SM Higgs Search at LEP in Channels other than 4 Jets

1
SM Higgs Search at LEP in Channels
other than 4 Jets
Enrico Piotto EP Division, CERN

• ee- --gt HZ--gtbb??, ee- --gt W- W ?e?e--gtH
  veve --gtbb ?e?e
• ee- --gt HZ--gtbb ee-
• ee- --gt HZ--gtbb µµ-
• ee- --gt HZ--gtqq ??-

2
SM Higgs branching ratios

• For mh 115 Gev/c2
• BR(H?bb) ? 78
• BR(H???) ? 7.5
• BR(H?gg) ? 7
• BR(H?WW) ? 5

3
Topologies _at_ LEP
2 Jets missing energy 16
2 jets 2 leptons 5
4 jets 56
qq?? 8
Search potential the probability to confirm a
signal if it is true.
z? qq, ??
h???,bb
4
Backgrounds

• For mh115 Gev/c2, assuming 100
• efficiency, 3.3 events are expected
• in a LEP experiment from 2000 data taking
• and using the final states other than 4 jets.
• Several hundred thousands of background
• events are expected.
• The strategy
• sequential cuts to remove the bulk of the
• background
• likelihood ratio or neural network to further
• discriminate the signal from the
• background.

5
B-tag the fundamental tool
B hadrons produced at LEP will fly for few mm and
their decays products will have large pT wrt jet
and large impact parameters with respect to light
quark hadrons. Combining different variables with
Neural Network or Likelihood a probability that a
jet comes from a b quark is derived.
Z--gtbb
OPAL
Z--gtcc
Z--gtuds
6
Missing energy channel
v
b
b
v
Main background near threshold
2 ISR escape undetected at low angle
b quark pair 2 photons ISR
the qq?? events will have 2 nearly coplanar jets
with low pT.
DELPHI
qqn(?)
Higgs m115
4 fermions
7
H?? kinamatic

• Double symmetric radiative events
• are nearly collinear.
• Higgs events may be acollinear even
• at the kinematic limit.
• The Higgs boost may come from
• widht of Z0
• WW fusion

Higgs mh115
8
Mass reconstruction
In the missing energy channel the energy flow
and the jet reconstruction are critical aspects
of the analysis. A mass resolution of ?3 GeV/c2
can be achieved assuming a Z boson
recoiling mass. But for an event at rest, the Z
recoil constraint artificially peaks the visible
mass at ?s-mz
DELPHI
9
Most significant H?? candidates
Selecting LEP candidates with (s/b)115 gt0.3
Observed 14 Expected (sb) 14 Signal 7
3rd most significant LEP candidate
10th most significant LEP candidate
14th most significant LEP candidate
10
L3 H?? candidate _at_ cme 206.6 GeV
Mh 114.4 GeV/c2 missing mass 94 GeV/c2
M5.5 GeV
7.3 mm from PV
1.4 mm from PV
Two b-tagged jets
11
Leptonic channel
e-,m ,?
b
e-,m,?
b

• Low background channel.
• Neural Network are used to identified ?.
• Good mass resolution also without constrained
  fit.
• Danger ISR photons, or photons from electron
  radiation,
• can artificially increase the di-jet mass if
  associated with one jet.

OPAL
12
Example
If the ? is associated to the muon -gt a perfect
ZZ
m
If the ? is included in the jet a very high
di-jet mass -gt good high mass Higgs candidate !
m
g
13
Leptonic channel
b
e-,m,?
e-,m,?
b
Hee candidate it is the 5th most significant LEP
candidates (ALEPH). Mh118GeV/c2,
Mrec78.8GeV/c2 b-tag1.4
S/b
H?? candidate it is the 8th most significant LEP
candidates (ALEPH). Mh115GeV/c2
14
ALEPH Hee candidate _at_ cme 205 GeV
15
ALEPH H?? candidate _at_ cme 208 GeV
16
S/b of candidates
Hqq 9 Hnn 3 Hll- 1 Htt 1
(70) (20) (7) (3)
Selecting candidates with (s/b)115gt0.3 observed
14
expected (sb)14
Htt
Hee-
Theoretical branchings
Hnn
Hnn
17
1-CLb
18
Conclusions

• bbvv, bb ee- ,bb µµ-, qq ??- together have
  the same potential discovery as the four jets
  channel
• a coherent view compatible with the expected
  branching ratios emerges from LEP leptonic and
  hadronic candidates
• BUT
• letponic candidates are consistent with both the
  background and
• signalbackground hypotheses.
• leptonic channels are affected by irreducible
  background
• all the data presented here are based on the
  papers published soon after the end of
  data-taking
• all LEP collaborations are reviewing their
  analysis taking into account final calibrations
  and data processing and more MC
• the final LEP word on higgs will be hopefully
  for summer 2001.

19
Backup
20
Backup qq?? cross section
21
Backup
22
Backup
Higgsstrahlung WW-fusion Total
Mh115 Gev Ecm206Gev