High-Luminosity LHC Upgrade

• High-Luminosity LHC is a comprehensive upgrade of CERN's LHC designed to boost collision rates for precision Higgs measurements and exploration beyond the Standard Model.
• The upgrade employs advanced technologies such as high-field superconducting magnets, compact crab cavities, and high-power superconducting links to achieve higher performance.
• It integrates extensive detector enhancements and innovative collimation, pileup mitigation, and radiation protection systems to sustain quality data under extreme conditions.

The High-Luminosity Large Hadron Collider (HL-LHC) is a major performance and technological upgrade of the LHC at CERN, designed to deliver a peak instantaneous luminosity of up to $7.5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$ and integrated luminosities of $\gtrsim 3000$ fb$^{-1}$ over a decade-scale run. Its aim is to enable precision studies of the Higgs boson, rare Standard Model processes, and physics beyond the Standard Model by significantly increasing event rates while sustaining or improving detector performance in an extreme radiation and pileup environment. The HL-LHC constitutes major innovations in accelerator components, power and cryogenics, collimation, beam instrumentation, and the integration of large-scale upgrades to all major experiments.

1. Luminosity Reach, Beam Parameters, and Physics Motivation

The HL-LHC upgrade is motivated by the need for five-fold higher peak and ten-fold higher integrated luminosity relative to the nominal LHC. Key baseline parameters include:

• Peak instantaneous luminosity: $\mathcal{L}_{\rm peak} \lesssim 7.5\times10^{34}$ cm$^{-2}$ s$^{-1}$ (ultimate), with the nominal HL-LHC at $5\times10^{34}$ cm$^{-2}$ s$^{-1}$ (Angal-Kalinin et al., 2017, Tomei, 2022, Burkhardt et al., 2017)
• Integrated luminosity: $\sim$300–400 fb$^{-1}$ yr$^{-1}$ per experiment, aiming for $>3000$ fb$^{-1}$ by the mid‐2030s (Angal-Kalinin et al., 2017, Burkhardt et al., 2017)
• Bunch structure: 2748–2808 bunches/beam, 25 ns spacing, $N_b \sim 2.2 \times 10^{11}$, $I_b \sim 1.1$ A
• Average pileup $\langle \mathrm{PU} \rangle \simeq 200$ (Tomei, 2022, 2512.04807)
• Stored beam energy: up to 700 MJ per beam (Apollonio et al., 2017)

The luminosity is given by: $$L = \frac{N_b^2 n_b f_{rev} \gamma}{4\pi \epsilon_n \beta^*} F$$ where $F$ is the geometric reduction factor due to the crossing angle.

These beam and luminosity parameters support a physics program targeting Higgs coupling measurement at the percent level, sensitivity to rare and BSM processes, and improved limits or measurements of flavor, CP-violation, and top-quark electroweak couplings (Burkhardt et al., 2017, Selvaggi, 2015, Sekmen, 2019, Slawinska, 2016).

2. Key Accelerator Upgrades and Enabling Technologies

2.1 High-Field Superconducting Magnets

• Deployment of Nb$_3$Sn low-$\beta$ triplet quadrupoles (Q1–Q3) with 150 mm coil bore and $140$ T/m gradient (up to $12$ T peak field), enabling $\beta^*$ reduction to $15$ cm and, ultimately, $10$–$7.5$ cm (Ambrosio et al., 2017, Angal-Kalinin et al., 2017).
• New 11 T Nb$_3$Sn dipoles in dispersion suppressor regions (IR2, IR7) create space for local collimation and improved protection (Angal-Kalinin et al., 2017, Appleby et al., 2017, Fessia et al., 2017).
• IR separation and matching dipoles (D1, D2) and Q4–Q6 are also upgraded to higher field or larger aperture specifications.
• Advanced corrector packages (orbit, sextupole, octupole, decapole, dodecapole) are integrated adjacent to the triplets for optics and dynamic aperture control (Ambrosio et al., 2017).

2.2 Compact Superconducting Crab Cavities

• Introduction of 400 MHz, $\sim 3.4$ MV deflecting voltage "compact" crab cavities—DQW, RFD, and four-rod variants—on both sides of IP1/5, restoring full geometric overlap at low $\beta^*$ (Baudrenghien et al., 2017, Angal-Kalinin et al., 2017).
• Cavity system: $8$ cavities per IP per beam, phase stability $\le 0.01^\circ$, total crab voltage per side $\sim 12$ MV.
• Cryogenic and RF integration require 2 K operation, active HOM damping, and fast LLRF feedback loops for $<1~\mu$s response (Baudrenghien et al., 2017).

2.3 High-Power Superconducting Power Links

• Semi-flexible MgB$_2$ and HTS hybrid links of up to $300$ m length deliver $\sim 150$ kA total current from surface power converters (moved out of the tunnel for radiation safety, reliability, and spatial constraints) (Ballarino et al., 2017, Fessia et al., 2017).
• Currents: $20$ kA per large circuit, $2$–$3$ kA per corrector, with multi-stage thermal intercepts, low-resistance stabilizers, and forced He cooling at $20$–$35$ K.
• Links demonstrate $\gtrsim$10 K temperature margin and $\lesssim 100$ W static+dynamic heat load per link in full-scale prototypes.

2.4 Advanced Collimation and Protection

• IR7 collimation system upgraded to include local DS collimators (TCLD) via 11 T dipole substitutions, low-impedance/robustness secondary collimators (MoGr, Mo coating, advanced cooling), and new tertiary absorbers (TCTPM) (Appleby et al., 2017).
• Machine protection interlocks: LHC Beam Loss Monitors (BLM), Fast Magnet Current Change Monitors (FMCM), and Beam Interlock System (BIS, Safety Integrity Level 3) provide $\leq 3$ turn ($<300\,\mu$s) response to fast faults (Apollonio et al., 2017).
• Quench protection upgrades: distributed heaters/CLIQ systems, energy extraction to DQR resistor blocks, and rad-hard cable/connector layout (Apollonio et al., 2017).

3. Integration, Installation, and Civil Engineering

The HL-LHC upgrade follows a staged installation approach:

• LS2 (2020–2022): Early deployment of superconducting links, new RF cryoplant, and ion collimation.
• LS3 (2025–2027): Core installation—triplet quadrupoles, 11 T DS dipoles, crab cavities, horizontal and vertical superconducting links, and large‐scale civil works at P1/P5 (new cryo/service/shaft infrastructure) (Fessia et al., 2017).
• Full integration requires precise mechanical tolerances ($\pm0.5$ mm magnet alignment), vacuum class cryostat splices ($<10^{-4}\,$mbar$\cdot$L/s), and alignment survey at 0.1 mm reproducibility.
• Commissioning proceeds in sequenced stages: vacuum and leak checks, cold-powering, beam-based alignment, and stepwise luminosity ramp (Fessia et al., 2017, Lamont et al., 2017).

4. Machine Operation, Data Acquisition, and Detector Upgrades

4.1 Commissioning and Operation

• Targeted physics fill cycle: magnet pre-cycle, injection (2748 bunches), energy ramp, optics squeeze ($\beta^*$ to $0.15$ m), collision adjustment, and extended "stable beams" (Lamont et al., 2017).
• Levelled luminosities at $5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$ per IP (or $7.5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$ ultimate).
• Availability metrics: $\sim$160 days Scheduled Proton Physics Time (target $>$70%), physics efficiency $>$50%, minimum turnaround 3 h (Lamont et al., 2017).

4.2 Experimental and DAQ Upgrades

• Both ATLAS and CMS receive comprehensive Phase-2 detector upgrades: all-silicon trackers (to $|\eta|<4$), precision-timing layers ($\sigma_t\sim30$ ps), high-granularity calorimeters (HGCal, sFCal), muon-system and trigger/DAQ system overhauls (Tomei, 6 Jan 2025, Tomei, 2022, Slawinska, 2016).
• CMS: Level-1 trigger latency up to 12.5 $\mu$s; input $750$ kHz; HLT farm at $37$ MHS06 ($\sim$2 million cores), with 50–80% code targeted for GPU/accelerator offloading (Tomei, 2022).
• CMS HLT input event size $\sim8.4$ MB, menu rates up to $7.5$ kHz (Phase-2), network throughput to $51$ Tb/s, processing time per event (full menu) $\sim8$ s (Tomei, 2022).
• ATLAS upgrades similarly include a $1$ MHz L0 and $400$ kHz L1 trigger, full digitization at L1, and FPGA-based hardware tracking with timing-based pileup suppression (Slawinska, 2016).

4.3 Pileup Mitigation Strategies

• Detector granularity, timing, combined calorimeter-tracker association (particle flow), precision timing layers, and ML-based pileup subtraction (e.g., PUPPI) are implemented to preserve jet, $E_T^{\mathrm{miss}}$, and lepton/b-tagging performance at $\langle \mathrm{PU}\rangle \sim 200$ (Tomei, 2022, Tomei, 6 Jan 2025, Selvaggi, 2015, Slawinska, 2016).
• Mitigation is crucial for systematic uncertainty control in SM and BSM precision measurements.

5. Radiation Environment, Machine Protection, and Reliability

• Dynamic heat loads in the IT region (cold mass + beam screen): $\sim$630 W+$\sim$615 W per side at full luminosity (Bignami et al., 2017).
• Peak triplet coil dose design limit $<30$ MGy over $3000$ fb$^{-1}$; neutron fluence $2$–$5\times 10^{17}$ n${}_{\text{>0.1 MeV}}$/cm${}^2$ (Bignami et al., 2017).
• Electronic racks in service alcoves experience $TID$ up to $50$–$70$ Gy/y, well above COTS tolerance, necessitating shielding, relocation, or rad-hard electronics (Bignami et al., 2017, Apollonio et al., 2017).
• Quench protection sensitivities, beam-dump and interlock systems are upgraded for $700$ MJ/beam operation; beam loss monitors provide $40~\mu$s detection, beam interlock response is $<3$ turns ($\sim 270~\mu$s) (Apollonio et al., 2017).
• Availability modelling indicates that to reach $\geq 300$ fb$^{-1}$/yr, the failure rate must be cut from $70\%$ (2012) to $50$--$60\%$ and average fault time to $5.2$–$6.9$ h (Apollonio et al., 2017).

6. Physics Impact and Future Prospects

• The HL-LHC enables physics reach far beyond current LHC limits, including:
  • $\mathcal{O}(1\%)$ precision on Higgs couplings, trilinear self-coupling sensitivity at $O(1)$ with $3000$ fb$^{-1}$ (Slawinska, 2016)
  • FCNC searches in top decays down to branching fractions $O(10^{-6})$, percent/sub-percent-level top property determinations (Selvaggi, 2015)
  • Sensitivity to SUSY, heavy exotic resonances, vector-like quarks, and dark matter mediators up to mass scales well beyond current TeV limits (Sekmen, 2019)
• The annual integrated luminosity is expected to ramp up from $50$ fb$^{-1}$ (Year 1) to $250$ fb$^{-1}$ (Year 3 and beyond) as reliability and operational efficiency mature (Lamont et al., 2017).
• Prospects for ultimate performance (luminosity $\sim7.5\times10^{34}$ cm$^{-2}$ s$^{-1}$, $4000$ fb$^{-1}$) are enabled by operational margins and design safety factors throughout the machine and experiments (Angal-Kalinin et al., 2017, Burkhardt et al., 2017, Lamont et al., 2017).

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The HL-LHC is characterized by a coordinated upgrade program—for the ring, injectors, protection systems, and every major detector—addressing increased particle fluxes, radiation fields, data throughput, and the challenge of ultra-high pileup. All major technical claims, performance metrics, and system parameters echo directly from the cited design reports and technical documents.