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

Moriond QCD 2001 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 +-


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 %


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.


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.


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


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


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


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


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


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 GeV7.3 mm from PV

1.4 mm from PV


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


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 !


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


ALEPH Hee candidate @ ALEPH Hee candidate @ cme 205 GeVcme 205 GeV


ALEPH HALEPH H candidate @ candidate @ cme 208 GeVcme 208 GeV


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


1-CL1-CLbb


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 after all the data presented here are based on the papers published soon after

the end of data-taking;the end of data-taking; all LEP collaborations are all LEP collaborations are reviewing their analysisreviewing their analysis taking into account final taking into account final

calibrations and data processing and more MC; 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.


BackupBackup


Backup: qqBackup: qq cross cross sectionsection


BackupBackup


BackupBackupHiggsstrahlung WW-fusion Total

Mh=115 GevEcm=206Gev

Documents

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