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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 -  p + p - p 0

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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
Outline

  • The Babar experiment

  • Initial state radiation (ISR) at Babar

  • Multihadron cross sections

    • e+e- p+p-p0

    • e+e- 2(p+p-), p+p-K+K-, 2(K+K-)

    • e+e- 3(p+p-), 2(p+p-p0), 2(p+p-)K+K-

    • pp

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


The pep ii b factory
The PEP-II B-factory

9.0 GeV e-

3.1 GeV e+

ECM=10.58 GeV

Peak Luminosity:

1034 cm-2 s-1


The babar detector
The Babar Detector

SVT:

97% efficiency

20mm z resolution

SVT+DCH:

s(pt)/pt=0.13%·pt+0.45%

EMC:

s(E)/E=2.32%·E-1/4+1.85%

1m



Isr at y 4s energy

gISR

e-

hadrons

s’

e+

ISR at Y(4S) energy

  • s of the machine fixed at 10.58 GeV

  • Radiation of a hard photon  s’=0-5 GeV

  • The gISR is detected in the analysis (acceptance ~10-15%)

    • Eg ~ 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”

  • m+m- used for luminosity normalization


Isr luminosity
ISR luminosity

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

x=2Eg/s

s’=s(1-x)

Probability of ISR photon emission:


Cross section calculation
Cross section calculation

dNfg = number of ISR f events

dNmmg = number of ISR mm events

dFSR = final state radiation correction (negligible for most f)


E e p p p 0
e+e-p+p-p0

c2 distribution in the range 1.05<M3p<3 GeV/c2

Selection:

  • particles inside fiducial volume

  • Eg>3 GeV for at least one g

  • kinematic fit applied

  • fit c2 used for event selection

    Backgrounds:

  • p+p-p0p0, p+p-, K+K-p0, t+t-, ...

    Bkg suppression cuts (improve S/B)

  • no track identified as K

  • Ep0>0.4 GeV

  • kinematical fit to 4p (to reject p+p-2p0)

  • Mpg>1.5 GeV/c2 (to reject tt)

Prel. selection (data/MC)

p+p-p0p0

other bkg processes

Selected

Bkg/efficiency study

89 fb-1

PRD 70, 072004 (2004)


E e p p p 01
e+e-p+p-p0

Inconsistent

with DM2

SND

Babar

DM2

w’’

M=1350±28 GeV/c2

G=450±100 GeV/c2

w’

w

M=1660±10 GeV/c2

G=230±36 GeV/c2

f


Systematic uncertainties
Systematic uncertainties

e = efficiency

eMC = eff. measured on MC

d = correction


E e 4h
e+e- 4h

p+p-p+p-

K+K-p+p-

K+K-K+K-

Selection:

  • particles inside fiducial volume

  • Eg>3 GeV for at least one g

  • 4 charged tracks

  • kinematical fits applied

    • 4p always applied

    • 2p2K and 4K applied if K are present

  • fit c2 used for event selection

    Background:

  • ISR t+t- and multihadron

    Separation of different channels based

    on the c2 for the different kinematical fit

4p candidates

4p MC

Non ISR bkg (JETSET)

89 fb-1

PRD 71, 052001 (2005)


E e p p p p
e+e- p+p-p+p-

Signal

Signal region (MC)

ISR background (control region)

Control region (MC)

Non ISR background (JETSET)

Selection efficiency vs mass (MC)

<1%

3-5%

10%

Syst. error due to bkg subtraction


E e p p p p1
e+e- p+p-p+p-

Comparison with previous experiments

Measured 4p cross section

Only statistical error shown


E e p p p p2
e+e- p+p-p+p-


E e p p k k
e+e- p+p-K+K-

c2(2p2K)<20

c2(4p)>30

c2(4K)>20

0.5% 4p contamination

negligible 4K contamination

  • 15% systematic uncertainty

  • Dominated by:

  • 10% uncertainty in the acceptance (MC)

  • 5% difference data/MC in K ID

Only statistical error shown


E e p p k k1
e+e- p+p-K+K-

2p2K channel dominated by intermediate K*0Kp

After removing events in K*0 bands

f

K*0(892)

r

“Other” Kp combination


E e k k k k
e+e- K+K-K+K-

c2(4K)<20

high purity sample

  • 25% systematic uncertainty

  • Contributions:

  • absence of detailed model for acceptance

  • uncertainty in bkg subtraction

  • difference data/MC in K ID

Only statistical error shown


E e 6h
e+e- 6h

3(p+p-)

2(p+p-)p0p0

K+K-2(p+p-)

Selection:

  • particles inside fiducial volume

  • Eg>3 GeV for at least one g

  • 6 tracks or 4 tracks and 4g (Eg>20 MeV)

  • kinematical fits applied

  • fit c2 used for event selection

    Background:

  • ISR t+t- and multihadron

232 fb-1


E e 3 p p
e+e- 3(p+p-)

Signal

ISR background (control region)

Signal region (MC)

Non ISR background (JETSET)

Control region (MC)

Selection efficiency vs mass (MC)

<3%

3-5%

Syst. error due to bkg subtraction


E e 3 p p1
e+e- 3(p+p-)

Intermediate state r0(770)2(p+p-) assumed in MC production

All pions

point = data

histogram = MC


E e 3 p p2
e+e- 3(p+p-)

r0(770)2(p+p-) model gives a very good description of the data

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

A 6p 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

MC

(J/y not included)

Data


E e 3 p p3
e+e- 3(p+p-)

Structure at 1.9 GeV also seen by FOCUS

FOCUS collab.

PLB 514, 240 (2001)

Only statistical error shown


E e 3 p p4
e+e- 3(p+p-)


E e 2 p p p 0
e+e- 2(p+p- p0)

  • Additional bkg reduction cuts:

  • require Eg>50 MeV

  • K veto on tracks

Structure at 1.9 GeV confirmed

in 2(p+p-p0)

Only statistical error shown


E e 2 p p p 01
e+e- 2(p+p- p0)


E e 2 p p p 02
e+e- 2(p+p- p0)

Data

MC

Presence of resonant structures

w

r+

J/y

w

h

w

r0

f0

p+p-p0 mass

“other” p+p-p0 mass

Evidence of wh production


E e 2 p p p 03
e+e- 2(p+p- p0)

Structure in wh channel

Fit to resonance:

m=1.645±0.008 GeV/c2

G=0.114±0.014 GeV/c2

peak cross section=3.08±0.33 nb

2(p+p-p0)

wp+p-p0

f(1680) ?

w(1650) ?

wh


Combining the 6 p channels
Combining the 6p channels

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

Continuum

Resonance

m = 1.88±0.03 GeV/c2

G = 0.13±0.03 GeV

f = 21±40˚

m = 1.86±0.02 GeV/c2

G = 0.16±0.02 GeV

f = -3±15˚


E e 2 p p k k
e+e- 2(p+p-)K+K-

  • At least one particle identified as K

  • Kinematical fit

  • c2(6p)>20

  • 15% systematic uncertainty

  • Dominated by:

  • 10% uncertainty in the acceptance (MC)

  • 5% difference data/MC in K ID

Only statistical error shown


E e p p
e+e- pp

  • Study of:

  • cross section (continuous spectrum from threshold, s=2mp, in a single measurement)

  • form factor in the time-like region

C = Coulomb correction factor

Allows s finite at threshold

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

Good PID and kinematical fit required

232 fb-1

hep-ex/0512023


E e p p1
e+e- pp

30% signal loss

ISR bkg suppression factors:

15·103 for pp

500·103 for mm

2·103 for KK

Selection:

  • particles inside fiducial volume

  • Eg>3 GeV for at least one g

  • 2 charged tracks, both with proton ID

  • kinematical fit to C+C-g applied

    • C = e, m, p, K, p

25% signal loss

ISR bkg suppression

factors:

50 for pp, mm

30 for KK

p

K

p

K


E e p p2
e+e- pp

1. ISR backgrounds

Background

Data

~4000 events

N. of bkg events

pp 5.9±2.5

KK 2.5±1.0

ee 2.5±1.0

mm <11

K

p

m


E e p p3
e+e- pp

Expected mpp distrib.

from ppp0 events

2. pp + photons background

Dominant background contribution from ppp0

Kinematical fit to ppgg performed


E e p p4
e+e- pp

Detection efficiency

10% systematic error

From the difference between pure electric

(GM=0) and pure magnetic (GE=0) values


E e p p5
e+e- pp

Cross section (systematic error included)


E e p p6
e+e- pp

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

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

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

Fit to the distribution:

  • Free parameters:

  • A (normalization)

  • |GE/GM|


E e p p7
e+e- pp

Angular distributions for different s regions

|GE/GM| measurement from

angular distributions

(stat. and syst. errors included)

GM=0

GE=0


E e p p8
e+e- pp

Definition of an “effective form factor” under the assumption |GE|=|GM|

Fit to asymptotic behavior

At threshold


Summary and perspectives
Summary and perspectives

  • Extensive ISR analysis program in Babar

  • Full spectrum from threshold to ~4.5 GeV

  • All main hadronic channels under study

    • pp, KK, LL

    • p+p-2p0, p+p-3p0

    • K+K-p0, K+K-2p0, fp0

    • p+p-p0h, fh

    • ...



Systematcs overlook
Systematcs overlook

  • Luminosity from mm: 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%


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