Effects of Jet Matchingin BSM Signals
Johan Alwall, SLAC
With Simon de Visscher and Fabio Maltoni(Université de Louvain)
arXiv:0810.xxxx (very soon!)

IPMU Seminar, Tokyo, Japan, 14 Oct 2008

Introduction – The Big Picture

How to describe QCD emissions●Matrix Element Generators●Parton Showers

Matrix Element Generators
2Diagrams forby MadGraph

Use full Matrix Element information● Different methods in different generators●‒ MadGraph, Grace, Sherpa: Feynman diags + Helicity ampls‒ AlpGen, Helac: Recursion relations‒ Whizard: 1POW/DAG optimizationParton Showers●●●

Based on soft­collinear approximation
Markov process to perform subsequent QCD
emissions (due to factorization)
Strict ordering of emissions in ordering variable– Q2 (Pythia < 6.3), pT (Pythia > 6.3, Ariadne)– θE (Herwig)

Jet matching/merging

Complementary approximations
Need to combine without double counting
Johan Alwall - Effects of Jet Matching in BSM Signals

Matching schemes

CKKW scheme (Catani, Krauss, Kuhn, Webber)
Lönnblad scheme
MLM scheme
Johan Alwall - Effects of Jet Matching in BSM Signals

MLM matchingKeep

Discard unless highest multiplicity

Johan Alwall - Effects of Jet Matching in BSM Signals

Impact of matching on Z+jets

Matching vital to get multijet backgrounds right!
Johan Alwall - Effects of Jet Matching in BSM Signals

Matching in MG/ME+Pythia[MG/ME = MadGraph/MadEvent, an automatized Matrix Element
generator and event generator, see arXiv:0706.2334]●

kT and cone jet MLM schemes●

New “shower kT” scheme●

Both Q2 and pT­ordered Pythia showers●●

Extensively validated in V+jets [arXiv:0706.2569],
VV+jets, t pair+jets, H+jets and inclusive jets
Only generator that allows matching in BSM
processes (e.g. gluino/squark production)
Johan Alwall - Effects of Jet Matching in BSM Signals

Smoothness of matchingkTmin=10 GeV

kTmin=30 GeV

Diff. jet rates by MadEvent+Pythia in W+jets prod. at the Tevatron
Johan Alwall - Effects of Jet Matching in BSM Signals

Comparison with Tevatron DatapT(W) at the Tevatron“Old”
Pythiashowers“New”
Pythiashowers

Tail given by ME, head by pure showers
Johan Alwall - Effects of Jet Matching in BSM Signals

Matching in gluino/squark production●●

We know that matching is vital for jet productionin SM backgrounds
But is it relevant for heavy QCD particle
production?
– Very hard jets from decays– Parton showers more accurate for larger masses●●Turns out there are many cases wherematching has a large impact!
Most important at hadron colliders: ISR matching
Johan Alwall - Effects of Jet Matching in BSM Signals

Double counting●Special difficulty in SUSY matching – double
counting between squark and gluino production

Johan Alwall - Effects of Jet Matching in BSM Signals

Double counting●Special difficulty in SUSY matching – double
counting between squark and gluino production

Double countedwith on­shellgluino prod with~→d~ +qg
R
Johan Alwall - Effects of Jet Matching in BSM Signals

Double counting●

Solved by keeping track of on­shell resonancesin the production event files– Allows to remove double­counted events afterproduction●

Double­check – perform generation without
resonant diagrams (gauge­inv. only in NWA!)– Excellent agreement
Johan Alwall - Effects of Jet Matching in BSM Signals

Shower parameter dependence●

Shower “tweakable”– Strength for fitting data (after­the­fact)– Weakness for predictivity●

Most important parameters used here:– Type of shower (Q2 or pT­ordered)– Shower starting scale●●Factorization scale ­ “wimpy”●

Total energy of collider ­ “power”

Matching quite insensitive to shower parameters
Johan Alwall - Effects of Jet Matching in BSM Signals

Shower parameter dependence
ISR jets for different Pythia shower params

600 GeV gluino pair production at the LHC
Johan Alwall - Effects of Jet Matching in BSM Signals

Shower parameter dependence
ISR jets after matching with MG/ME

600 GeV gluino pair production at the LHC
Johan Alwall - Effects of Jet Matching in BSM Signals

Shower parameter dependence
ISR jets after matching with MG/ME

This predictivity now allows us to make proper
investigations of dependence on different factors
Johan Alwall - Effects of Jet Matching in BSM Signals

–
Dependence on the initial state: gg, qq, qq
gg

qq–gg+qqAll ISR jets 1st ISR jet 2nd ISR jetNo single shower tune for all initial states!
Johan Alwall - Effects of Jet Matching in BSM Signals

Dependence on the produced particle mass
300 GeV

600 GeV

1200 GeVAll ISR jets 1st ISR jet 2nd ISR jet
Can ISR help determine overall mass scale?
Johan Alwall - Effects of Jet Matching in BSM Signals

Impact of matching with decay
Matching obviously important for radiated jets,
but what about including decay jets from thesquarks/gluinos?
Still important in many scenarios!
Example: Small mass splitting between gluino
and squark – gives gluino decay with 2 hard + 2soft jets
Johan Alwall - Effects of Jet Matching in BSM Signals

Impact of matching with decay
600 GeV gluino pair production
Mg­Mq=50 GeVsquarksheavy –3­body~g decay
HT(2)

HT(3)

Johan Alwall - Effects of Jet Matching in BSM Signals

HT(4)Risk for misinterpretation
Scenario: sqsq production only (gluinos heavy)
Matched production (data)

HT(2)

HT(3)

HT(4)

Unmatched production (simulation, Pythia default)

Looks like we're missing gluino component!
Johan Alwall - Effects of Jet Matching in BSM Signals

Risk for misinterpretation
Scenario: ~q~q production only (gluinos heavy)~q~g production, Mg = 700 GeV

HT(2)

HT(3)

HT(4)

Unmatched ~q~q + 25%~q~g (fits “data”)

Easy misinterpretation: ~q~g component!
Johan Alwall - Effects of Jet Matching in BSM Signals

Light gluinos at the Tevatron
JA, Le, Lisanti, Wacker [arXiv:0803.0019,arXiv:0809.3264]
●

●

●

●

Searches at the Tevatron have always been
done in the mSUGRA frameworkmSUGRA (and mGMSB) “special” scenarios:
fixed mass ratio mg:mW:mB ~ 6:2:1
Not representative for general MSSM (or otherBSM models!)
Study of projected exclusion region at Tevatronwith free ratio mg:mB (arXiv:0803.0019)
Johan Alwall - Effects of Jet Matching in BSM Signals

Light gluinos at the Tevatron
JA, Le, Lisanti, Wacker [arXiv:0803.0019,arXiv:0809.3264]

Special difficulty when decay products are soft(nearly degenerate masses):●No (small) missing transverse energy in decay●Need recoil agains jets to get ET signature

Johan Alwall - Effects of Jet Matching in BSM Signals

Light gluinos at the Tevatron
Matched

Unmatched

Mg=150 GeV
MB=40 GeV

Mg=150 GeV
MB=130 GeV

Tevatron, after 2-jet and missing ET cuts
Johan Alwall - Effects of Jet Matching in BSM Signals

Impact on collective exp. searches“SPS1a” SUSY production at the LHC ­ unmatched

Johan Alwall - Effects of Jet Matching in BSM Signals

Impact on collective exp. searches“SPS1a” SUSY production at the LHC ­ matched

Johan Alwall - Effects of Jet Matching in BSM Signals

Conclusions●●The LHC is a hadronic collider – busy andcomplicated QCD environment
Any search for new physics must take QCD
radiation effects into account– Extra jet production from ISR QCD emissions– Boost of the central production system●Many scenarios where jet matching between
matrix elements and parton showers is crucial
to get a good description of the signal as well
as SM backgrounds
Johan Alwall - Effects of Jet Matching in BSM Signals

Backup slides

Johan Alwall - Effects of Jet Matching in BSM Signals

CKKW matching

Johan Alwall - Effects of Jet Matching in BSM Signals

Boost­invariant kT measure:●●●For final­state showers: Combination of NLL
Sudakov factors and vetoed NLL showers
guarantees independenc of dini to NLL order
For initial­state showers: No proof but works okProblem in practice: No NLL shower
implementation! (Sherpa uses Pythia­like showers)
Johan Alwall - Effects of Jet Matching in BSM Signals

More about matching in MG
Shower kT scheme●

Keep/reject event based on kT of hardest shower emission(as reported by Pythia)●Highest multiplicity treatment as in CKKW, use min dparton
as cutoff●No jet clustering●No need of “fiducial region”, can use kTmatch = dcutME●Need similar kT definitions in ME and PS (only “new”, pTordered showers at present)

Johan Alwall - Effects of Jet Matching in BSM Signals

Comparisons between old and new Pythia showers

Johan Alwall - Effects of Jet Matching in BSM Signals