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The Quest The Quest for Supersymmetryfor Supersymmetry

Sabine Kraml Sabine Kraml (CERN, ÖAW APART)(CERN, ÖAW APART)

HabilitationskolloquiumHabilitationskolloquium

8 May 20078 May 2007

The Quest for Supersymmetry 2S. Kraml

Outline

Introduction The Standard Model of particle physics The hierarchy problem and need for New Physics

Supersymmetry (SUSY) What is SUSY The minimal supersymmetic model SUSY @ LHC SUSY dark matter

Conclusions


What is the world made of….

…. and what holds it together?


The Standard Model (SM) of elementary particle physics

Matter: 3 families of quarks and leptons, spin ½ fermions.

Forces mediated by spin 1 gauge bosons: , Z0, W±, g

Gauge group:

SU(3)c x SU(2)L

x U(1)Y

Interactions described as local gauge

theories

strong int., weak int., hypercharge

Q=T3-Y/2

O(MeV) 175 GeV

Masses

~100

GeV

mass-

less

c.f. mass of proton ~ 1 GeV

Arbitrary inclusion of masses spoils renormalizability Generate masses through gauge-invariant dynamics


The Standard Model (SM) of elementary particle physics

Matter: 3 families of quarks and leptons, spin ½ fermions.

Forces mediated by spin 1 gauge bosons: , Z0, W±

Gauge group:

SU(3)c x SU(2)L

x U(1)Y<Φ>≠0

Interaction with scalar background “Higgs” field breaks the symmetry at ~100 GeV to SU(3)c

x U(1)em

Higgs field

→ generation of particle masses


< 1973: theoretical foundations of the SM renormalizability of SU(2)xU(1) with Higgs mech. for EWSB asymptotic freedom, QCD as gauge theory of strong force KM description of CP violation

Followed by [more than] 30 years of consolidation experimental verification via discovery of

gauge bosons: gluon, W, Z (Europe) matter fermions: charm, 3rd family (USA)

experimental precision measurements of EW radiative corrections running of the strong coupling s

CP violation in the 3rd generation

technical theoretical advances (higher-order calculations, ....)


E ~ 1 MeV ↔ T ~ 1010 K ↔ t ~ 1 s after the Big Bang

(Nucleosynthesis)

The development of particle physics has also led tosignificant progress in astrophysics and cosmology,in particular in our description of the Early Universe.

E ~ 100 GeV ↔ T ~ 1015 K ↔ t ~ 10−10 s


The SM is tremendously successful; it continues to survive all experimental tests


Only missing piece: the Higgs!the particle most sought after …

2 fit of the Higgs boson mass from EW precision data as of Summer 2006


LEP Higgs search e+e− → ZH @ √s = 180-208 GeV

2 evidence of a 115 GeV Higgs until 2000, but then LEP had to stop operation

Transformed into lower limit of mH > 114.4 GeV

Only missing piece: the Higgs!the particle most sought after …

ALEPH

Aleph, Delphi, L3 and Opal collaborations

and the LEP Higgs Working Group


The SM is tremendously successful. Nevertheless it can’t be the ultimate theory!


Grand Unified Theory ?

GUTs attempt to embed the SM gauge group SU(3)xSU(2)xU(1) into a larger simple group G with only one single gauge coupling constant g.

Moreover, the matter particles (quarks & leptons) should be combined into common multiplet representations of G.

Prediction: Unification of the strong, weak and electro-magnetic interactions into one single force g at MGUT.

NB: If MGUT is too low → problems with proton decay



To break the electroweak symmetry and give masses to the SM particles, some scalar background field must acquire a non-zero VEV.

Elementary scalar “Higgs” boson of mass mH. However,

where is the scale (=cut-off) up to which the theory is valid.

The hierarchy problem

mH=O(mW)

mH2 ≤ mH

2

MGUT? MPlanck?


Beyond the SM (BSM)

The need to stabilize the electroweak scale, mH2 < mH

2, lets us expect new physics at TeV energies

Besides, neutrino masses as well as the dark matter and the baryon asymmetry of the Universe provide concrete experimental evidence for BSM physics.

The search for this new physics is

the genuine motivation to build the LHC


Supersymmetry (SUSY)Supersymmetry (SUSY)


What is SUSY? Supersymmetry (SUSY) is a symmetry between fermions and bosons.

The SUSY generator Q changes a fermion into a boson & vice versa

Extension of space-time to include anticommuting coordinates

x → (x, ) with

This combines the relativistic “external” symmetries (such as Lorentz invariance) with the “internal” symmetries such as weak isospin.

Actually the unique extension of the Poincare algebra *

* (the algebra of space-time translations, rotations and boosts)


... predicts a partner particle for every SM state


... predicts a partner particle for every SM state

Minimal supersymmetric standard model (MSSM)

gauge structure SU(3)xSU(2)xU(1)


Compare:

space-time symmetry

(special relativity)

Antiparticles

space-time

supersymmetry

Superpartners

doubling of

the spectrum


If SUSY were an exact symmetry,

SM particles and their superpartners would have equal mass.

This is obviously not the case (no superpartners found so far),

so SUSY must be broken

SUSY as a local gauge theory includes a spin-2 state,

the graviton (!) and its superpartner the gravitino.


Back to the hierarchy problem ...

In SUSY, every fermion has a bosonic partner (and vice versa)


... the SUSY solution

XX

XXXXsolves the hierachy problem

provided MSUSY < O(1) TeV !

+ −


Gauge coupling unification

SM SUSY

Again requires SUSY masses of < O(1) TeV!


MSSM particle spectrum

SM particles spin Superpartners spin

quarks 1/2 squarks 0

leptons 1/2 sleptons 0

gauge bosons 1 gauginos 1/2

Higgs bosons 0 higgsinos 1/2

gauginos +

higgsinos

mix to

2 charginos ±

4 neutralinos

2 Higgs doublets → 5 physical Higgs bosons:

3 neutral states: scalar h, H; pseudoscalar A

2 charged states: H+, H−

Lightest neutralino = lightest SUSY particle (LSP)

R parity: symmetry under which SM particles are even

while SUSY particles are odd.

If RP is conserved, superpartners can only be produced in pairs and

every spuperpartner will cascade-decay to the LSP, which is stable dark matter candidate!


SUSY breaking



Heavy top effect,

drives mH2 < 0

Minimal supergravity (mSUGRA)

charginos,

neutralinos,

sleptons

gluinos,

squarks

Universal

boundary

conditions

@ GUT scale

univ. gaugino mass

univ. scalar mass

Radiative electroweak symmetry breaking!


A light Higgs

tan = v2/v1


The beauties of SUSY Unique extension of relativistic symmetries

Solution to gauge hierarchy problem

Radiative EW symmetry breaking, light Higgs

Gauge coupling unification

Ingredient of string theories

Very rich collider phenomenology

Cold dark matter candidate

....

....


SUSY @ the LHCSUSY @ the LHC


Large Hadron Collider

New accelerator currently built at CERN, scheduled to go in operation this year

pp collisions at 14 TeV

Searches for Higgs and new physics beyond the Standard Model

„discovery machine“,

typ. precisions O(few%)


100 m underground27 km circumference

High Energy factor 7 increase w.r.t. present acceleratorsHigh Luminosity (#events/cross section/time) factor 100 increase

pp collisions at 14 TeV

The LHC machine and experiments

108 pp collisions per second, bunch spacing 24.95 nsevent size 1 MB, storage rate 1 Hz, data to tape: 106 GB/yr



SUSY searches at LHC

01

Z

q

q

02

q~g~

jet

jet

jets, l+l−

missing energy

Large cross sections ~100 events/day for M ~ 1 TeV

Spectacular signatures SUSY could be found early on

gggqqq ~~ ,~~ ,~~

Cascade decays into LSP

lead to typical signature:

multi-jets / multi-leptons

plus large missing energy

From Meff peak first+fast measurement of SUSY mass scale to 20% (ca 10 fb-1)


Compare with Higgs search


Mass measurements: cascade decaysET

miss → no peaks → mass reconstruction through kinematic endpoints

[ATLAS, G. Polesello]

Typical precisions: a few %


If TeV-scale SUSY is realized in Nature,

the LHC will discover a wealth of new states:

the superpartner world!

would also revolutionize our understanding of space-time


SUSY dark matterSUSY dark matter


0.094 < CDMh2 < 0.136 (95% CL)

[astro-ph/0611582]

WMAP+SDSS: CDMh2 = 0.105 ± 0.004[astro-ph/0608632]

multipole moment (l)

dark matter from BSM?

strong evidence for DM:

large-scale structures

rotation curves

CMB


WIMPs (weakly interacting massive particles)

Dark matter (DM) should be stable, electrically neutral,

weakly and gravitationally interacting WIMPs are predicted by most BSM theories

Stable as result of new discrete symmetries Thermal relic of the Big Bang Testable at colliders! Neutralino, gravitino,

axino, lightest KK state, T-odd little Higgs, etc., ...

BSM-DM

A neutralino LSP would indeed be

an excellent dark matter candidate


Relic density of WIMPs (weakly interacting massive particles)

(1) Early Universe dense and hot; WIMPs in thermal equilibrium

(2) Universe expands and cools; WIMP density is reduced through pair annihilation; Boltzmann suppression: n~e-m/T

(3) Temperature and density too low for WIMP annihilation to keep up with expansion rate → freeze out

Final dark matter density: h2 ~ v −1

Thermally avaraged annihilation cross section


Neutralino relic density

0.094 < h2 < 0.135 puts strong bounds on the parameter space

LSP as thermal relic: relic density computed as thermally avaraged

cross section of all annihilation channels → h2 ~ v −1


Neutralino relic density

0.094 < h2 < 0.135 puts strong bounds on the parameter space

LSP as thermal relic: relic density computed as thermally avaraged

cross section of all annihilation channels → h2 ~ v −1

mSUGRA


Prediction of v from collidersRecall LHC: large cross sections, ~100 gg, gq,... events/day~ ~ ~ ~

Z

q

q

02

q~g~

jet

jet

jets, l+l−

missing energyAbundant production

of our DM candidate LHC as „DM factory“

01


Prediction of v from colliders:

LSP mass and decompositionbino, wino, higgsino admixture

Sfermion masses (bulk, coannhilation)or at least lower limits on them

Higgs masses and widths: h,H,A tan

NB: determination of v also gives a prediction of (in)direct detection rates

Required precisions investigated in, e.g. • Allanach et al, 2004; • Belanger, SK, Pukhov, 2005; • Baltz et al., 2006

What do we need to measure?

Need precision measuremants of O(‰) !

ILC: international e+e− linear collider

LHC precision most likely not sufficient

to match WMAP/PLANCK accuracies


Direct detection rates Check by direct detection is indispensible to pin down the dark matter

[H.Baer et al, hep-ph/0611387]


Indirect searches:high energetic positrons or gamma rays from annihilation

[P. Gondolo, hep-ph/0501134]


Higgs?

SUSY?

1 GeV ~ 1.3 * 1013 K

LHC

There are exciting times

ahead of us !

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