A Surge of Seminars; Spying Mount Higgs?

One of my colleagues jokingly referred to July 4 as “Higgsdependence Day.” Of course, the Americans in my circle of colleagues find it both fun and frustrating that the big CERN seminar (announcing the state of ATLAS and CMS searches for the Standard Model Higgs Boson)  is on July 4, but at a time of day in Europe that corresponds to VERY early morning in the U.S. (9am CEST, 2am US Central). That’s OK; obviously, with the International Conference on High-Energy Physics (ICHEP) being in Melbourne, Australia – a full 7 hours ahead of CERN – it is critical to have an event that is both convenient at CERN and at ICHEP so people at both places can participate in the seminar.

But wait! CERN announced a week ago the seminar on ATLAS and CMS Higgs searches [1], and now Fermilab in Batavia, IL has announced THEIR OWN seminar on their Higgs searches on July 2 [2]. Their seminar is currently entitled, “CDF and DZero Higgs Results with the Full Tevatron Data Set.”

It’s worth reviewing a bit why all of this is so interesting. In December, 2011, ATLAS and CMS presented their first results in the search for the Standard Model Higgs Boson using a large data sample from the Large Hadron Collider (LHC). These results were preliminary at the time, and they were followed by internally reviewed and approved papers that were soon-after published in leading journals in the field. Here are two summary plots of what was in those publications (I will explain them in a clear and concise way in a moment):

Exclusion contours for the Standard Model (assuming there is no Higgs Boson). Wherever the black solid line rises above the yellow contour band, it means ATLAS were unable to exclude the existence of a Higgs Boson being produced at a certain rate at that point. The horizontal line at 1.0 indicates where ATLAS hit a sensitivity corresponding to the Standard Model prediction, and when the dashed line crosses that line it means ATLAS hit (or exceeded) sensitivity to the Standard Model.

The same plot as above, but produced independently by the CMS eperiment from their data.

The above plots are from Ref. [3] and [4]. But what do they mean?

We can understand these plots using my favorite analogy: science as an act of exploration. Particle physicists are engaged in a great act of exploration, not of land and sea but space, time, energy, and matter. This exploration is not without its maps. The Standard Model is our map. It makes a simple prediction: sail far enough, and you will eventually encounter Mount Higgs. Mount Higgs is the last unknown place that is hinted at in the Standard Model, but whose location is not exactly known. Sail far enough, and you might find it . . . or, the map may be wrong and you may never find it. It may not even exist at all.

And so we sail on a great ship, the LHC, in a vast sea of high energy. We can sail quite far in just a year, and every added nautical mile brings more data. Analyzing the data is a lot like scanning the horizon for signs of Mount Higgs poking up into view. The above plots are the result of aiming our spyglass on the data and asking the question: do we see land, or do we just see an endless ocean horizon unmarked by any hint of land?

When we aim our spyglass at the distant horizon, we see something that is not clearly ocean and is not clearly air. Is it land? Or, is it an optical illusion - a trick of heat and air and water? Only by sailing further can we know; only by refining the spyglass can we know. Photo licensed under Creative Commons, Ref. 5.

In December 2011, ATLAS and CMS independently sailing on these seas turned their spyglasses to the horizon, and there was one point on the horizon where they thought they spied a hint of  Mount Higgs poking up above the blurry line where ocean and sky meet. That is what we see in the plots above; the black line traces the contour of what we observe, while the colored bands trace the lines of what we expect to see if there is no Mount Higgs rising above the horizon. Both ATLAS and CMS see a hint of a mountain peak rising above the horizon.

But a hint is not enough. To know that Mount Higgs exists, we must see it clearly rising and indisputably above the horizon. To truly know that it is Mount Higgs, however, it is not enough to find a mountain at the end of a vast sea; we must land on Mount Higgs, explore its slopes, and see that indeed it is as predicted by the Standard Model. What if this is not Mount Higgs? What if this is another mountain, just as interesting but unpredicted by the Standard Model? Yes, it would be exciting to discover Mount Higgs; it would be even more exciting to discover something that is NOT Mount Higgs, something unpredicted and unexpected. There, new knowledge lies.

With the new data obtained in 2012 (so far), we have now sailed more than twice as far as we had in December, 2011. Will we now clearly spy Mount Higgs, or will the hazy hint of land on the horizon have been a mirage? On July 4, we'll know more about the answers to those questions. This photo is licensed under Creative Commons and is available from Ref. 6.

There is something even more exciting about all of this. What if we arrive at Mount Higgs and find that it is not, as the Standard Model says, just a solitary mountain in a vast ocean? What if Mount Higgs, as is more likely, marks a new continent? Then, there would be new and undiscovered lands to chart. As in the days of sea and land exploration, uncharted lands bring new opportunities. In science, unexpected finds bring such opportunity for fresh knowledge and a new way of looking at the cosmos.

So, what of the Tevatron Experiments – CDF and D0 – at the Fermi National Accelerator Laboratory? Back in March, CDF and D0 also reported hints of the Higgs Boson with a mass in the same range that could not be excluded by ATLAS and CMS [7].

Is it a race? Yes and no. One of the points I made above is extremely relevant here. What if we do, in fact, spy Mount Higgs . . . how can we be sure it’s THE Mount Higgs and not some other unpredicted mountain? The way you do it is by studying the slopes of the mountain up close. In physics, we do this by painstakingly cataloging the ways in which the Higgs interacts with other subatomic particles. It’s expected to interact differently with different particles; for instance, at the LHC the most accessible decays of the Higgs are those to bosons – particles with integer spin angular momentum (like the photon). The most sensitive channels in December, 2011, were the decays of the Higgs to either two photons or two Z bosons (with subsequent decays of the Z bosons to electrons and muons). However, at the Tevatron the CDF and D0 experiments are most sensitive to the decay of the Higgs to fermions – particles with half-integer spin angular momentum. So, for instance, a powerful decay channel brought to bear by the CDF and D0 experiments is that of the Higgs decaying into a pair of bottom quarks, or “b” quarks.

In order to know that we have found the Higgs, we must observe that the Higgs particle decays AS PREDICTED in the Standard Model. If it does not, either the Standard Model is wrong or this is not the Higgs boson (or both!). CDF and D0 are expected to squeeze every last drop of performance out of their data (they stopped taking data a year ago and won’t take any more ever again) while also adding the remaining data they did not analyze for March. ATLAS and CMS are expected to more than double their data samples AND improve their techniques (physicists are always clever and can always be expected to improve their methods).

So it’s both a race and not a race. Yes, ATLAS, CMS, CDF, and D0 are all angling to be the first to make a strong statement about the Higgs content of their data. But, we also know that all of the experiments have strengths and weaknesses; bringing their independent perspectives together is needed to make sense of any discoveries.

So polish your spyglass lens and unfurl those sails; we’re powering ahead to see what lies upon the horizon.

[1] http://press.web.cern.ch/press/pressreleases/Releases2012/PR16.12E.html

[2] http://vms-db-srv.fnal.gov/fmi/xsl/VMS_Site_2/000Return/video/r_live.xsl

[3] Combined search for the Standard Model Higgs boson using up to 4.9 fb−1 of pp collision data at sqrt(s) = 7 TeV with the ATLAS detector at the LHC. ATLAS Collaboration. Phys.Lett. B710 (2012) 49-66

[4] Combined results of searches for the standard model Higgs boson in pp collisions at sqrt(s) = 7 TeV. CMS Collaboration (Serguei Chatrchyan et al.). Phys.Lett. B710 (2012) 26-48

[5] http://www.flickr.com/photos/sheepies/4633031687/ [Photo licensed under Creative Commons]

[6] http://www.flickr.com/photos/michaelspencer/3100635548/ [Photo licensed under Creative Commons]

[7] http://www.nature.com/news/boost-for-higgs-from-tevatron-data-1.10167

About Stephen Sekula

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2 Responses to A Surge of Seminars; Spying Mount Higgs?

  1. Jeff K. says:

    The captions to the exclusion plots are incorrect, I believe. The fact that, at some mass points, the observed limits rise above the yellow bands does _not_ mean that ATLAS/CMS “were unable to exclude the existence of a Higgs Boson at that point”. Rather, it means (1) that the experiment(s) ruled out (at 95% CL) the production of the SM Higgs at rates corresponding to cross sections greater than the black line, and (2) that such a limit on the cross section is less than 5% likely in the case where there really is no Higgs production occurring. Neither of these statement are (direct) statements about the existence or non-existence of the Higgs.

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