First Thoughts On Parameters And Their Values
Parameters for the Linear Collider
September 30, 2003
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Parameters for the Linear Collider
1. Introduction
Over the past decade, studies in Asia, Europe and North America have
described the scientific case for a future electron-positron linear collider
[1,2,3,4]. A world-wide consensus has formed for a baseline LC project
with centre-of-mass energies up to 500 GeV and with luminosity above
1034 cm-2s-1 [5]. Beyond this firm baseline machine, several upgrades and
options are envisaged whose weight, priority and realisation will depend
upon the results obtained at the LHC and the baseline LC. This document,
prepared by the Parameters Subcommittee of the International Linear
Collider Steering Committee, provides a set of parameters for the future
Linear Collider and the corresponding values needed to achieve the
anticipated physics program. The membership and the charge to the
subcommittee are appended.
In the following, we define an equivalent integrated luminosity, LeqT, as that
which would be obtained if the LC were operated at its maximum available
energy. For LC operation at less than maximum energy, we assume that the
luminosity scales as L ~ √s. For example, in the 500 GeV baseline machine
described below, the actual ∫L dt collected at √s = 250 GeV would be
0.5xLeqT.
It should be noted that the overall time of running quoted in this document
by no means exhausts the full physics program expected. The numbers given
should only indicate a first pass of physics running, needed in order to
capitalize on the LHC and the LC operating simultaneously.
The document first discusses the parameters and their approximate values
for a world-wide agreed baseline machine [5], listed according to priority.
The physics results obtained in the first few years of running with this
machine, together with the results from LHC will then define the schedule
for upgrades or other modes of operation (options) of the baseline machine
and their respective priorities. We consider the timely realisation of the
baseline machine as very important particularly in view of the expected
synergy with the LHC.
2
We expect shutdowns to install the upgrades or options discussed in sections
3 and 4 to take not more than two years after an initial physics running time
of at least four years, including the commissioning of the upgrades or
options.
This document does not aim at making the physics case for the Linear
Collider and therefore does not repeat detailed physics arguments found in
the documents referenced above.
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2. Baseline Machine
• The maximum centre-of-mass energy should be 500 GeV. The
machine should allow for an energy range for physics between 200
GeV and 500 GeV, with operation at any energy value as dictated by
the physics (e.g. at the maximum of the Higgs production cross
section).
• Luminosity and reliability of the machine should allow the collection
of approximately Leq = 500 fb-1 in the first four years of running, not
counting year zero which is assumed to mainly serve for machine
commissioning and short pilot physics run(s).1
• The collider has to allow for energy scans at all centre-of-mass energy
values between 200 GeV and 500 GeV. The time needed for the
change of energy values should not exceed about 10% of the actual
data-taking time. Therefore, the down-time for switching between
energy values should not exceed a few shifts within a particular scan,
and should not take more than a few weeks when changing between
different energy scans.2
Energy scans might include the top quark pair threshold, Higgs
production threshold and the thresholds of various supersymmetric
particle reactions.
• Beam energy stability and precision should be below the tenth of
percent level, in the continuum as well as during energy scans. The
experiments and machine interface must allow measurements of the
beam energy and of the differential luminosity spectrum with a similar
accuracy. For example, precision measurements of the Higgs boson
and top quark masses call for this precision.
1 It is assumed here that the design luminosity and the efficiency/reliability of the
machine will only be reached gradually within the first years of operation and that the
design luminosity and reliability will be reached in year four of physics running.
2 Collection of 10 fb-1 at one energy value requires 1-2 weeks of data-taking at design
luminosity (1/25 of the year); a full scan of 100 fb-1 may take half a year.
4
• The machine should be capable of producing electron beams with
polarisation of at least 80% within the whole energy range used for
physics running.
• Two interaction regions should be planned, with space and
infrastructure provided for two experiments. Two experiments are
desired to allow independent measurement of critical parameters and
to provide better use of the beams thereby maximizing the physics
output. At least one interaction region should allow a crossing angle
compatible with a γγ interaction region (see also Options). Both
interaction regions should have the capability of similar energy reach
and luminosity. Switching the beam between experiments should be
accomplished with less than a few percent loss of integrated
luminosity.
• The machine should allow for an energy range for calibration that
extends down to 90 GeV. For calibration, large emittance and
consequently low luminosity are tolerable. The amount of calibration
data and the frequency of such calibration runs at the Z0 might depend
on the detector technologies. However, it is assumed that a similar
strategy as at LEP-2 will be appropriate for all technologies, where
calibration runs were taken after long shutdowns. The machine design
should allow such calibration runs without additional investment.
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3. Energy Upgrade beyond the Baseline machine
Independent of the results from the first few years of running there are
several reasons for an energy upgrade. Examples include higher sensitivities
for anomalous gauge boson couplings, measurement of the Higgs boson self
coupling, the coupling of the Higgs to the top quark, production thresholds
for new massive particles or exploration of extra spatial dimensions.
Consequently, the energy of the machine has to be upgradeable.
The strong likelihood that there will be new physics in the 500 – 1000 GeV
range means that the upgradeability of the LC to about 1 TeV is the highest
priority step beyond the baseline.
• The energy of the machine should be upgradeable to approximately 1
TeV.
• The luminosity and reliability of the machine should allow the
collection of order of 1 ab-1 (equivalent at 1 TeV) in about 3 to 4
years.
• The machine should have the capability for running at any energy
value for continuum measurements and for threshold scans up to the
maximum energy with the design luminosity (√s scaling assumed).
• Beam energy stability and accuracy should be as stated for the
baseline machine.
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4. Options beyond the Baseline machine
Timing and priorities of the options will depend on the results obtained at
the LC baseline 500 GeV machine and possibly at the energy upgraded
machine, together with the results from the LHC. An important issue here
will be LC/LHC synergy and the time budget for the different options.
Therefore, this list of options is not priority ordered.
• Luminosity and reliability of the baseline 500 GeV machine should
allow doubling the integrated luminosity to a total of 1 ab-1 within two
additional years of running, without requiring an additional shutdown.
This extension could become a high priority if there is rich new
physics discovered at ≤ 500 GeV.
• Running as an e-e- collider at any energy value up to the e+e-
maximum energy may be important for some physics measurements,
albeit with reduced luminosity. This option is also highly desirable if
γγ collisions are to be provided.
• Positron polarisation at or above 50% is desirable in the whole energy
range from 90 GeV to the maximum energy, depending on the loss of
luminosity. Specific studies of the Higgs boson, electroweak
parameters, QCD, supersymmetric particles and new non-
supersymmetric physics would benefit from positron polarisation (P+).
The exact gain differs for different measurements, but roughly one
expects gains in event yields that are proportional to (1+P+). Such a
gain should not be overcome by the loss of luminosity with polarised
positrons. Some measurements are only possible if the positrons are
polarised, and should these become essential, then polarised positrons
will be desired even with significant loss of luminosity. Some studies
are enabled by transverse polarisation of both beams.
• Running at the Z0 with a luminosity of several 1033 cm-2s-1 (GigaZ
running) would allow high precision tests of the Standard Model,
within a year of data taking. Positron polarisation and frequent flips
of polarisation states are essential for GigaZ, as is energy stability and
calibration accuracy below the tenth of percent level.
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• Running at the WW threshold with a luminosity of several 1033 cm-2s-1
will allow the most precise determination of the W-mass, within a
year of data taking. Positron polarisation is not required. Beam
energy calibration is required with an accuracy of a few 10-5 (still to
be demonstrated by the experimental community).
• Several physics measurements are uniquely enabled through collisions
of (polarized) photons, or electrons and photons, from backscattered
laser beams. High polarization of both electron beams is required.
This option will require transformation of one interaction region to
run as a γγ or eγ collider at any energy up to 80% of the e+e-
maximum energy, with reduced luminosity (some 30-50%) with
respect to the e+e- luminosity.
5 References
[1] GLC Report 2003
[2] TESLA Technical Design Report 2001
[3] Linear Collider Physics: Resource Book for Snowmass 2001
[4] World-wide Study of Physics and Detectors for Future Linear e+e-
Colliders, http://blueox.uoregon.edu/~lc/wwstudy
[5] “Understanding Matter, Energy, Space and Time: The Case for the e+e-
Linear Collider”,
http://sbhepnt.physics.sunysb.edu/~grannis/lc_consensus.html
6 Appendices
6.1 List of subcommittee members
Asia: Sachio Komamiya, Dongchul Son
Europe : Rolf Heuer (chair), Francois Richard
North America: Paul Grannis, Mark Oreglia
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6.2 Charge to the subcommittee
The Parameters Subcommittee has been set up by the ILCSC and will report
to it, the first report being expected at the meeting in August during the 2003
Lepton Photon Conference.
The group comprises two members each from Asia, Europe and North
America. It shall produce a set of parameters for the future Linear Collider
and their corresponding values needed to achieve the anticipated physics
program. This list and the values have to be specific enough to form the
basis of an eventual cost estimate and a design for the collider and to serve
as a standard of comparison in the technology recommendation process. The
parameters should be derived on the basis of the world consensus document
“Understanding Matter, Energy, Space and Time: The case for the e+e-
Linear Collider” using additional input from the regional studies. The final
report will be forwarded to the ILCSC for its acceptance or modification by
end of September, 2003.
The parameter set should describe the desired baseline (phase 1) collider as
well as possible subsequent phases that introduce new options and/or
upgrades.
For all phases and options/upgrades priorities should be discussed wherever
possible and appropriate, and the description should include at least the
following parameters:
• Operational energy range
• Minimum top energy
• Integrated luminosity and desired time spent to accumulate it, for selected
energy values
(e.g. at the top energy, at the Z-pole, at various energy thresholds…)
• Polarisation and particle type for each beam
• Number and type of interaction regions
The committee may include any other parameter that it considers important
for reaching the physics goals of a particular phase, or useful for the
comparison of technologies, subject to the approval of the ILCSC.
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