Results for the K0+ photoproduction off the proton in the ... · Results for the K0 +...
Transcript of Results for the K0+ photoproduction off the proton in the ... · Results for the K0 +...
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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 2 / 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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 2 / 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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 2 / 19
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)
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 3 / 19
Experimental Setup/Analysis
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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 5 / 19
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
↓
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 6 / 19
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
↓
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 6 / 19
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
↓
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 6 / 19
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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 7 / 19
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
ΛK* +ΣK*thresholds
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
ΛK* +ΣK*thresholds
: Ewald: CastelijnsM: SAPHIR
: Nanovaγ(p,K∗)Σ+
Eur. Phys. J. A 35 (2008) 333
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 8 / 19
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
ΛK* +ΣK*thresholds
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
ΛK* +ΣK*thresholds
: Ewald: CastelijnsM: SAPHIR
: Nanovaγ(p,K∗)Σ+
Eur. Phys. J. A 35 (2008) 333
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 8 / 19
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)
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 9 / 19
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
ΛK* +ΣK*thresholds
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
ΛK* +ΣK*thresholds
K-MAID
SAID
K-MAID (mod., with K*)
K-MAID (mod., no K*)
: Ewald: CastelijnsM: SAPHIR
: Nanovaγ(p,K∗)Σ+
Eur. Phys. J. A 35 (2008) 333
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 10 / 19
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
ΛK* +ΣK*thresholds
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
ΛK* +ΣK*thresholds
K-MAID
SAID
K-MAID (mod., with K*)
K-MAID (mod., no K*)
: Ewald: CastelijnsM: SAPHIR
: Nanovaγ(p,K∗)Σ+
Eur. Phys. J. A 35 (2008) 333
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 10 / 19
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)
)
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 11 / 19
Results
Photon Asymmetry — Results
• Ewald (PhD thesis)
K-MAID
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 12 / 19
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
O. Jahn (PI, Bonn) Results for γp → K0Σ+ 14 / 19