Another Luminosity Record at the Large Hadron Collider

Three weeks ago we reported that the LHC had achieved a record luminosity by squeezing the proton beams to get a factor of 10 improvement. Now they have upped the numbers once again to get a theoretical increase by a further factor of about three. The new configuration is 3.5TeV/2m/20Billion/4-bunches compared to the previous 3.5TeV/2m/12Billion/3-bunches.

Increasing the number of bunches normally increases the luminosity but it depends on how the bunches are organised in buckets to make them collide at the right points of the collider ring. The new arrangement is bucket numbers (1,3231,21081,26731) for beam 1 and (1,12141,17791,26731) for beam 2. Bunches in the same bucket number of both beams will collide in the CMS and ATLAS experiments so from these numbers you get 2 collisions per turn. The other numbers provide the same collisions rates for the other two points where LHCb and ALICE are situated. In fact these bucket numbers are providing the same number of collisions as the previous 3-bunch configuration so for now this is not providing an increase in luminosity.

However they have also increased the intensity of the beams from 12 billion to about 20 billion protons por bunch. This gives a theoretical increase in luminosity of about 3 times. The actual figure will depend on factors such as how much the beam spreads out and how well they can be aimed at each other in the collision points.

The plan is to increase to 6 bunches per beam in the next few days. Depending on the buckets they use this could provide 4 collisions per turn which means another doubling of the luminosity. At the same time they have been doing test runs at much higher intensities of 100 billion protons per bunch but so far this has not been used in combination with multiple bunches and squeezed beams. This means much improved luminosities should be possible soon.

What does this mean for the physics? With the luminosities they have been running at so far they have already announced observations of W bosons and bottom quarks. They must have already seen Z bosons too which are more rare and perhaps they will have seen some top quarks. It takes time for them to review and approve any announcements so we have not heard much about that yet. All of these particles are well-known and are routinely produced in large numbers at the Tevatron in the US. The LHC needs to keep improving its luminosities so that it has enough collisions to see new particles that are only produced very rarely. Of course they are also using energies three times higher than the Tevatron which means they could see new heavier particles that the Tevatron could never produce.

Update Saturday Morning: They have just succeeded in ramping “nominal bunches” to 3.5 TeV. This means bunches with 100 billion protons which is 5 times the number used in the latest physics runs. This is exciting because luminosity is increased by the square of this number. In fact if they used nominal bunches on a physics run right now they could have a new luminosity record even without the squeezing and multiple bunches. On this occassion they lost 40% of the beams during ramping due to “excitation of synchrotron sidebands” but it was still a good step forward. It seems that using such high intensities can lead to problems with instabilities and beam spreading, but they nearly have it cracked. Looking forward to seeing physics with squeezed nominal bunches, hope that is not too far off now.

Highest luminosity recorded so far in Atlas is 30·1027 cm-2s-1

Update Saturday Afternoon: After the nominal intensity run of the morning they started a 6 bunch physics run with configuration 3.5TeV/2m/20Billion/6-bunches. The fill pattern provides 3 collisions per turn in each experiment. It would have been possible to get 4 collisions per turn with six bunches but it looks like they are now opting for schemes that avoid displaced collisions. These are collisions that happen 11.5 m away from the collision point of one of the experiments and these are impossible to avoid in the more efficient filling schemes. 

In any case the injection and ramp for this fill went very well with no sharp beam losses. The run is still continuing after 18 hours and by now they must have doubled the total integrated luminosity of the LHC.  Highest luminosities have now been reported as 60·1027 cm-2s-1 which is double the previous record from last week and 6 times the earlier record. It is also 60 times the earlier runs before they squeezed the beams a few weeks back.

The LHC is now only a factor of 6000 behind the peak luminosities seen at the Tevatron. They can make up another factor of 25 if they use the nominal intensity bunches that they tested in the morning. The rest of the factor can be made up by increasing the number of bunches in the fill. They also have the option to further squeeze the beams down to the nominal beta of 0.5m rather than the current 2.0m. This would give another factor of four.

The plan of a few days ago was to reach this stage late on Sunday so they have really had a good few days


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