Searches for long-lived particles, lepton-jets, stable and meta

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Searches for long-lived particles, lepton-jets, stable and meta-

stable particles with the ATLAS detector

Matthew King IFIC, Valencia

03/07/2014

1

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Overview of analyses

1. Metastable gluinos [8TeV, 20fb^-1] ATLAS-CONF-2014-037

2. Muon + Displaced vertex [8TeV, 20fb^-1] ATLAS-CONF-2013-092

3. Disappearing tracks [8TeV, 20fb^-1] Phys. Rev. D 88 112006

4. Long-lived neutral particles [8TeV, 20fb^-1] ATLAS-CONF-2014-041

5. Long-lived sleptons [8TeV, 16fb^-1] ATLAS-CONF-2013-058

6. Long-lived, stopped, out of time R-hadrons [7TeV, 5fb^-1 + 8TeV, 23fb^-1]

Phys. Rev. D 88 112003

7. Non-pointing photons [7TeV, 4.8fb^-1] Phys. Rev. D 88, 012001

8. Long-lived weakly-interacting particles [7TeV, 1.9fb^-1] Phys.Rev.Lett. 108 251801

9. Displaced muonic jets [7TeV, 1.9fb^-1] Phys. Lett. B 721 32-50

10. Prompt lepton-jets [7TeV, 5fb^-1] Phys. Lett. B 719 299-317

03/07/2014 Matthew King, IFIC

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Lepton-jet Long-lived particle

In backup slides

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Long-lived particles (LLPs)



LLP lifetimes can originate from:

 Nearly conserved symmetry

 Low coupling between the particle and the final state

 Mass degeneracy between the particle and the final state



LLPs typically decay into an undetectable exotic (dark matter candidate), SM particles, or some combination



Different lifetimes give rise to different detector signatures:



 e



Missing momentum primary

vertex LLP secondary vertex

low ^β

high dE/dx Short decay length (~10mm)

Displaced vertex

Medium decay length (~100mm) Disappearing/Kinked track

Long decay length (>~1000mm) Stable massive particle (SMP)

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Metastable Gluinos

 Two ATLAS searches for promptly decaying SUSY are re-interpreted in the context of models with metastable ( ~ps – ns lifetime) gluinos

 “7 – 10 jets” analysis arxiv:1308.1841

 19 signal regions based on: [7, 8, 9, ≥10] jets, [0,1, ≥2] b-tagged jets , E_T^miss

 “2 – 6 jets” analysis arxiv:1405.7875

 15 signal regions based on: [2, 3, 4, 5, ≥6] jets, E_T^miss

 Decay models considered

 stop is lightest squark



 squarks are mass degenerate



4

0 1

~

~ tt g 

0 1

~ qq~ g 

0 1

~

~ g



g 

8TeV , 20fb

^-1

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Metastable Gluinos

 For ttbar decay model, both analyses can be used to set limits

 For qqbar/g decay model the 7-10 jet analysis has no sensitivity

 Jet multiplicity too low

 Drop of signal acceptance with longer lifetime is mostly caused by the

“Charged fraction” jet requirement

5

Excluded for τ=1ns, = 100GeV:

ttbar: < 900GeV qqbar/g: < 850

mg^~

0 1

~

m

mg^~

8TeV , 20fb

^-1

ttbar

qqbar/g

ATLAS-CONF-2014-037

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Muon + Displaced vertex

 Analysis conducted in the framework of long-lived neutralino decay in RPV SUSY

 Search for displaced vertices inside the inner detector

 Vertex requirements:

 Must be inside r<180mm, |z|<300mm fiducial region

 Must have matching muon candidate with p_T>55GeV and |d₀|>1.5mm

 Signal region requires:

 High vertex mass to reduce background

 High number of tracks to reduce fake vertex rate

 Exclusions depend on model parameters (masses, cτ)

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Excluded:

σ×BR > 0.8 ― 5.4fb

8TeV , 20fb

^-1

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Disappearing tracks

 Analysis considers AMSB models with mass degenerate charginos and neutralinos

 Leads to chargino LLPs that “disappear”

 Search for tracks with:

 Inner silicon detector: Well measured

 Outer straw tracks (TRT): Low (<5) number of hits

 Must be isolated, p_T>75GeV

 Background sources

 Interacting hadrons

 Non-identified leptons

 Mismeasured p_T tracks

 In this analysis results can be interpreted in terms of Δm_χ as well as cτ

7

Excluded (in decoupled AMSB): m^~^ ^ ²⁷⁰^GeV

8TeV , 20fb

^-1

Phys. Rev. D 88 112006

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Long-lived neutral particles

 In model considered; a Higgs boson decays to two long-lived neutral particles (π_v), which each decay to a fermion-antifermion pair

 Events are selected using the specialist LLP Cal-ratio trigger

 searches for jet with high Had:EM calorimeter energy ratio jets

 Require exactly 2 jets with:

 Both jets:

 log₁₀(E_H/E_EM) > 1.2

 No good tracks in ID with p_T>1GeV

 At least one jet matching the Cal-ratio trigger

 Leading jet E_T > 60GeV

8

8TeV , 20fb

^-1

v

v _f^f

h0

m_h0 (GeV)

m_πv (GeV)

Excluded cτ (m) with 30% BR

126 10 0.10 - 4.38 0.13 - 2.30

126 25 0.27 – 10.01 0.37 - 5.12

126 40 0.54 - 12.11 0.86 - 5.62

Exclusions for different BR ( ) h⁰ _v_v

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Long-lived sleptons

 Analysis searches for charged LLPs with cτ > 1m

 Interpreted in the context of GMSB stau

 Directly reconstruct the LLP and measure β and βγ

 βγ from dE/dx measurements in the pixel detectors

 β from Time of Flight measurements in calorimeter and MS

 Require two LLP candidate tracks with p_T>50GeV and β<0.95

 Signal region is set by calculating candidate masses and applying a mass cut

 Cut value depends on mass in GMSB model being evaluated

 Background is primarily muons with mis-measured β / βγ

 Excluded mass depends on model tanβ parameter

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Excluded: < 347 ― 402GeV ^m^^~¹

8TeV , 16fb

^-1

ATLAS-CONF-2013-058

β≈1 muons

1

~

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Long-lived, stopped, R-hadrons

 Analysis considers long-lived R-hadrons that “stop” in the ATLAS calorimeter

 Search for jets in “empty” bunch crossings from R-hadron decays

 Require:

 Veto events with reconstructed muon segments

 (Rejects cosmics)

 Veto events with spike-like signals in the calorimeter

 (Rejects random noise, also effective against cosmics & beam halo)

 Leading jet E > 50GeV, no more than 6 jets total

 E_T^miss / [leading jet p_T] > 0.5

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Leading jet E

Muon veto

Cosmic bkgd

Beam-halo bkgd

Total bkgd

Data

50 GeV No 4820 ± 570 900 ± 130 5720 ± 590 5396 50 GeV Yes 2.1 ± 3.6 12 ± 3 14.2 ± 4.0 10 100 GeV Yes 0.4 ± 2.7 6 ± 2 6.4 ± 2.9 5 300 GeV Yes 2.4 ± 2.4 0.5 ± 0.4 2.9 ± 2.4 0

Excluded (depends on other parameters):

< 545 ― 832 GeV < 344 ― 397 GeV ^m^q^~

mg~

before muon-veto

after muon-veto

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Summary

 Long-lived particle and Leptonic jet signatures, if observed, would be clear indications of new physics.

 However, there has been no hint of such signatures in the analyses conducted thus far.

 Further analysis of the LHC 2012 data is ongoing

 Analyses of these unusual signatures can be challenging to perform!

 Run-2 particularly represents new potential for discoveries

 Higher energy threshold and luminosity

 Detector upgrades open new avenues for analysis

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Backup slides

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Non-pointing photons

 Analysis model: SPS8 GMSB

 Long-lived decays to +

 Search for E_T^miss plus:

 Pairs of E_T > 75GeV photons not pointing back to primary vertex (can only use EM calorimeter)

 Crosscheck with any evidence of late photon detection

 Limits obtained by fitting template to the photon pointing distribution

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Pointing distribution Timing distribution

excluded for

GeV 230

0

~1  m

ns 0 . 2 4

.

0 0

~1 

_

7TeV, 4.8fb

^-1

Phys. Rev. D 88, 012001

0 1

~



_G^~

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Long-lived weakly interacting particles

 Same model as 8TeV paper

 , followed by

 Decays are identified by reconstructing vertices in the Muon Spectrometer (MS)

 Events triggered using the specialist LLP Muon RoI trigger

 searches for jet-like clusters of RoIs in MS

 Require 2 back to back, isolated vertices in MS

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v

v _f^f

h0

m_h0 (GeV)

m_πv (GeV)

120 20 0.5 - 20.65

120 40 1.60 - 24.65

140 20 0.45 - 15.8

140 40 1.10 - 26.75

v

h⁰ v _v  ff

no. of RoIs from punch-through jets

Exclusions for 100% BR ( ) h⁰ _v_v

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Lepton-jet signatures



Both recent lepton-jet analyses at ATLAS interpret results in the context of “Hidden Valley” models.



The models predict visible lepton jets arising from showers in their hidden sector.

 Final state results from mixing between “dark” photons and SM particles

 Dark sector particles are highly boosted and may produce multiple overlapping dark photons

=> 2 or more leptons produced in collimated jets



Initial decay into the hidden sector may be from:

 SUSY particles

 Higgs boson

 (Z boson)…

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Figure from: Cheung et al.,

Lepton Jets in (Supersymmetric) Electroweak Processes,

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Displaced muonic jets

 In model considered; a Higgs boson decays to two 2-muon jets via a hidden sector with long-lived γ_d

 Event requirements:

 Exactly 2 muon-jets, both with zero total charge

 Low total p_T in inner detector (Σp_T^ID<3GeV) of each jet (removes short decays)

 High isolation (E_T^isol<5GeV) of each jet, high separation (Δφ<2) between jets

 Moderate limits on impact parameters of each jet (|d₀|<200mm, |z₀|<270mm)

 Background from:

 Multi-jet events

 Secondary muons from K, π, etc…

 Exclusions:

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Higgs mass

excl. cτ(mm) at BR(100%)

excl. cτ(mm) at BR(10%) 100GeV 1 < cτ < 670 5 < cτ < 159 140GeV 1 < cτ < 430 7 < cτ < 82

7TeV, 1.9fb

^-1

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Prompt lepton-jets

 In the analysis model; pairs of squarks cascade decay through a dark sector into lepton jets

 Lepton pairs are produced from dark photons

 Energy and quantity of leptons in jet depend on:

 Dark photon mass

 Dark sector gauge coupling α_d (higher coupling → more overlapping dark photons)

 Analysis uses 3 channels:

 Single muon-jet with ≥4 muons

 Pairs of muon-jets with ≥2 muons

 Pairs of electron-jets with ≥2 electrons

 Main background contribution from fake muon-jets in multi-jet events

 Limits on applicable HV model couplings set:

(value depends on model parameters)

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2 e-jet 1 mu-jet 2 mu-jet

Data 15 7 3

bkgd (ABCD) 15.2±2.7 3.0±1.0 0.5±0.3

bkgd (tag-probe) 14.55±0.23 2.2±0.9

Excluded:

σ×BR > 0.017 ― 1.2 pb boson

q~ __~

h~d d

jet

lepton jet(s)

d

q~

7TeV, 5fb

^-1

Phys. Lett. B 719 299-317