Semileptonic tt decays with 0.1/fb

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

1/19


“1st physics run” scenario

2/19

• 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)


y

z

x

Myon chambers

Forward calorimeter

Superconductive coil (4 Tesla)

Electromagnetic calorimeter

(ECAL)

Silicon Tracker(Pixel+Strip)

Hadroniccalorimeter

(HCAL)

CMS detector

3/19


top pairs @ LHC

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

ttgg ttqq 13%:87%:

4/19

• 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

5/19

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

6/19

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

7/19

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

8/19

• 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: <

1‰

• 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

9/19

• 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

10/19

• 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.

i

2i

2iT

)p(

)ˆ(min2C

iT

npxy

l


Selection overview

11/19

• 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

p r

e l i

m i

n a

r y

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

12/19

• 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

13/19

• 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)

16/19

• 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

17/19

• 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?

18/19

Use net parameters at this point of training


Summary

19/19

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/

SIM

Semileptonic tt decays with 0.1/fb

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