1 / 21

Top Physics at the LHC

Top Physics at the LHC. Manchester Christmas Meeting 2006 Chris Tevlin. Outline. Experimental Work: Comparison of two jet algorithms for reconstructing the top mass Theoretical Work (to do!): Understanding/extending the dipole subtraction method Resummation

dorian-chan
Télécharger la présentation

Top Physics at the LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Top Physics at the LHC Manchester Christmas Meeting 2006 Chris Tevlin

  2. Outline Experimental Work: • Comparison of two jet algorithms for reconstructing the top mass Theoretical Work (to do!): • Understanding/extending the dipole subtraction method • Resummation • Application of these to ttbar cross section

  3. Experimental Work

  4. Introduction - Jet Algorithms • QCD - confinement (only colour singlets propagate over macroscopic distances) • No unique method of assigning (colourless) hadrons to (coloured) partons • Require a ‘sensible’ definition of a Jet - two of the main types of algorithm are: • Cone Algorithms • Cluster Algorithms

  5. Cone Algorithms • Geometrically motivated • Fixes the angular extent of a jet with Radius in - space (R invariant under boosts along the beam direction) • Requires some prescription for removing overlaps between jets • Not manifestly Infrared and collinear safe [Atlfast!] • Some Cone Algorithms Unsafe • ‘Mid-point’ Cone Algorithms Safe Clustering Algorithms (KtJet) • Kinematically motivated • [‘undoing’ the parton shower] • Theoretically favoured • Manifestly Infrared and Collinear Safe • All objects assigned exclusively to one jet

  6. Motivation (Theoretical Issues) • Infrared safety - the algorithm is insensitive to the addition arbitrarily soft partons • Collinear safety - the algorithm is insensitive to the replacement of any (massless) object by a pair of (massless) collinear objects • Infrared safety - IR and Collinear safety • Some Algorithms are classified as ‘Infrared Almost Safe’. The algorithm is rendered safe in the presence of a detector with finite energy resolution and angular granularity - this is dangerous for several reasons: • In order to perform a perturbative calculation one would need to know geometry of detector, cell thresholds etc • Since the angular extent of calorimeter cells, and cell energy thresholds are small, each term in such a calculation would be large - poor convergence!

  7. Motivation (Experimental Issues) • In the ‘Golden Channel’ the top mass reconstructed from 3 jets • By clustering to a specific jet multiplicity, one may hope to • Remove the soft underlying event (Exclusive Mode) • Solve Combinatorial issues • Increase the purity of the sample (pay in efficiency?)

  8. The algorithm (Exclusive Mode) • For each object, j, compute the closeness parameter djB, [(EjjB)2 for jB0] and for each pair of objects compute the parameters djk [min(Ej,Ek)2jk2 for jk0] • Find the smallest object from {djB,djk}. If this is a djB, remove it from the list of objects. Else, if it is a djk combine the two objects according to some recombination scheme [eg 4momentum addition] • Repeat stages 1 - 2 until some stopping criteria is fulfilled [eg some Jet Multiplicity]

  9. Analysis - Cuts • Require • >20GeV missing pT • At least one isolated lepton with pT>20GeV, ||<2.5 • Remove all isolated leptons from the list of objects, and run the jet finder: • Cone (Radii of 0.4 and 0.7) • KtJet (Exclusive Mode - Cluster to 4 jets) • Require at least 4 jets with pT>ptcut and ||<2.5 [Cone like] • Require 2 b-tagged jets [Truth]

  10. W reconstruction • Choice of two light jets as W candidate - for events with only two light jets, plot their invariant mass • Keep W candidates that lie within ±5 of the peak value, mjj. • From the remaining W candidates, the W which minimises 2 is chosen • If this W lies in a mass window of 2W then it is accepted [Cone like?] PxCone (R=0.4) KtJet

  11. W purity [Before the 2W cut] Seems to reconstruct the W better than PxCone

  12. Top Reconstruction • To reconstruct the top, choose the b quark which results in the highest pT top combined with the W candidate [later on use ‘leptonic top’ - missing pT] PxCone (R=0.4) KtJet

  13. Top purity/efficiency (Slightly) higher purity for low ptcut Lower efficiency

  14. Sub-Event Analysis • Merging scales - eg the scale at which the event changes from 5 jets to 4jets • One can cut on the different merging scales (peturbative observables) in the event Eg ptcut = 40GeV Red - good top candidates Blue - bad top candidates Cut? 

  15. A fifth jet (Hard Gluon emission) • So far always ran KtJet in the Exclusive mode, clustering until there were 4 jets • The signal (ttbar - Golden Channel) could include an additional jet from: • The emission of a hard gluon - O(S) effect • Extra jets from soft underlying event (In a fraction of events the ‘hardest’ 4 jets may not be from the signal) • Expect increase in efficiency • The emission of a hard gluon will alter the structure of the event - sub jet analysis

  16. Results - W purity [Before the 2W cut] • Drop in purity • expected! • Can we improve with • Subjet analysis?

  17. Results - Top purity/efficiency Significant increase in efficiency - factor of 2

  18. Theoretical ‘Work’

  19. Dipole subtraction Method • In general at NLO a jet observable will have two contributions: • Real emissions • Virtual loops • These ‘graphs’ are integrated over different phase spaces (m parton, m+1 parton) • A method for canceling all infrared and collinear divergences for a general jet observable, that could be implemented in a Monte Carlo [Nuc. Phys. B 485 (1997) 291-419] [Nuc. Phys. B 627 (2002) 189-265] • Possible extension is to a case with a massive parton in the initial state (eg tops) • Interesting phenomenology? Resummation • First measurement of bb cross section at Tevatron disagreed with NLO calculation by a factor of ~2

  20. Extras - (1) Mid-point Cone • The IR safety of an Iterating Cone Algorithm is ensured by considering the mid-point of any pair of proto-jets as a seed direction (Figure courtesy of Mike Seymour)

  21. Extras - (2) Infrared Safety • At NLO individual Feynman diagrams contain IR divergences - in any observable, these should cancel (eg the e+e-jets cross section) • When we define some observable, eg the 3 jet cross section, we must make sure that if a diagram with a divergence contributes to this, the diagram(s) which cancel it also contribute ‘3 jet’ ‘2 jet’

More Related