UC San Diego UC San Diego Previously Published Works Title Search for new physics in events with a leptonically decaying Z boson and a large transverse momentum imbalance in proton-proton collisions at s = 13 TeV. Permalink https://escholarship.org/uc/item/39g3v9qc Journal The European physical journal. C, Particles and fields, 78(4) ISSN 1434-6044 Authors Sirunyan, AM Tumasyan, A Adam, W et al. Publication Date 2018 DOI 10.1140/epjc/s10052-018-5740-1 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Eur. Phys. J. C (2018) 78:291 https://doi.org/10.1140/epjc/s10052-018-5740-1 Regular Article - Experimental Physics Search for new physics in events with a leptonically decaying Z boson and a large transverse√ momentum imbalance in proton–proton collisions at s =13TeV CMS Collaboration∗ CERN, 1211 Geneva 23, Switzerland Received: 1 November 2017 / Accepted: 16 March 2018 / Published online: 11 April 2018 © CERN for the benefit of the CMS collaboration 2018 Abstract A search for new physics in events with a Z cosmology has established that, in the total cosmic energy boson produced in association with large missing transverse budget, known matter only accounts for about 5%, DM cor- momentum at the LHC is presented. The search is based on responds to 27%, and the rest is dark energy [1]. Although the 2016 data sample of proton-proton√ collisions recorded several astrophysical observations indicate that DM exists with the CMS experiment at s = 13 TeV, correspond- and interacts gravitationally with known matter, there is no ing to an integrated luminosity of 35.9 fb−1. The results of evidence yet for nongravitational interactions between DM this search are interpreted in terms of a simplified model and SM particles. While the nature of DM remains a mys- of dark matter production via spin-0 or spin-1 mediators, a tery, there are a number of models that predict a particle scenario with a standard-model-like Higgs boson produced physics origin. If DM particles exist, they can possibly be in association with the Z boson and decaying invisibly, a produced directly from, annihilate into, or scatter off SM model of unparticle production, and a model with large extra particles. Recent DM searches have exploited various meth- spatial dimensions. No significant deviations from the back- ods including direct [2] and indirect [3] detection. If DM ground expectations are found, and limits are set on relevant can be observed in direct detection experiments, it must have model parameters, significantly extending the results previ- substantial couplings to quarks and/or gluons, and could also ously achieved in this channel. be produced at the LHC [4–9]. A promising possibility is that DM may take the form of weakly interacting massive particles. The study presented 1 Introduction here considers one possible mechanism for producing such particles at the LHC [10]. In this scenario, a Z boson, pro- In the pursuit of new physics at the CERN LHC, many sce- duced in proton-proton (pp) collisions, recoils against a pair narios have been proposed in which production of particles of DM particles, χχ. The Z boson subsequently decays into that leave no trace in collider detectors is accompanied also two charged leptons, producing a low-background dilepton miss by production of a standard model (SM) particle, which bal- signature, together with pT due to the undetected DM par- ances the transverse momentum in an event. The final state ticles. In this analysis, the DM particle χ is assumed to be considered in this analysis is the production of a pair of lep- a Dirac fermion. Four simplified models of DM production tons (+−, where = eorμ), consistent with originat- via an s-channel mediator exchange are considered. In these ing from a Z boson, together with large missing transverse models, the mediator has a spin of 1 (0) and vector or axial- miss momentum (pT ). This final state is well-suited to probe vector (scalar or pseudoscalar) couplings to quarks and DM such beyond the SM (BSM) scenarios, as it has relatively particles. The free parameters of each model are the masses small and precisely known SM backgrounds. mmed and mDM of the mediator and DM particle, respec- One of the most significant puzzles in modern physics is tively, as well as the coupling constant gq (gDM) between the the nature of dark matter (DM). In the culmination of over mediator and the quarks (DM particles). The vector coupling a century of observations, the “ΛCDM” standard model of model can be described with the following Lagrangian: μ μ Lvector = gDM Z μχγ χ + gq Z μqγ q, Electronic supplementary material The online version of this q article (https://doi.org/10.1140/epjc/s10052-018-5740-1) contains supplementary material, which is available to authorized users. where the spin-1 mediator is denoted as Z and the SM quark e-mail: [email protected] fields are referred to as q and q. The Lagrangian for an 123 291 Page 2 of 32 Eur. Phys. J. C (2018) 78 :291 Fig. 1 Feynman diagrams q χ χ illustrative of the processes gq Z gDM gq φ gDM beyond the SM considered in this paper: (upper left) DM production in a simplified model χ t χ with a spin-1 mediator Z; − − (upper right) DM production in Z Z a simplified model with a spin-0 mediator φ;(lower q + + left) production of a Higgs boson in association with Z boson with subsequent decay of the Higgs boson into invisible q particles; (lower q U G right) unparticle or graviton / production. The diagrams were Z drawn using the Z TikZ- Feynman package [11] − Z H q q + axial-vector coupling is obtained by making the replace- models [21–23] construct a generic connection between SM ment γ μ → γ 5γ μ. In the case of a spin-0 mediator φ, and DM particles via a Higgs boson mediator. This analysis the couplings between mediator and quarks are assumed considers decays into invisible particles of an SM-like Higgs to be Yukawa-like, with gq acting as a multiplicative√ mod- boson produced in association with a Z boson, as shown in ifier for the SM Yukawa coupling yq = 2mq/v (where Fig. 1 (lower left). v = 246 GeV is the SM Higgs field vacuum expectation Another popular BSM paradigm considered here is the value), leading to the Lagrangian: Arkani-Hamed–Dimopoulos–Dvali (ADD) model with large extra spatial dimensions [24–26], which is motivated by φ the hierarchy problem, i.e., the disparity between the elec- Lscalar = gDMφχχ + gq √ yqqq. 2 troweak unification scale (MEW ∼ 1 TeV) and the Planck q 16 scale (MPl ∼ 10 TeV). This model predicts graviton (G) → + The Lagrangian with pseudoscalar couplings is obtained by production via the process qq Z G. The graviton escapes inserting a factor of iγ 5 into each of the two terms (i.e., detection, leading to a mono-Z signature (Fig. 1, lower right). 5 5 In the ADD model, the apparent Planck scale in four space- χχ¯ → iχγ¯ χ and qq¯ → iq¯γ q). Example diagrams of + M2 ≈ Mn 2 Rn M DM production via spin-1 and spin-0 mediators are shown time dimensions is given by Pl D , where D is in Fig. 1 (upper left and right, respectively). the true Planck scale of the full n+4 dimensional space-time A primary focus of the LHC physics program after the and R is the compactification radius of the extra dimensions. discovery of a Higgs boson (H) [12–14] by the ATLAS and Assuming MD is of the same order as MEW, the observed CMS Collaborations is the study of the properties of this large value of MPl points to an R of order 1 mm to 1 fm for new particle. The observation of a sizable branching frac- 2 to 7 extra dimensions. The consequence of the large com- tion of the Higgs boson to invisible states [15–17] would pactification scale is that the mass spectrum of the Kaluza– be a strong sign of BSM physics. Supersymmetric (SUSY) Klein graviton states becomes nearly continuous, resulting models embodying R-parity conservation contain a stable in a broad Z boson transverse momentum (pT) spectrum. neutral lightest SUSY particle (LSP), e.g., the lightest neu- The final BSM model considered in this analysis is tralino [18], leading to the possibility of decays of the Higgs the phenomenologically interesting concept of unparticles, boson into pairs of LSPs. Certain models with extra spa- which appear in the low-energy limit of conformal field tial dimensions predict graviscalars that could mix with the theories. In the high-energy regime, a new, scale invariant Higgs boson [19]. As a consequence, the Higgs boson could Banks–Zaks field with a nontrivial infrared fixed point is oscillate to a graviscalar and disappear from the SM brane. introduced [27]. The interaction between the SM and Banks– The signature would be equivalent to an invisible decay of the Zaks sectors is mediated by particles of large mass scale Higgs boson. There could also be contributions from Higgs MU, below which the interaction is suppressed and can be boson decays into graviscalars [20]. With the same effect as treated via an effective field theory (EFT). The low-energy the simplified DM models presented earlier, “Higgs portal” regime will include unparticles, which have phase space fac- 123 Eur. Phys. J. C (2018) 78 :291 Page 3 of 32 291 tors equivalent to those of a noninteger number of ordinary couplings up to masses of 1.2 (1.25) TeV.