Geneva 2 June
Two months is a very short time in the life of a major particle physics project, but a lot can happen in that time as the LHC has shown since 30 March. Colliding beams at 3.5 TeV was an important milestone, a start to the LHC physics programme, but it was just a single step on a very long journey. Since then, we’ve lengthened our stride, and are progressing well towards the key objectives for 2010. The next major milestone came on 19 April with a ten-fold increase in luminosity – in other words, the machine started delivering ten times as many collisions to the experiments in a given period of time than had previously been possible. This came about thanks to two simultaneous developments: firstly the number of particles in each bunch was doubled, and secondly the beam size at the interaction point was squeezed down. The term you’ll hear used to describe the beam size at the interaction point is called beta-star, and the smaller the beta, the better. Before squeeze, beta is 11 m at ATLAS and CMS. The ultimate goal is to reduce it to 0.55 m. Today, we’re running with a beta of 2 m. That may not sound very small, and that’s because it’s not the size of the beam: beta is the distance from the interaction point that the beam is twice the size it is at the interaction point. What’s important for physics is that the lower the beta, the smaller the beam at the interaction point. With beta of 2 m, the beam is just 45 microns across at the interaction point, a quarter the width of a human hair, and its cross section is about five times smaller than with a beta of 11 m.
Four weeks of running under these conditions led to significant quantities of data being accumulated by the experiments, and then came the next big step. Over the weekend of 22 May, we started to run with 13 bunches in each beam.
The first collisions on 30 March were done with one bunch per beam, and the ultimate goal is to reach 2808, so there’s still some way to go. Nevertheless, we set a new luminosity record that weekend of 2 ´ 1029. To put that in context, we achieved 1027 on 30 March, the design figure for the LHC is 1034 and the objective for 2010 is to reach 1032.
Two months is a very short time in the life of a major particle physics project, but a lot can happen in that time as the LHC has shown since 30 March. Colliding beams at 3.5 TeV was an important milestone, a start to the LHC physics programme, but it was just a single step on a very long journey. Since then, we’ve lengthened our stride, and are progressing well towards the key objectives for 2010. The next major milestone came on 19 April with a ten-fold increase in luminosity – in other words, the machine started delivering ten times as many collisions to the experiments in a given period of time than had previously been possible. This came about thanks to two simultaneous developments: firstly the number of particles in each bunch was doubled, and secondly the beam size at the interaction point was squeezed down. The term you’ll hear used to describe the beam size at the interaction point is called beta-star, and the smaller the beta, the better. Before squeeze, beta is 11 m at ATLAS and CMS. The ultimate goal is to reduce it to 0.55 m. Today, we’re running with a beta of 2 m. That may not sound very small, and that’s because it’s not the size of the beam: beta is the distance from the interaction point that the beam is twice the size it is at the interaction point. What’s important for physics is that the lower the beta, the smaller the beam at the interaction point. With beta of 2 m, the beam is just 45 microns across at the interaction point, a quarter the width of a human hair, and its cross section is about five times smaller than with a beta of 11 m.
Four weeks of running under these conditions led to significant quantities of data being accumulated by the experiments, and then came the next big step. Over the weekend of 22 May, we started to run with 13 bunches in each beam.
The first collisions on 30 March were done with one bunch per beam, and the ultimate goal is to reach 2808, so there’s still some way to go. Nevertheless, we set a new luminosity record that weekend of 2 ´ 1029. To put that in context, we achieved 1027 on 30 March, the design figure for the LHC is 1034 and the objective for 2010 is to reach 1032.
All this was achieved during physics running, leading to incredible progress being made by the experiments. They have been running with 90% efficiency, a remarkable achievement for devices of such complexity. Billions of collisions have been recorded and successfully dispatched for analysis via the LHC Computing Grid. The rediscovery of the Standard Model, which is necessary before we can confidently say we’re ready for new physics, is well underway. There are even some intriguing observations about the properties of collisions at this new energy. As a measure of their success to date, the experiments have already published or submitted over a dozen papers to peer reviewed journals and conferences based on LHC collision data.
Physics running is interspersed with periods of machine development essential for further progress to be made. As a foretaste of what the experiments can expect over the next two months, the LHC operations team has notched up some impressive results over the last few machine development sessions. The first of these was to inject bunches with more than the LHC’s design intensity and collide them at 450 GeV. There’s nothing new about 450 GeV, but it’s an important milestone nevertheless since the difficulty of colliding bunches increases with intensity. By comparison, adding extra bunches is a relatively easier task. The icing on the cake of last week’s machine development came when design intensity bunches were brought into collision at 3.5 TeV on 26 May.
Behind this great progress is a guiding principle of caution. The masters of ceremony are those responsible for the systems that protect the LHC and the experiments from stray beam particles. Collimators absorb particles that wander from their intended orbits before they can impinge on LHC magnets or sensitive detector elements, while the LHC beam dump system is there to extract the beams safely in case of need. Any increase in intensity has to be approved by the LHC machine protection teams, and progress is incremental. Each increase in intensity, and therefore stored energy in the machine, is a learning process for the machine protection teams and only when they are ready do increases in intensity happen.
With all eyes on the amount of data being delivered to the experiments, it would be easy to overlook some of the pioneering systems that make the LHC possible. When I asked someone in the CERN Control Centre last week about the cryogenics, they replied that it’s working so well they’d almost forgotten it was there. For the operators of the world’s largest cryogenic installation that’s quite a compliment. And for anyone wondering whether large-scale cryogenics may have broader applications, the LHC is proving to be an interesting test case.
The same goes for the vacuum systems. Beam lifetimes of 1000 hours have been posted, which is truly exceptional for any particle accelerator. Of course, we don’t keep beams for that long: there are many reasons why beams are extracted long before they reach their theoretical lifetimes. So far in the LHC, the longest fill for physics has been 30 hours, which well exceeds my expectations for the first months of running.
A lot can happen in two months, and we are well on course to achieving our 2010 objectives for the LHC. The fact that the LHC’s availability for operation is already over 60% is testimony to the skills and professionalism of all those who operate the machine and its supporting infrastructure, and it is perhaps the one statistic that has made all the others possible. As I write, we’ve recently completed a rather frustrating weekend, with a short circuit in a cable terminal of an electrical cabinet stopping us from running. By Monday morning, however, we’d recovered and will resume LHC running tomorrow after a scheduled technical stop. Glitches such as this are a fact of life in a working lab, and do not detract from the fact that we have much to be pleased with from these two months. As the figures I’ve quoted above illustrate, however, we still have a long way to travel. My congratulations go to all involved with this great scientific adventure.
Source: IPM
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