Anyone desperately waiting to find out whether the particle discovered at the Large Hadron Collider (LHC) last year is indeed the Higgs boson, or something more surprising, will get little relief from the latest painful results.
Announced earlier today at an Italian ski resort, they follow up on one of the most intriguing aspects of the earlier results 鈥� an inconsistency in how the new boson decays into photons. Maddeningly, the new results still aren鈥檛 conclusive on this front. But the results do make several other quirks seem less important, increasing the likelihood of, but still not confirming, a garden-variety Higgs.
When the particle鈥檚 discovery was announced in July 2012, both big detectors at the LHC 鈥� CMS and ATLAS 鈥� reported that it was behaving a little bit strangely.
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The Higgs boson is not detected directly but by the slew of particles that it decays into. One of the most intriguing oddities to surface in 2012 was that the new particle appeared to decay into pairs of photon more often than our current best theory, the standard model, predicts the Higgs should.
Blip or new physics?
Some hoped this oddity might end up offering clues to areas that the standard model is silent on, such as dark matter and why there is an excess of matter over antimatter in the universe. But the effect wasn鈥檛 big enough to determine whether the photon result was a blip or due to new physics.
Since July, many more LHC readings have been collected and analysed, but two subsequent rounds of results in November and December failed to answer the photon question.
The LHC has now shut down for a two-year upgrade, but there is still plenty left to analyse. Earlier today, at the annual in La Thuile, Italy, Fabrice Hubaut reported that ATLAS had reduced the error bars associated with the apparent photon-pair excess.
If that was the only change, it would have made the excess look more real. But frustratingly, with that tightening of the statistics came a drop in the rate of decays to photon pairs, which muddies the waters. In December, ATLAS had the rate at about 1.8 times what is expected, with a possible error of 0.4. Today, with more data at hand, that was reduced to 1.65, with error bars of 0.3.
CMS meanwhile, is keeping mum about the photons. Guillelmo Gomez-Ceballos of the Massachusetts Institute of Technology announced that, despite having more data in hand, the ATLAS rival would not offer an update on the results.
Still under review
, one of the head Higgs-hunters at CMS, told New 杏吧原创 the results were simply not ready, and they weren鈥檛 going to be rushed by conference deadlines. 鈥淲e will only release results if the collaboration believes the analysis is fully ready,鈥� he said, noting it might be as early as next week. 鈥淲e are still in the review process.鈥�
The ATLAS results are allowing some to maintain their hope that the particle thought to be the Higgs is rather different from what the standard model predicts. 鈥淎TLAS鈥檚 error bar is shrinking, and the central value is still high,鈥� says ATLAS鈥檚 Martin White, at the University of Melbourne, Australia. 鈥淪o an optimist would say that over time we鈥檒l see that error bar shrink and we鈥檒l have room for new physics.
鈥淥f course, it remains too early to say,鈥� he says. 鈥淭his makes the lack of a CMS update all the more painful 鈥� I鈥檇 love to see what they get.鈥�
Parity boost
White鈥檚 caution seems warranted. ATLAS also reported on another property of the Higgs-like particle 鈥� its parity, which can be thought of as whether something looks the same when viewed in a mirror.
The Higgs predicted by the standard model has positive parity 鈥� it looks the same in the mirror 鈥� but there was some doubt over the parity of the particle announced in July.
With the latest data, Gomez-Ceballos reports that ATLAS now has 99.6 per cent confidence that it is positive, while CMS reported 98 per cent, upping its previous confidence of 97 per cent.
鈥淭hat really gives us confidence that the particle is going to be the standard-model Higgs or something very similar,鈥� says Raymond Volkas, a physicist from the University of Melbourne in Australia.
Meanwhile, one oddity has bitten the dust at Moriond. In July, the new particle did not seem to be decaying into tau leptons at all 鈥� another decay product predicted by the standard model. In November, CMS and ATLAS still weren鈥檛 able to say for sure that it was decaying into taus, but it was starting to look likely. Today, CMS reported an even stronger tau signal.
Volkas says that physicists and Higgs-watchers may have to prepare themselves for the possibility that the LHC data never establishes whether or not the particle is the Higgs predicted by the standard model. It may simply not have the sensitivity. 鈥淎t some point we鈥檒l be able to say it looks like the Higgs boson at the 80 per cent [confidence] level,鈥� Volkas says. That leaves open the possibility that more precise measurements will reveal the particle to be something different, but for now 鈥渋t is still pretty close to behaving as a standard Higgs鈥�, he says.
