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Narrowing in on the Higgs Boson at EPS

The following guest post is from Kostas Nikolopoulos, a postdoctoral researcher at Brookhaven National Laboratory. Nikolopoulos, who is analyzing data from the Large Hadron Collider at CERN, received his Ph.D. in experimental high-energy physics from the University of Athens in 2010.

Last Wednesday, I travelled three hours by train from Geneva, Switzerland to Grenoble, France to spend a week at the International Europhysics Conference on High Energy Physics. Here, I’m presenting some of the latest findings in the search for the Higgs boson at the Large Hadron Collider’s ATLAS detector, and joining the overarching conversation about the elusive particle.

Kostas Nikolopoulos

This is one of the major high-energy physics conferences this summer, and a much-anticipated one, given the excellent performance of the LHC over the last several months, which resulted in a rich yield of data.

Although a wide range of physics is being covered during the conference, one of the main topics is the hunt for the Higgs boson.

But let’s take a step back for a minute: the Higgs boson is a still-hypothetical particle postulated in the mid-1960s to complete what is considered the Standard Model (SM) of particle interactions. Its role within the SM is to provide other particles with mass. Specifically, the mass of elementary particles is the result of their interaction with the Higgs boson. The Higgs’ properties are defined in the SM, apart from its mass, which is a free parameter of the theory.

The SM is an impressively successful theory; it has been tested to unprecedented precision and, despite a few tension points between theory expectation and observation in data, it is still holding strong. (It is worth mentioning that one of the most stringent tests of the SM was performed using the g-2 experiment at BNL).

Despite the success of the SM, the Higgs boson has remained elusive for the last several decades. Clearly, this is rather annoying since the big picture of particle physics remains incomplete. Incidentally, demonstrating that the Higgs boson doesn’t exist would have equally fundamental implications to our understanding of elementary particles.

The search for the Higgs boson was one of the main motivations for building the LHC and its two general-purpose experiments, ATLAS and CMS. Brookhaven physicists and engineers have made key contributions in design and construction of the ATLAS detector, and now we gather the fruits of these efforts by analyzing the recorded data. In this experiment, the Higgs boson is sought after by examining its possible decay products. This is because the Higgs boson is very short-lived and decays almost instantly.

The most prominent Higgs boson decay channels are its decays to two Z bosons (H->ZZ) and its decay to a pair of oppositely charged W bosons (H->W+W). A group of BNL physicists is contributing to these searches.

Having set the landscape, let’s return to the conference. Last Friday, the ATLAS and CMS collaborations started presenting their results, decay channel by decay channel. This was a rather interesting procedure, since we’re trying to understand and compare each other’s results and the implications for the SM Higgs boson.

If it does exist, the Higgs boson is thought to have a mass in a certain range, about 100 to 600 times the mass of a proton. In my talk, I showed that ATLAS has excluded some of these masses – meaning that the Higgs boson is most likely not there – while constraining the rate of Higgs production in other possible masses. I could feel the excitement as questions and clarification were requested.

The combined upper limit on the Standard Model Higgs boson production cross section divided by the Standard Model expectation as a function of the Higgs boson mass is indicated by the solid line. This is a 95% confidence level (CL) limit using the CLs method in the entire mass range. The dashed line shows the median expected limit in the absence of a signal and the green and yellow bands reflect the corresponding 68% and 95% expected regions.

This coming Wednesday, the plenary talk on the Higgs searches will summarize the current status, taking into account everything that was presented at the conference. From the ATLAS side, this could be summarized with the figure on the left. The horizontal axis gives the possible Higgs boson masses, while the vertical axis is the production rate of Higgs boson in multiplies of the SM prediction. The continuous black line is the production rate upper limit at 95% confidence level based on the data; given our data, we are 95% certain that the Higgs boson production rate at this mass does not exceed the value given by the curve. When the upper limit is less than one, the production of a Higgs boson with the SM predicted rate is excluded; at 95% confidence level the Higgs boson is not there! The dashed line shows the expected sensitivity of the experiment: assuming that the Higgs boson does not exist, it estimates how stringent upper limits we could set. Finally, the green and yellow bands show the fluctuations of the expected limits if we were to repeat the experiment several times. If there was no SM Higgs boson, approximately 68% of our experiments would be within the green band and 95% would be within the yellow band. Overall, ATLAS is excluding the SM Higgs production for masses between 155 and 190 GeV (approximately 165 to 203 times the proton mass) and between 295 and 450 GeV (approximately 314 to 480 times the proton mass).

So, the Higgs boson hunting is well under way! On one hand, we have managed to exclude some possible values of its mass and constrain its production rate in other regions. On the other hand, there are certain mass regions where we observe higher event yields than expected, assuming that there is no SM Higgs boson. It will take more time and data to see how this picture will evolve. However, given the exceptional performance of the LHC – the collision rate is increasing day by day – and our better understanding of our detectors with time, we’ll be able to say more in the near future.


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