First results on MPGD prototypes in test beam for MPGD-based HCAL at a future Muon Collider experiment

: The Multi-TeV Muon Collider will allow significant advancement in particle physics and in the understanding of its Standard Model for the era after the High-Luminosity LHC. The Muon Collider physics program involves precise Higgs boson sector measurements and TeV-scale new physics exploration. These goals demand accurate full-event reconstruction. The Particle Flow algorithm, which utilizes tracking, calorimeter


Introduction
In particle physics, two main strategies for exploring new physics are usually pursued: one involves colliding electrons/positrons for precise measurements, and the other uses high-energy colliders like proton-proton machines for direct discoveries.Muon colliders [1], however, offer a unique advantage by combining the benefits of both approaches in a single machine.The high mass of muons allows for multiple ring passages and repeated collisions, mitigating synchrotron radiation emission.Besides, while protons can achieve high energy, their composite nature limits the available energy for short-distance physics.
Operating a muon collider poses a challenge due to the instability of muons.Their decay products and following interactions with machine components generate intense radiation known as "Beam Induced Background" (BIB) [2], which contaminates the otherwise clean collision environment.BIB is characterized by low-energy particles arriving in a broad temporal range.Designing a detector for a muon collider focuses on managing and mitigating the effects of BIB, requiring the ability to endure long-term radiation exposure and high-granularity measurements in space, time, and energy to distinguish between beam collision products and BIB particles.

Requirements for HCAL at Muon Collider
The muon collider aims at measuring the physics processes at the energy frontier.This requires excellent resolution, both in energy and space, of collimated high-energy jets.Specifically, for jets above 100 GeV, a jet energy resolution of 3-4% is required.The knowledge of the BIB distributions and rates in the calorimeter system is crucial to quantify the radiation levels and assess the optimal detector design, given the required jet energy resolution.The BIB in the calorimeter region is mainly formed by photons (96%) and neutrons (4%).The simulations show that at 15 kHz bunch crossing a photon flux of ∼ 7.5 kHz/cm 2 is expected at the surface of the hadronic calorimeter [2].An estimate of the radiation doses on the Muon Collider detector was released in [1,3] assuming a 2.5 km circumference ring, a bunch crossing frequency of 15 kHz, and 200 days of operation during a year.In the HCAL, -1 -the expected 1 MeV neutron-equivalent fluence is about 10 13 cm 2 per year, with a steeply decreasing radial dependence beyond it.The expected total ionizing dose in the HCAL is ∼ 10 3 kRad per year.
Choosing the right technology and the optimal design for the calorimeters is critical for mitigating BIB effects while maintaining optimal physics performance.High granularity would reduce BIB particle overlap within the same calorimeter cell.Time resolution on the order of nanoseconds would eliminate most out-of-time BIB components.A finely segmented longitudinal calorimeter can aid in distinguishing signal from those produced by BIB.These requirements align with the high granularity Particle Flow algorithm approach [4].The goal is to create an image of showers induced by various jet fragments, allowing accurate matching with charged particles in the tracker, thus resolving particle trajectories across the entire detector.This facilitates the precise identification and measurement of energy in showers induced by neutral hadrons.The CALICE collaboration has contributed to the development of calorimeter concepts for highly granular detectors in Particle Flow [5].The focus has been on gaseous detectors as the active element, offering higher granularity and radiation hardness compared to traditional scintillator-based calorimeters, and being more economically competitive.While Resistive Plate Chambers (RPCs) have historically been considered, recent proposals include Micro Pattern Gas Detectors (MPGDs) such as resistive Micromegas [6,7], and newer technologies like -rwell [8] and RPWELL [9].MPGDs are expected to outperform RPCs due to their higher rate capability, good energy resolution, detector stability, uniformity, and low pad multiplicity.

Strategy for MPGD-based HCAL studies
The strategy for studying HCAL with MPGD follows a comprehensive approach, starting with GEANT4 simulations [10] to assess the calorimeter response to individual pions.In this phase, the calorimeter geometry is optimized, and performance is evaluated to understand the detector behavior with single particles [11].Subsequently, the implementation of the geometry obtained through simulations is integrated into the context of the Muon Collider, allowing for an assessment of the impact on jet reconstruction in the Particle Flow environment and the presence of the BIB [12].This step is crucial for understanding how the detection system responds in realistic scenarios and to more intense particle fluxes.To further validate the simulations and demonstrate the detector capabilities, experimental tests are conducted using small-size calorimeter cells.These tests would provide a conceptual proof of the detector performance and allow for an experimental verification of the simulations.A small prototype of a calorimeter cell was built and tested in August 2023 at the CERN PS facility, with a pion beam of energy ranging from 2 GeV to 10 GeV.The prototype consisted of 8 layers with alternating stainless steel absorber and MPGD, reaching a depth of approximately one nuclear interaction length,   .The active layers have been instrumented with resistive MicroMegas, -rwell, and RPWELL.A preliminary test beam with a muon beam of 150 GeV was conducted at CERN SPS in July 2023 on the detectors alone to assess their performance.In the following paragraphs, some preliminary results, obtained from the data collected during the test beam at the CERN SPS, on these detectors are presented.

Characterization of the prototypes with muon beam at SPS
All the chambers under test have been designed and produced by the Micro-Pattern Technology Laboratory at CERN.The production batch counted twelve detectors employing three different -2 -resistive MPGD technologies: four resistive MicroMegas, with the Double-DLC scheme, seven -rwell, based on the high-rate layout scheme, and one RPWELL.The readout board consists of arrays of 1×1 cm 2 pads mounted on the PCB, covering a total active area of approximately 20×20 cm 2 .Figure 1 shows a picture of the setup at the SPS, in the H4 beam line.Besides the pad chambers, the setup includes two tracking chambers, which are currently not used for track reconstruction, and two scintillators, whose coincident signal gives the trigger for the data acquisition.The Micromegas All the chambers were read out through APV25 chips [13], connected in a master-slave configuration via HDMI cables to the ADC.The ADC was interfaced with the Front End Card, which managed data acquisition and communication between the front-end chips and the computer for data storage.

Track reconstruction and data analysis
The data were collected using the acquisition software mmdaq3 [14], which generates a root file organized in the form of a tree for each run.Information on the charge collected by each channel, expressed in ADC counts, is stored.The data are subsequently mapped to the corresponding chamber, chip, and channel of the front-end electronic.Given the dimensions of the readout pads (1×1 cm 2 ), it is expected that charge clusters are fully contained within a single pad if the charge is released close to the center of the pads.Alternatively, the clusters may span a maximum of four pads (2 in the x-direction and 2 in the y-direction) if the particle passes close to one of the corners.Therefore, the number of pads associated with the passage of a single particle (i.e., pad multiplicity) is determined as follows: for each event and each chamber, the pad with the maximum charge is identified.If one of the fired pads close to the first one collects a fraction of charge equal to or higher than 90% of that of the first pad, it is added to the pad multiplicity.The set of information related to the position in x and y within the detector, charge, and pad multiplicity constitutes a reconstructed hit for that detector.Muon tracks are then reconstructed using the chambers themselves, excluding the one under test each time.In the following section, some preliminary distributions for two of the pad chambers -3 -of the setup, which are referred to as the MicroMegas-Bari and the -rwell-Bari in the following, are shown, as representatives for all the other detectors.

Preliminary results
Figure 2 shows the residual distribution in the x-direction for the MicroMegas-Bari, on the left, and the -rwell-Bari, on the right.The distributions are derived for each test chamber by calculating the difference between the position of a hit propagated from the track and the position of the closest hit reconstructed on the test chamber.Subsequently, these differences are fitted with a Gaussian function.The sigma of the distributions is of the order of millimeters, in agreement with the pad dimensions.A reconstructed hit on the test chamber is considered matched with the track if its distance from the propagated hit falls within 3 times the sigma of the residual distribution.Figure 3 displays the distributions of the charge for the matched hits, with MicroMegas-Bari on the left and -rwell-Bari on the right.The charge distribution of the reconstructed hits matching the tracks follows a Landau distribution in both cases.However, a peak around 1700 ADC suggests saturation in some channels of the APVs.This peak is more prominent for MicroMegas, as this chamber was operating at a higher gain compared to -rwell.The distribution of the pad multiplicity (in both x and y directions) of the hits matching with the track is shown in figure 4, for the MicroMegas-Bari on the left and the -rwell-Bari on the right.For both chambers, the average value of the pad multiplicity is close to 1.

Conclusions
Some initial outcomes from prototype chambers, tested under a muon beam at CERN SPS in July 2023, are outlined.This preliminary test beam allowed us to become familiar with the behavior of the detectors.Although these results are preliminary, they are promising and indicate the detectors are properly functioning.At the time of writing this document, ongoing analysis of the test beam data is underway, requiring further investigation into detector performance.Furthermore, analysis of the calorimeter cell test beam data is still ongoing, and the experimental results will be integrated within the Monte Carlo simulation of the HCAL system with MPGD technology, as discussed in this contribution.

Figure 1 .
Figure 1.Picture of the setup, including the pad chambers, the tracking, and trigger system.

Figure 2 .
Figure 2. Residual distribution for pad chamber MicroMegas-Bari, on the left, and -rwell-Bari, on the right.

Figure 3 .
Figure 3. Distribution of the charge of the reconstructed hits matching with the track for pad chamber MicroMegas-Bari, on the left, and -rwell-Bari, on the right.

Figure 4 .
Figure 4. Distribution of the pad multiplicity of the reconstructed hits matching with the track for pad chamber MicroMegas-Bari, on the left, and -rwell-Bari, on the right.