Searches for RPV SUSY with ATLAS

Transcription

1 Max Goblirsch Max-Planck-Institut für Physik, München IMPRS Young Scientist Worksho 24, Ringberg

2 Reminder: Suersymmetry (SUSY) Idea: Introduce a new symmetry transformation Q with the following roerty: Q fermioni = bosoni; Q bosoni = fermioni Assign each article in the standard model a suersymmetric artner Sin different by /2, all other roerties identical SUSY can rovide a solution to the Hierarchy Problem No Suerartners observed so far - if it exists, SUSY must be a broken symmetry SUSY breaking introduces new arameters - rich arameter sace / 6

3 R-Parity Writing down all the allowed interaction terms in a (minimal) suersymmetric standard model, we get something new: W B,L = ijk Li Lj E k + ijk Li Qj D k + ijk U i D j D k + κi Li Hu L, Q: left-handed leton/uark suerfields (contain letons and sletons / uarks and suarks) E,D,U: right handed leton/ u-tye uark / down-tye uark suerfields Hu : Higgs suerfield couling to u-tye fermions Baryon and Leton number (accidentally conserved in SM) violated and can mediate raid roton decay d u e e+ π 2 s R L 2 ul dl s R 2 L 2 el ul s R u π 2 / 6

4 R-Parity One way out: R-Parity RP = ( ) 2s+3B+L ( = +(articles) (sarticles) R-Parity Conservation (RPC): Freuent assumtion in SUSY models Forbids the bi/trilinear W B,L terms SUSY articles only roduced in airs Lightest SUSY article (LSP): cannot decay dark matter candidate Conseuences for collider searches: Pair roduction of SUSY articles Decay cascades until 2 LSP are left LSPs escae the detector Resulting search strategy: Signs of escaing, undetected articles (Missing transverse momentum) 3 / 6

5 R-Parity violation (RPV) This session: Dro the assumtion of R-Parity conservation R-Parity Violation (RPV) Trilinear W B,L terms allowed (ossibly only some of them Proton) Decays of SUSY articles to SM articles Leton and Baryon number violation No dark matter candidate Why would we want to do this? R-Parity Violation may significantly alter collider signatures No escaing LSP - no missing transverse momentum! Potential generation of neutrino masses (Neutralino-Neutrino mixing) B and/or L violation: matter/antimatter imbalance Several other ways of stabilizing the roton exist Dedicated searches are starting to fill this ga 4 / 6

6 Phenomenology of R-Parity violation (RPV) New suerotential terms: WRPV = ijk Li Lj E k +ijk Li Qj D k + ijk U i D j D k + κi Li H2 {z } this talk Conseuences of a nonzero ijk : / couling - flavours determined by i,j,k Leton number and flavour violation Neutralino LSP: Oens dileton decay mode Examle: 2! = χ e µ+ e g Turn RPV on g 5 / 6

7 The ATLAS 4-Leton search Consider events with at least 4 charged letons include hadronic decay modes of the τ -leton Look for an excess in the number of events over the standard model background: Exloit roerties of SUSY signal to suress background and gain sensitivity Presence of Neutrinos - moderate Missing Transverse Momentum Cascade decays - additional SM articles (letons, jets) with high momenta Letons do not originate from Z decays reject events with leton airs of m mz < GeV ( Z veto ) 6 / 6

8 The ATLAS 4-Leton search Cascade decays: Tyically, high final state article multilicities X Sensitive uantity: Effective mass: Meff := T + ETMiss letons, Jets (a) low Meff (b) high Meff 4-Leton analysis: Meff reuirement owerful tool for signal/background discrimination 7 / 6

9 Observation on Data 4 (e,µ) 3 (e,µ) + τ 2 (e,µ) + 2 τ Σ SM Data.4 ± ± Events / 25 GeV 4 (e,µ) Event Category 3 ATLAS 2 s= 8 TeV SRnoZb - L dt = 2.3 fb s= 8 TeV SRnoZb - L dt = 2.3 fb Data 22 Total SM Reducible ZZ tt Z twz Higgs VVV ~ )=(8,4) GeV g~ g, 33, m(~ g,χ meff [GeV] 2 (e,µ) + 2 τ Events / 25 GeV Events / 25 GeV 3 (e,µ) + τ ATLAS twz Higgs VVV +χ -,, m(χ ±,χ )=(6,4) GeV χ 2 - Translate into exclusion limits in the SUSY arameter sace 2 ZZ tt Z Observation in solid agreement with SM exectation Total SM Reducible Note: simlified overview 3 Data 22 2 ATLAS s= 8 TeV SR2noZb - L dt = 2.3 fb Data 22 Total SM Reducible ZZ tt Z twz Higgs VVV ~+~~ llll, 33, m(ll,χ )=(225,) GeV meff [GeV] meff [GeV] 8 / 6

10 Exclusion Limits Examle : Strong roduction of Gluino airs g g Investigate 4 Coulings (only turn one on at a time): 2 (e,µ)-rich 2, 22 - LSP decays to ee and eµ, or µµ and eµ Tau-rich 33, LSP decays to eτ and τ τ, or µτ and τ τ Stronger exclusion for (e,µ)-rich coulings - reconstruction efficiency Close to the X axis: collimated letons lose efficiency High cross sections - strong limits u to m(g ) = 4/95 GeV 9 / 6

11 Exclusion Limits Examle 2: Electroweak roduction of Sneutrino airs No visible articles from the cascade - challenging signature Cross-sections lower than for strong roduction Limits u to m( ) = 4 GeV (light letons) Tau-rich coulings: exclude region at m( ) m( ) = GeV / 6

12 A look at the lifetime So far: assumed romt LSP decays What if we lay with the LSP lifetime(i.e. change )?. τ. s: Decay length below detector resolution 4 romt letons, 4 jets, low ETMiss (Neutrinos) 4-Leton results aly g g τ ~~ ~ gg roduction; g χ mχ [GeV] 4 ns. τ : LSP decays outside the detector zero letons, 4 jets, high ETMiss (LSPs) covered by existing RPC searches ATLAS Preliminary Observed limit (± σtheory) L dt = 2.3 fb, Exected limit (± σex ) - 2 SUSY s=8 TeV Observed limit (4.7 fb -, 7 TeV) -leton combined Exected limit (4.7 fb -, 7 TeV) m~g [GeV] ATLAS-CONF / 6

13 Medium lifetimes Interesting case: s. τ. ns Decays in the detector, but outside the rimary vertex g g 95% CL Limit on σsusy[b] 2 Exected limit ( ± σ ) ~ g~ g ATLAS-CONF ATLAS Work in Progress L dt = 2.3 fb, - s = 8 TeV ~~ gg χ χ ~ m( g) = 3 GeV, m(χ ) = GeV - -2 Signature: dislaced leton airs 4-leton search: restricted sensitivity new search channel τ LSP [s] 2 / 6

14 Dislaced Diletons - a short overview Ongoing effort - the analysis so far ATLAS Work in Progress.8 Both leton tracks reconstructed.6 arbitrary units Reconstruction efficiency Key ingredient: Retracking Issue: Highly dislaced tracks are missed by conventional ATLAS track reconstruction Solution: Rerun track reconstruction and vertex finding with secial algorithms Look for a dislaced vertex in the tracker volume of r <3cm, z <3cm Should contain two oositely charged letons Invariant mass: Reuire more than GeV to suress light resonances Reject any vertices in regions with detector material Exect very low background (< event) ~ g~ g χ χ χ ll ; 2 > Signal Region mll > GeV m( χ ) = 5 GeV Simulation s = 8 TeV ATLAS Work in Progress 2 Dileton Vertex reconstructed Vertex r [mm] XY mll [GeV] 3 / 6

15 Dislaced Diletons - the background Study otential background using MC simulation and low mass data r 2) 3) ) z Beam resonances - At low masses (delayed J/Ψ, KS ππ with 2 fakes) random crossings - Random crossing of two tracks to form a vertex. cosmic muons - might be interreted as back-to-back dileton vertex conversions/bremsstrahlung - ee/eµ vertex random crossings aear dominant Invariant mass: m2 ' 2E E2 ( cos(φ2 )) determined by (random) momenta and crossing angle Estimation techniue: Random combination of tracks arb. units Beam 4 Conversions Hadrons Crossings (PV+PV) Crossings (PV+PU) Crossings (PU+PU) vertex mass after track/leton t cuts [GeV] 4 / 6

16 Summary, Outlook R-Parity violation (RPV): Interesting class of suersymmetric models Collider signatures may evade conventional searches B and L violation, otential for Neutrino masses No SUSY dark matter candidate ATLAS: Probe RPV with a 4-Leton search Low background, sensitive u to high masses (low cross-sections) Final run results: No signs of SUSY yet Further ATLAS RPV analyses: also see next talk! Another loohole: Long-lived signatures RPC decays may occur with long lifetimes Not covered by existing searches Dedicated dislaced vertex search in rogress 5 / 6

17 One more thing... 6 / 6