Study%20of%20e e-?%20pp%20process%20using%20initial%20state%20radiation%20with%20BaBar - PowerPoint PPT Presentation

About This Presentation
Title:

Study%20of%20e e-?%20pp%20process%20using%20initial%20state%20radiation%20with%20BaBar

Description:

... ADONE and BEPC e e- experiments and from pp experiments at LEAR and Fermilab. ... We confirm near-threshold enhancement observed in LEAR experiment. ... – PowerPoint PPT presentation

Number of Views:60
Avg rating:3.0/5.0
Slides: 24
Provided by: nber6
Category:

less

Transcript and Presenter's Notes

Title: Study%20of%20e e-?%20pp%20process%20using%20initial%20state%20radiation%20with%20BaBar


1
Study of ee-? pp process using initial state
radiation with BaBar
  • V. Druzhinin (BINP,Novosibirsk) for BaBar
    Collaboration

2
ISR method
Mass spectrum of pp system in the ee- ? pp?
reaction is related to cross section of ee- ? pp
reaction at Em.
ee- ? pp cross section depends on two form
factors, electric GE and magnetic GM.
The ratio of form factors GE/GM can be obtained
from the analysis of the proton angular
distribution. The terms corresponding GM and GE
have angular dependence close to 1cos2? and
sin2?, respectively.
3
Existing data
The existing data come from DCI, ADONE and BEPC
ee- experiments and from pp experiments at LEAR
and Fermilab. All data were obtained under
GEGM assumption. The GE/GM ratio was
measured at LEAR and found to be compatible to
unity. We are going to measure the ee- ?pp from
threshold up to 4.5 GeV. The advantage of ISR
method over conventional ee- and pp measurements
is low dependence of the detection efficiency
on mass and proton angle.
4
BaBar detector
  • DIRC
  • Quartz Cherenkov radiator
  • Covers 80 of solid angle
  • Particle ID above 600 MeV/c
  • DCH
  • 40 layers, axial and stereo wires.
  • Covers 92 of solid angle
  • dpt/pt 0.5 -1.5
  • Particle ID up to 600 MeV/c
  • EMC
  • 6580 CsI(Tl) crystals
  • Covers 91 of solid angle
  • E resolution 2 at high E.

5
Event selection
Data 232 fb-1 collected during 1999-2004
Process Nexpect
pp? 4k
??-? 1000k
??-? 2000k
KK-? 100k
  • At least 2 tracks with
  • Rxy lt 2.5 cm
  • ?z lt 6cm
  • p? gt 0.1 GeV/c
  • 0.45 lt ? lt 2.4
  • A photon with ECM gt 3 GeV and 0.375 lt ? lt 2.4
  • Two tracks must pass proton selector
  • The kinematic fit is applied imposing energy and
    momentum conservation under different mass
    hypotheses

6
Event selection
K
p
p
K
7
KK-, ??-, ??-, ee- background
KK- background contribution is estimated from
the number of events with two identified protons
but with ?2K lt20. The scale factor is obtained
from the event sample with one identified photon
and MC simulation. The similar method is used
for ??- background. The ee- and ??-
background is estimated using the difference
between their mass distributions and pp mass
distribution for mass greater than 4.5 GeV.
data ??- KK- ??- ee-
4025 5.9 2.5 2.5 1.1 lt 11 0.8 0.8
8
KK-, ??-, ??-, ee- background
data
K
?
?
9
ee- ? pp? 0 background
The most considerable source of background is the
process ee-?pp?0 with lost soft photon or with
merged photons from ?0 decay. These events have
?2 distribution peaked at low ?2 and can not be
separated from the signal events. The events of
the ee-?pp?0 process were selected using ?2 of
the kinematic fit to ee-?pp?? hypothesis and
requirement on two-photon invariant mass. The
mass spectrum of pp?0 events passed our standard
selection is determined as KMC(dN/dM)data. The
scale factor found from simulation is about 3.
Mpp lt2.5 2.5-3.0 3.0-4.5 gt4.5
Ndata 3166 322 57 4
Npp? 171 29 33 11 25 8 5 3
10
ISR background
The main source of ISR background is ee-?pp?0?
process. Using the kinematic fit to pp?0?
hypothesis we select 84731 events of this
process. From the ratio of the detection
efficiencies for pp? and pp?0? selections,
0.0150.002, we estimate the number of ee-?pp?0?
events in pp? event sample is 133. The similar
procedure is used for ee-?pp2?0? process. The
56030 selected events of this process translate
to 0.5 0.3 events passed pp? selection.
N(1440)
ee-?qq with other than pp?0 final state
is studied with JETSET simulation. Simulation
yields 264 events. Two final states, pp2?0 and
pp?, dominate with 17 and 5 events, respectively.

11
Background subtraction
??-? KK-? pp?0 pp?0? uds pp? data
N1 5.9 2.5 2.5 1.0 229 32 13 3 26 4 3737 75 4025
N2 4.2 1.8 1.3 0.5 29 5 20 3 37 5 179 12 290
? 0.71 0.05 0.52 0.04 0.13 0.01 1.53 0.25 1.44 0.30 0.048 0.003
N1 and N2 are numbers of events with ?2 lt 30 and
30 lt ?2 lt60, respectively. ?N1/N2
Since the result of JETSET for rare processes
requires confirmation we use the method of
background subtraction based on the difference of
?2 distributions for signal and background
events.
pp?
Nbkg5020 is in an agreement with the
estimation from simulation, 395.
pp?0?
pp?0
12
Mass spectrum
13
Angular distribution
Mpp, GeV/c2 N Nbkg GE/GM
1.877-1.950 533 27
1.950-2.025 584 3712
2.025-2.100 602 5015
2.100-2.200 705 4214
2.200-2.400 592 6116
2.400-3.000 464 4512
The distribution over the angle between the
proton momentum in the pp rest frame and the
momentum of pp system in the ee- c.m. frame is
fitted by the sum of histograms, obtained from
two simulation event samples, one with GE0 and
the other with GM0. These distributions are
close to 1cos2?p and sin2?p.
14
Angular distribution
The simulated angular distribution are corrected
for data-MC difference in PID , tracking and
photon efficiencies. About 10 variation of
detection efficiency is explained by complex
momentum dependence of PID efficiency which has
minimum at momentum 1.5 GeV/c.
The sources of GE/GM systematic error
  • background subtraction (0.16)
  • MC statistics (0.08)
  • GE/GM mass dependence (0.01)
  • efficiency correction (0.02)

15
Angular distribution,cross-checks
To cross-check our method of GE/GM measurement
we compare the data and simulated distributions
over cos ?K for ee-? ?? ? KK- ? process. We
also study angular distribution for J/?? pp decay
which is described by 1?cos2?p with
?0.6720.034.
Our result is
16
GE/GM ratio
BaBar GE/GM measurements vs previous ones
and dispersion relation prediction (yellow) based
on JLab space-like GE/GM and analyticity
DM2FENICE
  • BaBar
  • LEAR
  • hep-ph/0507085

E835
17
Detection efficiency
To first approximation the detection efficiency
is taken from simulation. The model dependence
due to uncertainty in GE/GM ratio is about 1
for Mpplt3 GeV/c2, and 10 for higher masses where
we use GEGM assumption.
This MC efficiency must be corrected to account
for data-MC difference in detector response
18
Efficiency corrections
Effect 1.9 GeV/c2 3.0 GeV/c2 4.5 GeV/c2
?2 cuts -0.7 1.0 -1.1 1.0 -1.7 1.0
Track loss 0.8 3.0 1.1 3.0 1.0 3.0
PID 2.5 3.3 3.2 2.4 3.5 2.7
Photon inefficiency -1.3 0.1 -1.3 0.1 -1.3 0.1
Photon conversion 0.4 0.2 0.4 0.2 0.4 0.2
Trigger -0.6 0.3
Total 1.1 4.6 2.3 4.0 1.9 4.2
19
Cross section
With chosen mass bin the resolution correction
insignificantly changes the shape of mass
spectrum but leads to about 20 increase in the
errors and their correlation. The radiative
correction factor is calculated with use of
generator level simulation. We evaluate the ratio
of the mass spectrum generated with higher-order
radiative correction included (structure function
method) to pure Born mass spectrum. The
theoretical uncertainty is about 1.
20
Cross section
In reasonable agreement with ee- previous
measurements Negative steps at M 2.2 and 3 GeV
(!?)
21
Effective proton form factor
  • Steep behaviour at threshold confirmed
  • Similar behaviour at threshold is seen in other
    processes with different quantum numbers

22
J/? and ?(2S) decays
J/?
N ?, ?eeBpp, eV
J/? 438 22 17.20.7 12.00.60.5
?(2S) 22.2 5.5 16.50.7 0.700.170.03
?(2S)
23
Summary
  • The ee-? pp cross section and proton form factor
    have been measured from threshold up to 4.5 GeV.
  • The form factor have complex mass dependence. We
    confirm near-threshold enhancement observed in
    LEAR experiment. There are also two mass regions,
    near 2.25 and 3 GeV, with the rapid decrease of
    the form factor.
  • The GE/GM ratio has been measured from
    threshold to 3GeV. We have observe noticeable
    deviation of this ratio from unity, in
    disagreement with previous LEAR measurement.
Write a Comment
User Comments (0)
About PowerShow.com