Search for New Physics via η Rare Decays
Liping Gan
University of North Carolina Wilmington
Outline
 Physics Motivation
• Why η is unique for new physics search?
• η→0
• η→00
• η→0, η→3
 Suggested experiment in Hall D
 Summary
1
Why η is unique for new physics search?
The most massive member in the octet
pseudoscalar mesons (547.9 MeV/c2)
sensitive to QCD symmetry breakings
Due to the symmetries and conservation of angular momentum in the
strong and EM interactions, the η decay width Γη =1.3 KeV is
extremely narrow (comparing to Γρ= 149 MeV)
η decays have the lowest orders filtered out in the strong
and EM interactions, enhancing the contributions from higher
orders by a factor of ~100,000.
η decays provide a unique, flavor-conserving laboratory to
search for new sources of C, P, and CP violations in the regime
of EM and suppressed strong processes, and test the high order
χPTh predictions.
2
Examples of the η Rare Decay Channels
Mode
Branching Ratio
Physics Highlight
π0 π0
<3.5 × 10 -4
CP, P
π0 2γ
( 2.7 ± 0.5 ) × 10 − 4
χPTh, Ο(p6)
3γ
<1.6 × 10 − 5
C
π0 γ
<9 × 10 − 5
C
π0 π0 γ
<5 × 10 − 4
C
π+ π−
<1.3 × 10 − 5
CP, P
π0 π0 π0 γ
<6 × 10 − 5
C
π0 e+ e−
<4 × 10 − 5
C
4π0
<6.9 × 10 − 7
CP, P
3
Study of η→0 0 Reaction
 The Origin of CP violation is still a mystery
 CP violation is described in SM by Kobayashi-Maskawa (KM) mechanism
in Yukawa couplings (flavor-changing )→ a single phase in the CKM
quark mixing matrix. Deviations from the KM predictions would be
signatures of new physics.
 The KM mechanism fails by several orders of magnitude to explain the
observed matter-antimatter asymmetry of the Universe. Almost all
extensions of SM imply additional sources of CP violation. New source
of CP violation is also necessary for baryogenesis.
 The flavor-changing processes has been intensively investigated in K, B
and D meson decays, and no positive new source of CP violation has
been discovered yet. The flavor-conserving region remains much less
explored. The later represents a better chance for new physics due to
suppressed KM contribution.
 The η→0 0 is one of a few available flavor-conserving reactions listed
in PDG to test CP violation. The SM predicts: BR<2x10-27. An extended
SM calculation including spontaneous CP violation in the Higgs sector
and a θ-term in the QCD Lagrangian predicts: BR<2x10-15
 Unique test of P and CP symmetry violations, and search for new
physics beyond SM
4
η→0 γ and η→3γ Decays
 Both channels are forbidden by C invariance in the Standard
Model
 Offer an unprecedented opportunity for searching new C
violation in the electromagnetic interaction of hadrons.
 Current experimental limits:
BR(η→0 γ )<9x10-5, BR(η→3 γ )<1.6x10-5
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 by 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 value in PDG is
BR(η→0 )=(2.7±0.5)x10-4
6
Theoretical Status on η→0
By E. Oset et al.
Average of χPTh
0.42
7
History of the η→0 Measurements
After 1980
A long standing “η” puzzle is still un-settled.
8
High Energy η Production
GAMS Experiment at Serpukhov
D. Alde et al., Yad. Fiz 40, 1447 (1984)
 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 calorimeter
1981
1984
Major Background
 -p→ 00n
 η →000
Final result:
(η→0 ) = 0.84±0.17 eV
40 η→0  events
9
Low energy η production
CB experiment at AGS
S. Prakhov et al. Phy.Rev.,C78,015206 (2008)
NaI(T1)
η →000
 The η’s were produced
with 720 MeV/c - beam
through the -p→ηn
reaction
 Decay ’s energy range:
50-500 MeV
Final result:
(η→0 ) = 0.285±0.031±0.061 eV
92±23 η→0  events
10
Advantages of Jlab
 High energy tagged photon beam to reduce the background from η→ 30
 Lower relative threshold for -ray detection
 Improve calorimeter resolution
 Detecting recoil p’s to have an independent way to reduce non-resonant
p→00p and other combinatory backgrounds
 High resolution, high granularity PbWO4 Calorimeter
 Higher energy resolution → improve invariant mass and elasticity spectrum
 Higher granularity→ better position resolution and less overlap clusters to
reduce background from η→ 30
 Fast decay time (~30ns)→ less pile-up clusters
 High statistics to provide a precision measurement of Dalitz plot
High energy η-production
E  30 GeV/c
Low energy η-production
E  720 MeV/c
 production s  1020 MeV
  
GAMS
CB
KLOE
11
Suggested Experiment in Hall D
Counting
House
GlueX
FCAL
75 m
Simultaneously measure the η→0, η →00, η→3, and η→0
FCAL
 η produced on LH2 target with 11 GeV
tagged photon beam γ+p → η+p
 Further reduce p→ 00p and other
combinatory background by detecting
recoil p’s with GlueX detector
 Forward calorimeter (upgraded with
high resolution , high granularity
PbWO4) to detect multi-photons from
the η decays
•
•
Kinematics of recoil protons:
Polar angle ~55o-80o
12
Momentum ~200-1200 MeV/c12
Probability of two-cluster separation vs. distance between hits
by I. Larin
PWO
Separation distance (cm)
Reconstruction efficiency (100%)
Reconstruction efficiency (100%)
Study done by using PrimEx-II snake scan data
First cluster: “permanent” with energy 5 GeV
Second cluster: “moving” with energy 1-5 GeV
Artificially split events are counted as
missing
Pb glass
Separation distance (cm)
13
S/N Ratio vs. Calorimeter Types
0
0
signal:     , background:   3 , 100 days beam time
PWO
Major improvement:
1. Granularity
2. Resolutions.
dmin=2.5cm
Pb Glass
dmin=5.5cm
S/N=7.74
S/N=8.83x10-2
Invariant Mass of 4γ (GeV)
Invariant Mass of 4γ (GeV)
PWO
Invariant Mass of 4γ (GeV)
Elasticity
Elasticity
Event selection cuts:
1. Elasticity
2. Invariant mass.
Pb Glass
14
Invariant Mass of 4γ (GeV)
Invariant Mass and Elasticity Resolutions
σ=3.2 MeV
PWO
σ=6.9 MeV
M0
M
σ=0.0121
Elasticity
σ=6.6 MeV
Pb glass
σ=15 MeV
M0
σ=0.0257
M
Elasticity
15
Jlab vs. Low Energy Facilities (CB)
Low Energy Facility
Jlab
 p  p
  p  n

  3
0

  3 0
16
Acceptance
150x150 cm2 FCAL
118x118 cm2 FCAL
17
Rate Estimation
Np 
L
A
NA 
0.0708  30
 6.022 1023  1.28 1024 p/cm 2
1
The +p→η+p cross section ~70 nb (θη=1-6o)
Photon beam intensity Nγ~2x107 Hz (for Eγ~9-11.7 GeV)
N  N N p  2 107 1.28 1024  70 10 33
 1.79 Hz
 150k ( ' s/day)
 factory!
• The η→0 detection rate:
(a) BR(η→0 )~2.7x10-4
(b) average detection efficiency is :~26% (118x118 cm2 FCAL) and
~47% (150x150 cm2 FCAL)
N  0  150000  2.7 104  0.26  10.5 events/day (118 118 FCAL)
N  0  150000  2.7 104  0.46  18.6 events/day (150 150 FCAL)
18
Statistical Error on dΓ/dMγγ for ηπ02γ
Prakhov et al., PRC 78, 015206 (2008).
This figure gives a very
rough idea how statistics
limits our ability to probe the
dynamics of the ηπ02γ
decay.
Assumptions are 18.6
accepted events per live day,
negligible background, and 7
bins spanning 0.025-0.375.
12 days
112 days
19
20
Summary
• 12 GeV tagged photon beam with GlueX setup will provide a great
opportunity for precise measurements of various η rare decays
to test higher order χPTh, C, P and CP symmetry violations, and
search for new physics beyond the Standard Model
• Propose simultaneous measurements on BR(η→0), BR(η →00),
BR(η →3), and BR(η→0).
• Three experimental techniques will be combined:
1. 12 GeV high intensity tagged photon beam to produce η’s.
2. Further reduce the p→ 00p and other combinatory backgrounds
by detecting recoil p with GlueX detector
3. Upgrade FCAL with PbWO4 crystals
 High high granularity calorimeter to reduce the η →000
background due to over-lap showers.
 Fast decay time to reduce low energy pile-up clusters
 High energy and position resolutions to have precise invariant
mass and elasticity spectrum for event selection
21
The End
Thanks you!
22
Examples of the η’ Rare Decay Channels
Mode
Branching Ratio
Physics Highlight
π0 π0
<1.0 × 10 -3
CP, P
π0 e+ e−
<1.4× 10 −3
C
3γ
<1.0×10 − 4
C
ηe- e+
<2.4× 10 −3
C
23
Budget vs. Acceptance
$250 per crystal , $300 per PMT, $281 per ADC channel, $300 per HV channel
Size of
Cal.(cm2)
#crystals
Crystal
Cost
PMTs
ADC
HV
Total
118x118
3481
$0.87 M
$1.04 M
$0.98 M
$1.04 M
$3.93 M
150x150
5625
$1.41 M
$1.69 M
$1.58 M
$1.69 M
$6.37 M
FCAL
11304
$2.83
$3.39M
$3.18 M
$3.39 M
$12.79 M
(r=120cm)
Possible cutting
price
PrimEx
(1200 channels
of crystal and PMT)
$0.66 M
FCAL
$0.84 M
ADC )
total
(2800 channels of
$1.50 M
24
Detection of recoil p by GlueX
25
Reconstructed missing mass and efficiency
  p   p
26
Kinematics of Recoil Proton
Recoil θp vs θη (Deg)
Angle θη (Deg)
Recoil θp (Deg)
Recoil Pp (GeV)
• Polar angle ~55o-80o
• Momentum ~200-1200 MeV/c
Recoil Pp (GeV) vs θp (Deg)
27
Comparison of different crystals
(From R. Y. Zhu)
28
How Many η’s Can We Make?
A year of Jlab operations is about 32 weeks. Assuming 50% efficiency for the
accelerator and end-station, that is 112 live days.
With a 30 cm LH2 target, 70 nb cross section, and 2.0E7 gammas/second, we can
produce 1.7E7 η’s per year.
The number of accepted η decays per year would be ~1/3 this, or ~4E6 per year.
η production is
conservatively
similar to
KLOE
29
Selection Rule Summary Table:
η Decay to π’s and γ’s
Gamma Column
implicitly
includes
γ*e+e-
ηX
0π
1π
2π
3π
4π
L=0
0γ
P, CP
P, CP
L=1
1γ
C, CP C, CP C, CP
C, CP
L = even or
odd (no parity
constraint)
.
.
.
.
.
4γ
C and P
allowed,
observed
C and P
allowed,
upper limits
only
C, CP
2γ
3γ
Key:
C
C
C
C
C
C violating,
CP
conserving,
etc.
Forbidden by
energy and
momentum
.
conservation
30
Major background in CB-AGS experiment
MC
data
MC
 p  n


  3 0
MC
  p   0 0 n
31
Low Energy η Production Continue
KLOE experiment
B. Micco et al., Acta Phys. Slov. 56 (2006) 403
 Produce Φ through e+e- collision at
√s~1020 MeV
 The decay η→0γγ proceeds through:
Φ→η, η→0γγ, 0→γγ
Final result:
(η→0γγ)=0.109±0.035±0.018 eV
12 η→0  events
7
6
32
Can we use existing FCAL located at 10 m?
Z=5.5 m
PWO
σ=6.7 MeV
Pb
σ=14.5 MeV
Invariant Mass of 4γ (GeV)
Z=5.5 m
Pb
σ=16.2 MeV
Invariant Mass of 4γ (GeV)
Invariant Mass of 4γ (GeV)
33
Resolution of Elasticity
PWO
σ=0.0121
Elasticity
Pb
σ=0.0257
Elasticity
34
Figure of Merit
150x150 cm2 PWO
Cal.
118x118 cm2 PWO
Cal.
NS
FOM 
NB
•Signal is η→π0γγ
•Background is η→3π0
•Signal window is ±3σ
35
Experiment Figure of Merit
for “Forbidden Branch” Searches
In C and CP violation searches in η decays to date, it’s been true that Bkg Events >>
Signal Events. Since the background fluctuations are sqrt(N), the upper limit for the
branching ratio at ~95% CL is then approximately
where
BR upper limit ≈ 2*sqrt(fbkg*NMε)/NMε = 2*sqrt(fbkg/NMЄ)
NM= number of mesons decaying into the experimental acceptance
Є = efficiency for detecting products from the signal branch
fbkg = fraction of NM which remains in the signal box after all cuts
The figure of merit for experiments is therefore
NMЄ /fbkg.
This means that to reduce the BR upper limit by one order of magnitude, one must
either
•Increase NMЄ by TWO orders of magnitude, or
•Decrease fbkg by TWO orders of magnitude.
While maintaining a competitive η production rate, Jlab would reduce BR upper limits
by one order of magnitude using background reduction alone.
36
Collaboration with Chinese Institutes
• One week visit Beijing in Oct 2011: Peking University,
Chinese High Energy Physics Institute, Chinese
Theoretical Institute.
• Peking University group showed strong interests in
making a significant contribution to the FCAL upgrade.
MOU between Peking University and Jlab is in
process.
37
Island algorithm for the PWO calorimeter
by I. Larin
Island algorithm:
1.
Find maximum energy deposition cell
2. Declare all simply connected area
around as initial “raw” cluster
3. Try to split “raw” cluster into many
hits based on the shower profile
function
38
PWO Transverse Shower Profile
Experimental electron
scan data (Ee~4 GeV)
extracted shower
profile function
39