CMS Upgrades for the High-Luminosity LHC Era

Thiago R. F. P. Tomei

CMS Upgrades for the High-Luminosity LHC Era

T0 review · 0 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read This paper describes the CMS Phase-2 upgrade and claims that the redesigned detector will keep its selection efficiency, signal resolution, and background rejection under HL-LHC conditions, allowing CMS to fully exploit the planned 4000…

desk verdict A competent, derivative proceedings summary of the CMS Phase-2 upgrade, accurate to the TDRs but with no new science and one sloppy verbatim block. read the letter →

arxiv 2501.03412 v1 pith:TKJM5DWO submitted 2025-01-06 hep-ex physics.ins-det

Thiago R. F. P. Tomei This is my paper · ORCID

classification hep-exphysics.ins-det

keywords CMSPhase-2upgradeHL-LHCpileupmitigationhigh-granularitycalorimeterMIPtimingdetectorsilicontrackertriggeranddataacquisitionluminosity

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reviews the CMS Phase-2 upgrade programme for the High-Luminosity LHC and argues that it will preserve the detector's performance under extreme conditions. The HL-LHC will deliver up to 200 simultaneous proton-proton collisions per bunch crossing and a large integrated radiation dose, so every major subsystem is being replaced or overhauled. The central assertion is that the upgraded detector will be able to fully exploit the planned 4000 fb-1 dataset, enabling the collaboration's Standard Model precision programme and searches for new physics. If correct, CMS can record the entire HL-LHC run with efficiency, resolution, and background rejection comparable to earlier running.

What carries the argument

The carrying mechanism is the Phase-2 upgrade's coordinated set of replacements rather than any single device. The tracker's pT modules are a key trigger innovation: track stubs compatible with pT > 2 GeV trajectories are reconstructed at 40 MHz and fed into the Level-1 trigger, so tracking information enters the trigger decision for the first time. The HGCAL is a 5D calorimeter (position, energy, and time) with ~25 ps HGCROC timing and 2 MGy radiation tolerance, while the MTD adds 30-65 ps per-track time in the barrel and 35 ps in the endcap, allowing pileup vertices to be separated in time as well as in z. Together these provide the granularity, timing, and trigger bandwidth that the paper argues will hold performance at 200 pileup.

What would settle it

During early HL-LHC commissioning with an average of 200 pileup, measure the integrated detector's time-resolved vertex separation and Level-1 trigger output; if the in-situ MTD resolution is materially worse than ~65 ps in the barrel or the trigger cannot sustain 750 kHz within 12.5 microseconds latency, the claim of full exploitation of 4000 fb-1 would be falsified.

Watch

Extended reading notes

Core claim

The core claim is that the Phase-2 upgrade will keep CMS's selection efficiency, signal resolution, and background rejection at the level needed to exploit the HL-LHC data. The paper walks subsystem by subsystem through the design choices intended to achieve this: a fully replaced silicon tracker with pT-module track stubs feeding the Level-1 trigger, a high-granularity 5D endcap calorimeter with silicon sensors tolerant to 2 MGy, a new MIP timing detector that spreads vertices in time as well as space, upgraded barrel calorimeters and muon systems with new GEM and iRPC stations, and a redesigned two-level trigger and DAQ that sustains 750 kHz L1 output with 7-10 MB events and 37M HS06 of compute. The paper's conclusion is that, with these systems, CMS will fully achieve its physics programme and make the most of the HL-LHC era.

Load-bearing premise

The performance targets measured on components, prototypes, and test beams (such as the HGCROC's ~25 ps timing and 2 MGy tolerance, the MTD's 30-65 ps resolution, and the DAQ's 750 kHz throughput) will survive full system integration and operation at 200 pileup.

Editorial extensions

If this is right

The upgraded detector will keep selection efficiency, signal resolution, and background rejection at HL-LHC conditions, so CMS can record the full 4000 fb-1 dataset instead of losing events to pileup or trigger limits.
With Outer Tracker pT stubs in the Level-1 trigger, tracking information enters the 40 MHz trigger decision for the first time, preserving efficiency for low-pT objects needed for rare Standard Model processes such as H to mu mu and double Higgs production.
The MTD's 30-65 ps timing turns pileup separation from a longitudinal (z) problem into a (z,t) problem, which the paper argues will preserve vertex identification and improve particle identification in heavy-ion running.
L1 scouting will record intermediate trigger data for offline use, opening diagnostics, monitoring, and associated physics streams from data that was previously discarded.
The redesigned DAQ and HLT with 750 kHz input, 7-10 MB events, and 37M HS06 computing power will sustain the HL-LHC data rate, while 10 kHz output and 70-100 GB/s bandwidth feed offline storage.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Beyond the paper's listed examples, achieving the MTD and HGCAL timing targets in situ would make precision time-tagging a general analysis tool, potentially improving tau and heavy-flavour identification across the CMS programme.
The tightest test is integration: the paper's numbers come from TDRs, prototypes, and test beams, so one natural extension is to benchmark full-system timing and trigger rates during HL-LHC commissioning against the specs quoted here.
If the L1 scouting streams work as designed, they could also be used to train trigger-level machine-learning corrections on real full-rate data, a use beyond the diagnostic and monitoring roles the paper names.
The radiation-hard techniques developed for HGCAL and the timing-layer LGADs are likely to be reused by any future hadron-collider detector facing similar dose and pileup, although the paper does not say this.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

This is a conference proceedings summary, not a research paper. It accurately compiles the CMS Phase-2 upgrade specifications from the TDRs and adds a few test-beam results. No new physics, no new analysis, but it's a serviceable overview for someone who wants a single reference.

What it does well: the numbers match the cited TDRs (tracker channels, HGCAL layers, MTD timing, DAQ rates), the figures are informative, and it includes some useful test-beam data (BTL and ETL time resolutions). The structure is logical, walking through each subsystem and ending with the conclusion that Phase-2 will maintain CMS performance.

Soft spots: the most concrete flaw is in Section 2.5, where a large block from the L1/DAQ TDR is pasted verbatim, complete with 'Chapter 1' headers and figure captions. That's an editorial mistake that should be cleaned up, though it doesn't affect the technical content. The broader limitation is the central claim: that the upgraded detector will 'fully achieve' the physics programme. That rests on component-level specifications (HGCROC timing, MTD resolution, DAQ throughput) that are projected from test beams and TDR simulations, not from a full-system integrated run. The paper explicitly cites the TDRs where those integrated studies are documented, so this is an inherent feature of a summary of an ongoing project, not an internal contradiction.

Who is this for: a graduate student or a physicist outside CMS who wants a quick, reliable overview of the Phase-2 plan. Someone who already reads the TDRs will learn nothing new.

Recommendation: as a proceedings article, it's acceptable. If it were submitted as a research paper to a regular journal, I'd desk reject for lack of new content. But if the venue peer-reviews proceedings, it deserves a light referee check for accuracy and to fix the copy-paste block. I'd accept it with minor revisions.

Referee Report

0 major / 5 minor

Summary. This paper is a conference proceedings summary of the CMS Phase-2 upgrade programme for the High-Luminosity LHC. It describes the planned replacement of the silicon tracker, the new High-Granularity Calorimeter, the MIP Timing Detector, upgrades to the barrel calorimeters and muon systems, and the redesign of the trigger, DAQ, and beam radiation/luminosity instrumentation. The central claim, stated in the Conclusions, is that the Phase-2 upgrades will allow CMS to maintain very high performance and fully exploit the 4000 fb^-1 HL-LHC dataset.

Significance. The paper provides a concise and generally accurate overview of the CMS upgrade plans, compiled from the collaboration's Technical Design Reports and supplemented with some recent component-level test-beam results (e.g., BTL time resolution and ETL LGAD test beam, shown in Figure 5). It is a useful reference for the community, particularly as a proceedings summary of LHCP 2024. The main limitation is that no system-level integrated performance validation is presented; the projected figures, such as the HGCROC timing and the DAQ throughput, are quoted from TDRs and prototypes. This limitation is intrinsic to a status report of an ongoing upgrade, and the paper appropriately cites the full TDRs for the integrated studies. The paper does not claim to present new physics analyses, and the cited numbers are consistent with the referenced TDRs.

minor comments (5)

[Section 2.5] The paper reproduces a long verbatim excerpt from the CMS L1/DAQ TDR, including the 'Chapter 1' headers and figure captions for TDR Figures 1.3 and 2.1. The stray references in that block to 'Section 4.7' and 'Chapter 5' are nonsensical in the present paper and should be removed or rewritten in the authors' own words.
[Abstract] There is a grammatical error: 'fully profit from on the HL-LHC data' should read 'fully profit from the HL-LHC data'.
[Section 2.2] The word 'cassetes' is misspelled and should be 'cassettes'; it appears twice in the description of the HGCAL mechanical assembly.
[Figure 7] The left panel of Figure 7 appears to be a direct screenshot of a page from the L1 Trigger TDR, complete with the original figure number and caption text in the margin. The figure should be redrawn or cropped to be a clean schematic that matches the paper's own figure style.
[Section 2.5] The quantity '37M HS06' is introduced without defining the HS06 unit; a brief parenthetical explanation (e.g., 'HS06 is a standardized computing benchmark unit') would improve readability for a non-specialist audience.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper is a TDR-based status summary with no derived prediction that reduces to its inputs.

full rationale

This paper is a conference proceedings summary of the CMS Phase-2 upgrade. It does not present a derivation of a new result from prior claims; it reports design specifications and performance targets whose sources are explicitly cited CMS Technical Design Reports, prototype measurements, and test-beam results (e.g., HGCROC timing and 2 MGy tolerance from [5], MTD resolution from [6], BCP from [7], trigger/DAQ parameters from [9,10], BRIL from [11]). The central claim in the Conclusions—that with Phase-2 CMS will keep high performance and fully achieve the physics programme—is a forward-looking engineering projection, not a mathematical consequence reducible to the inputs of this paper. No parameter is fitted to data in this paper and then rediscovered as a prediction. The only self-citation pattern is that the references are CMS TDRs authored by the same collaboration; that is normal for a project-status report and is not load-bearing in the sense of importing an unexamined uniqueness theorem or smuggling in an ansatz. The paper is explicitly self-aware that the details live in the TDRs: 'The complete information on the Phase-2 L1T and HLT is available the respective Technical Design Reports [9, 10].' The verbatim TDR block in Section 2.5 is a layout artifact and does not change the argument. No circular step can be quoted, so the score is 0.

Assumptions & free parameters 0 free parameters · 2 assumptions · 0 invented entities

A review paper; it introduces no new free parameters and no new theoretical entities. The only implicit postulates are the HL-LHC parameters and the validity of TDR performance projections.

assumptions (2)

domain assumption HL-LHC will achieve L = 7.5e34 cm^-2 s^-1 at 14 TeV with average pileup 200 and deliver 4000 fb^-1 by the end of the 2040s.
Quoted from the CERN Yellow Report [1] and used as the design basis in all sections; the upgrade requirements follow from these parameters.
domain assumption The quoted performance specifications from the TDRs and test-beam measurements (HGCROC ~25 ps, MTD 30-65 ps, iRPC 0.5 ns timing, DAQ rates) will hold in production modules and integrated operation.
The central claim that CMS will maintain high performance depends on these projections; they are not re-derived or demonstrated at system level in this paper.

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Cite this review

Pith. "Pith review of CMS Upgrades for the High-Luminosity LHC Era." pith.science (2026). https://pith.science/paper/TKJM5DWO

@misc{pith2026250103412,
  author       = {Pith},
  title        = {Pith review of: CMS Upgrades for the High-Luminosity LHC Era},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TKJM5DWO}},
  note         = {Machine review of arXiv:2501.03412}
}

abstract

The High-Luminosity LHC (HL-LHC) era, set to begin in 2029, will provide the general-purpose experiments with an instantaneous luminosity of up to $\mathcal{L} = 7.5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$ from pp collisions at a centre-of-mass energy of 14 TeV. To fully exploit this unprecedented data set, the experimental setups must be upgraded to withstand the challenging conditions of the HL-LHC, including up to 200 simultaneous collisions per bunch crossing and a substantial radiation dose delivered to the detectors. The CMS collaboration is currently undertaking the Phase-2 upgrade, which aims to enhance the detector's capabilities to maintain high performance under these conditions. This upgrade includes significant improvements to the muon spectrometer and barrel calorimeter, a complete replacement of the silicon tracker, endcap calorimeter and beam radiation and luminosity subsystems, the introduction of a new MIP timing detector layer, and a redesigned trigger and data acquisition system. These enhancements will ensure that the CMS experiment can fully profit from on the HL-LHC data, maximising its physics potential and expanding its ability to make the most precise measurements.

Figures

Figures reproduced from arXiv: 2501.03412 by the authors.

**Figure 1.** Left: CMS Phase-2 Tracker System geometry, divided in Inner Tracker (orange and green modules) and Outer Tracker (red and blue modules). Right: Reconstruction of 𝑝T modules in the Outer Tracker, which are input to the Level-1 Trigger decision. lateral development, whilst displaying fine lateral and longitudinal granularity; the capability of precisely measuring the time of the showers is also required. Finally, the … view at source ↗

**Figure 2.** Left: arrangement of silicon sensors into an hexagonal module. Middle: layout of layer 40, with hexagonal modules and scintillator cells. Right: HGCAL geometry overview, showing layers that comprise the CE-E and CE-H sections in an 𝑟 − 𝑧 view. The CE-E section comprises 26 layers of silicon sensors interspersed with Cu, CuW and Pb absorbers, totalling 27.7 𝜒0 and ∼1.5 𝜆. On the other hand, the CE-H section comprises… view at source ↗

**Figure 3.** Left: The HGCROC ADC signal peak is well-behaved and stable even after 3.45 MGy irradiation. Right: As an example of the HGCAL performance, it displays good reconstruction quality of 60–80 GeV photons, even without dedicated energy corrections but after a PCA-based cleaning of the shower constituents. 2.3 The MIP Timing Detector The MIP Timing Detector (MTD) is a specialised tracking layer that aims to measure the p… view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: Left: the pp interactions are spread both in the longitudinal coordinate and in time. Leveraging the separation in the two dimensions leads to a better identification of the pileup vertices. Right: particle identification of protons, kaons and pions with the ETL. The M…

**Figure 5.** Figure 5: shows some of the latest characterisation studies for the MTD components [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

**Figure 6.** Figure 6: Left: Block diagram describing the upgraded EB electronics architecture. Right: overview of the CMS Phase-2 muon systems, with the new GEM chambers (ME0, GE1/1 and GE2/1) and iRPCs (RE3/1 and RE4/1). The new stations will enhance CMS’s detection capabilities of forward…

**Figure 1.3.** Figure 1.3: Functional diagram of the CMS L1 Phase-2 upgraded trigger design. The Phase-2 L1 [PITH_FULL_IMAGE:figures/full_fig_p006_1_3.png]

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Forward citations

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Reference graph

Works this paper leans on

11 extracted references · 6 canonical work pages · cited by 1 Pith paper

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Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.

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