MIP Timing Detector (MTD)

Chapter 4: A MIP Timing Detector (MTD) for CMS at HL-LHC

4.2 MIP Timing Detector (MTD)

The MTD will provide a track-time information for charged particles and is projected to have a timing resolution of 30-40 ps at the beginning of the HL-LHC, which will degrade to 50-60 ps by the end of the HL-LHC operations, because of radiation damage. The time resolution of the MTD is physics motivated. At the HL-LHC, there will be 140-200 pileup interactions with an RMS spread of 180-200 ps per bunch crossing. If one slices a bunch crossing in the temporal phase space with a 30-40 ps resolution, each snapshot in time has a reduced pileup of about ∼50 vertices, which is similar to the current LHC levels. Thus, the MTD will help maintain the current levels of particle reconstruction efficiency at the Phase-2 CMS detector.

Figure4.2shows a simplified implementation inGEANT4[93] of the proposed layout of the MTD in the CMS detector. The MTD is made of a barrel and an endcap region, with different technologies based on different requirements, for e.g., radiation dose, schedule constraints, cost effectiveness, etc. The barrel region of the MTD (described in more detail in Sec.4.2.1), will be placed in between the tracker and the ECAL and occupy the region where|𝜂|< 1.5. Since there is less time for R&D of the barrel timing layer (BTL), one needed a mature and established technology, and Silicon Photomultipliers (SiPMs) with crystal scintillators [94–96] were chosen as the photosensors. At an integrated luminosity of 3 ab⁻¹(full lifetime of HL-LHC), the BTL will have received radiation doses of up to 1.9×10¹⁴ 𝑛_{𝑒 𝑞}/𝑐𝑚2 (1 MeV neutron equivalent fluence) or 30 kGy and the endcap timing layer (ETL) will have received doses of up to 1.6×10¹⁵𝑛_{𝑒 𝑞}/𝑐𝑚²or 450 kGy. Due to the higher radiation rates, SiPMS are not a viable technology for the ETL (1.6 <|𝜂|< 3.0), and the best performance is achieved with low gain avalanche diodes or LGADs [97–99], which are silicon sensors with internal gain of about 10–30. The ETL will be placed as a disk shaped sub-detector, before the calorimeter endcaps, at a distance of 3m from the interaction point (along the beam axis). This thesis will however only focus on the BTL and more details on the ETL can found be in Ref. [92].

Extensive studies have been performed that have looked at the impact of TOF information from MTD, including the reconstruction of physics objects (like b-jets, missing transverse momentum, etc.) as well as the overall effect on the sensitivity of various physics analyses (Ref. [92]). One special physics case is the production of a pair of Higgs bosons decaying to a 4-bottom quark (𝐻 𝐻 → 𝑏 𝑏 𝑏 𝑏) final state. The CMS Run 2 search for this final state when both Higgs bosons are highly Lorentz

Figure 4.2: A schematic view of the GEANT geometry of the MTD, comprising a barrel layer (grey cylinder), at the interface between the tracker and the ECAL, and two silicon endcap (orange and light violet discs) timing layers in front of the endcap calorimeter [92].

boosted will be discussed extensively in Chapter6. Studying the HH production and measuring the Higgs trilinear self-coupling (𝜆_{𝐻 𝐻 𝐻}) is one of the main physics goals of the HL-LHC. It is projected that for 𝐻 𝐻 → 𝑏 𝑏 𝑏 𝑏, the signal yield increases by 14% due to the BTL alone, and 18% from the combined power of BTL and ETL, at a constant background rate, as shown in Fig.4.3.

4.2.1 Barrel Timing Layer (BTL)

The barrel timing layer will cover the pseudorapidity region up to |𝜂| = 1.48 with a total active surface of about 38 m². The sensor unit consists of Lutetium Yt- trium Orthosilicate crystals doped with Cerium ((Lu₁^−𝑥Y^𝑥)₂SiO₅:Ce), abbreviated as LYSO:Ce, and read out with SiPMs. LYSO:Ce crystals were chosen because of their high density (7.1 g/cm³), high light yield (∼40000 photons/MeV), fast scintil- lation rise time (< 100 ps), short decay time (∼43 ns), very high radiation tolerance (less than few percent loss in transparency over the 10 year HL-LHC operations) and also, LYSO:Ce crystals produce scintillation light at 420 nm wavelength which matches the optical range of SiPMs. SiPMs are widely used in particle physics

Figure 4.3: Projections for yield enhancement in𝐻 𝐻 → 𝑏 𝑏 𝑏 𝑏as a function of the Higgs boson rapidity, for a 200 pile-up scenario at the HL-LHC. The distributions are normalized to the no-timing case [92].

based experiments, are compact, not affected by magnetic fields, very robust (can be exposed to room light without damage), operate at relatively low voltages (30-77 V), and have a low power consumption. They also have a high photo-detection efficiency, PDE, of 20-40% in devices with small cell size (15𝜇m square pixels). In the BTL, the SiPMs will operate above their breakdown voltage in Geiger mode with a gain of the order of 10⁵. The LYSO:Ce crystals have a bar-like geometry with 57 mm length and 3.12 mm width. The radial thickness is varied along𝜂to ensure the slant depth crossed by particles coming from the interaction point remains constant, irrespective of where it hits the BTL. The slant depth varies as 3.7 mm for|𝜂|< 0.7, 3.0 mm for 0.7≤ |𝜂| ≤1.1 and 2.4 mm for |𝜂|> 1.1 . Each bar will be coupled to two SiPMs (at the two ends), whose dimensions will be 3 mm along𝜙and a variable thickness, equal to the bar’s radial thickness in each|𝜂|interval.

The time resolution per track, measured from two SiPMs at the two ends of a crystal, depends on the following uncertainties added in quadrature:

• CMS clock distribution: 15 ps

• Digitization: 7 ps

• Electronics: 8 ps

• Photo-statistics: 25–30 ps

• Noise (SiPM dark counts): negligible at startup, 50 ps after 3000 fb⁻¹

The electronics and digitization jitter have negligible impact on the overall time resolution, and the key factors affecting the time resolution are the photostatistics and the Dark Count Rate (DCR). The photostatistics term is described by

𝜎

𝑝 ℎ𝑜𝑡

𝑡 =

√︂𝜏_𝑟𝜏_𝑑 𝑁_{𝑝 ℎ𝑒}

∝

√︂ 𝜏_𝑟𝜏_𝑑

𝐸dep∗ 𝐿𝑌∗ 𝐿𝐶 𝐸∗𝑃 𝐷 𝐸 (4.1) where𝜏_𝑟 (100 ps) and𝜏_𝑑(43 ns) are the rise time and decay time of the scintillation pulse in the LYSO:Ce crystal, respectively. The energy deposited by a MIP in a thin LYSO:Ce crystal, 𝐸_{𝑑𝑒 𝑝}, follows a Landau distribution with the most probable value (MPV) of 0.86 MeV/mm. The number of photoelectrons,𝑁_{𝑝 ℎ𝑒}, scales linearly with 𝐸

dep, the crystal light yield (LY) as determined by the crystal thickness and scintillation properties, the light collection efficiency (LCE), i.e., the probability that a photon reaches the SiPM without escaping from lateral faces of the crystal or being absorbed within the material, and with the PDE of the SiPM. These parameters have driven the optimization of the sensor layout (crystal and SiPM configuration), as discussed later in Sec.4.3. The noise contribution from the DCR in the SiPM scales proportionally to √︁

𝐷𝐶 𝑅/𝑁_{𝑝 ℎ𝑒}. The magnitude of the DCR increases with integrated luminosity due to radiation damage creating defects in the silicon, increasing the probability of generating thermal electrons [100].

The BTL will be placed in the Tracker Support Tube (TST) and will share CO₂ cooling with the tracker. Figure 4.4 shows the layout of the BTL inside the TST.

72 trays consisting of LYSO:Ce+SiPM modules will be inserted into the TST, with 2 trays per 𝜙 interval (10^◦). Each tray will consist of 6 Readout Units (RU).

Each RU (see left of Fig. 4.6) has 12 detector modules, which are made of a copper housing and two BTL modules. Each BTL module (see right of Fig. 4.6) consists of an array of 16 LYSO:Ce bars with 32 SiPMs. The crystals will be wrapped with a reflective material, Enhanced Specular Reflector (ESR) Vikuiti by 3M, between adjacent channels to provide optical isolation. This reflector is chosen because it has a reflectivity for 420 nm light higher than 98.5%, and is sufficiently radiation tolerant. The BTL modules will be cooled to –35 ^◦C with CO₂ cooling and an additional –10 ^◦C with thermo-electric coolers (TECs), to limit SiPM self-heating and DCR. Additionally, annealing the irradiated SiPMs at room temperature is known to mitigate the DCR [100]. Annealing of the BTL

modules at room temperature will be scheduled during the yearly shutdowns, to help the SiPMs recover from the radiation induced defects. Fig.4.5shows the DCR rate as a function of time (increasing integrated luminosity), in different annealing scenarios [92]. Exploiting the full shutdown period (∼ 4 months) for recovery at room temperature provides a 30% reduction of DCR with respect to a two weeks only annealing scenario.

Figure 4.4: A schematic view of the structure and design of BTL [101].

Figure 4.5: Expected growth of DCR for various annealing scenarios at fixed OV of 1.5 V during the detector lifetime [92].

Each RU will consist of 4 front-end (FE) boards. Each FE board will consist of 6 ASICs, which are connected to their SiPMs via flex cables. The 4 FE boards plug into a Concentrator Card (CC), that provides low voltage power, bias voltage, and three low power Gigabit Transceivers (lpGBTs) that carry data and control signals to and from the ASIC.

Figure 4.6: Various BTL components. (Left) A BTL Readout unit, consisting of 24 BTL modules of LYSO:Ce bars + SiPMs. Each RU has a total of 768 SiPMs.

(Right) A BTL module consisting of 16 LYSO:Ce bars + 32 SiPMs. The module is connected to the FE boards using flex cables, as can be seen in the figure.

The modules will be read out with an ASIC, known as the TOFHIR (Time-of- flight, High Input Rate) chip (Ref. [102]). The TOFHIR chip is being designed to deliver precision timing information at the required rates for the BTL, and will be radiation tolerant, will satisfy the power requirements and will be able to operate at the low BTL temperatures (-35 ^◦C). It will also have a dedicated noise cancelling circuit to mitigate the effect of DCR noise in the SiPMs. Appendix B discusses the commissioning tests performed on the two preliminary versions of the BTL prototype readout electronics, i.e., the TOFPET and the TOFHIR2A ASIC.