Today I've been told from a friend that my blog contains too much Physics! I disagree, but can't help going in the "wrong" directions now, since I promised I'd post here my talk as soon as it was ready.
So here we go. For your eyes first, here are my slides. (Some corrections are still needed, but the meat is all there...).
Introductory slide. A summary of what I will discuss in the talk. A nice picture of the Hirise (the main building in the Fermilab complex).
An overview of the performance of the Tevatron accelerator complex. The all-important parameter here is the integrated luminosity: basically, a number which tells you how many proton collisions you are creating for the CDF and D0 detectors to analyze.
Now here I discuss the physics of the production and decay of the Higgs boson at the Tevatron. Graphs of the production mechanisms are shown in the bottom right, while above it there is a plot of the branching ratio - a number between zero and one, which says how frequently the Higgs creates something (say a pair of b-jets) rather than something else, as a function of the as-of-yet unknown Higgs mass. On the lower left, the production cross section of Higgs bosons is shown as a function of the Higgs mass. The cross section tells you how probable it is that two protons will produce something when they collide. You multiply it by the luminosity and Hoopla! You get the number of Higgs bosons contained in the dataset!
Here I discuss the knowledge on the Higgs boson characteristics that we have obtained from past experiments, especially at the LEP collider at CERN. The upper plot is a curve which has a minimum value where the probability of the Higgs mass is largest. It says the Higgs mass can't be far from 125 GeV!
Below, the plot shows that if you knew perfectly the W and top quark masses, you would not need to measure the Higgs - it would be known! However the tie between these values is only logarithmic, so we do need to measure the Higgs mass!
Here I discuss the results of a study we did in 2003 (I participated in it!). We re-assessed what chances the Tevatron experiments had to find the Higgs boson. The plot on the right shows that, with cunning, we can indeed reconstruct the Higgs mass from b-jet pairs with a 10% resolution (not an easy feat!). I did that plot and I am proud of the whole thing so I figured I would put it in! On the lower left, a much more important plot. This summarizes what are our chances to see the Higgs if we are given enough luminosity (DESIGN=what we might get by 2009). The purple curve is below the red hatched line up to masses of 180 GeV: that's GOOD, it means we can exclude the existence of the Higgs boson up to 180 GeV if it is not there. Otherwise, the blue curve is just touching the red hatched line at 115 GeV: it means we have a chance of discovery if the mass of the Higgs is 115 GeV!
This is basically a back-up slide which shows that CDF and D0 are measuring and understanding their high-Pt lepton samples very accurately, and extract precise physical information from them. On the left, the measurement of the W and Z cross sections are shown, and compared to theory (the lines).
Another back-up slide which describes what means we have to determine whether a jet was originated by b-quark decay - a very important thing in Higgs searches, since the Higgs decays to a pair of b-jets and to see it you have to identify these b-jets well!
The cartoon on the lower left shows the proton-antiproton pair colliding (yellow lines) and a B particle produced, which travels a bit before decaying into charged tracks. The tracks do not point towards the interaction point, so the jet containing these tracks can be identified as a b-jet.
Now another slide containing my own production (the results of my PhD thesis, which are still of some relevance here!). In Run I, I showed we could see the Z->bb decay in dijet events. It was important because finding the Higgs is basically the same thing, only harder. If we cannot find the Z, nobody would think we can find the Higgs!
Here are the freshly blessed results of my search for Z->bb in Run II. This is showing that we are in business! Also, it is sending a clear message that we can use this resonance to calibrate our detector very well, a thing of paramount importance for the accurate measurement of the Top quark mass - which, some of you will still remember, has implications for the Higgs mass besides being a fundamental parameter of the theory itself.
Here I describe some old results on Higgs searches by CDF, which however have just been combined in a new limit to the Higgs boson cross section. The lower plot shows the combined limit on the cross section as a function of the Higgs mass: The limit means that we exclude a Higgs boson of mass such and such has been produced with a cross section larger than that. The black curve at the bottom is the expected cross section for Higgs production: still much lower than the upper limit, which means we are not excluding the Higgs existence yet! (You should note that we do not particularly desire to exclude that the Higgs exists! If we can measure it, we do it, if not, we constrain its existence...)
Here I start describing real new results for the search of H->bb decays in association with a W boson decaying to a lepton-neutrino pair. Once one reconstructs the W decay, one looks for events with exactly two jets, makes the invariant mass of the two jets, and if they contain a b-quark the mass ends up in the lower left histogram. The distribution is understood as a sum of various backgrounds, and the absence of a bump (alas!) means the Higgs is not there in big numbers - so its cross section can't be too large (lower right plot: again a cross section limit).
The same business is demonstrated here by the D0 experiment. This slide also shows a nice event display of the production of a W + dijet event. The yellow bar is the measured neutrino transverse energy, the red bar is an electron, and the two clusters of bars are two jets. The detector is a cylinder, and it has been cut along its length and unrolled to show the energy deposition in each of its cells.
This slide discusses how to search for the decay H->WW. This entails finding WW events, which we do (as shows the plot on the upper right), but then we need to exploit the fact that if the WW pair was created in the decay of the Higgs, then the produced charged leptons from W decays will tend to fly away parallel to each other: a result of a peculiarity of the Higgs boson, the absence of spin, combined with simple conservation of helicity (I'm kidding you, not simple at all).
So D0 exploits the conservation of helicity by looking for pairs of leptons which travel close together in the azimuthal angle (in the plane transverse to the direction of protons and antiprotons). That is shown in the lower left plot, where the Higgs signal, in blue, is clustered towards the left, while all other backgrounds are towards the right. Another cross section limit ensues.
CDF did the same thing, and again you find here a delta phi plot. CDF uses the distribution in a fit which tries to extract the amount of signal from the scarce number of events collected. Again, a cross section limit versus Higgs mass is shown by the red curve in the lower left plot.
This plot is just to have a break, and have a look at the event display of a couple of interesting events by CDF, two events we believe are indeed the creation of WW pairs. One event has two electrons flying away not far away from each other, the other has an electron and a muon. Both events have large missing transverse energy, indicating the presence of neutrinos.
Next, CDF studied the WWW final state (no, nothing to do with internet!). Basically, when a WH pair is created, and the H decays to two more W bosons, you can get three of them. Since we do not see any of these peculiar events, again we set a limit on the cross section of WH production!
This slide is sure to get a lot of air time. It is basically a summary of all the cross section limits obtained by the Tevatron experiments in Run 2. The limits are compared to the theoretical cross sections (black lines lying below), to show we are still far from our goal - intersecating the limits with the black lines!. But we are getting there!
This is my conclusions slide. I summarize what I've shown in the past slides, and go as far as to claim that we have a shot at finding the Higgs boson if the Tevatron performs well in the next 4 years!
That is it. I hope my explanation was not too cryptic. If so, I am happy to explain details better. Post your comments below!
Tommaso, there is never enough physics in the blogs. (And being myself a manager of the physcomments.org site, I can tell how true this is, regretly).
A question surely nobody will ask you in your presentation: Can you see some traces of the disappearing H+ scalar of hep-ex/9909044, hep-ex/0009010, hep-ex/0105057? Averaging more events put its sigmas back to fluctuation, but it still worries me.
Posted by: Alejandro Rivero | March 05, 2005 at 03:42 AM
Hey Tommaso,
well this was really hard to read. Not sure, if I understood much. However, this is the first time, I feel like it was too much physics.
Just some questions on my side: How long will this talk take? It looks pretty impressive, when I look at it in my browser.
The second thing, I am curious about, is there anything you don't know about the Higgs Boson except its mass? And maybe its existence?
Posted by: Helge | March 05, 2005 at 04:53 AM
Unfortunately my physics is too weak to understand...;-D
Posted by: Marco | March 05, 2005 at 08:10 AM
Hi all, and thanks for trying to read through the post above!
Alejandro: I think the general agreement among the Physics community is that the signal was indeed a fluctuation. Besides, because of the way we analyze our data, these kinds of fluctuations are more likely to produce 3,4,5-sigma discrepancies than the absolute meaning of those figures would suggest. So worry not! However, we would have no sensitivity to that signal in proton-antiproton collisions, due to the huge QCD background.
Helge: thanks for your feedback! I am with you, the post above is too hard to digest. However, the audience of this site is composed in part by physicists (like Alejandro above :) and so I feel I can alternate easy and hard descriptions... As for the talk, I have 15'+5' to present it. I already know I can do it, although the slides do appear kind of thick. My style is to have on the slides more info than I am actually describing to the audience, because many will look at the slides but will not come to the talk. The Higgs boson: if it exists, and it is the standard model one, then we do know a lot about it, but we certainly need to measure those quantities that we know about, to make sure. We do not know its mass, and there might be other surprises behind the corner... One has to be alert.
Marco, thanks for the visit. As I explain above, not all my posts are for "general audience", but most are... But I think that the post is more "heavy" than "hard"...
Cheers to all,
Tommaso
Posted by: Tommaso Dorigo | March 05, 2005 at 08:49 AM
Just to follow up. You write "if it exists, and it is the standard model one". So I wonder: Are there others?
Thanks for the reply.
Helge
Posted by: Helge | March 06, 2005 at 04:40 AM
Hi Helge,
Thanks for your comment. Indeed, I was very quick in that sentence. The question is very important.
Yes, we are indeed not sure whether there is a Higgs boson (the one predicted in the Standard Model, SM), or if there is none (which is the most interesting case, since it would catch us with our pants down!), or if there are more than one! Actually, many believe that Supersymmetry (SUSY: a theory that modifies the SM to include a whole new series of particles, supersymmetric partners of all the particles so far known, quarks, leptons, and vector bosons alike) is the right theory of particle physics, since it has many a desirable feature, mending some shortcomings of the SM.
If SUSY is the theory, we do not know how many Higgs bosons we would see! But at the minimum, we would get FIVE of them. One of them would still behave quite similarly to the SM one, though, so in a sense finding a first Higgs boson would not allow us to immediately rule out one theory or another.
Maybe I should say a word or two about why the Higgs boson is important in the theory. Basically, it is called for by a property of space-time so important, one we cannot really do without: local gauge invariance.
If we are to both ensure that physical quantities which we expect to be conserved by the theory (think of the electric charge, for instance) do so in every point of space-time, that is locally, and not just at a global level, AND that the whole theory is consistent with other fundamental beliefs about the finiteness of the cross section of some particular processes, the easiest, cleanest way to do so is to introduce a Higgs boson in the theory.
I know the above is very fuzzy and unclear. Unfortunately, it takes about 5 years of courses in fundamental physics to really understand the whole thing!
Maybe I'll make a separate post of this later.
Cheers,
T.
Posted by: Tommaso Dorigo | March 06, 2005 at 08:52 AM
HTA big bang hall:
k = (2)*((t[a]*s+t[l]*b)/(t[c]*s+t[x]*b)) , (0
Day 0
Posted by: m. visaya | May 13, 2005 at 11:18 AM