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Proceedings sent November 1, 2006

Posted by dorigo in books, language, physics, science.

Ok, the final, camera-ready version of my proceedings to the “Quark Confinement and the Hadron Spectrum” conference held in the Azores islands last September has been sent.

All bells and whistles are there, including the preferred format specified by AIP guidelines, pacs numbers, keywords, figures and citations in the required format.

You can have a look at http://www.pd.infn.it/~dorigo/qchs.ps  … Happy reading!

Thanks to all who contributed here… This time I decided to thank specifically Helge in the acknowledgments section for his proofreading among those who sent corrections, since he did read the whole thing.


1. James Graber - November 1, 2006

Congratulations on finishing an excellent proceedings article! And now that you may have some time could you please explain the significance of the newly announced CDF WZ result for us laymen?
Jim Graber

2. dorigo - November 1, 2006

Hi James,

maybe you have not read my former post of a few days ago on the WZ discovery… If you scroll down a bit you’ll find it.

Anyway, the significance is not huge. It is a very fancy and rare process that the standard model predicted to happen, at a rate that was confirmed by the measurement. It is a final state very important for the discovery of the Higgs boson, since WH final states may produce the same signature, for instance if WZ-> lepton-neutrino plus two b-jets. One wants to understand how to best collect these events while reducing backgrounds, and seeing them it becomes much easier to do that.


3. Andrea Giammanco - November 1, 2006

I have a question, certainly very stupid (please don’t make fun of me!). By looking at Figure 2, Tevatron seems very far from being sensitive to the SM higgs.
Of course since most of the channels have been analyzed with a limited statistics, and only a few of them with 1 fb-1, the limits will improve greatly with time… but I don’t understand why in page 4 you say that by 2009 the sensitivity will reach the SM couplings, at least below 135 GeV.
My question is, to be precise: how is this extrapolation performed? Is the number of signal and background events passing the cuts simply rescaled to the expectations for 6 fb-1?
Or is also the control of systematics scaled in some way, maybe taking into account the increase in statistics of the calibration samples?
I’m particularly puzzled since you say (if I understood correctly) that it’s the range below 135 GeV which will benefit more from analyzing more data; but I see that in the range 140-180 GeV a single measurement with 1 fb-1 is more stringent than the combination of 3 measurements (with roughly 1 fb-1 each) in the 120-140 GeV range. So, why is a light higgs more close to discovery than a 140-180 GeV higgs, in your extrapolations? Is it for the expectations from the other measurements (the ones still relying on 200-300 pb-1) or for a different scaling of systematics?

4. dorigo - November 1, 2006

Hi Andrea,

your question is by no means a silly one, and deserves a thorough explanation. Indeed, if one takes Fig.2 at face value, it seems the first excluded region the Tevatron experiments have a chance to clip off is that around 160 GeV, where currently the limit is at 3.5x the SM expectation, with 1.3fb-1 total data analyzed (summing CDF and D0 datasets) – probably a factor of 10 below what we will have by end of 2009.

However, WW searches are really easy to carry out, and improvements on the other hand are hard to achieve – triggering is straightforward, and increasing the efficiency of analysis selections is not trivial. So these results are much more in line with what we think we can do at our best than those at low mass.

At low mass, in fact, two factors limit dramatically our discovery reach until they get perfected: the dijet mass resolution for pair of b-jets, and the tagging of b-jets. Work is continuing feverishly to improve these factors, and the margin of improvement is enormous. For instance, we are testing a Neural-network based B-tagger which would significantly increase the b-tagging efficiency, and keep backgrounds at bay. We have also learned how to achieve a 10% relative resolution on the Higgs boson mass from b-jets, but we are still not using these new algorithms in our analyses. Other improvements in primary lepton acceptance have not been included in the low mass searches either.

Systematics on the background at low mass are also important, as you correctly mention. This is not a big issue in high mass searches, but at low mass we are still learning how to best constrain our backgrounds.

All in all, there is a lot of work to do, but we believe once that is done the low mass prospects are better. We did a study in 2003 (the paper is referenced in my proceedings), and it transpired that we had a chance for discovery at 115-120 GeV with the total luminosity we will collect in Run II, and a chance for excluding the whole mass range up to 180 GeV. I however have an insider view of the works of that working group, and my personal belief is that we could probably exclude a light boson Higgs, but if it is there it is a matter of luck whether we see some 2 or 3 sigma fluke or if we just see nothing.

Hope that helps…


5. Andrea Giammanco - November 2, 2006

Thanks a lot, that’s exactly what I wanted to know. Thanks for you patience in answering:)

6. James Graber - November 2, 2006

Dr. Dorigo, Thanks for your reply. I guess what got me excited was the headline in Fermilab Today “WZ boson discovery inches Fermilab closer to Higgs”. I thought maybe it was more than just an important background. I have now also looked at your previous post and you seem to emphasize primarily the lack of deviation from SM predictions and the lack of (indirect?) evidence for low energy supersymmetry. I see that you attach Higgs related significance to the total mass of the one possible ZZ event. Is it possible to say roughly how many WW or WZ or ZZ events are needed to see a SM Higgs or an MSSM Higgs? By the way, is it still true a SM Higgs is supposed to be around 175GeV and an MSSM Higgs less than135GeV? Are there any other “popular” candidates other than these two? TIA Jim Graber

7. dorigo - November 3, 2006

Hi James,

well, yes, the WZ is an important background for the Higgs, but it had its own glory these days, since we had not seen any such events before. And they are spectacular, since there really is little in the event apart from the two massive particles, unlike what usually happens in a proton-antiproton collision -99.9999% of the times orchestrated by the strong interaction.

I do emphasize the fact that the more we dig, the more we find things exactly as the standard model predicts them to be, while according to exotic theories like SUSY there is a moment when we should start hitting on unknown fancy processes.

The ZZ is important because it is the cleanest possible signature of H production if 140180 GeV.

To see a H->WW decay you need hundred of such events, because the direct production of a WW pair through electroweak processes not involving the Higgs is 50 times as frequent as H production. Now, you can discriminate the two processes by looking at the kinematics of the decay, but it is fair to say you need about 500 WW events, where 10 of them can be isolated.

To see H->ZZ things are different because the four decay leptons in the final state will allow you to reconstruct a mass peak, and even a few events piling up at some mass will be a discovery. However, the branching ratio of Z bosons to leptons make event counts really small. The SM background of ZZ production has a cross section about two times larger than of the Higgs, and it does not peak in mass, so it is hardly a problem. But you really need a lot of data to see H->ZZ…


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