# Searching for long-lived particles at the LHC: Sixth workshop of the LHC LLP Community

Europe/Zurich
University of Ghent

#### University of Ghent

Campus Aula, Universiteitstraat 4, 9000 Ghent, Belgium https://goo.gl/maps/tH2rvK4SEPEki6XD7
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Description

Following the successes of the LHC Long-Lived Particle (LLP) Workshops in April and October of 2017, May and October of 2018, and May of 2019 -- and continuing the robust and rich tradition defined by prior workshops such as “LHC Searches for Long-Lived BSM Particles: Theory Meets Experiment", at U. Mass, Amherst, in November of 2015; “Experimental Challenges for the LHC Run II", at the Kavli Institute for Theoretical Physics in May of 2016; and the “LHC Long-Lived Particles Mini-Workshop" at CERN in May of 2016 -- the LHC LLP Community, composed of members of the CMS, LHCb, and ATLAS collaborations as well as theorists, phenomenologists and those interested in LLP searches with dedicated LHC detectors such as milliQan, MoEDAL, MATHUSLA, FASER, Codex-b, etc., convenes again -- this time at the University of Ghent, in Belgium -- to assess the state of searches for long-lived particles beyond the Standard Model at the LHC, brainstorm about new ideas for LLP searches, and focus on the future of LLP searches in HEP, both at the LHC and for future detectors and projects.

As with our previous workshops, we prioritize discussion and collaboration, with any talks functioning to set the stage for that collaboration.

The agenda is here, and, as you can see, this time we're focusing on the following topics:

• New results from ATLAS, CMS, and LHCb
• Heavy neutral leptons
• Machine learning topics for LLPs
• New models for LLPs
• Triggers, triggers, triggers (both for Run 3 and the HL-LHC)
• New ideas and overlooked avenues of LLP discovery

Finally, in the interest of being good global citizens, we're planning to make a group donation to an organization dedicated to combating anthropogenic climate change due to fossil fuel use.  The institutes of some attendees already compensate for research-related travel, but if yours doesn't, and you travel to the conference via a method that's known to be a large source of CO2 emissions, we'll ask you for a voluntary donation.

Note that while we hope to see you in person in Ghent, remote participation will indeed be available via Vidyo.

Sponsored by

Registration
Registration for the sixth workshop of the LHC LLP Community
Participants
• Albert De Roeck
• Alberto Mariotti
• Alessandro Biondini
• Alexander Izmaylov
• Alvaro Hernandez Cabezudo
• Amber Roepe
• Amin Aboubrahim
• Andreas Sogaard
• Aqeel Ahmed
• Artur Shaikhiev
• Basile Vermassen
• Carlos Vazquez Sierra
• Caroline Collard
• Caterina Doglioni
• Claudio Caputo
• Cristiano Alpigiani
• Cristiano Sebastiani
• Daniele Trocino
• Dave Casper
• David Curtin
• David Strom
• Didar Dobur
• Dylan Linthorne
• Else Lytken
• Emily Thompson
• Federico Leo Redi
• Freya Blekman
• Gerardo Vasquez
• Giovanna Cottin
• Giuliano Gustavino
• Gregor Kasieczka
• Guillermo Nicolas Hamity
• Haifa Rejeb Sfar
• Hugues Beauchesne
• Iacopo Longarini
• Igor Kostiuk
• Jack Setford
• Jackson Carl Burzynski
• James Beacham
• James Pinfold
• Jan Hajer
• Jan Heisig
• Jared Barron
• Jessica Prisciandaro
• JIA Liu
• Jingyu Luo
• Joel Jones-Perez
• John Stupak
• Jonathan Lee Feng
• Jose Miguel No
• José Francisco Zurita
• juan carlos helo
• Judita Mamuzic
• Kamila Kowalska
• Laura Molina Bueno
• Liam Wezenbeek
• Luka Lambrecht
• Manfredi Ronzani
• Marco Drewes
• Margaret Susan Lutz
• Martina Vit
• Martino Borsato
• Mason Proffitt
• Matthias Danninger
• Mohamed Darwish
• Nishita Desai
• Oleg Ruchayskiy
• Oliver Fischer
• Omar Giancarlo Suarez Navarro
• Pilar Coloma
• Piter Paye
• Rebeca Gonzalez Suarez
• Robin Newhouse
• Ruth Schäfer
• Saereh Najjari
• Sam Junius
• Sascha Mehlhase
• Sebastian Trojanowski
• Shohei Shirabe
• Stefano Giagu
• Steven Lowette
• Suchita Kulkarni
• Tamas Almos Vami
• Tom Cornelis
• Vilius Cepaitis
• Vinzenz Stampf
• Wenxing Zhang
• Xabier Cid Vidal
• Yamiel Abreu Alfonso
• Zeren Simon Wang
• Zhen Liu
• Wednesday, November 27
• 9:00 AM 9:30 AM
Registration 30m
• 9:30 AM 9:35 AM
Welcome and logistics 5m
Speaker: Didar Dobur (Ghent University (BE))
• 9:35 AM 9:45 AM
Introduction to LHC LLP Community 10m
Speaker: James Beacham (Duke University (US))
• 9:45 AM 10:05 AM
Theory overview 20m
Speaker: Zhen Liu (U of Maryland)
• 10:05 AM 10:25 AM
Experimental overview 20m
Speaker: Carlos Vazquez Sierra (Nikhef National institute for subatomic physics (NL))
• 10:30 AM 11:00 AM
Coffee break 30m
• 11:00 AM 11:20 AM
Summary of "LLPs and the third generation" theory workshop in Edinburgh from the previous week 20m
Speaker: Sinead Farrington (University of Edinburgh)
• 11:20 AM 11:40 AM
Heavy neutral leptons @ NA62 20m
Speaker: Artur Shaikhiev (Russian Academy of Sciences (RU))
• 11:40 AM 12:00 PM
Search for heavy neutrinos with the near detector ND280 of the T2K experiment 20m

Heavy Neutral Leptons (HNLs, heavy neutrinos) with masses below the electroweak scale are introduced in some extensions of the Standard Model to address consistently such effects as neutrino oscillations, light neutrino masses, dark matter and baryon asymmetry.
The talk presents the search for heavy neutrinos in the mass range of 140 < MHNL< 493 MeV/c^2 with the T2K neutrino oscillation experiment setup. The near detector complex ND280 is used to identify the products of decays of HNLs potentially originating from the kaon parents of the neutrino beam.
No events in the signal region were observed for the 2010-2017 T2K ND280 dataset. The limits on the mixing parameters between heavy neutrino and electron, muon- and tau- flavoured currents were extracted. The T2K data allow an improvement of the limits provided by the previous experiments such as the CERN PS191 which, together with the BNL E949 data, put the most stringent constraints in the mass region studied by T2K.

Speaker: Dr Alexander Izmaylov (INR RAS)
• 12:00 PM 12:20 PM
Connections between dark matter and LLP signatures at collider experiments 20m
Speaker: Caterina Doglioni (Lund University (SE))
• 12:30 PM 2:00 PM
Lunch 1h 30m
• 2:00 PM 2:15 PM
Uncovering quirk signal via energy loss inside tracker at the LHC 15m
Speaker: Wenxing Zhang
• 2:15 PM 2:30 PM
Searching for Dark Photons at the LHeC and FCC-he 15m

Extensions of the Standard Model (SM) gauge group with a new $U(1)_X$ predict an additional gauge boson. Through kinetic mixing with the SM photons featured by a coupling $\epsilon$, the ensuing so-called dark photons $\gamma'$, which acquire mass as a result of the breaking of the gauge group $U(1)_X$, can interact with the SM field content. These massive dark photons can therefore decay to pairs of leptons, hadrons, or quarks, depending on their mass $m_{\gamma'}$. In this work, we discuss searches for dark photons in the mass range around and below one GeV at the LHeC and FCC-he colliders. The signal is given by the displaced decays of the long-lived dark photon into two charged fermions. We discuss the impact of conceivable irreducible (SM and machine-related) backgrounds and different signal efficiencies. Our estimates show that the LHeC and FCC-he can test a domain that is complementary to other present and planned experiments.

Speaker: Zeren Simon Wang (APCTP, Korea)
• 2:30 PM 2:45 PM
Long live the Unification 15m

In light of null results from New Physics searches, we look at unification of the gauge couplings as a model-building principle. We present a comprehensive list of spectra that feature vector-like fermions in two distinct $SU(3)_C \times SU(2)_L \times U(1)_Y$ representations, in which precise gauge coupling unification is achieved. Complementarity of various experimental strategies allows us to probe many of the successful scenarios up to at least 10 TeV. We demonstrate that in this context the long-lived particle searches prove particularly effective, which provides strong motivation for their further development.

Speaker: Dr Kamila Kowalska (National Centre for Nuclear Research)
• 2:45 PM 3:00 PM
A long-lived stop with freeze-in and freeze-out dark matter in the hidden sector 15m

In extended supersymmetric models with a hidden sector the lightest $R$-parity odd particle can reside in the hidden sector and act as dark matter. We consider the case when the hidden sector has ultraweak interactions with the visible sector. An interesting phenomenon arises if the LSP of the visible sector is charged in which case it will decay to the hidden sector dark matter. Due to the ultraweak interactions, the LSP of the visible sector will be long-lived decaying outside the detector after leaving a track inside. We investigate this possibility in the framework of a $U(1)_X$-extended MSSM/SUGRA model with a small gauge kinetic mixing and mass mixing between the $U(1)_X$ and $U(1)_Y$ where $U(1)_Y$ is the gauge group of the hypercharge. Specifically we investigate the case when the LSP of MSSM is a stop which decays into the hidden sector dark matter and has a lifetime long enough to traverse the LHC detector without decay. It is shown that such a particle can be detected at the HL-LHC and HE-LHC as an $R$-hadron which will look like a slow moving muon with a large transverse momentum $p_T$ and so can be detected by the track it leaves in the inner tracker and in the muon spectrometer. Further, due to the ultraweak couplings between the hidden sector and the MSSM fields, the dark matter particle has a relic density arising from a combination of the freeze-out and freeze-in mechanisms. It is found that even for the ultraweak or feeble interactions the freeze-out contribution relative to freeze-in contribution to the relic density is substantial to dominant, varying between 30% to 74% for the model points considered. It is subdominant to freeze-in for relatively small stop masses with relatively larger stop annihilation cross-sections and the dominant contribution to the relic density for relatively large stop masses and relatively smaller stop annihilation cross-sections. Our analysis shows that the freeze-out contribution must be included for any realistic analysis even for dark matter particles with ultraweak or feeble interactions with the visible sector. A discovery of a long-lived stop as the lightest particle of the MSSM may point to the nature of dark matter and its production mechanism in the early universe.

Speaker: Amin Aboubrahim (Northeastern University)
• 3:10 PM 3:30 PM
Coffee break 20m
• 3:30 PM 3:45 PM
Higgsino; jet + soft displaced track 15m
Speaker: Hidetoshi Otono (Kyushu University (JP))
• 3:45 PM 4:00 PM
Searches for Atmospheric Long-Lived Particles 15m
Speakers: Pilar Coloma (Universidad Autónoma de Madrid) , Pilar Coloma (Fermilab) , Pilar Coloma (Instituto de Fisica Corpuscular) , Pilar Coloma (Virginia Tech University)
• 4:00 PM 4:15 PM
Classification of dark pion multiplets as dark matter candidates and displaced decays 15m

New confining sectors can contain a set of pseudo-Goldstone mesons that exhibit a complicated structure in terms of stability and relative masses. Stable ones can act as dark matter candidates, while their interactions with the unstable ones determine their relic abundances. The overall structure, by specifying which channels are kinematically forbidden or not, affects the cosmology, constraints and collider phenomenology. In this talk, I will show that these pseudo-Goldstone meson structures can be classified into three categories. I will also demonstrate that the unstable pions can lead to displaced vertices and that cosmology can be used to put an upper limit on their decay lengths that is roughly universal within a given category.

Speaker: Dr Hugues Beauchesne (Ben-Gurion University of the Negev)
• 4:15 PM 4:30 PM
LLP signatures of mirror stars 15m
Speaker: Dr Jack Setford (University of Toronto)
• 4:40 PM 5:00 PM
Coffee break 20m
• 5:00 PM 5:15 PM
Diagnosing Hidden Sectors with MATHUSLA and HL-LHC 15m

MATHUSLA is a proposed dedicated large-volume LLP detector, to be located on the surface above an LHC interaction point. Existing work suggests that in many circumstances MATHUSLA can determine the velocity of any detected LLPs with sufficient resolution to identify the bunch crossings at the main detector from which they originated. Under the assumption that MATHUSLA can identify the LLP production event, we study the possibility of combining MATHUSLA and main detector information to extract information about the LLP production topology and estimate BSM particle parameters. Using only a few simple, event-level variables, we show that in most circumstances the LLP production model can be correctly categorized into one of a few simplified topologies with high probability, with O(100) detected LLPs.

Speaker: Jared Barron (University of Toronto)
• 5:15 PM 5:30 PM
Searching for long-lived particles with the CMS High Granularity Calorimeter 15m

We focus on a new tool, the Phase-2 Upgrade of the CMS endcap calorimeter which is a high granularity silicon-based calorimeter (HGCAL). It is a nice stand-alone detector, which has its own trigger, tracker, calorimeters. It also has intrinsic high-precision timing capabilities due to silicon sensor. We study the long-lived particle signal from Higgs decay at the HGCAL, from gluon fusion and vector boson fusion Higgs production. The track based variables have been developed to suppress QCD and fake track backgrounds. The final results depends on trigger performance and are generally promising.

Speaker: Jia Liu (University of Chicago)
• 5:30 PM 5:45 PM
Emerging Triggers: Creating a Safe Space for Dark Matter 15m

The phenomenology of a dark SU(3) sector, within the context of a Hidden Valley, can lead to novel collider signatures. If accessible at the LHC, the dark quarks will shower throughout the detector volume, eventually fragmenting into jets of dark hadrons with a high multiplicity. The resulting unstable dark pions will decay back into visible hadrons, producing displaced tracks within a jet cone, termed $\textit{emerging jets}$. These signals, although unique, are limited by the dedicated triggers used at collider experiments, potentially throwing away an interesting signal if not properly optimized. In this talk, I will consider $\textit{emerging jet}$ events with additional radiation, where lower mediator mass scales can be probed with higher trigger efficiencies. Outside of the dedicated trigger menu, collaborations are looking at new ways to exploit the information at trigger level. We attempt to forgo track reconstruction, and consider only hits on various layers of an ideal tracking detector. Using these low level variables, simple machine learning techniques can provide discriminating power between QCD backgrounds.

Speaker: Mr Dylan Linthorne (Carleton University)
• 5:45 PM 6:00 PM
LLP signatures of non-thermal dark matter 15m
Speakers: Jose Miguel No Redondo, Jose Miguel No Redondo (Conseil Europeen Recherche Nucl. (CERN)-Unknown-Unknown)
• 6:00 PM 6:15 PM
Identifying Exclusive Displaced Hadronic Signatures in the Forward Region of the LHC 15m
Speaker: Xabier Cid Vidal (Universidade de Santiago de Compostela)
• 6:15 PM 6:30 PM
Probing dark sectors with long-lived particles at BELLE II 15m

I will present a new search for light scalar singlets in rare meson decays. For tiny interactions, the scalar is long-lived at detector scales and decays into displaced pairs of leptons or light mesons. I will show that Belle II has a remarkable potential to probe scalars in the GeV range with couplings as small as $10^{−5}$. The predicted sensitivity is higher than at the long-baseline experiments FASER and NA62. I will also discuss signatures of invisibly decaying scalars in rare meson decays with missing energy.

Speaker: Ruth Schäfer (Universität Heidelberg)
• Thursday, November 28
• 9:00 AM 9:20 AM
[Trigger] Triggers for LLP signatures at CMS 20m
Speaker: Cristián Peña (Fermi National Accelerator Lab. (US))
• 9:20 AM 9:40 AM
[Trigger] Triggers for LLP signatures at ATLAS 20m
Speaker: John Stupak (University of Oklahoma (US))
• 9:40 AM 10:00 AM
[Trigger] Triggers for LLP signatures at LHCb 20m
Speaker: Carlos Vazquez Sierra (Nikhef National institute for subatomic physics (NL))
• 10:00 AM 10:30 AM
Trigger discussion 30m
• 10:30 AM 11:00 AM
Coffee break 30m
• 11:00 AM 11:20 AM
[ML] CMS Open Data and machine learning for LLP signatures 20m
Speaker: Freya Blekman (IIHE, Vrije Universiteit Brussel (BE))
• 11:20 AM 11:40 AM
[ML] Machine learning techniques for dark photons at ATLAS 20m

Several new physics models predict the existence of neutral particles with macroscopic life-times known as dark photons. These particles, decaying outside of the interaction region, will give rise to striking signatures in the detectors at the LHC. These can be detected through numerous unconventional signatures: long time-of-flight, late calorimetric energy deposits, or displaced vertices.
A new approach to identify dark-photon late decays into ATLAS calorimeter system is offered by Deep Learning pattern recognition algorithms. A novel selection based on convolutional neural network (CNN) algorithms running on multi-dimensional jet cluster images is presented. The use of low-level input allows to fully exploit the ATLAS calorimeter information.
A L0 muon RPC trigger for HL-LHC based on CNN algorithms that will run on the new FPGA boards is also presented, designed to reconstruct displaced non-pointing tracks and displaced vertices already at L0.

Speaker: Iacopo Longarini (Sapienza Universita e INFN, Roma I (IT))
• 11:40 AM 12:00 PM
[ML] A deep neural network-based tagger to search for new long-lived particle states decaying to jets at CMS 20m
Speaker: Vilius Cepaitis (Imperial College (GB))
• 12:00 PM 12:20 PM
[ML] Multidimensional transfer functions & fake rates with neural networks 20m

Fake leptons are important backgrounds in many analyses and known for being hard to control. Commonly, background estimations through fake rates introduce several shortcomings due to necessary binning of inputs and a limited number of input parameters. In particular, fake rates in searches for long-lived particles might strongly depend on displacement and other secondary vertex properties.

Practically speaking, one can think of fake rates as multidimensional transfer functions which can be approximated via neural networks.

In this talk a way of fitting fake rates with the help of neural networks is presented. The method is showcased in the context of fake lepton backgrounds for a long-lived heavy neutrino analysis exploiting multiple fake rate dependencies.

Speaker: Vinzenz Stampf (eth zürich)
• 12:30 PM 2:00 PM
Lunch 1h 30m
• 2:00 PM 2:20 PM
[New results] Displaced lepton vertex & HNLs at ATLAS 20m
Speaker: Shohei Shirabe (Tokyo Institute of Technology (JP))
• 2:20 PM 2:40 PM
[New results] Delayed photons at CMS 20m
Speaker: Zhicai Zhang (California Institute of Technology (US))
• 2:40 PM 3:00 PM
[New results] Dark photons at LHCb 20m
Speaker: Martino Borsato (Universidade de Santiago de Compostela (ES))
• 3:00 PM 3:20 PM
[New results] Neutral LLPs in the muon system or inner detector of ATLAS 20m
Speaker: Margaret Susan Lutz (University of Massachusetts (US))
• 4:00 PM 6:00 PM
Workshop picture + social event 2h
• Friday, November 29
• 9:00 AM 9:20 AM
[HNL] Latest News on HNLs 20m
Speaker: Marco Drewes (Ecole Polytechnique Federale de Lausanne (EPFL))
• 9:20 AM 9:40 AM
[HNL] Topical talk on heavy neutral leptons 20m
Speaker: Prof. Oleg Ruchayskiy (University of Copenhagen (DK))
• 9:40 AM 10:00 AM
[HNL] Displaced heavy neutrinos from Z′ decays at the LHC 20m

I will comment on the LHC reach to probe right-handed neutrinos from Z' decays in B-L models. I will discuss two search strategies, including single and two displaced vertex searches at ATLAS and CMS, with the power to constrain electron and muon mixing in the heavy neutrino sector.

Speaker: Giovanna Cottin
• 10:00 AM 10:20 AM
[HNL] Finding LLPs in MiniBooNE 20m
Speaker: Alvaro Hernandez Cabezudo (Karlsruhe Institute of Technology (KIT))
• 10:30 AM 11:00 AM
Coffee break 30m
• 11:00 AM 11:20 AM
FASER 20m
Speaker: Dave Casper (University of California Irvine (US))
• 11:20 AM 11:40 AM
The MoEDAL-MAPP Experiment - a Status Report 20m

The MoEDAL is the LHC's first and until LHC's Run-3, only dedicated search experiment. The current incarnation of MoEDAL is designed to search for highly ionzing avatars of new physics. Starting in the long shiutdown MoEDAl will be upgrdaded with a new sub-detector called MAPP (MoEDAL apparatus for Penetrating Particles). MAPP will give MoEDAl additional sensitivity to mini-charged (mQPs) as well as long lived neutral avatars (LLPs) on physics beyond the Standard Model. We will report results from the current MoEDAL detector and briefly present its program for Run-3. Additionally, we will discuss the MAPP detector which is currently under construction, illustrating its physics reach by several example scenarios involving mQPs and LLPs. Lastly, we will briefly discuss the physics possibilities for MAPP-2 for high luminosity LHC running.

Speaker: James Pinfold (University of Alberta (CA))
• 11:40 AM 12:00 PM
ANUBIS: Proposal to search for long-lived neutral particles in CERN service shafts 20m

Long-lived particles are predicted by many extensions of the Standard Model and have been gaining interest in recent years. In this Letter we present a competitive proposal that substantially extends the sensitivity in lifetime by instrumenting the existing service shafts above the ATLAS or CMS experiments with tracking stations. For scenarios with electrically neutral long-lived particles with $m > 1$ GeV, the lifetime reach is increased by 2-3 orders of magnitude compared to currently operating and approved future experiments at the LHC. A detector design proposal is outlined along with projected costs.

Speakers: Martin Bauer (Heidelberg University) , Oleg Brandt (University of Cambridge (GB)) , Lawrence Lee Jr (Harvard University (US)) , Christian Ohm (KTH Royal Institute of Technology (SE))
• 12:00 PM 12:20 PM
MATHUSLA 20m
Speakers: Cristiano Alpigiani, Cristiano Alpigiani (University of Washington, Seattle)
• 12:20 PM 12:40 PM
MilliQan 20m
Speaker: Steven Lowette (Vrije Universiteit Brussel (BE))
• 12:40 PM 1:00 PM
SOLID 20m
Speaker: Haifa Rejeb Sfar (University of Antwerp (BE))
• 1:00 PM 1:05 PM
Farewell 5m
Speaker: José Francisco Zurita (KIT)
• 1:05 PM 2:00 PM
Lunch 55m