Low mass diffractive systems at LHC

Diffractive reactions in proton-proton collisions are characterized by the presence of rapidity gaps and by forward scattered protons. A diffractive trigger can therefore be defined by the tagging of the forward proton or by the detection of rapidity gaps. I present a diffractive trigger scheme for the ALICE detector at the large hadron collider LHC and discuss some physics topics. In particular, I concentrate on the low mass sector in central exclusive diffraction which becomes accessible by a double gap trigger. 1 The ALICE detector The ALICE experiment at the LHC is designed as a general purpose experiment with a central barrel covering the pseudorapidity range −0.9 < η < 0.9 and a muon spectrometer covering the range −4.0 < η < −2.5 [1, 2]. The ALICE experimental program foresees data taking in pp and PbPb collisions at luminosities of L = 5x10cms and L = 10cms, respectively. An asymmetric system pPb will be measured at a luminosity of L = 10cms. The central detectors track and identify particles from ∼ 100 MeVc to ∼ 100 GeVc transverse momenta. Short-lived particles such as hyperons, D and B mesons are identified by their reconstructed secondary decay vertex. The detector granularity is chosen such that these tasks can be performed in a high multiplicity environment of up to 8000 charged particles per unit of rapidity. Tracking of particles is achieved by the inner tracking system (ITS) of two layers of silicon pixel (SPD), two layers of silicon strip (SSD) and two layers of silicon drift detectors (SDD). The global reconstruction of particle momentum uses the ITS information together with the information from a large Time-Projection-Chamber (TPC) and a high granularity Transition-Radiation Detector (TRD). Particle identification in the central barrel is performed by measuring energy loss in the tracking detectors, transition radiation in the TRD and time-of-flight in a high-resolution TOF array. A single arm High-Momentum Particle Identification Detector (HMPID) with limited solid angle coverage extends the momentum range of identified hadrons. Photons will be measured by a crystal PbWO4 PHOton Spectrometer (PHOS) and an electromagnetic sampling calorimeter (EMCAL). Additional detectors for trigger purposes and for event classification are placed on both sides of the central barrel such that the pseudorapidity range −3.7 < η < 5 is covered. Fig. 1 shows the pseudorapidity acceptance of ALICE resulting from the ALICE detectors as explained above. The event characterization detectors shown in this figure are quartz