LHC

Still numerous questions arising from the Standard Model could not be answered by previous accelerators due to energy limitations, motivating the development of an even bigger, new
machine. The LHC (Large Hadron Collider) at CERN is an unprecedented project opening new perspectives in high energy physics. It is the successor of the LEP (Large Electron Positron) collider and has no comparable counterpart worldwide. Currently (2001), the LEP collider is being dismounted, and parts of LHC and its experiments are already under assembly. In 2006, the construction will be finished and the experiments will be taking data for about 10 years.

The LHC will reuse the LEP ring tunnel with $27\,\rm km$ circumference, yet it will provide much higher particle energies because collision partners are protons on protons (and alternatively lead ions) instead of electron/positron pairs. While LEP was designed for a center of mass energy of $200\,\rm GeV$, LHC will reach $14\,\rm TeV$ with protons and $1312\,\rm TeV$ with lead ions. Furthermore, the LHC bunch crossing (bx) frequency of $40\,\rm MHz$ (corresponding to $25\,\rm ns$) will be approximately thousand times higher compared to LEP. At average, every bunch crossing will result in about 18 proton-proton collisions, generating 500 charged particle tracks. Compared to the LEP electron-positron collider, where collisions occurred rarely due to the low cross-sections of electrons and positrons, the collision rate will be almost $10^9$ times higher in LHC. Such enormous rates are necessary to acquire reasonable statistics on extremely rare particles and decay processes.

The event rate $R$ in a collider is proportional to the interaction cross-section $\sigma_{\rm int}$,

$$R=\mathcal{L}\,\sigma_{\rm int}\quad,$$ (1.1)

with the factor $\mathcal{L}$ called luminosity [3]. When two bunches, each containing $n$ particles, collide with the frequency $f$, the luminosity is given by

$$\mathcal{L}=f\frac{n^2}{4 \pi \sigma_x \sigma_y}$$ (1.2)

where $\sigma_x$ and $\sigma_y$ characterize the beam spread in horizontal and vertical directions.

An overview of LEP and LHC related figures is given in tab. .

Table: Properties of LEP and LHC. The LEP collision rate refers only to hard (i.e. central) collisions.

Machine	Beams		Energy	Luminosity	bx period	Collision rate
			$\rm [TeV]$	$\rm [cm^{-2}\,s^{-1}]$	$\rm [ns]$	$\rm [1/s]$
LEP	$\rm e^+$	$\rm e^-$	0.2	$10^{32}$	22000	1
LHC	$\rm p$	$\rm p$	14	$10^{34}$	25	$7.2\cdot 10^8$
	$\rm Pb$	$\rm Pb$	1312	$10^{27}$	125	$5\cdot 10^3$

Four detectors will be located at the collision points along the circular LHC. These are called ``experiments'' and will measure the enormous number of particles arising from proton-proton collisions. The two large detectors are ATLAS (A toroidal LHC apparatus) and CMS (Compact Muon Solenoid), while the smaller experiments are LHCb (B-meson experiment) and ALICE (A Large Ion Collider Experiment).