Topics

TESLA project goes public

24 May 2001

The DESY laboratory in Hamburg recently published plans for a superconducting linear
electron-positron collider: TESLA. This article amplifies these ambitious plans and outlines the
objectives of the project.

At a major event held at the DESY laboratory in March (see News May 2000), the international TESLA
collaboration, together with the members of various study groups, released the TESLA Technical
Design Report. This five-volume opus presented the final facts and figures concerning a grand plan
for the future: the “TeV-Energy Superconducting Linear Accelerator”, a 33 km electron-positron
linear collider with an integrated X-ray laser laboratory.

To be built near the DESY
laboratory in Hamburg, the facility would not only provide particle collision energies of 500 GeV –
which could be increased to 800 GeV – but also include powerful X-ray lasers that would open up
new research opportunities in a variety of fields, ranging from condensed matter physics through
chemistry and material science to structural biology.

It is widely acknowledged among
particle physicists that a linear accelerator colliding electrons and positrons is the ideal machine to
complement CERN’s Large Hadron Collider, which is due to start operation in 2006. As well as
the TESLA collaboration, plans for similar next-generation linear electron-positron colliders are
being worked on by other teams.

SLAC in the US and KEK in Japan are jointly developing
two similar designs – known respectively as the Next Linear Collider and the Japan Linear Collider
– which could be ready for construction at around the same time as TESLA. CERN is also
working on a next-generation collider, CLIC. However, the TESLA proposal is the first to be fully
costed and made public. It is also the only project to include an X-ray laser laboratory and thus to
address a large interdisciplinary research community.

Resources needed

More than
1100 scientists from 36 countries have contributed to the 1424-page report, which describes the
scientific and technical details of TESLA, including cost estimates and time schedule. Based on the
experience gained in building the TESLA test facility at DESY and on industrial studies, the cost of
the TESLA project in its baseline design of 500 GeV has been estimated at a total of Ý3877 million spread over a period of 10 years: Ý3136 million is earmarked for the 500 GeV electron–positron collider,
Ý241 million for the accelerator components for the X-ray free
electron laser, Ý290 million to equip the X-ray free electron laser
laboratory and Ý210 million for one detector for particle physics. The
costs are based on prices for the year 2000. The person-years required to build the accelerators
amount to 7000, and the total costs for the operation of the accelerators have been estimated at
Ý120 million per year, assuming current prices and an annual
operation time of 5000 h. Staff costs are not included in this evaluation.

The size and
complexity of the TESLA endeavour means that it requires international input. From its onset in
1992, therefore, TESLA was planned and developed by members of a sizeable collaboration that
now comprises 44 institutes from 10 countries. The intention is to build and operate TESLA as an
international project for a limited duration, initially of 25 years.

As a possible model for the
realization of TESLA as an international co-operation, the collaboration has proposed using a
“Global Accelerator Network” of many existing accelerator and research centres, which would
allow the facility to be maintained and run, to a large extent remotely, from the participating
laboratories (see Accelerators to span
the globe
; June 2000). This approach would allow participating institutes to share the
responsibility for the facility as a whole. It would effectively allow the project to draw on
worldwide skills, ideas, manpower and financial resources, with site selection becoming a less
critical issue. In this approach the host country would carry roughly half of the investment
cost.

Particle physics

In its baseline design, the TESLA electron-positron linear
collider will reach a centre-of-mass (collision) energy of 500 GeV, five times as high as that of the
first linear collider, SLC at Stanford, and 2.5 times as high as that of LEP at CERN. At the same
time the luminosity of TESLA – a measure for the event rate a collider can deliver – is about 1000
times as high as that of LEP at 200 GeV (3.4 x 1034 cm_2
s_1). In a second phase, the energy range of TESLA could be extended to about 800
GeV without increasing the length of the machine.

Together with the “clean” and well
defined experimental conditions provided by the collisions of point-like electrons and positrons, the
energy range and luminosity of TESLA will make it an ideal machine to measure the properties of
new particles unambiguously and with high precision. These precision measurements will be
essential to complement the experiments being carried out at the world’s next flagship machine,
CERN’s LHC proton collider. A telling example from the past is the Z boson, which was
discovered at a proton-antiproton collider, while its properties could be determined with high
precision only at electron-positron colliders. These measurements were crucial for establishing the
Standard Model. In particular, they allowed an indirect determination of the mass of the top quark
prior to its discovery, and are responsible for the present constraints on the Higgs
mass.

Higgs exploration

The Higgs boson will play a central role at TESLA. The
Higgs mechanism is a compelling way to give the particles a mass: a priori, massless
particles acquire “effective masses” by interaction with a background medium, the Higgs field.
Recently, events observed at the highest energy of LEP have given a tantalizing hint that the Higgs
particle might have a mass of around 115 GeV (Season of Higgs and
melodrama
; March 2001). The Higgs particle is likely to be discovered at the Tevatron or the
LHC. The precise measurements of its properties, however, which are indispensable for a complete
understanding of the mechanism by which masses are generated, require a lepton collider. TESLA
is ideally suited to produce the Higgs particle directly and to determine its mass, lifetime,
production cross-sections, branching ratios and the way it couples to itself and to the top quark.

A comparison with the predictions of the Standard Model will establish whether or not the
Higgs mechanism is responsible for electroweak symmetry breaking and test the self-consistency
of the picture. TESLA will achieve a precision of 50 (70) MeV on the mass of a 120 (200) GeV
Higgs, and will measure many of the branching ratios to an accuracy of a few percent. The Higgs
coupling to the top quark will be measured to 5%. The accuracy of all of these measurements is
vital to a full understanding of the origin of mass.

Today, particle
physics is in an excellent, yet curious, state: although practically all experimental observations are
perfectly accounted for by the Standard Model, it is still based on too many assumptions and
leaves too many facts unexplained. Supersymmetry is the favoured candidate for an extension of
the model. It provides a framework for the unification of the electromagnetic, weak and strong
forces at large energies, and it is deeply related to gravity, the fourth of the fundamental forces.
Supersymmetry predicts that each matter and force particle has a supersymmetric partner.

TESLA’s precision measurements are required to determine the parameters of this
supersymmetric theory accurately. By sweeping the well defined centre-of-mass energy of TESLA
across the thresholds for new particle production, it will be possible to identify the particles one by
one and to measure their masses with very high precision. At LHC, part of the supersymmetric
particle spectrum can be resolved. Many final states are, however, overlapping, which will
complicate the reconstruction of some of the supersymmetric particles. Therefore, only the
combination of the results from TESLA and LHC will provide a complete picture.

The
highest possible level of precision is needed to extrapolate the supersymmetric parameters
measured at the energy attainable with TESLA to even higher energy scales, where the
mechanism of supersymmetry breaking and the structure of a grand unified supersymmetric theory
may be revealed. This may be the best way to link particle physics with gravity through an
experiment.

bright-rec iop pub iop-science physcis connect