# **[[PHYSICS HIGHLIGHTS]{.underline}]{.mark}**

# []{.mark}

# **[[FROM THE DØ EXPERIMENT]{.underline}]{.mark}**

**1992 - 1999**

![](media/image1.wmf){width="6.6625in" height="4.894444444444445in"}

![](media/image2.png){width="0.27708333333333335in"
height="0.27708333333333335in"} **Fermi National Accelerator Laboratory,
Batavia, Illinois, U.S.A.**

1.  **INTRODUCTION\
    **

The DØ experiment was proposed for the Fermilab antiproton-proton
Tevatron Collider in 1983 and approved in 1984. After 8 years of design,
testing, and construction of its hardware and software components, the
experiment recorded its first antiproton-proton interaction on May 12,
1992. The data-taking period referred to as \"Run 1\" lasted through the
beginning of 1996. Collisions were studied mainly at an energy of 1800
GeV in the center of mass (the world\'s highest energy), with a brief
run taken at 630 GeV. The total luminosity collected during Run 1 was
equivalent to 125 events/pb of cross section. All results summarized
below are based on these data, and on the dedicated and imaginative
efforts of the undergraduate and graduate students, postdoctoral fellows
and senior scientists involved in the program. Currently, the DØ
Collaboration consists of more than 500 scientists and engineers from 60
institutions in 15 countries (see some of them in Fig. 1). Over 110
Ph.D. dissertations have been written so far on various aspects of DØ,
and more are anticipated over the next two years, as the analyses of
data from Run 1 wind down, and the next run, with both an upgraded
detector and improved accelerator, commences.

![](media/image3.jpeg){width="5.783333333333333in"
height="4.600694444444445in"}

Fig. 1: Members of the DØ collaboration gathered near the detector in
early 1996.

Among the highlights from Run 1 described in the following sections are
the discovery of the top quark and measurements of its mass and
production cross section; the precise determination of the mass of the W
boson and the couplings of the electroweak bosons (photon, W and Z);
numerous searches for new physics; measurements of bottom quark
production; and extensive studies of the strong \"color\" force,
quantum-chromodynamics (QCD). We have already published most of our
results from the past six years; to date, over 80 papers have appeared
in refereed journals. In addition, our publications are reprinted in
annual collections that are available from the library at Fermilab. The
published papers, as well as work presented in conferences, can be
accessed from our web pages (see <http://www-d0.fnal.gov/>). In this
summary, we only discuss some of the highlights of the results of Run 1.
We have also prepared \"plain English\" summaries, intended for a more
general audience, that can be found on the web at

<http://www-d0.fnal.gov/public/pubs/d0_physics_summaries.html>.

Much of our research benefited from insights and friendly competition
within our scientific community. In particular, interactions with our
colleagues at CDF (the other major Fermilab Collider experiment), as
well as SLD (at SLAC), the LEP experiments (at CERN in Geneva,
Switzerland), the HERA experiments (in Hamburg, Germany), and theorists
around the world have been both intellectually stimulating and
productive.\
\
This summary of the highlights from Run 1 can only provide a flavor of
some of the most interesting results. To gain a better understanding of
their significance, and for greater detail, we invite the reader to
consult our public web pages, as well as the members of the DØ
collaboration.

2.  **THE DØ DETECTOR**

For many years, our understanding of nature revolved around four
separate, unrelated forces \-- gravity (familiar to us all), the
electromagnetic force (involved in everything from the formation of
molecules to the pointing of the arrow of a compass northward), the weak
force (responsible for radioactivity), and the strong force (which holds
the nuclei of atoms together). Over the past three decades, many
experimental and theoretical advances have led to a coherent and
predictive picture of the strong, electromagnetic and weak forces called
the Standard Model (SM). In the SM, the elementary constituents of
matter, quarks and leptons, interact through forces, which are
transmitted through the exchange of particles called gauge bosons. Each
of these three microscopic forces is described by a gauge theory, in
which the interactions are invariant under changes in the complex phase
of the constituent fields at every point in space-time, thus requiring
the presence of a spin-1 massless gauge boson. Gravity remains outside
the SM framework.\
\
During the 1960s and 70s, it was recognized that the electromagnetic and
weak forces could be described through a unified picture, and the theory
of electroweak interactions was born. A set of four gauge bosons with
zero mass was introduced in the SM, together with two pairs of spin-0
\"Higgs\" particles, to provide the observed breaking of the symmetry in
the underlying electroweak force. As a result of the symmetry breaking,
two of the mediators of the electroweak force, the W and Z bosons,
acquire mass, while the photon remains massless. Three of the Higgs
particles are absorbed in giving the W and Z their masses, while the
last one remains to be discovered; its mass is not predicted, but can be
inferred in the framework of the SM from precision measurements of other
quantities.\
\
The strong force is mediated by a set of eight massless gauge bosons
called gluons, and is described by Quantum Chromodynamics (QCD). Of the
matter particles, only the quarks experience the strong force. In the
SM, the strong and electroweak interactions are specified separately,
but are not unified. There are compelling reasons to believe that the
SM, though remarkably predictive and extremely well tested, is only an
approximate theory to nature. Theories have been postulated that extend
the SM, provide unification of the forces, and give deeper understanding
of the Higgs particles. Seeking evidence for the path beyond the SM is
the major theme of future experimentation.

According to the SM (see Fig. 2), the particles created at the Tevatron
fall into two broad classes: leptons (electron, muon, tau, and neutrinos
associated with each) and hadrons (protons, pions, kaons, etc.), the
latter being composed of combinations of the six quarks. The quarks and
leptons are mirrored by their respective antiparticles. In addition, the
gauge bosons transmit the fundamental forces; these include the photon
(electromagnetic force), the gluons (QCD strong force), and the W and Z
bosons (weak force). Other particles, outside this framework, could
exist and are the subject of many of our searches. Most collisions
produce quarks or gluons, which evolve into collimated sprays of hadrons
called jets. These jets usually do not contain leptons, and many of the
studies of rare processes \-- such as the production of the top quark, W
and Z bosons, or searches for new phenomena \-- that would be swamped by
backgrounds from copious QCD processes with jets, can be realized only
by using decays of the interesting objects into leptons. Neutrinos and
certain newly proposed particles do not interact with matter often
enough to be detected, but can be inferred by an apparent imbalance in
momentum conservation. Because of such considerations, the detector was
optimized to measure jets, leptons, and \"missing\" transverse momentum.

![](media/image4.png){width="2.8805555555555555in"
height="3.4555555555555557in"}

Fig. 2: A table of the elementary particles and force carriers in the
Standard Model.

The physics results from DØ rest on the technical achievements of many
scientists and engineers. The Fermilab accelerator complex, with its
eight distinct major components, provides high intensity proton and
antiproton beams at the world\'s highest energy (900 GeV for each beam).
These beams collide at two locations in the Tevatron ring, where
experiments are performed by the CDF and DØ collaborations. The DØ
experiment contains many sophisticated components, which include not
only the particle detectors, but also the electronics needed to select
and digitize events, and the software necessary to monitor the
experiment and reconstruct events written to magnetic tape. Although a
full description is not appropriate in this note, it is useful to
provide a brief overview of the detector.

![](media/image5.wmf){width="3.759027777777778in"
height="4.1930555555555555in"}

Fig. 3: A schematic view of the DØ detector during Run 1. The tracking
chambers near the beam are shown in purple, gray and pink. The
calorimeters are shown in yellow, blue, and green. The muon chambers are
shown in orange, and surround the iron magnets (in red).

The DØ detector, as it existed in Run 1, is shown in Fig. 3. There were
three major subsystems: a collection of tracking detectors extending
from the beam axis to a radius of 30 inches; energy-measuring
calorimeters surrounding the tracking region; and, on the outside, a
muon detector that deflected muons using solid iron magnets. The entire
detector was about 65 feet long, about 40 feet wide and high, and
weighed 5500 tons. It rested on a moveable platform that permitted
detector assembly and commissioning in accessible areas, prior to
positioning in the collision hall for operation. The umbilical cord of
cables for carrying signals and services followed the detector, and
allowed the sensitive electronics for triggering and digitization to be
housed in outer control rooms. The detector was operated around the
clock by teams of about six physicists and technicians, working from the
control room, and using the hundreds of available displays to monitor
the flow and quality of data. In all, the detector had over 120,000
channels of individual electronic signals. Some of these were used to
take a fast \"snapshot\" of the properties of an event, and to decide
whether it was a candidate for further study. This \"triggering\"
process proceeded in stages: the first level was completed within 4
microseconds, before the next accelerator beam-bunches arrived at DØ. A
second level of trigger decision followed the digitization of all
information in a farm of dedicated microprocessors. Events that survived
this screening process were written to tape and reconstructed in detail
for subsequent analysis.

Figure 4 shows a "typical" event as observed in the DØ detector. The
directions of all charged particles were measured in tracking chambers
surrounding the collision point. These detectors relied upon the
ionization of a gas caused by the passage of charged particles; the
produced ionization was focussed electrically onto sensors that recorded
the amount of charge and its time of arrival, and permitted
reconstruction of the particle trajectory. In addition, the tracking
region contained a stack of hundreds of thin foils, called a transition
radiation detector. Particles traversing this detector emitted x-rays
with intensity that depended upon their velocity. This device was used
to enhance electron identification.

![](media/image6.wmf){width="4.127083333333333in"
height="3.4854166666666666in"}

Fig. 4: A side view of \"Event 417\" referred to in Section 3. The muon
track is shown as a green line, the electron track is shown as a short
red line, and the two main jet energy depositions in the calorimeters
are shown in different colors that represent the energies in the
contributing cells.

The energy of most particles (all but muons and neutrinos) was measured
in the three calorimeters that surrounded the tracking volume. Each was
composed of a stack of heavy metal plates (uranium, steel or copper)
interspersed between gaps containing liquid argon. Particles hitting
upon the calorimeters interacted, yielding secondary particles, which
also interacted, leading to a shower of particles that ultimately ended
when all the secondary particles lost energy and stopped. The passage of
the full set of showering particles through the argon gaps produced
ionization electrons that were collected on localized electrodes. The
observed signal was proportional to the incoming particle energy. The
pattern of energy deposition along the shower was used to distinguish
electrons or photons from hadrons. Clusters of deposited energies were
used to reconstruct the jets associated with quarks and gluons.

Muons penetrated the calorimeters, typically without a substantial
change in their energy or direction. They were detected in the outer
region of the detector using gas-filled tracking chambers, positioned
before and after magnetized blocks of iron. These chambers provided the
muon trajectories before and after the bend in the magnet, and thus
yielded the momentum or energy of the muons.\
\
The computer software for DØ was almost completely custom-written. It
was required for monitoring and control of the experiment, for the
microprocessors in the trigger system, for controlling the data flow to
the ultimate logging to tape, for the reconstruction of particles from
the signals measured in the detector, and for managing the large data
samples (70 million events, 3 Terabytes of data) acquired over the run.
Special attention was paid to graphical displays of events and detector
performance. Many millions of simulated events were created for study of
detector performance and specific physics processes through \"Monte
Carlo\" programs that mimicked the response of the detector.

3.  **PHYSICS OF THE TOP QUARK**

The four lightest quarks (called \"up\", \"down\", \"strange\", and
\"charm\") have been known to us for over 25 years; they come in pairs,
with members of each doublet having internal \"weak isospin\" quantum
numbers of ±1/2. In 1977, the \"bottom\" (or \" b\") quark was
discovered, and found to have weak isospin of −1/2, thus requiring a
partner called the \"top\" quark. Prior to the start of Run 1, the lower
limit on the mass of the top quark had been pushed up to about 90 GeV by
experiments at CERN and early data from CDF. Physicists had already
begun to puzzle over what the large mass difference between the b quark
(at about 5 GeV) and the top quark implied, suggesting the possibility
of a special role for the top quark in the scheme of particle
phenomena.\
\
From the beginning, the search for the top quark was a very high
priority at DØ. The Standard Model was explicit in predicting
top-production and decay characteristics. Specifically, the production
rate for top-antitop pairs could be calculated reliably from on QCD
theory, once the top-quark mass was specified. Similarly, the decays of
a top (or antitop) quark could be predicted because the top was expected
to decay nearly all the time to a W boson and a b quark, giving rise to
a final state with two Ws and two b-quark jets. The decays of W bosons
(either into charged leptons and their neutrinos or into quark-antiquark
pairs) were already well established. Thus the basic classes of final
states arising from top and antitop production were the following: (a)
six quark jets (four from the Ws and two from b quarks); (b) a lepton
and neutrino, accompanied by four quark jets (two from one W and two b
jets); or (c) two leptons and neutrinos and two b quark jets (see the
diagram in Fig. 5). Other final-state particles were expected from the
interactions of the rest of the quarks and gluons in the colliding
proton and antiproton, and also from the radiation of gluons from the
interacting quarks. Neutrinos could be sensed only through the missing
transverse momentum in the detector. Tau leptons are difficult to
identify, and consequently the electron and muon channels turned out to
be the preferred channels for studying leptonic final states.\
\
The experimental challenges differ for the three classes of events: the
six jet class, with no leptons, is the most likely, but suffers from
huge backgrounds due to ordinary strong production of jets; the
two-lepton class has relatively little background but a small rate. The
single lepton class is intermediate in both rate and background. The
measurement of jet energies and directions is crucial to the
determination of the mass of the top quark; this measurement is
complicated by the spatial spreading of particles in the jet, and by the
possibility of gluon radiation. It was generally believed that a
measurement of the mass could not be performed to better than 10%
accuracy, both because of the jet problems and the presence of missing
transverse momentum carried by the invisible neutrinos.

![](media/image7.png){width="4.164583333333334in"
height="3.620138888888889in"}

Fig. 5: A schematic of top-quark pair production, where both Ws decay
leptonically

The first portion of Run 1 (Run 1a) was completed in mid-1993 and
yielded an accumulated collider luminosity corresponding to 14 events
per 1 pb of production cross section (usually referred to as 14 pb^−1^).
From these data, DØ published its first search for the top quark in
early 1994, using the single lepton, electron (e) and muon (μ) channels,
and the ee and eμ channels. The selection criteria were set to optimize
the discovery of a top quark with a mass of about 100 GeV. Three events
were found: one eμ candidate, one ee candidate and one single-electron
candidate, all with accompanying jets. The expected backgrounds were
comparable to the number of observed events. Hence, a lower limit of 131
GeV at the 95% confidence level was set on mass of the top quark, based
upon the SM calculations for the expected yield as a function of mass.
This was the highest mass limit at the time (and, as it turned out, the
last lower limit reported on the mass of the top quark!). There was a
spectacular event (\"Event 417\") in this sample, containing an
electron, a muon, and missing transverse momentum, all above 100 GeV,
together with two well-identified jets and a small third jet. The
probability for background processes to produce this event was extremely
small. Our publication reported an analysis of the mass, based on the
assumption that this event was a top-antitop production, stating that:
\"The likelihood distribution is maximized for a top mass of about 145
GeV, but masses as high as 200 GeV cannot be excluded.\" This event,
shown in Fig. 4, survived subsequent signal-selection criteria that were
even more restrictive and ended up in our final Run 1 top-quark sample.\
\
With this mass limit in place, and in anticipation of much larger data
samples from Run 1b later in 1994, DØ optimized the search for top at
higher masses, and developed powerful techniques for determining its
mass. Several useful variables were developed to aid in separating
signal events from background. One was the \"aplanarity\" variable that
measured the isotropy of energy flow. Top quark pairs are expected to be
produced nearly at rest in the center of mass frame and to spray their
decay products uniformly in all directions, in contrast to the more
back-to-back topology of multi-jet background processes. Another
variable was the scalar sum of the transverse momenta of jets and lepton
in the event. This variable, resembling a measure of event temperature,
distinguished the energetic decay fragments of massive top quarks from
typically lower energy background from jet production. Refined methods
for estimating background rates were established using the observed
rates of background samples, and which decreased exponentially as the
number of jets in the sample increased. Simultaneously, methods were
developed for determining the mass of the top signal. Using data for
background events and Monte Carlo simulation of the top-antitop signal
events with a given assumed top mass, templates were made for the
expected distributions of reconstructed top masses. The template with
which the data agreed best gave the best estimator of true top quark
mass.\
\
In late spring of 1994, the CDF experiment submitted for publication a
publication showing evidence that the top quark may exist, with a mass
near 175 GeV. The CDF excess of events corresponded to a cross section
of more than a factor of two above the expected (and currently accepted)
value. Although suggestive, these data were insufficient to claim
discovery. At the same time, DØ presented its updated results at
conferences. New features of the DØ analyses included the use of
additional variables and channels in which the b quark was tagged
through its decay to a muon (and its accompanying neutrino and other
particles). The techniques were now tuned to optimize the discovery of
top in the mass range above 160 GeV. The sensitivities of both the CDF
and DØ experiments to possible top signal were very similar, but the DØ
sample contained only a modest excess over background estimates (7
events with an expected background of 3.2 events), and the top-antitop
production rate inferred was consistent with that predicted (and now
confirmed) by the Standard Model.

At the beginning of 1995, data samples had increased by a factor of
nearly three. On February 24, 1995, DØ and CDF simultaneously submitted
papers announcing the discovery of the top quark. The DØ sample had 17
events with an expected background of 3.8, and the odds for the
background to fluctuate to the observed sample were only 2 in 1 million.
For this sample, the mass of the top quark was estimated to be between
167 and 231 GeV. The cross section was measured to be 6.3 ± 2.2 pb for a
mass of about 200 GeV. The CDF results were consistent with those from
DØ, favoring a somewhat larger cross section and a lower mass. The
discovery of the top quark completed the roster of SM particles
comprising matter, and underscored the special nature of the top quark
\-- an elementary particle as heavy as a gold atom, and with a mass
commensurate with the energy scale of electroweak symmetry breaking.
These CDF and DØ papers on the discovery of the top quark have now
become the second most cited result in experimental high energy physics
(after the papers on the J/ψ discovery).\
\
By the end of Run 1 in early 1996, DØ had recorded about 125 pb^−1^ of
data. From the full data set, several more improvements were made in
understanding the top quark. Searches for anomalous behavior in top
production were sought, but none found. Searches for new particles in
top decay, such as charged Higgs bosons, came up empty-handed. But
several important advances were made in the measurement of the
top-antitop production cross section and the mass of the top quark. A
comprehensive new study of top production was carried out in the single
and two-lepton classes using carefully optimized selection criteria to
minimize the uncertainty on the cross section. A sophisticated analysis
of the cross section was completed in the six-jet channel, making
extensive use of neural networks that were sensitive to the differences
between signal and background. The backgrounds were determined from
data, without recourse to Monte Carlo simulations. The combination of
all analyses of the top-antitop cross section yielded 5.9 ± 1.6 pb, for
a top mass of 172 GeV, in excellent agreement with the theoretical
prediction from QCD.

![](media/image8.png){width="4.270138888888889in"
height="3.7423611111111112in"}

Fig. 6: The mass reconstructed for the top-candidate events with one
lepton, four jets and missing transverse momentum (yellow histogram).
The triangular symbols represent the expected backgrounds, whereas the
red circles represent the sum of signal and background for the best
fitted value of the top mass. The inset shows the quality of the fit as
a function of top mass, with the best value of 173 GeV being at the
minimum.

The mass analysis was improved in several ways. For the single-lepton
channels, neural networks and a likelihood discriminant were developed
to distinguish signal and background without biasing the mass
distribution. The final data sample is shown in Fig. 6, where the
separate contributions for expected background and total (signal and
background) are compared with the observed mass distribution. From this
channel alone, the mass was found to be 173.3 ± 7.8 GeV.

Powerful new methods were also devised to estimate the mass for the
dilepton samples, where the presence of two neutrinos precluded the
direct calculation of a mass. These new techniques were pioneered in DØ
at the beginning of 1993, following the excitement over the observation
of \"Event 417\". Probabilities for dilepton events to originate from
top production were calculated as a function of the assumed top mass,
and a maximum likelihood fit was then used to extract the best value.
Taken together with the single lepton channels, the final top mass from
DØ analyses is 172.0 ± 7.1 GeV (an uncertainty of about 4%), far
exceeding the initial expectation for precision, and making the top mass
the most precisely known of all quark masses. Combining all mass
measurements from both CDF and DØ, yields a mass of 174.3 ± 5.1 GeV (\<
3% uncertainty) for the top quark.

The discovery of the top quark was a major achievement and the highlight
of the DØ program in Run 1. Its very large mass suggests that it may
well play a special role in the breaking of the electroweak symmetry,
and could be partially responsible for the mechanism by which all
particles acquire mass. It provides a probe for seeking new forces in
which top and antitop quarks combine (annihilate) to make new particles,
and a vehicle for the search for new massive particles in its decays.
These are the themes that will dominate top-quark studies in the
forthcoming Run 2, where at least forty times more top events are
expected in a substantially improved detector with greater capability
for deciphering these complex signals.

4.  **ELECTROWEAK PHYSICS**

One consequence of the unification of the electromagnetic and weak
forces was the prediction of the existence of two new particles: the W
and Z gauge bosons. After several years of search by experiments around
the world, two collaborations at CERN, using the world\'s most powerful
accelerator at the time, announced in 1983 the first direct observation
of these elusive particles. With a total of ten W bosons and four Z
bosons, the experiments measured the masses of the particles to be \~ 80
GeV and \~ 90 GeV respectively, with an uncertainty of 5--10 %. While
the number of events was relatively small, the importance of this
observation was immense because the W and Z bosons were essential
ingredients in the SM.

One of the primary goals of DØ was to measure accurately many of the
properties that characterize these fundamental particles. The high
energy and the intensity of the proton and antiproton beams at Fermilab
make the Tevatron an ideal place to produce large samples of W and Z
events. During Run 1, DØ and CDF collected the world\'s largest sample
of W bosons, with DØ accumulating over 100,000 W particles, a far cry
from the handful observed in their discovery. With such a large sample,
DØ has made some of the best measurements of the properties of the W
boson, including its mass and couplings to other particles, as we
briefly describe below.\
\
W bosons are produced at the Tevatron mainly when a quark from a proton
and an antiquark from an antiproton collide head-on at the DØ detector.
Almost immediately after being produced, the W decays into other
particles within about 10^−24^ seconds. Roughly 10% of the time a W
decays into an electron and a neutrino, and it is this decay mode that
DØ uses to measure the W mass. While only one W boson with this decay
signature is produced for about every forty million collisions,
processes that mimic this decay are about 50 times less likely. Thus,
although it took three years to accumulate the W events, the sample is
nearly pure.

To extract the mass of the W boson, DØ first measures the momenta of its
decay particles. The energy of the electron is measured in the
liquid-argon calorimeter. Since neutrinos rarely interact with matter,
their momenta must be measured indirectly by invoking momentum
conservation. The sum of the momenta of all the particles produced in
the collision (in the plane transverse to the proton and antiproton beam
directions) must be balanced by the transverse momentum of the neutrino.
A quantity called the transverse mass of the W boson is then calculated
by combining the transverse momenta of the electron and neutrino, and
the mass of the W is extracted from the shape of this transverse mass
distribution. The DØ value for the W mass is 80.482 ± 0.091 GeV, the
world\'s most accurate measurement of this important parameter published
to date from any single experiment.

The experimental uncertainty of 0.091 GeV, or 0.11%, represents an
improvement of about a factor of 100 compared to the original set of
measurements, and required an extremely detailed understanding of the
experimental apparatus. For example, the mean calorimeter response to
the electron had to be known to better than one part in a thousand, and
energy depositions as small as 100 MeV had to be taken into account in
collisions with up to 1.8 TeV (1TeV=10^6^MeV) of total available energy.
To put this in perspective, it is as if you had to know whether you had
several grains of sand under each of your fingernails when you weighed
yourself on the bathroom scale. Luckily, Z bosons are produced in nearly
the same way as Ws, and their decay particles can be used to calibrate
the detector. With 10,000 Zs available, DØ was able to understand the
apparatus to the required level of accuracy.

![](media/image9.wmf){width="3.8180555555555555in"
height="3.152083333333333in"}

Fig. 7: Measured values of the top and W mass at DØ are shown
superimposed upon predictions from the Standard Model in which the Higgs
mass is varied between 100 and 1000 GeV.

The precision determination of the W mass, together with the mass of the
top quark discussed above, can be combined to estimate the mass of the
Higgs particle. The W mass receives contributions from its virtual
disassociations to top and antibottom quarks or to W boson and Higgs.
The properties of the Z boson, accurately measured at the CERN and SLAC
e^+^e^-^ colliders, also provide sensitivity to the mass of the Higgs.
The full set of these measurements thus constrains the Higgs mass in the
context of the Standard Model. Figure 7 shows the result of the DØ
measurements. The indirect measurements using the Z, obtained mainly at
LEP and SLC, and the directly measured top quark and W masses from DØ
and CDF agree well, and suggest that the Higgs boson has a mass below
200 GeV -- perhaps within reach of the next run at the Tevatron.

In addition to measuring the W mass, DØ used its large sample of Ws and
Zs to probe the strength of the couplings between these gauge bosons and
the photon. The unified theory of electroweak interactions makes unique
predictions for these couplings, which are quite different from
predictions one would derive from separate electromagnetic and weak
theories. By studying events containing both a W boson and a photon, DØ
was able to show directly for the first time that the unified theory was
indeed needed to describe the results. In addition, analyzing events
produced with a W boson and two jets allowed DØ to demonstrate directly
that Ws and Zs interact with each other as predicted by the Standard
Model. Such tests of the couplings between the bosons probe the very
heart of the electroweak theory, and any deviations from the predictions
would provide direct evidence of new physics. With some of the most
sensitive measurements to date, DØ has been a world leader in studying
these couplings, but so far has found no sign of anything new beyond the
SM.

While Run 1 was quite successful, the future for DØ is even brighter.
When improvements to the Fermilab Tevatron and the DØ apparatus are
completed, DØ will begin to take data and expects to collect over 2.5
million W events. The uncertainty on the W mass will be reduced by at
least a factor of two, and significant improvements will be made in the
measurements of the gauge boson couplings. Along with many other
interesting W and Z physics topics, the DØ experiment should be able to
confront the electroweak sector of the Standard Model with unprecedented
sensitivity and with the hope and possibility of discovering something
new.

5.  **QCD PHYSICS**

Quantum Chromodynamics (QCD) is the part of the Standard Model that
describes the strong interaction responsible for the nuclear force. The
quarks that make up the proton and all hadrons interact with gluon force
carriers by virtue of their \"color\" quantum number. Though the proton
can be viewed simplistically as a collection of three quarks, when
examined closely, it reveals substantially more complex internal
structure. The additional quarks and gluons appear with increasing
magnification, or at larger momentum transfers, commensurate with
smaller distances, and are described by phenomenological functions
called parton distribution functions (PDFs). These PDFs are derived from
data, and therefore have uncertainties that have to be taken into
account in any QCD-based prediction. Moreover, the basic coupling
strength between quarks and gluons, α~s~, decreases as the momentum
transfer in a process increases. Hence, perturbative calculations of
strong-interaction processes become more precise at large values of the
square of four-momentum transfer (q^2^), whereas at low q^2^ such
calculations are extremely difficult, and often not reliable.

Because of the excellent coverage for jets provided by the calorimeter,
DØ has made detailed and accurate measurements of strong interaction
processes that test the predictions of QCD in many domains. We have
already remarked on the great success of QCD in predicting the
production of top quarks, and we focus here on only a few processes that
pertain to the production of jets (quarks and gluons) and W and Z
bosons.

The elastic scattering of quarks (or gluons) within the colliding proton
and antiproton resembles classic Rutherford scattering of alpha
particles by gold nuclei. Both processes are well described by the
exchange of a spin 1 quantum (a photon or a gluon) for the case when the
interacting objects display no substructure. The inclusive production of
jets at very large transverse energy (E~T~) can be calculated with
confidence in QCD, given knowledge of the PDFs. Using Run 1 data, DØ has
published the inclusive jet E~T~ spectrum in the range 60 \< E~T~ \< 560
GeV. In this measurement, jets were detected in the central region of
the detector. Figure 8 shows the observed cross section, which drops by
six orders of magnitude over the measured range. Taking account of the
statistical and systematic uncertainties, DØ finds that the QCD
prediction, including its higher order corrections (and using standard
PDFs), agrees well with the data. This result attracted considerable
attention because CDF had published an inclusive jet cross section,
which showed possible excess above theoretical predictions at the
high-E~T~ end of the spectrum. If such excess were confirmed, it could
be interpreted as providing evidence for quark compositeness or the
presence of other new physics beyond the Standard Model. The DØ result
showed that Standard Model calculations do not need to be augmented with
new physics beyond expectations from QCD.

![](media/image10.wmf){width="4.16875in" height="3.375in"}

Fig. 8: The measured DØ inclusive jet cross section compared with QCD
calculations.

The inclusive jet cross section was also measured during a special
Tevatron run at lower center of mass energy of 630 GeV (where the
earlier CERN measurements had been made). Taking the ratio of the
inclusive jet cross sections at 630 and 1800 GeV cancels many
experimental and theoretical uncertainties. DØ measured this ratio to be
about 20% lower than expected. However, better agreement can be obtained
if the energy scales for the perturbative calculations are defined
differently at the two center of mass energies.

Using the two highest-E~T~ jets among those reconstructed in any event;
DØ calculated an invariant mass to search for possible new particles
that might decay into two jets. Such a state would appear as a bump
above the smooth background of ordinary QCD production. The slope of the
falling dijet mass distribution is also sensitive to possible
substructure of quarks and gluons. DØ has published the dijet mass
spectrum for the range of 200 to 1,400 GeV, and found no structures. A
quantitative limit on quark compositeness was determined from the shape
of this distribution. A possible substructure can be characterized by a
mass-scale parameter Λ, corresponding to bound states of any subunits
within quarks. For Λ \< 2.4 TeV, the slope of the predicted mass
spectrum would be inconsistent with DØ\'s measured result. This limit on
Λ indicates that there is no substructure within quarks or gluons down
to the attometer scale (10^-18^ m), and is the most stringent limit on
quark substructure determined by experiments to date.

W and Z bosons are created primarily through the annihilation of valence
quarks and antiquarks, and so a comparison of measured W and Z
production cross sections with theoretical predictions provides test of
QCD that is complementary to jet production. Using W and Z decays in
both electron and muon channels; DØ has measured the ratio of the W/Z
cross section multiplied by their respective branching fractions to
leptons. The resulting ratio of 10.49 ± 0.25 is in excellent agreement
with the QCD calculation to order α~s~^2^ of R = 10.73 ± 0.11, where the
theoretical uncertainty stems from choice of input PDF and variations
due to the uncertainty in M~W~ and energy reference-scale factors used
in the theory. The measurement of R was also used to extract the total
decay width of the W (Γ~W~ = 2.152 ± 0.066 GeV), and to determine that
no more than 8% of W decays could proceed into unexpected final states.
DØ has also measured the transverse momentum spectra for the production
of the W and Z bosons. The comparison of these distributions is the most
sensitive to non-perturbative effects from multiple gluon radiation
present in low-q^2^ QCD.

In data that contain at least two high-E~T~ jets, DØ has observed that a
small fraction of events have the striking feature of sizeable gaps in
energy deposition between the two jets, or between jets and the beam
direction. The gaps are characterized by the absence of particles in
extended regions of polar angle in the tracking detectors, calorimeters
or forward trigger counters. Such events are termed \"rapidity-gap\"
events (the rapidity variable is related to the polar angle), and fall
into three topological categories: jet-gap-jet, gap-jet-jet, and
gap-jets-gap, depending on the location of the gaps in the detector.
Events in the first two categories (jet-gap-jet and gap-jet-jet) were
observed about 1% of the time of events with similar jet topologies.
Events of the third category (gap-jets-gap) were observed about 1% of
the time of events in category 2 (gap-jet-jet). Dijet events of all
three topologies have been observed at both 1800 GeV and 630 GeV.
Similar topologies have also been reported at the e-p collider
experiments at HERA in Hamburg Germany.

Explanations for the gap events are based on the supposition of the
existence of a color-free object called the Pomeron. The Pomeron has
long been postulated as the exchanged object and force carrier
responsible for elastic and diffractive scattering of two hadrons. The
colorless property of the Pomeron is used to explain the presence of
rapidity gaps. Ordinary hadrons are produced due the color carried by
their constituents, hence their emission from the color-free Pomeron is
suppressed. The jets produced in these events have E~T~ distributions
similar to those in standard QCD (quark and gluon exchange) processes.
This leads to the view that the Pomeron may have an internal structure,
consisting at least partly of normal quarks and gluons arranged in such
a way as to make the Pomeron colorless. DØ\'s study of rapidity-gap
events will be enhanced during Run 2, when a set of detectors very close
to the beams will enable the experiment to intercept diffractively
scattered beam particles on either side of the interaction point. These
detectors will provide the full kinematic reconstruction of certain
gap-jet-jet and gap-jets-gap topologies, shedding more light on the
Pomeron\'s structure and dynamics.

6.  **PHYSICS OF THE BOTTOM QUARK**

Within the family of known quarks, the bottom (or b) quark is
characterized by a set of rather peculiar and often intriguing
properties, sufficiently so as to warrant dedicated facilities for its
study. Discovered in an experiment at Fermilab in 1977, its unexpected
appearance created an imbalance in the internal organization of the
existing quarks. The absence of a \"weak isospin\" partner represented a
theoretical discomfort that was only dispelled with the later discovery
of its missing companion, the top quark (see Section 3).\
\
When confronted with its earlier known siblings, the bottom quark is
considered heavy, with a mass about four times that of its next heaviest
colleague, the charm quark. Such relatively high mass grants the bottom
quark special status in the studies of QCD. Bottom quarks are produced
in proton-antiproton collisions dominantly by the strong QCD
interactions of gluons and light quarks that reside within the colliding
beam particles.\
\
The large value of α~s~ and the non-abelian nature of QCD are
responsible for the difficulty of making quantitative predictions.
However, the higher the mass of the involved quark, the more reliable
are the calculations. The mass of the bottom quark is high enough for
obtaining reliable QCD calculations, but still low enough to have
copious production at the Tevatron. This balance is one of the aspects
that single out bottom quarks as an excellent source of data for
confrontation with theory, a true \"laboratory\" for QCD studies.
Consequently, one of the ways we test the reliability of QCD in DØ is by
measuring the rate at which bottom quarks are produced. An added bonus
of heavy quark production is that the dependence of the production rates
has a direct correlation to the internal gluon distributions within the
colliding protons, which are not well measured, and can be extracted
from such data.

DØ has measured the production of bottom quarks in various kinematic
regimes, and through the observation of different reactions and final
configurations. DØ is especially well equipped for such studies, partly
because of its extensive angular coverage. Once produced, free colored
quarks do not exist for very long, but immediately initiate a process of
pulling light quarks from the vacuum and \"dressing\" themselves into
colorless bound-state hadrons. Bottom quark hadronization usually leads
to the production of an unstable B hadron that subsequently decays.
Muons are produced in such decays about 11% of the time, and can be used
to tag b quarks. DØ has a good muon detector, and the extended muon
coverage near the incident beams, the so-called forward rapidity region,
is unique to DØ, and has provided measurements of bottom-quark
production in new kinematic regions.

The process starts with a selection of collisions that contain one or
more muons, a promising signature of something interesting having
happened in that event. Weeding out background leaves a sample that can
be classified according to the number of muons present in the final
state, and how they relate to each other (if two are present) and to the
remainder of the collision products. For example, a muon moderately
close to the hadrons comprising a b jet provides a signature for a b
quark.

Such studies have yielded a wealth of valuable measurements. Resonant
and non-resonant final states, in different physical configurations and
kinematic regions, have been traced back to their origins in
bottom-quark production, enabling a multifaceted focus on production
rates, correlations, and confrontations with predictions of QCD.

The results of such measurements are intriguing. While the general
aspects of the QCD predictions are in agreement with DØ observations,
the calculated production rates systematically fall short of the
observed yields by roughly a factor of three. The data from several
related studies are shown in Fig. 9, and indicate the level of agreement
between theory and experiment as a function of transverse momentum.
Similar results have been obtained by CDF. Although there are
uncertainties in theory and experiment, the present status represents an
exciting challenge that is currently being addressed by theorists, and
motivates the program of increasingly accurate measurements for the next
Tevatron run.

![](media/image11.wmf){width="3.5625in" height="3.5069444444444446in"}

Fig. 9: The DØ inclusive b-quark cross section compared to theoretical
calculations.

We noted that the bottom quark is a heavy object when compared with its
earlier known siblings; in striking contrast, when confronted with its
companion top it is in fact remarkably light. This delicate placement in
the mass scale, together with the tendency of quarks to interact mainly
with their weak isospin partners, conspire to give the bottom quark yet
another set of very welcome properties. The b quark has an unusually
long lifetime (hadrons containing b quark travel typically a few
millimeters before decay), and clear signatures associated with its
decay products. Once an experiment is equipped to observe and analyze
specific bottom-quark decay modes, another entirely new and rich chapter
of physics is opened, which includes such fundamental topics as CP
violation, and windows of exploration into particle physics phenomena
beyond the scope of the Standard Model.

The installation of a superconducting solenoid and precision tracking
sensors in its interior, are two important features of the upgraded DØ
detector for the next Tevatron run. They will give us access to specific
bottom quark decay modes and an opportunity to focus on some of these
new topics.

7.  **SEARCH FOR PHYSICS BEYOND THE STANDARD MODEL**

It is an amazing feature of the Standard Model that, despite its
extraordinary predictive power, it is almost surely incomplete. There
are 26 parameters needed to specify the SM, and these can only be
supplied by experiment. The strong and electroweak interactions that
jointly make up the SM are seemingly unrelated entities; we would prefer
to see a unification of these forces but the SM does not do this. The
mechanism that breaks the underlying symmetry of the electroweak
interaction, and thereby provides disparate masses to W/Z bosons and the
photon, is not understood; in the SM the Higgs boson is inserted to
provide the symmetry breaking, but its mass is expected to be 10^14^
times larger than that of the W and Z bosons unless some fantastic
\"fine tuning\" is at work. Beyond these defects, the SM offers no clue
as to why there are three generations of quark and lepton families with
nearly identical properties apart from their mass. It can accommodate,
but not explain the existence of CP violation, or why the cosmological
constant that should be of order 10^100^ GeV is close to zero, or how to
get gravity into a unified framework with the other forces.

Twenty years of precision tests of this model have resulted in an
enormous number of successful comparisons of data and theory, with no
verified departure from the SM. Despite this impressive predictive
power, we firmly believe that the SM is nothing more than a low-energy
approximation to a more general theory, the one that explains our world
in its completeness and puzzling beauty. This is a very interesting
situation, comparable to instances in the past that foreshadowed a major
shift of paradigm. Are we completely blind in our search for this more
complete theory? The answer is \"probably not\". We have several
hypotheses that we consider as strong candidates for extensions beyond
the SM. At the same time, it is imperative that we look for any possible
deviations from predictions of the SM, and the DØ experiment has been a
pioneer in such studies.

One set of possible extensions of the SM, usually associated with a
postulated new super-strong force involving new massive families similar
to the quarks, require the presence of particles called leptoquarks. The
leptoquarks would have the properties of both leptons and quarks, and
thus would let quarks and leptons interact in a non-SM way. In 1997, the
possibility of existence of leptoquarks got a boost from experiments at
HERA. By colliding positrons and protons, the HERA experiments could
produce single leptoquarks. In February 1997, the experiments H1 and
ZEUS announced an excess of events over SM expectations at large q^2^,
with an invariant mass around 200 GeV, which could be interpreted as due
to leptoquark production. The evidence was not compelling, but the
possible sighting could have had revolutionary implication, and it
therefore set the Tevatron experiments in motion to add information.

At DØ and CDF, leptoquarks can be produced in pairs via the strong
interaction. This mechanism is well understood and is relatively model
independent. The high energy of the Tevatron offers the possibility of
searching for leptoquarks to masses higher than accessible at HERA. DØ
physicists immediately teamed up for the search. It took three months of
analysis to unambiguously establish that the excess that HERA saw was
not due to leptoquarks. DØ used advanced data-analysis techniques, such
as neural networks and other methods of multivariate analysis,
introduced earlier in top-quark studies at DØ. These novel techniques
allowed DØ to establish the world's best limits on the existence of
leptoquarks that could decay into electrons and quarks. The lower limit
on mass of the leptoquark from the DØ experiment alone was 225 GeV, more
than enough to rule out the possibility for the HERA event-excess of
being interpreted as evidence for leptoquark production. Combined with
the 213 GeV limit obtained by CDF, the two Tevatron experiments were
able to rule out the existence of these particles with masses below 242
GeV. More general DØ limits on the mass of the first generation
leptoquarks (M~LQ~), as a function of the probability that these
particles decay into electron and quark (β), are shown on the left side
of Fig. 10.

Supersymmetry (SUSY) has been suggested as a possible cure for many of
the shortcomings of the Standard Model. Space-time symmetries such as
those of translation or rotations of coordinates lead to momentum and
energy conservation. Supersymmetry postulates a further symmetry between
bosons (integer-spin particles) and fermions (half-integer-spin
particles), thereby generalizing the Poincare group describing space and
time. This radical reshaping of our understanding of space-time is also
a key ingredient in the theory of strings in multiple dimensions. When
used as a phenomenological ingredient of physics at the scale of
present-day experiments, it provides a natural solution to the
shortcomings of the SM involving the instability of the mass of the
Higgs boson, and permits the unification of the strong and electroweak
forces. Supersymmetry predicts that each known fermion and boson should
have a mirror \"superpartner\" of the opposite type. Clearly,
supersymmetry is broken, since there is no spin-zero superpartner for
the electron at 0.511 MeV. But to be self-consistent, supersymmetry
predicts that the superpartners should be found with masses below 1000
GeV, and some could be within reach of discovery at the Tevatron.

The DØ experiment has searched for traces of supersymmetry in a variety
of processes. So far, these searches have not been successful, and have
resulted only in limits on the existence of superpartners. Depending on
the model parameters, squarks and gluinos (the superpartners of quarks
and gluons, respectively) with masses less than about 260 GeV have been
excluded. The right side of Fig. 10 shows the region of supersymmetry
parameter space over which the DØ results have ruled out squarks and
gluinos. The parameters M~0~ and M~1/2~ refer to the unified masses of
the spin zero and spin one-half superpartners at the scale of
unification of forces. Limits were also set on masses of charginos and
neutralinos, the superpartners of the W, Z, and Higgs bosons. Despite
these negative results, hopes are high as capabilities for discovering
supersymmetry improve dramatically in the next Tevatron collider run.
The mass reach will be about 100 GeV higher than present limits on
superpartner masses, bringing DØ into a very interesting range of SUSY
parameter space.

Among other fundamental symmetries probed by the DØ experiment is the
\"broken\" symmetry between the electric and magnetic charges. We know
that free carriers of electric charge exist, but there is no trace of a
free magnetic charge, or magnetic monopole. If monopoles exist, one
would expect pairs of high-energy photons to be produced at the Tevatron
at a much higher rate than predicted by the Standard Model. This
indirect search, though unsuccessful, yielded the most restrictive limit
on the mass of a possible magnetic monopole.

Recently, a novel idea was introduced for physics beyond the SM. It
originates from string theory that views all known particles as
vibrations of tiny "strings" of energy. The recent success of string
theory in explaining entropy flow in black holes has drawn much
attention. String theory, or its subsequent elaboration as membrane or
M-theory, seeks to explain all physical phenomena using structures in a
universe with 10 or 11 spatial dimensions and time. The extra (beyond
the usual four) dimensions are believed to be \"curled up\" at a scale
of at most 10^--19^ cm. However, recent suggestions predict that some of
these extra dimensions may be confined to a much larger scale, perhaps
of the order of one millimeter. If this is correct, then the highest
energy scale we know of, the so-called Planck scale might be much lower
than initially realized (∼ 1 TeV, and not 10^16^ TeV). DØ is currently
looking for possible manifestations of this predicted signal in several
channels.

![](media/image12.wmf){width="3.3916666666666666in"
height="3.3916666666666666in"}
![](media/image13.wmf){width="2.948611111111111in"
height="3.397222222222222in"}

Fig. 10: Highlights of searches for new physics at DØ: limits on the
mass of first-generation leptoquarks (left), and limits on squarks and
gluinos in SUSY models (right).

The searches discussed above are just highlights of the many that DØ has
performed in its very successful first run. DØ has also looked for
leptoquarks of other generations, additional quarks and vector bosons,
quark-lepton compositeness, technicolor, non-standard Higgs bosons, and
more. We are closely following recent developments in theory, and
several searches for the manifestations of new theoretical concepts are
still ongoing. Although no new physics has as yet been observed, DØ will
continue hunting for the unknown.

8.  **CONCLUSIONS**

The studies by DØ, together with those by our companion experiment CDF
at the Tevatron, and the experiments at LEP, SLC, HERA, and other
accelerators, have taught us much about the character of particles and
forces at smallest-distance scales. These results have given a
qualitatively new understanding of the properties of matter, and have
thus far demonstrated the surprising resilience of the Standard Model of
particle physics. But the puzzles that this research has created make us
eagerly anticipate the next round of experiments. There is an almost
agreed expectation that the experiments of the coming several years will
make breakthrough discoveries. There are pointed questions that have
arisen from the past work that cry out for answers. Why is the top quark
so heavy in comparison with its partners? Where is the Higgs boson, or
whatever else nature has chosen to be the agent of electroweak symmetry
breaking? Can we find evidence for supersymmetry and thus pave the way
to unification of all the microscopic forces? Or, are the solutions to
the questions before us to be found in some hitherto unexpected quarter?
From the vantage point of the understanding obtained from the past run
at DØ, we look forward with eager anticipation to the enhanced
possibilities of the next run.

We note with pride the efforts of the many in the DØ collaboration whose
ingenuity and hard work have made the results presented in this overview
possible. We appreciate also the many contributions to our understanding
that have come from our experimental and theoretical colleagues
worldwide. And we are most grateful to our governments for the support
that has made this research possible. The new results have brought not
only new understanding of the structure of matter, but have also
benefited society through the novel techniques that have been developed,
and that over the course of time will enrich society in ways that are
presently unforeseen.