High-Luminosity LHC Upgrade

• High-Luminosity Large Hadron Collider (HL-LHC) is a major upgrade of CERN's LHC that increases luminosity via enhanced beam focusing and novel technologies.
• It integrates state-of-the-art superconducting magnets, crab cavities, and multi-stage collimation to achieve a five-fold boost in peak luminosity and manage high pile-up conditions.
• The upgrade underpins a robust physics program with precise Standard Model measurements and extended discovery potential for new phenomena.

The High-Luminosity Large Hadron Collider (HL-LHC) is an ambitious upgrade to the CERN Large Hadron Collider, targeting a five-fold increase in peak luminosity and a ten-fold gain in accumulated integrated luminosity relative to the original LHC design. By integrating advanced superconducting magnet technology, compact crab cavities, novel cryogenics, upgraded collimation, and high-power superconducting links, the HL-LHC forms the cornerstone of the global post-2029 energy-frontier physics program (Apollinari et al., 2017). The HL-LHC aims to deliver up to 7.5 × 10³⁴ cm⁻² s⁻¹ luminosity and approximately 4000 fb⁻¹ over its operational lifetime.

1. Luminosity Targets, Optics, and Performance Metrics

The HL-LHC luminosity formula is: $$L = \frac{N_b^2\,n_b\,f_{\rm rev}\,\gamma}{4\pi\,\varepsilon_n\,\beta^*}\,F,$$ where $N_b$ is the bunch population, $n_b$ the number of bunches, $f_{\rm rev}$ the revolution frequency (~11.245 kHz at 7 TeV), $\gamma$ the Lorentz factor, $\varepsilon_n$ the normalized emittance, $\beta^*$ the beta at the interaction point (IP), and $F$ accounts for the geometric crossing-angle reduction (Angal-Kalinin et al., 2017, Tomei, 6 Jan 2025).

Key HL-LHC parameters:

Parameter	Nominal LHC	HL-LHC Design
Beam energy	7 TeV/beam	7 TeV/beam
Number of bunches	2808	2748–2808
Bunch population	1.15 × 10¹¹	2.2 × 10¹¹ (standard)
Normalized ε	3.75 μm	2.5 μm
β* (IP1/IP5)	0.55 m	0.15 m (down to 0.15 m)
Crossing angle	285 μrad	590 μrad
Peak L (levelled)	1 × 10³⁴ cm⁻² s⁻¹	5 × 10³⁴ cm⁻² s⁻¹

Reducing $\beta^*$ (from 0.55 m to 0.15 m), increasing bunch charge, doubling beam current, and achieving higher field strengths are central to attaining the HL-LHC luminosity goal. The ATS optics scheme uses the adjacent arcs as telescopic squeezers to reach ultra-low $\beta^*$ (Angal-Kalinin et al., 2017). Instantaneous integrated luminosity is levelled at the design value to mitigate pile-up (targeting ⟨μ⟩ ≈ 200).

2. Superconducting Magnet Upgrades

The HL-LHC features a pioneering deployment of Nb₃Sn-based superconducting magnets (Ambrosio et al., 2017). The principal upgrades involve:

• Inner Triplet Quadrupoles (Q1–Q3): 150 mm aperture, 140 T/m gradient, 17.46 kA nominal current, operational peak field ≥12 T, two-layer coil geometry. The large aperture enables the squeeze to $\beta^*=15$ cm while preserving magnet distance from the IP (23 m), maximizing focusing and thus luminosity.
• Nb–Ti Separation/Recombination Dipoles (D1, D2): 150 mm aperture, B=5.6/4.5 T, field quality and stress-optimized coil design, operation at 1.9 K for margin.
• 11 T Nb₃Sn Dipoles: Used in the dispersion suppressor regions to create space for collimators. Each unit achieves B₀ = 11.23 T on-axis, I_nom = 11.85 kA, paired in series to match the integrated bending of standard MB dipoles (Karppinen et al., 2017).

All magnets utilize advanced mechanical support and pre-stress schemes (Al “bladder and key” for Nb₃Sn, collared iron yokes for Nb–Ti), robust quench protection via heaters and dump resistors, and high-capacity He II bath cryogenics (operating at 1.9 K).

3. Crab Cavities and RF System Innovations

A major HL-LHC innovation is the deployment of compact superconducting crab cavities (DQW, RFD, or four-rod types) operating at 400 MHz (Baudrenghien et al., 2017). Each IP side features four cavities per beam, each supplying up to 3.4 MV transverse kick, restoring head-on bunch collisions despite a 590 μrad total crossing angle. Crab operation raises the geometric reduction factor F from R₀ ≈ 0.305 (no crab) to R₁ ≈ 0.829, recovering up to 70% of the geometric luminosity loss (Apollinari et al., 2017).

Key features:

• Q₀ ≥ 1 × 10¹⁰ at nominal field, ~0.3 W dynamic loss/cavity (static and RF heating).
• LLRF control systems with vector-sum feedback, phase/amplitude stability to ~0.01°, fast interlock triggering (≤270 μs beam abort).
• Higher-order mode (HOM) damping with multipurpose couplers to avoid coupled bunch instabilities.

These systems enable stable luminosity levelling and serve as a knob for operational pile-up tuning.

4. Collimation and Machine Protection Strategy

The HL-LHC collimation architecture is a multi-stage system spread across IR3, IR7 (primary, secondary, tertiary/absorber), IR1/5 (physics debris), and DS regions (TCLDs). The collimators use robust, low-impedance materials: MoGr, Inermet-180, and CFC for primary/secondary units and W-alloy for DS collimators (Appleby et al., 2017).

Performance targets:

• Local cleaning inefficiency: η_total ≲ 5 × 10⁻⁵ at 7 TeV, scaling to ≲2 × 10⁻⁵ at 14 TeV.
• Jaw flatness Δf ≲ 20 μm; reproducibility ≤10 μm.
• Collimator impedance budget: Z_coll/Z_total < 20%; MoGr/Mo coatings drastically reduce impedance.

The DS upgrade replaces each MB (8.3 T dipole, 14.3 m) with two 6.25 m 11 T MBH dipoles and a 2.21 m bypass cryostat to accommodate collimators, doubling local phase advance and reducing beam-induced losses in cold magnets by up to 30% (Karppinen et al., 2017).

Machine protection and interlocks are enforced by the Beam Interlock System (SIL3), fast beam loss monitors (≤40 μs response), and coordinated quench protection (QPS), with availability and response-time budgets ensuring reliable luminosity production (Apollonio et al., 2017).

5. Cryogenics, Powering, and Vacuum Systems

The HL-LHC cryogenic system comprises new plants at points 1, 4, and 5, with dedicated He II circuits at 1.8–1.9 K supporting IT magnets, crab cavities, and HTS/MgB₂ superconducting links (Claudet et al., 2017). Static/dynamic heat loads include:

Subsystem	Static (W/m)	Dynamic (W/m/beam)
Arc Beam Screen	40 (upgrade)	55 (synch. rad.), 60 (image)
Inner Triplets	135	up to 23 (e-cloud, debris)

Cold powering uses flexible, multi-circuit MgB₂ links (≤500 m), rated for total currents up to ~165 kA per insert (20 kA/quadrupole, multi-kA corrector lines), with <0.2 W/m cryogenic load. Power converters and current leads are relocated to radiation-free areas (Ballarino et al., 2017).

The vacuum system achieves <10⁻⁷ Pa in the IRs, enabled by NEG/a-C coatings, sectorization with interlocked valves, and bake-out/cryopumping cycles. Conductance and pumping layouts are optimized via Test-Particle Monte Carlo simulations (Baglin et al., 2017).

6. Detector Upgrades and Physics Reach

The increased luminosity and pile-up (⟨μ⟩≈200) require major upgrades in experiment instrumentation (Tomei, 6 Jan 2025, Azzi et al., 2019):

• ATLAS/CMS trackers: fully replaced with all-silicon sensors (ITk, upgraded pixel/strip), extending η coverage to |η|=4.
• High-granularity calorimetry: e.g., CMS HGCAL (47 layers, 0.6–1.2 cm² hexagonal silicon cells), <5% energy resolution, 25 ps timing.
• Precision timing detectors (MTD, HGTD): <50 ps per track, enabling 4D vertexing and pile-up subtraction.
• Muon system extensions: GEMs, iRPCs extend acceptance to |η|≈2.8, lower thresholds.
• Trigger and DAQ: L1 rates increased to 750 kHz, output ≥10 kHz, FPGA/GPU-based pipelines, real-time ML deployment.

Physics impact:

• SM precision: σ(Z) ~0.2%, Δm_W = 7–10 MeV, Δm_top ≲ 200 MeV, tt̄Z couplings at 3–5%, rare decays (BR limits 10⁻⁵–10⁻⁶) (Azzi et al., 2019).
• BSM reach: gluino mass 5σ discovery to 2.5 TeV, Z' → tt̄ exclusion to 4 TeV, mono-jet σ₉₅ limits ~3 fb (Sekmen, 2019).
• Pile-up resilience: per-object resolutions (σ(d₀), σ_z) maintained/improved (e.g., σ(d₀) ≲ 10 μm at pT=10 GeV, <1% fake rate) at PU=200 (Tomei, 6 Jan 2025).

7. Integration, Protection, and Operational Strategy

Installation is staged across long shutdowns (LS2, LS3) with a modular approach maximizing existing infrastructure reuse (Fessia et al., 2017). Mechanical tolerances Δx,y≤0.15 mm, Δθ≤100 μrad ensure orbit and optics stability.

Safety and availability are anchored in:

• ALARP/ALARA principles for radiological and operational hazards (residual dose, cryogenics, quench management, see Table 5-1 in (Adorisio et al., 2017)).
• Device interlock and emergency response layers (beam/collimator/quench), real-time diagnostics, fast abort/dump (≤3 turns ≈270 μs).
• Radiation-hardened electronics and remote/waste-minimized maintenance pathways.

Commissioning phases include extensive hardware validation (magnets, cryogenics, collimation, powering), staged beam commissioning (pilot to full intensity), and operational ramp-up with dynamic luminosity levelling (Lamont et al., 2017). Key formulas governing operational planning include stored beam energy ($E_{\rm stored} \approx 360$ MJ/beam) and hourglass/geometric factors for luminosity control.

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The HL-LHC machine upgrade thus integrates novel superconducting, RF, cryogenic, protection, and beam instrumentation systems to sustain an unprecedented proton–proton luminosity and dataset. The resulting performance enables both precision Standard Model studies and substantially extended discovery horizons for new physics (Apollinari et al., 2017, Tomei, 6 Jan 2025, Sekmen, 2019, Azzi et al., 2019).