Search for direct photons in p+Pb and p+C collisions at sqrt(sNN) = 17.4 GeV
aa r X i v : . [ nu c l - e x ] A p r Search for direct photons in p+Pb and p+Ccollisions at √ s NN = 17 . GeV
C Baumann for the WA98 collaboration
University of M¨unster, Institut f¨ur Kernphysik, Wilhelm-Klemm-Str. 9, 48149M¨unster, GermanyE-mail: [email protected]
Abstract.
Upper limits on direct photon production were determined as a functionof the transverse momentum for 0 . < p T ≤ . /c with the WA98 experimentin p+C and p+Pb collisions at √ s NN = 17 . √ s NN = 17 . Submitted to:
J. Phys. G: Nucl. Phys.
1. Introduction
Direct photons are a unique probe of the early stage of a heavy-ion collision, as they leavethe interaction region without being influenced by strong interactions or hadronizationprocesses. They are therefore regarded as an important tool in the search for the quark-gluon plasma in ultrarelativistic heavy-ion collisions. As direct photons from differentsources, e.g. prompt and thermal direct photons, cannot be separated experimentally,further information is needed to quantify the different contributions.The WA98 collaboration was the first to publish a significant direct-photon excessin heavy-ion collisions at √ s NN = 17 . √ s = 19 . x T -scaling and a limited p T coverage in the region where a thermalcontribution would be most prominent, no limits on the prompt photon contributioncould be derived [3]. pQCD calculations have large uncertainties at √ s ≈
20 GeV [4],therefore no definite conclusions on a thermal contribution could be drawn based on theavailable information [5, 6].Here, we will present results on direct photon production in p+Pb and p+Ccollisions measured at √ s NN = 17 . earch for direct photons in p+Pb and p+C collisions at √ s NN = 17 . GeV
2. Experimental Results
In the WA98 experiment photons were measured using a highly segmented lead glasscalorimeter, positioned 21.5 m downstream of the target and covering the pseudorapidityrange 2 . ≤ η ≤ .
0. The minimum bias trigger was determined by measuring thetransverse energy ( E T ) with a hadronic calorimeter in the range 3 . ≤ η ≤ . σ mb for p+C (p+Pb) of 170 mb (1341 mb)corresponds to 74% (76%) of the total geometric cross section. To enhance the reachin p T , a high-energy photon (HEP) trigger based on the energy signal in the lead glasscalorimeter was used. (GeV/c) T p γ ) - G e V ( c d y T d p N d e v t N T p π -7 -6 -5 -4 -3 -2 -1 p+Cp+Pb preliminary Figure 1.
Invariant inclusive photon yields in minimum bias p+C and p+Pbcollisions. The error bars represent the quadratic sum of systematic andstatistical uncertainties.
The fully corrected inclusive photon yields in p+C and p+Pb collisions at √ s NN =17 . p T = 1 . /c .In order to get the direct-photon excess γ direct , the contribution γ decay from decayphotons has to be subtracted from the inclusive photon yield γ inclusive . This can bedone by calculating the direct-photon yield γ direct as a fraction of the inclusive photonspectrum: γ direct = γ inclusive − γ decay = − R γ ! · γ inclusive ; R γ = ( γ/π ) meas ( γ/π ) sim (1)The neutral pion yields have been presented in [7]. A Monte-Carlo simulation based on earch for direct photons in p+Pb and p+C collisions at √ s NN = 17 . GeV γ/π -ratio. Thesimulation uses a parameterization of the measured neutral pion spectrum as input andsimulates the photonic decays of the relevant mesons, e.g. η, η ′ , ω . π / γ Measured Ratio, p+PbSimulated Ratio s i m ) π / γ / ( m e a s ) π / γ ( p+Pb Measured Ratio, p+CSimulated Ratio (GeV/c) T p p+C preliminary preliminarypreliminary preliminarya)c) b)d) Figure 2. a) Ratio of the inclusive photon yield to the neutral pion yield inp+Pb collisions, the boxes indicate systematic uncertainties, the error bars thestatistical uncertainties. The black line indicates the ratio expected from thedecay of π , η and other hadrons determined with a Monte-Carlo simulation.b) The same for p+C collisions. c) The double-ratio of the measured to thesimulated γ/π -ratio for p+Pb collisions. The boxes indicate systematic, theerror bars statistical uncertainties. d) The same for p+C collisions. The simulated and measured γ/π -ratios and the corresponding double-ratio R γ for p+Pb and p+C collisions are presented in Figure 2. While the measured γ/π -ratiois systematically above the simulated one for both targets, R γ is consistent with unityin the p T range relevant for a thermal contribution. The resulting direct-photon yieldsare presented in Figure 3. Significant data points can only be quoted for the highesttransverse momenta.As the p+A data should not exhibit a thermal contribution, any significant excessof the direct photons from Pb+Pb collisions over the p+A results, scaled by the averagenumber of nucleon-nucleon collisions N coll as described in [7], can be attributed to athermal source. The yields are compared in Figure 3. As both data sets agree within earch for direct photons in p+Pb and p+C collisions at √ s NN = 17 . GeV (GeV/c) T p γ ) - G e V ( c d y T d p N d e v t N T p π -8 -7 -6 -5 -4 -3 -2 -1 p+Cp+Pb ) - G e V ( c d y T d p N d e v t N T p π -7 -6 -5 -4 -3 -2 -1 Pb+Pb Central scaled coll p+Pb, N T Pb+Pb shifted by 0.03 GeV in p (GeV/c) T p γ -7 -6 -5 -4 -3 -2 -1 Pb+Pb Central scaled coll p+C, N T Pb+Pb shifted by 0.03 GeV in p preliminary preliminarypreliminary b)c)a)
Figure 3. a) Invariant direct photon yields in p+C and p+Pb collisions.The error bars represent the quadratic sum of statistical and systematicuncertainties. The upper edges of the arrows indicate upper limits (bestestimate + 1 . σ ). b) Comparison of the direct photon results from p+Pb( N coll = 3 .
8) scaled with the number of binary collisions and the centralPb+Pb direct photon spectra ( N coll = 727 . N coll = 1 . errors, no further limit on the contribution of prompt direct photons can be set.
3. Summary
Direct photon yields in p+C and p+Pb collisions at √ s NN = 17 . p T could be derived from both data sets. When compared to the resultson direct photon production in Pb+Pb collisions by applying N coll scaling, no additionalconstraints on the prompt direct photon production in this system can be set from thenew p+C and p+Pb data. References [1] M. M. Aggarwal et al. [WA98 Collaboration], Phys. Rev. Lett. , 3595 (2000)[2] S. Turbide, R. Rapp and C. Gale, Phys. Rev. C , 014903 (2004)[3] M. M. Aggarwal et al. [WA98 Collaboration], arXiv:nucl-ex/0006007.[4] P. Aurenche, M. Fontannaz, J. P. Guillet, B. A. Kniehl, E. Pilon and M. Werlen, Eur. Phys. J. C , 107 (1999)[5] P. Stankus, Ann. Rev. Nucl. Part. Sci. , 517 (2005).[6] K. Reygers [PHENIX Collaboration], Eur. Phys. J. C , 393 (2005)[7] M. M. Aggarwal et al.et al.