R Value at BESIII Haiming Hu Workshop on Future PRC-US Cooperation in High Energy Physics Beijing, China June 11, 2006

BESII Detector VC :  xy = 100  m TOF:  T = 180 ps  counter:  r  = 3 cm MDC:  xy = 220  m BSC:  E/  E= 22 %  z = 5.5 cm  dE/dx = 8.5 %   = 7.9 mr B field: 0.4 T  p/p =1.7%  (1+p 2 )  z = 3.1 cm

BESII & BESIII  BESIII has similar components of the subdetectors with BESII.  BESIII has much better resolutions of position, time, momentum, deposit energy, particle identification, and large solid angle, etc.  It is expected that more precise R value, form factors and many QCD tests may be measured at BESIII.

R value The status of R value for E cm < 5 GeV after 2000 The error of the R value measured by BES decreased by a factor of than previous experiments. The issue is now: ☻ Central values deviate 1  in energy region GeV. ☺ Central values coincide at 2.0 and in GeV. R E cm (GeV)

Influence to QED  (s) The QED running electromagnetic coupling constant changes with s Contribution from the vacuum polarization Where

Influence to QED  ( s ) BEPC energy region

Influence to (g-2) ， e + e - and  experiments are incompatible SM predictions differ from experiments by 1.9  [e + e - ] and 0.7  [  ] BEPC energy region

Influence to Higgs mass fitting Before 2000After 2000 The results from BES and other’s were used The results of M H from the global fit of the Standard Model using all data

② Determination of  s ( s ) with R value R value in pQCD expression Three-loop : Four-loop : In fact, OCD gives strict restriction to the R value for the reasonable  s

② Determination of  s ( s ) with R R Solve Equation evolution J.H.kuhn : average

② Determination of  s ( s ) with R BES result (2002): World-average value : The prediction on the precision of  s changing with error of R value R  s (s)  The error of  s will be about one order larger than the error of R value

② Measurement of  s ( s ) with R The prediction on the precision of  s changing with error of R value

Future R measurement Two keys ① precision ~ 1.5 %  large challenge to the experiment ② 2 – 4.5 GeV scan  the wider energy region is scanned, the larger contribution (dispersive integral) to the precise test of the Standard Model. Two methods ① data taking at some energy points, just as did at BESII in 1999 and ② collision is fixed at one energy point, and using initial state radiative return data, just as did at BARBAR.

ISR Method : e + e -   hard f Cross Section for final state f (normalized to radiative dimuons) Detection efficiencies Corrections for final state radiation “effective c.m. energy-squared” = s(1-x) dL(s’) ISR luminosity FSR e-e- e+e+  ISR f = hadrons or     at lowest order

Photo energy in hadronic events The energies of the most photos in hadronic final states are smaller than 1 GeV. The simulation dose not include the photo from the initial state radiation. LUARLW E cm = 3.07 GeV Cut for final state photos

ISR Method : e + e -   hard f Distribution of the effective center-of-mass energy E’ cm for Ecm = 4.2 GeV If E cm is fixed at 4.2 GeV, the number of total hadronic events is 10 7 /year, then the numbers of events in following energy intervals (10 MeV) are (  = 0.7 – 0.5):  E’ cm 3.65 – – – – –2.00 N had Measurement energy region E’ cm < 3 GeV R Form factors Statistic error is larger than 1.1 – 2.2 % for the data taken in one year, these error are too large to be acceptable in the future. E’ cm

R value measurement So we have to wait for about 10 years to obtain the initial radiative samples with the large enough statistics (10 4 – 10 5 events) in each effective energy interval  E cm = 10 MeV This waiting is too long to wait for BESIII and everyone The practical scheme is to measure R value with the conventional method, i.e, use the data with the fixed colliding energies taken at a set of designed energy points in wider energy region (2  4.5GeV)

R value formula R value is measured by following formula In which, the quantities are obtained by  experimental analysis  theoretical calculations  Monte Carlo simulations Notice: this talk is based on the experiences of R value measurement at BESII, the method in the future experiment at BESIII will be different from the old one.

Main contents of the R measurement  Data quality check and correction all information from raw data  Realization and trigger efficiency true realization  Data taking energy points, statistics ~ 0.1%  Hadronic events selection  1-prong or even include 0-prong  Luminosity new Bhabha generator,  (?)  LUARLW tuning and hadronic efficiency distributions & Brs  Initial state radiative correction include multi  radiation  Error analyses systematic & statistic errors  R value ~ 1 - 2% Items Requirements

Strategy for hadronic event selection The inclusive hadronic final states do not have clear characteristics to be used in event selection, the strategies for selecting the hadronic sample from raw data are Raw data  Remove cosmic ray, beam-associated BGs, QED BGs  Obtain the candidate hadronic sample N had obs  Fit event vertexes to remove the remainder beam-associated BGs  Subtract the remainder QED backgrounds statistically, the more precise generators for QED processes, ee, , ,  , are needed.

Lund area law generator hep-ph/ The experimental factors which cause the loss of the produced hadronic events are estimated with the Monte Carlo. At BEPC energy region, the Lund area law based generator LUARLW is a better one.

LUARLW

Tuning of LUARLW parameters There are many free parameters in JETSET and LUARLW, which values needs to be tuned at intermediate energies by comparing with data. For the string fragmentation b: string tension constant For the inclusive particle spectrum in JETSET PARJ(1) : P(qq)/P(q); PARJ(2): P(s)/P(u); PARJ(3) : (P(SU)/P(du))/(P(s)/P(u)) PARJ(11): P(S=1) d,u ; PARJ(12): P(S=1) s ; PARJ(13): P(S=1) c PARJ(14): P(J P =1 + ;L=1;S=0); PARJ(15): P(J P =0 + ;L=a;S=1) PARJ(16): P(J P =1 + ;L=1;S=1); PARJ(17): P(J P =2 + ;L=1;S=1) …….. For the multiplicity of string fragmentation in LUARLW RALPA(15-20) ……. ………

LUARLW parameters tuning Some distributions related to hadronic 3070 MeV Dots : BESII data histograms : LUARLW multiplicity polar angle Feynman momentum x deposit energy vertex time of flight

Luminosity Ecm (GeV) L BSC (nb -1 ) L MDC (nb -1 ) ΔL/L (%) In 2004, BES took data samples at three energy points Preliminary values Two methods are used to calculate the integral luminosity : ① by Barrel Shower Counter (BSC) information ② by Main Drift Counter (MDC) information The typical systematic error is about 1.7% estimated by global way

Hadronic event selection General criteria for selecting hadronic events: Fiducial cuts for charged tracks Track fitting quality requirements Maximum and minimum energy deposition cuts TOF and momentum cuts which include the track level and event level cuts, and they are very similar to the criteria used in Phys. Rev. Lett. 84, 954 (2000) Phys. Rev. Lett. 88, (2002)

Hadronic event criteria For 2-prong or more-prong events Inclusive hadronic events may be classified as 0-prong, 1-prong, and more prongs

Hadronic event criteria For 1-prong hadronic events  Request: 1 good charged track +  0 + (n  0 +neutral tracks)  Use 1-C fitting to identify  0 by decay  0  Invariant mass of  0  of data and LUARLW 2200 MeV2600MeV3070MeV3650MeV

Vertex distribution of the events BESII BESIII + 10 cm+ 1 cm- 1 cm- 10 cm The vertex distribution of BESIII is much narrower than BESII, so the systematic error for the beam-associated background is improved significantly. Notice: some practical simulations do not perform for BESIII at present, so the real vertex distribution for BESIII will wider than 2cm, but much narrower than 10 cm. The number of the hadronic events is obtained by fitting the distribution of the event vertex with the Gaussian and polynomial form, so the narrow vertex distribution will be helpful to reduce the error of number of hadronic events.

Systematic Error R value formula: Where

Error of hadronic event criteria &  had The systematic errors for selecting 1-prong and more-prong events are estimated by the difference of the number of events between data and MC for using or non-using the every cuts

Hadronic efficiency and error If P 0 = 5% (estimated by MC), and the error by 1C fit for selecting  º between data and MC is about 10%, then the effect of the difference of the lost 0-prong events ( the only lost event in measurement) between data and MC to hadronic efficiency  (Ngood  1) was estimated conservatively as: 5%  10 % = 0.5 % 0-prong event

Systematic Error N had L  had  trg 1+  total previous ~ 5 present ~3 future ~1.5 The main relative systematic errors (in %) among the previous and present and future R measurements 3 GeV In order to avoid the bias in R measurement, the idea of the “blind analysis” is insisted !!! ??? The goal that the error of R to be reduced to 1.5% is great challenge.

Summary  The measurement error of R value at BESII with large statistic sample is expected to be reduced to about 3% compared with previous 6%.  The R value at BESIII with much better resolutions and huge statistics is hopeful to have smaller error, saying 2%.

Thank You