SUSY searches at LHC and HL-LHC

perspectives
Maximilian Goblirsch-Kolb, on behalf of the ATLAS and CMS collaborations

26.10.2017, LCWS 2017, Strasbourg

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 SUSY particle production in p-p collisions
Two main directions for searches

Strong production of squarks and gluinos Electroweak production

• QCD production – high cross-sections • Electroweak cross-sections – smaller

➢ High expected production rates even rates than for strong production
for massive particles (several TeV) • Can be dominant if squarks / gluinos
• May lead to jet-rich final states decoupled
• May lead to final states with leptons
and/or gauge/Higgs bosons

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 SUSY decay phenomenology
some exceptions!
LHC searches detect SUSY production via decay signatures
• Cascade decays to standard model particles and lighter sparticles
• Nature of cascades: Determined by initial sparticles, mass spectrum, couplings/mixing
Two distinct scenarios, based on R-Parity RP= ቊ−11 for Superpartners
for SM particles

R-Parity conservation (RPC) R-Parity violation (RPV)

• Lightest SUSY particle (LSP) stable • Only SM particles in final state
• typically assumed to be χ10
➢ escapes detector

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 LHC SUSY searches – common tools
Challenge: Separate a potential signal from vast standard model background (σinelpp(13TeV)~O(102) mb)

• RPC SUSY: Escaping Neutralino LSP – missing momentum

• p-p collisions: Total z momentum unknown - use transverse component, ETMiss
• complex observable: Rely on precise and efficient reconstruction of all physics objects
• particularly challenging: Assure robustness against pileup...

• Cascade decays and high SUSY mass scales: Object multiplicity and high momenta
• Often drives trigger strategy – (multi)leptons, high-pT multijets, jets+ETMiss
• But not always the case – compressed scenarios more challenging (triggers may rely on ISR jets)

• If top/bottom squarks accessible: Flavour tagging

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Today
• Try to showcase selection of recent results – representing the main areas
• small subset of the full range – much more to see/read!
• Mostly stick to simplified model interpretations

• Topics covered today

• Top squark searches – all-hadronic, leptons, RPV-specific searches
• Inclusive strong production searches – fully hadronic and with leptons
• Searches for electroweak gaugino production – leptonic final states
• Long-lived particle searches – displaced vertices
• Prospects studies with HL-LHC

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 One main LHC search target: The top squark
Natural supersymmetry (solution to hierarchy problem): Top squark expected to be light
• Mass expected not far above TeV scale (depending on amount of “acceptable” fine-tuning)
• In reach for direct production at LHC?
Simplified RPC SUSY model: Mass hierarchy between top squark and LSP defines final state

• Dedicated searches and search regions targeting different parameter-space regions

2017-10-26 SUSY searches at LHC and HL-LHC perspectives
 arXiv:1709.04183 / submitted to JHEP
All-hadronic final states see also:
JHEP 10 (2017) 005
CMS-PAS-SUS-16-050
• Background suppression through combination of
• Lepton veto
• requirement of b-tagged jets
• Large ETMiss and jet kinematics (b-b separation, transverse mass)
• High mass splittings: Use of large-R-jets to form top candidates
• For m(tሚ)~m(t)+m(𝜒10 ): Rely on ISR activity to boost system
• Ensures presence of ETMiss for background suppression

• Important backgrounds:
Z+jets and top

• Observations in agreement with

SM prediction

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 CMS-PAS-SUS-16-052
Compressed scenarios – working with soft leptons
• Challenging: m(tሚ)~m(𝜒10 ) – soft final state objects, no ETMiss
• Again rely on ISR jet to boost top squark system – restores ETMiss
• SM decay particles still very soft – but within acceptance for
leptons

• Select events with one ISR jet candidate, at least one soft lepton
and ETMiss
• Veto presence of hard (>20 GeV) leptons or more than one additional hard
(>60GeV) jet

• Main backgrounds: pT tails in W+jets and top, non-prompt leptons

• Observation in agreement with SM prediction

• Note: Several additional searches in this phase-space region

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Top squark decays with R-Parity violation
If R-Parity is violated, new decay signatures need to be considered

One interesting case: resonant top squark decays

• Final state depends on type of R-Parity violation
• Lepton+Quark, Quark+Quark possible
• B or L violated

• No ETMiss - elude searches discussed so far

• Strategy: pair jets with leptons/jets to reconstruct resonance

candidates

• Main observables for searching a signal: Mass asymmetry between

candidates, candidate masses

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 arXiv:1710.05544 (submitted to PRD)

R-Parity violation arXiv:1710.07171 (submitted to EPJC)

Main backgrounds:
• single top, tt, Z+jets for tሚlq
• Estimated by scaling MC in control regions
• QCD multijets and tt for tሚqq
• Fully data-driven QCD, MC-based tt
• SUSY signal expected to appear as resonance in mass
distributions
• Observations consistent with absence of a signal

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 General strong production searches
In addition to direct stop production: Wide set of inclusive searches for strong production
• One main interest: Gluino pair production
➢Large cross-section if mass accessible
• Main feature: multijet signatures
• Missing transverse momentum for RPC SUSY
• Additional activity possible through cascade

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Classical RPC strong production searches
arXiv:1705.04650 (accepted by EPJC)
See also:
Phys. Rev. D 96 (2017) 032003
ATLAS-CONF-2017-025
• Searches for events with jets, ETMiss and no leptons arXiv:1708.02794 (submitted to JHEP)
• Decay chains can include intermediate, virtual top/bottom squarks
➢ Exploit via b-tagging
• High number of signal regions binned in jet, b-tag multiplicity, HT
➢ sensitive to wide range of strong production SUSY scenarios
• Example: Stransverse mass based search

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 JHEP 09 (2017) 084
Leptonic signatures in strong production See also:
EPJC 77 (2017) 144
We can also use leptons to probe for strong production signals arXiv: 1708.08232 (Submitted to PRD)
Eur. Phys. J. C 76 (2016) 683
• Intermediate top squarks, sleptons or gauginos can lead to PRL 119 (2017) 151802
lepton-rich final states arXiv:1709.09814 (submitted to PLB)
Eur. Phys. J. C 77 (2017) 578
• Example: Search for events with at least 2 same-sign leptons CMS-PAS-SUS-16-041
and jets
• When targeting RPC SUSY: Require ETMiss
• Additional tool: Effective mass – universal between RPC and RPV
• Main backgrounds: non-prompt leptons, ttV, dibosons

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Multilepton search for charginos/neutralinos
Final state leptons are an essential tool when searching for gaugino pair production
• EW cross-sections lower than for strong production
• But: SM background in lepton-rich topologies also much lower
• Leptons can come from intermediate gauge / Higgs bosons or virtual
sleptons/sneutrinos
➢ Intermediate gauge bosons: More challenging - SM leptonic W/Z BR
• Of particular importance: Hadronic τ decays - H→ττ and higgsinos

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 General multilepton search for charginos/neutralinos
• Select events with at least 3 leptons (e/µ/τ)

arXiv:1709.05406 (submitted to JHEP)

• Background contributions depend on lepton multiplicity see also:
• More pronounced fake component for τ-rich regions ATLAS-CONF-2017-039
CMS-PAS-SUS-17-004
• Diboson and triboson relevant for light-lepton final states arXiv:1709.08908 (submitted to JHEP)
CMS-PAS-SUS-16-048
arXiv:1708.07875 (submitted to EPJC)
ATLAS-CONF-2016-075

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 General multilepton search for charginos/neutralinos
• Large set of signal regions covering the phase-space
• Set limits for several decay chains and branching ratio assumptions
• Important role of τ-rich regions for Higgsino production
• Challenging nature of τ-reconstruction

WZ WH slepton
mediated

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Long-lived particle signatures
• Highly degenerate gaugino mass eigenstates or special mass hierarchies (e.g. split SUSY, weak
RPV couplings) can lead to long-lived sparticles

• Signatures include disappearing tracks, displaced vertices and others

• Require dedicated techniques for candidate reconstruction and background estimation

𝜒10 Jet
𝜒10
y
PV 𝑔෤ DV
𝑔෤
heavy Jet DV
x

ISR
private
illustration
2017-10-26 SUSY searches at LHC and HL-LHC perspectives
 Example: Displaced vertices in the ATLAS Inner Detector
ATLAS-CONF-2017-026
• Assume RPC scenario: Trigger using jet+ETMiss (requires
ISR jet) see also:
PRD 93 (2016) 112015
• Reconstruct tracks up to high transverse impact parameters PLB 760 (2016) 647-665
ATLAS-CONF-2017-017
• Search for high-mass displaced vertices consisting of at least 5 tracks
• Backgrounds: Instrumental
• Material interactions
• multiple close-by hadron decays
• hadron decay vertices crossed by unrelated tracks
• After selection “Zero-background” (0.02 expected) – data observation of 0

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 CMS-PAS-SUS-14-012
Towards the future – HL-LHC ATL-PHYS-PUB-2016-021
see also:
CMS-PAS-FTR-13-014
• Prospects for SUSY searches with the HL-LHC under study in both ATL-PHYS-PUB-2016-022
experiments ATL-PHYS-PUB-2015-032

• Reminder: Expect to gain factor ~10 in statistics, but no large energy increase (1314 TeV)
➢ Main gain expected for statistically limited searches – low masses, challenging topologies
➢ One main goal: Extend reach to still-uncovered corners of the parameter space
• Example: Gaugino discovery potential – factor 2 beyond Phase-1 in mass reach for light LSP

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 CMS-PAS-SUS-14-012
Towards the future – HL-LHC ATL-PHYS-PUB-2016-021
see also:
• Example: Direct tau slepton pair production CMS-PAS-FTR-13-014
ATL-PHYS-PUB-2016-022
• 2τ + MET signature ATL-PHYS-PUB-2015-032
• Challenging: τ reconstruction and background (fake) suppression
• Comparably small slepton production cross-section doesn’t help!
➢ current sensitivity very limited (exclusion at m𝜏෤ ~100GeV, m𝜒10 = 0)

• With 3000 fb-1, expect 5σ discovery sensitivity to 𝜏ǁ L-pair or mixed

production up to 450-500 GeV for light LSP!
• Complement possible linear collider searches for compressed scenarios

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Summary and Outlook
• Search for BSM physics is one main goal of the LHC
• Supersymmetry used as framework for developing and interpreting searches

• Extensive search program already completed, sensitivity far beyond reach of previous
colliders
• Profit from centre-of-mass energy
• Showed only small subset of results

• HL-LHC will further extend sensitivity

• Become sensitive to new signatures (e.g direct
stau)
• Double mass reach in some scenarios
• Some scenarios only with small gains
• Complementarity between HL-LHC (direct
production reach, most sensitive to large
mass splittings) and linear colliders
(compressed scenarios, indirect constraints
through precision measurements)

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Backup

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Reminder: Supersymmetry in a nutshell
Supersymmetry (SUSY): A symmetry between fermions and
bosons
Q |Fermion> = |Boson> Q |Boson> = |Fermion>

Most obvious consequence: Double the SM particle content

• ’Superpartners’ for every SM degree of freedom (’sparticles’)

Q
Experimental data: SUSY must be a broken symmetry
• SUSY breaking allows masses of sparticles to differ from SM
partners
• Also introduces wide parameter space: > 100 free parameters!

SUSY models used as benchmark for guiding development and

interpretation of BSM collider searches

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 Motivation for SUSY searches
Why is SUSY one main branch of LHC new physics
searches?

• SUSY has compelling theoretical aspects ...

• Solution to Hierarchy Problem
• Potential for DM Candidate
• Potential for B/L violation
• Mechanism for generating neutrino masses
• Note: Some of these mutually exclusive!
• Feature of string theories

• Most important: Rich and varied spectrum of predicted

signatures
• Use to drive a systematic search for BSM physics
• SUSY models as benchmark for guiding development and
interpretation of collider searches

2017-10-26 SUSY searches at LHC and HL-LHC perspectives

 LHC SUSY searches – interpretation framework
Frequently use simplified models to guide development and interpretation of searches
Bottom-up approach:
• Consider a single production process, typically producing one given pair of sparticles.
• Also fix decay of the sparticles, typically assume 100% BR (manually assigned)
➢Sparticle lifetimes also manually assigned (most common: ’prompt’, τ = 0)
• Sparticle masses are usually free and independent parameters to scan over

Useful for driving collider searches ...

• One clearly defined physics signature to look for
• Low-dimensional phase-space for exploration
➢ Often just two sparticle masses

...but keep in mind when looking at results:

• Not a full-fledged physics model!
➢Limits within simplified models give measure of the search sensitivity
➢But are valid only within a strong set of assumptions
• Think of an approximate limit on σ · BR for one signature occuring within a full-scale model!

2017-10-26 SUSY searches at LHC and HL-LHC perspectives