Higgs at Fermilab

The Tevatron collider, which was shut down on September 30, 2011, on the orders of the U.S. Department of Energy.-BY SPECIAL ARRANGEMENT

Given the high-statistics data of the LHC, the Tevatron result has added some bass to its thunder rather than stealing some of it.

When on July 2, just two days before CERN announced its landmark results in its search for the Higgs boson, the United States-based Fermi National Accelerator Laboratory (Fermilab) announced the final results of its own hunt for the elusive particle at the Tevatron collider, it seemed that it may have stolen some thunder from CERN. The machine, after its decade-long run, was ordered to be shut down by the U.S. Department of Energy in September 2011 in spite of pleas from the U.S. high-energy physics community to allow its operation to be extended for a few more months so that it could collect some more data that could improve the statistics of their results obtained until then.

Given the undercurrent of a race between the two laboratories to get to the Higgs finishing line, Fermilab wanted to make a definitive and statistically strong statement on the existence of the Higgs before CERN. But that was not to be. Nevertheless, with greatly improved analysis, the two international collaboration experiments at the Tevatron, CDF and DZero, squeezed out the last bit of information from the data of 500 trillion collision events gathered at the Tevatron since March 2001 which has given the strongest indication so far of the Higgs particle. While the scientists could not settle the question of the existence of the Higgs boson from the results that were unveiled, they said, The Higgs boson, if it exists, has a mass between 115 GeV and 135 GeV [giga electron Volt].

Our data, said Rob Roser, the spokesperson for the CDF collaboration, strongly point toward the existence of the Higgs boson, but it will take results from the experiments at the Large Hadron Collider (LHC) to establish its discovery.

From the combined data of CDF and DZero, they found a signal for the Higgs boson with a statistical significance of 2.9 sigma. This means that there is a 1-in-550 chance that the signal is due to a statistical fluctuation of the background of non-Higgs events. While this is far from the gold standard of a 5-sigma level signal to say that something new was going which gave the bump in the data in that energy window, Dmitri Densov, the DZero spokesperson, said: It seems unlikely that the Tevatron collisions mimicked a Higgs signal. (Figures 1a & 1b.)

The latest results from Fermilab are a reaffirmation of the results that CDF and DZero scientists presented at the annual Recontre de Moriond Conference in March in Italy with much better statistics. At that time the results had a 2.2-sigma level of statistical significance, or a 1-in-250 chance of the signal being due to statistical fluctuation, which itself was a big jump from the Tevatron results in Mumbai last year when the significance was no better than a random signal.

The methods employed at Fermilab and CERN to create the Higgs boson are similar; both accelerate particles to high energies and smash them head-on to create other particles, in particular the Higgs boson. At CERN, the LHC at present accelerates two counter-rotating proton beams with an energy of 4 tera electron Volt (4 TeV) each, thus providing 8 TeV of total collision energy to create new particles. At the Tevatron, counter-rotating beams of protons and antiprotons were being accelerated to 1 TeV each, which means 2 TeV of total collision energy. But to find the Higgs particle among the many particles created, the scientists had different strategies and looked for different signals.

At the Tevatron, the best chance of seeing a Higgs with mass around 120 GeV is where the Higgs decays into a bottom quark (b) and an anti-bottom quark (anti-b). The process involves the annihilation of a quark and an antiquark from the colliding proton and antiproton respectively to produce a W boson (Figure 2). (W boson is one of the carriers of the weak nuclear force.) This heavy W boson has a chance to create a Higgs boson from the extra energy it has. The W boson and the Higgs boson would then decay into other particles that are picked by the detectors and identified. According to the Standard Model, a 120 GeV-like Higgs will decay into a bottom quark and its antiparticle (H b + anti-b) 68 per cent of the time. But there are other collisions which can also mimic this decay via processes not involving the Higgs.

The task is to identify the excess events from Higgs decay above the background of non-Higgs events that produce the same final state and evaluate the statistical significance of such excess events. These other processes in particular include collisions that produce pairs of heavy bosons (WW or WZ) that decay into heavy quarks, and some of them can mimic the process involving the Higgs which decays into a b quark and an anti-b quark. However, these are much rarer than the process involving Higgs.

Notwithstanding the statistical weakness of the new Tevatron results relative to the latest LHC results, which are definitely indicative of a discovery, they are important because the experiments have looked at one of Higgss decay channels which is not easily accessible to the LHC. Since the LHC, which smashes protons into protons, operates at higher collision energies, each collision on the average produces many more particles than at the Tevatron.

In particular, the detectors are flooded with final states involving heavy quarks, in particular b and anti-b quarks, created by many different types of processes. Because of this large background, it becomes more difficult to find Higgs from this particular signal at the LHC. For a light-mass Higgs of mass around 120 GeV, therefore, the LHC experiments can most easily look for a Higgs signal by searching for the other channel in which Higgs decays into two photons (H 2 photons).

The two gamma decay channel arises as follows. The colliding quarks from the protons produce two gluons, which in turn collide to produce a Higgs that decays into 2 photons. This is, however, an extremely rare process since the Higgs itself does not interact directly with the gluons. Instead, the Higgs production occurs through intermediate virtual heavy quark-antiquark loops (Figure 3) which can occur in accordance with the laws of quantum theory.

The intermediate loop is what makes the process rare. According to simulations, the decay of a 120 GeV Higgs into two photons occurs only once in 500 times. Hence at the LHC, data from a sufficiently number of collisions (high beam luminosity) are needed to observe the process. Therefore, while the quark-antiquark channel has the problem of the background, the H 2 photons channel would be highly luminosity-sensitive. What is remarkable about the LHCs finding is that the accelerator engineers could deliver such high luminosity levels within a short time of two years plus since the LHC started operating and enable the scientists to make a path-breaking discovery.

There is full consistency between the Tevatron findings and those from the LHC, said Aleandro Nisati of INFN and a member of the ATLAS collaboration at the LHC. The results from the Tevatron on H b(anti-)b are very good, but also the LHC is now performing very good studies on b(anti-)b. This channel is challenging, but it will be crucial for Higgs physics properties studies [on its couplings to other particles] at the LHC, he added. Given the high-statistics data of the LHC, the Tevatron result has actually added some bass to the LHCs high-sounding thunder rather than stealing some of it.

At the Tevatron, on the other hand, experiments can most easily see the events in which the Higgs decays into a pair of bottom quarks without a serious background problem and they could obtain good data in this channel with low luminosity itself. So in this sense, there was complementarity between the two machines in the low-energy region.

We will definitely be able to say something before the LHC can if the Higgs lies between 115 GeV and 125 GeV, Marco Verzocchi of the National Institute of Nuclear Physics (INFN) said after presenting results at the Lepton-Photon (LP11) conference in Mumbai in August 2011. And, as he said, the Tevatron scientists had since focussed entirely on this b-quark channel to get the maximum information on Higgs and have now provided the best results on the Higgs from a machine that is now an accelerator of the past. And these results (albeit statistically much weaker compared with the LHC results) have enabled an independent validation of the LHCs 5-sigma result and announcement of the discovery of a new boson.