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Discovering the Higgs Boson
J. PilcherTalk for Graduate Students
January 9, 2004
January 9, 2004Page 2
Introductions Research in experimental high energy
physics Work with Kelby Anderson, Ed Blucher, Frank
Merritt, Mark Oreglia, Mel Shochet Graduate students Francesco Spano, Martina
Hurwitz Upcoming experiment motivated by the
previous one High precision tests of the electroweak theory OPAL experiment at the LEP facility at CERN
January 9, 2004Page 3
Previous Work
Reaction studied
The collider CERN, Geneva e+e- collisions Ecm to ~200
GeV
January 9, 2004Page 4
Physics Prejudice The three families of “point-like” fermions
Unclear why there is this replication
Interactions via the gauge bosons
g8 γ W± Z0
νe
e−
⎛ ⎝ ⎜
⎞ ⎠ ⎟
νμ
μ−
⎛ ⎝ ⎜
⎞ ⎠ ⎟
ντ
τ−
⎛ ⎝ ⎜
⎞ ⎠ ⎟
u
′ d ⎛ ⎝ ⎜
⎞ ⎠ ⎟
c
′ s ⎛ ⎝ ⎜
⎞ ⎠ ⎟
t
′ b ⎛ ⎝ ⎜
⎞ ⎠ ⎟
January 9, 2004Page 5
Physics Prejudice Weak boson mass splittings from interaction
with the Higgs boson Leading order predictions of the electroweak
theory are very simple 3 “degrees of freedom” If MW, MZ, and em are known, many observables are
predicted Expected accuracy a few %
Higher order effects involve unseen states Top quark, Higgs boson Predictions are a function of the variables MT and MH Fit observables to obtain the unknowns
Cross sections, angular distributions, forward-backward asymmetries
January 9, 2004Page 6
How well does it work? The observed cross section
Extraction of properties of Z and W bosons
January 9, 2004Page 7
Properties of the Z Fit to resonance gives mass and width
January 9, 2004Page 8
Properties of the Z Quality of the measurements?
January 9, 2004Page 9
Properties of the Z Mass results
January 9, 2004Page 10
Properties of the W boson Cross section and mass determinations
January 9, 2004Page 11
Properties of the W boson W boson mass
January 9, 2004Page 12
Other observables
January 9, 2004Page 13
Determination of Top Quark Mass
Compare with direct measurements
January 9, 2004Page 14
Determination of Higgs Boson Mass
Fit gives MH = 81 +52/-33 GeV MH < 193 GeV (95% CL)
Limit from direct search MH > 114 GeV (95% CL)
Overall 114 < MH < 193 GeV
January 9, 2004Page 15
The end of the LEP Era
OPAL finished data taking in fall 2000 1000 pb-1 of data collected LEP collider was pushed to the highest
possible energy To ECM ~ 207 GeV
Many high precision measurements The electroweak model works remarkably
well Prediction of the Higgs mass No direct observation of the Higgs
January 9, 2004Page 16
What’s the Next Step? The CERN Large Hadron Collider (LHC)
New project approved in 1995 Proton-proton collider
ECM = 14 TeV (7000 + 7000 GeV) Actually this is for protons Each internal quark has only a fraction of this energy Effective ECM for qq collisions is ~ 14/3=4.7 TeV
– Factor of ~20 higher than LEP Cross sections fall like 1/E2
– Luminosity of collider must be 400 times larger than LEP Design luminosity 1034/cm2/sec
Expected to start operation in ~ 3 years The Chicago group is part of the ATLAS experiment for
this facility
January 9, 2004Page 17
What about Fermilab? Fermilab collider has ECM for proton-
antiproton collisions of 2 TeV ECM for qq collisions of 0.7 TeV Luminosity ~ 1031/cm2/sec
A little low
It is operating NOW
January 9, 2004Page 18
What is the LHC? Fill the 27 km circumference LEP tunnel
with superconducting magnets 1200 14-m-long dipole magnets with 8.3T
field
January 9, 2004Page 19
Where is it? Just outside Geneva
January 9, 2004Page 20
What is the LHC? Build new state-of-the-art detectors
ATLAS and CMS for high PT physics Also ALICE for heavy ion physics
Search for quark-gluon plasma Also LHCb for studying the physics of the b
quark VERY large data samples
Chicago has been working on the ATLAS experiment since 1995 Also 30 other US institutions
January 9, 2004Page 21
The ATLAS Detector
January 9, 2004Page 22
The ATLAS Detector
January 9, 2004Page 23
How to see the Higgs? Decays very rapidly
Observe decay products and calculate the mass of their parent
H decays to heaviest states accessible Specific modes depend on mass
€
H → γγ
Low probability but excellent mass resolution Can see signal as a narrow peak above background
Important for 80 < MH < 120 GeV Requires excellent EM calorimeter for
energy
January 9, 2004Page 24
How to see the Higgs?
Signal shown corresponds to integrated luminosity of 100 fb-1
1 year at design luminosity (but first year only 10 fb-1) Peak corresponds to mass resolution of ~ 2%
January 9, 2004Page 25
How to see the Higgs?
Important for 120 < MH < 170 GeV One W could be “off shell”
are not detected but appear as “missing energy”
Requires good ability to detect e and Requires good calorimeter to see “missing
energy” No distinctive mass peak
Broad excess of events over background
€
H 0 → W +W − → l +νl −ν
January 9, 2004Page 26
How to see the Higgs?
) cos-(1 Ep 2 m missT
TT ϕΔ= ll
mH = 160 GeV ATLAS
qqH qqWW qq e
qqH qq qqe +X
January 9, 2004Page 27
How to see the Higgs?
This is the “golden” mode All final state particles directly detected with
good resolution Narrow mass peak
Important for 150 < MH < 700 GeV
€
H 0 → Z 0Z 0 → l + l −l + l −
January 9, 2004Page 28
How to see the Higgs?
Signal for 300 GeV Higgs with 10 fb-1 of luminosity First year of operation
January 9, 2004Page 29
Charged Lepton DetectionMuon detection
•Toroid magnets
•High precision drift chambers
•material to shield against hadrons
Electron detection
•LAr EM calorimeter
•Magnetic tracker
•Compare E and p to reject hadrons
January 9, 2004Page 30
How to see the Higgs? Must be sensitive to many decay modes
A combination of channels may be needed
January 9, 2004Page 31
Can we build all this stuff?Most of the surface buildings handed over to ATLAS last year
Underground civil construction complete and detector being installed
January 9, 2004Page 32
Muon Toroid Design
8 superconducting coils
January 9, 2004Page 33
Muon Toroid Construction
January 9, 2004Page 34
Muon Toroid Construction
January 9, 2004Page 35
Calorimetry EM calorimeter measures energy of e and EM showers develop in 1.5-mm Pb sheets Ionization sampled with liquid Argon layers
Hadron calorimeter measures energy of quarks and gluons Hadronic showers develop in 4-mm Fe plates Ionization sampled with plastic scintillator
and photomultiplier tubes
January 9, 2004Page 36
CalorimetryHad Tiles
Had LAr
EM LAr
Forward LArSolenoid
Barrel cryostat
End-cap cryostat
January 9, 2004Page 37
Barrel EM Calorimeter
January 9, 2004Page 38
Hadron Calorimeter (TileCal)
Had Tiles
Had LAr
EM LAr
Forward LArSolenoid
Barrel cryostat
End-cap cryostat
Chicago involved hereWith signal processing electronicsWith mechanical construction
January 9, 2004Page 39
Hadron Calorimeter (TileCal)
Mechanical concept
Chicago Sub-module Construction
January 9, 2004Page 40
TileCal Module Construction
January 9, 2004Page 41
TileCal Readout
Electronics drawer in each module carries photomultiplier tubes and electronics Each cell of calorimeter viewed by 2
phototubes Electronics needs very wide dynamic range
(65,000:1)
January 9, 2004Page 42
TileCal Electronics
January 9, 2004Page 43
Current Chicago Activity Assembling calorimeter
Surface assembly this year Start underground installation 2004
Integrating electronics systems Calibration, control, signal processing
Preparing software to process the data Especially for Tile Calorimeter Studying physics signals Developing methods for in-situ calibration
Using physics signals
January 9, 2004Page 44
Opportunities for Students In the past mainly undergraduates
Assisting with construction work Getting research experience Senior theses on ATLAS physics
PhD theses need to be on publishable physics In past years has been a bit too long before data
We expect to start data taking in 2007
Now excellent time to get involved in commissioning the detector and developing analysis software