I do research on particle physics, which is the science of the smallest of matter.
Below two descriptions can be found - the first one very introductory (no knowledge
of physics required), while the second one is more technical (knowledge of physics
Current Research (introductory description)
Atoms are made of electrons circling around a nucleus consisting of protons and
neutrons, which again are made of quarks. It is the understanding of the proporties
and interactions of quarks and electrons (and related particles - see chart) that
is the goal of particle physics, which is intimately linked to the birth and current
state of the Universe we live in today.
While the length scale of atoms is about 1/1.000.000.000, the sizes relevant in
particle physics is 1/10.000.000.000.000.000.000 or ten billionth the size of
atoms (i.e. if atoms had the size of the Earth, particle physics would take place
at the submillimeter scale).
The study of such tiny objects is done with (large) accelerators and particle
detectors. Physicists accelerate particles to extreme energies, collide them,
and then study the debris flying through the detector.
Why? Well, an analogy is a world, where one suspects that everything is made of
small mechanical devices (for example a classic alarm clock) instead of atoms.
Objects much to small to study in a microscope. A way to test the suspicion and
further investigate the building blocks of clocks would be to collide the clocks
and study what comes out of these collissions.
At a certain force, one of the bells might come off, and one would then see a
bell in the detector. At greater force more inner workings such as springs and
wheels will be seen, and by repeating the process many times, one will slowly
get a picture of what a clock consists of.
This is exactly what particle physicists do!
By repeatedly colliding particles at increasing energies and studying the outcome,
the Laws of Nature governing the building blocks of the Universe have been mapped
However, since we live in a Univers of relativity and quantum mechanics, things
are slightly more complicated. First of all, in addition to looking into the
constituences of matter, new particles can be produced and thus discovered.
An example of such particles are the W and Z bosons, which are the particles
responsible for the Weak force, in the same way as the photon "carries" the
ElectroMagnetic force. However, contrary to the photon the W (and Z) is massive,
which is the very reason for the Weak force to be weak. The W and Z boson and their
masses were predicted by Glashow, Weinberg, and Salam (Nobel Prize 1979), and
discovered at CERNs SPS accelerator (invented by Rubbia and van der Mer) a few
years later (Nobel Prize 1983).
Interestingly, the mass of the W plays a new role in predicting the mass of an
even heavier particle, namely the Higgs particle, which is responsible for the
mass of all other particles. However, the prediction of the Higgs mass requires
very precise input values of the W boson and top quark mass.
At the ATLAS experiment I work on a precise measurement of the W boson mass.
Current Research (technical description)
The mass of the W has already been measured to be
MW = 80.425 ± 0.034 MeV (PDG 2004),
but a more precise result is desired for two reasons:
In addition, the W and Z bosons will be used for all kinds of calibration, alignment,
and cross checks, and will therefore play a central role in the first period of the
- The W mass is (along with the top mass) intimately linked to the mass of the
yet undiscovered Higgs boson, which is the most awaited and most wanted measurement
in particle physics. By measuring the W and top masses very precisely, one can
get an indirect measurement of the Higgs mass. Estimates already exist, which
bounds the Standard Model Higgs boson to weight less than 280 GeV (at 95%
confidence level). However, with the expected precision of the LHC, this
estimate will become much more precise.
- If and when the Higgs is discovered and its mass is measured, the indirect
measurement will serve as a strong check of the Standard Model. In case of
disagreement, this will indicate more things to discover beyond the Higgs,
and once again precisely determined masses will be the key to estimate what
to look for and at which mass scale.
The W mass can be measured in many ways, as both the methods and the observables are
several. The two principle methods are:
The two methods are not as different as they may sound, as both rely on the Z decays
for calibration. However, the ratio method avoids having to produce large amounts of
Monte Carlo, who's preciseness can be brought into question, and thus this will most
likely be the method used.
- Template method. Using Monte Carlo, templates are created with different
input W masses. These templates are then compared to the data sample, and
the mass of the best matching becomes the mass of the W. The Monte Carlo
behind the templates are tuned to the very similar Z data in order to make
sure that it matches the W data well.
- Ratio method. Instead of using Monte Carlo tuned to the Z decays,
one uses the Z data directly. If one disregards one of the two leptons that
the Z decays into, the decay is very similar to that of the W, and one can
measure exactly the same quantities for the Z as for the W. Rescaling the
sensitive quantities of the Z until they match those of the W gives a
measure of the W/Z mass ratio, which in turn (using the very precisely
measured Z mass) can be turned into a measurement of the W mass.
The quantities sensitive to the W mass are generally the following three:
Since only the transverse (and not the longitudinal) missing momentum can be
measured, the longitudinal momentum (and hence pseudo-rapidity) of the W is
unknown. For this reason all three quantities are transverse and therefore
Since the transverse mass is partially derived from the two others, the three are
correlated, which will have to be taken into account when combining results. However,
since they don't have exactly the same systematic errors, some may be diminished in
the combined result.
- Transverse W mass.
- Transverse lepton momentum.
- Transverse neutrino/missing momentum.
A further subdivision in pseudo-rapidity is also likely, as both the efficiency, the
precision, and the systematics vary in this dimension. Given that the first year of
low luminosity running of the LHC will yield 300 million W's and 50 million Z's, it
is clear that the measurement of the W mass will be limited by systematic errors.
The aim of the analysis is an overall error of less than 20 MeV.
Finally, a remark on how seemingly "useless science" has given so many technologies
to our world. Quoting Glashow, here is a list of ten not-so-useless things, which
owe there invention to modern "seemingly useless" science:
- Computers (technical advancement for building the atomic bomb).
- World Wide Web (coordination in experimental particle physics).
- Global Positioning System (atomic clocks for testing relativity).
- Radioactive isotopes (now essential in medicine and leak searching).
- Beam technology in medicine (quantum mechanics and X-rays).
- Medical imiging (invented "for fun" in researchers spare time).
- Superconductivity (commercially available due to accelerators).
- Syncrotron radiation (spin off of accelerators used in material science).
- Kryptology (abstract math).
- Neutron (power production).
Updated: 18th of October 2001.
Mail me: firstname.lastname@example.org