1. Flavour independent neutral Higgs boson searches at LEP Ivo van Vulpen NIKHEF On behalf of the LEP collaborations EPS conference 2005

2. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 2 The LEP collider: e + e - collider at CERN operated 1989-2000 (4 experiments: ALEPH, DELPHI, L3, OPAL) s  m Z (LEP1) and s = 130-209 GeV (LEP2) Most results (95% CL limits) based on LEP2: Int. Lum.  2.6 fb -1 LEP and the LEP data-set http://lephiggs.web.cern.ch/LEPHIGGS/www/Welcome.html LEP combined results on Higgs searches:

3. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 3 SM Higgs boson  Weak boson fusion In the SM we know everything about the Higgs boson m h > 115.3 GeV (expected) m h > 114.4 GeV (observed) (except its mass) B-tagging udsc-jets b-jets Higgs-strahlung h bb (>80%) qq (70%) (20%) l + l - (10%) Production:  hz (m h,√s) Decay:  h (m h ) 2 b-jets + a “Z”. Excluded SM m h :

4. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 4 Simplest SM extension: 2 complex Higgs Doublets  5 Higgs bosons (3 neutral) Neutral Higgs boson productionHiggs boson decay Depends on model parameters Neutral Higgs boson production at LEP Models: 2HDM(I): One doublet for all fermions 2HDM(II): One up-type doublet One down-type doublet mainly h  bb Associated productionHiggs strahlung MSSM h  bb decays suppressed (light quarks and gluons) No sensitivity in standard analyses

5. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 5 What if a neutral Higgs boson is produced, but … does not couple (strongly) to b-quarks … decays into light quarks or gluon pairs … has a cross section X times smaller or larger than SM prediction In general: Test predictions of any (new) model against LEP data Experimentalist: Exclude neutral Higgs boson production, with (H  hadrons) over m H range YESNO Cancel

6. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 6 Neutral Higgs boson production at LEP Details Higgs sector SU(2) x U(1) Consider Higgs boson decays into all quark pairs and gluons Exclude and Topology based (no b-tag information) Flavour independent (hadronic) Higgs boson search: Worst case scenario  Assumption:  h small (w.r.t detector resolution) e + e-  hZ e + e-  hA

7. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 7 hZ production  qq/qql + l - : Z-like object + recoil mass  Gluon pairs versus light-quark pairs: higher selection efficiency worse di-jet mass resolution Experimental issues:  4-jets: topology+ masses (3 pairings)  Weak boson fusion Higgs-strahlung h qq/gg (100%) qq (70%) (20%) l + l - (10%) 3 final states SM Backgrounds: qq(), W + W -, ZZ

8. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 8 hZ production: single experiment DELPHI (all channels) Topologies: qqqq, qq, qql + l - Mh range: 4 < M h <115 GeV M h (GeV) M qq (GeV) C 2 Z(h  hadrons) Single channel (qq  +  - ) only 3% of all decays, but very clean SM hZ cross section

9. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 9 hZ production: LEP combined Flavour Independent Even without b-tag information, still a strong limit Expected Observed (GeV/c 2 ) (GeV/c 2 ) LEP results on hZ production: SM 115.3 114.4 Flav.Ind. 113.0 112.9 M h (GeV) C 2 Z(h  hadrons)

10. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 10 hA production 4-jet event at LEP h A SM Backgrounds: qq() ~ 8 pb W + W - ~ 8 pb ZZ ~ 0.5 pb  Analyses depend on h and A masses Topology (small m h or m A: 4 jets  3 jets)  Gluon pairs versus light-quark pairs: higher selection efficiency worse di-jet mass resolution 4-jet final state Experimental issues:  Pairing ambiguities: 4 jets (3 pairings) qq/gg (100%) Associated production OPAL: also consider h  AA

11. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 11 Interpretation in 2HDM models (parameter scan): see talk by P. Akesson M h (GeV) M A (GeV) LEP Flavour Independent results on hA production For C 2 =1: m h +m A < 140 GeV h  AA taken into account (6 parton final state) Exclude hA(hadronic) cross-sections relative to SM: DELPHI hA  hadrons

12. Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 12 Conclusions Limits on hZ and hA production (h,A  gluons/quarks) LEP combined results on hZ (No signal observed) Cross section exclusion as function of m h (m A ) Important experimental LEP legacy: More model independent summary of LEP data Important test for (future) model builders … SUSY, 2-3-4-5HDM, CP (non-)conserving, etc. LHC cannot easily cover low mass regions (allows check for any LHC prediction)