CMS Collaboration Research Articles

Status of the upgrade project of the CMS Tracker for HL-LHC

The CMS collaboration is currently preparing the upgrade of the tracking system for the High-Luminosity LHC operations (HL-LHC). The HL-LHC is scheduled to start in 2027 and it will bring the instantaneous luminosity up to 7.5 × 1034 cm−2 s−1, with an average of 200 interactions per beam crossing, and an integrated luminosity up to 4000 fb−1 over a decade. To operate efficiently in these challenging conditions, the new detector has to provide tracking information to the first level trigger stage and maintain good and efficient offline tracking. In this contribution, the layout of the new detector, the main technology choices together with highlights on the current status of the main detector components will be presented.

Engineering challenges in mechanics and electronics in the world’s first particle-flow calorimeter at a hadron collider: The CMS high-granularity calorimeter

The CMS Collaboration is preparing to build replacement endcap calorimeters for the HL-LHC era. The new high-granularity calorimeter (HGCAL) is, as the name implies, a highly-granular sampling calorimeter with 47 layers of absorbers (mainly lead and steel) interspersed with active elements: silicon sensors in the highest-radiation regions, and scintillator tiles equipped with on-tile SiPMs in regions of lower radiation. The active layers include copper cooling plates embedded with thin pipes carrying biphase CO2 as coolant, front-end electronics and electrical/optical services. The scale and density of the calorimeter poses many engineering challenges that we discuss here. These include: the design and production of 600 tonnes of stainless-steel absorber plates to very high physical tolerances; the development of the CO2 cooling system to maintain each 220-tonne endcap at −35 °C whilst the electronics dissipate up to 125 kW; the need to cantilever the calorimeters from the existing CMS endcap disks, using titanium wedges; the production of a thin but strong inner cylinder to take the full weight but have little impact on physics performance; the development of low-power high-dynamic-range front-end electronics for over 6 million detector channels; the integration of all services in a restricted volume with only a couple of mm height. We give an overview of the design of HGCAL, focusing on the materials and techniques being used to overcome the many challenges for this world’s first calorimeter of its type at a hadron collider.

Sensitivity to new physics in final states with multiple gauge and Higgs bosons

We analyse the sensitivity to beyond-the-Standard-Model effects of hadron-collider processes involving the interaction of two electroweak and two Higgs bosons, VVHH, with V being either a W or a Z boson. We examine current experimental results by the CMS collaboration in the context of a dimension-8 extension of the Standard Model in an effective-field-theory formalism. We show that constraints from vector-boson-fusion Higgs-pair production on operators that modify the Standard Model VVHH interactions are already comparable with or more stringent than those quoted in the analysis of vector-boson-scattering final states. We study the modifications of such constraints when introducing unitarity bounds, and investigate the potential of new experimental final states, such as ZHH associated production. Finally, we show perspectives for the high-luminosity phase of the LHC.

Heavy quarkonium dynamics at next-to-leading order in the binding energy over temperature

Using the potential non-relativistic quantum chromodynamics (pNRQCD) effective field theory, we derive a Lindblad equation for the evolution of the heavy-quarkonium reduced density matrix that is accurate to next-to-leading order (NLO) in the ratio of the binding energy of the state to the temperature of the medium. The resulting NLO Lindblad equation can be used to more reliably describe heavy-quarkonium evolution in the quark-gluon plasma at low temperatures compared to the leading-order truncation. For phenomenological application, we numerically solve the resulting NLO Lindblad equation using the quantum trajectories algorithm. To achieve this, we map the solution of the three-dimensional Lindblad equation to the solution of an ensemble of one-dimensional Schrödinger evolutions with Monte-Carlo sampled quantum jumps. Averaging over the Monte-Carlo sampled quantum jumps, we obtain the solution to the NLO Lindblad equation without truncation in the angular momentum quantum number of the states considered. We also consider the evolution of the system using only the complex effective Hamiltonian without stochastic jumps and find that this provides a reliable approximation for the ground state survival probability at LO and NLO. Finally, we make comparisons with our prior leading-order pNRQCD results and experimental data available from the ATLAS, ALICE, and CMS collaborations.

The CMS high granularity calorimeter for the High Luminosity LHC

The CMS Collaboration is preparing to build replacement endcap calorimeters for the High Luminosity LHC (HL-LHC) era. The new high-granularity calorimeter (HGCAL) is, as the name implies, a highly granular sampling calorimeter with approximately six million silicon sensor channels (≈1.1cm2 or 0.5cm2 cells) and about four hundred thousand scintillator tiles readout with on-tile silicon-photomultipliers. The calorimeter is designed to operate in the harsh radiation environment at the HL-LHC, where the average number of interactions per bunch crossing is expected to exceed 140. Besides measuring energy and position of the energy deposits the electronics is also designed to measure the time of their arrival with a precision on the order of 50ps. In addition to the hardware of the HGCAL, developing a reconstruction sequence that fully exploits the granularity to achieve optimal electromagnetic and hadron identification, as well as a good energy resolution in the presence of pileup, is a challenging task.

Intermittency in pp collisions at $\sqrt{s}=$ 0.9, 7 and 8 TeV from the CMS collaboration

The intermittency-type fluctuations as outlined by Bialas and Peschanski in the 1980s is analysed in pppp collisions at \sqrt{s}=s= 0.9, 7 and 8 TeV from the CMS collaboration at CERN. Our preliminary analysis shows that the intermittency exponents in the bin-averaged scaled factorial moments decrease in magnitude with increasing collision energy at the TeV scale, which suggests that the cascading nature of multiparticle production described by the \alphaα-model is weakening. We outline possible areas planned for future studies.

Characterization of a high bandwidth readout chain for the CMS phase-2 pixel upgrade

The CMS collaboration is building a new inner tracking pixel detector for the High-Luminosity LHC. Each pixel readout chip will be controlled with a single serial input stream at 160 Mb/s and will send out data via four current mode logic (CML) 1.28 Gb/s outputs. The readout chips will be grouped in modules and connected with up to 1.6 meters long low-mass electrical links to Low-Power Gigabit Transceivers (lpGBT) and Versatile Link PLUS Transceiver (VTRx+) modules that send the data optically to off-detector electronics at 10 Gb/s. The characterization of these components and system tests of the readout chain are presented.

Recent Highlights on Higgs Physics from the LHC

Many of the Higgs boson production and decay channels, predicted by the Standard Model, have been observed since its 2012 discovery. Today, measurements of the Higgs boson’s properties in these channels deepen our understanding of the nature of this fundamental particle by providing better information about its mass, production kinematics, and CP structure. Increasing precision also enables targeting rare Higgs boson decay modes and probing di-Higgs-boson production. In addition, searches for beyond Standard Model physics in the Higgs sector are performed. This article presents the latest Higgs boson measurement highlights from the ATLAS and CMS collaborations using LHC proton-proton collision data at a centre of mass energy of 13 TeV.

Bottomonium suppression and elliptic flow in heavy-ion collisions

In this proceedings contribution I review recent progress concerning the suppression of bottomonium production in the quark-gluon plasma. Making use of open quantum system methods applied to potential non-relativistic quantum chromodynamics one can show that the dynamics of heavy-quarkonium bound states satisfying the scale hierarchy 1/a0 » πT ∼ mD » E obey a Lindblad equation whose solution provides the quantum evolution of the heavy-quarkonium reduced density matrix. To solve the resulting Lindblad equation we use a quantum trajectories algorithm which allows one to include all possible angular momentum states of the quark-antiquark probe in a scalable manner. We solve the Lindblad equation using a tuned 3+1D dissipative hydrodynamics code for the background temperature evolution. We then consider a large number of Monte-Carlo sampled bottomonium trajectories embedded in this background. This allows us to extract the centrality- and pT-dependence of the nuclear suppression factor RAA[Υ] and elliptic flow v2[Υ]. We find good agreement between our model predictions and available √ sNN = 5.02 TeV = 5.02 TeV Pb-Pb collision experimental data from the ALICE, ATLAS, and CMS collaborations.

A portrait of the Higgs boson by the CMS experiment ten years after the discovery

In July 2012, the ATLAS and CMS collaborations at the CERN Large Hadron Collider announced the observation of a Higgs boson at a mass of around 125 gigaelectronvolts. Ten years later, and with the data corresponding to the production of a 30-times larger number of Higgs bosons, we have learnt much more about the properties of the Higgs boson. The CMS experiment has observed the Higgs boson in numerous fermionic and bosonic decay channels, established its spin–parity quantum numbers, determined its mass and measured its production cross-sections in various modes. Here the CMS Collaboration reports the most up-to-date combination of results on the properties of the Higgs boson, including the most stringent limit on the cross-section for the production of a pair of Higgs bosons, on the basis of data from proton–proton collisions at a centre-of-mass energy of 13 teraelectronvolts. Within the uncertainties, all these observations are compatible with the predictions of the standard model of elementary particle physics. Much evidence points to the fact that the standard model is a low-energy approximation of a more comprehensive theory. Several of the standard model issues originate in the sector of Higgs boson physics. An order of magnitude larger number of Higgs bosons, expected to be examined over the next 15 years, will help deepen our understanding of this crucial sector.

Characterization of planar and 3D silicon pixel sensors for the high luminosity phase of the CMS experiment at LHC

The High Luminosity upgrade of the CERN Large Hadron Collider (HL–LHC) calls for an upgrade of the CMS tracker detector to cope with the increased radiation levels while maintaining the excellent performance of the existing detector. Specifically, new high-radiation tolerant solid-state pixel sensors, capable of surviving irradiation fluences up to 1.9×1016neq/cm2 at 3 cm from the interaction point, need to be developed. For this purpose an R&D program involving different vendors have been pursued, aiming at the development of thin n-in-p type pixel sensors. The R&D covers both planar (manufactured by Fondazione Bruno Kessler, FBK; Hamamatsu Photonics, HPK and LFoundry) and single-sided 3D columnar (manufactured by FBK and Centro Nacional de Microelectronica, CNM) pixel devices. The target active thickness is 150μm while two different pixel cell dimensions are currently investigated (25 × 100 and 50×50μm2). Sensors presented in this article have been bump-bonded to the RD53A readout chip (ROC), the first prototype towards the development of a ROC to be employed during HL–LHC operation. Test beam studies, both of thin planar and 3D devices, have been performed by the CMS collaboration at the CERN, DESY and Fermilab test beam facilities. Results of modules performance before and after irradiation (up to 2.4×1016neq/cm2) are presented in this article.

Search for a lighter neutral custodial fiveplet scalar in the Georgi-Machacek model * *Supported by National Natural Science Foundation of China (11875275, 12061141003, 11661141007), China Ministry of Science and Technology (2018YFA0403901) and partially by the France China Particle Physics Laboratory (FCPPL) and CAS Center for Excellence in Particle Physics (CCEPP)

Many researches from both theoretical and experimental perspectives have been performed to search for a new Higgs Boson that is lighter than the 125 Higgs boson, which was discovered at the LHC in 2012. In this study, we explore the possibility of constraining a lighter neutral custodial fiveplet scalar in the Georgi-Machacek (GM) model using the latest results of the search for a lighter Higgs boson decaying into two photons from LHC data. The custodial-singlet mass eigenstate h or H is considered to be the LHC observed 125 Higgs boson. A new set of constrained parameters that is favoured by low-mass is proposed to generate events efficiently. The production of from a scan based on the constrained parameters is compared to the latest results of the search for a lighter Higgs boson decaying into two photons by the CMS Collaboration after applying theoretical constraints from the GM model and constraints from all existing relevant experimental measurements, including the recent results of the Higgs boson searches by the LHC. Numerical analyses of the surviving GM parameter space are performed. The tendencies and correlations of the GM input parameters from phenomenological studies are summarized. In addition, the discovery potential of the other interesting decay channels of this low-mass neutral custodial fiveplet scalar are discussed.

Explaining the 96 GeV Di-photon anomaly in a generic 2HDM Type-III

Motivated by results recently reported by the CMS Collaboration about an excess in the di-photon spectrum at about 96 GeV, especially when combined with another long-standing anomaly at the same value in the bb¯ invariant mass spectrum in four-jet events collected at LEP, we show that a possible explanation to both phenomena can be found at 1σ level in a generic 2-Higgs Doublet Model (2HDM) of Type-III in presence of a specific Yukawa texture, wherein Lepton Flavour Violating (LFV) (neutral) currents are induced at tree level. Bounds from Higgs data play a major role in limiting the parameter space of this scenario, yet we find solutions with mH=125 GeV and mh=96 GeV consistent with current theoretical and experimental bounds.

Meson spectroscopy results from CMS

Recent results in meson spectroscopy with the CMS detector are presented. In particular, the first observation by the CMS Collaboration of two excited B + c states and the measurement of the B + c (2S) mass using proton-proton collisions at a center-of-mass energy of 13 TeV are reported.

Searching for New Physics in Hadronic Final States with Run 2 Proton–Proton Collision Data at the LHC

The symmetries of the Standard Model give rise to the forces that act on particles, and the corresponding force mediators. While the Standard Model is an excellent description of particle interactions, it has known limitations; it is therefore important to search for new physics beyond the Standard Model, potentially indicating as-of-yet unknown symmetries of nature. The ATLAS and CMS collaborations have detailed physics programmes, involving a large number of searches for new physics in hadronic final states. As the start of Run 3 of the LHC is imminent, now is a good time to review the progress made and the status of hadronic searches during Run 2 at a centre-of-mass collision energy of s=13TeV. This review provides an overview of the motivations and challenges of hadronic final states at the LHC, followed by an introduction to jet reconstruction, calibration, and tagging. Three classes of searches for new physics in hadronic final states are discussed: di-jet searches, searches for missing transverse momentum in association with another object, and searches for hadronic di-boson resonances. The complementarity of these different analysis strategies is discussed, emphasising the importance of a varied hadronic physics programme in the search for new physics.

New angular and other cuts to improve the Higgsino signal at the LHC

Motivated by the fact that naturalness arguments strongly suggest that the SUSY-preserving higgsino mass parameter mu cannot be too far above the weak scale, we re-examine higgsino pair production in association with a hard QCD jet at the HL-LHC. We focus on \ell^+\ell^-+\eslt+j events from the production and subsequent decay, \tchi_2^0\to\tchi_1^0\ell^+\ell^-, of the heavier neutral higgsino. The novel feature of our analysis is that we suggest angular cuts to reduce the important background from Z(\to \tau\tau)+j events more efficiently than the m_{\tau\tau}^2<0 cut that has been used by the ATLAS and CMS collaborations. Other cuts, needed to reduce backgrounds from t\bar{t}, WWj and W/Z+\ell\bar{\ell} production, are also delineated. We plot out the reach of LHC14 for 300 and 3000~fb^{-1} and also show distributions that serve to characterize the higgsino signal, noting that higgsinos may well be the only superpartners accessible at LHC14 in a well-motivated class of natural SUSY models.

CMS results on beauty baryon spectroscopy

Recent beauty baryon physics results from the CMS collaboration, obtained using the proton-proton collision data collected at √ s = 13 TeV, are discussed. Observation of the Λ 0 b → J/ψΛφ decay and measurement of its branching fraction, relative to the Λ 0 b → ψ(2S)Λ decay, is reported. The excited baryon states Λb(5912)0 , Λb(5920)0 , Λb(6146)0 , and Λb(6152)0 are confirmed and their masses are measured. Observation of a new excited beauty strange baryon, labeled as Ξb(6100)∗−, is presented.

Pseudorapidity dependence of the p T spectra of charged hadrons in pp collisions at s = 0.9 and 2.36 TeV

Abstract We report the predictions of different Monte Carlo event generators including HIJING, Pythia, and QGSJETII in comparison with the experimental data measured by the CMS Collaboration at CERN in proton–proton (pp) collisions at center-of-mass energy s = 0.9 and 2.36 TeV. The CMS experimental transverse momentum (p T or p ⊥) spectra of charged hadrons were measured for pseudorapidity range 0 ⩽ η ⩽ 2.4 with bin width of η = 0.2 (for p T from 0.1 to 2 GeV/c) and a single bin of η for |η| &lt; 2.4 (for p T from 0.1 to 4 GeV/c). Pythia reproduced the p T spectra with reasonable agreement for most of the p T range. It depicts better results in the case of the |η| &lt; 2.4 than HIJING and QGSJETII which could reproduce the spectra in a limited p T range. Furthermore, to analyze the p T spectra of charged hadrons measured by the CMS Collaboration, we used a three-component function (structured from the Boltzmann distribution) and the q-dual function (from the q-dual statistics) to extract parameter values relevant for the study of bulk properties of hadronic matter at high energy. We have also applied the two analytic functions over the model predictions. The values extracted by the functions from the HIJING and Pythia models are closer to the experimental data than the QGSJETII model. Although the models could reproduce the p T spectra of all charged particles in some of the p T range but none of them could reproduce the distributions over the entire p T range and in all the pseudorapidity regions.

Heavy and light flavor jet quenching in different collision systems at energies available at the CERN Large Hadron Collider

Recent experiments have observed large anisotropic collective flows in high multiplicity proton-lead collisions at the CERN Large Hadron Collider (LHC), which indicates the possible formation of mini quark-gluon plasma (QGP) in small collision systems. However, no jet quenching has been confirmed in such small systems so far. To understand this intriguing result, the system size scan experiments have been proposed to bridge the gap between large and small systems. In this work, we perform a systematic study on both heavy and light flavor jet quenching in different collision systems at the LHC energies. Using our state-of-the-art jet quenching model, which combines the next-to-leading-order perturbative quantum chromodynamics framework, a linear Boltzmann transport model and the ($3+1$)-dimensional viscous hydrodynamics simulation, we provide a good description of nuclear modification factor ${R}_{\mathrm{AA}}$ for charged hadrons and $D$ mesons in central and midcentral $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Xe}+\mathrm{Xe}$ collisions measured by the CMS collaboration. We further predict the transverse momentum and centrality dependences of ${R}_{AA}$ for charged hadrons, $D$ and $B$ mesons in $\mathrm{Pb}+\mathrm{Pb}$, $\mathrm{Xe}+\mathrm{Xe}$, $\mathrm{Ar}+\mathrm{Ar}$, and $\mathrm{O}+\mathrm{O}$ collisions at the LHC energies. Our numerical results show a clear system size dependence for both light and heavy flavor hadron ${R}_{AA}$ across different collision systems. Sizable jet quenching effect is obtained for both heavy and light flavor hadrons in central $\mathrm{O}+\mathrm{O}$ collisions at the LHC energies. Our study provides a significant bridge for jet quenching from large to small systems, and should be helpful for finding the smallest QGP droplet and the disappearance of QGP in relativistic nuclear collisions.

Effects of hydrodynamic and initial longitudinal fluctuations on rapidity decorrelation of collective flow

We investigate the interplay between hydrodynamic fluctuations and initial longitudinal fluctuations for their effects on the rapidity decorrelation of collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional integrated dynamical model in which we combine initial conditions with longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We analyse the factorisation ratio in the longitudinal direction to study the effect of these fluctuations on the rapidity decorrelation. We find an essential difference between the effects of the hydrodynamic fluctuations and the initial longitudinal fluctuations in the centrality dependence of the factorisation ratios. A combination of the hydrodynamic fluctuations and the initial longitudinal fluctuations leads to reproduction of the centrality dependence of the second-order factorisation ratio, r2(ηpa,ηpb), measured by the CMS Collaboration. Our model also qualitatively describes the centrality dependence of the third-order factorisation ratio, r3(ηpa,ηpb). These results demonstrate the importance of the hydrodynamic fluctuations, as well as the initial longitudinal fluctuations, in understanding the longitudinal dynamics of high-energy nuclear collision reactions.