Transcript Large Hadron Collider

Early Physics with the
Large Hadron Collider
Thomas J. LeCompte
High Energy Physics Division
Argonne National Laboratory
JLAB Users’ Meeting: 16 June 2008
First Order of Business
Thanks very much for the invitation.
I’ve wanted to visit Jefferson Lab for a
long time, both for the rich scientific
program, and because Nate Isgur was
very kind to me when I was an ignorant
graduate student.
2
Second Order of Business
The HEP community likes
Mont.
You will too.
3
Outline
 The Standard Model
– QCD
– Electroweak Theory
 The Large Hadron Collider and Why You Might Want One
– The problem with Electroweak Theory
 Detectors: ATLAS and CMS
 The problem with QCD
 More on the EWK problem
 Summary
4
The Traditional Opening Pitch
 Practically every HEP talk starts
with this slide.
 This isn’t the way I want to start
this talk.
5
Comparing Two Figures
Notes Used in Symphony #5
350
300
A histogram of the notes
used in Beethoven’s 5th
Symphony, first movement.
250
200
150
100
50
0
A
Bb
B
C
C#
D
Eb
E
F
F#
G
Ab
 Both plots focus on the constituents of a thing, rather than their interactions.
 While there is meaning in both plots, it can be hard to see.
– A plot of a composition by A. Schoenberg would look different
I’d like to come at this from a different direction.
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The Twin Pillars of the Standard Model
 Quantum Chromodynamics
– Quarks carry a charge called
“color” carried by gluons which
themselves also carry color
charge.
– A strong force (in fact, THE
strong force)
– Confines quarks into hadrons
 Electroweak Unification
– The electric force, the magnetic
force and the weak interaction
that mediates b-decay are all
aspects of the same
“electroweak” force.
– Only three constants enter into it:
e.g. a, GF and sin2(qw).
– A chiral theory: it treats particles
with left-handed spin differently
than particles with right-handed
spin.
A beautiful theory.
Unfortunately, it’s broken.
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Why Study The Standard Model?
 Understanding it is a necessary precondition for discovering anything beyond the
Standard Model
– Whatever physics you intend to do in 2011, you’ll be studying SM physics in
2008
• Rate is also an issue
 It’s interesting in and of itself
– It’s predictive power remains extraordinary (e.g. g-2 for the electron)
 We know it’s incomplete
– It’s a low energy effective theory: can we see what lies beyond it?
 We’ve lived with the SM for ~25 years
– Long enough so that features we used to find endearing
are starting to become annoying
 Think of the LHC as “marriage counseling” for the SM
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Local Gauge Invariance – Part I
 In quantum mechanics, the probability density is the square of the
wavefunction: P(x) = |Y|2
– If I change Y to –Y, anything I can observe remains unchanged
 P(x) = |Y|2 can be perhaps better written as P(x) = YY*
– If I change Y to Yeif anything I can observe still remains unchanged.
– The above example was a special case (f = p)
 If I can’t actually observe f, how do I know that it’s the same everywhere?
– I should allow f to be a function, f(x,t).
– This looks harmless, but is actually an extremely powerful constraint
on the kinds of theories one can write down.
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Local Gauge Invariance – Part II
 The trouble comes about because the Schrödinger equation
(and its descendents) involves derivatives, and a derivative
of a product has extra terms.
d
dv
du
uv  u  v
dx
dx
dx
 At the end of the day, I can’t have any leftover f’s – they all have to
cancel. (They are, by construction, supposed to be unobservable)
 If I want to write down the Hamiltonian that describes two electrically
charged particles, I need to add one new piece to get rid of the f’s: a
massless photon.
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Massless?
 A massive spin-1 particle has three spin states
(m = 1,0,-1)
 A massless spin-1 particle has only two.
– Hand-wavy argument: Massless particles
move at the speed of light; you can’t boost to
a frame where the spin points in another
direction.
 To cancel all the f’s, I need just the two m = ± 1
states (“degrees of freedom”)
– Adding the third state overdoes it and
messes up the cancellations
– The photon that I add must be massless
m = ±1 “transverse”
m = 0 “longitudinal”
Aside: this has to be just
about the most confusing
convention adopted since
we decided that the current
flows opposite to the
direction of electron flow.
We’re stuck with it now.
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A Good Theory is Predictive…or at least Retrodictive
 This is a theoretical tour-de-force: starting with Coulomb’s Law, and
making it relativistically and quantum mechanically sound, and out pops:
– Magnetism
– Classical electromagnetic waves
– A quantum mechanical photon of zero mass
 Experimentally, the photon is massless (< 10-22me)
– 10-22 = concentration of ten molecules of ethanol in a glass of water
• Roughly the composition of “Lite” Beer
– 10-22 = ratio of the radius of my head to the radius of the galaxy
– 10-22 = probability Britney Spears won’t do anything shameless and
stupid in the next 12 months
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Let’s Do It Again
 A Hamiltonian that describe electrically charged particles also gives you:
– a massless photon J
 A Hamiltonian that describes particles with color charge (quarks) also
gives you:
– a massless gluon (actually 8 massless gluons) J
 A Hamiltonian that describes particles with weak charge also gives you:
– massless W+, W- and Z0 bosons
– Experimentally, they are heavy: 80 and 91 GeV
L
Why this doesn’t work out for the weak force – i.e. why the W’s and Z’s are
massive – is what the LHC is trying to find out.
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Nobody Wants A One Trick Pony
 One goal: understand what’s going on with
“electroweak symmetry breaking”
– e.g. why are the W and Z heavy when
the photon is massless
 Another goal: probe the structure of matter
at the smallest possible distance scale
– Small l (=h/p) means high energy
 Third goal: search for new heavy particles
– This also means large energy (E=mc2)
 Fourth goal: produce the largest number
of previously discovered particles (top &
bottom quarks, W’s, Z’s …) for precision
studies
“What is the LHC for?” is a little
like “What is the Hubble Space
Telescope for?” – the answer
depends on who you ask.
A multi-billion dollar instrument
really needs to be able to do
more than one thing.
All of these require the highest energy we can achieve.
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The Large Hadron Collider
Design Collision
Energy = 14 TeV
The Large Hadron Collider is a 26km
long circular accelerator built at
CERN, near Geneva Switzerland.
The magnetic field is created by 1232
superconducting dipole magnets (plus
hundreds of focusing and correction
magnets) arranged in a ring in the
tunnel.
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Thermal Expansion and the LHC
x
 aT means that the LHC should shrink ~50 feet in radius when cooled down.
x
The tunnel is only about 10 feet wide.
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ATLAS = A Toroidal LHC ApparatuS
Length = 44m
Diameter = 22m
Mass = 7000 t
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CMS = Compact Muon Solenoid
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How They Work
 Particles curve in a central
magnetic field
p
– Measures their r 
qB
momentum
 Particles then stop in the
calorimeters
– Measures their energy
Different particles propagate differently
through different parts of the detector;
this enables us to identify them.
 Except muons, which
penetrate and have their
momenta measured a second
time.
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ATLAS Revisited
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What ATLAS Looks Like Today
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The ATLAS Muon Spectrometer – One Practical Issue
 We would like to measure a 1 TeV
muon momentum to about 10%.
– Implies a sagitta resolution of
about 100 mm.
 Thermal expansion is enough to
cause problems. x
x
 a T
x
T 
 0 .2 K
ax
Beam’s eye view: d= 22m
 Instead of keeping the detector in
position, we let it flex:
– It’s easier to continually measure
where the pieces are than to keep
it perfectly rigid.
Pictures from Jim Shank, Boston University
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CMS: The Other LHC “Large” Detector
 Different detector
technologies
– e.g. iron core muon
spectrometer vs. air
core
– Crystal calorimeter vs.
liquid argon
Similar in concept to ATLAS,
but with a different execution.
 Different design emphasis
– e.g. their EM
calorimeter is
optimized more
towards precise
measurement of the
signal; ATLAS is
optimized more
towards background
rejection
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The Problem with QCD
Calculations can be extraordinarily difficult – many
quantities we would like to calculate (e.g. the structure of
the proton) need to be measured.
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QCD vs. QED
QED
QCD
Symmetry Group
U(1)
SU(3)
Charge
Electric charge
Three kinds of color
Force carrier
1 Photon – neutral
8 Gluons - colored
Coupling strength
1/137 (runs slowly)
~1/6 (runs quickly)
a changes by about 7% from Q=0 to
Q=100 GeV. This will change the results
of a calculation, but not the character of a
calculation.
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The Running of as
 At high Q2, as is small, and QCD is in
the perturbative region.
– Calculations are “easy”
 At low Q2, as is large, and QCD is in
the non-perturbative region.
– Calculations are usually
impossible
• Occasionally, some symmetry
principle rescues you
– Anything we want to know here
must come from measurement
From I. Hinchliffe – this contains data from
several kinds of experiments: decays, DIS, and
event topologies at different center of mass
energies.
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An Early Modern, Popular and Wrong View of the Proton
 The proton consists of two up (or u) quarks and one
down (or d) quark.
– A u-quark has charge +2/3
– A d-quark has charge –1/3
 The neutron consists of just the opposite: two d’s and a u
– Hence it has charge 0
 The u and d quarks weigh the same, about 1/3 the
proton mass
– That explains the fact that m(n) = m(p) to about
0.1%
 Every hadron in the Particle Zoo has its own quark
composition
So what’s missing from this picture?
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Energy is Stored in Fields
 We know energy is stored in electric & magnetic fields
– Energy density ~ E2 + B2
– The picture to the left shows what happens when the
energy stored in the earth’s electric field is released
 Energy is also stored in the gluon field in a proton
– There is an analogous E2 + B2 that one can write down
– There’s nothing unusual about the idea of energy
stored there
• What’s unusual is the amount:
Energy stored in the field
Thunder is good, thunder is
impressive; but it is lightning
that does the work.
(Mark Twain)
Atom
10-8
Nucleus
1%
Proton
99%
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The Modern Proton
 99% of the proton’s mass/energy is due to this selfgenerating gluon field
The Proton
Mostly a very dynamic
self-interacting field of
gluons, with three quarks
embedded.
 The two u-quarks and single d-quark
– 1. Act as boundary conditions on the field (a
more accurate view than generators of the field)
– 2. Determine the electromagnetic properties of
the proton
• Gluons are electrically neutral, so they can’t
affect electromagnetic properties
 The similarity of mass between the proton and
neutron arises from the fact that the gluon dynamics
are the same
– Has nothing to do with the quarks
Like plums in a pudding.
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The “Rutherford Experiment” of Geiger and Marsden
a particle scatters from source, off the
gold atom target, and is detected by a
detector that can be swept over a
range of angles
(n.b.) a particles were the most energetic probes
available at the time
The electric field the a experiences
gets weaker and weaker as the a
enters the Thomson atom, but gets
stronger and stronger as it enters
the Rutherford atom and nears the
nucleus.
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Results of the Experiment
o
Geiger-Marsden Results
Scattering (arbitrary units)
100
1
Data
Thomson Model
0.01
0.0001
1E-6
1E-8
1E-10
0
1
2
3
4
5
Degrees
6
7
8
9
 At angles as low as 3 , the data show
a million times as many scatters as
predicted by the Thomson model
– Textbooks often point out that
the data disagreed with theory,
but they seldom state how bad
the disagreement was
 There is an excess of events with a
large angle scatter
– This is a universal signature for
substructure
– It means your probe has
penetrated deep into the target
and bounced off something hard
and heavy
 An excess of large angle scatters is
the same as an excess of large
transverse momentum scatters
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Proton Collisions: The Ideal World
1. Protons collide
2. Constituents scatter
3. As proton remnants separate
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What Really Happens
You don’t see the constituent scatter. You see a jet: a “blast” of particles,
all going in roughly the same direction.
2 jets
2 jets
2
2
3 jets
5 jets
3
5
Calorimeter View
Same Events, Tracking View
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Jets
Initial quark
 The force between two colored
objects (e.g. quarks) is
~independent of distance
– Therefore the potential energy
grows (~linearly) with distance
– When it gets big enough, it
pops a quark-antiquark pair out
of the vacuum
– These quarks and antiquarks
ultimately end up as a collection
of hadrons
 We can’t calculate how often a jet’s
final state is, e.g. ten p’s, three K’s
and a L.
Jet
 Fortunately, it doesn’t matter.
– We’re interested in the quark or gluon that
produced the jet.
– Summing over all the details of the jet’s
composition and evolution is A Good Thing.
• Two jets of the same energy can look
quite different; this lets us treat them the
same
What makes the measurement possible
& useful is the conservation of energy
& momentum.
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Jets after “One Week”
Jet Transverse Energy
ATLAS
5 pb-1 of (simulated) data:
corresponds to 1 week running at
1031 cm-2/s (1% of design)
This is in units of transverse
momentum. Remember,
large angle = large pT
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Jets after “One Week”
Jet Transverse Energy
ATLAS
5 pb-1 of (simulated) data:
corresponds to 1 week running at
1031 cm-2/s (1% of design)
New physics (e.g. quark
substructure) shows up
here.
 Number of events we expect to
see: ~12
 If new physics: ~50
 Number we have seen to date
worldwide: 0
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Outrunning the Bear
 Present limits on 4-fermion contact
interactions from the Tevatron are
2-4-2.7 TeV
 This may hit 3 TeV by LHC turn-on
– Depends on how many people work
on this
 If we shoot for 6 TeV at the LHC and only
reach 5 TeV, we’ve already made
substantial progress
 Note that there are ~a dozen jets that are
above the Tevatron’s kinematic limit: a
day at the LHC will set a limit that the
Tevatron can never reach.
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The Big Asterisk
 The first run will be at 10 TeV, not 14 TeV
– Magnet training took longer than
anticipated
– CERN wisely decided to give the
experiments something this year rather
than to wait.
 This increases the running time for a given
sensitivity by a factor of 3-4
– A week’s worth of good data in a 2-3
month initial run is much more likely
than a month’s worth
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Compositeness & The Periodic Table(s)
Arises because atoms have
substructure:
electrons
Arises because hadrons
have substructure:
quarks
The 9 lightest spin-0
particles
The 8 lightest spin-1/2
particles
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Variations on a Theme?
Does this arise because
quarks have substructure?
 A good question – and one that the LHC would
address
 Sensitivity is comparable to where we found
“the next layer down” in the past.
– Atoms: nuclei (105:1)
– Nuclei: nucleons (few:1)
– Quarks (>104:1) will become (~105:1)
 There are some subtleties: if this is
substructure, its nature is different than past
examples.
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The Complication
 Light quarks are…well, light.
– Masses of a few MeV
 Any subcomponents would be heavy
– At least 1000 times heavier
• Otherwise, we would have already
discovered them
 Therefore, they would have to be
bound very, very deeply. (binding energy ~ their mass)
A d-function potential has only one bound state – so the
“particle periodic table” can’t be due to them being
simply different configurations of the same components.
Something new and interesting has to happen.
I’m an experimenter. This isn’t my problem.
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The Structure of the Proton
Even if there is no new physics, the same
kinds of measurements can be used to
probe the structure of the proton.
Because the proton is traveling so close to the speed of light, it’s internal
clocks are slowed down by a factor of 7500 (in the lab frame) – essentially
freezing it. We look at what is essentially a 2-d snapshot of the proton.
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The Collision
What appears to be a
highly inelastic process:
two protons produce two
jets of other particles…
(plus two remnants that go
down the beam pipe)
… is actually the
elastic scattering of
two constituents of the
protons.
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Parton Densities
 What looks to be an inelastic collision of
protons is actually an elastic collision of
partons: quarks and gluons.
 In an elastic collision, measuring the
momenta of the final state particles
completely specifies the momenta of
the initial state particles.
 Different final states probe different
combinations of initial partons.
– This allows us to separate out the
contributions of gluons and quarks.
– Different experiments also probe
different combinations.
 It’s useful to notate this in terms of x:
– x = p(parton)/p(proton)
– The fraction of the proton’s momentum
that this parton carries
 This is actually the Fourier transform of the
position distributions.
– Calculationally, leaving it this way is best.
44
Parton Density Functions in Detail
 One fit from CTEQ and one
from MRS is shown
– These are global fits from
all the data
 Despite differences in
procedure, the conclusions
are remarkably similar
– Lends confidence to the
process
– The biggest uncertainty
is in the gluon
 The gluon distribution is
enormous:
– The proton is mostly
glue, not mostly quarks
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Improving the Gluon: Direct Photons
 DIS and Drell-Yan are sensitive to the quark
PDFs.
 Gluon sensitivity is indirect
– The fraction of momentum not carried
by the quarks must be carried by the
gluon.
– Antiquarks in the proton must be from
gluons splitting
 It would be useful to have a direct
measurement of the gluon PDFs
– This process depends on the (known)
quark distributions and the (unknown)
gluon distribution
q
g
g
q
Direct photon “Compton” process.
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Identifying Photons – Basics of Calorimeter Design
Not too much or too
little energy here.
You want exactly one
photon – not 0 (a likely
hadron) or 2 (likely p0)
Not too wide here.
One photon and not
two nearby ones
(again, a likely p0)
Not too much energy
here.
A schematic of an
electromagnetic shower
A GEANT simulation of an
electromagnetic shower
Indicative of a hadronic
shower: probably a
neutron or KL.
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Direct Photons & Backgrounds
CMS
Before event selection
CMS
After event selection
 There are two “knobs we can turn”
– Shower shape – does this look like a photon (last slide)
– Isolation – if it’s a fake, it’s likely to be from a jet, and there is likely to be some
nearby energy
 Different experiments (and analyses in the same experiment) can rely more on one
method than the other.
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More Variations on A Theme
 One can scatter a gluon off of a heavy quark in
the proton as well as a light quark
– This quark can be identified as a bottom
or charmed quark by “tagging” the jet
– This measures how much b (or c) is in
the proton
• Determines backgrounds to various searches, like Higgs
• Turns out to have a surprisingly large impact on the ability to
measure the W mass (ask me about this at the end, if interested)
 Replace the g with a Z, and measure the same thing with different
kinematics
 Replace the Z with a W and instead of measuring how much charm is in
the proton, you measure how much strangeness there is
…and so on…
49
Double Parton Scattering
 Two independent partons in the proton scatter:
 A B
 AB 
 Effective
might be better
characterized by
 AB
 A B
 Asˆ 
 Inelastic
 Searches for complex signatures in the presence of QCD background
often rely on the fact that decays of heavy particles are “spherical”, but
QCD background is “correlated”
– This breaks down in the case where part of the signature comes from
a second scattering.
– Probability is low, but needed background reduction can be high
 We’re thinking about bbjj as a good signature
– Large rate/large kinematic range
• 105 more events than past experiments
– Relatively unambiguous which jets go with
which other jets.
50
Three Subtleties
 These densities are not quite universal
– They depend on the wavelength of your
probe of the proton.
 A large fraction of the proton’s momentum is
carried by gluons at low x
– There is a halo around the proton of large wavelength gluons (and quarkantiquark pairs)
• This sounds a lot like a particle physicist’s description of a pion cloud
• Measurements of heavy flavor in the proton can be interpreted as a cloud
of flavored mesons (up to B’s)
– It’s a little paradoxical – one needs the highest energy (i.e. shortest
wavelength) to probe this large wavelength halo
 Double parton scattering delineates the breakdown of this simple model.
51
The Problem with Electroweak Theory
Here we have the opposite problem than QCD – here
calculations are easier, but there is a fundamental flaw in
the underlying theory.
52
The “No Lose Theorem”
 Imagine you could elastically scatter beams of W bosons:
WW → WW
 We can calculate this, and at high enough energies
the cross-section violates unitarity
– The probability of a scatter exceeds 1 - nonsense
– The troublesome piece is (once again) the longitudinal spin state
 “High enough” means about 1 TeV
– A 14 TeV proton-proton accelerator is just energetic enough to give you
enough 1 TeV parton-parton collisions to study this
The Standard Model is a low-energy effective theory. The LHC gives
us the opportunity to probe it where it breaks down. Something new
must happen.
53
Spontaneous Symmetry Breaking
What is the least amount
of railroad track needed to
connect these 4 cities?
54
One Option
I can connect them this
way at a cost of 4 units.
(length of side = 1 unit)
55
Option Two
I can connect them this
way at a cost of only 3
units.
56
The Solution that Looks Optimal, But Really Isn’t
This requires only 2
2
57
The Real Optimal Solution
This requires 1
3
Note that the symmetry of
the solution is lower than
the symmetry of the
problem: this is the
definition of Spontaneous
Symmetry Breaking.
+
n.b. The sum of the solutions has
the same symmetry as the
problem.
58
A Pointless Aside
One might have guessed at the
answer by looking at soap
bubbles, which try to minimize
their surface area.
But that’s not important right
now…
Another Example of Spontaneous Symmetry Breaking
Ferromagnetism: the Hamiltonian is
fully spatially symmetric, but the
ground state has a non-zero
magnetization pointing in some
direction.
59
The Higgs Mechanism
 Write down a theory of massless weak bosons
– The only thing wrong with this theory is that it doesn’t describe the
world in which we live
 Add a new doublet of spin-0 particles:
– This adds four new degrees of freedom
(the doublet + their antiparticles)
  
 0
 
 
 
 *0 
 
 
 Write down the interactions between the new doublet and itself, and the
new doublet and the weak bosons in just the right way to
– Spontaneously break the symmetry: i.e. the Higgs field develops a
non-zero vacuum expectation value
• Like the magnetization in a ferromagnet
– Allow something really cute to happen
60
The Really Cute Thing
 The massless w+ and f+ mix.
– You get one particle with three spin states
• Massive particles have three spin states
– The W has acquired a mass
m = ±1 “transverse”
 The same thing happens for the w- and f In the neutral case, the same thing happens for
one neutral combination, and it becomes the massive Z0.
m = 0 “longitudinal”
 The other neutral combination doesn’t couple to the Higgs, and it gives
the massless photon.
 That leaves one degree of freedom left, and because of the non zero
v.e.v. of the Higgs field, produces a massive Higgs.
61
How Cute Is It?
 There’s very little choice involved
in how you write down this theory.
– There’s one free parameter
which determines the Higgs
boson mass
– There’s one sign which
determines if the symmetry
breaks or not.
 The theory leaves the Standard Model mostly untouched
– It adds a new Higgs boson – which we can look for
– It adds a new piece to the WW → WW cross-section
• This interferes destructively with the piece that was already there and
restores unitarity
 In this model, the v.e.v. of the Higgs field is the Fermi constant
62
Searching for the Higgs Boson
Because the theory is so
constrained, we have very solid
predictions on where to look and
what to look for.
H → gg
H → ZZ → llll
ATLAS
Simulation
10 fb-1
ATLAS Simulation
100 fb-1
63
Two Alternatives
 Multiple Higgses
– I didn’t have to stop with one Higgs doublet – I could have added two
– This provides four more degrees of freedom:
• Manifests as five massive Higgs bosons: h0, H0, A0, H+,H– Usually some are harder to see, and some are easier
– You don’t have to stop there either…
 New Strong Dynamics
– Maybe the WW → WW cross-section
blowing up is telling us something:
• The p  p → p  p cross-section also
blew up: it was because of a
resonance: the .
• Maybe there are resonances among the W’s and Z’s which
explicitly break the symmetry
Many models: LHC data will help discriminate among them.
64
The Higgs Triangle
Two of the three necessary
measurements are SM measurements.
W+
W-
W-
W+
Loop Effects on m(W)
65
What is the Standard Model?
g
The (Electroweak)
Standard Model is the
theory that has
interactions like:
W+
W+
Z0
but not
W+
W+
Z0
g
Z0
Z0
&
W-
Z0
g
W-
Z0
Z0
Z0
g
Z0
g
&
but not:
Z0
g
Only three parameters - GF, a and
sin2(qw) - determine all couplings.
66
Portrait of a Troublemaker
 This diagram is where the SM
gets into trouble.
 It’s vital that we measure this
coupling, whether or not we see
a Higgs.
W+
W+
W-
W-
Yields are not all that great
From Azuelos et al. hep-ph/0003275
100 fb-1, all leptonic modes inside detector acceptance
67
A Complication
If we want to understand the
quartic coupling…
…first we need to
measure the
trilinear couplings
We need a TGC program that looks at
all final states: WW, WZ, Wg (present in
SM) + ZZ, Zg (absent in SM)
68
The Semiclassical W
 Semiclassically, the interaction between the W and the electromagnetic
field can be completely determined by three numbers:
– The W’s electric charge
• Effect on the E-field goes like 1/r2
– The W’s magnetic dipole moment
• Effect on the H-field goes like 1/r3
– The W’s electric quadrupole moment
• Effect on the E-field goes like 1/r4
 Measuring the Triple Gauge Couplings is equivalent to measuring the 2nd
and 3rd numbers
– Because of the higher powers of 1/r, these effects are largest at small
distances
– Small distance = short wavelength = high energy
69
Triple Gauge Couplings
 There are 14 possible WWg and WWZ couplings
 To simplify, one usually talks about 5 independent, CP conserving, EM
gauge invariance preserving couplings: g1Z, kg, kZ, lg, lZ
– In the SM, g1Z = kg = kZ = 1 and lg = lZ = 0
• Often useful to talk about g, k and l instead.
• Convention on quoting sensitivity is to hold the other 4 couplings at
their SM values.
– Magnetic dipole moment of the W = e(1 + kg + lg)/2MW
– Electric quadrupole moment = -e(kg - lg)/2MW2
– Dimension 4 operators alter g1Z,kg and kZ: grow as s½
– Dimension 6 operators alter lg and lZ and grow as s
 These can change either because of loop effects (think e or m magnetic
moment) or because the couplings themselves are non-SM
70
Why Center-Of-Mass Energy Is Good For You
Approximate
Run II Tevatron
Reach
Tevatron
kinematic limit
 The open histogram is the
expectation for lg = 0.01
– This is ½ a standard
deviation away from
today’s world average fit
 If one does just a counting
experiment above the Tevatron
kinematic limit (red line), one
sees a significance of 5.5
– Of course, a full fit is more
sensitive; it’s clear that the
events above 1.5 TeV have
the most distinguishing
power
From ATLAS Physics TDR:
30 fb-1
71
Not An Isolated Incident
 Qualitatively, the same thing
happens with other couplings
and processes
 These are from WZ events with
g1Z = 0.05
– While not excluded by data
today, this is not nearly as
conservative as the prior
plot
• A disadvantage of
having an old TDR
Plot is from ATLAS Physics TDR: 30 fb-1
Insert is from CMS Physics TDR: 1 fb-1
72
Not All W’s Are Created Equal
 The reason the inclusive W and
Z cross-sections are 10x higher
at the LHC is that the
corresponding partonic
luminosities are 10x higher
– No surprise there
 Where you want sensitivity to
anomalous couplings, the
partonic luminosities can be
hundreds of times larger.
 The strength of the LHC is not
just that it makes millions of
W’s. It’s that it makes them in
the right kinematic region to
explore the boson sector
couplings.
From Claudio Campagnari/CMS
73
TGC’s – the bottom line
Coupling
Present Value
LHC Sensitivity
(95% CL, 30 fb-1 one experiment)
g1Z
0.005-0.011
 0.01600..022
019
kg
0.03-0.076
 0.027 00..044
045
kZ
0.06-0.12
 0.07600..061
064
lg
0.0023-0.0035
 0.02800..020
021
lZ
0.0055-0.0073
 0.08800..063
061
 Not surprisingly, the LHC does best with the Dimension-6 parameters
 Sensitivities are ranges of predictions given for either experiment
74
Early Running
 Reconstructing W’s and Z’s quickly will not be hard
 Reconstructing photons is harder
– Convincing you and each other that we understand the efficiencies and jet
fake rates is probably the toughest part of this
 We have a built in check in the events we
are interested in
– The Tevatron tells us what is happening
over here.
– We need to measure out here.
 At high ET, the problem of jets faking
photons goes down.
– Not because the fake rate is
necessarily going down – because the
number of jets is going down.
75
Precision EWK:The W Mass
I am not going to try and sell you on
the idea that the LHC will reach a
precision of [fill in your favorite
number here].
Instead, I want to outline some of
the issues involved.
76
CDF Results: The State of the Art
These systematics are
statistically limited.
These systematics are not.
77
One Way Of Thinking About It
25 MeV
15 MeV
If we shoot for 5 MeV, how close
might we come?
5 MeV
What needs to happen to get
down to 5 (or 15, or 25) MeV?
(If you shoot for 5, you might hit 10. If
you shoot for 10, you probably won’t
hit 5)
See Besson et al.
arXiv:0805.2093v1 [hep-ex]
8 MeV is 100 parts per million.
78
Difficulty 1: The LHC Detectors are Thicker
 Detector material interferes with the
measurement.
– You want to know the kinematics of the
W decay products at the decay point,
not meters later
– Material modeling is tested/tuned
based on electron E/p
 Thicker detector = larger correction = better
relative knowledge of correction needed
CMS material budget
X~16.5%X0
(red line on lower plots)
ATLAS material budget
79
Difficulty 2 – QCD corrections are more important
q
q
W
g
q
W
q
 No valence antiquarks at the LHC
– Need sea antiquarks and/or
higher order processes
 NLO contributions are larger at
the LHC
 More energy is available for
additional jet radiation
 At the Tevatron, QCD effects are
already ¼ of the systematic
uncertainty
– Reminder: statistical and
systematic uncertainties are
comparable.
 To get to where the LHC wants to
be on total m(W) uncertainty is
going to require continuous
effort on this front.
80
Major Advantage – the W & Z Rates are Enormous
 The W/Z cross-sections at the LHC are an order of magnitude greater than the at
the Tevatron
 The design luminosity of the LHC is ~an order of magnitude greater than at the
Tevatron
– I don’t want to quibble now about the exact numbers and turn-on profile for the
machine, nor things like experimental up/live time
 Implications:
– The W-to-final-plot rate at ATLAS and CMS will be ~½ Hz
• Millions of W’s will be available for study – statistical uncertainties will be
negligible
• Allows for a new way of understanding systematics – dividing the W
sample into N bins (see next slide)
– The Z cross-section at the LHC is ~ the W cross-section at the Tevatron
• Allows one to test understanding of systematics by measuring m(Z) in the
same manner as m(W)
• The Tevatron will be in the same situation with their femtobarn
measurements: we can see if this can be made to work or not
– One can consider “cherry picking” events – is there a subsample of W’s where
the systematics are better?
81
200
200
150
150
150
100
50
Measurement
200
Measurement
Measurement
Systematics – The Good, The Bad, and the Ugly
100
50
0
50
0
0
2
4
6
8
10
12
Some variable
100
0
0
2
4
6
8
10
12
Some variable
Good
Bad
 Masses divided into
several bins in some
variable
 Masses are
consistent within
statistical
uncertainties.
 Clearly there is a
systematic
dependence on this
variable
 Provides a guide as
to what needs to be
checked.
0
2
4
6
8
10
12
Some variable
Ugly
 Point to point the
results are
inconsistent
 There is no
evidence of a trend
 Something is wrong
– but what?
82
So, When Is This Going To Happen?
The latest schedule
shows the LHC ready
for beam in about a
month.
Beam will be injected
into sectors as soon
as they are cold.
The plan is to have collisions at 10 TeV for
2-3 months in 2008, train the magnets during
the winter shutdown, and go to 14 TeV in 2009.
83
LHC Beam Stored Energy in Perspective
Luminosity
Equation:
L
2
fE nb N p
n
b*
 Luminosity goes as the
square of the stored
energy.
 LHC stored energy at
design ~700 MJ
– Power if that
energy is
deposited in a
single orbit: ~10
TW (world energy
production is ~13
TW)
– Battleship gun
kinetic energy
~300 MJ
 It’s best to increase the
luminosity with care
USS New Jersey (BB-62)
16”/50 guns firing
84
My Take on The Schedule
 If we only have the same old problems (i.e.
no new ones) there will beam in fall.
– Full energy will be in early 2009.
 We will turn on with very low luminosity and
this will grow slowly as we learn to handle
the stored energy
– Luminosity grows as the square of
stored energy
 After maybe a year, the luminosity will
shoot up like a rocket
– Luminosity grows as the square of
stored energy
85
Apologies
 I didn’t cover even a tenth of the ATLAS physics program
– Precision measurements
– Top Quark Physics
• Orders of magnitude more events than at the
Tevatron
– Search for new particles
• Can we produce the particles that make up the
dark matter in the universe?
– Search for extra dimensions
• Why is gravity so much weaker than other forces?
• Are there mini-Black Holes?
– B Physics and the matter-antimatter asymmetry
• Why is the universe made out of matter?
– Heavy Ions
• What exactly has RHIC produced?
86
Summary
 Electroweak Symmetry Breaking is puzzling
– Why is the W so heavy? Why is the weak force so weak?
 The Large Hadron Collider is in a very good position to shed light on this
– The “no lose theorem” means something has to happen. Maybe it’s a
Higgs, maybe it’s not.
– Finding the Higgs is not enough. Precision electroweak
measurements are needed to understand what’s going on.
 Any experiment that can do this can also answer a number of other
questions
– For example, addressing the structure of the proton
– And the dozens I didn’t cover
Thanks for inviting me!
87
The LHC:
Ready or Not, Here It Comes