Study  of  additional  radiation   in  top  pair  events  using  the   ATLAS  detector  at  the  LHC

Study  of  additional  radiation   in  top  pair  events  using  the   ATLAS  detector  at  the  LHC

Doug Benjamin Duke University

On behalf of the ATLAS Collaboration

Why  study  the  top  quark  ?

top quark does not hadronize and plays a special role in the Standard Model

o   heaviest particle M

= 173.07 ± 0.52 ± 0.72 GeV (PDG) o   short lifetime - no time to make top jets (

>> Λ

)

o   Decays predominated to b quarks |V

|= 0.89±0.07 (PDG) o   Top dominant in the Higgs self-coupling

•  Higgs self-coupling determines Higgs field stability up to Plank scale

•  Measurements of top mass very important in determining Higgs Field stability

top pair + vector boson events probe top EWK properties

•  top quark pairs vital for perterbative QCD (pQCD) studies at the top quark energy scale.

top pair + jet events can be used to tune Monte Carlo models

top  pair  production  and  decay

•  At the LHC top pairs dominantly produced via gluon fusion

o   can be used probe the gluon PDF at large x

•  σ

170 (245) pb @ √s = 7 (8) TeV

•  the LHC is a top quark production factory – even more so in Run 2.

2

Top  quark  decay

full   hadronic

,  45%

e+jets,   15%

mu+jets,   15%

tau+jets,   15%

di-­‐‑

lepton,   10%

b  jet b  jet

jets

 (hadronic  decay)

E_t^miss lepton

Event  Selection  used  in  these  results lepton  +  jets

•  one  high  p_T  isolated  lepton

•  ≥  4  jets

•  E_T^miss,  M_T(W)

•  b-­‐‑tagging

Dilepton

•  two  high  p_T  isolated  leptons

•  ≥  2  jets

•  E_t^miss

•  b-­‐‑tagging

decay  channels

(event  classification)

top  pair  production    w/  extra  jets

•  Test of pQCD , provides constraint to uncertainties associated with QCD radiation modeling

•  √s = 7 TeV , dilepton decay channel w/ 2 b-tagged jets.

o   Done so additional jets could easily be distinguished from ttbar decay products.

•  Have measured several final state observables that are sensitive to parton radiaton (‘gap fraction’)

f (Q

) = n(Q

)

N

n(Q₀) – subset of events w/ no additional jets with P_T > threshold Q₀. Minimum Jet P_T > 25 GeV

f (Q

) = n(Q

)

N

n(Q_sum) – subset of events w/ scalar sum of jet P_T Σ_jP_T < Q_sum.

4

top  pairs  w/  extra  jets  (cont)

•  Comparison with multi-leg NLO and LO multi-leg generators with different phase space(PS) and tunings

Central Rapidity -

•  All generators but MC@NLO +Herwig comparable to the data

Forward Rapidity –

•  All MC generators have

difficulty describing the data especially at low Q_0. Sherpa is closest to the data.

f (Q

) = n(Q

) N

central  rapidity

forward  rapidity

top  pairs  w/  extra  jets  (cont)

•  Comparison with multi-leg NLO and LO multi-leg generators with different phase space(PS) and tunings

Central Rapidity -

•  All generators comparable to the data. MadGraph+Phythia and Powheg+Pythia show

poorer agreement w/ data.

Forward Rapidity –

•  All MC generators have

difficulty describing the data especially at low Q_Sum.

MadGraph+Phythia and

Powheg+Pythia show poorer agreement w/ data. Sherpa is closest to the data.

f (Q

) = n(Q

) N

central  rapidity

forward  rapidity

6

top  pairs  (lepton+jets)  w/  extra  jets  

Global jet-multiplicity – top anti top events, full 2011 7 TeV dataset.

Lepton + jets decay channel

•  Jet multiplicities for different jet- P_T thresholds compared to

multi-leg NLO and LO MC generators.

•  Systematic uncertainties limit measurement precision.

(Bkg modeling at lower jet

multiplicities and Jet Energy Scale at higher jet-multiplicities)

•  Jet-P_T > 25 GeV, – agreement in the 3-5 jets bins.

•  MC@NLO+Herwig

underestimates the data 6 jets and above.

top  pairs  +  heavy  flavor  

•  Used to constrain models of heavy-flavor quark production at top quark mass scale

•  √s = 7 TeV , dilepton decay channel with at least one additional jet

•  2-D Template fit method – displaced vertex mass and jet p_T, using different b-tag operating points (avg b-jet selection efficiency) (High: b-jet effcy 60%, c-jet 17%, light flavor jets 0.43%;

Medium b-jet 10%, c-jet 7%, light jets 1%; Low: b-jet 5%, c-jet 6%, light jets 1.33%)

•  ALPGEN+HERWIG (LO ME) used to produce inclusive ttbar and dedicated ttbar+HF simulated samples w/ heavy flavor overlap removal (to avoid double counting)

•  POWHEG+HERWIG(NLO ME+LO extra jets) – produce inclusive ttbar samples

•  Measure ratio w/ respect to tt+jets

•  Largest systematics – heavy flavor tagging efficiency and fragmentation modeling

Irreducible  background  for  WH,  H      bb

High  purity Medium  purity

Low  purity

tt+HF

σ

tt+jets

σ = 6.2 ± 1.1(stat.) ± 1.8(syst.)%

ALPGEN+HERWIG  =  3.4%

POWHEG+HERWIG  =  5.2% ⁸

Associated  Vector  Boson  

production  (W,Z)  and  top  pairs

•  √s = 8 TeV Full 2012 data-sample

•  2 leptons (same sign ( μμ ) (SS) or opposite sign (OS))

•  3 leptons (e or μ )

•  Multijets and b-tagged jets required to increase sensitivity to ttV (V= W or Z)

•  OS dilepton search – small signal on large

backgroup (tt+jets and Z+jets) – Neural net used to separate signal from background

•  Trilepton and same sign dilepton – signal to

background comparable – cut and count

technique used

Associated  Vector  Boson  production   (W,Z)  and  top  pairs  (cont)

ATLAS-­‐‑CONF-­‐‑2014-­‐‑038

Process Signal  Strength Observed  σ   Expected  σ

WV 4.9 4.9

WW 3.1 2.4

WZ 3.2 3.8

−0.22 +0.23

0.89

−0.48 +0.57

1.25

−0.26 +0.29

0.73

Summary  and  Conclusions

t-tbar events rich in physics

•  Precise tests of pQCD at the top quark mass scale

o  These final states with extra QCD radiation are used to provide constraints on current MC generators

o  Import source of information to enable further tuning of the generators and constraining model uncertanties

•  Associated production of top pairs and Heavy Flavor measurements provide crucial measurements for an irreducible background in ttH (H->bb)

o  ttH cross-section - a fundamental measurement in Run 2 and beyond

•  Top pairs + Vector bosons

o  Test the Electroweak properties of the Top.

Run  2  will  allow  us  to  continue  to  exploit  the  top  quark  factory  that  is  the  LHC Important  source  of  physics  measurement  for  many  years  to  come

Backup  Slides

12

Associated  Vector  Boson  production   (W,Z)  and  top  pairs  (cont)

ATLAS-­‐‑CONF-­‐‑2014-­‐‑038

2-­‐‑dim  simultaneous  fit  WZ  and  WW  signal  strengths    along  with  the  68%  CL  and   95%  CL  contours  compared  to  2-­‐‑D  fit  results  in  OS  dilepton  &  trilepton  and  SS   dileptons    dash  area  corresponds  to  22%  uncertainty  on  NLO  QCD  theor.  Calc.

Associated  Vector  Boson  production   (W,Z)  and  top  pairs  (cont)

ATLAS-­‐‑CONF-­‐‑2014-­‐‑038

Two-­‐‑dimensional  fit

Channel µμ_WZ µμ_WW Observed  σ Expect  σ

OS  dilepton 0.77±0.65 0.71±2.41 0.4 0.6 Trilepton  &  

SS  dilepton 0.70^+0.30_{-­‐‑0.28} 1.37^+0.62_{-­‐‑0.51} 4.1 4.1 Combination 0.71^+0.28_{-­‐‑0.26} 1.30^+0.59_{-­‐‑0.48} 4.4 4.4

The  observed  signal  strength  for  WZ  and  WW  production   from  the  two-­‐‑dimensional  fit,  and  the  observed  and  

expected  significance  of  the  signals  for  each  analysis  and   combination

14

Associated  Vector  Boson  production   (W,Z)  and  top  pairs  (cont)

ATLAS-­‐‑CONF-­‐‑2014-­‐‑038

Summary  of  obs  and  exp.  Number  of  events  in  all  signal  

regions.  Shaded  bands  include  stat.  and  sys.  uncertanties ¹⁵

Associated  Vector  Boson  production   (W,Z)  and  top  pairs  (cont)

ATLAS-­‐‑CONF-­‐‑2014-­‐‑038

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