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.