M easurement of the η’N scattering length at LEPS2
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Measurement of the η’N scattering length at LEPS2
2014/2/20Keigo Mizutani
Kyoto Univ.
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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)]
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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
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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
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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.)
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Cross section near threshold
black : w/ interactionred : w/o interaction (T-matrix = 0)
a = -2.7 + i0.0 fmre = 0.25 fm
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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
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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
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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-
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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
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RPC
proton
± 7°
± 4.3°
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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
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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
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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)]
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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
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Expected spectra w/ and w/o interaction
Two histograms are different ( > 4σ ).
black : w/ interactionred : w/o interaction
a = -2.7 + i0.0 fmr = 0.25 fm
6 months
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Expected spectra w/ and w/o interaction
Two histograms are different ( > 3σ ).
black : w/ interactionred : w/o interaction
a = -2.7 + i0.0 fmr = 0.25 fm
6 months
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a = -2.7 ± i0.5 [fm]
black : w/ interactionred : w/o interaction
a = -2.7 + i0.5 fmr = 0.25 fm
distinguishable ( > 4σ )
a = -2.7 - i0.5 fmr = 0.25 fm
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black : w/ interactionred : w/o interaction
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
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How about a = -0.1 fm ?
a = -0.1 + i0.0 fmre = 0.0 fm
black : w/ interactionred : w/o interaction
Not distinguishable
black : w/ interactionred : w/o interaction
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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.
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backup
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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