dark matter in atlasmoriond.in2p3.fr/qcd/2016/mondayafternoon/resconi.pdfreinterpretation of di-jets...
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Dark Matter in ATLAS
Silvia Resconi
INFN Milano (on behalf of the ATLAS Collaboration)
Rencontre de Moriond: QCD and High Energy Interactions 21st March 2016
Ordinary Matter
Dark Energy
Dark Matter
Outline � Dark Matter at LHC
� Analysis strategy
� Benchmark Models
� Results at 13 TeV: � Mono-photon � Mono-jet � Mono-W/Z(had) � Mono-Higgs(γγ)
� Conclusions
Silvia Resconi 2 Moriond QCD 2016
Dark Energy
MET performance is critical for dark matter searches: • Studied in Z µµ events, no real MET expected
• MET tails and resolution well modeled
Dark Matter at LHC
Silvia Resconi 3 Moriond QCD 2016
Dark matter can be produced at LHC if it interacts with SM particles: • Invisible dark matter candidate:
• Missing Transverse Momentum (MET) • Need detectable physics object (X):
• X = γ, jet,W, Z, h “Mono-X” searches at colliders
X= γ,jet,W,Z,h
MET
φ
Analysis strategy
Silvia Resconi 4 Moriond QCD 2016
Similar analysis strategy in all “mono-X” searches: Event selection: • High MET, compatible with χχ production • If X=γ,jet è high pT(X) with quality criteria • If X=W,Z,h èreconstruct mass within a window • Δφ(X,MET) large • Remove events with mismeasured MET, e.g.:
• Δφ(jet,MET) large • Δφ(pT
miss,MET) small (pTmiss based on ID tracks)
• Veto events with other physics objects Background estimation: • Normalization factors (ki) from Control Regions (CR) • Tested in signal free Validation Regions (VR) • Propagated to Signal Region (SR)
X= γ,jet,W,Z,h
MET
φ
VR
CR1
CR2 CR3
k1
k2
k3
SR
k1 k2
k3
ATLAS-CMS Dark Matter (DM) Forum arXiv:1507.00966 è define benchmark models for kinematically distinct signals for Run-2 searches: “Simplified” models: • DM particle is a Dirac fermion, χ • Mediator (med) exchanged in the s-channel • 5 parameters: Mmed, mχ, gq,, gχ, Γmed • Physics objects (X) produced in ISR • Specific model for X=Higgs (h):
• Mediator radiates h and decays to χχ
EFT models: • Valid if the Mmed >> momentum transfer at LHC • Specific EW EFT model:
• direct coupling betweeen DM and EW bosons • motivate searches with EW final states
Benchmark Models
Silvia Resconi 5 Moriond QCD 2016
X=γ,W,Z,h
q
− χ
χ q
med
med
X=h
X=γ,jet,W,Z q
−
χ
χ q gq gχ
med
q
−
χ
χ q
X
Mono-photon
Silvia Resconi 6 Moriond QCD 2016
Clean signature: • high pT photon plus MET (+up to 1 jet) 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 single-bin fit to CRs No excess found in data
γ: pT>150 GeV |η|<2.37 tight, isolated
MET>150 GeV
>0.4
jet: up to 1 pT>30 GeV
>0.4
µ/e veto φ
MET (GeV)
N lept
110
1
0
85 150
2
SR
2µCR
1µCR
2eCR
γ-jet CR
kZ
kW
kγjet
MET in SR pTγ in SR
Mono-photon
Silvia Resconi 7 Moriond QCD 2016
Simplified Model: Axial-vector mediator 95% CL exclusion limit on (mχ, mmed) plane: • Area under the limit is excluded: for mmed < 710 GeV, mχ < 150 GeV [GeV]medm
100 200 300 400 500 600 700 800
[GeV
]r
m
50
100
150
200
250
300
350 PreliminaryATLAS
-1=13 TeV, 3.2 fbs
Axial-vector mediator
Dirac DM
=1DM
g=0.25, qg
Observed 95% CLtheom 1 ±Observed
Expected 95% CLm 1±Expected
Relic densityPerturbative limit
[GeV]rm1 10 210 310
[GeV
]*
95%
CL
low
er li
mit
on M
100200
300400500
600700
800900 observed limit
expected limitm 1±expected m 2±expected
truncated limits
ATLAS PreliminaryEW EFT model
= 13 TeV,s -13.2 fb
2
4 8
/4
EW EFT Model: • Lower limit on M* (effective mass scale) as a function of mχ:
• M* < 570 GeV excluded • Not valid if √s > Mcut (Mcut = g*M* )
• Truncation procedure effective for low values of couplings g*
γ q χ
χ q gq gχ
med
γ q
− χ
χ q
γ
−
Mono-jet
Silvia Resconi 8 Moriond QCD 2016
More complex signature but more statistics wrt mono-photon analysis: • high pT jet plus MET (+up to 3 jets) Main backgrounds normalized to data in specific CRs: • Z νν + jets (irreducible), W µν + jets è1µCR • W eν (τν) +jets, Z ττ + jets è1eCR • Z µµ + jets è2µCR Background in SR from simultaneous simplified shape fit SR and CRs divided into exclusive/inclusive MET bins No excess found in data
MET (GeV)
N lept
1
0
250
2
SR
1µCR
2µCR
1eCR
jet: pT>250 GeV |η|<2.4 tight cleaning
MET>250 GeV
>0.4
jets: up to 3 pT>30 GeV
>0.4
µ/e veto
φ
k1
k2
k3
MET in SR leading pTjet in SR
Mono-jet
Silvia Resconi 9 Moriond QCD 2016
Simplified Model: Axial-vector mediator 95% CL exclusion limit on (mχ, mmed) plane: • Enlarged area wrt mono-photon:
• mmed < 1 TeV, mχ <250 GeV Complementarity with direct detection: Translation into 90% CL exclusion limit on the spin-dependent χ-proton scattering σ : • comparison is model dependent • excluded σSD (χ-p) > 10-42cm2, low mχ
collider SM χ
χ SM
direct detection
jet q
−
χ
χ q gq gχ
med
Complementarity with di-jets searches
Silvia Resconi 10 Moriond QCD 2016
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: gq=0.25,, gχ =1 • 95% CL exclusion contour on (mχ, mmed) plane recasted from the limits
provided on Gaussian-shaped resonances
Complementarity between di-jets and mono-X searches.
Mono-W/Z(had)
Silvia Resconi 11 Moriond QCD 2016
Large R jet: boson tagging
MET>250 GeV
µ/e veto W/Z
pTmiss
< π/2
Signature: • Large R-jet plus MET Main backgrounds normalized to data in specific CRs: • 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 (mχ, mmed) plane
W/Z q
−
χ
χ q gq gχ
med
−
Mono-Higgs(γγ)
Silvia Resconi 12 Moriond QCD 2016
Signature: • 2 photons plus MET
Different analysis categories: MET>100 GeV, pT
γγ>100 GeV MET>100 GeV, pT
γγ<100 GeV 50 GeV <MET < 100 GeV pT
hard>40 GeV pT
γγ>40 GeV Total background evaluated directly from data: • Unbinned fit to the mγγ distribution • No significant excess observed Simplified model: Exclusion limits on σ x BR vs mmed
At least 2γ: 105 < mγγ <160 GeV |η|<2.37 tight, isolated
MET>100 GeV
h
[GeV]aam110 120 130 140 150 160
Even
ts /
GeV
1
2
3
4
5
6
7 Data Signal + total bkg Non-resonant bkg + h Non-resonant bkg
ATLAS Preliminary-1 = 13 TeV, 3.2 fbsaa
T, High pmiss
THigh E
[GeV]medm0 200 400 600 800 1000 1200 1400 1600 1800 2000
) [fb
]aa
A B
R(h
×) r r h
A
(pp
m
5
10
15
20
25
30
35
40
45Observed 95% CLExpected 95% CLTheory
m 1±Expected m 2±Expected
ATLAS Preliminaryaa Ah
-1 = 13 TeV, 3.2 fbsVector mediator
= 1 GeVDMDirac DM, m=1
DMg=1/3, qg
Most sensitive to simplified model
[GeV]aam110 120 130 140 150 160
Even
ts /
GeV
1
2
3
4
5
6
7 Data Signal + total bkg Non-resonant bkg + h Non-resonant bkg
ATLAS Preliminary-1 = 13 TeV, 3.2 fbs
aa
T, Low PmissT
High E
q
− χ
χ q
med med
h
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 !!
Silvia Resconi 13 Moriond QCD 2016
Dark Energy
Back-up
Silvia Resconi 14 Moriond QCD 2016
References MET perfomance:
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PLOTS/JETM-2016-003/
Mono-photon:
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-05/
Mono-jet:
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-03/
Mono-W/Z(had): ATLAS-CONF-2015-080
Mono-H(γγ): ATLAS-CONF-2016-011
Di-jets: PRD 91, 052007 (2015) , PLB 754 (2016) 302-322
ATLAS/CMS Dark Matter Forum: arXiv:1603.04156, arXiv:1507.00966
Silvia Resconi 15 Moriond QCD 2016
Trimming: • recluster kt subjects, remove those with pTi/pTjet< fcut • 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-kT with R=1.0 • Trimmed with Rsub=0.2, fcut= 5% • Classified as originating from W or Z: using pT-dependent
selection on jet mass and jet substructure variable D2 selecting jets with 2 concentrations of energies.
• D2 selection provides constant efficiency of 50% using simulated samples enriched with W or Z bosons.
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W/Z boson tagging
Silvia Resconi CHEF2013
Large R jets provides efficient reconstruction of massive boosted objects whose decay products are sufficiently collimated.
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
Ex(y)miss = Ex(y)
miss,e +Ex(y)miss,γ +Ex(y)
miss,τ +Ex(y)miss,jets +Ex(y)
miss, Soft Term+ Ex(y)miss, µ
ETmiss =√ ( Ex
miss )2 + ( Eymiss )2
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Missing Transverse momentum ET
miss is a complex event quantity: • Adding significant signals from all detectors • Asking for momentum conservation in the transverse plane
Fake MET can be due to: • Mismeasured hard objects • Pile-up • Detector noise
pTmiss 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
VBF Higgs(invisible)
Silvia Resconi 18 Moriond QCD 2016
Signature: • Two high pT jets plus MET (>150 GeV), lepton veto Three orthogonal SRs: Different mjj , Δηjj, pT 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
Mono-jet
Silvia Resconi 19 Moriond QCD 2016
@ 8 TeV
Simplified Model: • Axial-vector mediator • fixed couplings: gq=0.25,, gχ =1 excluded σSD (χ-p) > 10-42cm2, low mχ
@ 13 TeV
Complementarity with di-jets searches
Silvia Resconi 20 Moriond QCD 2016
Procedure for recasting dijet analyses at 8 TeV and 13 TeV: Test signals generated with MadGraph: • Simplified model: axial-vector mediator, fixed couplings: gq=0.25,, gχ =1 • Di-jet mass distribution smeared by experimental resolution • The core of the di-jets mass compared with limits on cross-section of equivalent Gaussian-shaped signal.
Off-shell region: Mmed< 2mχ
The mediator can decay only to di-jet On-shell region: Mmed > 2mχ:
The mediator can decay also to DM è Reduced sensitivity