meson with g12 and CLAS12 - Thomas Jefferson National ... · BESIII (2.13 +- 0.09(stat) +-...
Transcript of meson with g12 and CLAS12 - Thomas Jefferson National ... · BESIII (2.13 +- 0.09(stat) +-...
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Towards the Electromagnetic Transition Form Factor of the η' meson with g12 and CLAS12
22 October 2015 Hadron Spectroscopy Working Group
Michael C. Kunkel Forschungszentrum Jülich GmbH
IKP-1
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Motivation: • Transition form factors provides insight into the meson charge
radius, . • Ratio of η/η’ form factors provides information on η/η’ mixing angle. • For ω there is a discrepancy between the measurement and the
VMD model. • The η form factor is needed to interpret the muon g-2 experiment.
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Transition Form Factors Michaela Schever, Michael C. Kunkel, Susan Schadmand, Jim Ritman;
Institut für Kernphysik, Forschungszentrum Jülich
CLAS collaboration meeting - HSWG
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Towards(Measuring(the(Electromagne5c(Structure(of(eta‘(Mesons(with(CLAS((
• accessible(via(invariant(mass(distribu0on(of(dilepton(• feasibility(study(using((
• CLAS(g12(data(on((η‘(→(γ(e+(e?(• background(simula0ons((cocktail(plot)((
2/19/15(master(thesis(Michaela(Schever((Jülich/
Aachen)( 1(
both%smooth%and%in,peak%background%important%
→%simula6ons%underway%
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η, ω, η’ Yield
η→e+e-γ
η'→e+e-γ
ω→e+e-Χ
PRELIMINARY
CLAS data yield from γp→pX with |M2x(pe+e-)|<0.018 GeV2
ME(pe+e-)>0.075 GeV and electron/positron ID
Counts
Goal: Measuring transition form factors
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ω Transition Form Factor
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dq2d�!!⇡0�=
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d�!!l+l�⇡0
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PRELIMINARY
Recent results the ω transition form factor. Image Source: Conference Proceedings
CLAS data yield from γp→pX with M2x(pe+e-) = M2π0±0.01 GeV2
PRELIMINARY
CLAS data yield from γp→pe+e-X with Mx(p) = Mω ±0.031 GeV
π0
CLAS collaboration meeting - HSWG
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Current BESIII and CLAS data sets do not have enough statistics to determine which theoretical model fits the η′→ charge radius
Number of events
BESIII (η′→γe+e−) 1.60 ± 0.17(stat) ± 0.08(sys) GeV-2 [1]
864
CELLO (η′→γµ+µ−) 1.7 ± 0.4 GeV-2 [2] 75
CLAS (η′→γe+e−) TBD 89
Dispersion 1.53+0.15-0.08 GeV-2
ChPT 1.6 GeV-2
VMD 1.45 GeV-2
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Status of η’ charge radius
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[1]M. Ablikim et al., Phys.Rev. D92 (2015) 012001 [2]R. I. Dzhelyadi et al., Phys. Lett. B 88, 379 (1979)
Current statistical error cannot discern the correct theoretical model
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η′→γee : analysis
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• lepton skim MK: IsLepG7_g12.root • cuts:
• g12 fiducial cuts • event efficiency • q-factor signal extraction
evaluate smooth background event-by-event
➢ 359 event candidates ➢ 82 events (signal weight) full g12 beam energy range
(1) pip_IsLepG7==1 && pim_IsLepG7==-1 ..................................... g7 leptons (CC&EC cut) (2) NProt==1 ................................................................................... final state proton (3) abs(pip_Beta -1.)<0.05 && abs(pim_Beta -1.)<0.05 ................. ToF electron candidates (4) MM2P>=0.75 && MM2P<=1. .................................................... MMp : eta‘ mass window (5) MM2PEpEm<0.01 && MM2PEpEm>-0.01 ................................ MMpee : massless or no missing
particle (6) MEPEpEm>0.2 ........................................................................... MEpee : missing particle (has energy) (7) abs(pip_vx*pip_vx+pip_vy*pip_vy)<4. ..................................... vertex cut for positron (8) abs(pim_vx*pim_vx+pim_vy*pim_vy)<4. ................................. vertex cut for electron
solid: all data filled: background weighted
CLAS collaboration meeting - HSWG
PRELIMINARY
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fit to data, range (0.,0.1) FCN=0.298654 FROM MIGRAD STATUS=CONVERGED 34 CALLS 35 TOTAL EDM=3.39438e-020 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 9.90865e+003 4.62091e+003 1.92653e+000 4.30050e-014 2 p1 3.77484e+005 6.29736e+004 3.20771e+000 4.02354e-015
rel BR = p0/p1 = 0.0262492 +- 0.013001 (fit errors)
BESIII (2.13 +- 0.09(stat) +- 0.07(sys)) x 1e-2
η′→γee : Mee distribution
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symbols: data (signal weight) filled: η′→γee simulation dotted: η′→γγ simulation solid: summed simulations
next step: kinematic fit improved missing mass cuts will affect large-mass end
evaluate in-peak background via invariant mass distribution
sole background candidate: external conversion in the target
full g12 beam energy range
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PRELIMINARY
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fit to data, range (0.,0.1) FCN=0.130833 FROM MIGRAD STATUS=CONVERGED 34 CALLS 35 TOTAL EDM=7.68571e-019 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 9.24417e+003 4.60182e+003 1.79231e+000 -2.07859e-013 2 p1 3.45819e+005 6.21834e+004 2.99290e+000 -1.93127e-014
rel BR = p0/p1 = 0.0267312 +- 0.0141485 (fit errors)
BESIII (2.13 +- 0.09(stat) +- 0.07(sys)) x 1e-2
η′→γee : Mee distribution
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symbols: data (signal weight) filled: η′→γee simulation dotted: η′→γγ simulation solid: summed simulations
next step: kinematic fit improved missing mass cuts will affect large-mass end
evaluate in-peak background via invariant mass distribution
sole background candidate: external conversion in the target
g11 beam energy range
CLAS collaboration meeting - HSWG
PRELIMINARY
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cross check: angular distribution
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signal events -> cross section dsigma/dcosthcm in limited beam energy range compare to g11
.... not yet
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CLAS12 e+e- pair physics
Electromagnetic structure of mesons and baryons. Currently we are benchmarking the η′→γe+e− decay
Here is a list of initial physics to be studied
Meson Baryon
η′→γe+e− (Δ→Νe+e−)
ω→π0e+e− Λ→ne+e−
Λ(1520)→Λe+e−
J/ψ→π0e+e− Σ0→Λe+e−
Σ+→pe+e−
CLAS ξ(e+e−)/ξ(π+π−) can be range 105 - 1011 CLAS e+e− efficiency (ε) range 1 - 10-2
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CLAS12 η’ Measurement
Fully Exclusive γp→η′p→ γe+e−p
Inclusive γp→η′(p)→ γe+e−(p)
M(e+e��)[GeV ] M(e+e��)[GeV ]
PRELIMINARY PRELIMINARY
Counts
Counts
570K events generated 570K events generated
CLAS collaboration meeting - HSWG
Using CLAS12 simulation and reconstruction software (GEMC & coatjava-1.0)
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) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Accept
ance /
0.02 G
eV
0.002
0.004
0.006
0.008
0.01
0.012
pγ-e+ e→ p γExclusive pγ-e+ e→ p γExclusive
) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Accept
ance /
0.02 G
eV
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
(p) with ECγ-e+ e→ p γInclusive (p) with ECγ-e+ e→ p γInclusive
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CLAS12 η’ Acceptance
e+e- Acceptance
PRELIMINARY
PRELIMINARY
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) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Count per
100 days
/ 0.02 GeV
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pγ-e+ e→ p γExclusive
pγ-e+ e→' p η → p γ
CLAS12 ExpectationsVMD ModelQEDN = 21836 / 100 days
pγ-e+ e→ p γExclusive
) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Count per
100 days
/ 0.02 GeV
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p with ECγ-e+ e→ p γExclusive
pγ-e+ e→' p η → p γ
CLAS12 ExpectationsVMD ModelQEDN = 2426 / 100 days
p with ECγ-e+ e→ p γExclusive
) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Count per
100 days
/ 0.02 GeV
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(p)γ-e+ e→ p γInclusive
(p)γ-e+ e→' (p) η → p γ
CLAS12 ExpectationsVMD ModelQEDN = 53040 / 100 days
(p)γ-e+ e→ p γInclusive
) [GeV]-e+M(e0 0.2 0.4 0.6 0.8 1
Count per
100 days
/ 0.02 GeV
10
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(p) with ECγ-e+ e→ p γInclusive
(p)γ-e+ e→' (p) η → p γ
CLAS12 ExpectationsVMD ModelQEDN = 5893 / 100 days
(p) with ECγ-e+ e→ p γInclusive
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CLAS12 η’ Rates
PRELIMINARY
PRELIMINARY
Within 100 days of beam-time CLAS12 can measure the η’ transition form factor with a statistical uncertainty ~1%
CLAS collaboration meeting - HSWG
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Summary
• Current statistics of CLAS data enables measurements of transition form factor for ω but not η’
• The η’ branching ratio measurement is consistent with recent BESIII result
• Cross check of angular distribution underway
• Future CLAS12 data:
• Hadron transition form factors.
• Branching ratios of meson conversion decays.
• Fundamental properties of hadrons
• Currently seeking applicant for PhD for η transition form factor measurement
CLAS collaboration meeting - HSWG
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END
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ηʹ′→γee : MMp distribution
21. Oktober 2015 16
MMp
solid line: data signal filled histo: simu par(0)*gee+par(1)*gg