Ultraperipheral Collisions Research Articles

Search Strategy for Sleptons and Dark Matter Using the LHC as a Photon Collider.

We propose a search strategy using the LHC as a photon collider to open sensitivity to scalar lepton (slepton ℓ[over ˜]) production with masses around 15 to 60GeV above that of neutralino dark matter χ[over ˜]_{1}^{0}. This region is favored by relic abundance and muon (g-2)_{μ} arguments. However, conventional searches are hindered by the irreducible diboson background. We overcome this obstruction by measuring initial state kinematics and the missing momentum four-vector in proton-tagged ultraperipheral collisions using forward detectors. We demonstrate sensitivity beyond LEP for slepton masses of up to 200GeV for 15≲Δm(ℓ[over ˜],χ[over ˜]_{1}^{0})≲60 GeV with 100 fb^{-1} of 13TeV proton collisions. We encourage the LHC collaborations to open this forward frontier for discovering new physics.

Probing double parton scattering via associated open charm and bottom production in ultraperipheral pA collisions

In this article, we propose a novel channel for phenomenological studies of the double-parton scattering (DPS) based upon associated production of charm c{bar{c}} and bottom b{bar{b}} quark pairs in well-separated rapidity intervals in ultraperipheral high-energy proton–nucleus collisions. This process provides direct access to the double-gluon distribution in the proton at small-x and enables one to test the factorized DPS pocket formula. We have made the corresponding theoretical predictions for the DPS contribution to this process at typical LHC energies and beyond and we compute the energy-independent (but photon momentum fraction dependent) effective cross section.

Electromagnetic Interactions of Nuclei at the FCC-hh Collider

At the Future Circular Collider (FCC-hh) at CERN in addition to proton-proton collisions it is planned to study interactions of 208Pb nuclei with the collision energy $$\sqrt {{{s}_{{NN}}}} = 39.4$$ TeV, which is 7–8 times higher than that achieved at the LHC. The electromagnetic dissociation (EMD) of $$^{{208}}$$ Pb nuclei, along with the electron capture resulting from the multiple production of e $$^{ + }$$ e $$^{ - }$$ pairs in ultraperipheral collisions at the LHC, lead to a significant luminosity decay, and secondary ions impose a well localized thermal load on superconducting magnets. As expected, such effects are reduced in collisions of light and medium nuclei. In the present work the EMD cross sections for collisions of $$^{{40}}$$ Ar, $$^{{40}}$$ Ca, $$^{{63}}$$ Cu, $$^{{78}}$$ Kr, $$^{{84}}$$ Kr, $$^{{115}}$$ In, $$^{{129}}$$ Xe, $$^{{208}}$$ Pb and $$^{{238}}$$ U at the LHC and the planned FCC-hh were calculated in order to assess the operation conditions of the colliders. The results were compared with results of the RELDIS model, and the uncertainties of calculations of the EMD cross sections associated with using of various approximations of the total nuclear photoabsorption cross sections were estimated.

Dijet Photoproduction in Ultraperipheral Collisions at the LHC and Nuclear PDFs at Small x

Dijet Photoproduction in Ultraperipheral Collisions at the LHC and Nuclear PDFs at Small x

Heavy quarkonia production at energies available at the CERN Large Hadron Collider and future electron-ion colliding facilities using basis light-front quantization wave functions

We study exclusive charmonium and bottomonium production in ultra-peripheral heavy-ion collisions and electron-ion collisions within the dipole picture. We employ heavy quarkonium light-front wavefunctions obtained within the basis light-front quantization framework, which has some features of the light-front holographic QCD. We focus on comparison with measurements of exclusive charmonium and bottomonium production in ultraperipheral $pp$, $p$Pb and PbPb collisions at LHC and find reasonable agreement with the cross sections. We also discuss the coherent production cross-section ratios of excited states to the ground state for charmonium and bottomonium, which exhibit insensitivity to the dipole model parameters. We show that electron-ion collisions provide an opportunity for a quantitative study of heavy quarkonium structure.

Photoproduction of charged final states in ultraperipheral collisions and electroproduction at an electron-ion collider

Ultra-peripheral collisions (UPCs) of relativistic ions are an important tool for studying photoproduction at high energies. Vector meson photoproduction is an important tool for nuclear structure measurements and other applications. A future electron-ion collider (EIC) will allow additional studies, using virtual photons with a wide range of $Q^2$. We propose a significant expansion of the UPC and EIC photoproduction physics programs to include charged final states which may be produced via Reggeon exchange. We consider two examples: $a_2^+(1320)$, which is a conventional $q\overline q$ meson, and the exotic $Z_c^+(4430)$ state (modeled here as a tetraquark). The $Z_c^+(4430)$ cross-section depends on its internal structure, so photoproduction can test whether the $Z_c^+(4430)$ is a tetraquark or other exotic object. We calculate the rates and kinematic distributions for $\gamma p\rightarrow X^+n$ in $pA$ UPCs and $ep$ collisions at an EIC and in UPCs. The rates are large enough for detailed studies of these final states. Because the cross-section for Reggeon exchange is largest near threshold, the final state rapidity distribution depends on the beam energies. At high-energy colliders like the proposed LHeC or $pA$ collisions at the LHC, the final states are produced at far forward rapidities. For lower energy colliders, the systems are produced closer to mid-rapidity, within reach of central detectors.

Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at [formula omitted

Evidence for the light-by-light scattering process, γγ→γγ, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390μb−1 recorded by the CMS experiment at the LHC. Light-by-light scattering processes are selected in events with two photons exclusively produced, each with transverse energy ETγ>2GeV, pseudorapidity |ηγ|<2.4, diphoton invariant mass mγγ>5GeV, diphoton transverse momentum pTγγ<1GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 9.0±0.9(theo) events for the signal and 4.0±1.2(stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 3.7 standard deviations, and has properties consistent with those expected for the light-by-light scattering signal. The measured fiducial light-by-light scattering cross section, σfid(γγ→γγ)=120±46(stat)±28(syst)±12(theo)nb, is consistent with the standard model prediction. The mγγ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the ▪ process, in the mass range ▪.

Measurement of exclusive {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}\xa0photoproduction in ultraperipheral pPb collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {Te}text {V}

Exclusive {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0} photoproduction is measured for the first time in ultraperipheral pPb collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {Te}text {V} with the CMS detector. The cross section sigma ({upgamma _{}^{}} mathrm{p}rightarrow {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}mathrm{p}) is 11.0 pm 1.4,text {(stat)} pm 1.0,text {(syst)} mu b at langle W_{{upgamma _{}^{}} mathrm{p}}rangle = 92.6,text {Ge}text {V} for photon–proton centre-of-mass energies W_{{upgamma _{}^{}} mathrm{p}} between 29 and 213,text {Ge}text {V} . The differential cross section mathrm {d}sigma /mathrm {d}|t | is measured in the interval 0.025< |t | < 1,text {Ge}text {V} ^{2} as a function of W_{{upgamma _{}^{}} mathrm{p}}, where t is the squared four-momentum transfer at the proton vertex. The results are compared with previous measurements and theoretical predictions. The measured cross section sigma ({upgamma _{}^{}} mathrm{p}rightarrow {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}mathrm{p}) has a power-law dependence on the photon–proton centre-of-mass, consistent with electron–proton collision measurements performed at HERA. The W_{{upgamma _{}^{}} mathrm{p}} dependence of the exponential slope of the differential cross section mathrm {d}sigma /mathrm {d}|t | is also measured.

Probing the linear polarization of photons in ultraperipheral heavy ion collisions

We propose to measure the linear polarization of the external electromagnetic fields of a relativistic heavy ion through azimuthal asymmetries in dilepton production in ultraperipheral collisions. The asymmetries estimated with the equivalent photon approximation are shown to be sizable.

Inclusive dijet photoproduction in ultraperipheral heavy ion collisions at the CERN Large Hadron Collider in next-to-leading order QCD

We compute the cross section of inclusive dijet photoproduction in ultraperipheral Pb-Pb collisions at the LHC using next-to-leading order perturbative QCD. We demonstrate that our theoretical calculations provide a good description of various kinematic distributions measured by the ATLAS collaboration. We find that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of parton distribution functions (PDFs) at the level of 10 to 20%. Hence, this process can be used to reduce uncertainties in the determination of these nuclear PDFs, whose current magnitude is comparable to the size of the calculated nuclear modifications of the dijet photoproduction cross section.

Extending the constraint for axion-like particles as resonances at the LHC and laser beam experiments

We study the discovery potential of axion-like particles (ALP), pseudo-scalars weakly coupled to Standard Model fields, at the Large Hadron Collider (LHC). Our focus is on ALPs coupled to the electromagnetic field, which would induce anomalous scattering of light-by-light. This can be directly probed in central exclusive production of photon pairs in ultra-peripheral collisions at the LHC in proton and heavy-ion collisions. We consider non-standard collision modes of the LHC, such as argon-argon collisions at sNN=7 TeV and proton-lead collisions at sNN=8.16 TeV, to access regions in the parameter space complementary to the ones previously considered for lead-lead and proton-proton collisions. In addition, we show that, using laser beam interactions, we can constrain ALPs as resonant deviations in the refractive index induced by anomalous light-by-light scattering effects. If we combine the aforementioned approaches, ALPs can be probed in a wide range of masses from the eV scale up to the TeV scale.

Light-by-light scattering in ultraperipheral heavy-ion collisions at low diphoton masses

We present a study of photon-photon scattering in the mass range $W_{\gamma\gamma} < 5$ GeV. We extend earlier calculations of this cross section for $W_{\gamma\gamma}>$ 5 GeV into the low mass range where photoproduction of the pseudoscalar resonances $\eta$, $\eta^{'}(958)$ contributes to two-photon final states. We present the elementary photon-photon cross section as a function of diphoton mass $M_{\gamma\gamma}$ arising from lepton and quark loop diagrams, and the visible cross section obtained with the gamma-gamma decay branching fractions of the resonances $\eta$, $\eta^{'}(958)$, $\eta_c(1S)$, $\eta_c(2S)$, $\chi_{c0}(1P)$. We derive the corresponding cross sections in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV by folding the elementary cross section with the heavy-ion photon fluxes. We consider the dominating background of the two-photon final state which arises from gamma decays of photoproduced $\pi^{0}$-pairs. Such $\pi^{0}$-pairs contribute to the background when only two of the four decay photons are within the experimental acceptance, while the other two photons escape undetected. We reduce this background by applying cuts on asymmetries of transverse momenta of the two photons and indicate how the background can be further suppressed using a multivariate sideband analysis. We present the cross section for the signal and the background at midrapidity $|\eta| <$ 0.9, and in the forward rapidity range 2.0 $< \eta <$ 4.5.

Exclusive dilepton production in ultraperipheral PbPb collisions at the LHC

In this paper we perform a systematic study of the exclusive dilepton production by gamma gamma interactions in PbPb collisions at the LHC Run 2 energies considering different levels of precision for the treatment of the absorptive corrections and for the nuclear form factor. The rapidity and invariant mass distributions are estimated taking into account the experimental cutoffs and a comparison with the recent ALICE and ATLAS data for the e^+ e^- and mu ^+ mu ^- production is presented.

Hard diffraction in photoproduction with Pythia 8

We present a new framework for modeling hard diffractive events in photoproduction, implemented in the general purpose event generator Pythia 8. The model is an extension of the model for hard diffraction with dynamical gap survival in mathrm {p}mathrm {p} and mathrm {p}overline{mathrm {p}} collisions proposed in 2015, now also allowing for other beam types. It thus relies on several existing ideas: the Ingelman–Schlein approach, the framework for multiparton interactions and the recently developed framework for photoproduction in gamma mathrm {p}, gamma gamma , mathrm {e}mathrm {p} and mathrm {e}^+mathrm {e}^- collisions. The model proposes an explanation for the observed factorization breaking in photoproduced diffractive dijet events at HERA, showing an overall good agreement with data. The model is also applicable to ultraperipheral collisions with mathrm {p}mathrm {p} and mathrm {p}{mathrm {Pb}} beams, and predictions are made for such events at the LHC.

Constraints on nuclear parton distributions from dijet photoproduction at the LHC

Using QCD calculations of the cross section of inclusive dijet photoproduction in Pb–Pb ultraperipheral collisions in the LHC kinematics as pseudo-data, we study the effect of including these data using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear parton distribution functions (nPDFs). We find that, depending on the assumed error of the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small values of the momentum fraction x_A. Taking the error to be 5%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs reduces approximately by a factor of two at x_A=10^{-3}. At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form.

Geometry of ultraperipheral nuclear collisions

It is advocated that geometry of the interaction region of two heavy nuclei colliding at large impact parameters is important for the relative role of light-by-light scattering and QCD-initiated processes. Exclusive production of resonances is possible by dense electromagnetic fields in the interior space between the nuclei. The cross-section of the two-photon processes is evaluated and some examples are considered. It is speculated that the exclusive production of [Formula: see text]-mesons by “two-photon” processes forbidden by the Landau–Yang rule may be allowed within strong magnetic fields due to odd number of photons becoming involved.

Photoproduction of J/ψ in non-single-diffractive p+p collisions * * Supported by the National Natural Science Foundation of China (11775213, 11505180, 11675168), the U.S. DOE Office of Science (DE-SC0012704) and MOST (2016YFE0104800)

Significant enhancements of production at very low transverse momenta were recently observed by the ALICE and STAR collaborations in peripheral hadronic A+A collisions. The anomalous excess points to coherent photon-nucleus interactions in violent hadronic heavy-ion collisions, which were conventionally studied only in ultra-peripheral collisions. Assuming that the coherent photoproduction is the underlying mechanism responsible for the excess observed in peripheral A+A collisions, its contribution in p+p collisions with nuclear overlap, i.e. non-single-diffractive collisions, is of particular interest. In this paper, we perform a calculation of exclusive photoproduction in non-single-diffractive p+p collisions at the RHIC and LHC energies based on the pQCD motivated parametrization using the world-wide experimental data, which could be further employed to improve the precision of the phenomenological calculations for photoproduction in A+A collisions. The differential rapidity and transverse momentum distributions of from photoproduction are presented. In comparison with the production from hadronic interactions, we find that the contribution of photoproduction is negligible.

Coherent photoproduction of J/ψ in nucleus-nucleus collisions in the color dipole approach

We investigate the exclusive photoproduction of $J/\psi$-mesons in ultraperipheral heavy ion collisions in the color dipole approach. We first test a number of dipole cross sections fitted to inclusive $F_2$-data against the total cross section of exclusive $J/\psi$-production on the free nucleon. We then use the color-dipole formulation of Glauber-Gribov theory to calculate the diffractive amplitude on the nuclear target. The real part of the free nucleon amplitude is taken into account consistent with the rules of Glauber theory. We compare our results to recent published and preliminary data on exclusive $J/\psi$ corrections in ultraperipheral lead-lead collisions at $\sqrt{s_{NN}}=2.76 \, \rm{TeV}$ and $\sqrt{s_{NN}} = 5.02 \, \rm{TeV}$. Especially at high $\gamma A$ energies there is room for additional shadowing corrections, corresponding to triple-Pomeron terms or shadowing from large mass diffraction.

Single-transverse-spin asymmetries in exclusive photo-production of J/ψ in ultra-peripheral collisions in the fixed-target mode at the LHC and in the collider mode at RHIC

We investigate the potentialities offered by the study of J/ψ exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected cross sections in both set-ups. Studying Single-Transverse-Spin Asymmetries (AN) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) Eg(x,ξ,t). We evaluate the expected precision on AN for realistic conditions with the LHCb detector in pH↑ and PbH↑ collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.

Exclusive ρ and J/Ψ photoproduction in ultraperipheral pA collisions: Predictions of the gluon saturation models for the momentum transfer distributions

In this letter we complement previous studies on exclusive vector meson photoproduction in hadronic collisions presenting a comprehensive analysis of the t-spectrum measured in exclusive ρ and J/Ψ photoproduction in pA collisions at the LHC. We compute the differential cross sections considering two phenomenological models for the gluon saturation effects and present predictions for pPb and pCa collisions. Moreover, we compare our predictions with the recent preliminary CMS data for the exclusive ρ photoproduction. We demonstrate that the gluon saturation models are able to describe the CMS data at small – t. On the other hand, the models underestimate the few data point at large – t. Our results indicate that future measurements of the large – t region can be useful to probe the presence or absence of a dip in the t-spectrum and discriminate between the different approaches to the gluon saturation effects.