Search for contact interactions and large extra dimensions in the dilepton final state using proton-proton collisions at √

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ATL-PHYS-PROC-2014-161 02October2014

Nuclear Physics B Proceedings Supplement 00 (2014) 1–3

Nuclear Physics B Proceedings Supplement

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 scaleΛbetween 15.4 TeV and 26.3 TeV, and on the string scaleMS 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

Many theories beyond the Standard Model (SM) pre- dict new phenomena which give rise to dilepton final states. A search for new phenomena appearing as broad deviations from the SM in the dielectron and dimuon invariant mass distributions or in the angular distributions of these leptons is described here. Specifically, this search was performed with the ATLAS detector [1] for the theoretical models of contact interactions and large extra dimensions.

2. Theoretical Motivation

The presence of a new interaction can be detected at an energy much lower than that required to pro- duce direct evidence of a new gauge boson. A non- renormalizable description of this process was success- fully formulated by Fermi in the form of a four-fermion contact interaction (CI) [2]. A CI can also accommodate deviations from the SM in proton–proton scattering due to quark and lepton compositeness, where a characteris- tic energy scaleΛcorresponds to the binding energy between fermion constituents. A new interaction or com-

positeness in the processqq → `⁺`⁻can be described by a four-fermion contact interaction Lagrangian [3, 4].

This search investigates left-left (LL) and, in addition to previous searches, left-right (LR) and right-right (RR) chiral structures of this interaction as well as the con- structive (-1) and destructive (+1) interferences with the Drell-Yan (DY) process.

A solution to the vast hierarchy between the elec- troweak and Planck scales has been proposed by Arkani-Hamed, Dimopoulos and Dvali (ADD) [5]. In this model, gravity propagates into large flat extra spatial dimensions, thereby diluting its apparent strength in 3+1 spacetime dimensions. The flatnextra dimensions are of common sizeRand are compactified on an n-dimensional torus. The fundamental Planck scale in (4+n)-dimensions, MD, is related to the Planck scale, MPl, by Gauss’s law M_Pl² ∼ M_Dⁿ⁺²Rⁿ. The propaga- tion of gravitons into these extra dimensions results in a Kaluza-Klein (KK) tower of graviton modes with mass spacing proportional to 1/R. Dilepton production via virtual KK graviton exchange involves a sum over these modes which needs to be truncated, chosen to be the string scale MS [6]. Three formalisms are investi-

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G, Savage, T. Berry/Nuclear Physics B Proceedings Supplement 00 (2014) 1–3 2

Figure 1: Reconstructed dielectron invariant mass distributions for data and the SM background estimate. Also shown are the predictions for a benchmarkΛvalue in the LL contact interaction model and benchmarkM_Svalue in the GRW ADD model. The ratio is presented with the total systematic uncertainty overlaid as a band.

gated, provided by Giudice–Rattazzi–Wells (GRW) [7], Hewett [8] and Han–Lykken–Zhang (HLZ) [9]. Addi- tionally, then=2 HLZ case is investigated.

3. Event Selection

Events in theeechannel are required to have passed a two-electron trigger with transverse momentum (pT) thresholds of 35 GeV and 25 GeV. Events in the µµ channel are required to have passed at least one of two single-muon triggers withpTthresholds of 36 GeV and 24 GeV. In both channels, events are required to have at least one primary vertex with more than two tracks.

Further requirements are imposed to ensure highpTiso- lated electrons and muons and minimise fake electron and muon candidates from jets, as detailed in Ref [10].

The dominant DY background, Photon-Induced (PI) background, tt¯ and single top-quark production processes, and diboson processes, are all modelled with Monte Carlo (MC). The combined multi-jet andW+jets 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- electon channel is displayed in Figure 1.

4. Search Regions

In this analysis, the normalization, control, and search regions are defined using the dilepton invariant mass. The normalization region cover the Z resonance

Figure 2: ReconstructedA_FBdistributions for data and the SM background estimate as a function of dimuon invariant mass. Also shown are the predictions of different benchmarkΛvalues for the LL and LR contact interaction models. The ratio displays the background- subtracted data (∆) divided by the total uncertainty (σ) in each bin.

tom 80 – 120 GeV and the control region is defined between 120 – 400 GeV which is used to check the qual- 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 ADD search is performed in a single high mass bin between 1900 – 4500 GeV optimised for the highest expected limit.

The dilepton decay angle,θ^∗, defined in the Collins–

Soper frame [12], has high discriminating power from DY events in certain cases such as the LR model and is also used in the CI search. Each invariant mass search region in the CI investigation is further split into two cosθ^∗regions defined as forward (cosθ^∗>0) and backward (cosθ^∗<0), and an asymmetry is calculated ac- cording to

AFB= N_F−N_B NF+NB

. (1)

The distribution of the asymmetry in the dimuon final state is shown in Figure 2.

5. Systematic Uncertainties

The total background estimate is normalized by scal- ing to data in the dilepton mass normalization region.

This protects the analysis against mass-independent systematic uncertainties. However, mass-dependent systematic uncertainties affect the shape of the discriminating variables and are therefore introduced as nuisance parameters in the statistical interpretation.

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G, Savage, T. Berry/Nuclear Physics B Proceedings Supplement 00 (2014) 1–3 3

Figure 3: Summary of 95% C.L. lower exclusion limits onΛfor the combined dilepton contact interaction search. Previous ATLAS search results [13, 14] are also presented for comparison.

Experimental uncertainties originate from lepton trigger and reconstruction efficiencies, lepton energy and momentum scale and resolution, lepton charge misiden- tification, multi-jet and W+jets background estimate, beam energy scale, and MC statistics. Theoretical uncertainties originate from variations among the proton eigenvector sets (PDF variation), PDF choice, PDFα_S scale, EW higher-order corrections, PI contributions, and the DY cross-section uncertainty. The DY MC is calculated at next-to-next-to-leading order, with the largest uncertainty coming from PDF variation.

6. Results

The most significant deviation from the SM is seen in the dimuon channel: a p-value of 8% in the CI LL destructive interference model given the 1/Λ²prior, and ap-value of 6% in the ADD GRW formalism given the 1/M⁴_Sprior. In neither case is the deviation significant.

A Bayesian approach is used for the statistical interpretation of the results, using a uniform positive prior as a function of the parameter of interest (1/Λ², 1/M_S⁸) to quantify any observed excess. The expected (in the absence of signal) and observed 95% credibility level (C.L.) lower exclusion limits are set on the parameter of interest, between 15.4 TeV and 26.3 TeV on the CI scale Λand between 3.2 TeV and 5.0 TeV on the ADDMS

scale. Additional helicity models and formalisms are included in this ATLAS result as an improvement to the 7 TeV searches [13, 14], and are presented in Figures 3 and 4 for the CI and ADD searches, respectively.

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Figure 4: Summary of 95% C.L. lower exclusion limits onM_Sfor the combined dilepton ADD large extra dimensions search. Previous ATLAS search results [13] are also presented for comparison.

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