You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Hunting for the<br />
Higgs Boson<br />
<strong>Ulrich</strong> <strong>Heintz</strong><br />
<strong>Brown</strong> <strong>University</strong>
the standard model<br />
spin ½ spin 1<br />
electromagnetism<br />
acts on all charged particles<br />
strong force<br />
acts on all quarks<br />
weak force<br />
acts on all particles<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 2
gauge theories<br />
• gauge theories describe all three interactions<br />
Lagrangian invariant under continuous group of<br />
local transformations<br />
• quantum electrodynamics<br />
Lagrangian invariant under local phase transformations<br />
need interactions of charged particles with a gauge field<br />
exactly describes electromagnetic interactions<br />
the quantum of the gauge field is the photon<br />
photon is mediator of the electromagnetic force<br />
• electroweak theory (Glashow 1960)<br />
gauge theory of QED and weak interactions<br />
larger symmetry group<br />
additional gauge field quanta: W ± and Z 0 bosons<br />
weak force is short range W and Z are massive<br />
PROBLEM: cannot describe massive particles without breaking the symmetry<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 3<br />
e<br />
e<br />
ν<br />
νe<br />
γ<br />
ZW<br />
e<br />
e<br />
e<br />
eν
spontaneous symmetry breaking<br />
• 1965<br />
Higgs Guralnik Hagen Kibble Brout Englert<br />
• introduce a field with a special potential<br />
• continuously degenerate ground state at non-zero field<br />
• field equations have same symmetry as theory<br />
• every ground state breaks the symmetry<br />
• can describe massive particles by coupling<br />
them with the Higgs field<br />
• the quantum of this field is the Higgs boson<br />
Higgs mechanism<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 4
not just a nice idea<br />
• 1973<br />
– Gargamelle bubble chamber observes weak neutral current<br />
• 1979<br />
– Glashow, Salam, Weinberg receive Nobel Prize<br />
• for their … theory of the unified weak and electromagnetic interaction<br />
• 1983<br />
– UA1 and UA2 find the W and Z bosons at CERN<br />
the hunt for the Higgs boson starts<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 5
properties of the Higgs boson<br />
• spin 0<br />
• coupling to the Higgs gives particles mass<br />
• Higgs couples to pairs of all massive particles<br />
and their antiparticles<br />
+ −<br />
H → µ µ if 210 MeV < mH < 270 MeV<br />
H<br />
→<br />
bb<br />
if 9 GeV < m H < 135 GeV<br />
H →WW<br />
if 135 GeV < mH • coupling is proportional to particle mass<br />
• decays to most massive particles with 2m
the mass of the Higgs boson<br />
• not predicted by the theory<br />
constraints from theory<br />
Triviality<br />
Vacuum Stability<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 7
the mass of the Higgs boson<br />
• not yet measured by experiment<br />
constraints from experiment<br />
global ewk fit<br />
consistency test<br />
of all measured<br />
parameters with<br />
standard model<br />
preferred mass<br />
≈ 90 GeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 8
theoretical problems<br />
• there is no (other) elementary spin-0 particle<br />
• corrections to Higgs mass diverge as Λ∞<br />
→new physics must exist at TeV scale<br />
• does it have to be one elementary scalar?<br />
– Supersymmetry → at least 4 Higgs bosons<br />
– strong dynamics → role of Higgs is played by a<br />
bound state of the new strong interaction<br />
does the Higgs boson exist?<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 9
events/20 MeV<br />
early Higgs searches<br />
• in decays of other particles, e.g. YH+γ<br />
CUSB Collaboration @ CESR<br />
PRD 35, 2883(1987)<br />
photon energy (MeV)<br />
exclude m H < 5 GeV<br />
should see<br />
monochromatic<br />
line in photon<br />
energy spectrum<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 10
L3<br />
DELPHI<br />
e + e - collisions<br />
1989: E com = 91 GeV<br />
2000: E com = 209 GeV<br />
ALEPH<br />
LEP<br />
CERN, Geneva<br />
OPAL<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 11
Higgs production at LEP<br />
LEP 1<br />
Center of mass energy (GeV)<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD<br />
LEP 2<br />
Björken process Higgsstrahlung<br />
12
Higgs decay<br />
• look for Z+Higgs<br />
M<br />
M<br />
M<br />
H<br />
H<br />
H<br />
<<br />
<<br />
<<br />
2m<br />
2m<br />
π<br />
b<br />
2M<br />
W<br />
e, µ – directly detectable<br />
H<br />
H<br />
H<br />
τ - decays to other particles<br />
g,c,b – form jets of particles<br />
→<br />
→<br />
→<br />
gg<br />
bb<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 13<br />
e<br />
+<br />
e<br />
−<br />
or<br />
or<br />
cc<br />
µ<br />
+<br />
easy to identify<br />
µ<br />
−<br />
or τ<br />
+<br />
τ<br />
cannot measure total energy<br />
hard to identify<br />
−
and then…<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 14
signal or fluctuation?<br />
• 3 selections with increasing purity for m H = 115 GeV<br />
Data 17<br />
Bkg 15.8<br />
Sig 7.1<br />
Data 5<br />
Bkg 3.9<br />
Sig 3.8<br />
for m H > 109 GeV<br />
Data 4<br />
Bkg 1.2<br />
Sig 2.2<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 15
the final LEP result<br />
“the results of the experiment were inconclusive so we had to use statistics...”<br />
from Louis Lyon’s book “statistics for nuclear and particle physicists”<br />
CLs<br />
• small if data agree better with<br />
background only<br />
• large if data agree better with<br />
signal+background<br />
signal can be excluded at 95% CL if CL s < 0.05<br />
exclude m H < 114 GeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD<br />
16
2000: the end of LEP<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 17
Tevatron<br />
Fermilab, Chicago<br />
pp collisions<br />
E com ≈ 2 TeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 18
Higgs production at Tevatron<br />
g<br />
g<br />
q<br />
q<br />
t<br />
W/Z<br />
large cross section – huge background<br />
H<br />
H<br />
W<br />
Z<br />
small cross section – moderate background<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 19
Higgs search at Tevatron<br />
H →<br />
2 jets<br />
bb<br />
“low mass” Higgs “high mass” Higgs<br />
m H < 135 GeV (Hbb)<br />
background driven<br />
WHlνbb, ZHll/ννbb<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD<br />
H →<br />
e<br />
WW<br />
µ<br />
ν<br />
2 acollinear leptons<br />
m H > 135 GeV (HWW)<br />
rate driven<br />
all production modes<br />
ν<br />
20
Higgs search at Tevatron<br />
ZH ννbb H WW<br />
backgrounds<br />
signal x 500<br />
M. Cooke – this conference R. Lysak – this conference<br />
need to use many observables combined into a discriminant variable<br />
this is a tough job!<br />
backgrounds<br />
signal x 10<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 21
can we see such a small signal?<br />
• observation of single top quark and WW production<br />
WW<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 22
can we see such a small signal?<br />
WW production – same final state as WH<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 23
Tevatron Higgs limits<br />
excluded @ 95% CL<br />
m H < 109.5 GeV<br />
158 < m H < 173 GeV<br />
J. Hays– this conference<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 24
will the Tevatron find the Higgs?<br />
M. Cooke – this conference<br />
possibly exclude mH < 190 GeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 25
LHC<br />
CMS<br />
Alice<br />
pp collisions<br />
2010-13: E com = 7 TeV<br />
2015: E com ≈ 14 TeV<br />
ATLAS<br />
LHCb<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 26
Higgs production at LHC<br />
Higgs production cross sections at LHC<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 27
LHC projections for Higgs search<br />
• mH < 130 GeV<br />
– H γγ<br />
• 130 < mH < 190 GeV<br />
– H W+W- ℓℓνν<br />
• 190 < mH < 460 GeV<br />
– H ZZ ℓℓ ℓℓ<br />
• 460 < mH < 800 GeV<br />
– H W+W- ℓνqq<br />
E com = 14 TeV<br />
designed to discover Higgs with 100 < m H < 800 GeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 28
the CMS experiment<br />
• homogeneous electromagnetic calorimeter<br />
– 78,000 lead tungstate crystals<br />
– excellent photon energy resolution<br />
– measure Hγγ<br />
# of events / 0.5 GeV<br />
m H = 130 GeV<br />
L = 100 fb -1<br />
M γγ (GeV)<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 29
ATLAS<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 30
LHC Operations<br />
• 2010:<br />
– delivered 45 pb -1 at E com = 7 TeV<br />
• 2011:<br />
– already delivered 20 pb -1 at E com = 7 TeV<br />
– goal: 1-3 fb -1<br />
• 2012:<br />
– E com ≥ 8 TeV<br />
• 2015:<br />
– E com → 14 TeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 31
first Higgs limits from LHC<br />
W. Quayle – this conference<br />
observed upper limit<br />
standard model prediction<br />
exclude cross sections > 2.1 standard model prediction<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 32
LHC projections<br />
W. Quayle – this conference<br />
possibly exclude 130 < m H < 460 GeV<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD 33
the quest goes on<br />
• the hunt for the Higgs boson has shaped<br />
particle physics experiments for the last three<br />
decades<br />
• it still eludes detection…<br />
3/24/2011 <strong>Ulrich</strong> <strong>Heintz</strong> - Moriond QCD<br />
… LHC will find it (or exclude it)<br />
34
![List of participants 27/2/09 [pdf] - Rencontres de Moriond - IN2P3](https://img.yumpu.com/17975746/1/190x135/list-of-participants-27-2-09-pdf-rencontres-de-moriond-in2p3.jpg?quality=85)
Change language