Large Hadron Collider Experiments Research Articles

Signatures of vector-like top partners decaying into new neutral scalar or pseudoscalar bosons

We explore the phenomenology of models containing one Vector-Like Quark (VLQ), t′, which can decay into the Standard Model (SM) top quark, t, and a new spin-0 neutral boson, S, the latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the t′ can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming t′ pair production via strong interactions, we design signal regions optimised for one t′ → S t transition (while being inclusive on the other overline{t} ′ decay, and vice versa), followed by the decay of S into the two very clean experimental signatures S → γ γ and S → Z (→ ℓ+ℓ−)γ. We perform a dedicated signal- to-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR(t′ → S t) = 100%, we are therefore able to realistically quantify the sensitivity of the LHC to both the t′ and S masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on t′ decays into SM bosons (W±, Z , h).

Suppressing the scattering of WIMP dark matter and nucleons in supersymmetric theories

The continuously improving sensitivity of dark matter direct detection experiments has limited the interaction between dark matter and nucleons being increasingly feeble, while the dark matter relic density favors it to take part in weak interactions. After taking into account the constraints from the Large Hadron Collider (LHC) search for Higgs bosons and sparticles, it is becoming difficult for the neutralino dark matter in the Minimal Supersymmetric Standard Model and the Next-to-Minimal Supersymmetric Standard Model to possess these two seemingly paradoxical features in their most natural parameter space for electroweak symmetry breaking due to the limited theoretical structure. In contrast, the seesaw extension of the Next-to-Minimal Supersymmetric Standard Model, which was initially proposed to solve the neutrino mass problem, enables the lightest sneutrino to act as a viable dark matter candidate, readily has these features, and thus, it easily satisfies the constraints from dark matter and LHC experiments. Compared with the Type-I seesaw extension, the dark matter physics in the inverse seesaw extension is more flexible, allowing it to be consistent with the experimental results in broader parameter space. We conclude that weakly interacting massive particles (such as the sneutrino in this study) work well in supersymmetric theories as dark matter candidates.

Studies on tetrafluoropropene-based gas mixtures with low environmental impact for Resistive Plate Chambers

Gaseous detectors are widely used in high-energy physics experiments, and in particular at the CERN Large Hadron Collider (LHC), to provide tracking and triggering over large volumes. It has been recently estimated that Resistive Plate Chambers (RPC), used for muon detection, have the highest contribution on the overall greenhouse gas (GHG) emissions at the LHC experiments. Gas mixtures for RPCs are mainly made of C2H2F4, which is a greenhouse gas with a high environmental impact in the atmosphere. C2H2F4 is already phasing out of production, due to recent European Union (EU) regulations, and its cost is expected to increase in the near future. Therefore, finding alternative gas mixtures made of gas components with a low Global Warming Potential (GWP) has become extremely important for limiting the GHG emissions as well as for economic reasons. The novel hydrofluoroolefins are likely appropriate candidates to replace C2H2F4 due to their similar chemical structures. This study is focused on the characterization of innovative gas mixtures with tetrafluoropropene HFO1234ze(E) (C3H2F4) that is one of the most eco-friendly hydrofluoroolefins, thanks to its very low GWP. HFO1234ze(E)-based gas mixtures with the addition of Ar, N2, O2 and CO2 are extensively discussed in this paper as well as the role of i-C4H10 and SF6 in such mixtures.

Production, Quality Control and Performance of GE1/1 Detectors for the CMS Upgrade

The Large Hadron Collider (LHC) will be upgraded in several phases that will allow significant expansion of its physics program. After the long shutdown in 2019 (LS2) the accelerator luminosity will be increased to 2-3 ×1034 cm−2s−1 for Run 3 and later up to 5×1034 cm−2s−1 for Phase 2 (HL-LHC). The physics program of the LHC experiments will benefit from the augmented luminosity; the sensitivity of the CMS experiment to new physics and to the Standard Model will be fully exploited, providing that a suitable upgrade will enable the CMS detector to cope with future data-taking conditions. Among the upgrades, the installation of new muon stations based on Gas Electron Multiplier (GEM) technology in the endcap will start in early 2019 (GE1/1 station), followed by the installation of two additional stations (GE2/1 and ME0) in 2023. The CMS Muon Collaboration produced the 144 GEM detectors to be installed in the GE1/1 station, sharing the assembly and testing of the detectors among several production sites spread all over the world. A detailed common assembly protocol and quality control procedure (QC) has been deployed, with the ambitious goal to ensure standardization of the performance of the detectors produced by the different sites. The same procedure has been successfully adopted to test the first prototypes of the GE2/1 detectors. In this contribution, we present the final results of the QC tests performed on the GE1/1 chambers, assembled by all the production sites following the common specification parameters.

Diffraction and photon exchange processes at the LHC and parton saturation

We present a review of the recent theoretical and experimental developments related to the field of diffraction, parton saturation, and forward physics. We first discuss our present understanding of the proton structure in terms of quarks and gluons, the degrees of freedom of quantum chromodynamics. We then focus on some of the main results on diffraction at the HERA electron–proton collider in DESY, Germany, at the Tevatron proton–antiproton collider at Fermilab, Batavia, US, and at the CERN Large Hadron Collider (LHC) proton–proton and nucleus–nucleus collider, which is located in Geneva, Switzerland. We also present a selected amount of results on diffraction and photon exchanges that can be done at the LHC experiments and at a future Electron Ion Collider (EIC) to be built in the US at Brookhaven National Laboratory, New York.

Achieving Picosecond-Level Phase Stability in Timing Distribution Systems With Xilinx Ultrascale Transceivers

This article discusses the challenges posed on the field-programmable gate array (FPGA) transceivers in terms of phase-determinism requirements for timing distribution at the Large Hadron Collider (LHC) experiments. Having a fixed phase after startups is a major requirement, and the typical phase variations observed in the order of tens of picoseconds after startups while using the state-of-the-art design techniques are no longer sufficient. Each limitation observed in the transmitter and receiver paths of the high-speed transceivers embedded in the Xilinx Ultrascale FPGA family is further investigated and solutions are proposed. Tests in hardware using Xilinx FPGA evaluation boards are presented. In addition to a higher phase determinism, the techniques presented make it possible to fine-tune the skew of a link with a picosecond resolution, greatly simplifying clock-domain crossing inside the FPGAs and providing better short-term stability for the FPGA-recovered clock in a high-speed link.

Integrating LHCb workflows on HPC resources: status and strategies

High Performance Computing (HPC) supercomputers are expected to play an increasingly important role in HEP computing in the coming years. While HPC resources are not necessarily the optimal fit for HEP workflows, computing time at HPC centers on an opportunistic basis has already been available to the LHC experiments for some time, and it is also possible that part of the pledged computing resources will be offered as CPU time allocations at HPC centers in the future. The integration of the experiment workflows to make the most efficient use of HPC resources is therefore essential. This paper describes the work that has been necessary to integrate LHCb workflows at a specific HPC site, the Marconi-A2 system at CINECA in Italy, where LHCb benefited from a joint PRACE (Partnership for Advanced Computing in Europe) allocation with the other Large Hadron Collider (LHC) experiments. This has required addressing two types of challenges: on the software application workloads, for optimising their performance on a many-core hardware architecture that differs significantly from those traditionally used in WLCG (Worldwide LHC Computing Grid), by reducing memory footprint using a multi-process approach; and in the distributed computing area, for submitting these workloads using more than one logical processor per job, which had never been done yet in LHCb.

Particle Track Reconstruction with Quantum Algorithms

Accurate determination of particle track reconstruction parameters will be a major challenge for the High Luminosity Large Hadron Collider (HL-LHC) experiments. The expected increase in the number of simultaneous collisions at the HL-LHC and the resulting high detector occupancy will make track reconstruction algorithms extremely demanding in terms of time and computing resources. The increase in number of hits will increase the complexity of track reconstruction algorithms. In addition, the ambiguity in assigning hits to particle tracks will be increased due to the finite resolution of the detector and the physical “closeness” of the hits. Thus, the reconstruction of charged particle tracks will be a major challenge to the correct interpretation of the HL-LHC data. Most methods currently in use are based on Kalman filters which are shown to be robust and to provide good physics performance. However, they are expected to scale worse than quadratically. Designing an algorithm capable of reducing the combinatorial background at the hit level, would provide a much “cleaner” initial seed to the Kalman filter, strongly reducing the total processing time. One of the salient features of Quantum Computers is the ability to evaluate a very large number of states simultaneously, making them an ideal instrument for searches in a large parameter space. In fact, different R&D initiatives are exploring how Quantum Tracking Algorithms could leverage such capabilities. In this paper, we present our work on the implementation of a quantum-based track finding algorithm aimed at reducing combinatorial background during the initial seeding stage. We use the publicly available dataset designed for the kaggle TrackML challenge.

Real-time data analysis model at LHC and connections to other experiments and fields

With the upcoming increase of proton-proton collision rates at the Large Hadron Collider (LHC) experiments, and the corresponding increase of data volumes, real-time analysis becomes a key ingredient to be able to analyse and select the interesting data within the available computing resources. In this talk I will review the main features of the techniques followed by the ATLAS, CMS and LHCb experiments. Similar challenges have to be faced in other fields, such as astronomy and cosmology, and I will comment about them.

Impact of different compilers and build types on Geant4 simulation execution time

Experimental observations and advanced computer simulations in High Energy Physics (HEP) paved the way for the recent discoveries at the Large Hadron Collider (LHC) at CERN. Currently, Monte Carlo simulations account for a very significant amount of computational resources of the Worldwide LHC Computing Grid (WLCG). The current growth in available computing performance will not be enough to fulfill the expected demand for the forthcoming High Luminosity run (HL-LHC). More efficient simulation codes are therefore required. This study focuses on evaluating the impact of different build methods on the simulation execution time. The Geant4 toolkit, the standard simulation code for the LHC experiments, consists of a set of libraries which can be either dynamically or statically linked to the simulation executable. Dynamic libraries are currently the preferred build method. In this work, three versions of the GCC compiler, namely 4.8.5, 6.2.0 and 8.2.0 have been used. In addition, a comparison between four optimization levels (Os, O1, O2 and O3) has also been performed. Static builds for all the GCC versions considered, exhibit a reduction in execution times of about 10%. Switching to newer GCC version results in an average of 30% improvement in the execution time regardless of the build type. In particular, a static build with GCC 8.2.0 leads to an improvement of about 34% with respect to the default configuration (GCC 4.8.5, dynamic, O2). The different GCC optimization flags do not affect the execution times.

Testing non-standard sources of parity violation in jets at the LHC, trialled with CMS Open Data

The Standard Model violates parity, but only by mechanisms which are invisible to Large Hadron Collider (LHC) experiments (on account of the lack of initial state polarisation or spin-sensitivity in the detectors). Nonetheless, new physical processes could potentially violate parity in ways which are detectable by those same experiments. If those sources of new physics occur only at LHC energies, they are untested by direct searches. We probe the feasibility of such measurements using approximately 0.2 fb−1 of data which was recorded in 2012 by the CMS collaboration and made public within the CMS Open Data initiative. In particular, we test an inclusive three-jet event selection which is primarily sensitive to non-standard parity violating effects in quark-gluon interactions. Within our measurements, no significant deviation from the Standard Model is seen and no obvious experimental limitations have been found. We discuss other ways that searches for non-standard parity violation could be performed, noting that these would be sensitive to very different sorts of models to those which our method would constrain. We hope that our initial studies provide a valuable starting point for rigorous future analyses using the full LHC datasets at 13 TeV with a careful and less conservative estimate of experimental uncertainties.

Strong constraints on non-standard neutrino interactions: LHC vs. IceCube

We find the constraints on various non-standard interactions (NSI) of neutrinos from monojet+T searches at the Large Hadron Collider (LHC). Also, we show that the measurement of neutrino-nucleon cross-section from the observation of high energy astrophysical neutrino events at IceCube facilitates strong constraints on NSI as well. To this end, we pursue a comparative study of the prospects of LHC and IceCube in detecting NSI, also mentioning the role of low-energy experiments. We discuss the case of NSI with a new vector boson Z′ and it is found that for some range of mz′ LHC puts more stringent bound, whereas IceCube supersedes elsewhere. We also pay special attention to the case of Z′ of mass of a few GeVs, pointing out that the IceCube constraints can surpass those from LHC and low-energy experiments. Although, for contact-type effective interactions with two neutrinos and two partons, constraints from LHC are superior.

Inverse seesaw model with global U(1)H symmetry

We propose an inverse seesaw model based on hidden global symmetry $U(1)_H$ in which we realize tiny neutrino masses with rather natural manner taking into account relevant experimental bounds. The small Majorana mass for inverse seesaw mechanism is induced via small vacuum expectation value of a triplet scalar field whose Yukawa interactions with standard model fermions are controlled by $U(1)_H$. We discuss the phenomenology of the exotic particles present in the model including the Goldstone boson coming from breaking of the global symmetry, and explore testability at the Large Hadron Collider experiments.

Studying Proton–Proton Interaction at Large Hadrons Collider Using Genetic Programming

This paper describes how to use Genetic Programming (GP) as an evolutionary computational that is a family of algorithms for global optimization. GP, as a global optimization technique used by discovery of a new function for modeling physical phenomena. The p-p interactions are modeled at Large Hadron Collider (LHC) experiments, the number of charged particles multiplicity <n> and the total cross-section, σT, as functions of the total center of mass energy (from low to ultra-high energy), are discovered by using GP. In view of the discovered function for , the overall trend of the values predicted is consistent with LHC data [predicted values are 34.8638 and 35.3520 at and respectively]. The new function , trained on experimental data of Particle Data Group (PDG) demonstrates a nice match to the other models. The predicted values of the total cross section at , and 14 TeV are found to be 109.0381 mb and 111.8329 mb respectively. Furthermore, the values predicted are agreed with other models like Block

Gas mixture quality monitoring for the RPC detectors at the LHC experiments

Resistive Plate Chamber (RPC) detectors are widely employed in the muon trigger systems of three experiments at the CERN Large Hadron Collider (LHC) thanks to their excellent time resolution. The LHC RPC systems are operated under gas recirculation to reduce operation cost and greenhouse gas emissions since their gas mixture is based on C2H2F4, which has a global warning potential of 1430 and it is subject to the European F-gas Regulation. Extensive gas analysis campaigns have been performed during LHC Run 2 for the CMS RPC and ALICE Muon Trigger (MTR) systems to verify the gas mixture quality and possible accumulation of impurities. A particular attention has been addressed to the ALICE MTR system, which has been operated under gas recirculation from the end of 2015. In order to validate the system ensuring good detector operation, the gas recirculation has been increased in steps: 30%, 60% and 70%. Detector currents and gas mixture quality have been closely monitored. A gas chromatograph and mass spectrometer (GC/MS) station has been installed to analyze the MTR gas mixture in different points of the gas system: fresh gas from the mixer, detectors output and output of the purifier module. Several impurities have been found and identified. Most of the impurities are created inside the detector gas gap due to the fragmentation of the C2H2F4 molecule under the effects of electric field and radiation. The GC/MS analyses have been regularly performed in 2016 and 2017. It has been demonstrated that impurities concentration (at the level of tens of ppm) increases with the increase of the gas recirculation fraction. GC/MS analyses have been also performed after the purifier module showing that some impurities are filtered while others not. In parallel, the RPC currents have been constantly monitored, and their trend showed no correlation with the gas recirculation fraction. In 2017 an Ion Selective Electrode station was installed to measure the fluoride ions (F−) concentration in the MTR RPC gas mixture. Indeed the products of the C2H2F4 fragmentation not always recombine and F^− species can stay free in the gas mixture. The analyses show that F− are present in the mixture exiting the RPCs and their concentration increases with the increase of luminosity, even if it stays at the level of ppb per day. A comprehensive overview of the results obtained from the different types of gas analyses and possible correlation with RPC currents and LHC luminosity will be presented.

Probing novelty at the LHC: Heuristic appraisal of disruptive experimentation

Abtract In this paper, ‘novelty’ is explored through a recent historical episode from high energy experimental physics to offer an understanding of novelty as disruption. I call this the ‘750 GeV episode’, an episode where two Large Hadron Collider (LHC) experiments, CMS and ATLAS, each independently observed indications of a new resonance at approximately 750 GeV. With further data collection, the initial excess was determined to be a statistical fluctuation. The approach taken, in the analysis of interviews conducted with physicists who were involved in the ‘750 GeV episode’, is to consider novelty as a valued difference. Following this conceptually driven approach, disambiguate between several notions of novelty through the identification of varied differences. This disambiguation is achieved through exploring differences expressed in comparison to varied expressions of the standard model, and through exploring varied ‘types’ of difference (properties and entities) to introduce disruptive exploratory experimentation, a complementary understanding ‘exploratory experimentation’ (Elliott, 2007; Steinle, 1997, 2002). I show that the kinds of novelty framed as most valuable are those that violate expectations and are difficult to incorporate into the existing structures of knowledge. In such instances, disruption to the existing ontology or ways of knowing is valued. This positive appraisal of disruption, and contradiction over confirmation, is considered in the recent context of high-energy physics, where several physicists have claimed that there is a lack of promising directions for the future, or even that the field is in a ‘crisis’. I show that the role of disruption explains the differences between the differing notions of novelty. Furthermore, I show that the positive appraisal of disruption is based on forward looking assessments of future fertility, or heuristic appraisal (Nickles, 1989, 2006). Within the context of concerns of a lack of available promising future directions, disruption becomes a generator of alternative futures.

Model independent analysis of B^* rightarrow P ell {bar{nu }}_ell decay processes

Very compelling deviations in the recently observed lepton nonuniversality observables (R_{D^{(*)}}, R_{K^{(*)}}, R_{J/psi } ) of semileptonic B meson decays from their Standard Model predictions hint towards the presence of some kind of new physics beyond it. In this regard, we investigate the effect of new physics in the semileptonic {bar{B}}_{d(s)}^* rightarrow P ell {bar{nu }}_ell decay processes, where P=D,pi (D_s,K), in a model independent way. We consider the presence of additional vector and scalar type interactions and constrain the corresponding new couplings by fitting mathrm{Br(B_{u}^+ rightarrow tau ^+ nu _tau )}, {mathrm{mathrm Br}(B rightarrow pi tau {{bar{nu }}}_tau )}, mathrm{Br(B_{c}^+ rightarrow tau ^+ nu _tau )}, R_pi ^l, R_{D^{(*)}} and R_{J/psi } data. Using the constrained new parameters, we estimate the branching ratios, forward–backward asymmetry, lepton-spin asymmetry and lepton non-universality observables of {bar{B}}_{d,s}^{*} rightarrow P tau {{bar{nu }}}_tau processes. We find that the branching ratios of these decay modes are sizeable and deviate significantly (for vector-type couplings) from their corresponding standard model values, which are expected to be within the reach of Run III of Large Hadron Collider experiment.

Importance-based signal detection and parameter estimation with applications tonew particle search

One of the hardest challenges in data analysis is perhaps the detection of rare anomalous data buried in a huge normal background. We study this problem by constructing a novel method, which is a combination of the Kullback?Leibler importance estimation procedure based anomaly detection algorithm and linear discriminant classifier. We choose to illustrate it with the example of charged Higgs boson (CHB) search in particle physics. Indeed, the Large Hadron Collider experiments at CERN ensure that CHB signal must be a tiny effect within the irreducible W-boson background. In simulations, different CHB events with different characteristics are produced and judiciously mixed with the non-CHB data, and the proposed method is applied. Our results show that distribution parameters of weak CHB signals can be estimated with high performance. This anomaly detection method is general enough to apply to similar problems in other fields (e.g., astrophysical, medical, engineering problems).

A peak efficiency tracking technique to improve the efficiency of switched capacitor DC–DC converters

In this paper, a peak efficiency tracking technique to improve the efficiency of switched capacitor (SC) DC–DC converters as the load varies is presented. A peak efficiency tracking circuit based on feedback control over the switching frequency is implemented for this scope. The basic idea of the proposed technique is to adjust the switching frequency according to the load. The technique is successfully implemented in a SC DC–DC converter to be embedded in detector pixels for the Large Hadron Collider (LHC) experiment at the Conseil Europeen pour la Recherche Nucleaire (CERN) of Geneve. It is realized in 65 nm bulk CMOS technology with an occupied area of 1.31 mm2. This converter provides an 800 mV output voltage from a 1.2 V supply. The load of the DC–DC converters is modeled as a resistor, RLOAD, that has 4 Ω nominal value but it can range from 2.67 up to 10 Ω. At 10 Ω RLOAD, a 6% efficiency improvement is reached with respect to the typical approach consisting in keeping constant the switching frequency at the optimum value for RLOAD nominal value.

Extending the constraint for axion-like particles as resonances at the LHC and laser beam experiments

We study the discovery potential of axion-like particles (ALP), pseudo-scalars weakly coupled to Standard Model fields, at the Large Hadron Collider (LHC). Our focus is on ALPs coupled to the electromagnetic field, which would induce anomalous scattering of light-by-light. This can be directly probed in central exclusive production of photon pairs in ultra-peripheral collisions at the LHC in proton and heavy-ion collisions. We consider non-standard collision modes of the LHC, such as argon-argon collisions at sNN=7 TeV and proton-lead collisions at sNN=8.16 TeV, to access regions in the parameter space complementary to the ones previously considered for lead-lead and proton-proton collisions. In addition, we show that, using laser beam interactions, we can constrain ALPs as resonant deviations in the refractive index induced by anomalous light-by-light scattering effects. If we combine the aforementioned approaches, ALPs can be probed in a wide range of masses from the eV scale up to the TeV scale.