Semi-exclusive Production Research Articles

A new approach to modelling elastic and inelastic photon-initiated production at the LHC: SuperChic 4

We present the results of the new SuperChic 4 Monte Carlo implementation of photon-initiated production in proton–proton collisions, considering as a first example the case of lepton pair production. This is based on the structure function calculation of the underlying process, and focusses on a complete account of the various contributing channels, including the case where a rapidity gap veto is imposed. We provide a careful treatment of the contributions where either (single dissociation), both (double dissociation) or neither (elastic) proton interacts inelastically and dissociates, and interface our results to Pythia for showering and hadronization. The particle decay distribution from dissociation system, as well the survival probability for no additional proton–proton interactions, are both fully accounted for; these are essential for comparing to data where a rapidity gap veto is applied. We present detailed results for the impact of the veto requirement on the differential cross section, compare to and find good agreement with ATLAS 7 TeV data on semi-exclusive production, and provide a new precise evaluation of the background from semi-exclusive lepton pair production to SUSY particle production in compressed mass scenarios, which is found to be low.

Study of central exclusive production in proton-proton collisions at sqrt{s} = 5.02 and 13TeV

Central exclusive and semiexclusive production of pairs is measured with the CMS detector in proton-proton collisions at the LHC at center-of-mass energies of 5.02 and 13TeV. The theoretical description of these nonperturbative processes, which have not yet been measured in detail at the LHC, poses a significant challenge to models. The two pions are measured and identified in the CMS silicon tracker based on specific energy loss, whereas the absence of other particles is ensured by calorimeter information. The total and differential cross sections of exclusive and semiexclusive central production are measured as functions of invariant mass, transverse momentum, and rapidity of the system in the fiducial region defined as transverse momentum and pseudorapidity . The production cross sections for the four resonant channels , , , and are extracted using a simple model. These results represent the first measurement of this process at the LHC collision energies of 5.02 and 13TeV.

Semiexclusive production of vector mesons in proton-proton collisions with electromagnetic dissociation

We calculate distributions of different vector mesons in purely exclusive ($p p \to p p V$) and semi-exclusive ($p p \to p X V$) processes with electromagnetic dissociation of a proton. The cross section for the electromagnetic dissociation is expressed through electromagnetic structure functions of the proton. We include the transverse momentum distribution of initial photons in the associated flux. Contributions of the exclusive and semi-exclusive processes are compared for different vector mesons ($V = \phi, J/\psi, \Upsilon$). We discuss how the relative contribution of the semi-exclusive processes depends on the mass of the vector meson as well as on different kinematical variables of the vector meson ($y$, $p_t$). The ratio of semi-exclusive to exclusive contributions are shown and compared for different mesons in different variables.

The fusion of hard and soft Pomerons: 3-jet diffractive production

We consider the central exclusive production of high E_T jets pprightarrow p+(X+mathrm{dijet})+p. In particular we study the possible contamination of the purely exclusive signal by semi-exclusive production where no other secondaries are emitted in one hemisphere, between the highest E_T dijet and the recoil proton, while in the other hemisphere a third jet, plus possibly additional hadron activity, is allowed, but still separated from the incoming proton by a large rapidity gap. This process arising from the fusion of a hard and a soft Pomeron has not been considered before. It turns out that it gives a negligible contribution. The calculation involves a careful treatment of the QCD colour structure of the amplitudes.

Semiexclusive production of vector mesons in proton-proton collisions with electromagnetic dissociation of protons

We discuss the semiexclusive production of vector mesons in protonproton collisions with electromagnetic dissociation of one of the protons. Several differential distribution in missing mass (MX), or single-particle variables related exclusively to the produced vector meson are calculated for pp center-of-mass energies 7 and 13 TeV. The cross sections and some differential distributions are compared to their counterparts for purely exclusive reaction $pp \to pV{\rm{ }}p$. For electromagnetic dissociation the important property is that the $p\gamma^{\bigstar} \to Xp$ transitions are given by the electromagnetic structure function of proton. In our calculations we use different parametrizations of the structure function and discuss how it is constrained by the data on virtual photoabsorption on a proton.

Semiexclusive production of J/ψ mesons in proton–proton collisions with electromagnetic and diffractive dissociation of one of the protons

We calculate the cross sections for both electromagnetic and diffractive dissociation of protons for the semiexclusive production of J/ψ mesons in proton–proton collisions at the LHC. Several differential distributions in missing mass (MX), or single-particle variables related exclusively to the J/ψ meson are calculated for s=7and13 TeV. The cross sections and distributions are compared to the cross section of the purely exclusive reaction pp→ppJ/ψ. We show the corresponding ratio as a function of J/ψ meson rapidity. We compare the distributions for purely electromagnetic and purely diffractive proton excitations/dissociation. We predict cross sections for electromagnetic and diffractive excitations of similar order of magnitude.

Where does the X(5568) structure come from?

We study the semi-exclusive production of $\pi^\pm B_s^0$ pairs in hadron colliders which is associated with the $X(5568)$ structure observed by the D0 Collaboration in 2016, but that was not confirmed by LHCb and CMS later. The reason of its appearance in the D0 and absence in LHCb and CMS is discussed in this letter. In a semi-exclusive process, one might miss the third particle which is produced together with the $\pi^\pm B_s^0$ simultaneously. In the three-body Dalitz plot, once the remaining region is narrow enough after the kinematic cuts, its reflection to another invariant mass distribution will accumulate a large number of events within a specific energy region. If there is an enhancement in the remaining region, it will make the reflection structure more pronounced. The precise line shape of the reflection will depend on the specific interaction form. A combined study of different cone cuts and the low-energy dynamics, e.g. the Landau singularity, demonstrates that the $X(5568)$ structure could come from this kinematic reflection. This conclusion can be checked by both searching for the enhancement in another invariant mass distribution, such as $B_s^0\bar{B}^0$, and the cone cut dependence of the $X(5568)$ mass. Such a combined study can be used to distinguish the effects of the triangle singularity from a genuine state. We also propose how to avoid this kinematic reflection in future experimental analysis.

Semi-exclusive production of photons at HERA

We study the feasibility of measuring semi-exclusive photon production \gamma + p \to \gamma + Y at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts be simply expressed in terms of hard pQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of L = 100/pb. The subprocesses of lowest order in \alpha_s are suppressed at low recoil masses M_Y, compared to higher order gluon exchange, i.e. BFKL contributions. The distinct M_Y-dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate.

Hard exclusive and semi-exclusive meson production

I review recent theory developments for hard exclusive and semi-exclusive production of mesons, emphasising the variety of physics issues that can be studied in these processes.

Semiexclusive production in electron-positron annihilation.

We present a thorough analysis of the direct semiexclusive production in quantum chromodynamics (QCD) of single, highly isolated mesons from electron-positron or two-photon initial states. Corrections of higher order and of subleading twist are considered, and potential divergences in the naive calculation are contained. Monte Carlo methods are used to relate the QCD calculations to experimentally measureable quantities. We find that the study of semiexclusive production is the most sensitive experimental probe of the structure of mesons in the valence ([ital q[bar q]]) state at energies [radical][ital s] [approx gt]10 GeV.

Direct semiexclusive production of energetic mesons.

The formation of color-singlet hadrons in hard-scattering processes is governed by process-independent distribution amplitudes. Such amplitudes may be measured in a class of processes we call semiexclusive, where one hadron is formed directly in hard scattering and the other QCD partons hadronize in jets. These processes can be identified experimentally by requiring a large angular or rapidity gap around a directly produced meson. We present cross sections for such processes, and show that experimental results, especially the ratio of charged to neutral $K$ production, are highly sensitive to the hadronic distribution amplitude. We discuss methods and prospects for thus gaining information about hadronic wave functions at present and future colliders.

Peripheral interactions in the collision of 16O+238U at 110.4 MeV bombarding energy.

Inclusive and semiexclusive elemental production cross sections of projectilelike fragments emerging in the collision [sup 16]O+[sup 238]U at 110.4 MeV have been investigated. Energy spectra and angular distributions of projectile residues with charge from [ital Z]=3 to [ital Z]=8 have been measured. It has been found that, as expected, the dominant reaction mechanism in the peripheral collisions is a mass transfer from projectile to target mainly dominated by a transfer of an [alpha] particle. The in-plane coincidences between projectilelike and fission fragments suggest that 14% of the total fission cross section is associated with the fission of a targetlike nucleus.

A FULL-ACCEPTANCE DETECTOR FOR SSC PHYSICS AT LOW AND INTERMEDIATE MASS SCALES: AN EXPRESSION OF INTEREST TO THE SSC

A FULL-ACCEPTANCE DETECTOR FOR SSC PHYSICS AT LOW AND INTERMEDIATE MASS SCALES: AN EXPRESSION OF INTEREST TO THE SSC