l * (1520) photoproduction off proton and neutron from clas eg3 data set

L*(1520) Photoproduction off Proton and Neutron

from CLAS eg3 data set

Zhiwen ZhaoNSTAR 2011 2011/05/19

• Physics motivation•Data analysis•Results• Summary

1

Physics Motivation

• Its production mechanism is poorly understood due to lack of data.

• Existing data suggest dominance of t-channel processes and K, K* exchange.

• Several model predictions for total and differential cross sections are available.

• Measurement of cross section and decay angular distribution can provide

constraints on model prediction and insights into the production mechanism.

• Possible missing N* resonances may decay through strange channels.

Λ(1520) I( J P ) = 0( 3/2 −

) Mass m = 1519.5 ± 1.0 MeV Full width Γ = 15.6 ± 1.0 MeV

2

S. Nam et al. Phys. Rev. D, 71, 114012 (2005)S. Nam et al. Phys. Rev. D, 75, 014027 (2007)S. Nam et al. Phys. Rev. C, 81, 055206 (2010)A. Titov et al. Phys. Rev. C, 72, 035206 (2005)

Published Experiment 1. on Proton photoproduction measurements

[1] A. Boyarski et al., (LAMP2, Daresbury), (1971)[2] D. Barber et al., (SLAC), (1980)[3] N. Muramatsu et al., (LEPS), (2009)[4] H. Kohri et al., (LEPS), (2010)[5] F. W. Wieland et al., (SAPHIR), (2010)

electroproduction measurements[5] T. Azemoon et al., (DESY), (1975)[6] S. P. Barrow et al. (CLAS, JLab), (2001)[7] Y. Qiang et al. (Hall-A, JLab), (2010)

2. on Neutron photoproduction measurements

[3] N. Muramatsu et al., (LEPS), (2009)

Published Theory

3

>>

?

Cross Section photoproductionComparing Proton results between data and theory

Cross sections of Neutron much smaller than Proton

4S. Nam et al. Phys. Rev. D, 71, 114012 (2005)

Cross Section photoproduction

LEPS Results• Show both forward and backward angle

differential cross sections on Proton.• Enhancement close to threshold is

interpreted as a resonance structure.• Very small cross sections on Neutron from

indirect measurement.

5

Decay Angle photoproduction

Gottfried-Jackson framemz=3/2mz=1/2 interference

6

Λ* JP = 3/2 − mz=1/2 mz=3/2 β/α

N(1/2 +)K (0 −) Y N 0

N(1/2 +) K*(1 −) Y Y 3/1

Decay angle distribution

Decay angle distribution is related to production mechanism.

S. Nam et al. Phys. Rev. C, 81, 055206 (2010)

Decay Angle photoproduction

cosθK-GJ cosθK

CM

θKCM forward backward

eg3 run• Photon beam electron beam 5.77 GeV, tagged photon energy 1.15 < E < 5.5 GeV, 30

nA• Target 40 cm upstream, LD2

• Trigger Tagger 4.5 < E < 5.5 GeV, STxTOF (3 sectors and prescaled 2 sectors)• Torus field optimized to -1980 A, negative charged particles outbending• Run period 12/06/2004 – 01/31/2005, 29 days of production on LD2 target• Data 4.2 billion physics events, 32 TB raw data, average 2.7 tracks/event

p(n) → K+ Λ* (n) Proton n(p) → K0 Λ* (p) Neutron

(Λ* →p K- , K0 → Ks → + - )

exclusive

Reaction Channelsdeuteron target

8

Correction and Cuts Applied

9

Invariant Mass of pK-

Proton

Neutron

10

Invariant Mass of K+K-Proto

n

11

Invariant Mass of K+K-Proto

n

12

Eγ < 2.25 GeV

Kinematic DistributionProton

1.5 < Eγ < 5.5 GeV16 bins, bin width 250 MeV

0.25 < t* = -(t-t0) < 2.5 GeV2

6 bins, bin width varies

Eγ (GeV)

Data Simulation

Eγ (GeV)

t*=-

(t-t 0)

(GeV

2 )

t*=-

(t-t 0)

(GeV

2 )

13

Kinematic DistributionNeutro

n

Eγ (GeV)

Data Simulation

Eγ (GeV)

t*=-

(t-t 0)

(GeV

2 /c4 )

t*=-

(t-t 0)

(GeV

2 /c4 )

14

1.5 < Eγ < 5.5 GeV6 bins, bin width varies

0.0 < t* = -(t-t0) < 2.5 GeV2


Differential Cross SectionProton

• 1.5 < Eg < 5.5 GeV16 bins, bin width 250 MeV

• Fit with function of αe – βt*

• Extrapolate the function and integrate over t* to obtain total cross sections

dσ/

dt*

(µb)

t* (GeV2)

dσ/dt*

15

Preliminary

Differential Cross Section

dσ/

dt *

(µb)

Neutron

• 1.5 < Eg < 5.5 GeV6 bins, bin width varies.

• Fit with function of αe – βt*

• Extrapolate the function and integrate over t* to obtain total cross sections

16 t* (GeV2)

dσ/dt*

Preliminary

t-slope

17

Preliminary

Total Cross Section

18

Preliminary


40 < θKCM < 120o


Eγ (GeV)

Data Simulation

Eγ (GeV)

θ KC

M

θ KC

M

19



n

Eγ (GeV)

Data Simulation

Eγ (GeV)

20


30 < θKCM < 120o


θ KC

M

θ KC

M

Differential Cross SectionProton

• 40o < θKCM < 120o


• No sign of resonance structure within the statistics

dσ/d

θ KC

M (µ

b)

E

(GeV)

dσ/dθKCM

21

Preliminary

Differential Cross SectionNeutro

n

dσ/d

θ KC

M (µ

b)

E

(GeV)22

dσ/dθKCM

• 20o < θKCM < 120o


• No sign of resonance structure within the statistics

Preliminary

Decay Angle DistributionProton

• 1.5 < Eg < 5.5 GeV16 bins, bin width 250 MeV

• Mixture of K and K* exchange

cosθK-GJ

dσ/dcosθK-GJ

dσ/d

θ K-C

J (µ

b)

23

Preliminary

Decay Angle Distribution

dσ/d

θ K-C

J (µ

b)

cosθK-GJ

Neutron

24

dσ/dcosθK-GJ

• 1.5 < Eg < 5.5 GeV6 bins, bin width varies

• Mixture of K and K* exchange

Preliminary

Decay Angle DistributionProton

Mixture of K and K* exchange

cosθK-GJ

dσ/dcosθK-GJ

dσ/d

θ K-C

J (µ

b)

25

E γ = 2.185 2.75 4.35 GeV

θK

CM

= 50 70 100 DEG

Preliminary

Decay Angle Distribution

Mixture of K and K* exchange

cosθK-GJ

dσ/dcosθK-GJ

dσ/d

θ K-C

J (µ

b)

26

E γ = 2.0 2.625 4.35 GeV

θK

CM

= 42 62 95 DEG

Neutron Preliminary

Summary• The Λ*(1520) differential and total cross sections up to 5.5 GeV on Proton

are extracted. The total cross section is in good agreement with the world

data.

• The Λ*(1520) differential and total cross sections on Neutron are obtained

for the first time. The cross section is about 70% of the proton channel

result, which is much larger than what the theory predicted.

• There is no sign of resonance structures at the covered forward kaon

angles.

• Λ*(1520) decay angle distributions in Gottfried-Jackson frame show

complicated structures indicating that both K and K* exchanges contribute

to the two reaction channels. 27

Backup

28

• Electroproduction of L* off Proton has been studied at DESY and CLAS

• CLAS data (S. Barrow, e1c) showed– Dominance of t-channel process

confirmed– Decay angular distribution showed

significant contribution from mz=±1/2 spin projection

Existing Data electroproduction

29

Photon Selectionvertex time diff (ns)

+ Mom(GeV)vertex time diff (ns)

- Mom(GeV)

+ Mom(GeV)

- Mom(GeV)

vertex and tagger time diff (ns)

vertex and tagger time diff (ns)

Neutron

30

Event SelectionParticle timingτ

M2

Positive Negative

Cut misidentified pions

MM2 (p+-) (GeV2)

MM

2 (pK

+ K

- )(G

eV2 )

Mom (GeV)Before and after misid π+ π - cut

MM2 (pK+ K-)(GeV2)MM2 (pK+-) (GeV2)

Proton

31

Event Selection

Mom (GeV)Before K0 cut After K0 cut

Positive Negative Particle timingτ

Before and after K0 cut

MM2 (pπ+π–K-)(GeV2)

MM

2 (pπ

+ π– K

- )(G

eV2 )

MM2 (pπ+π–π–)(GeV2)

Ks cut

Neutron

32

M2

Missing Nucleon MassProton

mean mean error

width width error

33

Missing Nucleon MassProton

mean mean error

width width error

34

Missing Nucleon Momentum

DataSim

DataSim

Proto

n Neutron

35


-0.8 < cosθK-GJ < 0.9


Eγ (GeV)

Data Simulation

Eγ (GeV)

cosθ

K-C

J

cosθ

K-C

J

36



n

Eγ (GeV)

Data Simulation

Eγ (GeV)

cosθ

K-C

J

cosθ

K-C

J

37


-0.8 < cosθK-GJ < 0.9


Yield Extraction (data)Proton

M(pK-) (GeV)

M(pK-) (GeV)

38


0.25 < t* = -(t-t0) < 3.0 GeV2


Yield Extraction (data)Neutro

n

39


0.0 < t* = -(t-t0) < 3.0 GeV2


M(pK-) (GeV)

M(pK-) (GeV)

Yield and Acceptance Data Simulation

E,

t* b

in

t

* bin

E

bin

Proton

Yield # of generated AcceptanceYield

40

Yield and Acceptance Data Simulation

E,

t* b

in

t

* bi

n

E

bin

Neutron

Yield # of generated AcceptanceYield

41

SAPHIR

42

SAPHIR

43

l * (1520) photoproduction off proton and neutron from clas eg3 data set

Documents

function of e t

tt0 gev2c4141

tt0 gev2t

tt0 gev2c4t

dominance of t

total cross sections

small cross sections

measurement of cross