A New Top Mass Measurement and Why You Should Care About It

The CDF collaboration has recently released the results of a new measurement of the top quark mass, based on a larger dataset than those previously analyzed, featuring an improved measurement technique, and benefitting from a more precise calibration of the jet energy measurement.

It would be too ambitious to explain the details of the measurement, and where do improvements weigh in. Suffices to say that this determination is more precise than any previous one, and has a better precision than the previous world average on the quantity under study.

The top quark mass is a very important parameter in the Standard Model of particle physics. Why is it so large ? Does its heft mean anything for the theory, or is it an accident ? Can we infer anything on the model itself and on its extensions, by knowing the value of the top quark ? These are meaningful questions, and physicists have been working around them ever since the failure to discover the top quark at the UA1 and UA2 experiments in the eighties, when it became increasingly manifest that the top quark, if it existed, had to be extremely massive.

The former world average of the top quark mass was 178.0+-4.3 GeV. The new measurement by CDF is 173.5 +- 4.1 GeV. Why is this so relevant ? Because the mass of the Higgs boson, as of yet unknown, is tied to the W boson and top quark masses by a feature of the Standard Model called radiative corrections. Basically, virtual Higgs bosons are continuously emitted and reabsorbed by top quarks and W bosons, contributing to the observed mass of these particles. But if the top quark is found to be more massive, the Higgs also has to be, while the opposite holds for W bosons.

The situation is illustrated in the picture on the right. On the x axis is the top quark mass, on the y axis the W boson mass. The bands show allowed regions of this plane, each point being a particular measurement of top and W masses along with a definite value of the as-of-yet unknown Higgs boson mass. There are two bands, the green one showing the theoretically allowed region for an extension of the Standard Model called "minimum supersymmetric Standard Model", the red one the Standard Model proper. Direct measurement of top and W masses are shown by the larger ellipses. What the ellipses say is that there is a 68% chance of the two true values of top and W masses to lie within the boundaries.

There are different ellipses, corresponding to present and predicted precisions in the measurement of top quark and W boson masses. The blue ellipse, computed with the newest measurement by CDF, speaks in favor of the Supersymmetric extension of the Standard Model, and also advocates a low value for the Higgs boson mass (which increases as the W mass decreases).

So there is room to be excited - if you are a particle physics freak, that is. For the following is true:

1. the new measurement is stronger indication for supersymmetry;
2. the new measurement indicates that the Higgs boson is light, thereby promising larger chances for discovery by the Tevatron experiments (in fact, the larger the Higgs mass, the harder it is to produce and discover it).
3. the new measurement proves that the Tevatron can indeed reach a precision of 1% on the top quark mass in the coming future - something we promised a while ago but stayed uncertain for long time.

Comments

[Alejandro]

Is this an anniversary celebration? The previous D0 measure, the one driving t up to 178 +-4 GeV, was published 10 June 2004. To be precise,
Nature 429, 638 - 642 (10 June 2004); doi:10.1038/nature02589

[Tommaso Dorigo]

Yup... Well, the CDF measurement was released 1.5 months ago. I decided to describe this since I have this nice new plot and have just given a seminar on related topics last week.

BTW the plot is courtesy Beate Heinemann.

[socratus]

100 years of Einstein's SRT.
100 years have passed from the date of creation of SRT.
Millions of articles, reviews and books have been written and the
United Nations has decided to establish 2005 as the centennial year of SRT.
Considering all that is clear in this theory, one must still
continue to be surprised by its unusual aspects
Lets review it again:
1. SRT is based on two postulates:
a) According to classical mechanics, physical processes,
which occur in rest or in a rectilinearly driven reference system
are described under the same laws.
b) The rectilinear - uniform propagation of a quantum of light (c=1) in vacuo
has a constant magnitude and does not depend on the source of radiation.
These two postulates would be proven if in the final analysis, they corresponded
with Galilean transformations. However, the result appears negative.
Galilean transformations do not unite these two theories. Why?
2. The rectilinear - uniform motion of a quantum of light (c=1) is connected with
Maxwell's classical electrodynamics. SRT has grown from Maxwell's electrodynamics
and main component in it is the electron.
What describes the electron in Maxwell's electrodynamics?
It is natural, that this electron should be in motion, but it does not move rectilinearly.
It rotates around its own diameter ( spin of Goudsmit-Uhlenbeck)
and such a rotation creates electrical waves. In such rotation all geometrical
and physical parameters of the electron are changed.
It is for this reason Einstein utilized the Lorentz transformations.
And all that is sensible in SRT is that it examines two completely
different types of movement: rectilinear (quantum of light c=1)
and rotational (Maxwell's electron).
It examines the transformation of the electron into a quantum of light (photon)
or quantum of light into an electron .
3. According to classical electrodynamics, an electron in rectilinear motion
does not create electrical waves. Why?
Because the electron travels as a quantum of light (c=1).
In such movement its geometrical form is a circle.
In such movement its area of contact with the vacuum is minimal
and it is not capable of changing the uniformity of the vacuum.
4. When the electron rotates around its own diameter, its speed is more
than the rectilinear motion of a quantum of light. Its speed is c > 1.
For this reason physicists ascribe a huge frequency
to the electron which is the reason its energy E =ħ is higher.
* * *
If you have time and desire, I ask you to visit my site
http://www.socratus.com
Best regards.
Socratus.

[manuel visaya]

nice grub guitar playing is called for here? this was posted here earlier too. (a virtual sighting)

[Gordon T Watts]

You guys did a good job on the top mass! When I first saw the LEPEWWG's plots I couldn't help but wonder if anyone was running, gleefully, around the halls saying "Gee, I wonder if LEP missed the Higgs." Obviously, it is very unlikely as the LEP folks are very good, but I couldn't help but smile at the thought.

[Frank Morgan]

since my theory says all measurable energy is electron energy, and that all directly detectible particles are multiples of the helical string electron whose constant mass is equal to Planck's constant, h, in grams, I think the quark is dimensionally-inferred single degree of vibratory freedom of the electron wave motion as it proceeds as propagating helix of Higgs particles that give it mass. An up, down, right, left,forward and backward directions inference of an ever-vibratory movement of the Higgs wave array. Science is totally lost still in the white noise of the energy of the small with no way to really parse the detailed mechanics. FIND THE HIGGs by inference techniques as I have done in plain language