Dark Showers Workshop

Jan 21, 2025, 2:00 PM → Jan 23, 2025, 6:00 PM Europe/Zurich

Annapaola De Cosa (ETH Zurich (CH)), Deepak Kar (University of the Witwatersrand (ZA)), Felix Kahlhoefer (Karlsruhe Institute of Technology), Jackson Carl Burzynski (Simon Fraser University (CA)), Jannicke Pearkes (University of Colorado Boulder (US)), Kevin Pedro (Fermi National Accelerator Lab. (US)), Marcella Bona (Queen Mary University of London (UK)), Suchita Kulkarni (University of Graz), Sukanya Sinha (The University of Manchester (GB)), Tim Cohen (CERN)

Following up on the initial discussions during the Dark-showers session of the Roadmap of DM workshop at CERN earlier this year, we would like to organize a “distributed” workshop, with a first meeting on 21-23 January 2025, to start the community effort on Darkshowers benchmarks.

This 3-day online meeting in January (only afternoon CERN time) will recap state of the art and initiate discussion about our 2 objectives:

1. initiate studies on benchmark models for Dark jet resonances/Semi-visible jets/Emerging jets
2. Initiate studies on new signatures

The plan is to create working groups for those interested in working together (mixing theorists and experimentalists), but still letting people who prefer to work standalone on projects (that might become their own paper) to do it.

After the first workshop, we want to allot time for people to work, with some follow-up meetings, and plan a second part of the workshop around June/July (before the summer vacations) to get a summary of the work. We can then prepare a community report based on the studies and from there plan other short workshops. We can use as an example the tau-enriched semi-visible jet paper or any other paper that went out following the discussions we had at the Semi-Visible jet workshop in Zurich and follow the recommendations highlighted in the report from the MITP dark sector workshop.

Tue, January 21

2:00 PM → 4:00 PM Introductory Talks

Convener: Annapaola De Cosa (ETH Zurich (CH))

2:00 PM Workshop Welcome

2:10 PM Theory Landscape Overview

Speaker: Tim Cohen (CERN)

2:30 PM Questions

2:40 PM ATLAS Experimental Overview

Speaker: Jackson Carl Burzynski (Simon Fraser University (CA))

3:00 PM Questions

3:10 PM CMS Experimental Overview

Speaker: Dr Roberto Seidita (ETH Zurich (CH))

3:30 PM Questions & Discussion

4:00 PM → 4:25 PM Virtual Coffee

4:25 PM → 6:25 PM Flash Talks

Convener: Jannicke Pearkes (University of Colorado Boulder (US))

4:30 PM Long-lived glueballs as a test of early matter domination

I will show how collider searches for long-lived particles (LLPs) can test the dynamics responsible for matter domination in the early universe. In particular I will focus on long-lived glueballs from a GeV-scale confining dark sector, of relevance for this workshop. After a summary of their cosmology, I will prove that searches of glueballs, from decays of the Higgs or other particles, test increasing values of dilution at ATLAS and CMS, CODEX-b, ANUBIS and MATHUSLA. These results provide a quantitative physics motivation to test longer lifetimes.

Speaker: Filippo Sala (University of Bologna and INFN)

4:50 PM Probing photons-enriched signatures for Semi-visible jets at the LHC

The Hidden Valley scenario consists of a set of models where the Standard Model is accompanied by a Hidden Sector and connected with it via a heavy mediator or a weak coupling. If realised in nature, the Hidden Valley scenario may result in unusual and little-studied phenomena at the LHC. Under the assumption of a QCD-like confining dark sector, novel experimental signatures emerge, characterized by sprays of particles resembling hadronic jets containing stable invisible dark matter bound states. The resulting signature is characterised by missing momentum aligned with one of the jets, defining an orthogonal phase-space compared to traditional WIMPs searches. These semi-visible jets have been studied theoretically and experimentally in the fully hadronic signature, and more recently including also leptonic final states. We present new simplified models allowing the decays of the unstable dark bound states to photons via a pseudo-scalar portal, such as an axion-like particle. The new resulting signatures are semi-visible jets characterised by an enhanced presence of non-isolated photons, coming directly from dark bound states decays. We show that these exotic jets evade the phase space probed by current LHC searches exploiting jets or photons due to the expected high jet neutral electromagnetic fraction and photons candidates non-isolation, respectively. Moreover, we show that by leveraging the distinguishing features of the substructure of the final state BSM jets compared to SM ones, it is possible to develop identification criteria that can be used in real analyses at the LHC to enhance the sensitivity to these novel signatures.

Speaker: Cesare Tiziano Cazzaniga (ETH Zurich (CH))

5:03 PM Exploring Dark Showers with Radiations

e investigate the potential to search for semi-visible jets (SVJs) at the Large Hadron Collider (LHC) using initial-state radiation (ISR). Both photon ISR and jet ISR channels are considered, using a benchmark signal model with the decay of a leptophobic Z′ mediator forming two SVJs. We compare and extend several techniques to decompose the missing transverse momentum into per-jet contributions, in order to reconstruct the mediator mass and to define a new observable measuring the fraction of invisible dark hadrons. The presence of ISR facilitates the identification of the SVJs, and the resulting boost improves the resolution of the observables, especially for models with high invisible fractions. We combine the two observables to propose a complete search strategy and discuss an extension of the strategy to probe the whole model parameter space.

Speaker: Bingxuan Liu (Sun Yat-Sen University (CN))

5:15 PM Bridging the Higgs Portal to the Dark Sector: From FCCee to LHC

Dark showers arising from the Higgs coupling to a strongly confined dark
sector present a unique avenue for exploring new physics signatures.
Within the FCC-ee framework, this model is being studied,
highlighting the production of semi-visible jets (SVJs) enriched with
photons and leptons. These signatures, featuring displaced decays and
complex jet structures, demand tailored reconstruction techniques and
novel targeted analyses.
We aim to adapt this model for the LHC, leveraging vector boson fusion
production of the Higgs, which couples to dark quarks and subsequently
hadronizes into dark mesons. Standard jet identification at the LHC may fail
to capture such photon-enriched and lepton-enriched SVJs, necessitating
the development of new algorithms relying on jet substructure.
Key challenges include understanding how Standard Model (SM) analyses,
such as those targeting the Higgs-to-b-quark decay channel, overlap with
or constrain these novel signatures. While $H\to bb$ acts as a background,
assessing the sensitivity of SM analyses to dark sector scenarios or
highlighting their limitations in identifying photon- and lepton-enriched
SVJs is central to our exploration.
This work will focus on developing new analysis strategies tailored to the
LHC environment, where existing detector capabilities and triggers may
already offer opportunities to probe this signature. By uncovering an
unexplored parameter space, particularly in the low-mass regime around
the Higgs scale, we aim to refine the methods for identifying dark sector
signatures and enhance sensitivity to new physics within Higgs portal
frameworks.

Speaker: Emre Sitti (ETH Zurich (CH))

5:27 PM Constraints on resonant and non-resonant production modes of SVJs

We look at the constraints we get from SM measurements on resonant and non-resonant production modes of semi-visible jets by running CONTUR. By varying several parameters in both channels, we can investigate the kinematics behind the exclusions and see the merit in using reinterpretation tools in developing search strategies/phenomenological studies.

Speaker: Clarisse Prat

5:39 PM Reinterpreting LHC Dark QCD results

In this contribution, we will present our recent studies on the recasting of the different LHC darkQCD analyses using MadAnalysis5. The reinterpretation was done in the context of dark shower models focussing on semi-visible jet resonances.

Speaker: Thomas Wojtkowski (LPSC - Centre National de la Recherche Scientifique (FR))

5:51 PM A multi-boson door into compositeness

We consider the prospect of the SM as a composite of a new fundamental fermionic sector ($q_{TC}$). Such scenarios can be considered as QCD-like, where the hadronization scale $\Lambda_{TC}\sim$ few TeV. The study of $q\bar q\rightarrow q_{TC}\bar q_{TC} $ at 100 TeV at FCC-hh has a direct analogue to the study of $e^+e^-\rightarrow q\bar q$ at around few GeV at LEP. While the latter considered the production of few-pion final states formed due to the fragmentation and eventual hadronization of the SM quarks, we will consider the production of few scalars (TC-mesons) that are formed out of the new quarks $q_{TC}$. In its simplest manifestation, these scalars are identified as the Higgs and the longitudinal components of the gauge bosons. The resulting multi-TC-meson final state is an artefact of the underlying QCD-like dynamics. Using the Lund string model for producing these TC mesons, we use simple kinematic discriminants to establish a proof of concept for such an analysis to be a near undisputed sign of compositeness and, hence, a need for FCC-hh.

Speaker: Abhishek Iyer (IIT Delhi)

6:03 PM Search for top-coupled dark sector with emerging jets signatures

Current search efforts for flavored dark sectors that couple to multiple SM quarks flavors has only included the coupling to isospin –½ SM quarks using the emerging jet signature. A natural extension to such models would be to change the coupling to isospin +½ SM quark. In particular, the coupling to SM top quarks allows for electroweak-like event selection and triggers to be used, opening the possibility to explore a wider region in the dark sector phase spaces. The accompanying dark sector shower also means that such model may not be fully excluded by existing $t+X$ or $t\overline{t}+X$ search efforts at the LHC. This talk aim to present the projection of the potential sensitivity of CMS to such models at Run 3, as well as exploring additional information available in Run 3 data that can further boost sensitivity to dark show signals.

Speaker: Yi-Mu Chen (KIT - Karlsruhe Institute of Technology (DE))

Wed, January 22

2:00 PM → 2:40 PM Extended Talks

Convener: Marcella Bona (Queen Mary University of London (UK))

2:00 PM Dark showers from sneaky dark matter

We present a minimal composite dark matter model, based on a SU(Nd) dark sector with nf dark quarks and a heavy t-channel mediator. For nf≥4, the dark flavor symmetry guarantees the stability of a subset of the dark pions, which serve as our dark matter candidates. Their relic abundance is determined by co-scattering or co-annihilation with the remaining dark pions, which are unstable and decay. Due to their degenerate masses, the annihilation cross section is suppressed at low temperatures, thereby avoiding stringent constraints from indirect detection and opening up the GeV mass window. The decaying dark pions are naturally long lived. We obtain limits on the model from semi-visible or emerging jet searches and estimate the reach of future probes.

Speaker: Pedro Schwaller

2:20 PM Signals of Pure-Glue and Many-Species Dark Sectors

The pure-glue limit of confining dark sectors reveals new insights into dark shower phenomenology. When the dark quark masses and the scale of a collider's hard interactions that produce the dark sector are significantly larger than the dark confinement scale, there is a dark gluon shower that produces only dark glueballs. These glueballs can then decay to the Standard Model with lifetimes that vary greatly depending on the glueball species and the parameters that set the strength of the portal interaction. This hierarchy of lifetimes leads to different potential signals, including semivisible and emerging jets. In fact, the difference in lifetimes between different species reveals an overlap in the semivisible and emerging jet regimes, motivating search strategies that leverage observables from both signals for any dark shower model with several dark hadron species.

Speaker: Austin Batz (University of Oregon)

2:40 PM → 2:52 PM Flash Talks

2:40 PM Semivisible Jets from Dark Glueballs

We explored a darkQCD model of the hadronization of dark gluons into dark glueballs to generate semi-visible jet signature. We considered two portal particles, a Higgs and a Z’. Trying to generate the SVJ signature with the Higgs mediator, we found the expected signature, but with an extremely low cross-section, to the order of 10^-9 pb. It was not possible to generate the signature with the Z’ portal due to technical issues in the MG5 event generation and a Pythia shower.

Speaker: Ms Zofia Borowska

2:55 PM → 3:25 PM Hidden Valleys: New Signatures and Experimental Approaches

Speaker: Matthew Strassler (Rutgers University)

3:25 PM → 3:44 PM Virtual Coffee Break

3:45 PM → 4:25 PM Extended Talks

Convener: Deepak Kar (University of the Witwatersrand (ZA))

3:45 PM Dark Showers in Herwig 7 and Comparisons with Pythia 8

We present the new implementation of Dark Showers in the Herwig 7 generator. This implementation features for the first time an angular ordered dark shower and dark hadronisation implemented via the cluster hadronisation model. This talk will cover the scale hierarchies involved in dark shower models, and how to pick sensible values for generator parameters based on these scales. Secondly, the talk will explore the impact of hadronisation. This has not yet been much discussed in the context of dark showers, but is potentially a large effect since current event generators use semi-empirical hadronisation models tuned to SM data. A method for estimating the size of this effect on the current cluster hadronisation model will be presented and on-going work to move to more theoretically motivated models with fewer tunable parameters discussed.

AND

We present an ongoing effort to compare the new Herwig Dark Showers code with the existing Pythia Hidden Valley module. In addition to the physical parameters of the model, an event generator requires a number of semi-empirical settings to be tuned, particularly in the hadronisation. Since no dark showers have yet been observed, the best choices for these parameters can only be estimated using intuition from QCD. The aim of these studies is to determine which observables are consistent between generators and for variations of the tunable parameters, and hence are more reliable for use in future analyses, and identify pitfalls for parameter setting. This talk will introduce the theoretically motivated benchmarks proposed in the Snowmass white paper on dark showers, which are used in this study, and present initial lessons learnt from attempting consistent parameter setting between generators.

Speaker: Dominic William Stafford (Deutsches Elektronen-Synchrotron (DE))

4:05 PM Flavoured Emerging Jets

Speaker: Sophie Alice Renner (University of Glasgow (GB))

4:30 PM → 6:30 PM Discussion

Conveners: Annapaola De Cosa (ETH Zurich (CH)), Sukanya Sinha (The University of Manchester (GB)), Tim Cohen (CERN)

Thu, January 23

2:00 PM → 3:00 PM Extended Talks

Convener: Sukanya Sinha (The University of Manchester (GB))

2:00 PM Hidden valleys in CMS muon endcap detector

We demonstrate the sensitivity of the CMS search at the high luminosity LHC run for long-lived particles for Hidden Valley scenarios. We use a theoretically consistent parametrisation of the Hidden Valley parameters to establish our simulation setup. We also adopt a hybrid strategy and establish upper limits on the Hidden Valley production cross section as a function of the model parameters for a model independent interpretation of the search results. Our results demonstrate that the CMS search for long-lived particle decays in the CMS muon endcap is highly sensitive to the underlying theory parameter and it has a capacity to constrain overall scale as well as the ratio of the pion mass to this scale. Furthermore we demonstrate that the search is comparatively less sensitive to the Hidden Valley number of colors or the flavours.

Speaker: Wei Liu (Nanjing University of Science and Technology)

2:20 PM Cosmic ray antihelium in the Galaxy

The creation of anti-nuclei in the Galaxy has been discussed as a possible signal of exotic production mechanisms such as primordial black hole evaporation or dark matter decay/annihilation in addition to the more conventional production from cosmic-ray interactions. Tentative observations of cosmic-ray andideuteron and antihelium by the AMS-02 collaboration have re-energized the quest to use antinuclei to search for physics beyond the standard model.

In this talk, we show state-of-art predictions of the antinuclei flux from both astrophysical and standard dark matter annihilation models from combined fits to high-precision antiproton data as well as cosmic-ray nuclei measurements. Astrophysical mechanisms can explain the amount of antideuteron events detected by AMS-02, while their antihelium production lies far below the sensitivity of this experiment. In turn, standard dark matter models could potentially produce the detected antideuteron and antihelium-3 events, but the production of any detectable antihelium-4 flux would require more novel dark matter model building. As one of the few proposed ways to explain the detection of antihelium-4 events, we discuss that the annihilation/decay of a QCD-like dark sector could potentially explain the preliminary observations of AMS-02.

Speaker: Pedro De la Torre Luque (Institute of theoretical physics (IFT-UAM))

2:40 PM iDMEu and the ESCAPE DM Projects and Joint ATLAS and CMS HepMC

With this talk, we would like to introduce the initiative for Dark Matter in Europe and beyond (iDMEu) and the ESCAPE Dark Matter Science Project. iDMEu is an initiative hosted by JENA (Joint ECFA, NuPECC and APPEC), and takes shape as a collective effort by a group of particle and astroparticle physicists to set up an online resource meta-repository, a common discussion platform and a series of meetings on topics concerning Dark Matter. The Dark Matter Science Project is a set of analyses / reinterpretations from different experiments (including one on dark showers) that have been implemented on a virtual research environment.

We would like to hear suggestions on how these initiatives and their connection to different communities (including those providing computing/tools) can help the dark showers benchmarking effort.

Acronyms, because this is needed by the author as well:
- ECFA (the European Committee for Future Accelerators)
- NuPECC (the Nuclear Physics European Collaboration Committee)
- APPEC (the Astroparticle Physics European Consortium)
- JENAA/JENAS (Joint ECFA, APPEC and NuPECC Activities/Seminar)
- ESCAPE (The European Science Cluster of Astronomy & Particle Physics ESFRI Research Infrastructures)
- ESFRI (European Strategy Forum on Research Infrastructures)

AND

With this talk, we would like to introduce the idea of using the dark showers HepMC files produced in this effort as a test case to exercise the (distributed) computing machinery that would allow ATLAS and CMS to share HepMC files.

If this works and there are no drawbacks to implementation even for large-scale samples, there are the following benefits:

ATLAS and CMS are able to compare results more easily if they start from the same HepMC
theorists who want to reinterpret results can look at shared generation cards (rather than emailing each collaboration to find out what they are using)
we save storage space (which also has an environmental cost)

Speaker: Caterina Doglioni (The University of Manchester (GB))

3:00 PM → 6:00 PM Discussion

Conveners: Deepak Kar (University of the Witwatersrand (ZA)), Felix Kahlhoefer (Karlsruhe Institute of Technology), Kevin Pedro (Fermi National Accelerator Lab. (US))

3:00 PM CMS Parameter choices

Speaker: Kevin Pedro (Fermi National Accelerator Lab. (US))

3:08 PM ATLAS Parameter choices

Speaker: Deepak Kar (University of the Witwatersrand (ZA))