Search for supersymmetry with disappearing tracks and high energy loss at the CMS detector

Transcription

1 Search for supersymmetry with disappearing tracks and high energy loss at the CMS detector Teresa Lenz in Collaboration with Loic Quertenmont, Christian Sander, Peter Schleper, Lukas Vanelderen International School on Subnuclear Physics, Erice June / 11

2 Introduction: Supersymmetry Several reasons, why the Standard Model is not the ultimate theory. No dark matter candidate: Hierachy Problem: m ren2 H = m bare2 H + m 2 H Either: SM not valid up to Planck scale O( 19 GeV) Or: Fine tuning needed for cancellation unnatural Other reasons: Does not include gravity, does not unify gauge couplings,... 2 / 11

3 Introduction: Supersymmetry Doubling of particle content (N = 1) MSSM: Two Higgs doublets masses to u- and d-type fermions Supersymmetry is broken 5 additional free parameters Provides DM candidate (R-parity conservation) Search for new heavy particles 3 / 11

4 Introduction: The pmssm The phenomenological MSSM (pmssm): Additional assumptions: No new sources of CP violation Minimal flavor violation First two generation universality The lightest supersymmetric particle (LSP) is the neutralino Only 19 additional free parameters 4 / 11

5 Motivation So far, no evidence for Supersymmetry at the LHC look for more specific signatures Still allowed: models with long-lived charginos (χ ± ). Chargino almost mass-degenerate with the lightest neutralino. long lifetime of χ ± (phase-space suppression) low-energetic decay products q q γ, Z χ ± χ ± χ 0 f W f 5 / 11

6 Reinterpretation of existing searches at CMS in the pmssm Scan over pmssm parameter space ( points) Search for heavy stable charged particles (EXO ) excludes models with very long-lived charginos (cτ 1m) Hadronic and Leptonic CMS SUSY analyses together (SUS ) exclude models with short-lived charginos (cτ 0.01m) points in pmssm LHC subspace Excluded Fraction CMS Preliminary - -1 s = 8 TeV - L = 18.8 fb pmssm LHC subspace excluded by : - EXO SUS unexcluded log [ cτ (m) ] log [cτ(m)] CMS PAS: EXO Models with chargino lifetimes around mm cτ 1m still possible. 6 / 11

7 Signature in the detector Signatures dependent on the lifetime of the charginos: CMS event display Long-lived charginos can be reconstructed as muons. Charginos with shorter lifetimes decay in the detector (Calorimeter, Tracker). 12 m Disappearing tracks in the tracker 7 / 11

8 Signature in the detector Signatures dependent on the lifetime of the charginos: CMS event display Long-lived charginos can be reconstructed as muons. Charginos with shorter lifetimes decay in the detector (Calorimeter, Tracker). 12 m Disappearing tracks in the tracker 7 / 11

9 Signature in the detector Signatures dependent on the lifetime cτ of= the 0.5m charginos: CMS event display Long-lived charginos can be reconstructed as muons. Charginos with shorter lifetimes decay in the detector (Calorimeter, Tracker). 12 m Disappearing tracks in the tracker 7 / 11

10 Selection of a disappearing track Selection of tracks which are neither reconstructed as SM particles nor are fake tracks. Missing outer hits in the silicon tracker Some quality cuts on the tracks No missing inner hits Calorimeter isolation... a.u Work in progress Background tracks (tt) Signal,m=0GeV,cτ=0.5m Signal,m=800GeV,cτ=0.5m outer N Lost 8 / 11

11 Selection of heavy particles Background seperation with de dx de = Energy deposition in silicon strip dx = Thickness of silicon strip For 0.2 < βγ < 0.9: Approximation of Bethe-Bloch formula: de dx = K m2 + C p 2 Single de/dx measurement following Landau distribution large variation Difficulty: Find a good estimator for de, also for low numbers of dx measurements Harmonic estimator ( de dx = 1 N ( E/ x) 2 i N i=1 ) 1/2 de/dx [MeV/cm] Work in progress 40 Background tracks (tt) 35 Signal,m=0GeV,cτ=0.5m 30 Signal,m=800GeV,cτ=0.5m 25 Harm. estimator P [GeV] 9 / 11

12 Mass reconstruction No selection applied. Work in progress Work in progress 1 Background tracks (tt) Signal,m=0GeV,cτ=2.0m 1 Background tracks (tt) Signal,m=0GeV,cτ=2.0m -1 Signal,m=800GeV,cτ=2.0m -1 Signal,m=800GeV,cτ=2.0m a.u. -2 a.u reconstructed mass [GeV] ( N =14) Hits reconstructed mass [GeV] (3 Hits) Reconstructed mass seems to have good discriminating power (also for shorter lifetimes N Hits > 3) Shown: Proof of principle Ongoing work: Implementation and interpretation / 11

13 Conclusion Supersymmetry offers solutions to SM problems no particles discovered yet Still allowed: models with almost mass-degenerate charginos and neutralinos For long-lived particles decaying inside the tracker: disappearing tracks Further separation against background by energy loss measurement Thank You! 11 / 11

14 Backup 12 / 11

15 The pmssm Assumptions: No new sources of CP violation (all numbers in L are real) Minimal flavor violation (mass matrices and trilinear couplings are diagonal) First and second generation universality (m f1 = m f2, A e = A u = A d = 0) Parameters: tan β: The ratio of the vevs of the two higgs doublets M A : Mass of the pseudoscalar higgs boson µ: Higgs-higgsino mass parameter M 1, M 2, M 3 : Gaugino mass parameters m q, mũr, m dr, m l, mẽr : First and second generation sfermion masses m Q, m t R, m b R, m L, m τ R : Third generation sfermion masses A t, A b, A τ : Third generation trilinear couplings 13 / 11

16 Reinterpreting existing searches in the pmssm Analyses contained in SUS : Hadronic H T + H miss T Hadronic H T + E miss T search (CMS-SUS ) +b-jets search (CMS-SUS ) Hadronic H T + E T + τσ search (CMS-SUS ) Hadronic monojet + E T search (CMS-EXO ) Leptonic same sign (SS) 2l search (CMS-SUS-11-0) Leptonic opposite sign (OS) 2l search (CMS-SUS ) Leptonic electroweakino (EWKino) search (CMS-SUS ) 14 / 11

17 Mass degeneracy of chargino and neutralino (a) M 2 µ : The lightest chargino and neutralino are both higgsinos. The relation between M 1 and M 2 does not matter (m χ ± m χ 0 µ ). (b) µ M 2 : The lightest chargino and neutralino are both gauginos. Mass degeneracy can be present only if M 1 M 2 (m χ ± m χ 0 M 2 ). 15 / 11

18 Derivation of the approximated Bethe-Bloch formula Bethe-Bloch formula: de dx = kz2 Z A 1 β 2 [1 2 ln 3m ec 2 β 2 γ 2 T max I 2 β 2 δ(βγ) ] 2 Z=atomic number; A=mass number; I=average excitation potential (173eV for silicon); δ= density correction; T max= maximum kinetic energy transfer possible in a single collision Derivation: 16 / 11

19 More on de/dx Different functions: Bethe-Bloch : Mean loss for moderately relativistic charged heavy particles (only for 0.1 βγ 00) (accurace a few %) Landau-Vassilov: [ Most probable energy loss: ] ξ = ln 2mc2 β 2 γ 2 I + ln ξ I + j β 2 δ (βγ) 17 / 11

20 The CMS detector 18 / 11

21 Detection of events q q γ, Z χ ± W χ ± χ 0 Momentum of final fermions depend on mass difference between χ ± and χ 0 Fermions are low energetic hard to detect f f a.u Work in Progress m χ ± = 200 GeV ± 0 m(χ, χ ) 150 MeV p [GeV] T Trigger on initial state radiation to record events 19 / 11

22 Detection of events q q γ, Z χ ± W χ ± χ 0 Momentum of final fermions depend on mass difference between χ ± and χ 0 Fermions are low energetic hard to detect f f a.u Work in Progress m χ ± = 200 GeV ± 0 m(χ, χ ) 150 MeV p [GeV] T Trigger on initial state radiation to record events 19 / 11