Meson Transition Form Factors at - Indico · 2018. 11. 16. · April 9, 2014 C.F. Redmer - Meson...
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April 9, 2014 | Christoph Florian Redmer for the BES-III Collaboration
Meson Transition Form Factors at
SFB1044 Workshop“(g-2)µ : Quo vadis?”
Institut für Kernphysik, Mainz
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 2
Outline
The BES-III Experiment
Measurement of Meson Transition Form Factors
Time - like
Space - like
Summary
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 3
Beijing Electron Positron Collider BEPC-II
2004: start of BEPC-II construction
2008: first e+e- collisions
Since 2009: BEPC-II/BES-III data taking
Beam Energy1.0 - 2.3 GeV
Energy Spreadσ(E)/E = 5.16 · 10-4
Design Luminosity1033 cm-2 s-1 @ ψ(3770)
Achieved Luminosity0.7 · 1033 cm-2 s-1 @ ψ(3770)
BES-III Detector
Linac
Storage Ring
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 4
BESIII Detector
Main Drift Chamber (MDC) σ(p)/p = 0.5% σ
dE/dx = 6.0%
Time-of-flight system (TOF) σ(t) = 90ps (barrel) σ(t) = 110ps (endcap)
EMC 6240 CsI(Tl) crystals σ(E)/E = 2.5% σ
Z,Φ(E) = 0.5 – 0.7 cm
Muon Chambers 8 – 9 layers of RPC p>400 MeV/c δRΦ = 1.4 ~ 1.7 cm
Superconducting Magnet 1 T magnetic field
NIM A614 (2010) 345
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 5
Acquired Data Sets
1.23 · 109 J/Ψ
0.52 · 109 Ψ(2S)
2.9 fb-1 Ψ(3770)
0.5 fb-1 @ 4.04 GeV
1 fb-1 @ 4.23 GeV
0.8 fb-1 @ 4.26 GeV
0.5 fb-1 @ 4.36 GeV
24 pb-1 τ mass scan
800 pb-1 R scan (104 points from 3.85 to 4.6 GeV)
World's largest samples of J/Ψ, Ψ(2S), and Ψ(3770)
BES-III Physics Program
Charmonium Spectroscopy
Charm Physics
Light Hadron Spectroscopy
τ, R & QCD
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 6
How to measure TFF at BES-III
Time – like Transition Form Factors:
Dalitz decays
0 < q² < M²
Annihilation process
q² = s > M²
Space – like Transition Form Factors:
Two-photon production of mesons
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 7
Time-like TFF
Terschlüsen, LeupoldJ. Fu, H.B. Li, X. Qin and M.Z. Yang,Mod. Phys. Lett. A 27 1250223 (2012)
Dalitz decays J/Ψ → P e+e-
Unmeasured, rare processes
BRtheo = 10-5 – 10-7
Recent interest in V → Pγ transitions of light unflavored mesons
NA60 result on ω → π0 e+e- not described by VMD
Recent measurements at SND, CMD-2, and KLOE on Φ
➔ Test VMD
Important for understanding of P → γ transitions
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 8
Dalitz Decays of J/Ψ
J/ψ → P e+e-, with P = π0, η, η'
Background from other J/ψ decays removed by kinematic cuts
External conversion is main source of background
MC: J/ψ → π+π- π0
arXiv:1403.7042Submitted to PRD
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 9
Analysis
225 · 106 J/ψ decays analyzed
Background well under control
Good agreement of data and MC
arXiv:1403.7042Submitted to PRD
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 10
Transition Form Factor
TFF used to estimate systematic error
Example : η'
Fit: Λ = (3.1 ± 1.0) GeV/c2
Λ = mΨ'
= 3.686 GeV/c2
arXiv:1403.7042Submitted to PRD
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 11
Branching Ratios
First measurement of Dalitz decays J/ψ → P e+e-, with P = π0, η, η'
BR of decays with P = η, η' in agreement with Theory
2.5 σ difference for P = π0 J. Fu, H.B. Li, X. Qin and M.Z. Yang,Mod. Phys. Lett. A 27 1250223 (2012)
arXiv:1403.7042Submitted to PRD
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 12
Space-like TFF
Talk by Y. GuoTomorrow
Single Tag Technique Measure only
One scattered lepton
Decay products of meson
Reconstruct second lepton from missing momentum
Require small scattering angle
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 13
Space-like TFF
Measure only
One scattered lepton
Decay products of meson
Reconstruct second lepton from missing momentum
Require small scattering angle
Single Tag Technique
Example: Monte Carlo, Ψ(3770), Lint
: 927 pb-1 , Tagged Lepton: e-
π0 2-Octet Modelη 2-Octet Model
Ψ(3770) Rad. Return (J/Ψ,Ψ')rad. BhabhaMC Sum
zoom
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 14
Space-like TFF
Measure only
One scattered lepton
Decay products of meson
Reconstruct second lepton from missing momentum
Require small scattering angle
Q12 : high virtuality
Q22 : quasi real
| F(Q12,Q2
2)|2 |F(Q12,0)|2 |F(Q2)|2
Single Tag Technique
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 15
Previous Measurements
e+e- →e+e- η e+e- →e+e- η'e+e- →e+e- π0
CELLO: Z.Phys.C49 (1991) 401CLEO: Phys.Rev.D57 (1998) 33BaBar: Phys.Rev.D80 (2009) 052002
Phys.Rev.D84 (2011) 052001Belle: Phys.Rev.D86 (2012) 092007
Recent results from B-factories cover only large Q² ( 5 < Q² [GeV²] < 40)
Discrepancy for π between BaBar and Belle⁰
Data scarce at lowest Q²
Region of relevance for (g-2)µ
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 16
Feasibility Studies
e+e- →e+e- η
BSc Theses: A. Hahn, B. Kloss
Assumptions:
√s = 3.773 GeV
Lint = 10 fb-1
Only detector geometry
Result:
TFF measurable up to Q2 = 10 GeV2
Unprecedented accuracy below 4 GeV2
Above 4 GeV2 accuracy comparable to CLEO
e+e- →e+e- η'
e+e- →e+e- π0
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 17
Analysis Example: π0 / ηData
Ψ(3770) on-peak, available: 2.92 fb-1
Monte Carlo
Signal: Ekhara 2.1
Background: Babayaga 3.5, KKMC
Event Selection:
exactly one lepton candidate
At least two, max four photons
Expected Background Channels
Radiative Bhabha Scattering
Hadronic Final States
Two-Photon Production with ISR
Two-Photon Production of other mesons
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 18
Analysis StepsHelicity Condition for π0
Angle between γ in π0 rest frame and π0 in lab
Flat for signal
Peaked for background
Reject events with cos(θH) > 0.8
reduction of QED background
Lint
: 927 pb-1 , Tagged Lepton: e-
π0 2-Octet Modelη 2-Octet Model
Ψ(3770) Rad. Return (J/Ψ,Ψ')rad. BhabhaMC Sum
Polar angle difference of γ pair
Strongly peaked for QED background
Flat for hadronic background
Dip for signal
Reject events with |Δθγγ| < 1.5°
reduction of QED background
Lint
: 927 pb-1 , Tagged Lepton: e-
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 19
Analysis StepsPolar angle of π0
Background enhanced at large cos(θπ )
Signal almost evenly distributed
Reject events with cos(θπ ) · qtagged > 0.8
Data/MC difference
QED background reduced
Condition on ISR
ISR results in wrong Q2
Useful observable:
If ISR,
Reject events with rγ< -0.1 and rγ> 0.2
Hadronic background almost completely removed
Lint
: 927 pb-1 , Tagged Lepton: e-
Lint
: 927 pb-1 , Tagged Lepton: e-
rγ=√ s−Ee± π 0,η
CMS−pe± π0, η
CMS
√srγ=
2Eγ
√ s
π0 2-Octet Modelη 2-Octet Model
Ψ(3770) Rad. Return (J/Ψ,Ψ')rad. BhabhaMC Sum
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 20
Analysis Steps
Study differential cross section dσ/dQ2
Bin wise back ground subtraction
Statistics from Ψ(3770) data only sufficient for π0 TFF up to Q2 = 3 GeV2
Include large samples from XYZ searches
Clear signals from π0 / η →γγ
Data: Background underestimated
➔ Use better MC generators
modified Babayaga@NLO two-photon generator
Lint
: 927 pb-1 , Tagged Lepton: e-
π0 2-Octet Modelη 2-Octet Model
Ψ(3770) Rad. Return (J/Ψ,Ψ')rad. BhabhaMC Sum
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 21
Expectations for π0 TFF
Full Simulation
Lint: 2.92 fb-1
Single Tag with both, e±
Extract TFF for 0.3 ≤ Q2[GeV2] ≤ 3.1
Expected statistical precision:
Unprecedented below Q2 = 1.5 GeV2
➔ Important for (g-2)μ
Compatible with CLEO
(Ekhara, 2-Octet Model)
Next steps:
Study systematics
Largest contribution expected from background subtraction
Include high energy data
Other final states
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April 9, 2014 C.F. Redmer - Meson Transition Form Factors at BESIII SFB1044 Workshop, Mainz 22
Summary BESIII is a good place to study time- and space-like meson transition form factors
First measurement of Dalitz decays in the charmonium region
Physics program started at BES-III
Single tagged measurements of space-like TFF of π0, η, η' Significant contribution for Q² < 10 GeV²
First result for π0 expected soon 0.3 GeV2 < Q2 < 3.1 GeV2 covered Agreement with CELLO and CLEO measurements Unprecedented accuracy for Q2 < 1.5 GeV2
Analyses of η and η' ongoing
Long Term Plan
Measurements of scalar and tensor mesons
Measurements of polarization observables
Double tagged measurements
Talk by Yuping GuoTomorrow