r value at besiii haiming hu workshop on future prc-us cooperation in high energy physics beijing,...
TRANSCRIPT
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