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Analysis Meeting 2010-03-26

Update on ttbar background. Analysis Meeting 2010-03-26. OBSERVABLE DISTRIBUTIONS a(E T miss ) and f(E T miss ). Data in CR= 2mu+2j50+T 6 >0.05 modelled in MC by sum of all SM backgrounds + SUSY with CR cuts. Data in 2mu+2j50 signal + background contains signal and background

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Analysis Meeting 2010-03-26

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  1. Update on ttbar background Analysis Meeting 2010-03-26

  2. OBSERVABLE DISTRIBUTIONS a(ETmiss ) and f(ETmiss) Data in CR= 2mu+2j50+T6>0.05 modelled in MC by sum of all SM backgrounds + SUSY with CR cuts Data in 2mu+2j50 signal + background contains signal and background modelled in MC by sum of all SM backgrounds + SUSY b(ETmiss) is shape for the background s(ETmiss) is shape for the signal in 2mu+2j50 2mu+2j50 background only 2mu+2j50+T6>0.05 NOT DIRECTLY OBSERVABLE b(ETmiss) c(ETmiss) Background ETmiss shape in Signal Region b(ETmiss) modelled in MC by sum of all SM backgrounds 100 GeV a(ETmiss)=b(ETmiss) +s(ETmiss) b(ETmiss) Same shape of ETmiss b/w bkg CR and SR <=> b(ETmiss)=k c(ETmiss) Need to find k • All events in mu2+2j50 • a(ETmiss)=b(ETmiss) + s(ETmiss) and k = b(ETmiss) / c(ETmiss) but we do not have b(ETmiss) • If s(ETmiss)<< b(ETmiss) in a certain region NR (normalisation region) then for ETmiss in NR • a(ETmiss) ≈b(ETmiss) and • k = a(ETmiss) / c(ETmiss) ≈ b(ETmiss) / c(ETmiss) in NR • k ≈ ∫ b(ETmiss) dETmiss / ∫ c(ETmiss) dETmiss integration over NR

  3. What are the approximations being made? b(ETmiss) / c(ETmiss) = k => verify by plotting total background divided by ETmiss in CR s(ETmiss) / b(ETmiss) << 1 in NR => verify by plotting s(ETmiss) / b(ETmiss) • Exact equations • a(ETmiss)=b(ETmiss) + s(ETmiss) • k (1+ε(ETmiss)) = b(ETmiss) / c(ETmiss) • Formula effectively used: k = ∫ a(ETmiss) dETmiss / ∫ c(ETmiss) dETmiss over NR • k = ∫ (b(ETmiss) + s(ETmiss)) dETmiss / ∫ c(ETmiss) dETmiss o • => Presence of signal in the NR leads to overestimate of k • Nbkg,SR = ∫SRb(ETmiss) dETmiss • = k ∫SR c(ETmiss) (1+ε(ETmiss) dETmiss • = k ∫SR c(ETmiss) (1+ε(ETmiss) dETmiss • = k ∫SR c(ETmiss) dETmiss + k ∫SR c(ETmiss) ε(ETmiss) dETmiss • ≈ k ∫SRc(ETmiss) dETmiss • Size of the approximation due to shape assumption • Nbkg,SR - k ∫SRc(ETmiss) dETmiss = k ∫SR c(ETmiss) ε(ETmiss) dETmiss

  4. 2mu + 2j50 CR= 2mu + 2j50 + T6 > 0.05 SR = 2mu + 2j50 + ETmiss > 100 GeV NR = 2mu + 2j50 + 60<ETmiss<80< GeV 10pb-1 7 TeV

  5. b(ETmiss) / c(ETmiss) = k => verify by plotting total background divided by ETmiss in CR T6> 0.05 At least 1 solution T6> 0.10 Background shape from CR/ True background shape ETmiss spectrum from CR is harder than for real background if there is indeed SUSY, due to SUSY contamination.

  6. b(ETmiss) = ETmiss for background only c(ETmiss) = ETmiss in control region At least 1 solution NO SUSY if no SUSY b(ETmiss) / c(ETmiss) = k is a very good approximation In presence of SUSY, ETmiss spectrum from CR is harder than for real background if there is indeed SUSY, due to SUSY contamination.

  7. Problem with statistics in CR @ 10pb-1 Control Region: T6> 0.10 Signal Region Expect less than 1 event in the useful region with ETmiss>100 GeV  Useless => Either find a different approach or looser cuts.

  8. Normalisation Region SU4 fraction in NR SU4 NR=60 < ETmiss < 80 GeV

  9. Results Based on previous slides choose NR=40 < ETmiss < 60 GeV CR= 2mu + 2j50 + T6 > 0.05

  10. Results 10pb-1 Based on previous slides choose NR=40 < ETmiss < 60 GeV CR= 2mu + 2j50 + T6 > 0.05 SU4 If SUSY contamination of NR and CR lead to overestimate of background, poorer exclusion limit If no SUSY => excellent bkg prediction

  11. A quick look at supersoft cuts @ 7 TeV 10pb-1 Cut Signal Bkg EFF S/√(B) med[ZW] Nmax σ95(pb) FOM Opal ---------------------------------------------------------------------------------------------------------------------------------- mu2_2j50_met100 2.8094 1.5435 0.0059 2.2613 1.8459 3.9485 67.2 676.2 mu2_mz_1j30_1j20_met30 5.3055 12.8320 0.0111 1.4811 1.3934 8.2491 74.3 729.5 mu2_1j100_1j50_met100 2.5451 1.0672 0.0053 2.4637 1.9284 4.0681 76.4 695.1 mu2_mz_2j20_met30 5.3545 13.0703 0.0112 1.4811 1.3941 8.8498 79.0 728.6 mu2_2j20_met30 5.7167 18.8571 0.0120 1.3165 1.2572 9.7251 81.3 802.5 mu2_1j30_1j20_met30 5.6677 18.3854 0.0119 1.3218 1.2614 9.8994 83.5 800.3 mu2_3j50_met100 1.5858 0.5554 0.0033 2.1279 1.6147 3.0093 90.7 1019.6 mu2_1j100_2j50_met100 1.4781 0.4610 0.0031 2.1770 1.6171 3.0062 97.2 1073.8 med[ZW] : Discovery significance G.Cowan, E. Gross, Stat. Forum December 2nd 2009 Eq.23 better approximation than S/√(B) Nmax : median 95% CL upper limit on the number of signal events in SR σ95 : 95% CL upper limit on signal cross section given "Asimov dataset" FOM Opal : approximate FOM related to maximum possible number of signal events in SR follows Nmax Still no systematics included

  12. mu2_1j30_1j20_met30 At least 1 ttbar solution

  13. Composition of normalisation region Normalisation region 2mu 1j30 1j20 20<MET<40 - should be 20-30 GeV ...

  14. Soft Cuts (mu2_1j30_1j20_met30) mu2_2j50_met100 ------------------------------------------------------------------ Nevents in background region (MC) = 2.76907 Nevents in background region (from CR prediction) = 3.2448 Nevents in control region = 129.301 Nevents in normalisation region = 26.496 ------------------------------------------------------------------ ------------------------------------------------------------------ Nevents in background region (MC) = 1.54353 Nevents in background region (from CR prediction) = 0.519253 Nevents in control region = 25.6899 Nevents in normalisation region = 2.69708 ------------------------------------------------------------------

  15. Outlook • Complete ttbar bkg with supersoft cuts and mZ cuts • Include systematics for ttbar background in FOM • Include other ballpark systematics in FOM • Conclude on ttbar background at 10pb-1 • Conclude on cuts for 10pb-1 analysis • Discussion ongoing concerning a softer SUSY model

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