PHYS 5326 – Lecture #20
Wednesday, Apr. 25, 2007
Dr. Jae Yu
•Higgs Search Strategy
•Higgs Search Channels
•Backgrounds to Higgs Searches
•Requirement on Experiments for
Higgs Searches
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
1
Hadron Collider SM Higgs Production s
MH=130 GeV
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
2
Tevatron Run I Results
• Run I limits are not very stringent
Channel
WH l nbb
D (95% CL)
<28 pb
ZH nnbb
ZH ll bb
CDF(95% CL)
Theory
<27 pb
0.07
<8 pb
0.10
<7.5 pb
0.11
No real mass
limit here…
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
3
Tevatron Run II Higgs Physics
• s  BR @ 2 TeV
s (pb) for MH=100 GeV
– Use associated H+W/Z
production
• H  b`b
gg  H
1.17
qq,gg  WH
0.31
qq,gg  ZH
0.17
qq,gg  H+2jets
0.12
All others
<0.02
Backgrounds
Wednesday, Apr. 25, 2007
WZ+ZZ
4.4
Wbb+Zbb
14
tt
7.5
tb+tq_tbq
3.4
QCD
O(106) dijet
O(105) 4jet
W  en
2800
PHYS 5326, Spring 2007
Jae Yu
4
MH=130 GeV
For MH<130GeV
• H  b`b
• H  WW
• H  gg or t`t
•Wednesday,
H Apr.c`c
25, 2007
For MH>130GeV
• H  WW/ZZ
• H  b`b
• H  gg or t`t
• H  c`c
PHYS 5326, Spring 2007
Jae Yu
5
H (h) b`b
• 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
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
6
How does a displaced vertex look?
2” at Run II
1” at present
b
vertex
Wednesday, Apr. 25, 2007
Beampipe
PHYS 5326, Spring 2007
Jae Yu
7
H (h) WW
• W’s decay immediately
• Look for signature of W’s
– W branching ratios
• ~11% to leptonic final state (l+n) 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
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
8
H (h) t`t
t’s decay almost immediately
• Look for signature of t’s

– t branching ratios
• ~17% to leptonic final state (e,+n+n) 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
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
9
For HW or HZ
• Cross section is about 10% of ggH
• 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
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
10
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% b`b 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
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
11
Summary of Final States of Interest
• W decay:
– W  qq channel very hard
• depends on dijet mass resolution
– W  en, n, tn
• Isolated lepton plus missing ET
• Z decay
– Z  qq same as W – difficult
– Z  ee,  (probably not tt)
• Isolated lepton plus mass resolution
• Higgs decay:
– Look for b`b pairs
• Impact parameter resolution
• Silicon vertexing
• Di-jet mass resolution
Wednesday, Apr. 25, 2007
b-tagging is a MUST
PHYS 5326, Spring 2007
Jae Yu
12
Higgs Backgrounds To WH
• WH  ln b`b backgrounds
– Wb`b
• Where the W decays leptonically
• Cross section is 2.3nb*as2*mb-threshold factor for each lepton
– WZ:
• W decays leptonically while Z decays to b`b
• Cross section is 2.3nb*aEW2*b final state branching ratio
– t`t: Each t decays to W and b pairs. How would this become a
background?
– single t: How?
• WH  qq b`b overwhelmed by QCD background since it has
four jets in the final state
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
13
Higgs Backgrounds to ZH
• ZH  l l b`b final state for signature
– Zb`b
• Z decays to two leptons
• Cross section is 0.2nb*as2*mb-threshold factor for each lepton
– ZZ
• One Z decays to two leptons while the other decays to b`b
• Cross section is 2*0.2nb*aEW*b branching ratio
– t`t:How does this channel become background to ZH?
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
14
Higgs Backgrounds to ZH
• ZH  nn b`b
– QCD:How would this become background to this process?
– Zb`b: Z decays to neutrinos
– ZZ: One Z decays to neutrinos while the other decays to
b`b
– t`t: How would this process become background?
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
15
Higgs Backgrounds to gg  H  WW*
• 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 (q`qg*ll)
– t`t: How this could be the background to this process?
– WZ: Z decays to two leptons, while W decays to l+n where l
is lost
– tt: Both t’s decay leptonically
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
16
Higgs Backgrounds to gg  H  WW*
• 1l+MET+2jets final state
– WZ: W decays to l+n and Z decays to two jets
– ZZ: One Z decays to two leptons (of which one is lost) while
the other to two jets
– t`t : How is this become a background to this process?
– tW:How could this become a background to this process?
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
17
Summary of Higgs Physics Backgrounds
• The best discovery channel is with HW and HZ
• The background for mH < 130 GeV are
– WH  ln b`b backgrounds Wb`b, WZ, t`t, single t
– WH  qq b`b overwhelmed by QCD background since it
has four jets in the final state
– ZH  l l b`b
backgrounds Zb`b, ZZ, t`t
– ZH  nn b`b
backgrounds QCD, Zb`b, ZZ, t`t
• The background for mH > 130 GeV are
– gg  H  WW* backgrounds Drell-Yan, WW, WZ, ZZ, tt,
tW, tt
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
18
Tevatron Higgs Production s
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
19
TeVatron Run II Higgs S/N
• H  b`b
– 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
Better
1 fb-1 delivered
Wednesday, Apr. 25, 2007
PHYS 5326, Spring 2007
Jae Yu
20