Photon Structure Function F2γ Research Articles

Measurement of the electron structure function [formula omitted] at LEP energies

The hadronic part of the electron structure function F2e has been measured for the first time, using e+e− data collected by the DELPHI experiment at LEP, at centre-of-mass energies of s=91.2–209.5 GeV. The data analysis is simpler than that of the measurement of the photon structure function. The electron structure function F2e data are compared to predictions of phenomenological models based on the photon structure function. It is shown that the contribution of large target photon virtualities is significant. The data presented can serve as a cross-check of the photon structure function F2γ analyses and help in refining existing parameterisations.

Measurement of the photon structure function [formula omitted] with the L3 detector at LEP

The e+e−→e+e−hadrons reaction, where one of the two electrons is detected in a low polar-angle calorimeter, is analysed in order to measure the hadronic photon structure function F2γ. The full high-energy and high-luminosity data set, collected with the L3 detector at centre-of-mass energies 189 GeV⩽s⩽209 GeV, corresponding to an integrated luminosity of 608 pb−1 is used. The Q2 range 11 GeV2⩽Q2⩽34 GeV2 and the x range 0.006⩽x⩽0.556 are considered. The data are compared with recent parton density functions.

Measurement of the hadronic photon structure function F2γ at LEP2

The hadronic structure function of the photon F2γ(x,Q2) is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e− centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2γ are extended to an average Q2 of 〈Q2〉=780 GeV2 using data in the kinematic range 0.15<x<0.98. The Q2 evolution of F2γ is studied for 12.1<〈Q2〉<780 GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2γ with F2γ(Q2)/α=(0.08±0.02+0.05−0.03)+(0.13±0.01+0.01−0.01)lnQ2, where Q2 is in GeV2, for the central x region 0.10–0.60. Several parameterisations of F2γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

Measurement of the low- x behaviour of the photon structure function F2γ

The photon structure function F 2 γ ( x, Q 2) has been measured using data taken by the OPAL detector at e +e − centre-of-mass energies of 91 GeV and 189 GeV, in Q 2 ranges of 1.5 – 6.0 GeV 2 (LEP1), and 7.0 – 30 GeV 2 (LEP2), probing the lowest possible values of x. The latest Monte Carlo programs and new methods, such as multi-variable unfolding, have been used to reduce model dependent systematic errors in the measurement.

Measurement of the photon structure function F2 at low x

Abstract Deep inelastic electron-photon scattering is studied using e+e− data collected by the OPAL detector at centre-of-mass energies s =M Z 0 . The photon structure function F2γ(x, Q 2) is explored in a Q 2 range of 1.1 to 6.6 GeV2 at lower x values than ever before. To probe this kinematic region events are selected with a beam electron scattered into one of the OPAL luminosity calorimeters at scattering angles between 27 and 55 mrad. A measurement is presented of the photon structure function F2γ(x, Q 2) at 〈 Q 2〉 = 1.86 GeV2 and 3.76 GeV2 in five logarithmic x bins from 0.0025 to 0.2.

A measurement of the photon structure function F2γ at Q2 = 6.8 GeV 2

The photon structure function F 2 γ has been measured at an average Q 2 value of 6.8 GeV 2 using data collected by the AMY detector at the TRISTAN e + e − collider. The measured F 2 γ is compared with several QCD-based parton density models.

A high- Q2 measurement of the photon structure function F2γ

The photon structure function F 2 γ has been measured at average Q 2 values of 73 and 390 GeV 2 using data collected by the AMY detector at the TRISTAN e + e − collider. F 2 γ is observed to be increasing as ln Q 2. The x-dependence of F 2 γ , where x is the momentum fraction carried by the parton inside the photon, is also measured. The measurements are compared with several parton density models.

Measurement of the photon structure function F2γ and jet production at TRISTAN

We have measured the photon structure function F 2 γ in the reaction e + e − → e + e − hadrons for average Q 2 values from 5.1 to 338 GeV 2 by using data collected by the TOPAZ detector at TRISTAN. The data have been corrected for detector effects and are compared with theoretical expectations based on QCD. The structure function F 2 γ increases as ln Q 2, as expected. A sample of events with one or two distinct jets has been identified in the final state. Although two-jet events can be explained solely by the point-like perturbative part, one-jet events require a significant hadron-like part in addition.

Measurement of the photon structure functionF 2 ? in the reactione + e ??e + e ?+hadrons at LEP

We present measurements of the hadronic photon structure functionF2γ(x), in twoQ2 ranges with mean values of 5.9 GeV2 and 14.7 GeV2. The data were taken by the OPAL experiment at LEP, with\(\sqrt s\) close to theZ0 mass and correspond to an integratede+e− luminosity of 44.8 pb−1. In the context of a QCD-based model we find the quark transverse momentum cutoff separating the vector meson dominance (VMD) and perturbative QCD regions to be 0.27±0.10 GeV. We confirm that there is a significant pointlike component of the photon when the probe photon hasQ2>4 GeV2. Our measurements extend to lower values ofx than any previous experiment, and no increase ofF2γ(x) is observed.

A phenomenological analysis of the photon structure functionF 2 ? (x, Q 2 )

A new method of analysis of the photon structure functionF2γ(x, Q2) is presented. It is a generalisation of Rossi's ansatz for generating regularised solutions of the inhomogeneous Altarelli-Parisi equations which enables initial conditions in theQ2 evolution to be incorporated. Its equivalence to the normal solution is demonstrated. It is applied in a phenomenological analysis of the data on the structure functionF2γ. It is found that the recent TPCF2γ data in the range 0.3<Q2<1.6 GeV2 cannot be used as a starting point for a perturbative QCD analysis, despite being well described by the vector meson dominance model (VMD) and exhibiting scaling. This conclusion invalidates previous discussions of the effect of the regularisation of the asymptotic point-like QCD prediction on the sensitivity ofF2γ to the QCD mass scale Λ, because they incorporated VMD assumptions in the perturbative region. A perturbative QCD analysis using higherQ2 data as a starting point is presented but here the initial conditions necessarily involve more assumptions, and starting the evolution at a higherQ2 involves an inevitable decrease in the sensitivity to Λ.

Early scaling in two photon reactions?

We show that the interpretation given by the TPC/2γ collaboration that their results for the photon structure functionF2γ(x) indicates precocious scaling forQ2>0.3 GeV2 andx 20 GeV2 and all values ofW. This hard component is also responsible for the bulk of the cross section at lowQ2 andW 6 GeV for values ofQ2≤10 GeV2.

The photon structure function F2γ, vector meson dominance and the applicability of perturbative QCD

A phenomenological analysis of the Q 2 evolution of the photon structure function F 2 γ ( x, Q 2) is presented, using the recent TPC data in the range 0.3⩽ Q 2⩽1.6 GeV 2 as a starting point. It is found that although these data are well described by the vector meson dominance (VMD) model and exhibit scaling, they cannot be used as a starting point for an analysis using perturbative QCD. This conclusion that the region described by VMD is outside the perturbative region invalidates all previous arguments over the sensitivity of F 2 γ to the QCD mass scale Λ, which use VMD to estimate the effect of the boundary conditions.

The hadronic and point-like contributions to theF 2 photon structure function in perturbative QCD

A detailed discussion is given of the hadronic and point-like contributions to theF 2 photon structure function (F 2 γ ) in both the naive parton model and QCD. The non-singlet part of the leading order solution, first found by Witten, is re-derived using the QCD improved parton model, enabling the hadronic and point-like terms to be clearly identified and correlated with observed jet structure in the final state. When important non leading-log terms are included, the sensitivity of the solution to Λ is found to be weak for allQ 2 values, and the all orders solution to be well approximated by theO(α 2 solution. The approximations made in deriving the leading order solution are critically examined, and an approach enabling more quantitative tests of QCD to be made from measurements of the point-like (perturbative) component ofF 2 γ is suggested.

Measurement of the photon structure functionF 2 γ (x, Q2) in the region 0.2&lt;Q 2&lt;7 GeV2

We present a measurement of the photon structure functionF2γ in the reactionee→eeX forQ2 in the range 0.2<Q2<7 GeV2, using 9,200 multihadron events obtained with the TPC/Two-Gamma detector at PEP. The data have been corrected for detector effects using a regularized unfolding procedure and are presented as a function ofx andQ2. The structure function shows scaling in the region 0.3<Q2<1.6 GeV2,x<0.3 and rises for higherQ2. AtQ2=5.1 GeV2 the results are compared with QCD and, within the scheme of Antoniadis and Grunberg, rather conservative bounds for the QCD scale parameter of 133±50<\(\Lambda _{\overline {{\rm M}S} } \)<268±98 MeV are obtained. A study of the final state structure shows that the rise ofF2γ is consistent with being entirely due to the pointlike component of the photon.