Cracking the Particle Code of the Universe

by Moffat, John W.

  Also on the blogs, a consensus is now being reached that, among the five or six living theorists who came up with the idea of the Higgs mechanism in 1964, Englert, Higgs, and Kibble should be the three to be awarded the prize the following year. Hagen and Guralnik, as well as Anderson, are being left out, for reasons that are unclear. Indeed, Gerry Guralnik was one of the first to consider spontaneous symmetry breaking to be a fundamental part of relativistic particle physics. He had to weather a lot of tough criticism. Legend has it that when Guralnik gave a talk in Munich about this research, the celebrated Werner Heisenberg stood up in the back of the auditorium and called him an idiot.

  Historically, the Nobel Prize for the discovery of the W and Z bosons in January 1983 was awarded the next year to Carlo Rubbia and Simon Van der Meer, both experimentalists at CERN. Moreover, the 1979 Nobel Prize awarded to Glashow, Weinberg, and Salam occurred four years before the actual discovery of the W and Z bosons and, of course, the Higgs boson, an integral part of their electroweak theory, had not yet been discovered.

  Always lurking in the background of this story is Rubbia’s unfortunate announcement in 1984 that he had “discovered” the top quark at an energy of about 40 GeV. This announcement at CERN via a press conference was held, coincidentally, on July 4, 1984. This “discovery” was based on 12 events for the decay of the W boson into a top and an antibottom quark. There were two experiments at CERN that could detect this decay, at an energy of 630 GeV, in the proton–antiproton collider. The UA1 detector observed 12 events against a background of three and a half events. The eight or nine events above background were enough to produce a resonance peak of just more than 3 sigma.

  This was considered a double-header triumph for CERN and Carlo Rubbia, coming so close after the discovery of the W and Z particles. On July 12, 1984, the editor of Nature, John Maddox, published an article with the title, “CERN Comes Out Again on Top,” commenting on the discovery of the top quark by the group under the leadership of Rubbia.5 He began the article with the statement, “The Matthew principle—’to him who hath should be given’—is working in favour of CERN …” The article also stated, “The new development at CERN follows almost exactly along the lines expected.” However, when more data were collected, it was discovered that the QCD background calculations were incorrect. They underestimated the background, leading to the announcement of the false claim. Moreover, the competing UA2 detector group in 1984 at CERN could not confirm the observations of the UA1 group. This only proved again that the roads of particle physics are paved with the tombstones of 3-sigma events, fluctuations, and unexplained effects.

  Once I had time to digest the contents of the July 4 scientific talks by Gianotti and Incandela, the following became clear: The signal for the Higgs boson decay into two photons had gone from about 3 sigma to 4 sigma in strength. The signal in the other golden decay channel—Higgs into ZZ*, which then decay into four leptons—had reached a strength of about 3 sigma. When they combined these two channels, they got a signal strength of the gold-plated 5 sigma for a Higgs boson at a mass of 125.5 GeV at CMS and 126.5 at ATLAS. An important fact that confirmed that CERN had actually discovered a new boson at about 125 GeV was that both groups, CMS and ATLAS, had detected a 5-sigma signal at about the same mass. Yet, there was no official combination of the ATLAS and CMS results announced at the seminar and press conference on the fourth of July.

  Remember that the signals in the other channels, such as Higgs decaying into WW, b-bar-b, and tau–tau, were much weaker. In fact, the tau–tau channel decay had a signal of zero, with no examples of it during the collisions. In addition, the two-photon channel decay signal was about twice the predicted size from the standard model, corresponding to about a 2.5-sigma deviation from the standard-model prediction. So it could be said that the experimental result was consistent with a Higgs boson, within 2.5 to 3 standard deviations. However, as the experimentalists stressed at the press conference, the spin of the resonance and the all-important parity were unknown. Therefore, it could be that an “impostor,” as Gianotti put it, is simulating a Higgs boson.

  In contrast to the experimentalists, the theoretical particle physics community was heavily biased toward the LHC having discovered the Higgs boson. This is understandable because of the almost 50 years of publications on the Higgs boson by theorists, and the many textbooks used in universities in which the Higgs boson and the idea of spontaneous symmetry breaking play a prominent role. However, this bias on the part of the theorists had not deterred the experimentalists from taking a cautious and objective view of the situation until more data were collected and it could be established beyond a doubt that the new resonance is a Higgs boson.

  In view of all this excitement, I couldn’t help pondering the technical and fundamental consequences for particle physics of finding a standard-model Higgs boson as an elementary scalar particle. The Higgs mass hierarchy problem, and the absurd fine-tuning prediction of the vacuum density resulting from the Higgs field, are not going away. Indeed, given the lack of evidence in the LHC data for the minimal supersymmetric standard model (MSSM), which could solve the Higgs mass hierarchy problem, as well as the gauge hierarchy problem and its extreme fine-tuning, the discovery of a Higgs boson on its own could be something of a nightmare. Any new particles discovered with a mass of 1 TeV or more, pointing toward a BSM scenario could, because of the scalar nature of the Higgs boson, still make the standard model untenable. Such a new heavy particle mass produces corrections to the 125 GeV observed mass that require a fine-tuning correction of 33 decimal places if the energy cutoff in the calculations is the Planck energy. Moreover, there is another issue with the standard model and the Higgs boson—namely, the discovery that the neutrinos have a mass. The origin of the neutrino masses is a mystery, and if no new heavy neutrinos are discovered experimentally, such as “sterile neutrinos,” which do not interact with ordinary matter, then the standard model with the Higgs boson may not be a renormalizable theory, so the original justification for having a Higgs boson is not so strong.

  It still seemed to me that the neatest solution to these somewhat catastrophic consequences of the Higgs boson would be avoided most easily simply by not having a Higgs boson! My proposal that the bump could be a quarkonium resonance, which I had been thinking about for some time, is not the only alternative model suggesting that the discovery is not a simple standard-model Higgs boson, but a more complicated beast. For instance, several physicists have suggested that two Higgs particles, or maybe even three, interact to produce the observed data. Another alternative predicts the existence of new charged and neutral particles to boost the size of the two-photon decay channel signal in the theory to agree with experiment. But, if the future data at the LHC continue to support the discovery of an elementary Higgs boson, then these possible solutions to the Higgs boson problems, including mine, will have to be abandoned.

  JULY 11, 2012

  It is critical to distinguish the spin and parity of the elementary, fundamental Higgs boson from what can be called a Higgs impostor. Particle theorists recognized during the 1990s and early 2000s that the spin of the standard Higgs boson had to be zero and its parity had to be positive. That is, it has to be a scalar boson. The scalar Higgs and an impostor can be distinguished experimentally through careful analysis of the angular correlations in their decay products.6 We know that because the 125-GeV boson has been observed to decay into two photons, and into a Z and a virtual Z boson, that the spin of the newly discovered boson has to be either spin 0 or spin 2. It cannot be spin 1, according to a theorem published years ago by Landau and Yang, who showed that a spin-1 vector particle such as a photon cannot decay into two spin-1 vector particles. Particle physicists prefer the spin-0 boson over spin 2, because a spin-2 particle would have the characteristics of a graviton, and most physicists do not believe that the decay of such a spin-2 particle has been seen in the data.

  A distinct signature that distinguishes between a scalar spin-0 boso
n and a pseudoscalar spin-0 boson is that, at high enough energies, the decay products of a scalar boson become longitudinally polarized predominantly, whereas the decay products of a pseudoscalar boson become mainly transverse in polarization. These longitudinal and transverse polarizations constitute the degrees of freedom of the propagation of a particle in space. The longitudinal polarization or degree of freedom means that the particle has a spin direction oriented along its path of motion, whereas transverse polarization means that the particle waves are perpendicular to the direction of motion of the particle. Experimentalists are able to detect these polarizations in the data.

  It was during the week after the CERN press conference that I finished constructing my own Higgs impostor model, the quarkonium resonance called zeta that I have referred to several times in this book. This model can be used to discriminate between a Higgs boson and a non-Higgs boson. This can be done by a careful comparison of the decay rates and branching ratios of the standard-model Higgs boson and those predicted by my impostor model. In particular, the impostor boson is a pseudoscalar particle with negative parity compared with the positive parity of the standard-model scalar Higgs boson. Moreover, the impostor boson has spin 0, like the Higgs boson.

  An interesting element in my model is that it can predict the existence of an as-yet-undetected fourth generation of quarks. I figured that the effective constituent mass of the 125- to 126-GeV boson should be approximately twice the mass of a 63-GeV quark. Until now, such a fourth-generation quark does not appear to have been observed, because no quark has been seen between the bottom quark, with a mass of about 4.5 GeV, and the top quark, with a mass of 173 GeV. (The other quarks are all lower than 4.5 GeV in mass.)

  Because a quark with a mass of about 63 GeV has not yet been detected, I postulated that the zeta and zeta prime boson masses are determined by the combination of the masses of the bottomonium and toponium bound states. There is a formula from the mixing of the zeta and zeta prime resonances such that when you insert the masses of the bottomonium and toponium states, and identify the zeta with the 125-GeV boson, then the mass of the zeta prime is 230 GeV. The “mixing” of these states means they interact with one another in a particular way, because the bottomonium and toponium states have identical quantum numbers, although different masses. The same is true of the zeta and zeta prime mesons. Similar mixing of bosons has already been confirmed experimentally for the neutral K-mesons and their anti-K-mesons, and also for the mixing of neutrino flavors, such as the electron neutrino and the muon neutrino, which resolve the solar neutrino problem of not enough electron neutrinos being emitted from the sun. In addition, there is strong evidence for the mixing of the pseudoscalar eta and eta-prime mesons, which belong to the lower mass octet of pseudoscalar mesons. A problem with the assumption that the zeta and zeta-prime resonances are formed from a mixing of the bottomonium and toponium states is that the standard gluon forces in perturbative QCD are not strong enough to cause the mixing that will form the zeta and zeta prime. It may be that a new strong gluon force has to be assumed to be acting between top and antitop, and bottom and antibottom, quark states.

  This idea of mine differed from the standard quark flavor mixing, which is determined by what is called the Cabibbo and the Cabibbo–Kobayashi–Maskawa matrices, which mix the flavors of the six known quarks and leptons within the standard model. In my model, I, of course, keep these standard quark flavor mixes.

  My Higgs impostor particle, the zeta, is electrically neutral and is analogous to the well-known parapositronium particle that can decay into two photons. Otherwise, the rest of the model is based on standard QCD and quark–gluon interactions, enhanced by nonperturbative gluon interactions.

  The ability to produce a theoretical Higgs impostor such as the zeta underscores the critical need to determine the parity and spin of the new 125-GeV boson. If the experimentalists eventually find the new particle to have spin 0 and positive parity, then they will have found the Higgs boson, whereas if they identify the new boson as having spin 0 but negative parity, this indicates a pseudoscalar boson, and they will have possibly found the zeta particle. Other impostor models have been proposed. For example, Estia Eichten, Kenneth Lane, and Adam Martin have proposed that the 125-GeV boson be identified as a Techni-boson in a modified Technicolor model. (Eichten and Lane are members of the Fermilab group in Chicago, whereas Martin is a member of CERN.)

  The other way to distinguish the zeta and zeta-prime bosons from the Higgs boson is to analyze carefully the rates of their decays into lower mass particles. The decay rates of the zeta are predicted by quarkonium calculations in QCD, whereas the Higgs boson decay rates are determined by the standard electroweak theory, given a Higgs mass of 125 GeV. The decay rates of the two bosons, the zeta and Higgs, into fermion–antifermion pairs, such as bottom and antibottom quarks, will be significantly different.

  AUGUST 7, 2012

  The Perimeter Institute ran a conference titled LHC Search Strategies from the 2nd to the 4th of August, which I attended. There were more than 40 participants from different international laboratories and universities, equally divided between theorists and experimentalists. The purpose of the workshop was to discuss what experiments should be done next to investigate the properties of the new boson. Proposals to discover physics beyond the standard model, such as supersymmetric particles and other possible exotic particles, played a dominant role in the workshop.

  The first morning started with introductory talks by the two CMS group leaders, Joe Incandela and Greg Landsberg. Incandela, from the University of California at Santa Barbara, gave a clear presentation about how the CMS and ATLAS groups claim to have discovered a new particle at 125 to 126 GeV. As in the seminar he presented on July 4 at CERN, he was cautious about claiming that they had found the standard-model Higgs boson.

  Next, Landsberg, from Brown University, discussed the status of the CMS and ATLAS results. He stated that identifying the spin and parity of the new boson was a primary objective before the LHC closed down on February 17, 2013, for about two years. He claimed that by the end of 2012, they would have about 30 inverse femtobarns of integrated luminosity at an energy of 8 TeV, allowing them to determine the parity of the new boson to a sensitivity of 3 sigma. He also said that the decision regarding whether the spin of the boson is 0 or 2 would not be as easy to determine as the parity.

  Landsberg claimed that it is difficult to fit a Higgs that is a spin-0 pseudoscalar boson into the standard model as an elementary particle. One possibility is to have two Higgs bosons, with the pseudoscalar nature of the Higgs boson revealing itself through quantum loop corrections. In my opinion, this is a contrived model and unlikely to be correct. Another possibility that Landsberg discussed is that supersymmetry is discovered at the LHC. Then, in the MSSM there are three neutral Higgs bosons and two charged Higgs bosons. Two of the neutral Higgs bosons have positive parity and one has negative parity, and therefore is a pseudoscalar boson.

  At the coffee break, I talked to Joe Incandela and stressed that, before it is claimed that the new boson is a standard-model Higgs, it is critical to determine its parity, which will indicate whether it is scalar or pseudoscalar. He agreed, and repeated Landsberg’s claim that they expect to have enough data at the end of the year to determine the parity of the new boson to within 3 sigma or standard deviations. He did say they had a result already that favored a scalar boson. However, the result was only 1 sigma or less in statistical sensitivity, and therefore it was obviously inconclusive.

  Near the end of the coffee break, I talked to Greg Landsberg. Again I stressed the importance of determining the parity of the new boson. He repeated his assertion that they would know the parity within three standard deviations by the end of the year when they reach a luminosity of 30 inverse femtobarns. He did say that it would not be easy for them to determine the spin of the boson using the data accumulated by the end of the year. We agreed that the most likely spin of the boson would be spin 0, because t
he boson is observed to decay into two photons. The alternative—that it is a spin-2 boson with positive parity, which can decay into two photons—was not a likely interpretation of the new boson because it would not be easy to support this by the data. I told him I was happy to see experimentalists at CERN were being cautious about claiming the new boson is the standard-model Higgs. I said this did not seem to be the case with the majority of particle theorists, who seem to have decided it is the Higgs boson, without further experimental investigation of its properties. He laughed, and said, “The theorists already decided it was the Higgs boson before we had discovered anything!”

  After the coffee break, there was another introductory talk by Marcus Klute, an experimentalist from the Massachusetts Institute of Technology (MIT). The first slide of his talk showed a Quotation from the director-general of CERN, Rolf-Dieter Heuer, at the July 4 announcement: “We have it!” Klute added to the quote: “We do?” He is a member of the CMS group that discovered the new boson in the diphoton decay channel. He reviewed in detail the experimental findings related to the two golden channels. Klute’s first slide indicated again that the CERN experimentalists were being cautious about the claim that the standard-model Higgs boson had been discovered.