powheg in herwig++ for susy - durpowheg in herwig++ for susy alix wilcock ippp, durham university...
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Powheg in Herwig++ for SUSY
Alix Wilcock
IPPP, Durham University
27/04/2015
Based on work done with P. Richardson, S. Platzer and B. Fuks
Acronyms
SUSY = Supersymmetry
Want SUSY to solve the hierarchy problem
Supersymmetry
- it’s not dead
Require SUSY partner of the top quark to be light(mt1
. few TeV)
Still possible in compressed spectra scenarios
Supersymmetry - it’s not dead
Require SUSY partner of the top quark to be light(mt1
. few TeV)
Still possible in compressed spectra scenarios
Compressed spectra SUSY
Mass difference between SUSY particle and the decayproducts is small
No energetic Standard Model objects
Not a lot of missing ET
Look for SUSY particles recoiling against hard initial-stateradiation
χ01
χ01
q
qp p
Compressed spectra SUSY
Mass difference between SUSY particle and the decayproducts is small
No energetic Standard Model objects
Not a lot of missing ET
Look for SUSY particles recoiling against hard initial-stateradiation
χ01
p p
q χ01
q
Acronyms
Herwig++
= Hard emission reactions with interfering gluons
Acronyms
Herwig++ = Hard emission reactions with interfering gluons
Monte Carlo event generator
Takes theoretical model → simulates expected experimentaldata
Monte Carlo simulations split into several stages:
Hard process → Parton shower → Hadronisation
Matrix-element matching
Parton showers resum large logarithms
Good approx. in soft/collinear limit
Doesn’t describe hard emissions well
(Remember hard emissionsimportant when studyingcompressed spectra SUSY)
j
k
iMn
θ
1(pj+pk )2 = 1
EjEk (1−cos θ)
Improve simulation of hard radiation in the shower usingNLO matrix-element matching
Combines exact matrix elements with the parton shower
We use the POsitive Weight Hardest Emission Generator(Powheg ) formalism
Matrix-element matching
Parton showers resum large logarithms
Good approx. in soft/collinear limit
Doesn’t describe hard emissions well
(Remember hard emissionsimportant when studyingcompressed spectra SUSY)
j
k
iMn
θ
1(pj+pk )2 = 1
EjEk (1−cos θ)
Improve simulation of hard radiation in the shower usingNLO matrix-element matching
Combines exact matrix elements with the parton shower
We use the POsitive Weight Hardest Emission Generator(Powheg ) formalism
Powheg formalism
For a pT ordered parton shower, cross section for the first emission:
Normal parton shower
dσPS = B(ΦB)dΦB
[∆(pmin
T , pmaxT ) + ∆(pT , p
maxT )P(z) dΦR
]∆(pT , p
maxT ) = exp
(−∫ pmax
T
pT
P(z)dΦR
)
Powheg corrected parton shower
dσPO = B(ΦB)dΦB
[∆(pmin
T , pmaxT ) + ∆(pT , p
maxT )
RB dΦR
]
∆(pT , pmaxT ) = exp
(−∫ pmax
T
pT
RB dΦR
)
B(ΦB) = B(ΦB) + V(ΦB) +
∫R(ΦB ,ΦR)dΦR
Powheg formalism
For a pT ordered parton shower, cross section for the first emission:
Normal parton shower
dσPS = B(ΦB)dΦB
[∆(pmin
T , pmaxT ) + ∆(pT , p
maxT )P(z) dΦR
]∆(pT , p
maxT ) = exp
(−∫ pmax
T
pT
P(z)dΦR
)
Powheg corrected parton shower
dσPO = B(ΦB)dΦB
[∆(pmin
T , pmaxT ) + ∆(pT , p
maxT )
RB dΦR
]
∆(pT , pmaxT ) = exp
(−∫ pmax
T
pT
RB dΦR
)
B(ΦB) = B(ΦB) + V(ΦB) +
∫R(ΦB ,ΦR)dΦR
Matrix-element corrections
Powheg correction available in Herwig++ for large number ofStandard Model processes.
For BSM processes, limited by absence of virtual matrix elements→ Powheg style matrix-element correction
Generate hardest emission using RBut local normalization is B rather than B
Implement ME correction using Matchbox and MadGraph
MadGraph 5 - used to generate B and RMatchbox - framework for NLO calculations, MC@NLO andPowheg matching to the Herwig++ angular ordered anddipole showers
Matrix-element corrections
Powheg correction available in Herwig++ for large number ofStandard Model processes.
For BSM processes, limited by absence of virtual matrix elements→ Powheg style matrix-element correction
Generate hardest emission using RBut local normalization is B rather than B
Implement ME correction using Matchbox and MadGraph
MadGraph 5 - used to generate B and RMatchbox - framework for NLO calculations, MC@NLO andPowheg matching to the Herwig++ angular ordered anddipole showers
Top squark pair production
Simulated pp → t1t∗1 , stable t1 at
√s = 14TeV, mt1
= 700 GeV.Limit simulation to hard process + full parton shower
LO
MEC
10−6
10−5
10−4
10−3
10−2
10−1
Matchbox +MadGraph 5
dσ/dpT[fb/GeV
]
0 500 1000 1500 2000
0.6
0.8
1
1.2
1.4
pT,t1t∗1[GeV]
MEC/LO
Example: e+e− → qqg [hep-ph/0310083]
Uncorrected shower:
over populates hard regions of phase space in pT . mt1region
has unpopulated dead zone for pT & mt1
Effect of the matrix element correction: exclusion bounds
ATLAS search for direct production of the top squark inevents with missing ET and two b-jets
t1 → bχ+1 → bf f ′χ0
1 with mχ+1−mχ0
1= 5GeV
Selection criterion Signal region A Signal region B
EmissT > 150 GeV > 250 GeV
Leading jet, j1 pT > 130 GeV, |η| < 2.8 pT > 150 GeV, |η| < 2.8
Subleading jet, j2 pT > 50 GeV, |η| < 2.8 pT > 30 GeV, |η| < 2.8
Third jet, j3 veto if pT > 50 GeV, |η| < 2.8 pT > 30 GeV, |η| < 2.8
∆φ(pmissT , j1) - > 2.5
b-tagged jetsj1 and j2 b-tagged with
pT > 50 GeV, |η| < 2.5
j2 and j3 b-tagged with
pT > 30 GeV, |η| < 2.5
mink (∆φ(pmissT , jk )) for k ≤ 3 > 0.4 > 0.4
EmissT /(
∑ni=1(p
jetT
)i + EmissT ) > 0.25, n = 2 > 0.25, n = 3
mCT [ref] > 150, 200, 250, 300, 350 GeV -
HT,3 =∑
(pjT
)i for all i > 3 - > 50 GeV
mbb =√
(pb,1 + pb,2)2 > 200 GeV -
Effect of matrix element correction: before
ATLAS search for direct production of the top squark inevents with missing ET and two b-jets
t1 → bχ+1 → bf f ′χ0
1 with mχ+1−mχ0
1= 5GeV
Original signal simulated with MadGraph + PYTHIA 6
200 300 400 500 600 700
mt1 [GeV]
100
200
300
400
500
600
mχ
0 1[G
eV]
ATLAS-SUSY-2013-05
∆mχ+1 −χ0
1= 5 GeV
t 1→bχ+1
forb
idden ∫
L = 20.1fb−1
√s = 8TeV
ATLAS result
Herwig++
Effect of matrix element correction: after
ATLAS search for direct production of the top squark inevents with missing ET and two b-jets
t1 → bχ+1 → bf f ′χ0
1 with mχ+1−mχ0
1= 5GeV
Original signal simulated with MadGraph + PYTHIA 6
200 300 400 500 600 700
mt1 [GeV]
100
200
300
400
500
600
mχ
0 1[G
eV]
ATLAS-SUSY-2013-05
∆mχ+1 −χ0
1= 5 GeV
t 1→bχ+1
forb
idden ∫
L = 20.1fb−1
√s = 8TeV
ATLAS result
Herwig++
And now for something a little different...
We’ve looked at SUSY searches based on a hard ISR jet + EmissT
But this in not the only option...
“monojet” = ISR jet + EmissT
“monophoton” = energetic photon + EmissT
“monotop” = top quark + EmissT
χ01
p p
q χ01
q
t
Monotop search for SUSY
Study LHC sensitivity using Monte Carlo simulations of 300fb−1 of14TeV collisions
Search for pp → t1χ01t with t1 → cχ0
1
Experimental signal is t + EmissT
t1 and χ01 light
Other SUSY particles decoupled m ≈ 10 TeV
Simulation
Signal:
pp → t1χ01t with t → bqq′ simulated in MadGraph 5
t1 → cχ01, parton shower, hadronization done with Herwig++
Background:
Process Simulation details
ttHard process at NLO with
PowhegBox, matched to Herwig++
Single top As above
tW production As above
W (→ lν) + light-jetsW production at NLO matched to
LO W+ 1 or 2 jets using Sherpa
γ/Z (→ l l/νν) + jets As above
Wbb with W → lνHard process at LO with
MadGraph, matched to Herwig++
Diboson NLO using Powheg in Herwig++
Event Selection Criteria
Designed to reflect final state of signal events
Exactly zero leptons
Exactly one b-jet, pT > 30GeV
Three other jets with pT > min(pbT , 40GeV)
Impose further cuts to maximize sensitivity = S√S+B
S , B are number of signal and background events passing the cuts
EmissT > 200GeV
50GeV < mjj < 100GeV
100GeV < mbjj < 200GeV
∆φ(pmissT , pj1) > 0.6 and ∆φ(pmiss
T , pb) > 0.6
∆φ(pmissT , pt) > 1.8
Results - Scan
Scan (mt1,mχ0
1) plane
Superimpose ATLAS monojet search for pair-produced top squarks
150 200 250 300
mt1[GeV]
50
100
150
200
250
300
350
mχ
0 1[G
eV]
mt1< m
χ01
+mc
ATLAS: 95% CL, 20fb−1
Monotop: 2σ, 300fb−1
Can monotops provide competitive sensitivity to monojettechniques?
No!
Results - Scan
Scan (mt1,mχ0
1) plane
Superimpose ATLAS monojet search for pair-produced top squarks
150 200 250 300
mt1[GeV]
50
100
150
200
250
300
350
mχ
0 1[G
eV]
mt1< m
χ01
+mc
ATLAS: 95% CL, 20fb−1
Monotop: 2σ, 300fb−1
Can monotops provide competitive sensitivity to monojettechniques? No!
Summary
SUSY is not dead
Light top squarks could still exist in compressed spectrascenarios
Search for them using non-standard analysis techniques, e.g.monojet or monotop signals
Monojet searches require accurate simulation of hard radiationand work pretty well
Monotops are not competitive at LHC energies