Hunting for the Higgs Boson Ulrich Heintz Brown University
the standard model electromagnetism acts on all charged particles strong force acts on all quarks weak force acts on all particles spin ½ spin 1 3/24/2011 Ulrich Heintz - Moriond QCD 2
gauge theories gauge theories describe all three interactions Lagrangian invariant under continuous group of local transformations quantum electrodynamics Lagrangian invariant under local phase transformations need interactions of charged particles with a gauge field exactly describes electromagnetic interactions the quantum of the gauge field is the photon photon is mediator of the electromagnetic force electroweak theory (Glashow 1960) gauge theory of QED and weak interactions larger symmetry group additional gauge field quanta: W ± and Z 0 bosons weak force is short range W and Z are massive e e ν νe γ ZW e e e eν PROBLEM: cannot describe massive particles without breaking the symmetry 3/24/2011 Ulrich Heintz - Moriond QCD 3
spontaneous symmetry breaking 1965 Higgs Guralnik Hagen Kibble Brout Englert introduce a field with a special potential continuously degenerate ground state at non-zero field field equations have same symmetry as theory every ground state breaks the symmetry can describe massive particles by coupling them with the Higgs field the quantum of this field is the Higgs boson Higgs mechanism 3/24/2011 Ulrich Heintz - Moriond QCD 4
not just a nice idea 1973 Gargamelle bubble chamber observes weak neutral current 1979 Glashow, Salam, Weinberg receive Nobel Prize for their theory of the unified weak and electromagnetic interaction 1983 UA1 and UA2 find the W and Z bosons at CERN the hunt for the Higgs boson starts 3/24/2011 Ulrich Heintz - Moriond QCD 5
properties of the Higgs boson spin 0 coupling to the Higgs gives particles mass Higgs couples to pairs of all massive particles and their antiparticles H µ + µ if 210 MeV < m H < 270 MeV H bb H WW if 9 GeV < m H < 135 GeV if 135 GeV < m H coupling is proportional to particle mass decays to most massive particles with 2m<m H 3/24/2011 Ulrich Heintz - Moriond QCD 6
the mass of the Higgs boson not predicted by the theory constraints from theory Triviality Vacuum Stability 3/24/2011 Ulrich Heintz - Moriond QCD 7
the mass of the Higgs boson not yet measured by experiment constraints from experiment global ewk fit consistency test of all measured parameters with standard model preferred mass 90 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 8
theoretical problems there is no (other) elementary spin-0 particle corrections to Higgs mass diverge as Λ new physics must exist at TeV scale does it have to be one elementary scalar? Supersymmetry at least 4 Higgs bosons strong dynamics role of Higgs is played by a bound state of the new strong interaction does the Higgs boson exist? 3/24/2011 Ulrich Heintz - Moriond QCD 9
early Higgs searches in decays of other particles, e.g. Y H+γ events/20 MeV should see monochromatic line in photon energy spectrum CUSB Collaboration @ CESR PRD 35, 2883(1987) photon energy (MeV) exclude m H < 5 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 10
L3 ALEPH LEP CERN, Geneva OPAL DELPHI e + e - collisions 1989: E com = 91 GeV 2000: E com = 209 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 11
Higgs production at LEP LEP 1 LEP 2 Björken process Higgsstrahlung Center of mass energy (GeV) 3/24/2011 Ulrich Heintz - Moriond QCD 12
Higgs decay look for Z+Higgs M M H H < < 2m 2m π b H H e + e or gg or cc µ + µ or τ + τ M H < 2M W H bb e, µ directly detectable τ - decays to other particles g,c,b form jets of particles easy to identify cannot measure total energy hard to identify 3/24/2011 Ulrich Heintz - Moriond QCD 13
and then 3/24/2011 Ulrich Heintz - Moriond QCD 14
signal or fluctuation? 3 selections with increasing purity for m H = 115 GeV Data 17 Bkg 15.8 Sig 7.1 Data 5 Bkg 3.9 Sig 3.8 for m H > 109 GeV Data 4 Bkg 1.2 Sig 2.2 3/24/2011 Ulrich Heintz - Moriond QCD 15
the final LEP result the results of the experiment were inconclusive so we had to use statistics... from Louis Lyon s book statistics for nuclear and particle physicists CLs small if data agree better with background only large if data agree better with signal+background signal can be excluded at 95% CL if CL s < 0.05 exclude m H < 114 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 16
2000: the end of LEP 3/24/2011 Ulrich Heintz - Moriond QCD 17
Tevatron Fermilab, Chicago pp collisions E com 2 TeV 3/24/2011 Ulrich Heintz - Moriond QCD 18
Higgs production at Tevatron g t large cross section huge background H g q W/Z H q W Z small cross section moderate background 3/24/2011 Ulrich Heintz - Moriond QCD 19
Higgs search at Tevatron H bb H WW e ν 2 jets low mass Higgs m H < 135 GeV (H bb) background driven WH lνbb, ZH ll/ννbb µ ν 2 acollinear leptons high mass Higgs m H > 135 GeV (H WW) rate driven all production modes 3/24/2011 Ulrich Heintz - Moriond QCD 20
Higgs search at Tevatron ZH ννbb H WW backgrounds signal x 500 backgrounds signal x 10 M. Cooke this conference R. Lysak this conference need to use many observables combined into a discriminant variable this is a tough job! 3/24/2011 Ulrich Heintz - Moriond QCD 21
can we see such a small signal? observation of single top quark and WW production WW 3/24/2011 Ulrich Heintz - Moriond QCD 22
can we see such a small signal? WW production same final state as WH 3/24/2011 Ulrich Heintz - Moriond QCD 23
Tevatron Higgs limits J. Hays this conference excluded @ 95% CL m H < 109.5 GeV 158 < m H < 173 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 24
will the Tevatron find the Higgs? M. Cooke this conference possibly exclude m H < 190 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 25
LHC CMS pp collisions 2010-13: E com = 7 TeV 2015: E com 14 TeV Alice ATLAS LHCb 3/24/2011 Ulrich Heintz - Moriond QCD 26
Higgs production at LHC Higgs production cross sections at LHC 3/24/2011 Ulrich Heintz - Moriond QCD 27
LHC projections for Higgs search mh < 130 GeV H γγ E com = 14 TeV 130 < mh < 190 GeV H W+W- llνν 190 < mh < 460 GeV H ZZ ll ll 460 < mh < 800 GeV H W+W- lνqq designed to discover Higgs with 100 < m H < 800 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 28
the CMS experiment homogeneous electromagnetic calorimeter 78,000 lead tungstate crystals excellent photon energy resolution measure H γγ # of events / 0.5 GeV m H = 130 GeV L = 100 fb -1 M γγ (GeV) 3/24/2011 Ulrich Heintz - Moriond QCD 29
ATLAS 3/24/2011 Ulrich Heintz - Moriond QCD 30
LHC Operations 2010: delivered 45 pb -1 at E com = 7 TeV 2011: already delivered 20 pb -1 at E com = 7 TeV goal: 1-3 fb -1 2012: E com 8 TeV 2015: E com 14 TeV 3/24/2011 Ulrich Heintz - Moriond QCD 31
first Higgs limits from LHC observed upper limit standard model prediction W. Quayle this conference exclude cross sections > 2.1 standard model prediction 3/24/2011 Ulrich Heintz - Moriond QCD 32
LHC projections W. Quayle this conference possibly exclude 130 < m H < 460 GeV 3/24/2011 Ulrich Heintz - Moriond QCD 33
the quest goes on the hunt for the Higgs boson has shaped particle physics experiments for the last three decades it still eludes detection LHC will find it (or exclude it) 3/24/2011 Ulrich Heintz - Moriond QCD 34