R Value at BESIIIHaiming 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+p2) 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 valueThe status of R value for Ecm < 5 GeV after 2000

The error of the R value measured by BES decreased by a factor of 2-3 than previous experiments. The issue is now:

☻ Central values deviate 1 in energy region 2.2-2.7GeV.

☺ Central values coincide at 2.0 and in 2.8-3.73GeV.

R

Ecm (GeV)

Influence to QED (s)The QED running electromagnetic coupling constant changes with s

Contribution from the vacuum polarization

Where

Rres Rcon )s(R

Influence to QED (s)

BEPC energy region

Influence to (g-2)QED had weak2

2

ga a a a

2

2had

2

4

( ) ( )

3m

K sa ds R s

s

，

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 2000 After 2000

The results fromBES and other’swere used

GeVm

GeVm

H

H

170

62 5330

GeVm

GeVm

H

H

212

98 5838

The results of MH from the global fit of the Standard Model using all data

② Determination of s (s) with R value

R value in pQCD expressionThree-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 )(ss )0.5(s )(MzsSolve Equation evolution evolution

J.H.kuhn :

)0.5(saverage

② Determination of s (s) with R

001.01184.0)( 0087.00118.0

Mzs

0042.01186.0)( MzsBES 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 RThe 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 2004.

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

)'()1(

)1()'( sdN

dNs

eefradf

radff

)1(),(),(

xsxsWdx

xsdf

Cross Section for final state Cross Section for final state ff (normalized to radiative dimuons) (normalized to radiative dimuons)

Detection efficiencies Corrections for

final state radiation

“effective c.m. energy-squared” = s(1-x)

s

Ex

*2

dL(s’)ISR luminosity

FSR

FSR

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

Ecm = 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 Ecm is fixed at 4.2 GeV, the number of total hadronic events is 107/year, then

the numbers of events in following energy intervals (10 MeV) are ( = 0.7 – 0.5):

E’cm 3.65 – 3.64 3.64 –3.63 3.00 – 2.99 2.50 – 2.49 2.01 –2.00

Nhad 8000 7820 4000 5000 2000

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 (104 – 105

events) in each effective energy interval Ecm = 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 (24.5GeV)

R value formula

R value is measured by following formula

0 (1 )

obshad BG l ll

trg had

N N N NR

L

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 selectionThe 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 Nhadobs

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/9910285

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 parametersThere 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(JP=1+;L=1;S=0); PARJ(15): P(JP=0+;L=a;S=1)

PARJ(16): P(JP=1+;L=1;S=1); PARJ(17): P(JP=2+;L=1;S=1)

……..

For the multiplicity of string fragmentation in LUARLW

RALPA(15-20) ……. ………

LUARLW parameters tuning

Some distributions related to hadronic criteria @ 3070 MeV

Dots : BESII data histograms : LUARLW

multiplicity

polar angleFeynman momentum x

deposit energyvertex

time of flight

Luminosity

Ecm

(GeV)

LBSC

(nb-1)

LMDC

(nb-1)

ΔL/L

(%)

2.60 1231 1229 0.15

3.07 2273 2257 0.75

3.65 6450 6446 0.10

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, 101802 (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 + (n0+neutral tracks)

Use 1-C fitting to identify 0 by decay 0

Invariant mass of 0 of data and LUARLW

2200 MeV 2600MeV 3070MeV 3650MeV

Vertex distribution of the events

BESIIBESIII

+ 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

)1(L

N~

)1(L

NR

trg

had

hadtrg

had

R value formula:

Where

MCobs

hadMCgenMC

genMCobs

had

had

hadhad N

NN

N/N

NNN~

22

had

had

22

had

had

2

sys 1

]1[

L

L

N~N~

R

R

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 P0 = 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 (Ngood1) was estimated conservatively as:

5% 10 % = 0.5 %

0-prong event

Systematic Error

Nhad L had trg 1+ total

previous 3.30 2.30 2.66 0.5 1.32 ~ 5

present 1.5 1.5 1.7 0.5 1 ~3

future 0.5 0.5 1 0.3 0.5 ~1.5

The main relative systematic errors (in %) among the previous and present and future R measurements are

@ 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