Detector Systems
A typical detector beam looks something like:
BaBar, CDF, STAR, ATLAS, GLAST……
momentum
energy
particle ID
Let’s look at the ATLAS Detector as an example of a “real” system
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A Toroidal LHC ApparatuS
Turns on in 2007
ATLAS is one of the 4 major experiments planned for the LHC
Goal is to find Higgs + physics beyond the standard model (SUSY??)
The ATLAS detector stresses:
good charged particle tracking
pixel+SCT+TRT
good momentum resolution
2 magnets
good calorimetry (EM+Had)
electron ID
TRT+ECAL
muon ID
ATLAS has limited (none?)
p/K/p ID
ATLAS superimposed on
the 5 story building 40 at CERN
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ATLAS is Under Construction Now
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A Systems View of ATLAS
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ATLAS Data Flow
At the LHC at full luminosity the interesting event rates will be:
107 B’s/sec (but most go down the beampipe)
several thousand W’s/sec
several hundered Z’s/sec
~10 tops/sec
~1 Higgs/sec (mH=150 GeV)
many SUSY particles/sec
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write out ~ 100Hz of data
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How Can ATLAS Discover the Higgs?
Standard Model
makes predictions for
the Higgs production
cross section.
Many process contribute
depends on Higgs mass.
In 1st 3years of data
taking ATLAS is
expected to accumulate
100 fb-1 of data
#evts=Ls
L=luminosity=100x10-15 barns
s= cross section~30 pb@100GeV
#evts~3x107 Higgs
What the Higgs decays
into depends on the value
of it mass.
Best guess is mH~120GeV
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If mH ~120 GeV then
the discovery modes
may be HZZ, H
HZ
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How Can ATLAS Discover the Higgs?
What is so good about the decay modes HZZ, HZ and H
If we measure the energy/momentum of the decay products then we can measure (reconstruct)
the mass of the Higgs.
For these decay modes it is possible to measure the energy and/or momentum of
all of its decay products:
H measure ’s in EM calorimeter

mH2  ( E 1  E 2 ) 2  ( p 1  p 2 ) 2  2 p 1 p 2 (1  cos )
HZZ with Ze+e- or Zm+m- measure e’s and m’s tracking+calorimeter(+muon)
The HZZ is more complicated, we must reconstruct the 2 Z’s. Need Z decay modes that do
not involve neutrinos or jets. Use Z decays to electrons or muons:

mZ21  ( pl  1  pl  1 ) 2  ( El  1  El  1 ) 2  ( pl  1  pl  1 ) 2

mZ2 2  ( pl  2  pl  2 ) 2  ( El  2  El  2 ) 2  ( pl  2  pl  2 ) 2

mH2  ( p Z 1  p Z 2 ) 2  ( EZ 1  EZ 2 ) 2  ( p Z 1  p Z 2 ) 2
Other Higgs decay modes involve “jets” and or decays with neutrinos:
HW+W- or t+t-: Always a neutrino in the decay products as a W or t decay includes n’s
and n’s do not interact in ATLAS (not enough material)
Hbb or cc or gg: We can not measure the quarks (b, c) or gluons (g) directly.
The quarks and gluons turn into many other particles collectively
called a “jet”. The jet may contain neutrinos and/or some particles from
the jet may not be measured (efficiency and acceptance)
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How Can ATLAS Discover the Higgs?
Simulation of HZZ
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Simulation of H
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How Can ATLAS Discover the Higgs?
Simulation of Hbb
a b-quark “jet”
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How Can ATLAS Discover the Higgs?
mode
pixel
SCT
TRT
ECAL
HCAL
muon reconstruct mH
H
veto
veto
veto
yes
no
no
yes
HZZ
Ze+emm
yes
yes
yes
yes
yes/eID
yes
yes
no
no
no
no
yes
yes
yes
yes +
vertex
yes
yes
eID?
yes
if e’s
yes
yes
if m’s
maybe
yes
yes
yes
yes
yes
yes
yes/eID
yes
yes
yes
no
no
no
no
yes
no
yes
no
no
no
no
yes +
vertex
yes
yes
eID?
yes
if e’s
yes
if hadrons
yes
if m’s
no
Hbb
HW+WWen
Wmn
Wquark n
tt
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How Can ATLAS Discover the Higgs?
From ATLAS detector simulations:
The statistical significance of Higgs observation
as function of Higgs mass and decay mode
MC simulation of H with
3 years of data collection
There can be > 1 Higgs….
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How Well Can ATLAS Determine Higgs Properties?
It is not enough to just show that the Higgs particle(s) exist!
Must measure their properties too.
mH measurement
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Higgs Branching Fraction
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How Can ATLAS Find Supersymmetry?
Supersymmetry: A grand unified theory with symmetry between fermions and bosons
With SUSY the force
couplings unify at
very high energy
Also: string theories
no proton decay
Supersymmetry predicts lots of new particles:
bosons
squarks
sleptons
Higgs
gluon
W, Z
photon
fermions
quarks
sleptons
Higgsinos
gluinos
winos
photino
~
~
W   H   ~   charginos
~ ~
~
~  Z  h 0  H 0  ~ 0  neutralino s
SUSY particles have not yet been observed.
Either too massive to produce at today’s accelerators or they don’t exist
About 100 free parameters in Minimal SUSY (MSSM)
Because of “R-Parity” SUSY particles to be produced in pairs. Lightest SUSY particle stable.
R=(-1)3(B-L)+2S with B=baryon #, L=lepton #, S=spin, R=1 for SM, -1 for SUSY particles
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How Can ATLAS Find Supersymmetry?
Several SUSY particles might be stable and not interact with matter
Lightest SUSY particle might be responsible for dark matter…
Consider the following decay chain (one of many…)
assume
~g
~X
pp  g
m~ 0  97 GeV
2
~
~  bb
g
m~ 0  45 GeV
~
1
0
~
b  b 2
~0  
~ 0ee

2
1
We can measure the electrons (or muon) energy and momentum
Also look for the presence of two b-quark jets
Key is the invariant mass of the lepton pair:
me  e  ) max
there is a sharp cutoff in m(e+e-) that depends on the
masses of the neutralinos:
 m~ 0  m~ 0
2
1
p ~ 0  p ~ 0  pe   pe   p ~ 0  p ~ 0  pe   pe 
2
1
2
1
me2 e   ( pe   pe  ) 2  ( p ~ 0  p ~ 0 ) 2
1
2
Evaluate in the frame where 2 is at rest
me2 e   ( m~ 0  E ~ 0 ) 2  p 2~0  m 2~0  m 2~0  2m~ 0 E ~ 0
2
m2
1
1
2
1
2
1
will be max when E1 is min.p1=0 so E1=m1
me2 e  ) max  m 2~0  m 2~0  2m~ 0 m~ 0  ( m~ 0  m~ 0 ) 2
2
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1
2
1
2
1
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