Fragmentation Contribution Research Articles

Gluon fragmentation into heavy quarkonia

We present a calculation of the gluon fragmentation contribution to the production of J/ψ, χ c and γ mesons at high energies. Both colour-singlet and colour-octet terms are included in the gluon fragmentation functions, and the kinematical effects of initial and final state parton radiation are taken into account. The theoretical results are compared with recent measurements at Tevatron and at HERA, and an extrapolation to LHC energies is discussed.

A full next-to-leading order study of direct photon pair production in hadronic collisions

We discuss the production of photon pairs in hadronic collisions, from fixed target to LHC energies. The study which follows is based on a QCD calculation at full next-to-leading order accuracy, including single and double fragmentation contributions, and implemented in the form of a general purpose computer program of "partonic event generator" type. To illustrate the possibilities of this code, we present the comparison with observables measured by the WA70 and D0 collaborations, and some predictions for the irreducible background to the search of Higgs bosons at LHC in the channel $h \to \gamma \gamma$. We also discuss theoretical scale uncertainties for these predictions, and examine several infrared sensitive situations which deserve further study.

Photon fragmentation in large- $Q^2$ ep collisions at next-to-leading order QCD

We study the production of photons accompanied by jets in large-\(Q^2\) deep inelastic scattering. Numerical results for the cross section differential with respect to the fraction of momentum \(z_{\gamma}\) carried by a photon inside a jet at large \(z_{\gamma}\), up to \(O(\alpha \alpha_s)\) in perturbative QCD, are presented. The sensitivity to the fragmentation contribution allows one to study the quark-to-photon fragmentation function. Our results can be confronted with future experimental data from HERA.

Photon fragmentation in large-

We study the production of photons accompanied by jets in large-Q2 deep inelastic scattering. Numerical results for the cross section differential with respect to the fraction of momentum z_gamma carried by a photon inside a jet at large z_gamma, up to O(alpha alpha_s) in perturbative QCD, are presented. The sensitivity to the fragmentation contribution allows one to study the quark-to-photon fragmentation function. Our results can be confronted with future experimental data from HERA.

Isolated photons in perturbative QCD

I present a definition of the cross section for the production of an isolated photon plus $n$ jets which only depends upon direct photon production, and it is independent of the parton-to-photon fragmentation contribution. This prescription, based on a modified cone approach which implements the isolation condition in a smooth way, treats in the same way quarks and gluons and can be directly applied to experimental data in hadron-hadron, photon-hadron and $\epem$ collisions. The case of several, isolated photons in the final state can also be dealt with in the very same way.

Isolated photons at hadron colliders at O( ααs2) (I): Spin-averaged case

The cross sections for isolated and non-isolated prompt photon production with unpolarized hadron beams are studied at order αα s 2. Two methods of performing the calculations are compared. One uses purely analytic techniques and the second uses a combination of analytic and Monte Carlo techniques to perform the phase-space integrations. The results of the analytic and Monte Carlo methods are compared both before and after isolation cuts are placed on the photon. Fragmentation contributions are included at next-to-leading order in the analytic case, and the question of the infrared sensitivity of this contribution once isolation is implemented is addressed. Numerical results are presented and compared with the latest CDF data for p p → γ + X using the latest parton densities for the proton.

Gluon fragmentation to3DJquarkonia

We present a calculation of the leading order QCD fragmentation functions for gluons to split into spin-triplet D-wave quarkonia. We apply them to evaluate the gluon fragmentation contributions to inclusive ^3D_J quarkonium production at large transverse momentum processes like the Tevatron and find that the D-wave quarkonia, especially the charmonium 2^{--} state, could be observed through color-octet mechanism with present luminosity. Since there are distinctively large gaps between the contributions of two different (i.e, color-singlet and color-octet) quarkonium production mechanisms, our results may stand as a unique test to NRQCD color-octet quarkonium production mechanism.

CHARMONIUM PRODUCTION AT THE LHC

The analyses of large transverse momentum charmonium production at the Tevatron have shown that fragmentation of gluons is an important production mechanism. We study large-pT charmonium production in pp collisions at the LHC, and find that due to the copious gluon production at this energy, the gluon fragmentation contribution completely overwhelms the fusion contribution and the charm quark fragmentation contribution. Our analysis shows that for J/ψ production at the LHC, there is a significant event rate even for pT ~ 100 GeV. The measurement of the cross-section at such large values of pT will provide a very important test of the fragmentation mechanism.

Prospects for the extraction of |gn cb| fromB c →J/ψμ + ν decays at LHC

We discuss the possibility of extracting |ν cb| from the future measurement at LHC of theB c→J/ψμ + ν decay rate followed by theleptonic decayJ/ψ→μ + μ −. We examine in detail the influence of the measurement of the muons’ momenta and vertex reconstruction in the final uncertainty of |ν cb|, as well as the required theoretical inputs at both the production and weak decay level, and their reliability in this regard. We stress the relevance of a rigorous description of the fragmentation contributions to inclusive production at large transverse momentum of heavy quarkonia inpp collisions.

Formula omitted] production via fragmentation at HERA

We compute the contributions to large- p T J ψ production at DHEA coming from fragmentation of gluons and charm quarks. We find that the charm quark fragmentation contribution dominates over the direct production of J ψ via photongluon fusion at large- p T , while the gluon fragmentation is negligibly small over the whole range of p T . An experimental study of p T distributions of J ψ at HERA will providea direct probe of the charm quark fragmentation functions.

Constraints on the proton's gluon distribution from prompt photon production

We analyze the capability of prompt photon production in pp and p p ̄ collisions to constrain the gluon distribution of the proton, considering data from fixed-target experiments as well as collider measurements. Combined fits are performed to these large- P T direct-γ cross sections and lepton-proton deep-inelastic scattering data in the framework of next-to-leading order perturbative QCD. Special attention is paid to theoretical uncertainties originating from the scale dependence of the results and from the fragmentation contribution to the prompt photon cross section.

Prospects for the extraction of ∣ V cb ∣ from B c → J /ψμ + ν decays at LHC

We discuss the possibility of extracting ${\mid}V_{cb}{\mid}$ from the future measurement at LHC of the $B_c\ {\rightarrow}\ J/\psi\ {\mu}^+\ {\nu}$ decay rate followed by the leptonic decay $J/\psi\ {\rightarrow}\ \mu^+\mu^-$. We examine in detail the influence of the measurement of the muons' momenta and vertex reconstruction in the final uncertainty of ${\mid}V_{cb}{\mid}$, as well as the required theoretical inputs at both the production and weak decay level, and their reliability in this regard. We stress the relevance of a rigorous description of the fragmentation contributions to inclusive production at large transverse momentum of heavy quarkonia in $pp$ collisions.

Surprises from quarkonium decay into photons

The perturbative QCD approach to quarkonium decay into a photon and hadrons is reconsidered. It is shown that a consistent treatment within perturbative QCD calls for the introduction of a fragmentation contribution which has been neglected so far. The ensuing phenomenological implications are discussed, and, in particular, the possibility of measuring the gluon fragmentation function of the photon is addressed. *To appear in the proceedings of the workshop QCD94, Montpellier, July '94.

J/ psi production from electromagnetic fragmentation in Z0 decay.

The rate for $ Z^{0}\to J/ \psi + \ell^{+}\ell^{-} $ is suprisingly large with about one event for every million $Z^{0}$ decays. The reason for this is that there is a fragmentation contribution that is not suppressed by a factor of $M^{2}_{\psi}/M^{2}_{Z}$. In the fragmentation limit $ M_{Z}\to\infty$ with $E_{\psi}/M_{Z}$ fixed, the differential decay rate for $ Z^{0}\to J/ \psi + \ell^{+}\ell^{-} $ factors into electromagnetic decay rates and universal fragmentation functions. The fragmentation functions for lepton fragmentation and photon fragmentation into $J/\psi$ are calculated to lowest order in $\alpha$. The fragmentation approximation to the rate is shown to match the full calculation for $E_{\psi}$ greater than about $3 M_{\psi}$.

Fragmentation contribution to quarkonium production in hadron collision

We compute the contributions due to gluon and heavy-quark fragmentation to quarkonium production at large transverse momentum in pp and pp collisions. For inclusive J ψ production, there is a large contribution from the g → X c fragmentation. At large p T , this is comparable to the conventional charmonium model prediction via gluon fusion at the ISR and dominates over the latter at the Tevatron energy. This may help to explain, at least partly, the large J ψ production cross-section recently observed by the CDF experiment. However, the fragmentation contribution to ψ′ production is not large enough to explain the corresponding CDF data. We also present the results for Y production at ISR and Tevatron energies.

Next-to-leading order hadronic single-spin production of polarized prompt photons.

We present a calculation of the complete next-to-leading order [[ital O]([alpha][alpha][sub [ital s]][sup 2])] corrections to the inclusive production of polarized prompt photons in collisions of longitudinally polarized hadrons with unpolarized hadrons. We show that this process should offer a good possibility of deciding between presently suggested proton spin scenarios and shows good perturbative stability. Furthermore, we also estimate the effect of the leading order fragmentation contribution and discuss its influence on the cross section asymmetries.

Polarized and unpolarized isolated prompt photon production beyond the leading order.

We present a simple and accurate analytical method for calculating the cross section for polarized and unpolarized isolated prompt photon production in next-to-leading order, taking into account also the next-to-leading-order fragmentation contribution in the unpolarized case. We demonstrate the good accuracy of our method over a wide range of the isolation parameters and study the effects the isolation cuts have on the cross section at colliders.

Fragmentation production of [formula omitted] and ψ′ at the Tevatron

We present a calculation of the charm and gluon fragmentation contributions to inclusive J ψ and ψ′ production at large transverse momentum at the Tevatron. For ψ production, we include both fragmentation directly into ψ and fragmentation into χ c followed by the radiative decay χ c → ψ + λ. We find that fragmentation overwhelms the leading-order mechanisms for prompt ψ production at large p T, and that the dominant contributions come from fragmentation into χ c . Our results are consistent with recent data on ψ production from the CDF and D0 experiments. In the case of prompt ψ′ production, the dominant mechanism at large p T is charm fragmentation into ψ′. We find serious disagreement between our theoretical predictions and recent ψ′ data from the Tevatron.

High-pT photon production at pp-bar colliders.

It is demonstrated by consistent next-to-leading order (NLO) calculations for isolated prompt photon production, performed within the small-cone approximation, that previous discrepancies between NLO expectations for the direct [gamma] production rates at small [ital p][sub [ital T]][sup [gamma]]([approx gt]11 GeV) and isolated prompt photon data are removed. This is not only due to the use of experimentally confirmed steep small [ital x] parton distributions, but also due to a consistent inclusion of isolated photonic fragmentation contributions in NLO. The latter contributions are even more important for fully inclusive predictions.

Mass spectrum and sublimation pressure of sodium oxide vapor: Stability of the superoxide molecule NaO2

The total sublimation pressure of Na2O(c) has been measured in the range 1000–1050 K by the torsion-effusion method, and the mass spectra of vapors over Na2O(c) and Na2O2(c) have been examined in order to resolve several issues regarding the thermochemical stability of the superoxide molecule NaO2. Measured torsion-effusion pressures are completely consistent with the primary sublimation process Na2O(c)=2Na(g)+0.5 O2(g) and set an upper limit of 200 kJ mol−1 for the bond strength D0(Na–O2). The ionization efficiency curves of Na+ and O2+ in the mass spectrum of vapor over Na2O(c) show no evidence for fragmentation contributions from NaO2. In addition, the absence of a detectable Na+/NaO2 fragment ion signal in vapor over Na2O2(c) can be used to derive an even tighter upper bound of 180 kJ mol−1 for D0(Na–O2). These results are in conflict with various kinetic analyses yielding D0(Na–O2) values of 202 to 243 kJ mol−1, but are in accord with rigorous theoretical calculations giving values near 160 kJ mol−1.