Asymmetry in Photoproduction

CONG-FENG QIAO Graduate School of Chinese Academy of Sciences

In collaboration with G.Hao & L.Li(hep-ph/0504087)

17 June 2005, PKU-QCD, Beijing

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Outline of the talk

Introduction(Background)

A Model Prediction for Asymmetry Production

Conclusions and outlooks

Background

High energy charm production is important in the study of QCD.

In recent years, large production asymmetries between charmed and anti-charmed mesons have been measured in fixed-target hadro-production and photo-procution experiments.

[E791(1996)(1997)(2000);E769(1994)(1996);WA89(1999);

SELEX(2002);NA14/2(1993);E687(1996)].

Among these measurements, the photoproduction is thought to give more clean signature than the hadroproduction.

It is intriguing to notice that the experimental observation on the asymmetries of the charmed hadron productions are greatly in excess of the prediction of pQCD

For instance, the Ds production in pQCD approach can be expressed as the convolution of parton distribution function, the partonic cross section, and fragmentation function, like

(1) where the charm and anti-charm are produced s

ymmetrically at leading order.

The charm and anti-charm asymmetry appears only in the NLO, or higher, corrections.

However, the pQCD NLO result on asymmetry is one order of magnitude smaller than the observation in the photoproduction experiments.

[ E.Cuautle et al. hep-ph/0005023]

That is the charmed- and anti-charmed-hadron production asymmetries can not be explained by the charm and anticharm quark production asymmetry.

Moreover, the mechanism (1) can not account for the differences among the asymmetr

ies of D mesons with different light quark flavors.

In literature, there are some early attempts to explain the observed asymmetries.

[ Norrbin and Sjoestrand(1998); Bengtsson and Sjoestrand(1997)]

In these approaches, the asymmetry is supposed to appear due to the non-perturbat

ive hadronization effect.

In recent years, the heavy quark recombination model was proposed for explaining the measured asymmetries.

[Braaten, Jia, Mehen,(2002),(2003)] This model can give a more nature explanation

for the asymmetries in B, D mesons, and also Baryon production.

In the heavy-quark recombination mechanism, a light parton(q) that

participates in the hard-scattering process recombines with a heavy quark or an antiquark and subsequently hadronize into the final-state heavy-light meson.

According to this model, the recombination happens only when the lig

ht-quark in the final state has momentum of order in the heavy-quark, or antiquark, rest frame.

QCD

By the Recombination model, the production asymmetry in photoprodcution was al

so considered. [Braaten, Jia and Mehen (2002)]

In their consideration, the asymmetry of Ds meson comes from the process in which the

(c) and light valence-quark of nucleon recombine into a meson, while the recoiling c ( ) quark fragments to meson.

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C( )D D

C ( )S SD D

In this mechanism the asymmetry of Ds mesons has the opposite sign as that of meson and is relatively small.

( )D D

An Alternative Approach for the Asymmetry in PhotoProduction

In recent years, there are discussions about the Strange-antiStrange quark asymmetry inside the nucleon

[ Brodsky and Ma(1996);Ding, Xu, and Ma(2005); Cao and Signal(2003)]

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Considering of the possible Strang-antiStrange asymmetry inside the nucleon and the recombination model, the asymmetry in photoproduction can be readily estimated.

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The Ds photoproduction can be schematically formulated as:

(2)

In our consideration, the hard interaction kernel of the Ds meson production has two different forms:

(3)

The calculation of partonic subprocesses is straightforward:

(4)

In (4) only the leading configuration, and in color-singlet, is considered.1

0S3

1S

The total cross-section estimation needs

the Strange(antiStrange) distribution input, which we take the following two models:

Light-cone meson-baryon fluctuation model[Brodsky and Ma(1996)]

Effective Chiral Quark Model[Ding,Xu and Ma (2005)]

In the first model, the asymmetry stems from the intermediate configuration of the incident nucleon, which has the lowest off-shell light-cone energy and invariant mass.

[Brodsky and Robertson(1995)]

K

In the second model, the Strange (antiStrange) quark is determined by both the constituent quark distribution and the quark splitting function, like,

In our calculation, the two nonperturbative input parameters, and , the spin-matched and –flipped parameters, are extracted from experiments by fitting to the E687 and E691 data [E687(1996);E691(1989)]

SM

SF

and (5)

The Peterson fragmentation function of charm quark to the Ds meson are

employed:[Peterson, et al. (1983)] (6)

Here, is the fragmentation probability.[OPAL(1998)]

Taking CTEQ6L and the E687 average photon energy 200GeV, we have the predictions on the asymmetry.

In the following the asymmetry variable is defined as

(7)

The solid and dash-dotted lines for and from RM(LCMB model), the dotted line for FM

SD

SD

The notations of the line types are the

same as in Figure 1

The result of ECQM

The result of ECQM

The asymmetry versus xf. The dotted and dash-dotted lines correspond to the the LCMB fluctuation model and the FCQM, respe

ctively. The solid line is the Braaten’s result. [ ]Ds

[ ]Ds

Conclusions and Outlooks We calculate the asymmetry in phot

oproduction by a different approach from what in literature. And hence our results are different.

The nowadays experimental data on the asymmetry are still with large error, and can not distinguish the different schemes. [E687,E691]

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Future experiment measurement on this asymmetry can help us to understand the Recombination model more, and as well the Strange (antiStrange)quark distribution

inside the nucleon.

Our scheme is applicable to the asymmetry study on other Strange mesons or baryons as well.

There are large uncertainties existing in our prediction, however, the qualitatively unique nature is clear.

Thank You!