eps (july 17th-23rd 2003) in aachen, germany
DESCRIPTION
The Hadronic Cross Section Measurement at KLOE Marco Incagli - INFN Pisa on behalf of the KLOE collaboration. EPS (July 17th-23rd 2003) in Aachen, Germany. Im[ ] | hadrons | 2. Still measuring hadronic cross section: why?. - PowerPoint PPT PresentationTRANSCRIPT
The Hadronic Cross Section Measurement at KLOE
Marco Incagli - INFN Pisa
on behalf of the KLOE collaboration
EPS (July 17th-23rd 2003) in Aachen, Germany
Still measuring hadronic cross section: why?
The hadronic cross section is a fundamental tool to evaluate the hadronic contributions to a and to (MZ)
These quantities are not evaluable in pQCD, but one can use DATA by means of optical theorem + analyticity:
For example a can be evaluated with the dispersion
integral:
Im[ ] | hadrons |2
ahad =
K(s) ~ 1/s (kernel function)
243
, )()(4
1
m hadree
lohad dssKsa
The factors 1/s and (ee hadr) in the
integrand of the dispersion relation make the
low energy
region and the large resonances
particularly relevant
The ee channel accounts for ~70% of the
contribution both to ahad and to (a
had)
Contributions, as of today, to the error (a
had)
(s<0.5GeV)
(except )
( region)
rest (<1.8GeV)
rest (1.8-5 GeV)
pQCD (>5GeV)
(a(ahadhad))
(from Davier, Eidelman, Hoecker, Zhang)
The role of the
aHAD can also be evaluated
starting from data and using the
(approximate) isospin invariance
The recent very precise
BNL determination of a
and some discrepancies
between the value of aHAD
as evaluatated with ee energy scan and data, make a new measurement relevant
ee
(had) through the radiative return at KLOE
A way to get the hadronic cross section (e+e) vs Q2 at a fixed energy machine: the Radiative Return the Radiative Return
(Binner, Kuehn, Melnikov, Phys.Lett. B 459 (1999) (Binner, Kuehn, Melnikov, Phys.Lett. B 459 (1999) 279)279)
EE
QQ22
Radiation functionH(Q2,) M
s
QsE
s ;
2
2
),,()()( 22
22
MQHee
dQ
eedQ
Radiative ReturN: PROs and CONs
luminosity and energy scale is estabilished at s=M and
applies to all values of M2=Q2
do not need to run the collider at different energies
requires precise understanding of radiative processes
MC used by KLOE : PHOKHARA ver.2.0 (on Tuesday 10, Jul 2003 we have received ver.3.0 which includes FSR!)
DANE e+e- machine at Frascati (Rome)
• e+e s ~ m = 1019.4 MeV
• beams cross at an angle of 12.5 mrad • LAB momentum p ~ 13 MeV/c
BR’s for selected decays
K+K- 49.1%
KSKL 34.1%
+ 15.5%
ee+
KLOE detectorKLOE detector
Cross sections:
3.3 b
eeb
eebeob
b
KLOE detector and Fiducial Volume
Definition of fiducial volume:
50o<<130o
<15o or >165o
where is the two-pion system
This cut enhances the signal wrt ee events in which the photon is radiated from the pion (final state radiation: FSR)
6 m
7 m
The price is that the kinematic region below Q2=0.3GeV2 cannot be probed by these small photon angle events
Getting the cross section
L=140.7pb-1 of data collected in
2001
1.5106 evts ~11000 evts/pb-1
0.35<Q2<0.97 GeV2 (592-985
MeV)
Bin width = 0.01 GeV2 (~7 MeV)
To get the cross section must
evaluate:
background ; efficiencies ;
luminosity
d
dM 2
Nobs N bkg
M2
1
Select.
1
L
Background
Selection efficiency Luminosity
events
M2 (GeV2)
num
ber
of
even
ts (
x10
3)
Background rejection I - e/ separation
e/ separation using a likelihood method:
• electron and pion likelihood definition based on TOF and cluster shape
• the log of the ratio of the two likelihoods is the discriminating variable
eff() ~ 98%
eff(e) ~ 3%log(Lpion/Lelectron)
signal + bkgd events• events• e+e events
Kinematic separation between
signal and background in the
(M2,MTRK) plane where MTRK is
defined as:
(p-p-p)2=p2=0
with: p=( p2+MTRK
2,p)
this cut effects multiphoton
processes (ee)
efficiency evaluated using MC
Background rejection II - closing the kinematics
ee
signalregion
M (GeV2)
MTRK (MeV)
tail
Efficiency of kinematic separation and FSR
The efficiency of the (M2,MTRK) cut
has been evaluated by MCThis efficiency evaluation does not include events with a FSR photon M
TR
K e
ffic
ienc
y
M2(GeV2)
A preliminary run with the new PHOKHARA shows that the FSR contribution is at most 2-3%
As of now, we do not apply any correction for FSR and add a contribution of 2% to the systematic error
M2(GeV2)
1 - )(
)&
(2
2dQISR
ddQ
FSR
ISR
d
• without TrackMass cut• with TrackMass cut
A.Denig, H.Czyz
peak
Luminosity with Large Angle Bhabhas
Luminosity measured with Large Angle Bhabhas: 55o<e<135o
2 independent generators used for radiative corrections: BABAYAGA (Pavia group): eff = (428.80.3stat) nb
BHAGENF (Berends modified): eff = (428.50.3stat) nb
Systematics from generator claimed to be 0.5%
Experimental systematic error determined by comparing data and MC angular and momentum distributions
Systematics on Luminosity
Theory 0.5 %
Acceptance 0.3 %
Background () 0.1 %
Trigger+Track+Clustering 0.2 %
Knowledge of s run-by-run 0.1 %
TOTAL 0.5 % theory 0.4% exp = 0.6 %
Summary of systematics
Experimental Acceptance 0.3% Trigger 0.2% Tracking 0.3% Vertex 1.0% Likelihood 0.1% Track Mass 0.2% BKG subtr. 0.5% Unfolding 0.6%
TOTAL 1.4% (1%)
Theory Luminosity 0.6% Vacum Pol. 0.1%
TOTAL 0.7%
FSR (NNLO processes)2.0%
(<1%)
Systematic error can be reduced to • in a short time scale
Observed cross section
Absolute ee cross section after bkg subtraction
To get (ee) we need the H(Q2) function
eeISR(
Radiation function H(Q2)H(Q2) is obtained from
PHOKHARA MC setting F(Q2)=1
and swithcing off vacuum polarization
(ee) ~
d/d
Q2 (
nb/G
eV2 )
M2 (GeV2)
e
e
F(Q2)
e
e
V.P.
1
1.02
1.04
1.06
1.08
0.2 0.4 0.6 0.8 1.0
Hadronic cross section
Hadronic cross section after dividing by the function H(Q2)The cross section to be inserted in the dispersion integral is the bare cross section
ee
d/d
Q2 (
nb/G
eV2 )
M2 (GeV2)
M2 (GeV2)
Must correct for running of
)()()()(1
)()( 20
2
02 ssss
ss barehadlep
(s)
(co
rrec
tion
tos had(s) from F. Jegerlehner
Preliminary value for ahad
In order to see how our result compares with existing data, we have integrated the bare cross section in the same region covered by CMD2 (0.37<Q2<0.95):
ahad(0.37:0.95) = 374.1 1.1stat 5.2syst 2.6theo (+ 7.5
0.
FSR)
The published CMD-2 result is :
ahad(0.37:0.95) = 368.1 2.6stat 2.2syst+theo
The two numbers are compatible, given the systematic error, but FSR corrections must be included before performing a detailed point to point comparison
Comparison e+e vs data
Q2 KLOEahad CMD2* a
had
0.37:0.6 256.2 4.1 (+5.1-0FSR) 249.7 2.2
0.6:0.95 117.9 2.1 (+2.3-0FSR) 119.8 1.1
10-15% relative difference
The difference with CMD2 value is mostly below the peak
It is very difficult, with our data, to explain the discrepancy between e+e and data in the region above the resonance
* our evaluation based on CMD2 published table
Q2 (GeV2) peak
Summary and outlook
KLOE has shown the feasibility of using initial state radiation to obtain the hadronic cross section at low energies
Measurement using small angle photon events is almost
finalized we have a new MC for a more precise evaluation of FSR
Preliminary result on ahad slightly higher, but compatible with,
CMD2 value
Next steps: Finalize current analysis
Study events at large photon angles which
allow us to cover the region (2m)2<M2<0.35 GeV2
Use events as normalization sample to reduce the systematic error