dark matter in atlasmoriond.in2p3.fr/qcd/2016/mondayafternoon/resconi.pdfreinterpretation of di-jets...

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


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


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


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


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


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


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


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


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)


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)


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 !!


Dark Energy

Back-up


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


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.

16

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

17

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)


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


@ 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


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

dark matter in atlasmoriond.in2p3.fr/qcd/2016/mondayafternoon/resconi.pdfreinterpretation of di-jets...

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