meson transition form factors at - indico · 2018. 11. 16. · april 9, 2014 c.f. redmer - meson...

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