# Publication List

## Preprints

###### Abstract

Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments—as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER—to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity ‘dark showers’, highlighting opportunities for expanding the LHC reach for these signals.

## Journal Publications

###### Abstract

We show that heavy-ion collisions at the LHC provide a promising environment to search for signatures with displaced vertices in well-motivated new physics scenarios. Compared to proton collisions, they offer several advantages: (i) the number of parton level interactions per collision is larger, (ii) there is no pileup, (iii) the lower instantaneous luminosity compared to proton collisions allows one to operate the LHC experiments with very loose triggers, and (iv) there are new production mechanisms that are absent in proton collisions We focus on the third point and show that the modification of the triggers alone can increase the number of observable events by orders of magnitude if the long-lived particles are predominantly produced with low transverse momentum. Our results show that collisions of ions lighter than lead are well motivated from the viewpoint of searches for new physics. We illustrate this for the example of heavy neutrinos in the Neutrino Minimal Standard Model.

###### Abstract

We study the sensitivity of displaced vertex searches for heavy neutrinos produced in W boson decays in the LHC detectors ATLAS, CMS and LHCb. We also propose a new search that uses the muon chambers to detect muons from heavy neutrino decays outside the tracker. The sensitivity estimates are based on benchmark models in which the heavy neutrinos mix exclusively with one of the three Standard Model generations. In the most sensitive mass regime the displaced vertex searches can improve existing constraints on the mixing with the first two SM generations by more than four orders of magnitude and by three orders of magnitude for the mixing with the third generation.

###### Abstract

This document summarises proposed searches for new physics accessible in the heavy-ion mode at the CERN Large Hadron Collider (LHC), both through hadronic and ultraperipheral γγ interactions, and that have a competitive or, even, unique discovery potential compared to standard proton–proton collision studies. Illustrative examples include searches for new particles—such as axion-like pseudoscalars, radions, magnetic monopoles, new long-lived particles, dark photons, and sexaquarks as dark matter candidates—as well as new interactions, such as nonlinear or non-commutative QED extensions. We argue that such interesting possibilities constitute a well-justified scientific motivation, complementing standard quark-gluon-plasma physics studies, to continue running with ions at the LHC after the Run-4, i.e. beyond 2030, including light and intermediate-mass ion species, accumulating nucleon–nucleon integrated luminosities in the accessible fb−1 range per month.

###### Abstract

We show that heavy ion collisions at the LHC provide a promising environment to search for new long lived particles in well-motivated New Physics scenarios. One advantage lies in the possibility to operate the main detectors with looser triggers, which can increase the number of observable events by orders of magnitude if the long lived particles are produced with low transverse momentum. In addition, the absence of pileup in heavy ion collisions can avoid systematic nuisances that will be present in future proton runs, such as the problem of vertex mis-identification. Finally, there are new production mechanisms that are absent or inefficient in proton collisions. We show that the looser triggers alone can make searches in heavy ion data competitive with proton data for the specific example of heavy neutrinos in the Neutrino Minimal Standard Model, produced in the decay of B mesons. Our results suggest that collisions of ions lighter than lead, which are currently under discussion in the heavy ion community, are well-motivated from the viewpoint of searches for New Physics.

###### Abstract

Novelty detection is the machine learning task to recognize data, which belong to an unknown pattern. Complementary to supervised learning, it allows us to analyze data model-independently. We demonstrate the potential role of novelty detection in collider physics, using autoencoder-based deep neural network. Explicitly, we develop a set of density-based novelty evaluators, which are sensitive to the clustering of unknown-pattern testing data or new-physics signal events, for the design of detection algorithms. We also explore the influence of the known-pattern data fluctuations, arising from nonsignal regions, on detection sensitivity. Strategies to address it are proposed. The algorithms are applied to detecting fermionic ditop partner and resonant ditop productions at LHC, and exotic Higgs decays of two specific modes at a future e+e- collider. With parton-level analysis, we conclude that potentially the new-physics benchmarks can be recognized with high efficiency.

###### Abstract

The sensitivity of beam dump experiments to heavy neutral leptons depends on the relative strength of their couplings to individual lepton flavours in the Standard Model. We study the impact of present neutrino oscillation data on these couplings in the minimal type I seesaw model and find that it significantly constrains the allowed heavy neutrino flavour mixing patterns. We estimate the effect that the DUNE experiment will have on these predictions. We then discuss implication that this has for the sensitivity of the NA62 experiment when operated in the beam dump mode and provide sensitivity estimates for different benchmark scenarios. We find that the sensitivity can vary by almost two orders of magnitude for general choices of the model parameters, but depends only weakly on the flavour mixing pattern within the parameter range that is preferred by neutrino oscillation data.

###### Abstract

Solutions to the electroweak hierarchy problem typically introduce a new symmetry to stabilize the quadratic ultraviolet sensitivity in the self-energy of the Higgs boson. The new symmetry is either broken softly or collectively, as for example in supersymmetric and little Higgs theories. At low energies such theories contain naturalness partners of the Standard Model fields which are responsible for canceling the quadratic divergence in the squared Higgs mass. Post the discovery of any partner-like particles, we propose to test the aforementioned cancellation by measuring relevant Higgs couplings. Using the fermionic top partners in little Higgs theories as an illustration, we construct a simplified model for naturalness and initiate a study on testing naturalness. After electroweak symmetry breaking, naturalness in the top sector requires a$_{T}$  = − λ$_{t}^{2}$ at leading order, where λ$_{t}$ and a$_{T}$ are the Higgs couplings to a pair of top quarks and top partners, respectively. Using a multivariate method of Boosted Decision Tree to tag boosted particles in the Standard Model, we show that, with a luminosity of 30 ab$^{−1}$ at a 100 TeV pp-collider, naturalness could be tested with a precision of 10% for a top partner mass up to 2.5 TeV.

###### Abstract

We present strategies to search for heavy neutral Higgs bosons decaying to top quark pairs, as often occurs at low tan β in type II two Higgs doublet models such as the Higgs sector of the MSSM. The resonant production channel is unsatisfactory due to interference with the SM background. We instead propose to utilize same-sign dilepton signatures arising from the production of heavy Higgs bosons in association with one or two top quarks and subsequent decay to a top pair. We find that for heavier neutral Higgs bosons the production in association with one top quark provides greater sensitivity than production in association with two top quarks. We obtain current limits at the LHC using Run I data at 8 TeV and forecast the sensitivity of a dedicated analysis during Run II at 14 TeV. Then we perform a detailed BDT study for the 14 TeV LHC and a future 100 TeV collider.

###### Abstract

In this write-up, we summarize the production of non-SM Higgses in the Type II Two Higgs Doublet Model at a 100 TeV pp collider, as well as their decays. We present the reach for pp → bbH0∕A → bbtt, bbττ as well as pp → tbH±→ tbtb, tbτν at the 100 TeV pp collider and outline the possible search channels via Higgs exotic decays. We point out that a combination of these conventional channels potentially yields full coverage for tan β and pushes the exclusion limits from the 𝒪(1)TeV at the LHC to the 𝒪(10)TeV at a 100 TeV pp collider, whereas the exotic decays of a heavy Higgs into two light Higgses or one light Higgs plus one SM gauge boson provide alternative discovery channels.

###### Abstract

Searching for Higgs bosons beyond the Standard Model (BSM) is one of the most important missions for hadron colliders. As a landmark of BSM physics, the MSSM Higgs sector at the LHC is expected to be tested up to the scale of the decoupling limit of $\mathcal{O}$ (1) TeV, except for a wedge region centered around tan β ∼ 3-10, which has been known to be difficult to probe. In this article, we present a dedicated study testing the decoupled MSSM Higgs sector, at the LHC and a next-generation pp-collider, proposing to search in channels with associated Higgs productions, with the neutral and charged Higgs further decaying into tt and tb, respectively. In the case of neutral Higgs we are able to probe for the so far uncovered wedge region via pp → bbH/A → bbtt. Additionally, we cover the the high tan β range with pp → bbH/A → bbτ τ . The combination of these searches with channels dedicated to the low tan β region, such as pp → H/A → tt and pp → ttH/A → tttt potentially covers the full tan β range. The search for charged Higgs has a slightly smaller sensitivity for the moderate tan β region, but additionally probes for the higher and lower tan β regions with even greater sensitivity, via pp → tbH$^{±}$ → tbtb. While the LHC will be able to probe the whole tan β range for Higgs masses of $\mathcal{O}$ (1) TeV by combining these channels, we show that a future 100 TeV pp-collider has a potential to push the sensitivity reach up to ∼ $\mathcal{O}$ (10) TeV. In order to deal with the novel kinematics of top quarks produced by heavy Higgs decays, the multivariate Boosted Decision Tree (BDT) method is applied in our collider analyses. The BDT-based tagging efficiencies of both hadronic and leptonic top-jets, and their mutual fake rates as well as the faking rates by other jets (h, Z, W , b, etc.) are also presented.

###### Abstract

We investigate the LHC sensitivity to supersymmetric models with light higgsinos, small R-parity breaking and gravitino dark matter. The limits on decaying gravitino dark matter from gamma-ray searches with the Fermi-LAT put a lower bound on the higgsino-like neutralino NLSP decay length, giving rise to a displaced-vertex collider signature. Using publicly available tools for simulation of signal, background and detector response, we find that higgsinos with masses of 100 – 400 GeV and R-parity violation of ζ ~ 10(−8) – 10(−9) can show up in the 8 TeV LHC data with 10 – 30 fb(−1) of integrated luminosity. We demonstrate that in the case of a signal, the higgsino mass can be determined by reconstruction of the dimuon mass edge.

###### Abstract

A recently proposed class of supersymmetric models predicts rather light and nearly mass-degenerate higgsinos, while the other superparticles are significantly heavier. In this paper we study the early LHC phenomenology of a benchmark model of this kind. If the squarks and gluinos, and in particular the lighter stop, are still light enough to be within reach, then evidence for our model can be found in hadronic SUSY searches. Moreover, with dedicated searches it will be possible to distinguish the light higgsino model from generic SUSY models with a bino LSP. Search channels with b-jets and with isolated leptons play a crucial role for model discrimination.

###### Abstract

We study supersymmetric extensions of the Standard Model with small R-parity and lepton number violating couplings which are naturally consistent with primordial nucleosynthesis, thermal leptogenesis and gravitino dark matter. We consider supergravity models where the gravitino is the lightest superparticle followed by a bino-like next-to-lightest superparticle (NLSP). Extending previous work we investigate in detail the sensitivity of LHC experiments to the R-parity breaking parameter zeta for various gluino and squark masses. We perform a simulation of signal and background events for the generic detector DELPHES for which we implement the finite NLSP decay length. We find that for gluino and squark masses accessible at the LHC, values of zeta can be probed which are one to two orders of magnitude smaller than the present upper bound obtained from astrophysics and cosmology.

###### Abstract

Supersymmetric extensions of the Standard Model with small R-parity and lepton number violating couplings are naturally consistent with primordial nucleosynthesis, thermal leptogenesis and gravitino dark matter. We consider supergravity models with universal boundary conditions at the grand unification scale, and scalar tau-lepton or bino-like neutralino as next-to-lightest superparticle (NLSP). Recent Fermi-LAT data on the isotropic diffuse gamma-ray flux yield a lower bound on the gravitino lifetime. Comparing two-body gravitino and neutralino decays we find a lower bound on a neutralino NLSP decay length, $c \tau_{\chi^0_1} \gsim 30 cm$. Together with gravitino and neutralino masses one obtains a microscopic determination of the Planck mass. For a stau-NLSP there exists no model-independent lower bound on the decay length. Here the strongest bound comes from the requirement that the cosmological baryon asymmetry is not washed out, which yields $c \tau_{\tilde\tau_1} \gsim 4 mm$. However, without fine-tuning of parameters, one finds much larger decay lengths. For typical masses, $m_{3/2} \sim 100 GeV$ and $m_{NLSP} \sim 150 GeV$, the discovery of a photon line with an intensity close to the Fermi-LAT limit would imply a decay length $c\tau_{NLSP}$ of several hundred meters, which can be measured at the LHC.

## Conference Proceedings

###### Abstract

I present a proposal to search for long lived particles in heavy ion collisions using heavy neutrinos as an example. In comparison to proton collisions heavy ion collisions allow to lower the trigger thresholds significantly. On the other hand, there has been an inter- ested to operate the LHC using ions lighter than lead. We have shown that searches in intermediate ion collisions can have a higher significance than in proton collision if one assumes equal running time.

###### Abstract

I present our study on heavy neutral leptons in the low scale type I seesaw model at the fixed target experiment NA62. First, I show the constraints on the heavy neutrino mixing parameter from the neutrino oscillation data. Afterwards, I present sensitivity estimates of the NA62 experiment in the beam dump mode for different benchmark scenarios.

###### Abstract

The sensitivity of beam dump experiments to heavy neutrinos depends on the relative size of their mixings with the lepton flavours in the Standard Model. We study the impact of present neutrino oscillation data on these mixing angles in the minimal type I seesaw model. We find that current data significantly constrains the allowed heavy neutrino flavour mixing patterns. Based on this, we discuss the implications for the sensitivity of the NA62 experiment to heavy neutrinos when operated in the beam dump mode. We find that NA62 is currently the most sensitive experiment in the world for heavy neutrino masses between that of the kaon and the $D$-mesons. The sensitivity can vary by almost two orders of magnitude if the heavy neutrinos exclusively couple to the tau flavour, but depends only comparably weakly on the flavour mixing pattern within the parameter range preferred by light neutrino oscillation data.

## Reviews and Reports

###### Abstract

A study group was formed in Beijing in September 2013 to investigate the feasibility of a high energy Circular Electron Positron Collider (CEPC) as a Higgs and/or Z factory, and a subsequent Super proton-proton Collider (SPPC). A provisional organization structure and a management team, consisted mostly of Chinese physicists, were established to guide the initial study [1]. The group aims at establishing an international collaboration on CEPC-SPPC after sufficient progress has been made in China or it has been given government funding for R&D on CEPC-SPPC. The CEPC-SPPC study group undertook a preliminary Conceptual Design Report (preCDR) study, with an aim to address some of the critical questions about the CEPC-SPPC: identifying the most exciting and fundamental physics case, performing the initial design of the accelerator and of the detector, and selecting critical R&D projects for the Technical Design Report (TDR). The preCDR will be reviewed in early 2015 and in time for China’s 13th Five-Year Plan consideration, which dictated the time window for the preCDR study. Since September 2013 a series of international workshops on CEPC-SPPC have been held to study their physics potentials, the designs and technologies of the accelerators and the detector, and the laboratory facilities in China. This preCDR report contains two volumes: Volume 1 summarizes the theory, detector, software and simulation, and the physics potential of the CEPC-SPPC project; Volume 2 describes the CEPC accelerator design and SPPC consideration, and the associated civil engineering [2]. The contents presented are preliminary and are predominately focused on the CEPC. Formal CDR and TDR work will follow.

## Theses

###### Abstract

Supersymmetric extensions of the Standard Model with small R-parity and lepton-number violating couplings are naturally consistent with primordial nucleosynthesis, thermal lepto- genesis and gravitino dark matter. We consider both supergravity models with universal boundary conditions at the grand unification scale and a scalar tau or bino-like neutralino as the next-to-lightest supersymmetric particle (NLSP) as well as hybrid gauge-gravity mediation models with a higgsino-like neutralino as the NLSP. Fermi-LAT data on the isotropic diffuse gamma-ray flux yield a lower bound on the gravitino lifetime, which we translate into a lower bound of the NLSP decay length of several centimeters. Together with gravitino and neutralino masses, one obtains a microscopic determination of the Planck mass. For supersymmetric mass parameters that can be tested at the Large Hadron Collider (LHC), the discovery of a photon line with an intensity close to the Fermi-LAT limit would imply a NLSP decay length of several hundred meters, which can also be measured at the LHC. We conduct a detailed investigation of the sensitivity of LHC experiments to the amount of R-parity breaking for models with masses of the coloured particles, which allow for strong production, as well as masses which only allow for Drell-Yang production. We perform a simulation of signal and background events using tools that are publicly available, which we have extended in order to also simulate the finite NLSP decay length. We find that values of the overall scale of R-parity violation can be probed which are one to two orders of magnitude smaller than the present upper bound obtained from astrophysics and cosmology. Using the example of higgsinos, we demonstrate that, given a signal, the NLSP mass can be determined by reconstructing the di-muon mass edge