LLong-Lived Particles at Future Colliders. ∗ Rebeca Gonzalez Suarez
Uppsala University, SwedenLong-lived particles have significant enough lifetimes as to, when pro-duced in collisions, leave a distinct signature in the detectors. Driven byincreasingly higher energies, trigger and reconstruction algorithms at parti-cle colliders are optimized for increasingly heavier particles, which in turn,tend to be short-lived. This makes searches for long-lived particles difficult,usually requiring dedicated methods and sometimes dedicated hardware topspot them. However, taking upon the challenge brings enormous potential,since new, long-lived particles feature in a variety of promising new physicsmodels that could answer most of the open questions of the standard model,such as: neutrino masses, Dark Matter, or the matter-antimatter unbalancein the Universe.Currently, the international high energy physics community is plan-ning future facilities post-LHC, and various particle colliders have beenproposed. Crucial physics cases connected to long-lived particles will beaccessible then, and in this presentation, three interesting examples arehighlighted: Heavy Neutral Leptons, Hidden Sectors connected to DarkMatter, and exotic Higgs boson decays. This is followed by a small reviewof the preliminary studies assuming different future colliders, exploiting thecomplementary advantages that different colliding particles and acceleratortypes provide.PACS numbers: 13.66.-a, 14.80.B, 14.60.St, 14.60.Pq, 29.17.+, 29.20.Lq
1. Introduction
In the Standard Model (SM) of particle physics elementary and sub-atomic particles display different lifetimes. How long the lifetime of a par-ticle is depends on different factors, mass and couplings being the mostimportant. A particle with large mass, like the Z boson, will decay fast anda particle that only couples very feebly with others will have long decaytimes. ∗ Presented at XXVII Cracow EPIPHANY Conference on Future of particle physics (1) a r X i v : . [ h e p - e x ] F e b Epiphany˙RGS˙2021 printed on February 16, 2021
The concept of “long-lived particle” in an experimental context accountsfor the experimental setup itself. For example, whereas the Higgs boson,that has a lifetime of 10 − s is certainly short-lived at the LHC, the muon,that takes just microseconds to decay, is in fact detector-stable since it hastime to cross the whole volume of the LHC experiments during that time,and to all purposes, long-lived.It is then useful to understand “long-lived particles” as an umbrella termcovering particles with lifetimes long enough to travel measurable distancesinside the detectors before decaying, long enough to have distinct experi-mental signatures. And though both the above-mentioned muon and forexample a photon, which is stable as far as we know, are then technicallylong-lived particles, the term is more commonly used to refer to new, be-yond the Standard Model (BSM) particles that have not yet been observedexperimentally.New, Long-Lived Particles (LLP) are in fact not a prediction of a sin-gle theory, and fit into virtually all proposed frameworks for BSM physics.Their presence is strongly motivated, and they typically are feebly interact-ing. Understanding the Nature of Dark Matter is among the different SMquestions that LLP could give an answer to, together with Neutrino masses,Baryogenesis, and Naturalness to name a few; and they feature in differentSupersymmetry (SUSY), compositeness, and hidden sector models.Searches for LLP are not new but they are gaining traction [1], as analternative and complement to more mainstream new physics searches, andwill be at least as interesting, if no more, at future colliders. Moreover,models featuring LLP can potentially be tested not only at different collid-ers, but by astro-particle and non-collider (mainly accelerator) experiments,making them ideal for cross-disciplinary, collaborative work.
2. Three interconnected physics cases
In this section, three possible physics cases of LLPs that could be fullyexplored at future colliders are presented.
The discovery of Heavy Neutral Leptons (HNL) could answer centralopen questions of the SM, conceivable more than one at the same time.Just to pick one example, in the ν MSM [2, 3], light sterile neutrinos canaccomplish leptogenesis, a mechanism to dynamically generate the matter-antimatter asymmetry of the Universe via HNL decays during the first10 − s after the Big Bang; provide a Dark Matter candidate; and solv-ing the question on how neutrino acquire mass. piphany˙RGS˙2021 printed on February 16, 2021 | V eN | as a function of the sterile neutrino mass m N , from Ref. [4]. It is then easy to understand why HNL are among the most interestingnew physics to target not only at particle colliders, but also at astrophysicsand non-collider experiments. Most of the current limits, as presented forexample in Fig. 1, cover high neutrino mixing values. For low values of theneutrino mixing angle, most of the still available phase space, the decaylength of the heavy neutrino can be significant. This means that HNL withlow mixing angles are in fact LLP. For example, HNL produced in Z bosondecays as Z → ν N decaying into N → l W will produce a displaced vertexwith multiple tracks. For HNL with long enough lifetimes, oscillations canalso be studied [5].
At the moment, we know there is more to Nature than the SM, but asstated in the introduction, we have no way of predicting the energy scalewhere new physics may exist. The concept behind Hidden Sectors is thatfull collections of new particles and forces could be already accesible bythe current experimental energies, but just not interacting or doing it veryfeebly with the SM particles. If very small couplings, called “portals”, ofthese new collections of particles and forces with SM particles existed, theHiggs boson being one of the prime candidates, that would produce subtle
Epiphany˙RGS˙2021 printed on February 16, 2021 experimental signs to follow. Small couplings lead to long-lived signatures.Within the Matter-Energy content of the Universe, Dark Matter repre-sents about 27%, while regular matter made up of SM particles just about5%. In other words, Dark Matter makes up >
80% of all matter. Regularmatter is very complex and it would not be far-fetched to assume a simi-lar complexity for Dark Matter. Thus, Hidden Sectors connected to DarkMatter, also called “Dark Sectors”, are an attractive area of research in thecurrent experimental landscape of Dark Matter searches in direct, indirect,and collider experiments. In the context of Dark Sectors there are manylong-lived signatures that arise, some of them quite exciting, like, e.g.: DarkShowers.“Axion-Like Particles” (ALPs) provide a very-weakly-coupled windowto the Dark Sector that can be explored widely at future colliders. Forsmall couplings and light ALPs, the ALP decay vertex can be consider-ably displaced from the production vertex, that is, conforming a long-livedsignature [6].
Fig. 2. Projected sensitivity regions for searches for e + e − → γa → γ (left) and e + e − → Za → Z vis γγ (right) at future e + e − colliders for BR( a → γγ ) = 1. Theconstraints from Figure 4 are shown in the background. The sensitivity regions arebased on 4 expected signal events. From Ref. [6]. We are still getting to know the Higgs boson. At the moment, measure-ment and searches characterize a Higgs boson largely compatible with heSM, but the uncertainties are large in some measurements and there couldstill be exotic behaviour to be found, that would provide indications of whatcan lie beyond the SM. This is one of the reasons why a Higgs factory is thefirst priority for the European Strategy for particle physics [7]. piphany˙RGS˙2021 printed on February 16, 2021 Within the exploitation of the Higgs sector, exotic Higgs boson decays toLLPs can be explored at future lepton colliders. This kind of decay featuresin different BSM models, some related to Hidden sectors, like Twin Higgsor Hidden Valley models [8].
3. Technical challenges
High-energy particle colliders push the Energy Frontier, producing heav-ier particles with each increase of the center of mass energy. Heavier parti-cles tend, in exchange, to be shorter-lived. A primer example of this is theHiggs boson at the LHC. A Higgs boson is produced and decays about 10 − seconds after. It often decays in turn to heavy particles, for example, to twoZ bosons, each of them decaying in about 10 − s in e.g. a couple of muonseach, giving way to one of the clearest channels for discovery. In practicethis means that 4 muons coming from the collision point are observed. TheLHC detectors, trigger, and reconstruction methods are designed to findthis kind of signature.New, LLPs are not like that, and display a collection of different sig-natures which are quite unique. From the most intuitive displaced tracksand vertices to unconventional emerging signatures passing by disappearingor kinked tracks; tracks with anomalous energy distributions; or slow orstopped particles that appear out of time.The implications of this are various. First of all, it means that, by dis-playing uncommon signatures, LLP are affected by little to no background.The downsides of this are: potential instrumental background that could bedifficult to model, and the fact that they may require dedicated reconstruc-tion techniques, trigger algorithms, and even dedicated detector design tobe identified.This is a problem at current high and low energy colliders, and willnot be different at future colliders, independently of accelerator or particletypes. At this point, when discussing future facilities, one could designthe future detectors as usual and then try to make the best out of themfor LLPs, which presents difficulties, as we know from the current colliderexperience; or LLPs could be included in the design strategy for futurecollider detectors, prioritizing for example displaced tracking capabilities,or timing, and budgeting for unexpected signals. Following the second pathwill result in a boost of the discovery potential for LLP searches; but couldalso have additional benefits providing innovative methods.Figures like Fig. 3, that presents the sensitivity of different detectorcomponents to HNL as a function of the mixing parameter and mass, fromRef. [9], could provide important input for the detector design of the future.Different colliders offer sensitivity to different parameters in different Epiphany˙RGS˙2021 printed on February 16, 2021
Fig. 3. Schematic illustration of the sensitivity of the different detector componentsto heavy neutrino decays as a function of the active-sterile mixing parameter andthe heavy neutrino mass. The parameter r D the outer radius of the muon system.From Ref. [9]. models, which means that complementarity should be exploited. Certainproposed colliders offer high collision energies, other clean environments,with different access to ranges of mass and couplings. There is also com-plementarity with non-collider and astrophysics in many models. However,many challenges related to LLP will be common. There is then a clear ben-efit in developing searches together and establish common benchmarking.
4. Future particle colliders
The LHC is currently undergoing its second, planned, long shut-down(LS2). In order to further increase its discovery potential after the nextrunning period of the LHC, the current accelerator chain would eventuallyundergo an upgrade. The High Luminosity LHC (HL-LHC) is an upgradeof the LHC to achieve instantaneous luminosities a factor of five larger thanthe LHC nominal value, thereby enabling the experiments to enlarge theirdata sample by one order of magnitude compared with the LHC baselineprogramme. Following five years of design study and R&D, this challengingproject requires now about ten years of developments, prototyping, testingand implementation; hence operation is expected to start in the middle ofthe next decade.The timeline of the project is dictated by the fact that, at the beginningof the next decade, many critical components of the accelerator will reachthe end of their lifetime due to radiation damage and will thus need tobe replaced. The upgrade phase is therefore crucial not only for the fullexploitation of the LHC physics potential, but also to enable operation piphany˙RGS˙2021 printed on February 16, 2021 of the collider beyond 2025 [11]. The current schedule of the project ispresented in Fig. 4.New functionalities of the HL-LHC could have positive effects in searchesfor LLPs. For example, the new track triggers [12] that the LHC experi-ments will be equipped with could allow for example to trigger on displacedmuons from the same vertex to find dark photons [13]. Better timing in-formation will be available, and that would enable e.g. searches that targetpair-produced LLPs significantly delayed [14].A very exciting development related to the HL-LHC is a variety of ded-icated LLP detectors proposed around it. This complementary instrumen-tation in the caverns would offer low background environments to detectLLPs. • FASER [15], already approved, is a 1 m on-axis experiment, located480 m downstream from the interaction point of the ATLAS experi-ment. • MATHUSLA [16], is a proposed large-scale surface detector instru-menting about 8 × m , off-axis, above ATLAS or CMS. • CODEX-b [17], is a proposed 10 m detector in the LHCb cavern,off-axis. • AL3X [18], is a proposed cylindrical 900 m detector inside the L3magnet and the time-projection chamber of the ALICE experiment. Epiphany˙RGS˙2021 printed on February 16, 2021 • ANUBIS [19], proposes 1 × m units on top of ATLAS or CMS,off-axis.At the moment of writing, the HL-LHC is the only approved futurecollider. However, there are many proposed future options beyond thatand there will be briefly presented, together with some selected preliminarysearch proposals for LLPs, in the next subsections. As stated in section 2.3, the European Strategy Update highlights theneed to pursue an electron positron collider acting as a Higgs factory asthe highest-priority facility after the LHC. For LLPs this means the clearphysics case discussed previously of exotic Higgs boson decays [20], but thatis only the starting point.There are two kinds of proposed electron-position colliders: Linear andCircular. In terms of linear electron-position colliders, two projects canbe highlighted: CLIC [21] at CERN, and the ILC [22] in Japan. Hiddenvalley searches in Higgs boson decays with displaced vertices [23] or De-generate Higgsino Dark Matter via chargino pair production searches withdisappearing tracks [24] are proposed to be performed at CLIC.In terms of circular electron-positron colliders, the options are the CEPC [25]in China, and the FCC-ee [26] at CERN, the lepton collider proposed withinthe Future Circular Collider.The FCC-ee offers exciting potential for the study of LLP, where searchescan be not only complementary to similar searches at collider and non-collider experiments, but highly competitive. More specifically, the FCC-eeoffers an unbeatable reach for HNL at the Z-Pole, making it the flagship ofLLP searches in this collider.Circular electron-positron colliders are expected to provide the best sen-sitivity to low neutrino mixing angles via displaced vertex searches [27]. Thelarge statistics FCC run around the Z pole, producing 5 10 Z (the Tera-Zregime), is expected to be particularly powerful in this area [28, 26, 29, 30].The Tera-Z run would allow for sensitivity down to a heavy-light mixingof 10 − , covering a large phase-space for heavy neutrino masses between5 (the B mass) and 80 GeV (the Z mass) with displaced vertex searches.It is important to note that this kind of searches are affected by very lit-tle background, since the displacement of the vertex can be e.g. of 1 m.Sufficiently long-lived HNL could also potentially allow for oscillations intoantineutrinos to be observed [5].Evidence for a Dark Sector at the FCC-ee could come in the form ofALPS. For small couplings and light ALPs, the ALP decay vertex can beconsiderably displaced from the production vertex. Very long-lived ALPs piphany˙RGS˙2021 printed on February 16, 2021 would leave the detector before decaying, leaving a trace of missing energy.As presented in Fig 5, and it has been shown in [6], a high-luminosity runat the Z pole would significantly increase the sensitivity to ALPs producedin e + e − → γa with subsequent decays a → γγ or a → l + l − . Fig. 5. The limit on Λ aBB , ALP coupling to hypercharge field, from future Z-factory. More details at the source, Ref. [31].
Following the plans for different additional LLP experiments at the HL-LHC presented in 4.1 it is possible to also envision similar concepts atother future colliders. This is the premise for HADES: A long lived particledetector concept for the FCC-ee or CEPC [32]. The civil engineering of theFCC-ee will have much bigger detector caverns than needed for a leptoncollider in order to use them further for a future hadron collider. Extrainstrumentation could then be installed in the cavern walls to search for newLLPs. Figure 6 shows the improvement of sensitivity that can be expectedin HNL searches at the FCC-ee or CEPC by HADES.Finding valid motivations for LLP at the FCC-ee/CEPC is certainly nota problem. As previously discussed it would be possible to explore TwinHiggs models with displaced exotic Higgs boson decays, Hidden Valley mod-els with neutral, long-lived particles that the Higgs boson can decay to [8].Other BSM searches that could be competitive concern dark glueballs [20];Neutral naturalness [33]; or Neutralinos [34] among others. Epiphany˙RGS˙2021 printed on February 16, 2021
Fig. 6. Comparison of the sensitivities (9 events) that can be achieved at the FCC-ee with 2 . × Z-bosons (red) or CEPC with 3 . × Z-bosons (blue). Moredetails at the source, Ref. [32].
Several hadron colliders and electron-hadron colliders have been pro-posed: the LHeC at CERN [35]; the HE-LHC also at CERN [36]; the SppCin China [37]; and the FCC-eh/hh at CERN [35, 38].Among the available studies of LLPs in future hadron and lepton-hadroncolliders, there are a few examples to highlight. One of them is the potentialfor searches for Dark Photons in electron-hadron colliders [39] that couldbe quite competitive. LLP signals arising from Higgsinos or exotic Higgsdecays can also have good potential at lepton-proton colliders [40]. Winoand higgsino dark matter searches with a disappearing track signatureshave been proposed for the FCC-hh [41]. Finally HNL, Sterile Neutrinos inparticular, can be explored at future hadron colliders as well [27], and thecomplementarity between e − e + , pp , and e − p in these flagship searches is anasset to take in consideration, as presented in Fig. 7.
5. Conclusions and summary
Searches for long-lived particles are a very attractive complement tomainstream new physics searches at colliders. However, this kind of searchesalso challenge conventional reconstruction and trigger methods.There are many interesting lines involving new, long-lived particles to piphany˙RGS˙2021 printed on February 16, 2021
M < m W is obtained from the displaced vertex searches at the Zpole run of the FCC-ee shown by the blue line. More details at the source, Ref. [27]. explore at future colliders, some of them very well-known, like Heavy NeutralLeptons, Hidden sectors that could be connected to Dark Matter, or exoticHiggs decays. While in general most proposed future colliders can accessmany of these models, with common challenges, different colliders accessdifferent areas of the phase space and offer different sensitivities. In order toanswer the big questions of the SM, synergies across experimental facilitiesshould be maximized beyond colliders.Now is the time to organize and properly benchmark the most interestingoptions and dive-in into dedicated detector design; not only to just beingable to reconstruct and identify long-lived particles at future colliders, but tomaximize the experimental coverage and fully exploit the available discoveryopportunities. REFERENCES [1] J. Alimena et al. , “Searching for long-lived particles beyond the StandardModel at the Large Hadron Collider,” J. Phys. G , vol. 47, no. 9, p. 090501,2020.[2] T. Asaka and M. Shaposhnikov, “The ν MSM, dark matter and baryon asym-metry of the universe,”
Phys. Lett. B , vol. 620, pp. 17–26, 2005.2
Epiphany˙RGS˙2021 printed on February 16, 2021 [3] M. Shaposhnikov and I. Tkachev, “The nuMSM, inflation, and dark matter,”
Phys. Lett. B , vol. 639, pp. 414–417, 2006.[4] P. D. Bolton, F. F. Deppisch, and P. S. Bhupal Dev, “Neutrinoless double betadecay versus other probes of heavy sterile neutrinos,”
JHEP , vol. 03, p. 170,2020.[5] S. Antusch, E. Cazzato, and O. Fischer, “Resolvable heavy neu-trino–antineutrino oscillations at colliders,”
Mod. Phys. Lett. A , vol. 34,no. 07n08, p. 1950061, 2019.[6] M. Bauer, M. Heiles, M. Neubert, and A. Thamm, “Axion-Like Particles atFuture Colliders,”
Eur. Phys. J. C , vol. 79, no. 1, p. 74, 2019.[7] “2020 Update of the European Strategy for Particle Physics (Brochure),” Tech.Rep. CERN-ESU-015, Geneva, 2020.[8] S. Alipour-Fard, N. Craig, M. Jiang, and S. Koren, “Long Live the HiggsFactory: Higgs Decays to Long-Lived Particles at Future Lepton Colliders,”
Chin. Phys. C , vol. 43, no. 5, p. 053101, 2019.[9] S. Antusch, E. Cazzato, and O. Fischer, “Displaced vertex searches for sterileneutrinos at future lepton colliders,”
JHEP , vol. 12, p. 007, 2016.[10] “The HL-LHC project website.”[11] O. Br¨uning and L. Rossi,
The High Luminosity Large Hadron Collider .WORLD SCIENTIFIC, 2015.[12] M. M˚artensson, M. Isacson, H. Hahne, R. Gonzalez Suarez, and R. Brenner,“To catch a long-lived particle: hit selection towards a regional hardware tracktrigger implementation,”
JINST , vol. 14, no. 11, p. P11009, 2019.[13] Y. Gershtein, “CMS Hardware Track Trigger: New Opportunities for Long-Lived Particle Searches at the HL-LHC,”
Phys. Rev. D , vol. 96, no. 3,p. 035027, 2017.[14] J. Liu, Z. Liu, and L.-T. Wang, “Enhancing Long-Lived Particles Searchesat the LHC with Precision Timing Information,”
Phys. Rev. Lett. , vol. 122,no. 13, p. 131801, 2019.[15] J. L. Feng, I. Galon, F. Kling, and S. Trojanowski, “ForwArd Search ExpeR-iment at the LHC,”
Phys. Rev. D , vol. 97, no. 3, p. 035001, 2018.[16] D. Curtin et al. , “Long-Lived Particles at the Energy Frontier: The MATH-USLA Physics Case,”
Rept. Prog. Phys. , vol. 82, no. 11, p. 116201, 2019.[17] G. Aielli et al. , “Expression of interest for the CODEX-b detector,”
Eur. Phys.J. C , vol. 80, no. 12, p. 1177, 2020.[18] V. V. Gligorov, S. Knapen, B. Nachman, M. Papucci, and D. J. Robinson,“Leveraging the ALICE/L3 cavern for long-lived particle searches,”
Phys. Rev.D , vol. 99, no. 1, p. 015023, 2019.[19] M. Bauer, O. Brandt, L. Lee, and C. Ohm, “ANUBIS: Proposal to search forlong-lived neutral particles in CERN service shafts,” 9 2019.[20] K. Cheung and Z. S. Wang, “Probing Long-lived Particles at Higgs Factories,”
Phys. Rev. D , vol. 101, no. 3, p. 035003, 2020. piphany˙RGS˙2021 printed on February 16, 2021 + e − Collider (CLIC): Accelerator and Detector,” 12 2018.[22] P. Bambade et al. , “The International Linear Collider: A Global Project,” 32019.[23] M. Kucharczyk and T. Wojton, “Hidden Valley searches at CLIC,” Jun 2018.[24] R. Franceschini et al. , “The CLIC Potential for New Physics,” vol. 3/2018, 122018.[25] “CEPC Conceptual Design Report: Volume 1 - Accelerator,” 9 2018.[26] A. Abada et al. , “FCC-ee: The Lepton Collider: Future Circular ColliderConceptual Design Report Volume 2,”
Eur. Phys. J. ST , vol. 228, no. 2,pp. 261–623, 2019.[27] S. Antusch, E. Cazzato, and O. Fischer, “Sterile neutrino searches at future e − e + , pp , and e − p colliders,” Int. J. Mod. Phys. A , vol. 32, no. 14, p. 1750078,2017.[28] A. Blondel, E. Graverini, N. Serra, and M. Shaposhnikov, “Search for HeavyRight Handed Neutrinos at the FCC-ee,”
Nucl. Part. Phys. Proc. , vol. 273-275,pp. 1883–1890, 2016.[29] J. Klari´c, M. Shaposhnikov, and I. Timiryasov, “Uniting low-scale leptogene-ses,” 8 2020.[30] A. Abada, G. Arcadi, V. Domcke, M. Drewes, J. Klaric, and M. Lucente,“Low-scale leptogenesis with three heavy neutrinos,”
JHEP , vol. 01, p. 164,2019.[31] J. Liu, L.-T. Wang, X.-P. Wang, and W. Xue, “Exposing the dark sector withfuture Z factories,”
Phys. Rev. D , vol. 97, no. 9, p. 095044, 2018.[32] M. Chrzaszcz, M. Drewes, and J. Hajer, “HADES: A long lived particle de-tector concept for the FCC-ee or CEPC,” 11 2020.[33] D. Curtin and C. B. Verhaaren, “Discovering Uncolored Naturalness in ExoticHiggs Decays,”
JHEP , vol. 12, p. 072, 2015.[34] Z. S. Wang and K. Wang, “Long-lived light neutralinos at future Z − factories,” Phys. Rev. D , vol. 101, no. 11, p. 115018, 2020.[35] M. Kuze, “Energy-Frontier Lepton-Hadron Collisions at CERN: the LHeC andthe FCC-eh,”
Int. J. Mod. Phys. Conf. Ser. , vol. 46, p. 1860081, 2018.[36] E. Todesco and F. Zimmermann, eds.,
Proceedings, EuCARD-AccNet-EuroLumi Workshop: The High-Energy Large Hadron Collider (HE-LHC10):Villa Bighi, Malta, Republic of Malta, October 14-16, 2010 , CERN YellowReports: Conference Proceedings, (Geneva), CERN, 2011.[37] J. Tang et al. , “Concept for a Future Super Proton-Proton Collider,” 7 2015.[38] A. Abada et al. , “FCC-hh: The Hadron Collider: Future Circular ColliderConceptual Design Report Volume 3,”
Eur. Phys. J. ST , vol. 228, no. 4,pp. 755–1107, 2019.[39] M. D’Onofrio, O. Fischer, and Z. S. Wang, “Searching for Dark Photons atthe LHeC and FCC-he,”
Phys. Rev. D , vol. 101, no. 1, p. 015020, 2020.4
Epiphany˙RGS˙2021 printed on February 16, 2021 [40] D. Curtin, K. Deshpande, O. Fischer, and J. Zurita, “New Physics Opportu-nities for Long-Lived Particles at Electron-Proton Colliders,”