Hadronic Bremsstrahlung Research Articles

Soft-Dielectron Excess in Proton-Proton Collisions at s=13 TeV

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\rm ee}<0.6$ GeV/$c^2$ and for $p_{\rm T,ee} < 0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13\,(\rm{stat.})\pm 0.17\,(\rm{syst., data}) \pm 0.34\,(\rm{syst., cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\rm ee}$ and $p_{\rm T,ee}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

Exclusivepp→ppπ0reaction at high energies

The amplitudes for the $pp\ensuremath{\rightarrow}pp{\ensuremath{\pi}}^{0}$ process applicable at high energy are discussed in detail. Both diffractive bremsstrahlung (Drell-Hiida-Deck-type model), photon-photon and photon-omega exchange mechanisms are included in the calculation. Large cross sections of the order of mb are predicted. The corresponding differential cross sections in rapidities and transverse momenta of outgoing protons and pions as well as relative azimuthal angle between outgoing protons are calculated for RHIC and LHC energies. The hadronic bremsstrahlung contributions dominate at large (forward, backward) pion rapidities. The diffractive nonresonant background contributes at small ${\ensuremath{\pi}}^{0}p$ invariant mass and could be therefore misinterpreted as the Roper resonance. We predict strong dependence of the slope in $t$ (squared four-momentum transfer between ingoing and outgoing proton) on the mass of the supplementary excited ${\ensuremath{\pi}}^{0}p$ system. At high energies and midrapidities, the photon-photon contribution dominates over the diffractive components, however, the corresponding cross section is rather small. The photon-odderon and odderon-photon contributions are included in addition and first estimates (upper limits) of their contributions to rapidity and transverse momentum distribution of neutral pions are presented. We suggest a search for the odderon contribution at midrapidity and at ${p}_{\ensuremath{\perp},{\ensuremath{\pi}}^{0}}\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{GeV}$. Our predictions are ready for verification at RHIC and LHC. The bremsstrahlung mechanisms discussed here contribute also to the $pp\ensuremath{\rightarrow}p(n{\ensuremath{\pi}}^{+})$ reaction. Both channels give a sizable contribution to the low-mass single diffractive cross section and must be included in extrapolating the measured experimental single diffractive cross section.

Exclusive production ofωmeson in proton-proton collisions at high energies

First we calculate cross section for the $\gamma p \to \omega p$ reaction from the threshold to very large energies. At low energies the pion exchange is the dominant mechanism. At large energies the experimental cross section can be well described within the $k_{t}$-factorization approach by adjusting light-quark constituent mass. Next we calculate differential distributions for the $p p \to p p \omega$ reaction at RHIC, Tevatron and LHC energies for the first time in the literature. We consider photon-pomeron (pomeron-photon), photon-pion (pion-photon) as well as diffractive hadronic bremsstrahlung mechanisms. The latter are included in the meson/reggeon exchange picture with parameters fixed from the known phenomenology. Interesting rapidity distributions are predicted. The hadronic bremsstrahlung contributions dominate at large (forward, backward) rapidities. At small energies the photon-pomeron contribution is negligible compared to the bremsstrahlung contributions. It could be, however, easily identified at large energies at midrapidities. Absorptions effects are included and discussed. Our predictions are ready for verification at RHIC and LHC.

Study of the dependence of direct soft photon production on the jet characteristics in hadronic Z 0 decays

An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behaviour to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.

Soft photon anomaly and gauge/string duality

Motivated by the recent DELPHI report on the anomalous photon production in e + e − annihilation, we exactly calculate the inclusive cross section of soft photons in the strong coupling limit of N = 4 super Yang–Mills. We find that the energy distribution is that of the Bremsstrahlung, while the angular distribution is spherical. Our result elucidates a new non-perturbative source of soft photons not associated with the final state hadronic Bremsstrahlung.

Evidence for an excess of soft photons in hadronic decays of Z0

Soft photons inside hadronic jets converted in front of the DELPHI main tracker (TPC) in events of qq̄ disintegrations of the Z0 were studied in the kinematic range 0.2<Eγ<1 GeV and transverse momentum with respect to the closest jet direction pT<80 MeV/c. A clear excess of photons in the experimental data as compared to the Monte Carlo predictions is observed. This excess (uncorrected for the photon detection efficiency) is (1.17±0.06±0.27)×10-3 γ/jet in the specified kinematic region, while the expected level of the inner hadronic bremsstrahlung (which is not included in the Monte Carlo) is (0.340±0.001±0.038)×10-3 γ/jet. The ratio of the excess to the predicted bremsstrahlung rate is then (3.4±0.2±0.8), which is similar in strength to the anomalous soft photon signal observed in fixed target experiments with hadronic beams.

Further analysis of a direct soft photon excess in π−p interactions at 280 GeV/c

In this Letter we present additional experimental characteristics of anomalous soft photon radiation observed in π−p interactions at 280 GeV/c in the CERN experiment WA91, together with a re-calculated ratio of the observed direct soft photon signal to the expected level of the hadronic inner bremsstrahlung, which is found to be 5.3±1.0. The other presented characteristics, energy dependence and angular distributions of direct soft photons, show the similarity of the observed signal to the expected hadronic bremsstrahlung behaviour.

Low transverse momentum photon production in proton-nucleus collisions at 18 GeV/c.

Inclusive single photon spectra have been measured in the transverse momentum (${\mathit{p}}_{\mathit{T}}$) range from a few MeV/c up to approximately 1 GeV/c at various nucleon-nucleon center of momentum rapidities (${\mathit{y}}_{\mathit{NN}}$) in the interval from -2.4${\mathit{y}}_{\mathit{NN}}$+0.5 in 18 GeV/c p-Be and p-W collisions. These distributions were used to estimate the inclusive ${\mathit{p}}_{\mathit{T}}$ and rapidity distributions for ${\mathrm{\ensuremath{\pi}}}^{0}$ production over approximately the same kinematic range. The measured photon spectra are compared with the expected distribution of photons from ${\mathrm{\ensuremath{\pi}}}^{0}$ decays and hadronic bremsstrahlung in order to search for new sources of ``soft'' photons at low transverse momentum. The results indicate no anomalous sources of soft photons in either p-Be or p-W interactions at these energies. The p-W data at forward angles set a limit on any excess of low ${\mathit{p}}_{\mathit{T}}$ photons at 2.65 times hadronic bremsstrahlung at the 99% C.L. \textcopyright{} 1996 The American Physical Society.

Soft photons in hadron-hadron collisions: synchrotron radiation from the QCD vacuum?

We discuss the production of soft photons in high energy hadron-hadron collisions. We present a model where quarks and antiquarks in the hadrons emit ``synchrotron light'' when being deflected by the chromomagnetic fields of the QCD vacuum, which we assume to have a nonperturbative structure. This gives a source of prompt soft photons with frequencies $\omega <= 300 MeV$ in the c.m. system of the collision in addition to hadronic bremsstrahlung. In comparing the frequency spectrum and rate of ``synchrotron'' photons to experimental results we find some supporting evidence for their existence. We make an exclusive--inclusive connection argument to deduce from the ``synchrotron'' effect a behaviour of the neutron electric formfactor $G_E^n(Q^2)$ proportional to $(Q^2)^{1/6}$ for $Q^2 < 20 fm^{-2}$. We find this to be consistent with available data. In our view, soft photon production in high energy hadron-hadron and lepton-hadron collisions as well as the behaviour of electromagnetic hadron formfactors for low $Q^2$ are thus sensitive probes of the nonperturbative structure of the QCD vacuum.

Dilepton production from p-p to CaCa at the Bevalac

The DLS collaboration has recently completed a high statistics study of dilepton production at the Bevalac. In particular, we have measured dielectrons (e +e −) from p- p and p- d collisions to understand the basic dilepton production mechanisms in the energy range from 1.05 – 4.9 GeV. These data can be used to determine the basic processes which contribute to nucleon-nucleon dilepton production such as hadronic bremsstrahlung, vector meson processes, and hadronic Dalitz decay. The data show that a simple elastic bremsstrahlung calculation is insufficient to explain the data. Theoretical models are compared with the data. A new high statistics study of Ca Ca at 1.05 A·GeV has been made to study the collectivity of A- A collisions.

Low pT photon production in proton-nucleus collisions at 18 GeV/ c

The inclusive single photon spectrum has been measured in the p T range from a few MeV/ c up to approximately 1 GeV/ c at several rapidities in the interval from −2.1 < y cm < + 0.5 in 18 GeV/ c proton-beryllium and proton-tungsten collisions at the Brookhaven AGS. The measured photon distributions are compared with the expected distribution of photons from π 0 decays and hadronic bremsstrahlung in a search for new sources of “soft” photons at low transverse momentum.

Soft photon production in 450 GeV/cp-Be collisions

We have measured the inclusivepT spectra of soft photons produced at central and backward rapidities in 450 GeV/cp-Be collisions down to 1 MeV/c in transverse momentum. In the region 1<pT<20 MeV/c an excess of photons over those expected from hadronic decays is observed. This excess is comparable, within systematic errors, with estimates of direct photons produced via hadronic bremsstrahlung. An upper limit is derived on the presence of additional sources of direct photons at small transverse momentum.

Low-mass dileptons at high momenta in ultra-relativistic heavy-ion collisions

Abstract We calculate the emission rate of low-mass dileptons at high momenta from a quark-gluon plasma. We find a large enhancement compared with the naive Born rate. Numerical estimates for relativistic heavy ions collisions at RHIC and LHC suggest, however, that the thermal signal is dominated by the background coming from meson decays and from hadronic bremsstrahlung.

Results of the HELIOS Collaboration on low mass dilepton and soft photon production in pBe, pW and SW collisions

We report results of the HELIOS Collaboration on low-mass dilepton and soft photon production in 450 GeV/ c proton-beryllium interactions and preliminary results on sulphur tungsten collisions. Both electron and muon pairs were measured in a double spectrometer with similar kinematic acceptance for both parts. The observed dilepton yield is found to be in good agreement with the expected cumulative lepton pair production by meson decays. Based on the simultaneous direct measurements of the η, via the Dalitz decay channel, which represents the main component of the decay background, the remaining systematic errors of this experiment are as small as 20–30%. With a specially modified experimental set-up the HELIOS-3 collaboration measured low-mass muon pairs in 200 GeV/ c proton-tungsten and sulfur-tungsten collisions. We show first preliminary results. For this analysis the mesonic Dalitz decay contributions are evaluated using cross-section parametrization leading to larger systematic errors. No significant additional dilepton source is detected within the framework of this preliminary analysis. Soft ( p T ≤ 50 MeV/ c) photon production was measured with two special independent set-ups in 450 GeV/ c proton beryllium collisions. The observed production rates and spectra are well reproduced by hadronic bremsstrahlung calculations.

Dilepton radiation from nucleon-nucleon collisions.

We calculate the dilepton spectrum arising from hadronic bremsstrahlung and from the radiative decay of the delta in nucleon-nucleon collisions. Comparison is made with the recent p+Be data at 4.9 GeV obtained by the DLS Collaboration at the Bevalac. We argue that dileptons constitute a valuable probe of nuclear dynamics.

Anomalous soft photons from a coherent hadronic phase in high-energy collisions

Recent experiment indicates that the pT-spectra of inclusive photons produced at central rapidity in 450 GeV/c p-Be and p-Al interactions show a marked excess of direct photons in the region 4 MeV/c≲-pT≲-20 MeV/c, over the photons from decays of neutral mesons and the direct photons expected from hadronic bremsstrahlung. We show that soft photon emission from a pion condensate, which state can be approached in the nuclear medium during the collision, exhibits specific characteristics that can explain the experiment. The very soft photons appear to signal the proximity of a new, coherent dynamical phase.

Photon production in ultrarelativistic heavy-ion collisions at 200 GeV/u

A calculation of sources of inclusive photons in ultrarelativistic O-W and Pb-Pb collisions at 200 GeV/u is presented. The production of thermal photons in a quark-gluon plasma, a deconfined state of nuclear matter at high energy density, is addressed. In events with a large amount of energy deposited in the target the thermal contribution to the inclusive spectrum is found to be ∼5%, quadratically increasing with particle multiplicity, but only weakly dependent on the size of the projectile nucleus. These photons are predominantly produced in a coexistence phase of plasma and hadronic matter. Hadronic bremsstrahlung is investigated including the spatial and temporal coherence due to the large space-time extension of heavy-ion systems. For small photon momenta there is a coherent signal in the fragmentation regions proportional to the squared number of participant protons.

The production of prompt positrons at low transverse momentum increases with the square of the associated charged multiplicity: Axial field spectrometer collaboration

We have measured the production of prompt positrons in pp collisions at √ s = 63 GeV and y = 0 in the p T interval 0.12< p T<1.0 GeV/c. The results indicate that the production of positrons at low p T (<0.4 GeV/ c) is proportional to the square of the mean multiplicity in the central region | y| < 1. Such a quadratic dependence is not expected from final-state sources such as hadronic bremsstrahlung or hadronic decays, but is natural in models where low mass electron pairs are produced by interactions of constituents created during the collision.

Production of prompt positrons at low transverse momentum in 63 GeV pp collisions at the CERN intersecting storage rings

The production of inclusive prompt positrons in pp collisions at √ s = 63 GeV and θ cm = 90° has been measured over the p T interval 0.08 <p T < 1.0 GeV c . The e/ π ratio rises from ∼ 2 × 10 −4 at p T = 0.9 GeV c to ∼ 1 × 10 −3 at p T = 0.1 GeV c . This is a factor of 6 higher than would be expected from the sum of hadronic bremsstrahlung and the semileptonic decay of charm.

Measurement of direct electron-positron pair production from hadronic bremsstrahlung

We have made a measurement of very-low-energy ${e}^{+}{e}^{\ensuremath{-}}$ pair production from ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}p$ collisions at 18 GeV/c. Using the SLAC 1-m bubble chamber to study a sample of 178\ifmmode\times\else\texttimes\fi{}${10}^{3}$ interactions, we find a signal of centrally produced direct ${e}^{+}{e}^{\ensuremath{-}}$ pairs which accounts for (48\ifmmode\pm\else\textpm\fi{}13)% of all ${e}^{+}{e}^{\ensuremath{-}}$ pairs detected with $|x|&lt;0.02$ and ${P}_{T}&lt;10$ MeV/c. This measurement agrees with a calculation of ${e}^{+}{e}^{\ensuremath{-}}$ pair production from the internal conversion of hadronic bremsstrahlung.