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Low Energy R Measurements with ISR

Low Energy R Measurements with ISR. Su Dong Stanford Linear Accelerator Center CLEO-c/BES c/ t /QCD workshop, Beijing, Jan/15/2004. Content. The need for improved R measurements for muon g-2 and running a QED .

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Low Energy R Measurements with ISR

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  1. Low Energy R Measurements with ISR Su Dong Stanford Linear Accelerator Center CLEO-c/BES c/t/QCD workshop, Beijing, Jan/15/2004 Low Energy R Measurements with ISR

  2. Content • The need for improved R measurements for muon g-2 and running aQED. • Initial State Radiation (ISR) basic characteristics and experimental issues. • Current measurements at KLOE and BaBar. • Radiative corrections. • Prospects. Low Energy R Measurements with ISR

  3. R measurements R = s(e+e- Hadrons) / s0(e+e-m+m-) A large number of measurements scattered at various energy ranges, over the last 3 decades. Low Energy R Measurements with ISR

  4. R Measurements: the last progress CMD-2 BES Focus down running aQED => mHiggs from precision EW Prediction for muong-2 => Test physics beyond SM Foundation of the SM: prediction for precision EW tests relies on R Low Energy R Measurements with ISR

  5. The Need for R in Muon g-2 am= (g-2)m/2 am x1011 QED 11658406 + 3 Hadronic (LO) ~7000 + ~60 Hadronic (NLO) 101 + 6 Hadronic (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 p+p- mode contribution. Low Energy R Measurements with ISR

  6. The status of am Aug/03 review by: Davier, Eidelman, Hoecker, Zhanghep-ph/0308213 Muon g-2 measurement is still 2.7s from e+e- based prediction. e+e- and t data agree better after CMD2 rad corr bug fix (s 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 ! Jan/04 ! Low Energy R Measurements with ISR

  7. The running aQED Da(Mz) x104 Leptonic 314.98 Top -0.70 + 0.05 Hadronic 276.1 + 3.6* (* Burkhardt &Pietrzyk) a(s) = a/[1-Da(s)] Most useful at s=Mz2 to confront the ensemble of precision electroweak measurements. The hadronic vacuum polarization again mostly requires experimental R measurements at low-medium energies Most 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. 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 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->sg analyses…) Becoming truly competitive with the luminosity of the B/t-c/f factories. BaBar and DaFne already started working at s1/2=10.6 GeV and s1/2=~1 GeVrespectively. It’s still in the early days and there are more questions than answers. Low Energy R Measurements with ISR

  9. ISR cross section at Ecm=U(4s) ISR photon mostly along beamline. Only using ~5-10% events with photon in calorimeter fiducial (~|cosqg*|<0.8 for good containment), but still integrate to ~0.05nb of g+had events below s1/2~7 GeV. (compare to the non-radiative ~1nb BB and 3.4nb udsc at s1/2=10.58GeV) Low Energy R Measurements with ISR

  10. The ISR Experimental Approaches cosq*= - 0.92 cosq*= 0.72 cosq*= 0.89 g g p p KLOE @ Ecm=1.02 GeV gp+p-: untagged g q>165oORq<15o and pp in 45o<q<135o BaBar @ Ecm=10.58 GeV Tagged ISR g |cosq*|<0.85 Inclusive OR exclusive final states recon. 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). 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, |cosqg*|<0.80 g tag KLOE ISR: current L ~ 140 pb-1 ,|cosqg*|>0.966 (no tag) actual analysis c-t factory ISR: for L ~ 3 fb-1 ,|cosqg*|<0.90 Ecm= 0.61—0.96 GeV: Luminosity Hadrons CMD2 180K Bhabha 114K (more data on r peak) BaBar ISR350K mmg 1540K (more data at s½=0.8-1 GeV vs CMD2) KLOE ISR 20M Bhabha1500K (pp untagged) c-t factory 250K Ecm= 2—5 GeV: Hadrons BES-I ~85K BaBar ISR 2810K (=30 points at 100 MeV s½ step with 0.35% stat.) c-t factory 193K in range s’1/2=2.0-2.7 GeV for s1/2 =3.77 GeV 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 Low Energy R Measurements with ISR

  14. 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 mm, pp; [Binner et al.; Arbuzov et al.];3p,4p, and other modes without FSR [Czyz,Kuhn + BaBar extensions] PHOKHARA:ISR with NLO for mm, pp, +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-t/f factories is highly desirable ! Low Energy R Measurements with ISR

  15. BaBar: Exclusive R ratio measurements Fully reconstruct the final state and use e+e- -> mmg 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 am 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 mm efficiency separately. • Statistical precision dominated by the lower stat. mm sample. • Unlikely to be workable for high multiplicity modes. Low Energy R Measurements with ISR

  16. BaBar Radiative mmg: Luminosity Topology: two charged tracks + hard photon + Muon ID 1m tag 2m tag Compare data cross section with MC simulation after efficiency and radiative corrections. Calibrate muon efficiency from mmgdata itself: tag 1m and check efficiency of the other. Low Energy R Measurements with ISR

  17. BaBar Radiative mmg: Resolution Apply energy and momentum conservation using a 1C kinematic fit to improve final state reconstruction resolution. After fit 8 MeV  Use J/ signal to monitor the resolution directly Before fit 16 MeV Low Energy R Measurements with ISR

  18. BaBar +-  Interference with f Pion Form Factor with  -  interference 89 fb-1 BaBar preliminary BaBar data covering the full mass range. Systematic study in progress (toughgoal at ~1%) Low Energy R Measurements with ISR

  19. BaBar p+p-p0g BaBar Preliminary Publication being reviewed within BaBar Low Energy R Measurements with ISR

  20. BaBar p+p-p+p-g BaBar Preliminary 89 fb-1 BaBar covers the full mass range. Publication (also include KKpp, KKKK) submission imminent. Low Energy R Measurements with ISR

  21. MTRK (MeV) p+p-p0 p+p-gg tail signal region 140 105 m+m-g + e+e-g M2pp (GeV2) KLOEppg: analysis scheme • Untagged ISR g in beam pipe. • Use 2 track momentum and E,P conservation for single ISR g to define MTRK from • MTRK cut: Each of eeg, mmg, p+p-p0 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 mmg ? Low Energy R Measurements with ISR

  22. KLOEppg: preliminary result Selection efficiency 80% s0(e+e-  )(nb) 60% M2pp (GeV2) M2pp (GeV2) 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 Low Energy R Measurements with ISR

  23. 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 mmg as normalization. • Include tt (mm) 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. Low Energy R Measurements with ISR

  24. ISR photon resolution relevance • Dahad 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 am… Low Energy R Measurements with ISR

  25. ISR photon resolution effects LLow s’½ sensitive to photon energy resolution. JHigh E*g range covers wide range of s’. aUse e+e- -> gg events (and rad Bhabha) as g calibration source Low Energy R Measurements with ISR

  26. ISR photon resolution test: Smeared spectrum Using the R spectrum in KKMC as a toy model. Inject gg data resolution. Detector fiducial |cosq*|<0.8 Low Energy R Measurements with ISR

  27. ISR photon resolution test: Da 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% ! Low Energy R Measurements with ISR

  28. ISR inclusive analysis: background backgr ISR signal S’ (GeV2) BaBar MC test with KKMC for inclusive e+e- g+hadron selection, after photon cluster shape cut and p0->gg veto, but before any final state event shape cuts. Background mainly from high momentum p0 in uds events. 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. Low Energy R Measurements with ISR

  30. FSR effect magnitude in mmg S1/2=10.58 GeV S1/2=4 GeV S’1/2 (GeV) S’1/2 (GeV) Phokhara 3.0 mmg generator: compare turning FSR on/off 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 DaFne, 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… Low Energy R Measurements with ISR

  32. 50% •data •MC - N() - N() -50% Asymetry Pion polar angle Initial and Final State Radiation QED charge asymmetry due to initial state and final state interference BaBar mmg Data MC KLOE ppg Showing 20% discrepancy with MC (phokhara 3.0) Learning from data 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 am and Dahad. • 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 … Low Energy R Measurements with ISR

  34. Backup Slides Low Energy R Measurements with ISR

  35. The status of am Aug/02 Low Energy R Measurements with ISR

  36. Uncertainty dominated by R measurement errors in 1-7 GeV region still. 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 measurements of R (at ~+7% precision) had significant impact (more so for pure data driven calculations) Dahad shift –0.00030 Electroweak fit from sin2qw mH + ~23 GeV Current (data driven, inc. BES) d(Dahad) ~ +0.00036 Equivalent todsin2qw +0.00011 Compare to current experimental measurements:dsin2qw +0.00017 Burkhardt & Pietrzyk PLB B513, 46 (2001) Low Energy R Measurements with ISR

  38. The companion t spectral function measurements Conserved Vector Current (CVC) s(e+e- p+p-)  v(t- p-p0nt) Measurement of the t decay p-p0spectral function at LEP and CLEO. Similar comparison also work for 4p modes. Some isospin symmetry violating effects need to be corrected (difference in radiative corrections, charged vs. neutral r,p masses, r-w mixing, EM decays etc.), amount to -157+28 x10-11 on Dam. ( See Aug/02 review by DEHS: Davier, Eidelman, Hoecker, Zhang,hep-ph/0208177 ) Low Energy R Measurements with ISR

  39. BaBar Radiative mmg: Efficiency  Obtain the efficiencies directly from the DATA Other test samples: Pi:  events 0 K: B events D0 K • Example: Radiative Dimuons • Can be selected with very high purity ~98% FORWARD ECAP: X,Y BARREL: Z, MUONTAG  1 2 MUONID BINS in P,, Low Energy R Measurements with ISR

  40. ISR photon resolution test Use e+e- ->gg data: CM energy and cluster directions => predict g 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. Low Energy R Measurements with ISR

  41. (ISR+FSR) – ISR ISR BKG/TOT e e  +2% BKG/TOT    +    5% -2% • without TrackMass cut • with TrackMass cut M2(GeV2) Mpp2(GeV2) KLOE analysis details (s) from F. Jegerlehner d(s) (correction to s(s)) Low Energy R Measurements with ISR

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