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Compelling Scientific Questions
The International Linear Collider will
answer key questions about
matter, energy, space and time
We sample some of these questions in more detail here
The Terascale terrain
Increasing energy of particle collisions
in accelerators corresponds to earlier
times in the universe, when phase
transitions from symmetry to
asymmetry occurred, and structures
like protons, nuclei and atoms formed.
The Terascale (Trillion electron volts),
corresponding to 1 picosecond after
the Big Bang, is special. We expect
dramatic new discoveries there.
The ILC and Large Hadron Collider
(LHC) are like telescopes that view the
earliest moments of the universe.
The Terascale terrain
The Standard Model requires a Higgs field pervading the
universe to unify the Electromagnetic and Weak forces, and
to give mass to the quarks and leptons. Its mass should be in
the sub-Teravolt range.
The Standard Model is surely incomplete; holding the Higgs
mass at the terascale requires a miracle of ‘fine tuning’. New
phenomena should exist (supersymmetry, technicolor, extra
spatial dimensions etc.). Each has observable consequences –
new particles or new forces – at the terascale.
Dark Matter seems to be a particle (or particles) left over
from the Big Bang, whose mass is in the teravolt range.
The clues to unification of forces will lie at the terascale.
The Quantum Universe Questions
The “Quantum Universe” report
gives nine key questions in three
major areas.
I. Einstein’s dream
1.
Undiscovered principles, new
symmetries?
2.
What is dark energy?
3.
Extra space dimensions?
4.
Do all forces become the
same?
II. The particle
world
III. Birth of
universe
5.
New particles
8.
6.
What is dark
matter?
How did the
universe
start?
7.
What do neutrinos
tell us?
9.
Where is the
antimatter?
The ILC will provide answers for at least eight of these.
Examples of the ILC scientific program follow.
Revealing the Higgs
The Higgs field pervades all of space,
interacting with quarks, electrons etc.
These interactions slow down the particles, giving them
mass. The Higgs field causes the Electromagnetic (long
range) and Weak (short range) forces to differ at low
energy. It provides at least one unseen particle (the Higgs
boson) that has yet to be found.
Different theories predict
different types of Higgs bosons
with different properties.
The Higgs boson is somewhat like
the Bunraku puppeteers, dressed in
black to be ‘invisible’, manipulating
the players in the drama.
Revealing the Higgs
If the Higgs decays to visible particles, the LHC will see it
and measure its mass. But the LHC will not determine its
properties (intrinsic spin, etc.) and will not accurately
measure the strength of its interactions with other particles.
interaction rate
Curves denote different Higgs
boson spins; ILC data cleanly
discriminate.
collision energy
The ILC can ‘see’ the Higgs boson
even if it decays to invisible
particles, and determine its quantum
number properties, and thus point to
the theory explaining it.
Revealing the Higgs
The ILC can measure the fractions
of the Higgs decays into quarks,
leptons, gluons and bosons.
These decay fractions are the signatures that reveal the
origin of the Higgs field. The pattern of deviations from
the standard model expectations tells us about the
underlying theory.
Two possible examples:
supersymmetry
baryogenesis
Standard
model values
Understanding the Higgs could give insight into Dark Energy
Decoding Supersymmetry
Supersymmetry overcomes inconsistencies in the standard
model by introducing a new kind of space-time. But this
requires that every known particle has a supersymmetric
counterpart at the terascale.
Thus the partner of the
spin ½ electron is a
spinless ‘selectron’.
All quarks also have their
partners, as do the W
and Z bosons, etc.
Decoding Supersymmetry
The LHC is guaranteed to see the effects of supersymmetry,
assuming SUSY has relevance for fixing the standard model.
The counterparts of quarks and gluons will be produced
copiously, but the LHC will not be sensitive to the partners of
leptons, the photon, or of the W/Z bosons.
The ILC can produce the lepton, photon, and W/Z partners,
and determine their masses and quantum properties.
If the matter-antimatter asymmetry in the universe arises
from supersymmetry, the ILC can prove this to be the case.
Decoding Supersymmetry
There are hundreds of variants of
SUSY theories and only detailed
measurement of quantum numbers
and masses of SUSY particles can show us which one is
true. The measured partner-particle masses can be
extrapolated to high energy to reveal the theory at work.
These plots show how
the superpartner
masses vary with
energy for two
theories – quite
different patterns
for each
Understanding dark matter
Our own and other galaxies are gravitationally bound by
unseen dark matter, predominating over ordinary matter by
a factor of five. Its nature is unclear, but it is likely to be
due to very massive new particles created in the early
universe.
Supersymmetry provides a very attractive candidate
particle, the neutralino. All supersymmetric particles
decay eventually to a neutralino. At the LHC the neutralino
cannot be directly observed.
Understanding dark matter
e+
e-
g,Z
~
m+
~
m-
ILC would copiously produce the partners of
leptons. These decay to an ordinary lepton
plus neutralino, from which the neutralino
mass and spin can be deduced.
The sharp edges in the
lepton energy distribution
pin down the neutralino
mass to 0.05% accuracy.
Understanding dark matter
An aside: at the LHC, the mass of the neutralino and its
heavier cousin (called c20) are entangled. LHC can’t
measure either accurately.
c20
mass
c20 mass
error
with ILC
help
c20 mass
error with
no ILC help
neutralino mass
The precise ILC neutralino
mass measurement allows the
LHC to pin down the other
particle mass accurately – an
example of the synergy of the
ILC and LHC.
Understanding dark matter
ILC and satellite experiments WMAP and Planck provide
complementary views of dark matter. The ILC will identify
the dark matter particle and measures its mass; Planck will
be sensitive just to the total density of dark matter.
Together they establish the nature of dark matter.
Maybe ILC agrees with Planck;
then the neutralino is likely
the only dark matter particle.
Maybe ILC disagrees with
Planck; this would tell us that
there are different forms of
dark matter.
Finding extra spatial
dimensions
String theory requires at least 6
extra spatial dimensions (beyond
the 3 we already know). The
extra dimensions are curled up like lines on a mailing tube.
If their radius is ‘large’ (~1 attometer = billionth of an
atomic diameter), they could unify all forces (including
gravity).
Finding extra spatial
dimensions
If a particle created in an
energetic collision goes off into
the extra dimensions, it becomes
invisible in our world and the
event shows missing energy and
total momentum imbalance.
There are many possibilities for the number of large extra
dimensions, their size, and which particles can move in
them. LHC and ILC see complementary processes that will
help pin down these attributes.
Finding extra spatial
dimensions
collision energy (TeV )
→
Different curves are
for different numbers
of extra dimensions
production rate
The ILC with fixed (but
tuneable) energy of electronpositron collisions can provide a
separate measure that tells us
both the size and number of
dimensions.
→
The LHC collisions of quarks span
a range of energies, and
therefore measure the size and
number of the ‘large’ extra
dimensions separately.
Finding extra spatial
dimensions
The ILC can measure the two
ways this particle interacts with
electrons. The colored regions
indicate the expectation of three
possible theories; the ILC can tell
us which is correct!
production rate
axial coupling
vector coupling
Wavefunctions trapped inside a
‘box’ of extra dimensions yields a
series of resonance states that
decay into e+e- or m+m-. (But other
new physical mechanisms could
provide similar states.) LHC will
not tell us what an observed new
‘resonance’ is.
dimuon mass
Is there a plot showing
ILC errors?
Seeking Unification
At everyday energy scales, the 4
fundamental forces are quite distinct.
At the Terascale, the Higgs field unifies the EM and Weak
forces. LHC and ILC together will map the unified
‘Electroweak’ force.
The Strong force may join the Electroweak at the Grand
Unification scale. The ILC precision allows a view of this.
We dream that at the Planck scale, gravity may join in.
go here
sense whats happening here
force strength
Seeking Unification
energy
g2
g3
g1
Present data show that
the three forces (strong,
EM, weak) have nearly the
same strength at very
high energy – indicating
unification??
A closer look shows it’s a
near miss!
With
supersymmetry,
ILC and LHC
can find force
unification!
g3
g2
g1
Seeking Unification
Einstein’s greatest dream was finding unification of the
forces.
ILC will provide the precision measurements to tell us if
grand unification of forces occurs.
The ILC can provide a connection to the string scale
where gravity may be brought in.
Precision measurements at the ILC provide the
telescope for charting the very high energy character
of the universe instants after the Big Bang.
Conclusions
 We know the terascale is fertile ground for new
discoveries about matter, energy, space and time.
 We strongly believe new phenomena will be seen there,
but don’t know yet which they will be.
 The ILC allows precision measurements that will tell us
the true nature of the new phenomena.
 The ILC and the LHC together provide the binocular
vision needed to see the new physics in perspective and
view the terrain at much higher energies, and thus earlier
times in the universe.