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Top mass: 172.6 +- 1.4 GeV !! March 8, 2008

Posted by dorigo in news, physics, science.

The above number is the new CDF+D0 combination of their best top mass results. The top quark is now known with a relative uncertainty of 0.8%, a value that only six years ago was thought to be unreachable by the Tevatron. And instead, through hundreds of man-years of work and ingenuity by many of the most brilliant people I know in this field, that result is now real. Such an accuracy on the top quark is a real success of the Tevatron Run II, and I am not being rethoric if I say I feel deeply proud of having been part of it, even if in a minor role.

A 0.8% precision on the top quark means a lot for the Standard Model, but the Tevatron is not done yet: I expect this number to go below 0.5% by the time the Tevatron stops running. Despite the huge (800 pb) cross section for top production at the LHC, I have my doubt that ATLAS and CMS will ever be able to do sizably better. A real legacy for the Tevatron and, let me say it, for CDF, the longest lasting physics experiment in history, ever.

I will have more to say next Monday, when I wish to discuss the result in detail, the combination, the inputs used, and the names of the real heroes behind all this. I will also have something to say about the speculations of a top mass measurement which some argued to depend on the number of jets contained in the final state… Check back here in a couple of days. Hell, this is a weekend after all!


1. student - March 8, 2008

Is it a prerequisite to become an experimental high energy physicist to get excited about such an ordinary result? I understand it may be really hard to achieve it and this number is used in many predictions, but at the end it will change nothing.

2. Big Vlad - March 8, 2008

this is good stuff mate, I like it. From what I understand, uncertainty in the top mass is responsible for the uncertainties in a lot of Higgs production cross sections at the LHC.

3. Alejandro Rivero - March 8, 2008

Well, it seems we are leaving the yukawa=1 value. Now it is
0.9914 \pm 0.008. Actually it is a good thing, because it gives some room for radiative corrections of any (yet unknown) kind.

4. JustChecking - March 8, 2008

I’ll maybe sound stupid, but how is this related to the last year’s result from CDF+D0? I’m talking about the http://www.arxiv.org/abs/hep-ex/0703034 results, that find top quark’s mass at 170.9+-1.8 GeV.
My main question is – is this new result a more precise “version” of the same result, or a new – read, independent – result? The second option would give us precision of 171.2-172.7 GeV!

As for Big Vlad’s reply to student – check http://en.wikipedia.org/wiki/Top_quark for more details on this (plus of course tons of articles on arxiv, but those are bit harder to understand😀 )…

Btw, is it just my feeling, or is WikiPedia becoming and amazing source of scientific info?

5. Anonymous - March 9, 2008

Congratulations! — but still another ~50 years to go before reaching the title of longest lasting physics experiment ever:


If anyone can measure the top mass to such a precision with a funnel and some pitch — now _that_ I would consider to be quite a remarkable achievement!

6. dorigo - March 9, 2008

Hi Anon,

with all due respect, I think that funnel setup does not qualify as a physics experiment… At most, it is a nice demonstration, and a quite longeve one. But you cannot compare it to Homestake or CDF.


7. dorigo - March 9, 2008

Dear Student, it is quite extraordinary to be able to measure the mass of such a short-lived, QCD-interacting particle. In fact, the top mass is not even defined well enough to grant one order of magnitude more precision in the measurement…

Hi Vlad, yes, the top mass enters EWSB physics in multiple places with some weight. We are reaching a precision where it does not provide more input in the global electroweak fits (the W mass is better suited for that right now), but indeed, cross sections for rare processes do depend on the top mass, and many other things too if you take SUSY in consideration.

JustChecking, this is an updated average of the previous result, incorporating everything, Run I and Run II results from D0. Not all of them: only the best ones, the ones which is most effective and straightforward to average.

8. Andrea Giammanco - March 9, 2008

> I have my doubt that ATLAS and CMS will ever be able to do sizably better.

I bet that 6 years ago you said that a 0.8% precision would have been:

> unreachable by the Tevatron

9. dorigo - March 9, 2008

Andrea, I said people thought it unreachable, not me. I have always had the highest faith in the ingenuity of my colleagues in CDF. I am serious, no kidding here: I had seen Run I deliver much better results than earlier foreseen, and so I was -if you will- biased, in the right direction it turns out!


10. Not Even Wrong » Blog Archive » HEP and Politics News - March 9, 2008

[…] than refer you to Tommaso Dorigo for coverage and excellent discussions of a new, more accurate top mass measurement, reports of not very convincing deviations from the Standard model in B-mixing and charm decays, […]

11. dorigo - March 10, 2008

Hey Alejandro, welcome back! I had not heard from you in a while… How is your slow drifting to string theory going ?😉

Good point about the Yt=1 issue… It cannot get any better until we understand by which mechanism that value is close to unity in fact.


12. Andrea Giammanco - March 10, 2008

Well, I notice the following general trends in long-running experiments(*):
– yellow reports, TDRs and such quote some expected precision on some important flagship analysis
– in the first years, all those expectations are found to be, in retrospect, way too optimistic
– after years, so many ingenious people have proposed so many improvements that, at the end, with complicated strategies you do better than expected with simple strategies.(**)
So, I can understand why you say that you expected RunII to outperform expectations, based on you experience from RunI, but why don’t you expect the same to happen at LHC?
If the argument is that the cleverest guys were at Tevatron, I remind you that most of them are already drifting towards LHC😉

(*) sources: old yellow reports about LEP (e.g. the precision expected on the Z mass was pessimistic by orders of magnitudes), old TDRs from Tevatron, “private communications” by old farts, including you🙂
(**) probably an exception was single top. Wasn’t it expected to be discovered relatively soon in Run II, in some TDR?

13. dorigo - March 10, 2008

Andrea, yes, I have exactly that trend in mind. And take single top: true, we have done worse than what originally expected, but I expect we will do well in the end there too. Same with a low-mass Higgs. The problem, there, is that manpower is short, and some tools which we know would increase our discovery reach have lagged behind.

No, I do not think the LHC has less skilled people manning the experiments. I believe, however, that we are at the very beginning of a journey -a long and difficult one. I know CDF took two years to understand their triggers, for instance, and I wonder how lucky and smart we can possibly be to deal with the huge complexity of LHC collisions. On the blueprint, Atlas and CMS should easily be able to reach sub-GeV precision on the top mass. I think, however, that it will take quite some time to do so…


14. Andrea Giammanco - March 10, 2008

In that case I agree, I objected only to your original formulation:

> I have my doubt that ATLAS and CMS will ever be able to do sizably better.

I understood that a good rule for winning bets is to be pessimistic on the short term and optimistic on the long term. For every human enterprise, probably:)

15. island - March 10, 2008

This is amazing news, (yet again!). You all have racked up much to celebrate and a lot to be proud of.


16. CDF/D0 Combined Top Mass Result « Imaginary Potential - March 11, 2008

[…] March 11, 2008 · No Comments For those of you interested in HEP in general and Higgs searches in specific, you may have already heard of the Combined CDF/D0 top mass preliminary release. They’ve changed from last year to which is a total uncertainty of 0.8%. Tommaso Dorigo has a really nice post on the individual measurements and combined uncertainties that went in to the result. There’s also a bit of general discussion on how to combine measurements and an interesting point on how top mass measurements may-or-may-not be effected by jet multiplicities in the final state. He suggests that the full luminoisty might improve with to 0.5% . […]

17. dorigo - March 13, 2008

Hi Island, thank you – but I have to share those congratulations with those who desrve them more… Tens of skilled people who are very devout to the challenge of improving the top mass measurement.


18. A top mass measurement technique for CMS and ATLAS « A Quantum Diaries Survivor - July 8, 2008

[…] The top quark mass, one of the fundamental parameters of the Standard Model, is now known with 0.8% precision – a value way beyond the most optimistic pre-Run II estimates of technical design reports and […]

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