Aims of the project

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Our aim is to form in the long run a new international collaboration working on a fixed-target program using the 7 TeV proton and the 2.76 TeV lead beams of the CERN Large Hadron Collider. At the heart of the proposal is the integration of the expertise of experimentalists, theorists and engineers at the very beginning of the conception of this fixed-target experiment, which we believe will become the first of a new generation. Beam extraction by bent crystals offers an ideal way to obtain a clean and very collimated high-energy beam, without decreasing the performance of the LHC. This technique is now becoming mature with successful tests at SPS (450 GeV) and at the Tevatron (900 GeV) and with approved tests at the LHC (3.5 or 7 TeV). 2 goniometers ans 2 crystals have now been installed in the LHC beam pipe. Another possibility is to use an internal gas target following the success of the LHCb SMOG system, initially designed to monitor the LHC luminosity. Data taking period at low luminosities using the LHCb detector of a few hours did not interfere with the normal LHC functioning.

Using the unprecedented energies of the LHC beams, such an experiment, tentatively named AFTER for “A Fixed-Target ExperRiment”, gives access to new domains of particle and nuclear physics complementing that of collider experiments, in particular that of Brookhaven's Relativistic Heavy Ion Collider (RHIC) and the projects of Electron-ion colliders (EIC). We have already evaluated that the instantaneous luminosity achievable with AFTER using typical targets would surpass that of RHIC by more than 3 orders of magnitude (both in the extracted-beam and internal-gas-target modes). As simple as it seems, the multi-TeV LHC beams will also allow for the most energetic fixed-target experiments ever performed.

Position of the project

The multi-TeV energy of the LHC beams would make this fixed-target physics program unique. As simple as it seems, the high energy LHC beams will allow for the most energetic fixed-target experiments ever performed. We believe that such a facility will be of much interest to a wide range of hadron, nuclear and particle physicists. The collision of the high energy LHC beams with fixed targets, including polarized and nuclei targets will greatly expand the range of fundamental physics phenomena accessible at CERN. The fixed-target mode will permit us to carry out unprecedented precision measurements of hard QCD processes. In particular, our aim is to study: rare configurations of the proton wave function which contain gluon or heavy-quarks with high momentum fraction ;

  • the gluon content in the deuteron and neutron in a wide momentum-fraction range;
  • the correlation between the proton spin and the gluon angular momentum through the Sivers effect and novel spin correlations;
  • the production of W and Z bosons in their threshold domain;
  • the melting of excited heavy-quark bound states in the deconfined QCD phase in heavy-ion collisions;
  • the nucleus structure function for momentum fractions close to and above unity;
  • the deconfinement dynamics in the target-rest frame;
  • ultra-peripheral collisions in a fixed-target mode.

Compared to the RHIC experiments, which benefit from similar center-of-mass energies, our proposal will bear upon a huge luminosity –typical of a fixed-target set-up– and upon a complete versatility of target species. Compared to Electron-ion collider projects, our proposal will certainly be highly competitive in terms of cost and it will be of complementary design, with a specific focus on the study of parton content at large momentum fractions – in particular that in terms of gluons. High-energy fixed-target experiments have already been discussed in the 90's, both at the European LHC and the American SSC. The main differences between our proposal and earlier ones are :

  • the fact that the LHC is now built and runs –very well indeed–,
  • bent-crystal beam-extraction techniques have now been successfully tested at the SPS and the Tevatron up to nearly 1 TeV and they will be tested on the LHC beams,
  • a number of modern detection techniques have been developed in the meantime –in particular, ultra-granular detectors– and, finally,
  • our proposal is, in essence, a multi-purpose experiment, not only focusing on one specific aspect of particle physics, as it was the case for the LHB project, for instance.

We believe it is well worth exploring this option and bringing our nuclear and particle physicist colleagues' attention to all these new physics opportunities. To do so, we plan to work out the detail of the physics case in adequacy with the current experimental possibilities –and limitations– , to develop a first robust –but ambitious– design of the experiment and its assembly compliant to the physics case, and to advertise our project all over the world-physics community to create an experimental collaboration large enough to make this project viable and fruitful for the years to come.

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