PHYS 5326 – Lecture #20
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Transcript of PHYS 5326 – Lecture #20
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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PHYS 5326 – Lecture #20Wednesday, Apr. 25, 2007
Dr. Jae Yu
•Higgs Search Strategy•Higgs Search Channels•Backgrounds to Higgs Searches•Requirement on Experiments for Higgs Searches
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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Hadron Collider SM Higgs Production
MH=130 GeV
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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Tevatron Run I Results• Run I limits are not very stringent
Channel D (95% CL) CDF(95% CL) Theory
WH l bb <28 pb <27 pb 0.07
ZH bb <8 pb 0.10
ZH ll bb <7.5 pb 0.11
No real mass limit here…
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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Tevatron Run II Higgs Physics• BR @ 2 TeV
– Use associated H+W/Z production• H bb
(pb) for MH=100 GeV
gg H 1.17
qq,gg WH 0.31
qq,gg ZH 0.17
qq,gg H+2jets 0.12
All others <0.02
BackgroundsWZ+ZZ 4.4
Wbb+Zbb 14
tt 7.5
tb+tq_tbq 3.4
QCD O(106) dijetO(105) 4jet
W e 2800
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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MH=130 GeV
For MH<130GeV• H bb• H WW• H gg or • H cc
• H WW/ZZ• H bb• H gg or • H cc
For MH>130GeV
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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H (h) bb• Two b-quark jets in the final state• Look for signature of b-quark jets
– b-quark jets contain B mesons– B-mesons have finite lifetime and decay in flight
• Final state of interest– A displaced vertex from the primary vertex– Existence of secondary leptons from b-quark
semileptonic decay
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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How does a displaced vertex look?
b vertex
Silicon
Detectors
Beampipe
2” at Run II1” at present
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H (h) WW• W’s decay immediately • Look for signature of W’s
– W branching ratios• ~11% to leptonic final state (l+) for each lepton• ~69% to quark and anti-quark final states
• Final states of interest– One high PT lepton (e or ) + missing ET + two jets
– Two opposite charge high PT leptons (ee, , or e) + Missing ET
– Four jets
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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H (h) ’s decay almost immediately • Look for signature of ’s
– branching ratios• ~17% to leptonic final state (e,+) for each lepton• ~68% to hadrons (one or three charged hadrons)
• Final states of interest– One medium PT lepton (e or ) + missing ET + one
narrow jet– Two opposite charge medium PT leptons (ee, , or
e) + Missing ET
– Two narrow jets
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Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu
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For HW or HZ• Cross section is about 10% of ggH• Both H and W/Z decay immediately• Look for signature of vector bosons with H decay
final states• W or Z final state characteristics
– Decay to Leptons or quark jets• Final states of interest for HW
– Higgs decay + One high PT lepton – Higgs decay + two light quark jets
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HZ Final States• Z decay branching ratio
– ~3% for each charged lepton pairs– ~20% two neutrino final states– ~70% quark, anti-quark final states
• ~15% bb final states
• Final states of interest for HZ– Two high PT lepton + Higgs decay channels
– Large Missing ET + Higgs decay channels– Four b-jet final states
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Summary of Final States of Interest• W decay:
– W qq channel very hard• depends on dijet mass resolution
– W e, , • Isolated lepton plus missing ET
• Z decay– Z qq same as W – difficult– Z ee, (probably not )
• Isolated lepton plus mass resolution
• Higgs decay:– Look for bb pairs
• Impact parameter resolution• Silicon vertexing• Di-jet mass resolution
b-tagging is a MUSTb-tagging is a MUST
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• WH l bb backgrounds – Wbb
• Where the W decays leptonically• Cross section is 2.3nb*s
2*mb-threshold factor for each lepton
– WZ: • W decays leptonically while Z decays to bb• Cross section is 2.3nb*EW2*b final state branching ratio
– tt: Each t decays to W and b pairs. How would this become a background?
– single t: How?• WH qq bb overwhelmed by QCD background since it has
four jets in the final state
Higgs Backgrounds To WH
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• ZH l l bb final state for signature– Zbb
• Z decays to two leptons• Cross section is 0.2nb*s
2*mb-threshold factor for each lepton
– ZZ• One Z decays to two leptons while the other decays to bb • Cross section is 2*0.2nb*EW*b branching ratio
– tt:How does this channel become background to ZH?
Higgs Backgrounds to ZH
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• ZH bb – QCD:How would this become background to this process?– Zbb: Z decays to neutrinos– ZZ: One Z decays to neutrinos while the other decays to
bb– tt: How would this process become background?
Higgs Backgrounds to ZH
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• WW final state is a generic, inherent and irreducible background
• 2l+MET final state– ZZ: One Z decays to two leptons while the other to two
neutrinos– Drell-Yan (qq*ll)– tt: How this could be the background to this process?– WZ: Z decays to two leptons, while W decays to l+ where l
is lost : Both ’s decay leptonically
Higgs Backgrounds to gg H WW*
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• 1l+MET+2jets final state– WZ: W decays to l+ and Z decays to two jets– ZZ: One Z decays to two leptons (of which one is lost) while
the other to two jets– tt : How is this become a background to this process?– tW:How could this become a background to this process?
Higgs Backgrounds to gg H WW*
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• The best discovery channel is with HW and HZ• The background for mH < 130 GeV are
– WH l bb backgrounds Wbb, WZ, tt, single t– WH qq bb overwhelmed by QCD background since it
has four jets in the final state– ZH l l bb backgrounds Zbb, ZZ, tt– ZH bb backgrounds QCD, Zbb, ZZ, tt
• The background for mH > 130 GeV are– gg H WW* backgrounds Drell-Yan, WW, WZ, ZZ, tt,
tW,
Summary of Higgs Physics Backgrounds
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Tevatron Higgs Production
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TeVatron Run II Higgs S/N• H bb
– S/N drops as MH increases• Run out of events….• Ultimately limits the experimental reach
• H WW/ZZ (virtual)– Depends on di-jet mass resolution
1 fb-1 delivered
Bet
ter