search for new physics in mono-jet final states in pp collisions at … · 2017-03-14 · atlas...

Search for New Physicsin mono-jet final states

in pp collisions at √s = 13 TeV with the ATLAS experiment at LHC

Giuliano Gustavino27 October 2016

Seminario Conclusivo XIX Ciclo

Giuliano Gustavino 2

ATLAS & LHC

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator.

pp and PbPb collider Circumference: 27 km √s = 13 TeV Collision frequency: 40 MHz

Giuliano Gustavino

Multi-purpose experiment able to the detect

the wide spectrum of final states for high precision

Standard Model measurements and New Physics searches


Hadron collisions

๏ Signal process + jets production ๏ Parton shower ๏ Fragmentation ๏ Hadron decays ๏ Beam remnants ๏ Underlying event

Large numbers of vertexes reconstructed in the same events!

PILEUP


Mono-jet final stateThe monojet analysis is a search for events with a high transverse momentum jet and missing transverse momentum in the final state.

The monojet topology constitutes a clean and distinctive signature in searches for new physics beyond the Standard Model (SM) at colliders.

transverse planeWhat is MET?

MET measures the energy imbalance in the plane transverse to the colliding proton beams

What is a jet?

Hadrons are clustered together to make particle jets

high energy jet coming from

the initial states

large missing transverse

momentum (MET)


The existence of a Dark Matter (DM) particle is a well-

established hypothesis that explains a range of astrophysical

and cosmological measurements.

The presence of a non-baryonic component in the universe is

inferred from the observation of its gravitational interactions

How can we study the Dark Matter?

direct detection based on scattering interaction detections

(DAMA, LUX etc.)

indirect detection experiments that look for final states given

by the DM annihilation (AMS, Ice-Cube etc.)

Pair production at LHC with large missing transverse

momentum in the detector

The detection of DM candidates in a collider can give complementary results with respect to the other DM detections.

Dark Energy 69%

Baryonic Matter 5%

Dark Matter 26%

DM

DM

SM

SM

Annihilation

Sca

tterin

g

Production

5

The Dark Matter paradigm


Selection

MET > 250 GeV leading jet pT > 250 GeV , |eta| <2.4,

tight jet cleaning njet <= 4

|dɸ(MET, jets)| > 0.4 lepton veto MET>250 GeV

jet pT>250GeV, |η|<2.4,

tight quality

e & µ veto

Up to 3 other jets (pT>30GeV)

>0.4

SR

Residual dominant backgrounds given by the Z(νν)+jets and W(τhadν)+jets processes

Z(νν)+jets 58%W(τν)+jets 18%

W(µν)+jets 9%

W(eν)+jets 8%

top 4%others 3%

Giuliano Gustavino

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) Control Regionµµ AZ(>250 GeV miss

T>250 GeV, E

Tp

Data 2015Standard Model

) + jetsii AZ() + jetsio AW() + jetsiµ AW() + jetsi eAW(

ll) + jetsAZ(Dibosons

+ single toptt

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Analysis strategy3 control regions are defined (1µ, 2µ, 1e) to evaluate the V+jets backgrounds. ➡ to reduce the uncertainties due to the MC

modeling

Z(νν) W(µν)

~ Z(νν)*, W(µν) CR1µW(τν), W(eν), Zττ CR1eZ(µµ) CR2µZ(ee), diboson, top MCMultijet and NCB data-driven

systematic for the W/Z ratio vs pT and the EW/QCD corrections differences

MET ~ boson pT muons treated as

invisibles in the MET calculation

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610ATLAS

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>250 GeV missT

>250 GeV, ET

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+ single toptt

[GeV]missTE

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Shape binned fitMET binned simultaneous fit is performed to exploit the shape information increasing the sensitivity between signals and background

k1µ(i) x Z(νν)+jets W(µν)+jets

W(τν)+jets W(eν)+jets Z(ττ)+jets

Z(µµ)+jets + k2µ(i) x + k1e(i) x

[TeV]medM1−10 1

µU

pper

lim

it on

2−10

1−10

1

10

210-1=50GeV Exclusion 5 fbχm

Simplified shape fit 15 bins + overflow

Best counting

ATLAS SimulationInternal

The signal strength limit on DM samples improves by ~20% over

the best counting experiment (inclusive MET region).

where i = 1,..,Nbin

7 bins used in the final results: METϵ[250,300,350,400,500,600,700,∞]GeV

3 normalization factors for each bin are applied (1 for each CR)


ResultsEv

ents

/ 50

GeV

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Signal Region>250 GeV miss

T>250 GeV, E

Tp




+ single toptt) = (350, 345) GeV0

r¾, b~m()= (150, 1000) GeV

med, M

DM(m

=5600 GeVD

ADD, n=3, M

[GeV]missTE

400 600 800 1000 1200 1400

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Dominant uncertainties (total 4-12%): statistical (3-10%), top (~3%), boson+jet modeling (2-5%)

low MET bins ➡ systematics unc. dominates high MET bins ➡ statistical unc. dominates.

Interpret results as limits

Good agreement is observed between data and MC expectations.


DM interpretationResults interpretation: axial vector mediator, gq=0.25, gDM=1 (as recommended by the LHC Dark Matter Working group arXiv:1603.04156)

Contour Limit in the 2D plane DM vs Mediator mass

Limit on DM-proton scattering cross-section.

DM

DM

q

q jet

JP=1-

gq gDM

LHC limit gives complementary results wrt direct detection experiments

http://arxiv.org/abs/1603.04156


DM interpretationOn-shell

- high xsecs - LHC exclusion

Off-shell - low xsec

- relic DM underproduced

Heavy mediator - production suppressed (σSD~Mmed-4)

- relic DM overproduced

On-shell

Off-shell

Heavy mediator

Heavy mediator

On-shell Off-shell


Mono-jet vs Dijets searchesThe mediator can decay back to quarks

dijet signatures limit almost independent with the DM masses

Complementary results between the two kind of searches Lower mediator-SM coupling and higher mediator-DM coupling increase the relevance of the mono-jet results

q

q

q

q

Z’gq gq


Future improvements

[GeV]Am0 500 1000 1500 2000

[GeV

]χ

m

0

200

400

)expσ 1 ±Expected limit (

Perturbativity Limit

Relic Density

ATLAS Internal-1 = 25 fbintL

Axial Vector MediatorDirac Fermion DM

= 1.0χ

= 0.25, gqg95% CL limits

χ

= 2

mAm

b-tag

3.2 fb-1 25 fb-1

CR1ph, more bins

more statistics expected increase of more than a factor 10 for the end of the year

more MET bins increase sensitivity at high MET

reducing low MET uncertainty by using CR1mu+bjets to evaluate the top background

(10-15% improv. in the low MET unc.)

reducing high MET uncertainty by introducing a new CR1gamma (similar to CR1mu) to evaluate Z(vv)+jets (10% improv. in the high MET unc.)

Only a subset of the signal samples used to produce the projection limits


New fitting strategies

2D fit (MET, Njets) MET-dependent NFs for each MET bin Njet binning scheme: # jets: [1,2,3,4]

Nje

ts

MET

Goal: exploit the other useful informations of the final state

[GeV]Am0 500 1000 1500 2000

[GeV

]χ

m

0

200

400

Expected Limits with 1D Fit

Expected Limits with 2D Fit

ATLAS Internal-1 = 13 TeV, 3.2 fbs

Axial Vector MediatorDirac Fermion DM

= 1.0χ

= 0.25, gqg95% CL limits

Limits improvement of 20-35% observed wrt the standard strategy

2D fit (MET, BDT) BDT exploits also other final state informations (such as Njets, jets direction, angular variables between the jets and the MET)

to build a 1D discriminant MET-dependent NFs for each MET bin

BDT

MET

Average gains obtained on the limit of 18%


Conclusions

Many improvements have been ideated and implemented in the mono-jet analysis based on the data samples of 3.2 fb-1 at √s=13 TeV.

The level of agreement between SM predictions and data has been translated in the DM (and other) scenarios (arXiv:1604.07773).

Complementarity of mono-jet, di-jet analysis and DD experiments ➡ leading role in the search for DM particles with low mass (<10 GeV).

The new amount of data will give the unique opportunity to look at never reached high MET distribution.

Increase of sensitivity for the next analysis generation: the decrease of the main uncertainties in the low and high MET spectrum exploiting other information in the final state

Dark Matter may be around the corner, it's just us find it!

NEW

http://arxiv.org/abs/1604.07773

Backup Slides

Giuliano Gustavino

Other SR distributions

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ll) + jets→Z(Dibosons


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Giuliano Gustavino

Monojet results

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250 GeV < MET < 300 GeV MET > 700 GeV

Giuliano Gustavino

Monojet Limits

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Mono-jet limits with fixed mediator & DM mass and variable coupling

Contour Limit in the 3D plot with DM vs Mediator mass vs µ UL @95%


SUSY interpretationsSUSY compressed scenarios

e.g. stop decays with small Δm = mstop-mLSP: signature: ISR + decay of two squarks

stop (t+LSP), sbottom (b+LSP), light squark (q+LSP).

[GeV]1t

~m260 280 300 320 340 360 380 400 420 440

[G

eV

]10

χ∼m

150

200

250

300

350

400

450

-1 = 13 TeV, 3.2 fbs

1

0χ∼ c→

1t~

production, 1t~1t~

All limits at 95% CL

ATLAS

)theory

SUSYσ1 ±Observed limit (

)expσ1 ±Expected limit (

= 8 TeVsATLAS

c + m

0

1χ∼ < m1t~m

W + mb + m

0

1χ∼ > m1t~m

[GeV]1b

~m100 150 200 250 300 350 400

[G

eV

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χ∼-m 1

b~m

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)theory



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1

0χ∼ b→

1b~

production, 1

b~

1b~


ATLAS

[GeV]q~

m

400 450 500 550 600 650 700

[G

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χ∼-m q~

m

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ATLAS


LED interpretations

Large Extra Dimensions Extra spatial dimensions proposed as a way to

solve the hierarchy problem. Jet + graviton modes escaping detection

Number Of Extra Dimensions

2 3 4 5 6

Lo

we

r L

imit

[Te

V]

DM

3

4

5

6

7

8-1=13 TeV, 3.2 fbs


ATLAS)expσ 1 ±Expected Limit (

Observed Limit

Obs. Limit (after damping)

-1fb TeV, 20.3 = 8sATLAS


µCR1EM1µ

µCR1EM2µ

µCR1EM3µ

µCR1EM4µ

µCR1EM5µ

µCR1EM6µ

µCR1IM7µ

Valu

e af

ter t

he fi

t

0.4

0.6

0.8

1

1.2

1.4

1.6

ATLAS-1 = 13 TeV, 3.2 fbs

Internal

CR1eEM1µ

CR1eEM2µ

CR1eEM3µ

CR1eEM4µ

CR1eEM5µ

CR1eEM6µ

CR1eIM7µ

Valu

e af

ter t

he fi

t

0.4

0.6

0.8

1

1.2

1.4

1.6


Internal

µCR2EM1µ

µCR2EM2µ

µCR2EM3µ

µCR2EM4µ

µCR2EM5µ

µCR2EM6µ

µCR2IM7µ

Valu

e af

ter t

he fi

t

0.4

0.6

0.8

1

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1.4

1.6


Internal

Normalization factors


Strategy comparison

0

0,05

0,1

0,15

0,2

250-300 400-500 700-800 > 1000

2015-likeCR2lCR1phCR1mubtag

MET [GeV]

rela

tive

unce

rtain

ty

Binning [250,300,350,400,500,600,700,800,900,1000,inf] GeV

CR1ph based strategy improves the high MET bins total unc. (~10%)

add other high met bins?

BLIND

Lint = 25 fb-1

CR2l based strategy suffers of

low statistics

b-tag based strategy yields

to an improvement in the low

MET regions (~10-15%)

Standard strategy CR2l based b-tag based CR1ph based

Z(vv)+jets from CR1mu Z(vv)+jets from CR2l top bkg from CR1mubtag Z(vv)+jets from CR1ph

CR1mu,CR1e,CR2mu CR1mu,CR1e,CR2mu,CR2e CR1mubveto, CR1mubtag,CR1e, CR2mu CR1ph,CR1mu,CR1e,CR2mu

search for new physics in mono-jet final states in pp collisions at … · 2017-03-14 · atlas...

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