One interesting result that has been presented at the La Thuile/Moriond meeting is a new measurement of the W mass by the CDF and D0 detectors at the Tevatron machine. This remarkable measurement determines the W mass to an accuracy of 0.02 percent. This, combined with the accuracy of the top quark mass measurement, can limit the mass of a potential Higgs boson, because the formula for the radiative or quantum field theory correction to the Higgs boson mass is sensitive to the W mass. According to a formula derived originally by Martinus Veltman during the early 1970s, there is a logarithmic dependence on the ratio of the Higgs mass to the W mass in the quantum radiative correction. With the new, very accurate measurement of the W mass, the best fit to the Higgs mass is 94 GeV, with an error of +29 GeV or −24 GeV. If this result is taken seriously, a Higgs boson at 125 to 126 GeV would be excluded. However, this result has a statistical significance of only one standard deviation, allowing for a possible two-standard deviation 125-GeV Higgs boson.
JULY 3, 2012
At this time, excitement about a potential discovery of the Higgs boson at the LHC has reached fever pitch. Both the ATLAS and CMS groups will announce the latest results of their 2012 runs at the big high-energy physics conference in Melbourne, Australia, on July 9. Before those talks, CERN will hold a press conference on the morning of July 4, tomorrow, which is Independence Day in the United States. CERN wants any announcement of the discovery of the Higgs boson to take place at its own laboratory.
The speculations about the LHC announcement are rampant; the blogo-sphere churns with unsubstantiated rumors. The Associated Press published a leaked statement today with the headline “Proof of the Existence of the God Particle.” Hours later this title was changed to “Evidence of the Existence of the God Particle.” This is perhaps the first time in the history of science that a major news agency corrected the headline of a scientific discovery. The reason is that the experimentalists at CERN are being cautious.
Several days ago, Peter Woit, on his respected blog “Not Even Wrong,” quoted a leak from the CMS group saying that they had discovered the Higgs boson. This alerted the ATLAS group to the CMS results before the two groups communicated their final results to one another before the July 4 announcement. This blog leak was not taken kindly by physicists at the LHC, because it allowed potential bias to enter into the analysis of the data. Indeed, according to accounts, the two groups were still analyzing the data up to two or three days before the July 4 announcement.
There is good reason for the experimentalists to be cautious in their announcements. In September 2011, the CERN Oscillation Project with Emulsion-Tracking Apparatus (or OPERA) group announced that they had experimental evidence for neutrinos moving faster than the speed of light. However, in May 2012, that was proved to be incorrect; the dramatic claim turned out to be the result, in part, of a faulty fiber optic cable connection to their GPS receiver. Going back in history to another Independence Day, July 4, 1984, as we recall, Carlo Rubbia, who had discovered the W and Z bosons experimentally the year before, made a special announcement from CERN saying that he had discovered the top quark at about an energy of 40 GeV. This also turned out to be false. Much later, in 1995, the top quark was discovered at Fermilab at about 173 to 175 GeV. Moreover, in 2010, experimentalists at Fermilab announced the discovery of a new particle in the W boson two-jet channel at the CDF that was later invalidated by the D0 detector at Fermilab. This announcement produced predictably another flood of theoretical papers identifying the new particle in some speculative beyond-the-standard model prediction. The CERN group understandably wants to avoid a similar embarrassing situation with the Higgs boson.
An article in Nature by journalist Matthew Chalmers published July 2, 20122 claimed, on the basis of unidentified sources at CERN, that a new particle had indeed been observed at about 125 GeV, in the decay of this particle into two photons. The claim was that the signal was now close to the 5 sigma, or five standard deviations, necessary for an announcement of the discovery of a particle resonance. But Chalmers cautioned in his article that the CERN experimentalists still had to prove that what they were seeing was a Higgs boson and not an impostor—a new particle that no one had thought of before. Chalmers wrote, “Even as rumors fly in the popular media, physicists have begun quietly cheering at CERN…. ‘Without a doubt, we have a discovery,’ says one member of the team working on the ATLAS experiment, who wished to remain anonymous. ‘It is pure elation!’” As I described in a paper on the electronic archive,3 it is necessary to determine not only the decay products of this putative new particle, but also its spin and parity before declaring it is a Higgs boson. This is not an easy task, and it could take time to resolve. The predictions of the standard-model Higgs boson decay products and the size of the signal observed at the ATLAS and CMS detectors are fairly precise. Rumors are spreading that the observed signal in the two-photon decay channel is about two times bigger than it should be, according to the standard-model predictions.
Despite some cautionary statements about declaring the identity of the resonance bump at 125 GeV, other physics bloggers have already announced that the Higgs boson—or the God particle—has been discovered. However, they have begun to temper their statements as the CERN press conference approaches by saying that there is “possible” evidence for the Higgs boson. In other words, they are hedging their bets. Any bottles of champagne uncorked on the fourth of July at CERN could potentially turn into vinegar.
Peter Higgs is being flown to Geneva to be present at the press conference. The media will be at CERN en masse. North American enthusiasts who want to view what could be a truly historical moment in the history of science must be awake at 3:00 a.m. to view the announcement live on the webcast from CERN. I will not set my alarm for 2:30 a.m., but I plan to watch a repeat later during the day. I predict that the CMS and ATLAS detectors will indeed have evidence of increased strength in the diphoton signal already glimpsed in the 2011 data. However, because of the lack of significant evidence in the other decay channels, such as the WW, tau+–tau-, and b-bar-b channels, I believe they will not announce the actual discovery of the Higgs boson, but will say only that there is strong evidence for its existence.
One of the significant predictions of the Higgs boson is that its coupling strength to particles such as the W and Z bosons and quarks and leptons is proportional to their masses. Therefore, its coupling strength to the W boson must be observed as a significant enhancement of a signal of the Higgs decaying into W bosons at about 125 GeV. So far, there have been fewer events than expected in this decay channel observed at both the Tevatron and the LHC. Not to be left out of the excitement about the discovery of the God particle, the Tevatron group posted a long announcement online yesterday, July 2nd, claiming to see a broad enhancement of about 3 sigma of a Higgs boson decaying into bottom–antibottom quarks at about 125 GeV. Puzzlingly, however, the Tevatron group also sees a broad enhancement in this same decay channel above 135 GeV. Again, this discrepancy in energy levels raises the concern that these enhancements may be the result of statistical fluctuations in the data and problems with calibrating the large backgrounds. Also in its online announcement, the Tevatron group states they see a lack of evidence for a Higgs signal in the WW decay channel. These announcements have been made on the basis of further analysis of the Tevatron data, up to an integrated luminosity of about 10 inverse femtobarns, after the machine was shut down in September 2011.
JULY 4, 2012 (INDEPENDENCE DAY)
I wake up at 2:30 in the morning without any help from my alarm clock. Despite my drowsy state, I decide to watch the live video from Geneva of the scientific seminar with the two talks announcing new results in the search for the Higgs boson. The video streaming from Geneva is of excellent quality. Before the start of the conference, the cameras pan the main auditorium at CERN, which is packed with dignitaries. Evidently younger physicists have been standing outside all night, attempting to get there early in the morning to find a seat. At one point,
Peter Higgs enters the auditorium and greets Françoise Englert. They both look ebullient, anticipating the new results. The director-general of CERN, Rolf-Dieter Heuer, makes a brief announcement and then introduces the first speaker, Joe Incandela, who describes the CMS results.
Incandela seems quite excited and nervous. After a preliminary discussion of the workings of the CMS detector, he moves on to the results of the 2012 runs. These data result from a beam energy of 7 TeV and 8 TeV, and an integrated luminosity of about 5 inverse femtobarns, which is close to the beam intensity or luminosity of 4.7 inverse femtobarns from the 2011 experimental runs. He claims that combining the new 2012 data with the 2011 data increases the sensitivity for the Higgs search by about 20 percent. Combining the two years of data effectively doubles the amount of available data. Incandela concentrates on the decay channel of the Higgs decaying to two photons, which, except for the problem of the photon background, provides the cleanest data because of the little or no hadronic background to be concerned about. He discusses the data from other decay channels, in particular concentrating on the decay of the Higgs into two Z bosons, each of which then decays into a lepton and an anti-lepton. The signal to look for in that channel is four leptons, such as electrons and muons, coming from the neutral Z bosons. The signal for this decay channel does not show significant excess of events, but the result is still consistent with a Higgs boson.
For the “golden” diphoton decay channel, Incandela announces they measured the mass of the resonance bump as 125 ± 0.6 GeV. He says that, in this channel, the data reached a statistical significance of just above 4 sigma, whereas for the two Zs decaying into four leptons, the statistical significance was just above 3 sigma, less than the gold standard of 5 sigma. By combining the decay events in these two golden channels, Incandela states dramatically, they have reached the magic 5-sigma statistical significance. This warrants the claim that they have discovered a new boson. At this statement, the audience applauds loudly.
The next speaker is Fabiola Gianotti from the ATLAS group. She goes straight to the issue of how they improved the data analysis techniques, and then comments on the amazing work done by the worldwide computer grid that has been analyzing the data for only a few weeks, with a cutoff of June 15. It is amazing how the many hundreds of analysts managed to crunch through the huge volume of numbers to produce the remarkable result that she was about to announce. She, too, concentrates in her talk on the two golden channels: the Higgs decay into two photons and its decay into two Zs and then four leptons. Working through slide after slide of detailed analyses, showing how they separated background noise from the signal, Gianotti finally announces that with the combined data from 2011 and 2012 for the two golden decay channels, they have a 5-sigma or five standard deviation confirmation of a new particle at a mass of 126.7 GeV. The five standard deviations means there is only about a one in two million chance for this result to be a statistical fluctuation. This is a significant increase from the 2011 runs, which had only reached about 3 sigma. Again, when this slide is shown, the audience erupts into boisterous applause.
Watching the slides appear one by one on my computer, I take note of three facts, emphasized by Gianotti. The first is the signal strength of the new boson’s decay into two photons. This is measured by multiplying the total production cross-section of the new boson by the predicted branching ratio of the Higgs decay channel producing two photons, which turns out to be about two times larger than predicted by the standard Higgs boson model.4 If this discrepancy continues to hold up, then the LHC experimentalists will have to question whether this new particle is a standard Higgs boson, some variant of the Higgs boson, or a new kind of particle that has not yet been observed experimentally. However, Gianotti avoids commenting on this issue because her role at this stage is just to provide the experimental results, not to give theoretical interpretations.
The second issue I note is that the measured mass of the new particle at the CMS is different from the one at ATLAS by 1.5 GeV, which could be a significant difference. This was a problem with the results of the 2011 experimental data as well, and it was hoped that this mass difference would go away with the new results. In my opinion, this difference could just be the result of experimental error or statistical variance, and it will likely disappear with new data.
The third issue I note is that of the other decay channels; the Higgs decay into two tau leptons shows a deficit of events at 125 GeV—not a spike in the data, but a dip. This result, too, is not consistent with the standard-model Higgs boson predictions.
As I listen to Incandela and Gianotti make their announcements, I think about the next steps to come. To establish that this new particle is the Higgs boson, experimentalists will now have to determine the quantum numbers of the particle, such as the spin and parity, and attempt to obtain a precise determination of all the branching ratios for the other decay channels, in addition to those for the golden channels. When these experimental results are finalized, only then can we say definitely that the particle is a Higgs boson.
Recall that the standard-model Higgs boson is an elementary scalar particle with spin 0 and positive parity. Until now, such a particle has never been observed experimentally. However, there have been observations of mesons with intrinsic quark spin 1 for the quark–antiquark bound state, with the quark spins aligned parallel to one another, and with an orbital angular momentum of 1, which look like a resonance with spin 0 and positive parity. Such a scalar particle is composed of a quark and an antiquark, whereas the standard-model Higgs boson is an elementary particle with no constituent elements inside of it. The quantum numbers S (spin) and L (angular momentum) for quark–antiquark mesons are important to determine experimentally. What is remarkable is that, if indeed these new results from the LHC prove to be a Higgs boson with the quantum numbers of the vacuum or ground state, then this will be the first such particle ever observed in nature. It will be in a family of one.
The final comments by Heuer after the talks do not go quite so far as to say that they have definitely discovered the Higgs boson. He says, “As a layman, I would now say, ‘I think we have it.’ Do you agree?” The receptive audience again erupts with laughter and applause.
After almost 50 years of particle physics with a hypothesized Higgs boson, and the efforts over 40 years by many thousands of experimentalists to discover this elusive particle at the accelerators, it is clear that the audience is biased heavily toward believing this resonance bump is indeed the Higgs boson. If so, this would be a big coup for the LHC and CERN, and would seem to justify the $9 billion spent on the experiment so far.
JULY 9, 2012
After the televised scientific seminar, there was a press conference at CERN, with a panel of experimentalists moderated by the director-general, Rolf-Dieter Heuer. There was a large collection of international journalists from all the prominent newspapers, television stations, science journals, and magazines. Peter Higgs entered the auditorium for the press conference surrounded by photojournalists flashing their cameras. Amusingly, he himself was a caricature of the most popular description of how the Higgs boson gives mass to other particles. As soon as he entered the room, he attracted a crowd of journalists and physicists and, moving slowly along, seemed to impart mass to them. He soon joined Françoise Englert in the audience.
In response to many questions from the journalists about whether the new boson that had been discovered was the Higgs boson, the experimentalists on the panel were reticent about saying this was the case. Eventually, after the cool responses of the experimentalists to this question, Englert raised his hand and asked how long it would take them to make the decision that it was really the scalar particle that they were looking for, that fitted into the standard model. Fabiola Gianotti, who appeared to be an objective and level-headed experimentalist, said that it could take several years before a final decision was reached. Heuer echoed that it could take three to four years to confirm whether it was the Higgs boson. He then said i
n an off-hand way that it would not be decided by the end of 2012. This meant we would have to wait until 2015 when the LHC would start up again at 13 TeV. However, he also stated that they would continue running the machine 2 to 3 months longer than planned during this run to collect as much data as possible.
Despite the cautions from Heuer and the members of the panel, others present at the press conference had no qualms about expressing their enthusiasm:
Michel Spiro, who is president of the CERN Council, said, “If I may say so, it [the discovery] is another giant leap for mankind.”
Françoise Englert declared, “I am extraordinarily impressed by what you have done.”
Gerald Guralnik enthused, “It is wonderful to be at a physics event where there is applause like there is at a football game.”
Peter Higgs mused, “It is an incredible thing that it has happened in my lifetime.”
Getting into the spirit of things, Heuer concluded, “Everybody that was involved in the project can be proud of this day. Enjoy it!”
Almost immediately, and ignoring the more cautious stance of the experimentalists, the blogosphere went hysterical about the “discovery” of the Higgs boson. Even at my own institute, the claim was emblazoned on our home page: “The Higgs boson, sought for decades, has been discovered. What does that mean and where do we go from here?” Needless to say, I felt disturbed that the objectivity of the physics had been subverted, because according to the experimentalists at CERN, we could not yet say definitively that the Higgs boson had been discovered. Yet the physics blogs were already speculating about who should get the Nobel Prize and when. Indeed, at “Not Even Wrong,” Peter Woit proposed that the prize should be given to the experimentalists first, shared among the CMS and ATLAS groups (a total of about 6,000 physicists!). Indeed, in Woit’s blog he discussed the possibility that the experimentalists should receive the prize this coming October, only a few months after the discovery of the as-yet unidentified new boson. However, nominations for the prize would have to have been submitted the previous January, to follow the conventional Nobel procedure.
Cracking the Particle Code of the Universe Page 21
