Why QCD Explorer stage of the LHeC should have high(est) priority

S. A. Çetin$^{a}$, S. Sultansoy$^{b}$, G. Ünel$^{c}$

$^{a}$[Doğuş University, Istanbul, Turkey]{}

$^{b}$[TOBB University of Economics and Technology, Ankara, Turkey\
and ANAS Institute of Physics, Baku, Azerbaijan ]{}

$^{c}$[University of California, Irvine, USA]{}

*Abstract: The QCD Explorer will give opportunity to enlighten the origin of the 98.5% portion of the visible universe’s mass, clarify the nature of the strong intecartions from parton to nuclear level and provide precision pdf’s for the LHC. Especially the $\gamma{}$-nucleus option seems to be very promising for QCD studies.*

Linac-ring type colliders have two main goals: to explore TeV scale with lepton-hadron and photon-hadron collisions and to achieve highest luminosity at flavor factories (the history of corresponding proposals can be found in \[1\]). This note concentrates on the first goal which is represented by the linac-ring option of the Large Hadron electron Collider (LHeC), proposed to explore the highest energy proton and ion beams available at the LHC probed by energetic electron or gamma beams from a linac tangent to the LHC. The Conceptual Design Report (CDR) of the LHeC project which is published in \[2\], investigates two options for the collider: Linac-Ring (LR) type collider where electrons are provided by linac and Ring-Ring (RR) option which assumes an additional electron ring in the LHC tunnel.

The idea of the LHC based linac-ring type ep/$\gamma{}$p collider includes two stages: QCD Explorer (E$_{e }$= 60, 140 GeV) and Energy Frontier (E$_{e }$$\geq{}$ 500 GeV). The first stage is mandatory for a deeper understanding of the strong interactions and an adequate interpretation of the LHC data which requires precision pdf’s. The second stage, which actually depends on the outcomes of the LHC, hence called provisional, will mainly have great potential for BSM physics complementary to LHC and exceeding the possible ILC. It should be noted that the Energy Frontier as well as $\gamma{}$p options of both QCD Explorer and Energy Frontier can only be realised with the linac-ring option.

Today, LR option is considered as the basic one for the LHeC. Actually this decision was almost obvious from the beginning due to the complications in constructing by-pass tunnels around the existing experimental caverns and installing the e-ring in the already commisioned tunnel. Let us remind that the CDR sthgeaof the LHC assumed also ep collisions using the already existed LEP ring; but it turned out that LHC installation required dismantling of LEP from the tunnel.

Now that LR is the choice for the LHeC, Energy Recovery Linac (ERL) is being pushed as a basic choice instead of the single-pass option using the argument that it could provide an order of magnitude higher luminosity. Nevertheless, keeping in mind that such higher luminosity is not necesseary for the QCD Explorer, it is likely that the single-pass option will become dominant soon, however we believe the sooner the better. It should ne mentioned that a very important advantage of the LR option, namely the opportunity to construct $\gamma{}$p/$\gamma{}$A collider loses its strength at rhe ERL based LHeC, moreover the single-pass option will give the opportunity to increase the energy of the electrons by lengthening the linac further.

Concerning the physics program of the QCD Explorer, putting forward the search for SUSY or other BSM physics or even detailed study of the Higgs boson as the main goal would have serious drawbacks. The uniqueness of such a machine lies in its potential to probe the nature of the strong interactions from parton to nuclear level and provide precision pdf’s for the adequate interpretation of the LHC results. It is well known that big challenges still exist in the QCD part of the Standard Model like understanding confinement and quark-gluon plasma. QCD Explorer will give the opportunity to reach very small x$_{g}$ region \[3\] shedding light on confinement. Then according to vector meson dominance the $\gamma{}$A collider will act as a $\rho{}$A collider which will give an opportunity to investigate formation of the quark gluon plasma at very high temperatures and low densities.

In light of the discussions presented above we propose the following phases for QCD Explorer based on single-pass linac option. First phase: ep collider with luminosity of 10$^{32}$cm$^{-2}$s$^{-1}$ and eA collider with luminosity of AxL$_{eA}$=10$^{31}$cm$^{-2}$s$^{-1}$ which seems sufficient for QCD studies. Second Phase: $\gamma{}$p and $\gamma{}$A collider with similar luminosities. Third Phase: construction of a second single-pass linac for energy recovery \[4\] to achieve much higher luminosities. Fourth Phase: lengthening the single-pass linac to switch to Energy Frontier stage.

With the discovery of the long sought Higgs boson, the electroweak sector of the Standard Model has filled its gaps. At this point it is worth mentioning that the Higgs Mechanism accounts for only $\sim$1.5% of the mass of the visible universe and the rest, $\sim$98.5% is provided by the QCD. Hence another strong motivation of the QCD Explorer is to better understand the formation of the visible universe.

In conclusion, we hope that the presented qualitative arguments justify the necessity of the QCD Explorer for the future of the high energy physics.

**References**

1\. A.N. Akay, H. Karadeniz and S. Sultansoy, *Review of Linac-Ring Type Collider Proposals*, Int. J. Mod. Phys. A25 (2010) 4589-4602; e-Print: arXiv:0911.3314 \[physics.acc-ph\]

2\. J L Abelleira Fernandez et al. (LHeC Study Group), *A Large Hadron Electron Collider at CERN: Report on the Physics and Desigon Concepts for Machine and Detector*, J. Phys. G. 39 (2012) 075001; e-Print: arXiv:1206.2913 \[physics.acc-ph\]

3\. U. Kaya, S. Sultansoy, G. Unel, *Probing small x(g) region with the LHeC based gamma-p colliders*, Nov 2012, e-Print: arXiv:1211.5061 \[hep-ph\]

4\. V. Litvinenko, *LHeC with \~100% energy recovery linac*, 2nd CERN-ECFA-NuPECC workshop on LHeC, Divonne-les-Bains, 1-3 Sep (2009).
