iREPA Modifications in CMS iRPCs for HL-LHC

• iREPA Modifications are upgrades in the CMS muon system that employ double-gap iRPCs with reduced gap thickness and optimized materials to boost high-rate performance.
• The modifications increase rate capability by nearly threefold, enabling sustained operation at 2 kHz/cm² while maintaining cosmic muon efficiency ≥95%.
• Enhanced readout electronics and precise timing deliver sub-150 ps resolution and 1 mm spatial accuracy, critical for effective HL-LHC muon triggering and reconstruction.

During the HL-LHC (High Luminosity LHC) upgrade, the CMS muon system undergoes a substantial enhancement focused on the RE3/1 and RE4/1 endcap stations. Central to this is the deployment of improved Resistive Plate Chambers (iRPCs), which feature a suite of modifications targeting increased rate capability, precise time and spatial resolution, robust operation under high background conditions, and longevity in the HL-LHC environment. These modifications span chamber geometry, materials, electronics, and readout strategies, and are grounded in extensive irradiation and performance studies under conditions exceeding anticipated HL-LHC backgrounds (Kumari, 2020).

1. Geometry, Materials, and Structural Differences

The iRPCs implement a double-gap parallel-plate structure, consisting of two identical gas gaps, each of thickness $t_{\mathrm{gap}} = 1.4$ mm (reduced from 2.0 mm in the existing CMS RPCs). Electrode plates are fabricated from high-pressure-laminated (HPL) Bakelite, with $t_{\mathrm{elec}} = 1.4$ mm and specified bulk resistivity $$\rho_{\mathrm{HPL}} \in [0.9,\,3.0] \times 10^{10}\ \Omega \cdot \mathrm{cm}$$, approximately half the resistivity of currently deployed systems.

Pick-up strips are arranged on a central PCB and have an average strip pitch $w_{\mathrm{strip}} = 7.5$ mm, narrowed from $\sim$10 mm. The chambers are sectionalized into 72 modules per station and collectively cover $1.9 < |\eta| < 2.4$, specifically targeting the endcap acceptance enhancement required by HL-LHC physics goals.

Key differences relative to the original CMS RPCs are summarized in the table below.

Parameter	Current CMS RPCs	iRPCs
Gas/electrode thickness (mm)	2.0	1.4
Electrode resistivity ($\Omega$cm)	$2 \times 10^{10}$	$[0.9, 3.0] \times 10^{10}$
Average strip pitch (mm)	~10	7.5
Electronics threshold (fC)	$\gtrsim 100$	$20-50$

This configuration enables the iRPCs to combine reduced avalanche charge, improved recovery time, and higher local granularity, thus supporting efficient operation in the demanding HL-LHC environment (Kumari, 2020).

2. Rate Capability Enhancements

The iRPC design explicitly increases the sustainable hit rate ($r_{\mathrm{max}}$) in avalanche mode. The theoretical scaling is given by:

$$r_{\mathrm{max}} \propto \frac{1}{Q_{\mathrm{ind}} \, \rho_{\mathrm{HPL}}\, t_{\mathrm{elec}}}$$

where $Q_{\mathrm{ind}}$ is the mean avalanche-induced charge. Reducing $t_{\mathrm{gap}}$ proportionally lowers $Q_{\mathrm{ind}}$ at constant field, as:

$$Q_{\mathrm{ind}}(t_{\mathrm{gap}}, E) \propto \exp[\alpha(E)\, t_{\mathrm{gap}}]$$

where $\alpha(E)$ is the Townsend coefficient for the gas mixture at field $E$. The lower $\rho_{\mathrm{HPL}}$ further decreases electrode recovery time.

Empirically, existing CMS RPCs demonstrate efficiency loss above O(0.5–0.8) kHz/cm²; iRPC prototypes achieve stable, efficient operation at 2 kHz/cm², maintaining a safety margin (≥3) over the expected maximum background rate at HL-LHC (Kumari, 2020). This sustained rate capability is verified without anomalous current behavior or premature tripping.

3. Readout Electronics and Precision Timing

The iRPC system employs enhanced front‐end electronics based on the PETIROC ASIC (32 channels, SiGe preamplification, gain = 25, bandwidth = 1 GHz) with a digitization threshold currently set at 50 fC and a target of reaching 20 fC. Time stamping utilizes an Altera/Cyclone II FPGA with integrated time-to-digital conversion. Crucially, each pick-up strip is read out from both ends, supporting position measurement along the strip by exploiting time-difference between arrivals.

The timing architecture yields a single-hit time resolution better than 150 ps per channel. The position ($Y$) along the strip is computed as:

$Y = \frac{L}{2} - \frac{v}{2}\,(t_2-t_1)$

where $L$ is the total strip length, $v$ the signal propagation velocity (≈15–20 cm/ns), and $t_{1,2}$ the signal arrival times at each end. The resulting longitudinal spatial resolution is:

$$\sigma_Y = \frac{v}{2}\,\sigma_t,\quad \sigma_t \lesssim 150\ \mathrm{ps} \ \Rightarrow \ \sigma_Y \sim 1\ \mathrm{mm}$$

These modifications enable precise online positioning and time tagging, essential for muon reconstruction and background discrimination at HL-LHC event rates (Kumari, 2020).

4. Performance Validation Under Irradiation and Aging

Comprehensive performance studies at the CERN GIF++ facility, employing a 13.9 TBq $^{137}$Cs source alongside a muon beam, assess iRPC robustness at high background rates. Efficiency measurements define plateau behavior as a function of effective operating voltage $HV_{\mathrm{eff}}$ and maintain cosmic muon efficiency $\epsilon \geq 95\%$ at 2 kHz/cm², a value three times above anticipated HL-LHC backgrounds. Operating voltage shifts arising from enhanced rates are contained ($\sim$200 V between 0 and 2 kHz/cm²), confirming operational flexibility.

Current–rate relationships remain linear past 2 kHz/cm², without spurious tripping. HPL resistivity remains within the design band after integrated charges up to O(10 mC/cm²), with no observed signs of performance degradation due to aging or radiation dose equivalent to multiple HL-LHC years—a critical longevity requirement (Kumari, 2020).

5. Efficiency, Sensitivity, and Prototype Metrics

Key operational metrics and their governing relations are defined as follows:

• Efficiency versus voltage curve:

$$\epsilon (HV_{\rm eff}) = \frac{\epsilon_{\max}}{1 + \exp\left[-\lambda (HV_{\rm eff} - HV_{50})\right]}$$

where $\epsilon_{\max}\ \approx 1.0$, $\lambda$ is the slope parameter, and $HV_{50}$ corresponds to the 50% efficiency crossing.

• “Knee” voltage: $HV_{\rm knee}$ at $\epsilon = 0.95\,\epsilon_{\max}$.
• Working point voltage: $WV = HV_{\rm knee} + 150\ \mathrm{V}$.
• Background sensitivity as a function of energy:

$S(E) = \frac{N_{\rm Hit}(E)}{N_{\rm BG}(E)}$

Demonstrated prototype performance metrics:

• Efficiency at 2 kHz/cm²: $\epsilon \geq 95\%$
• Timing resolution per channel: $\sigma_t < 150$ ps
• Longitudinal spatial resolution: $\sigma_Y \approx 1$ mm
• Sustained full-efficiency rate capability: $\geq 2$ kHz/cm²

These parameters confirm that iRPCs satisfy the HL-LHC requirements for high-rate, high-efficiency, and precise muon triggering and identification (Kumari, 2020).

6. Implications for HL-LHC Muon Trigger and Reconstruction

The iRPC modifications ensure that during HL-LHC’s Phase-2 running (target instantaneous luminosity $5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$, integrated over $3000\ \mathrm{fb}^{-1}$ in 10 years), the CMS muon system retains high-fidelity triggering and reconstruction capabilities across $1.9 < |\eta| < 2.4$. Enhanced rate tolerance and spatial granularity reduce susceptibility to efficiency loss at high background, while the precise timing allows for accurate bunch-crossing assignment and improved background suppression.

A plausible implication is continued scaling of RPC-based muon detection technology to even higher rates or harsher environments in future collider scenarios, leveraging the performance envelope established by the iRPC design (Kumari, 2020).