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Meson form factors and P → γ∗γ∗ physics atBESIII
For the BESIII CollaborationSpeaker: Joachim Pettersson
[email protected] University
FAIRNESS,14-19 February 2016
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Outline
• Introduction• BESIII experiment• Recent results from BESIII, π+π− form factor:
γ∗
π+
π−
• Physics in the P → γ∗γ∗ vertex:
P
γ∗
γ∗
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-
Anomalous magnetic moment of the muonaµ = (g − 2)/2
-700 -600 -500 -400 -300 -200 -100 0
aµ – a
µ exp ×
10
–11
BN
L-E
82
1 2
00
4
HMNT 07 (e+e
–-based)
JN 09 (e+e
–)
Davier et al. 09/1 (τ-based)
Davier et al. 09/1 (e+e
–)
Davier et al. 09/2 (e+e
– w/ BABAR)
HLMNT 10 (e+e
– w/ BABAR)
DHMZ 10 (τ newest)
DHMZ 10 (e+e
– newest)
BNL-E821 (world average)
–285 ±
51
–299 ±
65
–157 ±
52
–312 ±
51
–255 ±
49
–259 ±
48
–195 ±
54
–287 ±
49
0 ±
63
Eur. Phys. J. C 71 1515 (2011)
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Contributions to aµ
Breakup of atheoµ
atheoµ = aQEDµ + a
weakµ + a
QCDµ
Breakup of aQCDµ
aQCDµ = aVP,LOµ + a
VP,HOµ + aLbLµ .
µ
µHad
(a)
µ
µHad Had
(b) µ µ
π0 η η′
(c)
Example diagrams: (a) Leading-, (b) higher order hadronicvacuum polarisation and (c) Light-by-light contributions.
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Theory vs. experiment aµ
Sector Contribution ×10−10 ReferenceaQEDµ 11658471.8± 0.0 [PRL 109, 111808 (2012)]aweakµ 15.3 ±0.1 [PRD 88, 053005 (2013)]aVP,LOµ 694.9±4.2 [J. Phys. G 38, 085003 (2011)]aVP,HOµ 98.4±0.7 [J. Phys. G 38, 085003 (2011)]aLbLµ 11.6±3.9 [Phys. Rept. 477, 1 (2009)]atheoµ 11659182.8±4.9 [J. Phys. G 38, 085003 (2011)]aexpµ 11659208.9±5.4±3.3 [PRD 73 072003 (2006)]aexpµ − atheoµ ≈30±8 3-4σ deviation
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Theory vs. experiment aµ
Sector Contribution ×10−10 ReferenceaQEDµ 11658471.8± 0.0 [PRL 109, 111808 (2012)]aweakµ 15.3 ±0.1 [PRD 88, 053005 (2013)]aVP,LOµ 694.9±4.2 [J. Phys. G 38, 085003 (2011)]aVP,HOµ 98.4±0.7 [J. Phys. G 38, 085003 (2011)]aLbLµ 11.6±3.9 [Phys. Rept. 477, 1 (2009)]atheoµ 11659182.8±4.9 [J. Phys. G 38, 085003 (2011)]aexpµ 11659208.9±5.4±3.3 [PRD 73 072003 (2006)]aexpµ − atheoµ ≈30±8 3-4σ deviation
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Vacuum polarisation
γ∗γ∗Had
(a) Photon self energye−
γ∗
e−e+
Had
(b) Hadronic cross section
σ(s)e+e−→hadrons =4πα
sIm Πγ(s) (1)
Im Πγ(s) is the photon vacuum polarization/self-energy function.
Calculating VP
• Calculating the contribution of low momentum hadrons notpossible in QCD.
• Optical theorem relates VP amplitude to final state crosssections.
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Hadronic contributions to aVP,LOµ
Relating cross section VP to aVP,LOµ
• Many hadronic final states contribute.
aVP,LOµ =1
4π3
∫ ∞0
ds K (s) σ(s)e+e−→hadrons (2)
π−π+
π−π+π0
π−π+π−π+
π−π+π0π0
K 0SK0L
K−K+
Rest
(a) Contributions to aVP ,LOµ .
π−π+
π−π+π0
π−π+π−π+
π−π+π0π0
K 0SK0L
K−K+
Rest
(b) Contributions to σaVP,LOµ
.
Eur. Phys. J. C 71 1515 (2011)
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Hadronic contributions to aVP,LOµ
2E, GeV0.5 1 1.5 2 2.5 3 3.5 4
cros
s se
ctio
ns, n
b
-210
-110
1
10
210
310
γ-π+π 0π-π+π
0π0π-π+π -π+π-π+π
0π-π+π-π+π 0π2-π2+π 2
-π3+π 3-+π+-K*0 K
γ-K+ K-
K+K-
K+ K0π-K+ K
0π0π-K+ K-π+π-K+ K
0π-π+π-K+ K-π+π-π+π-K+ K
-+π+-KS K ppbar
PoS Hadron 2013, 126 (2013) [arXiv:1402.0618 [hep-ex]].
• Most important: π−π+, π−π+π0, ,π−π+2π0, K−K+.• Largest errors π−π+, π−π+2π0, K−K+.
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Status of π−π+ form factor
Eur. Phys. J. C 71 1515 (2011)
BABAR and KLOE
• High precision measurements, do not agree.• New measurement needed, BESIII can provide!
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|F (s)ππ|2 ∝ σ(s)ππ
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Beijing Electromagnetic Spectrometer IIIBeijing Electron Positron Collider II
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BESIIICollaboration:12 Countries,
55 Institutions,400 Members.
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BESIII Detector
BESIII sub-system resolutions
• MDC: spatial 135 µm, σp ≈0.5 % @ 1 GeV.• TOF: 80 ps, barrel, 90 ps endcaps.• Calorimeter (CsI(Ti)): σE ≈2.5 %, barrel, σE ≈5 % endcaps
@ 1 GeV.
• PID using MDC, TOF.
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Event selection
[GeV]π2m0 0.5 1 1.5 2 2.5 3 3.5 4
even
ts /
20 M
eV
0
5000
10000
15000
20000
25000 MCγ-π+π MCγ-µ+µ
data
[GeV]π2m0 0.5 1 1.5 2 2.5 3 3.5 4
even
ts /
20 M
eV
0
2000
4000
6000
8000
10000 MCγ-π+π MCγ-µ+µ
data
Event selections
• Kinematic fit of π+π−γISR final state• Electrons rejected by BESIII standard PID system.• Artificial Neural Network for differentiating µ± − π±
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e−
e+ γISR
γ∗
-
Form Factor of π+π− (Gounaris-SakuraiParametrisation)
[GeV]s'0.6 0.65 0.7 0.75 0.8 0.85 0.9
2 | π|F
10
15
20
25
30
35
40
45 BESIII fit
BESIII
Parameter BESIII PDG14mρ[MeV/c
2 ] 776.0± 0.4 775.26± 0.25Γρ[MeV] 151.7± 0.7 147.8± 0.9mω[MeV/c
2 ] 782.2± 0.6 782.65± 0.12Γω [MeV] fixed to PDG 8.49± 0.08|cρ| [10−3 ] 1.7± 0.2 -|φω| [rad] 0.04± 0.13 -
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Comparison of form factor result
• New BESIII measurement inbetter agreement withKLOE than BaBar.
[GeV]s’0.6 0.65 0.7 0.75 0.8 0.85 0.9
|M/MB
ESIII
MfitM-
M12 | π
|F
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
BESIIIMfit
SND
CMD2-2006
CMD2-2004
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Comparison of contribution to aVP,LOµ
• Precision comparable with previous measurements• BESIII compatible with KLOE, confirms deviation of 3.4 σ• Phys.Lett. B753 (2016) 629-638, arXiv:1507.0818
Outlook
• Full low energy range: Improve g-2• High energy range: Meson spectroscopy, pion form factor.
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Physics in the P → γ∗γ∗ vertex
µ µ
π0 η η′
P
γ∗
γ∗
Light-by-light contribution to aµ
Data on process involving P → γ∗γ∗ needed for aLbLµ calculations.
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Pseudo scalars → leptons
P → γ∗γ∗ vertex in P → e+e−
Experiment 6= theory:
• π0 → e+e− branching ratio, ≈ 2− 3σ deviation[Masjuan15,Dorokhov07].
• What about other P → γ∗γ∗ processes involving η, η′, ηc?• P → e+e− door to new physics? U-bosons, leptoquarks etc.
[Phys. Rev. D78, 115002],[arxiv:0704.3498v2].
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Status of P → e+e−
Table : Pseudoscalar to lepton branching ratios.Theory from [Petri2010,Dorokhov2009].Experiments: [KTeV2007], [HADES2012], [CMD,SND2015].
Branching ratio Theory Experiment
B(π0 → e+e−) (6.23± 0.12)× 10−8 (7.49± 0.38)× 10−8B(η → e+e−) (5.2± 0.3)× 10−9 ≤ 5.6 · 10−6B(η′ → e+e−) (1.9± 0.3)× 10−10 ≤ 1.2 · 10−8B(ηc → e+e−) - -
No prediction or measurement for ηc !
Enter BESIII!
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Pseudo scalars → leptons
ηcγ∗
γ∗
l−
l+
(a) Decay.
γ∗
γ∗
ηcl+
l−
(b) Formation.
The P → l+l− process• Physics accessible in bot decay and formation.
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Modes of access: Decay
J/Ψηc
γ
(a) ηc in J/Ψ radiative decay.
ηcγ∗
γ∗
l−
l+
(b) ηc Decay.
• Decay product J/Ψ→ ηcγ → (e+e−)ηcγ.
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Modes of access: Decay
J/Ψηc
γ
(a) ηc in J/Ψ radiative decay.
ηcγ∗
γ∗
l−
l+
(b) ηc Decay.
• Decay product J/Ψ→ ηcγ → (e+e−)ηcγ.• BESIII have 109 J/Ψ on tape.B(J/Ψ→ ηcγ) ≈ 1.7%→≈ 107 ηc candidates.
• ηcγ → (e+e−)ηcγ → Background from (e+e−)bhabaγISR .• ηcγ → (µ+µ−)ηcγ → Background from
e+e− → µ+µ−γISR/(µ+µ−)J/ΨγISR
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Modes of access: Formation
γ∗
γ∗
ηcl+
l−
(a) ηc Formation in e+e−.
ηcX
(b) ηc Decay into final state.
• Direct production e+e− → ηc . Possible at BESIII!• Only background from em-continuum.• Choice of final state(s) to study.• C-even final states suppress direct γ∗ background.
Work in progress!
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Modes of access: Formation
γ∗
γ∗
ηcl+
l−
(a) ηc Formation in e+e−.
ηcX
(b) ηc Decay into final state.
• Direct production e+e− → ηc . Possible at BESIII!• Only background from em-continuum.• Choice of final state(s) to study.• C-even final states suppress direct γ∗ background.
Work in progress!
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Summary
• BESIII π+π− form factor and aVP,LOµ measurement precisioncomparable with previous measurements.
• BESIII compatible with KLOE, confirms deviation of 3.4 σ.• π+π− pater published: Phys.Lett. B753 (2016) 629-638,
arXiv:1507.0818.
• e+e− → ηc in progress.
Thank you for the attention!
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