Top PDF Search for supersymmetry in pp collisions at root s=7 TeV in final states with missing transverse momentum and b-jets,Search for supersymmetry in pp collisions at root s=7 TeV in final states with missing transverse momentum and b-jets with the ATLAS detector
We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONI- CYT, Chile; CAS, MOST and NSFC, China; COLCIEN- CIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Founda- tion, Denmark; EPLANET and ERC, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Geor- gia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Rus- sia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Can- tons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Lever- hulme Trust, United Kingdom; DOE and NSF, United States of America.
The dominant SM background processes in the signal regions are the production of W or Z bosons in association with heavy-flavour jets (referred to as W +hf and Z+hf) and the production of top-quarks. Events with Z +hf production followed by Z → ν ν ¯ decay have the same signature as the signal and are the dominant background in SRA. Top- quark (dominant in SRB) and W +hf production satisfy the signal region selections when a charged lepton is produced but the event is not rejected, either because the lepton is a hadronically decaying τ , or because the electron or muon is not reconstructed. The domi- nant backgrounds are normalised in dedicated control regions (CRs) and then extrapolated to the signal regions using MC simulation. The control regions, detailed below, are defined by explicitly requiring the presence of one or two leptons (electrons or muons) in the final state together with further selection criteria similar to those of the corresponding signal regions. In particular, events with additional lepton candidates are vetoed applying the same lepton requirements used to veto events in the signal regions. The single top-quark contribution accounts for 5% to 20% of the total top-quark background contribution, de- pending on the signal region considered, and is added to the t ¯ t background contribution with a relative normalisation corresponding to that predicted by the MC simulation, as described in section 3.
identiﬁcation in the region | η | < 2.0. Before the start of Run 2, the new innermost pixel layer, the insertable B-layer (IBL) , was inserted at a mean sensor radius of 3.3 cm. The ID is surrounded by a thin superconducting solenoid providing an axial 2 T magnetic ﬁeld and by a ﬁne-granularity lead/liquid-argon (LAr) electromagnetic calorimeter covering | η | < 3.2. A steel/scintillator-tile calorimeter provides coverage for hadronic showers in the central pseudorapidity range ( | η | < 1.7). The endcaps (1.5 < | η | < 3.2) of the hadronic calorimeter are made of LAr active layers with either copper or tungsten as the absorber material. The forward region (3.1 < | η | < 4.9) is instrumented with a LAr calorimeter for both the EM and hadronic measurements. A muon spectrometer with an air-core toroidal magnet system surrounds the calorimeters. Three layers of high-precision tracking chambers provide coverage in the range | η | < 2.7, while dedicated fast chambers allow triggering in the region | η | < 2.4. The ATLAS trigger system  consists of a hardware- based level-1 trigger followed by a software-based high-level trigger (HLT).
SM background samples were simulated using different MC event generator programs depending on the process. Events containing W or Z bosons with associated jets, and diboson events (W W, W Z and ZZ) were simulated using the Sherpa 2.2.1  event generator. The ME calculation is performed using Comix  and OpenLoops  for up to two partons at NLO and four partons at LO in W /Z + jets events and up to one parton at NLO and three partons at LO in diboson events, and merged with the Sherpa parton shower  according to the ME+PS@NLO prescription . The NNPDF3.0NNLO PDF set  was used in conjunction with dedicated parton shower tuning developed by the Sherpa authors. The W /Z + jets events were normalised with NNLO QCD cross- sections . For the generation of t ¯ t events, Powheg-Box v2  was used with the CT10 PDF  set in the matrix element calculations. Electroweak t-channel, s-channel and W t-channel single-top-quark events were generated with Powheg-Box v1. For s- channel and W t-channel production, the CT10 PDF set was used, while for the t-channel production the four-flavour scheme was used for the NLO matrix element calculations together with the fixed four-flavour PDF set CT10f4. For t ¯ t and single-top-quark processes, top-quark spin correlations were preserved. The parton shower, hadronisation, and the underlying event were simulated using Pythia 6.428  with the CTEQ6L1 PDF set and the corresponding Perugia 2012 set of tuned parameters . The top-quark mass was set to 172.5 GeV. At least one leptonically decaying W boson was required in all generated t ¯ t and single-top-quark events. Fully hadronic decays do not produce sufficient E T miss to significantly contribute to the background.
three hardest jets is b tagged. The QCD multijet control region is defined by demanding low missingtransversemomentum E miss T < 40 GeV and low transverse mass m T < 40 GeV . This QCD control region is used only to estimate the QCD multijet background contribution to other background regions but not to the signal region. Instead, the electron and muon identification criteria are relaxed, obtaining a ‘‘loose’’ control sample that is domi- nated by QCD jets. A loose-tight matrix method, in close analogy to that described in Ref. , is then used to estimate the number of QCD multijet events with fake leptons in the signal region after final selection criteria: 0 : 0 þ 0:5
as the zero-lepton channel (ﬁgure 1a). For mixed decays models (intended as models where both direct decays and decays through an intermediate stage are kinematically allowed), the ﬁnal state of bottom or top squark pair production depends on the branching ratios of the competing decay modes. If the decay modes are equally probable, a large fraction of the signal events are characterized by the presence of a top quark, a bottom quark, and neutralinos. Hadronic decays of the top quark are targeted by the zero-lepton channel, whilst novel dedicated selections requiring one charged lepton, two b-jets and E T miss are developed for semi-leptonic decays of the top quark, referred to as the one-lepton channel (ﬁgure 1b). A statistical combination of the two channels is performed when interpreting the results in terms of exclusion limits on the third-generation squark masses.
Three signal regions are then deﬁned: two “Z -depleted” re- gions (SR1a and SR1b), with no SFOS pairs having invariant mass within 10 GeV of the nominal Z-boson mass; and a “ Z -enriched” one (SR2), where at least one SFOS pair has an invariant mass within 10 GeV of the Z-boson mass. Events in SR1a and SR1b are further required to contain no b-tagged jets to suppress con- tributions from b-jet-rich background processes, where a lepton could originate from the decay of a heavy-ﬂavor quark. SR1b is designed to increase sensitivity to scenarios characterised by large mass splittings between the heavy gauginos and the LSP by requir- ing all three leptons to have p T > 30 GeV. In both SR1b and SR2, the transverse mass variable m T must take values greater than 90 GeV, where m T is constructed using the E miss T and the lepton not included in the lepton pair with invariant mass closest to the nom- inal Z -boson mass. The m T requirement is introduced to suppress background from W Z events. The SR1a/b regions target neutralino decays via intermediate sleptons or via off-shell Z bosons while SR2 targets decays via an on-shell Z boson. Table 1 summarises the selection requirements for the three signal regions.
K. Nagai 160 , K. Nagano 66 , Y. Nagasaka 60 , A.M. Nairz 29 , Y. Nakahama 29 , K. Nakamura 155 , I. Nakano 110 , G. Nanava 20 , A. Napier 161 , M. Nash 77 , c , N.R. Nation 21 , T. Nattermann 20 , T. Naumann 41 , G. Navarro 162 , H.A. Neal 87 , E. Nebot 80 , P.Yu. Nechaeva 94 , A. Negri 119a , 119b , G. Negri 29 , S. Nektarijevic 49 , A. Nelson 64 , S. Nelson 143 , T.K. Nelson 143 , S. Nemecek 125 , P. Nemethy 108 , A.A. Nepomuceno 23a , M. Nessi 29 , u , S.Y. Nesterov 121 , M.S. Neubauer 165 , A. Neusiedl 81 , R.M. Neves 108 , P. Nevski 24 , P.R. Newman 17 , R.B. Nickerson 118 , R. Nicolaidou 136 , L. Nicolas 139 , B. Nicquevert 29 , F. Niedercorn 115 , J. Nielsen 137 , T. Niinikoski 29 , A. Nikiforov 15 , V. Nikolaenko 128 , K. Nikolaev 65 , I. Nikolic-Audit 78 , K. Nikolics 49 , K. Nikolopoulos 24 , H. Nilsen 48 , P. Nilsson 7 , Y. Ninomiya 155 , A. Nisati 132a , T. Nishiyama 67 , R. Nisius 99 , L. Nodulman 5 , M. Nomachi 116 , I. Nomidis 154 , M. Nordberg 29 , B. Nordkvist 146a , 146b , P.R. Norton 129 , J. Novakova 126 , M. Nozaki 66 , M. Nožiˇcka 41 , L. Nozka 113 , I.M. Nugent 159a , A.-E. Nuncio-Quiroz 20 , G. Nunes Hanninger 86 , T. Nunnemann 98 , E. Nurse 77 , T. Nyman 29 , B.J. O’Brien 45 , S.W. O’Neale 17 ,∗ , D.C. O’Neil 142 , V. O’Shea 53 , F.G. Oakham 28 , e , H. Oberlack 99 , J. Ocariz 78 , A. Ochi 67 , S. Oda 155 ,
For the Gbb models, gluinos with masses below 1.78 TeV are excluded at 95% C.L. for LSP masses below 800 GeV. At high gluino masses, the exclusion limits are driven by the SR-Gbb-A and SR-Gbb-B signal regions. The best exclusion limit on the LSP mass is approximately 1.0 TeV, which is reached for a gluino mass of approximately 1.6 TeV. The exclusion limit is dominated by SR-Gbb-C for high LSP masses. For the Gtt models, gluino masses up to 1.8 TeV are excluded for massless LSP. For LSP masses below 700 GeV, gluino masses below 1.76 TeV are excluded. For large gluino masses, the exclusion limits are driven by the combination of SR-Gtt-1L-B and SR-Gtt-0L-A. The LSP exclusion extends up to approximately 975 GeV, corresponding to a gluino mass of approx- imately 1.5 TeV – 1.6 TeV. The best exclusion limits are obtained by the combination of SR-Gtt-1L-B and SR- Gtt-0L-C for high LSP masses. The ATLAS exclusion limits obtained with the full p ﬃﬃﬃ s ¼ 8 TeV data set are also shown in Fig. 7. The current results largely improve on the p ﬃﬃﬃ s ¼ 8 TeV limits despite the lower integrated luminosity. The exclusion limit on the gluino mass is extended by approximately 500 GeV and 400 GeV for the Gbb and Gtt models for massless LSP, respectively. This improvement is primarily attrib- utable to the increased center-of-mass energy of the LHC. The addition of the IBL pixel layer in Run 2, which improves the capability to tag bjets , also particularly benefits this analysis that employs a data set requiring at least three b-tagged jets. The sensitivity of the data analysis is also improved with respect to the ﬃﬃﬃ
A number of theories beyond the Standard Model (SM) of particle physics address the nat- uralness problem  and offer mechanisms through which the quadratic divergences, which arise from the radiative corrections to the Higgs boson mass, are resolved. A straight- forward extension of the SM is the inclusion of a heavy fourth generation. However, fourth-generation quarks with SM-like chiral couplings are excluded as they contribute through loops to the couplings of the Higgs boson, altering the Higgs boson production cross-sections to values incompatible with observation [2, 3]. These constraints on chiral quarks can be evaded by vector-like quarks (VLQs) [4, 5], hypothetical spin-1/2 coloured particles whose left-handed and right-handed states have the same electroweak coupling. Vector-like quarks could dampen the unnaturally large quadratic corrections to the Higgs boson mass by contributing significantly to loop corrections. They appear mainly in the “Little Higgs” [6, 7] and “Composite Higgs”  classes of models.
increasing masses. Even though the gluinos and squarks are produced strongly in pp colli- sions, if the masses of the gluinos and squarks are large, the direct production of charginos, neutralinos and sleptons through electroweak interactions may dominate the production of SUSY particles at the Large Hadron Collider (LHC). Such a scenario is possible in the general framework of the phenomenological minimal supersymmetric SM (pMSSM) [19– 21]. Naturalness suggests that third-generation sparticles and some of the charginos and neutralinos should have masses of a few hundred GeV [22, 23]. Light sleptons are expected in gauge-mediated [24–29] and anomaly-mediated [30, 31] SUSY breaking scenarios. Light sleptons could also play a role in the co-annihilation of neutralinos, allowing a dark matter relic density consistent with cosmological observations [32, 33].
The results of a search for the top squark, the supersymmetric partner of the top quark, in finalstates with one isolated electron or muon, jets, and missingtransversemomentum are reported. The search uses the 2015 LHC pp collision data at a center-of-mass energy of p ﬃﬃﬃ s ¼ 13 TeV recorded by the ATLASdetector and corresponding to an integrated luminosity of 3 . 2 fb −1 . The analysis targets two types of signal models: gluino-mediated pair production of top squarks with a nearly mass-degenerate top squark and neutralino and direct pair production of top squarks, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missingtransversemomentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and top squark masses are set at 95% confidence level. The results extend the LHC run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low top squark mass region and add an excluded top squark mass region from 745 to 780 GeV for the direct top squark model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vectorlike top quarks.
In summary, a search for stop pair production is pre- sented in finalstates with one isolated lepton, jets, and missingtransversemomentum in p ﬃﬃﬃ s ¼ 7TeVpp colli- sions corresponding to 4 : 7 fb 1 of ATLAS 2011 data. Each stop is assumed to decay to a top quark and a long-lived undetected neutral particle. No significant excess of events above the rate predicted by the standard model is observed and 95% CL s upper limits are set on the stop mass in the stop-LSP mass plane, significantly extending previous stop-mass limits.
antisquark pairs, each decaying via an intermediate chargino to two jets, a W boson and a neutralino LSP. The chargino mass is fixed halfway in between the squark and LSP masses in figure (a). In figure (b) the neutralino mass is fixed at 60 GeV; the y axis shows the ratio of the chargino-LSP mass splitting to the squark-LSP mass splitting. The black dashed lines show the expected limits, with the light (yellow) bands indicating the 1 excursions due to experimental uncertainties. Observed limits are indicated by medium (maroon) curves, where the solid contour represents the nominal limit, and the dotted lines are obtained by varying the cross section by the theoretical scale and PDF uncertainties. The 95% CL s upper limit on the cross section times branching ratio (in fb) is printed for
A. Kastanas 149 , Y. Kataoka 157 , C. Kato 157 , A. Katre 52 , J. Katzy 45 , K. Kawade 70 , K. Kawagoe 73 , T. Kawamoto 157 , G. Kawamura 57 , E. F. Kay 77 , V. F. Kazanin 111,c , R. Keeler 172 , R. Kehoe 43 , J. S. Keller 31 , J. J. Kempster 80 , J Kendrick 19 , H. Keoshkerian 161 , O. Kepka 129 , B. P. Kerševan 78 , S. Kersten 178 , R. A. Keyes 90 , M. Khader 169 , F. Khalil-zada 12 , A. Khanov 116 , A. G. Kharlamov 111,c , T. Kharlamova 111,c , A. Khodinov 160 , T. J. Khoo 52 , V. Khovanskiy 99,* , E. Khramov 68 , J. Khubua 54b,ab , S. Kido 70 , C. R. Kilby 80 , H. Y. Kim 8 , S. H. Kim 164 , Y. K. Kim 33 , N. Kimura 156 , O. M. Kind 17 , B. T. King 77 , D. Kirchmeier 47 , J. Kirk 133 , A. E. Kiryunin 103 , T. Kishimoto 157 , D. Kisielewska 41a , K. Kiuchi 164 , O. Kivernyk 5 , E. Kladiva 146b , T. Klapdor-Kleingrothaus 51 , M. H. Klein 38 , M. Klein 77 , U. Klein 77 , K. Kleinknecht 86 , P. Klimek 110 , A. Klimentov 27 , R. Klingenberg 46 , T. Klingl 23 , T. Klioutchnikova 32 , E.-E. Kluge 60a , P. Kluit 109 , S. Kluth 103 , E. Kneringer 65 , E. B. F. G. Knoops 88 , A. Knue 103 , A. Kobayashi 157 , D. Kobayashi 159 , T. Kobayashi 157 , M. Kobel 47 , M. Kocian 145 , P. Kodys 131 , T. Koffas 31 , E. Koffeman 109 , N. M. Köhler 103 , T. Koi 145 , M. Kolb 60b , I. Koletsou 5 , A. A. Komar 98,* , Y. Komori 157 , T. Kondo 69 , N. Kondrashova 36c , K. Köneke 51 , A. C. König 108 , T. Kono 69,ac , R. Konoplich 112,ad , N. Konstantinidis 81 , R. Kopeliansky 64 , S. Koperny 41a , A. K. Kopp 51 , K. Korcyl 42 , K. Kordas 156 , A. Korn 81 , A. A. Korol 111,c , I. Korolkov 13 , E. V. Korolkova 141 , O. Kortner 103 , S. Kortner 103 , T. Kosek 131 , V. V. Kostyukhin 23 , A. Kotwal 48 , A. Koulouris 10 , A. Kourkoumeli-Charalampidi 123a,123b , C. Kourkoumelis 9 , E. Kourlitis 141 , V. Kouskoura 27 , A. B. Kowalewska 42 , R. Kowalewski 172 , T. Z. Kowalski 41a , C. Kozakai 157 , W. Kozanecki 138 , A. S. Kozhin 132 , V. A. Kramarenko 101 , G. Kramberger 78 , D. Krasnopevtsev 100 , M. W. Krasny 83 , A. Krasznahorkay 32 , D. Krauss 103 , J. A. Kremer 41a , J. Kretzschmar 77 , K. Kreutzfeldt 55 , P. Krieger 161 , K. Krizka 33 , K. Kroeninger 46 , H. Kroha 103 , J. Kroll 129 , J. Kroll 124 , J. Kroseberg 23 , J. Krstic 14 , U. Kruchonak 68 , H. Krüger 23 , N. Krumnack 67 , M. C. Kruse 48 , T. Kubota 91 , H. Kucuk 81 , S. Kuday 4b , J. T. Kuechler 178 , S. Kuehn 32 , A. Kugel 60c , F. Kuger 177 , T. Kuhl 45 , V. Kukhtin 68 , R. Kukla 88 , Y. Kulchitsky 95 , S. Kuleshov 34b , Y. P. Kulinich 169 , M. Kuna 134a,134b , T. Kunigo 71 , A. Kupco 129 , T. Kupfer 46 , O. Kuprash 155 , H. Kurashige 70 , L. L. Kurchaninov 163a , Y. A. Kurochkin 95 , M. G. Kurth 35a , V. Kus 129 , E. S. Kuwertz 172 , M. Kuze 159 , J. Kvita 117 , T. Kwan 172 , D. Kyriazopoulos 141 , A. La Rosa 103 , J. L. La Rosa Navarro 26d , L. La Rotonda 40a,40b ,
In the GGM models considered in this Letter, the neutralino has higgsino or neutral wino (supersymmetric partners of the Higgs and neutral W bosons) components instead of being predomi- nantly bino-like, and therefore, in addition to its conventional de- cay to a gravitino and a photon, it may decay to a gravitino and a Higgs boson or to a gravitino and a Z boson. This GGM signature could be identiﬁed as an excess of events with pairs of neutralinos decaying to these bosons, in all combinations, associated with high E miss T . In particular, for a light Higgs boson (m h < 130 GeV), which decays predominantly to bb, one ﬁnal-state signature is the ¯ combination of an isolated high transversemomentum ( p T ) pho- ton, jets originating from bottom quarks, and high E miss T . Such a signature arises when one neutralino decays to a gravitino and a photon and the other to a gravitino and a Higgs boson. This decay mode is therefore signiﬁcant when both branching fractions are large, namely when the bino mass term M 1 approximately equals the higgsino mass parameter − μ .
Slovakia; ARRS and MIZŠ, Slovenia; DST /NRF, South Africa; MINECO, Spain; SRC and Wallen-
berg Foundation, Sweden; SERI, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, United Kingdom; DOE and NSF, United States of America. In addition, indi- vidual groups and members have received support from BCKDF, the Canada Council, CANARIE, CRC, Compute Canada, FQRNT, and the Ontario Innovation Trust, Canada; EPLANET, ERC, ERDF, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union; Investissements d’Avenir Labex and Idex, ANR, Région Auvergne and Fondation Partager le Savoir, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Greek NSRF; BSF, GIF and Minerva, Israel; BRF, Norway; CERCA Programme Generalitat de Catalunya, Generalitat Valenciana, Spain; the Royal Society and Leverhulme Trust, United Kingdom.
consists of a silicon pixel detector including the newly installed insertable B-layer [5,6], followed by silicon microstrip, and transi- tion radiation tracking detectors. Lead/liquid-argon (LAr) sampling calorimeters provide EM energy measurements with high granular- ity. A hadronic (steel/scintillator-tile) calorimeter covers the cen- tral pseudorapidity range ( | η | < 1 . 7). The endcap and forward re- gions are instrumented with LAr calorimeters for both the EM and hadronic energy measurements up to | η | = 4 . 9. The muon spec- trometer surrounds the calorimeters and is based on three large air-core toroid superconducting magnets with eight coils each. The ﬁeld integral of the toroids ranges between 2.0 and 6.0 Tm for most of the detector. It includes a system of precision tracking chambers, over | η | < 2 . 7, and fast detectors for triggering, over | η | < 2 . 4. A two-level trigger system is used to select events. The ﬁrst-level trigger is implemented in hardware and uses a subset of the detector information. This is followed by a software-based trigger system that reduces the accepted event rate to about 1 kHz.
0.4 GeV and to have a longitudinal distance less than 200 mm from the centre of the collision region. There are on average 20.7 interactions per event in the data used for this analysis. The primary vertex is defined to be the one with the highest summed track p 2 T . Spurious tails in the E T miss distribution, arising from calorimeter noise and other detector problems are suppressed by checking the quality of each reconstructed jet and discarding events containing reconstructed jets of poor quality, following the description given in ref. . In addition, the ID track associated with the electron or muon is required to be compatible with originating from the primary vertex by requiring that the transverse distance of closest approach, d 0 , satisfies | d 0 | < 1 (0.2) mm and longitudinal distance, z 0 ,
Astrophysical measurements indicate the existence of nonbaryonic dark matter [1,2]. However, collider-based searches, nuclear scattering experiments, and searches for particles produced from dark-matter annihilation have not yet revealed its particle nature nor discovered its non- gravitational interactions, if they exist . Collider-based searches for weakly interacting massive particles (WIMPs, denoted as χ ), specifically pp → χ χ ¯ þ X at the Large Hadron Collider (LHC) via some unknown intermediate state, are an important facet of the experimental program and provide sensitivity over a broad range of values of the WIMP mass m χ , including for low masses where direct detection experiments are less sensitive. The presence of dark-matter particles, not directly observable in a collider detector, can be inferred from their recoil against Standard Model (SM) particles. The LHC collaborations have reported limits on the cross section for the process that includes initial state radiation (ISR), pp → χ χ ¯ þ X, where the ISR component X is a hadronic jet [4,5], a photon [6,7], or a W or Z boson decaying hadronically . Limits on dark matter produced in the decay of the Higgs boson have also been reported . In this analysis, limits are set using the final state of a Z boson decaying to two oppositely charged electrons or muons, plus missingtransversemomentum E miss