The LHCb PicoCal

The High-Luminosity LHC era will enable the LHCb experiment to record an unprecedented dataset of up to 300 ~fb⁻¹, operating at instantaneous luminosities approaching 1.5 × 10³⁴ ~cm⁻² ~s⁻¹. Under such conditions, the electromagnetic calorimeter (ECAL) must withstand MGy-level radiation in its central region, cope with significantly higher occupancy, and maintain excellent energy and time resolution performance. The “PicoCal” concept, proposed for Upgrade II, combines high-density absorbers, radiation-hard scintillators, and timing capabilities at the level of O(10 ps) to mitigate pile-up effects. Several technology options are being investigated, including spaghetti calorimeter (SpaCal) modules with tungsten or lead absorbers coupled to plastic or garnet scintillating fibers, and upgraded Shashlik modules with faster wavelength-shifting fibers. A key material development is the use of ultra-fast, radiation-tolerant GAGG:Ce crystals, grown via the Czochralski method and optimized through high-level Mg/Ce co-doping. These crystals aim to achieve effective decay times below 10 ns and can withstand up to 1 MGy of radiation, enabling both fine granularity and precision timing in the most demanding innermost ECAL regions. Test-beam studies have confirmed that the proposed configurations can meet the target energy resolution (~ 10 % / √E ⊕ 1%) and achieve timing precision below 20 ps at high energies. The combined detector and materials R$&$D ensures readiness for high-luminosity operation with enhanced spatial, temporal, and radiation performance.