Diffractive reactions can be defined in terms of rapidity gaps. The inelastic proton-proton reaction rate is in principle the sum of rates of non diffractive, single diffractive and double diffractive processes. Particles emitted in diffractive reactions are mainly found at rapidities close to that of the parent proton. It is possible to differentiate between single, double or central diffraction processes depending on the actual location of the rapidity gaps. Figure 1 shows diagrams of possible processes in proton-proton interactions. Those diagrams in which a strongly interacting color singlet is exchanged, result in the scattering of a proton or the two colliding protons. The φ-η plot shows the distribution of particles. The diagram for single diffractive scattering is similar to elastic scattering except that the proton breaks up populating a limited region of rapidity. Soft diffractive processes are dominated by non-perturbative dynamics. They may account approximately 20% of the total inelastic cross section.

The  setup of two stations called AD (stands for ALICE Diffractive) on the  right and on the left of the interaction point enhances significantly the  efficiency to study diffractive physics and photon induced processes. It allows to collect more data at very low diffractive invariant masses and increases considerably  the purity of registered diffractive events.

Efficiency

ALICE profited from particularly favourable circumstances in its study of diffraction: a detector sensitive to particles with low transverse-momentum, down to about 20 MeV/c; data taken with low luminosity so that corrections for event overlap in the same proton bunch-crossing are small.

AD detector will increase the sensitivity to diffractive masses close to the threshold 31 (mp + mπ) and also partially compensate for the loss of trigger efficiency for Minimum Bias events and diffractive events when reaching the design LHC energies . This detector will provide a level zero trigger signal which will be useful for diffractive cross sections measurements. It will extend the pseudorapidity gap trigger, crucial in the study of central diffraction, where the physics reach is limited by statistics. In addition, the possibility of triggering on the charge deposition in the AD scintillator modules will provide an extended centrality trigger in both PbPb and pPb collisions studies.

The challenge is also to study diffraction while being unable to observe either the non-diffracted proton or events in which the diffracted system escapes the acceptance of the detector. Nevertheless, the ALICE detectors cover a sufficient range in pseudorapidity (8.8 units, from –3.7 to 5.1, for collisions at the origin of the co-ordinate system) to have ample sensitivity to the SD and DD processes. Two independent observables were identified that are sensitive to diffraction: the ratio of the numbers of SD-like (activity on one side of the detector only) to NSD-like (activity on both sides of the detector) events; and the width distribution of the pseudorapidity gap for events of NSD type.

Figure 2. Single- and double-diffraction (Δη > 3) proton–proton and proton–antiproton cross-sections, left and right, respectively, as a function of centre-of-mass energy, compared with current models: Gotsman et al. (short, dot-dashed blue line), Goulianos (dashed green line), Kaidalov-Poghosyan (solid black line), Ostapchenko (long, dot-dashed pink line) and Ryskinet al. (dotted red line).

Collaboration

The collaborating institutes in the AD Detector are:

- Czech Technical University of Prague, CTU, PRAHA

- Helsinki Inst. of Physics, HELSINKI

- IPN - Institut de Physique Nucléaire, Université Claude Bernard Lyon-I, CNRS/IN2P3, Lyon

- Centro Studi e Ricerche "Enrico Fermi", ROMA

- Yonsei University, SEOUL

- Benemerita Autonomous Univ. of Puebla, PUEBLA

- Centro de Investigacion y de Estudios Avanzados del IPN  CINVESTAV , Dep. de Fisica and Dep. de Fisica Applicada; MEXICO

- Universidad Autonoma de Sinaloa; CULIACAN

- Pontificia Universidad Catolica del Peru (PUCP), LIMA

- The Henryk Niewodniczanski Inst. of Nuclear Physics Polish Academy of Sciences, CRACOW

- Russian Academy of Sciences, Inst. for Nuclear Research (INR), Moscow

- Creighton University, OMAHA

 

Comissioning

The light produced by BC404 plastic scintillator material is collected by two Wave Length Shifting bars (WLS) attached, but not glued, on each side of the pads. Each WLS bars transfer the collected light to a bundle of 96 transparent optical fibers, which conducts the light to the PMTs (inside the ALICE cavern). The light is converted into an electric pulse by a fine mesh PMT from Hamamatsu R5946 (hybrid assembly H6153-70).

The signal from the PMT is sent to a preamplifier card which delivers two signals: one, amplified by a factor of 10 and clamped at about 300 mV which is used for timing measurement, the second, direct unmodified signal, is used for charge integration. The preamplifiers are installed close to the Front End Readout electronics. The Front End Electronics provides signals for the level 0 trigger of ALICE.

The readout electronics is the one presently used in the VZERO detector [4]. The trigger signals of the AD counters will expand the acceptance of Minimum Bias trigger. Moreover it will be possible to trigger on charge deposition in the two AD detectors providing an extended centrality trigger in both Pb-Pb and proton-Pb collision studies. The photomultipliers and scintillators were calibrated with cosmic ray data taking and LED signals in the laboratory. The measured time resolution is about 0.8 ns and a clear separation between the signal and the pedestals is shown in figure 1 for cosmic muons.