## The LHC is back for Run 2

I had not found the occasion of impetus to kick-off this blog until now. But today is a notable enough day, being the effective start of Run 2 at the Large Hadron Collider at CERN. The LHC is back in business. The 6.5 TeV proton beams are going and the experiments are taking data.

I had not found the occasion of impetus to kick-off this blog until now. But today is a notable enough day, being the effective start of Run 2 at the Large Hadron Collider at CERN. The LHC is back in business. The 6.5 TeV proton beams are going, and the experiments are taking data.

The following is a screen shot from the LHC Page 1, where you can monitor the status of LHC on the web realtime. I’ll try to explain these plots, as they are kind of hard to parse because they show multiple things.

The plot on the left shows the energy of each proton beam in black. One beam goes one way around the ring, and the other, the opposite way, but they are always ramped-up together to have the same energy. Here each beam achieves 6.5 TeV of energy per proton, so that the combined collision energy per proton is 13 TeV. In red and blue you see the intensities of each beam expressed as a number of protons per beam. In this case each beam has 3 bunches of protons with $\sim 10^{11}$ protons in each bunch so each beam has $\sim 3 \times 10^{11}$ protons.

You can see several periods this morning where they had beams and then dumped them, but beginning around 10 am, the blue and red lines climb up, indicating that beams were refilled with protons, and shortly after the energy was ramped-up, where the black line rises, from the 450 GeV of energy that protons have as they are injected into the LHC from the SPS, up to the current max energy of the LHC: 6500 GeV or 6.5 TeV.

Then the plot on the right shows the instantaneous luminosity delivered at each of the four collision points around the ring, where the ATLAS, ALICE, CMS, and LHCb experiments are. Luminosity is a measure of the number of protons squeezed to pass through a small cross sectional area in the center of each experiment, per area, per time. This instantaneous luminosity is directly proportional to the rate of us producing any particular physics process, so it is proportional to the data rate of interest we are taking. In this case the instantaneous luminosity is $\sim 3 \times 10^{30}~\textrm{cm}^{-2}\:\textrm{s}^{-1}$.

During Run 1 of the LHC (2009-2012), the collider improved in performance substantially, achieving a peak instantaneous luminosity of $\sim 7 \times 10^{33}~\textrm{cm}^{-2}\:\textrm{s}^{-1}$ in the later half of 2012.  But that was at a total collision energy1 ($\sqrt{s}$) of 8 TeV, and the LHC is now back online at a higher energy of 13 TeV.  This higher energy raises the production rates of many interesting and hypothetical physics processes, giving us a much larger reach to search for new physics.

In future runs of the LHC in the coming weeks, eventually each beam will be filled with a few thousand counter-rotating bunches instead of just 3 per beam, and the beams will be squeezed to collide in smaller cross sectional areas, which together will drive the instantaneous luminosity to some $\sim 10^{33}$$10^{34}~\textrm{cm}^{-2}\:\textrm{s}^{-1}$, which will open the torrent of interesting data by a few orders of magnitude of what is achieved today.

A special milestone today, since we had 13 TeV test collisions already some days ago, is that the LHC declared “stable beams” as indicated by the green boxes in the bottom right of LHC Page 1. This means that the innermost parts of the tracking detectors, being silicon-based and the most sensitive, can have their high voltage settings safely turned up to nominal operation because the LHC is confident they have tight control of the beams and that no stray particles in the orbiting beams could get dangerously sprayed, which could saturate and damage our detectors instead of allowing them to safely detect the collision products. This is a prerequisite for the silicon-based tracking detectors to start recording the data, which they are doing now.

This is the beginning of a lot more data and a lot more searching. If there weren’t reason enough for the physicists at CERN to get their analysis software rocking, we have it now.

## References

1. http://press.web.cern.ch/press-releases/2015/06/lhc-experiments-are-back-business-new-record-energy  “CERN press release: LHC experiments are back in business at a new record energy”
2. https://op-webtools.web.cern.ch/op-webtools/vistar/vistars.php?usr=LHC1  “LHC Page 1”
1. Yes, it does seem silly to denote the total energy as the square root of something, instead of just saying E = 13 TeV, or something similar. The history comes from the use of $s = (p_{1} + p_{2})^2$ as a Mandelstam variable, commonly used in high energy physics.