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

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SM Higgs Search at LEP in Channels other than 4 Jets

Enrico Piotto EP Division, CERN

- e+e- -->HZ-->bb, e+e- --> W- W+ ee-->Hveve -->bb ee
- e+e- -->HZ-->bb e+e-
- e+e- -->HZ-->bb µ+µ-
- e+e- -->HZ-->qq +-

Enrico Piotto EP division CERN

- For mh =115 Gev/c2:
- BR(Hbb) 78 %
- BR(H) 7.5 %
- BR(Hgg) 7%
- BR(HWW) 5 %

Enrico Piotto EP division CERN

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

Enrico Piotto EP division CERN

- For mh=115 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.

Enrico Piotto EP division CERN

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.

OPAL

Z-->bb

Z-->cc

Z-->uds

Enrico Piotto EP division CERN

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 m=115

4 fermions

Enrico Piotto EP division CERN

- 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 mh=115

Enrico Piotto EP division CERN

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

Enrico Piotto EP division CERN

Selecting LEP candidates with (s/b)115 >0.3:

Observed: 14

Expected (s+b): 14

Signal: 7

3rd most

significant

LEP candidate

10th most

significant

LEP candidate

14th most

significant

LEP candidate

Enrico Piotto EP division CERN

Mh= 114.4 GeV/c2

missing mass = 94 GeV/c2

M=5.5 GeV

7.3 mm from PV

1.4 mm from PV

Two b-tagged jets

Enrico Piotto EP division CERN

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

Enrico Piotto EP division CERN

If the is associated

to the muon ->

a perfect ZZ

m

If the is included in the jet:

a very high di-jet mass ->

good high mass Higgs candidate !

m

g

Enrico Piotto EP division CERN

e-,m,

b

e-,m,

b

Hee candidate: it is the 5th most

significant LEP candidates

(ALEPH).

Mh=118GeV/c2, Mrec=78.8GeV/c2

b-tag=1.4

S/b

H candidate: it is the 8th most

significant LEP candidates

(ALEPH).

Mh=115GeV/c2

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

Hqq: 9

Hnn: 3

Hl+l-: 1

Htt: 1

(70%)

(20%)

(7%)

(3%)

Selecting candidates with (s/b)115>0.3: observed: 14

expected (s+b):14

Htt

He+e-

Theoretical

branchings

Hnn

Hnn

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

- bbvv, bb e+e- ,bb µ+µ-, 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
- signal+background 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.

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

Enrico Piotto EP division CERN

Higgsstrahlung WW-fusion Total

Mh=115 Gev

Ecm=206Gev

Enrico Piotto EP division CERN