detector plane after scaling to an integrated luminosity of 1 fb-1 is shown in
Fig. 46. The two most obvious differences to the corresponding map for HL-LHC
(Fig. 45 top right) are

  * •

the slope of the dominant band due to the vertical dispersion at HL-LHC,
almost negligible in Run 2,

  * •

the diffuse cloud of points present in the LHC map due to machine backgrounds,
not included in the HL-LHC calculation.

Nevertheless, the fluences at the point ($x=4.42$ mm, $y=0$), i.e. the
detector edge position at HL-LHC, agree within a factor 2: $2.10\times
10^{13}\,\rm p/cm^{2}$ (from data taken in 2017, averaged over the six
detector planes in the XRP “56-220-F-H”) versus $0.98\times 10^{13}\,\rm
p/cm^{2}$ (HL-LHC calculation). This shows that despite differences between
the optical functions in Run 2 and at HL-LHC, the order of magnitude of
particle rates in a given detector position is similar.

Figure 46: Measured hit map from Run 2 (2017) in the horizontal XRP unit
“56-220-F”, pixel plane 2. The map taken with zero-bias trigger for a sample
with recorded integrated luminosity of 0.14 nb-1 was scaled to 1 fb-1 for
direct comparison with the calculation for HL-LHC (Fig. 45 top right). The
colour scale represents the fluence $\Phi[\rm p/cm^{2}]$. The violet line
shows where the detector edge would be located at HL-LHC; the sensitive area
lies to its right.

The predicted hit maps can also be used to calculate the occupancy per unit
area and bunch crossing, which in turn can contribute one of the arguments for
determining the necessary detector segmentation. Table 6 shows the peak
fluence per bunch crossing and the resulting occupancy for three example
segmentations. In the Run-2 measurement (2017) the peak fluence per bunch
crossing in the 220 m detectors was 14 times higher than expected for HL-LHC
in the same location. This is caused by the much smaller detector distance
from the beam centre due to the different optics: 1.9 mm in 2017 and 4.4 mm at
HL-LHC.

Table 6: Second column: peak fluence scaled to the integrated luminosity of one bunch crossing for a luminosity of $5\times 10^{34}\,\rm cm^{-2}s^{-1}$ (with levelling) and $k=2736$ colliding bunches (the beam parameters given in Table 3). Third to fifth columns: occupancy per detector pad for three example segmentations. The last line represents the measurement in 2017 with the XRP “56-220-F-H”.  Station |  $\Phi[\rm p/cm^{2}]$ (BX) | Occupancy
---|---|---
|  | $\rm 50\times 50\,\mu m^{2}$ | $\rm 100\times 100\,\mu m^{2}$ | $\rm 200\times 200\,\mu m^{2}$
196 m | 2.9 | $7.3\times 10^{-5}$ | $2.9\times 10^{-4}$ | 0.0012
220 m | 16 | $4.0\times 10^{-4}$ | $1.6\times 10^{-3}$ | 0.0064
234 m | 75 | $1.9\times 10^{-3}$ | $7.5\times 10^{-3}$ | 0.030
420 m | 32 | $8.0\times 10^{-4}$ | $3.2\times 10^{-3}$ | 0.013
220 m (2017) | 221 |  $\rm 150\times 100\,\mu m^{2}$: Occupancy = 0.03

This study indicates that at all four locations a segmentation with pads as
large as $200\,\mu$m can be afforded while keeping the occupancy at a similar
level as observed in Run 2 (2017). At the 196 m and 220 m stations, where
large areas need to be covered by detectors, this solution would keep the
number of channels manageable. At 234 m, where the occupancy is highest, it
might be prudent to reduce the pad size to $100\,\mu$m, given that no
backgrounds beyond single diffraction have been considered so far.

####  8.1.3 Tracking Requirements

The occupancy study in the previous section yielded pad sizes between 100 and
$200\,\mu$m. Considering the upper end of that range, the resolution of a
$200\,\mu$m pad without charge sharing and without plane inclination would be
$\rm 200\,\mu m/\sqrt{12}=58\,\mu m$. However, the present pixel detectors
with a horizontal (vertical) cell pitch of 150 $\mu$m (100 $\mu$m) and a plane
inclination of 18.4∘ have a spatial resolution of $\sim 25\,\mu$m per plane,
which is clearly better than the conservative estimate of pitch/$\sqrt{12}$.

With four to six planes per unit the total resolution per detector stack will
be better, depending on the projective alignment of the pixels in different
planes. This is fully adequate for the reconstruction of the proton
kinematics.

####  8.1.4 Timing Requirements

The required time resolution is given by the linear density of vertices at the
IP which in turn is determined by the longitudinal bunch profile and the
pileup multiplicity $\mu$. To first order, the bunch profile is Gaussian
within $\pm 2\sigma$ from the centre with a width of $\sigma_{\rm
b}=0.270\,$ns, corresponding to 8.1 cm 444All calculations in this section are
carried out in natural units with $c=1$. (Table 3 and Ref. [114]) under the
approximation of a constant bunch length throughout the fill. The mean pileup
multiplicity is expected to be as high as 140 at standard HL-LHC performance
and up to 200 at ultimate performance. With this input the linear vertex
density at the longitudinal position $z$ (relative to the ideal IP) and the
time $t$ (relative to the crossing time of the bunch centres) can be derived
as

$\rho_{\rm v}(z,t)=\frac{{\rm d}^{2}\mu}{{\rm d}z\,{\rm
d}t}=\frac{\mu}{\left(\sqrt{2\pi}\frac{\sigma_{\rm
b}}{\sqrt{2}}\right)^{2}}\,{\rm e}^{-\frac{z^{2}}{\sigma_{\rm b}^{2}}}\,{\rm
e}^{-\frac{t^{2}}{\sigma_{\rm b}^{2}}}\>.$ (30)

This vertex density distribution is a two-dimensional Gaussian with spatial
and temporal widths

$\sigma_{\rm v}=\sigma_{\rm b}/\sqrt{2}\>.$ (31)

After time integration (from $-\infty$ to $\infty$ for simplicity, without
using vertex time information in central CMS) the mean vertex distance as a
function of $z$ is given by

$\langle\Delta z\rangle(z)=\frac{1}{\rho_{\rm
v}(z)}=\frac{\sqrt{2\pi}\,\sigma_{\rm v}}{\mu}\,{\rm
e}^{\frac{z^{2}}{2\,\sigma_{\rm v}^{2}}}\>.$ (32)

The reconstructed vertex position is given by the difference of the times of
flight in the two arms, $t_{1}$ and $t_{2}$:

$z_{\rm v}=\frac{1}{2}(t_{2}-t_{1})$ (33)

To resolve the mean vertex distance $\Delta z$, each arm of the proton
spectrometer needs a time resolution

$\sigma_{t}=\sigma(t_{1,2})<\sqrt{2}\,\langle\Delta z\rangle\>.$ (34)

Figure 47 shows $\sigma_{t}$ as a function of $z$ for four different values of
$\mu$.

Figure 47: Time resolution required per spectrometer arm to resolve the mean
vertex distance at a position $z$ (in units of the longitudinal vertex width
$\sigma_{\rm v}$) from the IP centre. Four different pileup multiplicities are
shown: $\mu=50$ (LHC Run 2), 100, 140 (nominal HL-LHC performance), and 200
(ultimate HL-LHC performance). Left: for standalone PPS timing. Right:
combining the PPS timing with the MTD system, selecting a time-slice of $\pm
50\,$ps around the central bunch crossing time.

As the graphs demonstrate, for $\mu=50$ a time resolution of 10 ps is
sufficient to resolve vertices even in the densest part of the bunch crossing
region, whereas at HL-LHC pileup multiplicities of 140 and 200, resolutions of
4.8 ps and 3.4 ps would be needed, respectively, which has not yet been
achieved. Present detector technologies can reach a single-plane resolution of
$\sim 50-60$ ps (Section 8.2.1). With two movable vessel units containing
together about 10 planes, the single-arm resolution would be about
$\sigma_{t}=$15$-$20 ps. In such a scenario, vertices would only be resolved
in the periphery of the distribution. To give an example, for $\mu=140$ (200)
a single-arm time resolution $\sigma_{t}=20\,$ps would resolve vertices
outside $\pm 1.68\,(1.88)\,\sigma_{\rm v}$, i.e. 9% (6%) of all vertices.
However, for most physics analyses it is not necessary to fully resolve the
entire ensemble of vertices of a given bunch crossing:

  * •

The selection of a specific central event topology reduces the number of
vertices relevant for combination with leading protons.

  * •

The CMS Phase-2 upgrade for HL-LHC foresees a thin MIP timing detector layer,
MTD [115], between the tracker and the calorimeters, divided into a barrel and
two endcap sections. With this new detector an additional vertex separation in
the time dimension, i.e. by the vertex time, $t_{\rm v}$, rather than its
position, $z_{\rm v}$, will become possible. The time integration of the
density in Eq. (30) can then be carried out in slices compatible with the MTD
time resolution between 30 and 60 ps (without and with radiation damage,
respectively), reducing the effective linear vertex density in each time
slice. A time slice of $\pm 50\,$ps around the peak density time point would
relax the single-arm time resolution requirement to 23 ps and 16 ps for
$\mu=140$ and 200, respectively. This lies within the achievable range. To
assign the two protons to the correct time slice, the vertex time, $t_{\rm
v}$, has to be calculated from PPS as well. This is done by using the sum of
the times of flight:

$t_{2}+t_{1}=2t_{\rm v}+2z_{\rm d}\>,$ (35)

where $\pm z_{\rm d}$ are the detector positions in the two arms.

Note that Eq. (35) can also be used to suppress background from strongly out-
of-time particles in the two beams, e.g. debunched beam halo, by eliminating
events where the calculated $t_{\rm v}$ is several $\sigma_{\rm v}$ away from
the peak of the vertex time distribution.

####  8.1.5 Combination of Tracking and Timing

Given that

  * •

in all locations, except at 220 m, the space available for integrating movable
detectors is very limited,

  * •

in regular operation there is no acceptance overlap between the four
locations,

  * •

the number of movable devices should be kept at a minimum, both for impedance
minimization and for limiting the complexity of the system and its operation,

it is not possible to have separate movable units for tracking and timing
detectors. The most elegant solution is to have detectors performing both
measurements at the same time. Developments at present are exploring pixel
detectors that will allow a simultaneous measurement of the position and the
timing; they will be briefly described in Section 8.2.2. The concurrent use of
diamond detectors for timing and coarse tracking is possible. An alternative
initial solution would be to have in each movable vessel two different types
of detectors dedicated to only tracking and only timing, respectively. The
disadvantage would be the doubling of the total number of detector planes
which would increase the longitudinal space needed and thus limit the
reduction of the overall dimensions of the movable devices.

###  8.2 Detector Technologies

As already stated in Section 8.1.1, the dimensions of the detectors of the new
PPS spectrometer are very small, and each layer considered will have to cover
an area between 12 cm2 at 196 m and a few cm2 at 420 m.

While the search for suitable technologies will proceed in all directions,
there are already some candidates for which present operational experience at
the LHC exists and whose performance can be expected to improve until the Long
Shutdown 3. Needless to say, all possible synergies with the developments
currently ongoing for the Phase-2 upgrade of the central pixel system and for
the forward MIP timing detector will be carefully evaluated.

####  8.2.1 Diamond Detectors

Diamonds detectors are presently used by PPS for timing measurements [116,
117, 118]. The Double Diamond (DD) configuration, i.e. a double-layer of
detectors connected to the same amplifier input, is the best performing option
for diamond detectors aimed at timing and limited tracking. A first
measurement in a test beam [120] showed that a time precision of 50 ps per DD
plane could be obtained for a readout pixel size of $0.55\times 4.2$ mm2 when
read out with a waveform digitizer (SAMPIC [121]) or an oscilloscope.

The present timing precision for the DD used by PPS during Run 2 at the
highest luminosity shows that the improvement due to the double layer
configuration is not as significant as expected [118, 119]. A detailed check
was done at a test beam for sensors coming from the Run 2. The reasons of the
poorer precision have been analysed and understood; here they are briefly
explained.

These measurements show:

  * •

The loss due to radiation damage of the detector is minimal (loss of a few %
and limited to a small area near the most irradiated point) and can be ruled
out as the main cause of the large difference in precision observed during Run
2.

In fact, after the end of Run 2, the diamond detectors that had integrated the
100 fb-1 were mounted on a new hybrid and examined at a test beam. The
detector pixel that had been closest to the LHC beam was mapped with a silicon
tracker. It exhibited a small region of the order of a mm2 with a reduced
efficiency but still $>\,95\%$ [122, 123]. The mapping in Fig. 48 shows a
decrease in the rise time of the pulse and a minor decrease in pulse
amplitude.

Figure 48: Average signal amplitude (left) and rise time (right) as a function
of the position in the irradiated crystal that was closest to the LHC beam
during Run2, as mapped on a test beam in 2019. The sensor was mounted on a new
hybrid with non-irradiated electronics. In this test setup the $x$ and $y$
coordinates are swapped relative to the LHC configuration.

This agrees with measurements at earlier radiation damage tests in the CERN
irradiation facility [124]. These tests had indicated that single crystal
diamond detectors exposed to fluences up to $5\times 10^{15}$ protons/cm2
(corresponding to $3\times 10^{15}\,\rm n_{\rm eq}/cm^{2}$), which is reached
after 100 fb-1, exhibit a slightly lower signal amplitude and a shorter rise
time but without a significant deterioration of the time resolution. A
possible increase of the operation high voltage can be applied, if necessary,
to recover signal amplitude and extend the lifetime.

This result, obtained with never irradiated hybrids, indicates that the
decrease in timing precision with increased irradiation is essentially due to
the damage of the amplifier of the pads closer to the circulating beam.

These hybrids were initially developed for the conditions foreseen for the
TOTEM experiment: a new hybrid is being designed with a better optimization
and will be ready for Run 3.

  * •

Another cause of loss of precision is the fact that the arrival time of the
signal was measured with the existing PPS High Performance TDC (HPTDC [125]):
the signal from the hybrids was routed via a NINO chip [126] where the signal
coupling could not be perfectly optimized, which deteriorated the resolution.

After a further re-optimization of the hybrid and the NINO connections, a
resolution of 50$-$60 ps per plane is expected for Run 3.

Furthermore, some technical aspects of the present XRP detector vessel and its
movement and cooling systems have to be improved. In fact, some observed
shortcomings limiting the performance were induced by non-detector related
problems. The detectors and the on-board electronics are operated under a
secondary vacuum of 8$-$10 mbar and cooled at a temperature of
$-12\,^{\circ}$C in the very small volume of the Roman Pot.

It was observed that for the nominal signal amplifier gain, in a real LHC run,
crossboard pickup led to larger noise oscillations, forcing the reduction of
the preamplifier gain to obtain stable operation, which brought a
deterioration of the precision.

In addition, for some detectors, HV induced discharges forced the reduction of
the operating high voltage, again resulting in a worse resolution, bringing
the overall timing precision to 100$-$150 ps.

Work is in progress to remove these limitations for Run 3. However, the
secondary vacuum and the operating temperature of $-12\,^{\circ}$C will
continue to be required.

In conclusion, if diamonds are chosen as detectors for the new PPS
spectrometer and each station is equipped with eight measuring planes designed
with the present technology and optimized for these new running conditions,
the single-arm resolution will then be around 20 ps.

With improved detectors and electronics the goal of $\lesssim$10 ps per arm
with this technology does not seem out of reach.

####  8.2.2 3D Pixel Detectors

3D silicon pixel detectors are presently used by PPS for tracking [127, 128].
They operate at a temperature of about $-20\,^{\circ}$C in the secondary
vacuum of the XRP. The spatial resolution is already adequate for the future
PPS spectrometer: $\sim 25\,\mu$m for 150 $\mu$m wide pixels with an
inclination of 20∘ between the perpendicular to the detector plane and the
beam. The present technology, with its so-called 200 $\mu$m wide “slim edges”,
has an insensitive margin of only $\sim 50\,\mu$m because the depletion region
extends significantly into the edge area. This leads only to minor acceptance
losses at the lower end of the $|\xi|$ or mass range. Typically, an increase
of the insensitive margin width by 0.1 mm increases the minimum accepted mass
by about 10 GeV. An option for further improving the “edgelessness” would be
3D detectors with active edges [129].

All acceptance numbers given in the earlier sections of this document take
into account a distance $\delta=0.5$ mm from the outer surface of the movable
vessel to the point in the detector where full sensitivity is reached, which
comfortably accommodates the $\sim 50\,\mu$m wide insensitive margin.

The radiation hardness of these detectors was measured in irradiation tests
and operationally observed in Run 2 [128].

The present sensors can cope with a uniform irradiation of $5\times 10^{15}$
protons/cm2 (corresponding to $3\times 10^{15}\,\rm n_{\rm eq}/cm^{2}$)
reached after 100 fb-1 for the optics and XRP configuration of Run 2. Future
sensors, the same as used in the Phase 2 CMS Inner Tracker upgrade, are
required to withstand $2\times 10^{16}\,\rm n_{\rm eq}/cm^{2}$.

The radiation field in the PPS detector is highly non-uniform, showing a sharp
peak around the very localized regions crossed by the huge flux of diffractive
protons. This leads to a strong reduction of the detection capability in an
area of the order of less than a millimetre while a large fraction of the
detector area is still working properly.

For the PPS pixels in Run 2 the main problem was traced to the strongly non-
uniform irradiation exposure of the electronics chip bonded to the detector,
the PSI46 chip developed for the CMS Pixel detector.

It is possible to cope with this localized damage during the run by
periodically moving the detector planes vertically inside the XRP vessels,
i.e. tangentially to the beam. In Run 2 this strategy of diluting the
irradiation peak has been implemented by shifting the detector packages
manually during short technical stops of the LHC. For Run 3 a remotely
controlled vertical movement system based on piezoelectric motors has been
developed. PPS experience from operation in Run 2 indicates that the present
detector electronics need to be displaced by about 0.5 mm after having
integrated approximately 20 fb-1 of luminosity in a specific running position
near the beam. For a total integrated luminosity of 300 fb-1 per year at HL-
LHC, this strategy would imply 15 movement steps of 0.5 mm or a total
displacement amplitude of 7.5 mm per year. The peak fluences in Table 5 (last
column) would thus be divided by a factor 15 and reach a manageable level.

Furthermore, new pixel read-out chips, more radiation-hard than the ones
currently used, are being developed for CMS and ATLAS on the HL-LHC horizon.
PPS will profit from these developments.

Thanks to the unique structure of 3D detectors, it is possible to have a
uniform field in the depletion layer; by choosing small electrode spacing,
very short drift times can then be achieved without reducing the substrate
thickness, thus preserving the signal amplitude. This makes the detector
architecture potentially interesting also for timing.

Very first measurements [130] on prototype detectors with 50 $\mu$m electrode
spacing, a non-optimized electronics, and using a 90Sr source already
indicated time resolutions between 30 and 180 ps per plane depending on the
signal amplitude. A later experiment [131] with 90Sr electrons confirmed the
30 ps resolution.

Recent measurements [132, 133] on specially shaped 3D cells with laser beams
and 270 MeV pion beams yielded time resolutions of 20 and 30 ps, respectively.

If these results are confirmed, this detector technology, capable of providing
simultaneous tracking and timing, can become available rapidly for the very
small dimensions and the small number of the detectors needed for the new PPS.

####  8.2.3 Planar Silicon Detectors

Prototypes of the Low Gain Avalanche Detectors, LGAD or more simply Ultra-Fast
Silicon Detectors, UFSD [134], have been used in Run 2. They served mainly for
timing measurements in special TOTEM+CMS runs at $\beta^{*}=90\,$m and low
pileup ($\mu\sim 0.5$), but a few planes were also used in PPS timing XRPs,
complementing the diamond detectors.

This technology could be explored further when a new generation of more
radiation hard detectors, now under development for the CMS MTD, will be
available.

###  8.3 Reference Timing System

The clock distribution system presently used by PPS can be reimplemented for
HL-LHC. It is described in Refs. [4] (Section 6.3.2) and [5] (Section 5.7.2).
This system also offers the possibility of correlating the proton timing
information with the one of the upgraded central detectors.

The system has been used for PPS timing detectors since September 2017.

The linearity of the HPTDC used for the PPS readout has been checked by
leveraging the tunable phase shift.

The optical clock distribution jitter is of the order of 1 ps (measurement
limited by the scope bandwith).

Since the present system is synchronized to LHC via the Beam1 reference, the
beam cogging (phase delay between the two beams used to centre the interaction
point in IP5) can introduce a bias or shift in time.

The phase drift is a long term one and oscillates in a range of about 10 ps.

##  9 Trigger Strategy

The trigger strategy is expected to largely follow the lines of the present
PPS system, which does not provide a Level 1 (L1) trigger but can contribute
to the High Level Trigger (HLT). However, as already discussed in Ref. [5]
(Chapter 6), there are options for L1 proton triggers capable of reducing
event rates, based on proton multiplicity or the longitudinal vertex position
obtained from the timing detectors. Furthermore, the TOTEM XRP tracking
detector trigger transmitted via a fast electrical cable [135] could be
adapted to PPS use. The increased CMS L1 latency time in Run 4 [136] would
open up this possibility even for the most distant station at 420 m.

##  10 Movable Detector Vessels and Machine Integration

The development of movable detector vessels and their integration into the
machine can build on a wealth of experience gathered in the LHC Runs 1 and 2.
For three out the four preferred locations, advanced versions of the Roman Pot
technology, developed by the TOTEM experiment and later upgraded [4] for high-
luminosity operation by PPS, are conceivable. The fourth location, near 420 m
from IP5, suffers from special space and integration constraints and will need
a specific development.

The present section discusses the most important guidelines for further
optimizations and new developments. In the end, the presently known candidate
technologies are listed.

###  10.1 Impedance Effects and their Mitigation

Like every beamline element with resistive material around the beam, near-beam
detectors contribute to the impedance acting on the beam current and causing
two kinds of effects:

  * •

Effects on the beam: wake fields induced by the passing proton bunches perturb
subsequent bunches and thus lead to longitudinal and transverse beam
instabilities. To limit these effects, an impedance budget has been defined
for the whole LHC ring. Each beamline element is only allowed to contribute a
certain fraction of this budget.

  * •

Local impedance heating, i.e. the transfer of power from the beam to the wall
of a cavity.

In the original design of the TOTEM XRPs impedance reduction was less
important because operation had been foreseen only in special runs with very
low beam currents. Thus the original box-shaped XRPs exhibited both an abrupt
aperture restriction (when inserted) and cavities around the detector boxes
(even when retracted). The only impedance mitigation implemented was the
installation of ferrite plates around the detector boxes [137] in order to
dampen cavity-induced resonances. In the first high-luminosity test insertions
of the XRPs at the end of Run 1 these measures were found to be insufficient,
as significant heating was observed at the XRPs, despite the active cooling of
the detector packages, resulting in ferrite outgassing and vacuum degradation.

Consequently, to make the XRP system compatible with continuous high-
luminosity operation in Run 2, more detailed studies and tests were performed
during LS1, resulting in the following system upgrades [138]:

  * •

All existing XRPs were equipped with Radio Frequency (RF) shields filling most
of the cavities between the cylindrical flange walls and the box-shaped
detector housings. This removed the most dangerous high-amplitude resonances
below 1.4 GHz.

  * •

The new XRPs for timing detectors were designed with cylindrical rather than
box-shaped geometry, avoiding cavities almost entirely.

  * •

Like in other machine elements, the ferrites were replaced with a new type
made of a better material with a higher Curie temperature. They were baked out
under vacuum to minimize outgassing.

  * •

The mounting location of the ferrites was moved to the region most affected by
impedance heating; the ferrite geometry was adapted accordingly.

In Run 2, the XRPs were inserted regularly in almost all physics fills. During
operation the temperature in several points on the XRPs as well as the machine
vacuum in the respective sectors were continuously monitored. The temperature
stayed within comfortable limits, no vacuum degradation and no beam
instabilities were observed. This proved the effectiveness of the impedance
mitigation measures and the adequacy of the cooling system.

For the HL-LHC, where the current will increase by a factor of two and hence
the heating by a factor of four, the LHC impedance team sees “no indication
that the current PPS Roman Pot system would be an issue with HL-LHC beams,
provided cooling is sufficient” [139]. For the design of a new system the
following points need to be kept in mind:

  * •

The number of near-beam devices should be kept at the minimum necessary for
the physics.

  * •

For beam stability, the impedance is most critical in the initial fill phase
before collisions because of the absence of Landau damping. Therefore it is
important to be able to keep the devices as far from the beam as possible
during this phase, ideally totally retracted.

  * •

Devices that cannot be fully retracted should not have abrupt aperture steps
but tapers with an angle smaller than 11∘.

  * •

Bellows have to be shielded.

  * •

The design should avoid resonant cavities below 2 GHz.

  * •

The device surfaces facing the beam should be coated with a good conductor.

  * •

The cooling has to be strong enough to cope with the increased beam current
and hence heating power.

During all preparations a close cooperation with the Impedance Working Group
will be maintained. For all new devices impedance simulations have to be
carried out, profiting from tools that have significantly improved since LS1.
For this purpose, the modelling of impedance heating was recently benchmarked
with temperature probe measurements taken during an LHC fill in 2015 [141].

###  10.2 Showers

The insertion of movable devices close to the beam generates showers since
beam halo and collision debris scatter off the vessel and detector material.
While these processes will have to be studied in detail with FLUKA, as
previously done for the Run-2 PPS configuration, some mitigation and
monitoring guidelines can already be given.

  * •

The material exposed to the beam halo has to be minimized. In particular, the
windows separating the secondary vacuum of the detector vessels from the
primary beam vacuum have to be as thin as possible, but thick and strong
enough to limit the deformation in case of an accidental secondary vacuum loss
which could lead to the dangerous scenario of a window bulging into the beam.

  * •

It is advantageous to have TCL collimators downstream of movable devices to
absorb debris generated by the latter. For the 196 m and 220 m stations the
collimators TCL5 and TCL6 will play that role. For the 234 m and 420 m
stations the present layout does not foresee any downstream TCLs. FLUKA
studies will address possible problems from showers. Note, however, that in
2018 the PPS system operated at the highest Run-2 luminosities with TCL6 fully
open and did not induce any radiation problem for the downstream elements.

  * •

After each detector station (and ideally even after each unit) a BLM has to be
installed to monitor losses generated by the movable device. This is also
needed for the beam-based alignment.

###  10.3 Technologies for Movable Devices

####  10.3.1 The “Warm” Locations (196 m, 220 m, and 234 m)

The three non-cryogenic locations are easiest to equip with detectors:

  * •

No cryogenic bypass is needed.

  * •

The dispersion is negative, implying that scattered protons with $\xi<0$
arrive on the side radially away from the LHC ring centre, not inwards where
the second beam pipe lies at only 194 mm distance (beam centre to beam
centre). Hence the transverse space constraints are quite relaxed.

Since the beam pipes in these regions have the same diameters as in the
present-day LHC, it would be possible to install Roman Pots very similar to
the existing ones. However, one will nevertheless take advantage of the
opportunity to improve some minor shortcomings experienced in Runs 1 and 2,
notably:

  * •

A lack of rigidity in certain aluminium parts that should better be made of
steel. This would straighten the movement trajectory and thus allow a more
precise transfer of metrology data to detector alignment constants,
simplifying the track- and physics-based alignment steps.

  * •

A better control of the distance between the detector edges and the Roman Pot
window facing the beam. Also this improvement would help in the alignment
procedure.

  * •

A revised, more sturdy microswitch and electrical stopper system, less prone
to electrical contact problems and hence even more reliable than the present-
day system (that had on average one failure per year).

  * •

The BPMs, mechanically integrated in the Roman Pot beam pipes, have not
performed with enough precision to be usable for the detector alignment
relative to the beam. Improvements have to be found. An option to be explored
is the integration of BPM buttons directly into the XRP jaws as done in the
LHC collimators. A problem, however, is the single-jaw nature of the XRPs in
contrast to the double-jaw collimators.

Also, given the rather small detector dimensions and their small weights, one
might explore a lighter, more compact design.

####  10.3.2 The “Cold” Location 420 m

The 420 m location requires the replacement of the present-day empty cryostat
LEGR with a connection crystat to create some free space for a detector
system. An additional complication is the positive dispersion deflecting
protons with $\xi<0$ into the narrow space between the two beam pipes, which
precludes the use of Roman Pots with present-day dimensions. The first problem
could be solved by reusing or adapting the already existing design of a
connection cryostat for the new TCLD collimators to be installed in equivalent
locations in IR2 and IR7. Figure 49 shows this cryostat assembly, already with
the TCLD integrated. A zoom on the TCLD itself is shown in Fig. 50. For PPS
purposes, the TCLD could be replaced with any movable detector vessel fitting
in the same space (see Fig. 51 for the dimensions). An alternative solution to
be explored could be the connection cryostat designed for the old FP420
project [11] (Section 6 therein). The compatibility with the present HL-LHC
layout needs to be investigated.

Figure 49: Full connection cryostat assembly design, with new TCLD collimator
integrated (from Ref. [97]).

Possible technology options for the movable detector vessel presently under
consideration include:

  1. 1.

A modified TCLD Collimator: most of the collimator design would be maintained
but the solid jaws replaced with detector housings of the same geometry. Such
a device would have the same advantages as the collimator: very stable
movement on two axes per jaw, the vertical movement for irradiation dilution,
the redundant LVDTs (Linear Variable Differential Transformer [140]), the
button BPMs integrated in the jaws, and optimized impedance properties. A
difficulty, on the other hand, would be the significant amount of material in
front of the detectors due to the flat-angle taper of the jaw.

Figure 50: Left: TCLD integrated in the bypass cryostat; right: TCLD alone
(from Ref. [96]). Figure 51: Dimensions of the assembly of TCLD and bypass
cryostat (from Ref. [96]).

  2. 2.

New compact Roman Pots could be designed to fit even in the narrow space
between the two beam pipes. Such a design could have the advantages of the
well-established impedance properties of the present-day Roman Pots, and a
very thin window in front of the detectors.

  3. 3.

Movable Beam Pipe: the design of the “Hamburg Beam Pipe” that had once been
considered for PPS, could be revisited and improved. The main problem, the
impossibility of complete retraction implying a higher impedance before
collisions, might be overcome by adding an 11∘ taper and a copper coating; see
also the discussion in Ref. [5] (Section 3.6).

###  10.4 Movement and Interlock System

The present-day XRP movement system [142] is based on the functionality
principles and on hardware and software components as used by the LHC
collimation system. This implies in particular that all components exposed to
radiation are as radiation tolerant as their identical counterparts in the
collimation system.

However, in the system design phase, certain simplifications have been made
because of the originally intended limited use of the XRPs only for special
runs of the TOTEM experiment. After operational experience further
modifications were made during LS1 and in year-end technical stops. For the
HL-LHC phase the movement system should be fundamentally redesigned taking
into account the lessons learnt in Runs 1$-$3, but keeping it as close as
possible to the collimator system. The vertical movement capability of the
horizontal detector units, allowing the dilution of the most irradiated
region, should be integrated in the main movement system. Such an option is
already present in the collimator system, called “5th axis”.

The functionality and implementation of the interlock system [143], preventing
beam operation while any movable device is in a forbidden position, are
expected to be very similar to the present ones. A particular upgrade to be
envisaged is the introduction of redundant LVDTs (measuring the XRP positions
for interlock purposes) to be protected against malfunctions and hence false
interlocks.

The design will proceed in continued close cooperation with the Machine
Protection Panel.

###  10.5 Services

  * •

Cooling System:
The evaporative cooling system based on $\rm C_{3}F_{8}$ [144], successfully
used for the TOTEM/PPS XRPs, is a likely candidate technology to be adapted
for the new HL-LHC spectrometer. Because of the increased impedance heating
power (factor of four, see Section 10.1), the cooling power needs to be
increased accordingly, which is reachable with a new generation of more
efficient heat exchangers at the detector units [145, 146]. Because of the
high radiation environment of the LHC tunnel, the main part of this cooling
system is installed in the shielded underground service cavern USC55. From
there the cooling fluid is transported at ambient temperature to the detector
stations. The related services (cooling plant in USC55, cooling fluid lines)
have to be foreseen. The capacity and performance of the cooling system
presently installed at IP5 are compatible with the requirements and demands of
the new RP installation for HL-LHC. It will not need more space, but all
components (pumps, valves, DCS elements) need to be exchanged.

Alternatively, the Aircooler system [146] might be an option. The ongoing
development of innovative heat exchangers will allow the use of detector
package cooling based on the VORTEX principle even during HL-LHC operation.
The main advantage is the maintenance free operation in the LHC tunnel; in
addition, the overhead on the cooling system at IP5 is reduced to the
compressor, which has to provide compressed air at around 10 bar.

  * •

Secondary Vacuum:
Like in the present Roman Pot system, the vessels housing the detectors will
have to be under vacuum, separated from the primary machine vacuum. This
requires remote-controlled pumps and gauges located in the alcoves RR53 and
RR57.

  * •

Summary of Rack Space:
The following space needs to be reserved (see Fig. 52 for USC55, Fig. 53 for
RR57, and Fig. 54 for RR53):

    * –

The detector electronics and the vacuum equipment together will require 4
racks in each of the tunnel alcoves RR53 and RR57.

    * –

The motor control needs 2 racks in USC55.

    * –

The vacuum control needs 1 rack in USC55.

    * –

The interlock logic box and the interface to the LHC BIS system (CIBUs) are
located in one rack together with other machine interlock components (e.g. the
main CMS CIBUs) in USC55.

    * –

Additional racks for detector power supplies, trigger, DAQ, and DCS elements
(still to be determined) will be needed in the CMS sections of USC55.

Figure 52: Layout of the USC55 service cavern (adapted from Ref. [147])
showing the rack space occupied by TOTEM / PPS equipment in Runs 1$-$3.

Figure 53: Layout of the RR57 tunnel alcove in Sector 5-6 [148]. The racks
presently used by TOTEM are marked with green frames.

Figure 54: Layout of the RR53 tunnel alcove in Sector 4-5 [149]. The racks
presently used by TOTEM are marked with green frames.

##  11 Summary

We propose to build a new Precision Proton Spectrometer for operation in the
full HL-LHC phase.

  * •

The physics goal is to extend the central exclusive production studies of the
present-day PPS system to processes with lower cross sections and to a wider
mass range.

  * •

Four locations have been identified to be suitable for installing movable
proton detectors: 196 m, 220 m, 234 m, and 420 m, each on both sides of IP5.
Preliminary space reservations have been made.

  * •

The locations at 196 m, 220 m, and 234 m can be instrumented with Roman Pot
devices similar to the ones presently used. The 420 m location requires a
bypass cryostat (which has been developed for other locations in the LHC) and
a movable detector vessel approaching the beam from the narrow space between
the two beam pipes (still to be developed, with some candidate technologies
existing).

  * •

Acceptance studies resulted in a strong preference for a vertical crossing-
angle in IP5. The HL-LHC Executive Committee decided to implement it for HL-
LHC.

  * •

With this vertical beam crossing the central mass range covered will be:

    * –

133 GeV$-$2.7 TeV with only the stations at 196 m, 220 m, and 234 m;

    * –

43 GeV$-$2.7 TeV with all stations, including 420 m. For comparison: the Run-2
configuration covered approximately the mass range 350 GeV$-$2 TeV.

  * •

The intense radiation background implies radiation hardness requirements on
all components installed in the tunnel. Service work during short Technical
Stops will not be possible.

  * •

The irradiation dose rate is very strongly peaked near the beam. Hence the
most irradiated spot on a detector needs to be periodically shifted with a
remote-controlled vertical detector movement system. A shift by 0.5 mm every
20 fb-1 is sufficient.

  * •

The mean pileup multiplicity up to 200 makes a longitudinal vertex
identification via time-of-flight measurement necessary. The time resolution
requirement is not a discrete “all or nothing” criterion. A single-plane
resolution of 50$-$60 ps is already adequate.

  * •

The detector technologies envisaged are those that are already used and
continuously improved in the pre-LS3 PPS system:

    * –

Diamond detectors for time-of-flight measurements,

    * –

3D pixel detectors for tracking, possibly also for timing,

    * –

UFSD detectors as an alternative for timing if the ongoing developments are
successful.

  * •

The stations at 196, 220, and 234 m are similar to those currently in use at
PPS, both in their design and in the technology used. Based on this, a
preliminary budget estimation suggests that they could be built at a total
cost between 1 and 2 MCHF.

Conversely, the 420 m station requires the modification of a cryostat and a
novel approach for the detector vessel. More work is needed for an estimate of
the cost in this case.

##  Appendix A Glossary

ALP

Axion-Like Particles

AQGC

Anomalous Quartic Gauge Coupling

BBLR

Long-Range Beam-Beam compensation unit (machine acronym for “Beam Beam Long
Range”)

BIS

Beam Interlock System

BLM

Beam Loss Monitor

BPM

Beam Position Monitor

BSM

Beyond the standard model

CEP

Central Exclusive Production

CIBU

User interface to the interlock system (“Controls Interlocks Beam User”)

DCS

Detector Control System

DD

Double Diamond, i.e. double layer of diamond detectors

HPTDC

High Performance Time-to-Digital Converter

IP

Interaction Point

LSn

nth Long Shutdown of the LHC

LSS5

Long Straight Section of the LHC around IP5

LVDT

Linear Variable Differential Transformer [140]: used for an XRP position
measurement.

MIP

Minimum Ionising Particle

PPS

Precision Proton Spectrometer

Qn

Quadrupole magnet in LHC cell number n counted from the nearest IP, e.g. Q6.

RF shield

Radio Frequency shield

RR53 and RR57

Tunnel alcoves for electronics and services at about $\pm$240 m from IP5.

TAN

Neutral beam absorber (machine acronym for “Target Absorber Neutrals”)

TCL

Debris collimator (machine acronym for “Target Collimator Long”)

TCLn or TCL.n

TCL collimator in cell number n counted from the nearest IP, e.g. TCL5. In
cell 4 it is called TCLX.4.

TCLD

Machine acronym for “Target Collimator Long Dispersion suppressor”

TCT

Tertiary collimator (machine acronym for “Target Collimator Tertiary”)

UFSD

Ultra Fast Silicon Detector

USC55

Underground Service Cavern number 55 at IP5.

XRP

Roman Pot (machine acronym for “eXperiment Roman Pot”)

XRPH

Horizontal Roman Pot

XRPV

Vertical Roman Pot

##  Appendix B PPS Station Overview

###  B.1 Pre-LS3 Configuration

The PPS XRP units in the pre-LS3 configuration of Sector 5-6 are shown in Fig.
55 and listed in Table 7. The configuration of Sector 4-5 is mirror-symmetric
to the one in Sector 5-6. Not all units were used every year, and their
detector instrumentation varied.

Table 7: List of PPS XRP units in the pre-LS3 configuration of Sector 5-6. The units in Sector 4-5 have the same distances from IP5; in their names “56” is replaced with “45”. Unit | Distance from IP5 [m] | Comments
---|---|---
56-210-N-T/B | 203.377 | vertical XRP pair for calibrations
56-210-N-H | 203.826 | horizontal XRP for tracking detectors, removed in LS2
56-210-F-H | 212.551 | horizontal XRP for tracking detectors
56-210-F-T/B | 213.000 | vertical XRP pair for calibrations and TOTEM special runs
56-220-N-T/B | 214.628 | vertical XRP pair for calibrations and TOTEM special runs
56-220-N-H | 215.077 | horizontal XRP for timing detectors after LS2
56-220-C-H | 215.710 | horizontal XRP for timing detectors
56-220-F-H | 219.551 | horizontal XRP for tracking detectors
56-220-F-T/B | 220.000 | vertical XRP pair for calibrations and TOTEM special runs

Figure 55: Schematic overview of the PPS units in Sector 5-6 (outgoing Beam 1)
in the pre-LS3 configuration (top drawing adapted from Ref. [150]). The
instrumentation in Sector 4-5 (outgoing Beam 2) is mirror-symmetric.

###  B.2 HL-LHC Configuration

The PPS units tentatively foreseen for Sector 5-6 at HL-LHC are shown in Fig.
56 and listed in Table 8. The configuration of Sector 4-5 is mirror-symmetric
to the one in Sector 5-6.

Table 8: Tentative list of PPS detector vessel units in Sector 5-6 foreseen for HL-LHC. The units in Sector 4-5 have the same distances from IP5; in their names “56” is replaced with “45”. Unit | Distance from IP5 [m] | Comments
---|---|---
56-196-N-H | 192$-$198 | horizontal unit for tracking and timing detectors
56-196-F-H | 198$-$199 | horizontal unit for tracking and timing detectors
56-220-N-H | $\sim 216$ | horizontal unit for tracking and timing detectors
56-220-N-T/B | $\sim 217$ | vertical pair for calibrations
56-220-F-T/B | $\sim 219$ | vertical pair for calibrations
56-220-F-H | $\sim 220$ | horizontal unit for tracking and timing detectors
56-234-N-H | $\sim 234$ | horizontal unit for tracking and timing detectors
56-234-F-H | $\sim 237$ | horizontal unit for tracking and timing detectors
56-420-H | $\sim 420$ | horizontal unit for tracking and timing detectors

Figure 56: Schematic overview of the planned PPS stations in Sector 5-6
(outgoing Beam 1) in the HL-LHC configuration (adapted from Ref. [90]). The
instrumentation in Sector 4-5 (outgoing Beam 2) is mirror-symmetric.

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