Semileptonic tt decays with 0.1/fb

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Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B

Semileptonic tt decayswith 0.1/fb

Stefan Kasselmann

III. Physikalisches Institut B, RWTH Aachen

LHC/CMS schedule

Picture of the Tracker Inner/Outer Barrel from July 2006

• CMS closes at 31/08/07

• First beam: November 2007: - 0.9 TeV (CM)

- 43 vs. 43 bunches- 1028-1030 cm-2s-1

• Debugging machine/detector

• Then: Commissioning of all 8 sectors for full energy in winter 2008 shutdown

• First Physics: Spring 2008

- 14 TeV- 156 vs. 156 b.- 1032 cm-2s-1

• 0.1/fb : “A few weeks of data taking"

http://lhc-commissioning.web.cern.ch

“1st physics run” scenario

• 0.1/fb corresponds to about 48.800 ttbar (inclusive) events, taking the LO cross section from PYTHIA (CTEQ 5L) (http://cmsdoc.cern.ch/cms/PRS/gentools/www/xsec/cmsxsec.html)

• This analysis is (so far) based on the following assumptions:

• no pixel detector No b jet tagging were used!• no ECAL endcaps Electron identification only in || < 1.47

No cut on MET used

• Ideal: Use new MC data (CMSSW) in this specific detector configuration:

• Tracking algorithms work different without pixel detector (seeds)!• Less material in front of silicon detector (particle interactions)• …

• So far: Data (Pythia) from 2004 used (with pixel). Ongoing: Converting data files (ALPGEN) which better simulates gluon radiation processes

• Goal: Develop analysis to "see" tops in this scenario (e.g. invar. mass spectrum)

Myon chambers

Forward calorimeter

Superconductive coil (4 Tesla)

Electromagnetic calorimeter

(ECAL)

Silicon Tracker(Pixel+Strip)

Hadroniccalorimeter

(HCAL)

CMS detector

top pairs @ LHC

• tt production: produced via two processes (strong interaction):

ttgg ttqq 13%:87%:

• 10 top pairs/s @ 1034 cm-2s-1

• But: About 20 pile up events!

• Main background: W+jets, Z+jets, Dileptonic ttbar decay

2/3 1/3

Top pair decay

W+ u , d / c , s (3 colours)

W- u , d / c , s (3 colours)

BR(tt bW+bW-) ~ 100%

BR(W+W- l11 l22) ~ 11%BR(W+W- q1q2q3q4) ~ 44%BR (W+W- q1q2l) ~ 44%

(9/81)(36/81)(36/81)

• Only electrons (pT > 10 GeV, || < 1.47) and muons (pT > 10 GeV, || < 2.4) are used

Electrons: Likelihood based selection of electrons from candidates Muons: Are taken as they come out of the GlobalMuonReconstructor

• Lepton isolation consists of calorimeter and tracker isolation

For both a cone of R = srqt(2 + 2) = 0.2 is used around the track of the particle

Calorimeter isolation: No energy deposits > 15% of lepton energy Tracker isolation: No tracks > 10% of lepton momentum

Lepton identification

Some of the input variables for electron likelihood:

• E / P: super cluster energy / track momentum (for electrons close to 1)

• H / E: energy in HCAL (behind super cluster) / super cluster energy (for electrons close to 0)

= | SC - track | : Difference between super cluster position and extr. track pos. at ECAL

• E9 / E25 : ECAL energy 3x3 cell / 5x5 cell• …

R = 0.2

Generic preselection

Typical preselection: • L1 & HLT Trigger• 4 jets with pT > 10 GeV, || < 2.5 (low pT cut to be able to run different scenarios)

• At least one (tracker & calo) isolated lepton with pT > 10 GeV

Dataset Events 0.1/fb L1 HLT 4 jets lepton eff. (%)Signal:

semilept. (e/mu) 14.331 12.907 9.997 9.987 5.798 40,46

Background:semilept. (tau) 7.176 4.871 2.303 2.302 358 4,99

dileptonic 5.094 4.593 3.767 3.756 2.622 51,47fully hadronic 22.199 12.684 6.185 6.185 13 0,06

W + jets 34.823.250 11.274.140 9.539.255 7.275.019 4.736.267 13,60Z + jets 1.105.530 296.842 200.770 188.370 128.650 11,64QCD 230.938.958.000 481.056.332 17.475.944 17.341.508 797.681 3,5E-04

WW + jets 7.002 3.343 2.124 2.086 1.066 15,22ZW + jets 2.700 8.056 446 432 256 9,48ZZ + jets 1.100 306 153 150 86 7,82

Signal 14.331 12.907 9.997 9.987 5.798Background (scaled) 2,31E+11 4,93E+08 2,72E+07 2,48E+07 5,67E+06

S / B 6,20E-08 2,62E-05 3,67E-04 4,02E-04 1/1000

Selection

• First selection cut: Exactly one lepton

• Less efficient cuts (not used): - Two leptons with diff. charge- Two lepton mass (Z peak)

• The „one lepton cut“ is most efficient against Z+jets and dileptonic TTbar events

• Z+jets suppression: 35%• Dileptonic suppression: 23%• Signal loss: <

• In about 98.1% of the selected semileptonic events the lepton taken is the one from W decay (that means it matches the MC signal lepton with R < 0.01 and has correct charge)

dileptonic

Z+jets

logarithmic scale!

W+jets

Selection

• Second selection cut: 3rd jet pT > 45 GeV

• Tried many cut variations on the (pT sorted) four highest pt jets

• Most efficient against W+jets and dileptonic ttbar events (which only have two high energetic b jets from hard interaction)

• Dileptonic suppression: 67% W+jets suppression: 99,9%

Signal loss: 40%• In addition all other jets (4th, 5th) in the event must fullfill: pT > 30 GeV to

reduce the jet combinations for the Jet Parton Matching (JPM)

dileptonic

W+jets

Selection

• Third selection cut: Circularity > 0.3

• This variable has small values for planar events and high values for circular events.

• This cut is most efficient against QCD events

• QCD suppression: 99% W+jets suppression: 45%

Signal loss: 40%

• (But: Low statistics of QCD!)

• Result: After these three selection cuts one gets an S/B of about 0.9

• Now one has to find the three jets from top out of 4 or 5 jets. Therefore a likelihood was developed.

)ˆ(min2C

Selection overview

• 4 or 5 jets with pT > 30 GeV, 3rd jet pT > 45 GeV (pT sorted)

• Exactly one (tracker & calorimeter) isolated lepton with pT > 10 GeV

• Circularity > 0.3

DatasetPreselected

Events4 or 5 jets

pt > 30 GeVExactly

one lepton3rd pt jet >

45 GeVcircularity >

0.3 eff. (%)Signal:

semilept. (e/mu) 5.798 2.743 2.656 2.122 1.330 22,94

Background:semilept. (tau) 358 170 164 134 84 23,46

dileptonic 2.622 620 448 319 199 17,00fully hadronic 13 6 6 5 2 15,38

W + jets 4.736.267 6.730 6.705 1.482 962 0,02Z + jets 128.650 1.261 869 327 204 0,16QCD 797.681 1.982 1.956 1.913 2 0,00

WW + jets 1.066 86 83 56 23 2,16ZW + jets 256 10 8 4 3 1,17ZZ + jets 86 8 6 4 2 2,33

Signal 5.798 2.743 2.656 2122 1.330Background (scaled) 5.666.999 10.873 10.245 4.244 1.481

S / B 0,0010 0,2523 0,2592 0,5000 0,90

Jet Parton Matching

• For early top physics JPM, I use six (simple) variables which distinguish between right and wrong jet pairings, namely angles, masses and pT of jets.

• JPM criteria (All 4-jet-combinations out of 4 or 5 jets are used)

The sum of R(jet, parton) of all 4-jet-comb. is calculated, the lowest taken( -> best global matching)

Each jet then must fulfill: R(jet, parton) < 0.25 and |PT

MC – PTRec| / PT

MC < 0.5( -> definition of matching jet)

The top candidate itself must fulfill: R(Rec. top, MC top) < 0.25 ( -> reject badly reconstructed events)

The selected lepton must fulfill: R(Rec. lep., MC lep.) < 0.01 ( -> the right lepton must have been found)

• The permutation that fulfills all these requirements for 4 jets is declared as true jet pairing (black curves). All others are filled as wrong pairings (red curves). The normalized distributions are used as probability density functions (PDFs)

JPM PDFs

• Mass of 2-jet-permutations: True combinations: Both jets from W False combinations: All other permutations

(Right combinations of two jets peak at W mass)

• Angle between 2-jet-permutations: True combinations: Both jets from W False combinations: All other permutations

(The jets from a W tend to have a smaller angle)

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 14/19

JPM PDFs

between top and anti top: True combinations: 3 jets from top / one jet/lepton from other top False combinations: All other permutations of 3 to 1 jet+lepton

(Right combinations tend to be antiparallel in )

• Angle sum of 3-jet-permutations: True combinations: All jets from had. top False combinations: All other permutations

(Right combinations of three jets tend to have a smaller angle sum due to boost)

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 15/19

JPM PDFs

• Angle between lepton and b jet: True combinations: b jet lep. side / lepton False combinations: All other permutations

(Right comb. of lepton and b jet tend to have a smaller angle due to boost)

• pT sum of 2-jet-permutations: True combinations: Both jets are b jets False combinations: All other permutations

(The b jets tend to have a tiny higher transverse momentum than other jets)

JPM (likelihood cut)

• Final selection cut: Likelihood > 0.85

• This cut is mainly to have a good probability to choose the right three jets (from top)

• This cut obviously also reduces much of the remaining background

• After the final selection one gets an S/B of about 6

• For the following top mass plots only events with a LR of more than 0.85 are taken (207 semileptonic events remain).

Top signal 0.1/fb

• But high combinatorial background:

In about 50% the correct W was found In about 35% the correct top was found

• Problem: Higher purity needs higher cut on JPM likelihood, but too less statistics!

w/o in situ cal.

with in situ cal.

(Not stacked)

• Top signal clearly visible!

• Artificial Neural Networks (ANN) uses correlations between input variables! But: Need three times more MC (training and validation)

• First look at different network topologies (1/2/3 hidden layers and different number of perceptrons) using SNNS http://www-ra.informatik.uni-tuebingen.de/SNNS/

• Can an ANN improve the JPM (likelihood) efficiency? -> Studies ongoing…

• As an example:

Training (black) and validation (red) of an ANN:

Two important issues:

1.) For each net take configuration with minimum of validation error 2.) Of all nets take the one withthe smallest validation error

(empirically search for best net)

Outlook- Use ANN?

Use net parameters at this point of training

Summary

Real: MTCC

• The top quark can clearly be identified with 0.1/fb of data (within the „1st physics run“ ) which can be collected in a couple of weeks (1032 cm-2s-1) without using any b tagging

• Background can almost be eliminated using lepton isolation, jet pt, event shape variables like circularity and the JPM likelihood

• A final S/B of about 6 was achieved with the use of a likelihood

• Remaining problem so far: Combinatorical background is high (Can an ANN help?)

http://www.physik.rwthaachen.de/~cmsmgr/analysis/

Semileptonic tt decays with 0.1/fb

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