Pentaquark vs Higgs Boson

 On the allegedly Higgs boson claimed to be discovered by ATLAS and CMS Collaborations on July 4th 2012

By Israel Pérez
Postdoctoral Fellow at the University of Saskatchewan

THE CASE OF THE THETA^+(1540) PENTAQUARK

In 2003 the LEPS and CLAS collaborations reported the evidence of an exotic pentaquark known as Theta^+(1540) [1,2]. The event caused a lot of excitement among the community of high energy particle physicists. Surprisingly, the data analysis carried out by some other collaborations didn't corroborate the finding. This gave place to a great debate about the possible existence of this resonance. Thereafter hundreds of papers have been published on this topic. By 2005 there existed in the literature at least 33 data analyses from around 20 collaborations all over the world. From these 33 reports, 17 claimed the pentaquark observation whereas the rest show no evidence. The common factor among the positive results is the low statistics whilst, with relatively high statistics, the resonance turns out to be imperceptible. With the aim of settling down this issue, in 2006, the same CLAS collaboration repeated the experiment with a great number of data... This time however, the result was negative (see Fig. 1).

                   

Figure 1. Search for the Theta^+(1540) pentaquark according to the CLAS collaboration. Left: Very first report in 2003 with low statistics. A clear peak is observed around 1540 MeV. Right. Repetition of the experiment in 2006 with high statistics. The resonance no longer appears.

In 2010 an article presented an alternative explanation of these apparently contradictory results [3]. The authors argued that in experiments with low data the appearance of the peak can be attributed to both the method of momenta selection in the decay modes and the cuts realized to the data. They concluded the following: The study shows that the method used in the experiment to associate momenta to the undetected proton and neutron, together with the chosen cuts, necessarily creates an artificial broad peak in the assumed K^+ n invariant mass in the region of the claimed Theta^+(1540). The most recent study [4] conducted with a great number of events doesn't report any evidence of the exotic pentaquark (See Fig. 2). It appears that these works put an end to the debate.

                                        

Figure 2. Last report on the invariant mass of the theta^+(1540) pentaquark with high number of events [4]. No peak is observed.

THE CASE OF THE HIGGS BOSON

In july 4th 2012, the ATLAS and CMS collaborations claimed to have observed a boson that seems to match with the properties of the Higgs boson predicted by the standard model [5,6]. Fig. 3 shows the corresponding graph where one can observe a peak around 125 GeV with only 5 events. 

Figure 3. Report on the Higgs boson by the ATLAS collaboration. It is worth noting the poor statistics. Around 125 GeV only 11 events can be counted.

If we keep in mind the situation of the pentaquark, it turns out to be unavoidable to raise the questions: Could the Higgs boson be facing a similar situation to the theta^+(1540) pentaquark? How many experiments have to be run to establish the existence of a particle? Indeed, the lesson learned from the pentaquark is that even after 34 experiments and 17 "positive observations" the issue is not settled. We are then tempted to conclude that the claiming of the Higgs boson was certainly very premature. Scientific criticism demands that a great number of experiments have to be carried out before asserting the validity of the results.

References

[1] T. Nakano et al., Phys. Rev. Lett. 91, 012002 (2003)
[2] S. Stepanyan et al. Phys. Rev. Lett. 91 252001 (2003)
[3] A. Martinez and E. Oset, PRL, 105, 092001 (2010)
[4]  K. Shirotori et al.,  arxiv:nucl-ex 1203.3604 (2012)
[5] ATLAS Collaboration, arxiv:1207.7214, hep-ex (2012)
[6] CMS Collaboration, arxiv:1207.7235, hep-ex (2012)