Recent Publications

The high-luminosity upgrade of the CERN LHC requires the replacement of the CMS tracking detector to cope with the increased radiation fluence while maintaining its excellent performance. An extensive R&D program, aiming at using 3D pixel silicon sensors in the innermost barrel layer of the detector, has been carried out by CMS in collaboration with the FBK (Trento, Italy) and CNM (Barcelona, Spain) foundries. The sensors will feature a pixel cell size of 25 × 100 μm², with a centrally located electrode connected to the readout chip. The sensors are read out by the RD53A and CROCv1 chips, developed in 65 nm CMOS technology by the RD53 Collaboration, a joint effort between the ATLAS and CMS groups. This paper reports the results achieved in beam test experiments before and after irradiation, up to a fluence of approximately 2.6 × 10¹⁶ neq/cm². Measurements of assemblies irradiated to a fluence of 1 × 1016 neq∕cm2 show a hit detection efficiency higher than 96% at normal incidence, with fewer than 2% of channels masked, across a bias voltage range greater than 50 V. Even after irradiation to a higher fluence of 1.6 × 10¹⁶ neq∕cm², similar performance is maintained over a bias voltage range of 30 V, remaining well within CMS requirements.

• NIM A Volume 1088, August 2026, 171359

A search for new particles decaying to top quark-antiquark pairs is performed using proton-proton collision data at a centre-of-mass energy of 13 TeV. The data set recorded with the CMS detector between 2016 and 2018 is used, corresponding to an integrated luminosity of 138/fb  . Final states with 0, 1, and 2 leptons are analyzed, covering all decay modes of the top quark-antiquark pairs. Heavy Z’ bosons with relative widths of 1, 10, and 30% are excluded for masses in the ranges 0.4-4.8, 0.4–6.2, and 0.4–7.4 TeV, respectively. A Kaluza–Klein gluon in the Randall–Sundrum model and a dark-matter mediator are excluded for masses between 0.5–5.5 and 1.0–4.2 TeV, respectively. These results set the most stringent limits to date for the considered models in the ttbar final state. In addition, in the two-Higgs-doublet models, upper limits are set on the coupling strength modifier for scalar and pseudoscalar Higgs bosons with relative widths of 2.5, 10, and 25% in the mass range of 0.5–1.0 TeV.

• Submitted to JHEP March 2026

A measurement is presented of dijet angular distributions in proton-proton
collisions at a center of mass energy of 13 TeV, using data collected with the CMS detector
at the CERN LHC and corresponding to an integrated luminosity of 138/fb.
For the first time, the dijet angular distributions, corrected for detector
effects, are compared with the predictions of perturbative quantum
chromodynamics at next-to-next-to-leading order, including
next-to-leading-order electroweak corrections. While data are generally found
to be in agreement with predictions, a small difference in shape of the
normalized distributions is seen for dijet masses ranging from 2.4 to 4.8 TeV
and above 6 TeV. The distributions are used to search for proposed signatures
of quark compositeness, extra spatial dimensions, quantum black holes,
dark-matter mediators, axion-like particles, and anomalous gluon couplings. The
most stringent limits to date are set for most of these scenarios. Quark
contact interactions are excluded at 95% confidence level (CL) up to a scale of
17 (37) TeV for destructive (constructive) interference in a benchmark
scenario, valid to next-to-leading order in quantum chromodynamics, and in
which only left-handed quarks participate. The coupling of axion-like particles
to the gluon is constrained to be lower than 0.42 TeV 95% CL. The anomalous triple-gluon coupling in a standard model effective field theory is
constrained to be lower than 0.0076 TeV at 95% CL.

• Submitted to Physics Letters B March 2026

Highly granular pixel detectors allow for increasingly precise measurements of charged particle tracks. Next-generation detectors require that pixel sizes will be further reduced, leading to unprecedented data rates exceeding those foreseen at the High- Luminosity Large Hadron Collider. Signal processing that handles data incoming at a rate of O(40 MHz) and intelligently reduces the data within the pixelated region of the detector at rate will enhance physics performance at high luminosity and enable physics analyses that are not currently possible. Using the shape of charge clusters deposited in an array of small pixels, the physical properties of the traversing particle can be extracted with locally customized neural networks. In this first demonstration, we present a neural network that can be embedded into the on-sensor readout and filter out hits from low momentum tracks, reducing the detector’s data volume by 57.1%–75.7%. The network is designed and simulated as a custom readout integrated circuit with 28 nm CMOS technology and is expected to operate at less than 300 μW with an area of less than 0.2 mm². The temporal development of charge clusters is investigated to demonstrate possible future performance gains, and there is also a discussion of future algorithmic and technological improvements that could enhance efficiency, data reduction, and power per area.

• 2024 Mach. Learn.: Sci. Technol. 5 035047

The measurement of the charge asymmetry in top quark pair events with highly Lorentz-boosted top quarks decaying to a single lepton and jets is presented. The analysis is performed using proton-proton collisions at 13TeV with the CMS detector at the LHC and corresponding to an integrated luminosity of 138\fb. The selection is optimized for top quarks produced with large Lorentz boosts, resulting in nonisolated leptons and overlapping jets. The top quark charge asymmetry is measured for events with an invariant mass larger than 750 GeV and corrected for detector and acceptance effects using a binned maximum likelihood fit. The measured top quark charge asymmetry of (0.42−0.69+0.64)% is in good agreement with the standard model prediction at next-to-next-to-leading order in quantum chromodynamic perturbation theory with next-to-leading-order electroweak corrections. The result is also presented for two invariant mass ranges, 750–900 and >900GeV.

• Physics Letters B Volume 846, 10 November 2023, 137703
• Publication Explained for the general public: Symmetric or Not? That is the question.

The discovery of the Higgs boson was announced ten years ago on the 4^th of July 2012 — an event that substantially advanced our understanding of the origin of elementary particles’ masses. In this collection of articles from Nature, Nature Physics and Nature Reviews Physics we celebrate this groundbreaking discovery and reflect on what we have learned about the Higgs boson over the intervening years.

• CMS Article in Nature 607, 60–68 (2022).

The CMS detector at the CERN LHC features a silicon pixel detector as its innermost subdetector. The original CMS pixel detector has been replaced with an upgraded pixel system (CMS Phase-1 pixel detector) in the extended year-end technical stop of the LHC in 2016/2017. The upgraded CMS pixel detector is designed to cope with the higher instantaneous luminosities that have been achieved by the LHC after the upgrades to the accelerator during the first long shutdown in 2013–2014. Compared to the original pixel detector, the upgraded detector has a better tracking performance and lower mass with four barrel layers and three endcap disks on each side to provide hit coverage up to an absolute value of pseudorapidity of 2.5. This paper describes the design and construction of the CMS Phase-1 pixel detector as well as its performance from commissioning to early operation in collision data-taking.

• JINST16 P02027 (2021)

A search for production of the supersymmetric partners of the top quark, top squarks, is presented. The search is based on proton-proton collision events containing multiple jets, no leptons, and large transverse momentum imbalance. The data were collected with the CMS detector at the CERN LHC at a center-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 137  fb−1. The targeted signal production scenarios are direct and gluino-mediated top squark production, including scenarios in which the top squark and neutralino masses are nearly degenerate. The search utilizes novel algorithms based on deep neural networks that identify hadronically decaying top quarks and W bosons, which are expected in many of the targeted signal models. No statistically significant excess of events is observed relative to the expectation from the standard model, and limits on the top squark production cross section are obtained in the context of simplified supersymmetric models for various production and decay modes. Exclusion limits as high as 1310 GeV are established at the 95% confidence level on the mass of the top squark for direct top squark production models, and as high as 2260 GeV on the mass of the gluino for gluino-mediated top squark production models. These results represent a significant improvement over the results of previous searches for supersymmetry by CMS in the same final state.

• Phys. Rev. D 104, 052001 (2021)

A search for a heavy resonance decaying into a top quark and antiquark (tt¯) pair is performed using proton-proton collisions at √s= 13 TeV. The search uses the data set collected with the CMS detector in 2016, which corresponds to an integrated luminosity of 35.9 fb−1. The analysis considers three exclusive final states and uses reconstruction techniques that are optimized for top quarks with high Lorentz boosts, which requires the use of non isolated leptons and jet substructure techniques. No significant excess of events relative to the expected yield from standard model processes is observed. Upper limits on the production cross section of heavy resonances decaying to a tt¯ pair are calculated. Limits are derived for a leptophobic topcolor Z’ resonance with widths of 1, 10, and 30%, relative to the mass of the resonance, and exclude masses up to 3.80, 5.25, and 6.65 TeV, respectively. Kaluza-Klein excitations of the gluon in the Randall-Sundrum model are excluded up to 4.55 TeV. To date, these are the most stringent limits on top quark pair resonances.

• J. High Energ. Phys. (2019) 2019: 31

A search is presented for physics beyond the standard model, based on measurements of dijet angular distributions in proton-proton collisions at sqrt(s)= 13 TeV. The data collected with the CMS detector at the LHC correspond to an integrated luminosity of 35.9 fb−1. The observed distributions are found to be in agreement with predictions from perturbative quantum chromodynamics that include electroweak corrections. Constraints are placed on models containing quark contact interactions, extra spatial dimensions, quantum black holes, or dark matter using the detector-level distributions. In a benchmark model where only left-handed quarks participate, contact interactions are excluded at the 95% confidence level up to a scale of 12.8 or 17.5 TeV, for destructive or constructive interference, respectively. The most stringent lower limits to date are set on the ultraviolet cutoff in the Arkani-Hamed-Dimopoulos-Dvali model of extra dimensions. In the Giudice-Rattazzi-Wells convention, the cutoff scale is excluded up to 10.1 TeV. The production of quantum black holes is excluded for masses below 5.9 and 8.2 TeV, depending on the model. For the first time, lower limits between 2.0 and 4.6 TeV are set on the mass of a dark matter mediator for (axial-)vector mediators, for the universal quark coupling gq≥ 1.

• Eur. Phys. J. C 78 (2018) 789

A search for narrow resonances decaying to bottom quark-antiquark pairs is presented, using a data sample of proton-proton collisions at √s= 8 TeV corresponding to an integrated luminosity of 19.7 fb−1. The search is extended to masses lower than those reached in typical searches for resonances decaying into jet pairs at the LHC, by taking advantage of triggers that identify jets originating from bottom quarks. No significant excess of events is observed above the background predictions. Limits are set on the product of cross section and branching fraction to bottom quarks for spin 0, 1, and 2 resonances in the mass range of 325-1200 GeV. These results significantly improve on the limits for resonances decaying into jet pairs in the 325-500 GeV mass range.

• Phys. Rev. Lett. 120, 201801 (2018)

A search for supersymmetry is presented based on proton-proton collision events containing identified hadronically decaying top quarks, no leptons, and an imbalance pmissT in transverse momentum. The data were collected with the CMS detector at the CERN LHC at a center-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 35.9 fb−1. Search regions are defined in terms of the multiplicity of bottom quark jet and top quark candidates, the pmissT, the scalar sum of jet transverse momenta, and the mT2 mass variable. No statistically significant excess of events is observed relative to the expectation from the standard model. Lower limits on the masses of supersymmetric particles are determined at 95% confidence level in the context of simplified models with top quark production. For a model with direct top squark pair production followed by the decay of each top squark to a top quark and a neutralino, top squark masses up to 1020 GeV and neutralino masses up to 430 GeV are excluded. For a model with pair production of gluinos followed by the decay of each gluino to a top quark-antiquark pair and a neutralino, gluino masses up to 2040 GeV and neutralino masses up to 1150 GeV are excluded. These limits extend previous results.

• Phys. Rev. D 97, 012007 (2018)

This Interim Report briefly documents the current and planned research and development that will lead to the Phase-2 upgrade of the CMS Level-1 (L1) trigger. As such, this document represents a roadmap to the preparation of a future Technical Design Report (TDR). Taking full advantage of advances in Field Programmable Gate Array (FPGA) and optical link technologies as well as their maturation expected over the coming years, the TDR for the Phase-2 upgrade of the CMS L1 trigger is scheduled to be delivered in approximately two years from the time of this writing. The purpose of this document is thus to complement the detector TDRs and to provide an updated cost estimate.

• CERN-LHCC-2017-013, CMS-TDR-017

Recent results at the Large Hadron Collider (LHC) have pointed to enhanced physics capabilities through the improvement of the real-time event processing techniques. Machine learning methods are ubiquitous and have proven to be very powerful in LHC physics, and particle physics as a whole. However, exploration of the use of such techniques in low-latency, low-power FPGA hardware has only just begun. FPGA-based trigger and data acquisition (DAQ) systems have extremely low, sub-microsecond latency requirements that are unique to particle physics. We present a case study for neural network inference in FPGAs focusing on a classifier for jet substructure which would enable, among many other physics scenarios, searches for new dark sector particles and novel measurements of the Higgs boson. While we focus on a specific example, the lessons are far-reaching. We develop a package based on High-Level Synthesis (HLS) called hls4ml to build machine learning models in FPGAs. The use of HLS increases accessibility across a broad user community and allows for a drastic decrease in firmware development time. We map out FPGA resource usage and latency versus neural network hyperparameters to identify the problems in particle physics that would benefit from performing neural network inference with FPGAs. For our example jet substructure model, we fit well within the available resources of modern FPGAs with a latency on the scale of 100 ns.

• Journal of Instrumentation 13, P07027 (2018)

A search is presented for extra spatial dimensions, quantum black holes, and quark contact interactions in measurements of dijet angular distributions in proton-proton collisions at √s = 13 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 2.6 f b1. The distributions are found to be in agreement with predictions from perturbative quantum chromodynamics that include electroweak corrections. Limits for different contact interaction models are obtained. In a benchmark model, valid to next-to-leading order in QCD and in which only left handed quarks participate, quark contact interactions are excluded up to a scale of 11.5 and 14.7 TeV for destructive or constructive interference, respectively. The production of quantum black holes is excluded for masses below 7.8 or 5.3 TeV, depending on the model. The lower limits for the scales of virtual graviton exchange in the Arkani-Hamed-Dimopoulos-Dvali model of extra spatial dimensions are in the range 7.9–11.2 TeV, and are the most stringent set of limits available.

• JHEP 07 (2017) 013