monte carlos for lhc - luhome.thep.lu.se/~torbjorn/talks/cern04cms.pdf · monte carlos for lhc...
TRANSCRIPT
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CMS Generator WorkshopCERN
3 November 2004
Monte Carlos for LHCTorbjorn Sjostrand
CERN and Lund University
Generator and Physics OverviewMatrix Elements vs. Parton ShowersUnderlying Event and Hadronization
Outlook
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Event Generator Position
“real life”
Machine ⇒ eventsproduce
events
“virtual reality”
Event Generator
observe & store events
Detector, Data Acquisition Detector Simulation
what is
knowable?Event Reconstruction
compare real and
simulated dataPhysics Analysis
conclusions, articles, talks, . . .
“quick
and dirty”
where and why?
• detector requirements
• analysis strategies
• acceptance corrections
physics is complex
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Event Physics Overview
Structure of the basic generation process:
1) Hard subprocess:|M|2, Breit-Wigners,
parton densities.
q
q Z0 Z0
h0
2) Resonance decays:includes correlations.
Z0
µ+
µ−
h0
W−
W+
ντ
τ−
s
c
3) Final-state parton showers.
q → qg
g → gg
g → qq
q → qγ
4) Initial-state parton showers.
g
q
Z0
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5) Multiple parton–partoninteractions.
6) Beam remnants,with colour connections.
p
p
b
b
ud
ud
u
u
�
�
�
�
�
5) + 6) = Underlying Event
7) Hadronization
c
g
g
b
D−s
Λ0
n
η
π+
K∗−
φ
K+
π−
B0
8) Ordinary decays:hadronic, τ , charm, . . .
ρ+
π0
π+
γ
γ
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9) QCD interconnection effects:
e−
e+
W−
W+
q3
q4
q2
q1
�
�
π+
π+
�
� BE
a) colour rearrangement(⇒ rapidity gaps?);b) Bose-Einstein.
10) The forgotten orunexpected: a chain
is never stronger thanits weakest link!
Many aspects still poorly understood,but most good enough to work with
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Generator Landscape
Hard Processes
Resonance Decays
Parton Showers
Underlying Event
Hadronization
Ordinary Decays
General-Purpose
HERWIG
PYTHIA
ISAJET
SHERPA
Specialized
a lot
HDECAY, . . .
Ariadne/LDC, NLLjet
DPMJET
none (?)
TAUOLA, EvtGen
specialized often best at given task, but need General-Purpose core
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The Smaller Picture: Subprocess Survey
Kind Process PYT HER ISA
QCD & related Soft QCD ? ? ?Hard QCD ? ? ?Heavy flavour ? ? ?
Electroweak SM Single γ∗/Z0/W± ? ? ?
(γ/γ∗/Z0/W±/f/g)2 ? ? ?Light SM Higgs ? ? ?Heavy SM Higgs ? ? ?
SUSY BSM h0/H0/A0/H± ? ? ?SUSY ? ? ?R/ SUSY ? ? —
Other BSM Technicolor ? — (?)New gauge bosons ? — —Compositeness ? — —Leptoquarks ? — —H±± (from LR-sym.) ? — —Extra dimensions (?) (?) (?)
. . . but processes usually only in lowest nontrivial order
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The Bigger Picture
Process Selection
Resonance Decays
Parton Showers
Multiple Interactions
Beam Remnants
Hadronization
Ordinary Decays
Detector Simulation
ME Generator
ME Expression
SUSY/. . .spectrum
calculation
Phase Space
Generation
PDF Library
τ Decays
B Decays
=⇒ need standardized interfaces
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The Les Houches Accord
Specialized Generator
=⇒ Hard Process
Les Houches Interface
HERWIG or PYTHIA
(Resonance Decays)
Parton ShowersUnderlying Event
HadronizationOrdinary Decays
Some Specialized Generators:• AcerMC: ttbb, . . .• ALPGEN: W/Z+ ≤ 6j,
nW + mZ + kH+ ≤ 3j, . . .• AMEGIC++: generic LO• CompHEP: generic LO• GRACE+Bases/Spring:
generic LO+ some NLO loops• GR@PPA: bbbb• MadCUP: W/Z+ ≤ 3j, ttbb
• MadGraph+HELAS: generic LO• MCFM: NLO W/Z+ ≤ 2j,
WZ, WH, H+ ≤ 1j
• O’Mega+WHIZARD: generic LO• VECBOS: W/Z+ ≤ 4j
Apologies for all unlisted programs
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Matrix Elements vs. Parton Showers
ME : Matrix Elements+ systematic expansion in αs (‘exact ’)+ powerful for multiparton Born level+ flexible phase space cuts− loop calculations very tough− negative cross section in collinear regions
⇒ unpredictive jet/event structure− no easy match to hadronization p2
⊥,θ2,m2
dσdp2
⊥
, dσdθ2, dσ
dm2
real
virtual
PS : Parton Showers− approximate, to LL (or NLL)− main topology not predetermined
⇒ inefficient for exclusive states+ process-generic ⇒ simple multiparton+ Sudakov form factors/resummation
⇒ sensible jet/event structure+ easy to match to hadronization p2
⊥,θ2,m2
dσdp2
⊥
, dσdθ2, dσ
dm2
real×Sudakov
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Parton Shower Approach
3 common algorithms:HERWIG: θ-ordered emissions (ISR & FSR)PYTHIA: M2, Q2-ordered emissions (ISR & FSR)ARIADNE: p⊥-ordered emissions (FSR primarily)
Steady evolution:HERWIG: new angular evolution variable⇒ improved phase space coverage,
better massive quark treatmentPYTHIA: p⊥-ordered emissions (ISR & FSR)⇒ improved coherence,
interleaved multiple interactions,(to prove:) simplified vetoed parton showers
LDCMD, CASCADE: CCFM generators for ISR at small x
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Matrix Elements and Parton Showers
Marriage desirable! But how?
Problems: • gaps in coverage?• doublecounting of radiation?• Sudakov?• NLO consistency?
Much work ongoing =⇒ no established orthodoxy
Three main areas, in ascending order of complication:
1) Match to lowest-order nontrivial process — merging
2) Combine leading-order multiparton process —vetoed parton showers (cf. talk by M. Mangano)
3) Match to next-to-leading order process —MC@NLO (covered in talk by S. Frixione)
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Merging
= cover full phase space with smooth transition ME/PS
Want to reproduce WME =1
σ(LO)
dσ(LO + g)
d(phasespace)
by shower generation + correction procedure
wanted︷ ︸︸ ︷
WME =
generated︷ ︸︸ ︷
WPS
correction︷ ︸︸ ︷
WME
WPS
• Exponentiate ME correction by shower Sudakov form factor:
WPSactual(Q
2) = WME(Q2) exp
(
−∫ Q2
max
Q2WME(Q′2) dQ′2
)
• Normally several shower histories ⇒ ∼equivalent approaches
• Use dσ = K σ0 dWPS
where K = 1 + O(αs) is set separately (ambiguity of O(α2s ))
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PYTHIA performs merging with generic FSR a → bcg ME,in SM: γ∗/Z0/W± → qq, t → bW+, H0 → qq,and MSSM: t → bH+, Z0 → qq, q → q′W+, H0 → qq, q → q′H+,χ → qq, χ → qq, q → qχ, t → tχ, g → qq, q → qg, t → tg
g emission for different Rbl3 (yc): mass effects
colour, spin and parity: in Higgs decay:
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
1.14
1.16
0 0.02 0.04 0.06 0.08 0.1
R3bl
yc
VectorAxial vector
ScalarPseudoscalar
angle (degrees)
PYTHIA ISR: only qq → γ∗/Z0/W± and gg → H0 (for mt → ∞)(but K factor not implemented here)
HERWIG: fewer for FSR, comparable for ISR
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Vetoed Parton ShowersS. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063; L. Lonnblad, JHEP0205 (2002) 046;
F. Krauss, JHEP 0208 (2002) 015; S. Mrenna, P. Richardson, JHEP0405 (2004) 040;
M.L. Mangano, in preparation
Generic method to combine ME’s of several different ordersto NLL accuracy; will be a ‘standard tool’ in the future
Basic idea:• consider (differential) cross sections σ0, σ1, σ2, σ3, . . .,
corresponding to a lowest-order process (e.g. W or H production),with more jets added to describe more complicated topologies,in each case to the respective leading order
• σi, i ≥ 1, are divergent in soft/collinear limits• absent virtual corrections would have ensured “detailed balance”,
i.e. an emission that adds to σi+1 subtracts from σi
• such virtual corrections correspond (approximately)to the Sudakov form factors of parton showers
• so use shower routines to provide missing virtual corrections⇒ rejection of events (especially) in soft/collinear regions
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Veto scheme:1) Pick hard process, mixing according to σ0 : σ1 : σ2 : . . .,
above some ME cutoff, with large fixed αs0
2) Reconstruct imagined shower history (in different ways)3) Weight Wα =
∏
branchings(αs(k2⊥i)/αs0) ⇒ accept/reject
CKKW-L:4) Sudakov factor for non-emission
on all lines above ME cutoffWSud =
∏
“propagators′′
Sudakov(k2⊥beg, k2
⊥end)
4a) CKKW : use NLL Sudakovs4b) L: use trial showers5) WSud ⇒ accept/reject6) do shower,
vetoing emissions above cutoff
MLM:4) do parton showers5) (cone-)cluster
showered event6) match partons and jets7) if all partons are matched,
and njet = nparton,keep the event,else discard it
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Multiple Interactions
Consequence of composite nature of hadrons!
Evidence:
• direct observation: AFS, UA1, CDF
• implied by width of multiplicitydistribution + jet universality: UA5
• forward–backward correlations: UA5
• pedestal effect: UA1, H1, CDF
One new free parameter: p⊥min
1
2σjet =
∫ s/4
p2⊥min
dσ
dp2⊥
dp2⊥ ⇐=
∫ s/4
0
dσ
dp2⊥
p4⊥
(p2⊥0 + p2
⊥)2dp2
⊥
g g
d
resolved
g g
d
screened
λ ∼ 1/p⊥
Measure ofcolour screening length d
in hadron:p⊥min 〈d〉 ≈ 1(= h)
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Event Structure and Beam Remnants
(TS & P.Z. Skands, JHEP 03 (2004) 053)
pg
u
s
s
u
d
tohardint.
beamremn.
Need to assign:
• correlated flavours• correlated xi = pzi/pztot
• correlated primordial k⊥i
• correlated colours
for initiators and remnants
+ showers
Example: parton densities after first interaction:
• valence: scale by #remaining/#original
• sea: bookkeep ‘companion’ by
s(x′;x) ∝g(x + x′)
x + x′Pg→ss
(x
x + x′
)
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Interleaved Multiple Interactions
(TS & P.Z. Skands, hep-ph/0408302)
interactionnumber
p⊥
hard int.
1
mult. int.
2
mult. int.
3
mult int.
4
p⊥max
p⊥min
p⊥1
p⊥2
p⊥3
p⊥4
p⊥23
ISR
ISR
ISR
ISR
p′⊥1
Data comparisons:usually ∼ Tune Abut need good tuning
0.3
0.4
0.5
0.6
50 100 150nch
<p ⊥
>
Tevatron Run II: <p⊥>(nch)
Tune ARapSharp ISRLow FSRHigh FSR
〈p⊥〉(nch) problem:colour correlations?
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Hadronization: Lund String Model
In QCD, for large charge separation, field lines seem to be compressedto tubelike region(s) ⇒ string(s)
String tension: F (r) ≈ const = κ ⇐⇒ V (r) ≈ κr
Confirmed e.g. by quenched lattice QCDUnquenched =⇒ nonperturbative string breakings
q (r)
g (rb)
q (b)
snapshots of string position
strings stretched
from q (or qq) endpoint
via a number of gluons
to q (or qq) endpoint
Gluon = kink on string,
carrying energy
and momentum.
Force ratiogluon/ quark = 2,
cf. QCD NC/CF = 9/4
• Few parameters to describe energy–momentum structure!• Many parameters to describe flavour composition!
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Lund hadronization news: fragmentation of junction topology,in R-parity violating SUSY decays χ0
1 → uds,or when 2 valence quarks kicked out of proton beam(TS & P.Z. Skands, NPB659 (2003) 243)
lab frame
z
x
u (r)
d (g)
s (b)
J
junctionrest frame
u (r)
d (g)
s (b)
J
120◦
120◦
120◦
flavour space
q3
q4
q5q3 q2 q2 qq1 qq1 u
q4
d
q5
s
More complicated
(but ≈solved) with
gluon emission and
massive quarks
Also new: fragmentation of stable gluino
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Hadronization: HERWIG Cluster Model
Introduce forcedg → qq branchings:
●
subprocess
underlyingevent
p
jet jet
p
hard
●
+
0Z
ee −
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� � � �� � � �� � � �� � � �� � � �� � � �
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� � � � � � � � � �� � � � � � � � � �� � � � � � � � � �� � � � � � � � � �� � � � � � � � � �
� � � � �� � � � �
� �� �� �
� �� � � � � � �� � � � �� � � � �
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� �� �� �� � ��� ���
��� ��� ��� � �� � �� � �� � �� � �� � �
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! !" "# ## #$ $$ $
%�% &�&
'�' (�(
)*
+�+ ,�,
●
Large-mass clusters require special attention• Many parameters to describe energy–momentum structure!
• Few parameters to describe flavour composition!
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Standards and Interfaces
? ? ? PDG particle codes
? ? ? HEPEVT hadron-level Event Record
? ? ? Les Houches Accord User Process Interface
? ? ? LHAPDF: Les Houches Accord Parton Density Functions(supersedes PDFLIB)
? ? ? SLHA: SUSY mass/coupling spectrum calculator interface
?? HepMC hadron-level Event Record in C++
?? JetWeb/HZtools: automated data comparisons
? StdHep, StdHepC++: converts non-standard particle codes
? HepPDT particle data tables in C++
? For C++ era: (improved) Les Houches Interface for HO or NLO ME’s,standardized cuts, standard cone clustering algorithm, . . .
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On To C++
PYTHIA7 project =⇒ ThePEGToolkit for High Energy Physics Event Generation:
general-purpose framework, kinematics, ME machinery, decays, . . .(L. Lonnblad; S. Gieseke, A. Ribon, P. Richardson)
ARIADNE/LDC: to do ISR/FSR showers, multiple interactions(L. Lonnblad; N. Lavesson)
PYTHIA7 leftover: old showers + incomplete string fragmentation⇒ restart from scratch 2 months ago (TS)
HERWIG++: new final-state shower + improved cluster modeland decays ⇒ e+e− complete, pp underway
(B.R. Webber; S. Gieseke, A. Ribon, P. Richardson, M. Seymour, P. Stephens)
SHERPA: does pp, but partly wrappers to PYTHIA Fortran; has CKKW(F. Krauss; T. Gleisberg, S. Hoeche, A. Schaelicke, S. Schumann, J. Winter)
• Conversion effort: everything takes longer and costs more(as for LHC machine, detectors and software)
• The physics hurdle is as steep as the C++ learning curve
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Outlook
Generators in state of continuous development:• better & more user-friendly general-purpose
matrix element calculators+integrators• new libraries of physics processes, also to NLO
• more precise parton showers• better matching matrix elements ⇔ showers
• improved models for underlying events / minimum bias• upgrades of hadronization and decays
• moving to C++⇒ always better, but never enough
But what are the alternatives, when event structures are complicatedand analytical methods inadequate?