Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions New development in Monte Carlos for the LHC Frank Krauss IPPP Durham RAL, 28.5.2008 F. Krauss IPPP New development in Monte Carlos for the LHC
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Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
New development in Monte Carlos for the
LHC
Frank Krauss
IPPP Durham
RAL, 28.5.2008
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Outline
1 The next generation event generators
2 The new OO tools
3 Signals & backgrounds at the parton level
4 From parton level to exclusive studies at hadron level
5 Forthcoming attractions in Sherpa
6 Modelling hadron/tau decays
7 Conclusions
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Why simulate events?Many interesting signals at the LHC:Higgs (or alternative EWSB), SUSY, ED’s, . . .
But: Severe backgrounds in nearly all channels,(almost always with large influence of QCD)
=⇒ depend on detailed understanding of QCD.
Typically: Take backgrounds from data in some region,but extrapolate to signal region.
Examples:
Central jet-veto in VBF (Higgs)
Multi-jet backgrounds for SUSY (e.g. Z+jets)
Todays signals = tomorrows backgrounds.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Why does anyone write a new event generator?New tools on the market: Pythia8, Herwig++, SherpaReflecting increased needs (precision, new physics, etc.):
getting rid of old errors (having new ones)
easier implementation of new physics models
incorporate new, better methods!
systematic inclusion of HO QCD
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
“Classical” example: n-gluon amplitudes:Start with two on-shell gluons, represented by their polarization vectors,hence the currents associated with them are Jν (k) = ε
ν (k).
Then the two-gluon current reads (no colors) Jµ(k = k1 + k2) =ig3
(k1+k2)2V µνρJν (k1)Jρ(k2).
From this, larger and larger currents can be built recursively.
For quarks, the currents are given by spinors, and similar reasoning applies for the construction of theone-particle off-shell currents.
Treatment of color: Color-ordering the amplitudes
=⇒ C(1, ..., n) = Tr [T a1 . . . T an ], where T a are color matrices in fundamental representation.
Problem: Need to sum over all allowed permutations.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Parton level simulations
Integration methods: Multi-channelingBasic idea: Translate Feynman diagrams into channels
=⇒ decays, s- and t-channel props as building blocks.R.Kleiss and R.Pittau, Comput. Phys. Commun. 83 (1994) 141
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
COMIX - a new matrix element generator for SherpaT.Gleisberg & S.Hoeche, in preparation
Colour-dressed Berends-Giele amplitudes in the SM
Fully recursive phase space generation
Example results (phase space performance):
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Survey of public parton-level tools
Comparison of tree-level toolsTools:
Models 2 → n Ampl. Integ. public? lang.Alpgen SM n = 8 rec. Multi yes FortranAmegic SM,MSSM,ADD n = 6 hel. Multi yes C++CompHep SM,MSSM n = 4 trace 1Channel yes CHELAC SM n = 8 rec. Multi yes FortranMadEvent SM,MSSM,UED n = 6 hel. Multi yes FortranO’Mega SM,MSSM,LH n = 8 rec. Multi yes O’Caml
Typically fed into shower MC’s through LHA, methodthen MLM.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Nomenclature
Specifying higher-order corrections: γ∗ → hadrons
In general: NnLO ↔ O(αns )
But: only for inclusive quantities(e.g.: total xsecs like γ
NLO-Normalization and first (hard) emission correct,Soft emissions correctly resummed in PS.
Method:Modify subtraction terms for real infrared divergences,use first order parton shower-expression,this is process-dependent!
In practise much more complicated.
Implemented for DY, W -pairs, gg → H , Q-pairs.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
MC@NLO
Example results: W -pairs @ Tevatron
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Combining MEs & PS: LO-MergingS.Catani, F.K., R.Kuhn and B.R.Webber, JHEP 0111 (2001) 063
F.K., JHEP 0208 (2002) 015
Want:All jet emissions correct at tree level + LL,Soft emissions correctly resummed in PS
Method:Separate Jet-production/evolution by Qjet (k⊥ algorithm).Produce jets according to LO matrix elementsre-weight with Sudakov form factor + running αs weights,veto jet production in parton shower.
Process-independent implementation.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Combining MEs & PS
n-jet rates @ NLLS.Catani et al. Phys. Lett. B269 (1991) 432
At NLL-Accuracy
R2(Qjet) =ˆ
∆q(Ec.m., Qjet)˜2
R3(Qjet) = 2∆q(Ec.m., Qjet)
·Z
dq
"
αs (q)Γq(Ec.m., q)∆q(Ec.m., Qjet)
∆q(q, Qjet)
∆q(q, Qjet)∆g (q, Qjet)
#
Sudakov weightsExample: γ∗ → qqg
WSud =αs (q)
αs (Qjet)· ∆q(Ec.m., Qjet)
∆q(Ec.m., Qjet)
∆q(q, Qjet)∆q(q, Qjet)∆g (q, Qjet)
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Combining MEs & PS
Algorithm as scale-setting prescriptionExample: p⊥ distribution of jets @ Tevatron
Consider exclusive W + 1- and W + 2-jet productionComparison with MCFM; J.Campbell and R.K.Ellis, Phys. Rev. D 65 (2002) 113007
in : F.K., A.Schalicke, S.Schumann and G.Soff, Phys. Rev. D 70 (2004) 114009
20 40 60 80 100 120 140 160 180p
T (jet) [GeV]
10-5
10-4
10-3
10-2
10-1
1/σ
dσ/d
p T [
1/G
eV]
MCFM NLOSherpaLO
Wj @ Tevatron
PDF: cteq6lCuts: p
Tlep> 20 GeV, |ηlep
|<1
pT
jet> 15 GeV, |ηjet|<2
pT
miss> 20 GeV
∆Rjj> 1.0
20 40 60 80 100 120 140 160 180p
T (first jet) [GeV]
10-4
10-3
10-2
10-1
1/σ
dσ/d
p T [
1/G
eV]
MCFM NLOSherpaLO
Wjj @ Tevatron
20 40 60 80 100p
T (second jet) [GeV]
10-4
10-3
10-2
10-1
1/σ
dσ/
dpT [
1/G
eV]
PDF: cteq6l
pT
jet> 15 GeV, |ηjet|<2
Cuts: pT
lep> 20 GeV, |ηlep|<1
pT
miss> 20 GeV
∆Rjj> 1.0
Sherpa = tree-level matrix elements with αs scales and Sudakov form factors.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Combining MEs & PS
Vetoing the shower
WVeto =
(
1 +
Z
Ec.m.
Qjet
dq Γq(Ec.m., q) +
Z
Ec.m.
Qjet
dq Γq(Ec.m., q)
Z
q
Qjet
dq′Γq(Ec.m.q
′) + · · ·
)2
=
(
exp
Z
Ec.m.
Qjet
dq Γq(Ec.m., q)
!)2
= ∆−2q (Ec.m., Qjet)
=⇒ Cancels dependence on Qjet.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Combining MEs & PS: Independence on Qjet
F.K., A.Schalicke, S.Schumann and G.Soff, Phys. Rev. D 70 (2004) 114009
Example: p⊥ of W in pp → W + X @ Tevatron
Qjet = 10 GeV Qjet = 30 GeV Qjet = 50 GeV
/ GeV Wp20 40 60 80 100 120 140 160 180
[ p
b/G
eV ]
W
/dp
σd
-210
-110
1
10
210
SHERPA
W + XW + 0jetW + 1jetW + 2jetsW + 3jets
/ GeV Wp20 40 60 80 100 120 140 160 180
[ p
b/G
eV ]
W
/dp
σd
-210
-110
1
10
210
SHERPA
W + XW + 0jetW + 1jetW + 2jetsW + 3jets
/ GeV Wp20 40 60 80 100 120 140 160 180
[ p
b/G
eV ]
W
/dp
σd
-210
-110
1
10
210
SHERPA
W + XW + 0jetW + 1jetW + 2jetsW + 3jets
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Soft physics simulation in SherpaImplemented a new version of the cluster fragmentationmodel;
a new module for the simulation of hadron and τ decays(special emphasis on τ , B and D decays, includingmixing, CP-violation etc.);
a new module for the simulation of photon radiation inhadron decays based on the YFS approach;
all in current, new release, Sherpa 1.1.
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Example (1): Spin correlations in H → τ+τ−
Angle of planes of decay products (piν) in c.m.s
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.5 1 1.5 2 2.5 3
no spin correlationsspin correlations with scalar Higgs
spin correlations with pseudoscalar Higgs
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Example (2): Form factors in decays
B → D∗ℓν τ → ππν
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Example (3): BB-mixing and decay into J/ΨKS
-10 -5 0 5 10
Eve
nts
0
10000
20000
30000
40000
50000
60000
tag candidates0B
tag candidates0
B
t(ps)∆-10 -5 0 5 10
Can
did
ate
Asy
mm
etry
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
SherpaTheory C=0 S=0.725
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Example (4): Photon radiation in W → ℓν
[GeV]γE0 10 20 30 40 50
]-1
[G
eVγ
dEΓd
tot
Γ1
-610
-510
-410
-310
-210
WINDEC
PHOTONS++
AMEGIC++
total photon energy in decay frame
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Example (5): Photon radiation in J/ψ → ℓℓ
[GeV]γE0.5 1 1.5 2 2.5 3
]-1
[G
eVγ
dEΓd
tot
Γ1
-710
-610
-510
-410
-310
ee→(1S) ψJ/
µµ →(1S) ψJ/
(1S) rest frameψtotal photon energy radiated in the J/
θ0 0.5 1 1.5 2 2.5 3
θdΓd to
tΓ1
-510
-410
-310
-210
-110
approximated matrix element
exact matrix element
eikonal factors only
radiated in Z rest frame-e+e→photon angular distribution in decay Z
F. Krauss IPPP
New development in Monte Carlos for the LHC
Introduction New tools Parton level Parton to hadron level Forthcoming attractions Hadron decays Conclusions
Summary & outlookMany interesting signals at LHC “spoiled” by QCD.
Simulation tools mandatory for success of LHC
Various new OO-projects in C++.
New methods of merging of ME& PS extremely powerful(Complementary to MC@NLO)
Sherpa a versatile tool - new features becoming available:higher ME multis, new showers, new Underlying Eventmodel (based on k⊥-factorization), more BSM models.
Plan: Go to NLO
automatic dipole subtraction implemented and testedbuild library of virtual corrections.