Stefan E. Müller Laboratori Nazionali di Frascati The determination of  had with the KLOE detector Euridice Collaboration Meeting Kazimierz, August 2006 (for the KLOE collaboration)

Introduction Three important formulas for a  had : 1.Dispersion integral: 2.Application of Radiator Function: 3.Extracting the ISR cross section from the observed spectrum: I will (mainly) concentrate on the last one, and speak only about the  channel.

An experimentalist‘s guide to Measuring s ppg with the KLOE detector (short version) (For long version, see Phys. Lett. B606 (2005) 12 KLOE Notes 202, 189 KLOE Memos 308, 295, 294, 292, 291, 287, 283, 278 PhD Theses F. Nguyen, S. M., B. Valeriani)

 had via ISR: While the Energy scan seems the natural way to measure  had, the experience at DA  NE and PEP- II has shown that the Radiative Return via Initial State radiation (ISR) has to be considered as a complementary approach: Data comes as By-Product of the standard program of the machine, no dedicated runs necessary Overall energy scale √s=m  well known and applies to all values of M had Advantages: Systematic errors from Luminosity, √s, rad. corr., … enter only once and do not need to be studied for every point in s Issues: Requires precise theoretical cal- culations of the radiator function H - PHOKHARA MonteCarlo (see Talk of H. Czyż ) Requires good suppression (or understanding) of Final State Radiation (FSR) – find effective selection cuts – test model of scalar QED with data (charge asymmetry) Needs high luminosity

KLOE Detector Driftchamber s p /p = 0.4% (for 90 0 tracks) s xy  150 m m, s z  2 mm Excellent momentum resolution s p /p = 0.4% (for 90 0 tracks) s xy  150 m m, s z  2 mm Excellent momentum resolution

KLOE Detector Electromagnetic Calorimeter s E /E = 5.7% /  E(GeV) s T = 54 ps /  E(GeV)  50 ps (Bunch length contribution subtracted from constant term) Excellent timing resolution s E /E = 5.7% /  E(GeV) s T = 54 ps /  E(GeV)  50 ps (Bunch length contribution subtracted from constant term) Excellent timing resolution

Event Selection       Pion tracks at large angles 50 o <  p <130 o a) Photons at small angles   165 o  No photon tagging! High statistics for ISR photons Very small contribution from FSR Reduced Backgr. contamination

Event Selection      Pion tracks at large angles 50 o <  p <130 o a) Photons at small angles   165 o  No photon tagging! High statistics for ISR photons Very small contribution from FSR Reduced Backgr. contamination b) Photons at large angles 50 o <   < 130 o  Photon tagging possible! Threshold region accessible Increased contribution from FSR Contribution from  f 0 (980)       

The Threshold Region: Kinematics do not allow events with M 2   GeV 2 in the small angle selection cuts - a high-energy ISR photon (  small M 2   emitted at a small angle forces the pions also to be at low angles: p+p+  p-p-  pp  MC, L=800pb -1 M 2   50 o <   < 130 o   < 15 o or   > 165 o Monte Carlo (PHOKHARA):  a  had (s<0.35 GeV 2 )  100· from e + e -  +  -

The Threshold Region: Kinematics do not allow events with M 2   GeV 2 in the small angle selection cuts - a high-energy ISR photon (  small M 2   emitted at a small angle forces the pions also to be at low angles: MC, L=800pb -1 M 2   50 o <   < 130 o   < 15 o or   > 165 o Monte Carlo (PHOKHARA):  a  had (s<0.35 GeV 2 )  100· from e + e -  +  - If the high-energy photon is emitted at a large angle, also the pions will be emitted at large angles; and thus the event will be selected.  pp  pp  pp

Final State Radiation: The cross section for e + e -  +  - has to be inclusive with respect to final state radiation events in order to evaluate a   We consider two kinds of FSR contributions: ** ** LO-FSR: No initial state radiation, e + and e - collide at the energy M  =1.02 GeV NLO-FSR: Simultaneous presence of one photon from initial state radiation and one photon from final state radiation In both cases, the presence of  FSR results in a shift of the measured quantity M 2   towards lower values: M 2     

Final State Radiation:  Events end up in the wrong M 2   bin in the measured spectrum LO-FSR: Events with a M    s=1020 MeV contaminate signal region with M 2     GeV 2 subtract by means of MonteCarlo (scalar QED) small angle selection cuts highly suppress these events Interference between FSR- and f 0 -amplitude in large angle analysis NLO-FSR: Spectrum has to be “reweighted” in order to move events to the correct M 2   bin Reweighing function obtained by Phokhara MonteCarlo (sQED) % Small angle cuts: MC: FSR /(ISR + FSR) M  2 [GeV 2 ] % MC: FSR /(ISR + FSR) M  2 [GeV 2 ] Large angle cuts: Phokhara

Event selection (cont‘d): M  2 (GeV 2 ) M Trk (MeV)       signal region 0.3      region (+ e  e   To further clean the samples from radiative Bhabha events, a particle ID estimator for each charged track based on Calorimeter Information and Time-of- Flight is used. Experimental challenge: Fight background from –       – e  e   e  e   – e  e       , separated by means of kinematical cuts in trackmass M Trk ( defined by 4-momentum conservation under the hypothesis of 2 tracks with equal mass and a  ) and Missing Mass M miss ( defined by 4-momentum conservation under the hypothesis of e  e      x

Background eval.: a)First make sure that MC reproduces data distributions in a satisfactory way by matching individual datataking conditions run-by-run (int. Luminosity,√s, machine background,…) by tuning (smearing, shifting,…) MonteCarlo distributions Trkmass (MeV) - MonteCarlo  Data Before curing MC:After curing MC: dN/dM trk

Background eval.: b)Estimate background contribution by Summing all absolutely normalized background distributions from MC (works well for small background contamination) or Fit signal+background MC distributions (in slices of M 2  ) to data distribution with free normalization parameters M Trk [MeV] Data   Events 0.84< M 2  < 0.86 GeV 2 N  =0.951±0.004 N  =0.994±0.006 Normalization parameters:  

Efficiencies: Efficiencies are defined as    = N sel,ppg / N in,ppg (>>  bkg  = N sel,bkg / N in,bkg ), have to be evaluated for each selection step in each bin of M 2   (usually, they are not flat!). Always First step (and last resort…): trust detector simulation and use efficiencies from MonteCarlo Intermediate steps: Use downscaled events retained during the data taking try to find independent control samples from data Reprocess some data from “raw” data files try to prove that MC is describing the reality, then use MC Tune MC distributions till they match data distributions find someone else doing it for you! (e.g. Photon detection efficiency also needed by other analysis groups) … One needs to evaluate the efficiency on the signal events! (preferrably from data!)

Luminosity: At KLOE, Luminosity is measured using „Large angle Bhabha“ events (55 o <  e <125 o )  KLOE is its own Luminosity Monitor! 2 independent generators used for radiative corrections: –BABAYAGA (Pavia group):  eff = (431.0  0.3 stat ) nb C. M. Carloni Calame et al., Nucl. Phys., B584, 459, 2000 –BHAGENF (Berends modified):  eff = (430.7  0.3 stat ) nb F. Berends and R. Kleiss, Nucl. Phys., B228, 537, 1983 E. Drago and G. Venanzoni, Report INFN-AE-97-48, 1997 Both groups of authors quote a systematic uncertainty of 0.5% The luminosity is given by the number of Bhabha events divided for an effective cross section obtained by folding the theory (QED rad. Corr.) with the detector simulation.

Luminosity: (hep-ex/ ) Systematics on Luminosity Theory0.5 % Acceptance0.3 % Background (  ) 0.1 % Tracking+Clustering Energy Calibration 0.1 % Knowledge of  s run-by-run 0.1 % TOTAL 0.5 % theory  0.3% exp = 0.6 % Apart from the theory uncertainity, systematic effects on the luminosity measurement arise from: slight differences in momentum and polar angle distributions between data and MC,  events misidentified as Bhabhas, Differences in tracking and clustering efficiencies between data and MC Variation of EMC calibration, Changes of √s in the data taking w. r. to nominal value of GeV

Old results and New ideas

Small angle analysis: Acceptance0.3% Trigger0.3% Tracking0.3% Vertex0.3% Offline reconstruction filter0.6% Particle ID0.1% Trackmass cut0.2% Background0.3% Unfolding effects0.2% Total experim. systematics 0.9% Luminosity ( LA Bhabhas )0.6% Vacuum polarization0.2% FSR corrections0.3% Radiator function0.5% Total theoretical Error 0.9% TOTAL ERROR KLOE 1.3% (CMD-2: 0.9%, SND 1.3%) s ( e + e -  p + p - )  nb  M pp 2 (GeV 2 ) KLOE 2001 Data 140pb -1 Statistical error negligible (1.5 Million events) Published Result: Phys. Lett. B606 (2005) 12 Contr. to syst. error on a   :

Small angle analysis (cont‘d): A new analysis is carried out at small photon angles using 2002 data (240pb -1 ) with improved machine background conditions and calibration conditions Goals: a) reduction of the total systematic error <1% Acceptance0.3% Trigger0.3% Tracking * 0.3% Vertex * 0.3% Offline reconstruction filter0.6% Particle ID0.1% Trackmass cut0.2% Background * 0.3% Unfolding effects0.2% Exp. Syst. with 2001 data: 0.9% Error was limited by cosmic veto filter, which caused up to 30% inefficiency CURED by introducing L3-Filter, no cosmic veto inefficiency anymore Main syst. experimental error due machine background dependence of an offline-event filter CURED by changing reconstruction filter, error reduces to <0.1% * Reduction of error, larger data set allows more precise determination

Small angle analysis (cont‘d): M Trk [MeV] Data    No man‘s land Goals: b) perform normalization with   Luminosity ( LA Bhabhas )0.6% Vacuum polarization0.2% FSR corrections0.3% Radiator function0.5% Total theoretical Error 0.9% Some Effects will cancel out in the ratio: x 0.3%  requires to select µµ  events with similar precision as  Pions and muons are separated using a cut in trackmass: 

Test of Radiator function: d  OBS (nb/0.01GeV 2 ) d  OBS (data)/d  OBS (MC) PRELIMINARY! M  2 (GeV 2 )   ee  M  2 (GeV 2 ) Compare µµ  -yield in data with Monte-Carlo simulation (PHOKHARA generator), which is using identical radiative ISR-corrections as for the F  analysis (radiator function) Important cross check of radiator function; (very) preliminary Comparison looks promising!

Large angle analysis: Threshold region non-trivial due to irreducible FSR-effects, which have to be cut from MC using phenomenological models (interference effects unknown)    ffff      FSR f0f0   important!  small! important cross-check with small angle analysis threshold region is accessible photon tagging is possible (4-momentum constraints) lower signal statistics LO-FSR not negligible anymore large       background irreducible bkg. from  decays 50 o <   <130 o 50 o <   <130 o      MC       MC M  2 (GeV 2 )

Large angle analysis (cont‘d): 2002 Data L = 240 pb -1 M  2 [GeV 2 ] Nr. of events / 0.01 GeV 2 PRELIMINARY!       background removed Apply dedicated selection cuts: Exploit kinematic closure of the event  Cut on angle  btw. ISR-photon and missing momentum Kinematic fit in        hypothesis using 4-momentum and  0 -mass as contstraints  cut on  2  reduces background while having high signal efficiency (>98%); Reducible Background from       and µ + µ   well simulated by MC  subtract from data spectrum     Evaluation of all selection efficiencies almost finished! (N)LO-FSR estimated from PHOKHARA generator

Asymmetry: Binner, Kühn, Melnikov, Phys. Lett. B 459, 1999  check the validity of the FSR model used in the MonteCarlo comparing the charge asymmetry between data and MonteCarlo in the presence of FSR. Czyz, Grzelinska, Kühn, hep-ph/  in a similar way, radiative decays of the  into scalar mesons decaying to  +  - contribute to the charge asymmetry Possibility to study the properties of scalar mesons with charge asymmetry      Pion polar angle N () N ()N () N () 90 o In the case of a non-vanishing FSR contribution, the interference term between ISR and FSR is odd under exchange  +   -.This gives rise to a non-vanishing forward-backward asymmetry:

Asymmetry: Data Simulation FSR+ISR Simulation FSR+ISR+f 0 (980)  Clear signal ~ 980MeV but also huge threshold effect, even without f 0 (600)  On  -peak (where scalar amplitude is small) very good agreement btw. data and simulation  precision test of the model of scalar QED for FSR f 0 (980) region Asymmetry M (MeV) Data Simulation FSR+ISR Simulation FSR+ISR+ f 0 (980) *Monte-Carlo used: hep-ph/ G. Pancheri, O. Shekhovtsova, G. Venanzoni (see Talk by O. Shekhovtsova) Plotting the Asymmetry as a function of M      *    : (results from KLOE scalar analysis - PLB 634, (2006) see talk C. Bloise)  model dependence in f 0 (980) amplitude main limitation at threshold!!

DA  NE Off-peak runs: - √s=1020 MeV - √s=1000 MeV    Trackmass [MeV] Run-Nr  s ( MeV )   s = 1023  s = 1030  s = 1018  s = 1010  s = 1000 Physics Case: Backgroundfree Radiative Return Meas. 200pb -1 allow to be competitive with VEPP-2M at threshold   Scan to study model-dependence of f 0 (980) background-free  -program Run at √s=1000 MeV between Dec and March 2006   225pb -1 on tape Energy scan around  -peak  4 scan points, 10pb -1 each Preliminary

Conclusions: KLOE experiment has proven that the radiative return analysis is feasible and has its own merits! Update of small photon angle analysis with 2002 data in progress - the planned normalization to muon pairs will also allow to test radiator function H(s) Large angle analysis with tagged photon allows to access threshold region, result expected soon with improved precision of region around  - peak Main limitations due to contributions from       and  f 0 (980)   dedicated DA  NE-Off-Peak program at √s=1000 MeV will allow ultimate precision for   at DA  NE

Effort to create a Working group on MC generators for s had and Luminosity at low energies First Meeting: Date: October 2006 Place: LNF Frascati - Monte Carlo generators for luminosity; - Monte Carlo generators for e+e- annihilation into leptons and hadrons - Monte Carlo generators for e+e- annihilation into hadrons plus an energetic photon from initial state radiation (ISR) - Monte Carlo generators for tau production and decays Inviting has already been sent to potentially interested people (many of them inside EURIDICE!) Compare existing generators and tools for s had and luminosity If you are interested, please contact: