TRAPS: Topological Reconstruction Algorithm for Parton Scatters Katy Ellis 1. 4th of November 2011

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1 TRAPS: Topological Reconstruction Algorithm for Parton Scatters Katy Ellis 4th of November 20

2 Overview TRAPS Topological Reconstruction Algorithm for Parton Scatters An original stand alone algorithm written from scratch in C++. Developed using generator level MC output from Rivet. Now working in the ATLAS software 'Athena'. Aim: Reconstruct the final state partons in LO hadron hadron collisions. Usage: Fragmentation and structure funtions. QM seminar 04// 2

3 Jet finder? No!! Parton finder A 'jet' is a precisely defined cluster of final state particles. This definition generally involves pre defined parameters which may dictate the jet size and shape. When the jets are defined, they are labelled 'partons'. TRAPS sets out to measure partons. Parton candidates. QM seminar 04// 3

4 Personnel Entirely within QMUL: Myself, Graham Thompson, John Morris, Dan Traynor, Tom Macey. QM seminar 04// 4

5 TRAPS a brief summary A top down jet finding type algorithm which attempts to reconstruct the final state partons exiting the hard interaction. (Pythia particle numbers) Parton information is required for QCD measurements such as parton fragmentation functions and structure functions. QM seminar 04// 5

6 TRAPS in/out Inputs: Generator level Monte Carlo final state particle 4 vectors (so far Pythia, Herwig) Reconstructed level Monte Carlo. Data...from ATLAS (although theoretically any TeV scale hadron collider). QM seminar 04// 6

7 TRAPS a brief summary Inputs: Generator level Monte Carlo final state particle 4 vectors (so far Pythia, Herwig) Reconstructed level Monte Carlo. Data...from ATLAS (although theoretically any TeV scale hadron collider). Outputs: 2 'TRAPS objects' i.e. 'jets' as defined by the TRAPS algorithm. Their properties, such as 4 vectors, η, φ, pt (with QM seminar 04// and without background subtraction). Other event properties, such as iteration quantities. 7

8 Problems with FFs and LHC Traditionally, fragmentation and structure functions are not studied extensively at hadron hadron colliders and for good reason! High rates of background, compared to e+e, DIS QM seminar 04// 8

9 Problems with FFs and LHC Traditionally, fragmentation and structure functions are not studied extensively at hadron hadron colliders and for good reason! High rates of background, compared to e+e, DIS At the highest energies... Many events are approximately LO assume 2 jets Backgrounds are at their lowest QM seminar 04// 9

10 Problems with FFs and LHC Traditionally, fragmentation and structure functions are not studied extensively at hadron hadron colliders and for good reason! High rates of background, compared to e+e, DIS At the highest momentum transfers... Many events are approximately LO assume 2 jets Backgrounds are at their lowest Lack of information due to the composite nature of both colliding objects QM seminar 04// 0

11 Problems with FFs and LHC Traditionally, fragmentation and structure functions are not studied extensively at hadron hadron colliders and for good reason! High rates of background, compared to e+e, DIS At the highest energies... Many events are approximately LO assume 2 jets Backgrounds are at their lowest Lack of information due to the composite nature of both colliding objects Approximate the momentum transfer by the combined jet pt QM seminar 04//

12 Motivation The LHC is making accessible a new energy region. Study QCD at Q>00 GeV (or as low as we can) 00 GeV No agreement We must study QCD in this new kinematic region to test the standard model and look for new physics does the quark have structure? QM seminar 04// 2

13 TRAPS motivation Existing jet finders pre define jets using parameters. QM seminar 04// 3

14 TRAPS motivation Existing jet finders pre define jets using parameters. No fixed size No predisposition to circular shape Know the area so as to perform bg subtraction QM seminar 04// 4

15 TRAPS motivation Existing jet finders pre define jets using parameters. No fixed size No predisposition to circular shape Know the area so as to perform bg subtraction Many analyses perform sweeping selection cuts, vastly reducing the sample. QM seminar 04// 5

16 TRAPS motivation Existing jet finders pre define jets using parameters. No predisposition to circular shape No fixed size Know the area so as to perform bg subtraction Many analyses perform sweeping selection cuts, vastly reducing the sample. Bias-free analysis Use greatest possible sample of high-energy jets available QM seminar 04// 6

17 TRAPS ethos Be top down in our approach, rather than re combinatorial. Avoid parameters wherever possible and prove insensitive and arbitrary where not. Use geometry of individual events to decide how to treat them and their background subtraction. Be inclusive of as many events as possible, so as to have a large sample at high energy. QM seminar 04// 7

18 TRAPS Step by step QM seminar 04// 8

19 Step I For each event... Calculate the angle of the 'plane' of the event by finding the angle at which the pt out of the plane is y minimised. plane x 'pos' semi-cylinder 'neg' semicylinder Define a 'positive' and 'negative' semi-cylinder. QM seminar 04// 9

20 Step II Calculate a pt-weighted mean in η and φ for each semi-cylinder. Large energy deposits φ 'pos' semicylinder 'neg' semicylinder -ηrange 0.0 QM seminar 04// ηrange 20

21 Step III...floating boundary Cut out parts of the event (grey) away from the mean, so that the mean is in the centre of the available η - φ space for each semicylinder. φ 'pos' semicylinder 'neg' semicylinder -ηrange 0.0 η ηrange Re-calculate the means. Iterate until stable (until new mean old mean < 0.05). QM seminar 04// 2

22 Step IV...standard deviation Calculate the standard deviation within the remaining area and about the mean. φ Larger ση 'pos' semicylinder 'neg' semicylinder Smaller ση -ηrange 0.0 QM seminar 04// η ηrange 22

23 Standard deviation Jets come in all shapes and sizes... QM seminar 04// 23

24 Step V Place 2 ellipses with axes proportional to the standard deviation and centred on the mean. φ n*σφpos n*σηpos -ηrange 0.0 η ηrange n is some constant value (default = 2.0). Agreed, it is a parameter of sorts but we can show that our results are not sensitive to variation of n between e.g..5 and 3.0. QM seminar 04// 24

25 Step VI Sum px, py, pz of final state particles within the nσ ellipse. Re-calculate η and φ from these momentum components this is the new jet centre. New jet centre Old jet centre Iterate until the jet centre position is stable. Use the p enclosed in the ellipse as a starting T value for the parton pt (both scalar and vector pt are summed). Record which particles were summed this will be used for jet quality test in real data. QM seminar 04// 25

26 Step VII Subtract the background. The position of the background sampling area is chosen 2 regions for each jet, +/-π/2 in φ and not near to the jets in η. The p /LISA in each of the 4 regions is summed and T averaged as shown in the example below. This average is multiplied by the jet ellipse area then subtracted from the jet pt. Jet Take mean for Jet Jet 2 Take mean for Jet 2 QM seminar 04// 26

27 Step VIII Take a mean average of the vector and scalar sums this is our virtuality correction. This is a small correction (~2% on the pt) to enforce zero mass on our partons. Tight, narrow jets have a small correction; wide, more spread out jets have a larger correction. Smaller mass QM seminar 04// Greater mass 27

28 TRAPS: Accurate measurement? QM seminar 04// 28

29 'Delta plots' Assess how well TRAPS does, directly comparing to the MC parton information (Pragmatic Paradigm). Compare key quantities η, φ, p, e.g. (η ηtraps). T parton J3: GeV J4: GeV J5: GeV J6: GeV QM seminar 04// 29

30 'Delta plots' Pythia J3 J6 =sign of parton parton TRAPS = parton TRAPS pt / pt = pt pt pt parton Ap = TRAPS T parton QM seminar 04// pt pt parton parton pt pt TRAPS TRAPS 30

31 'Delta plots' Pythia J3 J6 (log scale) QM seminar 04// 3

32 The n-parameter n*σφpos n*σηpos QM seminar 04// 32

33 Alternative MC models Test TRAPS (developed with Pythia) on Herwig. Compare with the parton. J6 QM seminar 04// 33

34 Alternative MC models Test TRAPS (developed with Pythia) on Herwig. Compare with the parton. J3 QM seminar 04// 34

35 xbj Bjorken x is the proportion of momentum of the proton taken by the colliding parton. s = x x 2 s s Y x = e s s Y x2= e s QM seminar 04// 35

36 xbj inclusive differential cross-section QM seminar 04// 36

37 The FAPS method Fragmentation Algorithm for Parton Scatters. Tom Macey's algorithm for calculating fragmentation function using parton information from LHC energy collisions. To measure the correlated hadrons from the associated parton. Histogram the hadron p as a proportion of parton p. T T Use TRAPS as the parton, and also MC for comparison. xbj = 0.05 xbj = 0.5 xbj = 0. xbj = 0.35 xbj = 0.25 xbj = 0. (Naïve example only!) QM seminar 04// ph x p= momentum transfer / 2 9 xp ranges 37

38 Analysis cuts? Events are approximately leading order or at least many of them are. (Actually all events are NNNNN...LO!) Could make some cuts decision of end user. Higher momentum transfer = cleaner events. FAPS cuts: p balance within 0% of the mean T φ balance within 0% of п. QM seminar 04// 38

39 First FF measurement Fragmentation function as measured by past experiments and TRAPS (with Pythia). Plot made by Tom Macey. QM seminar 04// 39

40 Summary The TRAPS algorithm is an original tool written in C++ and available via CERN SVN (Path: Institutes/QMUL). It reconstructs final state partons resulting from high p LO T MC events, with good resolution and very high acceptance. Similar results with both Pythia and Herwig (gen. level). It has been adapted for use within the ATLAS software. Reconstructed MC is in use, but upgrades required. Not running on experimental data yet. QM seminar 04// 40

41 Current plans Objective ) Write an 'introduction to TRAPS' paper to submit to Computer Physics Communications (early Spring). Objective 2) Apply TRAPS to experimental data... Update TRAPS in Athena to latest version (end November). Apply the ATLAS Jet Energy Scale (December). Examine the effect of reconstructed data (December). Run on ATLAS data (January). More papers ATLAS note? fragmentation function measurement? (202). QM seminar 04// 4

42 Back up QM seminar 04// 42

43 Reconstructed data J3 QM seminar 04// 43

44 Reconstructed data J6 QM seminar 04// 44