Colliding lead ions brings a whole different set of complications.
What’s new for the ALICE heavy ion programme?
UK membership of the ALICE collaboration has been further strengthened.
Take a closer look at ALICE.
All four of the LHC experiments are taking data during the heavy ion run.
Celebrating the first test collisions after a long night shift in the CERN Control Centre
(Credit: J Jowett)
Ask any accelerator physicist and they will tell you that colliding protons is not without its challenges. Colliding lead ions brings a whole different set of complications.
The LHC is best known for colliding protons, the subatomic particles found in the nucleus of every atom. But one month is set aside in each year of operation for the programme of research using heavy ions; the nuclei of lead atoms with all their electrons stripped away.
A couple of hundred times heavier and with 82 times the charge of the protons, lead ion collisions generate a more extended, but still unimaginably concentrated, fireball of energy which can re-create tiny samples of the Quark Gluon Plasma, the primordial soup of particles that is thought to have existed just millionths of a second after the Big Bang. It later cooled into the matter we know.
But getting the lead ions to the point of collision is no mean feat.
“Heavy ion beams at these high energies experience a number of new physical effects that could limit the performance of the collider,” explains John Jowett, who leads the Heavy Ion programme at the LHC. “For example, when we collide the beams, the extremely strong electric fields seen around each ion in near-miss (’ultra peripheral’) collisions can transform some of them into secondary beams emerging from the collision point. These are lost in the superconducting magnets that steer the beams around the 27 km LHC. Now that the magnets are operating closer to their limits, for the higher energies of the LHC, there is a greater risk of magnet quenches. We are testing new strategies to deal with these problems.”
Any beam, whether it is protons or heavy ions will become ‘ragged’ at the edges as it travels around the LHC, and collimators are used to strip the edges off the beam and ensure ‘clean’, homogenous bunches of particles.
“With a proton beam, you know that the particles will retain their proton identity after interactions with a collimator,” says John. “With heavy ions, there’s more scope for a variety of other particles to be generated as they fragment, and this is another way in which lost particles can cause the magnets to quench.”
As if that isn’t enough of a problem, within their bunches, the ions can scatter off each other, potentially causing the bunches themselves to blow up in size, reducing the rate of collisions.
With only four weeks allocated for the heavy-ion programme before the winter technical stop, the pressure was very much on John and his colleagues to switch the LHC from protons to lead ions as quickly as possible. Various teams had to implement a new LHC optics system with slightly reduced magnetic field in the superconducting magnets, switch back to protons for several days to carry out a short run as a reference for the lead collisions, then ensure that all beam losses were under control before switching yet again to ion beams and delivering test collisions. Only once they were satisfied that everything was working smoothly could they declare ‘stable beams’ and the start of a new era of heavy-ion physics at the highest energies ever.
The energies of both the lead-lead collisions and the proton reference collisions were carefully tuned to match the effective energy of the proton-lead collisions that were carried out back at the start of 2013. This will allow the experiments to make precise comparisons of three different combinations of colliding particles.
Five days after the start of the heavy ion collisions, a major milestone was reached; the luminosity (essentially, the collision rate) reached the ‘Design’ value promised many years ago in the specification of the LHC. While some colliders have taken years to achieve this, John points out that the LHC has done it after only about ten weeks of operation in lead-lead collision mode since the start in 2010.
The first heavy ion event recorded by ALICE on
25 November 2015
“We’re ending the year with a bang!” says David Evans (Birmingham and ALICE), “or rather, a hundred million bangs!”
Heavy ion collisions are ‘where it’s at’ for the ALICE experiment; David and his colleagues on the ALICE experiment are eagerly awaiting the first lead ion collisions at the LHC’s new higher energy and intensity.
With double the beam energy, the tiny fireballs that are created by the two ions colliding will be bigger, hotter, denser and longer lived, allowing the system to develop more as the fireball expands and cools.
Each fireball creates minute amount of Quark Gluon Plasma (QGP), the exotic state of matter that existed a millionth of a second after the Big Bang.
“From the data that we collected in Run 1, we already know a lot more about QGP,” explains David, “but we’re now looking for rarer collision events especially ones producing heavy flavour charm quarks. There wasn’t enough data about these type of events from Run 1, but the higher energy beams in Run 2 means more collisions, and more data.”
The ALICE collaboration has already established that quarks lose lots of energy as they travel through the fireball; it’s a profound effect, “rather like stopping a supersonic jumbo jet with a sheet of tissue paper,” says David.
“We would expect light quarks to lose more energy than heavy quarks as they pass through the quark-gluon plasma but we don’t currently have enough statistics on particles made from the heavier charm quarks to see this. A major goal for Run 2 is to increase our statistics on particles made from charm quarks by a factor of ten.”
Whilst most physicists working on LHC experiments are interested in particle collisions – after all, the clue is in the name, Large Hadron Collider - a small group of researchers on the ALICE experiment is interested in near-misses, and specifically near-misses between two lead ions.
“What we do is unlike the rest of the ALICE researchers,” admits Orlando Villalobos Baillie (Birmingham).
Orlando and his colleagues are interested in what happens when two heavy ions pass at very close range and such high energy that the perturbation between them causes the nuclei to release particles including photons. This type of interaction is called an Ultra Peripheral Collision (UPC).
Due to their lack of charge, photons are the ‘cleanest’ type of particle for probing the atomic nucleus and discovering, for example, how quarks and gluons are distributed. But it’s also the photon’s lack of charge that has previously made them difficult to produce at the high energies required to do the job.
Photon collisions from UPCs have previously been seen at RHIC and their electron-proton analogues have been seen at HERA but these have been at much lower energies. With its new higher energy beams, the LHC is now capable of generating 1 TeV photon collisions - three times the energy of HERA - and this is opening up new areas of physics.
There are models that predict what particles should be produced in a UPC, e.g. when a high energy photon from one nucleus collides with two or more gluons from the other, but until the advent of the LHC it has been extremely difficult to test the models experimentally because the interactions are so rare. The LHC run 1 has yielded five exploratory papers from ALICE, showing convincingly that UPCs take place, but so far hampered by low statistics.
Orlando is hoping that that this heavy ion run will provide a significant increase in the number of UPCs. With ALICE’s detectors optimised to measure the UPC interactions more cleanly and clearer than ever before, it will be possible to investigate an area of physics that has, until now, remained poorly tested.
University of Derby team gets to know ALICE
(Credit: ALICE collaboration)
UK membership of the ALICE collaboration has been further strengthened with the University of Derby becoming an associate member.
The university, which is developing a reputation for cloud computing, big data and data science, will be contributing Grid computing resources, software researchers and engineers to the O2 project – a collaborative effort to develop ALICE data acquisition capabilities ahead of Run 3 when there will be a massive increase in collision rates and data generation. The university’s electronics engineers are already assisting with detector control electronics.
Membership of any LHC experiment brings mutual benefits – the experiment gains access to specific expertise, and the member institute often has to take that expertise to new levels to meet the technical challenges that it has been set.
It’s the development opportunities that interest Ashiq Anjum (Derby), “Being part of the ALICE collaboration will be an important part of the computational physics programme that the university is launching; our students will get training opportunities and our academics will get an opportunity to collaborate with other academics in cutting edge computing and engineering problems.”
The University joined the collaboration in November as an Associate Member, and will review its membership level over the next year.
You can read more about Derby’s role in ALICE Matters, the online newsletter for the Collaboration
Take a closer look at ALICE.
ALICE short clip
When it comes to colliding heavy ions, the spotlight is very much on ALICE, but for the first time, all four of the LHC experiments are taking data during the heavy ion run, including LHCb, which will record this kind of collision for the first time.
Results from Run 1, confirmed the perfect liquid nature of the quark-gluon plasma (the primordial ‘soup’ of particles that existed for a few millionths of a second in the early Universe), and the existence of ‘jet quenching’ in ion collisions, a phenomenon in which generated particles lose energy through the quark-gluon plasma. There’s still much to discover about this strange state of matter, and the higher energy collisions is Run 2, coupled with advances in analytical techniques, should lead to a greater understanding.
ATLAS Heavy Ion Event Display - November 2015
“The heavy-ion run will provide a great complement to the proton-proton data we've taken this year," said ATLAS spokesperson Dave Charlton (Birmingham). "We are looking forward to extending ATLAS' studies of how energetic objects such as jets and W and Z bosons behave in the quark gluon plasma.”
CMS Heavy Ion Event Display - 25 November 2015
For CMS, the relatively high numbers of heavy flavour particles that will be produced will open up unprecedented opportunities to study hadronic matter in extreme conditions. The detector is optimised to capture these rare probes, and to measure them with high precision.
LHCb Heavy Ion Event Display - 25 November 2015
"This is an exciting step into the unknown for LHCb, which has very precise particle identification capabilities,” explains LHCb Spokesperson, Guy Wilkinson (Oxford). “Our detector will enable us to perform measurements that are highly complementary to those of our friends elsewhere around the ring.”