<p>We examine the possibility to detect new SM-neutral vector bosons (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math>) that couple exclusively to leptons in the electron-positron mode of the Future Circular Collider (FCC-ee). Focusing on the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> production with a radiated photon search channel, we show that the FCC-ee can significantly extend the unprobed parameter space by increasing the exclusion in the coupling by one to two orders of magnitude in the kinematically allowed mass range (from 10 to 365 GeV), with the leading sensitivity being driven by the muon channel. In doing so, it outperforms other proposed lepton collider options such as CLIC and ILC in this range of masses. Further, we discuss the possibility of improving the sensitivity of the FCC-ee to this model through the modification of the dilepton invariant mass resolution and the photon energy resolution. The impact of systematic uncertainties on the expected sensitivities is also studied.</p>

Sweden

Rebeca Gonzalez Suarez

Spain

Baibhab Pattnaik

José Zurita

Leptophilic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> bosons at the FCC-ee: Discovery opportunities

In this work, we discuss the possibility to test Heavy Neutral Leptons (HNLs) using “Cosmic Ray Beam Dump” experiments. In analogy with terrestrial beam dump experiments, where a beam first hits a target and is then absorbed by a shield, we consider high-energy incident cosmic rays impinging on the Earth’s atmosphere and then the Earth’s surface. We focus here on HNL production from atmospherically produced kaon, pion and D-meson decays, and discuss the possible explanation of the appearing Cherenkov showers observed by the SHALON Cherenkov telescope and the ultra-high energy events detected by the neutrino experiment ANITA. We show that these observations can not be explained with a long-lived HNL, as the relevant parameter space is excluded by existing constraints. Then we propose two new experimental setups that are inspired by these experiments, namely a Cherenkov telescope pointing at a sub-horizontal angle and shielded by the mountain cliff at Mount Thor, and a geostationary satellite that observes part of the Sahara desert. We show that the Cherenkov telescope at Mount Thor can probe currently untested HNL parameter space for masses below the kaon mass. We also show that the geostationary satellite experiment can significantly increase the HNL parameter space coverage in the whole mass range from 10 MeV up to 2 GeV and test neutrino mixing |U  α4|2 down to 10 −11 for masses around 300 MeV.

United Kingdom

Oliver Fischer

Testing Heavy Neutral Leptons in Cosmic Ray Beam Dump experiments

We consider the phenomenological signatures of Simplified Models of Flavourful Leptoquarks, whose Beyond-the-Standard Model (SM) couplings to fermion generations occur via textures that are well motivated from a broad class of ultraviolet flavour models (which we briefly review). We place particular emphasis on the study of the vector leptoquark ∆ μ  with assignments (3 , 1 , 2/3) under the SM’s gauge symmetry, SU(3) C  × SU(2) L  × U(1) Y , which has the tantalising possibility of explaining both   <math> <msub> <mi>R</mi> <msup> <mi>K</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {\mathcal{R}}_{K^{\left(\ast \right)}} $$  and   <math> <msub> <mi>R</mi> <msup> <mi>D</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {\mathcal{R}}_{D^{\left(\ast \right)}} $$  anomalies. Upon performing global likelihood scans of the leptoquark’s coupling parameter space, focusing in particular on models with tree-level couplings to a single charged lepton species, we then provide confidence intervals and benchmark points preferred by low (er)-energy flavour data. Finally, we use these constraints to further evaluate the (promising) Large Hadron Collider (LHC) detection prospects of pairs of τ-flavoured ∆ μ , through their distinct (a)symmetric decay channels. Namely, we consider direct third-generation leptoquark and jets plus missing-energy searches at the LHC, which we find to be complementary. Depending on the simplified model under consideration, the direct searches constrain the ∆ μ  mass up to 1500-1770 GeV when the branching fraction of ∆ μ  is entirely to third-generation quarks (but are significantly reduced with decreased branching ratios to the third generation), whereas the missing-energy searches constrain the mass up to 1150-1700 GeV while being largely insensitive to the third-generation branching fraction.

Germany

Jordan Bernigaud

Monika Blanke

Portugal

Ivo Medeiros Varzielas

Denmark

Jim Talbert

LHC signatures of τ-flavoured vector leptoquarks

The measured branching fractions of B-mesons into leptonic final states derived by the LHCb, Belle and BaBar collaborations hint towards the breakdown of lepton flavour universality. In this work we take at face value the so-called   <math> <msub> <mi>R</mi> <msup> <mi>D</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {R}_{D^{\left(\ast \right)}} $$  observables that are defined as the ratios of neutral B-meson charged-current decays into a D (*)-meson, a charged lepton and a neutrino final state in the tau and light lepton channels. A well-studied and simple solution to this charged current anomaly is to introduce a scalar leptoquark S 1 that couples to the second and third generation of fermions. We investigate how S 1 can also serve as a mediator between the Standard Model and a dark sector. We study this scenario in detail and estimate the constraints arising from collider searches for leptoquarks, collider searches for missing energy signals, direct detection experiments and the dark matter relic abundance. We stress that the production of a pair of leptoquarks that decays into different final states (i.e. the commonly called “mixed” channels) provides critical information for identifying the underlying dynamics, and we exemplify this by studying the tτbν and the resonant S 1 plus missing energy channels. We find that direct detection data provides non-negligible constraints on the leptoquark coupling to the dark sector, which in turn affects the relic abundance. We also show that the correct relic abundance can not only arise via standard freeze-out, but also through conversion-driven freeze-out. We illustrate the rich phenomenology of the model with a few selected benchmark points, providing a broad stroke of the interesting connection between lepton flavour universality violation and dark matter.

France

Geneviève Bélanger

Aoife Bharucha

Benjamin Fuks

Andreas Goudelis

Belgium

Jan Heisig

South Korea

Adil Jueid

Brazil

Andre Lessa

United States

Kirtimaan Mohan

Italy

Giacomo Polesello

Priscilla Pani

Russia

Alexander Pukhov

Dipan Sengupta

Leptoquark manoeuvres in the dark: a simultaneous solution of the dark matter problem and the   <math> <msub> <mi>R</mi> <msup> <mi>D</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {R}_{D^{\left(\ast \right)}} $$  anomalies

We examine the sensitivity at a future 100 TeV proton-proton collider to compressed dark sectors whose decay products are invisible due to below-threshold energies and/or small couplings to the Standard Model. Such a scenario could be relevant to models of WIMP dark matter, where the lightest New Physics state is an (isolated) electroweak multiplet whose lowest component is stable on collider timescales. We rely on the additional emission of a hard on-shell Z-boson decaying to leptons, a channel with low background systematics, and include a careful estimate of the real and fake backgrounds to this process in our analysis. We show that an integrated luminosity of 30 ab−1 would allow exclusion of a TeV-scale compressed dark sector with inclusive production cross section 0.3 fb, for 1% background systematic uncertainty and splittings below 5 GeV. This translates to exclusion of a pure higgsino (wino) multiplet with mass of 500 (970) GeV.

Switzerland

Rakhi Mahbubani

Probing compressed dark sectors at 100 TeV in the dileptonic mono-Z channel

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.

Simon Kast

Jennifer Thompson

Susanne Westhoff

Spotting hidden sectors with Higgs binoculars

The future electron-proton collider proposals, LHeC and FCC-he, can deliver <math><mi>O</mi><mo>(</mo><mi>TeV</mi><mo>)</mo></math> center-of-mass energy collisions, higher than most of the proposed lepton accelerators, with <math><mrow><mi>O</mi><mo>(</mo><mrow><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow></math> luminosity, while maintaining a much cleaner experimental environment as compared to the hadron machines. This unique capability of <math><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>p</mi></mrow></math> colliders can be harnessed in probing beyond the Standard Model scenarios giving final states that look like hadronic noise at <math><mi>p</mi><mi>p</mi></math> machines. In the present study, we explore the prospects of detecting such a prompt signal having multiple soft jets at the LHeC. Such a signal can come from the decay of gluino in <math><mi>R</mi></math>-parity-violating or stealth supersymmetry, where there exists a gap in the current experimental search with <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mover><mrow><mi>g</mi></mrow><mrow><mo>˜</mo></mrow></mover></mrow></msub><mo>≈</mo><mn>50</mn><mi>–</mi><mn>70</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>. We perform a simple analysis to demonstrate that, with simple signal selection cuts, we can close this gap at the LHeC at the 95% confidence level, even in the presence of a reasonable systematic error. More sophisticated signal selection strategies and detailed knowledge of the detector can be used to improve the prospects of signal detection.

Canada

David Curtin

Kaustubh Deshpande

Closing the light gluino gap with electron-proton colliders

Future electron-proton collider proposals like the LHeC or the FCC-eh can supply ab−1 of collisions with a center-of-mass energy in the TeV range, while maintaining a clean experimental environment more commonly associated with lepton colliders. We point out that this makes e − p colliders ideally suited to probe BSM signatures with final states that look like “hadronic noise” in the high-energy, pile-up-rich environment of pp colliders. We focus on the generic vector boson fusion production mechanism, which is available for all BSM particles with electroweak charges at mass scales far above the reach of most lepton colliders. This is in contrast to previous BSM studies at these machines, which focused on BSM processes with large production rates from the asymmetric initial state. We propose to exploit the unique experimental environment in the search for long-lived particle signals arising from Higgsinos or exotic Higgs decays. At e − p colliders, the soft decay products of long-lived Higgsino can be explicitly reconstructed (“displaced single pion”), and very short lifetimes can be probed. We find that e − p colliders can explore significant regions of BSM parameter space inaccessible to other collider searches, with important implications for the design of such machines.

New physics opportunities for long-lived particles at electron-proton colliders

CERN

Pedro Schwaller

Erratum to: Closing the window for compressed dark sectors with disappearing charged tracks

We investigate the sensitivity at current and future hadron colliders to a heavy electrically-charged particle with a proper decay length below a centimetre, whose decay products are invisible due to below-threshold energies and/or small couplings to the Standard Model. A cosmologically-motivated example of a framework that contains such a particle is the Minimal Supersymmetric Standard Model in the limit of pure Higgsinos. The current hadron-collider search strategy has no sensitivity to the upper range of pure-Higgsino masses that are consistent with the thermal relic density, even at a future collider with 100 TeV centre-of-mass energy. We show that performing a disappearing track search within the inner 10 cm of detector volume would improve the reach in lifetime by a factor of 3 at the 14 TeV LHC and a further factor of 5 at a 100 TeV collider, resulting in around 10 events for 1.1 TeV thermal Higgsinos. In order to include the particles with the largest boost in the analysis, we furthermore propose a purely track-based search in both the central and forward regions, each of which would increase the number of events by another factor of 5, improving our reach at small lifetimes. This would allow us to definitively discover or exclude the experimentally-elusive pure-Higgsino thermal relic at a 100 TeV collider. Our results illustrate the importance of varying detector design when assessing the reach of future high energy colliders.

Closing the window for compressed dark sectors with disappearing charged tracks