The Quest for SUSY : issues for collider physics and cosmology
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Transcript of The Quest for SUSY : issues for collider physics and cosmology
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S. Kraml (CERN)
1-3 Dec 2006
The Quest for SUSY :issues for collider physics and cosmology
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Supersymmetry (SUSY)
is the leading candidate for physics
beyond the Standard Model (SM).
Symmetry between fermions and bosons
Q|fermion> = |boson>
unique extension of relativistic symmetries of space-time!This combines the relativistic “external” symmetries (such as Lorentz
invariance) with the “internal” symmetries such as weak isospin.
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recall Arkani-Hamed‘s comments on the unification of space and time...
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... predicts a partner particle for every SM state________________________________________________________
The motivations for TeV-scale SUSY include
the solution of the gauge hierachy problem the cancellation of quadratic divergences gauge coupling unification a viable dark matter candidate
________________________________________________________
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The search for SUSY is hence
one of the primary objectives
of the
CERN Large Hadron Colider
and a future int. e+e_ linear collider!
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This talk
1. SM problems and SUSY cures Naturalness and hierachy problems Gauge coupling unification
2. The minimal supersymmetric standard model Particle spectrum Collider searches: LHC, ILC
3. The cosmology connection Dark matter EW phase transition and baryon asymmetry
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SM problems
and SUSY cures
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The hierachy and naturalness problems To break the electroweak symmetry and give masses to the SM
particles, some scalar field must acquire a non-zero VEV.
In the SM, this field is elementary, leading to an elementary scalar `Higgs' boson of mass mH. However,
where is the scale (=cut-off) up to which the theory is valid.
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These large corrections to the SM Higgs boson mass,
which should be mH=O(mW), raise problems at two levels:
to arrange for mH to be many orders smaller than other fundamental mass scales, such as the GUT or the Planck scale ― the hierarchy problem,
to avoid corrections mH
2 which are much larger than mH
2 itself ― the naturalness problem.
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The supersymmetric solution
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A light Higgs
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c.f. talk by W. Hollik
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2 fit of the Higgs boson mass from EW precision data as of Summer 2006
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Heavy top effect,
drives mH2 < 0
Radiative electroweak symmetry breaking
EW scale GUT scale
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Grand unification . 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 MX.
NB: If MX is too low → problems with proton decay
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1-loop renormalization group evolution of gauge couplings:
SM:
MSSM:
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One can also re-write this as
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XX
Can also be turned into a prediction of the weak mixing angle .....
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The MSSM
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Minimal supersymmetric modelMSSM = minimal supersymmetric standard model
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:
neutral states: scalar h, H; pseudoscalar A
charged states: H+, H-
Lightest neutralino = LSP
1 superpartner
for each d.o.f.:
qL,R and lL,R
L-R mixing
~Yukawas
~ ~
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XXXX
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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
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Recall: Light Higgs
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c.f. talk by W. Hollik
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R parity: symmetry under which SM particles are even _ and SUSY particles are odd
If R parity is conserved
SUSY particles can only be produced in pairs Sparticles always decay to an odd number
of sparticles the lightest SUSY particle (LSP) is stable any SUSY decay chain ends in the LSP,
which is a dark matter candidate
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The scale of SUSY breaking
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Goldstino and Gravitino
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Gravitino mass
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SUSY @ colliders
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Large Hadron Collider
New accelerator currently built at CERN, scheduled to go in operation in 2007
pp collisions at 14 TeV
Searches for Higgs and new physics beyond the Standard Model
„discovery machine“, typ. precisions O(few%)
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SUSY searches at LHC
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Events for 10 fb-1 signalbackground
(GeV) )(jet p E M4
1iiT
missTeff
Tevatron reach
ET(j1) > 80 GeVET
miss > 80 GeV
signalEvents for 10 fb-1
background
(GeV) )(jet p E M4
1iiT
missTeff
ATLAS
From Meff peak first/fast measurement of SUSY mass scale to 20% (10 fb-1, mSUGRA)
Spectacular and large signal
Caution: also other BSM models lead to missing energy signature → need spin determination
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Compare with Higgs search
c.f. talk by G. Dissertori
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Mass measurements: cascade decaysMass reconstruction through kinematic endpoints
[ATLAS, G. Polesello]
[Allanach et al., hep-ph/0007009]
Typical precisions: (a) few %
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International Linear Collider e+e- collisions at 0.5-1 TeV Tunable beam energy and polarization Clean experimental env. Precision measurements of
O(0.1%), c.f. LEP Global initiative, next big accelerator after LHC?
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ILC: Precision measurements with tunable beam energy and polarization
[TESLA TDR] can reach O(0.1%) precision
see talk by H.-U. Martyn
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High-scale parameter determination
c.f. talk by W. Porod
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Higgs?
SUSY?
1 GeV ~ 1.3 * 1013 K
The cosmology connection
• dark matter• dark energy• baryon asymmetry• inflation• ....
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What is the Universe made of? Cosmological data:
4% ±0.4% baryonic matter 23% ±4% dark matter 73% ±4% dark energy
Particle physics: SM is incomplete; expect new physics at the TeV scale Hope that this new physics also provides the dark matter Discovery at LHC, precision measurements at ILC ?
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WIMPs (weakly interacting massive particles)
DM should be stable, electrically neutral, weakly and gravitationally interacting
WIMPs are predicted by most theories beyond the Standard Model (BSM)
Stable as result of discrete symmetries Thermal relic of the Big Bang Testable at colliders!
Neutralino, gravitino, axion, axino, LKP, T-odd Little Higgs, branons, etc., ...
BSM dark matter
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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 ~ 1/<v>Thermally avaraged cross section of all annihilation channels
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Neutralino LSP
as dark matter candidate
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Neutralino system
Neutralino mass eigenstates
Gaugino m´s
Higgsino mass
→ LSP
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Neutralino relic density
h2 = 0.1 with 10% acc. puts strong bounds on the parameter space
LSP as thermal relic: relic density computed as thermally avaraged
cross section of all annihilation channels → h2 ~ 1/<v>
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Annihilation into gauge bosons → WW / ZZ mainly through t-channel chargino / neutralino
exchange; typically also some annihilation into Zh, hh
Does not occur for pure bino; LSP needs
to be mixed bino-higgsino (or bino-wino)
Pure wino or higgsino LSP: neutral and charged states
are a mass-degenerate triplet, (co)annihilation too efficient
Right relic density for (-M1)/M1 ~ 0.3,
(M2-M1)/M1 ~ 0.1
[hep-ph/0604150]
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Coannihilations Occur for small mass differences between LSP and next-to-lightest sparticle(s); efficient channel for a bino-like LSP
Typical case: coann. with staus
Key parameter is the mass difference
= mNLSP−mLSP
Other possibilities: Coannihilation with stops (~20-30GeV), coann. with chargino and the 2nd neutralino (in non-unified models)
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mSUGRA parameter space
GUT-scale boundary conditions: m0, m1/2, A0
[plus tan, sgn()]
4 regions with right h2 bulk (excl. by mh from LEP)
co-annihilation Higgs funnel (tan ~ 50) focus point (higgsino scenario)
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Prediction of h2 from colliders:Requires precise measurements of
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
Required precisions investigated in, e.g. Allanach et al, hep-ph/0410091 andBaltz et al., hep-ph/0602187 c.f. talks by H.U. Martyn & B. Allanach
NB: determination of <v> also gives a prediction of the (in)direct detection rates
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For a precise prediction of h2
we need precision measurements
of most of the SUSY spectrum
(masses and couplings)
→ LHC+ILC ←WMAP
LHC
ILC
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Gravitinos
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Recall:
If m3/2 > mLSP, the gravitino does not play any role in collider phenomenology
However, it is possible that the gravitino is the LSPPhenomenology as before, BUT all SUSY particles will cascade decay to the next-to-lightest sparticle (NLSP), which then decays to the gravitino LSP.
Note 1: the NLSP may be charged Note 2: since the couplings to the gravitino are very weak,
the NLSP can moreover be long-lived
→ Gravitino as dark matter candidate
→ Collider pheno characterized by the nature and lifetime of the NLSP
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Implications from cosmology The most popular model for explaining the apparent baryon
asymmetry of the Universe is LEPTOGENESIS
→ out-of-equilibrium decays of heavy singlet neutrinos
Leptogenesis requires
a reheating temperature
TR > 109 GeV
At high TR an unstable G is
severely constrained by BBN
► Leptogenesis is OK
if the gravitino is the LSP
[Buchmüller et al]
~
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Gravitino dark matter
• Neutralino NLSP is excluded by BBN• Best studied alternative: stau NLSP
• Need to confirm spin-3/2 [L. Covi et al]
c.f. talk by H.-U. Martyn
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instead of conclusions ...
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„Since its discovery some ten years ago, supersymmetry has fascinated many physicists“
Hans-Peter Nilles, Phys. Rept. 110 (1984)
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„The discovery of supersymmetry is
tantamount to the discovery of
quantum dimensions of space-time“
David Gross, CERN Colloq., 2004
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whether or not it is SUSY ....
The exploration of the TEV energy scale
at the LHC and a future ILC
will lead to
fundamental new insights on physics
at both the smallest and the largest scales.
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PS: SUSY phenomenology is extremly rich,
and this talk could only scratch on the surface.
SUSY at this meeting:
MSSM predictions W. Hollik
Charginos at the ILC T. Robens
Parameter determination H.-U. Martyn, W. Porod
SUSY CP violation T. Kernreiter, K. Rolbiecki
Neutrino masses F. Deppisch
SUSY breaking N. Uekusa
SUSY dark matter A. Provenza, B. Allanach
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backups
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Assume we have found SUSY with
a neutralino LSP and made very precise
measurements of all relevant parameters:
What if the inferred
h2 is too high?
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Solution 1:
Dark matter is superWIMP
e.g. gravitino or axino
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Solution 2:
R-parity is violated after all
RPV on long time scales
Late decays of neutralino LSP reduce the number density; actual CDM is something else
Very hard to test at colliders
Astrophysics constraints?
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Our picture of dark matter as a thermal relic
from the big bang may be to simple
Universe after Inflation radiation dominated?
Non-thermal production?
Assumptions in WMAP data ↔ h2 ?
Solution 3:
Cosmological assumptions are wrong