Particle Physics (Phenomenology) . Lecture 2/2 Peter Skands - PowerPoint PPT Presentation

Particle Physics (Phenomenology) . Lecture 2/2 Peter Skands (Monash University) November 2019 Sydney Spring School

What can our (incoming and outgoing) states be? Quantum Fields of the each comes in 3 “colours” Standard Model Spin-1/2 Spin-1 Spin-0 + anti-quarks 8 “colours” of gluons Spin-1/2 The LHC collides protons ➜ OK! (factorisation; PDFs) … and observes (jets of) hadrons … + anti-leptons 2 � Peter Skands

What are Jets? Think of jets as projections that provide a universal view of events π π Illustrations by G. Salam p φ K LO partons NLO partons Parton Shower Hadron Level LO partons NLO partons parton shower hadron level Jet Definition Jet Definition Jet Definition Jet Definition Def n Def n Def n Def n Jet Jet Jet Jet jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 I’m not going to cover the many different types of jet clustering algorithms (k T , anti-k T , C/A, cones, …) - see e.g., lectures & notes by G. Salam. ➤ Focus instead on the physical origin and modeling of jets � 3 Particle Physics Peter Skands

Example of a “bad” algorithm: “Seeded Cone Algorithm” ICPR iteration issue Start from “hardest” seeds Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Simplifed “event” with three energy depositions, at different “rapidities” (essentially different angles to the beam) in the detector Collinear splitting can modify the hard jets: ICPR algorithms are Want to find how many jets of a fixed “cone size” there are. collinear unsafe = ⇒ perturbative calculations give ∞ Idea: start from largest energy deposition as seed, and iterate from there. Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” ICPR iteration issue Start from “hardest” seeds Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Looks ok but energy-weighted centre of jet ≠ jet axis. Move jet axis to energy-weighted centre, and iterate Collinear splitting can modify the hard jets: ICPR algorithms are until stable jet axis found collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” ICPR iteration issue Start from “hardest” seeds Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Stable. Jet axis now gives us energy-weighted centre of jet. Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” ICPR iteration issue Start from “hardest” seeds Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” ICPR iteration issue Start from “hardest” seeds Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Looks fair. Why is this bad? Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Here’s the same event, with the highest energy “seed” split into two separate (but almost “collinear”) cells Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Now we would use a different seed to start from Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 Collinear splitting can modify the hard jets: ICPR algorithms are collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 300 200 100 0 − 1 0 1 rapidity jet 1 jet 2 Collinear splitting can modify the hard jets: ICPR algorithms are This time, we found not one, but two jets collinear unsafe = ⇒ perturbative calculations give ∞ Slides from G. Salam

Example of a “bad” algorithm: “Seeded Cone Algorithm” QCD lecture 4 (p. 28) ICPR iteration issue Start from “hardest” seeds Jets Iterative Cone Progressive Removal Cones cone iteration cone axis 500 cone p T (GeV/c) 400 Why were seeded 300 algorithms 200 sometimes used in the past? For 100 efficiency reasons 0 and due to lack of − 1 0 1 understanding of rapidity the problems of jet 1 such algorithms jet 2 Problem with seeded algorithms in general: Not " collinear safe ”. Collinear splitting can modify the hard jets: ICPR algorithms are By splitting a parton into two, we got a different number of jets. collinear unsafe = ⇒ perturbative calculations give ∞ Why is this bad? One parton physically indistinguishable from two collinear ones (if they sum to same 4-momentum) ⟹ ill-defined jet number Slides from G. Salam