1. The ATLAS Experiment at the LHC
Exploring the high-Energy Frontier of Particle Physics
University of Tokyo, 25th January 2007 (P. Jenni, CERN)
University of Tokyo, Jan-2007, P. Jenni (CERN)

2. Search for the Higgs boson
ATLAS
H  ZZ(*)  4
Simulation of a H   ee event in ATLAS
University of Tokyo, Jan-2007, P. Jenni (CERN)

3. SM Higgs in ATLAS and CMS
Events / 0.5 GeV
ATLAS + CMS
preliminary 1 10
10-1
Needed Ldt (fb-1)
per experiment
mH (GeV)
 1 fb-1 for 98% C.L. exclusion
 5 fb-1 for 5 discovery
over full allowed mass range
% C.L. exclusion
here discovery easier with
gold-plated H  ZZ  4l
 by end 2008 ?
University of Tokyo, Jan-2007, P. Jenni (CERN)

4. Light Higgs: more difficult case….
K-factors  (NLO)/(LO)  2
for H  NOT included (conservative)
mH ~ 115 GeV fb-1 : S/B  4 ATLAS
3 (complementary) channels with similar (small) significance:
H   b
ttHttbb  blbjjbb H 
qqH  qq
S=130, B=4300, S/B=2
S=15, B=45, S/B=2.2
S=10, B=10, S/B=2.7
different production and decay modes
different backgrounds
different detector/performance requirements:
-- ECAL crucial for H   (in particular response uniformity) : /m ~ 1% needed
-- b-tagging crucial for ttH : 4 b-tagged jets needed to reduce combinatorics
-- efficient jet reconstruction over || < 5 crucial for qqH  qq :
forward jet tag and central jet veto needed against background
All three channels require very good understanding of detector performance and
background control to 1-10%  convincing evidence likely to come later than 2008 …
Note: WH lbb (dominant at the Tevatron) provides less sensitivity than ttH at LHC
University of Tokyo, Jan-2007, P. Jenni (CERN)

5. Conclusions
The LHC construction is on track for first collisions end of 2007, and for
starting operation at design energy in 2008
Many important milestones have been passed in the construction,
pre-assembly, integration and installation of the ATLAS detector
Very major software, computing and physics preparation activities are
underway as well, using the Worldwide LHC Computing Grid (WLCG) for
distributed computing resources
The LHC at CERN and ATLAS will open a new era of particle physics,
exploring for the first time directly the TeV mass scale
 The coming years will certainly be very exciting!
(Informal news on ATLAS is available in the ATLAS eNews letter at
University of Tokyo, Jan-2007, P. Jenni (CERN)