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

I. Einstein’s dream1. Undiscovered principles,

new symmetries?

2. What is dark energy?

3. Extra space dimensions?

4. Do all forces become the same?

II. The particle world

5. New particles

6. What is dark matter?

7. What do neutrinos tell us?

III. Birth of universe

8. How did the universe start?

9. Where is the antimatter?

The “Quantum Universe” report gives nine key questions in three major areas.

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.

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.

collision energy

inte

ract

ion r

ate 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

Curves denote different Higgs boson spins; ILC data cleanly discriminate.

supersymmetry baryogenesis

Standard model values

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:

Revealing the Higgs

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 SupersymmetryThere are hundreds of variants of

SUSY theories and only detailed measurement of quantum numbersand 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++~

~

,ZILC would copiously produce the partners of leptonsThese 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 2

0) are entangled. LHC can’t measure either accurately.

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.

20

mass

neutralino mass

20 mass

error with ILC help

20 mass

error with no ILC help

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.

Understanding dark matterILC 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.

Finding extra spatial dimensions

String theory requires at least 6 extra spatial dimensions (beyond the 3 we already know). Theextra 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).

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.

Finding extra spatial dimensions

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.

collision energy (TeV ) →

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duct

ion r

ate

→

The LHC collisions of quarks span a range of energies, and therefore measure the size and number of the ‘large’ extra dimensions separately.

The ILC with fixed (but tuneable) energy of electron- positron collisions can provide a separate measure that tells us both the size and number of dimensions.

Finding extra spatial dimensions

Different curves are for different numbers of extra dimensions

Wavefunctions trapped inside a ‘box’ of extra dimensions yields a series of resonance states that decay into ee or . (But other new physical mechanisms could provide similar states.) LHC will not tell us what an observed new ‘resonance’ is.

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!

axial coupling

vect

or

couplin

g

dimuon mass

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ate

Finding extra spatial dimensions

Is there a plot showing ILC errors?

Seeking UnificationAt 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

Seeking Unification 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!

forc

e s

trength

energy

With supersymmetry, ILC and LHC can find force unification!

g3 g3 g2

g2

g1 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.

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.

Conclusions