korea cms-theory meeting, cern, january 14, 2011

Supersymmetry at LHC

Hyun Min LeeCERN

Korea CMS-Theory meeting, CERN, January 14, 2011

Why is beyond the Standard Model?

Model Building

Observations

Predictions

Discrepancies of observed data with theoretical predictions call for “new models”.

Solving theoretical problems require “new ideas”, leading to predictions to be observed by experiments.

The SM has been successful in explaining all the known electroweak data. However, the SM is incomplete.

Electroweak symmetry breaking: weak or strong dynamics?

Unification of forces?

Dark matter?

Quantum theory of gravity?

Neutrino masses?

Baryon asymmetry?

Fermion mass hierarchies and mixings? …

The Standard Model

Based on gauge symmetry

3 copies of quarks and leptons with different masses

Force carriers: gluon,

is broken to

Higgs mechanism in the SM

Spontaneous symmetry breaking: through VEV of an operator that changes under the symmetry.

Higgs mechanism breaks electroweak symmetry by the weakly coupled scalar dynamics:

Higgs mechanism in the SM also gives masses to quarks and leptons.

Higgs mechanism leads to the neutral Higgs boson

Electroweak precision data are compatible with the SM predictions in the presence of the Higgs boson of weak-scale mass.

Higgs mass bound Weakly coupled until

high energies:

Unitarity bound:

Electroweak precision data, direct searches:

W

W

W

W

H

W

W

W

W

Light Higgs and hierarchy problem Suppose that the light Higgs boson is

present as suggested by electroweak data.

If the SM is valid at high energy scales,

Neutrino masses:

Gauge coupling unification:

Planck scale:

it is unnatural to maintain the light Higgs boson such that

: hierarchy problem.

Solutions to hierarchy problem Hierarchy problem consists in UV sensitive

quantum corrections to the Higgs mass:

Solutions:

i) : Strongly coupled dynamics like QCD, e.g. in technicolor, Higgs is composite,

ii) Cancellation between various contributions to the Higgs mass: “Supersymmetry”

Supersymmetry The additional contribution of superpartners cancels the

quadratic term in the one-loop Higgs mass by

SUSY is the mathematical extension of translation and Lorentz invariances: [Haag-Lopuszanski-Sohnius]

with

SUSY generator is a spinor under Lorentz transformation so its action is the exchange between a fermion and a scalar boson with same masses and charges.

MSSMName

Spin 1/2

Spin zero

SU(3),SU(2), U(1)

Q (3, 2, 1/6)

U (3, 1, 2/3)

D (3, 1, -1/3)

L (1, 2, -1/2)

E (1, 1, -1)

(1, 2, 1/2)

(1, 2, -1/2)

Name

Spin 1 Spin 1/2

SU(3), SU(2), U(1)

EM (1, 1, 1)

Weak W, Z (1, 3, 0)

Strong

g (8, 1, 0)

squarks

sleptons

Higgsinos

photinowino, zino

gluino

SUSY couplings A pair of superparticles in each vertex

R-parity & LSP Baryon & Lepton number conservation in MSSM is

not guaranteed by gauge symmetry only.

Imposing R-parity forbids dangerous B/L violations:

Consequences of R-parity LSP(Lightest superparticle) is stable. If neutral,

it becomes Dark matter (e.g. lightest neutralino). Non-LSP decays into odd # of LSPs, typically one

LSP. Superparticles are produced in pairs.

Spont. SUSY breaking If SUSY is exact, superparticles would have

equal masses to SM partners.

SUSY must be broken in the vacuum:

In order not to have massless Goldstino, there must be a gauge fermion : supergravity.

: super-Higgs mechanism

× ×

Goldstino interaction with superpartner pair

Squark contribute to the one-loop Higgs mass, meaning that for the light Higgs.

for well-defined limit

Any fermion in the MSSM cannot be a Goldtino.

i) SM fermions: observed as independent d.o.f.s

ii) Photino: scalar masses ,

Moreover, wino and zino remain massless.

iii) Sum rule:

SUSY breaking must occur in hidden sector, being mediated to the MSSM.

SUSY breaking is parametrized by soft mass parameters,

Gauge coupling unification

SM gauge couplings change in energy scale Q due to screening of charged particles in vacuum,

• Weak-scale superpartners contribute to the running of gauge couplings:

cf. SM:

• Gauge couplings are unified at much better than in SM.

Gaugino masses run in energy scales,

Universal gaugino masses at GUT scale lead to

Scalar masses also run. Equal scalar masses at

GUT scale become split. large top Yukawa coupling

drives up-type Higgs mass to negative for electroweak symmetry breaking.

SUSY mediation

SUSY mediation

LSP

mSugra Bino-like

mAMSB Wino-like

GMSB gravitino

Mirage (KKLT) Bino-Wino mixed

Initial superpartner masses at GUT scale depend on SUSY mediation mechanisms.

SUSY signals at LHC

2010-2011 LHC at plans to collect luminosity of

Superparticles produced in pair, each cascade decaying into an LSP.

Undetected LSPs: large missing transverse momentum

Typical final states: LSP + jets + leptons

Needs cuts on pt to reduce QCD multi-jets and SM backgrounds, e.g. neutrinos from W/Z bosons

mSugra limits from Tevatron:

Transverse massProcess:

Invariant mass

Missing transverse momentum

Transverse mass

Invariant under longitudinal boosts (: independent of lab and parton c.m.)

zero-lepton 2jet channel : Highest discovery channel

Cuts: Pt(jet)>[70,30]GeV, MET>40GeV

zero-Lepton

SU4 - SUSY benchmark point close tothe Tevatron bound m(q,g)~410GeV

1-lepton Electron, muon channels

Cuts: Pt(lepton)>20GeV (no further lepton with Pt>10GeV)),

Pt(jets)>30GeV (at least two jets), MET>30GeV

W,top background suppression

Discovery reach at LHC 0-lepton and 1-lepton channels most powerful

even with ATLAS and CMS can significantly surpass the Tevatron and LEP limits

With squark and gluino up to 600-700 GeV can be discovered

mSUGRA , A0=0, tanβ=10, μ>0

(ATL-PHYS-SLIDE-2010-495)

Update on jets at CMS

New result from CMS at 7TeV with (arXiv: 1101.1628 [hep-ex] )

jets + missing transverse energy

Agreement between estimated SM background and observed events exclude SUSY events more than 13.4 at 95% C.L.

In turn, constraints on msugra or CMSSM parameter space.

SUSY Higgs boson There are two Higgs doublets in MSSM:

3 would-be Goldstons: eaten by W/Z bosons

5 Higgs bosons:

give masses to up-type quarks and down-type quarks(charged leptons) by

Upper bound on the neutral light Higgs mass:

Couplings to fermions and gauge bosons go to the SM case in the decoupling limit, (LHC wedge)

(95% C.L.)

Conclusion Supersymmetry is a well-motivated scenario

beyond the Standard Model, solving the hierarchy problem.

In mSugra with R-parity conservation, gluino and squark of masses 600-700 GeV could be discovered at LHC with

SUSY mediation can be identified by the nature of LSP.

SUSY prefers the light Higgs boson, which is accessible at LHC.

Other Higgs bosons in the MSSM could be also discovered with integrated luminosity at LHC.