Event Topology Research Articles

Constraining Stealth SUSY with illuminated fat jets at the LHC

We investigate the discovery potential of a Stealth SUSY scenario involving squark decays by reconstructing the lightest neutralino decay products using a large-radius jet containing a high transverse momentum photon. Requirements on the event topology, such as photon and large-radius jet multiplicity result in less background than signal. We also estimated the sensitivity of our analysis and found that it has a better exclusion potential compared to the strongest existing search for the specific benchmark points considered here.

Single-layer Compton detectors for measurement of polarization correlations of annihilation quanta

Measurement of gamma-ray polarization can provide valuable insight in different areas of physics research. One possible application is in Positron Emission Tomography, where the annihilation quanta with orthogonal polarizations are emitted. Since polarization can be measured via Compton scattering, the initial orthogonality of polarizations can be translated to correlation of azimuthal scattering angles, and this correlation may be exploited as an additional handle to identify the true coincidence events. In order to examine the concept of utilizing the polarization correlations in PET, we have used a system of two compact, position and energy-sensitive Compton scattering detectors in coincidence mode. Each consists of a single matrix of scintillation pixels, read-out by a matching array of Silicon photomultipliers on the back side. The Compton events in each module are clearly identified and the scattering angles are reconstructed from the energy deposition and event topology. We have extracted the polarimetric modulation factors from the measured azimuthal scattering angles of the two Compton-scattered gammas and studied their dependence on Compton scattering angles θ and on azimuthal resolution Δϕ. For scattering angles around θ1,2=82∘, where the maximum modulation is expected, the modulation factors from μ=0.15±0.01 to μ=0.27±0.02 have been measured, depending on the azimuthal resolution, which is governed by event topology in the detectors. Analogously, for scattering around θ1,2=70∘, modulation factors from μ=0.12±0.01 to μ=0.21±0.02 have been obtained. The results show that the measurement of the polarization correlations of annihilation quanta are feasible with compact single-layer, single-side read-out detectors, which may be used to build cost-efficient systems for various applications where gamma-ray polarization information is of interest.

Double-beta decay of ^{130}hbox {Te} to the first 0^+ excited state of ^{130}hbox {Xe} with CUORE-0

We report on a search for double beta decay of ^{130}hbox {Te} to the first 0^{+} excited state of ^{130}hbox {Xe} using a 9.8,hbox {kg}cdot hbox {yr} exposure of ^{130}hbox {Te} collected with the CUORE-0 experiment. In this work we exploit different topologies of coincident events to search for both the neutrinoless and two-neutrino double beta decay modes. We find no evidence for either mode and place lower bounds on the half-lives: T^{0nu }_{0^+_1}>7.9cdot 10^{23}hbox {yr} and T^{2nu }_{0^+_1}>2.4cdot 10^{23}hbox {yr} (90%,hbox {CL}). Combining our results with those obtained by the CUORICINO experiment, we achieve the most stringent constraints available for these processes: T^{0nu }_{0^+_1}>1.4cdot 10^{24}hbox {yr} and T^{2nu }_{0^+_1}>2.5cdot 10^{23}hbox {yr} (90%,hbox {CL}).

Topology Classification with Deep Learning to Improve Real-Time Event Selection at the LHC

We show how event topology classification based on deep learning could be used to improve the purity of data samples selected in real time at at the Large Hadron Collider. We consider different data representations, on which different kinds of multi-class classifiers are trained. Both raw data and high-level features are utilized. In the considered examples, a filter based on the classifier's score can be trained to retain ~99% of the interesting events and reduce the false-positive rate by as much as one order of magnitude for certain background processes. By operating such a filter as part of the online event selection infrastructure of the LHC experiments, one could benefit from a more flexible and inclusive selection strategy while reducing the amount of downstream resources wasted in processing false positives. The saved resources could be translated into a reduction of the detector operation cost or into an effective increase of storage and processing capabilities, which could be reinvested to extend the physics reach of the LHC experiments.

Sub-GeV Atmospheric Neutrinos and CP Violation in DUNE.

We propose to use the unique event topology and reconstruction capabilities of liquid argon time projection chambers to study sub-GeV atmospheric neutrinos. The detection of low energy recoiled protons in DUNE allows for a determination of the leptonic CP-violating phase independent from the accelerator neutrino measurement. Our findings indicate that this analysis can exclude a range of values of δ_{CP} beyond the 3σ level. Moreover, the determination of the sub-GeV atmospheric neutrino flux will have important consequences in the detection of diffuse supernova neutrinos and in dark matter experiments.

Variational autoencoders for new physics mining at the Large Hadron Collider

Using variational autoencoders trained on known physics processes, we develop a one-sided threshold test to isolate previously unseen processes as outlier events. Since the autoencoder training does not depend on any specific new physics signature, the proposed procedure doesn’t make specific assumptions on the nature of new physics. An event selection based on this algorithm would be complementary to classic LHC searches, typically based on model-dependent hypothesis testing. Such an algorithm would deliver a list of anomalous events, that the experimental collaborations could further scrutinize and even release as a catalog, similarly to what is typically done in other scientific domains. Event topologies repeating in this dataset could inspire new-physics model building and new experimental searches. Running in the trigger system of the LHC experiments, such an application could identify anomalous events that would be otherwise lost, extending the scientific reach of the LHC.

Performance of top-quark and varvec{W}-boson tagging with ATLAS in Run 2 of the LHC

The performance of identification algorithms (“taggers”) for hadronically decaying top quarks and W bosons in pp collisions at sqrt{s} = 13 TeV recorded by the ATLAS experiment at the Large Hadron Collider is presented. A set of techniques based on jet shape observables are studied to determine a set of optimal cut-based taggers for use in physics analyses. The studies are extended to assess the utility of combinations of substructure observables as a multivariate tagger using boosted decision trees or deep neural networks in comparison with taggers based on two-variable combinations. In addition, for highly boosted top-quark tagging, a deep neural network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is presented. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1 fb^{-1} for the tbar{t} and gamma +text {jet} and 36.7 fb^{-1} for the dijet event topologies.

Spotting hidden sectors with Higgs binoculars

We explore signals of new physics with two Higgs bosons and large missing transverse energy at the LHC. Such a signature is characteristic of models for dark matter or other secluded particles that couple to the standard model through an extended scalar sector. Our goal is to provide search strategies and an interpretation framework for this new signature that are applicable to a large class of models. To this end, we define simplified models of hidden sectors leading to two different event topologies: symmetric decay, i.e., pair-produced mediators decaying each into a Higgs plus invisible final state; and di-Higgs resonance, i.e., resonant Higgs-pair production recoiling against a pair of invisible particles. For both scenarios, we optimize the discovery potential by performing a multi-variate analysis of final states with four bottom quarks and missing energy, employing state-of-the-art machine learning algorithms for signal-background discrimination. We determine the parameter space that the LHC can test in both scenarios, thus facilitating an interpretation of our results in terms of complete models. Di-Higgs production with missing energy is competitive with other missing energy searches and thus provides a new opportunity to find hidden particles at the LHC.

NuLat: A Compact, Segmented, Mobile Anti-neutrino Detector

Here we present NuLat (the Neutrino Lattice), a new detector designed to study electron anti-neutrinos a few meters from a nuclear reactor. NuLat features a Raghavan Optical Lattice (ROL) and consists of 3375 boron or 6Li loaded plastic scintillator cubical cells boundaries have a 0.127 mm (0.005) air gap, resulting in total internal reflection guiding most of the light down the 3 cardinal directions. NuLat gives excellent spatial and energy resolution, allowing for in-depth event topology studies. These features allow us to discern inverse beta decay (IBD) signals and oscillation patterns, even in the presence of other backgrounds. We discuss here motivation, efficiency, sensitivity and project status.

Multiple Coulomb scattering method to reconstruct low-energy gamma–ray direction in the GAMMA-400 space-based gamma–ray telescope

The GAMMA-400 currently developing space-based gamma-ray telescope is designed to measure the gamma-ray fluxes in the energy range from ∼20 MeV to several TeV in the highly elliptic orbit (without shadowing the telescope by the Earth) continuously for a long time. The physical characteristics of the GAMMA-400 gamma-ray telescope, especially the angular and energy resolutions (at 100-GeV gamma rays they are ∼0.01° and ∼1%, respectively), allow us to consider this space-based experiment as the next step in the development of extraterrestrial high-energy gamma-ray astronomy. In this paper, a method to improve the reconstruction accuracy of incident angle for low-energy gamma rays in the GAMMA-400 space-based gamma-ray telescope is presented. The special analysis of topology of pair-conversion events in thin layers of converter was performed. Applying the energy dependence of multiple Coulomb scattering for pair components, it is possible to estimate the energies for each particle, and to use these energies as weight in the angle reconstruction procedure. To identify the unique track in each projection the imaginary curvature method is applied. It allows us to obtain significantly better angular resolution in comparison with other methods applied in current space-based experiments. When using this method for 50-MeV gamma rays the GAMMA-400 gamma-ray telescope angular resolution is about 4°.

DyTER – Dynamic Track and Event Reconstruction

The future PANDA experiment at FAIR experiment aims to cover a wide range of processes in antiproton-proton collisions at event rates of up to 20 MHz. Such event rates make reconstruction a challenging task for the purely software-based event filter. Investigating complex event topologies with displaced vertices increases the difficulty even further. Here we present two attempts to meet these future challenges: an algorithm for track reconstruction based on pattern matching with pre-determined look-up tables, and as a continuation of this approach a system of neural networks for identifying specific particle track candidates and predicting their momentum.

IceCube High Energy Starting Events at 7.5 Years - New Measurements of Flux and Flavor

The IceCube Neutrino Observatory at the South Pole, which detects Cherenkov light from charged particles produced in neutrino interactions, firmly established the existence of an astrophysical high-energy neutrino component. Here I present IceCube’s High-Energy Starting Event sample and the new results obtained with a livetime of about 7.5 years. I will focus on the new measurement of the flavor composition performed using this sample. IceCube is directly sensitive to each neutrino flavor via the single cascade, track and double cascade event topologies, the latter being the topology produced in tauneutrino interactions above an energy threshold of ~100 TeV. A measurement of the flavor ratio on Earth can provide important constraints on sources and production mechanisms within the standard model, and also constrain various beyond-standard-model processes.

Characterisation of flame surface annihilation events in self excited interacting flames

This paper investigates the non-linear flame dynamics of two interacting, premixed, V-shaped flames by characterising the two-dimensional topology of flame annihilation events when the separation distance, S, between them is reduced and large-scale flame merging occurs. The equivalence ratio was varied to promote self-excited oscillations, with the oscillation frequency, heat release phase, and stability limits shown to be dependent on S. High-speed OH-PLIF measurements show that these changes are correlated with the break-up of the shear layers into structures that lead to large-scale flame annihilation events. In isolated flames the shear layers break-up independently, but as S is reduced the shear layers combine leading to large-scale flame merging resulting in the roll up of a single large-scale vortex structure altering the flame annihilation events compared with the case of isolated flames. A flame front event tracking algorithm is developed to characterise the two-dimensional topology and identify the number and spatial location of flame front annihilation events, which shows a strong correlation between these events and the fluctuating heat release rate. Compared with stable flames for the same S, it is found that self-excited instabilities do not significantly increase the number of annihilation events but rather affects their spatial distribution and phase within the oscillation cycle. It is also shown that flame merging significantly increases the probability of flame front annihilation events which alters the phase of the fluctuating heat release rate.

Invisible Neutrino Decay Could Resolve IceCube's Track and Cascade Tension.

The IceCube Neutrino Observatory detects high energy astrophysical neutrinos in two event topologies: tracks and cascades. Since the flavor composition of each event topology differs, tracks and cascades can be used to test the neutrino properties and the mechanisms behind the neutrino production in astrophysical sources. Assuming a conventional model for the neutrino production, the IceCube data sets related to the two channels are in >3σ tension with each other. Invisible neutrino decay with lifetime τ/m=10^{2} s/eV solves this tension. Noticeably, it leads to an improvement over the standard nondecay scenario of more than 3σ while remaining consistent with all other multimessenger observations. In addition, our invisible neutrino decay model predicts a reduction of 59% in the number of observed ν_{τ} events which is consistent with the current observational deficit.

Particle identification in camera image sensors using computer vision

We present a deep learning, computer vision algorithm constructed for the purposes of identifying and classifying charged particles in camera image sensors. We apply our algorithm to data collected by the Distributed Electronic Cosmic-ray Observatory (DECO), a global network of smartphones that monitors camera image sensors for the signatures of cosmic rays and other energetic particles, such as those produced by radioactive decays. The algorithm, whose core component is a convolutional neural network, achieves classification performance comparable to human quality across four distinct DECO event topologies. We apply our model to the entire DECO data set and determine a selection that achieves ≥ 90% purity for all event types. In particular, we estimate a purity of 95% when applied to cosmic-ray muons. The automated classification is run on the public DECO data set in real time in order to provide classified particle interaction images to users of the app and other interested members of the public.

Longitudinal double-spin asymmetries for dijet production at intermediate pseudorapidity in polarized pp collisions at s=200 GeV

We present the first measurements of the longitudinal double-spin asymmetry $A_{LL}$ for dijets with at least one jet reconstructed within the pseudorapidity range $0.8 < \eta < 1.8$. The dijets were measured in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}$ = 200 GeV. Values for $A_{LL}$ are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction $x$ down to $x \sim$ 0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on $\Delta g(x)$ in this poorly constrained region when included in future global analyses.

Event Topology Reconstruction in the CBM Experiment

One of the main purposes of the physics program of the future heavy ion experiment CBM (FAIR, Germany) is to understand the properties of strongly interacting matter at very high baryonic densities and to study the possibility of a phase transition to a deconfined and chirally restored phase of quark matter. The experiment will operate at high interaction rates up to 10 MHz, that requires a full event reconstruction in real time.In order to make an efficient event selection online a clean sample of particles has to be provided by the reconstruction package called First Level Event Selection (FLES). The FLES package operates in two stages. First, particles registered in the CBM detector system are reconstructed. Then short-lived particles decayed before or inside the setup are searched based on their charged and neutral daughter particles. Since the FLES package is developed to run on many-core computer architectures, the reconstruction of particles is done in parallel that provides a possibility for a global competition between particle candidates. Such a global event topology reconstruction significantly improves suppression of a combinatorial background and provides for further physics analysis a very clean sample of particles produced at different stages of heavy ion collision.The global event topology reconstruction procedure and the results of its application to simulated collisions in the CBM detector setup are presented and discussed.

2K-Capture in 124Хе: Results of Data Processing for an Exposure of 37.7 kg day

The results of the experimental search for two-neutrino $2K$-capture in $^{124}$Xe with a large copper proportional counter obtained by processing the data for an exposure of 37.7 kg$\times$day are presented. The experimental setup is located at the Underground Low-Background Laboratory of the Baksan Neutrino Observatory at a depth of 4900 m w.e. The combination of methods of selection of useful signals with a unique set of characteristics and the event topology taken into account allowed us to suppress the background in the energy region of interest. A new half-life limit for $2K(2\nu)$-capture in $^{124}$Xe was determined: T$_{1/2}\geq7.7\cdot10^{21}$ yrs (90\% C.L.).

Ionization electron signal processing in single phase LArTPCs. Part II. Data/simulation comparison and performance in MicroBooNE

The single-phase liquid argon time projection chamber (LArTPC) provides a large amount of detailed information in the form of fine-grained drifted ionization charge from particle traces. To fully utilize this information, the deposited charge must be accurately extracted from the raw digitized waveforms via a robust signal processing chain. Enabled by the ultra-low noise levels associated with cryogenic electronics in the MicroBooNE detector, the precise extraction of ionization charge from the induction wire planes in a single-phase LArTPC is qualitatively demonstrated on MicroBooNE data with event display images, and quantitatively demonstrated via waveform-level and track-level metrics. Improved performance of induction plane calorimetry is demonstrated through the agreement of extracted ionization charge measurements across different wire planes for various event topologies. In addition to the comprehensive waveform-level comparison of data and simulation, a calibration of the cryogenic electronics response is presented and solutions to various MicroBooNE-specific TPC issues are discussed. This work presents an important improvement in LArTPC signal processing, the foundation of reconstruction and therefore physics analyses in MicroBooNE.

Upgrade of the ICARUS T600 Time Projection Chamber

The ICARUS T600 detector, with about 500 tons of active mass, is the largest Liquid Argon Time Projection Chamber (LAr TPC) ever realised. In 2013 ICARUS concluded an about 4 years long experiment with the T600 detector at the LNGS underground laboratory, taking data both with the CNGS neutrino beam and cosmic rays. This very successful experiment demonstrated the high spatial and energy resolutions, electron/photon separation and particle identification capabilities (via dE/dx vs range measurements) of the LAr technology.ICARUS Collaboration refurbished the T600 at CERN, in order to move it to FNAL in the framework of the SBN experiment, to serve as far detector in studies on the short baseline neutrino oscillations.A fundamental part of ICARUS is the light collection system, made of 360 Hamamatsu R5912-MOD, 8 in. diameter, PMT’s. This system is dedicated to three tasks: the generation of a light based trigger signal, the identification of the time of occurrence (t0) of each interaction with high time precision and the initial recognition of event topology for fast event selection purposes.