- Email: [email protected]
Heavy-flavor measurements with the PHENIX experiment at RHIC
Nuclear
Physics
A734 (2004)
70-73 www.elsevier.comllocate/npe
Heavy-flavor
measurements
with
R. Averbecka
for the PHENIX
aDepartment Stony Brook,
of Physics and Astronomy, NY 11794-3800, USA
the PHENIX
experiment
at RHIC
Collaboration State University
of New York at Stony Brook.
The PHENIX experiment has measured the contribution of heavy-flavor decays to single-electron spectra in .4u + Au collisions at fi = 130 GeV and 200 GeV at RHIC, providing a first measure of the total charm yield and its dependence on collision centrality at these energies. In addition, J/11,charmonium states were reconstructed by PHEKIX in their dilepton decay channels in p+p and Au+Au collisions at e = 200 GeV. 1. Introduction Particles carrying heavy flavor, i.e. charm or beauty quarks, are sensitive probes of the hot and dense medium created in high energy heavy-ion collisions. The production of heavy flavor quark-antiquark pairs proceeds mainly via gluon-gluon fusion in the very early stage of the collision. Hence, the total yield is sensitive to the initial gluon density [1,2] as well as to nuclear effects such as shadowing. In addition, thermal production from gluongluon scattering processes in later stages of the collision might play a role. Once being produced, a quark-antiquark pair forms either a bound quarkonium state or separates and hadronizes into two particles carrying open heavy flavor. In case a deconfined phase of nuclear matter is formed during the collision, the yield of heavy quarkonia might either be reduced due to the expected screening of the QCD attractive potential [3] or possibly even enhanced due to quark-antiquark coalescence [4] or statistical recombination [5]. Furthermore, quarks can lose energy by gluon radiation while propagating through a deconfined medium [6], although this effect might be reduced with increasing quark mass [7]. This energy loss could lead to a softening of final state particle spectra. 2. The
PHENIX
experiment
The PHENIX experiment [8] comprises four spectrometer arms, i.e. two central spectrometers (each covering a4 = 90” in azimuth and 1~1 < 0.35 in pseudorapidity) and two forward muon spectrometers (each covering the full azimuth and 1.1 < 1~1 < 2.4). Electrons with transverse momenta pi > 0.2 GeV/c are measured in the central spectrometers as charged particle tracks in drift and pad chambers that are associated with electromagnetic showers in electromagnetic calorimeters and rings in ring imaging Cerenkov detectors. Muons with momenta above 2 GeV/c are measured in the forward arms as tracks in the muon tracker stations that are associat,ed with hit roads detected in t,he muon identifier panels. 037%9474/S see front matter doi:10.1016/j.nuc1physa.2004.01.014
0 2004 Elsevier
B.V.
All rights reserved.
R. Auerbeck/Nuclear
electrons mz >
1
$
10.'
from non-photonic
sources
in min. bias Au+Au collisions
V (e’+e‘)/Z @ &
z
Physics A734 (2004) 70-73
= 200 GeV
V (e’dj’2 @ 6; = 130 GeV p!IEH!X PRL811wxm1’124031 q sys. error (9% = 200 Ge
% lo-=
?-
g 1o'3 z R -4 & 10 6=; 10-S 1o'6 10.'
0
0.5
1
1.5
2
2.5
3
3.5 pT
4
IGeV/cl
Figure 1. Invariant pi spectra of electrons from non-photonic sources in minimum bias Au+Au collisions at JSNN = 200 GeV and 130 GeV compared with the expected contributions from semileptonic open charm decays as calculated with PYTHIA (left panel). The right panels show the same spectra for JSNN = 200 GeV for central and more peripheral collisions.
3. Open charm measurements
in Au + Au collisions
The direct reconstruction of heavy-flavor decays, e.g. Do + K-K+, is difficult in the high-multiplicity environment of a heavy-ion collision. An alternative is to determine the contributions from semileptonic heavy-flavor decays, e.g. D + eKv, to single lepton and lepton pair spectra. PHENIX follows this approach in the analysis of single electrons, (e++e-)/2, measured in Au+Au collisions at fi = 130 GeV [9] and 200 GeV employing two complementary methods to extract these contributions from non-photonic electron sources. For the analysis of the single electron spectra measured at JSNN = 130 Gel: _ t,hr contributions expected from light meson decays (dominated by 7r” Dalitz decays) and photon conversions were calculated with a hadron cocktail generator and subtracted from the inclusive electron spectrum. With increasing pr an excess of electrons beyond the light hadron cocktail was observed and interpreted as a contribution from charm decays [9], corresponding to a charm production cross section of about 400 pb per binary nucleonnucleon collision in agreement with the extrapolation from experiments at lower energies. At fi = 200 GeV collision energy, inclusive electron spectra have been measured with and without, the addition of a brass photon converter (X/X0 = 1%) to the PHENIX setup [lo]. This approach allows to directly measure the photonic contribution Taothe electron spectra, i.e. photon conversions and Dalitz decays, and then subtract, it from t,hc inclusive electron spectra. This method is complementary to the cocktail method and has confirmed the observations from the lower collision energy. Fig. 1 summarizes these results and shows the non-photonic electron spectra from min-
12
R. Averbeck/Nuclear
Physics A734 (2004) 70-73
9.5 P
PHEMX
ON
c2 a ” 1.5
1
.’
........
.’
0 -3
-2
COM
-1
0.5
(MRST2001NLO)
0
1
2
rapidity
3
i -Fit(p+qIn\I;)
0 I:‘:::/ 10
lo2 \I;
(GeV)
Figure 2. J/T) rapidity distribution in p + p collisions at & = 200 GeV (a). .I/$ mean p-, (b) and total cross section (c) compared to results from experiments at lower &. imum bias Au + .4u collisions at both energies as well as centrality selected spectra from JSNN = 200 GeV collisions together with results from PYTHIA calculations that were adjusted to charm data at lower fi and extrapolated to nuclear reactions at RHIC energy assuming binary collision scaling. Although the uncertainties are quite large, the agreement between data and the scaled PYTHIA calculation is reasonable for all centrality selections at both collision energies. Neither a significant enhancement of the total charm yield nor indications for a substantial charm energy loss are observed. 4. Jl$
measurements
at *
= 200 GeV
4.1. Results from p + p collisions J/T) production was studied both in the dielectron and the dimuon decay channel [ll]. The yields were determined by subtracting background like-sign pairs from unlikesign pairs. Dividing the dimuon sample into two forward rapidity bins and adding the midrapidity dielectron measurement results in the J/Q differential cross section shown in Fig. 2 (a). The shape of the distribution agrees reasonably well with Color Octet Model (COM) and PYTHIA calculations using different parton distribution functions. The J/$ mean pi and the total production cross section are compared to results from experiments at lower & in Fig. 2. 4.2. Results from Au + Au collisions The study of J/11, production in the dielectron decay channel suffered from limited statistics [12]. For three centrality selections the most probable J/T) yields as well as the 90 % confidence limits were determined as summarized in Fig. 3 (a). While square markers show the most probable yields, the arrows indicate the 90 Yo confidence limits, and the brackets include the systematic errors. For comparison, the grey band shows the p + p result scaled by the number of binary collisions. Fig. 3 (b) compares these findings with various model predictions. Models predicting enhancement relative to binary collision scaling are disfavored. Given the large statistical uncertainties no distinction can be made between models predicting suppression.
R. Auerbeck/Nuclear
:,
0.45 /
sE
0.4
g
0.35
n
@I
g 0.3 IO.25
n
0.2
Q m0.15 I z 0.1 5
13
o,5’10’5
g
p
Ph.vsics A734 (2004) 70-73
0.05 0 /,,,
n 1/,/,,/,
0
i,. 50
104
150
,,,,,,,, 200
Ii 254 Number
II(I,,IL 300 350 of PaRtctpantS
Number
of Participants
Figure 3. Centrality dependence of the J/g yield in Au + Au collisions at fi = 200 GeV (a) compared with various model predictions (b). The four top curves are from a coalescence model [4] with different charm rapidity distributions, the dotted curve is from a statistical model [5], and the two lowest curves are from a model including absorption (both curves) and recombination (higher curve only) [13].
5. Summary
and
Outlook
The PHENIX experiment has demonstrated its capability to study heavy-flavor production at RHIC in a multitude of channels. First results have been obtained on open charm and J/$J production. High statistics data samples from p + p and d + Au collisions collected in the 2003 run of RHIC are currently being analyzed. In particular, the d + Au data will allow to study the effects of shadowing in cold nuclear matter, thus providing an important baseline for the high luminosity Au + Au run eagerly expected for 2004 at RHIC. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
J.A. Appel, Annu. Rev. Nucl. Part. Sci. 42, 367 (1992). B. Miiller and X.N. Wang, Phys. Rev. Lett. 68, 2437 (1992). T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986). R.L. Thews et al., Phys. Rev. C63, 054905 (2001). A. Andronic et al., Phys. Lett. B571, 36 (2003). Z. Lin and M. Gyulassy, Phys. Rev. Lett. 77, 1222 (1996). Y.L. Dokshitzer and D.E. Kharzeev, Phys. Lett. B519, 199 (2001). K. Adcox et al. (PHENIX Collab.), Nucl. Inst. Meth. A499, 469 (2003). K. Adcox et al. (PHENIX Collab.), Phys. Rev. Lett. 88, 192303 (2002). R. Averbeck et al. (PHENIX Collab.), Nucl Phys. A715, 695 (2003). S.S. Adler et al. (PHENIX Collab.), act. by Phys. Rev. Lett.; hep-ex/0307019 S.S. .4dler et al. (PHENIX Collab.), act. by Phys. Rev. C; nucllex/0305030. L. Grandchamp and R. Rapp, Nucl Phys. A709, 415 (2002).
Physics
A734 (2004)
70-73 www.elsevier.comllocate/npe
Heavy-flavor
measurements
with
R. Averbecka
for the PHENIX
aDepartment Stony Brook,
of Physics and Astronomy, NY 11794-3800, USA
the PHENIX
experiment
at RHIC
Collaboration State University
of New York at Stony Brook.
The PHENIX experiment has measured the contribution of heavy-flavor decays to single-electron spectra in .4u + Au collisions at fi = 130 GeV and 200 GeV at RHIC, providing a first measure of the total charm yield and its dependence on collision centrality at these energies. In addition, J/11,charmonium states were reconstructed by PHEKIX in their dilepton decay channels in p+p and Au+Au collisions at e = 200 GeV. 1. Introduction Particles carrying heavy flavor, i.e. charm or beauty quarks, are sensitive probes of the hot and dense medium created in high energy heavy-ion collisions. The production of heavy flavor quark-antiquark pairs proceeds mainly via gluon-gluon fusion in the very early stage of the collision. Hence, the total yield is sensitive to the initial gluon density [1,2] as well as to nuclear effects such as shadowing. In addition, thermal production from gluongluon scattering processes in later stages of the collision might play a role. Once being produced, a quark-antiquark pair forms either a bound quarkonium state or separates and hadronizes into two particles carrying open heavy flavor. In case a deconfined phase of nuclear matter is formed during the collision, the yield of heavy quarkonia might either be reduced due to the expected screening of the QCD attractive potential [3] or possibly even enhanced due to quark-antiquark coalescence [4] or statistical recombination [5]. Furthermore, quarks can lose energy by gluon radiation while propagating through a deconfined medium [6], although this effect might be reduced with increasing quark mass [7]. This energy loss could lead to a softening of final state particle spectra. 2. The
PHENIX
experiment
The PHENIX experiment [8] comprises four spectrometer arms, i.e. two central spectrometers (each covering a4 = 90” in azimuth and 1~1 < 0.35 in pseudorapidity) and two forward muon spectrometers (each covering the full azimuth and 1.1 < 1~1 < 2.4). Electrons with transverse momenta pi > 0.2 GeV/c are measured in the central spectrometers as charged particle tracks in drift and pad chambers that are associated with electromagnetic showers in electromagnetic calorimeters and rings in ring imaging Cerenkov detectors. Muons with momenta above 2 GeV/c are measured in the forward arms as tracks in the muon tracker stations that are associat,ed with hit roads detected in t,he muon identifier panels. 037%9474/S see front matter doi:10.1016/j.nuc1physa.2004.01.014
0 2004 Elsevier
B.V.
All rights reserved.
R. Auerbeck/Nuclear
electrons mz >
1
$
10.'
from non-photonic
sources
in min. bias Au+Au collisions
V (e’+e‘)/Z @ &
z
Physics A734 (2004) 70-73
= 200 GeV
V (e’dj’2 @ 6; = 130 GeV p!IEH!X PRL811wxm1’124031 q sys. error (9% = 200 Ge
% lo-=
?-
g 1o'3 z R -4 & 10 6=; 10-S 1o'6 10.'
0
0.5
1
1.5
2
2.5
3
3.5 pT
4
IGeV/cl
Figure 1. Invariant pi spectra of electrons from non-photonic sources in minimum bias Au+Au collisions at JSNN = 200 GeV and 130 GeV compared with the expected contributions from semileptonic open charm decays as calculated with PYTHIA (left panel). The right panels show the same spectra for JSNN = 200 GeV for central and more peripheral collisions.
3. Open charm measurements
in Au + Au collisions
The direct reconstruction of heavy-flavor decays, e.g. Do + K-K+, is difficult in the high-multiplicity environment of a heavy-ion collision. An alternative is to determine the contributions from semileptonic heavy-flavor decays, e.g. D + eKv, to single lepton and lepton pair spectra. PHENIX follows this approach in the analysis of single electrons, (e++e-)/2, measured in Au+Au collisions at fi = 130 GeV [9] and 200 GeV employing two complementary methods to extract these contributions from non-photonic electron sources. For the analysis of the single electron spectra measured at JSNN = 130 Gel: _ t,hr contributions expected from light meson decays (dominated by 7r” Dalitz decays) and photon conversions were calculated with a hadron cocktail generator and subtracted from the inclusive electron spectrum. With increasing pr an excess of electrons beyond the light hadron cocktail was observed and interpreted as a contribution from charm decays [9], corresponding to a charm production cross section of about 400 pb per binary nucleonnucleon collision in agreement with the extrapolation from experiments at lower energies. At fi = 200 GeV collision energy, inclusive electron spectra have been measured with and without, the addition of a brass photon converter (X/X0 = 1%) to the PHENIX setup [lo]. This approach allows to directly measure the photonic contribution Taothe electron spectra, i.e. photon conversions and Dalitz decays, and then subtract, it from t,hc inclusive electron spectra. This method is complementary to the cocktail method and has confirmed the observations from the lower collision energy. Fig. 1 summarizes these results and shows the non-photonic electron spectra from min-
12
R. Averbeck/Nuclear
Physics A734 (2004) 70-73
9.5 P
PHEMX
ON
c2 a ” 1.5
1
.’
........
.’
0 -3
-2
COM
-1
0.5
(MRST2001NLO)
0
1
2
rapidity
3
i -Fit(p+qIn\I;)
0 I:‘:::/ 10
lo2 \I;
(GeV)
Figure 2. J/T) rapidity distribution in p + p collisions at & = 200 GeV (a). .I/$ mean p-, (b) and total cross section (c) compared to results from experiments at lower &. imum bias Au + .4u collisions at both energies as well as centrality selected spectra from JSNN = 200 GeV collisions together with results from PYTHIA calculations that were adjusted to charm data at lower fi and extrapolated to nuclear reactions at RHIC energy assuming binary collision scaling. Although the uncertainties are quite large, the agreement between data and the scaled PYTHIA calculation is reasonable for all centrality selections at both collision energies. Neither a significant enhancement of the total charm yield nor indications for a substantial charm energy loss are observed. 4. Jl$
measurements
at *
= 200 GeV
4.1. Results from p + p collisions J/T) production was studied both in the dielectron and the dimuon decay channel [ll]. The yields were determined by subtracting background like-sign pairs from unlikesign pairs. Dividing the dimuon sample into two forward rapidity bins and adding the midrapidity dielectron measurement results in the J/Q differential cross section shown in Fig. 2 (a). The shape of the distribution agrees reasonably well with Color Octet Model (COM) and PYTHIA calculations using different parton distribution functions. The J/$ mean pi and the total production cross section are compared to results from experiments at lower & in Fig. 2. 4.2. Results from Au + Au collisions The study of J/11, production in the dielectron decay channel suffered from limited statistics [12]. For three centrality selections the most probable J/T) yields as well as the 90 % confidence limits were determined as summarized in Fig. 3 (a). While square markers show the most probable yields, the arrows indicate the 90 Yo confidence limits, and the brackets include the systematic errors. For comparison, the grey band shows the p + p result scaled by the number of binary collisions. Fig. 3 (b) compares these findings with various model predictions. Models predicting enhancement relative to binary collision scaling are disfavored. Given the large statistical uncertainties no distinction can be made between models predicting suppression.
R. Auerbeck/Nuclear
:,
0.45 /
sE
0.4
g
0.35
n
@I
g 0.3 IO.25
n
0.2
Q m0.15 I z 0.1 5
13
o,5’10’5
g
p
Ph.vsics A734 (2004) 70-73
0.05 0 /,,,
n 1/,/,,/,
0
i,. 50
104
150
,,,,,,,, 200
Ii 254 Number
II(I,,IL 300 350 of PaRtctpantS
Number
of Participants
Figure 3. Centrality dependence of the J/g yield in Au + Au collisions at fi = 200 GeV (a) compared with various model predictions (b). The four top curves are from a coalescence model [4] with different charm rapidity distributions, the dotted curve is from a statistical model [5], and the two lowest curves are from a model including absorption (both curves) and recombination (higher curve only) [13].
5. Summary
and
Outlook
The PHENIX experiment has demonstrated its capability to study heavy-flavor production at RHIC in a multitude of channels. First results have been obtained on open charm and J/$J production. High statistics data samples from p + p and d + Au collisions collected in the 2003 run of RHIC are currently being analyzed. In particular, the d + Au data will allow to study the effects of shadowing in cold nuclear matter, thus providing an important baseline for the high luminosity Au + Au run eagerly expected for 2004 at RHIC. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
J.A. Appel, Annu. Rev. Nucl. Part. Sci. 42, 367 (1992). B. Miiller and X.N. Wang, Phys. Rev. Lett. 68, 2437 (1992). T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986). R.L. Thews et al., Phys. Rev. C63, 054905 (2001). A. Andronic et al., Phys. Lett. B571, 36 (2003). Z. Lin and M. Gyulassy, Phys. Rev. Lett. 77, 1222 (1996). Y.L. Dokshitzer and D.E. Kharzeev, Phys. Lett. B519, 199 (2001). K. Adcox et al. (PHENIX Collab.), Nucl. Inst. Meth. A499, 469 (2003). K. Adcox et al. (PHENIX Collab.), Phys. Rev. Lett. 88, 192303 (2002). R. Averbeck et al. (PHENIX Collab.), Nucl Phys. A715, 695 (2003). S.S. Adler et al. (PHENIX Collab.), act. by Phys. Rev. Lett.; hep-ex/0307019 S.S. .4dler et al. (PHENIX Collab.), act. by Phys. Rev. C; nucllex/0305030. L. Grandchamp and R. Rapp, Nucl Phys. A709, 415 (2002).