EXTRA SPACE NEEDED?

Scientists in the DØ (D-Zero) experiment at Fermilab have been using the
world's highest energy particle accelerator to see if any of the
particles produced in collisions between protons and antiprotons,
meeting head-on at almost the speed of light, can quite literally leave
the known universe. They have been checking some revolutionary new
suggestions from theoretical physicists who propose that some kinds of
matter or forces could exist outside our familiar (three dimensional)
space, travelling into "extra" invisible dimensions.

There is a relatively simple way to search for this. We have known for
hundreds of years that momentum is balanced. This is still true at the
particle level. When you fire a bullet from a gun, the gun recoils.
Similarly, in the high energy proton-antiproton collisions at Fermilab,
if something (like a jet of particles) shoots out to the left, we expect
something else to come out to the right and balance its momentum. If we
see nothing balancing in this way, there are three possibilities:

-   The balancing momentum was carried off by a mundane normal particle,
    but our detection apparatus missed it somehow. For example, it could
    have passed through a crack between two detector pieces or through a
    support structure.

-   The balancing momentum was carried off by a known particle called a
    neutrino, which interacts so weakly with normal matter that it can
    pass right through the detector. Neutrino properties are well-known
    and we can calculate how often they are produced.

-   The balancing momentum was carried off by something new, for example
    something escaping from three dimensional space. This is what we
    want to look for.

In the late 1970's, the first high energy collider was built at CERN in
Switzerland, and achieved great success in discovering the W and Z
particles that are responsible for radioactive decay. A year or two
after that discovery, one of the teams working at CERN started to
believe that they were seeing "monojets"  a name given to the kind of
unbalanced event where something seems to come out one side and nothing
on they other. The excitement was so great that they even announced that
they had seen unambiguous evidence for the production of a new kind of
particle. Unfortunately, this "discovery" was not confirmed by a more
cautious group working independently at the same accelerator, and it
later turned out that they had underestimated the many ways in which
well-known physics processes, both from missed particles and neutrinos,
could give rise to what looked like a discovery. This bad experience
showed how easy is to make a mistake in accounting for monojets, even
with the best detector and the best team working on it.

Fast forward to 1998 and enter Nima Arkani-Hamed, Savas Dimopoulos and
Georgi Dvali. These three theoretical physicists came up with one of the
most exciting new ideas in recent decades. They proposed that the
universe could be a much bigger place than we think, with our familiar
three dimensions being only part of a five or more dimensional universe.
These *extra dimensions* would be invisible to us because the particles
that we are made of (and the world around us too) are trapped in only
three dimensions. This new idea offered an explanation for why gravity
is roughly 100,000,000,000,000,000,000,000,000,000,000,000,000 (a
hundred billion billion billion billions) times weaker than other forces
of Nature, like the electric and nuclear forces. In this new paradigm
the answer is simple -- gravity is just as strong as other forces, but
appears very weak to us because gravity propagates through the entire
space, including extra dimensions, while all other particles, including
ourselves, are stuck to the three-dimensional world.

![](media/image1.jpeg){width="2.85in" height="3.0in"}

Figure 1. Artists view of a monojet event produced in a
proton-antiproton collision. The jet is recoiling against the graviton
that escapes in the extra space. Our entire three-dimensional world
gains an extra momentum in extra space to conserve total momentum.

Now an idea like this is intriguing, but there needs to be some way that
we can "test" it, by seeing if it makes a prediction for what we should
see in experiments.

One of the exciting predictions of this theory of extra dimensions was
that one should see plentiful monojets in proton-antiproton collisions.
The "missing" momentum would be carried off into the extra space by a
quantum particle of gravity, called a graviton.

Both big experiments at Fermilab, CDF and DØ, decided to go back and
look for monojets in the huge amount of data that they had already
recorded in 1992-1995. Given past experience at CERN it was obvious that
this wasn't going to be easy, and that it would be extremely important
to carefully estimate all the ways in which standard physics could fake
a monojet signal. These fakes are called "backgrounds". The DØ team, who
pioneered the search for extra dimensions at a hadron collider, has now
reported results on their search for the monojet signature. (Also, see
our [Plain English
Summary](http://www-d0.fnal.gov/public/pubs/extra_dim/index.html) on a
previous DØ search for extra dimensions in a different signature, which
resulted in stringent limits on their existence.)

The DØ physicists sifted through 60 million proton-antiproton collisions
and picked out about 300 interesting events that looked like monojets.
The next, most important step, was to identify possible backgrounds and
find ways to reduce some of them. As well as the known backgrounds from
neutrinos and from mismeasurements, it turned out that there is a
background not from the Fermilab accelerator at all but from very
energetic cosmic rays (particles from space). These cosmic rays collide
with the Earth's upper atosphere. Very rarely, some of the produts of
these collisions reach the surface of the Earth and leave energy in the
dense material of our detector just at the same time as we expected a
particle collision from the accelerator. If this energy deposition looks
like a jet, it mimics a monojet. Careful analysis of the characteristics
of these background events allowed the DØ team to come up with quality
criteria that reduce these backgrounds significantly. As a result, the
monojet-like sample was reduced to some 30 events, completely consistent
with the predicted backgrounds (see Fig. 2).

This means we didn't see any evidence for extra dimensions of space.
That doesn't mean they aren't there  just that if they do exist, they
are either too small or there are too many of them to see with the data
we have in hand.

![](media/image2.jpeg){width="3.6666666666666665in"
height="3.4166666666666665in"}

Figure 2. \"Missing\" momentum in the monojet candidate events. Data
(points) clearly agree with the predicted background (red bars), and not
with an extra dimensions hypothesis (open bars).

This non-observation of an excess of monojet events is, of course, less
exciting than a discovery. However, it is an important step towards the
possibility of a discovery of extra dimensions at Fermilab. DØ is now
taking data with a modified detector. Over the next five years we will
accumulate more than 100 times the amount of data that was used for this
search, meaning we will be able to look for much smaller effects. Watch
this "space!"

If you have any questions about this subject, please contact Hai Zheng
(<hzheng@und.edu>) or Greg Landsberg (<landsberg@hep.brown.edu>).