T he H eavy Q u ark S earch at the L H C · ¥ sequ ential fou rth fam ily (w ith a h eavy ! ) w...
Transcript of T he H eavy Q u ark S earch at the L H C · ¥ sequ ential fou rth fam ily (w ith a h eavy ! ) w...
BH, JHEP 0708 (2007) 069
BH, JHEP 0703 (2007) 063
The Heavy Quark Search at the LHC
Backgrounds: estimating and suppressing
• tt, multijets, ...
• jet mass technique
• NLO e!ects
• matrix elements for extra jets
• initial state radiation
• compare event generators
Basic picture
• sequential fourth family (with a heavy !) 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
"LO ! 900 fb
"NLO ! 1400 fb
Basic picture
• sequential fourth family (with a heavy !) 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
"LO ! 900 fb
"NLO ! 1400 fb
• 600 GeV roughly corresponds to when we stop talking about a light Higgs
Basic picture
• sequential fourth family (with a heavy !) 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
"LO ! 900 fb
"NLO ! 1400 fb
• concentrate on first process
• assuming mt! < mb! , or
• |Vt!b| > 0.01, 0.004, 0.001 for mt! ! mb! = 70, 50, 30
• 600 GeV roughly corresponds to when we stop talking about a light Higgs
• W ! jj from single jet invariant mass
energy deposit in calorimeter cell
pµ1 + pµ
2
!
i
pµi
J. M. Butterworth, B. E. Cox and
J. R. Forshaw 2002
P. Savard 1997
I. Borjanovic et al. 2005
Identifying energetic and isolated W ’s
• W ! jj from single jet invariant mass
energy deposit in calorimeter cell
pµ1 + pµ
2
!
i
pµi
b jetb jet
from boosted t from t!
J. M. Butterworth, B. E. Cox and
J. R. Forshaw 2002
P. Savard 1997
I. Borjanovic et al. 2005
Identifying energetic and isolated W ’s
• suppression of tt background
Sample jet mass plot
W-jet definition
• invariant mass of jet within ! 10 GeV of the peak
• peak can be experimentally determined—need not be exactly at MW
• “splash out” and “splash in” e!ects
60 70 80 90 100 110
10
20
30
40
signal
background
GeV
Njet
jet mass
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| < 5
0.099733101 ! 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)
0.000005 ! 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
pp ! t"t" ! W+W#bb
Event Selection
• scalar pT sum HT > !HT
• one b-tag jet with pT > !b
• one W -jet
pp ! t"t" ! W+W#bb
Event Selection
• scalar pT sum HT > !HT
• one b-tag jet with pT > !b
• one W -jet
• choose !HT= 2mt! and !b = mt!/3
• for real data keep !HT/!b = 6 and scan to optimize signal
pp ! t"t" ! W+W#bb
b-tagging e!ciencies
• account for large pT ’s, where e!ciencies worsen
1/2 for b’s
1/10 for c’s
1/30 for light quarks and gluons
• assume to vanish for pseudorapidity |!| > 2
Event Selection
• scalar pT sum HT > !HT
• one b-tag jet with pT > !b
• one W -jet
• choose !HT= 2mt! and !b = mt!/3
• for real data keep !HT/!b = 6 and scan to optimize signal
HT distribution
• signal peaks at high HT ! 2mt!
• why not just look for the signal bump on the tail?
• t! mass reconstruction is essential
• consider invariant mass of all W -b pairs
• pairs of plots: W -mass plot and t!-mass plot
1200 1400 1600 1800 2000 2200
1000
2000
3000
4000
signal
HTGeV
HT distribution
• signal peaks at high HT ! 2mt!
• why not just look for the signal bump on the tail?
Event Generators
• MC@NLO-Herwig
• stand-alone Herwig
• Alpgen-Herwig
• Alpgen-Pythia
• stand-alone Pythia
Event Generators
• MC@NLO-Herwig
• 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, (WW/ZZ/WZ) + jets, and (t/t)(b/b)W
Event Generators
• MC@NLO-Herwig
• stand-alone Herwig
• Alpgen-Herwig
• Alpgen-Pythia
• stand-alone Pythia
• in each case we use the same tool to calculate both signal and background
MC@NLO-Herwig
• good for getting absolute rates
• no description of fourth family
• use mt = 600 GeV and t ! bW to model t! ! bW
• produces some negative weights (" 15%)
• use CTEQ6M and scale to 2.5 fb"1
MC@NLO-Herwig
• good for getting absolute rates
• no description of fourth family
• use mt = 600 GeV and t ! bW to model t! ! bW
• produces some negative weights (" 15%)
• use CTEQ6M and scale to 2.5 fb"1
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+,-!"## !$## !%## !&## "### ""##
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!"#$%&'(
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012
HT distribution
• NLO gives the K-factor enhancement
• but it also suppresses the tail, and thus the background
Alpgen-Herwig
• choose!
s/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
Alpgen-Herwig
• choose!
s/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
Alpgen-Pythia
• good agreement with Alpgen-Herwig and MC@NLO-Herwig
• either way of going beyond leading order increases signal to background
Stand-alone Pythia
• initial-state radiation becomes an issue
• models of underlying event(old with Q2 ordered showers and new with pT ordered showers)
• note that MSTP(68)=3 is default (“power showers”)
• ISR has very high phase space cuto! in tt production
• MSTP(68)=0 gives a phase space cuto! more in line with the hard process itself
HT
MSTP(68)=3
MSTP(68)=0
Stand-alone Pythia
• initial-state radiation becomes an issue
• models of underlying event(old with Q2 ordered showers and new with pT ordered showers)
• note that MSTP(68)=3 is default (“power showers”)
• ISR has very high phase space cuto! in tt production
• MSTP(68)=0 gives a phase space cuto! more in line with the hard process itself
• Alpgen drastically reduces sensitivity to ISR
HT tails again: sensitivity to ISR
• turning ISR o! in stand-alone Pythia shows it large e!ect, and thus modeldependence
tt + jets with pminT
= 100 GeV W + jets with pminT
= 150 GeV
• Alpgen controls bulk of showering
• standardize on tune QW and CTEQ6.1
• let Alpgen generate samples for 0, 1 and 2 extra hard partons, using the MLMjet-parton matching scheme
• maximum jet pseudorapidity 2.5 and the minimum jet separation 0.7
QCD multijet background
• problem is that the cross section is enormous and only a tiny fraction survivesin 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 pminT
• 3-jet sample dominates 2-jet sample (in signal region) if pminT
is not toolarge
• cross sections are smaller if pminT
is large
• compromise by choosing pminT
= 200 GeV
• 2-jet sample still has enormous cross section
• remove b-tag to generate enough events, and then compare to the 3 + 4 jet
samples
• not bad then to only keep 3 + 4 jet samples with b-tag
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)
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)
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
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
b!b! production
• can add to previous signal through
pp " b!b! " W+W"t!t! " W+W"W+W"bb
• assume mb! = 700 GeV and mt! = 600 GeV
b!b! production
• can add to previous signal through
pp " b!b! " W+W"t!t! " W+W"W+W"bb
• assume mb! = 700 GeV and mt! = 600 GeV
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: mjet > 100, pT > 300 GeV
• !R < 2 constraint between W and t jets
• assume mb! = 600 GeV
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: mjet > 100, pT > 300 GeV
• !R < 2 constraint between W and t jets
• assume mb! = 600 GeV
Summary
• a sequential fourth family wrapped up with electroweak symmetry breaking isa 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 inagreement (and encouraging for S/B)
• Alpgen reduces sensitivity to initial state radiation
• why not develop tunes after incorporating the matrix elements?
Summary
• a sequential fourth family wrapped up with electroweak symmetry breaking isa 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