Physics for Super Z-Factory
Chao-Hsi Chang (Zhao-Xi Zhang)
ITP, CAS
A Report of Working Group for
Physics at Z-factory
本报告基于“超级Z-工厂工作组”的初步研究报告，若有
不当之处，请以正式的“超级Z-工厂工作组”的报告为准。
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Physics for Super Z-Factory (SZF)
1. Introduction
– Accessible Super Z-Factories –
Numerous Z-bosons
Resonance production for all fermions
Unique ‘super-factory’ is not occupied in the world so far
2. Physics at Super Z-Factory

A. Precision test of SM & new physics (Z-boson properties)
B. Fragmentation functions & QCD
C. Heavy flavor physics, spectroscopy & new hadrons (X,Y,Z etc)
D. τ-lepton physics (precision of test SM & new physics clues)
E. Neutrino physics
3. Roadmap (by steps if accepted)




Dividing into two sub-steps & suppressing price for the first step
Valuable and unique physics in progress
Comparative ‘low’ price
Various dimensional ‘upgrades’
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1. Introduction
The problems (frontiers) after Higgs discovery
for accelerator HEP:
 New physics beyond SM
 Direct search for (Energy frontiers: LHC, SppC)
 Indirect evidences (precision frontiers: LHC, CEPC,
SZF, HIEPA,)
 New phenomena & deeply understanding SM








EW breaking mechanism (CEPC, SZF through precision tests)
Properties of Higgs (CEPC, SZF through precision tests)
Accelerator neutrino physics
Heavy flavor physics (LHCb, Super-B, SZF, HIEP)
QCD (LHC, LHCb, Super-B, SZF, HIEP)
New hadrons (LHCb, Super-B, SZF, HIEP)
Double heavy hadrons etc (LHCb, Super-B, SZF)
etc
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电弱破缺机制—Higgs机制
Hy
2
Hx
 v  H  iH 2 

;
 H 3  iH 4 
Aug. 16, 2016
电动力学教学研讨会
v
0    .
 0
4
标准模型中的Higgs机制：弱玻色子获得质量
2mW= g2 V , 2mZ=(g12+g22)0.5 V
V =247GeV
由于Yukawa型与费米子作用，‘真空破缺’费米子（夸克和带电轻子）也获
得质量。
‘剩下’一中性标量Higgs粒子, 其质量mH需要实验确定，一旦
质量确定，Higgs粒子与规范场力粒子费米子（夸克、轻子）
及其自作用都定了:
2012年Higgs粒子（最后）发现：mH=125GeV
如果所发现的Higgs粒子是标准模型的，Higgs粒子的所有相互作用完全确
定了。
当前实验（LHC）的目标：检验这些相互作用！
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1. Introduction
The ‘precision frontiers’ for high energy
physics:
The criteria to be a facility for ‘precision
frontiers’ in a period:
‘Clean’ and ‘precision’ environment
e+e- collider
doubt or without doubt ?
High statistics
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High luminosity L ,
At a proper resonances !
A factory
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1. Introduction
‘High L and at a proper resonances’:
‘ High L ’
technique problem
‘a proper resonances’
physics problem
Possible resonances in physics:
i). Φ meson (φ-factory & resonance for s-quark)
ii). Charmonium (τ-charm factory & resonance for
c-quark)
iii). Bottomonium (B-factory & resonance for
b-quark)
iv). Z-boson (Z-factory & resonance for various flavor
quarks and all of flavor leptons !)
v). Higgs factory (no resonance eff. except μ-collider )
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1. Introduction
Resonance effects:
The production of hadrons @ e+e- -collider

Z-factory vs B-factory vs τ-charm factory: c, b-hadron physics
Sept.. 11, 2015
CEPC working meeting
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1. Introduction
Resonance effects:
-lepton (the heaviest lepton):
The production of τ-lepton @ e+e- -collider
With Lorentz
boost, it is
the best place
to study the
lepton !
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CEPC working meeting
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1. Introduction
The Z-Factories:
The old ones
LEP-I: L0= 2.4  1031cm-2s-1
SLC:
L0= 0.6  1031cm-2s-1
The super ones:
L =103~4L0 even higher
〜1010~12 Z/year or more etc
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1. Introduction
The Status (or Plan) in the World:
Փ-factory (DAՓNE)
τ-charm factory: BEPC+BESIII for 5 and more years
(Super τ-Charm ?)
B-factory: CESR, KEKB, PEP-II
Super-B L = 1035~36cm-2s-1 at Japan
Z-factory:
Older ones: LEP-I at CERN, SLC at SLAC
Super one: Giga-Z, a stage of ILC, TLEP?
Higgs-factory (no resonance eff. for CEPC)
(a μ-collider@Higgs mass has the effect)
The super Z-factory has not been occupied in the world !
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2. Physics @ Super Z-Factory
A. Z boson properties:
(Precision tests of SM & looking for new physics)
LEP-I: Scan 88GeV~94GeV
15.5▪106 hadronic events; 1.7 ▪106 leptonic events
Detectors: Aleph, Delphi, L3, Opal.
SLC: At Z-peak 0.6 ▪106 events
(Especially electron polarization beam: 70%)
Detector: SLD
• Very precision and rich physics for Z-boson properties:
Measurements vs SM prediction: SM works well ! But ……
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A. The Precision Measurements of Z-boson
The measurement errors are dominant in comparison with SM at 1-loop level .
Super Z-factory may improve the exp. results substantially (stat. & syst. ) , that
will put the tests of SM and looking for new physics forward to a higher level !
The total & invisible width of Z-boson constrains new physics etc.
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A. The Properties of Z Boson
• The Effective Couplings Z-ff’ (Especially f, f’ leptons) :
Flavor change couplings:
 ff
effective vertex specially the couples to lepton sector.
e.g. Z  Z 
• New CP violation (different from CKM phase):
Lagrangian:
Observables:
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A. The Properties of Z Boson
The CP violation (different from CKM phase):
Models: electric and/or weak dipole of leptons has
Z,γ
The quark sector may have the similar CPV :
e.g.
the ‘Chromo dipole’:
CPV in
Zbb
and
Zbb g
etc.
To measure the ‘Chromo dipole’ etc
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A. The Properties of Z Boson
LEP-I example:
CPV of VZττ :
(weak dipole)
If we define:
The limit means:
Statistics errors quite large, so there are rooms to improve the measurement(s) !
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B. Fragmentation Functions & QCD
In SM Z-boson is a gauge boson for weak
interaction & couples to all quarks and
leptons directly.
The effects of resonance at Z-factory for all flavors
of quarks and leptons
For the studies of fragmentation functions, the heavy
flavors c, b and τthe advantages are outstanding !
For light flavor, to measure  s ( m z ) via to measure
jet shape is unique !
2
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Fragmentation Functions
• Quark hadronization (from partons to hadrons):
Non-perturbative fragmentation models: LUND , Webber Cluster, Quark
Combination (ShangDong) Model etc. The best place to test the models.
• Fragmentation functions (FFs) & polarized ones
QCD fundamental quantities & useful in hadronic collider experiments
Very important for
flavor-tagging !
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 The Polarized fragmentation functions：
For example：b to
Frag. Func.

To measure polarization
Non-perturbative fragmentation models:
LUND , Webber Cluster, Quark Combination
(ShangDong) Model. It is the best place to
test the models.
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B. Fragmentation Functions &QCD
The best place for measuring FFs (accurate and various):
No ‘initial state effects’, but very high statistics and
very energetic jets!
Significances:
Theoretically:
To offer ‘polarized’ FFs, because Z boson decays to a quark
pair, that is polarized !
Perturbative QCD and non-perturbative QCD models.
Experimentally:
Various (mesons and baryons) and accurate FFs;
To increase the (heavy) flavor tag efficiency.
Interphase of perturbative and non-perturbative QCD (as PDFs).
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C. The heavy flavor physics
The studies of the spectroscopy for heavy and
double (triple) heavy hadrons:
Heavy meson: (cq ) , (bq ) etc
Heavy baryon: (cqq ) , (bqq ) etc
Double heavy meson: (cc ), (cb ) , (bb ) etc
Double heavy baryon: (ccq ) , (bbq ) , (cbq ) etc
The spectrum of the systems, except (cc ) and (bb ), and transitions
among the excited and ground states are not complete!
Super-B factory and LHCb may do a lot of the physics but LHCb may not
observe the excited states of the systems and Super-B factory cannot observe those
systems which involve a b-quark (except B, Bs).
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The heavy flavor physics
c, b-hadron physics
Heavy flavored hadrons: mesons and baryons
CKM elements，mixing, CPV，rare processes
Many baryon states need to be confirmed!
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The physics
 Double heavy hadrons :
HQQ’ :

Bc meson, ……, Ξcc， Ωcc， Ξbc, , Ωbc ， Ξbb, and
their excited states:
• Their production can be estimated by pQCD reliable;
• The ground states decay ‘weakly’ that they have a
comparatively long lifetime (1.0~0.1ps) and one can
trace the vertices in vertex detector from production to
decay (with the Lorentz boost).
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C. The spectroscopy
&
mass spectra :
(G.L. Wang et al; X.Q. Li et al)
The D-wave
dominant 1-states of
have not
observed
yet !
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The spetroscopy
Take example Bc meson & its excited states to illustrate :
The spectroscopy:

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The heavy flavor physics
It can ‘confirm’ B-factory results for flavor c:
•Ds meson: to confirm the excited states etc,
•c-baryons (Λc, Ξc, Ωc, etc): production
mechanism(s), to confirm the decays, excited
states etc,
•Double heavy baryons (Ξcc, Ωcc, etc):
production mechanism(s), decays, excited
states etc,
•To confirm the ISR production of X, Y, Z
particles: production mechanism(s), properties
etc.
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Double heavy hadron physics
Production (estimated reliably by NRQCD):

The cross sections in pb.
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Double heavy hadron physics
Production mechanisms for double heavy hadrons:
(C.H.
Chang, X.G. Wu et al)
At LEP-I: Bc meson just a few (J/Ѱ+π) events, thus at Z-factory a
few thousands of such events !
Many decay modes can be
observed, and excited states may be seen too. The cross-sections
for heavy quarkonia and excited states are similar to that of Bc
meson but they are easy to observe.
The cross-section of double heavy baryon production is the same
in magnitude, thus the situation is similar to Bc meson.
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Double heavy hadron physics
Z couples to fermions in
vector and psudo-vector that
makes the asymmetry in forward
and backward，thus the
asymmetry in production may be
used to measure SinΘW ！
Differential cross sections for various states.
The polarized e+ebeams make the
asymmetry stronger.
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Double heavy hadron physics

The dependence on the Wenberg angle SinΘW can be used
to measure the Wenberg angle !
It will be better to do Bc (Bc and its excited states) physics with
luminosity 1035 cm-2s-1 than 1034 cm-2s-1 .
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Heavy hadron physics
b-hadron excited states (beyond B-factory):
Bc meson and baryons Λb, Ξb, Ωb , Ξbc, Ωbc ,Ξbb etc and their
excited states, can be expected to observe at Z-factory, although
it is difficult.
Heavy quarkonium physics:
a. Puzzle or not ? for Υ(ηb) production mechanism
b. Υ(ηb) excited states (even hybrids)
c. Exotics Xb, Yb, Zb via ISR similar to B-Factory
d. Hybrid states? maybe the heavy quark hybrid states are
easier identified than light quark ones.
etc
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Heavy flavor physics
• c-hadron studies (competitions from Bfactories, Tevatron, LHCb etc) :
• D-meson: D0  D0 mixing (CKM and else)
Due the Lorentz boost and the lifetime of D
meson at Z-factory the CP violation in the
mixing can be observed, whereas it is
impossible at B-factory & τ- Charm factory.
(H.-B. Li & M.-Z. Yang)
• To confirm and/or discovery the excited
charmed states (mesons and baryons) etc
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Double heavy hadron physics
-like X, Y, Z particles (ISR):
The cross-section is
smaller than that of
X off Z-peak,
because charge Qb=
-1/2 Qc!
Structures
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Double heavy hadron physics
Production of heavy quarkonia and ‘X’-type particles via
two-body exclusive production with a photon:
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It will be better to do this kind of physics with luminosity
1035 cm-2s-1 than 1034 cm-2s-1 .
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近年来发现了众多
的‘新粒子’X,Y,Z
（可能与
有关）
见左表，衰变到粲
耦素，或多粲夸克
的分枝比十分大。
(cc )
它们是什么？是否
存在与
(的
bb )Xbb,
Ybb, Zbb etc.
重味杂混态？和
含重夸克的多夸
克态？……比轻
夸克的实验和理
论比较容易确定。
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D. τ-lepton physics
Very good source of τ-lepton ( @ Z-factory):
Based on SM: mZ,
Sin2θW, α,ΓZ, etc
σ(cross-section) @
Z-peak ~ 0.5 σ @ the
highest one ~ 2.3 σ
@ B-factory
The most important thing is the Lorentz boost effects:
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D.τ-Lepton Physics
The most important is the Lorentz boost effects:
Lifetime :
τ= 0.2906∙ 10-12 s (comparatively small), cτ≃ 87.11 μm
Lorentz boost @ Z-factory: γ Z-fac= 25.66, cτ γ
≃ 25.66 ∙ 87.11 ≃ 2235.2 μm
B+-meson:
τ= 1.638 ∙ 10-12 s
cτ≃ 491.1 μm
B0-meson:
τ= 1.525 ∙ 10-12 s
cτ≃ 457.2 μm
Bs-meson:
τ= 1.472 ∙ 10-12 s
cτ≃ 441 μm
D+-meson:
τ= 1.040 ∙ 10-12 s
cτ≃ 311.8 μm
D0-meson:
τ= 0.4101∙ 10-12 s
cτ≃ 122.9 μm
Ds-meson:
τ= 0.500∙ 10-12 s
cτ≃ 149.9 μm
With vertex detector the momentum-energy of the produced τ-lepton is
well measured, but the vertex detector works well for the τ-lepton only
when γ is comparative large ! @ Z-factory, γ is quite great indeed.
The polarization of the produced τ-lepton can be measured.
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D.τ-lepton physics
Very good source of τ lepton
Production at a much high
energy (much higher than threshold) thus cause greater boost
effects than B-factory (good for vertex detector)
•Rare decays for τ-lepton (sensitive to new physics):
  e ,    ,    ,   ee,   eee,
etc, crucial for
the models: baryon genesis via lepton number violation!
the upper bound for τ lepton rare decays will be suppressed much
• τ-lepton physics:
Axial current physics
Test of chiral effective theory
• CP violation (CPV) via various CP-odd observables:
The CPV in τ-decays may come from various sources but different from CKM.
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D. CP Violation in Leptonic Sector
Via measuring the CP(T)-odd observables:
in the process with un-polarized beam
and the CP odd observables:
in the process with polarized beam
To find CP violation purely in the leptonic sector and
precisely to set stringent bounds on CP violation.
All of the above CP-odd observables contain the
momentum of the produced τ-lepton, so @Z-factory is
crucial !
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D. CP Violation in Leptonic Sector
Effective L:
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E.ν-lepton physics
The neutrinos (ν) produced in Z decay:
• Monochromatic and high energy (〜45 GeV)
• Pair production and three flavored (almost equal)
Neutrino cross-section is proportional to the energy E ν , thus
an energetic neutrino is easier to observe comparatively.
About 20% of Z boson into neutrinos !
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E.ν-Lepton Physicsh
Monochromatic & pure interacting ones:
The angle distribution (forward in electron direction)
Unpolarized beam
Polarized beam
Note: the Super-Z factory luminosity reached to 1036-37cm-2s-1 ,
the neutrino experiments will be avilabe.
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3. Roadmap (If accepted)
 Dividing into two steps:
The first step (design is optimized for SZF only)
luminosity initiated 1034 cm-2s-1 (circle one & proper radius);
The second step (as a new project at proper time)
luminosity upgraded to 1035 cm-2s-1 .
 Valuable and unique physics in steps
• Precision tests of SM
• The properties of Z-boson
• Rare processes, flavor change NC, lepton NV
• CPV beyond CKM phase, CPV in the Z-vertex, CPV
in τ- lepton decays
• τ- lepton physics
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




Measuring FFs and non-pertubative QCD(1035 ↑)
Double heavy hadrons (1035 ↑)
Evidence for new physics (1035 ↑)
To open new approach to study of neutrinos (1036-37↑)
etc.
 To make price comparative ‘low’
 The first step itself is a project
 The second step is another project in future
 Various dimensional ‘upgrades’
 Upgrade in energy (180GeV, 240GeV,……)
 Used as an injector etc
 Fixed target or EIC etc
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In Summery
 SZF has not been ‘occupied’ in the world
(resonance effects make us to gain a lot required
products without payment);
 Many unique physics can be done @SZF & so rich
physics worth us to work decades;
 The cost of SZF itself should be made comparative
‘low’;
 The extensions (upgrades) of SZF for future
should be dimensional;
 etc
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Thanks !
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