Lepton+Jet+Missing ET based Search for Universal Extra Dimensions

Introduction

A possible scenario to solve the hierarchy problem in the Standard Model (SM) is to postulate extra spacial dimensions, where the fundamental Planck scale is at TeV range. One of the extra dimensions models, called Universal Extra Dimensions (UED), assumes all the SM particles can propagate to the bulk of extra dimension, which leads to an infinite number of Kaluza-Klein modes of SM particles. The mass of a KK mode is approximately given by sqrt(m0^2 + (n/R)^2), where m0 is the mass of the zero mode (= SM particle), n is the level of KK mode excitation, and R is the size of the extra dimension. The KK quantum number is conserved (KK parity) and hence the lightest KK particle (LKP) becomes stable, which provides a good candidate for the Dark matter. The mass spectrum of KK particles is degenerated and the quantum correction controlled by R makes the mass shift between KK particles ( is a cut-off scale of the theory). Usually KK gluons and KK quarks are the heaviest particles, and they undergo a cascade decay to the KK photon, which is LKP. The minimum UED model with one extra dimension is considered in this analysis.

UED Signature

At LHC, the KK gluons and quarks are predominantly produced in pairs by the strong interactions. In the case of 1/R=700 GeV and R = 20, the mass of KK gluon (quark) is about 865 (815) GeV. Since the LKP mass is 700 GeV, the mass difference between the KK gluon and LKP is only about 160 GeV so the SM particles produced in the cascade decay are usually soft. A significant fraction of KK quarks decay leptonically via KK partners of weak bosons. The signature used in the analysis is therefore the presence of 1 (or 2) lepton, multiple jets and a large missing energy.

Monte Carlo Studies

We have studied UED signal in the minimum UED scenario using Monte Carlo samples generated at the center of mass energy 7 TeV. The search for UED signal in lepton+jets+missing ET final state is in progress in Lepton+X sub-group. Here various validation plots on the cross section, kinematics and multiplicity distributions of jets, leptons and missing ET are provided.

Event Generation and Sample Details

UED processes were generated using Pythia 6.4.19 and simulated/reconstructed using Atlfast I in Athena release 14.4.40. The compactification scale 1/R is chosen between 300 and 900 GeV. The parameter is varied between 2 and 50 to cover the extremely degenerate case. In the Atlfast reconstruction, the transfer functions developed for SUSY analysis are applied to get the similar detector resolutions and lepton identification inefficiencies in the full simulation.

Signal Validation

Job options for event generation with Pythia 6.4.19 at Ecm=7 TeV:

The following table contains the preliminary plots of lepton and jet kinematic distributions at the truth level.

Release	E CM	1/R	Lambda R	Entries	xsec	Validation plots
14.4.20	7 TeV	300 GeV	20	333,360	666.8 pb	Truth Electron Pt Truth Muon Pt Truth Jet Pt

The Pythia samples are compared with the official UED samples generated by Herwig++ with the same parameter conditions. The table below gives the details of the Herwig++ samples and comparison plots. "UED(Full)" means the Herwig++ and "UED(Atlfast)" means the Pythia sample.

Release	E CM	1/R	Lambda R	Entries	xsec	Comparison with Pythia
15.6.10.6 (Evgen) 15.6.9.8 (Reco)	7 TeV	300 GeV	20	10,000	400.8 pb	Electron Pt Number of Electron Muon Pt Number of Muon Jet Pt Number of Jet Missing Et

Dataset name : mc09_7TeV.114650.UED_R300_RL20_herwigpp_susy.merge.AOD.e611_s765_s767_r1302_r1306

Analysis Information

The analysis at 7 TeV center of mass energy has been reported at Lepton+X sub-group meetings;

• 26, October, 2010 : Applied one lepton + ISR jet analysis for data
• 22 July, 2010 : Data-driven background estimation for UED model
• 15, June, 2010 : Comparison with CMS results on 4-lepton analysis
• 18, May, 2010 : Update on UED search with three analysis methods

-- HiroshiYamaguchi - 17-Dec-2010