Strangeness photoproduction at extremely forward angles at ...€¦ · Strangeness photoproduction at extremely forward angles at the BGO-OD experiment The BGO-OD experiment at ELSA,

Strangeness photoproduction at extremely forwardangles at the BGO-OD experiment

Tom JudeOn behalf of the BGO-OD Collaboration

Physikalisches InstitutUniversity of Bonn

Supported by the DFG PN 50165297

28th June 2018

Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 1 / 32

Strangeness photoproduction at extremely forward anglesat the BGO-OD experiment

The BGO-OD experiment at ELSA, Bonn

Motivation: Hadron structure - parallels in strange & charmed quarksectors?

Strangeness photoproduction - first results:

1 Λ(1405) & access to higher lying hyperons

2 Extreme forward cross sections for K+Λ & K+Σ0 photoproduction

3 K 0 photoproduction off proton & neutron (deuterium) targets

4 Future opportunities - Y-N interaction & hypernuclei studies

Summary


The Electron Stretcher Accelerator (ELSA)


The BGO-OD experiment at ELSA

BGO calorimeter (central region) & Forward Spectrometer combination

High momentum resolution, excellent charged & neutral particle ID










Motivation - Exotic phenomena in the charmed sector

Pentaquark candidates observed at LHCb -Pc(4450)+ & Pc(4380)+

R. Aaij et al, PRL 115, 072001 (2015)

Meson-baryon dynamically generated statesin the charmed sector - J.-J. Wu, R. Molina, E. Oset,

and B.S. Zou, Phys. Rev. Lett. 105, 232001 (2010)

Forsaken pentaquark particle spotted atCERN

Nature 523, 267 (2015)

X (3872) - a molecular D0D0∗ state?N.A. Tornqvist, Phys. Lett. B590, 209 (2004)

Close to D0D0∗ threshold, favouringDD∗ decay

B. Aubert et al (BARBAR Collaboration), Phys. Rev.

D77 111101 (2008)

JPC verified: 1++R. Aaij et al. (LHCb

Collaboration), PRL 110, 222001 (2013)

X(3872) discovery - most citedpaper from Belle, PRL91, 262001 (2003)


Motivation - Structure of the Λ(1405)

Between the πΣ & KN thresholds

Line shape (invariant mass)depends upon the πΣ decay mode

Distorted by interference of Isospin0 & 1 amplitudes J.C. Nacher et al., Nucl.

Phys. A 725, 181 (2003)

K. Moriya et al. (CLAS Collaboration) Phys. Rev. C 87,

035206 (2013)

Λ(1405) - dynamically generated bymeson-baryon interactions? J.C.

Nacher, E. Oset, H. Toki, A. Ramos, U.G. Meissner,

Nucl. Phys. A 725 (2003) 181

LQCD calculations: J.M.M Hall et al., Phys.

Rev. Lett. 114 (2015) 132002


Motivation - Cusp structure in γp → K 0Σ+ cross section

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

This data: Full squares, Previous CBELSA-TAPS data: open squares, Previous SAPHIR data: triangles (references therein)

Cusp-like structure dueto K∗0 subthresholdproduction rescatteringto π0 & K 0?

Grey points - K 0Σ+ + K∗Σ+M. Nanova et al., EPJ A35, 333 (2008)


Motivation - K 0 photoproduction off the neutron

A. Ramos and E. Oset, Phys. Lett. B 727, (2013) 287 Therole of vector-baryon channels andresonances in the γp → K 0Σ+ andγn→ K 0Σ0 reactions near the K∗Λthreshold

K 0Λ Complementary to K+Λ -relate hadronic coupling constants

Limited available data, K. Tsukada et al,

Phys. Rev. C 78, 014001 (2008)


Motivation - Parallels between charmed and strange sectors

Table and figures from H. Schmieden, private communication

c-sector s-sector


Motivation - Threshold dynamics

Figure from H. Schmieden, private communication


Λ∗ and Σ∗ spectra

Very limited succss with constituent quark models

Models including meson-baryon interactions as degrees of freedom - moresuccessful, L.Ya. Glozman and D.O. Riska, Physics Reports 268 (1996) 263, C. Garcia-Recio et al., Phys. Lett. B

582 (2004) 49, M.F.M. Lutz, E.E. Kolomeitsev, Phys. Lett. B 585 (2004) 243.

Hadronic molecules - predict new Y ∗ states, eg Σ∗ (JP = 1/2−) close toΣ(1385): Jia-Jun Wu et al., Phys. Rev. D 80 (2009) 017503

K−p → Λπ+π−

26 PDG Σ∗ states - 16 are listed as existence poor to fair. C. Patrignani et al.

(Particle Data Group), Chin. Phys. C 40, 100001 (2016)Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 11 / 32

Experimental requirements

Charged particle identification at extremely forward angles - reactiondynamics at very low momentum exchange (t-channel)

High forward momentum resolution

Reconstruction of complicated, mixed charge final states - egK+Λ(1405)→ K+(π0Σ0)→ K+π0γpπ−

K

N

K

Λ / Σ

π

K*γ

K+

p

K+

Λ(1405)

γ

Unique & complementary to existing facilities (eg CBELSA-TAPS neutralparticle reconstruction, CLAS charged particle reconstruction).

BGO-OD at the ELSA facility, Bonn (Germany)


Detector performance - central region

1st large data set taken in 2017. Neutral meson reconstruction (lH2 target):

invariant mass [MeV]γ20 100 200 300 400 500 600 700 800

[M

eV

]γ

mis

sin

g m

ass r

ecoili

ng fro

m 2

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1

10

210

310

410

510

Neutral meson reconstruction in the BGO ball

invariant mass [MeV]γ2100 200 300 400 500 600 700

Counts

0

20

40

60

80

100

610×

= 10 MeVσ: mean = 135 MeV, 0π

= 20 MeVσ: mean = 557 MeV, η

(scaled x50)

Missing mass recoiling from the proton [MeV]0 200 400 600 800 1000 1200 1400

Counts

0

2000

4000

6000

8000

10000

12000=15.1 MeVσ η

=17.5 MeVσ ω

=18.6 MeVσ’ η

Momentum [MeV]500 1000 1500 2000 2500 3000

β

0

0.2

0.4

0.6

0.8

1

1.2

0

2000

4000

6000

8000

10000

12000

Charged particle identification in the forward spectrometer invariant mass [MeV]γ2

0 100 200 300 400 500 600 700 800

[M

eV

]γ

mis

sin

g m

ass r

ecoili

ng fro

m 2

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1

10

210

310

410

510

Neutral meson reconstruction in the BGO ball

invariant mass [MeV]γ2100 200 300 400 500 600 700

Counts

0

20

40

60

80

100

610×

= 10 MeVσ: mean = 135 MeV, 0π

= 20 MeVσ: mean = 557 MeV, η

(scaled x50)

Missing mass recoiling from the proton [MeV]0 200 400 600 800 1000 1200 1400

Counts

0

2000

4000

6000

8000

10000

12000=15.1 MeVσ η

=17.5 MeVσ ω

=18.6 MeVσ’ η

Momentum [MeV]500 1000 1500 2000 2500 3000

β

0

0.2

0.4

0.6

0.8

1

1.2

0

2000

4000

6000

8000

10000

12000

Charged particle identification in the forward spectrometer

Demonstration (1 of many bench marks) - γp → ωp dσ/dΩ via the mixedcharge decay, ω → π0π+π− (G. Scheluchin)

[MeV]γE1200 1400 1600 1800 2000 2200 2400 2600 2800

b/s

r]µ

[Ω

/dσ

d

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

< 0.60CM

θ0.67 < cos

[MeV]γE1200 1400 1600 1800 2000 2200 2400 2600 2800

b/s

r]µ

[Ω

/dσ

d

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

< 0.33CM

θ0.44 < cos

)γ0π→ωCBELSA/TAPS (

)π+π0π→ωCLAS (

)π+π0π→ωBGOOD (

Preliminary

F. Dietz et al. (CB-ELSA/TAPS),

Eur. Phys. J. A 51, 6 (2015)

M. Williams et al. (CLAS), Phys.

Rev. C 80, 065208 (2009)


Detector performance - forward spectrometer

Top - Particle identification in the Forward Spectrometer

Bottom - “Missing mass” from forward proton (raw counts)

p [MeV]500 1000 1500 2000 2500 3000

β

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1

10

210

310

410

Forward particle reconstructed mass [MeV]

0 200 400 600 800 1000 1200 14000

20

40

60

80

1003

10×

+e+π

+K

protons

Forward proton missing mass [MeV]

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Counts

0

50

100

150

200

250

310×

η

ω

’ηφ

Forward proton missing mass [MeV]900 920 940 960 980 1000 1020 1040 1060

Counts

14000

16000

18000

20000

22000

24000

26000’η

φ



BGO-OD experiment at ELSA, Bonn







Outlook and conclusions


Mass recoiling from K+ in the forward spectrometer

The study of Y ∗ states in an extremely low momentum transfer region

missing mass [MeV]+

K1100 1200 1300 1400 1500 1600

mis

sin

g m

ass [

Me

V]

0π

+K

900

950

1000

1050

1100

1150

1200

1250

1300

(1116)Λ

(1193)0Σ

(1405)Λ(1385)/Σ

n

γn+

Λ

0Σ

) missing mass0π+

Discriminating between states with (K

missing mass [MeV]+K1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Co

un

ts

0

200

400

600

800

1000 (1116)Λ

(1193)0Σ (1405)Λ

(1385)Σ

(1520)Λ

momentum < 1 GeV+

All events for K

Identify Y ∗ states from K+π0 recoiling mass

K+Λ→ K+π0n (Missing neutron mass from K+π0 system)

K+Λ(1405)→ K+π0Σ0 (Missing Σ0 mass from K+π0 system)

K+Σ(1385)→ K+π0Λ (Missing Λ mass from K+π0 system)

Goal - Λ(1405) lineshape at low momentum transfer regionTom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 16 / 32

Λ(1405) over a broad kinematic range

G. ScheluchinComplements CLAS data - K. Moriya et al., Phys. Rev. C

88, 045201 (2013)

Triangle singularity can describe peak atW = 2110 MeV & cos θ = 0:

Full K+Λ(1405) & K+Λ(1520)reconstruction

K+Λ(1405)→ K+π0Σ0 → K+γγγπ−pE. Wang et al., Phys. Rev. C 95, 015205 (2017)

Preliminary












K+Λ at forward angles - motivation

Not only Y ∗ states at forward angles important - ground state hyperonphotoproduction at low t virtually unconstrained by data!

Bydzovsky and D. Skoupil, arXiv:1211.2684v1 (2012) Proceedings of SNP12

Crucial constraint for hypernuclei electroproduction!- eg Saclay Leon model for p(e, e′K+)ΛT. Mizutani, C. Fayard, G.-H. Lamot & B. Saghai, Phys. Rev. C 58, 75 (1998)


K+Λ at forward angles - motivation

M. E. McCracken, et. al (CLAS Collaboration),

Phys. Rev. C 81, 025201 (2010)D. Skoupil, P. Bydzovsky, Phys. Rev. C97, 025202 (2018)

At BGO-OD

Forward CM polar angle range approx. 3-260, 0.9 < cos θK+

cm < 1.0

Unprecedented θK+

cm resolution (< 10)

PΛ - recoil Λ polarisation accessed by angular distribution of Λ→ π0n & π−p


Extracting K+Λ & K+Σ0 signal from background

W = 1600 - 1700 MeV

[MeV]+

Mass recoiling from K0 200 400 600 800 1000 1200 1400 1600 1800 2000

Counts

0

50

100

150

200

250

300

350

400

W = 1600 1700 MeV

[MeV]+


Events

/ (

16 )

0

50

100

150

200

250

300

350

W = 1700 - 1800 MeV

[MeV]+


Co

un

ts

0

100

200

300

400

500

600W = 1700 1800 MeV

[MeV]+


Eve

nts

/ (

16

)

0

100

200

300

400

500

600

W = 1800 - 1900 MeV

[MeV]+


Counts

0

100

200

300

400

500

600

W = 1800 1900 MeV

[MeV]+


Events

/ (

16 )

0

100

200

300

400

500

600

Fitting with RooFit https://root.cern.ch/roofit

Signal

Simulated K+Λ events

Simulated K+Σ0 events

Background from pair productionin the beam


Preliminary forward K+Σ0 differential cross sections

Test of systematics for forward K+ detection

Highest statistics data set (& only 75% of available data)

Work in progress - binning into fine polar angle intervals (30 centre of mass)

Photon beam energy [MeV]1100 1200 1300 1400 1500

b/s

r]µ

[Ω

/dσ

d

0

0.05

0.1

0.15

0.2

0.25 > 0.9+

K

CMθcos

BGOOD, preliminary

SAPHIR (Glander ’04)

BnGa PWA

CLAS (Bradford ’06)

CLAS (Dey ’10)

Preliminary

R. Bradford et al., Phys. Rev. C73, 035202 (2006), B.Dey et al., Phys.Rev. C82, 025202 (2010),

K.H. Glander et al., Eur. Phys. J. A19, 251 (2004), CLAS data in cos θCMK+ 0.85 to 0.95 interval


Preliminary forward K+Λ differential cross sections

Highest statistics data set - distinguish discrepancies in existing data

Ongoing analysis - statistical error to be reduced by 1/2

Photon beam energy [MeV]900 1000 1100 1200 1300 1400 1500

b/s

r]µ

[Ω

/dσ

d

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 > 0.9CM+

Kθcos

BGOOD, preliminary

SAPHIR (Glander ’04)

CLAS (McCracken ’10)

CLAS (Bradford ’06)

BnGa PWA

Preliminary

R. Bradford et al., Phys. Rev. C73, 035202 (2006), M.E.McCracken et al., Phys.Rev. C81, 025201 (2010),

K.H. Glander et al., Eur. Phys. J. A19, 251 (2004), CLAS data in cos θCMK+ 0.85 to 0.95 interval



X-axis: Eγ , 900 - 1400 MeV. Y-axis: dσ/dΩ, 0 - 0.8 [µb/sr]

Unprecedented polar angle resolution

Ongoing analysis - statistical error to be reduced by 1/2

BnGa PWA

BGOOD, very preliminary

labelled inset+KCM

θ

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

03.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

06.5

Energy [MeV]γ900 1000 1100 1200 1300 1400

00.10.20.30.40.50.60.70.8

09.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

012.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

015.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

018.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

021.5

900 1000 1100 1200 1300 14000

0.10.20.30.40.50.60.70.8

024.5

Veryprelim

inary!!!!

Forward dσ/dΩ for K+Λ(1520), K+Σ−, & K+Σ−(1385) in preparationTom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 24 / 32











Preliminary identification of K 0Σ+ with BGO-OD

Study the Cusp-like structure in the cross section. K∗0 subthresholdproduction rescattering to π0 & K 0?

Access γp → K 0Σ+ via different decay modes at BGO-OD

Reduction of background & improved momentum resolution acheivable withthe MWPC (analysis underway)


Test beam time data (Deuterium target), 2 days

Peak predicted from coherent interference of dynamically generated resonancesA. Ramos and E. Oset, Phys. Lett. B 727, (2013) 287

K 0 → 2π0 reconstructed in theBGO

Total neutral particles < 6

Total charged particles < 3

K 0 → 2π0 reconstructed in theBGO

Proton in the Forwardspectrometer & select missing π0

mass from Σ+ → pπ0

Total neutral particles < 6

Total charged particles < 3

invariant mass [MeV]0π2

300 400 500 600 700 800 900

Counts

0

500

1000

1500

2000

2500

3000

Mean = 508 MeV

Height = 380

= 26 MeVσ

= 23753σ 2±Integral over

in the BGO, Missing mass between 1000 1200 MeV0π(a) 2

invariant mass [MeV]0π2

300 400 500 600 700 800 900

Counts

0

20

40

60

80

100

120

Mean = 512 MeV

Height = 296

= 18 MeVσ

= 5787σ 2±Integral over

and proton, Missing mass between 1000 1200 MeV0π(b) 2

Preliminary












Future opportunities: Hypernuclei production

Hypernuclei photoproduction, egγ +12 C → K+ +12

Λ B - natural laboratoryto probe the Y − N interaction

Forward K+, coherent production, leavingthe Λ within the Fermi surface of theresidual nuclei

P. Achenbach, arXiv:1101.4395 (2008)

Opportunities for first real photon beam photoproduction?In conversation with P. Achenbach & J. Pochodzalla, 2018

Mesic decay (Λ→ πN) identification in BGO - favour light nuclei

Identify specific states, eg γ +12 C → π0 +12 C∗ → π0 +12 C + γ′ (4.4 MeV)

Can use “sensitive” targets that are spoiled by intense e− beams (eg Li)

Hypernuclei lifetime measurements, angular K+ distributions, differentialcross sections...


Future opportunities: Hypernuclei production

Coherent pion production (“non-strange” example)

Method similar to C.M. Tarbert, D.P. Watts et. al (A2 Collaboration) Phys Rev. Lett. 100 (2008) 13

γ +12 C → π0 +12 C∗ → π0 +12 C + γ′ (4.4 MeV)

E2 transition, Jπ : 2+ → 0+, sin2(2α) distr. between nuclear recoil & γ’

∼ 2 hours of commissing data with BGO-OD:

energy [MeV]0πCalculated measured 300− 200− 100− 0 100 200 300

Counts

0

100

200

300

400

500

600

700

800

900 = 297 MeVγE

angles0πAll 0

< 400πθ

events0π1. Search for coherent

Photon energy [MeV]2 4 6 8 10

Counts

0

100

200

300

400

500

600

700

800

900Exponential + Gaussian fn.

4.5 MeV≈Mean

0.6 MeV≈ σ

2. Search for additional decay photon

[deg]α0 50 100 150

Counts

0

1

2

3

4

5

) fitα(22sin

angular distr.γ3. Verification decay

Not efficiency corrected


Future opportunities: Y-N studies with deuterium targets

Data being taken this very moment!

Final state interactions forγd → K+Λ + (n)

KN, YN & pion mediated

θK < 300 - FSI dominated by Y-N

Models of YN forces - cusps at ΣN &YN thresholds

K. Miyagawa, T. Mart, C. Bennhold, W. Glockle, Phys. Rev.

C74 (2006) 034002

Cross section & pol. obs. sensitive toYN interactions at these cusps!


Summary

Hadrons - molecular-like structure?

BGO-OD - strangenessphotoproduction - extreme forwardangles

Low momentum transfer region

1st preliminary results - K+Y atforward angles

Λ(1405), K 0 off protons & neutrons -analyses ongoing

Deuterium target - data being takennow!

Y-N interaction study opportunities -deuterium, hypernuclei...

missing mass [MeV]+

K1100 1200 1300 1400 1500 1600

mis

sin

g m

ass [

Me

V]

0π

+K

900

950

1000

1050

1100

1150

1200

1250

1300

(1116)Λ

(1193)0Σ

(1405)Λ(1385)/Σ

n

γn+

Λ

0Σ

) missing mass0π+


missing mass [MeV]+K1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Co

un

ts

0

200

400

600

800

1000 (1116)Λ

(1193)0Σ (1405)Λ

(1385)Σ

(1520)Λ

momentum < 1 GeV+

All events for K


Extra slides


Previous γp → K 0Σ+ measurements

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

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]


K 0 photoproduction off the neutron

T. Mart, Phys. Rev. C 83, 048203 (2011)

Complimentary to γp → K+Λ measurements: relate hadronic couplingconstant - predictions of n(γ,K 0)Λ cross section

gK+Λp= gK 0Λn

gK+Σ0p

= −gK 0Σ0n

gV ,TK∗+Λp

= gV ,TK∗0Λn

BGO-OD - 1st differential cross section measurement to Eγ = 3 GeV &polarisation observables

Higher statistics - K 0 identification via both main decay modesTom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 32 / 32

Separating K 0Λ and K 0Σ0 final states

Simulated data to demonstrate separation of channels

Identify the decay: Σ0 → Λγ

energy in Hyperon rest frame [MeV]γ0 20 40 60 80 100 120 140 160 180 200

Co

un

ts

0

50

100

150

200

250

300 0Σ0

Simulated K

Λ0

Simulated K

(d)

invariant mass [MeV]0π2300 350 400 450 500 550 600

[M

eV

]0

πM

issin

g m

ass r

eco

ilin

g f

rom

2

800

900

1000

1100

1200

1300

1400

1500

1600Λ

0 + K0Σ

0(a) Simulated K

[MeV]0

Missing mass recoiling from K800 900 1000 1100 1200 1300 1400 1500 1600

Co

un

ts

0

100

200

300

400

500

600

700

800

WithDecay_Lambda

Entries 2119Mean 1141RMS 112.7

NDEntries 31903Mean 1194

RMS 144.1

0(b) Missing mass recoiling from K

All data

γEvents with decay

γEvents with no decay

energy in Hyperon rest frame [MeV]γ0 20 40 60 80 100 120 140 160 180 200

[M

eV

]γ

0π

Mis

sin

g m

ass r

eco

ilin

g f

rom

2

800

900

1000

1100

1200

1300

1400Λ

0 + K0Σ

0(c) Simulated K


Tagger timing

Cluster Time [ns]600− 400− 200− 0 200 400 600

Co

un

ts

0

20

40

60

80

100

120

1403

10×

Cluster Time [ns]10− 8− 6− 4− 2− 0 2 4 6 8 10

Co

un

ts

0

20

40

60

80

100

120

1403

10×

RF bunch structure

from ELSA

~ 160 psσ


K 0 photoproduction off the neutron

No K 0 t-channel exchange (but still K∗) - s channel resonances moreprominent

Complementary to K+Λ - relate hadronic coupling constants

K. Tsukada et al, Phys. Rev. C 78, 014001 (2008)


Incident photon beam parameters

Energy tagged γ beam ≤ 3 GeV, 250 ps time resolution

Current ≤2 nA, 10 nA upgrade planned

Circularly and linearly polarised γ beams available

Coherent bremsstrahlung using a diamond radiator

30% polarisation at 1.8 GeV with an incident electron energy of 3.2 GeV


K+ identification in the BGO

Time delayed, K+ weak decay within the crystals of the BGO ball

T.C Jude, D.I. Glazier, D.P. Watts, et al, 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

+

+

Decay cluster energy [MeV]0 50 100 150 200 250 300 350 400

Co

un

ts

0

500

1000

1500

2000

2500

3000Simulated data

Real data

Decay time [ns]0 10 20 30 40 50 60 70 80 90 100

Co

un

ts

0

1000

2000

3000

4000

5000

6000

Fitted lifetime ~ 11 ns


Identifying the Σ0 → Λγ decay (Γ = 100%)

Boost all neutral particles in theBGO into missing hyperon restframe

If K+Σ0:

γ energy (Σ0 rest frame) =MΣ0 −MΛ = 77 MeV

Efficiency scale eventswith/without ID to separatemissing mass spectrum

Photon detection in the BGO toseparate different K+Y ∗ states!


K+Λ Recoil Polarisation at forward angles

Up-down asymmetry between decay proton/neutron direction in Λ restframe & normal to the reaction plane

Λ→ π0n identified in the BGO

PΛ =2

α

Nup − Ndown

Nup + Ndown, α = 0.642 (Λ decay parameter)

W [MeV]1600 1650 1700 1750 1800 1850 1900 1950 2000

ΛP

1−

0.5−

0

0.5

1 = 0.95, Preliminary)+

KθBGOOD (cos

= 0.9)CM+

KθCLAS (cos

= 0.85)CM+

KθGRAAL (cos

BonnGatchina PWA

“Dilution” - spatialresolutions/particle miss ID

Energy [MeV]γ

900 1000 1100 1200 1300 1400 1500

Effic

iency o

f corr

ect asym

metr

y0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

~ 0.89∈

Simulated data event by event comparison

M.E.McCracken et al. (CLAS), Phys.Rev. C81, 025201 (2010). A. Lleres et al. (GRAAL), Eur. Phys. J. A 31, 79 (2007)

Can now identify Λ→ pπ− for greater statisticsTom Jude (University of Bonn) Strangeness photoproduction at BGO-OD 28th June 2018 32 / 32

d∗(2380) dibaryon, Γ ≈ 70 MeV, JP = 3+

Decay branching ratios determinedM. Bashkanov, H. Clement, T. Skorodko, Eur. Phys. J. A 51, 87 (2015)

Hexaquark/molecule/dominated by diquark components?Md∗ = M∆∆ − 80 MeV

P. Adlarson et al., (WASA-at-COSY Collaboration), PRL 106,

242302 (2011)

γd → pπ0π0

solid (dashed) lines - Conventional background (with d∗(2380)contribution),M. Egorov and A. Fix, Nucl. Phys. A933, 104(2015)M. Guenther, Master thesis, Basel (2015), T Ishikawa et al.,Phys. Lett B 772, 398 (2017)


High statistics new dataset

Missing mass [MeV]+

K0 200 400 600 800 1000 1200 1400 1600 1800 2000

Counts

0

200

400

600

800

1000

1200

1400

Signal

Entries 28098Mean 961.2RMS 236.9

April 2017 beam time

< 1300 MeVγ914 < E

eventsΛ+

8200 K

0π events with Λ

+3500 K

Missing mass [MeV]+

K0 200 400 600 800 1000 1200 1400 1600 1800 2000

Counts

0

50

100

150

200

250

300

350

400

450Signal

Entries 8626Mean 916.8RMS 262.9

July 2015 beam time

< 1300 MeVγ914 < E

eventsΛ+

2450 K

0π events with Λ

+1000 K

30 bins in θK+

CM , 6-7 bins in angle

25 bins in energy

∼ 10% statistical error - consistent with PAC proposal (& higher than otherdatasets at more backward angles)

Small bug found in track-tagger correlations, will increase yield by ∼ 1.5


γp → K+Σ0 with no initial particle Identification

Preliminary analysis , 25 days data, (G. Scheluchin, PhD thesis work)

K+ & Λ→ pπ− over large acceptance region (10 < θ < 1550)

Σ0 decay photon in the BGO (Σ0 → Λγ)

Technique used for higher lying Y* states (eg Λ(1405)→ π0Σ0)

Reconstructing the Σ0 invariant mass

(a) Momentumconservation & K+

“missing mass” cut

(b) Kinematic fit &confidence level cut

(c) Neural network tosuppress background

Reconstructed invariant mass [MeV]1100 1200 1300 1400 15000

10000

20000

30000

40000

50000

(a) All combinations

Real data0

Σ+

Simulated K


10000

20000

30000

40000

50000

(b) With a kinematic fit

confidence level > 1%


10000

20000

30000

40000

50000

(c) Kinematic fit & neural network


γp → K+Σ0 with no initial particle Identification

BGO-OD (preliminary), CLAS: B. Dey, et al., Phys. Rev. C82 025202 (2010)SAPHIR: K.-H. Glander et al., Eur. Phys. J. A19 251 (2004), LEPS: M. Sumihama et al., Phys. Rev. C73 035214 (2006)

b/s

r]µ

[Ω

/dσ

d

1− 0.5− 0 0.5 1

0.2

0.4

15 MeV±=1690s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1810s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1930s

1− 0.5− 0 0.5

0.2

0.425 MeV±=2080s

1− 0.5− 0 0.5 1

0.2

0.4

15 MeV±=1720s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1840s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1960s

0.5− 0 0.5

0.2

0.425 MeV±=2130s

1− 0.5− 0 0.5 1

0.2

0.4

15 MeV±=1750s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1870s

1− 0.5− 0 0.5

0.2

0.4

15 MeV±=1990s

1− 0.5− 0 0.5

0.2

0.4100 MeV±=2255s

− −

0.2

0.4

0.2

0.4

15 MeV±=1780s

−

0.2

0.4

0.2

0.4

15 MeV±=1900s

1− 0.5− 0 0.5

0.2

0.4

0.2

0.4

25 MeV±=2030s

LEPS 2006

CLAS 2010

SAPHIR 2003

BGOOD 2016

+Kcmθcos

Preliminary


Preliminary identification of K 0Σ+ with BGO-OD

Analysis by S. Alef


Previous γp → K 0Σ+ measurements

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

This data: Full squares, Previous CBELSA-TAPS data: open squares, Previous SAPHIR data: triangles (references therein)


Access to rare Y ∗ decays

missing mass [MeV]+

K1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

mis

sin

g m

ass [M

eV

]η

+K

400

600

800

1000

1200

1400

) missing massη+


missing mass [MeV]+

K1100 1200 1300 1400 1500 1600

mis

sin

g m

ass [M

eV

]γ

+K

1000

1100

1200

1300

1400

1500

1600

1700

0Σ

Λ

) missing massγ+


Λ(1405)→ Λγ

ΓγΛ indication of compositeness:

T. Sekihara, S. Kumano, Phys. Rev C89, 025202 (2014)

K+η recoiling mass

eg, Λ(1670)1/2+ (PDG ****)Width 25 - 50 MeVΓΛη = 10− 25% (never observed)

Many poorly known Σ∗ and Λ∗ states


Motivation - Λ hyperons in a constituent quark model

Parity ordering of 3 lowest states agrees with CQM model, but Λ(1405)mass too low (compared to N∗(1535))

Y* spectrum - limited success with constituent quark models

Models including meson-baryon interactions as degrees of freedom - moresuccessful, L.Ya. Glozman and D.O. Riska, Physics Reports 268 (1996) 263, C. Garcia-Recio et al., Phys. Lett. B

582 (2004) 49, M.F.M. Lutz, E.E. Kolomeitsev, Phys. Lett. B 585 (2004) 243.

U. Loring, B. Ch. Metsch, H. R. Petry, Eur. Phys. J A 10, 395 (2001)


High statistics new dataset

Unprecedented angular resolution - 30 bins in θK+

CM

Fit to background: e+e− from the beam, misidentifed π+ at higher energy



X-axis: θCMK+ , 0 - 300. Y-axis: dσ/dΩ, 0 - 0.8 [µb/sr]

Magenta line - BnGa PWA, Beam energy labelled inset

π0 identification required - x4 stats anticipated without this

+K

θ

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

937 MeV

[deg]+

Kθ

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

976 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1016 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1049 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1076 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1103 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1129 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1156 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1183 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1210 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1237 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1263 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1290 MeV

+K

θ

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1317 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1344 MeV

0 5 10 15 20 25 300

0.10.20.30.40.50.60.70.8

1370 MeV

Preliminary


Future opportunities in K+Y studies: Y-N interaction

Forward kinematics ideal for BGO-OD

Increase OD magnetic field to improvemomentum resolution

Difficult measurement but veryimportant and not possible anywhereelse!

Right hand side plots:K. Miyagawa, T. Mart, C. Bennhold, W. Glockle, Phys. Rev. C74

(2006) 034002


Strangeness photoproduction at extremely forward angles at ...€¦ · Strangeness photoproduction at extremely forward angles at the BGO-OD experiment The BGO-OD experiment at ELSA,

Documents