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Sigma meson cloud and Proton’s light flavor sea quarks

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### Sigma meson cloudand Proton’s light flavor seaquarks

Peking University, China (北京大学)

Feng Huang (黄 峰）

Supervisor: Bo-Qiang Ma （马伯强）

outline

- Introduction
- Meson cloud model
- Adding sigma meson in this model
- Summary

Chiral quark model

Meson cloud model

(directly including mesons)

Lattice gauge approach

Instanton induced interaction

Statistical model

Chiral quark soliton model

(not directly including mesons)

Explanations for this asymmetryMeson cloud model

- The nucleon was viewed as a bare nucleon plus a series of baryon-meson Fock states which result from the fluctuation of nucleon to baryon plus meson then the physical proton wave function is

So the quark distribution functions q(x) in the proton is

the splitting function is

We use time-ordered perturbation theory in the infinite momentum frame to calculate this function

the quark distribution functions of Pi meson

with the splitting function convolution

In a full calculation, we should include all kinds of mesons and baryons. While the probability of baryon-meson fluctuation should decrease with the invariant mass of the baryon-meson Fock state increasing, we can neglect the effects of Fock states with higherinvariant mass

Pion, the lightest meson, plays the dominant role. Difference between u_bar and d_bar in virtual pion clouds can provide the large lightflavor asymmetry in proton naturally

Problem in description for

The dominant role is played by the pion, while itprovides the ratio either increases monotonically with x or turns back towards unity too slowly.

Flavor symmetric sea contribution

- bare nucleon sea quarks

(M. Alberg, E. M. Henley, Nucl. Phys. A663 (2000) 301)

- isoscalar meson, such as omega, sigma

(M. Alberg, E. M. Henley, G. A. Miller, Phys. Lett. B471 (2000) 396)

Using different bare sea quarks

Comparison of a harder bare nucleon sea quarks (thick solid line) with a traditional bare sea quarks (dashed line) .

The thin solid line is only Pi contribution to the ratio

Adding sigma meson in this model

- Sigma meson

a) isoscalar scalar meson

b) chiral partner of Pi

c) not well established in experiment

the quark distribution functions of sigma meson

We assume the valence and sea quark distribution of mesons related here are the same.

Adding sigma effects here

Our calculations illustrate that the larger value of tends to give small values of the ratio and decreasing causes the maximum value of the ratio to be large and to appear at higher value of x

Together with omega meson

- compare pion+omega (dashed curve, Alberg-Henley’s work) to pion+omega+sigma.

Parameters related omega meson

- Lambda_omega as 1.5GeV could cover over the sigma contribution
- we present the number of the related mesons in the proton with the different cutoff values

Parameters and meson numbers in the proton

- The larger cutoff value leads to a large meson number in proton
- A reasonable picture of proton favors smaller cutoff of omega

Summary

- The inclusion of the sigmameson cloud effects has an improvement of the description for light flavor sea quarks in the proton
- We also provides a picture of a reasonable small n_omega with a smaller cutoff in the proton.
- Sigma meson may play an important role in meson cloud model

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