Combined Tevatron Higgs limits from ICHEP 08 August 7, 2008Posted by dorigo in news, physics, science.
Tags: higgs limits, ichep 2008, Tevatron
To conclude the small series of posts bringing you up-to-date on the current status of seaches for the Standard Model Higgs boson, I post today the 95% CL limits for Higgs boson production at the Tevaton, obtained by the CDF and D0 collaborations using up to 3 inverse femtobarns of proton-antiproton collisions.
Now, I realize that the above paragraph may sound utterly incomprehensible to occasional readers of this blog, and I want to address the issue. Please understand that this blog is about explaining things to people who do not work in the field: if you are not a particle physicist, please give me a chance to make it easy to understand what this is all about.
The Standard Model is the theory which describes the physics of elementary particles, the constituents of matter and the carriers of forces. The model, constructed in the sixties and verified in the seventies, has withstood thirty years of more and more precise tests, which have brought particle physicists from excitement to the verge of suicide: that is because there appears to be nothing else to discover! The model works wonders, with some tweaking of free parameters needed here and there every once in a while, and yet we know that it must break down at some point. Yes, because the standard model is just an effective theory: it is expected to describe well the physics within a certain restricted domain of application, but fail outside of it.
The clearest reason why the SM is not the whole story is that it does not incorporate gravity: we know matter experiences an attractive force because of its mass, and yet we have not figured out yet the details of how this force manifests itself at a subatomic level. There are other reasons, but let’s stop here for now.
Now, the Standard Model requires the existence of a neutral, heavy particle called the Higgs boson. The Higgs boson is all what remains of the symmetry between electromagnetic and weak interactions, which is not apparent in nature but it exists in the mathematical structure of the theory. The Higgs boson has to exist, or else the Standard Model would not even be an effective theory, but just an illusory construction.
The Higgs has been sought at the Tevatron collider, a synchrotron with a diameter of two kilometers built at the Fermi laboratory near Chicago, in the US. This enormous machine accelerates protons and antiprotons to 1 TeV of energy in opposite verses along the same circular orbit, and brings them to collide at the total energy of 2 TeV. A TeV (tera-electronvolt) is a small energy in macroscopic units, but it is very high for particles: it is a hundred billion times the energy needed to ionize a Hydrogen atom. By producing these energetic collisions, physicists try to produce the Higgs boson, and detect it with huge detectors, CDF and D0, located in two of the three points where the proton and antiproton orbits cross each other along the ring.
So far, we know that the Higgs boson has a mass larger than 114.4 GeV – about 120 times the proton mass. We also know, if we believe the correctness of the Standard Model, that it cannot be much heavier than that, or the measurements of several particle reactions we have performed in the past would not make sense. The mass region which is favored by the indirect information is between 114.4 and 140 GeV, although the upper limit is rather imprecise and subject to modifications depending on what data is used and how errors are treated. Gory details that need not bother you.
The CDF and D0 experiments have recently completed about two dozen independent searches for a signal of Higgs boson production in their large datasets, collected since 2002 and still being increased by a few permille every day. The data collected by each detector correspond to roughly two hundred trillion collisions: that’s right, two hundred thousand billions. In such huge datasets, finding the Higgs is much worse than finding the needle in the haystack: but the two collaborations are strong with little less than a thousand very skilled physicists, and they have been able to say something meaningful about the existence of the Higgs, depending on its mass.
The mass of the Higgs boson affects the number of events one expects to produce in a given amount of proton-antiproton collisions of given energy: that is because the higher its mass, the harder it is to create it. But the mass also crucially defines the way the Higgs boson disintegrates, and how then it needs to be identified from its decay products. An example ? If the Higgs has a mass close to its lower bound, 115 GeV, it is expected to decay mostly to a pair of bottom-quark jets: two collimated sprays of particles, one of them containing a bottom quark -one of the six objects of which nuclear matter is composed. If instead the Higgs has a mass of 160 GeV or more, it is expected to mostly decay into a pair of W bosons -the particles discovered at CERN in 1983, which are the carriers of the weak force responsible for radioactive decays and nuclear fusion in the core of stars.
So, let us see what CDF and D0 find. In the two graphs below, you can see on the horizontal axis the unknown value of the Higgs mass. As a function of it, the black curve shows a 95% confidence level upper limit on the rate of production of Higgs bosons. That 95% confidence level basically says that there is only a one-in-twenty chance, or smaller, than the rate is higher than the number corresponding to the curve. Things are slightly more complex, however, because the vertical axis does not have units of production rate: it is the higgs production rate divided by the expectation of the Standard Model. What that means is that, as long as the upper limit is above the line at a vertical value of 1.0, the correspoding Higgs masses are still allowed in the Standard Model. Once the black line will get below 1.0 throughout the studied mass range of these plots, we will know that the Higgs boson does not exist… We however hope to discover before then!
Some additional information on these graphs: the hatched lines, and the green and yellow bands, show what result the experiments were expecting to find for their 95% CL limits. The fact that the CDF full black line is well above the hatched one for Higgs masses below 150 GeV is starting to be an indication that the Higgs might be hiding there. Incidentally, the same behavior is shown by the D0 curves.
The results of the two experiments have been combined in a single Tevatron exclusion limit, but so far only for the higher mass values. It is shown below, with exactly the same style and meaning.
You can see that the mass of 170 GeV has just been excluded by the combination of the two experiments’ results. This is important news, but admittedly it is much more interesting to speculate on what is going on at lower mass values…
Another plot which is interesting to study carefully is the one shown below. It shows the other side of the coin in the 95%CL limit extraction: it in fact displays the level of confidence with which the Higgs production rate is found to be smaller than the Standard Model. While at 170 GeV of mass that confidence reaches 95%, and thus decrees that the Higgs does not weigh that much -unless we have been 1-in-20 unlucky -, the confidence decreases sharply for other mass values. Still, it is interesting to note that were we happy with an 80% confidence level, we could say that the Tevatron data is incompatible with the Higgs mass being in the range 155-182 GeV.
It remains to say what will be the future development of this fascinating hunt. The Tevatron experiments will continue to collect data until year 2010, and by then they should be sitting on more than twice the data analyzed so far. In the meantime, the LHC experiments ATLAS and CMS at CERN are expected to pitch in with their higher sensitivity -due to the much higher energy of the LHC collider, 14 TeV. It is possible to foresee that by the time ATLAS and CMS get ready to say something meaningful, the whole high-mass range (140 GeV and above) will have been already excluded at the Tevatron. These are indeed exciting times for an experimental particle physicist!
One last plot, the one below, shows the Tevatron expected reach at 160 GeV of mass: as a function of integrated luminosity, you can see the decreasing upper limit on the Higgs production rate, normalized in SM units. The limit obtained (colored bullets) has become more and more stringent with time, at a rate much better than what the mere statistical increase of the datasets was predicting (colored lines passing for the bullets, obtained by scaling the corresponding result accounting for statistical power increase) -you can note that from the fact that the points have jumped from one line to the next one down, as they moved to the right. It really is just a matter of time before we finally solve the mystery of the electroweak symmetry breaking, or find an even more intriguing mystery lying beneath…