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TOTEM

Upgrade of the TOTEM T2 TelescopeTECHNICAL DESIGN REPORT

CERN-LHCC-2019-007TOTEM-TDR-004

4 June 2019

2

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

TOTEM TDR–0044 June 2019

CERN–LHCC 2019-0074 June 2019

TDR - Upgrade of the TOTEM T2 Telescope

The TOTEM Collaboration

Abstract

A new T2 detector for the TOTEM experiment is designed to measure the rate of inelas-tic proton-proton events in low luminosity special runs dedicated to the measurement ofthe total cross section at the highest LHC energy. With a pseudorapidity coverage of5.3 < |η | < 6.5, the new T2 will detect more than 90 % of the inelastic events at a center-of-mass energy of 14 TeV and thus allow a precise inelastic rate and total cross sectionmeasurement. The corresponding elastic cross section will be used to normalize elastic dif-ferential cross section measurements at the same energy. The present TDR describes thephysics motivation, the running scenarios, the technical requirements, the electronics andreadout system as well as the construction timeline of the new T2.

Keywords: pp Total Cross Section, Detector, Tracking.

TOTEM T2 1

Contents

1 Introduction – Physics Motivation 2

2 Running Scenario 3

2.1 Radiation doses in the region of the new T2 . . . . . . . . . . . . . . . . . . . 7

3 General Technical Requirements 8

3.1 Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Detector Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3.1 Scintillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3.2 Mechanical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Electronics and Readout system 14

4.1 Near Detector Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.1 nT2 Mezzanine Board . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1.2 The Digitizer Board . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.2 Electronics in the Counting Room . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2.1 DAQ & Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 Installation in CMS 20

6 Construction: Cost, Timeline and Sharing Within the Collaboration 21

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1 Introduction – Physics Motivation

The measurement of the total proton proton (pp) cross section, the ρ parameter, the ratio ofthe real to the imaginary part of the hadronic amplitude of the elastic scattering at momentumtransfer t = 0, and the elastic differential pp cross section at the highest available center-of-mass energy

√s of 14 TeV, is of large importance for the understanding of the structure of

the proton and strong interaction at small momentum transfer. Hadronic high energy elasticscattering processes are traditionally described only by t-channel crossing-even exchange of apair (or even number) of gluons, the so-called “Pomeron”. The TOTEM experiment at the LargeHadron Collider (LHC) [1] has demonstrated a significant rise of the total cross section overa√

s range larger than 10 TeV (from 2.76 to 13 TeV) [2–5] and an apparent decrease of ρ

with increasing√

s [6] that is conveniently explained by adding a crossing-odd exchange, theso-called “Odderon”, introduced in [7, 8] and predicted in QCD as t-channel exchange in theelastic scattering of a bound state of 3 or a higher odd number of gluons bound state [9]. On thecontrary, if shown that the three-gluon bound state t-channel exchange is not of importance forthe description of elastic scattering, the decreasing ρ value would represent a first evidence of aslowing down of the total cross section growth at higher energies. Precise measurements of thetotal cross section and of the ρ parameter at 14 TeV would on one hand confirm (or contradict)the observed trends and on the other, together with the measurement of the same quantities fromthe analysis of the 900 GeV data taken in 2018, allow to quantify the observed effects over alarger energy range.

TOTEM has also demonstrated the persistence of a diffractive minimum (“dip”) in the elasticdifferential pp cross section over the whole energy range from 2.76 to 13 TeV [10, 11]. Com-bined with the absence of any dip in the elastic differential proton-antiproton pp̄ cross sectionmeasurements at the Tevatron and earlier at the Spp̄S [12], these measurements constitute ev-idence for a significant difference between the pp and pp̄ differential cross section at the TeVscale that is generally regarded as (strong) evidence of the Odderon. At the TeV energy scale,any possible other contribution by Reggeons is expected to be below the percent level [13]. Aprecise measurement of the diffractive dip and the second maximum region at 14 TeV wouldallow to better characterize the energy evolution of the differential elastic pp cross section.

The total cross section measurements [2–5] were done using the luminosity-independent methodthat requires the determination of the elastic and inelastic rates as well as the differential elasticrate extrapolated to t = 0. The resulting elastic cross sections were used to provide the normal-ization for the elastic differential cross section measurements at the same

√s. At 13 TeV with

a β ∗ = 2.5 km optics, the very low-|t| reach allowed also to determine the absolute normaliza-tion using the Coulomb amplitude with almost the same precision as the luminosity-independentmethod [6]. In any case, to ensure a reliable total cross section measurement and an independentnormalization for the elastic differential cross section measurements, a precise measurement ofthe inelastic rate is needed.

Since the pseudorapidity range of the final state particles in inelastic collisions is ∼ ln√

s/mp,corresponding to about 9.5 at

√s = 14 TeV, the central CMS coverage (|η | < 4.7) is not suffi-

cient to measure the inelastic rate efficiently enough. In particular, low-mass diffractive events

TOTEM T2 3

having all final state particles at higher |η | than the detector acceptance will be lost. The newT2 acceptance edge at |η | = 6.5 corresponds to a diffractive mass of about 4.8 GeV (at 50 %efficiency) at

√s = 14 TeV. The loss at 14 TeV with the new T2 due to low-mass diffraction, i.e.

the contribution of events with all final state particles at |η | > 6.5, is estimated to be about (7.4± 3.7) % with QGSJET-II-03 [14] using a similar procedure as in [5, 15, 16]. The uncertaintiesof low-mass diffraction are large and therefore PYTHIA8 [17], QGSJET-II-04 [18] and EPOSLHC [19] were examined to estimate the uncertainty range. Contrary to the central CMS cover-age that would only detect 80-85 % of the inelastic events at

√s = 14 TeV, the new T2 will detect

more than 90 % of the inelastic events. Therefore an inelastic detector with a coverage beyondthe central CMS coverage like the new T2 (nT2) with a pseudorapidity coverage between 5.3and 6.5 is needed for an efficient and therefore precise inelastic rate measurement.

Fig. 1: The CMS experiment longitudinal section indicating the position of the new T2 detector.

2 Running Scenario

The new T2 detector is designed to contribute a measurement of the inelastic rate to the de-termination of the total cross section at

√s = 14TeV with the luminosity-independent method,

as performed several times by TOTEM at energies from 7 to 13 TeV with the original T2 de-tector [2–5]. This method is based on the optical theorem and requires the simultaneous mea-surement of the inelastic rate and of the nuclear elastic differential rate in the nearly exponen-tial region at four-momentum transfers |t| between ∼ 10−2 and a few ∼ 10−1 GeV2. To reach

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the lower limit of this range, a special beam optics providing parallel-to-point focussing in thevertical coordinate is needed. For past measurements an optics with β ∗ = 90m has been devel-oped [20] and has proven to have adequate properties. The optics for the 14 TeV run has notyet been created, but it is expected to be the same or very similar to the ones used in the previ-ous runs. Like in the past, the data will be collected in the same special fill as the beam-basedalignment. During this alignment, the vertical Roman Pots (RPs) will touch the beam at about4.5 σ distance from the center and then be retracted by 0.5 σ for data taking in order to bein the protected shadow of the primary collimators (TCP). Due to this particularly close – andpotentially unsafe – approach to the beam, the total beam current is limited to 3×1011 protons,and the beam mode “Stable Beams” is not declared. The minimum measurable |t| value (at the50% acceptance level) resulting from these conditions is given by

|t50|=

Ksin∆µy

√εnmp

√s

β ∗+

δ√

s√2Ly

2

, (1)

where K ≈ 5 is the vertical RP distance from the beam center in beam sigmas, ∆µy ≈ π/2 is thevertical betatron phase advance, εn = 3.5 µrad is the nominal normalized transverse emittance,mp is the proton mass, and δ ≈ 0.5mm is the width of the insensitive space from the outersurface of the RP window to the efficient area of the detectors. The values of |t50| for the twotracking RP units are given in Table 1 along with the optical functions for the β ∗ = 90m versionof the year 2018, but for

√s = 14TeV in Eqn. (1).

Table 1: Vertical optical functions and minimum measurable |t| values at the tracking RP locations forthe 2018 version of the β ∗ = 90m optics, but at

√s = 14TeV, assuming a vertical RP-to-beam distance

of 5 σy.

Location σy [mm] Ly [m] vy ∆µy |t50| [GeV2]

IP5 0.213 0 1 0210-F 0.565 238.5 0.0026 0.497 π 0.019220-F 0.643 271.5 0.0000 0.500 π 0.018

Note that in the optics optimization via magnet adjustments the vertical and horizontal compo-nents of the beta function are not independent. The vertical parallel-to-point focusing condition,∆µy ≈ π/2 with maximized Ly, leads to a horizontal phase advance ∆µx ≈ π with a small Lx.Thus the hits from elastically scattered protons are concentrated in a narrow band around x = 0in the vertical detectors. The horizontal RPs are irrelevant for elastic scattering and are there-fore only temporarily inserted to collect enough tracks (from diffractive and beam backgroundevents) in the overlap zone with the vertical detectors for the purpose of the track-based precisionalignment by software.

A crucial parameter for an inelastic rate measurement is the pileup level, quantified by µ , themean number of inelastic events per bunch crossing. Given that a forward telescope like nT2 isnot able to separate multiple vertices, µ should be small, from past experience not larger than

TOTEM T2 5

0.1, in order to limit the pileup corrections in the analysis. On the other hand, if µ and hencethe instantaneous luminosity and the event rate are too small, it is difficult to control systematiceffects with the necessary precision. From past experience, µ > 0.04 is an adequate lower limit.The µ parameter can be expressed as

µ =γσin

4π· N2

εnβ ∗, (2)

where N is the number of protons per bunch, σin = 82mb is the estimated inelastic cross sectionat√

s =14 TeV, and γ =Epmp

= 7462.69 is the relativistic factor of the proton. The three parame-ters N, εn and β ∗ can be varied within certain limits to control µ . Figure 2 shows the dependenceof µ on N for different values of the emittance εn and fixed β ∗ = 90m.

Fig. 2: Inelastic pileup µ as a function of the bunch population N for different values of the emittanceεn (contour lines). The allowed region lies to the right of the red line which represents the stability limitfrom beam-beam interactions, ξbb := re

4π· N

εn< 0.01. The violet horizontal lines delimit the desired µ

range, and the vertical lines indicate a realistic range of N (see text).

A technical complication is that bunches with less than 0.5 × 1011 can be missed by an inter-locked Beam Position Monitor, which then causes a beam dump. Furthermore, the total chargeof all bunches together must not exceed 3 × 1011 for special operations without validation for

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“Stable Beams”. An optimal configuration would consist of 4 bunches of (0.6÷0.7)×1011 pro-tons at injection time and an emittance between 2.5 and 3 µm rad. The emittance is determinedat the stage of the LHC injectors; if it naturally happens to be too low (which would push µ

over the 0.1 limit), an intentional blow-up can adjust it to a higher value. An alternative strat-egy for reducing µ would be to increase β ∗. For example, a configuration with β ∗ = 120m,εn = 2 µm rad and N = (0.6÷0.7)×1011 would also yield a µ in the right range. A last tool –if needed – would be a slight beam separation.

The final operating point will be decided after more discussions with machine experts and beamtests, but there are enough adjustment handles available.

The instantaneous and integrated luminosity can be expressed by µ , without committing to aspecific configuration (β ∗,N,εn):

L =γ f4π· k N2

εnβ ∗= k f

µ

σin, (3)

where f = 11.245kHz is the LHC revolution frequency. Assuming k = 4 bunches, the resultinginstantaneous and integrated luminosity are summarized in Table 2, along with the counting ratesand event numbers. The running time assumed is 6 hours, the typical length of the data-takingtime at the end of a beam-based alignment fill. The statistics collected in that time range issufficient for track-based alignment, optics calibrations and for controlling the systematic errorsof the cross section measurements. Given that the event rates quoted in the table do no includeany backgrounds, the total rate at µ = 0.1 could already be close to the HLT limit of about 15kHz (depending on the event size). Hence an even higher pileup level has to be avoided.

Table 2: Luminosities and counting rates for two different inelastic pileup levels µ , assuming k = 4colliding bunches per beam, 100% efficiency and a running time of 6 hours. For the inelastic and elasticcross sections extrapolated values of 82 and 32 mb, respectively, have been used.

µ = 0.04 µ = 0.1

L 2.2×1028Hz/cm2 5.5×1028Hz/cm2

inelastic rate 1.8 kHz 4.5 kHzelastic rate 0.7 kHz 1.8 kHz

Lint 0.5 nb−1 1.2 nb−1

inelastic events 41 M 98 Melastic events 16 M 38 M

The scheduling of the special run for the total cross section measurement will follow the require-ment to minimize the irradiation of nT2 in regular high-luminosity fills. The strategy envisagedis to install nT2 in the last technical stop of the first year of 14 TeV operation, and then to per-form the special run. During the remaining few weeks of proton-proton operation nT2 can stayin place. Typically the year will be concluded with an ion run for which nT2 might be useful,too. Finally, during the end-of-year technical stop nT2 will be uninstalled. This strategy also

TOTEM T2 7

minimizes the number of dedicated CMS magnet ramp-downs, a requirement imposed by mag-net preservation arguments. In this case only one extra ramp-down would be needed: the onefor nT2 installation in the technical stop.

2.1 Radiation doses in the region of the new T2

A specific calculation for the dose of radiation in the region of T2 has been performed by theCMS BRIL for the vacuum pipe configuration in the CMS forward region during Run 3 in 2021.The calculation is tailored to the foreseen run scenarios, which foresees the installation in CMSof the nT2 for a short period of time to take data during a special run for the total cross sectionmeasurement at low luminosity. The absorbed dose on the CASTOR table at 15m in the vacuumchamber configuration of the last run in 2018 had been extensively calculated. The presentsimulation considers the new vacuum pipe (v.5.4.1) that is being installed for the future runs.

The plot in Fig. 3 shows the distributions of the expected absorbed dose as a function of thedistance from the beam axis for the two vacuum pipe scenarios in the nT2 region (z=15 mfrom IP5) for p-p collisions at 7 TeV and for an integrated luminosity of 2.6 nb−1, that is foran uninterrupted beam presence of 3 days (2.592 × 105 s) at a low luminosity L = 1.0 ×1028 cm−2s−1.

The transport of the secondary particles from p-p collisions in the CMS experimental cavernis simulated with the FLUKA code using the parameters listed in Table 3. The CMS FLUKAgeometry models v.3.26.1.3 and v.3.7.2.30 are used. The value of the inelastic cross section usedfor normalization is 80 mb.

We are considering scintillator material provided either by BICRON or Protvino, that have beenmeasured to undergo a transmission loss of less than ≈ 10 % after having been irradiated by a

Fig. 3: Radial distribution of the expected absorbed dose in the T2 region.

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Table 3: Parameters used in FLUKA for the nT2 dose simulation.Units p-p quantities

nucleon energy E TeV/n 7CM energy

√sNN TeV 14

Crossing Angle µrad ±295Vertex Spread (along z) cm 5Luminosity Lo cm−2 s−1 1.0 × 1028

Irradiation Time Tirr s 2.592 × 105

Integrated Luminosity Lint nb−1 2.6Inelastic cross section σinel mb 80Rate R = σinel · L s−1 8 × 102

Nint pp int. 2.1 × 108

dose of 8 MGy [21]. It can be seen that the dose absorbed during one week of special run isquite small even at the minimum radius of the detector, and guarantees that a scintillator-basednT2 is able to perform the measurement.

3 General Technical Requirements

The new T2 (nT2) telescope detector is designed to run only in the low luminosity special runsdedicated to proton-proton total cross section measurement.

The new T2 telescopes, as the original ones, will be installed in the CMS Forward zone sym-metrically on both sides of the interaction point IP5 at a distance of 15 m (see Fig. 1). It willdo measurements in the pseudorapidity range η = 5.3− 6.5. Two identical telescopes will beplaced on each side of the CMS detector using the existing CASTOR tables as their main sup-port. Like the original T2, the new telescope is structurally divided into two complementaryhalves, called “T2 quarters”, to make possible a fast installation around the beam pipe on twosliding rails. A schematic representation is shown Fig. 4

One “T2 quarter” is the basic unit of the telescopes and, once assembled in the aluminum frameto support and position its 16 sectors organized in 4 planes, becomes a single detector togetherwith its fiber bundle.

Since the luminosity for these special runs will be low we don’t have special radiation hardnessconstraints (see section 2.1 for a discussion of the radiation dose in the region of the nT2). In-stead one of the most important requirements to allow installation (or removal) of the nT2 before(after) a special run is the fast installation (dis-installation) time. To fulfill this requirement were-designed a passive detector, made with plastic scintillators and optical fiber light guides.

The detector can be installed in a very short time without the need for special cranes. The fiberswill be routed for a couple of meters outside the CASTOR table on the legs of the same where aninterface board containing the light sensors will connect via the existing cables to the standardTotem R/O electronics.

TOTEM T2 9

Fig. 4: The layout of the nT2 scintillators around the beam pipe. A single plane is shown for clarity onthe left.

3.1 Location

The T2 telescope is mounted with sliding supports on the CASTOR tables present on each sideof CMS (see Fig. 5).

The installation and removal of the detector will require access to the CASTOR table, whichimplies switching off the CMS magnet, removal of Collar shield and the opening of the thinsection of the CMS Rotating Shielding.

The sliding supports, a solution that has already been used in the past, allow for a reliable, preciseand quick installation and removal of the detector. The support structure of the telescope is madeof standard mechanical camponents. The nT2 quarters are installed and adjusted separately.

An nT2 quarter complete with its light aluminum frame and the 16 scintillator segments weighsabout 5 kg and can be handled easily during the installation.

3.2 Geometry

Dimensionally the detectors fits the CMS geometry in the z = 15 m zone. At this location thebeam pipe diameter is 78 mm, and the sensitive part of the detectors will begin at approximately45 mm from the LHC circulating beam. The envelope of the plastic scintillator detector planeshas an inner diameter of 90 mm, leaving sufficient clearance for the beam pipe: the nominalclearance is 6 mm, and a “no-go zone” of 5 mm is added to take into account tolerances andpossible movements, as was done for the old T2-GEM detector. The outer diameter of thedetector is 600 mm, determined by the internal dimension of the existing CMS collar protection.The total length of the telescope along z is around 200 mm assuming 2 cm thick scintillators.

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3.3 Detector Description

Four planes of segmented plastic scintillator counters compose each of the two symmetric nT2telescopes. The light will be measured by SiPm or multi-anode photomultipliers outside theCMS Rotating Shielding in an easily accessible region less irradiated than the one around thebeam pipe, this will permit, if necessary, easy checks or servicing.

Each plane is made up of eight counters (segments) with small overlaps of the scintillators inthe plane for hermeticity. In this way the telescope will be sensitive over the full 2π azimuthalangle around the beam pipe (see Fig. 5).

Light from each segment, after the operation of a light signal wave length shifter, is routed vialarge diameter (1mm), large aperture clear optical fibers to a region with negligible magneticfield where the light sensors and the front end electronics are located.

The light fibers of the telescope are read by multi-pixel SiPm. The total number of channels tobe read out is 64, 32 for each CMS side.

3.3.1 Scintillators

For each T2 quarter there are four half planes and four scintillators for half plane, totaling six-teen counters. Each sector is a 2cm thick plastic scintillator of appropriate geometry. A greenwavelength shifter (WLS) bar positioned on the outside rim of the sector collects the light fromthe scintillator.

Fig. 5: Detailed view of the location of the new T2 telescope.

TOTEM T2 11

Fig. 6: The nT2 telescope in the CMS forward shielding.

Fig. 7: Dimensions (envelope) for the nT2 telescope.

The WLS light is sent via a clear fibers 1 mm in diameter to the SiPm a couple of metersaway. The fibers from each counter are routed to become a fiber bundle that ends in a specialmechanical interface that positions the bundle precisely on the SiPm.

A sketch of a counter with WLS and fiber is shown in Fig. 8, and Fig. 9 show a picture of thefirst prototype.

A special LED diode is added to the SiPm mechanical interface in order to have a calibration

12

Fig. 8: A scintillator sector, including the WLS layer and the clear optical fibers.

Fig. 9: Photo of the protoype of a scintillator for the nT2 complete with the WLS bar, the fibers and thePM adapter.

TOTEM T2 13

source for the readout electronics. The fibers arrive at Patch Panels fixed at the bottom of theCASTOR table, in a non-magnetic and low radiation zone.

The loss of light due to the WLS and the attenuation in the fiber to the SiPm, the precise countergeometry and the most efficient position for the WLS will be measured on test beams before thefinal design. A prototype has been built (see Fig. 9 ) and is now under characterization studies.Test beams for the optimization of the scintillator light collection geometry and fiber routing arealso foreseen in 2019 and 2020 outside CERN .

3.3.2 Mechanical structure

The nT2 telescope is structurally divided into two complementary halves, called “T2 quarters”.Each T2-quarter consists of four identical half-planes of scintillators covering slightly more than180 Degree in the azimuthal angle ϕ (in what follows they will be called simply “planes”) Fig. 10shows different perspective views of the telescope.

Four trapezoidal plastic scintillators with a thickness of 20 mm, are assembled together in a nT2quarter with an overlap of 10 mm in order to exclude ”dead zones” between the scintillators. Inthe final (closed for run) position, the two nT2-quarters will also have an overlap of 10 mm.

Fig. 10: Schematic drawings with the views of the support for the nT2 telescope.

The distance along the beam between the planes is of 55 mm and the corresponding 4 crystalsare aligned and fixed on a longitudinal spacer bar structure, which in turn is fixed on two semi-

14

annular plates at each end that define the front and back reference plane for the telescope at amutual distance of 200 mm. Each trapezoidal crystal is clamped at two corners to the aluminumframe, in a way that allows easy and independent access for replacement or maintenance to eachscintillator without affecting the other ones.

Spatial positioning for the lightweight aluminum bars is provided by the two half-rings at bothends. These half-ring frames of a T2-quarter are then mounted on a base plate, the interfacewith a previously used and standard rail mechanism which allow easy and precise closing andopening of the quarters around the beam pipe.

In order to minimize the installation time in the radiation zone and ensure good precision, thedesign foresees a precise fastening to the rails with two positioning pins and two screws. Fouradjustment screws on the base plate are used to pre-align with the Beam Pipe during the instal-lation tests foreseen at the end of LTS2 and when CMS has been reassembled to its data takingconfiguration.

Optical fibers from scintillators are threaded inside flexible protection ducts fixed on the alu-minum frame, to prevent any damage to the fibers during installation and maintenance.

Assembly of and fiber routing around the nT2-quarters will be first checked in the lab with themock-up model of the CASTOR table at present under construction. Given their limited weightthe nT2-quarters are installed in CMS by hand without using special lifting devices.

4 Electronics and Readout system

The electronics for the new T2 telescope is located both near the detector in UXC (SiPm mez-zanine and digitizer board) and in the counting room on level S2 of the USC. The descriptionthat follows is logically separated into two parts according to the physical location (see Fig. 11).Since the telescopes on each side are identical the same electronics is foreseen for both sides.

The new T2 detector will partially use the existing infrastructure from the old T2-GEM removedfrom CMS at the end of Run2 in 2017, namely the LV, HV, DCS/DSS and signal cables in thealready available cable chains. Connection of the different parts of the electronics and of allthe services (cables, fibers, etc.) is made via existing Patch Panels in the experimental cavernand the counting room. The signals between the two levels are sent via the existing fiber opticsbundles.

Low Voltage power is provided through the existing Maraton power supplies and the DCS inter-face will be updated for the new electronics.

4.1 Near Detector Electronics

The design of the front end electronics follows the existing old T2 electronics, and the electronicsdeveloped for the CT-PPS Roman pots readout. It will be assembled using system componentsalready developed.

The “Digitizer Board” is the main component of the electronic system and can be fitted with

TOTEM T2 15

Fig. 11: Schematics of the R/O Electronics with indication of the various Components.

several mezzanine modules grouped in two main types: the first deals with input data and trans-lation, and the second with output-data packing, transfer and tests.

The new mezzanine board receives the light signals from the scintillator detectors and performslight conversion. Then the digitizer transfers the resulting digitization in optical format to thecounting room.

It also distributes control information and collects the different board parameters. A picture ofan existing board is shown in Fig.13.

4.1.1 nT2 Mezzanine Board

To readout the nT2 detectors, a new “Mezzanine Board” (nMB) to fit the Digitizer Board isbeing developed. A block diagram of the board is reproduced in Fig. 12.

Two 16 channel SiPm (or alternatively one SiPm with 32 channels) and an HV DCDC converterare present on the nMB. The photosensors that will be mounted on the mezzanine are the socketmounted 16 channel Hamamatsu S13361 - 6050.

The fiber bundles from the T2 quarter carrying the light signals from the scintillator sectors will

16

Fig. 12: Block diagram of the new Mezzanine Board.

be connected directly through the plastic mechanical interface described previously to the SiPMinstalled on the mezzanine. On the same mezzanine board NINO discriminators will analyze thesignals from the SiPM.

Ancillary electronics as DACs and drivers will be interfaced with control electronics throughI2C serial links. The mezzanine is connected to a J8 connector on the Digitizer Board (DB) andtransfers the data from the detector channels to the FPGA in LVDS format. The MB has thefollowing parameters and functionalities:

4.1.2 The Digitizer Board

The TOTEM-PPS Digitizer board is an interface card hosting a Microsemi SmartFusion2 M2S150-FC1152 FPGA and implements the standard CMS slow control and fast command chain.

The present Digitizer board has two connectors for external inputs meant for users’ mezzaninecards.

Once the signals from the T2 mezzanine are digitized, they are coded in a 32 bit word andsent, with the standard CMS 40 Mhz clock synchronization, to the CMS and TOTEM dataacquisition systems through two Gigabit Optical Hybrids (GOH) optical or four Pixel OpticalHybrids (POH) data links. The GOH hybrid is a multi-protocol high-speed transmitter able towithstand high doses of radiation, supports two standard transfer protocols, G-Link and Gbit-Ethernet, and sustains transmission of data at both 800 Mbit/s and 1.6 Gbit/s. For tests in the labor on a test beam, a QuickUSB mezzanine can be used. The firmware will be adapted to prepare aDAQ Frame to be read at any L1 accept strobe, while the trigger path will run in parallel sendingat each clock cycle the trigger 16 bit trigger pattern to the TOTEM central trigger FPGA.

The Digitizer board is part of the token ring generated by the DOHM mezzanine and communi-cates to the central CMS control FEC. The DOHM is the one from the old T2 electronics. Thecontrol software for the FEC and DOHM control was already developed for the CT-PPS project.

Here is a detailed description of the basic building blocks of the board:

TOTEM T2 17

Fig. 13: Photo of an existing Digitizer Board.

Fig. 14: The near detector electronics (left) and a photo of an existing housing (right).

– FPGA – based on a radiation-hard SmartFusion2 ASIC from Microsemi Co. It serves as abridge between the input data stream and output data transfer section of the board. It alsoserves as event builder according to the specification. Another function is to configure theHPTDC registers using an I2C interface;

– Control Logic Block – based on a CCUM mezzanine. This module is connected to thecontrol loop via two 20 pin 3M high speed connectors placed on the front panel. Thecontrol logic block provides 16 I2C interface channels and one 8 bit parallel control port.This block provides also for the remote programming and configuration of the FPGA onthe board;

– Clock and Commands Distribution Block based on the PLL25 chip, QPLL, and additionalcircuitry for clocks and command distribution; It delivers a synchronous clock signal andcommands to every component on the board including the onboard FPGA;

– Data Transfer Block; data transfer is done via Gigabit Optical Hybrid (GOH) or PixelOptical Hybrid (POH) modules with speeds up to 800 Mb/sec. The data transfer betweenthe detector area electronics and the counting room electronics located 300m away is donevia optical fibers;

18

– Power Supply Block provides the necessary power supply voltages. Several LHC4913LHC rad-hard power regulator designed for the LHC are used.

The firmware for the FPGA is built with Libero SoC 11.7 tools as a hierarchical structure usingVHDL and Verilog programming languages. The firmware is devised around two functionalunits: the control loop and the data path. The control loop implements an I2C slave, which isconnected to the I2C lines of the CCU. The I2C communication is used to set the configurationand control the Digital Readout Board operation. The data path block formats and packs the dataand then transfer them to the GOH sender block. A last block calculates the Cyclic RedundancyCheck (CRC) of the complete frame, serializes it and sends it through physical GOH device at40 MHz.

Due to the radiation environment in which the Digital Readout Board will be installed we are de-veloping new firmware to mitigate the impact of single event upsets by detecting and correctingerrors.

Fig. 15: The Front-End Driver and Readout crate.

The Digitizer Board with the SiPM mezzanine and the “Digital Opto-Hybrid Module” (DOHM),needed for control, are contained in a 6U crate (475 x 177 x 228 mm), placed physically near thethe CASTOR table. Since the box is placed outside the Rotating Shielding and the Collar it willbe easily accessible. The DOHM board is housed in the same box and connected with cables tothe Digitizer board. The two panels on the box are the connection patch panels. Componentsand a photo are shown in Fig. 14

4.2 Electronics in the Counting Room

The Front-End Driver (FED) is the general electronic board in the counting room. It is locatedin the readout crate in USC S2. One FED is sufficient to read both sides of the new T2 de-tector. The FED connects the Front End Electronics (FE) and Front-End Readout Links (FRL)system components. Each of these FRLs is capable of taking a maximum average data rate of200MByte/s or 2kByte/event at a maximum L1 trigger rate of 100 kHz.

The FED is designed in modular form and may house a set of mezzanine cards plugged onto

TOTEM T2 19

a main (host) motherboard. The optical components are mounted specifically on mezzaninecards, so that they can be tested separately and preserved if the motherboard has to be replaced.Depending on need FED motherboards can be equipped with a variable number of mezzanines,also of different kinds. In particular, for some applications it is possible to associate the samenumber of incoming fibers to only one FRL using a mezzanine card which performs advancedzero suppression and pedestal subtraction to reduce the amount of data transmitted. For theTOTEM application the incoming data is distributed over three FRLs. The board operates in aVME environment but is also equipped with USB ports to allow standalone operation. A pictureof the FED and Readout crate are shown in Fig. 15.

4.2.1 DAQ & Trigger

The data flow for the new telescope follows the standard Totem data path through the GOHhybrids and OptoRX receivers in the Totem DAQ. The Totem DAQ is already interfaced to theCMS DAQ and doesn‘t require any special development. The DAQ FED is in the SRS crate andis already integrated in the CSM DAQ system.

Fig. 16: Schematic representation of the Electronic chain that shows the two quarters and the communi-cation with the Counting room.

The existing Totem trigger combines the Roman pots information with the T2 inelastic informa-tion. The early trigger decision is based on the coincidence of at least 2 or 3 of 4 correspondingsectors of the new T2. The bits generated in the FPGA firmware are assembled in a word andsent via a second GOH to the Totem trigger board in the counting room. This existing systemwas already developed for the previous GEM based inelastic T2 detector. The firmware will beupdated for the new bit format.

The current Trigger, developed in the past for Totem special runs, is integrated in the CMS L1and provides 4 bits that will be included in the L1 trigger menu.

A block diagram of the DAQ and Trigger flow is reproduced in Fig. 16.

20

5 Installation in CMS

Before the nT2 telescope will be installed on the CMS CASTOR table. Here we describe all thesteps necessary to have a tested and approved procedure for the quick installation of the new T2.

Installation/removal

position

Run configuration

Installation/removal

position

Run configuration

Fig. 17: The new T2 in the open and closed positions for installation and run respectively.

All the preliminary activities and tests will be performed on HF and the CASTOR tables in thealcoves at ground level. For reasons dictated by radio-protection this will be scheduled only inthe last part of LS2.

This preliminary activity allows to test the entire installation procedure and the safety clearances.

Preliminary operations includes:

1. the rails and the movable plates of the support will be mounted on the CASTOR table.

2. To check all the mechanical details, the support will first be tested with the mock-up modelof the nT2 that is now under construction.

TOTEM T2 21

3. The same mock-up will be later used, at the end of LS2 just before the beginning of therun period, to measure and adjust the exact position of T2 relative to the Beam pipe (CT2).

The nT2 halves have been assembled as a single unit with optical fibers and tested.

The final installation of the detector requires that the CMS solenoid be switched off and theRotating Shielding be open, the CASTOR table still being accessible with HF closed.

The radiation in the region where nT2 will be installed is evaluated to be about 40 µSv/h andthe entire installation procedure of nT2 has been studied to be safely performed by two peoplein less than one half hour.

In itself the installation procedure, described here for one side, requires in sequence the follow-ing operations (see Fig. 17):

1. while in the ‘Installation Open’ condition, place and fasten each T2 half on the movablesupport plates.

2. then one at the time move each T2 half to the final position around the beam pipe.

3. the optical fibers of each half are then positioned in the vertical slot in the CASTOR tableand the optical connectors are connected to the R/O board mezzanine installed below theCASTOR table in the low radiation and magnetic field region.

The procedure is the same for both CMS Ends. Once the detector is installed and after electricaland electronics test the closing of the shield may restart.

The time necessary for the installation, connections and electrical and R/O checks for the in-stallation of the nT2 is evaluated to 2 days. The opening or closing of the Rotating Shieldingrequires approximately 1 day. Hence the work needed for the installation of the new T2 can beperformed during one technical stop to be programmed just before the special low luminosityrun.

6 Construction: Cost, Timeline and Sharing Within the Collaboration

The scintillators are produced and tested in Helsinki, then shipped to CERN for the final assem-bly and installation. The electronics will be assembled and tested at CERN.

Table 4 gives the detail of the new costs for the nT2 and the sharing of the responsibilities forbuilding the new components.

Table 5summarizes the FTEs necessary for the construction, test and installation of the nT2.

Fig. 18 details the construction, tests and installation time required for the nT2.

22

Table 4: Cost breakdown and sharing for the nT2 detector.

Item Cost (CHF) provided by

Mechanical supports in CMS 2,500 CERNDetector mechanics (4 halves) 5,000 CERNPhotosensors 30,000 HelsinkiNew mezzanine board 7,000 INFNDigitizer Boards 18,000 INFNScintillators 12,000 HelsinkiWLS, optical fibers, PM adapter 16,000 HelsinkiTOTAL 90,500

Table 5: FTE: construction, test and installation of the nT2.

Institute FTEINFN 2.5CERN 1.5Helsinki 2.5Total 6.5

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References

[1] G. Anelli et al. (TOTEM). ‘The TOTEM Experiment at the CERN Large Hadron Collider’.JINST, 3:S08007, 2008. doi:10.1088/1748-0221/3/08/S08007.

[2] G. Antchev et al. (TOTEM). ‘Luminosity-independent measurements of total, elastic andinelastic cross-sections at

√s = 7 TeV’. Europhys.Lett., 101:21004, 2013. doi:10.1209/

0295-5075/101/21004.[3] G. Antchev et al. (TOTEM). ‘Evidence for non-exponential elastic proton–proton differ-

ential cross-section at low |t| and√

s=8 TeV by TOTEM’. Nucl. Phys., B899:527, 2015.doi:10.1016/j.nuclphysb.2015.08.010. 1503.08111.

[4] G. Antchev et al. (TOTEM). ‘Measurement of elastic pp scattering at√

s = 8 TeV inthe Coulomb–nuclear interference region: determination of the ρ -parameter and the totalcross-section’. Eur. Phys. J., C76(12):661, 2016. doi:10.1140/epjc/s10052-016-4399-8.1610.00603.

[5] G. Antchev et al. ‘First measurement of elastic, inelastic and total cross-section at√s=13 TeV by TOTEM and overview of cross-section data at LHC energies’. The Eu-

ropean Physical Journal C, 79(2):103, Feb 2019. ISSN 1434-6052. doi:10.1140/epjc/s10052-019-6567-0. URL https://doi.org/10.1140/epjc/s10052-019-6567-0.

[6] G. Antchev et al. (TOTEM). ‘First determination of the ρ parameter at√

s= 13 TeV –probing the existence of a colourless three-gluon bound state’. 2017. CERN-EP-2017-335v3 (submitted to Eur. Phys. J. C), 1812.04732.

[7] L. Lukaszuk and B. Nicolescu. ‘A Possible interpretation of p p rising total cross-sections’.Lett. Nuovo Cim., 8:405, 1973. doi:10.1007/BF02824484.

[8] P. Gauron, L. Lukaszuk and B. Nicolescu. ‘Consistency of the maximal odderon approachwith the QFT constraints’. Phys. Lett., B294:298, 1992. doi:10.1016/0370-2693(92)90698-4.

[9] J. Bartels, L. N. Lipatov and G. P. Vacca. ‘A New odderon solution in perturbative QCD’.Phys. Lett., B477:178, 2000. doi:10.1016/S0370-2693(00)00221-5. hep-ph/9912423.

[10] G. Antchev et al. (TOTEM). ‘Elastic differential cross-section dσ/dt at√

s =2.76 TeV andimplications on the existence of a colourless 3-gluon bound state’. 2018. CERN-EP-2018-341 (to be submitted to Eur. Phys. J. C), 1812.08610.

[11] G. Antchev et al. (TOTEM). ‘Elastic differential cross-section measurement at√

s = 13TeV by TOTEM’. (arXiv:1812.08283), Dec 2018. CERN-EP-2018-338 (to be submittedto Eur. Phys. J. C), URL https://cds.cern.ch/record/2651186.

[12] V. M. Abazov et al. (D0). ‘Measurement of the differential cross section dσ/dt in elasticpp̄ scattering at

√s = 1.96 TeV’. Phys. Rev., D86:012009, 2012. doi:10.1103/PhysRevD.

86.012009. 1206.0687.[13] L. Jenkovszky, I. Szanyi and C.-I. Tan. ‘Shape of Proton and the Pion Cloud’. Eur. Phys.

J., A54(7):116, 2018. doi:10.1140/epja/i2018-12567-5. 1710.10594.[14] J. Milke et al. ‘Investigation of hadronic interaction models with the KASCADE experi-

ment’. Nucl. Phys. Proc. Suppl., 151:469, 2006. doi:10.1016/j.nuclphysbps.2005.07.081.[,469(2006)].

TOTEM T2 25

[15] G. Antchev et al. (TOTEM). ‘Measurement of proton-proton inelastic scattering cross-section at

√s = 7 TeV’. Europhys.Lett., 101:21003, 2013. doi:10.1209/0295-5075/101/

21003. URL http://inspirehep.net/record/1220863.[16] G. Antchev et al. (TOTEM). ‘Luminosity-independent measurement of the proton-proton

total cross section at√

s = 8 TeV’. Phys. Rev. Lett., 111:012001, Jul 2013. doi:10.1103/PhysRevLett.111.012001. URL http://link.aps.org/doi/10.1103/PhysRevLett.

111.012001.[17] R. Corke and T. Sjostrand. ‘Interleaved Parton Showers and Tuning Prospects’. JHEP,

03:032, 2011. doi:10.1007/JHEP03(2011)032. 1011.1759.[18] S. Ostapchenko. ‘QGSJET-II: physics, recent improvements, and results for air showers’.

EPJ Web Conf., 52:02001, 2013. doi:10.1051/epjconf/20125202001.[19] T. Pierog et al. ‘EPOS LHC: Test of collective hadronization with data measured at the

CERN Large Hadron Collider’. Phys. Rev., C92(3):034906, 2015. doi:10.1103/PhysRevC.92.034906. 1306.0121.

[20] H. Burkhardt. ‘High-beta optics and running prospects’. Instruments, 3(1), 2019. ISSN2410-390X. doi:10.3390/instruments3010022. And references therein., URL http://

www.mdpi.com/2410-390X/3/1/22.[21] S. Liao et al. ‘A comparative study of the radiation hardness of plastic scintillators for the

upgrade of the tile calorimeter of the ATLAS detector’. Journal of Physics: ConferenceSeries, 645:012021, oct 2015. doi:10.1088/1742-6596/645/1/012021. URL https://

doi.org/10.1088%2F1742-6596%2F645%2F1%2F012021.

TOTEM-2014-005

CERN-LHCC-2019-007 ; TOTEM-TDR-004-ADD-19 September 2019

Addendum to the TOTEM TDR:Upgrade of the TOTEM T2 Telescope.

LHCC document CERN-LHCC-2019-007

including questions/answers from/to the referees

The TOTEM Collaboration

Abstract

This document details the answer to the questions and observation raised by the refereesafter the LHCC open presentation of the TOTEM T2 Telescope Upgrade TDR.

1

ContentsIntroduction: referees’ document 3

Question & Answer n. 1 5

Question & Answer n. 2 5

Question & Answer n. 3 5

Question & Answer n. 4 6

Question & Answer n. 5 6

Question & Answer n. 6 6

Question & Answer n. 7 6

Question & Answer n. 8 6

Question & Answer n. 9 7

Question & Answer n. 10 7

Question & Answer n. 11 7

Question & Answer n. 12 7

Question & Answer n. 13 8

Question & Answer n. 14 8

2

Introduction: referees’ documentThis is the document containing the questions and comments made by the TOTEM referees tothe TOTEM T2 TDR that were received by TOTEM on the 227th of August 2019:

27 August 2019

LHCC referees’ comments/questions on the TOTEM T2 Telescope Upgrade TDR.M.L. Mangano.

We think this is important for LHC proton cross section measurement at 14

TeV... We would like TOTEM to address the following points:

1. Page 5. The distance of the RP from the beam is about 5 sigma. What

is the expected background rate at such distance?

2. Page 6-7 To which extent have the running conditions (optics, luminosity,

bunch structure, RP positioning, etc.) been validated by the LHC experts?

Is there some � even informal � document confirming the feasibility

of the running scheme proposed in the TDR? Are there new challenges

w.r.t. the experience with the 13 TeV runs?

3. Page 7. The total rate at mu=0.1 can be close to HLT limit. What limits

the HLT? Can the limit be increased?

4. Page 7. Would the decision of the final operating point impact the

detector, in particular the electronics and DAQ?

5. Page 11. what are requirements for the scintillation counters (light

output, non-uniformity)?

6. Page 14: there is overlap between the scintillators to avoid dead zones.

How well does the overlap need to be known for the analysis? Is that

implemented in the simulation and reconstruction?

7. Page 15, What accuracy of the counter installation is necessary? How

is it supposed to be checked?

8. Page 15. Old optical cables are going to be used. Is any degradation

of the existing cables observed? Do you have enough spare cables?

9. Page 16 You consider the options of 16 and 32 channels SiPM. When is

it supposed to be decided?

10. Page 20. The nT2 signals fit to the current DAQ. Are there any limitations

from the present DAQ?

11. Section 5 What is the status of the various formal documents that may

need to be approved to enable the installation? We learned previously

that CMS has internally approved the plan. What about the approval

by radioprotection, etc? Any major pending item?

3

12. Page 22, item 3. What is the precision with which the alignment is

known and is it going to change when nT2 is really installed?

13. Page 23 I was wondering whether the items are shipped to the different

institutes, go through some initial checks and then parts of the detectors

are built, or it is all shipped to CERN and then built there

14. Page 24 is there any table with the FTEs?

4

Question & Answer n. 1

1. Page 5. The distance of the RP from the beam is about 5 sigma. What

is the expected background rate at such distance?

Based on the experience of a similar run at β ∗ = 90 m at 13 TeV in October 2015 with theRPs at 5σ from the beam, the background rates during data taking just after RP beam-basedalignment were low and the data taking was smooth. We see no reason to expect that runningat 14 TeV in similar conditions would be significantly different. After the elastic selection cuts,collinearity of outgoing protons in the two arms, the suppression of the diffractive events andthe equality of the horizontal vertex position at the IP (x∗) reconstructed from the left and rightarms, the background contamination in the selected elastic sample is expected to be less than0.1% (see Ref. [1]).

Question & Answer n. 2

2. Page 6-7 To which extent have the running conditions (optics, luminosity,

bunch structure, RP positioning, etc.) been validated by the LHC experts?

Is there some � even informal � document confirming the feasibility of the

running scheme proposed in the TDR? Are there new challenges w.r.t. the

experience with the 13 TeV runs?

The running conditions of the β ∗ = 90m run described in the present TDR, to be taken atvery close RP approach distance right after the beam-based alignment, will be very similar tothe earlier ones at lower energies (7, 8, 13 TeV) carried out since 2010. The tight collimatorhierarchy where the vertical RPs are placed only 0.5 σ behind the primary collimators (TCPs)has become one of the standard configurations in special fills with setup-beam intensity. Thestrategy for optics and beam properties have been informally discussed with Helmut Burkhardt.While no particular complications are expected, the optics details still need to be developedand a few beam commissioning fills will be required.

Question & Answer n. 3

3. Page 7. The total rate at µ = 0.1 can be close to HLT limit. What limits

the HLT? Can the limit be increased?

The limit is intrinsic to the capability of the CMS HLT, Totem is sending a trigger rate toHLT that unfiltered passes to the data writing. In practice, the HLT limit will not be a problemduring data taking, since the rate sent by the TOTEM trigger can be adjusted to be below theHLT limit by limiting the number of colliding bunches per beam that are triggered on. The totalcross-section measurement is typically made using the data from only one colliding bunch pairand the other colliding bunch pairs are essentially used for cross-checks of the analysis and itsresult.

5

Question & Answer n. 44. Page 7. Would the decision on the final operating point (of LHC?) impact

the detector, in particular the electronics and DAQ?

No, everything can tolerate larger rates than are expected for the total cross section mea-surement.

Question & Answer n. 55. Page 11. what are requirements for the scintillation counters (light

output, non-uniformity)?

The most important quantity is the number of photo electrons to the input of the PM whichis evaluated to be≈ 30. Given the number of planes in the telescope, the dis-uniformity that canbe tolerated can be up to 20 %. This in any case will be measured (mapped) before assembly,

Question & Answer n. 66. page 14. There is overlap between the scintillators to avoid dead zones.

How well does the overlap need to be known for the analysis? Is that implemented

in the simulation and reconstruction?

The overlap of 10 mm between the scintillators in the plane is controlled with very goodprecision (≈ 0.2mm) by the mechanical assembly and does not have any sizable effect on thedetection efficiency.

Question & Answer n. 77. Page 15, What accuracy of the counter installation is necessary? How

is it supposed to be checked?

The main critical point is the beam pipe distance from the active part of the telescope whichwill be easily maintained with an accuracy below 1mm. In particular the present installationforeseen on the Castor table allows a better control of the detector position when compared tothe old T2 mechanics.

Question & Answer n. 88. Page 15. Old optical cables are going to be used. Is any degradation

of the existing cables observed? Do you have enough spare cables?

6

In the check on the fibers done before removal all where performing well above the re-quested minimum level. The degradation observed was negligible. Since we have many sparesfibers the best performing ones will be connected.

Question & Answer n. 9

9. Page 16 You consider the options of 16 and 32 channels SiPM. When is

it supposed to be decided?

Present considerations bring us to consider the 16 channels option. We are designing thenew mezzanine with this option.

Question & Answer n. 10

10. Page 20. The nT2 signals fit to the current DAQ. Are there any limitations

from the present DAQ?

Not at all, the requirements are much less stringent for the nT2 than in the past.

Question & Answer n. 11

11. Section 5 What is the status of the various formal documents that may

need to be approved to enable the installation? We learned previously that

CMS has internally approved the plan. What about the approval by radio-protection,

etc? Any major pending item?

When all the details of the nT2 will be finalized an EDR will be held in collaboration withCMS, as it is done for any detector installed in CMS.

The ALARA principle had been considered in the present design: for example, the old T2required a larger (10 times more) removal time. RP requirements and other issues will be alsoaddressed during the Engineering Design Review.

Question & Answer n. 12

12. page 22, item 3. What is the precision with which the alignment is

known and is it going to change when nT2 is really installed?

The precision of the position will be stable and better than 1 mm. This will be checkedpractically before the end of the long Technical stop with the foreseen installation tests in theforward region of CMS.

7

Question & Answer n. 1313. page 23 I was wondering whether the items are shipped to the different

institutes, go through some initial checks and then parts of the detectors

are built, or it is all shipped to CERN and then built there?

The scintillators are produced and tested in Helsinki, then shipped to CERN for the finalassembly and installation. The electronics will be assembled and tested at CERN.

Question & Answer n. 1414. page 24 is there any table with the FTEs?

Here the table of FTEs necessary for the construction, test and installation of the nT2.

Table 1: FTE: construction, test and installation of the nT2.Institute FTEINFN 2.5CERN 1.5Helsinki 2.5Total 6.5

References[1] G. Antchev et al. (TOTEM). ‘First measurement of elastic, inelastic and total cross-section

at√

s = 13 TeV by TOTEM and overview of cross-section data at LHC energies’. Eur.Phys. J., C79(2):103, 2019. doi:10.1140/epjc/s10052-019-6567-0. 1712.06153.

8