Debora Leone Institut für Experimetelle Kernphysik Universität Karlsruhe on behalf of KLOE collaboration Measurement of the hadronic cross section at KLOE

Debora LeoneInstitut für Experimetelle KernphysikUniversität Karlsruhe

on behalf of KLOE collaboration

Measurement of the hadronic cross section at KLOE

Tau04Nara 14-17/09/2004

D. Leone Hadronic cross section @ KLOE

a theory vs. experimentT

HE

OR

Y

‘03

e+ e - Data: 2.7 - Deviation

– Data: 1.4 - Deviation

Exp

erim

ent

’02/

‘04 Experiment BNL-E821

Values for +(2002) and -(2004)in agreement with each other.Precision: 0.5ppm

Dispersion integral for hadroniccontribution to a evaluated for:

a) CMD-2 (Novosibirsk/VEPP-2M) channel with 0.6% precision < 1 GeVb) -Data from ALEPH /OPAL/CLEO

e+e- and data differ for +-

channel in a specific energy window above 0.6 GeV2

(above the peak)

a-11659000(10-10)

Status up to July ’04

The standard method to measure (e+e- hadrons) is the energyscan, i.e. the syst. variation of c.m. energy of the machine.Since at DANE the collision energy is fixed,we use a complementary approach:looking for e+e- +- events, where the photon is emitted in the initial state (ISR),we have a continuous variation of s, theinvariant mass of the hadronic system

hadr

d(e+ e- hadrons + )d

hadrons

(e+ e- hadrons) H(hadr )

Precise knowledge of ISR processRadiator function H(Q2,,M2

)MC generator: PhokharaH. Czyz, A. Grzelinska, J.H. Kühn, G. Rodrigo

4m2< s < m

2


The Radiative Return


selection

Pion Tracks at large angles 50o < < 130o

Photons at small angles < 15o and > 165o

are masked byquadrupoles near the I.P.

(no photon tagging)

)pp(pp miss

High statistics for ISR Photons

Low relative contribution of FSR

Reduced background contamination

e+ e-

EM calorimeter

Drift chamber


dN()/dM2

after acceptance cutsEvent analysis:

Efficiencies and background

Normalize to Luminosity

Radiative CorrectionsCross section (e+e-

cross section

M2 [GeV2]

dN

(

)/

dM

2 [

nb

-1/G

eV

2] 141 pb-1

No acceptance at threshold region

50000

40000

30000

20000

10000

Differential cross sectiond()/dM

2

Divide by Radiator Function

signalregion

tail

Step 1: e+e-are separated from by means of a Likelihood method(signature of EmC-clusters and TOF of particle tracks)


Background Rejection 1/2

Mtr

k [

MeV

]M

2 [GeV2]

0|q||pp|)M|p|M|p|(M 2221

22trk

22

2trk

21

Step 2: and e+e-rejected by means “Trackmass ”

m

m

m

ee


Background Rejection 2/2

Step 3: fit data trackmass distributions with MC ones (signal + background) with free normalization parameters

M2 [0.32,0.37] GeV2 Mtrk [MeV]

Mtrk [MeV]

KLOE uses large angle Bhabha events for the luminosity evaluation:

)1( BkgMC

NdtL

Theory precision (radiative corr.)

•BABAYAGA event generator (Pavia group)• syst. comparison among other generators (Bhagenf, BHWIDE, VEPP-2M ); max. = 0.7 %uncertainty 0.5% =BABAYAGA error

Experimental precisionExcellent agreement data-MC

N = events with 55<<125


Luminosity Measurement

55o 125o

Polar Angle [°]

• data

MC


Analysis (e+e-)

EFFICIENCIES: Trigger + Cosmic Veto Tracking + Vertexing /e separation Reconstruction filter Trackmass cut Unfolding procedure Acceptance

BACKGROUND e+e- e+e- e+e-

LUMINOSITY Bhabha at large angles

0.9 %

0.3 % d(

e+e

-

)

/dM

[

nb

/GeV

2 ]

M [GeV2]

Interference

140 pb-1 of 2001 data1.5 millions events

0.6 %


FSR Contribution

Two kinds of FSRThere is no initial state radiation andthe e+ and the e- collide at the energy M the virtual has Q2 = M

2

Background

Simultaneous presence of a

initial and final photon

The cross section e+e- +- has to be inclusive with respect to NLO FSR events:


FSR Treatment

“FSR Inclusive” approach

(e+e- FSR )

Event analysis Phokhara ISR+FSR

Luminosity

(e+e- ISRFSR )

Radiator H

N(e+e- ISRFSR)add back missing FSR

Correction for unshifting

s

e

e} s

Invariant mass of thesystem , s invariant mass of thevirtual photon s


FSR uncertainty

“FSR Exclusive” approach

N(e+e- ISRFSR)

(e+e- ISR)

(e+e- )

Radiator H

Event analysisPhokhara ISRLuminosity

subtract FSR contributionestimated by MC

FSR

0.8 .. 0.9% Schwinger ‘90

= 0.2% ± 0.1%

Rel. difference inclusive - exclusive approach

0.4 0.5 0.6 0.7 0.8 0.9

1

1.04

1.02

0.98

0.97

0.96

0.95

0.99

1.01

1.03

1.05

1.

The 2 methods are in excellent agreement Higher order FSR corrections negligible Proof of Factorization Ansatz

FSR systematic = 0.3%, coming from 2 contributions0.2% difference incl-excl correctionupper limit of 20% for scalar QED model

(point-like pions): 20% 1% = 0.2%


Cross Section (e+e-+-)

Final spectrum: after the correction for vacuum polarization2

bare )sα(

α(0)σ(s)(s)σ

had(s) from F. Jegerlehner, July 2003

s[GeV2]

Total error = 1.3 %


2 contribution to ahadr

We have evaluated the dispersion integral for 2 channel in the energy range 0.35 <s<0.95 GeV2

2

2

)()(4

13

0.95GeV

0.35GeV

sKds

eea

Comparison with CMD-2 in the energy range 0.37<s<0.97 GeV2

KLOE (375.6 0.8stat 4.8syst+theo) 1.3% ErrorCMD-2 (378.6 2.7stat 2.3 syst+theo) 0.9% Error

At large values of s (>m2) we are consistent with CMD-2 and

we confirm the deviation from -data.

• CMD-2• KLOE

s [GeV2]

|F(s)|

a= (388.7 0.8stat 3.5syst

3.5theo) 10-10


Conclusion

KLOE has proven the feasibility to use initial state radiation to measure hadronic cross section (hep-ex 0407048)

We expect to reduce the systematic error below 1% by repeating the analysis with 2002 data. Improvements from theory are also expected.

The analysis at large photon angle to study the region near the threshold is going on.

Evaluation of ratio R – the analysis has already begun


= data

= MC

preliminary

Polar angle []A

sym

metr

y Test of sQED model: at large photons angles the amount of FSR is large. Here it is possible to study the charge asymmetry. It comes out from the interference between ISR (C-odd) and FSR (C-even)

))

)))

(θN(θN

(θN(θNA(θ

ππ

ππ

[]

+

-

Outlook

Integrating asymmetrywe get a difference data-MC of (8.5 1.2)%

Large angle photon analysis to explore the threshold region tagged measurement


Backup slices


Unfolding the Mass Revolution

s (true) [GeV2]

s (

obs)

[G

eV

2]

Trackmass [MeV]

The smearing matrix “almost” diagonalInversion of smearing matrix possibleA more sophisticated unfolding techniqueis obtained by means of the unfoldingpackage GURU (A. Höcker et.al./ALEPH).

Issues: - Reliability of MC simulation - Correct choice of the regularization parameter

Systematics studied by varying meaningful values of the regularization parameter:

Due to nature of the dispersion integral the effect on a is almost negligible


Terms not included in Phokhara

Unshifting correction


Relative contribution of LO-FSR Relative contribution of NLO-FSR


MTRK

e e

before cutting on particle ID

after cutting on particle ID

m

Likelihood effect on Mtrk distribution

F=1

wit

h F

=1

s (GeV2)

Radiator Function



reconstructedkine

M2 [GeV2]

M2 [GeV2]


KLOE & DANE

e+e- collider @ S = M = 1019.4 MeV

achieved peak Luminosity: 8 1031cm-2 s-1

2000-2002 data set: 500 pb-1

KLOE detector designed for CP violation studies good time resolution t= 57 ps /E(GeV) 54 psin calorimeter andhigh resolution drift chamber (p/p is 0.4% for > 45°), ideal for the measurement of M.

pb

-

1

Documents

Debora Leone Institut für Experimetelle Kernphysik Universität Karlsruhe on behalf of KLOE collaboration Measurement of the hadronic cross section at KLOE