Available online at www.sciencedirect.com Nuclear and Particle Physics Proceedings 273–275 (2016) 2394–2396 www.elsevier.com/locate/nppp Search for contact interactions and large√ extra dimensions in the dilepton final state using proton-proton collisions at s = 8 TeV with the ATLAS detector Tracey Berry and Graham Savage Department of Physics, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, UK Abstract A search is conducted for non-resonant new phenomena in dielectron and dimuon final states, originating from either√ contact interactions (CI) or large extra dimensions (LED). The full LHC 2012 proton-proton collision dataset at s = 8 TeV recorded by the ATLAS detector is used, corresponding to 20 fb−1. The invariant mass spectrum is used as a discriminating variable and for the first time in ATLAS CI searches, angular information is used to construct a forward-backward asymmetry to increase the search sensitivity. Lower limits are set on the CI scale Λ between 15.4 TeV and 26.3 TeV, and on the string scale MS for large extra spatial dimensions, from 3.2 TeV to 5.0 TeV. This 8 TeV result includes additional CI helicity models and LED formalism results to the 7 TeV ATLAS search. Keywords: Contact Interaction, Graviton, ADD, Exotics, New Physics, EXOTICS 1. Introduction positeness in the process qq → +− can be described by a four-fermion contact interaction Lagrangian [3, 4]. Many theories beyond the Standard Model (SM) pre- This search investigates left-left (LL) and, in addition to dict new phenomena which give rise to dilepton final previous searches, left-right (LR) and right-right (RR) states. A search for new phenomena appearing as broad chiral structures of this interaction as well as the con- deviations from the SM in the dielectron and dimuon structive (-1) and destructive (+1) interferences with the invariant mass distributions or in the angular distribu- Drell-Yan (DY) process. tions of these leptons is described here. Specifically, this search was performed with the ATLAS detector [1] for A solution to the vast hierarchy between the elec- the theoretical models of contact interactions and large troweak and Planck scales has been proposed by extra dimensions. Arkani-Hamed, Dimopoulos and Dvali (ADD) [5]. In this model, gravity propagates into large flat extra spa- tial dimensions, thereby diluting its apparent strength 2. Theoretical Motivation in 3+1 spacetime dimensions. The flat n extra dimen- The presence of a new interaction can be detected sions are of common size R and are compactified on an at an energy much lower than that required to pro- n-dimensional torus. The fundamental Planck scale in duce direct evidence of a new gauge boson. A non- (4+n)-dimensions, MD, is related to the Planck scale, 2 ∼ n+2 n renormalizable description of this process was success- MPl, by Gauss’s law MPl MD R . The propaga- fully formulated by Fermi in the form of a four-fermion tion of gravitons into these extra dimensions results contact interaction (CI) [2]. A CI can also accommodate in a Kaluza-Klein (KK) tower of graviton modes with deviations from the SM in proton–proton scattering due mass spacing proportional to 1/R. Dilepton production to quark and lepton compositeness, where a characteris- via virtual KK graviton exchange involves a sum over tic energy scale Λ corresponds to the binding energy be- these modes which needs to be truncated, chosen to be tween fermion constituents. A new interaction or com- the string scale MS [6]. Three formalisms are investi- http://dx.doi.org/10.1016/j.nuclphysbps.2015.09.403 2405-6014/© 2016 Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). T. Berry, G. Savage / Nuclear and Particle Physics Proceedings 273–275 (2016) 2394–2396 2395 Figure 1: Reconstructed dielectron invariant mass distributions for Figure 2: Reconstructed AFB distributions for data and the SM back- data and the SM background estimate. Also shown are the predic- ground estimate as a function of dimuon invariant mass. Also shown tions for a benchmark Λ value in the LL contact interaction model are the predictions of different benchmark Λ values for the LL and and benchmark MS value in the GRW ADD model. The ratio is pre- LR contact interaction models. The ratio displays the background- sented with the total systematic uncertainty overlaid as a band. subtracted data (Δ) divided by the total uncertainty (σ) in each bin. gated, provided by Giudice–Rattazzi–Wells (GRW) [7], tom 80 – 120 GeV and the control region is defined be- Hewett [8] and Han–Lykken–Zhang (HLZ) [9]. Addi- tween 120 – 400 GeV which is used to check the qual- tionally, the n = 2 HLZ case is investigated. ity of the background modelling since the signal con- tribution is negligible in this region. Above 400 GeV, the CI search is split into 6 search regions, while the 3. Event Selection ADD search is performed in a single high mass bin be- tween 1900 – 4500 GeV optimised for the highest ex- Events in the ee channel are required to have passed pected limit. a two-electron trigger with transverse momentum (p ) T The dilepton decay angle, θ∗, defined in the Collins– thresholds of 35 GeV and 25 GeV. Events in the μμ Soper frame [12], has high discriminating power from channel are required to have passed at least one of two DY events in certain cases such as the LR model and is single-muon triggers with p thresholds of 36 GeV and T also used in the CI search. Each invariant mass search 24 GeV. In both channels, events are required to have region in the CI investigation is further split into two at least one primary vertex with more than two tracks. cos θ∗ regions defined as forward (cos θ∗>0) and back- Further requirements are imposed to ensure high p iso- T ward (cos θ∗<0), and an asymmetry is calculated ac- lated electrons and muons and minimise fake electron cording to and muon candidates from jets, as detailed in Ref [10]. The dominant DY background, Photon-Induced (PI) NF − NB AFB = . (1) background, tt¯ and single top-quark production pro- NF + NB cesses, and diboson processes, are all modelled with The distribution of the asymmetry in the dimuon final + Monte Carlo (MC). The combined multi-jet and W jets state is shown in Figure 2. background is estimated in the dielectron channel only using a data-driven method, as described in Ref [11]. The reconstructed invariant mass distribution for the di- 5. Systematic Uncertainties electon channel is displayed in Figure 1. The total background estimate is normalized by scal- ing to data in the dilepton mass normalization region. 4. Search Regions This protects the analysis against mass-independent sys- tematic uncertainties. However, mass-dependent sys- In this analysis, the normalization, control, and tematic uncertainties affect the shape of the discriminat- search regions are defined using the dilepton invariant ing variables and are therefore introduced as nuisance mass. The normalization region cover the Z resonance parameters in the statistical interpretation. 2396 T. Berry, G. Savage / Nuclear and Particle Physics Proceedings 273–275 (2016) 2394–2396 Figure 3: Summary of 95% C.L. lower exclusion limits on Λ for Figure 4: Summary of 95% C.L. lower exclusion limits on MS for the combined dilepton contact interaction search. Previous ATLAS the combined dilepton ADD large extra dimensions search. Previous search results [13, 14] are also presented for comparison. 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