Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B Semileptonic tt decays with 0.1/fb Stefan Kasselmann III. Physikalisches Institut B, RWTH Aachen

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B LHC/CMS schedule Picture of the Tracker Inner/ Outer Barrel from July 2006 CMS closes at 31/08/07 First beam: November 2007: TeV (CM) - 43 vs. 43 bunches cm -2 s -1 Debugging machine/detector Then: Commissioning of all 8 sectors for full energy in winter 2008 shutdown First Physics: Spring TeV vs. 156 b cm -2 s /fb : “A few weeks of data taking" 1/19

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B “1st physics run” scenario 2/19 0.1/fb corresponds to about ttbar (inclusive) events, taking the LO cross section from PYTHIA (CTEQ 5L) ( 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)

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B y z x  Myon chambers Forward calorimeter Superconductive coil (4 Tesla) Electromagnetic calorimeter (ECAL) Silicon Tracker (Pixel+Strip) Hadronic calorimeter (HCAL) CMS detector 3/19 

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B top LHC tt production: produced via two processes (strong interaction): 13%: 87%: 4/19 10 top cm -2 s -1 But: About 20 pile up events! Main background: W+jets, Z+jets, Dileptonic ttbar decay

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 2/31/3 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 -  l 1 1 l 2 2 ) ~ 11% BR(W + W -  q 1 q 2 q 3 q 4 ) ~ 44% BR (W + W -  q 1 q 2 l ) ~ 44% (9/81) (36/81) (36/81)

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B Only electrons (p T > 10 GeV, || 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

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B Generic preselection 7/19 Typical preselection: L1 & HLT Trigger 4 jets with p T > 10 GeV, || < 2.5 (low p T cut to be able to run different scenarios) At least one (tracker & calo) isolated lepton with p T > 10 GeV

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

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B Selection 9/19 Second selection cut: 3rd jet p T > 45 GeV Tried many cut variations on the (p T 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 (4 th, 5 th ) in the event must fullfill: p T > 30 GeV to reduce the jet combinations for the Jet Parton Matching (JPM) dileptonic W+jets

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

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B Selection overview 11/19 4 or 5 jets with p T > 30 GeV, 3 rd jet p T > 45 GeV (p T sorted) Exactly one (tracker & calorimeter) isolated lepton with p T > 10 GeV Circularity > 0.3 p r e l i m i n a r y

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 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 p T 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 |P T MC – P T Rec | / P T 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)

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

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

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

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 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 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 with the smallest validation error (empirically search for best net) Outlook- Use ANN? 18/19 Use net parameters at this point of training

Stefan Kasselmann Bad Honnef, August 2006 RWTH Aachen, III. Phys. Inst. B 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 (10 32 cm -2 s -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?) SIM