Common Reference Topologies for Early LHC Searches by ATLAS and CMS

Common Reference Topologies for Early LHC Searches by ATLAS and CMS New Physics Characterization Working Group December 13, 2010 Abstract The purpose of this document is to specify a proposal for common reference topologies to be used by the ATLAS and CMS Collaborations in presenting exclusion limits from new-physics searches in the first 50 100 pb 1 of data. This document focuses on searches for supersymmetry and theories with similar phenomenology. 1 Introduction With more than 40 pb 1 of data, both ATLAS and CMS are sensitive to the production of new strongly interacting particles with mass scales beyond the reach of any existing experiment. Now and in the future, a key issue is to quantify the sensitivity of LHC searches for new physics in as clear a manner as possible so that the physics community can identify what reactions have been probed well and which ones require new techniques or different optimization strategies. Over the past several years, a growing consensus has formed that a particularly fruitful way to evaluate searches and characterize positive signals of new physics is by using a topological description for new physics hypotheses, i.e. by working in simple models parametrized by a few physical masses, cross-sections and branching ratios for a small number of specific production/decay topologies. A series of joint experimenttheory workshops at CERN have focused on how to characterize new physics [1, 2, 3, 4], and further efforts by over 100 theorists at a SLAC workshop have investigated the question of finding a set of representative topologies that if used will help ensure that searches explore all relevant phase space [5]. Finally, the question of facilitating more effective communication of results from the LHC to the rest of the physics community has been discussed in detail at all of the above. In essence, the proposal that has emerged is to supplement the standard msugra benchmarks with reference topologies [6]. The reference topologies can be simulated either as modules from widely used model frameworks (like the MSSM) in Pythia or MadGraph, as new models in MadGraph, or as OSETs using Marmoset [7, 8] or recent versions of Pythia [9]. Search results as applied to these specific reference topologies can be used to evaluate search sensitivity over the full range of kinematics that the 1

topology represents. Constraints on a wide variety of models (that contain those or similar topologies) can then be deduced. See [11, 12, 13, 14, 15] for specific information about this application. 1.1 Proposal from the Characterization of New Physics at the LHC II Workshop To begin, a set of reference topologies that both ATLAS, CMS, and theorists have already been considering in the context of topology based searches is proposed. These reference topologies are all initiated by strong production (i.e. are relevant for early searches), and are common to a wide variety of broadly discussed theories (e.g. supersymmetry, extra dimensions, etc.). This note will focus on the reference topologies that involve missing transverse energy as a signature. For that reason, the topologies are all considered to belong to the category of SUSY or SUSY-like new physics scenarios. This initial reference set does not include everything that may be important. For further reference, a fairly comprehensive set of reference topologies, encapsulated by simplified models, have been assembled by the LHC New Physics Working Group [10, 16] at http://lhcnewphysics.org/. Anyone can register to have access to materials (simplified model definitions, Monte Carlo material, supporting contacts, etc.). We emphasize that in this note, the term topology refers to a particular reaction for particle production and decay, and thus is more specific than a collection of reconstructed objects in the final state. 2 SUSY and SUSY-Like New Physics In this section we select a few basic topologies, which characterize SUSY-Like New Physics with missing transverese energy and significant jet activity. Some of these topologies give rise to purely hadronic final states, while others can produce one or more isolated leptons. An initial reference set of topologies involving final-state photons are also outlined. 2.1 All-Hadronic Jets+MET For hadronic SUSY, we propose to focus on a few classes of production topology, with and without cascade decays: 1. Squark anti-squark pair production with decay q q + χ 2. Gluino pair production with decay g q q + χ 3. Gluino pair production with decay through on-shell squark (lower priority), g q + ( q q + χ). In each case, the product χ can be either the LSP (treated as missing transverse energy) or an intermediate state χ 2 which decays as χ 2 W had + LSP. We propose to use hadronic W s (excluding τ s) here as the resulting limits/fits can be readily translated to apply to Z s in vast majority of parameter space. 2

It should be noted that limits on the process with cascades χ 2 W had + LSP on both sides are very useful, but if the sensitivity to these is low then limits presented on the topologies with one χ 2 cascade and one direct decay may be more instructive. Figure 1 shows diagrams of the topologies of interest for hadronic and 1-lepton searches. Note that hadronic and 1-lepton searches complement each other in different regions of phase space. Figure 1: Diagrams of the topologies of interest for hadronic and 1-lepton topologies (note that only glue initial states are shown but all should be included, and that these do not include the third, optional mode of gluino decay through an on-shell squark). The diagrams encircled in blue are a minimal set for the hadronic searches, though the other two diagrams could also be added; the diagrams encircled in red are the proposed set for the 1-lepton search [15]. 2.1.1 Parameter Space There are several ways to study the parameter spaces of these processes. Excluding the case of gluino decays through on-shell squarks 1, the kinematics is specified by 1. m g or m q 2. m LSP 3. (for cascade decays on one or both sides) m χ2 For direct decays, a natural two-dimensional plot can be presented in the m g (or m q ) vs. m LSP plane, with shading indicating (σ BR) limit /σ ref and an exclusion line where (σ BR) limit = σ ref to indicate the scale of new physics that the study can probe. There reference cross sections, σ ref, for gluino and squark production are given in Figure 6 in the appendix. See [11] for further information about the use of 2D plots to illustrate the key kinematics for these reference topologies. 1 For this case, representative slices such as m q = 1 4 m χ + 3 4 m g and m q = 3 4 m χ + 1 4 m g may be considered, but the choice is somewhat arbitrary. 3

In the case of cascades, there is an additional mass m χ2 that affects the kinematics and search acceptance. The slice m χ2 = 1 2 (m LSP + m g ), i.e. the mean of the other two masses, is particularly useful because it s near the minimum of sensitivity. Shading and an exclusion line can be defined for this case, as described above. Additional interesting slices would be m χ2 = xm LSP + (1 x)m g ) for x = 1/4 and 3/4 m χ2 = m LSP + 82 GeV (forcing a very soft on-shell W had ). These extra slices can be omitted in the interest of reducing time spent on signal event generation; if generation time is not a constraint (but a small number of plots is desired), then the exclusion lines for each of these slices (without shading) can be overlayed on a single figure to give some indication of how different assumptions of affect the search sensitivity. m χ2 2.2 Jets+MET+1 lepton These models are quite similar to those discussed above, with a few key differences: W had should be replaced with a leptonically (e, µ, or τ) decaying W. Clearly, the decay modes with two direct decays to the LSP are not interesting for this search; only the cascade+direct and cascade+cascade topologies are of interest. With these changes, the parametrizations presented above can be carried over to this case. It is, however, important to emphasize that the ansatz m χ2 = 1 2 (m LSP + m g is no longer in any respect a worst-case scenario. Rather, as m χ2 is varied, the kinematics of leptons, jets, and the missing transverse energy will all depend on this mass scale, and the dependence of search sensitivity to each of these depends on the details of the search. As such, the m g vs. m LSP mass plane is still appropriate, furthermore it is especially important to vary m χ2. For example, by considering slices m χ2 = xm LSP +(1 x)m g ) with different choices of x. In addition, some exploration is warranted to see at what splitting m χ2 m B the efficiency for leptons above threshold drops dramatically (presumably near m χ2 m B 2p T,min (l)). 2.3 Jets+MET with Heavy Flavors There are two classes of reference topologies for heavy-flavor production: Gluino pair production with decays g b b + χ, g t b + χ, or g t t + χ. top/bottom partner t or b pair production with decay t t+ χ or b b+ χ In each case, the product χ can be either the LSP (treated as missing transverse energy) or an intermediate charged state χ ± which decays as χ ± W had ± + LSP. For a minimal set of heavy-flavor reference topologies, the χ ± LSP mass difference can be taken very small, so that the main impact of χ ± is that it allows the decay g t b+ χ ±, where the visible products of χ ± W had ± + LSP are soft and can be ignored. 4

2.3.1 Generalizations It is worth emphasizing that the above reference topologies are meant as a start this is the main reason for not including decays with an intermediate cascade decay. However, the intermediate cascade decays may be important in some situations. For example, the g t b topology is likely less visible than g t b + χ where χ W + LSP in some range of the available kinematics. This obviously depends on the way the search is designed, so this should be kept in mind. Likewise, we have not included decays of g through an on-shell quark partner, which would produce the same final states but with an additional free parameter that affects the distribution of momentum between the two final-state quarks in each cascade. In this regard, the on-shell quark scenarios are an interesting generalization of the basic reference topologies, and demonstrate the search s degree of sensitivity to the kinematics of the produced heavy quarks. Indeed, in the limiting case of on-shell squarks very close to g but smaller in mass, the quark in g b + b is soft. This means that g b b + LSP production has very similar kinematics to b b + LSP production, but with a cross section appropriate for g production. This is yet another reason to be careful about taking cross-section projections too seriously. Figure 2: Heavy-Flavor diagrams of interest for the initial reference topologies. Included are reactions for g b b + χ, g t b + χ, g t t + χ, and t/b t/b + χ [17]. 2.3.2 Heavy Flavor Parameter Space For the case of gluino decays through off-shell squarks, the kinematics is specified by 1. m g and m LSP for the g production processes. 2. m t or m b and m LSP for the t and b production processes. A natural 2d plot is m g (or m t/b ) vs. m LSP, with shading indicating (σ BR) limit /σ ref and an exclusion line where (σ BR) limit = σ ref, using the reference cross-sections 5

from App..1. Please see http://www.lhcnewphysics.org/bottoms for more information. With on-shell squarks in the decay of g, it may be appropriate to show the impact on search sensitivity of a couple choices of m g /m q, using the same plots recommended above. In general, as with other searches, the most important kinematics to vary and display are those masses that most dramatically impact the search sensitivity. 2.4 Two or More Leptons Multi-lepton final states can dominate leptonic signals in SUSY scenarios where chargeconservation guarantees production of two leptons in a cascade, and in those where single-lepton cascades have large rates. One scenario that gives rise to many multi-lepton events and has a simple parameter space is General Gauge Mediation (GGM) with flavour-degenerate sleptons at the bottom of the MSSM spectrum. In particular, right-handed slepton co-nlsps decay 100% of the time to a massless gravitino and a lepton. Thus, in this case almost all SUSY events in these models contain at least two high-p T leptons. Depending on the details of the heavier states in the spectrum, these leptons can be same sign or opposite sign, and there can be additional energetic leptons in the event from decays to the slepton co-nslp. Left-handed slepton or neutrino co-nlsp s can also give rise to events with a high frequency of single-lepton cascades. Examples of relevant diagrams for the leptonic stages of these decays are shown in Figure 3. When χ 0 is dominantly bino- or wino-like, it decays with equal frequencies to all flavors of leptons. In gaugemediated scenarios, the missing transverse energy at the end of the decay chain is massless; if however the lightest neutralino is stable on the detector length scale, very similar topologies can arise from cascades of winos or binos to the lightest neutralino. Figure 3: Diagrams of decay chains giving rise to multi-lepton final states [18]. Figure 4 illustrates one concrete choice of topologies for multi-lepton studies. This set includes two representative topologies for di-lepton production, one three-lepton topology and one four-lepton topology (it should be noted that lepton signs on the two 6

legs are uncorrelated, and that the roles of the neutrino and lepton in a single-lepton cascade can be reversed). Each diagram has an analogue for squark pair production that can also be considered. Figure 4: Four specific topologies of interest for 2, 3, and 4-lepton final states. Note that lepton signs between the two legs are uncorrelated, that analogous topologies exist in which the roles of l and ν within a cascade are reversed, and that similar topologies can also be written down for squark initial states. As noted above, the case of a massless (gravitino-like) invisible final state is both an interesting physical limit and a reasonable simplifying assumption under which to better explore the remaining parameter space, parametrized by the gluino (or squark) mass, χ 0 mass, and nearly degenerate slepton mass. A useful parametrization of this mass space is by The mass difference m( g) m( χ 0 ), which drives the typical hadronic jet energies, or equivalently m( g) itself The mass of χ 0 itself, which (for massless missing transverse energy) controls the sum of lepton energies The fraction m( l)/m( χ 0 ), which controls the division of energy between the two leptons in each cascade, and for which a small number of values can be chosen, e.g. 1/2, 3/4 and 7/8. 2.4.1 Comments on Stau NLSP A related scenario (not shown in the figure) arises for decays to a gravitino the so-called stau NLSP scenario in which the slepton shown in the far right two figures decays as: l± l s ± τ s ( τ ± τ ± G ; for l = e, µ), (1) where the subscript s indicates a lepton/tau that may be much softer than the other leptons because its momentum is set by the splitting m l m τ, which can be naturally small. As this is a rather long decay chain, many parameters are hidden in any presentation. It is quite likely that searches for 3 leptons are most sensitive to this mode, but quite sensitive to the splitting (which directly controls the observability of the softer leptons). It is valuable to include a parametrization of results on which is explicitly 7

presented on one axis. One possible way to do this is a limit plot on the gluino-mass vs. plane (and analogously for squark production), displaying (1) exclusion contours σ max = σ ref for several choices of m χ 0, and (2) for a single choice of m χ 0, a temperature plot of σ max /σ ref. 2.5 Di-Photons The primary scenario probed by di-photon searches is a spectrum with a light gravitino LSP (as in gauge-mediation) and neutralino NLSP. Such scenarios can be realized in general gauge mediation. In this case the lightest neutralino decays to its superpartner plus a gravitino, i.e. for a bino NLSP, B γ + G dominates. Figure 5: Decay chains leading to di-photon final states (full topologies include production and decays of two gluinos, two squarks, or one of each) [18] χ 0 γ + G (2) The principal production channels that could dominate at the early LHC are gluino or squark pair production (or associated production) in this context, each produced gluino or squark undergoes the decay chain shown in Figure 5. More generally, γ+leptons final states can arise from wino-like neutralino NLSPs. An upper limit on σ BR for g g jets + γγ+ E T ( q q jets + γγ+ E T ) through the decay chains in Figure 5, as a function of m( g) (m( q)) and m( B), provides a very concise summary of the sensitivity of this search. As before, it is also illuminating to present these as upper limits on σ BR/σ ref, where σ ref is the QCD production cross-section. 3 Exotica and SUSY signals without MET (RPV) In the first revision, we have chosen to focus on signals with MET these correspond to searches that are being pursued early, and where simplified model presentation really 8

clarifies the impact of a search..1 SUSY Gluino/Squark Production Reference Cross-Sections In Figure 6 we provide plots of reference cross-sections for gluino production (in the limit of decoupled squarks) and squark production (in the limit of decoupled gluinos), i.e. production through QCD interactions, referred to throughout the text. These were computed at NLO in Prospino by E. Izaguirre. 100 1000 10 σpp g g(nb) 1 0.1 0.01 10 0.1 0.001 100 200 300 400 m g 0.001 100 200 300 400 500 600 0 Figure 6: Cross-sections for gluino pair production (left panel) and squark pair production (right panel, in units of pb) from QCD interactions only. A Likelihood and Efficiency Information The very general point (to be elaborated on later in future revisions) is that it s somewhat more general to present separately an efficiency as a function of masses, and the likelihood as a function of number of events expected in an exclusive final state. So it might be advisable to think about producing both a cross-section limit for visual presentation (i.e. a readily interpretable result to put in talks and papers) and also an (efficiency+likelihood) presentation that technically contains more information, but is more difficult to use. References [1] Characterization of New Physics at the LHC, ATLAS/CMS/LPCC Workshop at CERN, June 4 2010, http://indico.cern.ch/conferenceotherviews.py?view=standard&confid=94910 [2] CNP Workshop at CERN, June 4 2010, Joe Incandela (University of California, Santa Barbara) http://indico.cern.ch/getfile.py/access?contribid=2&sessionid=0&resid=0&materialid=slides&confid=94910 [3] CNP Workshop at CERN, June 4 2010, Paul de Jong (Nikhef), http://indico.cern.ch/getfile.py/access?contribid=1&sessionid=0&resid=0&materialid=slides&confid=94910 9

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