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Introduction to Jet Finding and Jetography (1)

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Introduction to Jet Finding and Jetography (1) Introduction to Jet Finding and Jetography (1) Gavin Salam CERN, Princeton University and LPTHE/Paris (CNRS) 2011 IPMU-YITP School and Workshop on Monte Carlo Tools for LHC Yukawa Institute for Theoretical Physics, Kyoto University, Japan September 2011 [Introduction] [Background knowledge] Jets Jets are everywhere in QCD But not the same as partons: Our window on partons Partons ill-defined; jets well-definable Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 2 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: dE dθ α ≫ 1 quark Z s E θ At low scales: αs → 1 Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: dE dθ α ≫ 1 quark Z s E θ θ At low scales: gluon αs → 1 Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: dE dθ α ≫ 1 quark Z s E θ At low scales: αs → 1 Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: dE dθ α ≫ 1 quark Z s E θ At low scales: αs → 1 hadronisation non−perturbative Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: π+ dE dθ α ≫ 1 quark Z s E θ KL π0 At low scales: K+ − αs → 1 hadronisation π non−perturbative Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Why do we see jets? Parton fragmentation Gluon emission: π+ dE dθ α ≫ 1 quark Z s E θ KL π0 At low scales: K+ − αs → 1 hadronisation π non−perturbative High-energy partons unavoidably lead to collimated bunches of hadrons Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 3 / 30 [Introduction] [Background knowledge] Jets from scattering of partons Tevatron results Jets are unavoidable at hadron 1010 K D=0.7 colliders, e.g. from parton scat- T CDF data ( L = 1.0 fb-1 ) 7 tering 10 Systematic uncertainties NLO: JETRAD CTEQ6.1M 4 corrected to hadron level jet µ µ JET µ 10 R = F = max pT / 2 = 0 PDF uncertainties [nb/(GeV/c)] 10 JET T JET 6 p dp -2 × 10 |y |<0.1 ( 10 ) JET -5 JET 10 0.1<|y |<0.7 (× 103) / dy σ JET 2 -8 0.7<|y |<1.1 p d 10 1.1<|yJET|<1.6 (× 10-3) 10-11 jet 1.6<|yJET|<2.1 (× 10-6) 10-14 0 100 200 300 400 500 600 700 JET pT [GeV/c] Jet cross section: data and theory agree over many orders of magnitude ⇔ probe of underlying interaction Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 4 / 30 [Introduction] [Background knowledge] Jets from scattering of partons CMS results Jets are unavoidable at hadron colliders, e.g. from parton scat- tering jet p p jet Jet cross section: data and theory agree over many orders of magnitude ⇔ probe of underlying interaction Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 4 / 30 [Introduction] [Background knowledge] Jets from scattering of partons ATLAS results Jets are unavoidable at hadron colliders, e.g. from parton scat- tering jet p p jet Jet cross section: data and theory agree over many orders of magnitude ⇔ probe of underlying interaction Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 4 / 30 [Introduction] [Background knowledge] Jets from heavy decays Heavy objects: multi-jet final-states − ◮ 107 t¯t pairs for 1 fb 1 @ 14 TeV ◮ Vast # of QCD multijet events − # jets # events for 1fb 1 3 2 · 1010 4 5 · 109 5 1 · 109 6 3 · 108 7 1 · 108 8 4 · 107 Tree level pt (jet) > 20 GeV, ∆Rij > 0.4, |yij | < 2.5 Gleisman & H¨oche ’08 Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 5 / 30 [Introduction] [Background knowledge] Seeing v. defining jets Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets q q Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets Jets are what we see. How many jets do you see? Clearly(?) 2 jets here Do you really want to ask yourself this question for 109 events? Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 6 / 30 [Introduction] [Background knowledge] Seeing v. defining jets A jet definition is a fully specified set of rules for projecting information from an event’s partons or hadrons onto a handful of parton-like objects (jets): π π p φ K LO partons NLO partons parton shower hadron level Jet Def n Jet Def n Jet Def n Jet Def n jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 Projection to jets should be resilient to QCD effects Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 7 / 30 [Introduction] [Background knowledge] QCD jets flowchart Jet (definitions) provide central link between expt., “theory” and theory And jets are an input to almost all analyses Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 8 / 30 [Introduction] [Background knowledge] QCD jets flowchart Jet (definitions) provide central link between expt., “theory” and theory And jets are an input to almost all analyses Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 8 / 30 [Introduction] [Background knowledge] These lectures Aims: to provide you with ◮ the “basics” needed to understand what goes into current jet-based measurements; ◮ some insight into the issues that are relevant when thinking about a jet measurement Structure: ◮ General considerations ◮ Common jet definitions at LHC Today ◮ Physics with jets Tomorrow Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 9 / 30 Defining jets Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 10 / 30 [Introduction] [General considerations] There is no unique jet definition The construction of a jet is unavoidably ambiguous. On at least two fronts: 1. which particles get put together into a common jet? Jet algorithm + parameters 2. how do you combine their momenta? Recombination scheme Most commonly used: direct 4-vector sums (E-scheme) Taken together, these different elements specify a choice of jet definition Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 11 / 30 [Introduction] [General considerations] There is no unique jet definition The construction of a jet is unavoidably ambiguous. On at least two fronts: 1. which particles get put together into a common jet? Jet algorithm + parameters 2. how do you combine their momenta? Recombination scheme Most commonly used: direct 4-vector sums (E-scheme) Taken together, these different elements specify a choice of jet definition Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 11 / 30 [Introduction] [General considerations] Not all ambiguity is allowed Jets should be invariant with respect to certain modifications of the event: ◮ collinear splitting ◮ infrared emission Why? ◮ Because otherwise lose real-virtual cancellation in NLO/NNLO QCD calculations → divergent results ◮ Hadron-level ‘jets’ would become fundamentally non-perturbative ◮ Detectors can resolve neither full collinear nor full infrared event structure Known as infrared and collinear safety Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 12 / 30 [Introduction] [General considerations] Not all ambiguity is allowed Jets should be invariant with respect to certain modifications of the event: ◮ collinear splitting ◮ infrared emission Why? ◮ Because otherwise lose real-virtual cancellation in NLO/NNLO QCD calculations → divergent results ◮ Hadron-level ‘jets’ would become fundamentally non-perturbative ◮ Detectors can resolve neither full collinear nor full infrared event structure Known as infrared and collinear safety Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 12 / 30 [Introduction] [General considerations] Not all ambiguity is allowed Jets should be invariant with respect to certain modifications of the event: ◮ collinear splitting ◮ infrared emission Why? ◮ Because otherwise lose real-virtual cancellation in NLO/NNLO QCD calculations → divergent results ◮ Hadron-level ‘jets’ would become fundamentally non-perturbative ◮ Detectors can resolve neither full collinear nor full infrared event structure Known as infrared and collinear safety Jets lecture 1 (Gavin Salam) MC tools for LHC school September 2011 12 / 30 [Introduction] [General considerations] Two main classes of jet alg.
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