# . VLHC Experiments and Detector Issues

The Stage-1 VLHC with 40 TeV and Stage 2 with 175 TeV center of mass
energy will, as each stage comes on line, provide the world's highest
energy proton proton collisions. Experience with large-scale collider
detectors built over the last two decades demonstrates that at these
frontier exploration energies general-purpose detectors are best. The
major reasons for this choice is space in the collider ring for
experiments, their high cost, and the long time scale for construction
and operation. As discoveries are made physics priorities will change
over the detector lifetime. The number of general-purpose experiments
for the VLHC is a compromise between available resources and the need
for confirmation of new results and discoveries. We assume that the VLHC
will have two general-purpose detectors. These will be designed for
Stage 1 and then upgraded or replaced for Stage 2. In this chapter we
describe the major ex­perimental parameters for the Stage-1 detectors,
then outline the experimental challenges for Stage 2 and conclude with
the accelerator detector interface requirements for Stage 1.

## 8.1 Description of Experiment Parameters for Stage 1 {#description-of-experiment-parameters-for-stage-1 .unnumbered}

The goal of a general-purpose collider detector is to measure the
largest possible number of independent quantities for each trigger
event. These include e/μ identification, sign of charge and momentum,
detection and measurement of isolated photons, measurement of jet
energies and directions, identification of jets with b-hadrons,
determination of charged particle multi­plicities, and detection of
non-interacting neutrals. The major parameters for VLHC detectors have
been discussed at Snowmass 1996 \[[^1]\] and at the 1997 VLHC Physics
and Detector Work­shop held at Fermilab \[[^2]\]. The SSC detectors, SDC
\[[^3]\] and GEM \[[^4]\], were designed to achieve the above stated
goals, while operating at a luminosity of 10^33^ cm^-2^s^-1^. Detectors
currently under construction at LHC, CMS \[[^5]\] and ATLAS \[[^6]\],
are designed for a center of mass energy of 14 TeV and luminosity
10^34^cm^-2^s^-1^ equal to the Stage-1 VLHC design. Therefore, the major
parameters for the VLHC detectors can be based on the large amount of
R&D and design work done for the above experiments.

Experimentally detectable objects are charged tracks and clusters of
electromagnetic and/or hadronic energy. Detector parameters can be
optimized based on a set of benchmark physics processes, e.g. Higgs
particle production or the search for supersymmetry. We are looking for
the heaviest particles at an energy frontier machine. These are produced
almost at rest in the center of mass system and therefore the acceptance
for the decay products is basically propor­tional to the detector solid
angle coverage. Based on this assumption rapidity coverage up to \~3 is
sufficient for \~90% detection efficiency. An exception to this rule is
neutrino detection. In order to reach good missing energy resolution,
calorimetry acceptance up to a rapidity of \~5 is needed. For detection
of charged tracks we are aiming at good momentum resolution up to the
maximum energy of the decay objects. Typically this energy is 5 -- 10 %
of the center of mass energy; so for a 40 TeV machine we are interested
in reconstruction of particles with momen­tum up to \~ 3 TeV/c.

A hypothetical generic VLHC detector is described beginning from the
central region of the detector: the central tracking system. Typically
this region consists of two sub-detectors: a precision vertex detector
and a tracking system for momentum measurement. Vertex detection is very
important. Decay of many heavy objects occurs via decay into massive b
quarks. In order to reduce backgrounds from production of light objects
(copious at such energies) detec­tion of b quark decay a few mm to cm
away from the original vertex is necessary. For precision vertex
detectors, silicon detectors are mainly used. Some of them \[Error:
Reference source not found\] have areas of hundreds of square meters. As
the radiation dose in the central rapidity region depends weakly on the
colliding beam energy and linearly on luminosity LHC type detectors
(with some modifications) could be used for the Stage-1 VLHC. In order
to increase radiation hardness and improve coordinate resolution,
silicon pixel detectors are under extensive R&D study \[[^7]\].

Reconstruction of charged particle tracks and their momentum measurement
is performed in the tracking system. There are two major goals: pattern
recognition in a high multiplicity environment and precision momentum
measurement in the TeV region. Different types of gaseous detectors,
silicon or scintillation fiber detectors could be used in this region.
Momen­tum resolution about 5 -- 10 % could be reached with such
technologies for momenta up to \~3 TeV/c and three sigma charge
identification up to \~15 TeV/c. For momentum measurements there is a
solenoidal magnetic field in the central area of the detector. Typical
values for the field are in the range 2 -- 4 T. Mechanical forces in the
magnet limit this field. The presence of the superconducting solenoidal
magnet requires a liquid He supply to the detector area and appropriate
cryogenic plant, so special cavern(s) are required close to the
experimental halls (see Section 7.5).

Calorimetry is used to measure the energy of electrons, gammas and jets
as well as missing energy in the event. As the depth of the hadronic
shower (which defines calorimeter thickness) depends logarithmically
upon particle energy, the overall size of the calorimeters is similar to
SSC and LHC experiments. Calorimeter energy resolution is proportional
to 1/√(E) and im­proves with energy increase up to a constant limit,
typically a few percent or less. Therefore calorimetry becomes a
precision tool for measurement of event parameters at very high energy.
Currently most of the large collider detector calorimeters are liquid Ar
or scintillation/Cerenkov light type. As the collision energy increases,
heavy gas calorimeters could become efficient and be an inexpensive
solution for ionization calorimetry. The radiation dose for the Stage-1
VLHC in the central calorimeter region is manageable, although in the
very forward region special radiation hard techniques, like tungsten
with quartz fibers, will have to be used.

The muon detection system is located outside the calorimeter. Muon
tracks are bent in the muon magnet/absorber, enabling the trigger to
select high momentum muons. The muon magnets also improve muon momentum
resolution at very high energy and provide track points at large
distances from the interaction region. One of the most challenging
problems in muon detection in the TeV energy range is that at these
energies, muons start to behave like electrons, irradiating
electromagnetic debris along their tracks in the absorbers. This debris
complicates reconstruction of muon hits, especially at the trigger level
and requires multiple hit measure­ment along the muon track as well as
special shielding materials.

An important part of the detector design is the shielding necessary to
reduce background. A major source of background is interaction of proton
fragments with the beam pipe and other interaction region components.
Special shielding design can reduce background flux by orders of
magnitude \[[^8]\], reducing radiation dose to the detector components
and improving triggering and track reconstruction capabilities.

One of the major challenges for detector operation at such high
luminosity is the large num­ber of interactions per crossing. At a
luminosity of 10^34^ cm^-2^s^-1^ and beam crossing time of 18.8 ns, the
average number of interactions per crossing will be 19 (Section 1.3).
While most of these events are soft minimum bias events, they complicate
central tracker triggering and reconstruction. Calorimetry and muon
systems are less affected by high multiplicity events. It is basically
impossible to separate events from the same crossing based only on
timing informa­tion. On the other hand, in order to reduce overlap of
events from different crossings, achieving detector and electronics
resolution time less than the beam crossing time is at premium. This is
especially true at the triggering level where only limited information
about the event is avail­able. Ideally charge collection time from the
detector should also be less than the beam crossing time.

Radiation flux is another major challenge for the VLHC detectors. Table
8.1 compares the estimated dose at different distances from the beam
pipe for one year of running at design luminosity for the Stage-1 VLHC
and LHC detectors. In the central rapidity region the dose values are
comparable, so detector technologies developed for the LHC detectors
could be adopted for the VLHC as well.

Table 8.1. Radiation dose in Mrad per year for LHC and Stage-1 VLHC at
90^0^.

  ----------------------------------- ----------------- -----------------
                                      *LHC*             *VLHC*

  Vertex detector (\~10 cm)           3                 5

  Central tracking (\~50 cm)          0.25              0.3

  EM calorimeter (\~150 cm)           0.06              0.1

  Hadron calorimeter (\~250 cm)       2 × 10^-3^        \~ 3 × 10^-3^
  ----------------------------------- ----------------- -----------------

An important part of any experiment is the trigger system. It should
reduce the initial inter­action rate of about 50 MHz to a manageable rate
of about 50 Hz for writing events to tape. With a high occupancy of
detector elements and a large number of readout channels a typical event
size is in multi Mbyte range. This translates to a Gbyte per second tape
rate. It is impor­tant to select only interesting classes of events at
the trigger level and reduce the huge data sample manipulation after
events are written to tape. So, triggering capabilities are an important
consideration in detector design and special fast trigger electronics
need to be developed.

A general detector design based on the above considerations is shown in
Figure 8.1. This concept is well established and proven based on the
Fermilab and CERN collider experiments.

One of VLHC design parameters is the size of the detector cavern. This
will be mainly de­termined by the muon system. For high precision muon
momentum resolution, the required size of the hall is large. Table 8.2
shows hall sizes together with muon momentum resolution. Based on these
numbers and taking into account the evolution of the detectors from
Stage-1 to Stage-2, the experimental caverns should have a length of
about 60 m and diameter of about 30 m plus additional space for
personnel and equipment access (Section 7.5).

![](media/image1.png){width="5.21875in" height="2.8020833333333335in"}

Figure 8.1. Layout of a general purpose VLHC detector.

Table 8.2. Hall sizes and momentum resolution at momentum of 1 TeV/c.

  -------------------------- ----------------------- --------------------
  *Experiment*               *Hall size, m^3^*       *dp/p, %*

  SDC (SSC)                  29 × 29 × 50            11

  GEM (SSC)                  24 × 24 × 76            10

  CMS (CERN)                 26 × 26 × 60            5

  ATLAS (CERN)               25 × 25 × 53            8
  -------------------------- ----------------------- --------------------

Today's detector technology provides a wide selection of choices for the
Stage-1 VLHC detectors. Experience obtained in designing SSC and LHC
detectors is very important. ATLAS and CMS designs could be used for
Stage-1 VLHC with minor modifications. In order to reduce detector cost
and improve operational parameters, detector R&D is important. For each
of the detector subsystems R&D studies should concentrate on providing
high radiation tolerance, resolution time below 19 ns beam crossing
time, high segmentation as well as reliable long term operation and low
construction and operational costs. Development of very fast electronics
to digitize signals from detectors, select events with interesting
topology with an input rate of 50 MHz should be part of the R&D program
as well.

## 8.2 Description of Major Experimental Challenges for Stage 2 {#description-of-major-experimental-challenges-for-stage-2 .unnumbered}

The upgrade from Stage-1 to Stage-2 will increase the luminosity a
factor of two and the center of mass energy nearly a factor of four.
These increases in energy and luminosity provide a significant increase
in the collider physics reach \[Error: Reference source not found\], but
bring major challenges to detect and reconstruct events. In this section
we will discuss how the increase in energy and luminosity will affect
detectors.

From 40 to 175 TeV, both the charged track multiplicity and the
inelastic cross section in­crease \[[^9]\], and the luminosity is twice
as large. As a result, the radiation dose in the central region is a
factor of \~3 higher than in Stage-1. Figure 8.2 shows the energy flux
per unit of rapidity for 100 TeV center of mass. Most of the energy is
deposited in the forward region. Dose in the calorimeter located at a
rapidity of \~5 will be about 1 Trad per year, a serious experimental
challenge.

![](media/image2.png){width="2.975in" height="1.851388888888889in"}

*Figure 8.2. Calculated energy flux per unit of rapidity for pp
collisions at energy 100 TeV.*

The average number of minimum bias interactions per crossing for Stage 2
will be about 49, a factor of three increase from Stage 1. This large
number of interactions per crossing creates pile up of events in the
tracking detectors as well as in the calorimeter. Figure 8.3 shows the
energy deposition in a calorimeter cone, (∆φ^2^ + ∆η^2^)^½^ of 0.4 in
the central rapidity region for 1000 overlapped events at an energy of
100 TeV. At a luminosity of 2 × 10^34^ cm^-2^s^-1^ the average energy
deposition in a 0.4 cone will be about 350 GeV. This "pedestal" level
will fluctuate due to statistical fluctuation in the number of events
per crossing as well as with changes in lumi­nosity. While challenging
this task is similar to operation of calorimeters at LHC where the
energy flux will be about factor of \~5 lower. Muon system pile up of
events is less important, as the probability to a have high-energy muon
in a minimum bias event is \~10^-5^.

![](media/image3.png)

*Figure 8.3. Energy deposition in a cone 0.4 for 10^3^ overlapped events
at 100 TeV center of mass energy.*

With increase in energy the detectable decay products will have higher
energy as well. Calo­rimetry energy resolution is improving as 1/√(E) and
will be limited by the constant term and pile up fluctuations. The real
challenge will be momentum measurement of the charged particles and
muons. If the energy of the detectable objects we are looking for
increases proportional to the center of mass energy we will need
momentum resolution of about 10% for 5 -- 10 TeV particles. Presently
designed detectors at 10 TeV (see Table 8.2) will have 50% resolution at
best. So, improvements in charged particle detection (precision of
tracking detectors, value of magnetic field, and/or size of the lever
arm) are needed. Another approach is to think about detectors based
mainly on calorimeter information.

In summary, Stage 2 of the VLHC will be similar to Stage 1 in terms of
occupancies and radiation dose in the central region as luminosity,
inelastic cross section, and charged particle multiplicities each
increase by a small factor. Calorimetry will become very precise with
resolution limited by event pile up and the energy resolution constant
term, while charged particle momentum measurements in the region
1 -- 10 TeV will require serious design and R&D efforts.

## 8.3 Machine-Detector Interface Requirements for Stage 1 {#machine-detector-interface-requirements-for-stage-1 .unnumbered}

There is a set of accelerator and detector interface parameters,
important for efficient detector operation. The most important of these
are: the dimensions of the beam interaction region, beam abort due to
abnormal losses, location of the low beta quads, shielding from
background from collisions in accelerator components and background from
the accelerator tunnel.

The beam interaction region is described in two planes: perpendicular to
the beam pipe and along the beam direction. For triggering on displaced
vertices and missing energy it is prefer­able to keep the beam diameter
below 50 μm. It is also important to maintain beam center location
stable to within 1 mm during the store and locate the beam to within a
few mm with respect to the beam pipe center. Usually these requirements
are easily met. The size of the interaction region along the beam pipe
is not as well defined. A smaller interaction region has advantages and
disadvantages. Advantages are better missing E~t~ resolution and smaller
detector size. Disadvantages are overlap of events in the tracking
detector, and more difficult pattern recognition in the muon and
calorimeter systems. Usually the compromise length is σ \~ 30 cm, which
means that 99% of events occur in a region ± 1 m around the detector
center.

Some of the detector elements are sensitive to radiation, especially
those elements of the tracking system located close to the beam pipe. In
order to prevent excess irradiation of these components during
unexpected high loss a feedback loop to the accelerator control room
will be provided. If the integrated dose or the irradiation rate
increase above specific levels the beam will be aborted to prevent
damage to the detector. Such systems exist at all hadron colliders and
are well developed.

The requirement of hermetic calorimetry to measure missing energy sets
the calorimeter pseudorapidity coverage at about ± 5 \[Error: Reference
source not found,Error: Reference source not found\]. In order to locate
calorimeters close enough to the beam pipe as well as provide space for
access to the detector elements the low beta quads should be located at
a distance of about 20 m from the center of the interaction. A more
specific number for this parameter will be determined from specific
detector design.

There are two major sources of background hits in the muon detectors
which are difficult to reduce: (1) sprays from hadronic and
electromagnetic showers developed along detector cracks and along the
beam pipe from accelerator components and (2) backgrounds coming from
the tunnel produced by beam losses along accelerator. Cracks between
sub-detectors will be mini­mized. Nevertheless there will be background
from these cracks and originating from the collision point. This
background will be reduced with a tapered beam pipe as well as shielding
along the beam pipe. There is considerable experience in designing such
shielding \[Error: Reference source not found,Error: Reference source
not found,Error: Reference source not found,Error: Reference source not
found,Error: Reference source not found\]. Reduction in charged particle
flux up to two orders of magnitude, especially in the muon detectors,
can be achieved.

In order to define an acceptable background flux of particles from the
tunnel, this flux has to be compared with backgrounds coming from the
interaction region, taking into account the effect of shielding. Typical
measured/estimated background flux on the muon detectors from the
interaction region is (0.1 -- 1) × 10^6^ particles^.^m^-2.^s^-1^
\[Error: Reference source not found,[^10]\]. The flux of particles from
the tunnel should be below this number. In the case of the CDF and D0
experiments, 2 m of thick concrete shielding in the tunnel on both sides
of the detector was needed in order to reduce background to an
acceptable level. Results of MARS \[[^11]\] calculations of the
background flux in the Tevatron agree well with observations, so as soon
as the interaction region lattice and beam parameters are known,
backgrounds can be calculated and ways explored for reducing them.

#### **References**  {#references .Reference-heading .unnumbered}

[^1]: \[?\] D. Denisov and S. Keller, "Summary of the Very Large Hadron
    Collider Physics and Detector Subgroup," Proceedings, Snowmass 1996.

[^2]: \[?\] G. Anderson et al., "Summary of the Very Large Hadron
    Collider Physics and Detector Workshop," Preprint Fermilab 1997,
    FERMILAB-CONF-97/318-T.

[^3]: \[?\] E. Berger et al., *SDC Technical Design Report*,
    SSCL-SR-1215, 1992.

[^4]: \[?\] W. Lefmann et al., *GEM Technical Design Report*,
    SSCL-SR-1219, 1993.

[^5]: \[?\] G. Bayatian et al., *The Compact Muon Solenoid Technical
    Proposal*, CERN/LHCC 94-38, 1994.

[^6]: \[?\] W. Armstrong et al., *Atlas Technical Proposal*, CERN/LHCC
    94-43, 1994.

[^7]: \[?\] <http://wcc.oeaw.ac.at/>, 9^th^ Vienna Conference on
    Instrumentation, to be published, 2001.

[^8]: \[?\] V. Sirotenko et al., "D0 Shielding Optimization: Simulations
    and Measurements," D0 Note 3417, 1998.

[^9]: \[?\] W.A. Barletta and H. Leutz, eds., *Supercolliders and
    Superdetectors*, World Scientific, 1993.

[^10]: \[?\] D. Denisov, private communication about D0 detector tunnel
    shielding.

[^11]: \[?\] N. Mokhov, "The MARS Code System User's Guide, Version
    13(95)", Fermilab-FN-628, 1995.