Meson form factors and P → γ∗γ∗ physics atBESIII

For the BESIII CollaborationSpeaker: Joachim Pettersson

joachim.pettersson@physics.uu.seUppsala University

FAIRNESS,14-19 February 2016

1 / 25

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∗

2 / 25

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)

3 / 25

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.

4 / 25

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

5 / 25

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

6 / 25

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.

7 / 25

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)

8 / 25

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+.

9 / 25

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!

10 / 25

|F (s)ππ|2 ∝ σ(s)ππ

Beijing Electromagnetic Spectrometer IIIBeijing Electron Positron Collider II

11 / 25

BESIIICollaboration:12 Countries,

55 Institutions,400 Members.

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.

12 / 25

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±

13 / 25

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 -

14 / 25

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

15 / 25

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.

16 / 25

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.

17 / 25

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].

18 / 25

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!

19 / 25

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.

20 / 25

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.

21 / 25

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

22 / 25

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!

23 / 25

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!

24 / 25

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!

25 / 25