Quasi-real Photons Research Articles

Low-momentum direct-photon measurement in Cu + Cu collisions at sNN=200GeV

We measured direct photons for pT<5GeV/c in minimum bias and 0%–40% most-central events at midrapidity for Cu+Cu collisions at sNN=200GeV. The e+e− contribution from quasireal direct virtual photons has been determined as an excess over the known hadronic contributions in the e+e− mass distribution. A clear enhancement of photons over the binary scaled p+p fit is observed for pT<4GeV/c in Cu+Cu data. The pT spectra are consistent with the Au+Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the p+p baseline are 285±53(stat)±57(syst)MeV/c and 333±72(stat)±45(syst)MeV/c for minimum bias and 0%–40% most-central events, respectively. The rapidity density, dN/dy, of photons demonstrates the same power law as a function of dNch/dη observed in Au+Au at the same collision energy.1 MoreReceived 13 May 2018DOI:https://doi.org/10.1103/PhysRevC.98.054902Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasRelativistic heavy-ion collisionsPhysical SystemsLeptonsPhotonsTechniquesHadron collidersParticles & FieldsNuclear Physics

Photon structure studied at an electron ion collider

A future electron ion collider (EIC) will be able to provide collisions of polarized electrons with protons and heavy ions over a wide range of center-of-mass energies (20 GeV to 140 GeV) at an instantaneous luminosity of $1{0}^{33}\ensuremath{-}{10}^{34}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. One of its promising physics programs is the study of the partonic structure of quasireal photons. Measuring dijets in quasireal photoproduction events, one can effectively access the underlying parton dynamics of the photons. In this paper, we discuss the feasibility of tagging resolved photon processes and measuring the dijet cross section as a function of jet transverse momentum in the range of $0.01&lt;{x}_{\ensuremath{\gamma}}^{\mathrm{rec}}&lt;1$ at an EIC. It will be shown that both unpolarized and polarized parton distributions in the photon can be extracted, and that the flavor of the parton can be tagged at an EIC.

A study of the process e+ + e−→ e+ + e− + p + p̄ by the two-photon mechanism γγ → pp̄ at high energies

In this paper, we consider the exclusive production of proton–antiproton pairs in the interaction between two quasireal photons in [Formula: see text] collision. The differential and total cross-section of the process [Formula: see text] at a beam energy of photons from 2.1 GeV to 4.5 GeV in the center-of-mass and for different values of [Formula: see text] is calculated. At energy [Formula: see text] the total cross-section process of the [Formula: see text] is calculated by the two-photon mechanism. The results are in satisfactory agreement with the experimental data.

Transverse single-spin asymmetries in ℓp↑→hX within a TMD approach: Role of quasireal photon exchange

We present an updated study of transverse single-spin asymmetries for the inclusive large-$P_T$ processes $\ell \, p^\uparrow \to h\, X$ and $\ell\, p^\uparrow \to {\rm jet}\,X$, within a transverse momentum-dependent approach, including the contribution of quasireal (Weizs\"acker-Williams) photons. In the spirit of a unified transverse momentum-dependent scheme, predictions are obtained adopting the Sivers and transversity distributions and the Collins fragmentation functions as extracted from fits to the azimuthal asymmetries measured in semi-inclusive deep inelastic scattering and $e^+e^-$ annihilation processes. The description of the available data is extremely good, showing a clear general improvement with respect to the previous leading-order analysis. Predictions for unpolarized cross sections and single-spin asymmetries for ongoing and future experiments are also given.

Charmonium production in ultra-peripheral heavy ion collisions with two-photon processes

We calculate the production of large-pT charmonium and narrow resonance state (exotic charmonium) in proton–proton, proton–nucleus, and nucleus–nucleus collisions with the semi-coherent two-photon interactions at Relativistic Heavy Ion Collider (RHIC), Large Hadron Collider (LHC), and Future Circular Collider (FCC) energies. Using the large quasi-real photon fluxes, we present the γγ→H differential cross section for charmonium and narrow resonance state production at large transverse momentum in ultra-peripheral heavy ion collisions. The numerical results demonstrate that the experimental study of ultra-peripheral collisions is feasible at RHIC, LHC, and FCC energies.

Systematic estimation of theoretical uncertainties in the calculation of the pion-photon transition form factor using light-cone sum rules

We consider the calculation of the pion-photon transition form factor ${F}^{{\ensuremath{\gamma}}^{*}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}}({Q}^{2})$ within light-cone sum rules focusing attention to the low-mid region of momenta. The central aim is to estimate the theoretical uncertainties which originate from a wide variety of sources related to (i) the relevance of next-to-next-to-leading order radiative corrections (ii) the influence of the twist-four and the twist-six term (iii) the sensitivity of the results on auxiliary parameters, like the Borel scale ${M}^{2}$, (iv) the role of the phenomenological description of resonances, and (v) the significance of a small but finite virtuality of the quasireal photon. Predictions for ${F}^{{\ensuremath{\gamma}}^{*}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}}({Q}^{2})$ are presented which include all these uncertainties and found to comply within the margin of experimental error with the existing data in the ${Q}^{2}$ range between 1 and $5\text{ }\text{ }{\mathrm{GeV}}^{2}$, thus justifying the reliability of the applied calculational scheme. This provides a solid basis for confronting theoretical predictions with forthcoming data bearing small statistical errors.

The lowQ2chicane and Compton polarimeter at the JLab EIC

The JLAB EIC (JLEIC) design includes a chicane after the interaction point to detect electron associated with production of quasi-real photon at the interaction. This chicane layout can also be used for Compton polarimetry to measure the electron beam polarization. This proceeding will present the layout of the low Q 2 chicane and the implementation and current R&D of a Compton polarimeter which would be located in the middle of this chicane.

Photon structure functions at small x in holographic QCD

We investigate the photon structure functions at small Bjorken variable x in the framework of the holographic QCD, assuming dominance of the Pomeron exchange. The quasi-real photon structure functions are expressed as convolution of the Brower–Polchinski–Strassler–Tan (BPST) Pomeron kernel and the known wave functions of the U(1) vector field in the five-dimensional AdS space, in which the involved parameters in the BPST kernel have been fixed in previous studies of the nucleon structure functions. The predicted photon structure functions, as confronted with data, provide a clean test of the BPST kernel. The agreement between theoretical predictions and data is demonstrated, which supports applications of holographic QCD to hadronic processes in the nonperturbative region. Our results are also consistent with those derived from the parton distribution functions of the photon proposed by Glück, Reya, and Schienbein, implying realization of the vector meson dominance in the present model setup.

Search for Kaluza-Klein gravitons in extra dimension models via forward detectors at the LHC

We investigate contributions of Kaluza-Klein (KK) graviton in extra dimension models to the process $pp \to p\gamma p \to p\gamma j X$, where a proton emits a quasireal photon and is detected by using the very forward detectors planned at the LHC. In addition to the $\gamma q$ initial state as in the Compton scattering in the standard model, the $\gamma g$ scattering contributes through the $t$-channel exchange of KK gravitons. Taking account of pileup contributions to the background and examining viable kinematical cuts, constraints on the parameter space of both the ADD (Arkani-Hamed, Dimopoulos and Dvali) model and the RS (Randall and Sundrum) model are studied. With 200 fb$^{-1}$ data at a center-of-mass energy of 14 TeV, the expected lower bound on the cut-off scale for the ADD model is 6.3 TeV at 95% confidence level, while a lower limit of 2.0 (0.5) TeV is set on the mass of the first excited graviton with the coupling parameter $k/\overline{M}_{\rm Pl}=0.1$ (0.01) for the RS model.

Measurement of the charged-pion polarizability.

The COMPASS collaboration at CERN has investigated pion Compton scattering, π(-)γ→π(-)γ, at center-of-mass energy below 3.5 pion masses. The process is embedded in the reaction π(-)Ni→π(-)γNi, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, Q(2)<0.0015 (GeV/c)(2). From a sample of 63,000 events, the pion electric polarizability is determined to be α(π)=(2.0±0.6(stat)±0.7(syst))×10(-4) fm(3) under the assumption α(π)=-β(π), which relates the electric and magnetic dipole polarizabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambiguously known, was performed for an independent estimate of the systematic uncertainty.

The role of finite-size effects on the spectrum of equivalent photons in proton–proton collisions at the LHC

Photon–photon interactions represent an important class of physics processes at the LHC, where quasi-real photons are emitted by both colliding protons. These reactions can result in the exclusive production of a final state X, p+p→p+p+X. When computing such cross sections, it has already been shown that finite size effects of colliding protons are important to consider for a realistic estimate of the cross sections. These first results have been essential in understanding the physics case of heavy-ion collisions in the low invariant mass range, where heavy ions collide to form an exclusive final state like a J/Ψ vector meson. In this paper, our purpose is to present some calculations that are valid also for the exclusive production of high masses final states in proton–proton collisions, like the production of a pair of W bosons or the Higgs boson. Therefore, we propose a complete treatment of the finite size effects of incident protons irrespective of the mass range explored in the collision. Our expectations are shown to be in very good agreement with existing experimental data obtained at the LHC.

Кулонівські резонанси, квазіреальні фотони і феномен дипольного гігантського резонансу

Various aspects of the influence of Coulomb resonances and quasireal photons on the dynamics of nuclear electro-disintegration by high-energy electrons have been studied in the framework of the nuclear shell model. Some peculiarities of numerical methods used to study the inelastic scattering cross-sections of high-energy electrons are also considered.

Measurement of radiative widths of a2(1320) and $ \pi_{2}$ (1670)

The COMPASS Collaboration at CERN has investigated the reaction $\pi^- \gamma \rightarrow \pi^-\pi^-\pi^+$ embedded in the Primakoff reaction of $190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target, $\pi^- \text{Pb} \rightarrow \pi^-\pi^-\pi^+ \text{Pb}$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001~(\text{GeV}/c)^2$. Using a partial-wave analysis the amplitudes and relative phases of the $a_2(1320)$ and $\pi_2(1670)$ mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the $a_2(1320)$ to be $\Gamma_0(a_2(1320) \rightarrow \pi\gamma) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$. As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181 \pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot (\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}/\textrm{BR}_{f_2 \pi})$ is obtained for the radiative width of the $\pi_2(1670)$, where in this analysis the branching ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.

MESON SPECTROSCOPY AT JLAB AT 12 GEV

The 12 GeV upgrade to the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab will enable a new generation of experiments in hadronic nuclear physics, seeking to address fundamental questions in our understanding of QCD. The existence of exotic states, suggested by both quark models and lattice calculations, would allow gluonic degrees of freedom to be explored, and may help explain the role played by gluons in the QCD interaction. This article will review the meson spectroscopy program being planned at the lab following the 12 GeV upgrade, utilising real and quasi-real photon beams in two of the lab's four experimental halls, whose distinct capabilities will enable an extensive set of spectroscopy experiments to be performed at the same facility.

Tests of Chiral perturbation theory with COMPASS

The COMPASS experiment at CERN accesses pion-photon reactions via the Primakoff effect., where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. When a single real photon is produced, pion Compton scattering is accessed and from the measured cross-section shape, the pion polarisability is determined. The COMPASS measurement is in contradiction to the earlier dedicated measurements, and rather in agreement with the theoretical expectation from ChPT. In the same experimental data taking, reactions with neutral and charged pions in the final state are measured and analyzed in the context of chiral perturbation theory.

The Meson Spectroscopy Program Using the Forward Tagger with CLAS12 at Jefferson Lab

The 12 GeV upgrade to the Continuous Electron Beam Accelerator Facility (CEBAF) will enable a new generation of experiments in hadronic nuclear physics at Jefferson Lab, seeking to address fundamental questions in QCD. For example, confirming the existence of exotic states, suggested by both quark models and lattice calculations, would allow gluonic degrees of freedom to be explored, and may help explain the role played by gluons in the QCD interaction.In Experimental Hall B, meson spectroscopy (the MesonEx experiment) will be performed using low Q 2 electron scattering to produce quasi-real photons. The scattered electron is detected at small angles by the Forward Tagger device, determining the properties of the photon on an event-by-event basis. This technique has notable advantages over real photon beams, and over hadronic beam experiments, where most experimental data exists.This article will focus on the development of the Forward Tagger by the INFN Genova group, and the proposed MesonEx experiment.

The Forward Tagger facility for lowQ2experiments at Jefferson Laboratory

Low Q2 electron scattering is an efficient and competitive experimental technique to provide intense, quasi-real photon beams, with a high degree of linear polarization. Such a technique will be employed in Hall B at Jefferson Laboratory by having the primary 11 GeV electron beam from the CEBAF accelerator impinging on a liquid hydrogen target. Low-angle scattered electrons will be detected with the new Forward Tagger facility, while the final state hadrons will be measured with the CLAS12 spectrometer. The unique combination of the two detectors will permit to carry out a broad physics program, and to explore new possibilities for high quality physics.

Negative Charged Pion Production on a Deuteron by Quasi-Real Photons

Experimental differential cross sections of photoproduction of negative pions on a deuteron have been obtained as functions of the polar angle of emission of π– mesons. A comparison is made of the measured cross sections with the results of calculations in a model that takes account the interaction in the final state of the reaction.

Observing Light-by-Light Scattering at the Large Hadron Collider

Elastic light-by-light scattering (γγ→γγ) is open to study at the Large Hadron Collider thanks to the large quasireal photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The γγ→γγ cross sections for diphoton masses m(γγ)>5 GeV amount to 12 fb, 26 pb, and 35 nb in p-p, p-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies √(s(NN))=14, 8.8, and 5.5 TeV, respectively. Such a measurement has no substantial background in Pb-Pb collisions where one expects about 20 signal events per run, after typical detector acceptance and reconstruction efficiency selections.

Can we understand an auxetic pion-photon transition form factor within QCD?

A state-of-the-art analysis of the pion-photon transition form factor is presented based on an improved theoretical calculation that includes the effect of a finite virtuality of the quasireal photon in the method of light-cone sum rules. We carry out a detailed statistical analysis of the existing experimental data using this method and by employing pion distribution amplitudes with up to three Gegenbauer coefficients ${a}_{2}$, ${a}_{4}$, ${a}_{6}$. Allowing for an error range in the coefficient ${a}_{6}\ensuremath{\approx}0$, the theoretical predictions for ${\ensuremath{\gamma}}^{*}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}$ obtained with nonlocal QCD sum rules are found to be in good agreement with all data that support a scaling behavior of the transition form factor at higher ${Q}^{2}$, like those of the Belle Collaboration. The data on ${\ensuremath{\gamma}}^{*}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\eta}/{\ensuremath{\eta}}^{\ensuremath{'}}$ from CLEO and BABAR are also reproduced, while there is a strong conflict with the auxetic trend of the BABAR data above $10\text{ }\text{ }{\mathrm{GeV}}^{2}$. The broader implications of these findings are discussed.