Dark sectors and missing energy searches at the LHC Tongyan Lin UC - - PowerPoint PPT Presentation

Dark sectors and missing energy searches at the LHC

Tongyan Lin UC Berkeley / LBL
 October 9, 2015 GGI workshop, “Gearing up for LHC13”

with M. Autran, K. Bauer, D. Whiteson (1504.01386)
 with Y. Bai, J. Bourbeau (1504.01395)

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Is there dark matter? Does the dark matter have (non- gravitational) interactions? Is dark matter a new field/ particle?

Does dark matter have gauge interactions?

• Interactions mediated by SM gauge bosons are

highly constrained, if we want those same interactions to set a thermal relic abundance.

3

Thermal WIMP freezeout:

χ χ

SM SM

Ωcdm / 1 hσvi

Matches observed abundance when annihilation rate (interactions) are “weak-scale”…

hσvi / 1 M 2

W

Does dark matter have gauge interactions?

• The framework of WIMP dark matter has guided

many dark matter searches, but has not yielded any clear signals in direct detection, indirect detection, or colliders.

4

1 M 2

∗

¯ χγµχ¯ qγµq

χ χ q ¯ q

Initial state radiation to tag on dark matter events:

[GeV] [GeV]

χ

WIMP mass m 10

2

10

3

10 [GeV]

*

Suppression Scale M 200 400 600 800 1000 1200 1400 1600

) σ 2 ± σ 1 ± expected limit (

• bserved limit

Thermal relic truncated, coupling=1 truncated, max coupling

ATLAS

• 1

fb TeV, 20.3 =8 s q

γ q χ

γ χ D5: GeV >500

E

What if dark matter is charged under new gauge interactions?

5

Dark sector

Standard Model

• We can have a dark sector including dark matter (plus
• ther states) and dark gauge group.
• SM states neutral, talk to dark sector by weak

coupling or high mass scale.

• How can this scenario be probed in experiments?

New opportunities for dark sector searches with colliders

• Qualitatively new signals from hidden sector dynamics
• There can be radiation of new gauge bosons,

including from the dark matter itself.

• Many cases can be experimentally challenging…

motivates understanding of data, SM better.

6

Mass scale of dark sector: O(1) GeV - O(100) GeV

Search strategies

7

8

size of gauge group / dark sector coupling

multiplicity MET

Z0

χ χ

U(1)’: Focus of talk today

Cheung et al. 2009

Strassler 2008 Strassler & Zurek 2006

9

coupling to SM states prompt displaced

Non-pointing photons

Emerging Jet

Schwaller et al. 2015 Primulando et al. 2015

Invisible

Z’ decay

`+ `−

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mass of new mediator resonance

lepton jet/ narrow jet

Invisible

Z’ decay

π+ π−

[GeV]

jj

m

100 200 300 400 500 600 700 800 900 1000

events

200 400 600 800 1000 1200 1400

Z+jj W+jj =500 fb σ =800;

Z'

DH:m =500 fb σ =400;

Z'

DH:m

Hadronic decay: Z’-jet Dijet+MET

Benchmarks

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Dark matter and U(1)’

• Massive Z’ can decay to SM states
• Leptophobic couplings of light Z’,

could be generated by operator like
 


• Could also consider kinetically

mixed Z’ (epsilon constrained to 1e-3 for GeV mass)

Given CMB constraints, asymmetric DM for light vector,


• r symmetric if light scalar

σv ∼ π(αχ)2 m2

χ

αχ & 5 × 10−5 ⇣ mχ GeV ⌘

Relic abundance:

ψ ψ V V

−mχ ¯ χχ + gχZ0

µ ¯

χγµχ

1 Λ2

• φ†Dµφ
• (¯

uγµu)

Benchmarks

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• Production through heavy states (hidden valley)

Assume contact

• perator

OV = χγµχ uγµu Λ2

OA = χγµγ5χ uγµγ5u Λ2 .

q ¯ q χ ¯ χ

• Part 2 (later): add a splitting (inelastic dark matter)

gχ 2 Z0

µ

• ¯

χ2γµγ5χ1 + ¯ χ1γµγ5χ2

• q

¯ q ¯ χ χ∗ Adding dark higgs coupling / majorana mass:

Radiation from dark matter

• Final state radiation of DM in colliders - especially important

if the dark matter is light and there is also a light force carrier

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Analogous to radiation from charged particles:

γ e+ e−

Z0

χ χ

• I will focus on single emission of a somewhat high-pT Z’.

1 2 3 Number of Radiated Dark Photons 0.2 0.4 0.6 0.8 1 A.U.

αd = 0.01 αd = 0.03 αd = 0.1 αd = 0.3

0.5 1 1.5 2 2.5 3

0.05 0.10 0.15 0.20 100 200 300 400 500 Αd MN

• U(1)’ case
• Emissions of Z’ for large enough couplings and light

mass scales.

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Cheung, Ruderman, Wang, Yavin 2009

Number of dark photons
 per dark matter particle

N ∼ αχ 2π  log ✓ q2 m2

χ

◆2

Spectrum of Z’ emitted

15

Buschmann, Kopp, Liu, Machado 2015

Z’ can carry away O(1) fraction of momentum

Mono-Z’ jets

• GeV-scale Z’ decaying to

hadrons → new narrow jet signature in the highly boosted regime

• One way for GeV-scale

Z’s to couple to SM is through kinetic mixing. Expect both lepton jets, light Z’ jets

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with Yang Bai & James Bourbeau, 2015

p p ¯ χ χ Z′ OV = χγµχ uγµu Λ2

OA = χγµγ5χ uγµγ5u Λ2 .

π− π+

δθ ∼ MZ0 pT ∼ 10−3 − 10−2

Light Z’

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• Assume Z’ has small coupling to SM fermions, with a

prompt decay on collider scales as long as coupling is larger than roughly 1e-5

• Distinguishing variables not very sensitive to model (Z’

decay) specifics. For example: 


• Compare kinetic mixing, with constraints on ϵ down to

1e-3.

uγµu → π+∂µπ− − π−∂µπ+ + K+∂µK− − K−∂µK+

L ⊃ −1 4F 0

µνF 0µν − ✏

2F 0

µνBµν g ¯

ffZ0 '

8 > < > : ✏cweQf MB0 ⌧ M Z ✏gyYf MB0 M Z.

Coupling of Z’ to SM fermions:

Z’ coupling to quarks

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Low mass 
 leptophobic Z’s:

• Carone & Murayama 1994
• Frugiuele & Dobrescu 2014
• Tulin 2014
• …

500 1000 1500 2000 2500 0.0 0.5 1.0 1.5 2.0 2.5 MZ'B (GeV)

gB

UA2 CDF Run I CDF 1.1 fb1 CMS 0.13 fb1 ATLAS 1 fb1 CMS 4 fb1 CMS 5 fb1 CMS 20 fb1 ATLAS 13 fb1

Dobrescu & Yu 2013

Low mass Z’ is difficult to search for in dijets: Z0 χ ¯ χ q ¯ q

Z’ coupling to quarks

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Low mass 
 leptophobic Z’s:

• Carone & Murayama 1994
• Frugiuele & Dobrescu 2014
• Tulin 2014
• …

Z0 χ ¯ χ q ¯ q

0.3 0.5 1 2 3 5 10 20 30 50 0.03 0.05 0.1 0.2 0.3 0.5 1 2 3

MZ' (GeV)

gz

m f 100 GeV

Φ

RZ

Zds model '

Dobrescu & Frugiuele 2014

Final state radiation from dark matter

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10 20 50 100 200 500 1000 0.02 0.05 0.10 0.20 0.50 1.00

mΧ GeV cross section fb

monoZ monojet gΧ1.0 MZ'1 GeV pTj or Z500 GeV 14 TeV LHC 5 TeV Χ ΓΜ Χ u ΓΜ u 2

• Increased rate for Z’ radiation compared to 


initial state radiation (QCD jets)

monojet mono-Z’

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10 20 50 100 200 500 0.05 0.10 0.20 0.50 1.00

MZ' GeV cross section fb

m

• n
• Z
• m

Χ

• 1

G e V

• m
• n
• Z
• mΧ
• 1

G e V

• gΧ1.0

pTZ500 GeV 14 TeV LHC 5 TeV

Depending on DM mass, larger rate for a range of Z’ masses

mono-jet

2 4 6 8 10 12 14 16

Ntrack in leading ∆R=0.2 subjet

0.0 0.2 0.4 0.6 0.8

Fraction of events

Herwig

ps = 14 TeV, MZ0 = 1 GeV, pT > 500 GeV

Pythia 8.1

Z0 QCD jet

Light Z’-jet tagging

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Z’: Mostly 2-track decay due to mass scale, charge conservation

Track multiplicity - primary distinguishing variable

QCD: high track multiplicity

0.00 0.02 0.04 0.06 0.08 0.10

Rtrack

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Fraction of events

Herwig

ps = 14 TeV, MZ0 = 1 GeV, pT > 500 GeV

Pythia 8.1

Z0 QCD jet

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Light Z’-jet tagging

Z’: small cone of radiation QCD

Rtrack = P∆Ri0.4

i,tracks

pT,i ∆Ri P∆Ri0.4

i,tracks

pT,i

Track radius: pT-weighted track radius

10 20 30 40 50

Track jet mass (GeV)

0.00 0.05 0.10 0.15 0.20 0.25 Fraction of events

Herwig

ps = 14 TeV, MZ0 = 1 GeV, pT > 500 GeV

Pythia 8.1

Z0 QCD jet

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QCD: larger invariant mass at higher pT Z’: low mass preferred

Light Z’-jet tagging

0.75 0.80 0.85 0.90 0.95 1.00

fcore in ∆R=0.1

0.0 0.2 0.4 0.6 0.8 1.0

Fraction of events

Herwig

ps = 14 TeV, MZ0 = 1 GeV, pT > 500 GeV

Pythia 8.1

Z0 QCD jet

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Light Z’-jet tagging

fcore ⌘ P∆Ri<0.1

i

pi

T

P∆Ri<0.2

i

pi

T

pT-fraction of
 leading subjet

Jet-pT smearing

• Track-based observables, 5% uncertainty on track pT:

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Track radius Track jet mass

10 20 30 40 50 Track jet mass (GeV) 0.0 0.2 0.4 0.6 0.8 1.0

Events /2 GeV

√s = 14 TeV, MZ0 = 1 GeV, pT >500 Smeared No smearing/ISR/FSR

0.00 0.02 0.04 0.06 0.08 0.10 Rtrack 0.0 0.2 0.4 0.6 0.8 1.0

Events

√s = 14 TeV, MZ0 = 1 GeV, pT >500 No smearing/ISR/FSR Smeared

Z’-jet tagging

• For default cuts,

can reject QCD jets at high significance

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To estimate improvement in sensitivity, we take efficiencies as 
 50% signal, 
 1% background

200 300 400 500 600

pT [GeV]

10−2 10−1 100 Efficiency

Ntrack < 4 mtrack < 20 GeV Rtrack < 0.02 fcore > 0.9

ps = 14 TeV, MZ0 = 1 GeV Z0 QCD jet

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2 4 6 8 10

MZ0 [GeV]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Efficiency

mtrack < 20 GeV Rtrack < 0.02 fcore > 0.9

√s = 14 TeV, pT > 500 GeV Ntrack ≤ 6 Ntrack ≤ 4 Ntrack ≤ 2

Range of values: test of model- dependence 
 in PYTHIA

Z’-jet tagging

Tagging can be implemented for a range of “GeV-scale” Z’ masses e.g. axial vs vector coupling, isospin- violating vs coupling

• nly to u-quarks

2 4 6 8 10

MZ0 [GeV]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Efficiency

mtrack < 20 GeV Rtrack < 0.02 fcore > 0.9

√s = 14 TeV, pT > 500 GeV Ntrack ≤ 6 Ntrack ≤ 4 Ntrack ≤ 2

Hadronic 훕h

• (By definition) look very similar to GeV-scale Z’s
• W* →훕hν is also background, but sub-dominant for large

MET, and could be further suppressed by vetoing 1- and 3-prong events

• Single hadronic tau trigger → Z’-jet trigger?

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2 4 6 8 10 12 14 16 Ntrack in leading R=0.2 subjet 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Events

ps = 14 TeV, MZ0 = 1 GeV, MET>500 Z0 QCD jet hadronic τ

0.00 0.02 0.04 0.06 0.08 0.10 Rtrack 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Events

ps = 14 TeV, MZ0 = 1 GeV, MET>500 Z0 QCD jet hadronic τ

10 20 30 40 50 Track jet mass (GeV) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Events /2 GeV

ps = 14 TeV, MZ0 = 1 GeV, MET>500 hadronic τ QCD jet Z0

Z’+MET sensitivity

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More monojets With light Z’-tagging

Assumed 10% systematic uncertainty on background

• New collider signal
• Significant increase 


in sensitivity

• LHC probe of light

DM with dark force

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5 10 50 100 500 MZ'@GeVD 0.001 0.005 0.01 ac L=1.25 TeV, s =14 TeV, mc=10 GeV

• We can probe

perturbative gauge couplings.

• Projected coupling

sensitivity, fixing the contact operator scale at the monojet limit:

Plot assumes MET > 500 GeV plus light Z’ tagging

Z’+MET sensitivity

Inelastic dark matter

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• L ⊃ ¯

χ(i / D − Md)χ − Mm 2 (¯ χχc + h.c.).

M1,2 = |Mm ± Md|.

Off-diagonal coupling can be generated by Dirac fermion DM + Majorana mass:

Can probe co-annihilating thermal relic region 
 (w/ displaced leptons):

Izaguirre et al. 2015

p p j DM∗ DM DM , `+`− . . . ← − cDM∗

Displaced pion: 
 Bai & Tait 2011

—> primarily off-diagonal couplings

• f Z’, no associated monojet signal

gχ (χ⇤γµχ + χγµχ⇤) Z0

µ .

(χ⇤γµχ + χγµχ⇤) uγµu p 2 Λ2 p

Mm ⇠ yχhΦi

Splitting easily has similar scale as gauge bosons:

Inelastic dark matter

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10 20 50 100 200 500 1000 0.02 0.05 0.10 0.20 0.50 1.00 2.00

mΧ GeV cross section fb

monoZ monojet gΧ1.0 MZ'1 GeV pTj or Z500 GeV 14 TeV LHC 5 TeV Χ ΓΜ Χ Χ ΓΜ Χ u ΓΜ u 2 2

2 → 3 process (휒* is off-shell) 2 → 2 process 
 (휒* decays to Z’+휒)

q ¯ q χ1 χ2 χ1 Z′

Mass splitting is ∆ = 2 GeV

Improvement in sensitivity for inelastic dark matter

34

More monojets With light Z’-tagging

Assumed 10% systematic uncertainty on background

Dijet & dilepton resonances

• In the inelastic case, heavier Z’ can be produced

in the decay of the excited state without kinematic suppression

• We consider the mass range: MZ’ = 50-800 GeV
• The signal is not captured as efficiently by

existing MET-based searches, and the presence

• f a resonance would be a strong indication of

new particles/physics

35

with M. Autran, K. Bauer, D. Whiteson 2015

Dijet resonances plus MET

• We consider the mass range: MZ’ = 50-800 GeV

36

[GeV]

jj

m

100 200 300 400 500 600 700 800 900 1000

events

200 400 600 800 1000 1200 1400

Z+jj W+jj =500 fb σ =800;

DH:m =500 fb σ =400;

DH:m

jj+MET

• Not covered well with standard jets

+MET (squark) search, which has strong requirements on HT, etc.


!

• Possible gains with mass window,

different jet pT and MET cuts

Dijet resonances

Dijet resonance sensitivity

37

50 100 800 MZ0 [GeV] 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Λ [TeV]

jj+MET, 8 TeV limits, Inelastic EFT

mχ1 light mχ1 heavy

Scale of contact

• perator

Z’ mass

Z0

q ¯ q χ ¯ χ χ∗

“Light” 휒: “Heavy” 휒:

; ∆ = 25 GeV

Two benchmark spectra, allowing on-shell 휒* decay:

Mχ = 5 GeV, M

Mχ = MZ0/2, M

, Mχ⇤ = Mχ + MZ0 + ∆

, Mχ⇤ = 2MZ0

monojet bounds at Λ~TeV

• We consider the mass range: MZ’ = 50-800 GeV

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For more models and focus on dilepton resonance, see: 


• A. Gupta, R. Primulando, P. Saraswat arXiv:1504.01385

Dimuon resonances

• Overlap with chargino search


!

• Improvements possible with finer

mass window, more stringent MET

• r pT(μμ) cuts

[GeV]

ll

m

50 100 150 200 250 300 350 400

events

2 4 6 8 10 12 14 16 18

ZZ WW WZ γ Z t t =4 fb σ =50;

IFT:m =4 fb σ =150;

IFT:m =4 fb σ =200;

IFT:m

μμ+MET

Dilepton resonances plus MET

39 50 100 800 MZ0 [GeV] 1 2 3 4 5 6 7 8 Λ [TeV]

µµ+MET, 8 TeV limits, Inelastic EFT

mχ1 light mχ1 heavy

Shown for BR(muons)=100%
 For “kinetic mixing” case, 
 횲≈3 TeV instead

Dimuon resonance sensitivity

Z’ mass

Scale of contact

• perator

Summary

• Dark matter can radiate dark gauge

bosons in high energy collisions

• For small gauge coupling, still have

signals with substantial missing energy, but also other pheno:

• GeV-scale Z’ decaying hadronically

are a new collider object (mono-Z’ jet)

• Significant increase in sensitivity

compared to ISR monojets

40

흌

Z’

j j

흌

Z’

π− π+

41

Gauge interactions in the dark sector lead to novel LHC signals of radiation from the dark sector, where the data has not been fully

• analyzed. New opportunities

and challenges!

Conclusions

p p ¯ χ χ Z′