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8NL-62407 adF-q+@/4- -A/ IWMARY OF TXE PARALLEL SESSION ON HADRON DYNAh4ICS

M. D. Cotasran Rice U l l i V e n i t Y

DEC 2 7 %35 Brookhaven National Laboratory Upton, NY 11973

Abrtnct The hadron d y n q - a rari00, f w d on Adepeodent eflectr in b u d in-

utioar and heavy quark productioa, aa +ell u eolor t r q u e n e y &e&. Othet

5 '

INTRODU

and spin

CTION

ru.

&on d y n d c s is a broad topic, and m obviously could not coyu rlf upects of it in & coderenee. We chose to &v&p a few themes in detail, with the hope of gaining 1 overview of one or two partidu areas.

k The two areas that we covcnd in detail were -4-dependent deck and color trans- ICY. In (jl

Strghtly 1 oint with

mote b P t

than one session wbf on Probes) was dcvot

devoted edtocd

, to lot 1

,dent mcy.

and one

, i Adependent Ef€ects

'. Hard scattering

Adependent &ecti in hud p- have been known for 20 years. The &at ex- unpk ii the 'Cronin dcct' h which the QOII section for the production of single high

, f i partidm from a heavy nudmt increases faster than A. fn this d o n we discussed : Adependent dets in the production of high pt single hadrons, dihadrons, dije~, and ; direct photons. The beam partides includd protons, pions, real photons, and virtu4 photons.

1 The genedyauxpted picture of hard scattering inside a nucleus that h u emerged b u foflowr. Mer a hard process, one ot more energetic partoar propagates through the rudeus. The uncertainty priodpte leads to the cxpcctation that hadronization of the parton into a jet occurs w d l ouuide the nucleus, an expcctation that is consistent with the data. As the puton propgates through nudcu matter, it can rescatter, thereby changing its direction a d pocsibly aIso losing energy.

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O 3 Proc. 5 t h Intersections Between Particle and Nuclear Physics, 52 $ 5 E" 225 S t . Petersburg, FL, May 31-June 6 9 1994, Ed. S.J. Seestrom, --- e A I P Confl Proc. 338,-375-380 ( 1 9 9 5 p -- - . -

376 Parallel Session on Hadron Dynamics

Traditionally A-dependent effects have been- characterized by the parameter a the expression &A) = u(pp)Aa. If all of the nucleons act independently a would be 1. If the cross section f a h steeply with respect to the variable being considered (such a pt), smearing of the angle of the outgoing parton wil l lead to a > 1. A value of Q 1- than one indicates shadowing of some sort.

Another way to study nudear rescattering is through its contribution to the unb& ance in pt of the outgoing partides in dihadron or dijet final states. Define 6 as the vector unbalance in pt of the &jet or dihadron system. & has contributions from several sources, such as primordial Fermi motion, gluon radiation, and hadronization, as well as experimental effects. When the target is a heavy nucleus, nuclear rescattering can make a contribution to &. A variable in recent use to quantify nuclear rescattering effects is kt6, the 4 component of 6 in the transverse plane (see figure 1).

Figure 1. Definition of ktb.

Tom Fields of Argonne National Laboratory began the session on .4-dependent ef- fects with areview of hard-scattering data. The original Cronin data has been confirmed and extended to higher pt and CM energiea by several Fermilab experiments. Even though many experiments have been done, in general these effects are not well mapped- out as functions of pt and especially energy. Io addition to sjngle high p c hadrons, several experiments have looked at A-dependent effects in the production of high pt dihadron pairs. However, most of the dihadron experiments d e r e d from limited acceptance in azimuth angle 4 , which as described by Fields, affects the measurement of a. [l]

Two experiments looked at A-dependent eff'ts in dijet production in pA interac- tions, Fermilab E557 and E609. Both experiments saw substantial effects. E609 has published results on ktb for dijets, indicating a large smearing of the opening angle be- tween the jets (Ad ) due to the presence of the nucleus (Figure 2 [2]). From this plot it is clear why an experiment with limited acceptance in 9 would measure a different value of a than one with full acceptance.

C. C. Chang from Maryland presented results for Fermilab E683. This experiment used a real photon beam with mean energy 250 GeV to study jet production. kt. shows a definite nudear contribution, see figure 3 [3]. A similar nuclear contribution is seen for dijets produced in a pion beam. An energy dependence of the nuclear contribution to kt& was also shown.

1

c J.; I

M. D. Corcoran andk S. Carroll 377

:terizkd by the parame- -4 xt independently o wodd iable being considered (w 1 4 to > 1. A value o f a d

1 its contribution to the ad : final states. Define & a has contributions h m m,

1, and hadronization, a8 5 nuclear rescattering a a r nuclear rescatterine e f f i e - i me 1).

4 I ' Kta I I I

-- ----

2 session on A-dependent d- f mi.n data has been confirmed -:

'ermilab experiments. Even -3 effects are not well mapped- 4 ngle high pt hadrons, several lduction of high pt dihadron 3 . from limited acceptance in

1 measurement of a. [I] t production in pA interac- :)stantid effects. E609 has 2 ng of the opening angle be- i ignre 2 [2]). From this plot J

I would measure a different ,

..I

.1 B

lab E683. This experiment y jet production. kt+ shows tuclear contribution is seen of the nuclear contribution

- Y c 0 u -0

a

a !! - 0 E c 0 c

02' 7 solid-pp - dot-p Pb ,

-

0.20 - - --

0.15 - ; r - : -

0.10 -

0.05 -

0 50 100 150 0.00 ' ' '

A0 between J E t t (deqreasl

Figure 2. E609 data showing the broadening of the opening angle between the jets Ad due to the presence of a heavy nudeus.

M. Zielinski from Rochester presented results from Fermilab E706 on the A- dependence of high pt hadron and direct photon production in FA interactions. E706 results on pion production agree with the earlier data at lower energy of Frisch et al., a, = 1.08. E706 sees a similar d u e of a for single high p t 3 production. The inter- esting new result is that direct photon production is consistent with a=l. It would be expected that the outgoing photon would not undergo apy multiple scattering in the nudear medim, so this result is consistent with that expectation. E706 compared their &jet results on ku with those of E683. They are quite similar, with E706 perhaps seeing a somewhat stronger A-dependence.

0 IO0 101 102 IC

Atomic Weight

Figure 3. E683 data showing the increase of kto with increasing A.

--- GeV muon beam, Fermilab E665. In the region %ai-< .01 ,the so-called shade- region, the cross section ratios of heavy nuclei to deuterium,are less than one. E665 sees a dependence of this ratio on the fraction of the avaiIable energy observed in the detector, the first time shadowing has been seen to depend on details of the hadronic final state. E665 has studied both If1 jet events (a single hard-scattered parton p lu a target jet) and 2+1 jet events (two hard partons plus a target jet). For the 2+1 jet sample, E665 studies kt+, and they do not see a nudear enhancement. This result is in contrast to E683 results discussed above from a real photon beam and is another (new) puzzle.

Heavy Quark Production

Results were presented fiom Fermilab E772 and E789on the A-dependence of heavy quark production. The J/+ suppression is by now well-known. In nA -* J/+ 4 pp, Q

is less than one and falls with increasing Feynman-x The same effect has now been seen for +' and T(1s) production. -However, both E789. and another Fermilab experiment, E769, agree that a is equal to one within errors for open charm (D meson) production.

The difference in the A-dependence of J/$ production and Drell-Yan production has generated a great deal of theoretical work. Ideas discussed included parton energy loss coupled with a steeply falling Feynman x distribution. But it is safe to say that, on the theoretical side, there was no consensus.

Theoretical Contributions

Boris Kopeliovich from JINR, Dubna presented a calculation which linked kt+ and 01 in pA interactions. The model is in qualitative but not quantitative agreement with E609 data. '

John Ralston of Kansas gave the theoretical review A-dependence effects. At this time no theoretical model can successfully describe all of the existing experimental results, or even a substantial fraction of them. However, recent theoretical interest in this area gives one reason to believe progress will be made in the near future.

Color Transparency

If a hadron is to pass through a nucleus and stay intact, it must be a "small" hadron. A small color dipole should have small interactions and therefore a good chance of escaping a nucleus intact. The probability that a hadron passes through a nucleus intact is the transparency. A small size implies a large momentum transfer, so one expects the transparency to increases as Q2 increases. Color transparency is closely related to the QED effect called the Landau-Pomeranchuk-Mgdal effect, in which the

M. D. Comran and A. S. C m U 379

on from an energetic electron is suppressed if the formation time for the radiated is longer than the mean free path for another scatter.

b m LOS Alamos showed data from LAMPF experiments E1136 and E1174, with

epend on details of t jingle hard-scattered

toton beam and is anot - 5

up saw an interesting rise in transparency with incident momentum, followed by a p as the momentum continued to increase. The rise is consistent with a a simple

mplitudes other than the perturbative amplitudes are involved. This group is taking data with a new detector, EVA, during the current run at Bm, so new results are

n on the A-dependence o f h

le Same effect has now b

In and Drell-Yan pro

i

ition which linked kt+ and a in d a t i v e agreement with E609

A-dependence effects. At this 1 of the existing experimental ., recent theoretical interest in de in the near future.

it must be a "small" hadron. d therefore a good chance of ron passes through a nudeus ! momentum transfer, so one Color transparency is closely tk-Migdal effect, in which the 3

1

0.95

0.9

0.85

0.8

tS 0.75

0.7

0.65

0.6

0.55

0.5

Figure 4. (t vs Q2 for exclusive p production in EGG. The prcdictiorl of color transparency is that Q should approach 1 as Q' increases.

380 Parallel Session on Hadron Dynamics 4 Mark Strikman from Perm State gave a theoretical summary, insisting that t b theoretical underpinnings of the color transparency idea ate well established, even i f t h experimental situation is not so dearcnt.

!

Other topics

Spin Effects

Tom Roser from BNL summarized the plans for polarized beams in RHIC. 80% polar- ization should be achievable. A f k t round of tests in the AGS looks very promising. At the high interaction energies possible in this collider, polarization effects should be relatively easy to predict in perturbative QCD. The necessary snake magnets are now part of the construction plans, so that this capability should be present at turn-on.

David Prout of Ohio State gave results for spin observables measured in an 800 MeV proton beam at Los Alamos in '*C(p,n) reactions in the A resonance region. These involved both a polarized proton beam and the capability to measure the momentum and poIarization of the neutron in the final state. Measurements of this type can be compared to (e,e'p) reactions, and elucidate the nudear response functions for both transversely and longitudinally polarized protons.

Exclusive Processes

Robin Appel from Yale presented results for BNL E838. This experiment studied a wide range of exclusive processes in order to isolate the dominant quark and gluon diagrams. One conclusion is that gluon exchange does not dominate in exclusive processes, but rather quark exchange seems to be the most important mechanism.

Rex Tayloe from IUinois presented preliminary results from LEAR experiment PSI85 on pp --c M, C c and AC. Theoretical interpretation of this data was presented by J. Speth from Institut fur Kernphysik, Julich, based on a coupled-channel meson exchange approach, which seems capable of describing the data..

REFERENCES

1. T. H. Fields and M. D. Corcoran, Phys. Rev. Lett., 70, 143 (1993).

2. M. D. Corcoran et al., Phys. Lett., B259,209 (1991)

3. D. Naples et al., Phys. Rev. Lett., 72, 2341 (1994).

4. G. Y. Fang, FERMILAB-CONF-93-305.