In Figures3.12,3.13and3.11we have the unfolded pseudorapidity gap distributions projected using our F∗-projection algorithm, described in [44] and Appendix C.6.
The type I boundary conditions calculate gaps with respect to the detector edge and type II between any two particle within the detector. The ALICE detector has no strict detector capabilities for direct measurement of these such as large calorimetry or tracking with ptmeasurement, thus our novel MC projection algorithm based on the combinatorial cross sections, represent a state-of-the-art approach. It is not a pure measurement, but an overcomplete `basis' projection.
0 1 2 3 4 5 6 7 8 9 10 11 12 13
10-2 10-1 100 101 102
Figure 3.11: ALICE data at √
s = 13 TeV and MC event generators: Type I pseu-dorapidity gap size distribution measured with respect to the detector edge, and the maximum of the backward or the forward direction gap is chosen per event. The projection uncertainties denote MC model variations.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 10-2
10-1 100 101 102
0 1 2 3 4 5 6 7 8 9 10 11 12 13
10-2 10-1 100 101 102
Figure 3.12: ALICE data at √
s = 13 TeV and MC event generators: Type I pseu-dorapidity gap size distribution to the backward (top) and the forward (bottom) direction measured with respect to the detector edge. The projection uncertainties denote MC model variations.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 10-3
10-2 10-1 100 101 102
0 1 2 3 4 5 6 7 8 9 10 11 12 13
10-2 10-1 100 101 102
Figure 3.13: ALICE data at √
s = 13 TeV and MC event generators: Type II pseudorapidity gap size distribution (top), and the maximum of type I and type II gaps per event (bottom). The projection uncertainties denote MC model variations.
One can see that the low mass diraction, which dominates at large pseudorapid-ity gap sizes, is more prominent in data compared with Pythia 6 and Phojet event generators. The magnitude of low mass diraction cannot be calculated from the Regge power law asymptotics controlled by the Regge trajectories, thus its proportion in event generators is purely model and tune dependent. At low masses, the proton structure uctuations and baryonic resonances dominate. The values obtained here are in the line with the ATLAS measurement at √
s = 7 TeV, which however has data only up to ∆η ≈ 8 [56]. They have an explicit control on the transverse en-ergy cuto, although the calorimeter noise starts to dominate over the signal at very low-pt.
3.7 Conclusions
The main new results of the measurements presented in this section with the pp data at the cms energy √
s = 13 TeV are as follows.
The rst unfolded multidimensional ducial measurement of combinato-rial partial cross sections.
The rst multidimensional maximum likelihood extraction of the single, double and non-diractive cross sections. Re-extraction is possible a pos-teriori with future Monte Carlo models based on the measured unfolded partial cross sections.
The rst multidimensional extraction of the eective soft Pomeron in-tercept with ∆P = 0.094 ± 0.01(stat+syst).
Low mass diraction enhancement with respect to Monte Carlo event generator tunes is clearly visible in the forward pseudorapidity gap dis-tributions up to ∆η ' 13 units, projected using the newly developed F∗-algorithm.
Our new measurements can be used to get more understanding of the AGK rule type calculus, which is only partially understood theoreti-cally. These are used implicitly in the construction of the EPOS LHC and QGSJet-II-04 event generators, which give one of the best descrip-tions of the measured partial cross secdescrip-tions. However, the quality of MC parameter `tunes' plays always a big factor. The detector level distribu-tions in Appendix A.1 are also a step forward on this topic. A future measurement using FNq with q = 3 or more, with three dierent multi-plicity bins per rapidity slice, could make this topic more transparent.
This higher order measurement requires more statistics at least by a factor of 3N/2N.
We introduce new algorithms for the studies of exclusive and semi-exclusive central diraction (CED/CEP) and describe our measurements with ALICE data. Poten-tially the most interesting on this topic is the future identi cation of glueball res-onances, understanding the pomeron spin structure and the contribution of proton dissociative events. Glueballs, the resonant bound states of non-abelian gauge elds, are very di cult objects because they mix quantum mechanically with quark bound states. A curious mathematical fact was pointed out by Coleman already in 1977, that classical non-abelian gauge elds do not contain glueball like eld con gura-tions [57]. Some prediccon gura-tions for the quantum glueballs can be calculated using lattice QCD [58,59] for pure glue states within the `quenched approximation', or using holo-graphic approximations within Witten-Sakai-Sugimoto model based descriptions [60, 61]. Dierent mixing matrix scenarios have been considered also purely phenomeno-logically [62].
The experimental identi cation of glueballs is an extremely laborious task, re-quiring simultaneous measurements in multiply decay channels such as pion pairs, kaon pairs but also photon pairs. We know that gluons couple only to quarks or other gluons, so a (pure) glueball decay to photons should be suppressed via quark loops. Central exclusive production is in general considered to be the best glue-ball production channel at the LHC [63], pomerons being presumably objects with a high gluon content. Technically speaking all f-mesons are glueball candidates at some level, some more than others because of their position on the pomeron Regge trajectory such as f2(1950) or because of lattice QCD estimates, which is the case for example with f0(1710). This way we see that it is just natural to say
Central Production Spectrum ' Glueball Candidate Spectrum.
Also, we point out that there is no such strict spin-parity rule as JP C = 0++, 2++, ...
in central production per se, quite often erroneously claimed. It is just that the decay channels which are usually measured, provide no other options by conservation laws.
One may well produce resonance states with other quantum numbers too such as
axial vectors JP = 1+, as shown by the WA102 data [64]. In any case, the non-perturbative production couplings are currently theoretically incalculable.
The rst evidence for the existence of double pomeron exchange type processes pp → p + X + p was measured at the CERN ISR in the low-energy range 30 ≤
√s ≤ 62 GeV [65]. For a review of the pre-LHC era measurements see [66] and for recent measurements see Table 5.1. Our measurements do not include the forward protons but we work rather with the Babinet's complement: a forward veto type event selection. The event signature is a striking one for a high energy hadron collider: we select events with two reconstructed charge opposite tracks and require otherwise an empty detector. This semi-exclusive event selection will include also events with one or both of the forward protons excited into a low-mass state.