Long Physics Run at the LHC


Physicists at the Large Hadron Collider have just completed their longest physics run yet with about 20 hours of collecting events. Fill number 1022 which started yesterday evening and finished at lunchtime today will have more than doubled the total amount of data taken so far at CERNs new collider. The run finally ended when a glitch in the cryogenics caused the beams to be automatically dumped. The stability of the beams was so good that if such glitches can be avoided they would circulate for many days before gradual loss of the protons degrades the beam intensity. This is a great boost for the prospects of the collider.  

As with earlier fills this one used two bunches of protons circulating in each direction of the collider ring. The bunches were injected with timings carefully worked out so that the protons would collide at intersection points around the circuit where the different particle detectors are positioned. With just one bunch in each beam the protons will collide at two points diametrically opposed on the 27 kilometer circle. The two main experiments CMS and ATLAS are positioned at such points, but two other experiments LHCb and ALICE are at different locations about one eighth of the way round the ring in either direction starting from ATLAS. With two bunches per beam the protons can be made to collide at these points as well and that has been the configuration used since first collisions last week.  For this run all the LHC experiments were switched on and should have been able to collect data.

We will have to wait some time before we know what the results will be. The physicists have to collect vast amounts of collision data and analyse it before they can publish their results. Exactly how long depends on what physics is waiting to be discovered and how efficiently the accelerator engineers can increase the luminosity of the beams to generate more collision events in the detectors.

Already the proton beams are circulating with an energy of 3.5 TeV per proton and 20 billion protons in each beam. The experiments were collecting data at the rate of about 100 events per second on this latest run which means they already have some 10 million events to analyse from each detector. But the most interesting events are the very few where new particles never seen before are created. To separate those from the background the teams will have to collect many times more events. To achieve this we expect to see more bunches of protons injected into the collider before they are ramped up in energy. Last year during early test runs at lower energy they already had 16 bunches circulating in each beam but the ultimate target is to increase this figure to 2808. Further increases in collision rates can be achieved by adjusting and “squeezing” the beams using magnetic fields to make them collide in a smaller space. If all this can be accomplished the number of events seen during the latest long run will be seen in just a few seconds. Obviously they will want to get there as soon as possible and the stability of the latest fills should give them confidence to move forward quickly.

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