̽��ֱ�� of Cambridge - Higgs boson

Opinion: Large Hadron Collider sees tantalising hints of a new particle that could revolutionise physics

Anonymous — Thu, 17 Dec 2015 11:38:32 +0000

At the start of December a rumour swirled around the internet and physics lab coffee rooms that researchers at the Large Hadron Collider had spotted a new particle. After a three-year drought that followed the discovery of the Higgs boson, could this be the first sign of new physics that particle physicists have all been desperately hoping for?

Researchers working on the LHC experiments remained tight-lipped until December 14 when physicists packed out CERN’s main auditorium to hear presentations from the scientists working on CMS and ATLAS experiments, the two gargantuan particle detectors that discovered the Higgs boson in 2012. Even watching the online webcast, the excitement was palpable.

Everybody was wondering if we would witness the beginning of a new age of discovery. ̽��ֱ��answer is … maybe.

Baffling bump

̽��ֱ��CMS results were revealed first. At first the story was familiar, an impressive range of measurements that again and again showed no signs of new particles. But in the last few minutes of the presentation a subtle but intriguing bump on a graph was revealed that hinted at a new heavy particle decaying into two photons (particles of light). ̽��ֱ��bump appeared at a mass of around 760GeV (the unit of mass and energy used in particle physics – the Higgs boson has a mass of about 125 GeV) but was far too weak a signal to be conclusive on its own. ̽��ֱ��question was, would ATLAS see a similar bump in the same place?

̽��ֱ��ATLAS presentation mirrored the one from CMS, another list of non-discoveries. But, saving the best for last, a bump was unveiled towards the end, close to where CMS saw theirs at 750GeV – but bigger. It was still too weak to reach the statistical threshold to be considered solid evidence, but the fact that both experiments saw evidence in the same place is exciting.

̽��ֱ��discovery of the Higgs back in 2012 completed the Standard Model, our current best theory of particle physics, but left many unsolved mysteries. These include the nature of “dark matter”, an invisible substance that makes up around 85% of the matter in the universe, the weakness of gravity and the way that the laws of physics appear fine-tuned to allow life to exist, to name but a few.

Could supersymmetry one day crack the mystery of all the dark matter lurking in galaxy clusters? NASA/wikimedia

A number of theories have been proposed to solve these problems. ̽��ֱ��most popular is an idea called supersymmetry, which proposes that there is a heavier super-partner for every particle in the Standard Model. This theory provides an explanation for the fine-tuning of the laws of physics and one of the super-partners could also account for dark matter.

Supersymmetry predicts the existence of new particles that should be in reach of the LHC. But despite high hopes the first run of the machine from 2009-2013 revealed a barren subatomic wilderness, populated only by a solitary Higgs boson. Many of the theoretical physicists working on supersymmetry have found the recent results from the LHC rather depressing. Some had begun to worry that answers to the outstanding questions in physics might lie forever beyond our reach.

This summer the 27km LHC restarted operation after a two-year upgrade that almost doubled its collision energy. Physicists are eagerly waiting to see what these collisions reveal, as higher energy makes it possible to create heavy particles that were out of reach during the first run. So this hint of a new particle is very welcome indeed.

A cousin of Higgs?

Andy Parker, head of Cambridge’s Cavendish Laboratory and senior member of the ATLAS experiment, told me: “If the bump is real, and it decays into two photons as seen, then it must be a boson, most likely another Higgs boson. Extra Higgs are predicted by many models, including supersymmetry”.

Perhaps even more exciting, it could be a type of graviton, a hypothesised particle associated with the force of gravity. Crucially, gravitons exist in theories with additional dimensions of space to the three (height, width and depth) we experience.

For now, physicists will remain sceptical – more data is needed to rule this intriguing hint in or out. Parker described the results as “preliminary and inconclusive” but added, “should it turn out to be the first sign of physics beyond the standard model, with hindsight, this will be seen as historic science.”

Whether this new particle turns out to be real or not, one thing that everyone agrees on is that 2016 is going to be an exciting year for particle physics.

Harry Cliff, Particle physicist and Science Museum fellow, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Harry Cliff (Cavendish Laboratory) discusses the potential discovery of a new particle at the Large Hadron Collider and its implications for particle physics.

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̽��ֱ��Large Hadron Collider/ATLAS at CERN

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Firing up the proton smasher

fpjl2 — Mon, 16 Feb 2015 12:33:08 +0000

While it slept, we were allowed into the tunnels.

̽��ֱ��Large Hadron Collider (LHC) had shut down for two years to upgrade following the discovery of the Higgs boson. In the main ring, 175 m underground, chunks had been cut out of the snaking tubes for essential maintenance. These tubes fire protons in opposite directions, whipping them ever faster until they almost reach the speed of light. Along the 27 km run are four ‘experiments’: vast machines envelop the points at which tubes intersect and particles collide to capture the results. ̽��ֱ��largest of these, ATLAS, is the size of a six-storey building.

Each collision, known as an ‘event’, produces a splurge of elementary particles such as quarks, gluons and – as we now know – Higgs bosons. On average, events occur 40 million times a second in the LHC.

̽��ֱ��precision required for these events is exquisite. Our guide tells us to imagine two people standing six miles apart and each simultaneously firing a gun so that the bullets meet exactly head-on. Except instead of bullets, imagine needles. Inside the tunnels, engineers zip past on bicycles – the best way to get around underground unless you’re a proton. Next to every lift shaft is a bike rack.

In the next few months, the LHC will be switched back on. ̽��ֱ��2012 triumph of demonstrating the Higgs boson affirmed the Standard Model: the elegant solution to the building blocks of the Universe. Now, with an anticipated almost doubling of energy for the LHC’s second run, physicists are aiming to “go beyond” the Standard Model.

One of the central goals is to prove or disprove the theory of supersymmetry: the “prime candidate” theory for unlocking the mystery of the dark matter in our Universe.

“Observable matter only makes up 5% of the Universe; the rest is what we call dark matter. We know it’s there because we can see galaxies rotating at velocities which require surrounding matter for such gravitational pull – but, unlike the part of the galaxies that we can see, we cannot detect it optically,” said Professor Val Gibson, Head of the Cambridge High Energy Physics (HEP) group.

Supersymmetry theory essentially predicts that every particle in the Standard Model has a matching particle waiting to be found. These partner particles (or ‘sparticles’) could be candidates for dark matter, but we haven’t yet seen them – perhaps because they are heavier and take more energy to generate, a problem LHC Run II could overcome.

“Supersymmetry theory predicts there is a sister particle of the electron called a ‘selectron’, which would have integer ‘spin’: its intrinsic angular momentum. For the quark, there would be a supersymmetric ‘squark’, and so on for every elementary particle we know,” said Gibson. If supersymmetry is correct, there would also be a further four Higgs bosons for us to discover.

“Proton collisions in the LHC might produce a heavy supersymmetric particle which decays into its lightest form, a light neutral particle, but different from those we know about in the Standard Model,” said Gibson.

“We have been looking for supersymmetry particles throughout the first run of the LHC, and the increase in power for Run II means we can look at higher energies, higher mass, and gradually blot out more areas of the map in which supersymmetrical particles could be hiding.”

Will supersymmetry be proved by the end of next year, or will the data show it’s a red herring? For HEP research associate Dr Jordi Garra Ticó, what is really fundamental is experimental evidence. “I just want to see what nature has prepared for us, whether that’s consistent with some current theory or whether it’s something else that no one has ever thought about yet, outside of current knowledge.”

̽��ֱ��two experiments that Cambridge researchers work on are the mighty ATLAS and the more subtle LHCb – known as LHC ‘Beauty’ – which is Gibson and Garra Ticó’s focus. Beauty complements the power of ATLAS, allowing scientists to ‘creep up’ on new physics by capturing rare particle decays that happen every 100 million events.

Garra Ticó spent six months in Cambridge before taking up residence at CERN, where he works on LHCb. LHCb’s 10 million events a second create 35 kbyte of data each, a figure that is expected to go up to 60 kbyte during Run II – too much to ever imagine storing. “There is no guidebook,” he explained. “These machines are prototypes of themselves.”

ATLAS, the biggest experiment, feels like the lair of a colossal hibernating robot. Engineers perch in the crevices of the giant machine, tinkering away like tiny cleaner birds removing parasites. And sealed in the heart of this monster is layer upon layer of the most intricate electronics ever devised.

Dr Dave Robinson arrived in CERN as a PhD student in 1985, and joined the Cambridge HEP group in 1993. He went back to CERN in 2004 – expecting a stint of “one to two years” – and has remained. He is now Project Leader for the most critical detector system within ATLAS, the Inner Detector, which includes the ‘semi-conductor tracker’ (SCT), partially built
in Cambridge.

Each collision event inside ATLAS leaves an impression on the layers of silicon that make up the SCT like an onion skin – enabling scientists to reconstruct the trajectory of particles in the events. “ ̽��ֱ��sensitivity of the tracker is vital for making precise measurements of the thousands of particles generated by the head-on collisions between protons, including decay products from particles like b-quarks which only exist for picoseconds after the collision,” said Robinson.

He is currently working with Gibson and colleagues at the Cavendish Laboratory on the next generation of radiation-proof silicon technology in preparation for the LHC shutdown of 2020, the next time they will be able to get at the SCT, which is otherwise permanently locked in the core of ATLAS. ̽��ֱ��technology will have an impact on areas like satellite telecommunications, where cheaper, radiation-hardened electronics could have a huge effect.

This, for Gibson, is the way science works: solving technical problems to reveal nature’s hidden secrets, and then seeing the wider applications. She recalls being in CERN when she was a postdoc in the 1980s at the same time as Tim Berners-Lee, who was working on computer-sharing software to solve the anticipated data deluge from LHC-precursor UA1. He ended up calling it the World Wide Web.

Inset image – top: representation of the Higgs Boson particle; Credit: CERN.

Inset image – bottom: Professor Val Gibson.

̽��ֱ��Large Hadron Collider is being brought back to life, ready for Run II of the “world’s greatest physics experiment”. Cambridge physicists are among the army who keep it alive.

I just want to see what nature has prepared for us, whether that’s consistent with some current theory or whether it’s something else that no one has ever thought about yet, outside of current knowledge

Jordi Garra Ticó

CERN

Large Hadron Collider

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