Susan Schadmand - QNP2012

WASA-at-COSY 

Rare Meson Decays and 

Transition Form Factors 

Mitglied in der Helmholtz-Gemeinschaft 

Susan Schadmand, IKP & JCHP 

talk at QNP 2012 - 16-20 April 2012 

Palaiseau (France)

meson decays 

a laboratory for the strong interaction 

→ symmetries and symmetry violation 

→ isospin symmetry breaking, quark mass differences 

→ test predictions, obtain parameters 

→ electromagnetic properties, transition form factors 

→ non conventional effects beyond standard model 


MesonNet 

Meson Physics in 

Low-Energy QCD 

April 2012 WASA-at-COSY 

2

experimental approach and analysis 

beam from accelerator 

P ~ GeV/c 

target nucleus 

P=0 

detection of decay 

and recoil particles 

and their momenta 

Meson 

• reactions: p+d → 3 He+meson and p+p→p+p+meson 

• particle tracking & identification, reconstruction 

• missing mass method: meson tagging 

1.0GeV p+d → 3 HeX 

D.Coderre 

1.4GeV p+p →ppX trig 



3 

D.Coderre 

'inclusive ' 

η peak at 

MM=547MeV

experimental approach and analysis 

beam from accelerator 

P ~ GeV/c 

target nucleus 

P=0 

detection of decay 

and recoil particles 

and their momenta 

Meson 


• reactions: p+d → 3 He+meson and p+p→p+p+meson 

• particle tracking & identification, reconstruction 

• missing mass method: meson tagging 

• detection of all decay products 

• kinematic conditions / kinematic fitting 

• background suppression and subtraction (other decays) 

• background subtraction (bin-wise) of multi-pion production 

• systematic effects 

total & differential cross sections, , upper limits, 

Dalitz plots, branching ratios, form factors, ... 

1.0GeV p+d → 3 HeX 

1.4GeV p+p →ppX trig 


4 

D.Coderre 

D.Coderre

physics around transition form factors 

o 

electromagnetic structure / coupling 

o 

phenomenological vector meson dominance calculations fail to describe the transition form 

factor of the ω meson 

o 

new strategies for processes at higher energies with additional degrees of freedom 

o 

closely related: π o →γ e + e - and η→γ e + e - , and in e + e - →e+e-pi 0 production processes 

o 

π o form factor needed for the interpretation of g-2 and π o →e + e - experiments 

(plus possible search for U boson in Dalitz dileptons) 


o 

inportance of 3π final states (proper treatment of hadronic final-state interactions) 


5

motivation for Dalitz plot analysis π + π - π 0 

probing isospin violation (for pseudoscalar mesons) 

+ hadronic final-state interactions 

access to light quark mass ratios 

extract from decay width 

test with experimental Dalitz plot distributions 

decay width Dashen limit 

Q D = 24.2 

expansion 

Dalitz plot density 


Dalitz plot parameters: 

comparison with theory 

recent experimental result 


Slide 6

status η → π + π - π 0 

analysis Patrik Adlarson (Uppsala) 

goal: Dalitz plot analysis of the three pions 

‣10 7 η events from pd→ 3 Heη analyzed 

‣bin-wise background subtraction 

‣2 x 10 5 events in Dalitz plot 


preliminary results for Dalitz plot parameters pending 

(thesis writing in progress) 

more data available in pp → ppη 

Dalitz plot also studied with ω → π + π - π 0 


Slide 7

status ω→π 0 π + π - 

goal: Dalitz plot analysis of the three pions 

first step: missing mass analysis, with preliminary decay channel selection 

1.5 GeV p+d 

Lena Heijkenskjöld(Uppsala) 

Siddesh Sawant (IIT Bombay) 

2.063GeV p+p 

15% of available data sets 

9152 ω→π 0 π + π - 

50% of pilot run 

• simulation ppπ 0 π + π - 

(phase space, arb. norm.) 

• background subtracted signal 


3 

HeX(3π): peak is the result of careful tuning of the 

reconstruction and calibration for forward He particle 

(stopped in detector) 

expectation: ~1 ω/s 

ppX(3π): at this stage, ω only a shoulder above 

(multi pion) background in p+p reaction 

issue: high energy protons 

expectation: ~100 ω/s 


8

Dalitz (conversion) decays of mesons 

1 - F=1 (QED) 

2 - |F(q 2 )|> 1 (VMD) 

3 - |F(q 2 )|< 1 

affects branching ratio 

~ 


'standard VMD' 


9

world data set I: conversion decays with µ + µ - 

for ω meson, clearly additional mechanisms 

apart from standard VMD 


• confirmed by NA60 AA reactions, S. Damjanovic, PLB 677 (2009) 260 

• confirmed by NA60 pA reactions, A.Uras, J.Phys. Conf.Ser.270(2011)012038 

• recent theoretical advance: C. Terschlüsen and St.Leupold, Phys.Lett. B691 (2010) 191-201 

chiral Lagrangian including light vector mesons and Goldstone bosons 

different experimental approach: elementary reactions, using di-electrons 


10

world data set II: conversion decays with e + e - 

SND 

η→γe + e - 

b η =(1.6±2.0)GeV -2 

more η results: 

improved statistics, photon detection 

• Crystal Ball and TAPS at MAMI-C 

• Henning Berghäuser (γ+p) 

Phys.Lett. B701 (2011) 562 

• WASA-at-COSY 

• Himani Bhatt (IIT Bombay) (p+p) 

• Malgorzata Hodana (Krakow) (p+d) 

CMD-2 

ω→π 0 e + e - 

b ωπ =(2.5±3.1)GeV -2 

CMD-2 

φ→ηe + e - 

b φη =(3.8±1.8)GeV -2 


WASA-at-COSY 

• Farha Anjum Khan 

(Jülich) (p+d) 

• Ankita Goswami 

(IIt Indore) (p+p) 

KLOE 

• Jaroslaw Zdebik 

(Cracow) (e+e) 


11

status η→γ e + e - 

goal: transition form factor studies 

Malgorzata Hodana (Krakow) 

1.0GeV p+d 

based on 10 7 3 Heη 

background suppression 

based on 

← selective cuts 

525±26 events 

all data: 3∙10 7 3 Heη 

+kinematic fit → 

3084±55 events 

all data: 5∙10 8 ~10 9 η(trig) 

Himani Bhatt (IIT Bombay) 

1.4GeV p+p 

based on 10 7 ppη 

still background issues 

suspect background from2π 


to do: reconsider cuts and do systematic studies 

data set pd: full data set and kinematic fit 

data set pp: much more data, real chance for precision 

and more background, thus: 

fine-tune calibrations, re-tune kinematic fit, 

bin-wise background subtraction 


12

towards the ωπ transition form factor 

first step: reference channel ω→π 0 γ from p+d 

Farha Anjum Khan (Jülich) 


estimate: ~1.4 x 10 6 ω produced with both data sets (1.5 and 1.45 GeV p+d) 

fast simulation of ω→π 0 ee (background-free) would promise ~160 events (too optimistic...) 

further data sets: pilot runs using p+p reaction (analysis Ankita Goswami (IIT Indore) ) 

→ experimental conditions for high statistics run 


13

summary 

• light meson decays in the low energy regime of QCD 

• WASA has been taking valuable data on several key aspects 

• η meson decays: 

• 3x10 7 η mesons produced in pd → 3 HeX 

• 5∙10 8 ~ 10 9 η decays in pp → ppη 

• experimental goal: very rare meson decays η→e + e - and η→π 0 e + e - 

• down to branching ratios below 10 −8 (standard model) 


• ω decays in pd->3HeX and pp pilot runs 

• goal: precise determination of ωπ transition form factor 

• 3π final states 

• high-statistics production run planned 


14

Susan Schadmand - QNP2012

Create successful ePaper yourself

Delete template?

Save as template?