LHC Run II: What To Look Out For

The Large Hadron Collider is the world’s largest proton collider, and in a mere five years of active data acquisition, it has already achieved fame for the discovery of the elusive Higgs Boson in 2012. Though the LHC is currently off to allow for a series of repairs and upgrades, it is scheduled to begin running again within the month, this time with a proton collision energy of 13 TeV. This is nearly double the previous run energy of 8 TeV,  opening the door to a host of new particle productions and processes. Many physicists are keeping their fingers crossed that another big discovery is right around the corner. Here are a few specific things that will be important in Run II.


1. Luminosity scaling

Though this is a very general category, it is a huge component of the Run II excitement. This is simply due to the scaling of luminosity with collision energy, which gives a remarkable increase in discovery potential for the energy increase.

If you’re not familiar, luminosity is the number of events per unit time and cross sectional area. Integrated luminosity sums this instantaneous value over time, giving a metric in the units of 1/area.

lumi                          intLumi

 In the particle physics world, luminosities are measured in inverse femtobarns, where 1 fb-1 = 1/(10-43 m2). Each of the two main detectors at CERN, ATLAS and CMS, collected 30 fb-1 by the end of 2012. The main point is that more luminosity means more events in which to search for new physics.

Figure 1 shows the ratios of LHC luminosities for 7 vs. 8 TeV, and again for 13 vs. 8 TeV. Since the plot is in log scale on the y axis, it’s easy to tell that 13 to 8 TeV is a very large ratio. In fact, 100 fb-1 at 8 TeV is the equivalent of 1 fb-1 at 13 TeV. So increasing the energy by a factor less than 2 increase the integrated luminosity by a factor of 100! This means that even in the first few months of running at 13 TeV, there will be a huge amount of data available for analysis, leading to the likely release of many analyses shortly after the beginning of data acquisition.

Figure 1: Parton luminosity ratios, from J. Stirling at Imperial College London (see references.)


2. Supersymmetry

Supersymmetry theory proposes the existence of a superpartner for every particle in the Standard Model, effectively doubling the number of fundamental particles in the universe. This helps to answer many questions in particle physics, namely the question of where the particle masses came from, known as the ‘hierarchy’ problem (see the further reading list for some good explanations.)

Current mass limits on many supersymmetric particles are getting pretty high, concerning some physicists about the feasibility of finding evidence for SUSY. Many of these particles have already been excluded for masses below the order of a TeV, making it very difficult to create them with the LHC as is. While there is talk of another LHC upgrade to achieve energies even higher than 14 TeV, for now the SUSY searches will have to make use of the energy that is available.

Figure 2: Cross sections for the case of equal degenerate squark and gluino masses as a function of mass at √s = 13 TeV, from 1407.5066. q stands for quark, g stands for gluino, and t stands for stop.


Figure 2 shows the cross sections for various supersymmetric particle pair production, including squark (the supersymmetric top quark) and gluino (the supersymmetric gluon). Given the luminosity scaling described previously, these cross sections tell us that with only 1 fb-1, physicists will be able to surpass the existing sensitivity for these supersymmetric processes. As a result, there will be a rush of searches being performed in a very short time after the run begins.


3. Dark Matter

Dark matter is one of the greatest mysteries in particle physics to date (see past particlebites posts for more information). It is also one of the most difficult mysteries to solve, since dark matter candidate particles are by definition very weakly interacting. In the LHC, potential dark matter creation is detected as missing transverse energy (MET) in the detector, since the particles do not leave tracks or deposit energy.

One of the best ways to ‘see’ dark matter at the LHC is in signatures with mono-jet or photon signatures; these are jets/photons that do not occur in pairs, but rather occur singly as a result of radiation. Typically these signatures have very high transverse momentum (pT) jets, giving a good primary vertex, and large amounts of MET, making them easier to observe. Figure 3 shows a Feynman diagram of such a decay, with the MET recoiling off a jet or a photon.

Figure 3: Feynman diagram of mono-X searches for dark matter, from “Hunting for the Invisible.”


Though the topics in this post will certainly be popular in the next few years at the LHC, they do not even begin to span the huge volume of physics analyses that we can expect to see emerging from Run II data. The next year alone has the potential to be a groundbreaking one, so stay tuned!



Further Reading:



CMS evidence of a possible SUSY decay chain

Title: “Search for physics beyond the standard model in events with two leptons, jets, and missing transverse energy in pp collisions at sqrt(s)=8 TeV.”
Author: CMS Collaboration
Published: CMS Public: Physics Results SUS12019

The CMS Collaboration, one of the two main groups working on multipurpose experiments at the Large Hadron Collider, has recently reported an excess of events with an estimated significance of 2.6σ. As a reminder, discoveries in particle physics are typically declared at 5σ. While this excess is small enough that it may not be related to new physics at all, it is also large enough to generate some discussion.

The excess occurs at an invariant mass of 20 – 70 GeV in dilepton + missing transverse energy (MET) decays. Some theorists claim that this may be a signature of supersymmetry. The analysis was completed using kinematic ‘edges’, an example of which can be seen in Figure 1. These shapes are typical of the decays of new particles predicted by supersymmetry. 


Figure 1: Diagram of kinematic ‘edge’ effects in decay chains, from “Search for an ‘edge’ with CMS”. On the left, A, B, C, and D represent particles decaying. On the right, the invariant mass of final state particles C and D is shown, where the y axis represents the number of events.

The edge shape comes from the reconstructed invariant mass of the two leptons; in the diagram, these correspond to particles C and D. In models that conserve R-parity, which is the quantum number that distinguishes SUSY particles from Standard Model particles, a SUSY particle decays by emitting an SM particle and a lighter SUSY particle. In this case, two leptons are emitted in the chain. Reconstructing the invariant mass of the event is impossible because of the invisible massive particle. However, the total mass of the lepton pair can have any value, provided it is less than the maximum difference in mass between the initial and final state, as enforced by energy conservation. This maximum mass difference gives a hard cutoff, or ‘edge’, in the invariant mass distribution, as shown in the right side of Figure 1. Since the location of this cutoff is dependent on the mass of the original superparticle, these features can be very useful in obtaining information about such decays.


Figure 2 shows generated Monte Carlo for a new particle decaying to a two lepton final state. The red and blue lines show sources of background, while the green is the simulated signal. If the model was a good estimate of data, these three colored lines would sum to the distribution observed in data. Figure 3 shows the actual data distribution, with the relative significance of the excess around 20 – 70 GeV.

Figure 2: Monte Carlo invariant mass distribution of paired electrons or muons; signal shown in green with characteristic edge.
Figure 3: Invariant mass data distribution for paired leptons; excess between 20 and 70 GeV constitutes an estimated 2.6σ significance. 















This excess is encouraging for physicists hoping to find stronger evidence for supersymmetry (or more generally, new physics) in Run II. However, 2.6σ is not especially high, and historically these excesses come and go all the time. Both CMS and ATLAS will certainly be watching this resonance in the 2015 13 TeV data, to see whether it grows into something more significant or simply fades into the background.


Further reading: