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Andrea Signori | Nikhef | Netherlands

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AFTER@LHC: Going beyond the collider

While everybody is excited by the coming “phase 2” of the LHC, someone else is already looking beyond it, thinking: “what are the possible future scenarios for our beloved Large Hadron Collider?”

The community of “phenomenologists”, theoreticians who like to play with data, closely collaborate with experimentalists to plan new experiments. We are hoping to get the most out of a set-up and think about future stages and improvements.

In the last months there has been a lot of interest around a proposal for a new experiment at the LHC: “AFTER@LHC”, namely A Fixed Target ExpeRiment at the LHC. This means that we do not have particles running in opposite directions within two rings (the collider setting), crashing head-on; rather, there is just one ring where particles run coherently and are then extracted by means of a crystal and smashed against a fixed target, like hitting a wall.


You may actually wonder: “Why should I prefer this instead of the super nice and Nobel-prize-generator collider?”

In the LHC protons are accelerated at approximately the speed of light and collide along the ring. The protons are made out of quarks and gluons, so each proton-proton collision can be interpreted as a smashing among their elementary constituents. In particular, since gluons are the most relevant elementary constituents at the LHC energy, the latter can be thought as a collider of gluons.

As I partly discussed in a previous post, we can study the structure of the proton with 3D probability distributions (transverse-momentum-dependent distributions, TMDs) which allow you to access all the possible spin and momentum configurations of the constituents. For example, quark and gluons can be investigated with and without their spin state, and the proton where they live in can be polarized or not. There are several of these combinations and each one represents a fundamental piece in the puzzle of the proton structure.

The LHC is currently running with beam of unpolarized protons only. Meaning we do not consider their spin in analyses. For those who want to investigate the puzzle of a proton’s structure, this is a limitation. We are able to access only two out of the eight (under certain assumptions) configurations of polarizations, namely the unpolarized and the linearly polarized gluons. So there are six options we don’t get to study!

In this table the eight available TMD (transverse-momentum-dependent) distributions shaping the physics of (un)polarized gluons inside (un)polarized protons are listed. At the LHC we can access the first row only, at AFTER more combinations will be investigated.

In this table the eight available TMD distributions shaping the physics of (un)polarized gluons inside (un)polarized protons are listed. At the LHC we can access the first row only, at AFTER more combinations will be investigated.

And here is the answer to our question. The fixed target at AFTER could be easily polarized, allowing us to study the physics of gluons inside polarized protons, which would be impossible at the present collider! There is only another machine in the world where hadrons can be polarized: the Relativistic Heavy Ion Collider – RHIC at Brookhaven National Lab.

For this reason, AFTER could access novel phenomena intrinsically related to the polarization of hadrons and, at the same time, allow us to study processes already available at the LHC but in different physical regions. For example, there is the possibility of accessing the simple 1D probability distributions in a region where they are still poorly known.

A particularly interesting observable which AFTER could look at is the so-called “Sivers” distribution for gluons, namely the probability of extracting unpolarized gluons from a proton whose spin is transverse with respect to the direction of the beam. Part of its core features cannot be calculated from first principles in the theory, so a good way to explore it would be extraction from experimental data. In the past years physicists got indications that the Sivers effect for gluons could be small, but an experimental insight at AFTER would be really important.

As you can see, there could be a lot of cool physics going on. We are in the early stages, where all the possible (including economic) constraints need to be taken into account and where a good scientific motivational plan is fundamental.

When you try to give birth to an experiment you face a lot of problems, like “What’s a realistic estimate of its scientific impact? Do we really need a new machine or not?” Some of these questions have already been addressed and the answers are collected in scientific publications, which you can partly find here.

If everything goes according to plan and desires, AFTER@LHC will bring very good insight and contributions to the study of the proton structure: stay tuned for updates!


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