New CDF Combination of Higgs limits! February 11, 2009Posted by dorigo in news, physics, science.
Tags: CDF, Higgs boson, LHC, Tevatron
A brand-new combination of Higgs boson cross-section limits has been recently produced by the CDF experiment for the 2009 winter conferences. The results are almost one month old, but I decided to wait a bit before posting them here, in order to avoid arising bad feelings in a few of my CDF colleagues, those who believe I have no right to post here published results in too timely a fashion, because they feel those results should first be presented at conferences by the real authors of the analyses.
Now I think it is due time to have the most relevant plots here, since they are all available from a public web page of CDF anyway; so here we go, with the most updated information. Mind you, these are CDF-only results: a sizable improvement in the limits will come when they get combined with the findings of DZERO. I seem to understand that the Tevatron combination group folks are dragging their feet this year, so we have better to just as well take the CDF results and comment them alone.
The first graph is the most important one of all: it describes the combination of CDF results, in the usual “95% CL limit on times the SM cross section“. It is shown below.
On the x axis is the Higgs boson mass, and on the y axis the cross-section limit. Different colors of the curves refer to different analyses, which target the various decay channels of the sought particle; hatched lines show expected limits, and full ones show instead the limits actually obtained by the analysis.
As you can see by examining the thick red curve at the bottom, CDF by itself cannot rule out the 170-GeV point, which last summer was excluded by the CDF+D0 combination. However, a sizable improvement can be observed across the board in the results. The red curve, for one thing, is considerably flatter than it used to be, a sign that the low-mass searches have started to pitch in with momentum. Another thing to note is that these results correspond to 3.0/fb of analyzed luminosity or less (2.4/fb at low mass): there is already at least twice as much data waiting to be analyzed, and results are thus expected to sizably improve their sensitivity.
The above summary brings me to mention another important point. By looking at the graph, you might run the risk of failing to appreciate the enormous effort which CDF is putting into these searches. In truth, the name of the game is not “wait more data and turn the crank”. Quite the opposite! The most important improvements in the discovery reach have been achieved in the course of the last five years by continuously improving the algorithms, the search methods, by refining tools, by finding new avenues of investigation, and new search channels neglected before. This is summarized masterfully in the two graphs shown below.
Above, you can see that for a Higgs mass of 115 GeV, the limit that CDF was able to set on its existence, in terms of cross section (well, “times the SM cross section” units to be precise: the ones shown on the y axis) has improved much more than what one would have expected by scaling down the limit with a simple square root law -the one that Poisson Statistics would dictate, for statistically-limited measurements. Quite the opposite: as time went by, the actual limits (colored points) have moved down almost vertically, a sign that the data has been used better and better! Above, if you took the extrapolation expected after the first limit was published (the one in green), you would expect that the limit today, with 2.4/fb analyzed, was at 7xSM, while it in fact is at 3xSM: this corresponds to a 2.3x improvement in the limit, which would have been granted by a 5.2 times larger analyzed dataset!!
Above, the same information is shown for the value. In this case, CDF is now expected to be able to set an exclusion alone with 9/fb of data, but we still expect to see some improvements in the data analyses, which should move the points well into the brown band. In this case, 7/fb of data might be enough.
The last two plots I wish to discuss are shown below. BEWARE: This is information that LHC scientists would really, really not like to see – so, if your life depends on the success of ATLAS or CMS, please stop reading now, take my advice.
OK. The plot below shows the probability that the Tevatron experiments, by combining their datasets and results, may observe a 2-sigma evidence for SM Higgs production, with 5/fb and 10/fb of data collected by each. If the analyses will not perform better than what they have so far, you get the full lines -red for 5/fb, blue for 10/fb. If they improve as much as it is reasonable to predict, you get the hatched lines.
What to gather from the plot ? Well: it seems that, regardless of the Higgs boson mass, the Tevatron has a sizable chance to be able to say something good, by the time CDF and D0 will have analyzed the datasets they already possess (which are in excess of 5/fb each: the delivered luminosity of the Tevatron is passing the 6/fb mark as we speak, and the typical live time of the experiments is above 80%).
Below, we see what is the chance of a 3-sigma evidence. Again, there is a sizable chance of that happening, although if no additional improvements occur in the analyses, it seems that the Tevatron will need to get lucky!
I remember that in 2005 I gave a talk in Corfù (Greece) where I ventured to speculate on the possible scenarios for Higgs searches at the Tevatron and the LHC. One of the scenarios saw the two experiments competing to find the particle with roughly equal reach, and eventually producing a combined observation. That possibility does not seem too far-fetched any longer!
In the next few days I plan to discuss in some detail the most important analyses which contribute to the combination discussed above. Stay tuned…