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. 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.
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. 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.
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:
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.
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.
3)One can also study the production of the W and Z bosons in their threshold domain.
4)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.
5)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.
6)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.
7)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.
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.
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.
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.
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. Media:Example.ogg