If you haven’t already, I highly recommend you read The Wire‘s guide that will teach you all you need to know to get started on particle physics (and which is free as in beer) – although you can still read the article below.
The world’s largest science experiment, the Large Hadron Collider (LHC), sits in a 27-km-long tunnel about 150 feet below the border between France and Switzerland. Despite the experiment’s size, the tunnel itself wasn’t built for the LHC, though it may have been modified for it. There was a previous occupant: the Large Electron-Positron collider (LEPc), which operated from 1989 to 2000 in two phases, I and II. In both phases, the LEPc accelerated and then smashed together electrons with their antiparticles, the positrons. LEP I began to operate at an energy of about 45 GeV and, by 2000, LEP II had reached 209 GeV. However, because an energy of at least 216 GeV had to be reached to produce Higgs bosons, LEP II was dismantled to make way for the LHC.
Earlier this year, a physicist named Arno Heister revisited the archives of the data collected by the LEPc, presumably in the search for phenomena called ‘hidden valleys’. Because of the large number of particle-particle collisions generated by colliders, physicists can’t follow up each collision to see what happens. Instead, they train themselves – and their computers – to stay on the lookout for interesting phenomena that theoretical physicists have already predicted. ‘Hidden valleys’ are phenomena that haven’t been predicted to exist in much detail but could if only we went groping in the dark. And because of the way ‘hidden valley’ particles are thought to interact with other, known particles, proponents believe that new techniques of detection will have to be devised to properly look for them. And in the archives, Heister found some feeble data that seemed to suggest the presence of a hitherto unobserved particle.
Heister, however, is cautious and with good reason. Late last year, a hint of a possibly new particle in one of the LHC’s detectors’ datasets had physicists in a tizzy. But after a few months of detailed analysis, the data failed to show any signs of new particles, leaving the research community in a state one blogger called a “hangover”. So, in a paper uploaded to the arXiv preprint server, Heister only characterises the finding as the “observation of an excess” – referring to the way particles show up on experimental observations, as small and distinctive bumps on plotlines.
An untouched realm?
In its lifetime, the LEPc produced over 16 million particles called Z bosons. Together with the W+ and W- bosons, the Z boson mediates the weak nuclear force, a natural force infamous for its hand in radioactivity. Heister was able to access data from the detector called ALEPH, one of the four that straddled the LEPc ring and observed collisions. ALEPH was adept at observing how Z bosons decayed. And Heister saw that in one of the plots, the Z boson may have decayed to one or two intermediary particles that then decayed to a muon and an antimuon or an electron and a positron – in both cases alongside a quark and an antiquark as well. Muons are exactly like electrons, except 200-times heavier. The existence of these intermediary particles is not predicted by the Standard Model, a group of theories in particle physics that physicists are trying to disprove.
On October 20, Heister uploaded a (non-peer-reviewed) paper to the arXiv preprint server describing the context in which the data was obtained and if a particle could exist in it. According to him, the muon-antimuon pairs were being found at an energy of 30.4 GeV at a significance of 2.63 sigma (σ). This means that whatever was producing the pairs weighed about 30.4 GeV. However, the statistical significance is low, well below the 3σ threshold that signifies that a detection has a 1-in-100,000 chance of being wrong and is therefore equivalent to evidence; anything lower has a higher chance of being a fluke. Moreover, the width of the bump in the plot is 1.78 GeV, which it means it is more like a gentle upheaval than the pronounced spike typical of a real particle.
Nonetheless, some physicists have looked at Heister’s finding with hope because it revitalises the cause of hidden valleys, which they have claimed have been ignored for far too long. Matthew Strassler, a theoretical physicist, wrote, “It is intriguing that the bump in the plot … is observed in events with bottom quarks. It is common for hidden valleys to contain at least one spin-one particle … and at least one spin-zero particle.” A particle’s ‘spin’ is the value of a quantum mechanical number ascribed to the particle. Spin-zero particles are called scalar bosons, which, in Heister’s data, could have decayed into the quark-antiquark pair; and spin-one particles are called vector bosons, which could have decayed into either muon-antimuon or electron-positron pairs.
Strassler adds that it would have been wiser if ALEPH and other detectors’ data had been revisited from the late 2000s, when physicists really started to become concerned about hidden valleys – or if the LHC’s detectors had looked for them from when they began to operate in 2009. The LHC produces hundreds of times more Z bosons than the LEPc ever could. “But despite specific proposals for what to look for (and a decade of pleading), only a few limited searches have been carried out, mostly for very long-lived particles, for particles with mass of a few [GeV] or less, and for particles produced in unexpected Higgs decays. And that means that, yes, hidden physics could certainly still be found in old ALEPH data, and in other old experiments,” he speculated, adding that “searches at the LHC are far from complete, and that discoveries might lie hidden, for example in rare Z decays…”
Questioning the valleys
However, Tommaso Dorigo, a physicist who works with one of the detectors at the LHC, remained more skeptical of Heister’s claims. According to him, the ALEPH detector has had a record of finding strange phenomena that turned out to be not so strange. Dorigo also questioned Heister’s not having discussed any data from the LEP II phase in support – nor data between 1990 and 1992, as an ALEPH member has pointed out. “One thing I found strange is that the paper only discusses 91-GeV data, i.e. data collected by LEP, not LEP II. One would think that the higher … energy would help in the search of heavy particles, but maybe this need not be the case here,” Dorigo wrote on his blog. “Anyway, the analysis is rather thorough, and seems to address many of the questions one would normally ask in these cases.”
Another physicist, Lubos Motl, had different reservations from Dorigo’s but disagreed with Strassler or his hope for hidden valley models to persist. For starters, Motl wrote, “A blogger’s proposed set of new particles may be real, but worries about the blogger’s impartiality may potentially be justified, too” – referring to the fact that Strassler, along with a few other physicists, first conceived of hidden valley models in 2006. A bigger problem was that, from an ontological PoV, there are no reasons for hidden-valley models to exist at all. According to him: “Just to be sure, it’s not wrong or inconsistent … But I don’t feel that they’re needed for anything, that there really exists evidence that they should be right. There’s no strong evidence that they shouldn’t exist, either.” As a result, as far as Motl was concerned, it wasn’t clear what physicists will be able to do if hidden valleys are proven to exist down the road from Heister’s data.
The LEPc did and continues to hold the record for having been the most powerful lepton collider in existence (electrons and muons are both types of leptons). And while it is weird that Heister did not compare LEP I data with that obtained from LEP II, there were three other detectors operating at the same time as ALEPH, and whose data will now be combed for any signs of anomalous decays. As mentioned, the Standard Model does not predict their existence; if they are found to exist many months from now, they will become yet another phenomenon that physicists will use to explore new realms of physics, new landscapes built by the fundamental building blocks of nature.