Hadron Collider Experiments Research Articles

Improved extrapolation methods of data-driven background estimations in high energy physics

Data-driven methods of background estimations are often used to obtain more reliable descriptions of backgrounds. In hadron collider experiments, data-driven techniques are used to estimate backgrounds due to multi-jet events, which are difficult to model accurately. In this article, we propose an improvement on one of the most widely used data-driven methods in the hadron collision environment, the “ABCD” method of extrapolation. We describe the mathematical background behind the data-driven methods and extend the idea to propose improved general methods.

Minimal and nonminimal universal extra dimension models in the light of LHC data at 13 TeV

Universal Extra Dimension (UED) is a well-motivated and well-studied scenario. One of the main motivations is the presence of a dark matter (DM) candidate namely, the lightest level-1 Kaluza-Klein (KK) particle (LKP), in the particle spectrum of UED. The minimal version of UED (mUED) scenario is highly predictive with only two parameters namely, the radius of compactification and cut-off scale, to determine the phenomenology. Therefore, stringent constraint results from the WMAP/PLANCK measurement of DM relic density (RD) of the universe. The production and decays of level-1 quarks and gluons in UED scenarios give rise to multijet final states at the Large Hadron Collider (LHC) experiment. We study the ATLAS search for multijet plus missing transverse energy signatures at the LHC with 13 TeV center of mass energy and 139 inverse femtobarn integrated luminosity. In view of the fact that the DM RD allowed part of mUED parameter-space has already been ruled out by the ATLAS multijet search, we move on to a less restricted version of UED namely, the non-minimal UED (nmUED), with non-vanishing boundary-localized terms (BLTs). The presence of BLTs significantly alters the dark matter as well as the collider phenomenology of nmUED. We obtain stringent bounds on the BLT parameters from the ATLAS multijet plus missing transverse energy search.

Nonperturbative effects on electroweakly interacting massive particles at hadron collider

Electroweakly Interacting Massive Particles (EWIMPs), in other words, new massive particles that are charged under the electroweak interaction of the Standard Model (SM), are often predicted in various new physics models. EWIMPs are probed at hadron collider experiments not only by observing their direct productions but also by measuring their quantum effects on Drell-Yan processes for SM lepton pair productions. Such effects are known to be enhanced especially when the di-lepton invariant mass of the final state is close to the EWIMP threshold, namely twice the EWIMP mass. In such a mass region, however, we have to carefully take non-perturbative effects into account, because the EWIMPs become non-relativistic and the prediction may be significantly affected by e.g., bound states of the EWIMPs caused by the electroweak interaction. We study such non-perturbative effects using the non-relativistic effective field theory of the EWIMPs, and found that those indeed affect the differential cross section of the Drell-Yan processes significantly, though the effects are smeared due to the finite energy resolution of the lepton measurement at the Large Hadron Collider experiment.

Probing dark matter with disappearing tracks at the LHC

Models where dark matter is a part of an electroweak multiplet feature charged particles with macroscopic lifetimes due to the charged-neutral mass split of the order of pion mass. At the Large Hadron Collider, the ATLAS and CMS experiments will identify these charged particles as disappearing tracks, since they decay into a massive invisible dark matter candidate and a very soft charged Standard-Model particle which fails to pass the reconstruction requirements. While ATLAS and CMS have focused on the supersymmetric versions of these scenarios, we have performed here the reinterpretation of the latest ATLAS disappearing track search for a suite of dark matter multiplets with different spins and electroweak quantum numbers. More concretely, we consider the cases of the inert Two Higgs Doublet model (i2HDM), of Minimal Fermion Dark Matter (MFDM) and of Vector Triplet Dark Matter (VTDM). Our procedure is validated by using the same wino and higgsino benchmark models employed by the ATLAS collaboration. We have found that with the disappearing track signature one can probe a vast portion of the parameter space, well beyond the reach of prompt missing energy searches (notably mono-jets). We provide tables with the upper-limits on the cross-section upper limits, and efficiencies in the lifetime - dark matter mass plane for all the models under consideration. Moreover we make the recasting code employed here publicly available, as part of the LLP Recasting Repository.

Triple Higgs boson production at the Large Hadron Collider with Two Real Singlet scalars

We investigate the production of three Higgs bosons in the Two Real Singlet extension of the Standard Model, where the scalar sector is augmented by two additional real scalar fields which are singlets under the Standard Model gauge group. The model contains three neutral CP-even scalars, allowing for resonant production and asymmetric decay chains. We focus on the signature pp → h3→ h1h2→ h1h1h1, where we identify h3 as the heaviest scalar state, h2 as the second heaviest and the lightest, h1, as the Standard Model-like Higgs boson discovered by the Large Hadron Collider experiments. The dominant final state occurs when all three Higgs bosons decay to bottom-anti-bottom quark pairs, h1→ boverline{b} , leading to 6 b-jets. Taking into account all current theoretical and experimental constraints, we determine the discovery prospects for this channel in future runs of the Large Hadron Collider, as well as in the high-luminosity phase.

A relatively light, highly bino-like dark matter in the Z3-symmetric NMSSM and recent LHC searches

A highly bino-like Dark Matter (DM), which is the Lightest Supersymmetric Particle (LSP), could be motivated by the stringent upper bounds on the DM direct detection rates. This is especially so when its mass is around or below 100 GeV for which such a bound tends to get most severe. Requiring not so large a higgsino mass parameter, that would render the scenario reasonably ‘natural’, prompts such a bino-like state to be relatively light. In the Minimal Supersymmetric Standard Model (MSSM), in the absence of comparably light scalars, such an excitation, if it has to be a thermal relic, is unable to meet the stringent experimental upper bound on its abundance unless its self-annihilation hits a funnel involving either the Z-boson or the Standard Model (SM)-like Higgs boson. We demonstrate that, in such a realistic situation, a highly bino-like DM of the popular Z3-symmetric Next-to-Minimal Supersymmetric Standard Model (NMSSM) is viable over an extended range of its mass, from our targeted maximum in the vicinity of the mass of the top quark down to about 30 GeV. This is facilitated by the presence of comparably light singlet-like states that could serve as funnel (scalars) and/or coannihilating (singlino) states even as the bino-like LSP receives a minimal (but optimal) tempering triggered by suitably light higgsino states that, in the first place, evade stringent lower bounds on their masses that can be derived from the Large Hadron Collider (LHC) experiments only in the presence of a lighter singlino-like state. An involved set of blind spot conditions is derived for the DM direct detection rates by considering for the very first time the augmented system of neutralinos comprising of the bino, the higgsinos and the singlino which highlights the important roles played by the NMSSM parameters ‘λ’ and tan β in delivering a richer phenomenology.

The upper bound of the second Higgs boson mass in minimal gauge mediation with the gravitino warm dark matter

A keV-scale gravitino arising from a minimal supersymmetric (SUSY) Standard Model (MSSM) is an interesting possibility since the small scale problems that the ΛCDM model encounters in the modern cosmology could be alleviated with the keV-scale gravitino serving as the warm dark matter (WDM). Such a light gravitino asks for a low scale supersymmetry (SUSY) breaking for which the gauge mediation (GM) is required as a consistent SUSY-breaking mediation mechanism. In this paper, we show upper bounds of the masses of the second CP-even Higgs boson H and the CP-odd Higgs boson A, assuming the keV-scale gravitino to be responsible for the current DM relic abundance: the upper bound on the mass of H/A is found to be ∼ 4 TeV for the gravitino mass of mathcal{O}left(10-100right) keV. Interestingly, the mass of H/A can be as small as 2–3 TeV and the predicted tan β is as large as 55–60 for the gravitino mass of mathcal{O}(10) keV. This will be tested in the near future Large Hadron Collider (LHC) experiments.

A Timing, Trigger, and Control System With Picosecond Precision Based on 10 Gbit/s Passive Optical Networks for High-Energy Physics

The Large Hadron Collider (LHC) experiments will have their timing, trigger, and control (TTC) system upgraded as a consequence of the need for higher bandwidth and components which are obsolete. In this article, we present a TTC based on passive optical networks (PONs). TTC-PON is a point-to-multipoint bidirectional TTC system based on the 10 Gbit/s International Telecommunications Union (ITU) XG-PON technology and modern field-programmable gate array (FPGA) devices. Each master can handle up to 64 slaves through a fully passive network, delivering a fixed-phase recovered clock to all the destinations with less than 5-ps jitter. TTC-PON pushes the limits of the PON technology by exploiting cutting-edge custom protocols on top of the commercially available XG-PON optical modules. It can potentially reuse the current optical fiber infrastructure already installed in the experiments and allows for high flexibility in terms of partitioning, which can ease future upgrades of the TTC network. The system features a picosecond-level on-the-fly phase monitoring for each slave's recovered clock by exploiting the bidirectionality of the network. In addition, a full set of link-quality monitoring tools was developed, allowing real-time performance monitoring. An overview of the tailored protocols will be given together with the details on the system implementation, operation, and performance. A discussion on the characterization campaign of more than 1000 optical modules delivered to the first implementation of the TTC-PON system will be drawn.

A simple method to determine charge-dependent curvature biases in track reconstruction in hadron collider experiments

A new data-driven method, using Zrightarrow mu ^+mu ^- decays, is proposed to correct for charge-dependent curvature biases in spectrometer experiments at hadron colliders. The method is studied assuming a detector with a “forward-spectrometer” geometry similar to that of the LHCb experiment, and is shown to reliably control several simplified detector mis-alignment configurations. The applicability of the method for use in measurements of precision electroweak observables is evaluated.

Machine Learning in Calorimeter optimization

The optimization of big industrial setups and the accompanying detailed simulations of internal physical processes require complex and time-consuming simulation calculations. We propose a versatile approach that can alleviate difficulties in solving this problem and show this using an example of electromagnetic calorimeter optimization at a Large Hadron Collider experiment. Our approach consists of a block representation of the calorimeter optimization process from setting sensitive characteristics of modules and their locations to obtaining a quality metric and applying machine learning methods. The main blocks are signal and background particles generation and their propagation to the calorimeter, the generation of electromagnetic showers of signal and noise in modules with a given technology, the combination of signal and noise with the simulation of different luminosities, the energy and spatial reconstruction of the signal and obtaining the final metric. This approach allows us to evaluate the operational characteristics of the calorimeter and find a more suitable configuration with the necessary quality without extensive use of time-consuming resources.

Comparative analysis of machine learning methods by the example of the problem of determining muon decay

The history of using machine learning algorithms to analyze statistical models is quite long. The development of computer technology has given these algorithms a new breath. Nowadays deep learning is mainstream and most popular area in machine learning. However, the authors believe that many researchers are trying to use deep learning methods beyond their applicability. This happens because of the widespread availability of software systems that implement deep learning algorithms, and the apparent simplicity of research. All this motivate the authors to compare deep learning algorithms and classical machine learning algorithms. The Large Hadron Collider experiment is chosen for this task, because the authors are familiar with this scientific field, and also because the experiment data is open source. The article compares various machine learning algorithms in relation to the problem of recognizing the decay reaction + + + at the Large Hadron Collider. The authors use open source implementations of machine learning algorithms. We compare algorithms with each other based on calculated metrics. As a result of the research, we can conclude that all the considered machine learning methods are quite comparable with each other (taking into account the selected metrics), while different methods have different areas of applicability.

Δϕ distributions between final state particles as a criterion of the pile-up background mismodeling and its impact on Z(vv)γ process

In the case of the pile-up background (background from the neighboring interactions inside the bunch crossing) for Z(vv)γ process at the Large Hadron Collider (LHC) experiments, its accurate calculation is very challenging. If the impact of the pile-up background is negligible, as it is expected in the case of Z(vv)γ process selection, some global uncertainty from this source can be used. This report studies the pile-up background and shows that it is expected to have a completely different shape for Δϕ distributions between final state particles in comparison to the signal and other background processes. Thus, the absence of Δϕ-mismodeling in the analysis proves that the pile-up background is indeed negligible.

Signature-Based Model-Independent Searches at the Large Hadron Collider: An Experimental Strategy Aiming at Safeness in a Theory-Dependent Way

Signature-based model-independent (SBMI) searches for new physics form an explorative component of High Energy Physics experiments that aims at avoiding biases toward beyond-the-standard-model theories. In order to maximize their chances of meeting their objectives, however, SBMI searches must be guided by theory about the target phenomena. They correspond to strategy-driven experiments as defined by Koray Karaca, in contrast to Friedrich Steinle’s conception of explorative experiments. A concept of type II safeness is proposed to explain how the objectives of the experiment introduce theory dependence in the search strategy. Empirical evidence obtained from Large Hadron Collider experiments can therefore not avoid dependence on theory.

What can a heavy boson do to the muon anomaly and to a new Higgs boson mass? * *J.P.R is supported by the project PTDC/FIS-PAR/31000/2017. The work of A.P.M. has been performed in the framework of COST Action CA16201 “Unraveling new physics at the LHC through the precision frontier” (PARTICLEFACE). A.P.M. is supported by the Center for Research and Development

The minimal extension of the Standard Model (B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism; it also offers two new physics states, namely an extra Higgs boson and a new gauge boson. The emergence of a second Higgs particle as well as a new gauge boson, both linked to the breaking of a local symmetry, makes the B-L-SM rather constrained by direct searches in Large Hadron Collider (LHC) experiments. We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data. Taking into account the current bounds from direct LHC searches, we demonstrate that the prediction for the muon anomaly in the B-L-SM yields at most a contribution of approximately , which represents a tension of standard deviations, with the current uncertainty, by means of a boson if its mass is in the range of to , within the reach of future LHC runs. This means that the B-L-SM, with heavy yet allowed boson mass range, in practice, does not resolve the tension between the observed anomaly in the muon and the theoretical prediction in the Standard Model. Such a heavy boson also implies that the minimal value for the new Higgs mass is of the order of 400 GeV.

Gas R&D on gas recirculation and recuperation for the Resistive Plate Chamber detectors

Resistive Plate Chamber (RPC) detectors at the Large Hadron Collider (LHC) experiments are operated with a gas mixture containing C2H2F4 (R134a) and SF6. These gases are used because they allow to achieve specific detector performance. However, due to their high Global Warming Potential (GWP) they are currently subject to a phase down policy that started to affect the market with price increase and it may cause a decrease in their long-term availability. To minimize these risks, four different strategies have been identified for optimizing their usage. As immediate actions and where possible, during the LHC Long Shutdown 2 (LS2) the RPC gas systems will be upgraded to cope with new detector requirements and, in parallel, extensive campaigns for fixing leaks at detector level will be performed. Since R134a dominates the greenhouse gas consumption, the development of a R134a gas recuperation plant is going to be the next step and it might have an important positive impact on the overall optimization process. Recently a prototype system was tested with encouraging results. For future long-term detector operation, R&D studies are ongoing for finding “green” alternatives to the currently used gases. Unfortunately, new gases developed by industry as refrigerant fluids are not behaving as the R134a in particle detectors which makes the replacement for present experiments difficult. The last strategy consists in the possibility of using industrial plants for the disposal of greenhouse gases by decomposition in harmless compounds. This solution avoids the emission in the atmosphere but it does not optimize the gas usage and problems like gas availability and price for detector operation might become the challenge in the coming years due to the greenhouse phase down policy.

Effects of saturation temperature on the boiling properties of carbon dioxide in small diameter pipes at low vapour quality: Pressure drop

The thermal management of most of the silicon pixel detectors at the heart of the Large Hadron Collider experiments at CERN relies on boiling carbon dioxide inside compact heat exchangers of small hydraulic diameter at saturation temperatures between +15 ∘C and −30 ∘C. Due to the current lack of suitable predictive models, a long-term study has been launched to create a consistent and reliable experimental database studying the peculiarities of boiling carbon dioxide in mini- and micro-channels. This study presents results on the total pressure drop of boiling carbon dioxide at low vapour quality (0 < x < 0.4) in 200 mm-long stainless steel tubes with an inner diameter of 2.15 mm, 1 mm and 0.5 mm. By means of a dedicated test setup equipped with high precision sensors, a wide range of saturation temperatures (+15 ∘C to −25 ∘C) and mass fluxes (100 to 1800 kg/m2s) have been explored and diabatic tests were carried out for heat fluxes from 5 to 35 kW/m2. The data presented focus on the influence of the saturation temperature on the two-phase pressure drop. It is suggested that the combination of shifting physical properties of carbon dioxide and different confinement conditions causes a change in the phenomenological behaviour of the flow and that a transition between macro- and micro-scale most likely occurs within the range of test parameters. It is further shown that a shift in the applicability of existing prediction methods is caused by those effects and no correlation is able to predict the experimental data and trends in the whole temperature range. A selective approach to the use of existing correlations is also proposed.

Di-Higgs production in SUSY models at the LHC

We study the modification to di-Higgs production via gluon fusion within the context of the minimal supersymmetric standard model and the next-to-minimal supersymmetric standard model in the parameter space allowed by current experimental and theoretical constraints, and also relevant to the Large Hadron Collider (LHC) experiments in the near future. The calculation is based on the analytical expression of the leading-order Feynman amplitudes (which includes both quark and squark loops). We separate the di-Higgs production cross section into resonant, non-resonant, and interference parts, in order to better understand the mechanisms that are responsible for the modification to di-Higgs production rate in different regions of the allowed parameter space. We also investigate the sensitivity of high-luminosity LHC (HL-LHC) to the di-Higgs production in these low-energy supersymmetry models. Furthermore, we examine the complementarity between di-Higgs searches and direct searches for BSM particles and precision Higgs couplings measurements at the HL-LHC. We found that the di-Higgs production cross section can be enhanced significantly through resonant production. In the region where the resonant production cross section is small, di-Higgs production receives a moderate enhancement due to the modifications in the Higgs couplings. In addition, there is a strong correlation between di-Higgs production and single Higgs production, and di-Higgs production is not as sensitive as single Higgs production at the HL-LHC.

Leptonic scalars at the LHC

We explore the collider prospects of neutrino non-standard interaction with a Standard Model (SM) gauge-singlet leptonic scalar ϕ carrying two units of lepton-number-charge. These leptonic scalars are forbidden from interacting with the SM fermions at the renormalizable level and, if one allows for higher-dimensional operators, couple predominantly to SM neutrinos. For masses at or below the electroweak scale, ϕ decays exclusively into neutrinos. Its characteristic production signature at hadron collider experiments like the LHC would be via the vector boson fusion process and leads to same-sign dileptons, two forward jets in opposite hemispheres, and missing transverse energy, i.e., ppto {mathrm{ell}}_{alpha}^{pm }{mathrm{ell}}_{beta}^{pm } jj+{E}_T^{mathrm{miss}}left(alpha, beta =e,mu, tau right) . Exploiting the final states of electrons and muons, we estimate, for the first time, the sensitivity of the LHC to these lepton-number-charged scalars. We show that the LHC sensitivity is largely complementary to that of low-energy precision measurements of the decays of charged leptons, charged mesons, W, Z and the SM Higgs boson, as well as the neutrino beam experiments like MINOS, and searches for neutrino self-interactions at IceCube and in cosmological observations. For ϕ mass larger than roughly 10 GeV, our projected LHC sensitivity would surpass all existing bounds.

New challenges for distributed computing at the CMS experiment

The Large Hadron Collider (LHC) experiments soon step into the next period of run-3 data taking with an increased data rate and high pileup requiring an excellent working computing infrastructure. In the future High-Luminosity LHC (HL-LHC) data-taking period, the compute, storage and network facilities have to be further extended by large factors and flexible and sophisticated computing models are essential. New techniques of modern state-of-the-art methods in physics analysis and data science, Deep Learning and Big Data tools, are crucial to handle high-dimensional and more complex problems. Beside flexible cloud computing technologies the usage of High Performance Computing (HPC) at the LHC experiments are explored. In this presentation, I will discuss the LHC run-3 and future HL-LHC runs computing technologies and the utilization of modern physics analysis and data science methods for the increasing and complex demands of large-scale scientific computing.

Debated Models for Galactic Rotation Curves: A Review and Mathematical Assessment

Proposed explanations of galactic rotation curves (RC = tangential velocity vs. equatorial radius, determined from Doppler measurements) involve dramatically different assumptions. A dominant, original camp invoked huge amounts of unknown, non-baryonic dark matter (NBDM) in surrounding haloes to reconcile RC simulated using their Newtonian orbital models (NOMs) for billions of stars in spiral galaxies with the familiar Keplerian orbital patterns of the few, tiny planets in our Solar System. A competing minority proposed that hypothetical, non-relativistic, non-Newtonian forces govern the internal motions of galaxies. More than 40 years of controversy has followed. Other smaller groups, unsatisfied by explanations rooted in unknown matter or undocumented forces, have variously employed force summations, spin models, or relativistic adaptations to explain galactic rotation curves. Some small groups have pursued inverse models and found no need for NBDM. The successes, failures, and underlying assumptions of the above models are reviewed in this paper, focusing on their mathematical underpinnings. We also show that extractions of RC from Doppler measurements need revising to account for the effect of galaxy shape on flux-velocity profiles and for the possible presence of a secondary spin axis. The latter is indicated by complex Doppler shift patterns. Our findings, combined with independent evidence such as hadron collider experiments failing to produce non-baryonic matter, suggest that a paradigm shift is unfolding.