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New Tevatron average of top mass March 16, 2007

Posted by dorigo in news, physics, science.

The Tevatron electroweak working group has produced a new average of CDF and D0 results on the top quark mass, which includes the most recent new determinations of that quantity.

So, the new Tevatron-average of the top quark mass is…

M(top) = 170.9 +- 1.8 GeV.

What is left to me is to comment that the apparently small improvement – from 1.2 to 1.1% of relative uncertainty since last year – is the result of painstaking work from the two experiments, and the fruit of 1/fb datasets. With the 5-6 times more data the two experiments expect to collect by the end of 2009, the Tevatron might arrive at a precision in the top mass measurement that LHC would not be able to match for a long time…

On the other hand, think positive: CMS and ATLAS will have a reference point to calibrate their jet energy scale to better than one percent accuracy!

A further comment is about the value itself: last August the top mass was quoted at 171.4+-2.1 GeV . The decrease to 170.9 GeV is small but significant, if you note that the two averages share most of the input. The effect of this small downward shift will be to push further down the fitted value of Higgs boson mass from global electroweak fits, which currently quote it at 76 GeV with +33 and -24 GeV errors . The upper limit, at 95% confidence level, is at 144 GeV – and the inclusion of the new top mass average will bring that number further down.

If the trend should continue, or even if the measured top quark mass stayed the same but the error got to shrink by a factor of two through more data and more great work, it is easy to predict that we would end up in a situation when the “tension” becomes a plain disagreement. What if the Standard Model Higgs gets to be indirectly excluded to be above 114 GeV by global electroweak fits, but it is directly excluded below the same value by LEP II ?

No worry. The limits are just saying there is “only” a 5% chance that the mass is beyond the bounds. We would still be ok for a while…


1. a - March 16, 2007

Notice that the dominant uncertainty in the Higgs mass fit no longer comes from the top mass. Even if we could measure the top mass with zero error, the uncertainty on the (log of) the Higgs mass would decrease only by about 10%.

2. dorigo - March 16, 2007

Dear A,

This is news to me. Where do you get the 10% ? The slope of the constant-Mh lines shows the relative sensitivity of Mh on Mt and Mw, but of course more information is needed to obtain what you quote above.

Let us know!

3. dorigo - March 16, 2007

Oh, and indeed, what I write in the post above is not correct. The LEP electroweak working group has already run its fitter, so the 76 GeV quoted are including the new M(top) measurement! This is a downward shift from 80-ish GeV, but the most recent top mass average is included.


4. Kea - March 16, 2007

It’s not OK! If we can derive lepton (and maybe baryon) masses from a theory with NO Higgs boson, then maybe there is NO Higgs boson.

5. a - March 16, 2007

hi, my computer said it. And it roughly agrees with what suggested by the LEPEWWG plot in the (mh, mt) plane (e.g. http://lepewwg.web.cern.ch/LEPEWWG/plots/winter2007/w07_mh_mt_contours.eps): the variation in log(mh) due to a 1sigma variation in mt is about 2 times smaller than the horizontal half-thickness of the blue curve, that measures all other uncertanties.
Combining errors in quadrature, one gets the “about 10%” (2/sqrt(2^2+1^2) = 1 – 0.1)

6. a - March 16, 2007

hi Tommaso, apparently my answer was too technical and got blocked by your spam filter.

7. dorigo - March 17, 2007

Hi Kea,

well, I have to agree. If there is a different mechanism to provide masses other than the Higgs, whoever needs it then. I think it would in fact be much, much more interesting if we found no such particle at the Tevatron or the LHC. It would also be ironic – the US congress killed the SSC, and got heat from scientists for that, but it would turn out to be the right decision! 😉
Of course, though, finding no Higgs and being left with nothing else would be dramatic too. Go category theory!


8. dorigo - March 17, 2007

Hi A,

it is too bad. Maybe you can send it to me and I will post it here with your name ?


9. dorigo - March 17, 2007


hmmm, on a second thought, I think you are right. Indeed, we have gone too far with the precision of the top with respect to other electroweak parameters, it seems!


10. dorigo - March 17, 2007

Dear a,

ok, on the issue of the top mass contribution to M(higgs): I talked
to “the” expert, who says that should we leave the precision of all other measurements unchanged , then a 0 error on M(top) would only improve the knowledge of M(higgs) from electroweak fits only by 15% or so – close to what you guess above.

However, as other measurements are improved (M(w), delta(alpha)), then the need of a still more accurate M(top) value will remain: clearly, the inverse argument does apply. That is, make all other measurement errors 0, and the top mass uncertainty would be the one of largest impact in the indetermination of M(higgs).

Of course, there is not much room for improvement on many of the electroweak parameters entering the global fits. In particular, those measured at the Z pole by LEP and SLC will practically not change in the future. Still, an improvement in those that can be, will keep the top mass error in the way.


11. Euclidistheway - March 17, 2007

What if the machines (LEP, SLC) are coincidentally just at the edge of limitation to well define the potential at 114-115 GeV. Could the Higgs be hiding in this range? Would not the delineation between the SM and MSSM therefore be sharp?

12. dorigo - March 17, 2007

Hi Euclid,

I am confused by your comment… Maybe I was not too clear in my post. The 114 GeV lower limit obtained by LEP is a direct one and it is rather strict, meaning that if the particle had a significantly lower mass it would have most certainly be found.
Upper limits such as the one I was envisioning above would instead be quite “elastic”, being based on indirect observations and on some subtle assumptions. The possibility exists, however, that we come to a real inconsistency.

The distinction between SM and MSSM has nothing to do with the mass of the (lightest) neutral Higgs boson. For some particular value of some parameters of the MSSM there indeed is NO distinction; but the value of M(h) may still be in a wide range of masses.


13. Euclidistheway - March 17, 2007

Hi T,
Yes, discerning between the SM and MSSM vailidity will be next to impossible in the above scenario. What i should say is that it can still be one or the other. I happen to favor MSSM.

What options are there with the “real inconsistency”?


14. dorigo - March 18, 2007

Well, if the global fits will start to really fight the lower limits, and we still see no Higgs (nor anything else) I would predict a crisis in the field. No directions, no funds. Unless in the meantime Kea finds the solution with Category theory.


15. Bryan - March 18, 2007

Isn’t it true that there could be no Higgs boson but there could still be a Higgs field?
See, e.g. http://www.phy.uct.ac.za/courses/phy400w/particle/higgs3.htm
If so then are there any plans to try to detect the effect of the field?

16. dorigo - March 18, 2007

Hi Bryan,

the effects of the field you mention are there for all of us to see: the masses of the fermions and bosons. If that is the cause, of course.

I am not sure how we would detect the Higgs field if there was not a field carrier. The whole concept is confusing me… Other readers here could maybe be more helpful ?

I had read the text you linked above, and found as illuminating as a torchlight in the fog: it only allows to see that far, but no further…


17. Bryan - March 19, 2007

Honestly I’m confused also but had read that page recently and found it interesting. I think a “real theorist” would have to chime in to tell us whether it’s true that the Higgs mechanism can exist but there could be no (detectable?) Higgs particle.
I’m reading the early papers by Higgs and Weinberg but am finding them to be more like lasers in that I can only see one point at a time and not the whole picture 😉

18. Tony Smith - March 19, 2007

Bryan asks “… whether it’s true that the Higgs mechanism can exist but there could be no (detectable?) Higgs particle …”.

It is true in some sense. For example, Barger and Phillips say in their book Collider Physics (updated edition Addison-Wesley 1997) at page 451:

“… the width of the Higgs boson grows rapidly with mass above 2 M_W, reflecting the fact that the coupling to longitudinal W or Z becomes strong … The values of the width for some representative Higgs masses are given below:

m_H (GeV)…….[width]_H (GeV)
200…………… 2
400…………… 25
600…………… 100
1,000…………. 450

A very broad Higgs boson may be difficult to identify as a particle. …”.

Tony Smith

19. dorigo - March 19, 2007

Good point, Tony.
I would add that whether or not we are able to detect a particle with our technology does not affect its existence.

20. jeff - March 19, 2007

Tommaso. At face value the sentence you wrote “whether or not we are able to detect a particle with our technology does not affect its existence” sounds very medieval. A modern scientific theory has an obligation to furnish observables. We are obliged to detect the particle, at least produce a very tight set of quantitative observations that point to it. Otherwise an outsider would say “these physicists are no better than those that believe in supernatural ectoplasms”.

Years ago there was an old lady in a nearby town that would go into trances and she claimed she could see the Virgin Mary. The local priest set up a whole set of cameras and tape recorders to catch the event. The next time the lady went to trance the priest was ready. When he developed the films he saw no indications of a signal. He then claimed: a miracle! Its must have been truely supernatural event else the films and tapes – natural and ordinary – would have recorded it.

21. Alejandro Rivero - March 20, 2007

Jeff, my friends from philology inform me that “existence” is a medieval concept on itself.

22. dorigo - March 20, 2007

Hi Jeff,

well, let outsiders say what they prefer. There are things we can observe and things we still cannot, and there is not much we can do about it; but for sure whether we understand our universe or not does not affect it much.

I concur, a modern scientific theory needs to furnish observables. But some things are harder to observe, that is all…


Alejandro, philologists are a kind of players I would prefer to leave out of our game 🙂


23. Bryan - March 21, 2007

We haven’t detected the graviton yet either, nor have we been able to incorporate it into the Standard Model. Someday I think it will be detected but it may not be during my lifetime. Hopefully the Higgs is different but I guess there is no guarantee.
And yes, it will remain a hypothesis until proven by experiment so there is no crisis of the scientific method here. There is nothing wrong with searching for something which the existing evidence strongly points to, even if it takes a long time (the top quark took decades to find).

24. jeff - March 21, 2007

Tommaso. Avoid bad philosophy and bad relations with outsiders. Bad philosophy can and IS used as an excuse by hostile outsiders to argue the emptiness and hence uselessness of the scientific enterprise.

Regards evidence and existence you guys can do better than that! Maybe most of you are too young and haven’t read enough history or have forgot it. How about the neutrino! Far more paradigmatic than the recent top quark. From the mid 30s to mid 50s, the neutrino seemed to elude direct detection, BUT is was leaving finger prints everywhere! The neutrino was an essential ingredient to allow physicist to make sense of weak interactions. It allowed them to be quantitative and scientifically creative: data could be accounted for and new ideas and experiments could be concieved to mount further evidence that the theory was on the right track. Ultimately these further experiments pointed to further sublte and deeper problems.

Consilience: a network of loose evidence collectively furnishes a tight argument. Single experiments are not water tight proofs for any theory; it is the network of experiments and accounted phenomena that make modern science. It is “detection of existence” in the broad sense of the word. Most physicts were convinced of the existence of the neutrino before the “direct” Reines and Cowan 1956 experiment. The top quark was a replay. After the discovery of the bottom quark the evidence for the top mounted in a anticlimatic crescendo. Is the Higgs story another example? A replay of a story alread told. Do you really think it is?

I have left HEP for some years now so I am not updated (was I ever? No, just cog in a big machine.). I personally believe that physicist formulate effective theories – the Fermi point theory of weak interactions with neutrinos was the first modern example. I mentioned above consilience. Let me add that quantitative and profound qualitative changes may occur: a theory evolves to become signicantly more solid and profound when new unexpected phenomena become available as when energy thresholds are crossed. Higher energies required the Fermi effective theory be transformed to incorporate the heavy W and Z bosons and the theoretical and experimental work eventually led to the Standard Model.

In the past 100 years energy thresholds have uncovered suprises: existence of nucleus, of nucleons and nuclear structure, of nucleon substructure (partons and quarks), heavy intermediate bosons, jets, gluons, etc… To my knowledge NONE of these were predicted in any water tight way and when they were observed there was a great hustle of theory building and killing. I do fing it remarkable that a consistent Standard Model emerged. It truely is a magnificent showpiece of how modern science works and it should be taught to outsiders emphasizing the hard work, the wrong turns, the great insights, the good and the bad philosophy, hoping to learn from mistakes, the predicatables (hard work) and the unpredictables (the unexpected discoveries), but absolyely leaving out the the “gee-whiz”. That can sound like hocus-pocus to hostile outsiders.

From this point of view the real risk of accelerator physics is that the energy thresholds for uncovering new and welcomed unexpected phenomena are out of reach. But that is not to argue that the LHC should not have been built. There is no gain without pain. There is no thrill with a risk. There is the possibility that the Higgs might not make a show. Now that would be a discovery!!! Astrophysics and ultimately cosmology are probably the best realms for finding unaccountable and new phenomena.

25. dorigo - March 21, 2007

Hi Jeff,

I am electing your above comment to a post, since I think you make a couple of points worth noting which would be wasted if left in this column.


26. dorigo - March 21, 2007


I agree with your statement above… But I think we will see a graviton one day, only – I am not sure we can detect it with big particle guns…


27. Beliefs and proofs in particle physics « A Quantum Diaries Survivor - March 21, 2007

[…] physics, science, personal. trackback Today Jeff posted a reply to a long thread originated by  my post on the new Tevatron average of the top quark mass , and I think he makes a few interesting points, which can be appreciated without reading all the […]

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