WG2 Summary: Small-x, Diffraction and Vector Mesons
Here we briefly summarize the contributions to Working Group 2: Small-x, Diffraction and Vector Mesons, at the DIS2021 Workshop.
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arXiv subjects
Marta Luszczak
Publications and source records attributed to Marta Luszczak.
Production of dileptons via photon-photon fusion in proton-proton collisions with one forward proton measurement
We discuss the mechanism of dilepton production in proton-proton scattering via fusion of virtual photons with identification of one proton on either side. This is relevant for the ATLAS+AFP and CMS+PPS at the LHC. Transverse momenta of the photons are taken into account via photon unintegrated fluxes. The latter ones are expressed in terms of proton electromagnetic form factors and structure functions. We include different categories of such processes (double elastic, single dissociative). Some differential distributions are shown explicitly and differences with respect to the results without proton identification are discussed. A soft gap survival factor is calculated using SuperChic-4 code.
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Central dileption production in proton-proton collisions with rapidity gap and with forward protons
We discuss photon-photon fusion mechanisms of dilepton production in proton-proton collisions with rapidity gap in the main detector and one forward proton in the forward proton detectors. This is relevant for the LHC measurements by ATLAS+AFP and CMS+PPS. Transverse momenta of the intermediate photons are taken into account and photon fluxes are expressed in terms of proton electromagnetic form factors and structure functions. Both double-elastic and single-dissociative processes are included in the analysis. Different parametrizations of the structure functions are used. Some differential distributions are presented. Some differences with respect to the results without proton measurement are discussed.
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Production of $W^+ W^-$ and $t \bar t$ pairs via photon-photon processes in proton-proton collisions
We review our recent results for production of $W^+ W^-$ and $t {\bar t}$ pairs via photon-photon fusion. A theoretical approach is presented in short. We include transverse momenta of photons when calculating fluxes of photons. Then we discuss our results for cross section (total and differential) for $W^+ W^-$ production. Results for different parametrizations of proton structure functions are used to calculate inelastic fluxes of photons. A discussion on rapidity gap survival probability due to remnant fragmentation is presented. A similar discussion is presented for $t {\bar t}$ production.
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Probing the photonic content of the proton using photon-induced dilepton production in $p+\textrm{Pb}$ collisions at the LHC
We propose a new experimental method to probe the photon parton distribution function inside the proton (photon PDF) at LHC energies. The method is based on the measurement of dilepton production from the $γp\rightarrow\ell^+\ell^-+X$ reaction in proton--lead collisions. These experimental conditions guarantee a clean environment, both in terms of reconstruction of the final state and in terms of possible background. We firstly calculate the cross sections for this process with collinear photon PDFs, where we identify optimal choice of the scale, in analogy to deep inelastic scattering kinematics. We then perform calculations including the transverse-momentum dependence of the probed photon. Finally we estimate rates of the process for the existing LHC data samples.
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Production of $W^+ W^-$ pairs via $γ^*γ^* \to W^+ W^-$ subprocess with photon transverse momenta
We discuss production of $W^+ W^-$ pairs in proton-proton collisions induced by two-photon fusion including, for a first time, transverse momenta of incoming photons. The unintegrated inelastic fluxes (related to proton dissociation) of photons are calculated based on modern parametrizations of deep inelastic structure functions in a broad range of their arguments ($x$ and $Q^2$). In our approach we can get separate contributions of different $W$ helicities states. Several one- and two-dimensional differential distributions are shown and discussed. The present results are compared to the results of previous calculations within collinear factorization approach. Similar results are found except of some observables such as e.g. transverse momentum of the pair of $W^+$ and $W^-$. We find large contributions to the cross section from the region of large photon virtualities. We show decomposition of the total cross section as well as invariant mass distribution into polarisation states of both W bosons. The role of the longitudinal $F_L$ structure function is quantified. Its inclusion leads to a 4-5 % decrease of the cross section, almost independent of $M_{WW}$.
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Single-diffractive dijet production at high energies within the $k_t$ -factorization approach
We discuss single-diffractive production of dijets. The cross section is calculated for the first time in the $k_t$-factorization approach, neglecting transverse momentum of the pomeron. We use Kimber-Martin-Ryskin unintegrated parton (gluon, quark, antiquark) distributions (UPDF) both in the proton as well as in the pomeron or subleading reggeon. The UPDFs are calculated based on conventional MMHT2014nlo PDFs in the proton and H1 collaboration diffractive PDFs used previously in the analysis of diffractive structure function and dijets at HERA. We try to describe the existing data from Tevatron and make detailed predictions for possible LHC measurements.
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Single-diffractive production of dijets within the $k_t$-factorization approach
We discuss single-diffractive production of dijets. The cross section is calculated within the resolved pomeron picture, for the first time in the $k_t$-factorization approach, neglecting transverse momentum of the pomeron. We use Kimber-Martin-Ryskin unintegrated parton (gluon, quark, antiquark) distributions (UPDF) both in the proton as well as in the pomeron or subleading reggeon. The UPDFs are calculated based on conventional MMHT2014nlo PDFs in the proton and H1 collaboration diffractive PDFs used previously in the analysis of diffractive structure function and dijets at HERA. For comparison we present results of calculations performed within collinear-factorization approach. Our results remaind those obtained in the NLO approach. The calculation is (must be) supplemented by the so-called gap survival factor which may, in general, depend on kinematical variables. We try to describe the existing data from Tevatron and make detailed predictions for possible LHC measurements. Several differential distributions are calculated. The $\overline{E}_T$, $\overlineη$ and $x_{\bar p}$ distributions are compared with the Tevatron data. A reasonable agreement is obtained for the first two distributions. The last one requires to introduce a gap survival factor which depends on kinematical variables. We discuss how the phenomenological dependence on one kinematical variable may influence dependence on other variables such as ${\overline E}_T$ and ${\overline η}$. Several distributions for the LHC are shown.
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Interpreting the 750 GeV diphoton signal as technipion
We discuss whether the enhancement in the diphoton final state at $M_{γγ}$ = 750 GeV, observed recently by the ATLAS and CMS Collaborations could be a neutral pseudoscalar technipion $\tildeπ^0$. We considered two distinct minimal models for the dynamical electroweak symmetry breaking. Here we concentrate only on two-flavor vector-like technicolor model and we assume that the two-photon fusion is a dominant production mechanism. We include contributions of $2 \to 1$, $2 \to 2$ and $2 \to 3$ partonic processes. All the mechanisms give similar contributions to the cross section. With the strong Yukawa (technipion-techniquark) coupling $g_{TC} \simeq 20$ we roughly obtain the measured cross section of the "signal". With such value of $g_{TC}$ we get a relatively small total decay width $Γ_{\rm tot}$. We discuss also the size of the signal at lower energies (LHC, Tevatron) for $γγ$ final states, where the enhancement was not observed. We predict a measurable cross section for neutral technipion production associated with one or two soft jets. The technipion signal is compared with the Standard Model diphoton background contributions. We observe the dominance of inelastic-inelastic $γγ$ processes. We predict the signal cross section for purely exclusive $p p \to p p γγ$ procesess at $\sqrt{s}$ = 13 TeV to be about 0.2 fb. Such a cross section would be, however, difficult to measure with the planned integrated luminosity. We conclude that in all considered cases the signal is below the background or/and below the threshold set by statistics although some tension can be seen.
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Diffractive production of heavy mesons at the LHC within $k_t$ - factorization approach
We discuss diffractive production of heavy mesons at the LHC. The differential cross sections for single- and central-diffractive mechanisms for $c\bar c$ pair production are calculated in the framework of the Ingelman-Schlein model corrected for absorption effects. Here, leading-order gluon-gluon fusion and quark-antiquark anihilation partonic subprocesses are taken into consideration. Both pomeron flux factors as well as parton distributions in the pomeron are taken from the H1 Collaboration analysis of diffractive structure function and diffractive dijets at HERA. The extra corrections from subleading reggeon exchanges are also taken into consideration. In addition to standard collinear approach, for the first time the differential cross sections for the diffractive $c\bar c$ pair production are calculated in the framework of the $k_t$-factorization approach, i.e. effectively including higher-order corrections. The unintegrated (transverse momentum dependent) diffractive parton distributions in proton are calculated with the help of the Kimber-Martin-Ryskin prescription where collinear diffractive PDFs are used as input. Some correlation observables, like azimuthal angle correlation between $c$ and $\bar c$, and $c \bar c$ pair transverse momentum were obtained for the first time. The hadronization of charm quarks is taken into account by means of fragmentation function technique.
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From deep-inelastic structure functions to two-photon dilepton production in proton-proton collisions
We compare two different approaches used for calculating cross sections for the two-photon $p p \to l^+ l^- X$ process. In one of the approaches photon is treated as a collinear parton in the proton. In the second approach a recently proposed $k_T$-factorization method is used. In this presentation we discuss sensitivity of the results to the choice of structure function parametrization and experimental cuts in the $k_T$-factorization approach. We compare results of our calculations with recent experimental data for dilepton production and find that in most cases the contribution of the photon-photon mechanism is rather small. We discuss how to enhance the photon-photon contribution. We also compare our results to those of recent measurements of exclusive and semi-exclusive $e^+ e^-$ pair production with certain experimental data by the CMS collaboration.
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Single-diffractive production of charmed mesons at the LHC within the $k_t$-factorization approach
We discuss the single diffractive production of $c \bar c$ pairs and charmed mesons at the LHC. For a first time we propose a $k_t$-factorization approach to the diffractive processes. The transverse momentum dependent diffractive parton distributions are obtained from standard (collinear) diffractive parton distributions used in the literature. In this calculation the transverse momentum of the pomeron is neglected with respect to transverse momentum of partons entering the hard process. We also perform the first evaluation of the cross sections at the LHC using the diffractive transverse momentum dependent parton distributions. The results of the new approach are compared with those of the standard collinear one. Significantly larger cross sections are obtained in the $k_t$-factorization approach where some part of higher-order effects is effectively included. The differences between corresponding differential distributions are discussed. Finally, we present a feasibility study of the process at the LHC using proton tagging technique. The analysis suggests that the measurement of single diffractive charm production is possible using ATLAS and CMS/TOTEM detectors.
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Can the diphoton enhancement at 750 GeV be due to a neutral technipion?
We discuss a scenario in which the diphoton enhancement at $M_{γγ}$ = 750 GeV, observed by the ATLAS and CMS Collaborations, is a neutral technipion $\tildeπ^0$. We consider two distinct minimal models for the dynamical electroweak symmetry breaking. In a first one, two-flavor vector-like technicolor (VTC) model, we assume that the two-photon fusion is a dominant production mechanism. We include $γγ\to {\tilde π}^0$ and production of technipion associated with one or two jets. All the considered mechanisms give similar contributions. With the strong Yukawa (technipion-techniquark) coupling $g_{TC}$ = 10 - 20 we obtain the measured cross section of the "signal". With such values of $g_{TC}$ we get a relatively small $Γ_{\rm tot}$. In a second approach, one-family walking technicolor (WTC) model, the isoscalar technipion is produced dominantly via the gluon-gluon fusion. We also discuss the size of the signal at lower energies (LHC, Tevatron) for $γγ$ (VTC) and jet-jet (WTC) final states and check consistency with the existing experimental data. We predict a measurable cross section for ${\tilde π}^0$ production associated with one or two soft jets. The technipion signal in both models is compared with the SM background diphoton contributions. We observe the dominance of inelastic-inelastic processes for $γγ$ induced processes. In the VTC scenario, we predict the signal cross section for purely exclusive $p p \to p p γγ$ processes at $\sqrt{s}$ = 13 TeV to be about 0.2 fb. Such a cross section would be, however, difficult to measure with the planned integrated luminosity. In all considered cases the signal is below the background or/and below the threshold set by statistics.
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Recent progress in some exclusive and semi-exclusive processes in proton-proton collisions
We present the main results of our recent analyses of exclusive production of vector charmonia ($J/ψ$ and $ψ'$) in $k_t$-factorization approach and for $γγ$ production of charged dilepton pairs in exclusive and semiinclusive processes in a new approach, similar in spirit to $k_t$-factorization. The results for charmonia are compared with recent results of the LHCb collaboration. We include some helicity flip contributions and quantify the effect of absorption correction. The effect of $c \bar c$ wave function is illustrated. We present uncertainties related to $F_2$ structure function which are the main ingredient of the approach. Our results are compared with recent CMS data for dilepton production with lepton isolation cuts imposed.
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Subleading processes in production of $W^+ W^-$ pairs in proton-proton collisions
We discuss new subleading processes for inclusive production of $W^+ W^-$ pairs. We focus mainly on photon-photon induced processes. We include elastic-elastic, elastic-inelastic, inelastic-elastic and inelastic-inelastic contributions. The inelastic photon distributions in the proton are calculated in two different ways: naive approach used already in the literature and using photon distributions by solving special evolution equations with the photon being a parton in the proton. The results strongly depend on the approach used. The resolved photon contribution was calculated in addition and found to be small. We also calculate the cross section for single-diffractive production of $W^+ W^-$ pairs including pomeron and reggeon exchanges in the Ingelman-Schlein model. Finally we only mention here about double parton contribution which is interesting by itself.
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Single- and central-diffractive production of open charm and bottom mesons at the LHC
We discuss diffractive production of open charm and bottom mesons at the LHC. The differential cross sections for single- and central-diffractive mechanisms for $c\bar c$ and $b\bar b$ pair production are calculated in the framework of the Ingelman-Schlein model corrected for absorption effects. The LO gluon-gluon fusion and quark-antiquark anihilation partonic subprocesses are taken into consideration, which are calculated within standard collinear approximation. The extra corrections from reggeon exchanges are taken into account. The hadronization of charm and bottom quarks is taken into account by means of fragmentation functions. Predictions for single- and central-diffractive production in the case of $D$ and $B$ mesons, as well as $D\bar D$ pairs are presented, including detector acceptance of the ATLAS, CMS and LHCb Collaborations.
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Two-photon dilepton production in proton-proton collisions: two alternative approaches
We investigate different methods to incorporate the effect of photons in hard processes. We compare two different approaches used for calculating cross sections for two-photon $p p \to l^+ l^- X$ process. In one of the approaches photon is treated as a collinear parton in the proton. In the second approach recently proposed a $k_T$-factorization method is used. We discuss how results of the collinear parton model depend on the initial condition for the QCD evolution and discuss an approximate treatment where photon is excluded from the combined QCD-QED evolution. We demonstrate that it is not necessary to put photon into the evolution equation as often done recently but it is sufficient to use a simplified approach in which photon couples to quarks and antiquarks which by themselves undergo DGLAP evolution equations. We discuss sensitivity of the results to the choice of structure function parametrization and experimental cuts in the $k_T$-factorization approach. A new optimal structure function parametrization is proposed. We compare results of our calculations with recent experimental data for dilepton production and find that in most cases the contribution of the photon-photon mechanism is rather small. We discuss how to enhance the photon-photon contribution. We also compare our results to those of recent measurements of exclusive and semi-exclusive $e^+ e^-$ pair production with certain experimental data by the CMS collaboration.
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Open charm meson production at BNL RHIC within $k_{t}$-factorization approach and revision of their semileptonic decays
We discuss inclusive production of open charm mesons in proton-proton scattering at the BNL RHIC. The calculation is performed in the framework of $k_t$-factorization approach which effectively includes higher-order pQCD corrections. Different models of unintegrated gluon distributions (UGDF) from the literature are used. We focus on UGDF models favoured by the LHC data and on a new up-to-date parametrizations based on the HERA collider DIS high-precision data. Results of the $k_t$-factorization approach are compared to next-to-leading order collinear predictions. The hadronization of heavy quarks is done by means of fragmentation function technique. The theoretical transverse momentum distributions of charmed mesons are compared with recent experimental data of the STAR collaboration at $\sqrt{s} = 200$ and $500$ GeV. Theoretical uncertainties related to the choice of renormalization and factorization scales as well as due to the quark mass are discussed. Very good description of the measured integrated cross sections and differential distributions is obtained for the Jung setB$0$ CCFM UGDF. Revised charm and bottom theoretical cross sections corresponding to those measured recently by the STAR and PHENIX collaborations for semileptonic decays of $D$ and $B$ mesons are presented. Significant improvement in theoretical description of the non-photonic electrons measurements is clearly obtained with respect to the previous studies within the $k_{t}$-factorization.
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