SUSY searches at the LHC * and Dark Matter

SUSY searches at the LHC * and Dark Matter Elisabetta Barberio ATL-PHYS-SLIDE-2009-025 26 February 2009 School of Physics The University of Melbourne/Australia Dark 2009: Seventh International Heidelberg Conference on Dark Matter in Astro and Particle Physics * ATLAS

Evidence of Dark Matter So far only from astronomical observations DARK MATTER Is it a fundamental particle? What are its properties? How does it interact? What is the symmetry origin of the dark matter particle? Is dark matter composed of one particle species or many? How and when was it produced? 1

Experiments looking for Dark Matter Astrophysical experiments direct detection χ χ indirect detection - land-based - high altitude - space-based χ _ p e + γ ν χ Collider experiments (LHC) Measurements at LHC are complementary to direct and indirect astro searches / measurements 2

Complementarity σ χp si (cm 2 ) EDELWEISS DAMA ZEPLIN-I CRESST-II ZEPLIN-2 EDELWEISS 2 XENON CDMS CDMS-II σ χp si ZEPLIN-4 GENIUS No. MC Experiments Polesello et al JHEP 0405 (2004) 071 σ χp si Ω χ h 2 ZEPLIN-MAX m χ log 10 (σ χp si / 1pb) Ω χ h 2 Dark Matter particle mass m χ (GeV) If the LHC discovers e.g. SUSY we can study the compatibility of e.g. SUSY signal with Dark Matter hypothesis Use SUSY model parameters and ATLAS measurements to predict Dark Matter parameters Ω χ h 2, m χ, σ χp si etc. and compare with astro measurements

What we know about DM? Neutral Cold Stable Not Baryonic Weakly interacting ρ χ 0.3GeV/cm 3 V 220 km/s Nature of DM particle? Observational constraints are no match for the creativity of theorists Many hypothesis, but not all are equally motivated WIMPs and SuperWIMPS can be produced by colliders 4

Why WIMP? Naturally predicted in many new physics models Supersymmetry, Extra-dimension, Little Higgs Naturally give the correct Dark Matter relic density 10-34 The amount of dark matter remaining is inversely proportional to the annihilation cross section: / Ω DM <σ A v> 1 0.1 5

Supersymmetry Supersymmetry (SUSY) fundamental continuous symmetry connecting fermions and bosons SUSY stabilises Higgs mass against loop corrections (gauge hierarchy /fine-tuning problem) Leads to Higgs mass 135 GeV

Supersymmetry In principle the SUSY partners to have same masses as SM states Not observed! SUSY must be a broken symmetry at low energy h u c t γ g G A d s b Z H 0 e µ τ W ± H ± ν e ν µ ν τ Minimal Supersymmetric Standard Model (MSSM) Hχ 0 1 u u d c χh 0 2 0 d t b τ γ Z χ ± 1 g G e s µ χ ± W ± 2 χ 0 H 3 + u ν e ν µ ν τ χ 0 H4 d

In constructing SUSY models, a symmetry called R-parity can be imposed under which SM particles are even, SUSY particles are odd consequences: SUSY Dark Matter sparticles are produced in pairs at the LHC the lightest sparticle (LSP) is absolutely stable LSP neutral/weakly interacting naturally provides a solution to Dark Matter problem Which particle is the LSP depends on the point in parameter space, e.g. LSP neutralino (WIMP) gravitino (SuperWIMP) R-Parity violating models not covered here. 8

focus-point region above 7 TeV for m t = 178 GeV bulk' region: t- channel slepton exchange. 'Bread and Butter for LHC. Neutralino (WIMP) Dark Matter Within msugra parameter space, only a few regions are compatible with WMAP constraint Favoured by g µ 2 disfavoured by BR (b sγ) χ 0 1 χ 0 1 l R l l msugra A 0 =0, tan(β) = 10, µ>0 + funnel region at large tanβ χ 0 1 τ 1 τ τ 1 γ/z/h Charged LSP co-annihilation region. Small slepton-lsp mass difference. Measurements difficult 0.094 Ω χ h 2 0.129 (Ellis et al., Phys. B565 (2003) )

LARGE HADRON COLLIDER protons protons (or Pb ions) Starts this year! E COM = 14 TeV Design Luminosity 10 34 cm -2 s -1 Bunch crossing rate 40 MHz Events per bunch crossing 20 4 main experiments CMS, ATLAS (multi purpose) ALICE (heavy ion physics) LHCB (low-p T B physics)

Dark matter production at LHC Hard Scattering χ χ χ model-dependent study Underlying Event 11

ATLAS Length : 46 m Radius : 12 m Weight : 7000 tons 10 8 electronic channels 3000 km of cables Inner Detector (B=2T) : - Si pixels and strips - Transition Radiation Detector Calorimetry: - EM : Pb-LAr - HAD: Fe/scintillator (centr.), Cu/W-LAr (fwd) Muon Spectrometer: air-core toroids with muon chambers 12

First beams, 10 September 2008 2x10 9 protons at 450 GeV 13

Variables at a hadron collider Transverse momentum p T and Energy: p T = p sin θ Component of momentum of particles in the plane perpendicular to the beam axis non-interacting particles (WIMP, neutrinos) do not leave signature in a detector they are detected by using energy and momentum conservation: sum up the momenta of everything measured, what is left to get back to zero (missing energy) is the neutrino(s)/wimp energy. In a proton-proton collision, only the transverse missing energy (E T miss ) is measured since the energy along the beam-pipe direction is undetected

Sparticles production at the LHC Squarks and gluinos produced via strong processes large cross-section e.g. for m(q, g) 1 TeV 100 events produced with 100 pb -1 Charginos, neutralinos, sleptons direct production occurs via electroweak processes much smaller rate (produced more abundantly in squark and gluino decays), e.g. 15

What the LHC actually sees e.g., q q pair production: q are heavy (Tevatron limits: m > 300-400 GeV) cascade decays favoured each q neutralino χ which escapes detection apparent missing energy in the final state Spectacular events with many jets, missing transverse energy, leptons relatively easy to extract SUSY signal from SM backgrounds at LHC (in most cases ) 16

Background processes Very difficult to observe light objects (e.g.w, Z,..) in final states with only jets rely on lepton, γ Mass resolutions of 1% (10%) needed for lepton, γ (jets) to extract tiny signals from backgrounds, and excellent particle identification 17

SUSY Dark Matter strategy 1 st Step Look for deviations from the Standard Model Example: multi-jet + E T miss signature 2 nd Step: Is it SUSY? If so establish the SUSY mass scale using inclusive variables, e.g. effective mass distribution Relevance to Dark Matter Inclusive studies: Verify if the discovered signal provides a possible Dark Matter candidate Exclusive studies: Model-independent calculation of LSP mass, compare with direct searches 3 rd Step: Which SUSY flavour? Determine model parameters (difficult) Strategy: select particular decay chains and use kinematics to determine mass combination Relevance to Dark Matter model-dependent calculation of relic density, σ(χp si ), etc

SUSY Signatures p p q g χ 0 2 l χ 0 1 q q l l Strongly interacting sparticles (squarks, gluinos) dominate production. Heavier than sleptons, gauginos etc.: cascade decays to LSP. Long decay chains and large mass differences between SUSY states Many high p T objects observed (leptons, jets, b-jets). If R-Parity is conserved LSP Large E T miss signature

Search for 0,1,2 leptons plus jets Inclusive searches With leptons, smaller signal rates, but better S:B conditions More robust discovery potential, specially at the beginning, when uncertainties on the backgrounds are large 0-lepton 1-lepton bulk region 5σ ATLAS

Background Estimate Standard Model background using data-driven techniques e.g.. Studying Z νν + n jets, W lν + n jets, W τν + (n-1) jets Select Z ll e.g. Z l + l - + n jets (e or µ) can be use to validate MC / estimate E T miss replace charged leptons by neutrinos

LHC SUSY discovery reach Multi-jet+missing energy signatures with 1 fb -1 : sensitive to m(g)< 0.5-1.5 TeV Current limit> 300-400 GeV

Is it SUSY? Need to demonstrate we have SUSY, not another model: e.g. spin measurements One possibility use two-body slepton decay chain charge asymmetry of lq pairs measures spin of χ 0 2 shape of dilepton invariant mass spectrum measures slepton spin Spin-0 Measure Angle Point 5 Spin-½ m lq spin-0=flat Polarise Spin-½, mostly wino Spin-0 Spin-½, mostly bino 150 fb -1 ATLAS

Which SUSY? Symmetry Breaking mechanism determines phenomenology at colliders Constrained models: msugra/cmssm: Neutralino is the LSP Many different final states Common scalar and gaugino masses GMSB: AMBS: Gravitino is the LSP Photon or tau final states expected If R-Parity is conserved R-Parity Violated LHC experiments sensitive only to LSP lifetimes < 1 ms ATLAS Physics TDR R-Parity Conserved

Which SUSY? SUSY spectroscopy.measure weak scale SUSY parameters through exclusive decay measurements Search for kinematic endpoints in the invariant mass distributions of visible decay products Solve edge equations to reconstruct sparticle masses edge of kinematic endpoint Use the mass spectrum to reconstruct SUSY parameters assuming a particular breaking framework Weiglein et al. (2004)

Dilepton edge measurements χ 0 2 l l l χ 0 1 kinema(c endpoint: When kinematically accessible χ 0 2 can undergo sequential two-body decay to χ 0 1 via a right-slepton edge of kinematic endpoint Results in sharp dilepton invariant mass edge sensitive to combination of masses of sparticles. Position of edge measured with precision 0.5% (30 fb -1 ) 1 fb -1 di lepton mass (GeV) 26

Coannihilation signatures Small slepton-neutralino mass difference gives soft leptons Decays of χ 0 2 to both l L and l R kinematically allowed. Double dilepton invariant mass edge structure; edges expected at 57 / 101 GeV Stau channels enhanced (tanβ) edge expected at 79 GeV; less clear due to poor tau visible energy resolution. Point chosen within region: m 0 =70 GeV; m 1/2 =350 GeV; A 0 =0; tanß=10 ; µ>0; 27

Bulk Signatures Dilepton edge starting point for reconstruction of decay chain. Make invariant mass combinations of leptons and jets. Gives multiple constraints on combinations of invariant masses. q L χ 0 2 q χ 0 1 1 fb -1 l l l llq edge llq threshold lq edge Point chosen within region: m 0 =100 GeV; m 1/2 =300 GeV; A 0 =-300; tanß=6 ; µ>0;

Heavy Gaugino Measurements - Crucial input for the reconstruction of MSSM neutralino mass matrix independent of the SUSY breaking scenario ATLAS - Potentially possible to identify dilepton edges from decays of heavy gauginos. Requires high stats. SPS1a ATLAS 100 fb -1 ATLAS 100 fb -1 ATLAS 100 fb -1 SPS1a

Measuring Model Parameters e.g. msugra/cmssm model and perform global fit of model parameters to observables ex: msugra 2 edge ( m ll ) m 0, m 1/2, A 0, meas tanβ, sgn(µ) 2 edge ( m ll ) pred. Fit: χ 2 Point m 0 m 1/2 A 0 tan(β) sign(µ) Bulk 100 Gev 300 GeV -300 6 +1 m χ 2 0, m l R ±, m χ1 0 sign(µ)=+1 expected unc. 1 fb -1 m 0 98.5 GeV ± 9.3 GeV m 1/2 317.7 GeV ± 6.9 GeV tan(β) 7.4 4.6 A 0 445.0 GeV ± 408 GeV

CP violation So far considered CP conserving MSSM: What if CP is violated? Need new sources of CP violation beyond the SM for baryogenesis In the general MSSM, gaugino and higgsino mass parameters and trilinear couplings can be complex: Important influence on sparticle production and decay rates Expect similar influence on <σν> NB1: M 2 can also be complex, but its phase can be rotated away. NB2: CPV phases are strongly constrained by dipole moments; we set φ m =0 and assume very heavy 1st+2nd generation sfermions 31

Electroweak Baryogenesis LHS-3 M A = 1000 GeV arg(µ) = π/2 LHS-1 Representative benchmarks LHS-1: strong light stop-neutralino coannihilation to produce correct relic density; small mass difference makes it hard to find at the LHC LHS-2 LHS-2: resonant annihilation of neutralinos via Higgs resonances lowers the neutralino abundance; the most promising benchmark for discovery at the LHC C. Balázs et al., Les Houches 2005 LHS-3: lightest neutralino coannihilates with lightest stop and chargino to lower abundance and produce correct relic density; could be studied at the LHC

Final state is very similar to top pair production events. As before, after the discovery: Light Stop χ 0 q hadronic leg _ χ b ν _ e,µ q t 1. Reconstruct the kinematic endpoints in the invariant mass distributions of visible decay products 2. Solve the edge equations to reconstruct sparticle masses 3. Find the model parameters T. Lari and G. Polesello hep-ph/0602198 t b leptonic leg χ + Mass Spectrum t 1 χ 0 1 χ + 1 LHS-2 χ 0 137 GeV 89 GeV 129 GeV M(bjj) 1.8 fb -1 M(bl) 1.8 fb -1 GeV GeV

Dark Matter Parameters Can use parameter measurements m 0, m 1/2, A 0, tanβ, sgn(µ) derived from the previous fit to estimate the LSP dark matter properties Calculate rate for all possible neutralino annihilation processes a) slepton exchange, slepton masses < 200 GeV b) annihilation to vector bosons (LSP has a wino or higgsino component) c) coannihilation with light sleptons d) annihilation to third-generation fermions. Need of all the masses and couplings of sparticles contributing to neutralino

Ultimate LHC precision msugra bulk region, 300 fb -1 Polesello, Tovey JHEP 0405 (2004) 071 best-case scenario (old fast simulation) Micromegas 1.1 (Belanger et al.) + ISASUGRA 7.69 Ω χ h 2 300 fb -1 ATLAS DarkSUSY 3.14.02 (Gondolo et al.) + ISASUGRA 7.69 σ χp 300 fb -1 ATLAS Ω χ h 2 = 0.1921 ± 0.0053 log 10 (σ χp /pb) = -8.17 ± 0.04 This study aggressively targets those (weak scale) parameters needed for relic density calculation

Dark Matter in the MSSM To obtain a more model-independent relic density estimate is much harder: much more measurements are needed measure relevant (co-)annihilation channels and exclude all irrelevant ones also Stau, higgs, stop masses/mixings are important as well as gaugino/higgsino parameters Nojiri, Polesello & Tovey, JHEP 0603 (2006) 063 Ω χ h 2 300 fb -1 Ω χ h2 300 fb -1 σ(ω χ h 2 ) vs σ(m ττ ) SPA point σ(m ττ )=5 GeV σ(m ττ )=0.5 GeV

Outlook 37

Collider + Astrophysical expts: Baer,Tata (2008) Baer, et al (2004) 38

Collider + Astrophysical expts: LCC1 Battaglia (2005) 39

Summary The LHC and direct astro-particle experiments are complementary Following a SUSY discovery, the LHC experiments will aim to test the SUSY Dark Matter hypothesis. Conclusive result only possible in conjunction with astroparticle experiments Ultimate goal: observation of neutralinos at LHC confirmed by observation of e.g. signal in Dark Matter experiment (in-direct and direct detection) as predicted mass and cross-section. LHC begins in 08-09, direct and indirect detection are improving rapidly this field will be transformed soon