Search for Minimal Universal Extra Dimensions in 8 TeV pp collisions in the ATLAS detector

Transcription

1 JPS, Kochi September Search for Minimal Universal Extra Dimensions in 8 TeV pp collisions in the ATLAS detector Ljiljana Morvaj, Naoko Kanaya, Makoto Tomoto, Takuya Nobe 3 & ATLAS SUSY group Nagoya University, ICEPP, Tokyo Tech. 3

2 Introduction Minimal universal extra dimensions (mued) is an interesting candidate for physics beyond the standard model (BSM) All the SM fields propagate in the compactified extra dimensions (only ED assumed in this case) Tower of Kaluza-Klein (KK) states KK masses given by (at the tree level): m n = n R + m SM KK particle GeV~TeV R = size of the ED n = excitation level (n=0 => SM particle) MeV~GeV => very small contribution to mn mkk The KK particle spectrum is naturally compressed Low momentum (soft) particles produced in the decays Experimentally challenging signature R ( 700GeV) m γ q Z ± γ m~o(0) GeV soft SM decay products The lightest KK particle is stable Dark maer candidate Parameters of the theory: ultra-violet cut-off ΛR [3, 40] small m larger m

3 3 Typical signature Two KK quarks (or gluons) are produced in proton-proton collisions One typically decays hadronically (producing only jets) The other decays leptonically, often to KK Z which, in turn, gives two opposite sign same-flavour leptons KK gamma is the lightest particle of the model and a dark-maer candidate It doesn t interact with the detector leaving a missing transverse (ET miss ) energy signature jets opposite-sign soft leptons large ET miss Large branching fraction to leptons KK W & Z decay to leptons with Br~00% Dilepton channel is a promising signature

4 4 Event selection Full 0 ATLAS dataset (L=0. fb - ) The selection determined by optimising S/ B in the signal region (SR) Mainly focus on smaller m region ( ΛR 0 ) Simple -bin counting experiment, no shape fiing Baseline selection: Trigger: ET miss > 80 GeV Two soft muons: 6 GeV < pt < 5 GeV Two (or more) jets - often the leading jet is coming from the initial state radiation (ISR) Large ET miss SR definition: High transverse mass (mt) region calculated with nd µ ISR jet Events ET miss jet p jets T (GeV) N b tag (GeV) E miss T ATLAS Work in progress - Ldt=0. fb, m T (GeV) miss / incl signal Data (0) DY barv R900_ R40 >70,5 0 >70 > 80 Data (0) DY barv R900_ R40 B-jet veto (among 3 leading jets) These cuts mainly reject the dominant -bar background Data/SM N b-jets >5 GeV N b-jets >5 GeV Nb-jets>5 GeV

5 5 Backgrounds Misidentified (fake) muon background - µ from b- or c-hadron decays or jets misidentified as µ dominant source: muons form b-jet decays in semileptonic -bar events suppressed by requiring µ to be well isolated define a cone around jet => R to nearest µ > 0.4 fake µ significant at small µ pt and small mt suppressed further by requiring high mt Dileptonic -bar suppressed by puing an upper cut on muon pt and by requiring 0 b-jets in the SR Events/0 GeV ATLAS Work in progress - Ldt=0. fb, Data (0) Data (0) DY DY barv R900_ barv R40 R40-0, Drell-Yan, single-top, diboson & -bar+v minor backgrounds estimated using only MC simulation Data/SM (µ) M T (µ) M T mt (µ)[gev]

6 6 Misid. µ background Estimated in a fully data-driven way (the matrix method) Based on inverting the muon track-isolation and impact parameter cuts: pt tracks in a dr = 0.3 cone p µ T < 0. Longitudinal IP : z PV 0 sinθ 0.4 mm Transverse IP : d PV 0 /σ(d PV 0 ) 3 Fake rate: FR = Nµ isolated,fake N µ total,fake Measured in the di-jet data sample Low ET miss and mt region N iso = RR N real + FR N fake N non iso =( RR) N real +( FR) N fake Real rate: Events / 0 GeV RR = Nµ isolated,real N µ total,real Measured in the Z->µµ data sample ATLAS Work in progress Ldt=0. fb -, Data (0) Data (0) DY DY barv R900_ R40 barv R900_ R40 Invert the equations above to extract Nfake The result is validated in the whole ET miss range Good agreement with the data Data/SM measure miss ET [GeV] miss ET validate ET miss [GeV]

7 7 Dileptonic -bar Estimated in the semi data-driven way -bar normalised to the data in the control region (CR) => orthogonal to the SR -bar purity enhanced by requiring at least b- tagged jet Events/0 GeV ATLAS Preliminary Ldt=0. fb -, Data (0) Data (0) misid. lepton Single Top Other contribution mued (900 GeV,40) Other contribution mued (900 GeV,40) soft dimuon top control region MC transfer factor is used to extrapolate to the signal region: N SR,est.data = N CR,obs.data N SR,MC N CR,MC The result of the normalisation is checked in the 3 validation regions (VR) Defined in between the CR and SR ) [GeV] Data/SM m T(µ SR no req. on b VR lepton p T no req. on b VR [GeV] M T (µ) [GeV] normalise 80 GeV b-veto validate 5 GeV b-tag CR TR m T mt [GeV] VR3 b-veto

8 8 -bar estimation validation Very good agreement observed in all the validation regions channel VR VR VR3 Observed events Fied Estim bkg events 68.7 ± ± ± 4.63 Fied Estim -bar events 8.6 ± ± ± 0.5 Misid. lepton events.5 ± ± ± 5.87 Diboson events 9.45 ± ± ± 9.5 Single-top events.70 ± ± events 5.99 ± ± ± 0.8 -bar+v events.4 ± ± ± 0. ) [GeV] m T(µ 80 GeV b-veto SR no req. on b VR 5 GeV lepton p T no req. on b [GeV] VR b-tag TR CR VR3 b-veto Events / 0 GeV Data/SM ATLAS Work in progress - Ldt=0. fb, normalisation factor not applied Data (0) Data (0) Single DYTop DY barv barv R900_ R40 R900_ R subleading lepton p [GeV] T subleading µ pt [GeV]

9 9 Result No significant deviation between predicted SM background and observed number of events in the SR Disagreement is at the level of.3 sigma Observed events Fied Estim. bkg events Fied Estim. -bar events Misid. lepton events Diboson events Single-top events events -bar+v events Signal region 7.60 ±.04.0 ± ± ± ± Events / 0 GeV ATLAS - Ldt = 0. fb, Preliminary p-value for bkg only hypothesis Data (0) W+jets Other contribution mued (900 GeV,40) soft dimuon signal region m T [GeV] mt [GeV] Dominant sources of systematic uncertainty +/- Neve The uncertainties can be correlated and don t necessarily add up quadratically to the total uncertainty -bar parton-shower uncertainty MC statistics in SR B tagging -bar yield Systematics ±0.8 [50.6%] ±0.45 [7.9%] ±0.30 [9.0%] ±0.0 [.7%] ±0.9 [.%]

10 0 Exclusion Model-independent limits: 95% CL upper limits on: Derived using CLS prescription visible x-section N signal events The cross-section upper limits are set for the mued model in the D parameter space Λ R ~ mass spliing between the states Universal Extra Dimension ATLAS Preliminary dimuon signal region L dt = 0. fb SUSY Observed limit (± σtheory ) Expected limit (± σ exp ) /R [GeV] expected observed ~ mass of the KK photon, excluded model cross sections [pb] Numbers give 95% CL

11 Conclusion The analysis performed using the full 0. fb - of ATLAS data at The signal region definition optimised specifically for mued model soft leptons/jets in the decay chains dimuon channel is used No significant deviation form the standard model expectation is observed The limit on the compactification radius of up to /R=800 GeV is set, depending on the compression scale ( ) ΛR This extends the previous ATLAS limit set by the s=7 TeV 3-lepton analysis into ΛR 0 region

12 Backup

13 3 The matrix-method Data-driven method to estimate the misidentified lepton background Based on inverting the muon isolation: pt tracks in a dr = 0.3 cone p µ T < 0. Longitudinal IP : z PV 0 sinθ 0.4 mm Transverse IP : d PV 0 /σ(d PV 0 ) 3 Fake rate: FR = Nµ isolated,fake N µ total,fake Measured in the di-jet data sample: ET miss <30 GeV mt<40 GeV Real rate: RR = Nµ isolated,real N µ total,real Measured in the events in the data

14 4 The matrix-method Number of tight = isolated & loose = non-isolated muons in our data sample count it Fake & real rates measured Number of real & fake muons in our data sample invert the matrix to extract this number Events / 0 GeV ATLAS Work in progress Ldt=0. fb -, Data (0) DY barv R900_ R40 The result is validated in the whole ET miss region Good agreement with the data Data/SM measure miss ET [GeV] miss E [GeV] validate T