1. Hunting the Last Missing Particle of the Standard Model
Shufang Su • Caltech

2. Particles and Forces Building blocks of matter -- elementary particles
smaller distance : higher energy
Fundamental interactions
Gravity
Electromagnetic force
Weak interaction nuclear -decay, burning of the sun …
Strong interaction -decay, holds proton and neutron …
forces ?
S. Su
U. Arizona - Colloquium

3. Exp Discovery and Theory Development- c b t
1 GeV = 109 eV=1.8x10-24 g  p n  Z W s d
Electromagnetic 
Weak interaction Z,W
Strong interaction g
Standard Model u e
mass (GeV) e    g
theory
S. Su
U. Arizona - Colloquium

4. Standard Model mW=80 GeV mZ=91 GeV Simply impose mass-
mW=80 GeV mZ=91 GeV
Simply impose mass
 theory not self-consistent
Create mass for “gauge boson”
 predict a Higgs particle
However, we have not find this particle yet …
Is there anything missing ?
S. Su
U. Arizona - Colloquium

5. Outline prediction  evidence confirmation establishment-
prediction 
evidence
confirmation
establishment
Why need a Higgs ?
How to search the Higgs ?
Is it really a (Standard Model) Higgs ?
How to probe new physics using Higgs study ?
S. Su
U. Arizona - Colloquium

6. Why need a Higgs ?
S. Su
U. Arizona - Colloquium

7. Electroweak Symmetry Breaking- EM
Photon 
m=0
Weak
W+,W- Z
mW=80 GeV
mZ=91 GeV
2x10-18 m
Begin with a unified theory of EM and weak interaction
We want something that
not disturb electromagnetic force
make weak interaction short-range
Higgs mechanism
P.W. Higgs (1964, 1966)
Weinberg(1967), Salam (1968)
Universe filled with background
Higgs field
Particle get mass via interaction
with the background Higgs field
How to give mass to W and Z boson ?
How to give mass to quarks and leptons ?
S. Su
U. Arizona - Colloquium

8. Higgs Mechanism (Particle Physics)-
Potential V= - m2 H2 + ½  H4
left-over degree of freedom
 physical Higgs particle
minimize the potential
mHSM2 v2  (100 GeV)2
H0=v
Higgs
degree of freedom eaten by
W and Z
 longitudinal modes
 W ,Z obtain mass
W,Z mass W H g X
H0=v
mW  gv
 v=246 GeV
gauge transformation
S. Su
U. Arizona - Colloquium

9. Higgs Property However, Higgs is still missing ... Go look for it!-
Advantage of Higgs mechanism
quark and lepton mass
SM (with Higgs) agrees well
with experiments f H y X
H0=v
mf  y v
Higgs property v=246 GeV
CP even scalar
coupling  mass
fermion mf/v
gauge boson mW/v , mZ/v
mass : mHSM2 v2  (100 GeV)2
mass not predicted
Other model:
composite Higgs
hard to fit with
experimental data
hard to build model
However,
Higgs is still missing ...
Go look for it!
S. Su
U. Arizona - Colloquium

10. Theoretical Constraint on mHSM-
V= - m2 H2 + ½  H4
mH2= v2
K. Riesselmann (1997)
105 GeV
(ytyt  …
Landau pole
 = Mpl  1019 GeV
130 GeV < mH < 180 GeV
116 GeV
potential unbounded from below
SM valid up to scale 
S. Su
U. Arizona - Colloquium

11. Indirect Constraint from Electroweak Data-
indirect
direct
without NuTeV
LEP EWWG
indirect bounds: mt= GeV
direct search: mt=174.3  5.1 GeV
+11
mHSM= GeV
mHSM<193 GeV at 95% C.L.
-33
S. Su
U. Arizona - Colloquium

12. How to search the Higgs ? Decay right after it is produced
does not exist in nature anymore
produced it at high-energy colliders
look for its decay products at detectors
S. Su
U. Arizona - Colloquium

13. Higgs Decay (SM) mHSM (GeV) mHSM  135 GeV mHSM  135 GeV-
mHSM  135 GeV
mHSM  135 GeV
Branching ratio
gold-plated mode for LHC seaches:
Ze+e- / +-
BrHSM =  (HSM  final state)
 (HSM  everything)
mHSM (GeV)
M. Spira (1998)
S. Su
U. Arizona - Colloquium

14. LEP Search : Current mHSM Limit-
Large Electron Positron Collider (CERN)
Ecm  189 GeV 2461 Pb-1
Ecm  206 GeV 536 Pb-1
1 pb= b; 1b=10-28 m2 e- e+
four jets
Hbb, Zqq
missing energy
Hbb, Z
leptonic
Hbb, Zll
tau lepton
Hbb, Z
H, Zqq
e+e ZHSM
Add plot
possible signal mHSM  116 GeV
signal+background 37% C.L.
background 8% C.L.
exclusion bound
mHSM  GeV 95% C.L.
S. Su
U. Arizona - Colloquium

15. Higgs Prodcution at Tevatron (Run II)- -
Tevatron (Fermilab) pp collider CDF, D0 /
Ecm = 2 TeV
L= 2 fb-1 / year
events/year
2x104
huge background
W,Z  leptons
mHSM  135 GeV
HSM  bb
mHSM  135 GeV
HSM  W+W-
cross section (pb)
2x102 2
M. Spira (1998)
S. Su
U. Arizona - Colloquium

16. Higgs Search at Tevatron (Run II)-
Tevatron (Fermilab) Ecm = 2 TeV, L= 2 fb-1 / year
If no Higgs is found
95% C.L. exclusion limit
(15 year)
(5 year)
(1 year)
Run II Higgs working group
S. Su
U. Arizona - Colloquium

17. Higgs Search at Tevatron (Run II)-
Tevatron (Fermilab) Ecm = 2 TeV, L= 2 fb-1 / year
Discovery reach
(15 year)
130
20 fb-1
(5 year)
(1 year)
Run II Higgs working group
S. Su
U. Arizona - Colloquium

18. Higgs Producction at LHC-
Large Hadron Collider (CERN) pp ATLAS, CMS
events/year
Ecm = 14 TeV
L= 10 fb-1 / year
102
106
mHSM  135 GeV
HSM  
mHSM  135 GeV
HSM  W+W-, ZZ
ggH 10 p 1
104
cross section (pb)
HSM  , ,
W+W-, ZZ
120 GeV HSM  180 GeV
10-1
10-2
102
10-3
M. Spira (1998)
10-4 1
mHSM (GeV)
S. Su
U. Arizona - Colloquium

19. Higgs Search at LHC Large Hadron Collider (CERN) pp-
Large Hadron Collider (CERN) pp
L=10 fb-1 / year
ggH
ggHtt
5 
cover entire Higgs mass region of SM with
more than 5  significance.
CMS, ATLAS (2002)
S. Su
U. Arizona - Colloquium

20. Need precise study of Higgs properties at a e+e- collider
If we see something at LHC looks like a Higgs …
Is it really a Higgs ?
Is it the Standard Model Higgs ?
Need precise study of Higgs properties at a e+e- collider
basic properties: mass, width, spin and CP quantum number
origin of particle mass  Higgs coupling  particle mass
reconstruct Higgs potential: Higgs self-coupling
S. Su
U. Arizona - Colloquium

21. Higgs Prodcution at LC Linear Collider e+e- clean environment
Ecm = 500 GeV / 1 TeV ; L= 500 / 1000 fb-1 / year
H HZ
H
cross section (fb) HZ
Hee
Hee
mHSM (GeV)
LC source book
copiously produced, thousands of events
S. Su
U. Arizona - Colloquium

22. Higgs Mass, Decay Width and JPC-
Miller et. al. (2001)
Garcia et al. (2001)
Higgs mass: mHSM = 40 MeV
reconstruct Higgs in ZH production
compare with indirect bounds from
global fit e+ e- Z H
Higgs decay width: HSM/ HSM = 6%
HSM =  (HSM  W+W-)
Br (HSM  W+W-)
Higgs production H
Higgs decay HSM  W+W-
e+e-  Z
final state angular distribution
angular and polarization asymmetry
S. Su
U. Arizona - Colloquium

23. Higgs Coupling mHSM  150 GeV Higgs decay branching ratio-
mHSM  150 GeV
Higgs decay branching ratio
Higgs production cross section
mHSM  150 GeV
HSM  cc, gg, 
too small
HSM  W+W-,ZZ
precisely measured
HSM  bb
precision reduced when
mHSM  200 GeV
HSM  tt
possible when mHSM  2 mt
reconstruct Higgs potential v
Carena et. Al. (2002)
mHSM = 120 GeV
S. Su
U. Arizona - Colloquium

24. Photon Collider e+e-collider operating in  mode Ecm=0.8 Ecmee
measure  (HSM  )
 bb  (HSM  ) Br(HSM  bb)
sensitive to heavy states
Higgs decay width HSM =  (HSM  ) / Br(HSM   )
mHSM  200 GeV , directly measure HSM
photon polarization  CP quantum number
every particle (get mass
from Higgs) contributes
S. Su
U. Arizona - Colloquium

25. There must be new physics beyond minimal Standard Model
If we find deviation from Standard Model prediction …
What can we learn ?
There must be new physics beyond minimal Standard Model
What is it ?
How to probe it using Higgs study ?
S. Su
U. Arizona - Colloquium

26. What New Physics ? Minimal Supersymmetric Standard Model (MSSM)-
SM is an effective theory below some energy scale 
Hierarchy problem: MEW100 GeV , Mplank 1019 GeV ?
Naturalness problem: mass of a fundamental scalar (like Higgs) receive huge quantum corrections:
(mH2)physical  (mH2)  (100 GeV)2 H
- 2 H
-(1019 GeV)2
precise cancellation
up to 1034 order
(1019 GeV)2
Supersymmetry
SM particle superpartner
Spin differ by 1/2
Minimal Supersymmetric Standard Model (MSSM)
S. Su
U. Arizona - Colloquium

27. Higgs Sector in MSSM CP-even Higgs h0 decoupling limit: mA0  mZ
h0 similar to SM Higgs
h0 mass
tree level: mh0mZ
loop corrections: mh0 135 GeV SM
MSSM
HSM
Hd, Hu
HSM0=v
v=246 GeV
Hd0=v1 ,Hu0=v2
v2=v12 +v22
tan=v2/v1
 H4
g12+g22
(mHSM2 v2)
h0 (mh0)
H0 ,A0 ,H (mA0)
gauge
coupling
SM Higgs searches at low mass
region could be applied to the
light MSSM Higgs h0
MSSM Higgs sector determined
by tan and mA0
S. Su
U. Arizona - Colloquium

28. LEP Search Limit (MSSM)-
e+e Zh0
e+e A0h0
cos2(-eff)
 sin2(-eff)
A0h0
mA0  91.9 GeV
mh0  91.0 GeV
Zh0
0.5  tan  2.4
S. Su
U. Arizona - Colloquium

29. Bounds on mA0 From Higgs Coupling Measurements-
 Br(HW+W-)=5%
mA0 controls the degree of decoupling
h0 coupling mZ2
HSM coupling mA02
-1 
mA0  mZ
decoupling region
no deviation
mA0  GeV
non-zero deviation 
constraints on mA0
Carena et. al. (2002)
S. Su
U. Arizona - Colloquium

30. Three SUSY Breaking Scenarios-
supersymmetry must be broken
electron: m=0.511 MeV, no scalar-electron
MSSM: 105 new SUSY breaking parameters out of control
mechanism for SUSY breaking: (flavor blind)
greatly reduce SUSY parameters
low energy MSSM
(visible sector)
105 parameters
SUSY-breaking
(hidden sector)
a few parameters
gravity
gravity-mediated SUSY breaking (SUGRA)
gauge-mediated SUSY breaking (GMSB)
gauge interaction
anomaly-mediated SUSY breaking (AMSB)
anomaly
something
S. Su
U. Arizona - Colloquium

31. Distinguish SUSY Breaking Scenario-
LEP search bound
Dedes, Heinemeyer, SS, weiglein (2001, 2002)
MSSM
SUGRA
GMSB
AMSB
mh0max (GeV)
135
127
123
125
tanmin
0.5 or 2.4
2.9
3.1
3.8
deviation from SM Higgs coupling  constrains on mA0
mA0  500 GeV
mA0  1000 GeV
S. Su
U. Arizona - Colloquium

32. GMSB and AMSB mA0  1300 GeV mA0  1100 GeV S. Su-
mA0  1300 GeV
mA0  1100 GeV
S. Su
U. Arizona - Colloquium

33. Can we distinguish various SUSY breaking scenario?
Distinguish SUSY Breaking Scenario -
500 GeV  mA0  600 GeV , tan  30
AMSB
if we know ranges of
mA0, tan from LHC
if we see deviations
of coupling at LC
Can we distinguish various SUSY breaking scenario?
GMSB
SUGRA
S. Su
U. Arizona - Colloquium

34. Conclusion Why need a Higgs ?-
Why need a Higgs ?
How to search the Higgs ?
Is it really a (Standard Model) Higgs ?
How to probe new physics using Higgs study ?
Higgs mechanism provides a simple and elegant way
to explain the origin of the mass
Tevatron Run II exclude Higgs up to 180 GeV (10 fb-1)
LHC would find the Higgs if it is there
detail study at Linear Collider
measure Higgs mass, decay width, coupling… at high precision
Minimal Supersymmetric Standard Model (MSSM)
constrain parameter space (mA0)
distinguish various SUSY breaking scenarios
Heavy Higgs search
S. Su
U. Arizona - Colloquium

35. The hunting is continuing … Tevatron p LHC p - p 2007-- Now -- S. Su
U. Arizona - Colloquium