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Prospects of Searches for Long-Lived Charged Particles with Moedal

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Prospects of Searches for Long-Lived Charged Particles with Moedal Prospects of searches for long-lived charged particles with MoEDAL B.S. Acharyaa;b, A. De Roeckc;d, J. Ellisb;e;f , D.K. Ghoshg, R. Mase lekh, G. Panizzoa;i, J.L. Pinfoldj, K. Sakuraih, A. Shaah, A. Wallk aThe Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, Trieste, Italy bTheoretical Particle Physics and Cosmology Group, Department of Physics, King's College London, Strand, London WC2R 2LS, UK cExperimental Physics Department, CERN, CH1211 Geneva 23, Switzerland dAntwerp University, B2610 Wilrijk, Belgium eTheoretical Physics Department, CERN, CH1211 Geneva 23, Switzerland f National Institute of Chemical Physics & Biophysics, R¨avala 10, 10143 Tallinn, Estonia gSchool of Physical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Kolkata 700 032, India hInstitute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, PL{02{093 Warsaw, Poland iINFN Gruppo Collegato di Udine, Sezione di Trieste, Udine jPhysics Department, University of Alberta, Edmonton, Alberta T6G 2E4, Canada kUniversity of Alabama, Department of Physics, Tuscaloosa, Alabama, USA Abstract We study the prospects of searches for exotic long-lived particles with the MoEDAL detector at the LHC, assuming the integrated luminosity of 30 fb−1 that is expected at the end of Run 3. MoEDAL incorporates nuclear track detectors deployed a few metres away from the interaction point, which are sensitive to any highly-ionizing particles. Hence MoEDAL is able to detect singly- or doubly-charged particles with low velocities β < 0:15 or < 0:3, respectively, and lifetimes larger than (1) m=c. We examine the MoEDAL sensitivity to various singly- O charged supersymmetric particles with long lifetimes and to several types of doubly-charged long-lived particles with different spins and SU(2) charges. We compare the prospective MoEDAL mass reaches to current limits from arXiv:2004.11305v2 [hep-ph] 30 Apr 2020 ATLAS and CMS, which involve auxiliary analysis assumptions. MoEDAL searches for doubly-charged fermions are particularly competitive. KCL-PH-TH/2020-18, CERN-TH-2020-060 1. Introduction been the object of many other searches at the LHC [10, 11]. The minimal supersymmetric ex- One of the key priorities of the LHC experi- tension of the SM (the MSSM) does not con- mental programme is to search for new particles tain any doubly-charged particles, but many of beyond the Standard Model, or at least set robust its singly-charged particles are candidates to be constraints on their possible existence. Priorities long-lived, and several mechanisms suggest this during Runs 1 and 2 of the LHC included scans possibility [12{15]. The lightest supersymmetric for bumps in invariant-mass spectra, searches for particle (LSP) would be stable if R-parity is con- excesses of missing transverse energy in events served, but could be unstable with a long life- with various topologies, and precision tests of time if R-parity is weakly broken [16,17]. Even if Standard Model (SM) predictions. With the R-parity is conserved, the next-to-lightest super- striking exception of the discovery of the Higgs symmetric particle (NLSP) could be long-lived if boson, none of these searches has yet borne any it is very nearly degenerate with the LSP, as in fruit. coannihilation scenarios, or if the supersymmet- In parallel with these mainstream searches for ric particle(s) mediating its decay is (are) very new particles, there has been growing interest heavy, as in scenarios with split supersymmetry in less-orthodox searches. These have included [18, 19]. Another possibility is that the LSP has searches for long-lived particles (LLPs), which we only very weak interactions, e.g., if it is the grav- review in Section2. These long-lived particles itino or some other particle with gravitational- may arise in various scenarios beyond the SM that strength interactions, or if it is located in a well- satisfy any of the following conditions: (i) small sequestered hidden sector. In either case, the couplings, (ii) nearly degenerate masses or (iii) lightest supersymmetric particle in the visible sec- heavy intermediate (virtual) particles mediating tor could be long-lived [20{22]. decays [1{3]. We expect such searches to receive Prospects for discovering long-lived sparticles increased attention during LHC Run 3. with MoEDAL was first discussed in [8,9]. These During Runs 1 and 2 of the LHC the search studies considered a double long-lived (LL) cas- for magnetically-charged particles was the main 0 0 cade chain: pp g~g~,g ~ jj[~χ1]LL, [~χ1]LL objective of the MoEDAL experiment, which ! ! 0 ! τ[~τ]LL, assuming that theχ ~1 andτ ~ are both has published limits from searches for magnetic long-lived. In this paper we extend this discus- monopoles [4] and dyons [5]. The ATLAS Col- sion to consider direct pair production of gen- laboration has also published limits on magnetic eral meta-stable sparticles, including charged R- monopole production [6]. hadrons containing a gluino, squark or stop, as As we discuss in Section3 one of the key well as Winos, Higgsinos and sleptons, without features of the MoEDAL detector [7] is its ar- making any specific assumptions about the mod- ray of panels of nuclear track detectors (NTDs), els in which they appear [23]. which can register anomalously heavy ionization A second scenario that may lead to a long- as would be produced by singly-charged particles lived heavy fermion is a Type-III seesaw model with velocities β v=c < 0:15, or by doubly- [24{26], in which the SM is augmented with at charged particles with≡ β < 0:3. The MoEDAL least two SU(2)L-triplet fermion fields (Σ) with NTD panels are located at distances 2 m from ∼ Y = 0. The observed neutrino mass is given by the interaction point, and are therefore sensitive 2 2 mν Y v =mΣ, where Yν is the Dirac Yukawa to long-lived heavily-ionizing particles with life- ≈ ν coupling, v is the SM vev and MΣ is the mass times τ longer than (1) m/c [8,9]. We discuss O of the heavy triplet fermion. In this model, in Section4 the analysis framework that we use to radiative corrections generate a mass splitting study long-lived charged particles, which we ap- mΣ± mΣ0 > mπ± [27,28] so that the decay Σ± ply to a couple of well-motivated theoretic models 0 − ! Σ + π± is kinematically allowed, with a lifetime in Sections5 and6. 8 (10− ) s that leads to very soft charged pion One such model is supersymmetry, which has butO no long-lived charged-particle track. How- 2 3 ever, if there are additional contributions to the prospective sensitivities of MoEDAL for a wide ;0 Σ± masses that reduce the mass difference so selection of supersymmetric candidates with the that the two-body decay is not allowed, the dom- results of ATLAS and CMS, and Table2 shows a inant decay of the Σ± would be three-body with a similar comparison for a wide selection of doubly- 6 lifetime (10− ) s. The latter case would lead to charged candidates. a detectableO long-lived charged particle signature We note in passing that the parameter spaces similar to that of the Wino, as we discuss later in of both the Type-II and-III seesaw models, as the paper. well as L-R symmetric models, are constrained by A long-lived doubly-charged scalar particle various theoretical considerations and low-energy may appear in a number of models. One exam- experimental data [35{37, 45, 46, 54]. Since our ple is a Type-II seesaw model of neutrino masses, goal in this paper is to estimate the MoEDAL in which the SM is supplemented by a complex sensitivities on these exotic particles in a model- SU(2)L triplet of scalar fields with hypercharge independent way, we refrain from discussing fur- Y = 2 [29{37], and other scenarios for doubly- ther these indirect model-dependent limits. charged scalars are mentioned later. Another pos- The outline of our paper is as follows. In sibility for a doubly-charged particle is a spin-1/2 Section2 we review previous searches for long- particle, such as a doubly-charged Higgsino as lived charged particles at the LHC by ATLAS appears in supersymmetric L-R models [38{42]. and CMS. Then, in Section3 we review rele- One can also simply add doubly-charged scalars vant aspects of the MoEDAL detector, and in and fermions in various SU(2)L representations Section4 we describe the analysis framework we to the particle content of the SM, then write down use. The prospective sensitivities of MoEDAL for the SM gauge-invariant interaction terms involv- long-lived supersymmetric particles are obtained ing these new fields and study their phenomenol- in Section5 and those for doubly-charged parti- ogy [43, 44]. We consider here doubly-charged cles in Section6. Finally, Section7 summarises particles that may be scalars or fermions and ei- our conclusions. ther singlets or triplets of the SM SU(2)L gauge group, without making any specific assumptions 2. Previous LHC Searches about the models in which they appear. 1 Both ATLAS [49, 50] and CMS [51] have set There have been previous searches for long- limits on certain species of long-lived super- lived particles (LLPs) at the LHC by the AT- symmetric particles, ATLAS has also considered LAS [49, 50, 55, 56] and CMS [51, 53, 57, 58] Col- multi-charged particles [52], and CMS [51] has laborations. These searches are based on ob- also considered one example of a doubly-charged servables related to ionization loss, dE=dx, dis- fermion as well as multi-charged particles [53], as placed tracks, displaced vertices, delayed decays we review in Section2.
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