Transcript Precision Measurements of Radiative Decay width and

Search for New Physics via η Rare
Decay
Liping Gan
University of North Carolina Wilmington
March, 2008
Liping Gan, UNCW
Contents
 Physics Motivation
• Why η meson is interesting?
• η→0
• η→00
 Experimental Design
 Summary
March, 2008
Liping Gan, UNCW
Why η Meson Is Interesting?
• One of the Goldstone bosons
due to spontaneous chiral
symmetry breaking
• The heaviest member in the
octet pseudoscalar mesons
• Provide a rich program to
investigate evidences of
symmetry breaking and the
new physics beyond Standard
Model
March, 2008
Liping Gan, UNCW
The Neutral η Decay
(from B.Nefkens)
March, 2008
Liping Gan, UNCW
Study of the η→0 Decay
• A stringent test of the χPTh prediction at Ο(p6) level
 Tree level amplitudes (both Ο(p2) and Ο(p4)) vanish;
 Ο(p4) loop terms involving kaons are suppressed
large mass of kaon
 Ο(p4) loop terms involving pions are suppressed by
G parity
 The first sizable contribution comes at Ο(p6) level
• A long history that experimental results have large
discrepancies with theoretic predictions.
• Current experimental limits in PDG is
BR(η→0 )<4.4x10-4
March, 2008
Liping Gan, UNCW
Theoretical Status on η→0
By E. Oset et al.
Unitarized χPTh with new PDG inputs
March, 2008
0.33±0.08
Liping Gan, UNCW
Study of η→0 0 Reaction
• The Origin of CP violation is still a mystery
• CP violation is described in SM by the phase in the CabibboKobayashi-Maskawa quark mixing matrix. A recent SM
calculation predicts BR(η→0 0)<3x10-17
• The η→0 0 is one of a few available flavor-conserving
reactions listed in PDG to test CP violation.
• Current experimental limit in PDG is
BR(η→0 0)<4.3x10-4
• Unique test of P and PC symmetries, and search for new physics
beyond SM
March, 2008
Liping Gan, UNCW
History of the η→0 Measurements
After 1980
  1.29 KeV
A long standing “η” puzzle is
still un-settled.
March, 2008
Liping Gan, UNCW
GAMS Experiment on η→0 at Serpukhov
•
•
•
Experimental result was first
published in 1981
The η’s were produced with 30
GeV/c - beam in the -p→ηn
reaction
Decay ’s were detected by leadglass wall
Final result (D. Alde et al.)
~40 of η→0 events
BR(η→0γγ)=(7.1±1.4)x10-4
(η→0γγ)=0.84±0.17 eV
Major Background
• -p→ 00n
• η →000
March, 2008
Liping Gan, UNCW
CB experiment on η→0 at AGS
•
•
•
•
March, 2008
About 3x107 η’s data were
collected in 1998 at AGS, BNL
The η’s were produced with 720
MeV/c - beam through the
-p→ηn reaction
NaI crystal ball calorimeter with
672 modules
Decay ’s energy range: 50-500
MeV
Liping Gan, UNCW
CB Data Analysis I
(by S. Prakhov et al. in 2005)
η →000
-p→ 00n
Final result
1600 of η→0 events
BR(η→0γγ)=(3.5±0.9)x10-4
(η→0γγ)=0.45±0.12 eV
March, 2008
Liping Gan, UNCW
CB Data Analysis II
(by N. Knecht et al. in 2004)
Final result
121±37 of η→0 events
BR(η→0γγ)=(2.7±0.9±0.8)x10-4
(η→0γγ)=0.32±0.15 eV
March, 2008
Liping Gan, UNCW
What can be improved?
•
•
Tagged η to reduce non-resonance 00 background
Higher beam energy to reduce the background from η→ 00 0
•
Calorimeter with PWO4 insertion
•
Large statistics to provide a precision measurement of Dalitz plot
 Lower relative threshold for -ray detection
 Improve calorimeter resolution
 Higher energy resolution → improve 0γγinvariance mass
 Higher granularity→ better position resolution and less pile-up clusters
E  720 MeV/c
E  30 GeV/c
March, 2008
Liping Gan, UNCW
Experimental Design
• η produced on LH2
target with 11 GeV
tagged photon beam
• Tag η by measuring
recoil p with GlueX
• Forward calorimeter
with PWO4 insertion
to detect multiphotons from the η
decay
March, 2008
Liping Gan, UNCW
Kinematics of Recoil Proton
• Polar angle ~65o-80o
• Kinetic energy ~20-300 MeV
• Momentum ~200-800 MeV/c
March, 2008
Liping Gan, UNCW
Acceptance
March, 2008
Liping Gan, UNCW
Invariance Mass of 0
March, 2008
Liping Gan, UNCW
Rate Estimation
• The η production rate:
LH2 target length L=30cm, ρ=0.0708 g/cm3
Np 
L
A
NA 
0.0708  30
 6.022 1023  1.28 1024 p/cm 2
1
The +p→η+p cross section ~1 μb
Photon beam intensity Nγ~1.5x107 Hz
N  N p N   1.28 1024 1.5 107 1.1030  20 Hz
• The η→0 detection rate:
BR(η→0 )~4x10-4 , detection efficiency ~11.4%
N  0  20  4. 104  0.114  9 10 4 Hz
 80 events/day
March, 2008
Liping Gan, UNCW
Summary
• High energy and high intensity tagged photon
beam
• First tagged η experiment by measuring recoil
p with GlueX
• High resolution calorimeter with PWO4 crystal
insertion to reduce the major backgrounds
• Precision measurement on (η→0) to test
the χPTh prediction at Ο(p6) order
• Measurements of branching ratio/upper limits
for various rare and forbidden η decays
involving photons in the final states to test P,
PC and C symmetries, and search for new
physics beyond Standard Model
March, 2008
Liping Gan, UNCW