKKK) submission imminent. BaBar Preliminary 89 fb -1

Jan/15/04 Su DongLow Energy R Measurements with ISR21 KLOE  analysis scheme Untagged ISR  in beam pipe. Use 2 track momentum and E,P conservation for single ISR  to define M TRK from M TRK cut: Each of ee       0 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  ?       e  e   signal region M   (GeV 2 ) M TRK (MeV)       tail 

Jan/15/04 Su DongLow Energy R Measurements with ISR22 KLOE  preliminary result Selection efficiency 80% 60%  0 ( e + e -      ) (nb) KLOE result: a   ( ) =  0.8 stat  4.5 syst  3.0 theo  3.8 FSR CMD-2 revised: a   ( ) =  2.7 stat  2.3 syst+theo M 2  GeV 2 

Jan/15/04 Su DongLow Energy R Measurements with ISR23 BaBar: Inclusive R Measurements It 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 DongLow Energy R Measurements with ISR24 ISR photon resolution relevance  had integrand fortunately similar to ISR cross section at low s ½  ~just counting ISR photons and s ½ resolution hardly matters ! but not so lucky with g-2 a  …

Jan/15/04 Su DongLow Energy R Measurements with ISR25 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 DongLow Energy R Measurements with ISR26 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 DongLow Energy R Measurements with ISR27 ISR photon resolution test:  integral The absolute total effect of integral at 6 GeV compared to perfect detector: ~7% All clusters ~3% Crystal center ~3% Rough edge corr. ~0.5% Gaussian We can surely calibrate from data to a small fraction of the 3% !

Jan/15/04 Su DongLow Energy R Measurements with ISR28 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’ (GeV 2 ) ISR signal backgr

Jan/15/04 Su DongLow Energy R Measurements with ISR29 NLO Radiative Corrections G. Rodrigo et al. hep-ph/ : Higher order corrections to radiative spectrum at low s’ is smaller and flatter if e + e - Ecm is higher.

Jan/15/04 Su DongLow Energy R Measurements with ISR30 FSR effect magnitude in  S’ 1/2 (GeV) S 1/2 =10.58 GeVS 1/2 =4 GeV Phokhara 3.0  generator: compare turning FSR on/off

Jan/15/04 Su DongLow Energy R Measurements with ISR31 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 Da  ne, 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 DongLow Energy R Measurements with ISR32 Initial and Final State Radiation DataMC 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 DongLow Energy R Measurements with ISR33 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 DongLow Energy R Measurements with ISR34 Backup Slides

Jan/15/04 Su DongLow Energy R Measurements with ISR35 The status of a  Aug/02

Jan/15/04 Su DongLow Energy R Measurements with ISR36 Uncertainty dominated by R measurement errors in 1-7 GeV region still.

Jan/15/04 Su DongLow Energy R Measurements with ISR37 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 measurements of R (at ~+7% precision) had significant impact (more so for pure data driven calculations)  had shift –  Electroweak fit from sin 2  w m H + ~23 GeV Current (data driven, inc. BES)  had ) ~  Equivalent to  sin 2  w Compare to current experimental measurements:  sin 2  w Burkhardt & Pietrzyk PLB B513, 46 (2001)

Jan/15/04 Su DongLow Energy R Measurements with ISR38 The companion  spectral function measurements Conserved Vector Current (CVC)  (e + e -   +  - )  v (  -   -  0  ) Measurement of the  decay  -  0  spectral function at LEP and CLEO. Similar comparison also work for 4  modes. Some isospin symmetry violating effects need to be corrected (difference in radiative corrections, charged vs. neutral  masses,  mixing, EM decays etc.), amount to x on  a . ( See Aug/02 review by DEHS: Davier, Eidelman, Hoecker, Zhang, hep-ph/ )

Jan/15/04 Su DongLow Energy R Measurements with ISR39 BaBar Radiative  : Efficiency  Obtain the efficiencies directly from the DATA  1111 2222 BINS in P, ,  ID MUONID TAG MUONTAG BARREL: Z,  FORWARD ECAP: X,Y Example: Radiative Dimuons  Can be selected with very high purity ~98% Other test samples: Pi:  events  0 K: B events D 0  K 

Jan/15/04 Su DongLow Energy R Measurements with ISR40 ISR photon resolution test Use e + e - ->  data: CM energy and cluster directions => predict  energy E0 and examine measured E/E0. All cluster E/E0 tail dominated by crystal edge effects. Can check using clusters hitting center of crystal.

Jan/15/04 Su DongLow Energy R Measurements with ISR41 KLOE analysis details BKG/TOT e  e   M  2 (GeV 2 ) 5% BKG/TOT      +        (s) from F. Jegerlehner  (s) (correction to  s)) M  2 (GeV 2 )   without TrackMass cut with TrackMass cut (ISR+FSR) – ISR ISR