1. Search for New Physics via η Rare Decay
Liping Gan
University of North Carolina Wilmington
March, 2008
Liping Gan, UNCW

2. Contents Physics Motivation Why η meson is interesting? η→0 η→00
Experimental Design
Summary
March, 2008
Liping Gan, UNCW

3. 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

4. The Neutral η Decay (from B.Nefkens)
March, 2008
Liping Gan, UNCW

5. 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

6. Theoretical Status on η→0
By E. Oset et al.
Unitarized χPTh with new PDG inputs ±0.08
March, 2008
Liping Gan, UNCW

7. 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 Cabibbo-Kobayashi-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

8. History of the η→0 Measurements
After 1980
A long standing “η” puzzle is still un-settled.
March, 2008
Liping Gan, UNCW

9. 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 lead-glass 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

10. CB experiment on η→0 at AGS
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: MeV
March, 2008
Liping Gan, UNCW

11. 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

12. 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

13. 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
Lower relative threshold for -ray detection
Improve calorimeter resolution
Calorimeter with PWO4 insertion
Higher energy resolution → improve 0γγinvariance mass
Higher granularity→ better position resolution and less pile-up clusters
Large statistics to provide a precision measurement of Dalitz plot
March, 2008
Liping Gan, UNCW

14. 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 multi-photons from the η decay
March, 2008
Liping Gan, UNCW

15. Kinematics of Recoil Proton
Polar angle ~65o-80o
Kinetic energy ~ MeV
Momentum ~ MeV/c
March, 2008
Liping Gan, UNCW

16. Acceptance
March, 2008
Liping Gan, UNCW

17. Invariance Mass of 0
March, 2008
Liping Gan, UNCW

18. The η production rate: LH2 target length L=30cm, ρ=0.0708 g/cm3
Rate Estimation
The η production rate: LH2 target length L=30cm, ρ= g/cm3
The +p→η+p cross section ~1 μb
Photon beam intensity Nγ~1.5x107 Hz
The η→0 detection rate: BR(η→0 )~4x10-4 , detection efficiency ~11.4%
March, 2008
Liping Gan, UNCW

19. 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