Download Higgs boson and EW symmetry breaking

APS plenary talk
Paul Grannis
April 7, 2003
Sailing the uncharted seas
beyond the Standard Model
New findings in experiments at the highest energy
will expand our knowledge of the structure of
matter on the shortest scales of distance, and at
the earliest times in the expanding universe.
The accelerators and experiments at the energy
frontier are costly, so we need confidence that
they will bring important new understanding.
Map fragment discovered on the previous voyages in Mare SM: Is it a
faithful guide for future exploration, or product of a delusional seaman?
Grand Unification
Gondwandaland
EWSB- land
Cliffs of
Dark Matter
Beagle
Bay of SUSY
Planck Dragon
n Oscillations
Mare SM
Dark Energy Maelstrom
Quark mixing
n CP
e→m
GZK Atolls
B≠ B
The Flavor Archipeligo
Muon
g-2
Quark-gluon plasma
volcano
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Gravitational
Waves
To boldly go … !
In the Mare Standard Model voyages so far, our explorations
showed:
“Four fundamental forces”
QCD (quantum chromodynamics – the strong nuclear force)
}
Electromagnetic
Weak
become the unified Electroweak (EW)
force for ECM > 250 GeV
Gravity (still no quantum theory)
QCD and EW forces are Yang Mills gauge theories – a local invariance
(at each space-time point) to generalized phase transformations of
the matter fields leads to forces mediated by spin-1 (vector)
massless gauge bosons.
The fundamental forces have a common nature
3
QCD: mediated by massless gluons coupling to ‘color’. Quarks
have color. Leptons do not, so do not feel the strong force.
red
quark
Red + yellow
gluon
3 pairs of quark flavors,
each coming with 3 colors.
u
yellow
quark
c
t
(d) (s ) ( b)
Gluons carry ‘color charge’ also (an octet of
colored gluons), so they couple to themselves.
(Making QCD much more complex than EM
interaction where the photon couples to
charge but carries no charge.
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Color charge neutralizes in observed particles today, but the
early universe had unconfined color – Quark Gluon plasma. The
details of the phase transition affect the subsequent universe.
EM: mediated by massless
photons coupling to ‘electric
charge’. All quarks and three
leptons e, m, t have charge and
feel the EM force
quark, e, m, t
photon, g
quark, e, m, t
Charged current
The underlying WEAK
interaction is similar – charged
and neutral currents mediated
by massless isovector ‘w’ and
isoscalar ‘b’ bosons that couple
to ‘weak isospin charges’
n, u quark
w±
e, d quark
Neutral current
n, lepton,
quark
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n, lepton,
quark
b0
But the range of the observed Weak force is ~ 1 fm, so the physical
W± and Z0 bosons must be massive. Experiments of the past decade
show MW = 80.450 ± 0.034 GeV and MZ = 91.1875 ± 0.0021 GeV.
So observed W and Z cannot be the Yang Mills gauge bosons, and EW
symmetry is broken.
In the Standard Model, we invoke the spontaneous symmetry breaking
of the unified EW force:
Massless gauge
bosons in
symmetry limit
Complex spin 0
Higgs doublet
(
b
w- w0 w+
( )
f1
f2
)
(
g
W- W+
Z0
)
=
Physical
bosons
Electroweak
symmetry breaking
In the symmetry breaking induced by the Higgs fields, 3 of Higgs
degrees of freedom go to provide the missing longitudinal
polarization state needed by massive vector bosons.
Before symmetry breaking: 4x2 gauge boson d.o.f. and 4 Higgs d.o.f.
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After, 3x3 for W±/Z, 2 for g and 1 Higgs d.o.f. left over
In the Standard Model, the 4th Higgs field results in the
HIGGS BOSON.
All the parameters of the EW interaction
are fixed by experiment, except for the Higgs boson mass.
Higgs gives mass to W and Z – and quarks & leptons too
e.g.
t quark
t quark
l
Higgs
l
generates the
top quark mass
The more massive the quark, the larger its Yukawa coupling l.
Many observables of the Z and W bosons are dependent upon
the mass of the Higgs boson.
In the context of the SM, precision measurements of the 100 or
so properties of Z, W, top quark indicate:
114 < MH < ~200 GeV
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Of course, Nature may choose some other way than SM
to break Electroweak symmetry!
But why the up quark (mass = 5 MeV)
and the top quark (mass = 174,000
MeV) have such different couplings
is still a big mystery !
‘Who ordered that’
The phase transition that breaks the EW symmetry also
governs the evolution of the early universe. We know the
energy scale of that transition to be the vacuum expection
value of the Higgs field ≈ 250 GeV.
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Something new must occur at this energy scale, maybe the
Higgs or perhaps something else, and experiments should find
the footprints and enough clues to figure out how it works.
The gauge couplings vary with the size of
the probe (or momentum transfer q2 ).
The QCD coupling decreases with q2 while
EM and Weak couplings increase. Were
they to converge at a common point, we
would have unification of the gauge forces.
In the SM, the couplings come close
at around 1016 GeV but do not unify
g1
g2
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g3
The flavor archipelago has many exotic islands:
 Particle-antiparticle (CP) symmetry is violated for the quarks;
the SM prescription fails however to explain the baryon-antibaryon
asymmetry in the universe.
 Quarks ‘ mix ’ (i.e. the quark QCD eigenstates differ from the
weak states): a linear combination of down, strange and bottom
quarks couple to the up quark in producing b decay.
 Neutrinos have mass, mix (hence flavor species oscillate). They
could have CP-violation as well. The mixing pattern is bizarre.
 The difference of fermion masses from the lightest neutrino at
about 10-3 eV to the heaviest quark above 1011 eV is a mystery!
 We do not know if quarks and leptons are immutable – and thus
if protons are stable.
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The energy scale at which fundamental new insights will
emerge is less clear for the flavor issues than for
Electroweak Symmetry breaking.
The SM explains much, but seems flawed:
 Failure to unify Strong, Electromagnetic and Weak forces
 The masses (W, Z, Higgs) are unstable to quantum corrections
and should rise to the Planck scale (1019 GeV) if not
protected (the “hierarchy problem”)
 Gravity is left out
 Baryon-antibaryon asymmetry of the universe is not explained
by SM sources of CP violation
 There is no good dark matter candidate in the SM
 The ad hoc mass and mixing parameters have no explanation
and are puzzling
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Our map fragment from the SM experiments of the past two
decades may give a sensible guide for future exploration – not
for details, but to tell us that we are poised to make new
discoveries on the coming voyages.
Possible New Physics – Supersymmetry
Postulate extension to Poincare group to include a
symmetry between bosonic (commuting) and
fermionic (anticommuting) space-time coordinates
For every SM boson there is a supersymmetry
partner fermion with all other quantum numbers
the same (color, charges, chirality …) and a
superpartner boson for SM fermions.
electron
spin =1/2
selectron
spin = 0
Susy expands the Higgs sector to 5 physical Higgs
states, one with mass likely below 130 GeV.
 String theories have supersymmetry as a necessary ingredient
 Cancellation of quantum corrections between boson and fermion
partners prevents mass renormalizations to Planck scale
 Lightest Susy particle (LSP) stable and non-interacting: an
excellent candidate for dark matter
 Potential for CP violations in supersymmetry sector to explain B≠B
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 A large spectrum of new particles to discover and measure
Possible New Physics –
Large Extra Dimensions
If the 7 extra spatial dimensions needed in
string theory are curled up, not at the Planck
length, but at mm or fm scales, one could cure
the hierarchy problem by reducing the true
energy (Planck) scale of gravity to few TeV.
Possible signatures include:
 Production of mini-black holes when collision energy exceeds
the inverse extra dimension size (mini-BH’s decay
democratically to all particles)
 Resonances from ‘standing wave nodes’ between two branes in
extra dimensions – Kaluza Klein states resemble excited Z bosons
 Apparent mono-photon production when graviton escapes into
extra dimension (qq or e+e- → g [unseen graviton] )
 Modifications to e+e- or qq scattering at high energy
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Many model variants exist, depending on the number & size of large
extra dimensions, and the particles that penetrate into them.
Possible New Physics – Strong Coupling
A new interaction patterned on QCD gauge theory, in which there is
a new spectrum of fundamental particles above the TeV scale.
Condensates of the new particles can form scalar particles that
generate EW symmetry breaking and give mass to W and Z .
 Observe the Goldstone boson condensates, measure their
properties (different from SM Higgs)
 Seek the new fundamental particles at higher energy
 Measure modifications to W/Z/g couplings and anomalous form
factors of the top quark
 Measure deviations to e+e- or qq scattering at high energies
 Detectable differences in precision Z/W properties from SM
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Past precision W,Z, top measurements constrain
Strong Coupling models tightly
The voyages of exploration now planned or proposed:
The Fermilab Tevatron:
CDF
DØ
Present luminosity is 4x1031 cm-2s1; expect to reach 3x1032 cm-2s-1 .
Total accumulation of 10 – 15 fb-1
by the end of the program.
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Integratef fb-1
Counter-rotating p and p,
colliding at 1.96 TeV. In effect
the Tevatron is a quarkantiquark collider with qq energy
up to ≈ 1 TeV. Two experiments
CDF and DØ will operate until
the LHC program is producing
physics results (2009?)
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14
12
10
8
6
4
2
0
Base
Stretch
Projection
FY02 FY03 FY04 FY05 FY06 FY07 FY08 FY09 FY10
End FY
The LHC at CERN
The Large Hadron Collider (LHC) at
CERN will collide protons with
protons at 14 TeV. This will provide
collisions of the constituent quarks
and gluons to about 5 TeV.
General purpose experiments
ATLAS and CMS.
First collisions are expected in
2007; first physics run in 2008 and
first results in 2009??
Mt. Blanc
Lake Geneva
ATLAS
CMS
Luminosity should reach 1x1034 cm-2 s-1
The LHC will reach the energy scale where current experiments tell us
that new physics should surely exist – LHC is the primary discovery vessel
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The e+e- Linear Collider (proposed)
The first phase of the LC should collide e+ and eat energies up to 500 GeV. Luminosity up to a few
x 1034 cm-2 s-1. Electrons polarized to above 80%.
First operation in 2015 ???
Upgrades and options:
 Polarized gg collisions (backscattered laser
light); also e-g and e-e- collisions)
 Polarized positrons
Two technology proposals exist – the TESLA
proposal (Germany) using superconducting rf
cavities, and the room temperature rf proposals
of Japan (JLC) and US (NLC). Both judged
feasible, but expensive (~$5B), so world
cooperation to build is necessary. R&D
continues on 5 TeV linear collider (CLIC at CERN).
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~30 km
 energy increase to about 1 TeV
US version of the Linear
Collider
The complementarity of the accelerators
Tevatron: operating now with seasoned detector collaborations.
Mainly qq collisions at high energy. Energy is limited however and will
access only a part of the terra incognito where new physics can lurk.
LHC: being built and fully funded and has the largest energy reach, so
should span the TeV scale new physics regime. The colliding quarks and
gluons within the protons have a range of energies, and the initial state
quantum numbers are not fixed. The collision rates are very high (Giga
Hz) and the backgrounds from SM and new physics processes are large;
experiments are challenging.
LC: proposed, with technical and political challenges remaining to be
solved. 500 GeV energy is large enough to explore the EW symmetry
breaking (Higgs) physics and some part of the new physics (SUSY etc.),
but not all. However, the initial state has a fixed cm energy and welldefined quantum state. The processes are simple and rates and
backgrounds are low. The LC will provide the detailed and precise
information to sort out the new physics.
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What will the different accelerators provide us?
Higgs boson and EW symmetry breaking
Past experiments at the CERN LEP collider,
the SLAC SLC and the Tevatron have
constrained the SM Higgs boson to have a
mass below about 200 GeV through
precision measurements of the Z, W
bosons and top quark.
LEP measurements set a lower limit for
Higgs mass at 114 GeV
19
Only if the Higgs mass is in the range
140 – 180 GeV can the SM be a valid
theory up to the Planck scale. If we
find the Higgs expected in SM or
SUSY will require new physics!
Higgs boson and EW symmetry breaking
Tevatron will seek Higgs via qq → WH or ZH
now
with H → bb (for MH below about 135 GeV)
or gg → H with H → WW above 135 GeV. Can obtain
3s evidence for Higgs over much of this range, or
rule out SM Higgs for MH < 185 GeV.
with Tevatron Run 2
excluded
by LEP
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from precision W
and top mass
Tevatron experiments can discover the Higgs, measure its mass to a
few %, determine its dominant decay mode and branching fraction
times cross section to 10’s of percent.
Higgs boson and EW symmetry breaking
LHC will be able to discover SM-like
Higgs boson (> 5s) from the current
limit up to 1000 GeV. Low mass region
expected in SM is hardest; only H → gg
observed. Determine Higgs mass to
fraction of %. Can measure width if
MH > 200 GeV. Determine the ratio of
branching fractions of Higgs for some
decay channels to ~25%.
1 yr run at full
design luminosity
Susy Higgs states: can always observe
at least 1, typically 2 or more, but
require several years at full
luminosity.
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Higgs boson and EW symmetry breaking
LC produces Higgs from e+e- → ZH or Hnn.
Higgs of several possible
mass values seen as recoil
to Z
Seeing Higgs recoil from Z gives ‘bias free’ Higgs
laboratory in which one can measure Higgs
branching ratios (bb, cc, t+t-, gg, W+W-) to a few %.
These BRs are crucial for establishing whether the
Higgs seen is SM, Susy or other model.
SM value (decoupling limit)
b
Possible BR
measurements
Allowed MA
W
t
g
c
The LC will establish the spin-parity of
the Higgs state; measure the ttH
coupling; measure the Higgs self-coupling
gHHH which is related to the shape of the
Higgs potential and to the Higgs mass,
thus is a crucial constraint on the model.
Susy models
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Tevatron or LHC discover the Higgs: LC tells us what it really is
Supersymmetry
Tevatron can sense the presence of Susy through such processes as
pair production of partners of W and Z, leading to multiple lepton
final states, or squark/gluino production with decays into ordinary
quark/gluon jets plus missing energy carried by the LSP.
LHC will see Susy if it exists and has anything to do with EWSB. Primarily
produce the strongly interacting squarks and gluinos, but can have a range
of particles in the decay chains of these. LHC can measure the masses of
Susy particles, but which ones and the precisions depend on the nature of
the Susy model. The nature of the mass spectrum will give reasonable
indication of the type of Susy model.
Rate vs. sum of 4 leading jet
transverse energies and missing
transverse energy:
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Solid points are for SM processes
only; Open points include the
production of supersymmetric
squarks and gluinos. An order of
magnitude enhancement.
Supersymmetry
LC can make precision measurements of Susy particles that are
light enough to be produced; unlikely that 500 GeV LC will access
the full spectrum, but 1000 TeV will do much better.
 Kinematic distributions and threshold scans determine Susy masses
to typically O(0.1 %)
 Electron beam polarization and angular distributions allow
determination of spin, parity, chirality of observed states.
 Accurate masses & cross sections will give the
mixing matrix angles and phases for the states
of similar quantum numbers (e.g. 4 partners of Z,
g and neutral higgs).
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These measurements, together with the LHC
determinations of squark and gluino, allow
extrapolation of the Susy parameters to very
high energy, and should indicate the type of
Susy model Nature chooses, without model
assumptions. Connect to string theories?
Large Extra Dimensions
If there are additional large (> Planck length)
dimensions, new phenomena appear, depending
on the size and number of extra dimensions,
and the fields (gravitons, fermions, gauge
bosons) that move in them. True Planck scale
M* is lowered to near the TeV region.
Tevatron: Gravitons moving in mm sized ED’s
would modify the cross section for pp → e+eor gg at high mass and small angles.
Sensitive to M* to about 2 TeV,
depending on the number of
extra dimensions.
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Large Extra Dimensions
LHC : qq → g Gn , where Gn is a graviton
escaping into extra dimensions results in a
monojet event. The LHC will be sensitive to
such processes for fundamental Planck scales,
M*, up to ~ 8 TeV.
Gravity propagating in
g
q
q
Gn
usual 3+1 dim. brane PLUS
N extra (small) bulk
dimensions.
n=2
n=7
g+X
Drell-Yan
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Mini Black Hole production
If collision energy exceeds the
reduced Planck scale, can
produce mini-black holes of
mass MBH > MPlanck . If the real
Planck scale is several TeV, then
mini-black hole production at
LHC is very large for BH mass
up to >10 TeV. LHC would be a
Black Hole factory.
Large Extra Dimensions
Hawking radiation from mini-black holes is very rapid. Since the
coupling is to mass/energy, there is a democracy in the particles
produced, so get sprays of particles including W,Z bosons, Higgs
bosons, top quarks in equal abundance with ordinary light quarks.
s = 15 nb
MP = 1 TeV, 1 LHC-hour (!)
W/Z
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h
t
Gives an alternate way for LHC to
produce Higgs bosons (copiously)
Example spectra of two jets in a
multijet event from black hole
evaporation, for 1 hour of running
and MPlanck = 1 TeV.
Peaks show the W/Z, 130 GeV
Higgs and top quark (unresolved
jets from W/Z). Tagging bquarks improves the cleanliness
but is not necessary.
Large Extra Dimensions
LC: The process e+e- → g Gn and the
qq → g Gn process at LHC have
comparable reach for effective Planck
scale for 500 GeV LC (1 TeV LC is
better). These measurements depend
both on M* and on d = # extra
dimensions. Varying the energy of the
LC determines d.
e+e- → m+m-
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s(ee
gGn)
d=6
5
4
3
d=2
400
600
ECM
800
In the case of TeV-1 sized extra
dimensions with a warped metric,
resonances that resemble excited Z
bosons (KK states) appear and can be
seen directly at LC or LHC.
mini Black Hole
evaporation event
at 5 TeV CLIC
The complementarity of the programs
The LHC has higher energy,
larger backgrounds and
a less well controlled
initial quantum state
than the LC.
The LHC is thus
LHC
more sensitive to
direct discovery of new
phenomena at high mass but
with less incisive detail.
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The LC with precision measurements
can indirectly sense new
phenomena at very large
energy. The detailed
measurements of
LC
couplings and quantum
numbers of new
particles adds much to
our understanding of the
new physics, and the nature
of the uncharted territory at
even higher energy.
As for past programs with e+e- and hadron collisions,
the LHC and LC offer a complementary view of Nature
at the energy frontier. The two operating together is
more than the sum of the individual programs.
Conclusions
The experiments outlined here will help triangulate
the terra incognito that we have now glimpsed only
darkly. The richness of the program can give us a
huge leap in understanding:
 The way Nature unifies its forces (or doesn’t)
 How the underlying symmetric theory of EM and weak
forces is broken into the pattern that we observe
 Point to the way the universe was at 10-32 to 10-12
seconds old when our future fate was sealed
 See the dark matter
 Quite possibly transform our understanding of space
and time itself