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Probing iT/z Asymmetry in the Proton via Quarkonium and W/Z Production

J. C. Peng, D. M. Jansen, and Y. C. Chen

Baryons '95, 7th Striicture of Baryons Santa Fe, New Mexico

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PROBING iT/z ASYMMETRY IN THE PROTON VIA QUARKONIUM AND W/Z PRODUCTION

J. C. PENG and D. M. JANSEN Los Alrmos Ndonn l Laboratory, bs Alums, NM 87545, USA

Y. C . CHEN Institute qf Physics, Acuclemiu Sinica, Tuipei, Tuiwun

The experimental and theoretical status of the flavor dependence of the sea quark distributions in the proton is summarized. Sensitivities of .I/I,Y, Y , and W/Z production to the E/a asymmetry are then discussed.

The subject of parton distribution in the nucleons and nuclei has been actively studied for the past two decades. Despite many advances, the sea quark contents in the nucleon are still rather poorly known. In particular, the tlavor dependence of the sea quarks in the proton are not well deter- mined. It was assumed by many authors that the ii and d sea quark distributions are identical in the proton. It came as a surprise when the NMC collaboration reported' a measurement of the Gottfried Sum, showing evidence that ii f in the proton. In this note, we first summarize the experimental and theoretical status on this subject, and we then discuss how several ongoing and possible future experiments can help to determine the ii/z ratios in the proton accurately.

The Gottfried Sum is defined as IG(xl + x2) = J:12 [F { (x ) - F$(x)J /xdx . Assuming isospin symmetry in the nucleons and E = in the proton, it can be shown straightforwardly that IG(O + 1) = 1/3, called the Gottfried Sum Rule (GSR).2 Early SLAC data3 gave zG(0.02 + 0.8) = 0.20 f 0.04, indicating that GSR could be violated. Indeed, these data had prompted Field and Feynman4 to suggest that Pauli-blocking effect causes suppression of the gluon + uii process relative to the gluon + d z process, hence 2 > ii in the proton. The NMC collaboration' extended the measurement of I, to the small x region. By extrapolating the experimental result IG(0.004 + 0.8) = 0.221 k 0.008 f 0.019 to the

= 0.235 k 0.026, significantly lower than the va

that recent E665

SR. However, an 8% CSB effect, integrated over the inconsistent with the smaller CSB effe

~ ~~

I

1

Many authors consider the NMC result as evidence for an asymmetric ii, distributions in the proton. Using the NMC result for ZG and assuming no CSB effect, one obtains /$z(x)- i7(x)]dx = 0.14 k 0.02, showing an excess of ;t over ii in the proton. An interesting interpre- tation of this result is given by Levelt et aZ.,10 who define T = ~ o [ u ( x ) - d ( x ) ] d x and T = j i [ Z ( x ) - F(x)]crX. One can regard T/2 and T/2 as the proton isospin carried by the quarks and antiquarks, respectively. The NMC result implies 2" = 0.86 & 0.02 and T = 0.14 k 0.02; hence suggesting that - 14% of the proton isospin is carried by antiquarks, a situation reminiscent of the proton spin crisis.

What are the mechanisms that would lead to 2 > Z in the proton? As mentioned earlier, the Pauli-blocking effect favors 2 over E . Unfortunately, it is difficult to make quantitative calculations. Another mechanism, advocated by many authors," invokes the pion cloud in the proton. In this so-called 'Sullivan process,' the z' in p + n + IC+ would contribute to an excess of 2. Detailed calculations taking into account both the p + n + IG+ and the p + A" + n- processes show that - 50% of the observed GSR violation is accounted f0r.l' Using a Generalized Sullivan process, which also contains the N + K + Y processes, Hwang and Speth" were able to explain the NMC data well.

the possible E / z asymmetry in the proton.12 In fact, the E772 Drell-Yan data obtained with tungsten and isoscalar targets have been compared" with predictions from various models. More recently, the NA51 experiment reported14 2oDY(p+p) /oDY(p+d) = 0.91+ 0.02 & 0 measured at 450 GeV near xF = 0 and x = 0.18, showing a large asymmetry of ii/z = 0.5 0.04 k 0.05 in the proton. The E866 experiment at Fermilab is designed to measured the r 2oDy(p+p)/crDy(p+d) over a wide x range (0.05 < x < 0.3) at 800 GeV and it should provide a definitive test for the various models.

In addition to the DIS and the Drell-Yan processes, J /y and Y production could also be sensitive to the sea-quark distributions in the nucleon. Using the semi-local duality model and the lowest-order QCD cross sections for the qq annihilation and the gg fusion processes, the sensitivit of the proton-induced J / y and T production to the possible ii/z asymmetry has been studied.'- The ratio R ( x F ) , defined as

1

It has been proposed that the Drell-Yan process provides an independent and sens

Y

would be equal to 1 for all models

beam. Five different structure function sets have been used in the calculations. The solid curves correspond to the Z / d symmetric DOL1 structure functions, and R ( x F ) is identically one in this case. Figure 1 shows that the measurement of R ( x F ) for I' production at 800 GeV provides a sensitive test of models that predict different iT/z symmetry in the proton. In comparison, J/tl/ production is less sensitive to the sea quark distributions due to the dominance of gg fusion at this energy. However, at lower beam energies, J/v production is more s distributions as the qZj annihilation stam to be important.

p and p + A collisions at the future heavy i

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3. S . Stein e t al., Phys. Rev. D 12, 1884 (1975).

6. M. R. Adams et al., Phys. Rev. Lett 7 5 , 1466 (1995).

9. E. N. Rodionov, A.

14. A. Baldit et al., Phys. Lett. 332B, 244 (1994).