189 DIQUARK EFFECTS IN PROTON FRAGMENTATION David Hanna, Harvard University, Cambridge, Mass., USA (Aachen-CERN-Harvard-Munich-Northwestern-Riverside Collaboration) ABSTRACT Some results from an ISR experiment looking at pp collisions with a high-pT pion at (y = are presented. Forward particle production in the same hemisphere30° as the 1.tr3)igger indicates that when a valence quark is removed from the proton, the remaining valence quarks behave as a bound state rather than fragmenting individually. Preliminary evidence for charmed baryon production is also presented. RESUME Nous presentons des resultats d'une experience aux ISR. Nous y avons etudie des collisions pp dans lesquelles il y avait un pion energetique (y La production de particules petits angles dans le meme hemispherea 30indique° = que1,3), . lorsqu'un quark de valence sorta du proton, ceux qui restent produisent des parti­ cules d'une maniere coherente. Des preuves preliminaires pour la production de baryons charmes sont presentees . 190 The fragmentation of quarks has been extensively studied in e+e- annihila�ion as we ll as in hadron collisions and deep inelastic leptoproduction experiments1l, but data on the "diquark" system which remains when a valence quark is ejected from a nucleon is considerably more limited2). In order to understand more about the diquark fragmentation3l mechanism, we have undertaken a study of small-angle hadron production in pp collisions triggered by a high-momentum pion at 30° in the same hemisphere. In the naive quark-parton model, the trigger pion is thought to be the decay product of a quark e ected in a hard scattering process, as shown in Fig. "or j 1. Y1 TRIGGER p = -Js12 Fig. 2.5 a pion of sufficiently high pT GeV/c) , the parent is likely to be a valence quark (x 0.2), implying the (>product ion of a forward diquark. Moreover, the bj charge of the> trigger pion should reflect the flavour of the quark (u for n+ ; d for TI-) and thereby give a clue to the flavour content of the diquark (ud or uu) . A morE' realistic calculation using the QCD-inspired mc,del of Feynman, Field and Fox4) indicates that hard gluons may be the origin of many of our triggers . Table shows the fractional composition of a pT 3 GeV/c trigger as given by this calcu­ latior5 ) . = Table 1 h h- r ° 63Origi GeVn, of ig3 GePV/c,trigger 8 = 30particle' 0.26 Pr = Xbj Fraction of the trigger Parent parton n+ - 1T u quark 0.57 ± 0.03 0.17 ± 0.03 d quark 0.07 0.03 0.37 0.03 ± ± gluon 0.32 0.03 0.40 ± 0.03 ± anti-quark 0.04 ± 0.03 0.05 0.03 ± 191 Most of the apparatus has been described previously6) . Briefly, it consisted of two coaxial spectrometers surrounding Beam just downstream of intersection region I6 (Fig. 2) . The outer spectrometer was used to define and detect the trigger particle, whilst the inner one measured forward fragments from 1° to 6° with respect to the beam axis. R-606 Generacut throughl layout (Arm (vertical beam ax1 s} B , Magnet mwp.c U 1 ) IT'Wpc Bock 16 - ,/ �I _ :::-:i ; : } ���9n9:!�s \ I \H EMt 01 81 Im TF; chambers EM 1 = Spectrometer magnet B z Dr1It 3 = Lampshade magnet 8, EM 8; c, U1Dt F Sc intillation counters C2 =Cerenkov counters =Shower counters l ��,C, ru 18, S, 018, F ig. 2 The trigger spectrometer was based on an air-core toroid known as the Lamp­ shade Magnet (LSl!) , which had a l/r field dependence with a maximum value of 3 kG and an average field integral of 1.5 kG•m. Twelve coils divided the azimuth into 30° sectors, 10 of which were instrumented with a Cerenkov (C) and shower (S) coun­ ter combination as well as trigger scintillators (TF, TB) . The Cerenkov counters contained C02 at atmospheric pressure, which has a pion threshold of 5 GeV/c. Large drift chambers7) , before and after the magnet, measured the particle trajectories. The six sectors used in the trigger covered the range 45° 135° and 235° 315°, and were equipped with a paddle scintillator (B) bei< ng� < the shower counter< � , < <which was used to restrict the polar angle (8) of the trigger particle to 30° 1°. ± The forward spectrometer made use of two identical septum magnets (jB d£ 12.9 kG•m) which sandwiched the beam pipe. Particle tracks were recorded by 2 MWPCs, and hodoscopic Cerenkov counters built into the front half of each magnetnnn provided particle identification . Each counter had four cells and was filled with freon-114 at atmospheric pressure. 192 The trigger was designed to select events in which a fast TI± was emitted at 30° with respect to Beam 1, so a pulse from an LSl1 Cerenkov counter was required. To reduce background from pairs or delta-rays firing the Cerenkov, we demanded single-particle ionization in TB and the shower counter . This trigger rejected , on line, 99% of hadrons below pion threshold, and a later software cut eliminated all but a small sample of pT GeV/c triggers which were saved for comparison purposes. At of 1 GeV/c the< 1 trigger particle may only be identified as an Pr "inclusive hadron'', but above Cerenkov threshold (pT GeV/c) the fraction of pions approaches 1. = 2.5 Th·e data presented here were taken during 100 hour.> of running (at of 63 GeV) , corresponding to an integrated luminosity of � 1.4 pb-1• Track/S recons­ truction and fiducial cuts in the LSM left a total of 1·40,000 positive and 107,000 negative trigger tracks with pT 1 GeV/c. > About half of the selected events were found to have one or more charged par­ ticles in the forward spectrometer, and these particles were identified using in­ formation from the Cerenkov counter. The K, and p thresholds in freon-114 are TI, 2.6, 9.3, and 17.7 GeV/c, respectively, which makes TI/K separation very difficult over much of the momentum range of interest here. On the other hand, it is easy to distinguish between mesons and baryons since the former are almost non-existent above proton threshold. For our purposes, the structure, qq or qqq, is more funda­ mental than the flavour, so we label forward hadrons as either mesons or protons, using the notation to denote mesons since pious are the dominant component . TI The data were first examined for evidence of the ¢ correlation between the trigger and the forward particles. This correlation, which has been seen in pre­ ) vious ISR studies8•9 of high-pT phenomena, is related to the intrinsic transverse momentum of the partons . The trigger preferentially selects partons with some motion in the trigger direction. Figure 3 shows the difference in azimuth between the trigger and the forward hadron, for protons6¢ , of x > 0.4 for three ranges Tr igger < 0.5GeV/c Trigger GeV/c c) Trigger GeV/c a) P,- bl P,- >I P,- >2 2 ltt+f Fig. 3 193 of trigger P · The data have been subjected to fiducial cuts and have been r weighted by the inverse of the geometrical acceptance, Acc(p1 ,pT ,¢) . The fitted curve is a simp le cosine, and the growth and saturation of its amplitude with trigger P as well as the peaking at 6¢ 180° , are consistent with the notion of r • a recoiling system of partons balancing the= Fermi motion of the trigger parton. Using a simple model, we infer from the amplitude a value of 300 MeV/c for the intrinsic P of the constituents • r . we proceed in analogy with quark fragmentation studies, defining the invariant quantity zdnh f(z) Ndz = where nh is the number of hadrons of type h observed with fraction z of the di­ quark' s momentum, and N is the number of triggers taken. If D� (z) is the fragmen- 1 tation function for a diquark of type i into hadrons of type h and all forward par- ticles came from diquark fragmentation, then f(z) = zEE.D� (z) , is the fractional population of type1 1 i diquarks . As mentioned earlier, where Ei we expect this picture to be only approximately true, owing to the presence of gluon triggers . Since the diquark is defined to be the proton minus a valence quark, its momentum is given by p - xbj)/S/2 where xbj is the fraction of the incident = (1 proton's momentum carried by the trigger quark. To calculate xbj ' we assume a sub-process like that shown in Fig. 1 and obtain, from simple kinematics, 2 xbj P (eY1 e Y )//S = r + In this experiment y 1 1.3 and y2 is not measured. Since previous measurements8) = indicate that (y2) 0.05 for a similar kinematical configuration, we use y2 0 in our calculation with= the result = x 0.075 bj Pr , where P refers to the trigger parton. Assuming that we trigger on hadrons taking r 80% .to 90% of the jet momentum, we use 0.08 xbj P , = r where P is now the transverse momentum of the triggering hadron. r Figure 4 shows f(z) versus (1 - z) for p, and TI- for positive and negative n+ , triggers in trigger P ranges 1.0-1.5 GeV/c and 2.5-4.0 GeV/c. Diquark effects are r expected to be more pronounced in the higher Pr plot. Arguments based on phase­ space considerations1 0 ) suggest that f(z) should behave as a power of (1 - z) , so we use a log-log scale where such behaviour appears as a straight line.