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Direct Photons in p+p, d+Au and Au+Au Collisions at sqrt(s_NN) = 200 GeV

arXiv:nucl-ex/0605005v1 9 May 2006

DIRECT PHOTONS IN p+p, d+Au AND Au+Au COLLISIONS AT 200 GeV

√ sN N =

B. SAHLMUELLER for the PHENIX Collaboration University of M¨nster, Institut f¨r Kernphysik, Wilhelm-Klemm-Str. 9, 48149 M¨nster, Germany u u u

√ The PHENIX experiment has measured direct photons at sNN = 200 GeV in p + p, d+Au and Au+Au collisions. For pT < 4 GeV/c, the internal conversion into e+ e? pairs has been used to measure the direct photons in Au+Au.

1

Introduction

Direct photons are a unique probe to study the hot and dense matter produced in heavy ion collisions at RHIC. They provide information about the thermalized state of such collisions. Moreover, high-pT direct photons can be used as a measure of the rate of initial hard partonparton scatterings. In p + p collisions, direct photons are produced by hard scattering processes. A direct photon measurement thus is an important baseline for the understanding of the hard scattering contribution in Au+Au collisions as well as - in its own right - a good test of pQCD calculations. Measurements in d+Au collisions allow a quanti?cation of possible initial state e?ects. In (central) heavy ion collisions, the temperature of the collision can be obtained via thermal direct photons from the QGP that are expected to be the dominant source of direct photons at 1 GeV/c < pT < 3 GeV/c 1 while at high transverse momenta hard direct photons can help understanding the observed hadron suppression 2 . There are also theoretical predictions for direct photons produced in the interaction of a jet and the created medium 3 . 2 Measurement of Direct Photons

The measurement is challenging due to a large background from decaying hadrons such as the π 0 and the η. Two methods have been applied to measure the direct photon signal: The so-called subtraction or cocktail method has been used for all collision systems. The idea of this method

1/[2πpTNevt ]d N/dpTdη (GeV/c)

p+p PHENIX Preliminary
NLO-pQCD CTEQ6M ?=0.5pT,pT,2pT

d2N (GeV/c)-2 1 2π pTNevt dpTdy

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direct photon Au+Au sNN = 200GeV PHENIX preliminary
min. bias × 10 3 0-10% × 10 0 10-20% × 10
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20-30% × 10 -4 30-40% × 10 -6 40-50% × 10 -8 50-60% × 10
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Bands represent sytematic error

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-11

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16 18 20 pT (GeV/ c )

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Figure 1: Preliminary direct photon invariant yield as a function of pT as measured in p + p √ and d+Au at sNN = 200 GeV. The solid curves are pQCD predictions for three di?erent scales 9 .

Figure 2: Preliminary direct photon invariant yield as a func√ tion of pT as measured in Au+Au at sNN = 200 GeV for di?erent centrality classes and minimum bias. The solid curves are pQCD predictions 9 , scaled with the corresponding nuclear overlap function TAB .

is to subtract the photons from hadronic decays from all inclusive photons; it has been well established in di?erent analyses. 4 , 5 . This method has been applied to the 2003 p + p and d+Au dataset as well as to the large 2004 Au+Au dataset with about 10 times more events than in the 2002 dataset. In the Au+Au case, direct photons have also been measured via their internal conversion into e+ e? pairs. This measurement has already been described elsewhere 6 and has lead to a signi?cant improvement of the measurement at lower transverse momenta. The idea of the internal conversion method is to use a process corresponding to the π 0 (or η) Dalitz decay where one decay photon is a virtual photon decaying into an e+ e? pair. The invariant mass distribution of these electron-positron pairs is given by 7 2α 1 dNee = Nγ dmee 3π 1? 4m2 e m2 ee 1+ 2m2 e m2 ee 1 | F (m2 ) |2 ee mee 1? m2 ee M2
3

.

(1)

The same formula applies to any source of real photons as each such source also produces virtual photons at very low masses. In the case of direct photons, there is no phase space limitation when mee << pphoton . T The great advantage of this method is, that the e+ e? pairs from the π 0 Dalitz decay are suppressed at higher invariant masses due to the limited phase space. By measuring the pair yield in such a mass region, e.g. for 90 < mee < 300 GeV/c2 , the decay photon background ? ? can be mostly eliminated. As γdirect /γincl. = γdirect /γincl. must be satis?ed to derive real photons from the measured virtual photons, one has to relate the obtained yield to the yield in a region with an unrestricted phase space. The term Rγdata = Ndata (90 ? 300 MeV)/Ndata (0 ? 30 ? ? MeV) is the ratio of the measured yields in the two intervals, it is used to calculate γdirect /γincl. . ? ? Therefore, the ratios Rγdirect and Rγhadron are precisely calculated from Equation 1 that de-

γ direct inv. yield [(GeV/c) -2 ]

1.8

Run4 γ / πmeasured γ / πbackground 0 - 10%
1.6

0

/

0

10

γ direct = γ incl ? γ *direct /γ *incl.
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γ direct/γ incl. +1 from γ * 0 - 20% (150 MeV/c shifted)

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γ direct Au+Au (0-20%)
PHENIX Preliminary

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thermal

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-6

b)
0 0.5 1

pQCD + thermal

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2.5 3

3.5 4 4.5 5 pT [GeV/c]

Figure 3: a) Direct photon excess for the most central events above the decay photon background for the subtraction and the virtual photon method in Au+Au collisions. b) Direct photon yield for the same centrality from the virtual photon measurement in comparison with di?erent theory calculations: scaled pQCD 9 , thermal model 10 and the sum of both calculations.

scribes the form of the corresponding electron-positron pair spectra. With the knowledge of these ratios, the ratio of the virtual direct photons and the virtual inclusive photons can be calculated as
? γdirect ? γincl.

=

0?30 Nγ ?

0?30 Nγ ?

direct incl.

=

Rdata ?Rhadron Rdirect ?Rhadron .

To get the ?nal direct photon yield, the calcu-

lated γdirect /γincl. is applied to the real inclusive photon yield from the measurement with the subtraction method 4 . The uncertainty of the internal conversion method is much smaller than the uncertainty of the classical method as most errors cancel in the ratio. Therefore, the systematic error is dominated by the 20 % uncertainty of the η/π 0 -ratio 8 that translates to the same uncertainty in the direct photon yield. Other contributions to the total systematic error of 25 % are the error of the inclusive photon yield (10 %) and the acceptance for e+ e? pairs (5 %). 3 Results

The preliminary results for p + p and minimum bias d+ Au collisions are shown in Figure 1 and compared with a next-to-leading-order perturbative-QCD (NLO pQCD) calculation 9 . The data are consistent with the calculation for the whole shown pT range between 5 and 16 GeV/c. Therefore it is justi?ed to use the pQCD calculation as reference for hard direct photons, i.e. direct photons at high transverse momenta, also in Au+Au collisions at the same energy. Preliminary Au+Au results from recent measurements using the large 2004 data sample, are shown for a pT region of 4 GeV/c to 13 GeV/c in Figure 2 for eight di?erent centrality classes and

for minimum bias. The data agree with the pQCD expectation for all centralities. Compared to earlier measurements 4 , the errors are smaller and the pT region has been extended. Further work is done to improve the signi?cance and to further extend the pT range. For the lower pT region, the direct photon spectrum has also been obtained with the internal conversion method. The signi?cance of the preliminary result is compared to the conventional subtraction method in Figure 3 a). The result from the subtraction method is shown as the so-called double ratio (γ/π 0 |meas )/(γ/π 0 |background ) while the virtual photon result is shown as γdirect /γinclusive +1 which is not exactly equivalent. In contrast to the subtraction method where no signi?cant excess above the hadronic background could be seen for pT < 3 GeV/c, a clear signal of direct photons is measured using the new method, being about 10 %. Both results are consistent. The preliminary direct photon invariant yield for the 20 % most central events from the internal conversion measurement is shown in Figure 3 b) compared to di?erent theoretical calculations. The direct photon signal has been measured with large signi?cance for the shown pT range. The TAB scaled pQCD calculation 9 underpredicts the measurement at low transverse momenta pT < 3 GeV/c while a 2+1 hydrodynamical model for the emission of thermal phoave. tons with a formation time τ0 = 0.15 fm/c and an average initial temperature T0 = 378 MeV max = 590 MeV) 10 is signi?cantly below the data at p > 3 GeV/c. A combination of both (T0 T photons sources, however, describes the data, but the temperature obtained for the thermal model has only a meaning if the observed direct photon signal is of thermal origin. Therefore it is necessary to measure the direct photons with the same analysis technique in p + p and d+Au collisions at the same energy for pT < 3 GeV/c as well, where the excess will be much smaller if it is - in Au+Au collisions - from thermal photons. The d+Au measurement will also help understanding the in?uence of possible initial state e?ects to the direct photon yield. 4 Summary

The PHENIX experiment has measured direct photons in p + p, d+Au and Au+Au collisions at √ sN N = 200 GeV. The measured yields in all systems are consistent with a NLO pQCD calculation at high transverse momenta (pT > 4.5 GeV/c). Direct photons at pT < 4.5 GeV/c have been measured in Au+Au collisions via their internal conversion into e+ e? pairs. A signi?cant signal could be extracted for 1 < pT < 4.5 GeV/c that is signi?cantly above the expectation from NLO pQCD. However, also taking into account thermal photon emissions, the measurement is in agreement with calculations. References S. Turbide, R. Rapp, C. Gale; Phys. Rev. C69:014903; 2004. S.S. Adler et. al.; Phys. Rev. Lett. 91:072301; 2003 S. Turbide et. al.; Phys. Rev. C72:014906; 2005. S.S. Adler et. al.; Phys. Rev. Lett. 94:232301; 2005. K. Okada; hep-ex/0501066; 2005. S. Bathe; nucl-ex/0511042; 2005. N.M. Kroll and W. Wada; Phys. Rev. 98:1355-59; 1955. S.S. Adler et. al. Phys. Rev. Lett 94:082301; 2005. L.E. Gordon and W. Vogelsang; Phys. Rev. D48:3136-59; 1993; also P. Aurenche et al.; Phys. Lett. B140:87; 1984. 10. D. d’Enterria and D. Peressounko; Eur. Phys. J. C46:451; 2006. 1. 2. 3. 4. 5. 6. 7. 8. 9.


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