arXiv subjects

Malte Buschmann

Publications and source records attributed to Malte Buschmann.

29 records · Page 2Linked to original sources

arXiv2019

Direct Acceleration: Cosmic and Exoplanet Synergies

Direct measurement of acceleration is a key scientific goal for both cosmology and exoplanets. For cosmology, the concept of redshift drift (more than 60 years old by the 2020s) could directly establish the Friedmann-Lema{\^ı}tre-Robertson-Walker model. It would increase the dark energy figure of merit by a factor of 3 beyond Stage 4 experiments, in combination with cosmic microwave background measurements. For exoplanets, the same technology required provides unprecedented radial velocity accuracy, enabling detection of Earth mass planets in the habitable zone. Other science cases include mapping the Milky Way gravitational potential and testing its dark matter distribution.

astro-ph.IM↗

arXiv2019

Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider

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.

hep-ex↗

arXiv2018

Revisiting Gluinos at LHC

We examine the experimental signature of a UV complete Supersymmetry (SUSY) theory, the $G_2$-MSSM. This model predicts that only some superpartners will be produced in possibly detectable amounts at LHC: $p p \rightarrow \tilde{g}\tilde{g}$, $p p \rightarrow \tildeχ^{\pm}_1 \tildeχ^{\mp}_1$, and $p p \rightarrow \tildeχ^{0}_2 \tildeχ^{\pm}_1$. We exclude spectra with $m_{\tilde{g}} \approx1.5$ TeV. While spectra with $m_{\tilde{g}} \approx1.7$ TeV and $m_{\tilde{g}} \approx1.9$ TeV are currently allowed (contrary to what is often claimed), data in hand could exclude these spectra. This is not in tension with reported exclusion limits due to the difference in decay topologies between simplified models and a UV motivated ($G_2$-MSSM) model.

hep-ph↗

arXiv2017

Stellar Wakes from Dark Matter Subhalos

We propose a novel method utilizing stellar kinematic data to detect low-mass substructure in the Milky Way's dark matter halo. By probing characteristic wakes that a passing dark matter subhalo leaves in the phase space distribution of ambient halo stars, we estimate sensitivities down to subhalo masses $\sim 10^7\,M_\odot$ or below. The detection of such subhalos would have implications for dark-matter and cosmological models that predict modifications to the halo-mass function at low halo masses. We develop an analytic formalism for describing the perturbed stellar phase-space distributions, and we demonstrate through simulations the ability to detect subhalos using the phase-space model and a likelihood framework. Our method complements existing methods for low-mass subhalo searches, such as searches for gaps in stellar streams, in that we can localize the positions and velocities of the subhalos today.

astro-ph.GA↗

arXiv2017

Collider constraints and new tests of color octet vectors

We analyze the collider sensitivity for new colored resonances in $t\bar{t}$, $b \bar{b}$, and $jj$ final states. While searches in the single production channel are model-dependent, the pair production rate is model independent and the existing $(JJ)(JJ)$ and $4t$ searches impose strong constraints on the relevant branching fractions, where $J = j$ or $b$. We point out the missing, complementary searches in the mixed decay modes, $t\bar{t}(jj)$, $t\bar{t}(b\bar{b})$, and $(b\bar{b})(jj)$. We propose analysis strategies for the $t\bar{t}(jj)$ and $t\bar{t}(b\bar{b})$ decays and find their sensivity surpasses that of existing searches when the decay widths to tops and light jets are comparable. If no other decays are present, collective lower limits on the resonance mass can be set at 1.5~TeV using 37~fb$^{-1}$ of 13~TeV data.

hep-ph↗

arXiv2017

Angular observables for spin discrimination in boosted diboson final states

We investigate the prospects for spin determination of a heavy diboson resonance using angular observables. Focusing in particular on boosted fully hadronic final states, we detail both the differences in signal efficiencies and distortions of differential distributions resulting from various jet substructure techniques. We treat the 2 TeV diboson excess as a case study, but our results are generally applicable to any future discovery in the diboson channel. Scrutinizing ATLAS and CMS analyses at 8 TeV and 13 TeV, we find that the specific cuts employed in these analyses have a tremendous impact on the discrimination power between different signal hypotheses. We discuss modified cuts that can offer a significant boost to spin sensitivity in a post-discovery era. Even without altered cuts, we show that CMS, and partly also ATLAS, will be able to distinguish between spin 0, 1, or 2 new physics diboson resonances at the $2σ$ level with 30 fb$^{-1}$ of 13 TeV data, for our 2 TeV case study.

hep-ph↗

arXiv2016

New Signatures of Flavor Violating Higgs Couplings

We explore several novel LHC signatures arising from quark or lepton flavor violating couplings in the Higgs sector, and we constrain such couplings using LHC data. Since the largest signals are possible in channels involving top quarks or tau leptons, we consider in particular the following flavor violating processes: (1) $pp \to thh$ (top plus di-Higgs final state) arising from a dimension six coupling of up-type quarks to three insertions of the Higgs field. We develop a search strategy for this final state and demonstrate that detection is possible at the high luminosity LHC if flavor violating top--up--Higgs couplings are not too far below the current limit. (2) $pp \to tH^0$, where $H^0$ is the heavy neutral CP-even Higgs boson in a two Higgs doublet model (2HDM). We consider the decay channels $H^0 \to tu, WW, ZZ, hh$ and use existing LHC data to constrain the first three of them. For the fourth, we adapt our search for the $thh$ final state, and we demonstrate that in large regions of the parameter space, it is superior to other searches, including searches for flavor violating top quark decays ($t \to hq$). (3) $H^0 \to τμ$, again in the context of a 2HDM. This channel is particularly well motivated by the recent CMS excess in $h \to τμ$, and we use the data from this search to constrain the properties of $H^0$.

hep-ph↗

arXiv2016

Hunting for Dark Matter Coannihilation by Mixing Dijet Resonances and Missing Transverse Energy

Simplified models of the dark matter (co)annihilation mechanism predict striking new collider signatures untested by current searches. These models, which were codified in the coannihilation codex, provide the basis for a dark matter (DM) discovery program at the Large Hadron Collider (LHC) driven by the measured DM relic density. In this work, we study an exemplary model featuring $s$-channel DM coannihilation through a scalar diquark mediator as a representative case study of scenarios with strongly interacting coannihilation partners. We discuss the full phenomenology of the model, ranging from low energy flavor constraints, vacuum stability requirements, and precision Higgs effects to direct detection and indirect detection prospects. Moreover, motivated by the relic density calculation, we find significant portions of parameter space are compatible with current collider constraints and can be probed by future searches, including a proposed analysis for the novel signature of a dijet resonance accompanied by missing transverse energy (MET). Our results show that the $13$ TeV LHC with $100~\mathrm{fb}^{-1}$ luminosity should be sensitive to mediators as heavy as 1 TeV and dark matter in the 400--500 GeV range. The combination of searches for single and paired dijet peaks, non-resonant jets + MET excesses, and our novel resonant dijet + MET signature have strong coverage of the motivated relic density region, reflecting the tight connections between particles determining the dark matter abundance and their experimental signatures at the LHC.

hep-ph↗

arXiv2015

Lepton Jets from Radiating Dark Matter

The idea that dark matter forms part of a larger dark sector is very intriguing, given the high degree of complexity of the visible sector. In this paper, we discuss lepton jets as a promising signature of an extended dark sector. As a simple toy model, we consider an $\mathcal{O}(\text{GeV})$ DM fermion coupled to a new $U(1)'$ gauge boson (dark photon) with a mass of order GeV and kinetically mixed with the Standard Model photon. Dark matter production at the LHC in this model is typically accompanied by collinear radiation of dark photons whose decay products can form lepton jets. We analyze the dynamics of collinear dark photon emission both analytically and numerically. In particular, we derive the dark photon energy spectrum using recursive analytic expressions, using Monte Carlo simulations in Pythia, and using an inverse Mellin transform to obtain the spectrum from its moments. In the second part of the paper, we simulate the expected lepton jet signatures from radiating dark matter at the LHC, carefully taking into account the various dark photon decay modes and allowing for both prompt and displaced decays. Using these simulations, we recast two existing ATLAS lepton jet searches to significantly restrict the parameter space of extended dark sector models, and we compute the expected sensitivity of future LHC searches.

hep-ph↗

arXiv2014

Mass Effects in the Higgs-Gluon Coupling: Boosted vs Off-Shell Production

In the upcoming LHC run we will be able to probe the structure ofthe loop--induced Higgs--gluon coupling through kinematics. First, we establish state-of-the-art simulations with up to two jets to next-to-leading order including top mass effects. They allow us to search for deviations from the low-energy limits in boosted Higgs production. In addition, the size of the top mass effects suggests that they should generally be included in Higgs studies at the LHC. Next, we show how off-shell Higgs production with a decay to four leptons is sensitive to the same top mass effects. We compare the potential of both methods based on the same top--Higgs Lagrangian. Finally, we comment on related model assumptions required for a Higgs width measurement.

hep-ph↗

arXiv2014

Resolving the Higgs-Gluon Coupling with Jets

In the Standard Model the Higgs coupling to gluons is almost entirely induced by top quark loops. We derive the logarithmic structure of Higgs production in association with two jets. Just like in the one-jet case the transverse momentum distributions exhibit logarithms of the top quark mass and can be used to test the nature of the loop-induced Higgs coupling to gluons. Using Higgs decays to W bosons and to tau leptons we show how the corresponding analyses hugely benefit from the second jet in the relevant signal rate as well as in the background rejection.

hep-ph↗