measurement of e + e - to multihadron cross sections using initial state radiation in babar
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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%