The Electron-Positron Linear Collider

Probing the Secrets of the Universe

R.-D. Heuer (Univ. Hamburg) Colloquium Brookhaven, May 25, 2003

History of the Universe

LHC, LCRHIC,HERA

● Particle Physics today

● The case for the Linear Collider

● Status of the TESLA project

● The LC as a global project

The physical world is

composed of Quarks and Leptons

interacting via force carriers (Gauge Bosons)

Last entries: top-quark 1995 tau-neutrino 2000

What have we learned the last 50 yearsor

Status of the Standard Model

Standard Model

Precision measurements 1990-2000

(LEP,SLD,Tevatron, NuTeV,…)

Standard Model testedto permille level

Precise and quantitative description of subatomicphysics

Indirect determination of the top mass

possible due to

• precision measurements

• known higher order

electroweak corrections

)ln(,)( 2

W

h

W

t

M

M

M

M

Proves high energy reach through virtual processes

Test of the SM at the Level of Quantum Fluctuations

Top quark mass LEP indirect

)ln(,)( 2

W

h

W

t

M

M

M

M

Higgs boson mass LEP indirect

Test of the SM at the Level of Quantum Fluctuations

Indirect determination of the Higgs mass

possible due to

• precision measurements

• known higher order

electroweak corrections

HERA ep collider

Electroweak Unification

Weak and electromagneticInteractions:

Similar magnitudebeyond ~ 100 GeV

Open questions

Standard Model

• What is the origin of mass

Open questions

Ultimate Unification

• Do the forces unify, at what scale• Why is gravity so different• Are there new forces•••

Open questions

Hidden Dimensions

• Are there more than four space-time dimensions• What is the quantum theory of gravity•••

Open questions

Cosmic Connections

• What is dark matter• What is dark energy• What happened to antimatter• ••

Particle Physics and Cosmology

Particle Physics and Cosmologyboth point to New Physics at the TeV scale

Electroweak unification

Dark Matter

Dark Energy

Inflation

Neutrino Masses

CP Violation

The next steps

There are two distinct and complementary strategies for gaining new understanding of matter, space and timeat future particle accelerators

HIGH ENERGY direct discovery of new phenomena i.e. accelerators operating at the energy scale of the new particle

HIGH PRECISION interference of new physics at high energies through the precision measurement of phenomena at lower scales

Both strategies have worked well together → much more complete understanding than from either one alone

ex: top quark , prediction of Higgs boson mass range (LEP/Tevatron)

We know enough now to predict with great certainty that fundamental new understanding of how forces are related, and the way that mass is given to all particles, will be found with a Linear Collider operating at an energy of at least 500 GeV.

Experimental limits on the Higgs boson mass

indirect

direct

The next steps

MH between 114 and ~200 GeV

Electron-Positron Linear Collider offers

● well defined initial state collision energy √s well defined collision energy √s tuneable precise knowledge of quantum numbers polarisation of e- and e+ possible

● Clean environment collision of point-like particles → low backgrounds

● precise knowledge of cross sections

● Additional options: e-e- , eγ , γγ collisions

Machine for Discoveries and Precision Measurements

The power of e+e- Colliders

q q

LEP(OPAL)

e+

e-

Physics Potential of a Linear Collider (highlights)

• Higgs bosons

• Supersymmetry

• Structure of space-time

• Precision tests of electroweak interactions top W Z

√s: 91…500…~1000 GeV

Precision physics of Higgs-Bosons

Discovery and first measurements at the LHC (perhaps at Tevatron?)

The Higgs boson is a new form of matter a fundamental scalar a new force coupling to massTherefore, need to establish Higgs mechanism as the mechanismresponsible for giving mass to elementary particles breaking of the electroweak symmetry

Task of the Linear Collider:

Precision measurements to determinemass(es)quantum numbers (spin zero)couplings (proportional to masses of bosons, quarks, leptons)self-coupling (→ reconstruction of Higgs potential)

Dominant production processes at LC:

Precision physics of Higgs bosons

Precision physics of Higgs bosons

model independent measurement

Recoil mass spectrum

ee -> HZ with Z -> l+l-

~ 3%

m ~ 50 MeV

sub-permilleprecision

mH = 40 MeV

Precision physics of Higgs bosons

mH = 120 GeV

ee -> HZdiff. decay channels

mH = 150 GeV

mH = 70 MeV

Reconstructed Higgs Mass (GeV)

mH=240 GeV

Precision physics of Higgs bosons

ΔmH = 400 MeV (0.2%)

Δ σ (HZ) = 4%

Results availableforMH up to 320 GeV

e e ZH qqqq

Precision physics of Higgs bosons

Determination of quantum numbers

Spin from thresholdmeasurement

CP-quantum numbersfrom H,Z angular distributionsorpolarisation analysisof Higgs decays to taus

Precision physics of Higgs bosons

Higgs field responsible for particle masses→ couplings proportional to masses

Precision analysisof Higgs decays

ΔBR/BR

bb 2.4%cc 8.3%gg 5.5%tt 6.0%gg 23.0%WW 5.4%

For 500 fb-1

MH = 120 GeV

Standard Model Higgs vs

MSSM Higgs

Precision physics of Higgs bosons

High precision allowssensitivity to new effectse.g. additional heavy Higgs bosons

Reconstruction of the Higgs-potential

Φ(H)=λv2H2 + λvH3 + 1/4λH4

SM: gHHH = 6λv, fixed by MH

gHHH

20% (1 ab-1)

Precision physics of Higgs bosons

The precision measurements at the Linear Collider are crucial to establish the Higgs mechanismresponsible for the origin of mass and for revealingthe character of the Higgs boson

If the electroweak symmetry is broken in a more complicated way then foreseen in the Standard Modelthe LC measurements strongly constrain the alternative model

Conclusion

Beyond the Higgs

Why are electroweak scale (102 GeV) and the Planck scale (1019 GeV) so disparate ?

Are there new particles ? → supersymmetry new forces ? → strong interactions

hidden dimensions ?

Supersymmetry

● unifies matter with forces for each particle a supersymmetric partner (sparticle) of opposite statistics is introduced

● allows to unify strong and electroweak forces

● provides a link to string theories

Supersymmetry

● Predicts • light Higgs boson ( + additional heavier Higgs bosons) • spectrum of sparticles (→doubling number of particles)● Contains • many new parameters connected to SUSY breaking

● High precision measurements of • masses • couplings • quantum numbers needed to • extract fundamental parameters (few) • determine the way Supersymmetry is broken i.e the underlying supersymmetric model

Mass spectra depend on choice of models and parameters...

Supersymmetry

well measureable at LHC

precise spectroscopyat the Linear Collider

Supersymmetry Production and decay ofsupersymmetric particlesat e+e- colliders

charginos s-muons

Lightest supersymmetric particle stable in most models

candidate for dark matter

Experimental signature: missing energy

Energy spectrum of muons

Production and decay of smuons:

320 GeV, 160 fb-1

Mass errors (MeV):

smuon Χ10

end points: 300 100

threshold: 90 50

Supersymmetry Sleptons

3 ‰ 1 ‰

Cross section rises as

Shape of X-section -> spin

Produced in pairs

350 GeV

160 fb-1

m ~ 50 MeV

Easy detection through their decays

Supersymmetry Charginos

Di-jet inv mass (500 fb-1, E = 800 GeV)

Mass peak (50 fb-1, E = 800 GeV)

MSSM: one additional Higgs doublet

→ h0,H0,A0,H+,H-Supersymmetry

HA: 5σ discovery possible up to Σm = √s – 30 GeV

Extrapolation of SUSY parameters from weak to GUT scale (within mSUGRA)

Gauge couplings unify at high energies,

Gaugino masses unify at same scale

Precision provided by LC for slepton, charginos and neutralinos will allow to test if masses unify at same scale as forces

Gluino (LHC)

SUSY partners of electroweak bosons and Higgs

Supersymmetry Extrapolation to GUT scale

Supersymmetry Conclusions

The Linear Collider will be a unique toolfor high precision measurements

● model independent determination of SUSY parameters

● determination of SUSY breaking mechanism

● extrapolation to GUT scale possible

but what if ……

No Higgs boson(s) found….

WLWL scattering:

Standard Model mathematically inconsistent unless new physics at about 1.3 TeV

Experimental consequence: New strong interaction measurable in triple and quartic gauge boson couplings

Sensitivity at a TeV Linear Collider: ~ 8 TeV (TGC) ~ 3 TeV (QGC)

Hidden dimensions

String theory motivates brane models in which our world is confinedto a membrane embedded in a higher dimensional space

Extra dimensions provide an explanation for the hierarchy problem

e.g. large extra dimensions:

Emission of gravitons into extra dimensions

Experimental signature

single photons

In how many dimensions do we live?

measurement of cross sections at different energies allows to determine number and scale of extra dimensions

(500 fb-1 at 500 GeV,

1000 fb-1 at 800 GeV)

cross section for anomalous single photon production

Energy

= # of extra dimensionse+e- -> G

Hidden dimensions

Effects from virtual graviton exchange:

can prove Spin-2 exchange!

Hidden dimensions

Hidden dimensionsRandall-Sundrum Model

cross-sectionfor

e+e-→μ+μ-

includingexchange ofa KK-towerof gravitons

Direct observation…

Precision electroweak tests

σ (pb)

Energy scan of top-quark threshold

As the heaviest quark, the top-quark could play a key role inthe understanding of flavour physics…..

…requires precise determination of its properties….

ΔMtop ≈ 100 MeV

together with

ΔMW = 7 MeV(threshold scan)

And

ΔMtop = 100 MeV

high luminosity running at the Z-pole

Giga Z (109 Z/year) ≈ 1000 x “LEP” in 3 months

with e- and e+ polarisation

Precision electroweak tests

ΔsinΘW = 0.000013

We know enough now to predict with very high confidence that theLinear Collider, operating at energies up to 500 GeV, will be neededto understand how forces are related and the way mass is given to all particles.

We are confident that the new physics that we expect beyond the standard model will be illuminated by measurements at both the LHC and the LC, through an intimate interplay of results from the two accelerators.

The physics investigations envisioned at the LC are very broad and fundamental, and will require a leading edge program of research for many years.

Summary: key scientific points

The Linear Collider Report of the Worldwide Physics and Detector Study Group:

The challenges:

Luminosity: high charge density (1010), > 10,000 bunches/s

very small vertical emittance (damping rings, linac)

tiny beam size (5x500 nm) (final focus)

Energy: high accelerating gradient (> 25 MV/m, 500 - 1000 GeV)

To meet these challenges: A lot of R&D on LC’s world-wide

different technologies: NLC/JLC…..TESLA……CLIC

For E > 200 GeV need to build linear colliders

Proof of principle:

SLC

General layout of a Linear Collider

The TESLALinear Collider

superconducting Nb cavities

Decisions of the German Ministry for Education and Research concerning the TESLA Linear Collider

5 February 2003

● Today no German site for the TESLA linear collider will be put forward.

● This decision is connected to plans to operate this project within a world-wide collaboration

● DESY will continue its research work on TESLA in the existing international framework, to facilitate and assure German participation in a future global project

● DESY will remain a world-wide leading centre of particle physics.

● The decision to not propose a site today is not meant as a reduction of the importance of particle physics in Germany

The statement by the German government

• is positive on a linear collider in general,

• approves continued R&D on TESLA,

• encourages the German participation in a global project,

• but leaves the site selection open for the time being.

Essence:

Routine production of cavities exceeding 25 MV/m(TESLA goal for 500 GeV)

New surface treatment, gradients of > 40 MV/m (single cells) -> clear energy upgrade

The TESLALinear Collider

TESLA

April 2003: 38 MV/m

High powertest ofelectropolishednine-cellcavity

The path to higher energies….

1. unite first behind one project with all its aspects,

including the technology choice, and then

2. approach all possible governments in parallel in order to

trigger the decision process and site selection.

Next steps or How to arrive at a LC as a Global Project

International LC Steering Group:

How:

• Gather a committee of wise persons, who use criteria to be developed by the ILCSG, to recommend a technology choice to the ILCSC.

• The regional steering committees will each nominate 4 persons from which the ILCSG will choose three from each list for a total of 9 wise persons.

First discussion of the make-up of the committee in August.

Aim at joint selection of one technology in spring 2004

Next steps 1. Technology Recommendation

The ILCSG agrees that it would be highly desirable to form a precursor to the Global Linear Collider Center:

Core group to begin making an international design, based on accumulated work to date, but reexamined in a completely international context.

In parallel to the work of the design group:

preparation of political decision, definition of organisational structure, site analysis

Aim at approval of LC around 2006/2007

Next steps 2. Global Linear Collider Center

Conclusion

We have a convincing scientific case and a world consensus on the importance of a Linear Collider and on its timing w.r.t. the LHC

LC and LHC offer complementary view of Nature at energy frontier

We have the technology/ies for the LC at hand

We are developing detector technologies to do the physics at the LC

We have a great dynamics in the international coordination and are gaining political attention

We have an exciting and promising future for discoveries and for

understanding the universe and its origin

Let’s make it happen