Experiments in Particle Physics Discovery of the top quark

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Experiments in Particle Physics
Discovery of the top quark
Finding the fundamental constituents of matter
has been a theme of natural science since the
Greeks original proposal of atomos the
building blocks of the universe. Ever since
Newton showed us the fundamental role of Forces
in describing motions and behaviors of systems of
objects, scientists have tried to find the
underlying simple forces at work in the
Universe. Only in the 19th century with the
advent of modern experimental science was there
progress in understanding these two related
questions.
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The Building Blocks of Matter
19th century chemistry showed us the periodic
table of elements from which more complex
molecules could be understood. These atomic
elements were seen to have substructure when
Thomson identified the electron, a negative
particle that could be extracted from an atom and
formed into streams in a cathode ray
tube. Rutherfords experiments scattering a
particles on Au showed that the atom contained
not only the electrons, but a equal amount of
positive charge that was concentrated in a
nucleus, whose size was tiny ( 10-5 Ratom) but
very massive (99.99 of atomic mass). The atom
is mostly empty space with electrons orbiting far
from the nucleus. Nuclear physics in the early
20th century showed that the nucleus is
constructed from proton and neutrons in roughly
equal numbers the proton and neutron have very
similar properties nearly equal mass, same spin
(½) but 1 and 0 electron charges respectively.
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High energy accelerators in the 1960s showed
that very energetic collisions of protons and
neutrons could produce a bewildering zoo of new
particles p, K, r, w mesons (lighter than
proton and integer spin), L, S, X, W baryons
(spin ½ partners of the proton and neutron), plus
many, many more. Some of the particles (K, L, S
etc.) were strange ! The idea of a few
simple elementary particles seemed to being
lost. In the late 1960s the quark idea was
postulated by Gell-Mann and Zweig only three
quarks, if combined in various combinations, were
needed to form the whole zoo of known particles.
u up quark charge 2/3, spin 1/2, baryon
1/3, strangeness 0 d down quark charge
-1/3, spin 1/2, baryon 1/3, strangeness 0 s
strange quark charge -1/3, spin 1/2, baryon
1/3, strangeness -1
Each quark has anti-quark partner (u,d,s) with
opposite quantum numbers.
Proton (uud) neutron (udd) p meson (ud)
, etc. Could make all then observed particles
with simple u,d,s combinations.
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The lepton family in the late 60s consisted of
the charged electron and muon and neutral ne and
nm . All nuclei are made of quarks atoms
include nuclei and electrons muons and neutrinos
occur in nuclear beta decay, solar reactions,
etc. But theory is untenable unless the number of
quarks and leptons are the same, so it seemed
something was missing. 1974 discovery of a new
meson Y that is very heavy (3 proton masses)
found to be a bound state of a new charm quark Y
(cc), so order was restored! However, in 1976,
a yet heavier meson U was found to be a bound
state of another new quark called bottom U
(bb), AND two new leptons, t and nt, were found,
destroying the symmetry again! We need one
more quark! (name it top t). People guessed
that since b was (3-4) x mass of c, that t
would be a few times the mass of b. But
experiments through 1990 at several accelerators
failed however to find it, at masses up to 20
times that of b.
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The forces of nature
Following Newtons identification of forces as
the cause of motion, people saw many types of
force e.g. gravity, tension in a string,
friction, normal contact forces, air resistance,
viscous drag, electric attraction and repulsion,
magnetic forces from currents, etc. Is there any
simplification? Over the 19th century much
progress was made to understand that most of
these forces were due to the electrical charges
and currents in atoms, explaining friction,
contact forces as well as the simple Coulomb and
Lorentz forces. The big breakthrough was by
Maxwell who showed that electric and magnetic
forces are inextricably intertwined are a
unified Electromagnetic force. Gravity was
then the only really distinct force. When the
nucleus was discovered, a new Strong (nuclear)
force was needed to overcome the electrical
repulsion of protons and keep them bound in the
nucleus. Strong force is very short range (10-13
cm) The beta decays (e.g. n ? p e- ne ) and
reactions in the suns interior required also a
short range (lt 10-16 cm) Weak force. Weak force
is much weaker than EM or Strong.
4 forces?
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Force carriers
The modern view of forces in relativistic quantum
field theory has force carrier particles called
Vector Bosons. Each force has its own
characteristic boson(s) that respond to different
properties of the matter particles (quarks and
leptons). Analogy with two skaters exchanging a
puck, with the momentum transferred causing a
deflection of the skaters (momentum conservation)
A
puck
B
Force skaters (matter particles)
puck (force carrier) EM charged q
and charged leptons photon Gravity
all massive particles
graviton? Strong quarks
gluons Weak quarks
and all leptons W? and Z0 bosons
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Puzzle!
The vector bosons have to have zero mass to work
theoretically. The very short range of the Weak
force means that the W and Z bosons must be very
massive (100 x proton mass). (uncertainty
principle DE Dt h/2p ). The solution to this
was symmetry breaking the basic theory has
massless bosons, but Nature has broken symmetry
with massive W and Z. (Higgs mechanism). This
idea also tells us that EM and Weak forces are in
fact unified into a single Electroweak EW
force at short distance (high energy), but in our
everyday low energy world, EM and Weak appear
different. So now, just 3 fundamental forces!
Strong, EW, Gravity. We suspect that in future
we will find that the Strong and EW are unified
(at even higher energy grand unification,
supersymmetry), and maybe even all the forces
will be found to be unified (String theory).
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The building blocks of matter (Standard Model) 6
Quarks, 6 Leptons These come in three families
of two objects (u d, e and ne etc.)
4 Force carrying vector bosons Where is
the top quark? How do EM and Weak get unified
(find the Higgs boson?) Is there further
unification? If so, why are the unification
energy scales so very different? And why
asymmetry in matter and antimatter? How does
Gravity work? Extra spatial dimensions? Supersymme
try? Strings?
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DØ Experiment at the Fermilab proton antiproton
Collider and the top quark discovery
We need a very powerful microscope to find the
top quark a very high energy accelerator.
(Uncertainty principle p l h/2p again).
Tevatron 2 counter-rotating beams of 1000 GeV
(109 eV) protons and antiprotons in a 4 km
circumference tunnel. p and p collide head on in
two locations where there are large
detectors. Head-on collisions mean that the
center of mass frame energy available for
creation of new particles (Emc2) is much larger
than if we direct beam on stationary target.
Here get 2000 GeV available energy for particle
creation ? many particles created and many are
very energetic (10s of GeV)
CDF
DØ
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The DØ Detector
Size scale is set by the energy of the particles
from the collisions (many GeV each) and the
number of them in a collision (typically 100 or
so particles).
Use a nested set of subdetectors to measure
particle identity and their energy and directions
(aim to fully reconstruct events). a) Inner layer
is set of tracking detectors that show the
trails of ionization left by charged particles.
Particles go through these with little energy
loss. b) Surrounding calorimeter that absorbs
protons, neutrons, pions, electrons, photons and
gives their energies and directions. c) Outer
layer of detectors that show the ionization
trails of the only penetrating particles the
muons, and get added momentum information by
bending them in solid iron magnets.
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(Half) of the tracking detector region
4 separate subdetectors Nearest the collision,
silicon strips give 10m resolution for
distinguishing particles that travel few 100m
distance before decay Outside that, scintillating
fiber tracker in a magnetic field to determine
momentum Preshower detectors that help identify
electrons and photons.
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Silicon strip detector 840,000 strips of 50m
width, 300m thick. Detector has 4 concentric
barrels around the beam and disks perpendicular
and a set of disks perpendicular to beam.
Particles leave ionization on traversing the
detector which is collected on electrodes at the
surfaces and digitized. Excellent spatial
resolution (12m)
barrel stave
disk
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Assembled silicon tracker
Readout chip amplifies and digitizes 128 strips
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Scintilating fiber tracker 8 layers of axial
and stereo fibers (835m diameter) 74,000 in all.
Scintillation light from traversal of particles
transmitted on 12m optical fiber to high gain
photo diodes and digitized for offline
reconstruction of tracks.
Waveguide bundles entering electronics area under
detector.
Inserting the fiber tracker into the calorimeter.
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Preshower detectors made from triangular strips
of scintillator with wavelength shifting fiber in
center. Waveguide to same photodiodes used for
fiber tracker
One octant of the forward preshower detector,
with 144 strips and waveguide fibers
(green). This module made by Stony Brook and BNL
was included in an exhibit of Museum of Modern
Art in summer 2003.
Preshower installed on calorimeter
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Calorimeter 50,000 cells of liquid argon
interspersed with uranium plates. Particles
interact with the material and create a shower of
particles. The ionization from the shower is
proportional to the total energy of the incoming
particle. Read out the ionization on electrodes
in each of the cells and transmit to the offline
reconstruction.
Central calorimeter
End calorimeter
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Central calorimeter with three rings of modules,
before closing cryostat
End calorimeter
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View from end of (end) calorimeter beam tube
through middle. Liquid argon plumbing above.
Iron toroids for muon bending in red, with muon
chambers inside and outside.
Space opened in calorimeter for service
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Scintillator paddles for registering muons that
penetrate the calorimeter
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Detector view showing calorimeter and beam tube
surrounding muon chambers and iron toroids
outside muon chambers. Free standing structure
is the end muon toroid and chambers ready to be
pushed into position.
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Muon detectors after installing all chambers and
scintillators the outside box of the detector.
The whole detector is on rollers and moved from
assembly hall to collision hall after completion.
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Control room for experiment about 6 people on
shift at all times.
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Seeking top quarks (1 possibility of
several) Quarks (in the proton) and antiquarks
(in the antiproton) collide to produce top
and anti-top quark pair (and other particles). qq
? tt t ? W b t ? W- b W ? u d
W- ? e- ne
The quarks do not emerge freely, but create a
spray of many particles moving along the original
quark direction JET
Result is final state of e-, 4 quark jets (2 are
b-quark jets), unseen n, and a large number of
other particles produced together with the top
quark pair. Infer the n by imbalance of
momentum ? beam axis.
b-quark jets can often be identified, either
because the b-quark travels few 100m before decay
(silicon detector!) or because it decays into an
electron or muon.
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• Identification of objects
• Electron characteristic well-confined deposit
  of energy in first section of calorimeter, with a
  track pointing to it.
• Muon particle track in inner tracking, minimal
  energy deposit along the line through the
  calorimeter and a track in the outer muon
  chambers.
• Jet a collection of energy deposits in the
  full depth of the calorimeter, associated with a
  spray of tracks pointing to it.
• b-quark jet a jet with an associated muon or
  electron, or evidence for secondary displaced
  vertex due to the long b-quark lifetime.
• Neutrino unseen, but detected by imbalance of
  momentum perpendicular to the beam, measured in
  the detector (momentum components along the beam
  axis are smeared by the loss of small angle
  particles down the beam pipe.
• (Photon like an electron, but without a
  pointing track)

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Side view of top quark candidate event (full
azimuth rolled into picture). Colored cells are
the energy deposits in calorimeter (color coded).
Red blob is concentrated energy deposit from
electron. See four jets, one tagged with a muon
(green line) near it, marking it as a b-jet.
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End view of the m 4 jet event, with one jet
identified as a b-jet.
m
ET
m
mjet event with b-tag( m )
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• Energy and direction of electron measured using
  the calorimeter and track
• Energy and direction of muon measured from
  curvature of track in magnetic field using inner
  and outer tracking detectors
• Energy and direction of jets taken from the
  calorimeter
• Transverse energy and transverse direction of
  neutrino from the overall imbalance of observed
  momentum perpendicular to the beam
• The produced t decays into 3 final particles (u d
  ) b or (e n) b
• Get mass of top quark from the energy and
  momenta of the particles that it decays into
  e.g.
• Mt v(Eu Ed Eb)2 (pu pd pb)2
• (There are errors on all these measurements, and
  often even the association of the objects is
  wrong)
• Need 18 quantities measured 3 momentum
  components for 6 objects
• Have 3 components for 5 objects (lepton jets)
  and 2 for n in addition, know M(en)MW and
  M(ud)MW. Also know that Mtop Mantitop. So 17
  measured quantities and 3 constraints.
  2-constraint (2C) fit

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• Backgrounds
• Even with the electron (muon) and four jets plus
  missing transverse momentum, there are copious
  backgrounds. The 2 main categories are
• W plus 4 jets produced in a quark-antiquark
  collision. (W decay into en or mn, as in the top
  quark events).
• Quark-antiquark collisions giving five final
  jets one jet is misidentified as an electron
  (or muon) and mismeasurements lead to missing
  transverse energy.

The background events tend to differ from top
antitop The top quark is very massive, so its
decay products are energetic. Use a quantity HT
Sum of transverse energy of all objects. HT
tends to be large for signal and small for
background. The background from misidentified
jets arises from production of two jets and
subsequent splitting into multiple jets. Thus
this background tends to give objects aligned in
azimuth. Use a quantity called aplanarity that
measures the tendency of objects to be
distributed spherically.
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• Select events that have electron or muon, missing
  transverse energy and at least 4 jets (all with
  minimum transverse energy of 20 GeV).
• Select events with large values of HT and
  aplanarity to improve the signal to background
  ratio.
• For each candidate event, make the best
  association possible for the 5 observed decay
  products from top and anti-top.
• Compute the mass of the top (and antitop) with
  the 2C fit. Call this mass the fitted mass
• Using Monte Carlo simulations of top pair signal
  events with the experimental resolutions as in
  data, and selection cuts as used for the data,
  compute the fitted mass distribution expected for
  a set of assumed True top masses.

• Add these signal templates to the measured
  distribution of background events vs. fitted
  mass. For each choice of true top mass, compare
  the sum of signal and background expectation to
  the data.
• Select the best top mass by minimizing the
  chisquare of the fit between data and hypothesis.

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Experiment result for top mass
Yellow shaded histogram is data. Triangle points
are the expected background, with normalization
taken from the fit. Red circle points are the sum
of best fit top mass distribution plus
background. The blue inset is the distribution of
c2 vs. true top mass.
The best fit for the true top quark mass was 199
? 28 GeV in the discovery paper (March
1995). With more data and refined analysis
techniques, the event sample was about doubled,
and the mass measurement was refined to Mtt
179.0 ? 5.2 GeV
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The original top discovery paper yielded a data
set of 8 e4 jet events, 6 m4 jet events, and 3
events with 2 leptons (e or m) (that come from
both W bosons decaying to leptons). The
backgrounds were estimated to contribute 3.79 ?
0.55 events, compared to the 17 observed events.
The observed events corresponded to a 4.6s excess
over background, and thus the discovery of the
top quark.
The number of signal events, and the known
acceptance and efficiency fractions, allowed
calculation of the production cross section for
tt. The result s 5.9 ? 1.7 pb (from the full
data set) agrees well with the expected
theoretical cross section for tt.
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Companion CDF experiment made simultaneous
discovery. Both papers submitted to Physical
Review Letters on March 2, 1995.
Pressing the button to submit Observation of the
top quark electronically
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• What does the discovery mean?
• It completes the list of expected fundamental
  particles fills out the quark sector with the
  sixth quark, the companion of the bottom quark
• The top quark is amazingly massive about the
  mass of a gold atom! Why should a fundamental
  constituent of matter be so heavy? This
  seems to be giving us a clue, since the expected
  energy at which the Electromagnetic and Weak
  forces are unified is around 200 GeV, about the
  energy of the top quark. Does the top play a
  special role in the unification?
• The Higgs boson is the agent of the EM-Weak
  unification in the Standard Model. The
  measurement of the top quark mass, together with
  the W boson mass and properties of the Z boson,
  predicts the mass of the Higgs boson in the
  Standard Model. Thus we have illuminated the
  window of discovery for the Higgs.

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Predicting Higgs mass
Two of the most sensitive parameters to constrain
the Higgs mass are the top quark and W boson
masses. For example, the W mass depends
quadratically on Mt through virtual tb loops and
logarithmically on MH through Higgs loops. So,
measuring W mass (DØ, CDF and LEP experiments)
and top mass gives a prediction of Higgs mass
The top and W mass measurements tell us that the
SM Higgs should be between 115 and 150 GeV
within reach of DØ and CDF perhaps, or surely at
the CERN LHC starting in 2008. The present DØ and
CDF run should reduce the top and W mass errors
by a factor of 2 or more and give much more
powerful constraints on the Standard Model.
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Building, operating the experiment, doing all the
analyses (not only top, but studies of EW
interaction, strong interactions, b-quark,
searches for other new particles) requires many
people working collaboratively.
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The students doing their PhDs are key to the
success of the experiment.
Stony Brook graduate students played a key role.
And even one NYU student who was a SB undergrad
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Conclusion

• The experiments to find the smallest
  constituents of matter are large in
  accelerators, detectors, size of collaborations,
  money, and effort. Doing this research is
  enormously fun and rewarding.
• We now understand a great deal about the way the
  universe is put together at the basic level, and
  that knowledge has told us much about the
  evolution of the universe in the first moments
  after the big bang.
• Our understanding is very clearly incomplete.
  The next round of experiments should reveal
  exciting new phenomena such as supersymmetry,
  extra dimensions, and tell us much about how the
  fundamental forces are unified. There is much
  to do!