Incoherent φ photo-production from deuteron in SPring-8/LEPS

Incoherent φ photo-production from deuteron in SPring-8/LEPS

M. Miyabe 　 for LEPS collaboratorsRCNP Osaka University

Baryons’10

Physics overview Experiment Result and discussion Summary

Contents

2010/12/7Baryons’10

Vector Meson Photo-production

● Vector Meson Dominance

● Pomeron Exchange

● Meson Exchange

N

q_q

_qq =

Decreasing with energy.Dominant at low energies

Slowly increasing with energyAlmost constant around threshold

N

(~ss)

uud

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Titov, Lee, Toki Phys.Rev C59(1999) 2993

Data from: SLAC('73), Bonn(’74),DESY(’78)

Natural parity

exchangeUnnatural parity

exchange

It is important to distinguishthe natural parity exchanges from unnatural ones

P2: 2ndpomeron ~ 0+glueball(Nakano, Toki (1998) 　

EXPAF97)

=0

deg

ree)

photo-production near threshold

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Polarization observables with linearly polarized

photonDecay Plane // natural parity exchange (-1)J (Pomeron, Scalar mesons)

Polarizationvector of

K+

K+

K-

In meson rest frame

Decay Plane unnatural parity exchange -(-1)J

(Pseudo scalar mesons )

Relative contributions from natural, unnatural parity exchanges

Decay angular distribution of

meson2010/12/7Baryons’10

Decay Angular distribution

Prediction by A. Titov (PRC,2003) -

ρ3=-0.5Pure unnatural

parity exchange

0

P, glueball, , f2’

p

)(2cos~121)( 3

PW

ρ3=0.5 forPure natural

parity exchange

~

~

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Baryons’10

Polarization angle of γ

K decay plane angle

Result from LEPS with proton target

Non monotonic behavior around 2GeV.Natural parity exchange is dominant　→　 0+ glueball ?Pseudo scalar meson exchange is not negligible.

T. Mibe, et al. PRL95 182001(2005)

ρ3=0.199+/-0.052

ρ3=0.189+/-0.030

E1E1

E2

E2

~

~

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Is the bump structure candidate of the exotic process 　　　　 (2nd Pomeron (glueball))? The model based on Pomeron exchange and pseudo scalar

exchange failed to explain such a non-monotonic behavior. This suggests that unknown natural parity process exist. But natural parity exchange process is comparable to pseudo

scalar exchange process from the value of ρ3.

To extract the natural parity process, detailed study for pseudo scalar π-η exchange is important.

Study of incoherent photo-production from deuteron is unique tool for this purpose.

Explore the exotic process

~

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φ photo-production from Deuteron

1. Coherent production Interact with deuteron itself.

Deuteron is iso-scalar target Iso-vector π exchange is

forbidden. Pure natural parity exchange

except for η-exchange process.

2. Incoherent production Interact with proton or

neutron in deuteron. Estimation the neutron

contribution.2010/12/7 Baryons’10

Due to isospin effect, gπnn = - gπpp → destructive gηnn = gηpp → constructive π-η interference effectDetailed Information for unnatural (π/η)

exchange process

Incoherent production

g(π, η)NN

gφγ (π, η)π,η

φγ

N N2010/12/7Baryons’10

Differential cross section as a function of energy and angle.

Due to πη interference effect, cross section from neutron decrease as low energy and forward angle.2010/12/7Baryons’10

Decay asymmetry as a function of energy and η-exchange strength

Decay asymmetry Σφ=2ρ3

Eγ=2 GeV

if η-exchange contribution is large --> Large difference in decay asymmetry

neutron

protonneutronproton

~

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EXPERIMENT

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The LEPS beamline

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LEPS spectrometer for charged particles

1m

TOF wall

MWDC 2MWDC 3

MWDC 1

Dipole Magnet (0.7 T) Start

counter

Silicon VertexDetector

AerogelCerenkov(n=1.03)

K-

K+

• Eγ~2.4GeV• Polarization ~95%• > 1 Mcps

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Liq. D2

result

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Invariant mass K+K-

Fit with Gaussian convoluted breit-weigner Resolution~1.5MeV

Cut point for invariant mass is +/-10MeV from peak position

Total Φ event ~ 17k.

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Minimum momentum Pmin spectator approximation

Pmiss

np

φ

np

PCM

γ PKKEKK

PγEγ

PmissEmiss In LAB system, the spectator

momentumbecome minimum when the direction of proton and neutron is anti-parallel to pmiss

In quasi-free event, spectator nucleon has a small momentum such as fermi motion momentum

np

Pmin

= Pmin2010/12/7Baryons’10

Characteristics of Pmin

Quasi-free process makes a peak around zero. coherent process Pmin ~ +0.15 Dominant Pmin ≧0.1 Other inelastic events are

distributed at large negative value.

GeV

GeV

MMD

Pmin2010/12/7 Baryons’10

Deuteron Mass

Differential cross section

Differential cross section at t=tmin About 30% reduction from

free proton Not a simple nuclear

density effect since deuteron is loosely bounded.

Red : incoherent γN→φNBlack: free proton

Lower Histogram Td = (dσ/dt)N/2*(dσ/dt)p

Eγeff (GeV)

dσ/d

t(t=t

min)

(μb)

Tran

spar

ency

ratio

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Differential cross section in KKp mode

Similar degree of reduction such as incoherent process reduction of incoherent

process is not only neutron. π-η 　 interference is small

Red : exclusive KKp eventBlack: free proton

Lower Histogram Td = (dσ/dt)KKp/(dσ/dt)free p

Td

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np

K+ K-

detectNot detect

Spin density matrix element

ρ3N is little bit higher than free proton.

Theoretical prediction of ρ3n is 0.25~0.30. ρ3N is 0.23~0.25 good agreement

Small difference ρ3

p and ρ3n

η-exchange is smallRed : γ+N→φ+NBlack : γ+p→φ+p

Eγeff (GeV)

ρ3 　 as a function of Eγ~

~

~

~

~ ~

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Non monotonic structure in cross section with Eγ increase at Deuteron target. Differential cross section for incoherent φ photo-production shows a significant

reduction from free proton Some effect other than nuclear density is necessary. Reduction is significant independent with coherent exclusion cut and Eγeff estimation.

From analysis for exclusive KKp event, Reduction is not only neutron but also proton in deuteron. π-η interference is small.

Decay asymmetry ρ3 is similar with free proton one η 　 exchange component is weak.

Bump like structure around Eγ= 2GeV for γ+p→φ+p→Another natural parity process candidate

to cancel out the increasing pseudo-scalar π . More statistic and new data is ready. 2006~7 about 3 times event for proton and deuteron target. 2009 ~10 maximum ~2.9GeV photon beam for proton. detail analysis and extrapolate the energy region.

Summary

~

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Backup

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Differential cross section

Differential cross section at t=tmin About 30% reduction from

free proton Not a simple nuclear

density effect since deuteron is loosely bounded.


Lower Histogram Td = (dσ/dt)N/2*(dσ/dt)p

Eγeff (GeV)

dσ/d

t(t=t

min)

(μb

)

Td

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p p

p p

M.A. Pichowsky and T.-S. H. LeePRD 56, 1644 (1997)

Prediction from Pomeron exchange

Prediction from meson exchange

Data from: LAMP2('83), DESY('76), SLAC('73), CERN('82),FNAL('79,'82), ZEUS('95,'96)

Prediction : dominant contribution frompseudo scalar meson exchangenear threshold

Vector Meson Photo-production

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K+

K+

K-

p’

meson rest frame (Gottfried-Jackson(GJ) frame)

K+

K+

K-

pol Productionplanez

Decayplane

z-axis

K+-pol

direction of linear polarization

Decay angular distribution of meson

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Coherent production with deuteron Deuteron is iso-scalar target Iso-vector π exchange is forbidden.

Pure natural parity exchange except for η-exchange process.

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Result of coherent production off deuterons from LEPS

Differential cross section Decay asymmetry

Differential cross section at t=tmin shows increasing with energy. Dashed line shows theoretical calculations.

Decay asymmetry showsnatural parity exchange is dominant

W. Chang et al., Physics Letter B 658, 209 (2008).

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Differential cross section Increase with energy Model prediction including Pomeron and η-exchange is

under estimate. No bump structure. Decay asymmetry Pure natural-parity exchange η-exchange is weak?

Additional natural parity process is required!

Summary of results of coherent production

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Linearly polarized Photon

• Backward Compton scattering by using UV laser light• Intensity (typ.) : 2.5 * 106 cps• Tagging Region : 1.5 GeV< E < 2.4 GeV• Linear Polarization : 95 % at 2.4 GeV

E (Tagger) (GeV) E (GeV)

Cou

nts

Line

ar p

olar

izat

ion

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Check for coherent exclusion

Without pmin cut. Reduction is

significant Eγ< 2.0 GeV,

coherent contamination is small.

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Exclusive KKp event in LH2

Eγ

dσ/dt t=tmin (μb)

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Spin density matrix elements

)(2cos~121)( 3

PW

2

12

1 cos~sin~121

23)(cosW

22 cos~21

21)( W

)(2cos~121)( 4

PW

2cos~121)( 5 PW

1-dimensional projections

100

1115

211

1114

211

1113

0112

0001

2~Im2/1~Im2/1~

~

~

Relations to standard definition

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Alvin Kiswandhi 　 Phys. Lett. B(2010)2010/12/7Baryons’10

Comparison with proton

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Measure the Incoherent φ photo-production from deuteron target γ+N→φ+N. Differential cross section(bump structure) (π 、 η)-interference Decay asymmetry η-exchange process magnitude

Extract quasi-free γ+N→φ+N events clearly.

Explore in the bump structure observed in φ photo-production from free protons.

The objectives of this thesis

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Nuclear transparency ratio TA=σA/(A*σN ) (=Pout) Mass number

dependence is larger than theoretical calculation. Large σφN in nuclear

medium. How about deuteron

case?

T. Ishikawa et al, Phys. Lett. B 608, 215 (2005) 2010/12/7Baryons’10

Summary of data taking● Trigger condition : TAG*UpVeto*STA*AC*TOF● Run period (150mm-long LH2) 2002,May - 2002.July (150mm-long LD2) 2002,July – 2003 Feb, Apr-

Jun● Total number of trigger

2.26*108 trigger 4.64*108 trigger

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Number of reconstructed track ≧2 Particle identification of K+ and K- particles(PID) Decay in flight cut (DIF) Vertex cut to select events produced at the deuteron

target. Tagger cut to select reconstructed track at Tagger Invariant Mass K+K- to select phi events Missing Mass cut to select γ + N → φ X events.

φ Event selection

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Particle identification and decay in flight cut

Consistency of TOF hit position Difference of y-position of

TOF ≦80mm Difference TOF slat

number ≦ 1 Number of outlier Noutl ≦ 6 χ2 probability Prob(χ2)≧0.02

Kaon identification is 4σ

Decay in flight cut

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Vertex cut

-1120. < Vertex z < 880. -30< Vertex(x,y) < 30 2010/12/7Baryons’10

Missing Mass cut

Missing mass distribution for

γ + p → φ X (MMp)

Smearing due to the fermi motion effect. Cut region for MMp

with LD2 is 80 MeV.

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Summary of φ selection

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LEPS spectrometer has designed for forward φ→K+K- event Exclusive γ+n→φ+n event can’t be accepted.Precise analysis forCoherent processFinal State Interaction(FSI)Fermi motion effectFortunately, we have acceptance of exclusive

γ+p→φ+p →K+K-p too.

Procedure analysis for quasi-free like Incoherent γ+N→φ+N production

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Energy Definitions

For cross section100MeV step

For decay asymmetry 200MeV step

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Minimum momentum spectator approximation

Pmiss

np

np

φ

np

PCM

np

γ PKKEKK

PγEγ

PmissEmiss

In lab system, the missing momentumbecome minimum when the direction of proton and neutron is anti-parallel to pmiss

The momentum of pn system as

In quasi-free event, spectator nucleon has a small momentum such as fermi motion momentum

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Coherent production

Pcm ～　 0 Pmin ～ γβMpn ～ 0.5 Pmiss Positive momentum

around > 0.1 GeV/c

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Pmin by Monte-Carlo simulation

Peak due to the quasi-free process is symmetric around zero.　　　　　 σ ～４４ MeV 　 Coherent process Dominant at Pmin

> 0.1GeV

cut point is Pmin ~ 0.1GeV.

Pmin distribution in MC

GeV

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Effective photon energy

Due to Fermi-motion, total energy of KKN system was smeared. Pmin strongly correlated to z-

component of fermi momentum inside deuteron. Pmin could be used for

estimating Fermi momenta of target nucleons. Total center of mass energy

s of KKN system

Pmin vs. Fermi momentum z

GeV

GeV

Pmin

Fermi momentum

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Conversion Eγeff from Eγ

Effective photon energy Eγeff as

s is the center of mass total energy of KKN system.

Resolution for Eγ is improved. (~50%) Eγ resolution GeV

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Contamination of coherent process into incoherent process

Real data(black) MC(blue/red)Contamination ratio of coherent to incoherent

Contamination of coherent process is large at High energy region. less than 10% at Pmin≦0.09

Pmin cut

Pmin

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Validity check of Pmin cut-point for incoherent process

slope Differential cross section

Distributions are flat at |Pmin|=<0.09 except for E1 and E2. about 10% fluctuation from the tighter cut.

Pmin cutPmin cut

E9E9 E8 E7

E6 E5 E4

E3 E2

E1

E2E1

E3

E4E5 E6

E7E8

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Conversion to effective photon flux

Photon flux ωγ for each Eγ as ωγ(Eγ)=Frac(Eγ) ＊ Ntag Frac(Eγ) : photon fraction for EγNtag : corrected tagger scalar count

In nucleon at rest frame,

At one specific Eγeff, it depends on some Eγ which spread over because of fermi motion.Conversion ratio from each Eγ bin is calculated using Monte-Carlo.

Eγeff

Eγeff (GeV)

Pmin<0.09Pmin : all

ωγ

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Conversion ratio

At one specific Eγeff

Conversion ratio from each Eγ bin 10 MeV bining for each

Eγeff and Eγ Eγ resolution ~

10MeV

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Estimate the Final state interaction

Momentum of outgoing nucleon is small, and FSI effect is enhanced near threshold .

Strength of FSI is enhanced in small relative momentum p and n. (similar kinematics in coherent)

np

n

φγ

p

FSI enhancement factor

pn relative momentum kk = |pn – pp|/2 2010/12/7Baryons’10

Monte Carlo simulation

Real data are fitted by distributions of coherent, inciherent and FSI processes produced by MC. FSI contribution is

very weak at all energy bin.

<< 1%

Missing Mass distribution MMD

Black :real, red coherent green: incoherent, blue: FSI

GeV2010/12/7Baryons’10

P-N relative momentum fitData were fitted by results FSI and coherent prodeced by MC.

χ2/ndf distributionas a function of FSI strength

χ2 is better with smaller FSI effect. weak

FSI effect is negligible small

GeV

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Background from non-resonant KK events Backgrounds under the KK

invariant mass spectrum are mainly from non-resonant KK events. Estimate the background

contamination into phi events by using the side band region

Non-resonant K+K- invariant massGeV

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Differential cross section t dependence.

Fitting function : dσ/dt = C*exp(-b*t) (red curve)

Slope parameter b as a function of Eγeff

nonmonotonic behavior of slope,Slope b = 3.74 +/- 0.12 (free proton b=3.38+/-0.23)

GeVEγefft (GeV2)

dσ/dt (μb)

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Energy dependence of differential cross section at t=tmin

Cross section does not simply increase with energy

at Eγ ～ 2.2 GeV

dσ/dt at (t=tmin) with Constant slope b=3.742010/12/7 Baryons’10

Differential cross section with tighter Pmin cut

nonmonotonic behavior of slope,Slope b = 3.45 +/- 0.13 (free proton b=3.38+/-0.23)

dσ/dt at (t=tmin) with b=3.45 t dependence Pmin ≦50

Cross section decreases at E1~E3

Eγeff (GeV)Eγeff (GeV)

dσ/dt(t=tmin) (μb)

Black histogram : 90MeV, Red : 50MeV2010/12/7Baryons’10

3-particle KKp mode All particles (KKp) in the

final state were detected to select the exclusive proton events. The statistics is very limited

but, the bump-like structure is seen as well as the results from free-proton target.

dσ/dt at (t=tmin) with Constant slope b=3.38


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Decay angular distribution

ρ1~5 as a function of EγDecay angular distribution 2010/12/7Baryons’10

Differential cross section at Eγ In the original Eγ, similar

reduction appeared Eγ ～ 1.9GeV Photon flux is similar for

both Eγ and Eγeff .

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Eγeff at KKp mode

Eγeff(KK) : calculated with K+K- Eγeff(KKp) :calculated with K+K- p Estimation of Eγeff works well.

Eγeff(KK) Eγeff(KKp)

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Tighter Pmin cutDifferential cross section at t=tmin


Red : γ+N→φ+NBlack : γ+p→φ+p

ρ3 　 as a function of Eγ

Decrease in Highest energy region

Eγeff (GeV) Eγeff (GeV)

ρ3

Td


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Backup figures

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Nuclear transparency ratio

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Eγ vs. Eγ(n)

Eγ

Eγ(n)

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Spin density matrix

Horz Vert

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Spin density matrix

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ρ

Horz, Vert Horz+Vert

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Vert

Horz

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Pmin dependence

slope Differential cross section

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Photon flux

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Comparison to LEPS result

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Pmin<50MeV

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Tagger cut

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Coherent contamination

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Lambda(1520)

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Non-resonant BG

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Experiment at Spring-8

•8GeV electron storage ring Harima Hyogo

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Liquid hydrogen target150mm

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Drift Chamber calibration

Depth of the multi hit TDC ~3 (before 8) High Voltage value of

sense wire is large. Threshold for discri-

amp is low. Noisy level is high

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t0 calibration

Large fluctuation in TDC offset value t0 . Every 10 run

calibration.

DC-t0 run dependencerun

tdc

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xt-calibration Xdrift = c1t+c2t2+c3t3+c4

Correct the origin-point Calcurate parameters

when resolution and efficiency are down

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Result of calibration

Number of outlier Χ^2 probability

LH2(short)

LH2(long)

LH2(long)2010/12/7Baryons’10

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Decay angular distribution

● W0,W1,W2 are parameterized by the 9 spin density matrix elements.

Re() Im() andIm()

),(),,( 0 WW

),()2sin(),()2cos( 21 WPWP Unpolarized part

Polarized part

K.Schilling et al. Nucl. Phys. B15(1970) 408

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Final state interaction

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Incoherent φ photo-production from deuteron in SPring-8/LEPS

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