PART 3
F. Gianotti, LHC Physics
F. Gianotti, LHC Physics
F. Gianotti, LHC Physics
Search for
SUperSYmmetry
SUSY
F. Gianotti, LHC Physics
SUPERSYMMETRY
(see R. Rattazzi lectures)
Symmetry between fermions (matter)
and bosons (forces)
for each particle p with spin s,
there exists a SUSY partner ~p
with spin s-1/2.
q (s=1/2) 
g (s=1) 
Ex. :
q~ (s=0)
squarks
gluino
g~ (s=1/2)
Motivations:
• Unification fermions-bosons and matter-forces is “sexy”
• Solves some problems of SM, e.g. divergence of mH :
~
f
f
H
f
~
f
Fermion and boson loops cancel, provided
m ~f  TeV.
F. Gianotti, LHC Physics
• Measured coupling constants unify at GUT scale
in SUSY but not in SM.
SM
SUSY
• Provides candidate for the universe cold
dark matter (LSP)
F. Gianotti, LHC Physics
• Does not contradict predictions of SM at low
energy  not ruled out by present experiments.
Predicts a light Higgs (mh < 135 GeV)
• Ingredient of string theories that many consider
best candidate for unified theory including
gravity
However: no experimental evidence for
SUSY as yet
Either SUSY does not exist
OR
mSUSY large (>> 100 GeV)  not accessible
to present machines
LHC should say “final word” about (low E)
SUSY since theory predicts mSUSY  a few TeV
F. Gianotti, LHC Physics
Many new particles predicted !
Here : Minimal Supersymmetric extension of
the Standard Model (MSSM) which
has minimal particle content
MSSM particle spectrum :
5 Higgs bosons : h, H, A, H
quarks 
leptons 
W

H

g

Z

h, H 
g

squarks
sleptons
winos
charged higgsino
photino
zino
neutral higgsino
gluino
u~, d, etc.
~
e~, ~ , ~, etc.
  1,  2
2 charginos
  01,2,3,4
4 neutralinos
g~
Masses not known. However charginos/neutralinos
are usually lighter than squarks/sleptons/gluinos.
Present limits : m ~l ,   > 90-100 GeV LEP
m q~, ~g > 250 GeV Tevatron Run 1
400 GeV expected in Run 2
F. Gianotti, LHC Physics
SUSY phenomenology
There is a multiplicative quantum number:
+1
R-parity
SM particles
Rp=
-1
SUSY particles
which is conserved in most popular models
(considered here).
Consequences:
• SUSY particles are produced in pairs
• Lightest Supersymmetric Particle (LSP)
is stable.
In most models LSP is also weakly interacting:
LSP  01
 LSP is good candidate for cold dark matter
 LSP behaves like a   escapes detection
 ETmiss (typical SUSY signature)
F. Gianotti, LHC Physics
Production of SUSY particles at LHC
• Squarks and gluinos produced via strong processes
 large cross-section
q
a
Ex.:
s
a
g
q
s
q~
~
q
g
~
q
q~
q
g~
s ~ 1 pb  104 events per year
produced at low L
m ~ ~ ~ 1 TeV
q, g
• Charginos, neutralinos, sleptons produced via
electroweak processes  much smaller rate
q
Ex.
q’
q~q~, q~g~, g~g~
q~
+
0
s  pb m  150 GeV
are dominant SUSY processes at LHC
if kinematically accessible
F. Gianotti, LHC Physics
Decays of SUSY particles : some examples

W

~
l
0 2
01= LSP
0
Z
01
Z
2
01
~, g
~
q
heavier  more complicated decay chains
q
Ex.
g~
q~
q
0 2
Z
0 1
F. Gianotti, LHC Physics
Cascade decays
involving many
leptons and /or jets +
missing transverse energy
(from LSP)
 such spectacular
signatures are easy to
extract from the SM
background
F. Gianotti, LHC Physics
Conclusion on SUSY
If SUSY exists, it should be easy (and fast)
to discover at LHC.
Mass reach : up to m  3 TeV.
Thanks to large cross-section and spectacular
signatures  small background
If nothing found at LHC : (low-E) SUSY will
be most likely dead
If SUSY found : several measurements
of sparticle masses can be performed at LHC
F. Gianotti, LHC Physics
F. Gianotti, LHC Physics
G
G
If gravity propagates
in 4 + n dimensions,
a gravity scale MD  1 TeV is possible
 hierarchy problem solved
Bulk
1 1
2
M Pl r
1
1
V4 n (r) ~
n 2
MD R n r
V4 (r) ~
MPl2  MDn+2 Rn
at large distance
n, R = number and size
of extra-dimensions
If we want MD  1 TeV, then:
n=1 R  1013 m  excluded by macroscopic gravity
n=2 R  0.7 mm  limit of small- scale gravity experiments
….
n=7 R  1 Fm
A gravity scale as low as ~ 1 TeV is possible,
provided that there exist n additional dimensions
with n  2 compactified over R < mm
R
• Only one scale in particle physics : EW scale
• Can test gravity and the geometry of
the universe in the lab
F. Gianotti, LHC Physics
F. Gianotti, LHC Physics
F. Gianotti, LHC Physics
CONCLUSIONS
LHC : most difficult and ambitious high-energy
physics project ever realized (human and
financial resources, technical challenges,
complexity, ….)
Very broad and crucial physics goals:
understand the origin of masses,
look for physics beyond the SM,
precision measurements of known particles.
In particular: can say the final word about
-- SM Higgs mechanism
-- low-E SUSY and other TeV-scale predictions
It will most likely modify our
understanding of Nature
F. Gianotti, LHC Physics
Enrico Fermi, preparatory notes for a talk on
“What can we learn with High Energy Accelerators ? ”
given to the American Physical Society, NY, Jan. 29th 1954
F. Gianotti, LHC Physics
End of
lectures
F. Gianotti, LHC Physics