Aims of the project

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The LHC accelerates protons and lead nuclei to multi-TeV energies. These high intensity beams, which have unprecedented energy up to 7 TeV, could be extracted from the circulating collider beams and brought into collision with fixed targets.  
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The principal goal of this proposal is to form a new collaboration of high energy particle experimentalists, accelerator physicists and theorists who will explore the physics opportunities and feasibility of extracting the 7 TeV LHC proton and 2.75 TeV lead beams to provide a viable high-energy fixed-target experimental program at CERN.
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=Aim=
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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 allow an intensive study of rare processes, novel spin-correlations, high xF dynamics, diffractive physics, nuclear phenomena as well as the novel spectroscopy of hadrons carrying multiple heavy quarks.
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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.
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The extraordinary energy of the LHC beams would make this fixed-target physics program unique. We believe that such a facility will be of much interest to a wide range of hadron, nuclear and particle physicists.
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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.  
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The scope of the physics program at such a fixed-target facility is extraordinary. A major goal of this proposal  will be to map out many of the possible physics avenues. Here are a few examples:
 
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1)The collision of the LHC beams with polarized fixed targets will allow the investigation of novel correlations of produced hadrons with the target spin  –  dynamics beyond the usual domain of perturbative QCD (pQCD). Present data indicates that these correlations are much larger than expected from pQCD.
 
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2)Fixed-target experiment are particularly advantageous for studying rare configurations of the proton wavefunction which contain heavy quarks with high momentum fractions. The D0 experiment at the Tevatron has reported a strong excess of events of photons and charm quark jets at high transverse momentum, signaling that the charm quark distribution in the proton has been significantly underestimated at high momentum fraction. This is consistent with  large range of fixed target and ISR experiments which report the production of hadrons with heavy quarks at high momentum. In fact, the SELEX fixed-target experiment at FermiLab has shown that one can produce baryons with two charmed quarks at very high momentum. One thus expects that the LHC beams colliding on fixed targets can be used to discover a large array of hadrons with heavy charm and bottom quarks – even the Ω(bbb),  a baryon with three bottom quarks, the heaviest hadron of the Standard Model.
 
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2)One can also study the production of the W and Z bosons in their threshold domain. In fact, if the Higgs particle is found at the LHC to have a mass below 110 GeV, it can also be studied in novel ways in the fixed-target mode.
 
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3)The use of nuclear targets provides a new window on nuclear phenomena, such as the study of the quark-gluon plasma in the target rest frame. Nuclear targets also allow the exploration of a wide range of novel nuclear physics such as hidden-color configurations of the nuclear wavefunction and high momentum quarks and gluons in the nucleus, far beyond the conventional Fermi momentum domain. 
 
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4)The anomalous factorization-breaking nuclear target dependence of J/psi production at high xF observed at Fermilab, the breakdown of the pQCD Lam Tung relation observed by NA10 in Drell-Yan reactions at CERN,  the onset of longitudinal polarization of the virtual photon in Drell-Yan reactions at high xF  observed by the Chicago Princeton experiment at FermiLab are just a few examples of unexpected and not well-understood phenomena seen in past fixed-target experiments.   
 
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==Position of the project==
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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.
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The fixed-target mode will permit us to carry out unprecedented precision measurements of hard QCD processes. In particular, our aim is to study:
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rare configurations of the proton wave function which contain gluon or heavy-quarks with high momentum fraction ;
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*the gluon content in the deuteron and neutron in a wide momentum-fraction range;
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*the correlation between the proton spin and the gluon angular momentum through the Sivers effect and novel spin correlations;
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*the production of W and Z bosons in their threshold domain;
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*the melting of excited heavy-quark bound states in the deconfined QCD phase in heavy-ion collisions;
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*the nucleus structure function for momentum fractions close to and above unity;
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*the deconfinement dynamics in the target-rest frame;
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*ultra-peripheral collisions in a fixed-target mode.
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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.
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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 :
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*the fact that the LHC is now built and runs –very well indeed–,
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*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,
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*a number of modern detection techniques have been developed in the meantime –in particular, ultra-granular detectors–  and, finally,
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*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.
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5)RHIC experiments have observed a remarkable phenomenon in heavy-ion collisions called the``baryon anomaly" -- the baryon to meson ratio increases at high transverse momentum in central collisions where the nuclei have maximal overlap. This phenomenon is opposite to expectations that baryons should be more absorbed in the nuclear medium than mesons; it could point to a new QCD mechanism where color-transparent hadrons are  produced directly from the hard quark and gluon subprocess rather than jet fragmentation.
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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
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6)Nuclear shadowing and antishadowing have novel features which can be probed at the LHC fixed-target experiments. One can also study diffractive dynamics, where the target or beam hadron remains intact even in extraordinarily disruptive reactions.  The E791 diffractive dijet fixed target experiment at Fermilab has shown how one can measure  fundamental features of hadronic light-front wavefunctions as well as demonstrate QCD color transparency of high transverse momentum reactions.
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to work out the detail of the physics case in adequacy with the current experimental possibilities –and limitations– ,  
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to develop a first robust –but ambitious– design of the experiment and its assembly compliant to the physics case, and  
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A central aim of this proposal will thus be to explore these and other new physics opportunities at the extraordinary laboratory energies which would be accessible using the LHC beams in a fixed-target mode.
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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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In addition to outlining the physics opportunities, the collaboration will also look into prospective designs of experimental facilities capable of making measurements over the full range of fixed-target kinematics, including the target and beam rapidity domains. 
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The target-rapidity domain is challenging experimentally, but it is particularly interesting for ion-nucleus collisions since it cannot be accessed in ion-ion colliders and may thus reveal new insights into the formation of the quark-gluon plasma.  In addition, detectors need to be designed to detect new baryons containing heavy quarks appearing in the target-rapidity domain. The aim is to be able to detect single and double diffractive reactions where the target proton or nucleus remains intact in order to access the gluon-rich phenomenology of diffractive processes. A forward detector with high resolution could also be designed to measure the nuclear diffractive process p A → Jet Jet Jet A' where a proton diffracts to three quark jets leaving the nucleus in the target intact. As outlined by Frankfurt, Miller and Strikman,  such an experiment can measure the three-quark wavefunction of the proton as well as test color transparency.  The fixed target facility could also be designed to provide high energy secondary hadron beams.
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The extraction of the high-energy LHC beams in a form usable for fixed-target experiments brings extraordinary challenges for particle and accelerator physics, including efficient extraction, and radiation issues. A principal aim of this proposal will be to bring in experts in accelerator physics, as well as target detector design to evaluate the feasibility of a viable fixed-target program at CERN and optimize its characteristics, while minimizing costs.
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Latest revision as of 08:43, 12 October 2015

Aim

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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