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

27 Nov 2019, 09:00 → 29 Nov 2019, 17:30 Europe/Zurich

University of Ghent

Albert De Roeck (CERN), Basile Vermassen (Ghent University (BE)), Carlos Vazquez Sierra (Nikhef National institute for subatomic physics (NL)), Didar Dobur (Ghent University (BE)), Freya Blekman (IIHE, Vrije Universiteit Brussel (BE)), Giovanna Cottin (National Taiwan University), James Beacham (Duke University (US)), José Francisco Zurita (KIT), Juliette Alimena (Ohio State University (US)), Martina Vit (Ghent University (BE)), Michelangelo Mangano (CERN), Rebeca Gonzalez Suarez (Uppsala University (SE)), Ryu Sawada (University of Tokyo (JP)), Steven Lowette (Vrije Universiteit Brussel (BE)), Tom Cornelis (University of Ghent (BE))

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

 

 

 

Wednesday 27 November

09:00 → 09:30 Registration

09:30 → 09:35 Welcome and logistics

Speaker: Didar Dobur (Ghent University (BE))

09:35 → 09:45 Introduction to LHC LLP Community

Speaker: James Beacham (Duke University (US))

Beacham_LHC_LLP_Sixth_Ghent_Intro_2019Nov27.pdf

09:45 → 10:05 Theory overview

Speaker: Zhen Liu (U of Maryland)

16_LLPTheory_ZhenLiu_Ghent_6thLLP.pdf

10:05 → 10:25 Experimental overview

Speaker: Carlos Vazquez Sierra (Nikhef National institute for subatomic physics (NL))

CVS_LLPs_experimental_overview.pdf

10:30 → 11:00 Coffee break

11:00 → 11:20 Summary of "LLPs and the third generation" theory workshop in Edinburgh from the previous week

Speaker: Sinead Farrington (University of Edinburgh)

LLP3gen_Farrington_271119.pdf

11:20 → 11:40 Heavy neutral leptons @ NA62

Speaker: Artur Shaikhiev (Russian Academy of Sciences (RU))

shaikhiev_ghent_long_lived19.pdf

11:40 → 12:00 Search for heavy neutrinos with the near detector ND280 of the T2K experiment

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 → 12:20 Connections between dark matter and LLP signatures at collider experiments

Speaker: Caterina Doglioni (Lund University (SE))

20181127_Doglioni_LLPCommunityWorkshop.pdf

12:30 → 14:00 Lunch

14:00 → 14:15 Uncovering quirk signal via energy loss inside tracker at the LHC

Speaker: Wenxing Zhang

Uncovering quirk signal via energy loss inside tracker.pdf

14:15 → 14:30 Searching for Dark Photons at the LHeC and FCC-he

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)

slides.pdf

14:30 → 14:45 Long live the Unification

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)

2019.11.27_Kowalska.pdf

14:45 → 15:00 A long-lived stop with freeze-in and freeze-out dark matter in the hidden sector

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)

LLPstop_Ghent.pdf

15:10 → 15:30 Coffee break

15:30 → 15:45 Higgsino; jet + soft displaced track

Speaker: Hidetoshi Otono (Kyushu University (JP))

Hide_191127_LLP.pdf

15:45 → 16:00 Searches for Atmospheric Long-Lived Particles

Speakers: Pilar Coloma (Universidad Autónoma de Madrid), Pilar Coloma (Fermilab), Pilar Coloma (Instituto de Fisica Corpuscular), Pilar Coloma (Virginia Tech University)

coloma-LHC-longlived.pdf

16:00 → 16:15 Classification of dark pion multiplets as dark matter candidates and displaced decays

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)

Beauchesne_LLP_Ghent.pdf

16:15 → 16:30 LLP signatures of mirror stars

Speaker: Dr Jack Setford (University of Toronto)

Signatures_of_Mirror_Stars_LLP.pdf

16:40 → 17:00 Coffee break

17:00 → 17:15 Diagnosing Hidden Sectors with MATHUSLA and HL-LHC

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)

MATHUSLA_Slides_20191126_GhentWorkshop.pdf

17:15 → 17:30 Searching for long-lived particles with the CMS High Granularity Calorimeter

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)

2019-11-27-LLP-6th-v3.pdf

17:30 → 17:45 Emerging Triggers: Creating a Safe Space for Dark Matter

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)

llp_emerging_jets.pdf

17:45 → 18:00 LLP signatures of non-thermal dark matter

Speakers: Jose Miguel No Redondo, Jose Miguel No Redondo (Conseil Europeen Recherche Nucl. (CERN)-Unknown-Unknown)

Ghent_LLP_Workshop_DM_JMN.pdf

18:00 → 18:15 Identifying Exclusive Displaced Hadronic Signatures in the Forward Region of the LHC

Speaker: Xabier Cid Vidal (Universidade de Santiago de Compostela)

LLP_workshop_Ghent19.pdf

18:15 → 18:30 Probing dark sectors with long-lived particles at BELLE II

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)

Schaefer_LLPs_Ghent.pdf

Thursday 28 November

09:00 → 09:20 [Trigger] Triggers for LLP signatures at CMS

Speaker: Cristián Peña (Fermi National Accelerator Lab. (US))

llp_workshop_Ghent_2019_trigger_cpena_v4.pdf

09:20 → 09:40 [Trigger] Triggers for LLP signatures at ATLAS

Speaker: John Stupak (University of Oklahoma (US))

Stupak_281119_ATLASLLPTriggers.pdf

09:40 → 10:00 [Trigger] Triggers for LLP signatures at LHCb

Speaker: Carlos Vazquez Sierra (Nikhef National institute for subatomic physics (NL))

LLP_triggers_at_LHCb.pdf

10:00 → 10:30 Trigger discussion

10:30 → 11:00 Coffee break

11:00 → 11:20 [ML] CMS Open Data and machine learning for LLP signatures

Speaker: Freya Blekman (IIHE, Vrije Universiteit Brussel (BE))

freyasLoopOverOpenData.py

LLP-CMSopendata-fblekman.pdf

LLP-CMSopendata-fblekman.pptx

11:20 → 11:40 [ML] Machine learning techniques for dark photons at ATLAS

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))

longarini_LLPWorkshop.pdf

11:40 → 12:00 [ML] A deep neural network-based tagger to search for new long-lived particle states decaying to jets at CMS

Speaker: Vilius Cepaitis (Imperial College (GB))

LLP_28Dec_vc.pdf

12:00 → 12:20 [ML] Multidimensional transfer functions & fake rates with neural networks

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)

llp_28Nov19.pdf

12:30 → 14:00 Lunch

14:00 → 14:20 [New results] Displaced lepton vertex & HNLs at ATLAS

Speaker: Shohei Shirabe (Tokyo Institute of Technology (JP))

LHCLLPWorkShop.v3.pdf

14:20 → 14:40 [New results] Delayed photons at CMS

Speaker: Zhicai Zhang (California Institute of Technology (US))

Zhicai_DelayedPhoton_LLP_20191128.pdf

14:40 → 15:00 [New results] Dark photons at LHCb

Speaker: Martino Borsato (Universidade de Santiago de Compostela (ES))

LLPGhent_Borsato.pdf

15:00 → 15:20 [New results] Neutral LLPs in the muon system or inner detector of ATLAS

Speaker: Margaret Susan Lutz (University of Massachusetts (US))

LLP_Ghent_dijets.pdf

16:00 → 18:00 Workshop picture + social event

Friday 29 November

09:00 → 09:20 [HNL] Latest News on HNLs

Speaker: Marco Drewes (Ecole Polytechnique Federale de Lausanne (EPFL))

LLP_Ghent_2019.pdf

09:20 → 09:40 [HNL] Topical talk on heavy neutral leptons

Speaker: Prof. Oleg Ruchayskiy (University of Copenhagen (DK))

LLP_Ruchayskiy_Ghent.pdf

09:40 → 10:00 [HNL] Displaced heavy neutrinos from Z′ decays at the LHC

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

LLPWorkshopGhent_Nov2019.pdf

10:00 → 10:20 [HNL] Finding LLPs in MiniBooNE

Speaker: Alvaro Hernandez Cabezudo (Karlsruhe Institute of Technology (KIT))

Finding_LLPs_in_MiniBooNE.pdf

10:30 → 11:00 Coffee break

11:00 → 11:20 FASER

Speaker: Dave Casper (University of California Irvine (US))

FASER_LLP_20191129.pptx

11:20 → 11:40 The MoEDAL-MAPP Experiment - a Status Report

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))

Ghent-2019.pdf

11:40 → 12:00 ANUBIS: Proposal to search for long-lived neutral particles in CERN service shafts

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))

2019_11_29_Anubis_LLP.pdf

12:00 → 12:20 MATHUSLA

Speakers: Cristiano Alpigiani, Cristiano Alpigiani (University of Washington, Seattle)

LLPWorkshop_MATHUSLA_CristianoAlpigiani_Ghent_November2019.pdf

12:20 → 12:40 MilliQan

Speaker: Steven Lowette (Vrije Universiteit Brussel (BE))

Lowette_191129_LHCLLPWorkshop_milliQan.pdf

12:40 → 13:00 SOLID

Speaker: Haifa Rejeb Sfar (University of Antwerp (BE))

HNL@SoLid LLP Workshop.pdf

13:00 → 13:05 Farewell

Speaker: José Francisco Zurita (KIT)

Farewell_LLP_Ghent.pdf

13:05 → 14:00 Lunch