Strangeness photoproduction at the BGO-ODexperiment

Tom JudeOn behalf of the BGO-OD Collaboration

University of BonnSupported by the DFG / SFB tr-16

NSTAR 2015

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 1 / 16

The BGO-OD experiment at ELSA

Energy tagged bremsstrahlung photon beam up to 3 GeVBGO calorimeter (central region) & Forward Spectrometer combinationHigh momentum resolution, excellent charged and neutral particle ID

scintillator barrel

Goniometer

Tagger magnet

Tagger

ARGUSMøller

Beam dump

Cryotarget, LH2 or LD2silicon

γ monitoring Open Dipole magnet MRPC BGO ball

MWPC

MOMOSciFi2 detector

Drift chambers

ToF 1ToF 2ToF 3ToF 4

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 2 / 16

Incident photon beam parameters

ELSA e− beam ≤ 3.5 GeV, BGO-OD experiment - Tagged γ beam ≤ 3 GeV

Current ≤2 nA (ideal for tagged photon beam), 10 nA upgrade planned

Linearly polarised γ beam - coherent bremsstrahlung using a diamondradiator

Circularly polarised γ beam - ELSA achieves full spin rotationmaximum polarisation at Ee ∼1.7 GeV (W ∼ 2 GeV)

Energy region ofunresolved/missing resonances

K ∗Y thresholds

ω and η′ thresholds

Non-understood “bump”structure in φ photoproduction

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 3 / 16

Analysis overview (Preliminary Data)

Example of recent commissioning data (preliminary calibrations)Ee− = 2.9 GeV, linearly polarised beam

invariant mass [MeV]γTwo 0 100 200 300 400 500 600 700

[M

eV

]γ

γ X

→

p

γM

issin

g m

ass:

500

600

700

800

900

1000

1100

1200

1300

1400

1500Neutral meson reconstruction in the BGO

invariant mass [MeV]γTwo 0 100 200 300 400 500 600 700

Counts

0

100

200

300

400

500

600

700

800

900

Mean = 137 MeV

= 15 MeVσ

Mean = 528 MeV

= 31 MeVσ

peak0πFit to

peakηFit to

Momentum (p) [MeV]0 500 1000 1500 2000 2500 3000

β

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1Charged particle ID in the Forward Spec.

pX [MeV]→pγMissing mass: 0 200 400 600 800 1000 1200 1400

Counts

0

1000

2000

3000

4000

5000

ηM ωM ’ηM

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 4 / 16

Strangeness Photoproduction - Physics Motivation

Acceptance at forward angles with BGO-OD - ideal for dominantlyt-channel exchange mechanisms

Identify final states of mixed charge - investigation of Y ∗ resonances,eg K+Λ(1405)→ K+π0Σ0 → K+π0γΛ→ K+γγγpπ−

Differential cross sections & polarisation observables

K+Λ, K+Σ0

K+Σ∗ states

K 0Σ+

K 0Λ (deuteron target)

Intense production beam times starting now

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 5 / 16

Previous γp → K 0Σ+ measurements

R. Ewald et al.,Phys. Lett. B 713 (2012) 180 (CBELSA/TAPS Collaboration)

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 6 / 16

Previous γp → K 0Σ+ measurements

Cusp structure due to K ∗0

subthreshold productionrescattering to π0 & K 0?

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 7 / 16

Previous γp → K 0Σ+ measurements

The 1st beam asymmetry measurementR. Ewald et.al, PLB 738 (2014) 268(CBELSA/TAPS Collaboration)

Black line - Bonn-Gatchina solution (BG2011-02)Black dashed - BG2011-02m (includes this Σ data & CLAS P data)Red line - Kaon-MAIDRed dashed- Kaon-MAID modified (K∗ t-channel switched off)

BGO-OD Proposal

Extend the range of Σ over the cuspstructure (1700-1800 MeV)

Identify charged & neutral decay modes(higher stats.)

K 0 → π0π0, Γ = 31%K 0 → π+π−, Γ = 69%Σ+ → pπ0, Γ = 52%Σ+ → nπ+, Γ = 48%

­1

­0.5

0

0.5

1

= ­0.8K

cmθcos

1200 1400 1600 1800 2000 2200­1

­0.5

0

0.5

1

= ­0.4K

cmθcos

1500 2000­1

­0.5

0

0.5

1

= +0.4cm

Kθcos

1200 1400 1600 1800 2000 2200­1

­0.8

­0.6

­0.4

­0.2

0

0.2

0.4

0.6

0.8

1

= 0.0K

cmθcos

1500 2000­1

­0.8

­0.6

­0.4

­0.2

0

0.2

0.4

0.6

0.8

1

= +0.8cm

Kθcos

γΣ

Photon beam energy [MeV]

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 8 / 16

Strangeness photoproduction with BGO-OD

γp → K+Λ - cover important range at forward angles & disputed peakstructure at W = 1900 MeV

Bydzovsky and D. Skoupil,

arXiv:1211.2684v1 (2012)

Proceedings of SNP12

Many Y ∗ states poorly established & CQM do not explain hyperon spectrum

Hadronic molecules - predict new Y ∗ states, eg Σ∗ close to Σ(1385)

B. S. Zou, Eur. Phys. J. A 35 (2008) 325 & Jia-Jun Wu, S. Dulat & B. S. Zou, Phys.

Rev. D 80 (2009) 017503Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 9 / 16

K+ identification in the BGO

Time delayed, K+ weak decay within the crystals of the BGO ballT.C Jude, D.I. Glazier, D.P. Watts, PLB, 735 (2014) 112

Lifetime 12 ns,2 main decay modes:

K+ → µ+νµK+ → π+π0

µ

3ns18ns

16ns

19nsBGOcrystal

Incident sub−clusterDecay sub−cluster

νµ

Κ

+

+

Energy of decay sub­cluster [MeV]50 100 150 200 250 300 350

0

200

400

600

800

1000 Simulated data

Experimental data

Time between incident and decay sub­cluster [ns]0 10 20 30 40 50 60 70 80 90 100

0

1000

2000

3000

4000

5000

11 ns≈ lifetime +

Fitted K

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 10 / 16

Identifying the Σ0 → Λγ decay

Boost all neutral particles in the BGO into missing hyperon rest frameIf K+Σ0:

Missing mass from K+γ(lab frame) = MΛ = 1116 MeV (Y axis)γ energy (hyperon rest frame) = MΣ0 −MΛ = 77 MeV (X axis)

energy in hyperon rest frame [MeV]γ20 40 60 80 100 120 140

γ+

Mis

sin

g m

ass f

rom

K

600

800

1000

1200

1400

1600

1800Experimental data

energy in hyperon rest frame [MeV]γ20 40 60 80 100 120 140

γ+

Mis

sin

g m

ass f

rom

K

600

800

1000

1200

1400

1600

1800Λ

+K→pγSimulated

energy in hyperon rest frame [MeV]γ20 40 60 80 100 120 140

γ+

Mis

sin

g m

ass f

rom

K

600

800

1000

1200

1400

1600

1800

0Σ+K→pγSimulated

energy in hyperon rest frame [MeV]γ

20 40 60 80 100 120 140

Co

un

ts

0

100

200

300

400

500 Experimental data

Λ+

Simulated K0Σ

+Simulated K

Missing hyperon masses from K+ detection in the BGO

Data subset where Σ0 → Λγ is taggedΣ0 → Λγ detection efficiency from sim. (∼ 60%, K+Λ “false” eff. < 10%)Scale data by efficiency - separate K+Λ and K+Σ0

Missing (hyperon) mass [MeV]600 800 1000 1200 1400 1600 1800

0

100

200

300

400

500

All Data

TaggedγΛ→0

ΣWith

= 1116 MeVΛM

= 1193 MeV0Σ

M

Missing (hyperon) mass [MeV]600 800 1000 1200 1400 1600 1800

0

100

200

300

400

500

All Data

efficiency scaledγΛ→0

Σ

Missing (hyperon) mass [MeV]600 800 1000 1200 1400 1600 1800

0

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100

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200

250

300

350

subtracted0Σ

+K

Missing (hyperon) mass [MeV]600 800 1000 1200 1400 1600 1800

0

50

100

150

200

TaggedγΛ→0

ΣWith

false eff. scaledΛ+

K

subtractedΛ+

K

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 12 / 16

K+ identification in the forward spectrometer

β determined from momenta measured inthe forward spectrometerCharacteristic loci of protons & π± →

To enhance K+ signal

π0 mass reconstructed in the BGOBGO total energy deposition < 250 MeV

p [MeV/c]0 500 1000 1500 2000 2500 3000

β

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

p [MeV/c]0 500 1000 1500 2000 2500 3000

β

0.3

0.4

0.5

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0.7

0.8

0.9

1

1.1

p [MeV/c]0 500 1000 1500 2000 2500 3000

β

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

βWith Calculated

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 13 / 16

Missing (Y) mass from K+ in the forward spectrometer

Missing (hyperon) mass [MeV]800 1000 1200 1400 1600 1800

0

20

40

60

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100

120

140

160

180

200No extra selection cuts

Missing (hyperon) mass [MeV]800 1000 1200 1400 1600 1800

0

20

40

60

80

100

reconstructed in the BGO0π

(1116)Λ

(1193)Σ

(1385)Σ

(1405)Λ

(1520)Λ

Missing (hyperon) mass [MeV]800 1000 1200 1400 1600 1800

0

20

40

60

80

100

120

Energy deposition in the BGO < 250 MeV

Missing (hyperon) mass [MeV]800 1000 1200 1400 1600 1800

0

10

20

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40

50

60

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80

reconstructed0πEnergy deposition in the BGO < 250 MeV &

Photon tagger hodoscope with x4 energy resolution installedTom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 14 / 16

Summary & Collaborating Institutions

K 0 identificationAnalysis in preparation

High statistics for K 0Σ+

polarisation observables

Basis for “neutral channels”, egγn→ K 0Λ

K+ identificationID reaction channels with BGO &forward spectrometer

K+Y cross sections at forwardangles

Σ∗ states

Ready for high statistics data taking

Physikalisches Institut, Bonn, Germany

Helmholtz-Institut fur Strahlen und Kernphysik, Bonn,Germany

Justus-Liebig-Universitat Giessen, Germany

Institut fur Physik, Basel, Switzerland

INFN sezione di Pavia, Pavia, Italy

The University of Messina,Messina, Italy

INFN sezione di Catania, Catania, Italy

INFN Roma Tor Vergata, Rome, Italy

The University of Rome “Tor Vergata”, Rome, Italy

INFN sezione di Torino, Torino, Italy

The University of Torino, Torino, Italy

INFN sezione di Roma, Rome, Italy

INFN - ISS, Rome, Italy

INFN - LNF, Frascati (Rome), Italy

National Science Center Kharkov Institute of Physicsand Technology, Kharkov, Ukraine

Russian Academy of Sciences Institute for NuclearResearch, Moscow, Russia

Petersburg Nuclear Physics Institute, Gatchina, Russia

The University of Edinburgh, Edinburgh, UK

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 15 / 16

Extra slide

Missing mass from forward protons & selecting π0/η in the BGO ball

pX [MeV]→pγMissing mass: 0 200 400 600 800 1000 1200 1400

Counts

0

1000

2000

3000

4000

5000

All Protons in the forward spectrometer

pX [MeV]→pγMissing mass: 0 200 400 600 800 1000 1200 1400

Counts

0

100

200

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400

500

600

700

reconstructed in the BGOηWith an

pX [MeV]→pγMissing mass: 0 200 400 600 800 1000 1200 1400

Counts

0

200

400

600

800

1000

1200

1400

1600

reconstructed in the BGO0πWith a

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR 2015 16 / 16