Physics and the CMS Detector at the LHC

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Physics and the CMS Detector at the LHC
Roger Rusack The University of Minnesota
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Physics and the CMS Detector at the LHC
The high energy physics community will soon begin
to explore a new energy domain with a new machine
the Large Hadron Collider. What will we see
there and what might we learn?
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What do we know?
The Standard Model of Particle Physics
One major still-unobserved component the Higgs
Boson
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How good is the standard model
SU(3) SU(2) U(1)

• Divide Standard Model into three sectors
• QCD which describes the interaction among quarks
  and gluons.
• Electroweak which is the symmetric world of weak
  interactions and electromagnetism, whose symmetry
  is broken by the Higgs Field.
• Higgs sector Interactions of quarks and leptons
  (matter) with the Higgs condensate that give
  their masses and the mixing angles.

? Works very well
? Works very well too
? Ramshackle but works very well
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Electroweak
Governed by a very tight theory with just three
parameters. Describes excellently all the
properties of the gauge bosons and the
interactions they mediate.
LEPEWWG compilation of all data from many
different experiments
Another major triumph for quantum field theory.
Summer 2005
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An old problem in the Standard Model
At 1 TeV WLWL scattering violates unitarity.
(not causal).
This can be fixed by introducing a boson which
exactly cancels out unwelcome contributions. Now
identified in the SM as the Higgs boson.
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Higgs Sector

• The most ad hoc part of the Standard Model.
• Inspired by the Landau-Ginzberg mechanism to
  explain the Meisner effect in superconductors
• A massless photon acquires mass by coupling to
  the Cooper pairs inside a superconductor.
• Massless particles acquire mass (inertia) by
  coupling to the Higgs Field.
• Fundamental field with an interaction potential
• Interactions of this field with fermions and
  bosonsgives them a mass.
• Field permeates all of space.
• Has a vacuum energy density lltfgt4

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Higgs Sector
Interactions of quarks and leptons (matter) with
the Higgs condensate give them their masses and
mixing angles. No deep principle at play just
a large number of extra parameters 6 masses 4
mixing parameters. Mixing between the quarks
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The Higgs Mechanism - Summary

• Essential ingredient of the Standard Model
• Complex scalar field with potential
• This has a minimum at a non-zero energy.
• Used to break the el. weak symmetry......
• ..... and to generate fermion masses
• And strength of coupling is proportional to mass.

There is only one free parameter for the Higgs in
the Standard Model the mass. Knowing the mass
everything else is calculable.
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Standard Model Higgs

• Experimental challenges
• Higgs boson discovery - observation.
• Measurement of Higgs boson parameters (couplings
  to bosons and fermions)
• Measurment of the Higgs self coupling ? Higgs
  potential.
• Mass limits
• lower 114.4 GeV/c2 and
• upper up to 1TeV from theory and
• 200 GeV from precision EW measurements

Finding the Higgs and measuring its properties
is a central challenge for experiments at the
highest energies.
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Where do we expect to find it?
Calculations of standard model process include
corrections due to the Higgs. These effects show
up in varying degrees in the electroweak
parameters.
http//lepewwg.web.cern.ch/LEPEWWG/
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• Experimentalists worldview of particle physics
  today
• QCD works very well at high energies. But
  difficult to calculate useful quantities in low
  energy region.
• Electroweak sector has been (monotonously)
  successful. No obvious chinks in the armor
  there.
• Higgs sector
• We still havent found direct evidence for the
  Higgs.
• Once found (if its there) we need to
  characterize it.
• And understand better the structure within the
  SM.

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BUT

1. Why are there three and only three light
   generations?
2. What is the reason for the pattern of quark
   mixing?
3. Why is the universe made mostly of matter? CP in
   baryon sector not enough.
4. Why are there so many input parameters. (17).

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However.

• The Higgs coupling with other particles is
  proportional to their mass.
• We know that at the really high mass scales this
  model must break down to include gravity.
• Scale known as the Planck scale MP
  (8pGNewton)-½ 2.41018 GeV.

f
S
H
H
H
H
The natural mass for Higgs is on the order of
1016 GeV
S. Martin hep-ph/0709356
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A Candidate Theory - Supersymmetry
Restore low energy behavior by postulating a
fermion-boson symmetry ? a complete new set of
mirror particles. Fermion lt-gt Boson. High
mass contributions to the Higgs are cancelled
out. This is a broken symmetry with the masses
of the supersymmetric particles are much
higher. Coupling between ordinary and
supersymmetric matter is like weak interactions
so above energy threshold BIG EFFECTS.
Lots of parameters so there is a wide range of
predictions
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Supersymmetry
Squarks
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SUSY

• Many models of SUSY
• Different symmetry breaking mechanisms
• Gravity SUGRA
• SM gauge interactions GMSB
• Anomalies AMSB
• Many new parameters (gt120).
• Minimal model (MSSM) has 3 neutral Higgs and two
  charged ones.

Very large phase space for predictions since so
many parameters. Use physics intuition to make
reasonable guesses.
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SUSY
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Constrained MSSM
At GUT Scale the scalar and gaugino masses take
on only two values m0 and m½.These are projected
back to lower Q2.
At low energies the lightest Particle is the
neutralino which is a mixing of ci ai B bi W
gi H1 di H2
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CMSSM Spectra
Unification to rich spectrum EWSB
.and it predicts (like other variants) a stable
light neutral particle which could be the answer
to another very interesting mystery.
Falk
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Dark Matter - A Mini-Review
Rotational Velocity of Galaxies.
NGC 3198
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Whats the universe made of?
Precision measurement of the Cosmic Wave
Background radiation.
Measures non-uniformity of universe where photons
decoupled from the plasma tU 380,000 years.
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Whats the universe made of?
Precision measurements of the microwave
background.
W WM WL WB WCDM WL
Inflation W 1.000
WMAP result of angular distribution.
WB0.047 0.006. WM0.29 0.07. From
Big-Bang-Nucleo-Synthesis WB0.040 0.005
Nn3.
So baryonic matter accounts for only 4 of
W? There is a dark component WCDM 25.
Also age of the universe 13.7 0.2 109 and W
1.02 0.2
astro-ph/0302209, astro-ph/0001318
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Put together with other data
There is a lot of dark matter out there
And most of it is non-baryonic

• Big Bang Nucleosynthesis ?b 0.040 0.005
• Cosmic Microwave Background ?b 0.0470.006

And WMAP rules out that it is neutrinos must be
higher mass stuff.
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Dark Matter Candidates.

1. Baryons.
2. Massive Neutrinos.
3. Jupiter like objects.
4. Remnant Stars.
5. Remnant Black Holes.
6. Axions
7. Lightest Supersymmetric neutral particle.

Currently strongly favored
Other candidates are axinos gravitonos
K.A.Olive. astro-ph/0301505
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SUGRA
Densiity of LSP from WMAP data limits choices of
parameters.
WCDM h2 0.09 - 0.12
tanb is the ratio of the vacuum expectation
values of the Higgs doublets.
Allowed
Scalar mass.
Mass Lightest Supersymmetric Particle 0.4 m½
Ellis, Olive, Santoso, Spanos
Gaugino Mass
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Signatures of SUSY

• Large amounts of unseen energy like neutrinos
  in beta decay.
• Clusters of isolated electrons.
• More than one Higgs. (h,H,A and H)

Some variants of SUSY will be seen in a week of
running at 14 TeV. Others will take a lot longer.
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How do you look for something at 1 TeV

1. You have to create it so you need an
   accelerator with enough energy and luminosity.
2. You build a detector that measures for the
   products of the collisions and stores the event
   data.
3. You find in the reconstruction of the events
   anomalies that cannot be explained by standard
   physics.

Each of these steps is very hard and expensive.
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Build a Machine with Enough Energy
Large Hadron Collider Project at CERN Started in
1995, will turn on in 2007.
x
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at CERN
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Colliding Beams
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CERN Accelerator Complex
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Some LHC parameters
Two counter-rotating 0.5 A beams of 7 TeV
protons. Ring circumference 27 km. Steered by
8.33 Tesla magnets. Stored energy in each beam is
350 MJ. Beams are bunched each bunch separated
by 25nsec. 20 Minimum bias events every bunch
crossing.
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Build a Detector
Two general purpose detectors are under
construction to take data when the LHC turns on
Summer 2007. CMS ATLAS One detector to look
at heavy ion collisions. ALICE Another one
dedicated to B-physics. LHC-B
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Production Rates at the LHC at design luminosity.
Inelastic background events produced at a rate of
1 GHz.
Detectable Higgs production 1 mHz.
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The Old Days
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Today

• CMS
• Compact
• Muon system
• One magnet coil (Solenoid)
• Tracking and calorimetry inside the magnet coil
• Physics performance
• Excellent tracking
• Excellent electromagnetic calorimetry (photons,
  electrons)over large acceptance
• Excellent muon detection

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What is detected
In a HEP detector you detect
Photon ? (ECAL shower, no
track) electron e (ECAL shower,
with track) muon ?
(ionization only) gluon Jet in
ECAL HCAL quark u, d, s Jet (narrow) in
ECALHCAL quark c, b Jet (narrow) Decay
Vertex top --gt W b W b ?e????
Et missing in ECALHCAL ?--gtl ??
?l Et missing charged lepton W --gt l
?l Et missing charged lepton,
EtM/2 Z --gt l
l- charged lepton pair --gt ?l
?l Et missing in ECALHCAL
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How is it detected.
Charged particles loose energy through ionization
this is used to generate some electronic signal
that is converted to a number.
Lead Tungstate
Silicon Detector
Al Strips
Oxide
OV
2cm

P implants



N Bulk






-
HV
25cm
N Implants
Electromagnetic cascade.
Charged particle tracker
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Hadron calorimetry
Measure energy of a hadron in a sampling
calorimeter
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Muons
Use property of muons that they go further in
material than all other charged particles.
Track muons through to the outside of the
detector.
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The ATLAS Detector
Diameter 25 m Barrel toroid length 26
m End-cap end-wall chamber span 46 m Overall
weight 7000 Tons
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The CMS Detector
Electromagnetic and hadronic calorimeters.
Muon Chambers
Tracker
4T Solenoid Magnet
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Slice through CMS
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Because we have to build both the detector and
the experimental hall at the same time - build
detector on surface and lower large parts into
the cavern 100 m below.
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Experimental Cavern UXC5
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Meanwhile upstairs
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The Electromagnetic Crystal Calorimeter
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Elsewhere The Tracker
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Very Far Forward Calorimeter
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Quartz Calorimeter
Iron calorimeter Covers 5 gt h gt 3 Total of
1728 towers, i.e. 2 x 432 towers for EM and HAD
h x f segmentation (0.175 x 0.175)
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A Few of the Challenges Building the Detector

• Collaborate with 1700 scientists from 37
  different countries.
• Invent whole new radiation hard crystal
  scintillator technology PbWO4 and make 80 tons of
  it.
• Develop and industrialize radiation hard
  avalanche photodiodes and make 120,000 of them.
• Develop radiation hard electronics and
  industrialize.
• Data Analysis of etabytes of data distributed
  globally.
• Everything else you didnt think of when you
  designed it.

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pp collisions at 14 TeV at 1034 cm-2s-1

• 20 min bias events overlap
• H?ZZ
• Z ?mm
• H? 4 muonsthe cleanest (golden) signature

And this (not the H though) repeats every 25 ns
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Physics Selection at the LHC
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How to make the Higgs at the LHC.
The dominant production mechanism for the Higgs
is gluon-gluon fusion
The gluon couples to the Higgs through a top
loop.
Stolen from G. Dissatori
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W - Fusion
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Low mass Higgs (MHlt140 GeV/c2)

• H?gg decay is rare (B10-3)
• But with good resolution, one gets a mass peak
• Motivation for crystal calorimeters
• Resolution at 100 GeV, s?1GeV
• S/B ? 120

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Intermediate mass Higgs

• H?ZZ?ll ll (l e,m)
• Very clean
• Resolution better than 1 GeV (around 100 GeV
  mass)
• Valid for the mass range 130ltMHlt500 GeV/c2

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High mass Higgs

• H?ZZ? ll jet jet
• At the limit of statistics

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Finding one or more SUSY Higgss

• Most promising modes for H,A
• ts identified either in hadronic or
• leptonic decays
• Mass reconstruction take
• lepton/jet direction to be the t direction

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Finding SUSY particles
JET
JET
JET
Look for decay chains like this with missing
energy
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SUSY _at_ LHC

• Simplest SUSY
• A SUSY factory
• Gauginos produced in their decay example
  qL?c20qL

M500 GeV
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Experimentally spectacular signatures
Events/(2 GeV/c2)
M(ll-) (GeV/c2)
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Other resonances/signatures (I)

• New vector bosons

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Heavy Ion Physics
There will be Pb Pb collisions with ÖsNN 5 TeV
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Summary

• Symmetry Breaking in the SM (and beyond!) still
  not really understood
• LHC (and ATLAS/CMS) initially motivated to find
  it
• Physics at the LHC will be extremely rich
• SM Higgs (if there) should be in the pocket
• Turning to measurements of properties (couplings,
  etc.)
• Supersymmetry (if there) ditto
• Can perform numerous accurate measurements
• Large cms energy new thresholds
• TeV-scale gravity? Large extra dimensions?
  Black Hole production? Rotons?
• And of course, compositeness, new bosons, excited
  quarks
• All we need to do now is to build the machine and
  the experiments and to take data.

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