Hunting for QCD instantons at the LHC in events with large rapidity gaps

We outline a strategy of how to search for QCD instantons of invariant mass 20–60 GeV in diffractive events in low-luminosity runs at the LHC. We show that, by imposing appropriate selection criteria on the final states, one can select the kinematic regime where the instanton signal exceeds the background by a factor of at least 8. In spite of the relatively strong cuts that we impose on the total transverse energy and the number of charged tracks, ${\sum }_i{E}_{T,i}>15\mathrm{GeV}$ and $N_{\mathrm{ch}}>20$ measured within the $0<\eta <2$ interval and excluding events with high-$p_T$ particles, the expected cross section is sufficiently large to study the instanton production in events with large rapidity gaps at low luminosities, thus avoiding problems with pileup. The paper also includes an updated computation of instanton cross sections and other parameters relevant for ongoing studies.

Figure 1

Schematic diagram for the triple parton scattering process.

Figure 2

Instanton production in a diffractive process with an LRG. The Pomeron exchange is shown by the thick doubled line. The red bar shows the range of $\eta$ considered in this paper. $Y$ indicates the incoming proton position in rapidity. As shown in the diagram, secondaries will be produced outside this range but will not be used when calculating $E_T$ or $N_{ch}$.

Figure 3

Multiplicity distribution of charged hadrons produced in the events with the instanton (green line) in comparison with the expected background (red line). The number of events is normalized to the integrated luminosity $L=1{\mathrm{pb}}^{-1}$ and $\mathrm{\Delta }\ln (x_{\mathrm{Pom}})=1$ interval. We required events to have ${\sum }_i{E}_{T,i}>15\mathrm{GeV}$ and $N_{ch}>20$, summing only over charged particles in the region $0<\eta <2$ with $p_T>0.5\mathrm{GeV}$, with an additional constraint that there is no charged particle in this region with $p_T>2\mathrm{GeV}$ (left figure) or no charged particle in the region $-2<\eta <2$ with $p_T>2.5\mathrm{GeV}$ (right figure).

Figure 4

Distribution over the transverse sphericity $S_T$ [Eq. (8)] of the charged hadrons produced in the events with the instanton (green line) in comparison with the expected background (red line). The selection criteria used are the same as those described in Fig. 3.

Figure 5

Distribution over the number of neutral kaons produced in the events with the instanton (green line) in comparison with the expected background (red line). The selection criteria used are the same as those described in Fig. 3.