Download Document

Measurement of Differential Cross
Section of φ-meson Photoproduction
From Deuterons near Threshold at
SPring-8/LEPS
Advisors: Rurng-Sheng Guo
Wen-Chen Chang
Graduate: Po-Ju, Lin
2006/05/29, NKNU
Outline





Introduction
Experimental Apparatus
Data Analysis
Results and Discussions
Summary
Introduction
The flow chart
Physics motivation
Raw data
Data analysis
Physics events
DAQ
Detectors
Electronic devices
Physics Results
Cross section
The cross section σ is defined as：
Density of final states,
phase space
2
2

M fi   E ' V
  va
Transition matrix element,
dynamical information contained.
And can be understand as：
Number of reactions per unit time
Beam particles per unit time × scattering centers per unit area
Cross section
The differential cross section measured in the experiment is referred to：
 d 
 d 
2

 
 Fq
 dt  exp  dt 
*
t = four-momentum transfer squared
q = three-momentum transfer
 
2
Differential Form
cross factor
section for a
point-like scattering case.
Form factor, which is the Fourier transform of the charge distribution, determines
how the scattering rate is reduced from its value for a point-like scatting.
We measure the cross section experimentally and
confront the result with the theoretical presications.
The Standard Model
The Standard Model is the most widely accepted approach describing
the fundamental particles and the interaction between them. It consists
of two major parts: the spin ½ fermions, and the integer-spin bosons.
Particles participating
strong interaction
Force carriers
Constituents of Matter.
The hadrons
Under the framework of Quantum Chromo-dynamics, quarks interact with each
other by exchanging gluons and are bonded to form hadrons.
(強子)
Consist of three quarks
(重子)
Consist of quark, anti-quark pair
(介子)
Intermediate-energy nuclear physics
The perturbative QCD (pQCD) approach which
works well in the extremely high energy regime is
not applicable in the relatively low energy region.
Modifications and Model construction emphasizing
the most aspects of QCD need to be made and
tested with experimental data.
GeV-photons are chosen to be our
probe to investigate the property of
strong interaction in intermediateenergy resion.
Vector meson photoproduction and vector
meson dominance model
Vector-meson photoproduction
with
J 1
small four-momentum transfer squared
has originally been described with
vector meson dominance (VMD) model,
which assumes that photons interact
with hadrons by first changing into
neutral vector mesons having the same
quantum number with photon, J PC  1 
such as ρ,ω, and φ.
PC

  uu  dd  / 2
  uu  dd / 2
  ss
φ-meson photoproduction
Why is φ- meson photoproduction of special interest?
Vector Meson Photoproduction
Pomeron exchange
g
, , φ
Pomeron
p
uud
Meson exchange (OZI suppressed for φ)
g
, 
,h,・・・
p
Speculate new Pomeron trajectory
by 0+ glueball exchange ??
g
p
, , φ
???
uud
What’s the Pomeron
In the pre-QCD approach, Regge theory,
particles exchanged are summed coherently
to give the exchange of so-called Regge
trajectories.
Small-angle scattering is characterized by
the exchange of the Pomeron trajectory
which dominates in the high total energy
region
 pp  2.17 s 0.08  56.1s 0.45mb
 pp  2.17 s 0.08  98.4 s 0.45mb
Differential Cross section of g p  φ p
(t=
gp p
)
0
Meson
gp fp
Pomeron
M.A. Pichowsky and T.-S. H. Lee
PRD 56, 1644 (1997)
A.I.Titov et. al.
PRC 59, R2993 (1999)
OZI rule
Observed by Okubo, Zweig, and Iizuka：If the Feynman diagram expressing
a specific reaction can be cut in two by slicing only gluon lines, the process is
suppressed.
Allowed
Suppressed
Photoproduction of φ-mesons at forward region
  uu  dd  / 2
h  uu  dd  2ss / 2
• Pomeron:
• Pseudo-scalar particle:
–
–
–
–
Positive power-law scaling of s.
– Negative power-law scaling of s.
Dominating at large energy.
– Showing up at small energy.
Natural parity (=+1).
– Un-natural parity (= –1).
Exchange particles unknown;
– Exchange particles like ,h.
likely to be glueball : P1(J=2+),
– OZI suppressed

+
P2 (J =0 , negative power-law
scaling
of s,
T. Nakano and
This fact
makes
theRef:
f-photoproduction
an unique process to determine
H.Toki, 1998)
Pomeron contribution and possible new trajectories near threshold!
Φ-photoproduction using LD2 target.
γ
γ
φ
φ
d
φ
φ
n
d
d
p
• Incoherent Interaction：
• Coherent Interaction：
g + d  φ + pn
g+dφ+d
Deuteron breaks up to be
a proton and a neutron in
the final state.
Deuterons remains intact.
Isospin
Isospin is a symmetry of the strong interaction introduced by Heisenberg as it
applies to the interactions of the proton and neutron.
this symmetry was in certain respects similar to the mathematical formulation
of spin, the proton and the neutron are therefore assign to a doublet:
p：isospin ½ with I3=1/2
n：isospin ½ with I3=-1/2
In the framework of the Standard Model, the isospin symmetry of the proton
and neutron are reinterpreted as the isospin symmetry of the up and down
quarks.
Coherent production
Primary components of meson-exchange channel：π, η
f : isoscalar
D : isoscalar
η: isoscalar
Allowed
π: isovector
Forbidden
Pseudo-scalar Exchange
Conjecture：
By isospin conservation we should be able to extract isovector
part (π) from the whole exchanging channels.
Isotopic effect in incoherent interaction
Incoherent
LH2
The destructive inteference would
occur between π and η exchange in
the reaction g n  φ n
Isotopic effect can be investigated by comparing with results
from proton target experiments.
Previous experiment
d  d 

exp bt  | t |min 

dt  dt  t  |t |
min
Solid curve :
Pomeron + Pseudo scalar
exchange by A. Titov, et al.
(PRC 67 (2003), 065205)
New production mechanism
near threshold?
Previous experiment
There was only one measurement of deuteron φ photoproduction at Eg= 6.4-9.0
GeV and no separation of in coherent and incoherent contribution was made.
Experimental Apparatus
Super Photon Ring at 8 Gev (SPring - 8)
SPring-8 beamline map
Backward-Compton scattering
El 1    cos 1 
Eg 
1    cos  2  El 1  cos 2  1  / Ee
By shooting a few eV photons to 8 GeV electrons, photons
with maximum energy 2.4 GeV are produced.
LEPS beamline
The laser system
Ar ion laser
(MLUV, 5W)
Polarization
rotator
Focusing lens
Eγ collision in storage ring
Straight
section
Laser
Bending
magnet
g
Tagging
counter
e’
e- (8GeV)
The charged particle spectrometer
Start counter
Dipole Magnet
(0.7 T)
TOF wall
Target 160mm-long
g
Aerogel
Cerenkov Silicon Vertex
(n=1.03) Detector
MWDC 3
MWDC 1
MWDC 2
1m
Particle identification


 2 1  
m  p

2



2
1/ 2
β = particle velocity
p = particle momentum
Momentum
Path of flight Time of Flight
Length
Time
Velocity
Mass
Data Analysis
Data samples


LH 2 target runs
r23690 (2002.05.23) – r24058 (2002.07.09)
r25453 (2003.02.27) – r25968 (2003.04.14)
LD2 target runs
r24095 (2002.10.18) – r24841 (2002.12.18)
r25015 (2003.01.30) – r25447 (2003.02.21)
r26001 (2003.04.20) – r26338 (2003.06.02)
Invariant mass and missing mass
This study focus on the reaction, g d  φ X, followed by the φ K+K-
M inv  ( EK 

 2
 EK  )  ( PK   PK  )
2
Invariant mass：reconstruct the mass of the mother
particle by the detectable decay produced daughters.
  2
MM p  ( Eg  m p  Ef )  ( Pg  Pf )
2
Missing mass：identify the undetected particle by all the
other particles relating to the reaction
Disentanglement by MMd
Coherent interaction
  2
MM d  ( Eg  md  Ef )  ( Pg  Pf )
2
Clear peak around 1.875 GeV/c2 is shown in LD2 events, but absent in LH2 one.
Coherent events and incoherent events can be disentangled by
fitting the MMd distribution with Monte-Carlo-simulated events
Results and Discussions
Disentanglement of Coherent and Incoherent
events
Fitting of MMd of LD2 real data
using Mote-Carlo generated
incoherent, coherent events.
coherent
incoherent
sum
The Acceptance
LD2 coherent
LD2 incoherent
Acceptance obtained by
Monte Carlo simulation：
Accep 
N accep
N gene
LH2
Implemented t-distribution：
d
 ~
 a  exp  b t 
dt
 
~
t  t  t min
Fitting of t distribution
LD2 coherent
LD2 incoherent
Fitting Formula：
d
 
 N 0 exp  b t   Accep
dt
 
~
LH2
N0
b
intercept
slope
The slope parameter
The trend of steady slope distribution is
observed in three different reaction
machenisms.
The averaged slope parameters：
Data set
b (GeV-2)
χ2/ndf
LD2 coherent
13.784 ± 0.830
2.2215
LD2 incoherent
4.136 ± 0.136
1.8940
LH2
3.619 ± 0.192
1.6520
LD2 coherent
The slope parameter
LD2 incoherent
LH2
Differential cross section
N0 need to be background-subtracted to obtain N 0

N BG 
N 0  N 0  1 

N signal 

The differential cross section can than be
deduced:
d
dt
~
t 0
N 0  Fnorm

Rbranch  N t arg et  N beam  Fbeam htrans  Pntag1 
LD2 coherent
Differential cross section
LD2 incoherent
LH2
Discussions
•No strong energy dependence is shown.
•Slope of LD2 coherent events is significantly
higher than the LD2 incoherent and LH2
events. This can be understood by the form
factor of deuteron.
 d 
 d 
2


F
q




 dt  exp  dt 
*
 
2
Incoherent
Coherent
LH2
Discussions
•Constantly increase with photon
energy in coherent interaction
•Clear isotopic effect is seen in the
LD2 incoherent events.
•Peak structure is seen in LD2
incoherent and LH2 events.
Incoherent
Coherent
LH2
Previous
SLH2 exp.
Discussions
• Differential cross section of LD2
coherent events corrected with form
factor comparing with the results
with LH2 events.
• No dropping with decreasing
photon energy is seen.
：LH2
：Corrected coherent
：Previous SLH2 exp.
Summary
Summary
•Differential cross section of the photoproduction of φmeson from deuterium
target has been studied and compared with the results from hydrogen target in
the energy range from the production threshold to Eγ=2.4 GeV.
•By fitting the MMd spectra with those of Monte-Carlo-simulated coherent and
incoherent events, disentanglement was achieved.
•No strong energy dependence was observed in three different reaction
mechanisms.
•LD2 coherent events
•The LD2 coherent differential cross section was observed to have a large tslope. And the differential cross section at t=-|t|min shows a constantly
increase with photon energy.
Summary
•LD2 incoherent and LH2 events
•Consistent exponential slope were obtained for the differential cross
section of LD2 and LH2 events.
•Strong Isotopic effect of amplitude interference is shown.
•Peaking structure around Eγ=2.2 GeV observed in the previous LEPS
analysis is seen.
•Together with the study on decay symmetry, the presence of the peaking
structure may be interpreted as the manifestation of additional natural-parity
exchange process, such as the daughter Pomeron trajectory.
Summary
•Future works
•Comparison of the predicted differential cross section of g p  φ p from
Pomeron exchange and that of the LD2 coherent interaction can be
proceeded to discriminate the possible natural-parity contribution.
•By using polarized target, more informative observables can be obtained.
•New measurement at higher energies is crucial to establish the nonmonotonical-increase structure of the cross section near threshold.
Thank you for your paying attention
Backup slide