Dark Matter in ATLAS

Transcription

1 Dark Matter in ATLAS Silvia Resconi INFN Milano (on behalf of the ATLAS Collaboration) Ordinary Matter Dark Matter Dark Energy Rencontre de Moriond: QCD and High Energy Interactions 21st March 2016

2 Outline Dark Matter at LHC Analysis strategy Benchmark Models Dark Energy Results at 13 TeV: Mono-photon Mono-jet Mono-W/Z(had) Mono-Higgs(γγ) Conclusions 2

3 Dark Matter at LHC Dark matter can be produced at LHC if it interacts with SM particles: Invisible dark matter candidate: Missing Transverse Momentum (MET) X= γ,jet,w,z,h Need detectable physics object (X): X = γ, jet,w, Z, h φ Mono-X searches at colliders MET performance is critical for dark matter searches: Studied in Z µµ events, no real MET expected MET tails and resolution well modeled MET 3

4 Analysis strategy Similar analysis strategy in all mono-x searches: Event selection: High MET, compatible with production If X=γ,jet è high pt(x) with uality criteria If X=W,Z,h è reconstruct mass within a window Δφ(X,MET) large Remove events with mismeasured MET, e.g.: Δφ(jet,MET) large Δφ(p T,MET) small (p T based on ID tracks) Veto events with other physics objects Background estimation: Normalization factors (k i ) from Control Regions (CR) Tested in signal free Validation Regions (VR) Propagated to Signal Region (SR) k 1 k 2 k 3 X= γ,jet,w,z,h φ MET CR1 CR2 CR3 k 1 k 2 k 3 VR SR 4

5 Benchmark Models ATLAS-CMS Dark Matter (DM) Forum arxiv: è define benchmark models for kinematically distinct signals for Run-2 searches: Simplified models: DM particle is a Dirac fermion, Mediator (med) exchanged in the s-channel 5 parameters: M med, m, g,, g, Γ med Physics objects (X) produced in ISR Specific model for X=Higgs (h): Mediator radiates h and decays to EFT models: Valid if the M med >> momentum transfer at LHC Specific EW EFT model: direct coupling betweeen DM and EW bosons motivate searches with EW final states X=γ,W,Z,h 5 X=γ,jet,W,Z g med med X g X=h med

6 Mono-photon Clean signature: high p T photon plus MET (+up to 1 jet) jet: up to 1 p T >30 GeV γ: p T >150 GeV η <2.37 tight, isolated >0.4 >0.4 φ µ/e veto Main backgrounds normalized to data in specific CRs: Z νν + γ (irreducible) è 2µCR + 2eCR W lν + γ è 1µCR γ + jets è γ+jetcr Background in SR derived from simultaneous singlebin fit to CRs No excess found in data N lept γ-jet CR MET>150 GeV k Z k W k γjet 2µCR 2eCR 1µCR SR MET (GeV) 6 MET in SR p γ T in SR

7 Mono-photon Simplified Model: Axial-vector mediator 95% CL exclusion limit on (m, m med ) plane: Area under the limit is excluded: for m med < 710 GeV, m < 150 GeV γ med g g [GeV] m ATLAS Preliminary -1 s=13 TeV, 3.2 fb Axial-vector mediator Dirac DM g =0.25, g =1 DM Observed 95% CL Observed ± 1 theo Expected 95% CL Expected ± 1 Relic density Perturbative limit EW EFT Model: Lower limit on M * (effective mass scale) as a function of m : M * < 570 GeV excluded Not valid if s > M cut (M cut = g * M * ) Truncation procedure effective for low values of couplings g * γ γ [GeV] 95% CL lower limit on M * m med 900 ATLAS Preliminary observed limit 800 EW EFT model expected limit expected ± 1 s = 13 TeV, 3.2 fb expected ± 2 truncated limits m [GeV] [GeV]

8 Mono-jet More complex signature but more statistics wrt mono-photon analysis: high p T jet plus MET (+up to 3 jets) jets: up to 3 p T >30 GeV Main backgrounds normalized to data in specific CRs: Z νν + jets (irreducible), W µν + jets è 1µCR W eν (τν) +jets, Z ττ + jets è 1eCR Z µµ + jets è 2µCR Background in SR from simultaneous simplified shape fit SR and CRs divided into exclusive/inclusive MET bins No excess found in data jet: p T >250 GeV η <2.4 tight cleaning φ N lept >0.4 >0.4 k 1 k 2 MET>250 GeV k 3 µ/e veto 2µCR 1µCR 1eCR SR 250 MET (GeV) 8 MET in SR leading p jet T in SR

9 Mono-jet Simplified Model: g g Axial-vector mediator 95% CL exclusion limit on (m, m med ) plane: Enlarged area wrt mono-photon: m med < 1 TeV, m <250 GeV jet med Complementarity with direct detection: Translation into 90% CL exclusion limit on the spin-dependent -proton scattering σ : comparison is model dependent excluded σ SD (-p) > cm 2, low m SM SM collider direct detection 9

10 Complementarity with di-jets searches Reinterpretation of di-jets analysis at 8 TeV and 13 TeV in the context of simplified model studied in mono-x searches: Axial-vector mediator, fixed couplings: g =0.25,, g =1 95% CL exclusion contour on (m, m med ) plane recasted from the limits provided on Gaussian-shaped resonances Complementarity between di-jets and mono-x searches. 10

11 Large R jet: boson tagging Mono-W/Z(had) Signature: Large R-jet plus MET W/Z µ/e veto < π/2 p T Main backgrounds normalized to data in specific CRs: MET>250 GeV Z+jets 2µCR, W+jets 1µCR, no b-jets, tt 1µCR, up to 1 b-jets Background in SR from simultaneous shape fit to MET distribution: No excess found in data wrt SM prediction Simplified Model: Exclusion limit on signal strengh µ, in (m, m med ) plane W/Z med g g 11

12 Mono-Higgs(γγ) Signature: 2 photons plus MET At least 2γ: 105 < m γγ <160 GeV η <2.37 tight, isolated h Different analysis categories: MET>100 GeV, p γγ T >100 GeV Most sensitive to MET>100 GeV, p γγ T <100 GeV simplified model 50 GeV <MET < 100 GeV p hard T >40 GeV p γγ T >40 GeV Events / GeV ATLAS Preliminary s = 13 TeV, 3.2 fb High E, High p T T MET>100 GeV -1 Data Signal + total bkg Non-resonant bkg + h Non-resonant bkg Total background evaluated directly from data: Unbinned fit to the m γγ distribution No significant excess observed Simplified model: Exclusion limits on σ x BR vs m med BR(h ) [fb] ) m [GeV] h 20 2 med 15 1 med m med [GeV] (pp h ATLAS Preliminary h -1 s = 13 TeV, 3.2 fb Vector mediator Dirac DM, m g =1/3, g =1 DM = 1 GeV DM Observed 95% CL Expected 95% CL Theory Expected ± 1 Expected ± 2 Events / GeV ATLAS Preliminary s = 13 TeV, 3.2 fb High E, Low P T T -1 Data Signal + total bkg Non-resonant bkg + h Non-resonant bkg m [GeV]

13 Conclusions Dark matter searches at 13 TeV have just begun: First results of mono-x searches in ATLAS. Common interpretation between ATLAS and CMS provided by DM forum. Complementarity with direct detection searches and di-jets searches. In 2016 more data expected ( 25 fb -1 ): Increase discovery and exclusion power of DM searches. No sign of DM so far but it could be just around the corner!! Dark Energy 13

14 Back-up 14

15 References MET perfomance: Mono-photon: Mono-jet: Mono-W/Z(had): ATLAS-CONF Mono-H(γγ): ATLAS-CONF Di-jets: PRD 91, (2015), PLB 754 (2016) ATLAS/CMS Dark Matter Forum: arxiv: , arxiv:

16 W/Z boson tagging Large R jets provides efficient reconstruction of massive boosted objects whose decay products are sufficiently collimated. Trimming: recluster k t subjects, remove those with p Ti /p Tjet < f cut improve resolution of jet mass of W/Z wrt multijets No dependence of jet mass vs Npv after trimming Criteria adopted in mono-w/z(had) Large R jets: anti-k T with R=1.0 Trimmed with R sub =0.2, f cut = 5% Classified as originating from W or Z: using p T -dependent selection on jet mass and jet substructure variable D 2 selecting jets with 2 concentrations of energies. D2 selection provides constant efficiency of 50% using simulated samples enriched with W or Z bosons. Silvia Resconi CHEF

17 Missing Transverse momentum E T is a complex event uantity: Adding significant signals from all detectors Asking for momentum conservation in the transverse plane Reconstructed physics objects: e, γ, τ, jets, muons Soft energy: Tracks unmatched to physics objects from primary vertex overlap removal based on to the association map E x(y) = E x(y),e +E x(y),γ +E x(y),τ +E x(y),jets +E x(y), Soft Term + E x(y), µ E T = ( E x ) 2 + ( E y ) 2 p T based ID tracks: Jets, µ and Soft energy from ID tracks Fully calibrated e, è Neutrals are lost, limited ID acceptance è complementarity to MET for some topologies Fake MET can be due to: Mismeasured hard objects Pile-up Detector noise 17

18 VBF Higgs(invisible) Signature: Two high p T jets plus MET (>150 GeV), lepton veto Three orthogonal SRs: Different m jj, Δη jj, p T of leading/subleading jets Main backgrounds Zνν+jets and W+jets normalized in leptonic CRs Higgs-portal DM scenario: results from the BF(H invisible) limit translated into upper limits on WIMP-nucleon cross section 18

19 @ 8 TeV 13 TeV Simplified Model: Axial-vector mediator fixed couplings: g =0.25,, g =1 excluded σ SD (-p) > cm 2, low m 19

20 Complementarity with di-jets searches Procedure for recasting dijet analyses at 8 TeV and 13 TeV: Test signals generated with MadGraph: Simplified model: axial-vector mediator, fixed couplings: g =0.25,, g =1 Di-jet mass distribution smeared by experimental resolution The core of the di-jets mass compared with limits on cross-section of euivalent Gaussian-shaped signal. Off-shell region: M med < 2m The mediator can decay only to di-jet On-shell region: M med > 2m : The mediator can decay also to DM è Reduced sensitivity 20