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Compatibility of CDF single top results February 19, 2007

Posted by dorigo in news, physics, science.
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The CDF experiment has been looking for single top production for quite a while now, and the latest results, based on larger statistics and more refined analysis methods than ever before, have been puzzling.

Single top production occurs at the Tevatron proton-antiproton collisions mainly through two electroweak processes, which have a combined cross section of about 2.9 picobarns – that is to say, they happen once every 20 billion collisions, or twice per hour at the highest Tevatron luminosity. The process is quite hard to distinguish from background processes, and sophisticated analysis techniques have to be devised to see a signal.

A priori calculations show that the data collected in Run II is by now enough to show single top production, and indeed our cousins at the competitor D0 experiment claimed to see a nice signal of single top production a couple of months ago, but CDF struggled, hard-pressed to explain why its three analyses (now four) show conflicting results: while one analysis sees a signal compatible with expectations, the others see nothing at all, at the point of being close to ruling out the existence of the process!

The mesmerizing paradox spurred a deep investigation of the correlation of the four analyses, performed with pseudo-experiments: you simulate all known processes contributing to your data samples in large amounts with Monte Carlo generators, and then fish randomly from the simulated samples respecting the proportions of the various processes, creating a large number of sets of events (“pseudo-data”) each of which represent the possible outcome of your data selection process.

Once you have pseudo-data sets,  you can forget about their origin, and concentrate on the search for single top with your four methods in each of the sets, coming up with four independent measurements of single top production cross section for each pseudo-experiment. The sets of results allows you to see how likely it was to see such a poor compatibility in the four cross section measurements as the one you saw in the real data.

CDF finds that they have been unlucky, although not overly so: the probability of the puzzling result in the data is 0.65%. That is to say, you should expect to get such poor agreement between the four analyses only twice in three hundred trials.

I am not sure what is the plan now. I suppose we will collect more data, planning on a measurement with 2 inverse femtobarns, which should allow to come up with a measurement after all.   

Comments

1. island - February 19, 2007

Tommaso, could this have any effect on the recalculated expectation for the Higgs mass, or is this strictly a CDF related problem?

2. Tony Smith - February 19, 2007

Tommaso said: “… the data collected in Run II is by now enough to show single top production … CDF … three analyses (now four) … one analysis sees a signal … the others see nothing …”.

In earlier posts, Tommaso described CDF using a Likelihood Function (LF) method and a Matrix Element (ME) method, and D0 using a Decision Tree (DT) method.

Were those the three methods of analysis first used by CDF?
What was the fourth method of analysis used by CDF?
Which analysis showed the signal?

Were the methods of analysis that see nothing primarily looking for a single-T near 175 GeV?
Was the method of analysis that saw a signal looking over a wider range of T-quark masses?

Tony Smith

3. dorigo - February 19, 2007

Hi Island,

well, I believe this indeed is just a statistical fluke. Not a fault of CDF – once you do many analyses, sometimes you are bound to find inconsistencies of this kind.

If, anyway, one were to doggedly try and associate any significance to the discrepancy among the analyses, one would be hard-pressed to fit it in whatever model. Tony has reminded us of his model a while ago, and indeed, one of the CDF analyses -the one not using much of a mass information, thus not assuming 172 GeV for Mt – is more in line with expectations.

Your question, though, is more focused on the Higgs. Well, I believe that if Tony were right – i.e., a single top of 145 GeV of mass present in the data – then the indirect Higgs limit would change quite significantly! But the whole SM would be shaken, so I doubt one could conclude that the predictions on the Higgs mass would still decrease (as you know, dMh/dMt>0, dMh/dMw

4. dorigo - February 19, 2007

Hi Tony,

wow, you force me to do quite a bit of homework today! I am still struggling with the KM post – which I think I will put out tomorrow – and there you go with yet another question that deserves a full answer.

My problem is that I have not seen the latest blessed results of single top searches in CDF out yet. I only saw this 0.56% number today. I will have to dig into our public web pages. Let me do that. In the meantime, what I can say is that CDF has a matrix element, a global likelihood, and a one- and two-dimensional Neural Network analysis.
The one finding a signal is the ME one. But I will be more precise later. Right now I have to go upstairs to put the telescope out – Saturn observation in order tonight, and the mirror needs some cooling!

Cheers,
T.

5. dorigo - February 19, 2007

Ok, the matrix element analysis indeed is the one which sees a signal, and measures sigma(t) = 2.7 +1.5 -1.3 pb (theory predicts 2.9 pb). The measurement assumes Mt=175 GeV to estimate the selection efficiency. You already saw the mass plot in https://dorigo.wordpress.com/2006/11/20/a-low-mass-top-in-single-top-events/

The other analyses are the 1-dim and 2-dim Neural Network searches, who find a cross section compatible with zero (the first determines sigma(t) less than 2.6 pb at 95%CL, the second sigma(t, s-channel)=0.7 pb, sigma(t,t-channel)=0.2 pb.

Finally the Likelihood analysis finds 0.1 and 0.2 pb, respectively for the two production channels.

As far as I understand, the Matrix element is the analysis which makes less use of prior knowledge of the top quark mass. You can find more detail in the following pages:
http://www-cdf.fnal.gov/physics/new/top/2006/SingleTop/LF_1FB/public_page.html (likelihood analysis),
http://www-cdf.fnal.gov/physics/new/top/2006/SingleTop/ME_1FB/index.html (matrix element analysis), and
http://www-cdf.fnal.gov/physics/new/top/2006/SingleTop/NN_1FB/index.html (neural networks).

Cheers,
T.

6. Tony Smit - February 19, 2007

Tommaso said: “… if Tony were right – i.e., a single top of 145 GeV of mass present in the data – then the indirect Higgs limit would change quite significantly! …”.

I am not sure about that, because in my model
the single-T mass state around 145 GeV is mostly coupled to other quarks (such as the B-quark) and is not the T-quark mass state that forms the Higgs through T-Tbar condensate
while
the Higgs is primarily a T-Tbar condensate of T-quarks in a mass state around 175 GeV.

My model is speculative and may be wrong, but it may not be very non-standard with respect to the conventional standard model predictions involving the Higgs and the 175 GeV T-quark state of the T-Tbar condensate that gives the Higgs.

Tony Smith

PS – Happy Saturn viewing. A nice planet to see on Mardi Gras.

7. dorigo - February 19, 2007

Hi Tony,

I remember your idea of the 175 GeV top being the one which counts for coupling with the Higgs, but if there was a 145 GeV top in the data it would be a 1450 pound Gorilla. We might have missed it, sure, but it would have affected our other measurements quite substantially in that case. In that sense, the effect on Higgs predictions would be quite unpredictable.

Cheers,
T.


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