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New combined Higgs limits from CDF: <1.9xSM! August 16, 2007

Posted by dorigo in news, physics, science.

A combination of all but one of the CDF analyses looking for a Standard Model Higgs boson has been produced by my colleagues in time for the Lepton-Photon 2007 conference. Mark Neubauer showed it at a Wine and Cheese seminar at Fermilab a week ago. Here is the “money plot”:

In the plot the x axis is the unknown Higgs boson mass, and on the y axis there is a multiplicative factor to apply to the Standard Model predicted cross section for Higgs boson production. The black line is the combined limit, the yellow band the limit one should have expected to place with the applied techniques and data, and the various colored lines show individual limits set with different decay channels.

As you see, CDF improved their limits quite sizably from last summer. With analyzed datasets of 1 to 2 inverse femtobarns, the excluded region is now at less than twice the Standard Model predicted rate, if the Higgs mass is 160 GeV. The limit at lower masses has also been improved quite sizably, but it is still wanting. Note that the result of the search for ZH decays to two b-jets and two neutrinos has not been included in the combination yet. That channel is not the most sensitive, but everything helps…

D0 also has combined its results, and a Tevatron combination is in the making, as far as I know. I am sure no mass region will be excluded this summer, but the game is getting less and less academic. Chances are that next summer we will see the first chipping off of mass values… Unless the Higgs is right there, of course!

I will have more to tell about each of the ingredients to this combination next week…


1. J - August 16, 2007

That’s beautiful.

I’m somewhat surprise HZZllll isn’t even listed. Does that not give anything very useful at CDF (or D0)? It’s certainly one of the more important channels at the LHC. Or is that analysis just also a little behind in getting results?

2. Kea - August 16, 2007

Wonderful, thanks! Slowly but surely onwards.

3. dorigo - August 17, 2007

Hi J,

I believe there is nobody in CDF seriously attempting to extract a Higgs cross section limit from the H->ZZ decay mode. That final state is hard but actually not unfeasible. To give an order of magnitude, back-of-the-envelope kind of estimate: CDF looks for ZZ production from regular electroweak processes, which have a cross section of 1.4+-0.1 picobarns. After the selection of 4-lepton events from 1.5/fb of data, CDF predicts to see 2.5 signal events (and small backgrounds), and 11 (with 150-event backgrounds) in the 2-leptons plus missing Et final state.

Now, if we take into account the prediction for H xs at 180 GeV, which is 0.3 pb, we get a prediction of 0.5 4-lepton events and 2 2-lepton 2-neutrino events.

As I look at these numbers, I wonder why nobody is trying it, as much as you do. Now, it’s early morning here, and I haven’t had a cup of coffee here, so I might be neglecting something important in my computation. I will read back what I wrote here later…


4. dorigo - August 17, 2007

Lol! Of course it was early morning! I forgot the branching fraction, duh!

A 180 GeV Higgs goes to ZZ only about 6% of the times. So, in 1.5/fb of data, assuming the same selection efficiency for H->ZZ and qq->ZZ, one gets 2.5 SM ZZ, 0.03 H->ZZ in four leptons.
In the dilepton-dineutrino final state one gets 11 SM ZZ and 0.15 H->ZZ (with 150 event backgrounds).

Hopeless ? Well, yes, even if one accounts for the fact that in the 4-lepton mode a mass window search cut (say 175-185 for a 180 GeV Higgs) would reduce everything but the H by a factor of three to five. In the dilepton-dineutrino final state instead there is no easy way of reconstructing the H mass, although one might try with neural networks.


5. dorigo - August 17, 2007

Hi Kea,

I read between the line(s) of your comment above, and can’t help smiling… Of course, you are not alone in secretly hoping the Higgs is not there, or plainly believing it isn’t. It would mark the start of little short of a revolution in theoretical particle physics.

I have been looking at that wigglish line for ten years now, first as a projection, then as a limit far from the unity line, and now getting close to it… I now want badly the lines to intersect sizably!


6. J - August 17, 2007

Thanks Tommaso

7. C - August 18, 2007

Out of interest, does CDF produce a 68% confidence limit plot? Would this exclude a 160GeV Higgs?

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