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Moriond QCD 2001 Enrico Piotto EP division CERN
SM Higgs Search at LEP in Channels other than 4 Jets
Enrico Piotto EP Division, CERN
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-->H veve -->bb ee
•e+e- --> HZ-->bb e+e- •e+e- --> HZ-->bb µ+µ-
•e+e- --> HZ-->qq +-
Moriond QCD 2001 Enrico Piotto EP division CERN
SM Higgs branching SM Higgs branching ratiosratios
For mh =115 Gev/c2:
•BR(Hbb) 78 %
•BR(H) 7.5 %
•BR(Hgg) 7%
•BR(HWW) 5 %
Moriond QCD 2001 Enrico Piotto EP division CERN
Topologies @ LEPTopologies @ LEP
4 jets : 56%2 Jets &missing energy : 16%
2 jets &2 leptons: 5%
z qq,
h,bb
qq : 8% Search potential:the probability to confirma signal if it is true.
Moriond QCD 2001 Enrico Piotto EP division CERN
BackgroundsBackgrounds
For mh=115 Gev/c2, assuming 100%efficiency, 3.3 events are expectedin a LEP experiment from 2000 data takingand using the final states other than 4 jets.
Several hundred thousands of backgroundevents 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.
Moriond QCD 2001 Enrico Piotto EP division CERN
B-tag: the fundamental B-tag: the fundamental tooltool
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-->bb
Z-->cc
Z-->uds
OPAL
Moriond QCD 2001 Enrico Piotto EP division CERN
Missing energy channelMissing energy channel
v
v
b
b
Main background near threshold:
b quark pair2 photons ISR
2 ISR escape undetected at low angle:the qq events will have 2 nearly coplanar jets with low pT.
DELPHI
Higgs m=115 qqn()
4 fermions
Moriond QCD 2001 Enrico Piotto EP division CERN
HH kinamatic kinamatic
Higgs mh=115
Double symmetric radiative eventsare nearly collinear.Higgs events may be acollinear evenat the kinematic limit.The Higgs boost may come from:•widht of Z0
•WW fusion
Moriond QCD 2001 Enrico Piotto EP division CERN
Mass reconstructionMass reconstruction
DELPHIIn the missing energy channel the energy flow and the jet reconstructionare critical aspects of the analysis.A mass resolution of 3 GeV/c2 can beachieved assuming a Z boson recoilingmass.But: for an event at rest, the Z recoil constraint artificially peaks the visible mass at s-mz
Moriond QCD 2001 Enrico Piotto EP division CERN
Most significant HMost significant H candidatescandidates
Selecting LEP candidates with (s/b)115 >0.3:
(s/ b)115 Mrec(GeV/ c2) Chanel Exp.
2.0 114 H L3
0.49 114 H L3
0.32 104 H OPAL
3rd mostsignificant LEP candidate
Observed: Observed: 1414Expected (s+b): Expected (s+b): 1414Signal: Signal: 77
10th mostsignificant LEP candidate
14th mostsignificant LEP candidate
Moriond QCD 2001 Enrico Piotto EP division CERN
L3 HL3 H candidate @ cme candidate @ cme 206.6 GeV206.6 GeV
Two b-tagged jets
Mh= 114.4 GeV/c2
missing mass = 94 GeV/c2
M=5.5 GeV
7.3 mm from PV
1.4 mm from PV
Moriond QCD 2001 Enrico Piotto EP division CERN
Leptonic channelLeptonic channele-,
e-,
b
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
Moriond QCD 2001 Enrico Piotto EP division CERN
ExampleExample
If the is associated to the muon ->a perfect ZZ
If the is included in the jet:a very high di-jet mass ->good high mass Higgs candidate !
Moriond QCD 2001 Enrico Piotto EP division CERN
Leptonic channelLeptonic channelb
be-,
e-,
S/b
Hee candidate: it is the 5th mostsignificant LEP candidates (ALEPH). Mh=118GeV/c2, Mrec=78.8GeV/c2
b-tag=1.4
H candidate: it is the 8th mostsignificant LEP candidates(ALEPH).Mh=115GeV/c2
Moriond QCD 2001 Enrico Piotto EP division CERN
ALEPH Hee candidate @ ALEPH Hee candidate @ cme 205 GeVcme 205 GeV
Moriond QCD 2001 Enrico Piotto EP division CERN
ALEPH HALEPH H candidate @ candidate @ cme 208 GeVcme 208 GeV
Moriond QCD 2001 Enrico Piotto EP division CERN
S/b of candidatesS/b of candidates
Selecting candidates with (s/b)115>0.3: observed: 14 expected (s+b):14
Hqq: 9Hqq: 9HH: 3: 3HlHl++ll--: 1: 1HH: 1: 1
(70%(70%))
(20%(20%))
(7%)(7%)(3%)(3%)TheoreticalTheoretical
branchingsbranchings
HH
HH
HeHe++ee--HH
Moriond QCD 2001 Enrico Piotto EP division CERN
1-CL1-CLbb
Moriond QCD 2001 Enrico Piotto EP division CERN
ConclusionsConclusions bbvv, bb ebbvv, bb e++ee-- ,bb µ ,bb µ++µµ--, qq , qq ++-- together together have the same potentialhave the same potential discovery discovery
as the as the four jets channelfour jets channel;; a a coherent viewcoherent view compatible with the expected branching ratios emerges compatible with the expected branching ratios emerges
from LEP leptonic and hadronic candidates; from LEP leptonic and hadronic candidates;
BUTBUT
letponic candidates are letponic candidates are consistent with both the background andconsistent with both the background and signal+backgroundsignal+background hypotheses. hypotheses. leptonic channels are affected by leptonic channels are affected by irreducible backgroundirreducible background;; all the data presented here are based on the papers published soon all the data presented here are based on the papers published soon
after the end of data-taking;after the end of data-taking; all LEP collaborations are all LEP collaborations are reviewing their analysisreviewing their analysis taking into account taking into account
final calibrations and data processing and more MC; final calibrations and data processing and more MC; the final LEP word on higgs will be hopefully for summer 2001. the final LEP word on higgs will be hopefully for summer 2001.
Moriond QCD 2001 Enrico Piotto EP division CERN
BackupBackup
Moriond QCD 2001 Enrico Piotto EP division CERN
Backup: qqBackup: qq cross cross sectionsection
Moriond QCD 2001 Enrico Piotto EP division CERN
BackupBackup
Moriond QCD 2001 Enrico Piotto EP division CERN
BackupBackupHiggsstrahlung WW-fusion Total
Mh=115 GevEcm=206Gev