Now I felt that I had arrived at a satisfactory electroweak theory without a Higgs boson. Within the local quantum field theory formalism on which it was based, it would prove to be self-consistent and agree with accurate electroweak experiments.
However, as experimental results were being accumulated at the LHC, it was becoming clear that evidence was beginning to confirm that a Higgs-like boson had been observed at the CMS and ATLAS detectors. A confirmation of the existence of the standard-model Higgs boson would obviously nullify the need for a model without the Higgs boson. However, the theoretical attempts to find a non-Higgs boson electroweak model produced a “null hypothesis” against which the experimental evidence in search of a Higgs boson could be compared.
It is of historical interest that after Salam had proposed a solution to electroweak theory in his paper of 1962, he inserted a footnote explaining that, in a collaboration with Weinberg and Jeffrey Goldstone, they had demonstrated that spontaneous symmetry breaking would produce massless Goldstone bosons, which would prevent a successful completion of electroweak theory. The issue of the massless Goldstone bosons, which cannot exist in nature, had triggered a hiatus in the search for an electroweak theory. Then, in 1964, the Group of Six proposed that gauge bosons such as the W and Z “ate” the massless Goldstone bosons, producing massive gauge bosons. This came to be understood as the Higgs mechanism. It is interesting that Salam got caught up in the spontaneous symmetry-breaking fever and abandoned his approach of 1962, adopting instead an electroweak theory with a Higgs boson, which, as we recall, he published in 1968 independently of Weinberg’s paper a year earlier.
CONFRONTING THEORIES WITH DATA
Developments in physics often have sociological implications. People devote their entire careers to certain speculative paradigms, only to discover when the experimental data come in that they were wrong from the beginning. Right now, particle physicists are being confronted by data from the LHC, which is, in effect, a killing machine. For example, the whole supersymmetry paradigm appears to be on life support now, with the onslaught of new data coming out of the LHC.
As I often remind my physics colleagues, an elementary scalar particle with spin 0 and positive parity has never been detected experimentally since the construction of the first accelerators during the early 1930s. Possibly the reason is that an electrically neutral spin-0 scalar particle with positive parity (and positive charge conjugation) such as the standard-model Higgs boson has the quantum numbers of the vacuum state. All the other flavor quantum numbers, such as strangeness and charm, are zero in both the vacuum and for the Higgs boson. No other observed elementary particle has all these particular quantum numbers of the vacuum. This implies that the Higgs boson is closely related to the properties of the vacuum, such as vacuum fluctuations.
In the observed meson spectroscopy of particle resonances, resonances with spin 0 and positive parity (as well as negative parity) are composed of quark and antiquark states, and thus, in contrast to the standard-model Higgs boson, they are not elementary particles. Elementary scalar particles just seem not to exist in nature. The pi meson is a pseudoscalar particle; that is, it is a scalar spin-0 particle with negative parity. It is not an elementary particle, but a composite made up of a quark and an antiquark. It is amazing that so much of modern physics is based on the idea of an elementary scalar particle (and field) even though such a particle has never been seen, until perhaps now.
Another obvious example of an elementary scalar particle occurs in inflation theory, the most popular theory explaining how to resolve the initial value problems in the early universe in the Big Bang model. This theory has gained enormous popularity since 1981. Yet, it is difficult, if not impossible, to develop a successful inflation cosmology scenario without one of these never-detected scalar particles—in this case, the inflaton. Attempts have been made to use spin-1 particles that do exist experimentally, such as the W and Z bosons and the photon, to develop inflation models, but they have not been successful. One of the most remarkable things about modern physics is the way theoretical physicists often turn a blind eye to experimental facts, such as the experimental exclusion of constrained models of supersymmetry. In a sense, they are busy digging their own graves, and eventually they may find themselves buried with a headstone on which is inscribed: THE ELEMENTARY SCALAR PARTICLE, RIP. On the other hand, if the higher-energy LHC does confirm the discovery of the standard-model Higgs boson, then this would be the first time such a particle has been shown to exist in nature, and it would truly be something to celebrate.
A SCALAR HIGGS BOSON VERSUS A PSEUDOSCALAR MESON
Let us consider some of the prominent features of the standard-model Higgs boson. The Higgs boson couples to other elementary particles proportional to their masses. Therefore, experimentalists have to show that the new boson decays into the most massive spin-1 heavy bosons, which are the W and the Z bosons. Indeed, up until March 2013, the experimentalists at the LHC have apparently shown the decay of the new boson into Ws and Zs, albeit one of the pairs of W and Z bosons is a virtual particle, and the other a real particle, and both subsequently decay into real leptons.
Because the boson that carries the electromagnetic force, the photon, has zero mass, the Higgs boson cannot decay directly into two photons. It must decay through the intermediary of two Ws and a top and antitop quark. The probability for this decay to occur is only about 0.3 percent, yet the CMS and ATLAS groups have detected the two-photon decay, which is one of the golden decay channels.
In addition, the new boson should not be seen to decay into very light fermions such as positrons and electrons. However, it should be observed to decay into much heavier leptons—namely, the tau+–tau-, because the tau lepton has a mass of about 1.8 GeV. So far, the decay into tau+–tau- has not been substantiated convincingly by the LHC.
The most dominant decay of the Higgs boson into fermions and antifermions is the decay into a bottom quark and an antibottom quark, because the bottom quark has a mass of about 4.5 GeV. The experimental results up through January 2013 do not confirm that the new boson decays into bottom and anti-bottom quarks. The new boson cannot be detected to decay into the heaviest quark and antiquark pair, the top and antitop quarks, because the top quark has a mass of 173 GeV, and therefore a pair of top quarks would have a mass well above the mass of the new boson at 125 GeV.
My quarkonium model predicts the existence of a resonance called the zeta meson at 125 GeV with spin 0 and negative parity, which must be a pseudoscalar meson. The second, and heavier, resonance in my model is the zeta prime boson, which has a mass of 230 GeV and mixes with the lighter zeta through an angle of 36 degrees. The masses of the zeta and zeta prime bosons are determined by a mixing of the known bottomonium and toponium energy eigenstates. I emphasize that my zeta quarkonium is in no way a Higgs boson or a pseudoscalar Higgs boson. It is quite a different animal.
Let us now compare the predictions of my quarkonium model with the predictions of the Higgs boson model. The ground state decay of the zeta resonance, which is a composite of quarks and antiquarks, is a spin-0 boson with negative parity. That is, it is a pseudoscalar boson. In contrast to the elementary Higgs boson, which decays into two photons through the mediation of a decay into two top/antitop quarks or a W+/W- loop, my bound state quarkonium, which is not an elementary particle, decays directly into two photons. We recall that a calculation of its decay strength is consistent with the latest observational data and comparable with the prediction of the Higgs boson decay into two photons. Moreover, a calculation of the decay of the zeta boson into a pair of Z bosons, one of which is a virtual Z, yields a result comparable with the Higgs boson prediction. The same is true of the decay channel of the zeta boson into a pair of W bosons, of which one W is a virtual boson. However, the calculated decays of the zeta boson into tau+–tau- leptons, bottom and antibottom quarks, and charm and anticharm quarks are suppressed, or much less than the decay of the Higgs boson into these particles. The
refore, an important prediction of my model is that the experimentalists should not observe a strong signal of the decay of the new boson into fermion/antifermion pairs, which up until now appears to be the case.
A critical problem faced by the CERN experimentalists is to determine the quantum numbers of the resonance bump they have seen at 125 GeV. Primarily, they have to confirm that the particle has zero spin and positive parity. There are only two possibilities for the spin, for a spin-1 particle cannot decay into two photons and conserve spin, so that the particle has to have either spin 0 or spin 2. Technically speaking, the experimentalists have to determine the angular distribution of the two photons in the decay process: H to two photons or H to ZZ* to four leptons. This is not an easy task, and requires a significant number of particle events to decide the issue.
The data analysis performed up until March 2013 compares the standard Higgs boson scalar model with a pseudoscalar Higgs boson model. This latter model, which I emphasize is not the same particle as my pseudoscalar zeta meson, does not follow directly from the fundamental standard Higgs boson gauge theory. In addition, this effective pseudoscalar Higgs model gives a very suppressed decay into a pair of Z bosons or W bosons, compared with the standard-model Higgs boson. Such an effective pseudoscalar boson model, in contrast to the standard Higgs boson model, is not renormalizable and therefore leads to unwanted infinities in calculations of amplitudes and cross-sections.
The heavy, composite quarkonium model of a 125-GeV pseudoscalar resonance acts as a non-Higgs boson “null” hypothesis for the experimental determination of the spin and parity of the new boson. In particular, the confirmation of the parity of the 125-GeV boson—which determines whether it is a scalar boson or a pseudoscalar boson—is of critical importance in confirming that it is the standard-model Higgs boson.
One of the most difficult problems faced by the CMS and ATLAS collaborations is the analysis of the immense amount of data produced by the proton–proton collisions. The algorithms used to analyze the data have to be able to distinguish between a real signal and the background. From rumors, one gathers that the results of the analysis of the fermion/antifermion decays of the new boson, such as the tau+–tau- decays, have undergone significant changes since July 2012. One hopes that the analysts are not falling into the psychological trap of simply “seeing” in the data what the Higgs boson predictions require. However, every effort is made by the LHC experimental analysts not to fall into this trap.
As I often say, physics is a brutal business. No mercy is shown to our theories by experimental apparatus such as the LHC high-energy accelerator, which is the way it should be. We are trying to discover how nature works, and nature is indifferent to our ideas and our imagined scenarios of how it behaves. Nature has its own rules, and our goal should be to try to understand nature’s laws, which make our universe what it is.
9
The Discovery of a New Boson: Is It the Higgs or Not?
MARCH 2012
The latest developments in the saga of the Higgs search are occurring at the Rencontre de Moriond meeting at the ski resort La Thuile, in the Aosta Valley, Italy, from March 3 to 10, 2012. These annual meetings, organized by the French National Institute of Nuclear and Particle Physics, have been held for 30 years or more. Their purpose is to present the results of high-energy experimental and theoretical investigations from the previous year. The Alpine town of La Thuile is famous for its winter skiing and summer hiking. Subtracting the large number of tourists, and physicists turning up from many different countries to attend the Moriond meetings, the small town has a permanent population of only about 800. The meetings are usually organized so that the physicists can ski early in the day and then attend talks in the late afternoon, which can continue past 7:00 in the evenings. I am not attending this particular meeting, but the slides of the talks are released electronically shortly thereafter.
According to results presented at the Moriond meeting, new analyses of the 2011 data by the CMS and ATLAS collaborations at the LHC have sharpened the standard-model Higgs boson search, but nothing seems dramatically different from the presentation at the CERN press conference in December 2011, when there were “hints” of the discovery of a new boson. On the other hand, at Moriond there are new, interesting results from the Tevatron group at Fermilab. Even though the Tevatron machine shut down in September 2011, the experimental physicists working with the accelerator have been completing the analyses of their 2011 data.
Searching for the Higgs boson consists of tracking down possible decay products of the Higgs particle in the proton–proton collisions in the LHC and the proton–antiproton collisions at the Tevatron. The two so-called golden channels are the decay of the Higgs into two photons and its decay into two Z bosons and then into four leptons. The other, dirtier channels that are difficult to interpret because of background noise are the Higgs decaying into bottom–antibottom (b-bar-b) quarks, charm–anticharm (c-bar-c) quarks, and into two tau leptons (tau–antitau pair). In practice, the probability for detecting the charm–anticharm decay is small because the charm quark is lighter than the bottom quark. The coupling of the Higgs boson to a fermion is proportional to the fermion’s mass, so the lighter the mass, the smaller the coupling between the Higgs boson and the fermion, and the more difficult the detection of this decay. Therefore, experimentalists concentrate on detecting the b-bar-b and tau–antitau decay channels.
A significant fact has emerged at this Moriond meeting: The ATLAS and CMS detectors do not see any excess of events in the low-energy mass range (between 115 GeV and 140 GeV) for the Higgs decay in the difficult fermion channels. However, it is important, in the end, to verify that the Higgs boson is seen in these channels because they are the dominant decay channels compared with the golden channels. So far, the LHC results have shown only an excess of events—the so-called hints of the Higgs boson—around 125 GeV in the golden channels.
From the theoretical standard-model calculations, we know that the Higgs boson should decay through all the channels that are being investigated in the LHC and Tevatron detectors. Many people have felt that the Tevatron might have a better chance than the LHC of seeing the Higgs boson in its low-mass range for the difficult and dominant decay channel, the Higgs decaying into the bottom and antibottom quarks.1 The results being presented at the Moriond meeting by the Tevatron groups show a broad excess of events between 115 GeV and 135 GeV, consistent with a Higgs boson around 125 GeV, as already revealed by the CMS and ATLAS data for the two-photon decay channel. However, this excess being observed at the Tevatron accelerator is only about 2 sigma in statistical significance, which is not strong enough to confirm the existence of a Higgs boson in this mass range. In particular, the Tevatron representative said at the meeting, the data do not show a “peak” in the low-mass range that would correspond to a Higgs resonance. The lack of events in the difficult Higgs decay channels at the CMS and ATLAS detectors does not support the overall evidence for a low-mass Higgs boson. In particular, when the ATLAS group showed a combined plot for all channels, the evidence for a Higgs boson dropped from about 3 sigma to 2 sigma. Indeed, the results from the ATLAS group are a bit of a shock. The CMS collaboration already reported a small excess for the two difficult channels (bottom–antibottom and tau–tau)—namely, a one standard deviation—which is indeed small. The reporting by the ATLAS group that they see essentially no excess in either of these channels, and even a deficit in the tau–tau channel, is not good news for the Higgs hunters.
At this point, the ATLAS group results exclude a standard-model Higgs boson in the range of 114.4 GeV (the old LEP bound) to 122 GeV, except for a small window of about 1 GeV wide, centered at 118 GeV. The mass range from 129 GeV all the way up to 600 GeV has now been excluded by the ATLAS and CMS combined data. The only window left for the standard-model Higgs boson to exist is between 122 GeV and 129 GeV, more or less centered around the hinted mass value of 125 to 126 GeV that was last seen in the December 2011 data. The b
ottom line is that the December hints of a Higgs boson at around 125 to 126 GeV have, from the new data analysis of the difficult fermion–antifermion decay channels, dropped by a full standard deviation or sigma to about 2 sigma.
This means that by now, March 2012, the Higgs search has reached a pause. We have received some positive information and some negative information. However, it appears that we no longer have to worry about a Higgs boson below 122 GeV.
One worrying aspect in the data analysis is that the hints of a Higgs boson at around 125 GeV as reported in December 2011 are only based on 20 percent of the data having been analyzed. A skeptic might be led to believe that these hints of a Higgs boson at 125 GeV in the two golden channels are purely a statistical fluke. The new data analysis presented at this Moriond meeting, which has somewhat weakened the evidence for a low-mass Higgs boson, only strengthens this skepticism.
There has been a great deal of discussion about the Higgs boson in the media and on popular physics blogs. One blog is run by Matt Strassler, a theoretical physicist at Rutgers University. He has expressed a healthy skepticism about whether the standard-model Higgs boson has been confirmed definitely to exist. Many in the media, including blogs, are showing anger and resentment toward any skepticism about the so-called 125-GeV Higgs boson discovery. Indeed, papers are now appearing on the electronic archive (arXiv.org) discussing the Higgs boson at 125 GeV as if the LHC data have already established its existence beyond doubt.
As scientists, we have to strive to make convincing statements on the basis of the data as they appear, rather than on the basis of our emotional beliefs. Of course we hope that the Higgs boson’s existence, or nonexistence, will be revealed by new data emerging during the running of the LHC and, indeed, that by the end of the year enough data will have been gathered at a high enough luminosity to establish the existence of the Higgs boson or exclude it. There has even been skepticism about this being the case. Unfortunately, it may take a lot more data to confirm the existence of the Higgs boson through direct searches at the LHC; the background problems must be removed convincingly and the gold-plated 5-sigma excess of events showing a definite peak at the Higgs mass must be achieved.
Cracking the Particle Code of the Universe Page 20
