Photon-proton Collisions Research Articles

Search for photoproduction of axionlike particles at GlueX

We present a search for axion-like particles, $a$, produced in photon-proton collisions at a center-of-mass energy of approximately 4 GeV, focusing on the scenario where the $a$-gluon coupling is dominant. The search uses $a\to\gamma\gamma$ and $a\to\pi^+\pi^-\pi^0$ decays, and a data sample corresponding to an integrated luminosity of 168 pb$^{-1}$ collected with the GlueX detector. The search for $a\to\gamma\gamma$ decays is performed in the mass range of $180 < m_a < 480$ MeV, while the search for $a\to\pi^+\pi^-\pi^0$ decays explores the $600 < m_a < 720$ MeV region. No evidence for a signal is found, and 90% confidence-level exclusion limits are placed on the $a$-gluon coupling strength. These constraints are the most stringent to date over much of the mass ranges considered.

Measurement of exclusive {varvec{{{{pi ^+pi ^-}}}}} and {varvec{{{{rho ^0}}}}} meson photoproduction at HERA

Exclusive photoproduction of {{rho ^0}} (770) mesons is studied using the H1 detector at the ep collider HERA. A sample of about 900,000 events is used to measure single- and double-differential cross sections for the reaction gamma p rightarrow pi ^{+}pi ^{-}Y. Reactions where the proton stays intact ({{{m_Y}} {=}m_p}) are statistically separated from those where the proton dissociates to a low-mass hadronic system (m_p{<}{{m_Y}} {<}10~{{text {GeV}}} ). The double-differential cross sections are measured as a function of the invariant mass m_{pi pi } of the decay pions and the squared 4-momentum transfer t at the proton vertex. The measurements are presented in various bins of the photon–proton collision energy {{W_{gamma p}}} . The phase space restrictions are 0.5le m_{pi pi } le 2.2~{{text {GeV}}} , vert tvert le 1.5~{{text {GeV}^2}} , and 20 le W_{gamma p} le 80~{{text {GeV}}} . Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the {m_{pi pi }} dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the {{rho ^0}} (770) meson mass and width at m_rho = 770.8{}^{+2.6}_{-2.7}~({text {tot.}})~{{text {MeV}}} and Gamma _rho = 151.3 {}^{+2.7}_{-3.6}~({text {tot.}})~{{text {MeV}}} , respectively. The model is used to extract the {{rho ^0}} (770) contribution to the pi ^{+}pi ^{-} cross sections and measure it as a function of t and {W_{gamma p}}. In a Regge asymptotic limit in which one Regge trajectory alpha (t) dominates, the intercept alpha (t{=}0) = 1.0654 {}^{+0.0098}_{-0.0067}~({text {tot.}}) and the slope alpha ^prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({text {tot.}}) ~{{text {GeV}^{-2}}} of the t dependence are extracted for the case m_Y{=}m_p.

Possible generation of π-condensation in a free space by collisions between photons and protons

A sharply peaked structure is found in the angular distribution of emitted π+ mesons from photon–proton collisions. This offers a possible way to generate a π+-condensation in free space. To make the stimulated emission of π+-mesons efficient, a ring resonator is designed.

Exclusive photoproduction ofJ/Ψ and Ψ′ mesons in proton-proton collisions

We present results based on our new paper (1). The amplitude for the γp → Vp reaction is calculated in a pQCD kt-factorization approach. The corresponding total cross section for different unintegrated gluon distributions is calculated and compared with new HERA data for photon-proton collisions. We also compare the total cross section for γp → Vpreaction with recent data extracted by the LHCb collaboration. The amplitude for γp → Vpis used to predict cross section for exclusive photoproduction of J/ψ and ψ � mesons in proton-proton collisions. Both Dirac and Pauli electromagnetic form factors are included in the calculation and results are compared with results when only Dirac form factor is included. The effect of Pauli form factor is quantified. Absorption effects are included and their uncertainties are discussed. Different differential distributions e.g. in J/ψ(ψ � ) rapidity and transverse momentum are presented and compared with existing experimental data for the Tevatron and LHC energy.

Probe of extra dimensions inγq→γqat the LHC

We have examined TeV scale effects of extra spatial dimensions through the processes gamma q-> gamma q where q=u,d,c,s,b, anti-u, anti-d, anti-c, anti-s, anti-b. These processes have been treated in a photon-proton collision via the main reaction pp-> p gamma p-> p gamma qX at the LHC. We have employed equivalent photon approximation for incoming photon beams and performed statistical analysis for various forward detector acceptances.

Exclusive production of vector mesons in γp and pp collisions

The dominant mechanism for the central exclusive production of vector meson in $p p$ and $p \bar p$ collisions is the $\gamma$ $I P$ fusion. As a building block for the pp reaction, the amplitude for photoproduction $ \gamma p \to V p $ is calculated in a pQCD $k_{T}$- factorization approach. We will present results for several vector mesons: $\rho$, $\omega$, $\phi$, $J/\Psi$ and $\Upsilon$. The total cross section for diffractive mesons production as a function of energy and photon virtuality is calculated. We will present dependence on the mass of the quark for light mesons. The results for $ \gamma p \to V p $ photoproduction depend on the model of the meson wave function. We compare our results with a HERA data for photon-proton collisions. Finally we turn to pp collisions, and we present distribution in rapidity, transverse momentum of vector mesons and azimuthal angle between outgoing protons for Tevatron and LHC energies. The absorption effects will be discussed.

Measuring C-odd correlations at lepton–proton and photon–proton collisions

We consider the charge-odd correlations (COC) in cross sections of charged particle production. The cases of a muonic pair and pion systems π+π−, π+π−π0 production are considered in detail for electron–proton or photon–proton collisions in the proton fragmentation region kinematics. COC arise from the interference of amplitudes which describe the different mechanisms of charged leptons (pions) production. One of them corresponds to the production of particles in the charge-odd state (one virtual photon or vector meson annihilation to this system of particles) and the other corresponds to the charge-even state of produced particles (creation by two photons). COC for a muon–antimuon pair creation have a pure QED nature and can be considered as a normalization process. The processes with pion production are sensitive to some characteristics of proton wavefunctions and, besides, can be used for checking the anomalous and normal parts of the effective pionic Lagrangian. The matrix element of three electromagnetic current operators can be measured in photon–proton interactions with lepton pair production. For this aim a charge-odd combination of cross sections can be constructed as a conversion of leptonic 3-rank tensor with hadronic ones. These experiments can be considered as an alternative to deep virtual Compton scattering.

TESTING THE DYNAMICS OF HIGH ENERGY SCATTERING USING VECTOR MESON PRODUCTION

I review work on diffractive vector meson production in photon–proton collisions at high energy and large momentum transfer, accompanied by proton dissociation and a large rapidity gap. This process provides a test of the high energy scattering dynamics, but is also sensitive to the details of the treatment of the vector meson vertex.The emphasis is on the description of the process by a solution of the non-forward BFKL equation, i.e. the equation describing the evolution of scattering amplitudes in the high-energy limit of QCD. The formation of the vector meson and the nonperturbative modeling needed is also briefly discussed.

The Top-Charm Associated Production Within the R-Parity Violating Supersymmetric Model in Photon-Proton Collisions**The project supported by National Natural Science Foundation of China

The top-charm associated production with the effects from both B- and L-violating interactions in TeV scale photon-proton collisions is investigated in the framework of minimal supersymmetric standard model. Within the bounds on the relevant R-parity violating couplings, the total cross section will reach the order of 10 fb in some parts of the parameter space.

Energy flow and rapidity gaps between jets in photoproduction at HERA

Dijet events in photon-proton collisions in which there is a large pseudorapidity separation $\Delta \eta > 2.5$ between the two highest $E_{T}$ jets are studied with the H1 detector at HERA. The inclusive dijet cross sections are measured as functions of the longitudinal momentum fractions of the proton and photon which participate in the production of the jets, $x_{p}^{jets}$ and $x_{\gamma}^{jets}$ respectively, $\Delta \eta$ , the pseudorapidity separation between the two highest $E_T$ jets, and $E_T^{gap}$ , the total summed transverse energy between the jets. Rapidity gap events are defined as events in which $E_T^{gap}$ is less than $E_T^{cut}$ , for $E_T^{cut}$ varied between 0.5 and 2.0 GeV. The fraction of dijet events with a rapidity gap is measured differentially in $\Delta \eta$ , $x_{p}^{jets}$ and $x_{\gamma}^{jets}$ . An excess of events with rapidity gaps at low values of $E_T^{cut}$ is observed above the expectation from standard photoproduction processes. This excess can be explained by the exchange of a strongly interacting colour singlet object between the jets.

Photoproduction of jets and prompt photons

Jet and prompt photon production in the photon-proton collisions has been studied for the center of mass energy interval 120 < W γp < 270 GeV. The data were collected with the ZEUS detector at HERA. Recent results are presented.

Leading proton spectrum from DIS at HERA

The QCD hardness scale for secondary particles ($h$) production in semi-inclusive deep inelastic scattering (DIS), $e p \to e' X h$, gradually decreases from $Q^{2}$, the photon virtuality which determines the hard scale in the virtual photon (current) fragmentation region to a soft, hadronic, scale in the proton fragmentation region. This suggests similarity of the inclusive spectra of leading protons and neutrons, $h=p,n$, in high energy hadron-proton and virtual photon-proton collisions. We explore this similarity extending to the DIS regime the nonperturbative peripheral mechanisms of inelastic scattering traditionally used in hadronic interactions to explain fast nucleons production. While the production of leading neutrons is known to be exhausted by DIS off charged pions, the production of leading protons by DIS off neutral pions must be supplemented by a substantial contribution from isoscalar reggeon ($f_{0}$) exchange extrapolated down to moderate values of $x_{L}$. We comment on the $x$ and $Q^{2}$ dependence of leading proton production as a probe of a universal pattern of the $x,Q^{2}$ evolution of the nucleon and meson (reggeon) structure functions at small $x$.

A unitarity bound and the components of photon-proton interactions

We show how and why the short distance (“hard”) interaction, which is calculated in perturbative QCD, provides a mass cutoff in Gribov's formula for photon-proton collisions. This enables us to find a new and more restrictive unitarity bound for this process, $\sigma(\gamma^{*}p)\leq C(ln\frac{1}{x})^{\frac{5}{2}}$ . We develop a simple model that consists of “soft” and “hard” contributions, which yields a qualitative description of the published experimental data over a wide range of photon virtualities ( $Q^2$ ) and energies ( $W$ ). This model provides a quantitative way of evaluating the relative rate of the short and long distance contributions, in the different kinematic regions. The main results of the analysis are (i) that even at $Q^2 =0$ and high energies the short distance contribution is not small, and it provides a possible explanation for the experimental rise of the photoproduction cross section; and (ii) at large values of $Q^2$ , the long distance processes still contribute to the total cross section.

Measurement of jet shapes in photoproduction at HERA

The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range 134–277 GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the \(\eta - \phi\) plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies \(E^{jet}_T>14\) GeV are presented. The jet shape broadens as the jet pseudorapidity (\(\eta^{jet}\)) increases and narrows as \(E^{jet}_T\) increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high \(\eta^{jet}\) and low \(E^{jet}_T\). The observed broadening of the jet shape as \(\eta^{jet}\) increases is consistent with the predicted increase in the fraction of final state gluon jets.

Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity $C=+1$ neutral meson production in virtual photon proton collision at high energies. Due to the opposite $C$ -parities of photon and meson $M^+$ ( $M^+ = \eta_C, \pi^0, a_2$ ) this process probes the $t$ -channel $C=-1$ odderon exchange which is described here as noninteracting three–gluon exchange. Estimates for the cross section of inelastic diffractive process $\gamma^* p\to \eta_C X_p$ are presented. The total cross section of diffractive $\eta_C$ meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons ( $\pi^0, a_2$ ) in $\gamma^* p$ collisions are of the same order if the photon virtuality $Q^2$ is $ m^2_C$ .

Measurement of jet shapes in photoproduction at HERA

The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range 134–277 GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the $\eta - \phi$ plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies $E^{jet}_T>14$ GeV are presented. The jet shape broadens as the jet pseudorapidity ( $\eta^{jet}$ ) increases and narrows as $E^{jet}_T$ increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high $\eta^{jet}$ and low $E^{jet}_T$ . The observed broadening of the jet shape as $\eta^{jet}$ increases is consistent with the predicted increase in the fraction of final state gluon jets.

A unitarity bound and the components of photon-proton interactions

We show how and why the small distance ("hard") interaction, calculable in perturbative QCD, provides a mass cutoff in Gribov's formula for photon-proton collisions. This enables us to find a new and more restrictive bound for this process, $\sigma(\gamma^{*}p)\leq C(ln 1/x)^{5/2}$.A simple model the published experimental data over a wide range of photon virtualities ($Q^2$) and energies ($W$), is developed. This model provides a quantitative way of evaluating the relative rate of the small and large distance contributions, in the different kinematic regions. The main results of the analysis are (i) that even at $Q^2 =0$ and high energies the short distance contribution is not small, and it provides a possible explanation for the experimental rise of the photoproduction cross section; and (ii) at large values of $Q^2$, the long distance processes still contribute to the total cross section.

Thrust jet analysis of deep-inelastic large-rapidity-gap events

A thrust analysis of Large-Rapidity-Gap events in deep-inelastic $ep$ collisions is presented, using data taken with the H1 detector at HERA in 1994. The average thrust of the final states $X$ , which emerge from the dissociation of virtual photons in the range $10 < Q^2 < 100 {\rm GeV}^2$ , grows with hadronic mass $M_X$ and implies a dominant 2-jet topology. Thrust is found to decrease with growing $P_t$ , the thrust jet momentum transverse to the photon-proton collision axis. Distributions of $P_t^2$ are consistent with being independent of $M_X$ . They show a strong alignment of the thrust axis with the photon-proton collision axis, and have a large high $-P_t$ tail. The correlation of thrust with $M_X$ is similar to that in $e^+e^-$ annihilation at $\sqrt s_{ee}=M_X$ , but with lower values of thrust in the $ep$ data. The data cannot be described by interpreting the dissociated system $X$ as a $q\bar q$ state but inclusion of a substantial fraction of $q\bar q g$ parton configurations leads naturally to the observed properties. The soft colour exchange interaction model does not describe the data.

Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity C=+1 neutral meson production in virtual photon proton collision at high energies. Due to the opposite C-parities of photon and meson M (M = eta_C, pi^0, a_2) this process probes the t-channel C=-1 odderon exchange which is described here as noninteracting three-gluon exchange. Estimates for the cross section of the inelastic diffractive process gamma^* p -> eta_C X_p are presented. The total cross section of diffractive \eta_C meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons (pi^0, a_2) in \gamma^* p collisions are of the same order if the photon virtuality Q^2 is m^2_C.

Color-octet contributions to J/ psi photoproduction.

We have calculated the leading color-octet contributions to the production of $J/\psi$ particles in photon-proton collisions. Using the values for the color-octet matrix elements extracted from fits to prompt $J/\psi$ data at the Tevatron, we demonstrate that distinctive color-octet signatures should be visible in $J/\psi$ photoproduction. However, these predictions appear at variance with recent experimental data obtained at HERA, indicating that the phenomenological importance of the color-octet contributions is smaller than expected from theoretical considerations and suggested by the Tevatron fits.