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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

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Why we shouldn’t combine Higgs searches across experiments

Right now both the ATLAS and CMS experiments are working around the clock to get results ready for the upcoming International Conference on High Energy Physics (ICHEP). What happens when we have a big conference around the corner? We try to analyze as much of the data we have, of course! With all this pressure to get as much out of the data as possible it’s tempting to move too quickly and do what we can to get a discovery, but now is not the time to rush things.

A typical Higgs-like event at CMS (CMS experiment)

A typical Higgs-like event at CMS (CMS experiment)

In order to declare a new discovery we need to have a 5 sigma excess (see this post to explain what we mean by sigma) and projecting our sensitivities using results from the 2011 data, to the data we have accumulated so far suggests that either experiment might see something close to 5 sigma at ICHEP. In this scenario there is an option to combine Higgs searches in order to increase the sensitivity of the datasets even further. This is already what each experiment does for the different final states, and since each experiment understands their detectors and the correlations between the measurements this is the best way to get the most from the datasets.

So if neither experiment gets 5 sigma, and we would like a discovery, what can be done? The next obvious step would be to combine the results from the two experiments and count the sigma. Despite being an obvious next step, this is the worst thing we could do at the moment. The Higgs field was postulated nearly 50 years ago, the LHC was proposed about 30 years ago, the experiments have been in design and development for about 20 years, and we’ve been taking data for about 18 months. Rushing to get a result a few weeks early is an act of impatience and frustration, and we should resist the temptation to get an answer now. Providing good quality physics results is more important than getting an answer we want.

The status of the ATLAS exclusion with 2011 data.  Now our focus is on the remaining space. (ATLAS experiment)

The status of the ATLAS exclusion with 2011 data. Now our focus is on the remaining space. (ATLAS experiment)

The reason we have two experiments at the LHC looking for the Higgs boson is because if one experiment makes a discovery then the other experiment can confirm or refute the discovery. This is why we have both D0 and CDF, both Belle and BaBar, both ATLAS and CMS, both UA2 and UA1 (where in the interest of fairness the orders the names are chosen at random.) Usually these pairs of experiments are neck and neck on any discovery or measurement, so when one experiment sees an effect but its counterpart doesn’t then it’s likely due to a problem with the analysis. Finding such a problem does not indicate poor scientific practice or incompetence, in fact it’s part of the scientific method to catch these little hiccups. (A good example is the dijet anomaly that CDF saw last year. In an experiment as complicated as CDF it’s not unsurprising that something subtle would get missed. Everything that the CDF hardware and software was telling the physicists was there was a bump in their distribution. The easiest way to see if this is wrong is to see what the D0 hardware and software tell us. It turns out they disagreed in this instance and we got the crosscheck we needed.)

If we combine measurements from two different experiments we end up losing the vital crosscheck. The best way to proceed is two wait a few more weeks until both experiments can produce a 5 sigma discovery and see if these results agree. If the results are consistent then we celebrate victory! So let’s resist the temptation to get too excited about combined results between experiments. If we wait a few more months the discovery will be all the sweeter. Then again we may get lucky at ICHEP!

Related articles:
What next for the Higgs?
Higgs Update CIPANP 2012
December 2011 Higgs Seminar liveblog

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