Search for squarks and gluinos with the ATLAS detector Jeanette Lorenz (LMU München) Research Area B Science Day,
Supersymmetry Symmetry between fermions and bosons Only possible extension of Poincare space-time symmetry. In the Minimal Supersymmetric Standard Model, each Standard Model particle has a supersymmetric partner Interesting extension of the Standard Model, as: Can provide a Dark Matter candidate. Can solve the Higgs hierarchy problem (if SUSY particles not too heavy). Unification of gauge couplings possible. 2
Searches for Supersymmetry in Run 2 Extensive search program during LHC Run 1, but nothing found. LHC Run 2: Cross sections of gluinos and squarks significantly increased with respect to Run 1. 8 TeV 13 TeV: σ(~g~g) x 30 for m~g = 1.4 TeV σ(~t~t) x 8 for m~t = 800 GeV σ(~χ~χ) x 4 for m~χ = 500 GeV (while background: σ(tt) x 3.3) [http://inspirehep.net/record/1326406] Primary target gluino and squark (including stop) production, but thanks to large int. luminosity already available also sensitive to charginos/neutralinos 3
Example signatures Strong production: Possibly leptons Lightest supersymmetric particle (LSP): stable in RPC theories ETmiss Jets Gluino-mediated stop quark production Direct stop quark production Chargino production + RPV decay 4
Signal models We usually interpret in simplified models......however exclusion limits in realistic SUSY models are much weaker. [https://atlas.web.cern.ch/atlas/groups/physics/combinedsummaryplots/susy/atlas_ SUSY_PMSSM/ATLAS_SUSY_PMSSM.png] 5
Distinguish signal from background Use kinematic variables to discriminate signal from background. mef = HT + ETmiss [ATLAS-CONF-2016-095] [ATLAS-CONF-2016-078] Due to large sensitivty gain, most analysis try to use simple combination of cuts on kinematic variables ' cutand-count' 6
Background estimation methods Reducible backgrounds: backgrounds with another final state in comparison to the signal Irreducible backgrounds: backgrounds show the same final state as the signal reducible irreducible Matrix method, jet smearing... 7
Vast publication program Significant LMU (4) and MPI (2) contributions. 2015: 3.2 f-1 @ 13 TeV 13 SUSY results for winter conferences (most published as paper by now) mostly strong production 18 results for summer conferences including 13 18 fb-1 2015 + 2016 data @ 13 TeV first results of searches for charginos and neutralinos Significantly more data on tape already! 8
Searches for gluino and squark pair production
0-lepton + 2-6 jets + ETmiss [ATLAS-CONF-2016-078] Very general search sensitive to both direct decays of gluinos and squarks and to more complicated decays Signature: 0-lepton + 2-6 jets + high ETmiss/mef Two analysis parts: Mef analysis jet multiplicity, ETmiss, HT = sum of all jet pt, mef = HT + ETmiss, spherical variables New: analysis using RJigsaw variables special reconstruction technique to define kinematic variables on event-by-event level by imposing decay rules/hypotheses on the system The RJigsaw analysis part is more sensitive towards signals with small mass diferences between gluino/squark and LSP. 10
0-lepton + 2-6 jets + ETmiss: backgrounds [ATLAS-CONF-2016-078] Dominant backgrounds: tt, single top, W+jets, QCD multi-jet, Z νν Estimated using control regions with the cuts inverted or relaxed that suppress the background. SR 11
0-lepton + 2-6 jets + ETmiss: results [ATLAS-CONF-2016-078] No significant excess seen. Results of all signal regions combined by always taking the signal region with the highest sensitivity. Limits: Up to 1.35 TeV for squark pairproduction Up to 1.86 TeV for gluino pairproduction, assuming direct decays Example interpretation analysis sensitive to many more models. 12
1-lepton + 2-6 jets + ETmiss LMU contribution [ATLAS-CONF-2016-054] Cascade decays with leptons in the final state 10 diferent signal regions (6 for gluino pairproduction, 4 for squark pair-production): requiring >= 2 6 jets, ETmiss, l miss mt = 2 p lt Emiss, m ef, (1 cos([δ φ( p, p T T T )])) aplanarity + 1 isolated electron or muon, Low pt lepton (pt > 6/7 GeV) in 2-jet signal region to address compressed scenarios Main backgrounds: tt, W+jets and diboson Estimated using control regions 13
1-lepton + 2-6 jets + ETmiss: Results [ATLAS-CONF-2016-054] No excess seen. Limits up to 1.75 TeV on gluino mass, and up to 1.05 TeV on squark mass. 14
0/1 lepton + 3 b-jets [ATLAS-CONF-2016-052] Searching for sbottom and stops in gluino decay using 3 b-tagged jets. Three sets of signal regions: 0 leptons + >= 4 jets + ETmiss and mef etc. sbottom 0 leptons + >= 8 jets + ETmiss and mef etc. stop 1 lepton + >= 6 jets + ETmiss and mef etc. stop Data in good agreement with background expectation, largest deviation 2 σ. Limits reaching up to 1.9 TeV in the gluino mass. 15
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2 opposite-sign leptons Signature: 2 opposite-sign leptons (e or μ) + ETmiss 2 same-flavour leptons + >= 2 jets + HT + ETmiss + mll compatible with Z mass 'on-z search' Signal shows a kinematic edge determined by may be on, below or above Z mass searching for edge in mll, 'of-z' search (other cuts similar to 'on-z search' overlap!) 17
2 opposite-sign leptons 3 σ in ee channel in 'on-z search' in ATLAS, CMS sees nothing 2.6 σ in 'of-z' search in CMS, ATLAS sees nothing [JHEP 04 (2015) 124] Run 1, 2012 [Eur. Phys. J. C75 (2015) 318] Long history of (coming and going) excesses in these searches CMS sees no excess 'on-z', small excesses 'of-z' [CMS-PAS-SUS-16-021] 2015 ATLAS analysis sees 2.2 σ sigma excess in 2012 'on-z' SR [ATLAS-CONF-2015-082] Run 2, 2015 + 2016 18
2 opposite-sign leptons [ATLAS-CONF-2016-098] 2015+2016: 14.7 f-1, new, unchanged 'on-z' signal regions, 'of-z' optimised Extensive background estimation studies, validation regions look all very fine: Z + jets from γ + jets control region, top and diboson events from diferent-flavor control region. No excess seen in 'on-z search'. No significant excess seen in 'of- Z search'. 19
Searches for stops
Signatures with stops Dark matter interpretations by all main stop searches Compressed scenarios: Soft leptons Mono-jet searches 0-2 leptons, multiple b-jets: Stop 0-lepton Stop 1-lepton Stop 2-lepton 21
Example for stop searches: 1 lepton [ATLAS-CONF-2016-050] Direct and more complex decays of the stop + dark matter interpretation 7 signal regions (2 for DM, 5 for stop) >= 2-4 jets, >= 0 2 b-jets, various variants of mt for background suppression, top specific variables, large R-jets for boosted signal signatures Backgrounds estimated using control regions for all main backgrounds: tt, single top Wt, tt + Z and W+jets 22
Example for stop searches: 1 lepton [ATLAS-CONF-2016-050] Modest excesses in three signal regions (correlated), largest 3.3 σ in a DM SR 23
Stop to stau LMU contribution Inspired by models with gauge-mediated SUSY breaking/ natural gauge-mediation SR region requiring 1 tau + 1 electron or muon Main discriminating variable mt2 No excess seen, limits reaching up to 870 GeV on stop mass. 24
Stop summary Closing gaps at diagonals m(b) + m(w) and m(t). Limits reaching up to 850 GeV in the stop mass. Searches are complementary 25
Summary of limits 26
Summary on excesses Single photon: 2 σ Few modest excesses to look at with more data: 3b: 2 σ Stop 1-lepton: 3.3 σ 27
Summary and what to expect next Excellent LHC performance + significantly increased cross sections for many SUSY particles in Run 2 => Vast harvest of results on searches for SUSY by ATLAS and CMS shown at summer conferences. Already about twice the statistics on tape => Analyses undergo R&D phase to redesign signal regions, address more complex models and to explore more sophisticated background estimation techniques. Stay tuned to see results at the Moriond conferences! 28
Backup 29
Background estimations using control regions [Eur.Phys.J. C75 (2015) 153] 30
0-lepton: cuts of the signal regions 31
0-lepton: cuts of the signal regions 32
0-lepton: More interpretations 33
Inclusive 1-lepton: results and interpretation 34
RPV 1-lepton (and no ETmiss cut) [ATLAS-CONF-2016-094] 1 electron or muon + >= 8-10 jets + (0-4) b-jets sensitive to R-parity violating decays Standard background estimation methods using control regions not reliable enough data-driven estimate: Assumption: probability of additional jet radiation (V+jets) or of additional b-jet (tt)is constant extrapolate from n to n+1 jets, and in b-jet multiplicity Global likelihood fits separately for W+jets, Z+jets and tt Validation of V+jets templates in multijet and γ+jets events 35
RPV 1-lepton [ATLAS-CONF-2016-094] Limits reaching up to 1.75 TeV and 1.4 TeV. 36
MC in multi-b 37
Stop 1-lepton: best signal region 38
Single photon [ATLAS-CONF-2016-066] Optimized for GGM scenarios. Signature: 1 photon + jets + mef + ETmiss Main backgrounds: γ+jets, W+γ, Z+ γ, estimated via control regions. 39
Searches for electroweakinos
2 taus LMU contribution [ATLAS-CONF-2016-093] Searching for chargino/neutralino production decaying to multiple taus. 2SRs, selection: at least 2 opposite-sign taus, Z veto, b-jet veto, ETmiss, mt2 Dominant backgrounds: multi-jet (ABCD method), W+jets (from CR) and diboson 41
Example: 4 light leptons MPI/LMU contribution [ATLAS-CONF-2016-075] Looking for chargino/neutralino production with RPV decays to four electrons/muons. Low background level, thus only further cutting on mef + Z veto to define SRs. Main backgrounds ZZ and tt+z with 4 real leptons, taken from simulation. Backgrounds with two fake leptons (tt, Z+jets) estimated in data-driven way. 42