measurement of the η’n scattering length at leps2 2014/2/20 keigo mizutani kyoto univ

Measurement of the η’N scattering length at LEPS2

2014/2/20Keigo Mizutani

Kyoto Univ.

η’N interaction

scattering length : order of 0.1 fm.

[P. Moskal et al., Phys. Lett. B 482, 356 (2000)]

• Few experiments have measured the η’N interaction.absorption of η’ is relatively small.

[M. Nanova et al., Phys. Lett. B 710, 600 (2012)]

γpη’p process

Re(a) = -2.7 fm, Re(re) = 0.25 fm(bound state w/ 〜 6 MeV B.E.)

[S. Sakai, D. Jido, Phys. Rev. C88, 064906 (2013)]

γ

p

η’

p

Tη’pη’p Vγpη’p

γ

p

η’

p

γ

p

η’

p

+ = Vγpη’p Tγpη’p

scattering length

Imaginary parts are not calculated.

phase shift effective range

Chiral effective theory

ωN scattering lengthω photoproduction near threshold at FOREST

[ref. Hashimoto’s doctoral dissertation (2011)]

We are planning η’ version of this experiment.

interaction is repulsive

preliminary

preliminary

Measurements of the scattering length

• The large η’N scattering length (-2.7 fm) will make threshold enhancement of total cross sections.

• Near-threshold σtot measurement w/ high Eγ resolution and statistics is necessary.

[V. Crede et al., Phys. Rev. C 80, 055202 (2009)]

σtot spectrum by CBELSA

( E⊿ γ is not sufficiently small.)

Cross section near threshold

black : w/ interactionred : w/o interaction (T-matrix = 0)

a = -2.7 + i0.0 fmre = 0.25 fm

Dependency on imaginary part

black : w/ interactionred : w/o interaction

a = -2.7 + i0.5 fmre = 0.25 fm

a = -2.7 - i0.5 fmre = 0.25 fm

a = -2.7 – i1.0 fmre = 0.25 fm

a = -2.7 + i1.0 fmre = 0.25 fm

BGO experiment at LEPS2• BGO experiment has

started data-taking from Dec. 2013.

• Maximum 107 cps photon beam (1.4 GeV – 3 GeV) is available.

• BGO egg detector has 1320 crystals, and it covers from 24° to 144°.

e-

γ

γ-beam production

• UV laser (351 nm or 266 nm) is injected to the storage ring.

• Maximum 107 cps photon beam (1.4 GeV – 3 GeV) is available.

Tagger

e-

Experimental setup

BGOEGG

BGOEGG

target(LH2, 4cm)

γ

24°144°

InnerPlastic Scinti. 4 m 12 m

7°TOF wall( 〜 200 ps)

γ

p

BGOEGGDrift Chamber

RPC( 〜 50 ps)

γp η’p

RPC

proton

± 7°

± 4.3°

Detection of protonγ p η’ p detected by

RPC (or TOF wall)η π0 π0 (br: 22 %)

γ γ (br: 39 %)γ γ (br: 2.2 %)

RPC(50 ps)

TOF wall(200 ps)

RPC(50 ps)

TOF wall(200 ps)

Eγ = 1.45 GeV

Eγ = 1.46 GeV

(br = branching ratio)

• Around Eγ = 1.45 GeV, protons are emitted at forward angle.

• detection efficiency of p : 〜 100 %

by RPC

Detection of η’

γ p p η’

η π0 π0 (br: 22 %)

γ γ (br: 39 %)

γ γ (br: 2.2 %)

BGO egg

BGO egg

γ γ

BGO egg

acceptance for 2 γ

p θ

0° 20°

0.5 GeV/c 0.73 0.75

1.0 GeV/c 0.55 0.63

acceptance for 6 γ

p θ

0° 20°

0.5 GeV/c 0.34 0.35

1.0 GeV/c 0.26 0.29

Yield estimation• cross section of γppη’ : 0.3 ub (at Eγ = 1.45 GeV)

• target : 4 cm length LH2

• beam 2.7 * 105 cps (1430 – 1500 MeV)• branching ratio * acceptance : 〜 0.04

〜 10000 events / 6months

invariant mass of 2 γ

invariant mass of 6 γ

[V.Crede et al.(CBELSA), Phys. Lett. C 80, 055202 (2009)]

Photon energy resolution• High photon energy resolution is necessary.• Kinematic Fitting– 6γ (or 2γ) invariant mass = η’ mass (1 constraint)– four-momentum conservation (4 constraints)

• δP/P = 0.5 %– ( δt = 100 ps, L = 12 m for 0.7 GeV/c proton)

• δθ = 0.005,• δφ = 0.005/tanθ

( δx = 50 mm )• tagger resolution :

– 15 MeV

• BGOEGG resolution :

Eγ = 1.45 GeV

⊿Eγ = 0.9 MeV

by RPCresolutions of proton

black : 6γred : 2 γ

Expected spectra w/ and w/o interaction

Two histograms are different ( > 4σ ).


a = -2.7 + i0.0 fmr = 0.25 fm

6 months

Expected spectra w/ and w/o interaction

Two histograms are different ( > 3σ ).


a = -2.7 + i0.0 fmr = 0.25 fm

6 months

a = -2.7 ± i0.5 [fm]


a = -2.7 + i0.5 fmr = 0.25 fm

distinguishable ( > 4σ )

a = -2.7 - i0.5 fmr = 0.25 fm


distinguishable ( > 3σ )

a = -2.7 - i1.0 fmr = 0.25 fm

a = -2.7 ± i1.0 [fm]

a = -2.7 + i1.0 fmr = 0.25 fm

How about a = -0.1 fm ?

a = -0.1 + i0.0 fmre = 0.0 fm


Not distinguishable


Summary• Production cross section of η’ near threshold η’N cross

section may enhance due to η’N interaction.• Large scattering length (-2.7 fm) has been predicted

theoretically and we can measure with over 3σ confidence level at LEPS2. The sensitivity for small scattering length will be investigated.

• 104 η’ (near threshold) / half year at BGO experiment at LEPS2

• ⊿Eγ 〜 1 MeV ( Kinematic Fitting )

• σtot depends on imaginary part of a and re.– Theoretical help to evaluate imaginary part is necessary.

backup

Effects of virtual π Nη’p

• Diagrams as above are possible, and they may distort the shape of near-threshold cross section even if η’ and N do not interact.

• γpπN ( few μbarn ) , πNη’p are small?

γ

p N

π η’

p

measurement of the η’n scattering length at leps2 2014/2/20 keigo mizutani kyoto univ

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