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Volume 244, number 2

PHYSICS LETTERS B

19 July 1990

Longitudinal energy flow in hard proton-nucleus collisions at 400 GeV/c E 609 Collaboration R.C. Moore, R.K. Clark, M. Corcoran, K. Johns, M. Marcin, M.E. Miettinen, C. Naudet, J.B. Roberts T. W.. Bonner Nuclear Laboratories, Physics Department. Rice University, Houston, TX 77251, USA

M.W. Arenton, W.R. Ditzler, T.H. Fields, G. Thomas Argonne National Laboratory, Argonne, IL 60439, USA

M. Harrison Fermilab, Batavia, IL 60510. USA

A. Kanofsky LeHigh University, Bethlehem, PA 18015, USA

R. Gustafson University of Michigan, Ann Arbor, M148109, USA

L. Cormell, M. Dris, J. Fleischman, W. Kononenko, B. Robinson, G. Theodosiou, W. Selove, B. Yost University of Pennsylvania, Philadelphia, PA 19104, USA

H.F. Chen, A.R. Erwin, M.A. Hasan, C.E. Kuehn, K.S. Nelson and M.A. Thompson University of Wisconsin, Madison, W153706, USA Received 27 March 1990

Using the E609 calorimeter system, we have measured longitudinal energy flow in hard pA collisions which yield dijet final states. The mean laboratory energy flow into the nuclear target (backward) region, nlab<2.83, minus the value for pp collisions is found to vary as A °'33-+°-12,and reaches a value of 50_+8 GeV for Pb. The observed probability distribution of energy flow indicates that the energy transfer to a heavy nucleus seldom fluctuates to small values. We also find that the A dependence of the dijet production cross section depends sensitively upon the energy flow to the target region.

Collisions o f high energy hadrons with nuclei provide an o p p o r t u n i t y to study experimentally the propagation o f high energy partons within nuclear matter. The final state o f the incident partons after they traverse nuclear m a t t e r can be inferred by making measurements upon particular types o f final states which are reached either via hard scattering pro-

cesses such as D r e l l - Y a n or j et production, or via soft processes such as the inclusive p r o d u c t i o n o f fast forward secondary hadrons at small values o f Pt. F o r the D r e l l - Y a n process, the p r o d u c t i o n cross section per nucleus has been measured to be nearly directly proportional to A, and only small nuclear effects u p o n the x and Pt distributions o f the produced

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dilepton system have been observed ~1. Thus the Drell-Yan experimental results indicate that the active incident parton seems to be little affected by traversing nuclear matter. For inclusive production of fast forward hadrons, the cross section per inelastic collision depends only slowly upon A [ 2 ]. This property of hadron production has been interpreted using various parton-like models in which a high energy parton first traverses nuclear matter and then fragments after emerging practically undeflected by its passage through the nucleus. By contrast, larger cross section enhancements which are presumably due to nuclear rescattering effects have been observed in two different kinematical regions of inclusive single hadron production. These two cases are, first, particles which have low energies in the nuclear target rest system [2], and second, particles with large transverse m o m e n t u m Pt which are presumed to originate from hard scattering [2 ]. These two kinds of nuclear enhancements are well measured and established, and are are believed to be caused by rescattering processes inside the nucleus, but they are not understood quantitatively. In this paper, we study the effects of nuclear matter upon high m o m e n t u m partons by measuring energy flow in hard scattering events ~2. For this purpose, we nl See re£ [ 1 ] for a r e v i e w of D r e l l - Y a n production. For a review o f e x p e r i m e n t a l results o n high energy collisions w i t h nuclei, see ref. [2]. ~2 See ref. [ 3 ] for a r e v i e w o f concepts a n d d a t a on energy flow in pA collisions.

19 July 1990

used the E609 calorimeter system at Fermilab to obtain data on hard pA collisions at 400 GeV/c. Throughout this paper, we define a hard scattering event as one which has two jets o f P t > 4 GeV/c in the CM angular range of 60 °-110 o, selected using methods to be described below. These dijet events are a small subclass of the highly inelastic events with large total transverse energy which have been studied previously [4 ]. To a first approximation, the dijet events which we are studying may be expected to be similar to Drell-Yan events with the outgoing leptons replaced by outgoing quarks or gluons. We have previously reported [ 5 ] that the forward "beam jet" in these hard collisions has a width which varies little with A. This would be expected for a beam parton which fragmented only after emerging from the nucleus. Here we report the results of a detailed analysis ~3 of the A dependence of the longitudinal energy flow, emphasizing the backward (target) angular region. We present new results on the backward energy flow into the target region - both its mean value and its probability distribution. We also present data on the relationship between the magnitude of the backward energy flow and the A dependence of dijet production. A schematic drawing of the E609 calorimeter layout is shown in fig. 1. The main calorimeter, consisting of 132 individual towers, covered the full azimuth over a CM polar angular range of 2 5 ° < 0* < 120 °. The corresponding range of laboratory po-

n3 Further details are given in ref. [ 6 ]. MAIN CALORIMETER 152 TOWERS

400 GeV

PROTON BEA~

>

| NUCLEAR TARGET

BEAM HOLE CALORIMETER

• METERI

Fig. 1. E609 calorimeter system (plan view).

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lar angle is 15-118 mrad (4.9>~hab>2.8). (All equivalent CM angles in this paper are given in the pp CM system, for particles with Pt >> m. ) Energy flow into the forward region, 0*<25 ° or ~71,b>4.9, was measured by the beam calorimeter shown in fig. 1. Energy flow into CM polar angles greater than 120 °, which we shall call the target region, was not measured directly but was calculated for each event by subtracting the energy observed in the main and beam calorimeters from the incident beam energy of 400 GeV. Targets with A > 4 were mounted on a wheel which automatically changed targets after every beam spill in order to minimize A-dependent systematic errors due to electronics drifts or other changing conditions. Most target thicknesses were 2% of an interaction length. Additional tin and Pb targets of 1% interaction length were also used; which allowed us to show that our results are not affected by secondary interactions (with other nuclei) in the target. The hydrogen target was located 100 cm farther downstream than the other targets; small corrections were applied to the hydrogen data to correct for this. All events were tracked using a drift chamber chamber system not shown in fig. 1, allowing background events occuring outside the target to be removed from the data set. A special trigger called the "two-high" trigger was used to obtain the data presented here. This trigger required that the transverse energy detected in each of any two towers of the main calorimeter exceeded a threshold of ~ 1.5 GeV. As shown in extensive studies which we have described elsewhere ~, this trigger is expected to have reasonable efficiency for dijets with P~ (jet)>~4 GeV/c. At the same time, it will have low efficiency for non-jet events which have a broad angular distribution of transverse energy. As stated above, the dijet events used in all of the results described in this paper were further required to contain at least two jets of mean P t > 4 GeV/c, 6 0 ° < 0j*et< 110 °, as determined offiine using two alternate j etfinder algorithms. One of these software jetfinders searched for P~ concentrations within a cone of 45 ° CM half-angle. The other used an E T a gaussian weighting search procedure. Both algorithms yielded similar results. As described elsewhere [ 7,8 ], we have See re£ [7] for pp events and ref. [8 ] for pA events.

19 July 1990

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fit). found that these jetfinding procedures yield event samples which are quite free of non-jet background events, particularly for P,> 5 GeV/c. Extending our earlier analysis [ 5 ] of nuclear target data, we have now studied the dependence of EB, the (laboratory) energy flow into the backward angular region (0"> 120 ° ) upon A. We have computed EB for each event using EB = 400 G e V - E C A L - BCAL,

(1)

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(2)

as a function of A. The brackets refer to a mean over all of the (dijet) events from a given target. Fig. 2 also shows the dependence of < ECAL> and < BCAL> upon A. is just the (laboratory) energy of the (triggering) dijet system.
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imate measure of the forward beam jet energy, and decreases dramatically as A increases. The dashed line shown in fig. 2 is a fit of ( E N > to the form b A '~ for targets with A>~4. This fit yields a = 0.33 + 0.12, indicating that the nuclear target energy flow is consistent with being proportional to A ~/3 and hence to the mean nuclear path length traversed by the partons. The value of the fitted mean nuclear target energy flow is 50 + 8 GeV for Pb. This transfer o f beam energy to the nuclear target rapidity region seems similar to what was observed for minim u m bias events by the NA5 collaboration [ 9 ]. Using a detailed theoretical model of nuclear stopping power at high energies, Datr, Gyulassy and Sumiyoshi [ I 0] have predicted a m a x i m u m value of 45 GeV for the energy loss of high energy protons traversing a Pb nucleus at zero impact parameter. However, it should be noted that our experiment measures the backward energy flow into the fixed angular region r/lab< 2.83. This energy flow is closely related to the "nuclear stopping power" but the quantitative relationship is model-dependent [ 3] so that we are unable to make a direct comparison with theory. Another theoretical discussion of parton energy losses arising from a color charge traversing hadronic matter has been given by Sivers [-11 ]. A value o f 45 GeV for the energy loss to the Pb nucleus (radius ~ 7 fin) seems much larger than might be expected from the simple Q C D string tension effects ( ~ 1 G e V / f m ) which were considered by Sivers. Recent theoretical papers by Bodwin, Br0dsky, and Lepage [ 12 ] and by Brodsky and Hoyer [ 13 ] also use Q C D ideas to estimate the energy loss experienced by a fast hadronic system traversing a nucleus, but do not give specific predictions for the process which we are studying. We turn next to the event-by-event distribution of EB, the energy flow into the target region. This is shown in fig. 3 for H and Pb targets. An interesting feature of the distribution for Pb is that there are few events which have a target region energy flow which is H-like, e.g., which are in the left hand part of the H distribution in fig. 3. Such H-like events might be expected for peripheral collisions with a quasi-free proton at the edge o f a Pb nucleus. It is difficult to obtain more detailed information on the energy flow distribution probability from fig. 3, since the calorimeter energy resolution ( ~ 7%) is making a d o m i n a n t con350

19 July 1990

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tribution to the widths of the observed distributions o f EB. Fig. 4 shows the measured dijet event cross section par ametrized in terms of A", as a function of EB, the backward energy flow into the target region on an event-by-event basis. All eight targets from H to Pb were used in these fits. Fig. 4 indicates that for these (rare) events with little energy transfer to the target nucleus, the jet production cross section is roughly proportional to A (i.e., each target nucleon scatters

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independently), By contrast, much faster A dependence is observed for dijet events with substantial energy transfer to the target. This might be expected in a multiple scattering model o f the parton scattering processes within the nucleus, and is o f course closely related to the different energy flow (loss) distributions for light nuclei and heavy nuclei as shown in fig. 3. We have carried out several studies to search for possible sources o f systematic error in the above results on energy flow. A first issue concerns the extent to which these results might be influenced by our particular selection criteria for "dijet" events. This is a potential concern, since, not only are hadron jets difficult to define precisely in any experiment, but at our energy o f x / ~ = 2 7 GeV. We have shown [7,8] that selecting a clean sample o f dijet pp events requires careful analysis and Monte Carlo modelling. In selecting jet events from heavy nuclei, the potential backgrounds are greater. However, as we have described previously [ 5,6,14 ], the pattern o f longitudinal energy flow (e.g. figs. 2 and 3) appears to depend very little upon the details o f the hard scattering trigger or jetfinder requirements. For example, results similar to those of fig. 2 are obtained using raw "two-high" triggered events with no use o f a jetfinder, so that the energy flow seems to be reflecting some general features of high energy p-nucleus collisions. For the dijet cross section A-dependence shown in fig. 4, there may well be some residual dependence upon details o f the jet definition, so that the systematic error in ot is perhaps 0.1-0.2. We have also investigated effects of non-linearities in the energy response of our calorimeters. A particular concern is that such non-linearities could lead to an underestimate o f the forward energy flow from heavy nuclei shown in fig. 2, because of the somewhat increased n u m b e r o f soft particles emerging from interactions with heavy nuclei. We based our analysis o f this effect primarily on our own data concerning particle energy spectra from various nuclei [ 6 ] and our energy calibration data for calorimeter towers using pions and electrons o f energy from 10 GeV to 50 GeV. Our conclusion was that such nonlinearities result in small systematic corrections, probably less than 5 GeV of the 50 GeV value which we measured for Pb. Finally, we have studied the accuracy o f the energy scales of our calorimeters. We have previously esti-

19 July 1990

mated the overall energy scale o f the ECAL calorimeter towers to be uncertain by about 7%. [ 7,8 ]. This uncertainty does not seem likely to seriously affect our results, since our results depend on a comparison between nuclear target data and H target data. We have made a correction o f - 5% to both ECAL and BCAL energy scales in order that fig. 4 be consistent with energy conservation within the resolutions of the calorimeters: The corresponding systematic uncert a i n t y in ( E N ) is estimated to be less than 10 GeV for Pb. In summary, we have presented three new experimental measurements concerning energy transfer to the "target region" in hard collisions which yield dijet events in pA collisions at 400 GeV/c. T h e s e results show that large changes in longitudinal energy flow occur when a hard scattering takes place within a nucleus, and should be useful for testing theoretical models o f parton propagation and energy loss in nuclear matter. We thank the Fermilab staff for their dedicated efforts in Supplying the beam to E609, and D. Sivers, G. Bodwin, J. Collins, and M. Gyulassy for valuable discussions. This work was supported in part by the US DOE.

References [ 1] C. Grosso-Pilcher and M. Shochet, Ann. Rev. Nucl. Part. Sci. 36 (1986) 1. [2] S. Fredriksson, G. Eilam, G. Berlad and L. Bergstrom, Phys. Rep. 144 (1987) 188. [3] W. Busza and R. Ledoux, Ann. Rev. Nucl. Sei, 38 (1988) 119. [4] R. Gomez et al., Phys. Rev. D 35 (1987) 2736; A. Samburtini et al., Phys. Rev. D 41 (1990) 1371, and references therein. [ 5 ] H.E. Miettinen et al., Phys. Lett. B 207 ( 1988) 222. [6JR. Moore, Ph.D. thesis, Rice University (1989), unpublished. [ 7 ] M. Arenton et al., Phys. Rev. D 31 (1985) 984. [ 8 ] M. Corcoran et al., to be published. [9] C. DeMarzo et al., Phys. Rev. D 26 (1982) 1019, [ 10] S. Date', M. Gyulassy and H. Sumiyoshi, Phys. Rev. D 32 (1985) 619. [11 ] D. Sivers, Ann. PhYs. 182 (1988) 157. [12] G. Bodwin, S. Brodsky and G. Lepage, Phys. Rev. D39 (1989) 3287. [ 13] S. Brodsky and P. Hoyer, Phys. Rev. Lett. 63 ( 1989) 1566. [ 14] J. Rice, Ph.D. thesis, Rice University (1983), unpublished. 351