Supersymmetry at LHC

Shaaban KhalilCenter for theoretical Physics at BUE

& Ain Shams University

Outline 1- Introduction2- Motivation for supersymmetry3- What is supersymmetry3- Supersymmetry breaking4- The minimal supersymmetric standard model5- Searches for Supersymmetry 6- Conclusions.

The SM, based on the gauge symmetry SU(3)C x SU(2)L x U(1)Y , is in excellent agreement with experimental results.

Strong indications that it is just an effective of fundamental one:

1) In the SM we can not predict a particle mass.

2) We do not know why there are six kinds of quarks and leptons

3) The electroweak and the strong forces are partially unified and gravity is not included.

4) In the SM neutrinos are massless & why matter dominates antimatter.

6) In the SM we do not know what is the dark matter made of.

Introduction

Supersymmetric theories are promising candidate for unified theory beyond the SM.

SUSY is a generalization of the space-time symmetries that links the matter particles with the force carrying particles, and leads to additional 'superparticles'.

SUSY is a new symmetry which relates boson and fermions

SUSY ensures the stability of hierarchy between the week and the Planck scales.

Motivation For Supersymmetry

Local supersymmetry leads to a partial unificationof gravity of the SM with gravity 'supergravity'.

In supersymmetric theories, the mechanism of the electroweak symmetry breaking is natural.

Supersymmetry is a necessary ingredient in string theory.

With supersymmetry, the SM gauge couplings are unified at GUT scale MG ≈ 2 x1016 GeV.

Hierarchy problem and SUSY String and GUT unification -> A cutoff scale ~ Planck scale (1019 GeV). SUSY is a symmetry to avoid the fine tuning in the renormalization of the Higgs boson mass at the level of O(1034).

In SUSY, the loop diagrams that are quadratically divergent cancel, term by term against the equivalent diagrams involving superpartners.

If mH ~ O(100) GeV, the masses of superpartners should be ≤ O(1) TeV.

Thus, some of the superpartners will be detected at the LHC.

What is Supersymmetry

• Supersymmetry (SUSY): a symmetry between bosons and fermions.• Introduced in 1973 as a part of an extension of the special relativity.• Super Poincare algebra.

PQQ )(},{

• SUSY = a translation in Superspace.

• Supergravity was formulated in 1976.

Superfields• We define superfied as function of the superspace

coordinates.),,( x

)()(2)(),( xFxxx 1- Chiral Superfields :

2- Vector Superfields V=V+:

)(2

1)()(),,( 2222 xDxixivxV m

m

0D

3- Linear Superfields DDL=0: contains a real scalar field, a Majorana spinor and antisymmetric tensor. It represents the gravity multiplet of superstrings which contains antisymmetric tensor & dilaton and dilataino.

Supersymmetric Lagrangian• The most general supersymmetric lagrangian takes the form:

.2

1

..2

))(2

1)((

4

1

)|||(|

2

*

222

2*2

jiji

ij

aaiaia

aaaamna

iimm

iii

AA

W

chATgi

DDivg

FDiDL

a

• Eliminating the Da and Fi fields give rise to the scalar potential

ia

iiaa

i

i

iaSUSY

ATAgDA

WF

FDV

*

22

,

||||2

1

Supersymmetry Breaking

SUSY must be broken symmetry or else SUSY particles should have been observed (with same mass as SM-partners)

What happens to the cancellation of quadratic divergences?– SUSY partners must be not heavier than ~TeV.

From the definition of the SUSY algebra:

0)(4

122221111 QQQQQQQQH

If the vacuum is supersymmetric the Evac=<0|H|0>=0 and if SUSY is broken Then Evac >0. Hence SUSY is broken if <0|F|0>≠0 or <0|D|0> ≠0

Spontaneous SUSY breaking

• One may introduce terms in the Lagrangian which break SUSY softly.

• The general structure for the SUSY breaking includes three sectors:

1) Observable sector: which comprises all the ordinary particle and their SUSY particles,

2) Hidden sector: where the breaking ofSUSY occurs,

3) The messengers of the SUSY breakingfrom hidden to observable sector.

The soft SUSY breaking terms are: Masses for the scalars, Masses for the gauginos, cubic couplings for scalars.

The minimal supersymmetric standard model

The particle content of MSSM = Two Higgs doublet SM + scalar SUSY partners and fermionic SUSY partners

Two Higgs doublets are necessary for fermion Yukawa couplings.

H1: down-type-quark and lepton Yukawa couplingsH2: up-type-quark Yukawa couplings

The MSSM is a straightforward supersymmetrization of the SM with minimal number of new parameters.

Different assumptions about SUSY breaking are often made. This leads to quite different phenomenological predictions.

A new symmetry, called R-symmetry is introduced to rule out the terms violate baryon and lepton number explicitly and lead to proton decay at unacceptable rates.

SUSY Particle Spectrum

MSSM Lagrangian

222

21

22

2123

22

22

21

21 )|||(|

8.).(|||| HH

ggchHHmHmHmV

SUSY invariant Lagrangian: Coupling constants g1,g2,g3 & yij

The potential of the Higgs neutral components is given by:

m1& m2 are soft scalar masses. The running from MGUT to weak scale reduce their squared values until the condition of the EW symmetry breaking are satisfied.

22

21

23

2

2

2222

22sin

,21tan

tan21

mm

m

MmmZHH

Constrained MSSM• Unconstrained SUSY would bring in 105 parameter (on top

of the SM ones)

• mSUGRA: simple boundary conditions at GUT scale reduce the number of parameters to ~5!

– Common scalar mass m0

– Common gaugino mass m1/2

– Common trilinear scalar interaction A– Ratio of vevs of two Higgs fields tan– Sign of Higgs mass parameter

Can predict SUSY spectrum at our energies

In CMSSM (mSUGRA), universality of soft SUSY breaking terms is assumed at GUT scale.

The RGE are used to calculate the parameters at the electrweak scale.

Higgs in MSSM• The SM Higgs mechanism is very economical

• In SUSY, two Higgs doublets are needed.

• This means: 8 degrees of freedom, 3 eaten up by the W± and Z => 5 Higgs fields: h0,A0,H0,H±

• Connection between Higgs masses and gauge boson masses:

M2H±= M2

A + M2W => MH± > MW

MA2 =m1

2+m22

M2H,h =1/2 [M2

A+M2Z ± ((M2

A+M2Z)2- 4 M2

AM2Z cos2β)0.5]

Mh < MZ

• This is only true at tree level. When including higher order contributions: Mh < ~ 130 GeV (or so…)

R-parity and the LSP• A totally unconstrained SUSY would lead to the proton

decaying with lifetime of ~hours!!!

• Introduce R-parity

R = (-1)3(B-L)+2s

• All SM particles have R=+1, all sparticles R=-1

• R-parity conservation implies that:1) SUSY particles are produced or destroyed only in pairs

2) The LSP is absolutely stable and it is a candidate for Dark Matter.

Interactions of SUSY particles

A major signature for R-parity conserving models is represented by events with missing Energy:

For instance e+ e- jet + missing energy

Search for SUSY particles• Today's negative SUSY search

provide the following lower maas limits.

GeVmGeVmGeVmGeVm

g

q

1541762390

~

~

0

The LHC at CERN is the machine that will take particle physics into a new phase of discovery.

The LHC is a proton-proton collider which will explore energies around 1 TeV.

The Higgs boson is the last missing particle in the SM and its discovery is in favoured of SUSY.

At LHC, the search for the Higgs boson will be one of the primary goals

Summary Supersymmetry is one of the most interesting candidate for physics beyond standard model.

SUSY is a key to unification and cosmology.

Discovery of SUSY, if it occurs, would be a revolution of physics in 21st century.

If SUSY exists at the electroweak scale, then these particles could be easily observed at the LHC.