Tag archives: LHC
By Hamish Johnston
It’s been quite a rollercoaster ride for physicists working on the Large Hadron Collider (LHC) at CERN. When the collider was first switched on in 2008 it suffered a major explosion when a superconducting connector failed – and was shut down for over a year for repairs. Then in 2010 the LHC began taking data and the excitement about the imminent discovery of the Higgs boson grew and grew – and then on 4 July last year, CERN physicists announced the discovery of a Higgs-like particle.
The graph shows the distribution of the “w-jj excess” as seen by the CDF experiment (Courtesy: Punzi/Fermilab)
By Tushna Commissariat
Two months ago, in early April, the particle-physics community was rife with speculation and excitement over a “bump” – a possible new particle – in the data that Fermilab’s CDF experiment was looking at. On Monday 30 May Giovanni Punzi, a CDF collaborator, presented an update on what is now referred to the “ W-jj” bump, as a part of his talk at the 23rd Rencontres de Blois Particle Physics and Cosmology conference currently being held in France. The update says that the bump is seen in more recent data with an even larger statistical significance.
At the time, CDF was looking for slightly rare di-boson pairs – W bosons produced in association with another W or a Z boson. It noted a bump between 120 and 160 GeV /C2 in the jets produced in the collisions with a statistical significance of about “three-sigma”, which meant that the result would not be considered valuable until a “five-sigma” statistical significance could be established. The new data, however, have established a significance that is officially “closer to five sigma” (unconfirmed sources suggest it is as close as 4.8) and that “it was not just a statistical fluctuation” and that it is now a “serious issue for CDF to understand this”, according to Punzi.
Interestingly, Punzi’s slides also say that it is almost impossible that bump is due to the Standard Model top-quark background, as suggested by some theoretical papers, as that would imply that previous measurements for SM top-quark background had huge errors. The next step forward would be if CDF’s sister experiment D0 or the LHC’s ATLAS or CMS experiments, none of which have found the bump in their data so far, manage to detect it.
This updated result has seen a variety of responses from physicists.
Adam Falkowski, who writes the Resonaances blog, seems rather jubilant. “In a collider experiment, such a huge departure from a Standard Model prediction is happening for the first time in the human history,” he writes. “I don’t have to stress how exciting it is.”
Peter Woit, author of the Not Even Wrong blog, feels that while a five-sigma significance is important, problems with background modelling might thwart the result. “The signal is being extracted from a huge background, so a small misunderstanding of the background could be its cause.” Only a detection of the same result by another experiment would make the case more compelling, according to him.
Tommaso Dorigo, a blogger and CDF collaborator, is still sceptical of the result and chalks it up to bad background modelling, like Woit. “No, it is not the Higgs. And it is not a new particle. It is, in my humble opinion, a problem in the modelling of backgrounds, one which was unnoticed before only because it is small enough to have escaped previous attempts at “tuning” the simulations.”, he writes in his blog.
So while it seems like we the path to “new physics” is full of “bumps”, the field of particle physics is a rather exciting one right now! Take a look at the slides Punzi used for his talk here.
Rolf-Dieter Heuer talking to journalists at the Royal Society, London.
(Courtesy: Tushna Commissariat)
By Tushna Commissariat
The Large Hadron Collider (LHC) at CERN has had its share of good and bad press over the past few years. Controversy and rumours abounded when the machine was switched on in September 2008. The mood then turned quickly to disappointment when its magnets failed and finally to euphoria when the first beams collided at 7 TeV in March 2010.
This week, a meeting to discuss the LHC and all things related was held at the Royal Society in London. The “Physics at the High Energy Frontier – the Large Hadron Collider Project” meeting took place on 16–17 May and saw leading lights of the project come together to discuss the collider and its future.
I was at the meeting for the second day, when a press briefing was held where CERN director Rolf-Dieter Heuer, plus Fabiola Gianotti and Guido Tonelli of the ATLAS and CMS experiments respectively, answered all of the questions that the Higgs-hungry reporters could throw at them!
The three speakers described how the collider has “surpassed all expectations” – experimental and computational. Talking about how the LHC is the very essence of global co-operation, Tonelli stressed that “no country could have done it as a stand-alone”. Heuer boasted that every year about 1000 students get their PhDs thanks to the LHC, while just the ATLAS experiment involves about 3000 researchers.
Explaining how things work at the LHC, Tonelli said, “We [experimental scientists] try to test the theory without prejudice. We ask our friends the theorists to come up with something that we can observe.” The collider has already produced the top quark in Europe for the first time and now it is poised to begin a regime of “new physics”, to look for supersymmetry (SUSY), multiple dimensions, matter–antimatter disparity and, of course, the Higgs boson.
The Higgs…or something else?
“We will have an answer to the Shakespeare question for the Higgs – ‘To be or not to be’ – by the end of 2012” declared a confident Heuer. While he did show a great deal of enthusiasm about discovering the Higgs, Heuer was also keen to point out that not finding the particle would be a great result in itself. “Not finding [the Higgs] when it does not exist is a success,” he exclaimed. “If it does not exist, we need to find something else that takes up the job of the Higgs and gives mass to elementary particles,” he added.
The LHC will run until the end of 2012 without any major breaks and Heuer is confident that it will decide the fate of the Higgs by the end of this run. “Physics will not be the same after 2012.” declared Tonelli. “It will change the view of the world.”
One of the first questions, asked by BBC reporter Pallab Ghosh, was about the recent ”leak” of an unconfirmed sighting of the Higgs by ATLAS. A sighting that was later denied by a paper released by the ATLAS team and in interviews with physicists on various media channels.
“Unfortunately we live in a world of WikiLeaks, so it leaked!” said a grinning Gianotti. On a more serious note, she explained that such leaked results have not undergone the scientific scrutiny that is necessary, and hence are almost always insubstantial.
“The CERN management was not amused by the leak” said Heuer. He went on to ask journalists not to believe leaked results in the future. “Don’t trust it on first sight” he said. Although Heuer’s displeasure was clear, the leak did put the LHC back in the public eye after a few quiet months. Also, the media interest did provide the public with a rare insight into the vetting process that all scientific discoveries undergo. So perhaps the CERN management should lighten up and enjoy the renewed interest in the LHC!
Rolf-Dieter Heuer giving a talk about the future of the LHC at the Royal Society, London. (Courtesy: Tushna Commissariat)
Bumps and jumps
When asked about the Higgs-like ‘bumps’ seen at other experiments like the Tevatron and CERN’s Large Electron Positron Collider (LEP) the panel had mixed replies. The Tevatron bump was dismissed by Gianotti and Tonelli, as they both explained that it was too small, statistically speaking, and was only seen by one of the Tevatron’s two detectors. Would the LHC have a look for the Tevatron signal? “No”, was their reply.
However, “interesting events” seen at 115 GeV by the LEP just before its closure in 2000 are of interest to them. While Heuer did say that it is very difficult to determine if it was anything more than a “hint”, the LHC will be looking for the Higgs at that energy soon.
The International Liner Collider – a possible successor to the LHC – is another project that Heuer is excited about. He feels that CERN, with the LEP and now the LHC under its belt, would be the perfect host for the collider. “I think CERN has huge potential, not only on the human side, but on its experience side. We have all the instruments. So I see CERN in a very good position.” he said.
But what about the money? “If you have an excellent science case, you will get the money. Don’t ask for the money until you have the science figured out.” he said. He pointed out that, compared to the US, in Europe the politics of funding are more stable and for that reason CERN would be a better host.
Right: prototype microwave cavity for the ILC, illuminated for a “Science Night” in Hamburg. (Courtesy: DESY)
By Tushna Commissariat
Looks like the Large Hadron Collider (LHC) at the CERN particle physics lab had an interesting few days last week, just before everybody left for Easter, and the Internet is now abuzz with rumours of an impending discovery.
But before we get into any of the highly interesting and debatable stuff, let’s look at one thing that has definitely happened at the LHC.
Around midnight on Friday 22 April, the LHC set a new world record for beam intensity when it collided beams with a luminosity of 4.67 × 1032 cm–2s–1. This was significantly more than the previous luminosity record of 4.024 × 1032 cm–2s–1 held by the US Fermi National Accelerator Laboratory’s Tevatron collider in 2010.
This new beam intensity was achieved after two weeks of planning and readying the collider. The machine is now moving into a phase of continuous physics scheduled to last until the end of the year when, after a short technical stop, the machine will resume running for 2012.
“Beam intensity is key to the success of the LHC, so this is a very important step,” said CERN director Rolf-Dieter Heuer in a statement. “Higher intensity means more data, and more data means greater discovery potential.”
But didn’t I read all about some record being broken by the LHC last year, you ask? Yes, but that was the LHC accelerating its proton beams to 3.5 TeV each, leading to later collisions at 7 TeV. Now it is the beam intensity or the “luminosity” that is record breaking. Luminosity gives a measure of how many collisions are happening in a particle accelerator. So the higher the luminosity, the more particles are likely to collide which is necessary while looking for rare particles like the infamous Higgs boson.
“There’s a great deal of excitement at CERN today,” said CERN’s director for research and scientific computing, Sergio Bertolucci, “and a tangible feeling that we’re on the threshold of new discovery.”
Well, it looks like Bertolucci spoke a tad too soon, as on the same day a leaked memo posted by an anonymous commenter on mathematician Peter Woit’s blog, claimed that certain researchers at the ATLAS experiment at CERN had seen firm evidence for the Higgs particle in recent data.
The memo, though not official by any means, was authored by four ATLAS members who claimed to have seen an excess number of photons produced at energy of 115 GeV that could be caused by the decay of the Higgs particle into photons.
Surprisingly, only a few websites and blogs mentioned the news for the first day or so, before slowly more people seemed to notice this juicy story of physics, Higgs and betrayal!
On 25 April, Nature reported on its blog, an official statement from ATLAS spokeswoman Fabiola Gianotti. Gianotti said “Only official ATLAS results, i.e. results that have undergone all the necessary scientific checks by the collaboration, should be taken seriously.” She went on to say that signals of the kind reported in the memo show up often during data analysis and are later falsified after more detailed scrutiny.
But the damage had already been done as physicists and others began to comment on the legitimacy of the claim made in the memo and the ethics of such an internal memo being posted and talked about online.
As people began to look deeper into the memo, interesting facts began to creep up.
Tommaso Dorigo, from the University of Padova in Italy wrote an initial post on his blog A Quantum Diaries Survivor that turned into a debate and eventually a bet! His post was sceptical from the start and he gave his reasons for why he was sure it as nothing more than a blip in the data, then went on to explain in more detail what other data already exists.
After that, a regular reader of his blog pointed out that the authors of the ATLAS study are actually physicists from Wisconsin, and include a Professor Wu, “who was among those less happy of the decommissioning of LEP [the Large Electron-Positron Collider] at the time when they were claiming a possible Higgs signal at 115 GeV. So maybe these guys have been looking for some confirmation of the 115 GeV Higgs all along”.
Woit too was quick to distance himself from the memo saying that “it should be made clear that, while members of ATLAS work here at Columbia, I have no connection at all to them, and they had nothing to do with this. The source of the abstract posted here anonymously as a comment is completely unknown to me.”
As more people debated and commented over the memo, Dorigo came back to say that he would bet anyone who “has a name and a reputation in particle physics (this is a necessary specification, because I need to be sure that the person taking the bet will honour it) that the signal is not due to Higgs boson decays” and then updated that comment by saying that if he is wrong he would pay $1000 but that if he is right he would be given only $500.
Meanwhile, Channel 4 conducted an interview with Jon Butterworth, a particle physics professor at University College London, who also works at ATLAS. He went on to say the same thing; that nothing would be definitive until it was scrutinized by CERN officially (look above).
So at the end of the day, it looks like the world is going to have to wait a while longer before Higgs boson gets its official post in the Standard Model hall of fame.
Data from ATLAS are the black dots that follow a relatively smooth curve. Excited quarks would appear as bumps that are illustrated by the three coloured curves. (Courtesy: PRL)
By Hamish Johnston
Unlike most people, particle physicists can get very excited about seeing absolutely nothing.
Indeed, one of the world’s most prestigious journals has published a letter from the ATLAS collaboration at the Large Hadron Collider (LHC) about how the multibillion-euro facility has found no evidence for an “excited quark” or any other unexpected particles.
And furthermore, it’s done a much better job at not finding an excited quark than its less energetic competitor, Tevatron at Fermilab in the US.
In the Standard Model of particle physics, quarks are fundamental particles. This means that they have no internal structure and therefore no excited states. By failing to find excited quarks (or other surprise particles) at masses up to 1.26 TeV, the ATLAS team have provided invaluable guidance to physicists who are developing theories beyond the Standard Model.
Tevatron had previously excluded excited quarks to about 0.87 TeV, which required 3500 times more data than the ATLAS measurement. This performance bodes well, and we can look forward to the LHC not finding many more particles – and who knows, it might even manage to find a new particle or two.
But what’s really incredible about this paper in Physical Review Letters is the author list, which begins with G Aad and ends with V Zutshi. Normally, letters are restricted to four pages, but this one stretches to 19 pages to include all the authors and their organizations. I gave up counting authors (I think there are about 3000), but there are 177 organizations listed.
Sadly, 19 authors passed away before seeing ATLAS’s first publication of results in PRL.
The letter is published at Phys. Rev. Lett. 105 161801.
By James Dacey
First there was the LHC rap then the media bonanza for the big September switch-on; also playing their part were the harbingers of doom – foretelling apocalypse from Geneva’s ‘black hole machine’.
Now CERN’s (in)famous experiment is about to get even more dramatic as it provides the fictional setting for a new theatre production.
The Gentlemen’s Tea Drinking Society is produced by Ransom Theatre Company who bill it as “a fast and funny exploration of science, friendship, sexuality and the end of everything as four men face the truth on one fateful night”.
The play was written by Richard Dormer who made a name in 2003 with his internationally-acclaimed portrayal of the talented-yet-troubled snooker legend Alex “Hurricane” Higgins. It also contains an original score by Belfast born DJ David Holmes who produced the music for Ocean’s Twelve and Out of Sight.
Dormer and Co haven’t revealed much about the plot other than it centres around four men in a room, one of whom is a physicist harbouring a very big secret – he’s found the Higgs boson.
This is not the first time physics has taken to the stage. Famous examples include Tom Stoppard’s Arcadia (1993) — a look at the life of Byron which incorporated ideas from thermodynamics and chaos theory; and Michael Fryan’s Copenhagen (1998) — a play built around a 1941 conversation between Neils Bohr and Werner Heisenberg about the nature of the quantum world.
More recently American composer John Adams created an opera based on The Bomb and its creation at the Manhattan project. Dr Atomic premiered in 2005 and finally comes to London this February.
The Gentlemen’s Tea-Drinking Society launches on 4 February at Belfasts’s Old Museum before going on tour across Ireland until 10th March. Later in the year it will appear in Glasgow and London.