The Heavy Quark Search at the LHC

Transcription

1 The Heavy Quark Search at the LHC

2 The Heavy Quark Search at the LHC Backgrounds: estimating and suppressing tt, multijets,... jet mass technique NLO effects matrix elements for extra jets initial state radiation compare event generators BH, JHEP 0708 (2007) 069 BH, JHEP 0703 (2007) 063

3 Basic picture sequential fourth family (with a heavy ν) with at least some CKM mixing fourth family quarks with mass 600 GeV pp t t W + W bb and/or pp b b W + W tt since colored fermions are involved, cross sections are decent at the LHC σ LO 900 fb σ NLO 1400 fb

4 Basic picture sequential fourth family (with a heavy ν) with at least some CKM mixing fourth family quarks with mass 600 GeV pp t t W + W bb and/or pp b b W + W tt since colored fermions are involved, cross sections are decent at the LHC σ LO 900 fb σ NLO 1400 fb 600 GeV roughly corresponds to when we stop talking about a light Higgs

5 Basic picture sequential fourth family (with a heavy ν) with at least some CKM mixing fourth family quarks with mass 600 GeV pp t t W + W bb and/or pp b b W + W tt since colored fermions are involved, cross sections are decent at the LHC σ LO 900 fb σ NLO 1400 fb 600 GeV roughly corresponds to when we stop talking about a light Higgs concentrate on first process assuming m t < m b, or V t b > 0.01, 0.004, for m t m b = 70, 50, 30

6 Identifying energetic and isolated W s W jj from single jet invariant mass energy deposit in calorimeter cell J. M. Butterworth, B. E. Cox and J. R. Forshaw 2002 p µ 1 + pµ 2 i p µ i P. Savard 1997 I. Borjanovic et al. 2005

7 Identifying energetic and isolated W s W jj from single jet invariant mass energy deposit in calorimeter cell J. M. Butterworth, B. E. Cox and J. R. Forshaw 2002 p µ 1 + pµ 2 i p µ i P. Savard 1997 I. Borjanovic et al suppression of tt background b jet b jet from boosted t from t

8 Sample jet mass plot 40 N jet 30 signal background GeV jet mass

9 Sample jet mass plot 40 N jet 30 signal background GeV jet mass W-jet definition invariant mass of jet within 10 GeV of the peak peak can be experimentally determined need not be exactly at M W splash out and splash in effects

10 Detector Simulation PGS4 with cone-based jet finder option ATLAS! parameter set name 81! eta cells in calorimeter 63! phi cells in calorimeter 0.1! eta width of calorimeter cells eta < ! phi width of calorimeter cells 0.01! electromagnetic calorimeter resolution const 0.1! electromagnetic calorimeter resolution * sqrt(e) 0.8! hadronic calolrimeter resolution * sqrt(e) 0.2! MET resolution 0.00! calorimeter cell edge crack fraction cone! jet finding algorithm (cone or ktjet) 3.0! calorimeter trigger cluster finding seed threshold (GeV) 0.5! calorimeter trigger cluster finding shoulder threshold (GeV) 0.6! calorimeter kt cluster finder cone size (delta R) 1.0! outer radius of tracker (m) 2.0! magnetic field (T) ! sagitta resolution (m) 0.98! track finding efficiency 0.30! minimum track pt (GeV/c) 2.5! tracking eta coverage 3.0! e/gamma eta coverage 2.4! muon eta coverage 2.0! tau eta coverage

11 pp t t W + W bb Event Selection scalar p T sum H T > Λ HT one b-tag jet with p T > Λ b one W -jet

12 pp t t W + W bb Event Selection scalar p T sum H T > Λ HT one b-tag jet with p T > Λ b one W -jet choose Λ HT = 2m t and Λ b = m t /3 for real data keep Λ HT /Λ b = 6 and scan to optimize signal

13 pp t t W + W bb Event Selection scalar p T sum H T > Λ HT one b-tag jet with p T > Λ b one W -jet choose Λ HT = 2m t and Λ b = m t /3 for real data keep Λ HT /Λ b = 6 and scan to optimize signal b-tagging efficiencies account for large p T s, where efficiencies worsen 1/2 for b s 1/10 for c s 1/30 for light quarks and gluons assume to vanish for pseudorapidity η > 2

14 H T distribution signal peaks at high H T 2m t why not just look for the signal bump on the tail?

15 H T distribution signal peaks at high H T 2m t why not just look for the signal bump on the tail? signal H T GeV t mass reconstruction is essential consider invariant mass of all W -b pairs pairs of plots: W -mass plot and t -mass plot

16 Event Generators stand-alone Herwig Alpgen-Herwig Alpgen-Pythia stand-alone Pythia

17 Event Generators stand-alone Herwig Alpgen-Herwig Alpgen-Pythia stand-alone Pythia in each case we use the same tool to calculate both signal and background

18 Event Generators stand-alone Herwig Alpgen-Herwig Alpgen-Pythia stand-alone Pythia in each case we use the same tool to calculate both signal and background Backgrounds consider the tt irreducible background first then consider QCD multijet backgound dealing with this basically eliminates other backgrounds: (W/Z) + jets, bb + jets, (W/Z)bb, (W W/ZZ/W Z) + jets, and (t/t)(b/b)w

19 good for getting absolute rates no description of fourth family use m t = 600 GeV and t bw to model t bw produces some negative weights ( 15%) use CTEQ6M and scale to 2.5 fb 1

20 good for getting absolute rates no description of fourth family use m t = 600 GeV and t bw to model t bw produces some negative weights ( 15%) use CTEQ6M and scale to 2.5 fb 1

21 stand-alone Herwig increases background and decreases signal!

22 H T distribution NLO gives the K-factor enhancement but it also suppresses the tail, and thus the background!"#### $###!#####!"#$%&'(!"#$%&'( %"####!"#$)*#%&'( '###!"#$)*#%&'( +",-./ %##### "### "####!###!"# "## $"# %### %!"# +,- %"##!"##!$##!%##!&## "### 012 ""##

23 Alpgen-Herwig choose ŝ/2 as the renormalization scale ignore K-factors generate samples involving tt + 0, tt + 1 and tt + 2 partons use MLM parton-jet matching scheme model t decay the same way and use CTEQ6M again

24 Alpgen-Herwig choose ŝ/2 as the renormalization scale ignore K-factors generate samples involving tt + 0, tt + 1 and tt + 2 partons use MLM parton-jet matching scheme model t decay the same way and use CTEQ6M again

25 Alpgen-Pythia good agreement with Alpgen-Herwig and

26 Alpgen-Pythia good agreement with Alpgen-Herwig and either way of going beyond leading order increases signal to background

27 Stand-alone Pythia initial-state radiation becomes an issue models of underlying event (old with Q 2 ordered showers and new with p T ordered showers) note that MSTP(68)=3 is default ( power showers ) ISR has very high phase space cutoff in tt production MSTP(68)=0 gives a phase space cutoff more in line with the hard process itself

28 Stand-alone Pythia initial-state radiation becomes an issue models of underlying event (old with Q 2 ordered showers and new with p T ordered showers) note that MSTP(68)=3 is default ( power showers ) ISR has very high phase space cutoff in tt production MSTP(68)=0 gives a phase space cutoff more in line with the hard process itself MSTP(68)=3 MSTP(68)=0 H T

29 DW (R. Field) and S0A (Sandhoff-Skands) tunes share value of PARP(90)

30 m t = 800 GeV increase Λ top5 and Λ b by 4/3 MC@NLO-Herwig and Pythia(DW)

31 k T jet vs. cone jet finder Pythia(DW)

32 H T tails again: sensitivity to ISR turning ISR off in stand-alone Pythia shows it large effect, and thus model dependence Alpgen drastically reduces sensitivity to ISR

33 standardize on tune QW and CTEQ6.1 let Alpgen generate samples for 0, 1 and 2 extra hard partons, using the MLM jet-parton matching scheme maximum jet pseudorapidity 2.5 and the minimum jet separation 0.7 tt + jets with p min T = 100 GeV W + jets with p min T = 150 GeV Alpgen controls bulk of showering

34 QCD multijet background problem is that the cross section is enormous and only a tiny fraction survives in the signal region how to generate enough events? use Alpgen to divide and conquer generate 2, 3, and 4-jet samples, with relative sizes determined by a p min T 3-jet sample dominates 2-jet sample (in signal region) if p min T large cross sections are smaller if p min T is large is not too compromise by choosing p min T = 200 GeV 2-jet sample still has enormous cross section remove b-tag to generate enough events, and then compare to the jet samples not bad then to only keep jet samples with b-tag

35 dealing with QCD background R < 2.5 between W and b look for the leptonic decay of the other W in the signal events consider a loose cut and a tight cut (isolated electron or muon) or (> 200 GeV missing energy) (isolated electron or muon) and (> 50 GeV missing energy)

36 dealing with QCD background R < 2.5 between W and b look for the leptonic decay of the other W in the signal events consider a loose cut and a tight cut (isolated electron or muon) or (> 200 GeV missing energy) (isolated electron or muon) and (> 50 GeV missing energy)

37 try without b-tag made possible by the lepton/missing energy cuts may be useful for early running of LHC covers the possibility that b is not in the dominant decay

38 try without b-tag made possible by the lepton/missing energy cuts may be useful for early running of LHC covers the possibility that b is not in the dominant decay

39 b b production can add to previous signal through pp b b W + W t t W + W W + W bb assume m b = 700 GeV and m t = 600 GeV

40 b b production can add to previous signal through pp b b W + W t t W + W W + W bb assume m b = 700 GeV and m t = 600 GeV

41 pp b b W + W tt use jet masses to identify both the t and the W no b-tags again problem is that a single cone size is not optimal for either choose cone size 0.8 t-jet: m jet > 100, p T > 300 GeV R < 2 constraint between W and t jets assume m b = 600 GeV

42 pp b b W + W tt use jet masses to identify both the t and the W no b-tags again problem is that a single cone size is not optimal for either choose cone size 0.8 t-jet: m jet > 100, p T > 300 GeV R < 2 constraint between W and t jets assume m b = 600 GeV

43 Summary a sequential fourth family wrapped up with electroweak symmetry breaking is a predictive scenario the process pp t t W + W bb is most promising jet mass technique is a promising way to identify W s and enhance S/B

44 Summary a sequential fourth family wrapped up with electroweak symmetry breaking is a predictive scenario the process pp t t W + W bb is most promising jet mass technique is a promising way to identify W s and enhance S/B improved matrix elements from MC@NLO and Alpgen give results that are in agreement (and encouraging for S/B) Alpgen reduces sensitivity to initial state radiation why not develop tunes after incorporating the matrix elements?