Physics and Outlook for Eta Rare Decays at Jlab

1
Physics and Outlook for Eta Rare Decays at Jlab

• Liping Gan
• University of North Carolina Wilmington

• Physics Motivation
• Why the ? is unique for symmetry tests
• ChPT, C-violating and P-conserving new physics
• Proposed ? rare decay experiment in Hall D
• CLAS data mining in Hall B
• Summary and discussion

Outline
2

• We have to remember that what we observe is not
  nature herself, but nature exposed to our method
  of questioning.
• Werner Heisenberg

3
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

4
Why ? 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

• ? decay width G? 1.3KeV is narrow (relative to
  G?8.5 MeV)
• The lowest orders of ? 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
  Study violations of discrete symmetries

• The ? decays are flavor-conserving reactions
  effectively free of
• SM backgrounds for new physics search.

5
The ? Decay Modes
The light blue sliver represents BR 0.7. All
other ? rare decays would be invisible on this
pie chart.
PDG 2011
6
? Neutral Decays
Mode Branching Ratio (PDG) Physics Highlight
p0 2? ( 2.7  0.5 )  10 - 4 ?PTh _at_ ?(p6)
3? lt1.6  10 - 5 C
2p0 lt3.5  10 -4 CP, P
4? lt2.8  10 -4 Suppressed (lt10-11)
2p0 ? lt5  10 - 4 C
p0 ? lt9  10 - 5 C, L, gauge inv.
3p0 ? lt6  10 - 5 C
4p0 lt6.9  10 - 7 CP, P
3p0 (32.57?0.23) Quark mass md-mu
2? (39.31?0.20) Anomaly, ??? mixing
7
Allowed Rare Decay ???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 (LECs) of the O(p6) counter-terms
  in the chiral Lagrangian.

8
???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 ??0ll-

CP conserving background
CP violating
?
L.M. Sehgal, Phys. Rev., D38, 808 (1988)
KL ?p0 2? was recently measured by KTeV to
estimate the CP conserving contributions

• ? Sector C and CP search ? ??0ll-

C and CP conserving background
C and CP violating
J.N. Ng, et al., Phys. Rev., D46, 5034 (1992)

• A cross-check of LEC's with different processes
• test the foundations of ChPT.

9
Status of ???0?? Partial Decay Width
There have been about 20 experiments since 1966.
Experiments After 1980
Average 0.44 eV
?PTh by Oset et al., Phys. Rev. D77, 07300 (2008)
10
Status of ???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 LECs of
the O(p6) counter- terms in the
chiral Lagrangian
11
Measurement of ??3?0
Phys. Lett., B694, 16 (2010)

• ? ?3?0 is the most promising channel
• to determine an accurate light quark
• mass ratio.
• Recent experimental results are from
• 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.

Slope ?
Exp.
Theory
12
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) )

13
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 ? 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 right-handed
  singlets.

C Violating ? neutral decays
Final State Branching Ratio (upper limit) Gammas in Final State
3? lt 1.610-5 3
p0? lt 910-5 3
2p0? lt 510-4 5
3?p0 Nothing published 5
3p0? lt 610-5 7
3?2p0 Nothing published 7
14
Experimental Improvement on ??3?

• SM contribution
• BR(??3?) lt10-19 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?)lt 10-2 ( )
• (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
15
P and CP Violating ???0 ?0
Proj. JEF

• The nEDM measurements access the static property
  of the particle. A non-
• zero 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.

16
World Competition in ? Decays
ee- Collider
Low energy ?-facilities
High energy ?-facility
hadroproduction
Fixed-target
CBELSA/TAPS at ELSA
JEF at Jlab
photoproduction
17
Filter Background with ? Energy Boost (???0??)
CB-AGS Experiment ?-p?? p (E?730 MeV)
Jlab ?p??p (E? 9-11.7 GeV)
? ??0?0?0
GAMS Experiment ?-p?? p ( E? 30 GeV )

• Major Background
• ? ??0?0?0?6?
• ?-p? ?0?0 neutron

18
Proposed Experiment in Hall D
FCAL
Simultaneously measure ? neutral decays ???0??,
??3?, and

• ? produced on LH2 target with 9-11.7 GeV tagged
  photon beam
• ?p ? ?p

• Reduce non-coplanar backgrounds by detecting
  recoil ps with GlueX detector

• Upgraded Forward Calorimeter with High
  resolution, high granularity
• PbWO4 (FCAL-II) to detect multi-photons
  from the ? decays

18

19
Detection of Recoil Proton with GlueX

• Recoil proton kinematics
• Polar angle 55o-80o
• Momentum 200-1200 MeV/c

20
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 Typessignal
, background
FCAL (Pb glass)
FCAL-II (PbWO4) vs. FCAL (Pb glass)
S/N0.11
Property Improvement factor
Energy s 2
Position s 2
Granularity 4
Radiation-resistance 10
FCAL-II (PbWO4)
S/N101
21
Hadronic Backgrounds Reduction in 4? States

• Event Selection
• Elasticity is
• ELSE?/ Etagged-?
• Energy conservation
• for ?p ? ?p reaction
• ?EE(?)E(p)-E(beam)-M(p)
• Co-planarity ?? ?(?)- ?(p)

• 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.

Signal ???0??
22
Rate Estimation

• The ?p??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 N?4x107 Hz (for
  E?9-11.7 GeV)

Jlab Eta Factory (JEF)

• The ???0?? detection rate
• BR(???0?? )2.7x10-4
• Average geometrical acceptance is 20 (118x118
  cm2 FCAL-II)
• Event selection efficiency 70

23
Beam Time Requirement
Run type Beam Time (days)
LH2 Production 100
Empty target and target out 7
Tagger efficiency, TAC runs 3
FCAL-II commissioning, Calibration 12
Luminosity optimization 14
Total 136
24
Projected JEF Results
BR Upper Limit
BR Upper Limit
Proj. JEF
25
?? Charged Modes
26
Extension of Physics

• ??' decays
• Dark photon search ????U (U ?ee-)

??
27
CLAS data mining in Hall B(by M. Amaryan et. al.)
28
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 ?
  decays compared to other facilities in the world.
  
• Perform a simultaneous measurement of ? decays to
  all neutral final states
• ???0??, measure BR (4 precision) and Dalitz
  distribution to determine two O(p6) LECs in the
  chiral Lagrangian.
• Improve BR upper limits by 1-2 orders of
  magnitude for SM forbidden decays (potentialy
  2-3 orders of magnitude)
• ? ?3? and other C-violating neutral channels
  offer the best window for C-violating and
  P-conserving new physics.
• ? ??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 ? ?3?0 with a significant
  different systematics to constrain the light
  quark mass ratio.
• Extend to ? charged decay channels and ? decays.
• Dark photon search ????U (U ?ee-)

29
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 ? ? p0
  2? decay (as well as the main background channel
  ? ? 3p0 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.

30

• The End
• Thanks you!

31
Tests of C Invariance (PDG 2012)