Results for the K0+ photoproduction off the proton in the ... · Results for the K0 + photoproduction o the proton in the energy range from threshold to 2250 MeV Oliver Jahn / Ralf

Results for the K0Σ+ photoproduction off the proton inthe energy range from threshold to 2250 MeV

Oliver Jahn / Ralf Ewald

Physikalisches Institut, Universitat BonnFor the CB-ELSA/TAPS Collaboration

DPG Spring Meeting 2012, Mainz

supported by

1 Motivation2 Experimental Setup/Analysis

3 Results4 Summary

O. Jahn (PI, Bonn) Results for γp → K0Σ+ 1 / 19

Motivation

Motivation

Investigate nucleon structure

Baryon spectroscopy → excitationspectra (”missing resonances”)Strange meson photoproduction

Here investigated:γ + p −→ K0 + Σ+

↓ ↓π0π0 pπ0

(Γ ≈ 31%)

from threshold (Eγ = 1048 MeV) to 2250 MeV

(a)

(b)

”Clean” probe for s-channel contributions (photon doesnot couple to K0)

t-channel K∗ exchange may contribute, though

Tool to study contributions (b) close to K∗ threshold


Motivation

Motivation




↓ ↓π0π0 pπ0

(Γ ≈ 31%)


(a)

(b)





Motivation

Motivation




↓ ↓π0π0 pπ0

(Γ ≈ 31%)


(a)

(b)





Experimental Setup/Analysis

Electron Stretcher Accelerator (ELSA)

Energy Range: 0.5–3.5 GeV

typ. extracted Intensity: 1 nA

Electron Polarisation: typ. 55–65 %

see Talk: HK 65.3 (Friday, 15h)



CB-ELSA/TAPS Setup



Analysis Overview

Select events meeting these conditions:

Six photons and max. one charged particle

Bremsstrahlung photon of energy Eγ ≥ 1047 MeV

Construct three π0 with individual invariant masses110 MeV < mγγ < 160 MeV

Reject dominant η → 3π0 background→ Anti-cut on 3π0 inv. mass region 460 MeV < m3π0 < 640 MeV

Kinematic fit to test hypothesis γp→ pπ0π0π0:confidence level > 0.1

Hyperon and Kaon reconstruction: cut on Σ+ mass1170 MeV < mΣ+ < 1210 MeV

Background subtraction (uncorrelated 3π0 production) by simulation


Results

Differential Cross Section

: Ewald (PhD thesis)to be published

: CastelijnsEur. Phys. J. A 35, 39 (2008)

4: SAPHIREur. Phys. J. A 24 (2) 275 (2005)

Observation:

Sudden decrease of cs especially in forwarddirection (≈ factor 4) close to K∗ threshold

Indication of transition from t-channel exchangemechanism to s-channel K0Σ+ productionwhen K∗ channel opens

↓


Results





Observation:



↓


Results





Observation:



↓


Results

Differential Cross Section — Discussion

Plot forwardmost bins only

W / MeV1700 1800 1900 2000 2100 2200 2300

b/sr

µ /

Ω/dσd

0

0.02

0.04

0.06

0.08

0.1

0.12

/ MeVγE1200 1400 1600 1800 2000 2200

ΛK* +ΣK*thresholds

: Castelijns

: Ewald


Results

Total Cross Section

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


: Ewald: CastelijnsM: SAPHIR

: Nanovaγ(p,K∗)Σ+

Eur. Phys. J. A 35 (2008) 333


Results

Total Cross Section

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200




Eur. Phys. J. A 35 (2008) 333


Results

Total Cross Section

Strength of the dip in the cross section for K0Σ+ contributes to theK∗Σ+ channel

Speculate that right below threshold, K∗ couples strongly to a K0

and π. The K0 is observed in the final state, the π re-absorbed by thehyperon.

Diagram (b) contributes to K0Σ+ below K∗ threshold. Above, itadds to the K∗ channel.

(a) (b)


Results

Total Cross Section

Adjust MAID: change rel. S31(1900) couplings to G1 = G2 = 0.3

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


K-MAID

SAID

K-MAID: www.kph.uni-mainz.de/MAID/

SAID: gwdac.phys.gwu.edu/analysis/

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


K-MAID

SAID

K-MAID (mod., with K*)

K-MAID (mod., no K*)



Eur. Phys. J. A 35 (2008) 333


Results

Total Cross Section

Adjust MAID: change rel. S31(1900) couplings to G1 = G2 = 0.3

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


K-MAID

SAID

K-MAID: www.kph.uni-mainz.de/MAID/

SAID: gwdac.phys.gwu.edu/analysis/

W / MeV1800 1900 2000 2100 2200 2300

bµ/to

tσ

0

0.2

0.4

0.6

0.8

1

1.2

/ MeVγE1200 1400 1600 1800 2000 2200


K-MAID

SAID

K-MAID (mod., with K*)

K-MAID (mod., no K*)



Eur. Phys. J. A 35 (2008) 333


Results

Photon Asymmetry Σ

Lin. polarised photons on unpolarised target

dσ

dΩ=

(dσ

dΩ

)0

(1−PlinΣ cos(2φ))

Photon Asymmetry Σ

Determined from fit function:

f(φ) = A

(1− B

2A· cos(2φ)

)


Results

Photon Asymmetry — Results

• Ewald (PhD thesis)

K-MAID


Results

Photon Asymmetry — Results

/ MeVγE1100 1200 1300 1400 1500 1600

Σ

-1

-0.5

0

0.5

1 = 0.6 to 1.0Θcos

• Ewald (PhD thesis)

K-MAID

Σ positive except in highest energy bin

Supports the idea of formation of a K∗-hyperonquasibound state in s-channel close to K∗ thresholds

No data at and above K∗ thresholdO. Jahn (PI, Bonn) Results for γp → K0Σ+ 13 / 19

Summary

Summary

Latest results for γ + p→ K0 + Σ+ from threshold to 2250 MeVphoton beam energy presented

Cross section shows cusp-like structure at forward angles (drop byfactor 4)

Photon asymmetry shows drop to negative values at forward angles inhighest energy bin

Indication for cross-over from t-channel mechanism to s-channelK0Σ+ production close to K∗ threshold and formation of aK∗-hyperon quasibound state in K∗ sub-threshold production

Interest in extending measurements for photon asymmetry to andabove K∗ threshold


Summary

Thank you!


Summary


Summary


Summary


Summary


Documents

Results for the K0+ photoproduction off the proton in the ... · Results for the K0 + photoproduction o the proton in the energy range from threshold to 2250 MeV Oliver Jahn / Ralf