International Journal of Modern Physics A

During the past 40 years, the production of pairs of $J∕\psi$ mesons in high-energy hadron collisions has been studied by several experiments. Despite the experimental and theoretical efforts, the origin of the process and the relative weight of different production mechanisms remain unknown. Depending on the energy scale, the double $J∕\psi$ production can be described by single- and/or double-parton scattering sub-processes and gluon–gluon fusion or quark–antiquark annihilation mechanisms. The process can also be related to the hypothesis of the intrinsic charm of hadrons and the existence of exotic tetraquark states which were predicted by various theoretical models and have recently been observed by the LHCb experiment.

To study dimuon pair production, the COMPASS experiment at CERN uses a 190GeV/$c$ negative pion beam impinging on different nuclear targets. First preliminary COMPASS results on $J∕\psi$ pair production will be presented. The search for possible signals from exotic resonances and the study of double $J∕\psi$ production mechanisms will be discussed.

The results from the CMS search for supersymmetric particles based on Run-2 data recorded at a center-of-mass energy of 13TeV are summarized. Strong and weak production of SUSY scenarios are considered. Results presented include the searches for squarks and gluinos, direct production of charginos, neutralinos and sleptons. These searches involve final state objects comprising jets, missing transverse momentum, electrons or muons, taus or photons, as well as long-lived particles. The data in these searches are found to be consistent with Standard Model predictions and no significant excess is observed. Upper limits have been set on the masses of supersymmetric particles from a variety of search channels.

The Compact Muon Solenoid (CMS) electromagnetic calorimeter (ECAL) is a high granularity lead tungstate crystal calorimeter operated at the CERN LHC. The future High Luminosity upgrade of the LHC will provide unprecedented instantaneous and integrated luminosity with up to 200 collisions per bunch-crossing [pileup (PU)]. The CMS detector is therefore undergoing an extensive upgrade program to prepare for these demanding conditions. In the ECAL barrel region, the crystals will continue to perform well. The Avalanche Photodiodes (APDs) detecting the scintillation light will also continue to be operational, with some increase in noise that will be mitigated by reducing the operational temperature. Nonetheless, the entire readout and trigger electronics will need to be replaced to cope with the harsh conditions and increased trigger latency requirements. We plan a 25-fold improved readout granularity and a factor 4 sampling rate increase. The timing precision is used in important physics measurements and it is speculated that further improved time information could be exploited for PU mitigation and for the photons assignment to the correct collision vertex. As presented in this paper the endcaps region calorimeter needs to be replaced due to irrecoverable radiation damage of the crystals in the forward region. The upgraded ECAL will preserve the energy resolution, and will improve on time resolution for photons and electrons. In this paper, the ongoing R&D activities for the ECAL barrel readout electronics upgrade are presented.

In this paper, we study the effects of anisotropy on phase transitions in graphene. We work with a low-energy effective field theory which is strongly coupled, and solves a coupled set of Schwinger–Dyson equations. We show that the effect of anisotropy is to reduce the critical coupling.

Many theories beyond the Standard Model (SM) predict new phenomena, such as $Z^{\prime }$ and vector-like quarks, in final states containing bottom- or top-quarks. It is challenging to reconstruct and identify the decay products and model the major background processes. Nevertheless, such final states offer great potential to reduce the SM backgrounds due to their characteristic decay signature. The latest searches in the third-generation quark final states using the full Run-2 proton–proton collision data collected by the ATLAS experiment are presented. Particularly, the recent results of di-bjet and top-antitop resonance searches and dark matter produced in association with a top-quark are discussed in these proceedings. The associated improvements in $b$-quark and top-quark identification techniques are also highlighted.

Advanced machine learning (ML) methods are increasingly used in CMS physics analyses to maximize the sensitivity of a wide range of measurements. The landscape is diverse in terms of both methods and applications. Deep learning methods, from recurrent long short-term memory (LSTM) architectures for classification tasks to deep autoencoders for data quality monitoring, have greatly improved the physics results delivered from the CMS experiment. Algorithms are developed both for collaboration-wide use as well as for individual physics analyses. Results from CMS, such as the measurement of the Higgs boson’s properties in the diphoton decay channel, exploit a variety of ML algorithms to reduce uncertainties on measurements.

Many beyond the Standard Model signatures predict new physics phenomena that lead to final states containing both leptons and jets. The latest search results for massive particles or lepton flavor violation using the full Run 2 dataset recorded by the ATLAS experiment are presented. A particular focus is given to searches for leptoquarks, that offer an attractive potential explanation for the lepton flavor anomalies observed at $B$-physics factories; searches for heavy leptons, such as the ones appearing in Type-III seesaw model that can explain the smallness of neutrino masses; and lepton flavor violating $Z$ decays, which is an excellent probe for new physics that predict direct lepton flavor violating processes.

This paper addresses two unusual scaling regimes (types of critical behavior) predicted by the field-theoretic renormalization group analysis for a self-organized critical system with turbulent motion of the environment. The system is modeled by the anisotropic stochastic equation for a “running sandpile” introduced by Hwa and Kardar in [Phys. Rev. Lett. 62, 1813 (1989)]. The turbulent motion is described by the isotropic Kazantsev–Kraichnan “rapid-change” velocity ensemble for an incompressible fluid. The original Hwa–Kardar equation allows for independent scaling of the spatial coordinates $x_{\parallel }$ (the coordinate along the preferred dimension) and $x_{\perp }$ (the coordinates in the orthogonal subspace to the preferred direction) that becomes impossible once the isotropic velocity ensemble is coupled to the equation. However, it is found that one of the regimes of the system’s critical behavior (the one where the isotropic turbulent motion is irrelevant) recovers the anisotropic scaling through “dimensional transmutation.” The latter manifests as a dimensionless ratio acquiring nontrivial canonical dimension. The critical regime where both the velocity ensemble and the nonlinearity of the Hwa–Kardar equation are relevant simultaneously is also characterized by “atypical” scaling. While the ordinary scaling with fixed IR irrelevant parameters is impossible in this regime, the “restricted” scaling where the times, the coordinates, and the dimensionless ratio are scaled becomes possible. This result brings to mind scaling hypotheses modifications (Stell’s weak scaling or Fisher’s generalized scaling) for systems with significantly different characteristic scales.

In this paper, we study the confinement/deconfinement phase transition of the radion field in a warped model with a polynomial bulk potential. The backreaction of the radion on the metric is taken into account by using the superpotential formalism, while the radion effective potential is obtained from a novel formulation which can incorporate the backreaction. The phase transition leads to a stochastic gravitational wave background that depends on the energy scale of the first Kaluza–Klein resonance, $m_{\mathrm{\text{KK}}}$. This work completes previous studies in the following aspects: (i) we detail the evaluation of the radion spectrum; (ii) we report on the mismatches between the thick wall approximation and the numerical bounce solution; (iii) we include a suppression factor in the spectrum of sound waves accounting for their finite lifetime; and (iv) we update the bound on $m_{\mathrm{\text{KK}}}$ in view of the O3 LIGO and Virgo data. We find that the forthcoming gravitational wave interferometers can probe scenarios where $m_{\mathrm{\text{KK}}}\lesssim 10^9\mathrm{\text{TeV}}$, while the O3-run bounds rule out warped models with $10^4\mathrm{\text{TeV}}\lesssim m_{\mathrm{\text{KK}}}\lesssim 10^7\mathrm{\text{TeV}}$ exhibiting an extremely strong confinement/deconfinement phase transition.