Search for supersymmetry via resonant final states with the ATLAS detector
The Search For Supersymmetry
description
Transcript of The Search For Supersymmetry
The Search For Supersymmetry
Liam Malone and Matthew French
SupersymmetryA Theoretical View
Introduction
Why do we need a new theory? How does Supersymmetry work? Why is Supersymmetry so popular? What evidence has been found?
The Standard Model
6 Quarks and 6 Leptons.
Associated Anti-Particles.
4 Forces – but only successfully describes three.
Symmetries and Group Theory
Each force has an associated symmetry. This can be described by a group. The group SU(N) has N2-1 parameters. These parameters can be seen as the amount of
mass-less bosons required to mediate the force. Ideally the standard model is a
SU(3)×SU(2)×U(1) model.
Weak Force
Weak force is very short range due to its massive bosons.
Have difficulty adding massive bosons and keeping the gauge invariance of the theory.
Yet scalar bosons are proposed. Some other process is taking place.
The Higgs Mechanism
Higgs mechanism solves this problem. Uses SPONTANEOUS SYMMETRY
BREAKING. Mix the SU(2) and U(1) symmetry into one
theory. Creates three massive bosons for the weak
force, the Higgs and the mass-less photon.
Renormalisation
Used to calculate physical quantities like the coupling constants of each force or the mass of a particle.
Sum over all interactions. Have to use momentum cut-off. Results in the quantity being dependant on
the energy scale it is measured on.
The Hierarchy Problem
Renormalizing fermion masses gives contributions from:
Renormalising the Higgs mass gives contributions from:
2
2
ln4
3~
fff m
Lm
π
αdm
)4
(~ 22 Lπ
αOdm
H
Other Problems with the Standard Model
No one knows why the electroweak symmetry is broken at this scale.
Why are the three forces strengths so different?
Why the 21 seemingly arbitrary parameters?
History of Supersymmetry
First developed by two groups, one in USSR and one in USA.
Gol’fund and Likhtmann were investigating space-time symmetries in the USSR.
Pierre Ramond and John Schwarz were trying to add fermions to boson string theory in the USA.
Supersymmetry
In renormalisation fermion terms and boson terms have different signs.
Therefore a fermion with the same charge and mass a boson will have equal and opposite contributions.
The basis of supersymmetry – every particle has a super partner of the opposite type.
Supersymmetry
In Quantum Mechanics this could be written as:
The operator Q changes particle type. Q has to commute with the Hamiltonian because
of the symmetry involved:
| |
| |
Q fermion boson
Q boson fermion
[ , ] 0Q H
Supersymmetry
The renormalised scalar mass now has the contributions from two particles:
2 2 2 2 2 2 2~ ( )( ) - ( )( ) ( )( - )4 4 4H B F B Fdm O L m O L m O m m
The only thing that this requires is the stability of the weak scale:
222 1- TeVmm FB
Constraints on SUSY
124 parameters required for all SUSY models.
However some phenomenological constraints exist.
These mean some SUSY models are already ruled out.
Minimal Supersymmetric Standard Model
In supersymmetry no restrictions are placed on the amount of new particles.
Normally restrict the amount of particles to least amount required.
This is the Minimal Supersymmetric Standard Model (MSSM).
MSSM
All particles gain one partner.
Gauge bosons have Gauginos: E.g The Higgs has the Higgsinos.
Fermions have Sfermions: E.g Electron has Selectron and Up quark has the
Sup.
Constrained MSSM
A subset of the MSSM parameter space.
Assumes mass unification at a GUT scale.
This gives only five parameters to consider.
The Five Parameters
M1/2 the mass that the gauginos unify at.
M0 the mass at which the sfermions unify at. Tan β is the ratio of the vacuum values of the
two Higgs bosons. A0 is the scalar trilinear interaction strength. The sign of the Higgs doublet mixing
parameter.
Figure showing the mass unification at grand scales. The five parameters m1/2=250 GeV, m0 = 100 GeV, tan β= 3, A0=0 and μ>0.
Local or Global?
Supersymmetry could be local or global symmetry.
Local symmetries are like the current standard model.
If SUSY is global has implications on symmetry breaking mechanisms.
SUSY Breaking
SUSY has to be broken between current experiment scales and Planck scale.
Natural to try and add in Higgs mechanism but this reintroduces Hierarchy problem.
Two possible ways: Gravity Interactions of the current gauge fields and the
superpartners
Gravity mediated breaking
In super gravity get graviton and gravitino.
Gravitino acquires mass when SUSY is broken.
If gravity mediates the breaking, LSP is the neutalino or sneutrino.
Gauge Mediated Breaking
If SM gauge fields mediate the SUSY breaking then SUSY is broken a lower scale.
Gravitino therefore has a very small mass and is the LSP.
Other Models do exist.
R-Parity Conservation
R-parity is a new quantity defined by:
All SM particles have R-parity 1 but all super partners have -1.
It is this that makes the LSP stable.
SLBR 2)-(31-
Dark Matter
Cosmologists believe most matter is dark matter.
Inferred this from observing motions of galaxys.
No one’s sure what it is.
Dark Matter
If R-parity is conserved then the Lightest Super Partner (LSP) will be stable.
Could explain the Dark Matter in the universe.
Depends on SUSY parameters whether the LSP is a gaugino or a sfermion.
Which LSP?
Graph showing regions of different LSP’s.Tan β =2
Proton Decay
The best GUT prediction is 1028 years.
Current best guess is greater than 5.5×1032
years.
SUSY can be used to fix this problem.
Other Advantages of SUSY
Grand Unified Theories (GUTs). Current understanding is just a low energy
approximation to some grand theory. On a large energy scale all forces and
particles should essentially be the same. Coupling constants should equate at high
energy.
Figure (a): Coupling constants in the standard model
Figure (b): Coupling constants a GUT based on SUSY
Possible GUTs
The main competitor is a theory based on SU(5) symmetry.
Has 24 gauge bosons mediating a single force.
Others as well like one on SO(10) with 45 bosons!
Conclusions
The Standard Model has problems when considered above the electroweak scale.
Supersymmetry solves some of these problems.
Supersymmetry can also be used to explain cosmological phenomena.
SupersymmetryExperimental Issues and
Developments
Outline
Motivation for SUSY (continued) Detecting SUSY Current and future searches Results & constraints so far
Motivation for SUSY
Convergence of coupling constants Proton lifetime Dark matter (LSP) Anomalous muon magnetic moment Mass hierarchy problem
Convergence of Coupling Constants 1
In a GUT coupling constants meet at high energy
GUT gauge group must be able to contain SU(3)xSU(2)xU(1)
SU(5) best candidate Three constants:
21 5 /(3cos )W
22 / sin W
23 /(4 )sg
Convergence of Coupling Constants 2
Sou
rce:
Kaz
akov
, D I
; arx
iv.o
rg/h
ep-p
h/00
1228
8
Dark Matter
A leading candidate is the LSP SM has R=1 & SUSY has R=-1 Conservation of R-parity R-parity conservation ensures SUSY
particles only decay to other SUSY particles so LSP is stable
3( ) 2( 1) B L SR
WMAP 1
Sou
rce:
http
://m
ap.g
sfc.
nasa
.gov
WMAP 2
Sou
rce:
http
://m
ap.g
sfc.
nasa
.gov
WMAP 3
73% dark matter in universe Total matter density Improves prospect of discovery at LHC Within reach of 1TeV linear collider
2 0.01610.01810.1126CDMh
400 500m GeV
2
1
tan
WMAP 4
Adapted from: J. Ellis et al, Phys, Lett B 565, 176-182
Anomalous Muon Magnetic Moment
Experiment
Dirac theory:
QED corrections: virtual particles Deviation from SM of
1.00116592032muon
e
m
h
1.6 2.6
12muon
e
m
h
Anomalous Muon Magnetic Moment 2
Anomalous Muon Magnetic Moment 3
Sou
rce:
http
://ar
xiv.
org/
hep-
ex/0
4010
08
Who is looking for SUSY particles?
LEP Tevatron LHC – from 2007? ILC
Currently no experimental evidence found Can only constrain models
LEP
Sou
rce:
http
://in
tran
et.c
ern.
ch/P
ress
/Pho
toD
atab
ase/
LEP
Sou
rce:
http
://in
tran
et.c
ern.
ch/P
ress
/Pho
toD
atab
ase/
s-fermion searches
Production
Decay
Events with missing energy ~ 0
1fM m m
LEP Results 1
sleptons: selectron, smuon, stau Decay of sleptons Mass of s-lepton depends on mass of
neutralino
~
1R ll
LEP Results 2
Sou
rce:
LE
P2
SU
SY
Wor
king
Gro
up
LEP Results 3
s-lepton lower mass limit
neutralino mass
selectron 99.9 GeV 0 GeV
99.9 GeV 40 GeV
smuon 94.9 GeV 0 GeV
96.6 GeV 40 GeV
stau 86.6 GeV 0 GeV
92.6 GeV 40 Gev
Source: LEP2 SUSY Working Group
LEP Results 4
Sou
rce:
LE
P2
SU
SY
Wor
king
Gro
up
Tevatron
Sou
rce:
ww
w.f
nal.g
ov/p
ub/p
ress
pass
/vis
med
ia/in
dex.
htm
l
Tevatron
Sou
rce:
ww
w.f
nal.g
ov/p
ub/p
ress
pass
/vis
med
ia/in
dex.
htm
l
Tevatron Results 1
CDF & D0 Searches for bottom squarks
Photon + missing energy searches
Search for R-parity violation
NLSP LSP
01b b % %
Tevatron Results 2
Sou
rce:
htt
p://
ww
w.d
pf99
.libr
ary.
ucla
.edu
/ses
sion
7/H
ED
IN07
09.P
DF
LHC
Starting 2007 14TeV proton-proton collider ATLAS & CMS
ATLAS
Sou
rce:
http
://at
las.
ch
SUSY at ATLAS
Assuming MSSM & R-parity conservation SUSY production at LHC dominated by
gluino and squark production Decay signature is distinctive cf SM Large missing energy & multiple jets
SUSY at ATLAS 2
Sou
rce:
SU
SY
at A
TL
AS
talk
, Fra
nk P
aige
CMS
Source: http://cmsinfo.cern.ch
ILC
International linear collider Election-positron
Large electron polarisation Clean beams Beam energy can be tuned
Verifying SUSY at ILC
Pair production Precise study: mass, spin, coupling, mixing Look of SUSY breaking mechanism
Highly polarised source means background can be reduced to ~0
Mass and Spin
SUSY: and
Electron :: spin ½ :: light Selectron :: spin 0 :: heavy
Higgs :: spin 0 :: heavy Higgsino :: spin ½ :: light
0 0 0, , , ,h H H H A 0 0 0, , , ,h H H H A % %% % %
If SUSY is not Found
Summary SUSY Particle Masses
46m GeV01%02%03%
62.4m GeV
99.9m GeV
1 % 94m GeV
Source: Particle Date Group: http://pdg.lbl.gov/2004/tables/sxxx.pdf
e%%%
73m GeV
94m GeV
81.9m GeV
q%b%
t%
250m GeV
89m GeV
95.7m GeV
Summary
WMAP, LEP, Tevatron have placed limits If SUSY exists LHC expected to find it ILC – detailed examination of SUSY
particles