publications

2021

2021

1. An Exploration of Learnt Representations of W Jets

   Jack H. Collins

   In , Sep 2021

   Abs arXiv PDF Poster

   I present a Variational Autoencoder (VAE) trained on collider physics data (specifically boosted W jets), with reconstruction error given by an approximation to the Earth Movers Distance (EMD) between input and output jets. This VAE learns a concrete representation of the data manifold, with semantically meaningful and interpretable latent space directions which are hierarchically organized in terms of their relation to physical EMD scales in the underlying physical generative process. A hyperparameter βcontrols the resolution at which the VAE is sensitive to structures in the data manifold. The variation of the latent space structure with β, and the scaling of some VAE properties, provide insight into scale dependent structure of the dataset and its information complexity. I introduce two measures of the dimensionality of the learnt representation that are calculated from this scaling.

2. Comparing weak- and unsupervised methods for resonant anomaly detection

   Jack H. Collins, Pablo Martı́n-Ramiro, Benjamin Nachman, and 1 more author

   Eur. Phys. J. C, Sep 2021

   Abs PDF

   Anomaly detection techniques are growing in importance at the Large Hadron Collider (LHC), motivated by the increasing need to search for new physics in a model-agnostic way. In this work, we provide a detailed comparative study between a well-studied unsupervised method called the autoencoder (AE) and a weakly-supervised approach based on the Classification Without Labels (CWoLa) technique. We examine the ability of the two methods to identify a new physics signal at different cross sections in a fully hadronic resonance search. By construction, the AE classification performance is independent of the amount of injected signal. In contrast, the CWoLa performance improves with increasing signal abundance. When integrating these approaches with a complete background estimate, we find that the two methods have complementary sensitivity. In particular, CWoLa is effective at finding diverse and moderately rare signals while the AE can provide sensitivity to very rare signals, but only with certain topologies. We therefore demonstrate that both techniques are complementary and can be used together for anomaly detection at the LHC.

3. The LHC Olympics 2020: A Community Challenge for Anomaly Detection in High Energy Physics

   Gregor Kasieczka, and  others

   Rept. Prog. Phys., Sep 2021

   Abs arXiv PDF

   A new paradigm for data-driven, model-agnostic new physics searches at colliders is emerging, and aims to leverage recent breakthroughs in anomaly detection and machine learning. In order to develop and benchmark new anomaly detection methods within this framework, it is essential to have standard datasets. To this end, we have created the LHC Olympics 2020, a community challenge accompanied by a set of simulated collider events. Participants in these Olympics have developed their methods using an R&D dataset and then tested them on black boxes: datasets with an unknown anomaly (or not). This paper will review the LHC Olympics 2020 challenge, including an overview of the competition, a description of methods deployed in the competition, lessons learned from the experience, and implications for data analyses with future datasets as well as future colliders.

2019

2019

1. Extending the search for new resonances with machine learning

   Jack H. Collins, Kiel Howe, and Benjamin Nachman

   Phys. Rev. D, Sep 2019

   Abs PDF

   The oldest and most robust technique to search for new particles is to look for “bumps” in invariant mass spectra over smoothly falling backgrounds. We present a new extension of the bump hunt that naturally benefits from modern machine learning algorithms while remaining model agnostic. This approach is based on the classification without labels (CWoLa) method where the invariant mass is used to create two potentially mixed samples, one with little or no signal and one with a potential resonance. Additional features that are uncorrelated with the invariant mass can be used for training the classifier. Given the lack of new physics signals at the Large Hadron Collider (LHC), such model-agnostic approaches are critical for ensuring full coverage to fully exploit the rich datasets from the LHC experiments. In addition to illustrating how the new method works in simple test cases, we demonstrate the power of the extended bump hunt on a realistic all-hadronic resonance search in a channel that would not be covered with existing techniques.

2. R-parity Violating Supersymmetric Explanation of the Anomalous Events at ANITA

   Jack H. Collins, P. S. Bhupal Dev, and Yicong Sui

   Phys. Rev. D, Sep 2019

   Abs PDF

   The ANITA balloon experiment has observed two EeV-energy, up-going events originating from well below the horizon. This is puzzling because (i) no standard model (SM) particle is expected to survive passage through Earth at such energies and incident angles, and (ii) no such events were reported by IceCube. In this paper, we address both these issues by invoking a beyond-SM interpretation of the EeV events as due to the decay of a long-lived bino in the R-parity violating (RPV) supersymmetry. In particular, a TeV-scale slepton/squark can be resonantly produced through the interaction of the EeV neutrino with electrons/nucleons inside Earth, which decays to a light, long-lived bino that survives the propagation through Earth matter before decaying back to neutrinos, leptons, and/or quarks, thus producing up-going air showers in the atmosphere near ANITA. We find that the ANITA events can be explained with a GeV-scale bino and O(0.1) RPV couplings, which are consistent with all existing high- and low-energy constraints. We also find that an isotropic neutrino flux is inadequate for a beyond-SM explanation of this kind, and an anisotropic flux must be invoked. Finally, we also address the apparent tension of these observations with IceCube. Various aspects of our interpretation are testable in the near future at different frontiers, such as by the LHC, Belle II, ANITA-IV and IceCube.

3. Dedicated Strategies for Triboson Signals from Cascade Decays of Vector Resonances

   Kaustubh Agashe, Jack H. Collins, Peizhi Du, and 3 more authors

   Phys. Rev. D, Sep 2019

   Abs PDF

   New colorless electroweak (EW) charged spin-1 particles with mass of a few TeV arise in numerous extensions of the standard model (SM). Decays of such a vector into a pair of SM particles, either fermions or EW bosons, are well studied. Many of these models have an additional scalar, which can lead to (and even dominate in certain parameter regions) a novel decay channel for the heavy vector particles instead—into a SM EW boson and the scalar, which subsequently decays into a SM EW boson pair. In this work, we focus on the scalar being relatively heavy, roughly a factor of 2 lighter than the vector particles, rendering its decay products well separated. Such a cascade decay results in a final state with three isolated bosons. We argue that for this “triboson” signal the existing diboson searches are not quite optimal due to combinatorial ambiguity for three identical bosons, and in addition, due to a relatively small signal cross section determined by the heaviness of the decaying vector particle. In order to isolate the signal, we demonstrate that tagging all three bosons, followed by use of the full triboson invariant mass distribution as well as that of appropriate subsets of dibosons, is well motivated. We develop these general strategies in detail within the context of a specific class of models that are based on extensions of the standard warped extradimensional scenario. We also point out that a similar analysis would apply to models with an enlarged EW gauge sector in four dimensions, even if they involve a different Lorentz structure for the relevant couplings.

2018

2018

1. Detecting a Boosted Diboson Resonance

   Kaustubh Agashe, Jack H. Collins, Peizhi Du, and 3 more authors

   JHEP, Sep 2018

   Abs PDF

   New light scalar particles in the mass range of hundreds of GeV, decaying into a pair of W/Z bosons can appear in several extensions of the SM. The focus of collider studies for such a scalar is often on its direct production, where the scalar is typically only mildly boosted. The observed W/Z are therefore well-separated, allowing analyses for the scalar resonance in a standard fashion as a low-mass diboson resonance. In this work we instead focus on the scenario where the direct production of the scalar is suppressed, and it is rather produced via the decay of a significantly heavier (a few TeV mass) new particle, in conjunction with SM particles. Such a process results in the scalar being highly boosted, rendering the W/Z’s from its decay merged. The final state in such a decay is a “fat” jet, which can be either four pronged (for fully hadronic W/Z decays), or may be like a W/Z jet, but with leptons buried inside (if one of the W/Z decays leptonically). In addition, this fat jet has a jet mass that can be quite different from that of the W/Z/Higgs/top quark-induced jet, and may be missed by existing searches. In this work, we develop dedicated algorithms for tagging such multi-layered “boosted dibosons” at the LHC. As a concrete application, we discuss an extension of the standard warped extra dimensional framework where such a light scalar can arise. We demonstrate that the use of these algorithms gives sensitivity in mass ranges that are otherwise poorly constrained.

2. Anomaly Detection for Resonant New Physics with Machine Learning

   Jack H. Collins, Kiel Howe, and Benjamin Nachman

   Phys. Rev. Lett., Sep 2018

   Abs PDF

   Despite extensive theoretical motivation for physics beyond the standard model (BSM) of particle physics, searches at the Large Hadron Collider have found no significant evidence for BSM physics. Therefore, it is essential to broaden the sensitivity of the search program to include unexpected scenarios. We present a new model-agnostic anomaly detection technique that naturally benefits from modern machine learning algorithms. The only requirement on the signal for this new procedure is that it is localized in at least one known direction in phase space. Any other directions of phase space that are uncorrelated with the localized one can be used to search for unexpected features. This new method is applied to the dijet resonance search to show that it can turn a modest 2σ excess into a 7σ excess for a model with an intermediate BSM particle that is not currently targeted by a dedicated search.

2017

2017

1. A generic anti-QCD jet tagger

   J. A. Aguilar-Saavedra, Jack H. Collins, and Rashmish K. Mishra

   JHEP, Sep 2017

   Abs PDF

   New particles beyond the Standard Model might be produced with a very high boost, for instance if they result from the decay of a heavier particle. If the former decay hadronically, then their signature is a single massive fat jet which is difficult to separate from QCD backgrounds. Jet substructure and machine learning techniques allow for the discrimination of many specific boosted objects from QCD, but the scope of possibilities is very large, and a suite of dedicated taggers may not be able to cover every possibility — in addition to making experimental searches cumbersome. In this paper we describe a generic model-independent tagger that is able to discriminate a wide variety of hadronic boosted objects from QCD jets using N -subjettiness variables, with a significance improvement varying between 2 and 8. This is in addition to any improvement that might come from a cut on jet mass. Such a tagger can be used in model-independent searches for new physics yielding fat jets. We also show how such a tagger can be applied to signatures over a wide range of jet masses without sculpting the background distributions, allowing to search for new physics as bumps on jet mass distributions.

2. LHC Signals from Cascade Decays of Warped Vector Resonances

   Kaustubh S. Agashe, Jack Collins, Peizhi Du, and 3 more authors

   JHEP, Sep 2017

   Abs PDF

   Recently (arXiv:1608.00526), a new framework for warped higher-dimensional compactifications with “bulk” standard model (SM) was proposed: in addition to the UV (Planck scale) and IR (a couple of TeV) branes, there is an intermediate brane, taken to be around 10TeV. The SM matter and Higgs fields propagate from the UV brane down to this intermediate brane only, while gauge and gravity fields propagate in the entire bulk. Such a configuration renders the lightest gauge Kaluza-Klein (KK) states within LHC reach, simultaneously satisfying flavor and CP constraints. In addition, the usual leading decay modes of the lightest KK gauge bosons into top and Higgs bosons are suppressed. This effect permits erstwhile subdominant channels to become significant. These include flavor-universal decays to SM fermions and Higgs bosons, and a novel channel — decay to a radion and a SM gauge boson, followed by radion decay to a pair of SM gauge bosons. In this work, we first delineate the parameter space where the above mentioned cascade decay of gauge KK particles dominates, and thereby can be the discovery mode at the LHC. We then perform a detailed analysis of the LHC signals from this model, finding that 300/fb suffices for evidence of KK-gluon in tri-jet, jet + di-photon and jet + di-boson channels. However, KK photon in photon + di-jet, and KK-W in leptonic W + di-jet require 3000/fb. The crucial feature of this decay chain is a “double” resonance, i.e. 3-particle and 2-particle invariant mass peaks, corresponding to the KK gauge boson and the radion respectively.

2016

2016

1. Traces of a triboson resonance

   J. A. Aguilar-Saavedra, J. H. Collins, and S. Lombardo

   JHEP, Sep 2016

   Abs PDF

   We show that the relatively small but coincident excesses observed around 2 TeV in the ATLAS Run 1 and Run 2 hadronic diboson searches — when a cut on the number of tracks in the fat jets is not applied — and the null results of all remaining high-mass diboson searches are compatible with the decay of a triboson resonance R into WZ plus an extra particle X. These decays can take place via new neutral (Y^0) or charged (Y^±) particles, namely R \to Y^0 W, with Y^0 \to ZX, or R \to Y^± Z, with Y^± \to WX. An obvious candidate for such intermediate particle is a neutral one Y^0, given a 3.9σ excess found at 650 GeV by the CMS Collaboration in searches for intermediate mass diboson resonances decaying to ZV, with V = W, Z. We discuss discovery strategies for triboson resonances with small modifications of existing hadronic searches.

2. Novel kinematics from a custodially protected diphoton resonance

   Jack H. Collins, Csaba Csaki, Jeff Asaf Dror, and 1 more author

   Phys. Rev. D, Sep 2016

   Abs PDF

   We study a simple, well-motivated model based on a custodial symmetry which describes the tree-level production of a 750 GeV diphoton resonance from a decay of a singly produced vector-like quark. The model has several novel features. The identification of the resonance as an SU(2)R triplet provides a symmetry explanation for suppression of its decays to hh, WW, and gg. Moreover, the ratio of the 13 TeV to 8 TeV cross sections can be larger than single production of a 750 GeV resonance, reaching ratios of up to 7 for TeV scale vectorlike quark masses. This eliminates any tension between the results from Run I and Run II diphoton searches. Lastly, we study the kinematics of our signal and conclude that the new production mechanism is consistent with available experimental distributions in large regions of parameter space but, depending on the mass of the new vectorlike quarks, can be differentiated from the background with more statistics.

3. A 2TeV W_R , supersymmetry, and the Higgs mass

   Jack H Collins, and Wee Hao Ng

   JHEP, Sep 2016

   Abs PDF

   A recent ATLAS search for diboson resonances and a CMS search for eejj resonances which both show excesses with significance around 3σ have generated interest in SU(2)_R gauge extensions of the Standard Model with a W^′ mass around 2 TeV. We investigate the possibility that an SU(2)_R gauge extension of the MSSM compatible with an explanation of the diboson anomaly might give rise to a significant enhancement of the Higgs mass above the MSSM tree level bound m_h,\mathrmtree < 90 ; \mathrmGeV due to non-decoupling D-terms. This model contains a vector-like charge −1/3 SU(2)_R singlet quark for each generation which mixes significantly with the SU(2)_R doublet quarks, affecting the W_R phenomenology. We find that it is possible to achieve m_h,\mathrmtree > 110 ; \mathrmGeV, and this requires that the Z^′ mass is close to 3 TeV.

4. Odd Top Partners at the LHC

   Archana Anandakrishnan, Jack H. Collins, Marco Farina, and 2 more authors

   Phys. Rev. D, Sep 2016

   Abs PDF

   LHC searches for fermionic top partners T focus on three decay topologies: T→bW, T→tZ, and T→th. However, top partners may carry new conserved quantum numbers that forbid these decays. The simplest possibility is a conserved parity, under which the top partner is odd and all SM states are even. In this case, decays of top partners may involve new particle-odd scalars, leading to signal topologies more commonly associated with supersymmetry, either with or without R-parity conservation. We study a simplified model in which this possibility is realized, and estimate the bounds on the top partner mass in this model implied by LHC searches for supersymmetry. We find that the bounds can be significantly weaker than in the conventional top partner decay scenario. For example, if the new parity is exact, a 500 GeV top partner is allowed as long as the lightest parity-odd scalar mass is between 325 and 500 GeV. The lower allowed top partner mass reduces the need for fine-tuning in the Higgs mass parameter, compared to the conventional decay scenario. We also present an explicit model, the oddest little Higgs, which exhibits this phenomenology.

2015

2015

1. Mixed Stops and the ATLAS on-Z Excess

   Jack H. Collins, Jeff Asaf Dror, and Marco Farina

   Phys. Rev. D, Sep 2015

   Abs PDF

   The ATLAS experiment has recently observed a 3σ excess in a channel with a leptonically decaying Z, jets, and ETmiss. It is tantalizing to interpret the signal as the first sign of a natural supersymmetric spectrum. We study such a possibility in a minimal model containing light stops and a neutralino lightest supersymmetric particle. The signal is characterized by a novel topology (compared to previous attempts) where the Z is emitted from a colored particle in the first step of a decay chain, namely t˜2→t˜1Z, which is characteristic of mixed stops. We show that the excess is compatible with a compressed stop spectrum and is not excluded by any other relevant search, finding some regions of parameter space with signal strength within 1σ of that measured by the ATLAS Collaboration. In addition, we notice that the corresponding CMS search could be prone to background contamination in unexpected topologies of this kind.

2014

2014

1. Spin-One Top Partner: Phenomenology

   Jack H. Collins, Bithika Jain, Maxim Perelstein, and 1 more author

   JHEP, Sep 2014

   Abs PDF

   Cai, Cheng, and Terning suggested a model in which the left-handed top quark is identified with a gaugino of an extended gauge group, and its superpartner is a spin-1 particle. We perform a phenomenological analysis of this model, with a focus on the spin-1 top partner, which we dub the "swan". We find that precision electroweak fits, together with direct searches for Z^\prime bosons at the LHC, place a lower bound of at least about 4.5 TeV on the swan mass. An even stronger bound, 10 TeV or above, applies in most of the parameter space, mainly due to the fact that the swan is typically predicted to be significantly heavier than the Z^\prime. We find that the 125 GeV Higgs can be easily accommodated in this model with non-decoupling D-terms. In spite of the strong lower bound on the swan mass, we find that corrections to Higgs couplings to photons and gluons induced by swan loops are potentially observable at future Higgs factories. We also briefly discuss the prospects for discovering a swan at the proposed 100 TeV pppp collider.

2012

2012

1. Hidden-Sector Higgs Bosons at High-Energy Electron-Positron Colliders

   Jack H. Collins, and James D. Wells

   Oct 2012

   Abs PDF

   The possibility of a scalar messenger that can couple the Standard Model (SM) to a hidden sector has been discussed in a variety of contexts in the literature in recent years. We consider the case that a new scalar singlet charged under an exotic spontaneously broken Abelian gauge symmetry mixes weakly with the SM Higgs resulting in two scalar mass states, one of which has heavily suppressed couplings to the SM particles. Previous phenomenological studies have focussed on potential signatures for such a model at the Large Hadron Collider (LHC). However, there are interesting regions of the parameter space in which the heavier Higgs state would be out of reach for LHC searches if its mass is greater than 1 TeV. We therefore investigate the discovery potential for such a particle at a 3 TeV electron-positron collider, which is motivated by the recent developments of the Compact Linear Collider (CLIC). We find that such an experiment could substantially extend our discovery reach for a heavy, weakly coupled Higgs boson, and we discuss three possible search channels.

2. Identifying the colour of TeV-scale resonances

   S. Ask, J. H. Collins, J. R. Forshaw, and 2 more authors

   JHEP, Oct 2012

   Abs PDF

   We explore how the colour of any new TeV-scale resonances that decay into top quark pairs can be identified by studying the dependence of the observed cross-section on a central jet veto. To facilitate this study, colour octet resonance production was implemented in Pythia8 and colour singlet resonance production is simulated after minor modifications. We find that the colour of a 2 TeV resonance can be identified with 10/fb of data at a centre-of-mass energy of 14 TeV for a wide range of couplings, but only if the uncertainty in the theoretical prediction is dramatically reduced from its current level.

2010

2010

1. Construction of a Prototype Spark Chamber

   Jack Collins

   Oct 2010

   Abs PDF

   A small demonstration spark chamber is to be built at the Cavendish laboratory. A prototype chamber consisting of five 20x22.5cm plates has been built and descriptions of its properties and construction are given, while a second chamber with a somewhat novel design is nearly complete. A discussion of the issues surrounding the final design is presented, and recommendations are made in light of the results of this work.