Powheg in Herwig++ for SUSY

Transcription

1 Powheg in Herwig++ for SUSY Alix Wilcock IPPP, Durham University 27/04/205 Based on work done with P. Richardson, S. Plätzer and B. Fuks

2 Acronyms SUSY = Supersymmetry Want SUSY to solve the hierarchy problem

3 Supersymmetry Require SUSY partner of the top quark to be light (m t few TeV)

4 Supersymmetry - it s not dead Require SUSY partner of the top quark to be light (m t few TeV) Still possible in compressed spectra scenarios

5 Compressed spectra SUSY Mass difference between SUSY particle and the decay products is small No energetic Standard Model objects Not a lot of missing E T q χ 0 p q p q q χ 0

6 Compressed spectra SUSY Mass difference between SUSY particle and the decay products is small No energetic Standard Model objects Not a lot of missing E T Look for SUSY particles recoiling against hard initial-state radiation q χ 0 p q p q q χ 0

7 Acronyms Herwig++

8 Acronyms Herwig++ = Hard emission reactions with interfering gluons

9 Monte Carlo event generator Takes theoretical model simulates expected experimental data Monte Carlo simulations split into several stages: Hard process Parton shower Hadronisation

10 Matrix-element matching Parton showers resum large logarithms Good approx. in soft/collinear limit Doesn t describe hard emissions well (Remember hard emissions important when studying compressed spectra SUSY) Mn i (p j +p k ) 2 = E j E k ( cos θ) θ j k

11 Matrix-element matching Parton showers resum large logarithms Good approx. in soft/collinear limit Doesn t describe hard emissions well (Remember hard emissions important when studying compressed spectra SUSY) Mn i = (p j +p k ) 2 E j E k ( cos θ) θ j k Improve simulation of hard radiation in the shower using NLO matrix-element matching Combines exact matrix elements with the parton shower We use the POsitive Weight Hardest Emission Generator (Powheg ) formalism

12 Powheg formalism For a p T ordered parton shower, cross section for the first emission: Normal parton shower dσ PS = B(Φ B )dφ B [ (pt min (p T, pt max ) = exp ], pt max ) + (p T, pt max )P(z) dφ R ( ) p max T p T P(z)dΦ R

13 Powheg formalism For a p T ordered parton shower, cross section for the first emission: Normal parton shower dσ PS = B(Φ B )dφ B [ (pt min (p T, pt max ) = exp ], pt max ) + (p T, pt max )P(z) dφ R ( ) p max T p T P(z)dΦ R Powheg corrected parton shower dσ PO = B(Φ B )dφ B [ (pt min, p max ) + (p T, p max ) R ] B dφ R ( ) p max (p T, pt max T R ) = exp p T B dφ R B(Φ B ) = B(Φ B ) + V(Φ B ) + R(Φ B, Φ R )dφ R T T

14 Matrix-element corrections Powheg correction available in Herwig++ for large number of Standard Model processes. For BSM processes, limited by absence of virtual matrix elements Powheg style matrix-element correction Generate hardest emission using R But local normalization is B rather than B

15 Matrix-element corrections Powheg correction available in Herwig++ for large number of Standard Model processes. For BSM processes, limited by absence of virtual matrix elements Powheg style matrix-element correction Generate hardest emission using R But local normalization is B rather than B Implement ME correction using Matchbox and MadGraph MadGraph 5 - used to generate B and R Matchbox - framework for NLO calculations, MC@NLO and Powheg matching to the Herwig++ angular ordered and dipole showers

16 Top squark pair production Simulated pp t t, stable t at s = 4 TeV, m t = 700 GeV. Limit simulation to hard process + full parton shower dσ/dpt [fb / GeV] Matchbox +MadGraph LO MEC Example: e + e q qg [hep-ph/030083] MEC / LO p T, t t [GeV] Uncorrected shower: over populates hard regions of phase space in p T m t region has unpopulated dead zone for p T m t

17 Effect of the matrix element correction: exclusion bounds ATLAS search for direct production of the top squark in events with missing E T and two b-jets t b χ + bf f χ 0 with m χ + m χ 0 = 5 GeV Selection criterion Signal region A Signal region B ET miss > 50 GeV > 250 GeV Leading jet, j p T > 30 GeV, η < 2.8 p T > 50 GeV, η < 2.8 Subleading jet, j 2 p T > 50 GeV, η < 2.8 p T > 30 GeV, η < 2.8 Third jet, j 3 veto if p T > 50 GeV, η < 2.8 p T > 30 GeV, η < 2.8 φ(p miss T, j ) - > 2.5 b-tagged jets j and j 2 b-tagged with p T > 50 GeV, η < 2.5 j 2 and j 3 b-tagged with p T > 30 GeV, η < 2.5 min k ( φ(p miss T, j k )) for k 3 > 0.4 > 0.4 ET miss /( n i= (p jet T ) i + ET miss ) > 0.25, n = 2 > 0.25, n = 3 m CT [ref] > 50, 200, 250, 300, 350 GeV - H T,3 = (p j T ) i for all i > 3 - > 50 GeV m bb = (p b, + p b,2 ) 2 > 200 GeV -

18 Effect of matrix element correction: before ATLAS search for direct production of the top squark in events with missing E T and two b-jets t b χ + bf f χ 0 with m χ + m χ 0 = 5 GeV Original signal simulated with MadGraph + PYTHIA ATLAS-SUSY m χ + = 5 GeV χ0 ATLAS result Herwig++ m χ 0 [GeV] t b χ + forbidden s = 8TeV L = 20.fb m t [GeV]

19 Effect of matrix element correction: after ATLAS search for direct production of the top squark in events with missing E T and two b-jets t b χ + bf f χ 0 with m χ + m χ 0 = 5 GeV Original signal simulated with MadGraph + PYTHIA ATLAS-SUSY m χ + = 5 GeV χ0 ATLAS result Herwig++ m χ 0 [GeV] t b χ + forbidden s = 8TeV L = 20.fb m t [GeV]

20 And now for something a little different... We ve looked at SUSY searches based on a hard ISR jet + E miss T But this in not the only option... monojet = ISR jet + ET miss monophoton = energetic photon + ET miss monotop = top quark + ET miss t q χ 0 p q p q q χ 0

21 Monotop search for SUSY Study LHC sensitivity using Monte Carlo simulations of 300fb of 4 TeV collisions Search for pp t χ 0 t with t c χ 0 Experimental signal is t + ET miss t and χ 0 light Other SUSY particles decoupled m 0 TeV

22 Simulation Signal: pp t χ 0 t with t bq q simulated in MadGraph 5 t c χ 0, parton shower, hadronization done with Herwig++ Background: Process t t Single top tw production W ( lν) + light-jets γ/z( l l/ν ν) + jets Wb b with W lν Diboson Simulation details Hard process at NLO with PowhegBox, matched to Herwig++ As above As above W production at NLO matched to LO W + or 2 jets using Sherpa As above Hard process at LO with MadGraph, matched to Herwig++ NLO using Powheg in Herwig++

23 Event Selection Criteria Designed to reflect final state of signal events Exactly zero leptons Exactly one b-jet, p T > 30 GeV Three other jets with p T > min(pt b, 40 GeV) S S+B Impose further cuts to maximize sensitivity = S, B are number of signal and background events passing the cuts E miss T > 200 GeV 50 GeV < m jj < 00 GeV 00 GeV < m bjj < 200 GeV φ(p miss T, pj ) > 0.6 and φ(p miss T, pb ) > 0.6 φ(p miss T, pt ) >.8

24 Results - Scan Scan (m t, m χ 0 ) plane Superimpose ATLAS monojet search for pair-produced top squarks ATLAS: 95% CL, 20fb Monotop: 2σ, 300fb m χ 0 [GeV] m t < m χ 0 + m c m t [GeV] Can monotops provide competitive sensitivity to monojet techniques?

25 Results - Scan Scan (m t, m χ 0 ) plane Superimpose ATLAS monojet search for pair-produced top squarks ATLAS: 95% CL, 20fb Monotop: 2σ, 300fb m χ 0 [GeV] m t < m χ 0 + m c m t [GeV] Can monotops provide competitive sensitivity to monojet techniques? No!

26 Summary SUSY is not dead Light top squarks could still exist in compressed spectra scenarios Search for them using non-standard analysis techniques, e.g. monojet or monotop signals Monojet searches require accurate simulation of hard radiation and work pretty well Monotops are not competitive at LHC energies

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