12.50pm: Cern, the European particle physics lab near Geneva, has called a special seminar at 1pm GMT at which scientists working on the two main detectors on the Large Hadron Collider (LHC) will share their results. They will hold a press conference at 3.30pm when they are expected to announce they have good evidence for the existence of the Higgs boson.

Follow events here as they unfold during the afternoon, including the announcement, the key data and reaction from around the world.

12.56pm: In case you've been distracted by other news over the past few months, here's a quick catch-up.

The Higgs boson is a subatomic particle that was predicted to exist nearly 50 years ago. Scientists have been searching for the particle for decades, but so far have no solid proof that it is real.

Although the Higgs boson grabs headlines – unsurprising, given its nickname, the God particle – it is important only because its discovery would prove there is an invisible energy field that fills the vacuum throughout the observable universe. Without the field, or something like it, we would not be here.

Scientists have no hope of seeing the field itself, so they search instead for its signature particle, the Higgs boson, which is essentially a ripple in the Higgs field.

According to theory, the Higgs field switched on a trillionth of a second after the big bang blasted the universe into existence. Before this moment, all of the particles in the cosmos weighed nothing at all and zipped around chaotically at the speed of light.

When the Higgs field switched on, some particles began to feel a "drag" as they moved around, as though caught in cosmic glue. By clinging to the particles, the field gave them mass, making them move around more slowly. This was a crucial moment in the formation of the universe, because it allowed particles to come together and form all the atoms and molecules around today.

But the Higgs field is selective. Particles of light, or photons, move through the Higgs field as if it wasn't there. Because the field does not cling top them, they remain weightless and destined to move around at the speed of light forever. Other particles, like quarks and electrons – the smallest constituents of atoms – get caught in the field and gain mass in the process.

The field has enormous implications. Without it, the smallest building blocks of matter, from which all else is made, would forever rush around at the speed of light. They would never come together to make stars, planets, or life as we know it.

12.58pm: The Higgs field is often said to give mass to everything. That is wrong. The Higgs field only gives mass to some very simple particles. The field accounts for only one or two percent of the mass of more complex things like atoms, molecules and everyday objects, from your mobile phone to your pet llama. The vast majority of mass comes from the energy needed to hold quarks together inside atoms.

1.01pm: Cern's live webcast has begun, but the seminar has yet to start. The expressions on some of the faces in the audience suggest Christmas is about to come early for the physics community.

1.02pm: Ok the seminar has started, but traffic to the webcast is obviously heavy, breaking up the transmission.

1.04pm: Fabiola Gianotti, in charge of the Atlas experiment at the LHC, is presenting the new data.

1.15pm: Still trying to get onto the Cern webcast and failing. This is like trying to buy tickets for the Stone Roses or something. Anyone in the Cern vicinity or who can get online, do tweet me @alokjha or leave a comment below

1.25pm: While Fabiola Gianotti goes through the slides from the Atlas experiment, excluding various energies for the Higgs signal, here are some thoughts from Prof Stephan Söldner-Rembold, head of the particle physics group at the University of Manchester:

Atlas and CMS have presented an important milestone in their search for the Higgs particle, but it is not yet sufficient for a proper discovery given the amount of data recorded so far. Still, I am very excited about it, since the quality of the LHC results is exceptional.

The Higgs particle seems to have picked itself a mass which makes things very difficult for us physicists. Everything points at a mass in the range 115-140GeV and we concentrate on this region with our searches at the LHC and at the Tevatron. The results indicate we are about half-way there and within one year we will probably know whether the Higgs particle exists with absolute certainty, but it is unfortunately not a Christmas present this year. The Higgs particle will, of course, be a great discovery, but it would be an even greater discovery if it didn't exist where theory predicts it to be. This would be a huge surprise and secretly we hope this might happen. If this is case, there must be something else that takes the role of the "standard" Higgs particle, perhaps a family of several Higgs particles or something even more exotic. The unexpected is always the most exciting.

1.33pm: Watching Fabiola's presentation, particle physicist @TaraShears has tweeted: "So this channel excludes 114-115, and 135-136 GeV. #higgsupdate"

It's worth following Tara (and our own @jonmbutterworth) if you want real-time updates on Twitter. Jon's latest: "126 GeV... (probably) #higgsupdate"

Also from Jon: "That 126 GeV was where the ATLAS excess Fabiola just showed is. Not conclusive, but suggestive. ZZ and CMS to come..."

1.43pm: From Cern: "#ATLAS sees a small excess at a Higgs mass of 126 GeV coming from 3 channels. Local significance: 3.6 sigma but only 2.4 sigma globally"

That's not enough for a "discovery" (which techically needs 5 sigma) but it is very interesting evidence for the Higgs.

Also: "#ATLAS excludes a #Higgs mass between 131 and 453 GeV at 95% confidence level at #CERN Higgs seminar"

1.49pm: Fabiola Gianotti has finished her presentation. So far, we know that Atlas seems to have found evidence for a bump around 126GeV for something that looks like the Higgs.

Next up is Guido Tonelli, spokesperson for Cern's other main detector, the Compact Muon Solenoid (CMS). As @iansample says, "So. What we're looking for now is whether CMS detector has seen Higgs-like signals around the same mass (126GeV)."

1.56pm: Via the Science Media Centre, Dr Claire Shepherd-Themistocleus, head of the CMS group at the STFC Rutherford Appleton Laboratory, said: "We are homing in on the Higgs. We have had hints today of what its mass might be and the excitement of scientists is palpable. Whether this is ultimately confirmed or we finally rule out a low mass Higgs boson, we are on the verge of a major change in our understanding of the fundamental nature of matter."

(Also: Hooray, I can hear the webcast properly! Well done for inventing the web, Cern folk, and also for sorting out your IT)

2.15pm: CMS has released an image of the results of a proton-proton collision (main pic above) in which you can see four high-energy electrons (green lines and red towers). This shows the characteristics expected from the decay of a Higgs boson.

2.19pm: From @stevengoldfarb, who is a physicist, outreach, education and communication coordinator on Atlas experiment at Cern: "Looks like our colleagues from #CMS see similar excesses near 125 GeV. 2012 will be fun! #ATLAS #LHC #CERN #Higgs"

2.23pm: A succinct update of information so far by Eugenie Samuel Reich over at Nature News:

The latest results narrow the field even more: Atlas has excluded all masses outside the range of 115–130 GeV, and the CMS team has revised the range to 117–127 GeV. Raising anticipation still further, each experiment separately reports that the LHC's high-energy collisions between protons generated an excess of particles that could be the products of Higgs particle production. The ATLAS result is consistent with a 125–126 GeV Higgs at a statistical level of at most 3.6 standard deviations, and the CMS team reports a 124GeV signal of at most 2.6 standard deviations. In particle physics, a statistical significance of five standard deviations is considered to be proof of a particle's existence, and three standard deviations to be evidence that a particle may exist. The Atlas and CMS results have not yet been combined, so a joint probability is not available.

The situation is further complicated, says Samuel Reich, by another signal seen by both experiments at around 119 GeV. Though this is a weaker signal, its presence has made scientists cautious in interpreting what is real and what is not in the new data. A sighting of the Higgs boson at either energy is consistent with the Standard Model of particle physics, and also with its extension, known as supersymmetry.

2.32pm: Guido Tonelli's presentation on CMS data seems to suggest that scientists cannot exclude a Higgs below 127GeV. This complicates the picture from Atlas results slightly, which seemed to favour a Higgs around 127GeV.

From @Cern: "All channels combined, #CMS excludes a #Higgs mass from 127 GeV to 600 GeV; sees small excess at 1.9 sigma level below 130 GeV"

2.36pm: Guido Tonelli finishes his talk with a dedication to his father, who died on Sunday.

2.36pm: Now over to questions …

2.46pm: Over at the New York Times, Dennis Overbye writes that there have been "tantalising hints" but no direct proof of the Higgs.

The putative particle weighs in at about 125 billion electronvolts, about 125 times heavier than a proton and 500,000 times heavier than an electron, according to one team of 3,000 physicists, known as Atlas, for the name of their particle detector. The other equally large team, known as CMS – for their detector, the Compact Muon Solenoid – found bumps in their data corresponding to a mass of about 126 billion electronvolts. If the particle does exist at all, it must lie within the range of 115 to 127 billion electronvolts, according to the combined measurements. "We cannot conclude anything at this stage," said Fabiola Gianotti, the Atlas spokeswoman, adding, "Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012."

Also, Overbye has a nice detail about a room of scientists in New York:

As seen on the Webcast, the auditorium at Cern was filled to standing room only. At New York University, dozens of physicists gathered in a physics lounge burst into applause.

2.54pm: Cern director Rolf Heuer winds up the seminar: "These are preliminary results, we're talking small numbers, and remember that we are running [the LHC] next year. The window for the Higgs mass gets smaller and smaller but it is still alive. We have not found it yet. Stay tuned for next year."

Cern has also published its press release:

The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130GeV by the Atlas experiment, and 115-127GeV by CMS. Tantalising hints have been seen by both experiments in this mass region, but these are not yet strong enough to claim a discovery.

"We have restricted the most likely mass region for the Higgs boson to 116-130GeV, and over the last few weeks we have started to see an intriguing excess of events in the mass range around 125GeV," said Atlas experiment spokesperson Fabiola Gianotti. "This excess may be due to a fluctuation, but it could also be something more interesting. We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012."

Guido Tonelli, spokesperson for the CMS experiment, said: "We cannot exclude the presence of the Standard Model Higgs between 115 and 127GeV because of a modest excess of events in this mass region that appears, quite consistently, in five independent channels. The excess is most compatible with a Standard Model Higgs in the vicinity of 124GeV and below but the statistical significance is not large enough to say anything conclusive. As of today what we see is consistent either with a background fluctuation or with the presence of the boson. Refined analyses and additional data delivered in 2012 by this magnificent machine will definitely give an answer."

Both experiments will be further refining their analyses in time for particle physics conferences in March 2012. A definitive statement on the existence or non-existence of the Higgs boson will need more data, and is not likely until later in 2012.

3.00pm: Here's the New Scientist take by Lisa Grossman on this afternoon's seminars:



The ultra-shy Higgs boson may have finally shown itself at the LHC. Both of the main detectors, Atlas and CMS, have uncovered hints of a lightweight Higgs. If it pans out, the only remaining hole in the standard model would be filled. Even more exciting, a Higgs of this mass, about 125 gigaelectronvolts, would also blast a path to uncharted terrain. Such a lightweight would need at least one new type of particle to stabilise it. "It's very exciting," says CMS spokesman Guido Tonelli. "This could be the first ring in a chain of discoveries."

Grossman continues:



The Atlas data restricts the Higgs to within 115 and 131GeV; CMS rules out a Higgs heavier than 127GeV. Most excitingly, Atlas saw a tantalising hint of the Higgs at 126GeV; CMS saw one at 124GeV. It is the first time both experiments have seen a signal at nearly the same mass. "We're very competitive, but once I see they're coming with results, I'm happy," Tonelli says. "Their results are important for us. They're obtained in a completely independent manner." That mass also paves the way for physics beyond the Standard Model. Thanks to subtle quantum mechanical effects, a lightweight Higgs needs a heavier companion particle "acting as a sort of bodyguard", Tonelli says. Otherwise, the quantum vacuum from which particles appear would be unstable, and the universe would long ago have disintegrated. If the Higgs is lightweight, the fact that we are here today suggests there is at least one extra particle beyond the Standard Model.

3.25pm: While we wait for the press conference to start, here's some more analysis. This article on "Higgs mania" from the excellent In The Dark blog starts with the writer being woken at 7am with hints on the radio that the Higgs would be announced later today:



Evidence soon emerged however that this particular squib might be of the damp variety. Consistent with previous blogospheric pronouncements, a paper on the arXiv this morning suggested no convincing detection of the Higgs had actually been made by the Atlas experiment.

From Cern's rival particle accelerator, Fermilab, a press release outlining the results as they see them:



The experiments' main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130GeV by the Atlas experiment, and 115-127 GeV by CMS. Tantalising hints have been seen by both experiments in this mass region, but these are not yet strong enough to claim a discovery. Higgs bosons, if they exist, are short-lived and can decay in many different ways. Just as a vending machine might return the same amount of change using different combinations of coins, the Higgs can decay into different combinations of particles. Discovery relies on observing statistically significant excesses of the particles into which they decay rather than observing the Higgs itself. Both Atlas and CMS have analysed several decay channels, and the experiments see small excesses in the low mass region that has not yet been excluded. Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126GeV. It's far too early to say whether Atlas and CMS have discovered the Higgs boson, but these updated results are generating a lot of interest in the particle physics community.

That last point, about how different today's Cern results are from chance, is crucial in working out how robust the data is. Conclusion: more data needed.

3.37pm: Rolf Heuer sits down with Guido Tonelli of CMS to begin the press conference. Camera bulbs flashing everywhere. I assume this is what it's like at all science seminars, yes?

3.40pm: Press conference begins. If you have questions, you can tweet them to the Cern press office with the hashtag #higgsupdate

"We need many more collisions to get the Shakespeare answer to the Higgs: to be or not to be," says Heuer.

Fabiola Gianotti – leader of the Atlas experiment – speaks first, says that what we have seen today is only part of the Atlas science programme. "It's too early to tell if the success is due to the fluctuations in the backgtround or if it's due to something more interesting."

Guido Tonelli, in charge of the CMS experiment, says: "We are discussing the last chapter, we hope, of a story that has lasted 47 years. There are people in the audience who have dedicated decades to this goal … We know from today that, in the next year, very likely, we might get an annoucement that is solid."

He adds that the scientists at Cern will speak more solidly about the science in forthcoming research papers, hopefully published in January or February.

4.09pm: Press conference ends with Rolf Heuer stating: "See you next year with a discovery."

Scientists making predictions, eh? Fingers crossed he's right.

Reacting to today's announcements from Cern, Columbia University physicist Brian Greene said:

The researchers' confidence in this result, while fairly strong, does not yet rise to the level at which a definitive discovery is claimed (there's roughly a chance of a few in a thousand that the data is a statistical fluke, sort of like the chance of getting 8 to 9 heads in a row when you flip a coin; the protocol for claiming a definitive discovery is more like 1 in a million, similar to getting heads about 20 times in a row). But within the next few months, or surely within the next year, the teams should know whether or not they've found the Higgs particle.

5.24pm: Our man at Cern, Guardian science correspondent Ian Sample, has filed his story about today's announcement, including physicists' reactions.

Early next year, the Atlas and CMS teams will pool their results, a move that should see the signals strengthen. Both teams are expected to need around four times as much data before they can finally confirm whether or not the Higgs boson exists. "There is definitely a hint of something around 125GeV but it's not a discovery yet. We need more data! I'm keeping my champagne on ice," said Jeff Forshaw, a physicist at Manchester University. "It should be said this is a fantastic achievement by all concerned. The machine has been working wonderfully and it is great to be closing in on the Higgs so soon."

Read Ian's full story here.