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A low-mass top in single top events ? November 20, 2006

Posted by dorigo in news, physics, science.

I am by nature a very gullible person in my private life, but when it comes to physics I am utterly skeptical. It helps in my job… So when Tony Smith (http://valdostamuseum.org/hamsmith/) contacted me to discuss the possibility of low-mass top quark events in the Tevatron data, and his hypothesis of several states, one lighter and one heavier than the regular one found at 172 GeV, I automatically frowned.

However hard to believe, the idea has some interesting aspects. It entails believing that the 172 GeV top is related to the Higgs as a ttbar condensate, and the 140 GeV top is related to other quarks and therefore to single top things such as t-bbar. If that is the case, the single top should show up at the Tevatron at 140 GeV rather than at 172.

The idea is not new… And it has some basis in an old diatriba that went on in the early days before top discovery, in Run I at CDF. I do not remember the details correctly any more, but I seem to recall that a CDF collaborator had analyzed together with two outsiders (non-CDF particle physicists) the early data and claimed not authorized that the top quark could be there, at a mass of 140 GeV or so.

Later, there were some speculations about the dileptonic top candidates used to show a lower top mass than the other events. But frankly, in Run II the data is such in beautiful agreement to Monte Carlo predictions, that I would be tempted to shrug my shoulders…

However, CDF has now two public analyses of single top searches out, and while one sees no signal after constructing a global likelihood, the other does measure a meaningful cross section – but a part of the excess of signal events clusters at low mass, indeed.

With a single top event, one cannot really run a kinematical fitter very effectively to extract the most probable top quark mass. So one usually looks at things such as the mass of lepton,neutrino, and b-tagged jet, which has a sizable probability of being the triplet of objects emitted in top decay. If one looks at the distribution shown below, one does observe an excess of candidates with respect to non-single-top processes, but indeed part of the excess clusters at low mass (if the legend does not read well, orange and red are the contributions from t-channel and s-channel single top production in the selected sample).

To be fair, this is only one of many distributions that show beautiful agreement with Standard Model expectations. See, for instance, the signed product of lepton charge by rapidity of the b-jet, a variable that shows asymmetry in the s-channel production process:

As always, time will tell… For now, the result of this analysis are not unequivocal but beautifully in line with the SM, with a single top cross section of 2.72+1.46-1.30 pb.

And by the way, in June I was asked by Michael Schmitt to make a prediction on what exciting new result CDF might produce before the end of 2006. I said single top observation, and the above plots show I was right… Almost. No observation yet…. But more to the point, CDF has discovered several other exciting things in the last few months! Exclusive diphoton production (see a post I wrote here a couple days ago), new Sigma_b baryons… Our experiment is not a boring place!



1. Andrea Giammanco - November 20, 2006

One of the two outsiders that you are mentioning was Dalitz, the one who gave the name to Dalitz plots.
I would like to have your opinion, some time, on “The Evidence for the Top Quark: Objectivity and Bias in Collaborative Experimentation”, by Kent Staley. The author is a philosopher of science who took the CDF paper with the 3 sigma evidence for top quark, and its aftermath, as a case study for his theory of how a consensus is formed about a discovery in modern physics.
In making this book, he interviewed several members of the CDF collaboration of the time and had access to the minutes of all the internal meetings.
Usually physicists are very hostile against philosophers of science, since some of them tend to assign a weight to subjectivity, in the process of scientific reasoning, that we consider an overestimation. But Staley’s analysis of the top discovery proves to the cultured layman that subjectivity was properly wiped out during the lenghty and painful process of approval of the result.

2. Kea - November 21, 2006

Go Tony! There are certainly some exciting times ahead. I’m not at all sure how the single top will come out in a rigorous mathematical analysis – we need more people to work on this. But the tt Higgs idea is looking pretty solid.

3. dorigo - November 21, 2006

Hi Andrea,

🙂 I did not want to name names… Of course I know the names -I was there in 1992- but since you started, here is it. Back then, Krisztof Sliwa analyzed the ttbar candidate by CDF in the dileptonic final state with an analysis called “neutrino weighting technique” which has later become a standard, and worked with Dalitz and Goldstein on a paper which was not authorized by the CDF collaboration…

I will look at the paper if I have some time, thank you for pointing it out.


4. dorigo - November 21, 2006

Hi Kea,

I am not in the position to comment meaningfully on whether the ideas of Tony are solid or not, but I indeed find the whole thing stimulating, and will keep posting on it here.


5. Alejandro Rivero - November 21, 2006

Tomasso, I think that it is enough that you are keeping track of these ideas in the blog. Perhaps nobody can seriouusly to claim to know what things will be worthwhile after the LHC (and your 2nd round).

As for Tony, I am grateful of his own bookkeeping about the top history. Somehow I was in my own ivory tower -or cellar- between computers and theoretists and I missed all the developments. Had I known better back in 1990, I could have though aboing doing the doctorate in some of the experimentalist groups (on other hand, I was told they were afraid whenever I was near a cryostat in the ILL neutron beam).

6. dorigo - November 21, 2006

Well Alejandro, you certainly might have missed a lot of exciting developments in the top sector as you stayed locked in your ivory tower in the nineties, but in a nutshell, things have not evolved much as far as our knowledge is concerned since then… Of course we know that the top quark exists, that its mass is 172 GeV give or take 1%, that it is produced as it should. But as you well know we still cannot answer the very basic questions we could not answer in 1990. And I cannot claim we can disprove Tony’s bold idea yet. At least, I know nobody who took the matter seriously enough to run a simulation yet.

As for your aborted early branching into experimental physics, well, we seem to have so many skilled experimentalists around and so few free thinking theoreticians, that I am glad you populate the latter.


7. Tony Smith - November 21, 2006

Tomasso, thanks for posting this blog entry.
As you said in an e-mail message, you “… I think one more year -two times the data we have analyzed so far- will give … a more clear answer. ..”.
I look forward to more data, as it is experimental observation data that should determine which models are useful and which are not.

You and Andrea both mention the Sliwa – Goldstein – Dalitz situation
back around 1992 in which personalities got involved in controversy.

I met Richard Dalitz at a meeting in Waco, Texas, in 2002.
He told me that he had written a letter to the New Scientist expressing his concerns about the events around 1992, but that “nobody ever read it”.
When I told him that I was at least one counterexample who had read his letter, we had extensive conversation about it. He made it clear to me that he was very unhappy with the personal conflicts that had arisen, and that he felt that the conflict was primarily due to irresponsible journalism by the New Scientist writer William Bown.
Unfortunately, Richard Dalitz is now deceased. So that his views might get a wider audience, here is a copy of his letter to the New Scientist (15 August 1992, page 47) regarding the matter, captioned “Top quark”:

“With regard to William Bown’s article on the so-called discovery of a top quark (This Week, 27 June), when I spoke with him I did not claim to have found the top quark.
That is a job for an experimenter, whereas I am a theoretical physicist.
The “earlier paper” he mentions gave a speculative analysis of an event already published by the collider detector (CDF) group at Fermilab, but there was no claim that this event was due to top-antitop production and decay.
We were completely open and told Bown the current situation in this research, and even sent him copies of our three papers on top-antitop event analysis.
The CDF group at Fermilab is not blocking the publication of any paper of ours.
I should note here that we would never publish data from any group,
unless it has given us formal permission to do so or has already published it itself. We have never done so in the past, and will not do so in the future.
When Bown asked me what, supposing that a top quark were found now,
would be the effect on the Tevatron Main Injector project, I told him that this upgrading programme would then have the highest priority, since the Tevatron would be the only accelerator capable of top quark studies before the next century. His statement in the last paragraph that money spent on the Tevatron upgrade would be wasted is opposite to what I said.
Richard Dalitz
Department of theoretical physics
University of Oxford”.

I should mention that, back in 1992, I was interested in the Sliwa – Goldstein – Dalitz analysis because it seemed to support my theoretical calculation of Tquark mass.

When the book mentioned by Andrea, “The Evidence for the Top Quark: Objectivity and Bias in Collaborative Experimentation”, by Kent Staley, appeared, I noticed that it mentioned the Sliwa – Goldstein – Dalitz situation, and I had some e-mail correspondence with Kent Staley, who said:
“… Krys Sliwa … certainly was skeptical of the standard CDF
interpretation of the data, even after the 1995 Observation of Top
Quark paper (PRL 74(14): 2626), in particular suggesting that, even if
the top quark was contributing to the excess, there might be something
more there as well. I did not get specifics from him, however (he would
not agree to be interviewed in person, but only to answer by letter
questions that I submitted to him). …”.

I also had e-mail discussion with Krzysztof Sliwa, in which he said that he “… always felt that the “official” interpretation of the CDF top candidate
events in Run-I could be too simplistic …”.

That line of thought seems to be in the epilogue of Staley’s book, which says:
“… Sliwa struck a hopeful note with regard to just those things in the data that threaten our current understanding. Even when experimenters find that they have achieved experimenter’s success, they look more closely to see the interesting flaw in their achievement – the discrepancy that will mean, not failure necessarily, but the possibility of some new success to strive for. Just as the prelude to discovery should not be seen in terms of a monotonic preparation for the discovery that occurs, so the aftermath of discovery should not be seen as the straightforward unfolding of the consequences of the knowledge thus gained. CDF and D-zero went to great lengths to establish that they now know some things about the world that were previously unknown. Just as important is the probing of this new realm of information to find out just what it is that we do not yet know. …”.

Tony Smith

8. dorigo - November 21, 2006

Dear Tony,

thank you for this very informative comment. Despite its mention of several physicists – I’m glad to say many of which are still in good health, and not just the deceased as poor Rich Dalitz – I will give it more visibility by pasting it as an independent post. Of course, if the involved parties have anything to say about the whole thing, they are welcome to comment…


9. Tony Smith - November 21, 2006

Since my web site is big and can be hard to navigate,
and since I don’t have all the individual images for my html page
on singeTquark at CDF uploaded yet, I should say that my ideas
about singleTquark at CDF can be found at


Tony Smith

10. Single top: new results from CDF! « A Quantum Diaries Survivor - October 11, 2007

[…] top quark mass of candidate events with a high value of EDT, a plot which last year caused some discussion (echoed  for the D0 analysis), given that it showed some excess at 140 GeV which could fit […]

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