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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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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-
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
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
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
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
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