1. Measurement of the ttbar Interaction Cross Section in the tau+jets Decay Channel via Multivariable Template Fitting of 4.6 fb-1 of data collected at √s = 7 TeV by the ATLAS Detector
William P. Edson
Adviser Muhammad Sajjad Alam
Albany High Energy Physics Laboratory (AHEPL)
Nov

2. Contents The Large Hadron Collider (LHC)
ATLAS (A Toroidal LHC Apparatus) Detector
The Standard Model
Protons
ttbar production and decays
tau decays
Event Selection
QCD Multijet Data-Driven Analysis
Fitting Function
Ensemble and Linearity Test
Uncertainties
Final Results
Comparisons with Previous Results and Future Extensions
Nov

3. CERN aerial view from Geneva airport (© CERN) Photograph: Jean-Luc Caron Date: Jun 1986
Nov

4. The ATLAS Detector
ATLAS detector was designed to look for new physics in the highest currently possible energy range (14 TeV center of mass). This analysis’ results are based upon collisions at 7 TeV.
Nov

5. A detailed computer-generated image of the ATLAS detector and it's systems [1]
Nov

6. Liquid Argon Calorimeter
The Liquid Argon Calorimeter (LAr Calorimeter) is used to acquire precise measurements for hadronic jets stemming from quark or tau lepton production or gluon emission, as well as measure Missing Transverse Energy (MET).
Design Resolution [11]:
𝜎 𝐸 𝐸 = 50% 𝐸(𝐺𝑒𝑉) ⊕3% HEC
𝜎 𝐸 𝐸 = 100% 𝐸(𝐺𝑒𝑉) ⊕10% Fcal
Tile barrel
Tile extended barrel
LAr hadronic end-cap (HEC)
LAr electromagnetic end-cap (EMEC)
Energy resolution (after noise subtraction): 𝜎 𝐸 𝐸 = 𝑎 𝐸(𝐺𝑒𝑉) ⊕𝑏 [1]
a: stochastic term – governed by photostatistics and sampling fluctuations
b: constant reflecting non-uniformities in calorimeter response
⊕: Quadratic summation (ex. Pythagorus’ Theorem: 𝑎 2 = 𝑏 2 + 𝑐 𝑠 )
Liquid Argon:
stability of response over time
intrinsic radiation hardness
intrinsic linear behavior
Lar electromagnetic barrel
LAr forward (FCal)
Detailed computer generated view of the ATLAS Liquid Argon Calorimeter [1]
Nov

7. Tile Calorimeter
The Tile Calorimeter is used to make measurements for both MET and hadronic jets.
Design Resolution [12]:
𝜎 𝐸 𝐸 = 50% 𝐸(𝐺𝑒𝑉) ⊕5%
The information concerning the jets (from HEC, FCal and Tile Calorimeter) is then used for jet reconstruction and identification.
Tile barrel
Tile extended barrel
LAr hadronic end-cap (HEC)
LAr electromagnetic end-cap (EMEC)
Energy resolution (after noise subtraction): 𝜎 𝐸 𝐸 = 𝑎 𝐸(𝐺𝑒𝑉) ⊕𝑏 [1]
a: stochastic term – governed by photostatistics and sampling fluctuations
b: constant reflecting non-uniformities in calorimeter response
⊕: Quadratic summation (ex. Pythagorus’ Theorem: 𝑎 2 = 𝑏 2 + 𝑐 𝑠 )
Liquid Argon:
stability of response over time
intrinsic radiation hardness
intrinsic linear behavior
Lar electromagnetic barrel
LAr forward (FCal)
Detailed computer generated view of the ATLAS Liquid Argon Calorimeter [1]
Nov

8. ATLAS coordinate System
We define the azimuthal angle φ as the angle off the positive x-axis ranging from [-π, +π] and the polar angle θ measured from the positive z-axis.
The pseudorapidity is defined by the equation: y x z φ θ
η≡−𝑙𝑜𝑔 𝑡𝑎𝑛 𝜃 2
Nov

9. Standard Model (SM)
Table displaying the SM quarks and leptons with their properties. [4] The corresponding antiparticles are not given, however they display the same mass but negated charge values.
Nov

10. Protons
Protons are the most stable baryons which exist. They have a positive elementary charge of +1 and a mass of ± MeV. [4]
The valence quarks of the proton are the first generation quarks uud. Flowing between and around the quarks is a “sea” of gluons and quark anti-quark pairs.
Parton distribution functions at 10 GeV2 energy. X is the longitudinal momentum fraction of the proton. [8]
Nov

11. ttbar Production
Due to the high energy at the LHC, the most prolific particles will be the gluons.
~87% of the desired production is by gluon fusion. [9]
Remaining (~13%) will be mainly by qqbar annihilation. [9] g t g g g t t g t t g t g q t g q t
Nov

12. ttbar Decays
The top particle immediately decays via the weak interaction to Wb (BR ~ 0.91 ± 0.04). [4]
The resulting interactions of these two daughter particles are what are truly visible in the detector. t W+ b t W- b
Nov

13. ttbar Decays Continued
b quark production kicks of a hadronization process which is visible in the detector as a “jet”.
W boson production likewise results in quarks (seen as jets), or leptons and neutrinos.
Look for in b-jets:
A secondary vertex
A large number of charged tracks within them
A nearby lepton
Why? B hadron properties:
Large mass
A long life-time
Large cτ
High number of charged particles per decay
Chance of leptonic decay
Top Pair Branching Fractions for ttbar decays [10]
Nov

14. tau Decays
Due to the large mass of the tau, the tau has an extremely short lifetime.
((290.3 ± 0.5)*10-15 s [4])
Just like the top (or tbar) the tau decays via the weak interaction and must be reconstructed from the daughter particles of its decay.
μ-νμντbar 17.41±0.04% e-νeντbar 17.83±0.04% π-ντbar 10.83±0.06% π-π0ντbar 25.52±0.09% π-2π0ντbar 9.30±0.11% π-3π0ντbar 1.05±0.07% h-h-h+ντbar 9.80±0.07% h-h-h+π0ντbar 4.76±0.06%
Main branching ratios of tau decays h± can stand for either K or π. [4]
Nov

15. tau Decays Continued
Based on the branching ratios, we expect tau to decay hadronically approximately 65%. [4] In addition, the leptonic decays are impossible to distinguish as being produced via the tau decay versus any other process.
Due to these facts, the taus are identified through their hadronic decay jets. τ- W-
ν τ l
ν l τ- W- d u
ν τ
Nov

16. tau Hadronic Jets
The hadronic jets produced by taus come in 1 (~77%) or 3 (~23%) [4] prong type named for the number of charged hadron tracks (π’s or K’s) contained within the jet and recorded by the inner detector.
Tau jets are highly collimated resulting in a narrow particle shower recorded in the calorimeter.
The jets can also contain any number of neutral mesons (π0’s or K0’s) allowed by conservation of momentum and the tau neutrino. π0 π- π+
Example 3-prong jet
Nov

17. Backgrounds W + jets Z + jets Diboson (WW, ZZ, WZ)
ttbar → (μ, e) + jets
Single Top
QCD multijet
Feynman Dia.
Feynman Dia.
Feynman Dia.
Feynman Dia.
Feynman Dia.
Feynman Dia.
Nov

18. Why Study These Interactions?
tau is an expected final state of previously undiscovered particles both within and beyond the Standard Model (SM):
extra gauge bosons:
Z’ → τ+τ-
charged Higgs (possibly of mass less than top quark):
H+ → τ+ντ Z’ τ H+ τ ντ
Nov

19. Analysis Event Selection QCD Multijet Data-Driven Analysis
Remove events from samples which do not correspond to the desired event based on exhibited physical properties
QCD Multijet Data-Driven Analysis
Separate QCD multijet events remaining in sub-set of sample for use in predicting behavior in remainder
Template Fitting Method
Predict fraction of desired events from those in remaining data sample for use in cross section calculation
Ensemble and Linearity Test
Validate result utilizing toy data created with predefined signal fraction
Uncertainty Determination
Nov

20. Cross Section
Measure of the likelihood that some specific interaction will occur
𝜎= 𝑁 ℒ , [17]
where N is the event rate and ℒ is the luminosity (number of collisions per second per cm2)
Detectors and the selection process introduce errors to the event count which is corrected by applying additional terms.
Final equation:
𝜎= 𝑁 𝑜𝑏𝑠 − 𝑁 𝑏𝑎𝑐𝑘 ε∗ℒ∗𝐵𝑅 ,
where Nobs are the number of observed events, Nback are the number of background events which can fake the signal event, ε is the efficiency of the selection, ℒ is the measured luminosity and BR is the branching ratio of the interaction
Nov

21. Event Selection
Good Runs List and remove events collected during known LAr calorimeter instabilities
Trigger: medium identified tau with ET > 29 GeV, MET > 30 GeV, medium identified tau with ET > 29 GeV, MET > 30 GeV, 3 jets with ET ≥ 10 GeV
Primary vertex with greater than 4 primary tracks
Jet cleaning and remove events collected coinciding with noise bursts
Lepton Veto
At least 1 1-prong tau
At least 4 jets within|η| < 2.5
MET greater than 60 GeV
MT less than 80 GeV 𝑀 𝑇 = 2∗ 𝑝 𝑇 𝜏 ∗𝑀𝐸𝑇∗ 1−𝑐𝑜𝑠 ∆𝜑 𝜏,𝑀𝐸𝑇
At least one jet has been tagged as corresponding to production of a b quark
At least one event trigger matched tau with transverse momentum greater than 40 GeV
Truth matching of tau object to true tau for isolation of the signal sample from ttbar leptonic Monte Carlo sample
∆𝑅= ∆η 2 + ∆𝜑 2 <0.2
MET MT
2 triggers -> necessary to keep the HLT efficiency rate consistent at the higher L1 trigger rate in the later periods (B-K and L-M)
LAr error -> removes lumi blocks coinciding to times of known instabilities in LAr calorimeters
LAr noise -> removes lumi blocks coinciding to times of noise bursts in LAr calorimeters
Jet cleaning -> removes jets not actually associated with energy deposits in the calorimeter (stems from hardware problems)
bjets
Trigger
Nov

22. Event Selection Cutflow
Event Selection cuts
Event counts for 2011 data sample
Event counts for MC signal sample
Initial Count
Good Runs List and LAr Error
Trigger
Primary Vertex
Jet Cleaning and LAr Noise
Lepton Veto
963980
Tau Amount
269607
Number of Jets
566145
MET
74394
MT(W)
38312
Number of b-tagged jets
10842
Trigger Matching
1654
Truth Matching
Not applicable to data
Nov

23. QCD Multijet Data Driven Analysis
QCD Multijet events are not well modelled in MC simulation.
Instead: Create templates which symbolize expected QCD multijet behavior for the variables in question.
Divide the samples into two regions:
Signal: region which is expected to be signal rich, and so is where the resultant measurement is taken.
Control: region which is expected to be enriched with background events from which the template can be derived.
Nov

24. Boosted Decision Tree (BDT) Score
BDT Details
BDTs use multiple variables which alone do not have much discernment power.
The final score is determined by all decision trees in the collection.
The BDT used here is designed to determine the likelihood that a given jet is in fact truly a hadronic tau decay jet.
A true tau which would fail a particular tree may still receive a high score.
This choice is based on the fact that true QCD multijet events lack taus. Therefore any tau objects in a QCD multijet event would in reality be a misidentified jet.
BDT Variables
Nov

25. Normalized Signal and Background vs. Jet BDT Score
Nov

26. Template Fitting
Nov

27. Correlation Plots
Nov

28. QCD Templates
Nov

29. Chi-squared minimization
Equation:
χ 2 = 𝑖 𝑛 𝐷𝑎𝑡𝑎 𝑖 − 𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 ∗𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 𝑖 +𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 ∗𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 𝑖 + 𝑛 𝑀𝐶 𝑏𝑎𝑐𝑘 𝑖 𝜎 𝑠𝑢𝑚,𝑖 2
𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 =𝑁 𝐷𝑎𝑡𝑎 −𝑁 𝑀𝐶 𝑏𝑎𝑐𝑘
𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 =1−𝑓𝑟𝑎𝑐 𝑠𝑖𝑔
N(x): total number of events of type x for distribution
n(x)i: number of events in bin of type x
frac(sig): fraction of N(Remain) expected to be Signal
binfrac(x)i: fraction of events in bin of type x expected from MC or template
σsum,i: uncertainty in bin i
Nov

30. Chi-squared minimization cont.
𝜎 𝑠𝑢𝑚,𝑖 = 𝑗∈(𝐷𝑎𝑡𝑎, 𝑀𝐶 𝑏𝑎𝑐𝑘, 𝑠𝑖𝑔, 𝑄𝐶𝐷) 𝛿𝑠𝑢𝑚 𝛿 𝑛 𝑗 𝑖 2 𝜎 𝑛 𝑗,𝑖 𝛿𝑠𝑢𝑚 𝛿 𝑁 𝑗 𝑖 2 𝜎 𝑁 𝑗 2
𝛿𝑠𝑢𝑚 𝛿 𝑛 𝐷𝑎𝑡𝑎 𝑖 =1, 𝛿𝑠𝑢𝑚 𝛿 𝑛 𝑀𝐶 𝑏𝑎𝑐𝑘 𝑖 =−1
𝛿𝑠𝑢𝑚 𝛿 𝑛 𝑠𝑖𝑔 𝑖 =− 𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 ∗ 𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 𝑁 𝑠𝑖𝑔 , 𝛿𝑠𝑢𝑚 𝛿 𝑛 𝑄𝐶𝐷 𝑖 =− 𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 ∗ 𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 𝑁 𝑄𝐶𝐷
𝛿𝑠𝑢𝑚 𝛿 𝑁 𝑠𝑖𝑔 𝑖 = 𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 ∗ 𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 ∗ 𝑛 𝑠𝑖𝑔,𝑖 𝑁 𝑠𝑖𝑔 2
𝛿𝑠𝑢𝑚 𝛿 𝑁 𝑄𝐶𝐷 𝑖 = 𝑁 𝑅𝑒𝑚𝑎𝑖𝑛 ∗ 𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 ∗ 𝑛 𝑄𝐶𝐷,𝑖 𝑁 𝑄𝐶𝐷 2
𝛿𝑠𝑢𝑚 𝛿 𝑁 𝐷𝑎𝑡𝑎 𝑖 =− 𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑠𝑖𝑔,𝑖 + 𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑄𝐶𝐷,𝑖
𝛿𝑠𝑢𝑚 𝛿 𝑁 𝑀𝐶 𝑏𝑎𝑐𝑘 𝑖 = 𝑓𝑟𝑎𝑐 𝑠𝑖𝑔 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑠𝑖𝑔,𝑖 + 𝑓𝑟𝑎𝑐 𝑄𝐶𝐷 ∗ 𝑏𝑖𝑛𝑓𝑟𝑎𝑐 𝑄𝐶𝐷,𝑖
Nov

31. Iterative Process Validation
Nov

32. Minimization Results Signal Fraction: 0.69149 ± 0.04117
𝜎=165.8±9.9 𝑠𝑡𝑎𝑡. pb
Chi2/NDF: 1.0
Chi2/NDF: 0.81
Nov

33. Ensemble and Linearity Test
Nov

34. Ensemble and Linearity Test Results
Nov

35. Systematic Uncertainties
Nov

36. Systematic Uncertainties
Unc. (pb)
Unc. (%)
Linear Correction
± 1.0
±0.6
JER
± 3.9
± 2.4
JRE
± 0.1
± 0.03
JES
+19.8/ -18.9
+12.0/ -11.4
JVF SF
+0.8/ -0.7
+0.5/ -0.4
b-tag SF
+4.3/ -3.9
+2.6/ -2.4
MET
+2.9/ -2.0
+1.7/ -1.2
Tau Trigger SF
+4.3/ -5.5
+2.6/ -3.3
TES
± 1.8
± 1.1
Tau Eveto SF
± 0.04
W+jets Normalization
+11.6/ -11.1
+7.0/ -6.7
QCD Selection
± 4.9
± 3.0
QCD Template Bin/Range
+4.9/ -9.2
+2.9/ -5.5
MC Generator
± 11.3
± 6.8
ISR/FSR
± 7.3
± 4.4
PDF
± 1.5
TauID
Total
+29.7/ -30.0
+17.9/ -18.1
Lumi
Systematic Uncertainties
Nov

37. Final Results Final Result:
Sample
W+jets
Z+jets
ttbar leptonic
Diboson
Single top
QCD Multijet
Signal
Total
2011 Data
Events
100.3 ± 9.1
28.6 ± 2.2
26.5 ± 1.0
0.3 ± 0.1
50.8 ± 2.0
276.5 ± 22.2
608.2 ± 5.4
± 24.8
1091 ± 33
Final Result:
𝜎 𝑡 𝑡 →𝜏+𝑗𝑒𝑡𝑠 = ±9.9 𝑠𝑡𝑎𝑡. − 𝑠𝑦𝑠𝑡. ±3.0 𝑙𝑢𝑚𝑖. pb
Nov

38. Comparison with SM Prediction and Previous Results
Nov

39. Possible Future Extensions of the Analysis
Determine same value in 3-prong case either individually or in combination with current 1-prong analysis.
Perform analysis utilizing the 8 TeV center of mass energy collected in 2012.
Perform analysis utilizing the upcoming data to be collected at 14 TeV with the stable beam collection projected in early 2015.
Nov

40. Thank You Special Thanks: Dr. Muhammad Sajjad Alam
Dr. Dick Greenwood – Louisiana Tech U.
Dr. Patrick Skubic – University of Oklahoma
Dr. Brad Abbott – University of Oklahoma
Dr. Serban Protopopescu - BNL
Nov

41. References
ATLAS Collaboration, “The ATLAS Experiment at the CERN Large Hadron Collider”, Journal of Instrumentation, vol 3, S08003 (2008), doi: / /3/08/S08003.
ATLAS Collaboration, “Measurement of the ttbar production cross section in the tau+jets channel using the ATLAS detector”, The European Physical Journal C, vol 73, 2328 (2013), arXiv: v2 [hep-ex].
CMS Collaboration, “Measurement of the top-antitop production cross section in the tau+jets channel in pp collisions at sqrt(s) = 7 TeV”, The European Physical Journal C, vol 73, 2386 (2013), arXiv: v2 [hep-ex].
K.A. Olive et al. (Particle Data Group), Chin. Phys. C, 38, (2014).
ATLAS Collaboration, “Measurement of the top quark pair production cross-section with ATLAS in the single lepton channel”, Physical Letters B, vol 711 (3-4), pp (2012), arXiv: v2 [hep-ex].
CMS Collaboration, “Measurement of the t t-bar production cross section in pp collisions at sqrt(s) = 7 TeV with lepton + jets final states”, Physics Letters B, vol 720 (1-3), pp (2013), arXiv: v2 [hep-ex].
Michal Czakon, Paul Fiedler, Alexander Mitov, “The total top quark pair production cross-section at hadron colliders through O(alpha_S^4)”, Physical Review Letters, vol 110 (25), (2013), arXiv: v1 [hep-ph].
Placakyte, Ringaile. “Parton Distribution Functions.” Proceedings of XXXI Physics in Collision 28 Aug-1 Sep Vancouver, BC Canada. Web. 28 Oct arXiv: v4 [hep-ph].
Lambacher, Marion. Study of fully hadronic ttbar decays and their separation from QCD multijet background events in the first year of the ATLAS experiment. Diss. LMU München, Electronic Theses of LMU Munich. Web. 27 Oct
Collins, Neil. “Top Cross Section (Current Status and Early LHC Prospects).” Birmingham Particle Physics Seminars. University of Birmingham Particle Physics group. University of Birmingham, Birmingham, UK. 10 March Seminar. 27 Oct
Lampl, W. “Status of the ATLAS Liquid Argon Calorimeter and its Performance after Three Years of LHC Operation.” 14th ICATPP Conference on Astroparticle, Particle, Space Physics and Detectors for Physics Applications 23 – 27 Sept , Como, Italy. Ed. S. Giani, C. Leroy, L. Price, P.-G. Rancoita, and R. Ruchti. Hackensack: World Scientific, Web. 29 Oct doi: / _0094.
Bertolucci, Federico. “The ATLAS Tile Calorimeter performance at LHC in pp collisions at 7 TeV.” EPJ Web of Conferences, vol 28, (2012), doi: /epjconf/
Lefevre, C. LHC: the guide. CERN Geneva: CERN, 17 February Web. CERN-Brochure Eng.
ATLAS Collaboration. ATLAS muon spectrometer : Technical Design Report. Geneva : CERN, Print.
Ruber, Roger. “ATLAS Magnet System.” ATLAS Magnet System. ATLAS Magnet Group, Web. 29 Oct
Nave, C. R. “Fundamental Forces.”HyperPhysics. C. R. Nave, Web. 29 Oct
Griffiths, David. Introduction to Elementary Particles 2nd Ed. Federal Republic of Germany: Wiley-VCH Verlag GmbH & Co., KGaA, Weinheim, Print.
The ATLAS collaboration, “Performance of the Reconstruction and Identification of Hadronic Tau Decays with ATLAS”, 13 November 2011, ATLAS-CONF
ATLAS Collaboration,
Nov

42. Back-ups
Nov

43. ATLAS coordinate System
The point of interaction (collision) is taken as the origin. The z-axis is defined by the direction of the incoming beam. The x-axis points to the center of the LHC and the y-axis points skyward. y x z
Nov

44. Rapidity Rapidity (𝜑) is defined by the equation: tanh 𝜑 =v
where v is the velocity of the particle
As the particle’s velocity is related to the momentum of the particle (which is determined by the recorded track in the inner detector and energy deposited in the calorimeters), it can be calculated but not as easily as the pseudorapidity based on the polar angle (θ).
Nov

45. Particle Interactions and Gauge Bosons
At the particle physics scale, three forces are responsible for the interactions which occur and each has corresponding mediating particles (the gauge bosons):
Strong force: mediated by the massless and elementary chargeless gluons (8 total), is (as its name suggests) the dominate force at this scale and has a range of approximately m. [16] Only occurs between color charged particles.
Electromagnetic force: mediated by the photon which also has no mass or charge. It is the second strongest of these forces and has an infinite range. Occurs between particles carrying charge.
Weak force: mediated by the intermediate vector bosons W±, and Z. These bosons actually have mass and the W carries charge. Due to the mass of these particles, the range of the weak force is actually the shortest at m. [16] Occurs between all quarks and leptons.
Nov

46. Proton Interactions
The primary interaction for ttbar production is the strong interaction due to the quarks and sea gluons carrying color charge.
As the energy of the interaction increases the amount of gluons within the proton (the sea) increases.
Parton distribution functions at GeV2 energy. X is the longitudinal momentum fraction of the proton. [8]
Nov

47. The Top Quark
Quarks are the elementary particles which come together to form hadrons. These particles are fermions (having spin 1 2 ), and also carry color charge and fractional elementary charge.
All quarks also have an associated antiparticle which has the same mass and spin but opposite charge. These particles are differentiated via the use of a bar above their symbol ( t ) or following their name (tbar).
The most massive of the known quarks is called the top quark (t). It is a member of the third generation of quarks (with bottom) having a charge of and a mass of ± 0.51 ± 0.71GeV. [4]
Nov

48. Taus
Leptons are elementary particles that are similar to quarks but vary in some very drastic ways. Like quarks, all leptons are fermions and have corresponding antiparticles. However, leptons do not carry color charge and have integral (-1) elementary charge.
Each generation of leptons includes a corresponding neutrino with neutral charge and extremely small mass (~0).
Taus are the most massive of the known leptons ( ± 0.16 MeV) [4] and are located in the third generation of leptons and have a charge of -1.
Nov

49. Tau Reconstruction
Current reconstruction method for the tau uses either calorimeter-seeded, track-seeded algorithms or a combination of the two:
Calo shower
Calo-seeded: Begins with the detection of a particle shower within the calorimeter and then searches for associated tracks within a narrow cone.
Track-seeded: Selects a good quality track (pT > 6 GeV) and then looks for other tracks around the seed track within an isolated cone. The energy deposited in the calorimeter within this cone is also collected.
Calo seed
Seed track
Track cone
Isolation cone
Nov

50. Finding the Interaction
When searching for ttbar → τ events, we can select events from that data that fit the required criteria.
2 b-jets
Missing transverse energy (MET)
the presence of a lepton (muon or electron) or hadronic jets.
And, of course, the reconstructed tau
l, q
νl, q t+ W+ b- t- W- b+ ντ
τ -
Nov

51. Ensemble Test Fits of QCD Template
Nov

52. Linearity Results to Full Fraction Range
Nov

53. Linearity Test Results
𝑥(𝑓𝑖𝑛𝑎𝑙)= 𝑦(𝑜𝑢𝑡𝑝𝑢𝑡)−𝑦(0) 𝑠𝑙𝑜𝑝𝑒
Nov