News From LHC, CERN: Record Beam Bunch, Record Beam Intensity
By on May 23rd, 2011

News of new records seem to be tumbling out of CERN in the past two days, with both the number of bunches per beam and the beam luminosity in the Large Hadron Collider (LHC) hitting a record high. Though this is still a long way away from the theoretical maximum predicted by the engineers for LHC, this is a significant step.

In this post, we discuss about the records only. In a subsequent post, we shall discuss what LHC has managed to uncover about quarks the basic building block for all almost matter you see around you.

First: bunches per beam

Yesterday, i.e. on 22nd May, 2011, CERN announced that the LHC had crossed the mark of 900 bunches per beam, creating a new record for itself with 912 bunches. This betters the previous count of 480 bunches, which was achieved only a month back. But exactly what is a bunch?

What is a bunch?

The particles in a beam in LHC is not a continuous stream, as you’d have thought. They are broken down into small collections each containing the same number of particles. These collections are called ‘bunches’. So each beam is made up of a number of bunches, each bunch being made up of a huge number of particles. Bunches arrive with a certain time delay. The present time delay between two bunches is 50 nanoseconds, while the theoretical upper limit (the one permitted by the detector) is 25 nanoseconds. Also, the number of bunches in a beam, currently 912, will also increase. The theoretical upper limit is 2808 bunches per beam.

Upgrading the LHC to a higher number of bunches per beam is a delicate process, since increase in the number of bunches dramatically increases the beam energy. Engineers have to be sure that not only is the LHC under no danger from the increased energy, the detectors function properly and the calibration remains true.

Take a bunch of protons. This may consist of a hundred billion protons (that’s one followed by 11 zeros), each separated by a distance much greater than the theoretical radius of protons. When such beams from opposite sides collide, the number of collisions is really small owing to the huge separations (the beams pass through each other mostly as ghosts). It’s the relatively small number of collisions that happen that are important.

Next, luminosity of a beam:

The next milestone achieved by the LHC early today was the attainment of a huge luminosity of 1033. But what does this really mean? Roughly, this corresponds to the collision rate, but it is not exactly so! It refers to the probability of collisions rather than the rate of actual collisions happening. The difference is a subtle and important one. Physicists call this the ‘cross-section’ of the process. Think of cross-section as an area (in fact, this is where the name comes from). The bigger the cross-section, the bigger the probability of a collisions happening. (Think of a door and a small slit in a postbox. If you randomly throw small rocks at each one from a good distance, the probability of hitting the door is much higher than that for hitting the mailbox slit. The door is said to have a higher ‘cross-section’.)

A Higgs event simulated at CMS, a detector at LHC, CERN

Each process has its own cross-section. For example, the cross-section for the production of a muon from an electron-positron collision is much higher that the same for the production of a Higgs from a proton-antiproton collision. Multiplying the cross-section with the luminosity gives the number of events to expect (call it, ‘expectation’) at that luminosity. (This is indeed different from cross-section, if you’re wondering. Cross-section is an inherent quantity, fixed for a certain process. Expectation depends on the number of particles in the beam being fired.)

Most protons in the LHC beams, as mentioned above, don’t collide at all and some collide only at glancing angles. The head-on collisions are the ones that produce the most exotic phenomenon (utilizing their full energy in the collision), and this is what interests scientists the most.

The highest luminosity, set last year, was 1032, with the theoretical highest being 1034. Currently, there is predicted about a 100 million collisions.

The LHC is expected to operate at 3.5 TeV this entire year set to hit 7 TeV in the next couple of years. Exciting physics is round the corner. Watch this space for more…

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Author: Debjyoti Bardhan Google Profile for Debjyoti Bardhan
Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.

Debjyoti Bardhan has written and can be contacted at debjyoti@techie-buzz.com.
 
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