Soft-photon Bremsstrahlung Research Articles

Central exclusive diffractive production of a single photon in high-energy proton-proton collisions within the tensor-Pomeron approach

We discuss central-exclusive production (CEP) of photons via different fusion processes in the reaction $pp \to pp \gamma$ at high energies, available at RHIC and LHC, within the tensor-pomeron model. We consider two types of processes, the photoproduction contribution via the photon-pomeron and photon-reggeon fusion reactions, and the purely diffractive contribution via the reggeon-pomeron and odderon-pomeron fusion reactions. We present predictions for the measurements of photons at midrapidity, $|{\rm y}| < 2.5$, and at relatively low transverse momentum, $0.1~{\rm GeV} < k_{\perp} < 1~{\rm GeV}$. To check the main results of our study the measurement of the outgoing protons is not necessary. This is of relevance, e.g., for the present version of the ALICE detector at the LHC. Several differential distributions, for instance, in ${\rm y}$, $k_{\perp}$ and $\omega$, the rapidity, the absolute value of the transverse momentum, and the energy of the photon, respectively, are presented. We show that the photoproduction is an important process in the kinematic region specified above. There it gives a much larger cross section than diffractive bremsstrahlung. This is remarkable as the CEP cross section is of order $\alpha_{\rm em}^{3}$ whereas the bremsstrahlung one is only of order $\alpha_{\rm em}$. On the other hand, the soft-photon bremsstrahlung where the basic $pp \to pp$ reaction is due to strong interaction diffraction is more important than CEP in the forward rapidity range, $|{\rm y}| > 4$, and/or at very low $k_{\perp}$. We leave it as a challenge for the planned ALICE 3 experiment at the LHC to study these two contributions to soft photon production in $pp$ collisions. This could shed new light on the so called ``soft photon puzzle'' in hadron-hadron collisions.

Soft photon bremsstrahlung at next-to-leading power

The emission of soft radiation provides a fundamental probe of the consistency of the underlying quantum field theory. Correspondingly, the measurement of the soft photon bremsstrahlung, such as the one proposed with the planned future upgrade of the ALICE experiment at the LHC, is of great interest. In this letter we explore the possibility to implement analytic techniques that have been recently developed for soft gluon resummation at Next-to-Leading-Power (NLP) in the context of the soft photon spectrum. We provide a formula for the differential cross-section with shifted kinematics that is particularly suitable for numerical implementations. We also discuss the impact of loop corrections to Low's theorem due to radiative jet functions.

Bremsstrahlung Calculations of Single Gauge-Boson Production on Linear Colliders in Context of "New Physics" Searching

Differential and total cross sections of single gauge boson production in high energy electron-photon collisions obtained within the Standard Model in leading order and next-to-leading order are presented. Soft photon bremsstrahlung as well as hard photon bremsstrahlung parts were considered using the dimensional regularization procedure. Special features of receiving the hard bremsstrahlung convergent contribution are discussed. The corresponding anomalous gauge boson couplings were studied in the effective Lagrangian approach. Best conditions for registration of effects beyond the Standard Model are determined.

Modern Technique for Real Photon Radiative Correction Calculations

The problem of the bremsstrahlung contribution calculation as a part of the radiative corrections in the case of single gauge boson production was discussed. It was shown that the hard photon bremsstrahlung contribution can be divided into the finite and divergent terms. The exact calculation of soft photon bremsstrahlung and infrared part of hard photon bremsstrahlung was presented in frame of the dimensional regularization scheme. Numerical analysis of radiative corrections to the cross sections of single gauge boson production was performed.

Modern methods of calculations of bremsstrahlung in the interaction of elementary particles

. The problem of real bremsstrahlung calculation is considered using the modern methods of regularization of divergencies. In particular, we calculate soft photon bremsstrahlung in the most general form using the method of dimensional regularization of infrared divergences. The general calculation algorithm of hard photon bremsstrahlung is described. It is shown that the contribution of hard bremsstrahlung can be separated into the finite and divergent parts. The divergent part can be factorized with the contribution of the initial process in the Born approximation. It is shown that a good choice of kinematic variables makes an analytic covariant calculation of the divergent part of the hard bremsstrahlung possible. In a particular case, an algorithm for determining the kinematic constraints on the invariants is described. A numerical analysis of the radiative corrections for gauge bosons production processes in the case of electron-photon collisions is performed. It is discovered that the contribution of the finite part of bremsstrahlung at high collision energies reaches 20 per cent and must be taken into account in calculations of radiative corrections. The results obtained can be used in various calculations, including covariant ones, performed in the context of confirmation of the Standard Model theoretical predictions or searching for manifestations of alternative gauge models.

Resonant production of dark photons in positron beam dump experiments

Positrons beam dump experiments have unique features to search for very narrow resonances coupled superweakly to $e^+ e^-$ pairs. Due to the continue loss of energy from soft photon bremsstrahlung, in the first few radiation lengths of the dump a positron beam can continuously scan for resonant production of new resonances via $e^+$ annihilation off an atomic $e^-$ in the target. In the case of a dark photon $A'$ kinetically mixed with the photon, this production mode is of first order in the electromagnetic coupling $\alpha$, and thus parametrically enhanced with respect to the $O(\alpha^2)$ $e^+e^- \to \gamma A'$ production mode and to the $O(\alpha^3)$ $A'$ bremsstrahlung in $e^--$nucleon scattering so far considered. If the lifetime is sufficiently long to allow the $A'$ to exit the dump, $A' \to e^+e^-$ decays could be easily detected and distinguished from backgrounds. We explore the foreseeable sensitivity of the Frascati PADME experiment in searching with this technique for the $17\,$MeV dark photon invoked to explain the $^8$Be anomaly in nuclear transitions.

Two-Loop Corrections To Bhabha Scattering

The two-loop radiative photonic corrections to Bhabha scattering are computed in the leading order of the small electron mass expansion up to the nonlogarithmic term. After including the soft photon bremsstrahlung, we obtain the infrared-finite result for the differential cross section, which can directly be applied to a precise luminosity determination of the present and future e+ e- colliders.

Complete one-loop corrections to e+e−→W+W− in the MSSM

The complete O (α) corrections including soft-photon bremsstrahlung to the process e + e − → W + W − in the MSSM are calculated for on-shell W bosons. The relative difference between the MSSM and Standard Model corrections is generally quite small. The maximum deviation from the Standard Model within the scanned region of parameter space is ≲1.5% for unpolarized and transversally polarized W bosons, and ≲2.7% for longitudinal W bosons.

Multiple soft pion production in nonlinear chiral sigma model

Multiple soft pion production in the baryon scattering reactions is considered in the framework of chiral nonlinear sigma model neglecting the pion mass. Treating baryons in the eikonal approximation as classical sources, a set of analytical solutions for the pion field is found. A tree S-matrix is constructed on the basis of these solutions describing the emission (or absorption) of any number of soft pions. Then the contribution of soft virtual pions is taken into account in a closed form. It is shown that the loop corrections strongly suppress the pion radiation, and for the two limiting cases of nonrelativistic and ultra-relativistic baryon scatterings there is no pion emission from the "ends". Thus, the mechanism similar to the soft photon bremsstrahlung in the quantum electrodynamics seems to be unable to create a state with a large number of the soft pions.

Soft-photon bremsstrahlung in elastic pp and np scattering and the problem of dibaryon resonances

Soft-photon bremsstrahlung in elastic pp and np scattering and the problem of dibaryon resonances

The soft-photon bremsstrahlung in an electron and a positron collision with a hydrogen atom

The process of bremsstrahlung, formed in the fast e+or-+H collisions with a simultaneous excitation or ionization of the target, is considered in the low-frequency limit. It is demonstrated that in the case of the positron scattering the intensity of the 'inelastic' bremsstrahlung greatly exceeds the intensity of the radiation formed during the collision without the change of the target's state. The role of 'inelastic' channels in the formation of the total bremsstrahlung spectrum of the electron is relatively small.

Supersymmetric electroweak radiative corrections toe+e−→W+W−. I

This is the first of a series of three papers in which we give a complete analysis of one loop quantum corrections to the W pair production in the context of supersymmetric electroweak theory. We have adopted the on-mass-shell subtraction scheme of Sakakibara and previously demonstrated the consistency of this scheme. The relevant analytic results are given, and the one-loop-corrected differential cross section incorporating the finite corrections is written out. A complete computer program for the calculations of these corrections has been developed. This program has been checked in several ways to ensure against errors in this long calculation where many subtle cancellations are involved. In this paper we give an outline of our model, sketch our renormalization procedure, and give analytic and numerical results for the self-energy insertions, wave function renormalization, and soft-photon bremsstrahlung. We find that the percentage (with respect to the tree-level) of virtual loop corrections (considered in this paper) due to the addition of SUSY particles varies approximately from -5.93% to -5.21%. As a comparison the percentage virtual corrections due to self-energy insertions and wave function renormalization in the SM varies typically from 27.9% to 20.3%. The SM total percentage virtual loop corrections varies typically from 17.4% to 19%. The above comparison is made at the same center-of-mass energy (200 GeV). The first percentage in this comparison is for center-of-mass angles of 10\ifmmode^\circ\else\textdegree\fi{}, the second being at 90\ifmmode^\circ\else\textdegree\fi{}.

Electroweak radiative corrections to e −γ → W −νe

The cross section for e − γ → W − ν e with arbitrary lepton and boson polarizations is calculated within the electroweak standard model including the complete O(α) virtual and soft-photon bremsstrahlung corrections. The relevant analytical results are listed and the influence of the electroweak radiative corrections is discussed both for the polarized and unpolarized cross sections. The corrections are usually of the order of 10%. Where the lowest-order cross section is suppressed such as in the backward direction or for unequal polarizations of the photon and the W-boson they reach, however, values of ∼ 100%. An angular cut reduces the total cross section considerably and enhances the corresponding radiative corrections. Important non-universal corrections arise from box diagrams in particular at high energies. As a consequence including only the leading universal corrections does not give a good approximation to the corrections.

Radiative corrections for [formula omitted] in the standard model

The partial widths of the fermionic Higgs-boson decays are calculated in the one-loop approximation of the minimal standard model of electroweak interactions. The soft-photon bremsstrahlung is resummed by means of Yennie-Frautschi-Suura exponentiation and the hard-photon spectrum is integrated analytically for a variable maximum photon energy. A general expression for the weak corrections in the modified on-mass-shell scheme is presented. For the charged-lepton and light-quark modes the formulae may be greatly simplified by neglecting the masses of all fermions, except for the top quark. For the b b and t t channels individual approximation formulae are derived assuming that the Higgs boson and the top quark are heavy as compared to the weak gauge bosons. Below the t t production threshold the electroweak corrections are generally at the few-percent level but for a 1 TeV Higgs boson they may become as large as 15%.

Radiative corrections for H→W +W −( γ) in the standard model

The HW +W − three-point function with all external particles being off-mass-shell is calculated in the one-loop approximation of the minimal standard model of electroweak interactions. The effect of radiative corrections on the Higgs-boson decay mode H→W +W −( γ) is examined. The soft-photon bremsstrahlung is resummed using Yennie-Frautschi-Suura exponentiation and the hard-photon spectrum is integrated analytically. The impact of a fourth generation of heavy fermions is investigated. Approximation formulae are presented for the light-fermion contribution and for the case that the Higgs boson is heavy as compared to the weak vector bosons. In the modified on-mass-shell scheme the partial decay width may be enhanced by up to 15% if the masses of both the Higgs boson and the top quark assume values close to their theoretical upper bounds.

Electroweak radiative corrections to e +e − → Z 0Z 0

The cross section for e +e − → Z 0Z 0 with arbitrary polarizations of the leptons and bosons is calculated in the standard electroweak model including the complete one-loop virtual and soft-photon bremsstrahlung corrections. All necessary analytical formulas are presented; numerical results are discussed for the unpolarized case and for the polarization asymmetry. At LEP 200 energies the weak corrections are dominated by the self-energy of the external Z 0-bosons which contributes about +15%. With increasing energy, the weak corrections decrease up to -18% at 1 TeV due to other contributions.

Radiative corrections to Bhabha scattering at high energies (I): Virtual and soft photon corrections

The complete set of formulas for the differential Bhabha cross section including all the one-loop virtual corrections and soft photon bremsstrahlung emission is presented for the standard model in an on-shell renormalization scheme. The influence and structure of the corrections are discussed below, on top and around, and far above the Z 0 resonance.

Electroweak radiative corrections to e +e − → W +W −

The cross section for e +e − → W +W − with arbitrary polarizations of the leptons and bosons is calculated in the standard electroweak model including the complete one-loop virtual and soft-photon bremsstrahlung corrections. The relevant analytic results are listed and the influence of the radiative corrections on the unpolarized cross section is discussed. We find moderate corrections at LEP II energies. The corrections increase continuously with energy up to − 50% of the Born cross section at 1 TeV.

Radiative corrections to polarized e −e + annihilation in the standard electroweak model

The cross section for e − e + → mf f ( f ≠ e) including the 1-loop electromagnetic corrections and soft photon bremsstrahlung, thereby taking account of the finite width of the Z boson. The observable quantities like angular distribution. change asymmetry A C, longitudinal polarization asymmetry A L and azimuthal asymmetry with natural polarization A T are discussed in the energy range 40 GeV ⩽ √ s ⩽ 140 GeV for a Z boson with mass and couplings of the standard electroweak model. We find sizable corrections to all observables especially in the interference region, but only small corrections to A L and A T on the Z resonance.