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)

Is there dark matter? Is dark matter a new field/ particle? Does the dark matter have (non- gravitational) interactions? 2

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. Thermal WIMP freezeout: SM χ 1 1 h σ v i / Ω cdm / M 2 h σ v i W Matches observed abundance SM χ when annihilation rate (interactions) are “weak-scale”… 3

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. [GeV] 1600 ATLAS expected limit ( ± 1 σ ± 2 σ ) q χ -1 s =8 TeV, 20.3 fb observed limit 1400 µ Initial state D5: q q χ γ χ γ * Suppression Scale M µ Thermal relic miss E >500 GeV truncated, coupling=1 T 1200 truncated, max coupling radiation to 1000 tag on dark 800 600 matter events: ¯ q χ 400 1 χγ µ χ ¯ ¯ q γ µ q 200 M 2 ∗ 0 3 2 10 10 10 4 [GeV] WIMP mass m [GeV] χ

What if dark matter is charged under new gauge interactions? • We can have a dark sector including dark matter (plus other 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? Dark Standard Model sector 5

New opportunities for dark sector searches with colliders Mass scale of dark sector: O(1) GeV - O(100) GeV • 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

Search strategies 7

size of gauge group / dark sector coupling Z 0 χ Strassler 2008 Strassler & Zurek 2006 χ U(1)’: Focus of talk today Cheung et al. 2009 MET multiplicity 8

coupling to SM states Invisible Emerging Jet Schwaller et al. 2015 ` + Non-pointing ` − photons Z’ decay Primulando et al. 2015 displaced prompt 9

mass of new mediator Z+jj Hadronic decay: events 1400 W+jj DH:m =800; =500 fb σ Z' Z’-jet 1200 DH:m =400; =500 fb σ Z' Invisible 1000 800 600 π + 400 Dijet+MET π − 200 Z’ decay 0 0 100 200 300 400 500 600 700 800 900 1000 m [GeV] jj lepton jet/ resonance narrow jet 10


 Benchmarks Relic abundance: Dark matter and U(1)’ σ v ∼ π ( α χ ) 2 m 2 χχ + g χ Z 0 χγ µ χ − m χ ¯ µ ¯ χ α χ & 5 × 10 − 5 ⇣ m χ ⌘ GeV • Massive Z’ can decay to SM states • Leptophobic couplings of light Z’, ψ could be generated by operator V like 
 1 φ † D µ φ � � (¯ u γ µ u ) V ψ Λ 2 • Could also consider kinetically Given CMB constraints, mixed Z’ (epsilon constrained to asymmetric DM for light vector, 
 1e-3 for GeV mass) or symmetric if light scalar 11

Benchmarks • Production through heavy states (hidden valley) Assume contact χ q operator O V = χγ µ χ u γ µ u Λ 2 ¯ O A = χγ µ γ 5 χ u γ µ γ 5 u ¯ q χ . Λ 2 • Part 2 (later): add a splitting (inelastic dark matter) χ ∗ q Adding dark higgs coupling / majorana mass: g χ χ 2 γ µ γ 5 χ 1 + ¯ χ 1 γ µ γ 5 χ 2 � � 2 Z 0 ¯ µ ¯ ¯ q χ 12

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 γ e + Z 0 χ Analogous to radiation from charged particles: χ e − • I will focus on single emission of a somewhat high-pT Z’. 13

U(1)’ case • Emissions of Z’ for large enough couplings and light mass scales. ✓ q 2 ◆� 2  N ∼ α χ log m 2 2 π χ 500 1 α d = 0.01 1 2 α d = 0.03 0.8 α d = 0.1 400 α d = 0.3 3 1.5 0.6 A.U. M N � 300 2.5 0.5 0.4 200 0.2 Number of dark photons 
 per dark matter particle 100 0 0.05 0.10 0.15 0.20 0 1 2 3 Number of Radiated Dark Photons Α d Cheung, Ruderman, Wang, Yavin 2009 14

Spectrum of Z’ emitted Z’ can carry away O(1) fraction of momentum Buschmann, Kopp, Liu, Machado 2015 15

with Yang Bai & James Bourbeau, 2015 Mono-Z’ jets δθ ∼ M Z 0 ∼ 10 − 3 − 10 − 2 • GeV-scale Z’ decaying to p T π + hadrons → new narrow π − jet signature in the highly Z ′ χ boosted regime p p • One way for GeV-scale Z’s to couple to SM is through kinetic mixing. ¯ χ O V = χγ µ χ u γ µ u Expect both lepton jets, Λ 2 light Z’ jets O A = χγ µ γ 5 χ u γ µ γ 5 u . Λ 2 16

Light Z’ • 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: 
 u γ µ u → π + ∂ µ π − − π − ∂ µ π + + K + ∂ µ K − − K − ∂ µ K + • Compare kinetic mixing, with constraints on ϵ down to 1e-3. Coupling of Z’ to SM fermions: L ⊃ − 1 µ ν F 0 µ ν − ✏ 8 4 F 0 2 F 0 µ ν B µ ν � ✏ c w eQ f M B 0 ⌧ M Z > < g ¯ ffZ 0 ' � ✏ g y Y f M B 0 � M Z . > : 17

Z’ coupling to quarks χ q Low mass Z’ is difficult to Z 0 search for in dijets: ¯ ¯ q χ 2.5 CDF Run I CMS 5 fb � 1 CMS 2.0 0.13 fb � 1 Low mass 
 ATLAS 1 fb � 1 UA2 leptophobic Z’s: 1.5 ATLAS g B CMS 13 fb � 1 4 fb � 1 1.0 CDF 1.1 fb � 1 Carone & Murayama 1994 • CMS 20 fb � 1 Frugiuele & Dobrescu 2014 0.5 • Tulin 2014 • … • 0.0 0 500 1000 1500 2000 2500 M Z ' B (GeV) Dobrescu & Yu 2013 18

Z’ coupling to quarks χ q Z 0 3 R Z 2 Φ 1 m f � 100 GeV ¯ ¯ q χ 0.5 0.3 g z 0.2 Low mass 
 0.1 leptophobic Z’s: 0.05 ' Z ds model 0.03 0.3 1 2 3 10 20 30 0.5 5 50 Carone & Murayama 1994 M Z ' (GeV) • Frugiuele & Dobrescu 2014 • Tulin 2014 Dobrescu & Frugiuele 2014 • … • 19

Final state radiation from dark matter mono-Z’ 1.00 p T � j or Z � � � 500 GeV 14 TeV LHC g Χ � 1.0 0.50 M Z ' � 1 GeV cross section � fb � mono � Z � 0.20 mono � jet 0.10 monojet 0.05 Χ Γ Μ Χ u Γ Μ u �� 5 TeV � 2 0.02 10 20 100 200 1000 50 500 m Χ � GeV � • Increased rate for Z’ radiation compared to 
 initial state radiation (QCD jets) 20

1.00 p T � Z � � � 500 GeV 14 TeV LHC g Χ � 1.0 �� 5 TeV 0.50 m o n cross section � fb � o � � Z � m � 1 0 Χ G e V � 0.20 Z � m o n o � � m Χ � 1 0 0 G e V � mono-jet 0.10 0.05 10 20 100 200 50 500 M Z ' � GeV � Depending on DM mass, larger rate for a range of Z’ masses 21

Light Z’-jet tagging p s = 14 TeV, M Z 0 = 1 GeV, p T > 500 GeV Pythia 8.1 Z 0 0 . 8 QCD jet Z’: Mostly 2-track Fraction of events decay due to mass 0 . 6 scale, charge conservation 0 . 4 QCD: high track 0 . 2 multiplicity Herwig 0 . 0 2 4 6 8 10 12 14 16 N track in leading ∆ R=0.2 subjet Track multiplicity - primary distinguishing variable 22

Light Z’-jet tagging p s = 14 TeV, M Z 0 = 1 GeV, p T > 500 GeV 0 . 35 P ∆ R i  0 . 4 p T,i ∆ R i Pythia 8.1 i, tracks R track = 0 . 30 Z 0 P ∆ R i  0 . 4 p T,i i, tracks QCD jet 0 . 25 Fraction of events Track radius: pT-weighted 0 . 20 track radius 0 . 15 QCD 0 . 10 Z’: small cone of 0 . 05 radiation Herwig 0 . 00 0 . 00 0 . 02 0 . 04 0 . 06 0 . 08 0 . 10 R track 23

Light Z’-jet tagging p s = 14 TeV, M Z 0 = 1 GeV, p T > 500 GeV Pythia 8.1 Z 0 0 . 25 QCD jet Z’: low mass Fraction of events 0 . 20 preferred QCD: larger 0 . 15 invariant mass at higher pT 0 . 10 0 . 05 Herwig 0 . 00 0 10 20 30 40 50 Track jet mass (GeV) 24

Light Z’-jet tagging p s = 14 TeV, M Z 0 = 1 GeV, p T > 500 GeV 1 . 0 Pythia 8.1 Z 0 P ∆ R i < 0 . 1 p i i T 0 . 8 f core ⌘ P ∆ R i < 0 . 2 QCD jet p i i T Fraction of events 0 . 6 pT-fraction of 
 leading subjet 0 . 4 0 . 2 Herwig 0 . 0 0 . 75 0 . 80 0 . 85 0 . 90 0 . 95 1 . 00 f core in ∆ R=0.1 25

Jet-pT smearing • Track-based observables, 5% uncertainty on track pT: √ s = 14 TeV, M Z 0 = 1 GeV, p T > 500 √ s = 14 TeV, M Z 0 = 1 GeV, p T > 500 1 . 0 Smeared No smearing/ISR/FSR 1 . 0 No smearing/ISR/FSR Smeared 0 . 8 0 . 8 Events /2 GeV 0 . 6 Events 0 . 6 0 . 4 0 . 4 0 . 2 0 . 2 0 . 0 0 . 0 0 10 20 30 40 50 0 . 00 0 . 02 0 . 04 0 . 06 0 . 08 0 . 10 Track jet mass (GeV) R track Track jet mass Track radius 26

Z’-jet tagging p s = 14 TeV, M Z 0 = 1 GeV • For default cuts, 10 0 can reject QCD jets at high Z 0 significance QCD jet E ffi ciency 10 − 1 N track < 4 m track < 20 GeV To estimate R track < 0.02 f core > 0.9 improvement in sensitivity, we take 10 − 2 efficiencies as 
 50% signal, 
 200 300 400 500 600 1% background p T [GeV] 27