Heavy-ion Results from CMS

The High Luminosity upgrade of the LHC (HL-LHC) at CERN will provide unprecedented instantaneous and integrated luminosities of around 5×1034 cm−2s−1 and 3000 fb−1, respectively, from 2025 to 2035. During this operational period, an average of 140 to 200 collisions per bunch-crossing (pile up) is expected, posing a challenge to the event reconstruction. In order to cope with these extreme pile up conditions, harsh environment, and increased data rates, the Compact Muon Solenoid (CMS) detector is undergoing a radical Phase II upgrade program. In the barrel region of the CMS electromagnetic calorimeter (ECAL), the lead tungstate crystals and avalanche photodiodes (APDs) will keep performing well and will therefore be maintained, while the entire readout and trigger electronics will be replaced. A dual gain trans-impedance amplifier and an ASIC providing two 160 MHz ADC channels, gain selection, and data compression will be installed. The noise increase in the APDs, due to radiation-induced dark current, will be contained by reducing the temperature at which ECAL is operated. The trigger decision will be moved off-detector and performed by powerful and flexible FPGA processors, allowing for more sophisticated trigger algorithms to be applied. The upgraded ECAL will be capable of high-precision energy measurements and will greatly improve the time resolution for photons and electrons above 10 GeV. Together with the introduction of a new timing detector designed to perform timing measurements with the resolution of a few tens of picoseconds for minimum ionizing particles, the CMS detector will be able to precisely reconstruct the primary interaction vertex under the described pile up conditions.

Quarkonia are most important probes of the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. The Compact Muon Solenoid (CMS) detector is well suited to measure quarkonia via their decays to dimuon channel. This writeup summarizes quarkonium measurements in pp and PbPb collisions by CMS detector at \(\sqrt{s_{NN}} = 2.76~\mathrm {TeV}\) and \(\sqrt{s_{NN}} = 5.02~\mathrm {TeV}\).

The Compact Muon Solenoid (CMS) detector is one of the 4 detectors at the CERN LHC to study the Standard Model (SM) and even searching for signatures of new physics produced in high-energy collisions in the LHC. As many of the signatures in both SM and new physics include muons, it is crucial to possess high performance of muon identification and reconstruction. The performance of CMS muon identification and reconstruction is studied based on the run 2 data collected with the CMS detector at $\sqrt{s} = 13 ~\mathrm{TeV}$. The identification and reconstruction efficiencies are measured using tag and probe method. As High-Level Trigger (HLT) has made improvements in its algorithms during the run 2, we also highlight its upgraded feature by comparing with the previous algorithm.

The leading part of electroweak radiative corrections to the Drell-Yan process in the CMS (compact muon solenoid detector) experiment at LHC were calculated. Analytic and numerical tests indicate that the results are independent of all nonphysical parameters. A numerical analysis of radiative effects was performed by using the FORTRAN code READY with allowance for experimental constraints at the CMS detector. The results presented for the relative correction to the fully differential cross section for the Drell-Yan process over the entire kinematical region of the CMS detector would hopefully be helpful in handling experimental data.

During the first 3 years of operation at the Large Hadron Collider, the Compact Muon Solenoid (CMS) experiment has collected data across evolving conditions of center of mass energy, instantaneous luminosity and collisions pile up. Following this evolution, the CMS collaboration has constantly strived to guarantee the prompt availability of high quality reconstructed data, in order to ensure early and sound physics results. This has relied on a few key areas of constant attention covering careful preparation and maintenance of the event simulation and reconstruction algorithms; efficient and robust strategies and algorithms for the calibration and the alignment of the detector elements; continuous scrutiny of the data quality and the validation of any changes to the software or calibrations which were introduced during the operations. This contribution covers the major development and operational aspects of the CMS offline workflows during the 2010-2013 data taking, underlying its essential role towards the main physics achievements and discoveries of the CMS experiment.

We report on a scaling behaviour in the transverse momentum () distributions for charged hadrons produced in proton–proton (pp) collisions with different centre of mass energies ( = 0.9, 2.76 and 7 TeV) at the Compact Muon Solenoid detector. This scaling behaviour appears when the is replaced by , where K is a parameter and depends on . A similar scaling behaviour is observed in the pT distributions of charged hadrons produced in proton–antiproton () collisions with = 0.63, 1.8 and 1.96 TeV at the Collider Detector at Fermilab. The particle production mechanism behind the scaling behaviour in the pp or collisions could be explained by the model of percolation of strings.

One burden of high energy physics data analysis is uncertainty within the measurement, both systematically and statistically. Even with sophisticated neural network techniques that are used to assist in high energy physics measurements, the resulting measurement may suffer from both types of uncertainties. Fortunately, most types of systematic uncertainties are based on knowledge from information such as theoretical assumptions, for which the range and behaviour are known. It has been proposed to mitigate such systematic uncertainties by using a new type of neural network called adversarial neural network (ANN) that would make the discriminator less sensitive to these uncertainties, but this has not yet been demonstrated in a real-life LHC analysis. This work investigates ANNs using as a benchmark the search for the production of four top quarks, an extremely rare physics process at the LHC and one of the important processes that can prove or disprove the Standard Model. The search for four top quarks in some cases is sensitive to large systematic uncertainties. The expected cross section upper limit for four top quark production is calculated using traditional neural networks and adversarial neural networks based on simulated proton-proton collisions within the Compact Muon Solenoid detector within Large Hadron Collider, and are compared to existing results. The improvement and further considerations to the search for rare processes at the LHC will be discussed.

We report the results on the search for exclusive production of W pairs in the LHC with data collected by the Compact Muon Solenoid detector in proton-proton collisions at √s = 7 TeV. The analysis comprises the two-photon production of a W pairs, pp → pW+ W− p → p νe± νµ∓ p. Two events are observed in data for pT(ℓ) > 4 GeV, |η(ℓ)| < 2.4 and m(µ± e∓) > 20 GeV, in agreement with the standard model prediction of 2.2 ± 0.4 signal events with 0.84 ± 0.15 background events. Moreover, a study of the tail of the lepton pair transverse momentum distribution is performed to search for an evidence of anomalous quartic gauge couplings in the γγ → W+ W− vertex. As no events are observed in data, it results in a model-independent upper limits for the anomalous W quartic gauge couplings aW0,C/Λ2, which are of the order of 10−4.

This thesis presents two research works carried out within the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC): the system validation of the scintillator section of the High Granularity Calorimeter (HGCAL), and a search for a heavy resonance coupling to top quarks and produced in association with top quark pairs in proton–proton collisions at center-of-mass energies of 13 and 13.6 TeV. For the High-Luminosity LHC (HL-LHC) phase, the endcap calorimeters of CMS will be replaced by the HGCAL, a high-precision sampling calorimeter designed to cope with the extreme radiation and pileup conditions expected at the HL-LHC. In regions where radiation levels remain moderate, the hadronic section employs scintillator tiles coupled to silicon photomultipliers (SiPM-on-tile) as active materials. Each HGCAL tilemodule, the basic detector unit of this region, accommodates up to 144 SiPM-on-tile channels read out by dedicated front-end electronics. The performance of several tilemodules was evaluated during test-beam campaigns at the DESY-II facility using 3 GeV electron beams. The measurements serve to validate the response and uniformity of the detector components and to confirm that they meet the design specifications for HL-LHC operation. A search for a new heavy resonance that couples exclusively to top quarks and is produced in association with a top quark pair is presented. The analysis uses proton–proton collision data collected with the CMS detector at center-of-mass energies of 13 TeV (2016–2018) and 13.6 TeV (2022). The search targets the final state with two oppositely charged leptons from the W boson decays of the nonresonant top quarks, and two high-energy jets reconstructing the hadronically decaying resonant top quarks. In this scenario, the two top quarks from the resonance decay are expected to be highly Lorentz-boosted. Their hadronic decays are reconstructed using jets clustered with a variable-radius algorithm and identified with a dedicated top quark tagger based on a boosted decision tree (BDT). Events are selected in a final state containing opposite-sign leptons and b-tagged jets, and the resonance mass is reconstructed from pairs of tagged top jets. The analysis probes resonance masses between 500 GeV and 4 TeV and searches for local excesses in the reconstructed mass spectrum. The results are interpreted in the context of models with vector-like, scalar and pseudoscalar resonances produced in association with top quarks, representing the first such interpretation performed with CMS data.

The High Luminosity upgrade of the LHC (HL-LHC) at CERN will provide unprecedented instantaneous and integrated luminosities of around 5 x $10^{34}$ cm$^{-2}$ s$^{-1}$ and 3000 fb$^{-1}$, respectively. An average of 140 to 200 collisions per bunch-crossing (pileup) is expected. In the barrel region of the Compact Muon Solenoid (CMS) electromagnetic calorimeter (ECAL), the lead tungstate crystals and avalanche photodiodes (APDs) will continue to perform well, while the entire readout and trigger electronics will be replaced. The noise increase in the APDs, due to radiation-induced dark current, will be mitigated by reducing the ECAL operating temperature. The trigger decision will be moved off-detector and performed by powerful and flexible FPGA processors. The upgraded ECAL will greatly improve the time resolution for photons and electrons with energies above 10 GeV. Together with the introduction of a new timing detector designed to perform measurements with a resolution of a few tens of picoseconds for minimum ionizing particles, the CMS detector will be able to precisely reconstruct the primary interaction vertex under the described pileup conditions. We present the status of the ECAL barrel upgrade, including time resolution results from beam tests conducted during 2018 and 2021 at the CERN SPS.

A prospective analysis for the discovery of a light Higgs boson in the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) is presented. The associated production channels WH and ZH of a Higgs boson decaying into a photon pair are studied using a full detector simulation. The method of analysis here employed should permit the utilization of real data once they become available in order to optimize the analysis performance and to estimate background rates. Minimizing in this way reliance on simulated data should allow a significant reduction in systematic errors. One year of LHC running at high luminosity (integrated luminosity of 100 fb−1) should allow an observation at 3σ of the Standard Model Higgs boson from the LEP lower limit of 114.4 GeV/c2 up to 146 GeV/c2. Three years of running at high luminosity should allow a 5σ discovery from the LEP lower limit up to 148 GeV/c2. In the context of supersymmetric models, the dominant gluon fusion Higgs boson production process could be strongly suppressed. This light Higgs gluophobic scenario could occur when the mixing in the stop sector is maximal. In such a case, the associated production channels WH and ZH may be recovery channels.

High-Luminosity Large Hadron Collider (HL-LHC) upgrade aims to increase performance. In order to efficiently handle the larger particle flux and increased radiation, an upgrade is planned for the inner Tracker of the Compact Muon Solenoid (CMS) detector, which is located at the innermost part of the CMS. To efficiently reconstruct and track particles, the existing pixel tracker will be upgraded with novel sensors, readout chips, and front-end electronics designed to handle increased data rates. These improvements will allow the detector to track particles more precisely. In the United States, we are working on assembling new sensor modules to be inserted into disks, referred to as TFPX, for installation in the CMS detector for the HL-LHC era. We are currently testing the prototype of TFPX modules to ensure the upgrade is carried out in the best way possible. The University of Ilinois Chicago has been tasked with testing the final versions of the TFPX sensor modules before they are sent to CERN to be installed into the CMS detector. This work is supported by NSF.

The homogeneous lead tungstate electromagnetic calorimeter for the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider operates in a challenging radiation environment. The central region of the calorimeter uses large-area avalanche photodiodes to detect the fast blue-violet scintillation light from the crystals. The high hadron fluence in the forward region precludes the use of these photodiodes and vacuum phototriodes are used in this region. The constructional complexity of the calorimeter, which comprises 75848 individual crystals, plus the activation of material make repair during the lifetime of the detector virtually impossible. We describe here the key features and performance of the photodetectors and the quality assurance procedures that were used to ensure that the proportion of photodetectors that fail over the lifetime of CMS will be limited to a fraction of a percent.

The synchronization of the trigger and data acquisition systems for the Cathode Strip Chambers (CSCs) in the Compact Muon Solenoid (CMS) detector at CERN is described. The CSC trigger system is designed to trigger CMS on muons with high efficiency (~99% per chamber) and is able to accurately identify its 25ns proton bunch crossing. To date, asynchronous cosmic ray data have been used to define the protocol and to refine timing algorithms, allowing synchronization to be realized within and between chambers to within ±10 ns. Final synchronization of the CSCs requires timing parameters to be accurate to 2 ns. This goal will be readily achieved from the cosmic ray baseline using data taken with the synchronous beam structure of the Large Hadron Collider.

An aging study with intensive radiation sources was performed for the forward resistive plate chambers (RPCs) of the Compact Muon Solenoid (CMS) detector in the Large Hadron Collider experiment (LHC). The expected rate of the radiation background to be induced in the forward RPCs ranges from a few tens of Hz/cm to as much as ∼1 kHz/cm. Therefore, the radiation hardness for the forward RPCs should be sufficient to ensure the safe operation of the CMS and successful achievement of the LHC physics. Two oil-coated and one plain double-gap RPCs were manufactured and tested with the neutron beam provided by the MC50 cyclotron of the Korea Institute of Radiological and Medical Science (KIRAMS) and with gamma rays from a 200-mCi Cs source. The estimated radiation dose induced by the fast neutrons was approximately 50 Gy. No significant aging effect was found to be a consequence of the neutron irradiation, However, a degradation of the RPC performance was observed after intense gamma irradiation for a total of 32.5 days. The mean gamma-signal rate drawn in each RPC was 4.0 kHz/cm, which was approximately two times the maximum rate expected in the highest η-region covering the particle trigger. Increases in the noise rates and the currents accounted for the degradation in the efficiencies for the RPCs. As a result, safe operation of the forward RPCs in the avalanche mode was ensured for signal rates up to 2 kHz/cm.