DRAFT
CLIC Study Projects
0. Introduction
In 2004, the CLIC Physics Working Group has summarized in a comprehensive Yellow Report the
expected conditions for experiments at CLIC and other topics. Of key importance are the luminosity
spectrum, accelerated induced and beam-beam backgrounds and their impact on the detector. Despite
impressive understanding, very often further and deeper studies are needed. Also, the recent change of
parameters of CLIC will affect luminosity and backgrounds. The impact of this on the detector design and
physics needs to be understood in more detail. The following is a first attempt to list important studies
which still needs to be done.
1. Luminosity Spectrum
1.1 Impact on Physics
For many physics goals, the luminosity integrated over the peak of the spectrum (typically within 1% of
Ecm) is more important than the peak (or total) luminosity. The optimization of the integrated luminosity
rather than the peak luminosity may relax the technological challenges of the design. The background
levels would also be reduced; hence it will simplify the design of the low angle and low radius detectors.
The impact on the physics reach needs to be addressed, a first attempt to define a non-complete but
significant list of physics topics that may be interesting to cross check:
1.1.1 Supersymmetry
If SUSY exist, the first priority of a multi-TeV linear collider is to complete the discovery reach of SUSY
particles. It only depends on a minimum luminosity to achieve a 5 significance. What is this minimum.
Second priority is to measure accurately the parameters of these new particles. A good example of the
balance between having more luminosity or having a better luminosity spectrum is the determination of
the sfermion masses. For instance, the precision achieved in the smuon mass measurement for a given
luminosity could be improved if the effect of the beamsstrahlung on the luminosity spectrum is reduced.
How much is the gain in the sparticle mass measurements with a better luminosity spectrum?
The determination of the mass of the neutralino using the dilepton invariant mass distribution should not
depend much on the luminosity spectrum, and just scale with the number of events, hence with the
luminosity. However, if the energy scan method is used in addition, the luminosity spectrum may be
again relevant. It would be interesting to evaluate the precision on the neutralino mass using the energy
scan method combined with the invariant mass method, using a better luminosity spectrum.
1.1.2 Higgs Physics
If the Higgs boson has been discovered at the LHC, a multi TeV LC would attempt to measure the Higgs
couplings, e.g. To fermions. As an example, the measurement of the branching ratio of the rare decay of
the Higgs Hµµ requires high luminosity. The accuracy on the Higgs coupling would probably just scale
with luminosity.
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Another fundamental test of the Higgs sector with a light Higgs boson, which would benefit significantly
from multi-TeV data, is the study of the Higgs self-couplings and the reconstruction of the Higgs
potential. The double Higgs production, e+e-Hh depends strongly on the CM-energy; hence the
determination of the triple Higgs coupling using this channel will depend on the luminosity spectrum.
What is the effect of a better luminosity spectrum but lower integrated luminosity on such Higgs
measurements?
A good example of physics analysis affected by different background conditions, may be e+e-  H+H-,
with H decaying into t and b quarks, hence having 4 b-jets and 2 W bosons in the final state, or e+e-  H0
A0, with 4 b-jets in the final state. How does the H mass reconstruction depend on background conditions
and the luminosity spectrum?
1.2 Composition of Luminosity Spectrum
Due to beam-beam interaction the luminosity and background is severely affected at CLIC. The so-called
pinch effect enhances the luminosity but since it bends the particle trajectories it also leads to the
emission of beamstrahlung and reduces the particle energy. The average number of photons emitted is of
the order of one, depending on the values of the machine parameters. The effect of the new CLIC
parameters on the luminosity spectrum needs to be evaluated.
The main source of energy spread is the single-bunch energy spread and the beamstrahlung.
Bunch-to-bunch as well as pulse-to pulse energy variations should be small, but together they
cause about 0.3% rms spread in the energy. To reduce this spread would decrease the luminosity.
Is there a physics case which needs better peak energy resolution?
2. Luminosity Stabilisation
2.1 Integration of final Doublet
what is present design?
questions: integration, space for stabilisers
2.2 Vibrations from Experiment
The quadrupoles of the last doublet in the final focus need to be stabilized to avoid sizable luminosity
reduction. In a noisy (electrical, mechanical) site, significant luminosity loss can be experienced. Further
studies to determine the size of different dynamic effects coming from the detector or experimental hall,
their impact on the luminosity, and the possible counter measures remain to be done.
2.3 Beam Coupling to Detector B-field
In the experimental solenoidal field the beam particles will emit synchrotron radiation, which modifies
their energy and thus their trajectory slightly. As a consequence the beam-spot size will increase in the
vertical direction which reduces the luminosity. This effect can be reduced by decreasing either the
detector solenoidal field or the crossing angle. A preliminary study indicates that for Bz = 4T a crossing
angle of c =20 mrad is still acceptable. However, the results depend significantly on the magnetic field in
the detector, in particular the endcap.
It is necessary to perform more precise simulations for a realistic design of the detector solenoid and
verify the size of the effect on the luminosity .
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2.4 Beam Pipe design
3. Luminosity Optimisation
3.1 Low Angle Tagger to measure Pairs
3.2 Detector for Coherent Pairs in Extraction Line
3.3 Beamstrahlung Detector
4. Backgrounds
4.1 Hit Density in the first detector layer
A crucial constraint on the detector design arises from the hits of incoherent pairs in the innermost vertex
detector layer. The distribution of hits per train needs to be update for the new CLIC parameters as
function of radius and longitudinal position for different solenoidal fields.
4.2 Dead Cone (Mask)
A potentially important source of hits in the vertex detector is that of low-energy electrons and positrons
from coherent pair creation; these are backscattered in the final quadrupoles and are then guided by the
main solenoidal field back into the vertex detector. This effect can be suppressed by using a mask that
covers the side of the quadrupole facing the detector.
The inner mask also serves as a shield against neutrons, which are produced by the spent beam and
backscattered into the detector. It has been shown that such a shield can reduce the neutron flux in the
vertex detector by three orders of magnitude. For this purpose, the opening in the mask needs to be
smaller than the vertex detector.
Optimization of the mask remains to be done, and in particular the instrumentation of this area needs to
be considered.
4.3 Hadronic Backgrounds
Two-photon collisions can also lead to the production of hadrons. The cross section for this process is not
very well established at higher centre-of-mass energies. Simulations indicate about 4 events with a centreof-mass energy above 5 GeV per bunch crossing. The hadronic background contributes also to the
number of hits in the innermost VDET layer, similar but with lower rate than from incoherent pairs. These
backgrounds need to be studied further.
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4.4 Secondary Photons from Pairs
4.5 Neutron Background
There might be a sizable neutron flux at the detector from the spent beam hitting the material of the beam
dump. A simple estimate of the neutron flux can be based on the giant resonance production of neutrons.
The low energy neutrons from this process tend to fly in all spatial directions with an almost equal
probability. The high energy neutrons from other processes tend to move more in the direction of the
incoming particle beam and will thus fly away from the detector. While a good fraction of the neutrons
can be shielded, this shielding needs to have a hole to let the spent beam pass.
Furthermore, the secondary neutron flux from hadronic background events is another background source.
It has a maximum flux of cm-2 per year (= 107 s) of operation. The flux is highest around the
masks and smaller around the IP.
A detailed study of backscattering of neutrons remains to be done.
4.6 Muon Background
The rate of muons, produced as secondary particles in the collimation of high-energy (1.5 TeV) electrons,
can be substantial and requires a reliable simulation. With a muon protection system of three tunnel
fillers, their number could be reduced to 4000 muons per train, or 26 per bunch crossing with energies
going well above 100 GeV. In particular the effect could be substantial for calorimetric measurements
due to catastrophic radiation events.
A dedicated muon protection system will be needed, its properties and locations have not yet been
optimized.
The GEANT4 program has been extended to perform the tracking through the machine lattice and
materials in a combined, flexible manner. The simulation of muon background in the detector can then be
built from an existing machine description, allowing for background optimization.
4.7 Synchrotron Radiation
Synchrotron radiation emitted by the beam before entering the detector is a potentially dangerous
source of background. The radiation emitted in the final doublet defines the collimation
requirements. Radiation emitted before the final doublet can lead to secondary particles which
may be harmful due to their larger number.
The impact of the synchrotron radiation needs to be evaluated.
4.8 Spent Beam
Losses of beam particles, beamstrahlung and pairs in the post collision line can lead to the
backscattering of particles that can generate background in the detector.
The background level due to these processes needs to be evaluated.
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5. Instrumentation Issues
CLIC
5.1 Spent Beam Profile
5.2 Relative Luminosity
Detector
5.3 Track Time Stamping
5.4 Dual Readout Calorimetry
6. Simulation Tools
6.1 BDSIM
A detailed simulation program BDSIM [5] has been developed, based on GEANT4 [48], to model the beam
delivery system (BDS). BDSIM incorporates efficient accelerator-style tracking based on transfer matrix
techniques, together with the standard shower generation and physics processes of GEANT4. BDSIM
continues to be upgraded and now also incorporates hadronic processes, including neutron production and
tracking.
6.2 GuineaPig
The GUINEAPIG code simulates the beam–beam interaction in a linear collider. The production of
incoherent pairs, bremsstrahlung and hadronic background, is achieved using the Weizäcker–Williams
approach. Each beam particle is replaced by a number of virtual photons. In the collision these photons are
treated as real and the cross sections for →ee and → hadrons are used. Secondary electrons and
positrons are tracked through the field of the beams. Beam-size effects and those due to the strong field on
the virtual photon spectrum can also be taken into account
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