Physics and Outlook for Eta Rare Decays at Jlab
Liping Gan
University of North Carolina Wilmington
Outline
 Physics Motivation
• Why the η is unique for symmetry tests
• ChPT, C-violating and P-conserving new physics
 Proposed η rare decay experiment in Hall D
 CLAS data mining in Hall B
 Summary and discussion
1
“We have to remember that what we observe is
not nature herself, but nature exposed to our
method of questioning.”
Werner Heisenberg
2
Challenges in Physics
 Confinement QCD


Chiral perturbation theory (ChPT)
Lattice QCD
QCD at different energies
 New physics beyond the Standard
Model (SM)



New sources of symmetry violation
Dark matter
Dark energy
3
Why η is a unique probe
 A Goldstone boson due to spontaneous breaking of
QCD chiral symmetry
 is one of key mesons bridging our
understanding of low-energy hadron dynamics
and underlying QCD
 η decay width Γη =1.3KeV is narrow (relative to Γ=8.5 MeV)
The lowest orders of η decays are filtered out, enhancing the
contributions from higher orders by a factor of ~7,000 compared
to  decays.
 Eigenstate of P, C, CP, and G: IG J PC =0 0
Study violations of discrete symmetries
 The η decays are flavor-conserving reactions effectively free of
SM backgrounds for new physics search.
4
The η Decay Modes
PDG 2011
The light blue sliver
represents BR = 0.7%.
All other η rare decays
would be invisible on
this pie chart.
5
η Neutral Decays
Mode
Branching Ratio
(PDG)
Physics Highlight
π0 2γ
( 2.7 ± 0.5 ) × 10 − 4
χPTh @ Ο(p6)
3γ
<1.6 × 10 − 5
C
2π0
<3.5 × 10 -4
CP, P
4γ
<2.8 × 10 -4
Suppressed (<10-11)
2π0 γ
<5 × 10 − 4
C
π0 γ
<9 × 10 − 5
C, L, gauge inv.
3π0 γ
<6 × 10 − 5
C
4π0
<6.9 × 10 − 7
CP, P
3π0
(32.570.23)%
Quark mass md-mu
2γ
(39.310.20)%
Anomaly, ’ mixing
By PrimEx
Allowed Rare Decay η→0
ChPT is highly developed and well-tested in the domain of pionic and kaonic
reactions.  →0 is one of a few important channels to benchmark the success
of ChPT in the -sector.
The major contributions to  →0 are two O(p6) counter-terms in
the chiral Lagrangian
a rare window for the high order ChPT contributions.
L. Ametller, J. Bijnens et. al., Phys. Lett., B276, 185
O(p6)
counter-term
Precision measurements of both the branching ratio and the Dalitz distribution
of  →0 are critical to model-independently determine two Low Energy
Constants (LEC’s) of the O(p6) counter-terms in the chiral Lagrangian.
7
η→0 and Other Rare Decays
 →0 is an important “door-way” channel for interpretation of other rare
decays searching for new sources of C- or CP-violation
 KL Sector: CP violation search KL →0l+lCP conserving background
CP violating

L.M. Sehgal, Phys. Rev., D38, 808 (1988)
KL π0 2γ was recently measured by KTeV to estimate the CP conserving contributions
  Sector: C and CP search  →0l+lC and CP violating
C and CP conserving
background
J.N. Ng, et al., Phys. Rev., D46, 5034 (1992)
 A cross-check of LEC's with different processes
test the foundations of ChPT.
8
Status of η→0: Partial Decay Width
There have been about 20
experiments since 1966.
Experiments After 1980
0.33  0.08
Average
~0.44 eV
PTh by Oset et al., Phys. Rev. D77, 07300 (2008)
9
Status of η→0: Dalitz Distribution
Prakhov et al., Phys. Rev. C78, 015206 (2008)
CB-AGS
Projected JEF
Combined BR and Dalitz measurement
model-independent determination of two LEC’s of the O(p6)
counter- terms in the chiral Lagrangian
10
Measurement of η→30
is the most promising channel
to determine an accurate light quark
4
mass ratio.

→30
 QDT
  
 Q
1 md2  mu2
 2 2,
2
Q
m s m
m
Phys. Lett., B694, 16 (2010)
A000 ( z )  1  2  z
2

 

2 3 Ti
2
Z= （
-1）
3 i=1 T
md  mu
2
 Recent experimental results are from Exp.
the low energy -production facilities
and more sensitive to the threshold
effect in the -detection.
 Proposed measurement at high
energy will be comparable to existing
data in statistics but significant
different systematics.
 Offers an independent experimental
verification.
Slope 
Theory
11
The Four Classes of C, P, and T Violations
(Assuming CPT Invariance)
B. Nefkens and J. Price, Phys. Scrip., T99, 114 (2002)
Experimental probes
P-violating exp.,
-decays,
K-, B-, D-meson decays
EDM, even ’s
tests involving , ’, ,
, J/ decays




For class 4:
few experimental probes available
not well tested experimentally for EM and strong interactions
The current constraint:  1 GeV
EDMs place an ambiguous constraint on the new TVPC physics;
“new TVPC physics could arise at scales as light as a few GeV.”
(M. Ramsey-Musolf et. al., phys. Rev., D63, 076007 (2001) )
12
C Invariance
 Maximally violated in the weak
interaction and is well tested.
 Assumed in SM for both the
electromagnetic (EM) and strong
interactions, but it is not
experimentally well tested.
 C-violating η decays will provide
unambiguous, direct constraints on
new C-violating and P-conserving
physics (class 4).
 Testing C-invariance will provide a
better understanding of
 new source of CP violation
 asymmetry in SM:
left handed doublets and righthanded singlets.
C Violating η neutral decays
Final
State
Branching Ratio
(upper limit)
3γ
< 1.6•10-5
π0γ
2π0γ
<
9•10-5
Gammas
in Final
State
3
< 5•10-4
5
3γπ0
Nothing
published
3π0γ
< 6•10-5
3γ2π0
Nothing
published
7
13
Experimental Improvement on η→3γ
 SM contribution:
BR(η→3γ) <10-19 via P-violating
weak interaction.
A new C- and T-violating, and
P-conserving interaction was
proposed by Bernstein, Feinberg
and Lee
(Phys. Rev., B139, 1650).
 A calculation due to such new
physics by Tarasov suggests:
BR(3)< 10-2 ( f new ~ 1)
(Sov.J.Nucl.Phys., 5, 445)
Improve BR upper limit by one
order of magnitude to tighten
the constraint on C-violating,
P-conserving new physics
Proj. JEF
The upper limit for the branching ratio at
~90% CL is estimated by:
BR upper limit
2
fbkg
N   accep
14
P and CP Violating η→0 0
Proj. JEF
The nEDM measurements access the static property of the particle. A nonzero nEDM violates P and T directly, and indirectly violates CP under the
assumption of CPT conservation.
The  →20 decay is related to a dynamic process and it violates P and CP
directly.
 →20 is flavor-conserving counterpart of the corresponding
flavor-changing CP-violating KL →20.
15
World Competition in η Decays
e+eCollider
Fixed-target
Low energy
-facilities
hadroproduction
CBELSA/TAPS at ELSA
High energy
-facility
JEF at Jlab
photoproduction
16
Filter Background with η Energy Boost (0)
CB-AGS Experiment
-p→η p (E=730 MeV)
Jlab: p→ηp (E = 9-11.7 GeV)
η →000
GAMS Experiment
-p→η p ( E= 30 GeV )
Major Background
 η →0006
 -p→ 00 + neutron
17
Proposed Experiment in Hall D
FCAL
Simultaneously measure η neutral decays: η→0, η→3, and …
 η produced on LH2 target with 9-11.7 GeV tagged photon beam:
γ+p → η+p
 Reduce non-coplanar backgrounds by detecting recoil p’s with GlueX
detector
 Upgraded Forward Calorimeter with High resolution, high granularity
PbWO4 (FCAL-II) to detect multi-photons from the η decays
18
18
Detection of Recoil Proton with GlueX
Recoil proton kinematics
 Polar angle ~55o-80o
 Momentum ~200-1200
MeV/c
19
New Equipment: FCAL-II
PrimEx HyCal
FCAL-II:
118x118 cm2 in Size (3445 PbWO4)
2cm x 2cm x 18cm per module
S/N Ratio vs. Calorimeter Types
0
0
signal:     , background:   3
FCAL (Pb glass)
FCAL-II (PbWO4) vs. FCAL (Pb glass)
Property
Improvement
factor
Energy σ
2
Position σ
2
Granularity
4
Radiationresistance
10
S/N=0.1:1
FCAL-II (PbWO4)
S/N=10:1
20
Hadronic Backgrounds Reduction in 4 States
Event Selection
Elasticity is
EL=ΣE/ Etagged-
Energy conservation
for γ+p → η+p reaction:
ΔE=E()+E(p)-E(beam)-M(p)
Co-planarity Δ= ()- (p)
Signal: 0
Note:
Statistics is normalized to
1 beam day.
BG will be further reduced
by requiring that only one
pair of ’s have the 0
invariant mass.
21
Rate Estimation
L
0.0708  30
 6.022 1023  1.28 1024 p/cm 2
A
1
The +p→η+p cross section ~70 nb (J.M. Laget , Phys.Rev. , C72, 022202 (2005)
and A. Sibirtsev et al. Eur.Phys.J., A44, 169 (2010))
Photon beam intensity Nγ~4x107 Hz (for Eγ~9-11.7 GeV)
Np 
NA 
N  N N p  4 107 1.28 1024  70 10 33
 3.6 Hz
 3.1105 ( 's/day)
•
Jlab Eta Factory (JEF)
The η→0 detection rate:



BR(η→0 )~2.7x10-4
Average geometrical acceptance is ~20% (118x118 cm2 FCAL-II)
Event selection efficiency ~70%
N  0  3.1105  2.7 104  0.2  0.70  12 events/day
22
Beam Time Requirement
Run type
LH2 Production
Beam Time (days)
100
Empty target and target out
7
Tagger efficiency, TAC runs
3
FCAL-II commissioning,
Calibration
12
Luminosity optimization
14
Total
136
23
BR Upper Limit
BR Upper Limit
Projected JEF Results
Proj. JEF
24
𝜂 Charged Modes
25
Extension of Physics
 𝜂ˈ decays
 Dark photon search: 𝜂→γU (U →e+e-)
𝜂
26
CLAS data mining in Hall B
(by M. Amaryan et. al.)
27
Summary
 12 GeV tagged photon beam with GlueX setup will provide a great
opportunity for precise measurements. It offers two orders of
magnitude reduction of the backgrounds of neutral rare η decays
compared to other facilities in the world.
 Perform a simultaneous measurement of η decays to all neutral final
states:
 η→0, measure BR (~4% precision) and Dalitz distribution to
determine two O(p6) LEC’s in the chiral Lagrangian.
 Improve BR upper limits by 1-2 orders of magnitude for SM
forbidden decays (potentialy 2-3 orders of magnitude):
 η →3 and other C-violating neutral channels offer the best window for C-violating
and P-conserving new physics.
 η →00 is a direct P- and CP-violation test, and a flavor-conserving counterpart of
the corresponding flavor-changing P- and CP-violating KL →20 .
 A new measurement on η →30 with a significant different
systematics to constrain the light quark mass ratio.
 Extend to η charged decay channels and η’ decays.
 Dark photon search: 𝜂→γU (U →e+e-)
28
Call for theoretical support
Jlab PAC40 report on the JEF proposal
 Feasibility:
“The proposed measurements appear to be feasible and the experiment
is well suited for the tagged Hall D photon beam.”
 Issues:
 “The PAC recognizes the scientific interest of performing new measurements
of rare eta decays with improved sensitivity to test the SM. However, the
PAC identified some issues, mainly related to the theoretical implications of
these measurements.”
 “For the SM forbidden decays more work should be done to identify physics
scenarios which could imply branching ratios closer to the experimental
sensitivities. The PAC suggests that these issues be addressed in close
collaboration with the theory community working in this field, which should
be involved in helping strengthen the physics case.”
 “Similar remarks apply to the impact the η → π0 2γ decay (as well as the main
background channel η → 3π0 which is offered as a means to constrain the
light quark mass ratio from the slope of the Dalitz distribution) would have
on chiral perturbation theory. “
29
The End
Thanks you!
30
Tests of C Invariance (PDG 2012)
31