measurement of e + e - to multihadron cross sections using initial state radiation in babar

Measurement of e+e- to multihadron cross sections

using Initial State Radiation in Babar

Matteo Negrini

Frascati, Jan 19, 2006

Outline• The Babar experiment• Initial state radiation (ISR) at Babar• Multihadron cross sections

– e+e- +-0 – e+e- 2(+-), +-K+K-, 2(K+K-) – e+e- 3(+-), 2(+-0), 2(+-)K+K-

– pp

Disclaimer: extensive study of J/ decays in the same channels is not presented here

The PEP-II B-factory9.0 GeV e-

3.1 GeV e+ ECM=10.58 GeV

Peak Luminosity:1034 cm-2 s-1

The Babar Detector

1m

SVT:97% efficiency 20m z resolution

SVT+DCH:(pt)/pt=0.13%·pt+0.45%

EMC:(E)/E=2.32%·E-1/4+1.85%

Particle Identification

ISR at Y(4S) energy

s of the machine fixed at 10.58 GeV• Radiation of a hard photon s’=0-5 GeV s’

e-

e+

ISR

hadrons

• The ISR is detected in the analysis (acceptance ~10-15%)– E ~ 3-5 GeV small fake beam gas background

• The hadronic state is collimated on the recoil• Final state radiation contribution small and well separated

s’ determined from the final state– energy resolution depends on tracking– entire energy scan of the cross sections in a single “shot”

• +- used for luminosity normalization

ISR luminosity

))1((),( xsxsWdxd

ff

211ln22),(

2xxms

xxsW

e

ISR cross section for the production of the final state f:

x=2E/ss’=s(1-x)

Probability of ISR photon emission:

Cross section calculation

)1()'(1)'( FSR

ff

ff

dNsdL

s

)1()'(

)'( FSR

sdNsdL

dNf = number of ISR f eventsdN = number of ISR eventsFSR = final state radiation correction (negligible for most f)

)'()1()1(

)'( sdNdN

s FSRff

FSRf

f

e+e- +-0

89 fb-1 PRD 70, 072004 (2004)

Selection:• particles inside fiducial volume• E>3 GeV for at least one • kinematic fit applied • fit 2 used for event selection

Backgrounds:• +-00, +-, K+K-0, +-, ...

Bkg suppression cuts (improve S/B)• no track identified as K• E0>0.4 GeV• kinematical fit to 4 (to reject +-

20)• M>1.5 GeV/c2 (to reject )

other bkg processes

Prel. selection (data/MC)

2 distribution in the range 1.05<M3<3 GeV/c2

+-00

Selected Bkg/efficiency study

e+e- +-0

’’’

Babar

SND

DM2

M=1350±28 GeV/c2

=450±100 GeV/c2

M=1660±10 GeV/c2

=230±36 GeV/c2

Inconsistentwith DM2

Systematic uncertainties)1/( MC

= efficiencyMC = eff. measured on MC = correction

e+e- 4h +-+-

K+K-+-

K+K-K+K-

89 fb-1 PRD 71, 052001 (2005)

Selection:• particles inside fiducial volume• E>3 GeV for at least one • 4 charged tracks• kinematical fits applied

– 4 always applied– 22K and 4K applied if K are present

• fit 2 used for event selection

Background:• ISR +- and multihadron

Separation of different channels based on the 2 for the different kinematical fit

4 candidates4 MC

Non ISR bkg (JETSET)

e+e- +-+-Signal

ISR background (control region)Non ISR background (JETSET)

10% <1% 3-5%Syst. error due to bkg subtraction

Signal region (MC)Control region (MC)

Selection efficiency vs mass (MC)

e+e- +-+-

Only statistical error shown

Measured 4 cross section Comparison with previous experiments

e+e- +-+-

e+e- +-K+K- 2(22K)<202(4)>302(4K)>20

0.5% 4 contaminationnegligible 4K contamination

Only statistical error shown

15% systematic uncertaintyDominated by:• 10% uncertainty in the acceptance (MC)• 5% difference data/MC in K ID

e+e- +-K+K- 22K channel dominated by intermediate K*0K

K*0(892)

After removing events in K*0 bands

“Other” K combination

e+e- K+K-K+K- 2(4K)<20

high purity sample

25% systematic uncertaintyContributions:• absence of detailed model for acceptance• uncertainty in bkg subtraction• difference data/MC in K ID

Only statistical error shown

e+e- 6h 3(+-)2(+-)00

K+K-2(+-)

232 fb-1

Selection:• particles inside fiducial volume• E>3 GeV for at least one • 6 tracks or 4 tracks and 4

(E>20 MeV)• kinematical fits applied • fit 2 used for event selection

Background:• ISR +- and multihadron

e+e- 3(+-) ISR background (control region)

Non ISR background (JETSET)

Signal

Signal region (MC)Control region (MC)

Selection efficiency vs mass (MC)Syst. error due to bkg subtraction

<3% 3-5%

e+e- 3(+-) Intermediate state (770)2(+-) assumed in MC production

All pions

point = datahistogram = MC

e+e- 3(+-)

0(770)2(+-) model gives a very good description of the data

No other significant structures observed (but full partial wave analysis not done)

A 6 phase space simulation does not produce deviations in the angular distribution

The acceptance varies by less than 3%, which is taken as a systematic uncertainty

Data MC (J/ not included)

e+e- 3(+-) Structure at 1.9 GeV also seen by FOCUS

FOCUS collab.PLB 514, 240 (2001)

Only statistical error shown

e+e- 3(+-)

e+e- 2(+- 0)

Additional bkg reduction cuts:• require E>50 MeV• K veto on tracks

Structure at 1.9 GeV confirmedin 2(+-0)

Only statistical error shown

e+e- 2(+- 0)

e+e- 2(+- 0)Presence of resonant structures

J/

DataMC

+

f0

0

+-0 mass “other” +-0 mass

Evidence of production

e+e- 2(+- 0)Structure in channel

2(+-0)+-0

Fit to resonance:m=1.645±0.008 GeV/c2

=0.114±0.014 GeV/c2peak cross section=3.08±0.33 nb

(1680) ?(1650) ?

Combining the 6 channels

m = 1.88±0.03 GeV/c2

= 0.13±0.03 GeV = 21±40˚

m = 1.86±0.02 GeV/c2

= 0.16±0.02 GeV = -3±15˚

Fit to Breit-Wigner structure by several states decaying to the same mode

2

2

2

2/3

24

cont

i

Asims

egms

a

msb

cont mseccA

20

)/(

10 )(

0

Continuum

Resonance

e+e- 2(+-)K+K- • At least one particle identified as K• Kinematical fit• 2(6)>20

Only statistical error shown

15% systematic uncertaintyDominated by:• 10% uncertainty in the acceptance (MC)• 5% difference data/MC in K ID

e+e- pp

2

22

2

|)(|2

|)(|3

4)( sGsm

sGsCs E

pMpp

yeyC

1

hep-ex/0512023232 fb-1

Study of:• cross section (continuous spectrum from threshold, s=2mp, in a single measurement)• form factor in the time-like region

sm

y p

C = Coulomb correction factorAllows finite at threshold

Experimental challenging because of larger 2-body backgrounds: , , KK

Good PID and kinematical fit required

e+e- pp Selection:• particles inside fiducial volume• E>3 GeV for at least one • 2 charged tracks, both with proton ID• kinematical fit to C+C- applied

– C = e, , , K, p

p

p

K

K25% signal loss

ISR bkg suppressionfactors:50 for , 30 for KK

30% signal loss

ISR bkg suppression factors:15·103 for 500·103 for 2·103 for KK

e+e- pp

Data Background

K

N. of bkg events 5.9±2.5KK 2.5±1.0ee 2.5±1.0 <11

1. ISR backgrounds

~4000 events

e+e- pp 2. pp + photons background

Dominant background contribution from pp0

Kinematical fit to pp performed

Expected mpp distrib.from pp0 events

e+e- pp Detection efficiency

10% systematic error

From the difference between pure electric(GM=0) and pure magnetic (GE=0) values

e+e- pp Cross section (systematic error included)

e+e- pp

*22

2*22

2sin)(4)cos1()(

4

sG

smsG

sC

dd

EN

M

|GE|/|GM| measurement from p angular distribution

Two samples of pp events generated: one with GE=0 the other with GM=0

Angular distributions HM(cos*,s) and HE(cos*,s) obtained from the simulation

Fit to the distribution:

),(cos),(cos

cos**

* sHGGsHA

ddN

EM

EM

Free parameters:• A (normalization)• |GE/GM|

e+e- pp

|GE/GM| measurement from angular distributions(stat. and syst. errors included)

Angular distributions for different s regions

GM=0GE=0

e+e- pp Definition of an “effective form factor” under the assumption |GE|=|GM|

22

2

|)(|3

4)( sFsCspp

At threshold

)/(log/~ 222 ssCFFit to asymptotic behavior

Summary and perspectives

• Extensive ISR analysis program in Babar• Full spectrum from threshold to ~4.5 GeV• All main hadronic channels under study

– , KK, – +-20, +-30

– K+K-0, K+K-20, 0

– +-0, – ...

BACKUP

Systematcs overlook

• Luminosity from : 3%• Background subtraction: ~1-10%

– larger where the cross section is small• Acceptance from simulation: 1-3%

– ~10% in some cases (model dependence)• Data/MC differences in tracking/PID: ~3-5%