Recent results from the TOTEM collaboration at the LHC

We describe the most recent results from the TOTEM collaboration at the LHC,namely the elastic cross section measurements at a center-of-mass on 2.76, 7, 8and 13 TeV. No structure or resonance is observed at high t at high center-of-mass energies. A pure exponential form of dσ/dt is excluded both at 8 and 13TeV. Accessing the very low t region allows measuring the ρ parameter at 13 TeV.1 Method to measure elastic events at the LHC The TOTEM collaboration measuredthe elastic pp → pp cross sections by detecting both intact protons in the finalstate and vetoing on activities in the main CMS detector. It installed sets ofvertical roman pot detectors at about 147 and 220-240 meters from theinteraction point that have good acceptance in detecting intact protons inelastic interactions. The elastic event trigger requires the presence of oneintact proton on each side of the interaction point on UP-DOWN or DOWN-UPconfigurations. In addition to the roman pots detectors, the TOTEM collaborationinstalled two inelastic telescopes called T 1 and T 2 covering respectively theregion 3.1 < |η| < 4.7 and 5.3 < |η| < 6.5 for charged particle p T above 100and 40 MeV, respectively. Requesting no activity in T 1 and T 2 allowssuppressing 92% of inelastic background. Before the TOTEM measurements, it isworth noticing that the predictions of elastic cross sections were showing largedifferences by orders of magnitude especially at large t. Some models evenpredicted the existence of diffractive structures at medium t at LHC energies.Elastic events allow probing soft diffraction and Pomeron exchange at low t,diffractive structures at medium t and parton scattering and perturbative QCDeffects at larger t. In Fig. 1, we display the different domains of measurementfor the dσ/dt elastic cross section. At very low t, the cross section isproportional to 1/t 2 (Coulomb region) and at higher t, we reach the nuclearregion where the cross section is proportional to e bt. Between these tworegions, we need to take into account the interferences between the nuclear andCoulomb cross sections (called C-N interference in the figure or CNI) where wecan measure the ρ parameter, the ratio of the imaginary part to the real part ofthe elastic cross section. At higher t, we reach the resonance or 131