Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in √s=13 TeV pp collisions with the ATLAS detector
Aaboud, Morad; Aad, Georges; Abbott, Brad; Abdinov, Ovsat Bahram oglu; Abeloos, Baptiste; Abidi, Syed Haider; AbouZeid, Hass; Abraham, Nadine L.; Abramowicz, Halina; Abreu, Henso; Buanes, Trygve; Dale, Ørjan; Eigen, Gerald; Fomin, Nikolai; Lipniacka, Anna; Martin dit Latour, Bertrand; Mæland, Steffen; Stugu, Bjarne; Yang, Zongchang; Zalieckas, Justas; Bugge, Magnar Kopangen; Cameron, David Gordon; Catmore, James Richard; Feigl, Simon; Franconi, Laura; Garonne, Vincent; Gramstad, Eirik; Hellesund, Simen; Morisbak, Vanja; Oppen, Henrik; Ould-Saada, Farid; Read, Alexander Lincoln; Røhne, Ole Myren; Sandaker, Heidi; Serfon, Cédric; Stapnes, Steinar; Vadla, Knut Oddvar Høie; Abulaiti, Yiming; Acharya, Bobby S.; Adachi, Shunsuke; Adamczyk, Leszek; Adelman, Jareed; Adersberger, Michael; Adye, Tim; Affolder, Anthony Allen; Afik, Yoav; Agheorghiesei, Catalin; Aguilar Saavedra, Juan Antonio; Ahmadov, Faig; Aielli, Giulio; ATLAS, Collaboration
Peer reviewed, Journal article
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Original versionAaboud M, Aad G, Abbott B, Abdinov OBo, Abeloos B, Abidi SH, AbouZeid H, Abraham NL, Abramowicz H, Abreu H, Buanes T, Dale Ø, Eigen G, Fomin N, Lipniacka A, Martin dit Latour B, Mæland S, Stugu B, Yang Z, Zalieckas J, Bugge MK, Cameron DG, Catmore JR, Feigl S, Franconi L, Garonne V, Gramstad E, Hellesund S, Morisbak V, Oppen H, Ould-Saada F, Read AL, Røhne OM, Sandaker H, Serfon C, Stapnes S, Vadla KOH, Abulaiti Y, Acharya BS, Adachi S, Adamczyk L, Adelman J, Adersberger M, Adye T, Affolder AA, Afik Y, Agheorghiesei C, Aguilar Saavedra JA, Ahmadov F, Aielli G, ATLAS C. Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in √s=13 TeV pp collisions with the ATLAS detector. European Physical Journal C. 2018;78:625(8):1-38 https://doi.org/10.1140/epjc/s10052-018-6081-9
Supersymmetry (SUSY) [1, 2, 3, 4, 5, 6] is an extension to the Standard Model (SM) that introduces partner particles (called sparticles), which differ by half a unit of spin from their SM counterparts. For models with R-parity conservation , strongly produced sparticles would be pair-produced and are expected to decay into quarks or gluons, sometimes leptons, and the lightest SUSY particle (LSP), which is stable. The LSP is assumed to be weakly interacting and thus is not detected, resulting in events with potentially large missing transverse momentum ( 𝑝𝑝missT , with magnitude 𝐸missT ). In such a scenario the LSP could be a dark-matter candidate [8, 9]. For SUSY models to present a solution to the SM hierarchy problem [10, 11, 12, 13], the partners of the gluons (gluinos, 𝑔̃ ), top quarks (top squarks, 𝑡̃ L and 𝑡̃ R ) and Higgs bosons (higgsinos, ℎ̃ ) should be close to the TeV scale. In this case, strongly interacting sparticles could be produced at a high enough rate to be detected by the experiments at the Large Hadron Collider (LHC). Final states containing same-flavour opposite-sign (SFOS) lepton pairs may arise from the cascade decays of squarks and gluinos via several mechanisms. Decays via intermediate neutralinos ( 𝜒̃ 0𝑖 ), which are the mass eigenstates formed from the linear superpositions of higgsinos and the superpartners of the electroweak gauge bosons, can result in SFOS lepton pairs being produced in the decay 𝜒̃ 02→ℓ+ℓ−𝜒̃ 01 . The index 𝑖=1,…,4 orders the neutralinos according to their mass from the lightest to the heaviest. In such a scenario the lightest neutralino, 𝜒̃ 01 , is the LSP. The nature of the 𝜒̃ 02 decay depends on the mass difference Δ𝑚𝜒≡𝑚𝜒̃ 02−𝑚𝜒̃ 01 , the composition of the charginos and neutralinos, and on whether there are additional sparticles with masses less than 𝑚𝜒̃ 02 that could be produced in the decay. In the case where Δ𝑚𝜒>𝑚𝑍 , SFOS lepton pairs may be produced in the decay 𝜒̃ 02→𝑍𝜒̃ 01→ℓ+ℓ−𝜒̃ 01 , resulting in a peak in the invariant mass distribution at 𝑚ℓℓ≈𝑚𝑍 . For Δ𝑚𝜒<𝑚𝑍 , the decay 𝜒̃ 02→𝑍∗𝜒̃ 01→ℓ+ℓ−𝜒̃ 01 leads to a rising 𝑚ℓℓ distribution with a kinematic endpoint (a so-called “edge”), the position of which is given by 𝑚maxℓℓ=Δ𝑚𝜒<𝑚𝑍 , below the Z boson mass peak. In addition, if there are sleptons ( ℓ̃ , the partner particles of the SM leptons) with masses less than 𝑚𝜒̃ 02 , the 𝜒̃ 02 could follow the decay 𝜒̃ 02→ℓ̃ ±ℓ∓→ℓ+ℓ−𝜒̃ 01 , also leading to a kinematic endpoint, but with a different position given by 𝑚maxℓℓ=(𝑚2𝜒̃ 02−𝑚2ℓ̃ )(𝑚2ℓ̃ −𝑚2𝜒̃ 01)/𝑚2ℓ̃ ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾√ . This may occur below, on, or above the Z boson mass peak, depending on the value of the relevant sparticle masses. In the two scenarios with a kinematic endpoint, if Δ𝑚𝜒 is small, production of leptons with low transverse momentum ( 𝑝T ) is expected, motivating a search to specifically target low- 𝑝T leptons. Section 3 and Fig. 1 provide details of the signal models considered. This paper reports on a search for SUSY, where either an on-Z mass peak or an edge occurs in the invariant mass distribution of SFOS ee and 𝜇𝜇 lepton pairs. The search is performed using 36.1 fb−1 of pp collision data at 𝑠√=13 TeV recorded during 2015 and 2016 by the ATLAS detector at the LHC. In order to cover compressed scenarios, i.e. where Δ𝑚𝜒 is small, a dedicated “low- 𝑝T lepton search” is performed in addition to the relatively “high- 𝑝T lepton searches” in this channel, which have been performed previously by the CMS  and ATLAS  collaborations. Compared to the 14.7 fb−1 ATLAS search , this analysis extends the reach in 𝑚𝑔̃ /𝑞̃ by several hundred GeV and improves the sensitivity of the search into the compressed region. Improvements are due to the optimisations for 𝑠√=13 TeV collisions and to the addition of the low- 𝑝T search, which lowers the lepton 𝑝T threshold from >25 to >7 GeV .