Search for direct photons in heavy-ion collisions

305c

Nuclear Physics A.525 (1991) 305c-310~ North-Holland, Amsterdam

Search for Direct

Photons

in Heavy-Ion

Collisions

WA80 Collaboration L. Dragon4, R. Albrecht’, T. C. Awes’, P. Beckmann4+, F. Berger4, D. Bock4, R. Bock’, G. Claesson3, G. Clewing4, A. Eklund3, R. L. Ferguson5, A. Franz*, S. Garpman3, R. Glasow4, H. A. Gustafsson3, H. H. Gutbrod’, G. Hiilker4, J. Idh3, P. Jacobs’, K. H. Kampert4, B. W. Kolb’, H. Lijhner 4+‘, I. Lund’, F. E. 0benshain5,*, A. 0skarsson3, I. 0tterlund3, T. Peitzmann’, F. Plasi15, A. M. Poskanzer2, M. Purschke4, H. G. Ritter’, S. &in?, R. Santo4, H. R. Schmidt’, S. P. Sanense&*, K. Steffens 4, E. Stenlund3, D. Stiiken4, M. L. Tincknel15, and G. R. Young5 i Gesellschaft fiir Schwen’onenforschung, D-6100 Darmstadt, Fed. Rep. of Germany 2Laulrence Berkeley Laboratory, Berkeley, CA 94720, USA 3 University of Lund, S-22362 Lund, Sweden 4 University of Munster, D-4400 Miinster, Fed. Rep. of Germany 50ak Ridge National Laboratory, Oak Ridge, TN 37831, USA Onow at: KVI Groningen, Groningen, Netherlands *University of Tennessee, Knoxville, TN 37996, USA Abstract. A search for direct photon production has been performed using p and 160 projectiles at 60 and 200 A.GeV interacting with a C and a Au target. Photon and m” spectra have been measured in the pseudorapidity range 1.5 5 7) 5 2.1 for the transverse momentum region 0.4 GeV/c 5 M 5 3 GeV/c with the lead-glass spectrometer SAPHIR. An upper limit of 15 % at the 90 % CL for the direct photon signal relative to the neutral pion production is obtained from the measured photon spectra and the Monte Carlo simulations of the hadronic background based on reconstructed a0 and n spectra. Consequences for a possible phase transition to a quark-gluon plasma (QGP) are discussed.

1

Introduction

Direct photons, i. e. photons not produced by resonance decays but on the quark and gluon level, are proposed to be an excellent means to study the QGP since direct photons are essentially uninfluenced by the hadronization process because of their electroweak interaction [l]. Therefore, their interpretation is by far less ambiguous than other possible signals of the QGP. Direct photons from a QGP are expected to be emitted in the transverse momentum region 1 GeV/c 5 pr 5 3 GeV/ c and will be called thermal in contrast to hard direct photons ( pi 1 3 GeV/c ) which are well established experimentally [2] and understood theoretically [3] for p + p and p + light nuclei reactions.

2

Experiment

and data analysis

The complete WA80 experimental setup is described in Ref. (4,5]. For the measurement of photons the lead-glass detector SAPHIR has been used [5,6], which is located at a distance of 342 cm from the target and measures photons from 0.2 to 20 GeV in about 1/6th of the azimuth ‘p in the pseudorapidity range 1.5 5 n < 2.1. Charged particle rejection is achieved by two layers of streamer tube arrays with an overall efficiency of 98 %. For the data analysis only photons with an energy above 500 MeV are used. Neutral pions are identified by their decay photons (no -+ 7~). The method to calculate a 7r”pr distribution 0375-9474/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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L. Dragon et al. / Direct photons in heavy-ion collisions

is described in Ref. [7]. An effective energy resolution of UE/E z 8.6%/dand position resolution of 5.9 mm at 10 GeV is obtained by a fit to the observed no mass resolution during the operation of the calorimeter at the CERN SPS. By investigating the variation of the ~0 mass peak position as a function of one photon energy a maximum nonlinearity in &Y/E of 3 % can be derived from the data. A minimum ionizing peak has been found at 540 MeV photon equivalent energy for vetoed hits, i. e. charged particles.

3

Efficiencies

The e5ciency to identify a photon a~ a neutral particle has been determined from a ?y” analysis and from the method of superimposing events. The combination of both results leads to the total photon reconstruction efficiency which has been found to be dependent on the mean hit multiplicity poll. The probability L, to identify a photon as a neutral hit has been determined by meiLns of 7r” invariant mass calculations. This method has already been described in detail in Ref. [5,7,8] and leads to a total photon conversion probability in the vacuum chamber, streamer tubes, and the target of about 8 %, which turns out to be in qualitative agreement with calculations of the conversion probability. The overlapping probability has been determined by the technique of superimposing raw data events with low particle multiplicity and by comparing the particle identification from superimposed events with the identification of the low particle multiplicity events. From this analysis two different overlap probabilities are deduced, namely the probability ICCthat a neutral hit overlaps with a vetoed one and the probability KM that two or more neutral hits merge to a cluster which cannot be resolved. While the probability KC denotes the decrease of neutral hits, the probability IEM introduces different efficiencies for the photon and 7r” spectra. Due to the overlap of neutral particles the total number of neutral hits will be reduced and at the same time neutral hits with an increased energy will be created. The reconstruction e5ciency for inclusive photon spectra drops from 89 % at ~~~11 = 2 to 66 % at P,,,~ = 20 and the reconstruction efficiency for photons contributing to the ?y” mass peak from 88 % at P,,J = 2 to 52 % at ~,,II = 20. The effect that the detector response of a neutron can be identified as a photon has been found to be negligible, mainly due to only one interaction length of the lead-glass modules. 4

Background

from resonance

decays

The direct photon to 7r” ratio (r/n’) is extracted measured pion yield N,o according to the following

from the measured expression [2] :

photon

yield NY and

where the R,a and I$ are the Monte Carlo calculated ratios of observed background 7’s to reconstructed no’s for earh of the sources, and A ro and A-, are products of geometric acceptance and reconstruction efficiencies for 7’s and 7r”s. Beside photons

originating from x0 decays the following mesons are taken into account : measures neutral pions, the pi distribution [7] is used in the Monte For the other hadrons the pT spectra are derived from the aopT spectrum

7,~’ and w. As SAPHIR Carlo calculation. assuming

scaling with the transverse f(mT;h)

mass mT = JW =

c ' f( mT;n');

Fl as h = q,v',u.

(2)

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L. Dragon et al. / Direct photons in heavy-ion collisions

T 2

1.4 1.2 1.

.;

0.6

“I 0.6 2

0.4

*- 0.2 1.4 1.2

1. 0.6 0.6 0.4 0.2

200

A*GeV

60

i*

0. 0.

0.8

1.6

2.4

A*GeV 0.8

r*O+A 1.6

2.4

p,( GeV/c) Figure 1: Ratio of inclusive photon cross section to u O cross section as a function of transverse momentum for 200 GeV p+C , 200 GeV p+Au , 200 A.GeV i60+C , and 60 A.GeV 160+Au events under minimum-bias trigger conditions. The squares indicate the data, and the shaded histograms are the Monte Carlo results representing hadronic decays It has been found that another frequently used parameterization same mr-scaling as eq. 2 in the pi region of consideration here.

[lo] leads to effectively

the

The 7) production is measured with SAPHIR in the pr region 2 GeV/c 5 pr(rr) 5 The measured q/a0 ratio of 2.4 GeV/c for minimum-bias 200 A.GeV i60+Au reactions. (61 f 26)% is consistent with the r,r/a’ ratio from p + p data within the statistical errors. Since our detector setup does not allow vertex reconstruction the target is always assumed as the vertex. Therefore, neutral pions decaying several cm from the target cannot be reconstructed even if both decay photons are in SAPHIR’s solid angle. The dominant source '.Monte Carlo simulations lead to an upper limit of for those ?y”‘s are the decays Ki + T'?r r/a0 5 low3 for fake direct photons. 5

Results

A possible excess of direct photons can be more easily observed by comparing the indata and clusive photon spectrum divided by the inclusive rr” spectrum from experimental from the Monte Carlo calculations. This is shown in Fig. 1 for the systems 200 GeV p+C , 200 GeV p+Au , 200 A.GeV 160+C , and 60 A.GeV “O-l-Au under minimum-bias trigger conditions. In this figure the squares indicate the experimental data, and the shaded histograms are Monte Carlo results based on measured ?y” spectra and represent the hadronic background. The difference between data and the histograms can be attributed to the contribution of direct photons. For all four systems the pT dependence of the data is reproduced

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.;

L. Dragon et al. / Directphotons in heavy-ion collbions

0.8

z ‘;; 0.6

c: *- 0.4 0.2 0.

0.

0.8

1.6

2.4

0.8

1.6

2.4

p,(GeV/c) Figure 2: The same as in Fig. 1 but now for central and peripheral reactions

200 A.GeV i60+Au

by the Monte Carlo simulations. QGP’s and hence direct photons are expected to occur more likely in central than in peripheral reactions. Therefore, the 200 A.GeV lsO+Au data (sample with the highest statistics) is divided into peripheral and central events, 8s has been achieved by cuts in the energy measured by the zero degree calorimeter. The data in Fig. 2 for central 200 A.GeV lsO+Au events show a slight overall enhancement over the hadronic background, which is, however, within the statistical and systematic errors. Collecting all sources of uncertainties and assuming them as gaussian, we arrive at an upper limit of about 15 % for the contributions of direct photons to the r/u0 signal corresponding to a 90 % confidence level.

6

Discussion and summary

The observed similarity in the invariant ?r cross sections between the peripheral 200 A.GeV ‘sO+Au and p + p spectra including the onset of hard scattering [7] suggests a similarity in direct photon production. In p + p reactions the direct photon yield was found to be vanishingly small at pi M 2-3 GeV/c [ll]. In addition direct photons have been measured for high transverse momenta in p-induced reactions on light mass targets [12]. These data agree with our data from p-induced events in the limited pi region of overlap and enlarge the direct photon analysis to lower pr-values. The most pessimistic interpretation of the data would be that in p- and 160-induced reactions there is no quark-gluon plasma at all. However, it is also possible that the direct photon signal is smaller than our present experimental sensitivity. In addition the selection of central events done in the present analysis might not be optimized to enrich the number of quark-gluon plasma events in the analyzed sample. In the present experiment it has been demonstrated that with a finely granulated leadglass detector a direct photon analysis based on rr” reconstruction is feasible even in the high multiplicity environment of heavy-ion reactions. The absence of a direct photon signal in the 16C+Au experiment within the error of 15 % is in accordance to theoretical predictions 1131.

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L. Dragon et al. / Direct photons in heavy-ion collkions

With larger detector coverage ( as already implemented in the WA80 setup ), higher number of measured events and an aimed direct photon trigger, the error can be decreased to the level of some %.

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