- Email: [email protected]
PHENIX results on low pT direct photons in Au + Au collisions
Available online at www.sciencedirect.com
Nuclear Physics A 956 (2016) 417–420 www.elsevier.com/locate/nuclphysa
PHENIX results on low pT direct photons in Au+Au collisions Richard Petti (for the PHENIX Collaboration)1 Brookhaven National Laboratory, Upton, New York 11973-5000, USA
Abstract Since soft photons are unmodified once produced in heavy ion collisions, they give information about the entire thermal evolution of the medium. Excess photon yield over the expectation from initial hard scattering has been measured by PHENIX. In addition, PHENIX has measured a large azimuthal anisotropy, v2 , of these soft photons with respect to the reaction plane. The large yield and v2 have been difficult to describe quantitatively and raise important questions about the early time dynamics in the medium. It is thus important to make more differential measurements to distinguish various potential explanations for this thermal photon puzzle. In this proceedings, we present yields, v2 , and v3 of direct √ photons from sNN = 200GeV Au + Au collisions.
1. Introduction Direct photons are viewed as an ideal probe to study the quark gluon plasma, as they have a negligible interaction cross-section with the strongly interacting partonic matter and escape virtually unmodified. In this way direct photons allow the study of the entire space-time evolution of a heavy ion collision, not just at the freeze-out surface as is the case for hadronic probes. The study is complicated by multiple sources of direct photons. Detailed models are required to fold the time dependent emission rate of each source with the dynamic evolution of the expanding medium, as that expansion can distort the observed photon spectrum through the Doppler shift. It is important to note that the term direct photon does not indicate photons from a specific mechanism of production, but instead refers to any photon that is not the product of a long lived hadron decay. Traditionally, it is thought that the dominant contributions to the direct photon yield is thermal radiation from the quark-gluon plasma and hadron gas stages of the collision at low transverse momentum and from hard parton scattering at higher momentum. Model calculations assuming only these sources underestimate the measured yield from the PHENIX experiment [1, 2] below about 2GeV in pT by about a factor of two or more [3]. This has prompted theorists to propose other more exotic production mechanisms as well as modify some of the existing more traditional thermal models [4, 5, 6, 7]. In addition to the yield of direct photons, PHENIX has also measured the azimuthal asymmetry of direct photon production with respect to the reaction plane, quantified as v2 and v3 [2, 8]. These measurements can further elucidate the dynamics of the collisions. Current theoretical calculations expect a small v2 , in √ contrast to the data. The latest results of the direct photon yield, v2 , and v3 from sNN = 200GeV Au + Au collisions will be discussed in these proceedings, with particular interest at low pT . 1 For
the full PHENIX Collaboration author list and acknowledgments, see Appendix “Collaboration” of this volume
http://dx.doi.org/10.1016/j.nuclphysa.2016.04.004 0375-9474/© 2016 Elsevier B.V. All rights reserved.
418
R. Petti / Nuclear Physics A 956 (2016) 417–420
2. Experimental techniques for photon identification at PHENIX The PHENIX detector was specifically designed to make good measurements of photons and electrons. Since PHENIX has multiple techniques for identifying photons, it is particularly well suited to making the direct photon measurements described here. The central region of the PHENIX detector is a two arm spectrometer. The details of the detector are discussed elsewhere [9]. The main subsystems used for the analysis of direct photons are the electromagnetic calorimeters, the drift chambers and pad chambers, and the ringimaging Cherenkov detector (RICH). The beam-beam counters (BBC) and the reaction plane detector are also used to provide information about the event vertex, collision centrality, and reaction plane orientation. PHENIX employs three techniques for identifying photons in the detector. Two of these techniques involve measuring di-electrons that come from a photon conversion, either from a real photon via an external conversion [2], or a virtual photon via an internal conversion [1]. Electrons are identified primarily with the RICH detector. The third photon identification technique identifies photons that directly deposit their energy into the calorimeters [8]. The external conversion and calorimeter techniques will be briefly described here, as they form the basis for the latest measurements. For a more detailed explanation on the methods, see [2, 8]. The external conversion technique relies on measuring e+ e− pairs from photons converting in material in the experimental aperture. We focus on conversion in the back of the Hadron Blind Detector (HBD) as this portion of the detector provides a localized source of photon conversions at a radius of about 60cm. At the year of data taking, PHENIX performed all of its charged particle tracking outside of the magnetic field in the drift chambers and pad chambers and thus could not track a particle to its vertex, but rather assumed the event vertex to be the origin for all tracks. This has the effect of giving conversion pairs at the HBD an artificial mass of 12MeV. The momentum of each track is reconstructed with two different vertex assumptions, with origins at the event vertex and at 60cm. A strong correlation in the e+ e− pair mass calculated from the two vertex assumptions is observed for physical pairs. We place a two-dimensional cut on the reconstructed masses to isolate the conversions in the HBD. In an alternate identification technique, photons are identified in the electromagnetic calorimeter directly. Cuts are placed on the shower shape of the cluster in the calorimeter, ensuring that the shape is consistent with an electromagnetic shower, rather than with a hadron. A charged track veto cut is also placed, where any cluster in the calorimeter is rejected if a charged track is projected to point to that cluster based on the tracking in the DC and PCs. Remaining hadron contamination is corrected for by Monte Carlo simulations. 3. Data analysis for direct photon signal extraction A first step in the measurement of direct photons is to establish that there is a signal of direct photons produced in the collisions. This is quantified with the ratio known as Rγ which is defined as the ratio of the inclusive photon yield to the yield of photons from hadron decays. A value above one means that there is a direct photon signal while a value equal to one indicates the absence of direct photons. A value below one is unphysical. As a more detailed example, we consider the external conversion analysis chain. In this analysis, we directly measure the inclusive photon yield (the di-electron pairs from conversions) as well a tagged subset of those inclusive photons as coming from a π0 decay. This is done by reconstructing π0 s as one converted photon and one photon in the calorimeter. Rγ is expressed as a double ratio in Eqn. 1 to reduce systematic uncertainties. In Eqn. 1, γ represents the true yield produced by nature, N represents the experimentally observed yield, and εγ f represents the correction for inclusive photons that we fail to tag as coming from a π0 decay due to detector dead area or inefficiency. Note that each yield is expressed as a function of the converted photon pT . The corrections for the di-electron acceptance and reconstruction efficiency explicitly cancel in the double ratio, leaving only the corrections dealing with the partner photon reconstruction in the calorimeter. The numerator of the double ratio (labeled ”data”) is the ratio of the number of inclusive photons (di-electron pairs) to the number of inclusive photons that are tagged as coming from a π0 decay measured in the data. The denominator of the double ratio (labeled ”sim”) accounts for the decay photons from hadrons other than π0 s (η, η , ω) as calculated in a decay cocktail simulation based on published results.
R. Petti / Nuclear Physics A 956 (2016) 417–420
419
√ Figure 1: Left: The invariant yield of direct photons in sNN = 200GeV collisions in Au + Au and p + p for the minimum bias centrality selection. Various data points represent results from various measurements as indicated in the legend. A fit to the p + p data represented by the dashed black line is shown, as well as the Ncoll scaled fit as the green line. The shading around the line represents the estimated systemic uncertainty on the fit. Center: The invariant yields from Au + Au collisions in four centrality bins as measured with the external conversion method. The error bars represent the statistical uncertainty, the error boxes represent the correlated systematic uncertainty. Right: The integrated yield of the isolated excess after subtracting the estimated hard scattering component as a function of Npart . The integration is done with different lower pT bounds, shown by the various colors. All plots reproduced from [2].
Rγ =
γdirect γhadron
εγ f =
Nγinc
0 Nγπ tag
γhadron γπ0 sim
data
(1)
The measured Rγ is used to extract the direct photon yield and the direct photon v2,3 via Eqns. 2 and 3. The hadron decay photon yield, γhadron , and the decay photon vdec 2,3 are calculated in a decay photon cocktail based on previously published measurements. γdirect = Rγ − 1 γhadron vn =
− Rγ − 1
Rγ vinc n
vdec n
(2) (3)
4. Results and discussion The left panel of Fig. 1 shows a comprehensive set of measurements from four different data sets (2004, 2005, 2007, 2010), two different collision systems (p+ p, Au+Au) and three photon identification techniques (internal conversions, external conversions, direct calorimeter) for the minimum bias selection. In all cases, the different measurement techniques agree very well where the measurements overlap, which is a great success and gives confidence in the result. Also shown in the figure is a fit to the p + p data (dashed black line) that is used to quantify the expected contribution from hard scattering. This fit is scaled by the number of binary collisions (Ncoll ) for each corresponding centrality bin, as hard processes are expected to scale by this number. At a pT greater than 4GeV, there is good agreement between the scaled p + p fit and the data. The center panel of the figure shows the invariant yield as measured with the external conversion method in four centrality bins as labeled in the figure. In the right panel of the figure, the integrated yield of the isolated excess (after subtracting the estimated hard scattering component) is shown as a function of the number of participants, Npart in a given centrality bin. The plots in the figure are referenced from [2].
R. Petti / Nuclear Physics A 956 (2016) 417–420
420
v2
0.3 0.2
(a) 0-20% Direct photon vn Conversion method Calorimeter method
(b) 20-40% PHENIX 2007 and 2010 Au+Au 200 GeV
(c) 40-60%
(d) 0-20%
(e) 20-40%
(f) 40-60%
0.1
0.15
v3
0.1
0.05
0
0
1
2
3
4
0
1
2
3
p GeV/c
4
0
1
2
3
4
T
Figure 2: The direct photon v2 (panels (a), (b), (c)) and v3 (panels (d), (e), (f)) shown as a function of pT in three centrality bins indicated in the columns. Two different measurements are shown. The green circles represent the conversion method, the black squares represent the calorimeter method. The error bars represent the statistical uncertainty, the error boxes represent correlated systematic uncertainties. All plots reproduced from [3].
Fig. 2 shows the results of the direct photon v2,3 as a function of pT in three centrality bins as labeled in the figure (reproduced from [3]). The top row shows the v2 measurement, the bottom row shows the v3 measurement. The figure shows the results from two different methods (external conversions and direct calorimeter). The two methods agree well where they overlap. 5. Summary The results presented in these proceedings provides important differential measurements of the direct photon yield, v2 , and v3 . These measurements provide critical constraints to theoretical models which seek to describe simultaneously both the production as well as the dynamic time evolution of the heavy ion collision. Currently, models still fail to simultaneously describe the observed yield and v2 , although much progress has been made as calculations have become more detailed and rigorous. PHENIX has observed a large excess of direct photons above what is expected from hard processes which persists in all centrality bins. Looking at the excess further, it is observed that the shape of the excess is the same in all centrality bins, while the yield drops by two orders of magnitude in going from most central to most peripheral collisions. Additionally, the excess is observed to follow a power law as a function of the number of participants in the collision (N part ). A large v2 for direct photons is observed, which is at a similar magnitude to that for hadrons in this low pT range. A sizable non-zero v3 is also observed. √ In the near future, PHENIX is expecting to release results for sNN = 62GeV and 39GeV, which will provide further constraints to models. References [1] [2] [3] [4] [5] [6]
A. Adare, et al., Phys. Rev. Lett. 104 (2010) 132301. A. Adare, et al., Phys. Rev. C 91 (2015) 064904. A. Adare, et al., submitted to Phys. Rev. C (2015). arXiv:1509.07758. H. van Hees, C. Gale, R. Rapp, Phys. Rev. C 84 (2011) 054906. O. Linnyk, W. Cassing, E. L. Bratkovskaya, Phys. Rev C 89 (2014) 034908. C. Gale, Y. Hidaka, S. Jeon, S. Lin, J.-F. Paquet, R. D. Pisarski, D. Satow, V. V. Skokov, V. Vujanovic, Phys. Rev. Lett. 114 (2015) 072301. [7] C. Shen, U. Heinze, J.-F. Paquet, C. Gale, Phys. Rev. C 89 (2014) 044910. [8] A. Adare, et al., Phys. Rev. Lett. 109 (2012) 122302. [9] K. Adcox, et al., Nucl. Inst. Methods Phys. Res. Sect. A A499 (2003) 469–479.
Nuclear Physics A 956 (2016) 417–420 www.elsevier.com/locate/nuclphysa
PHENIX results on low pT direct photons in Au+Au collisions Richard Petti (for the PHENIX Collaboration)1 Brookhaven National Laboratory, Upton, New York 11973-5000, USA
Abstract Since soft photons are unmodified once produced in heavy ion collisions, they give information about the entire thermal evolution of the medium. Excess photon yield over the expectation from initial hard scattering has been measured by PHENIX. In addition, PHENIX has measured a large azimuthal anisotropy, v2 , of these soft photons with respect to the reaction plane. The large yield and v2 have been difficult to describe quantitatively and raise important questions about the early time dynamics in the medium. It is thus important to make more differential measurements to distinguish various potential explanations for this thermal photon puzzle. In this proceedings, we present yields, v2 , and v3 of direct √ photons from sNN = 200GeV Au + Au collisions.
1. Introduction Direct photons are viewed as an ideal probe to study the quark gluon plasma, as they have a negligible interaction cross-section with the strongly interacting partonic matter and escape virtually unmodified. In this way direct photons allow the study of the entire space-time evolution of a heavy ion collision, not just at the freeze-out surface as is the case for hadronic probes. The study is complicated by multiple sources of direct photons. Detailed models are required to fold the time dependent emission rate of each source with the dynamic evolution of the expanding medium, as that expansion can distort the observed photon spectrum through the Doppler shift. It is important to note that the term direct photon does not indicate photons from a specific mechanism of production, but instead refers to any photon that is not the product of a long lived hadron decay. Traditionally, it is thought that the dominant contributions to the direct photon yield is thermal radiation from the quark-gluon plasma and hadron gas stages of the collision at low transverse momentum and from hard parton scattering at higher momentum. Model calculations assuming only these sources underestimate the measured yield from the PHENIX experiment [1, 2] below about 2GeV in pT by about a factor of two or more [3]. This has prompted theorists to propose other more exotic production mechanisms as well as modify some of the existing more traditional thermal models [4, 5, 6, 7]. In addition to the yield of direct photons, PHENIX has also measured the azimuthal asymmetry of direct photon production with respect to the reaction plane, quantified as v2 and v3 [2, 8]. These measurements can further elucidate the dynamics of the collisions. Current theoretical calculations expect a small v2 , in √ contrast to the data. The latest results of the direct photon yield, v2 , and v3 from sNN = 200GeV Au + Au collisions will be discussed in these proceedings, with particular interest at low pT . 1 For
the full PHENIX Collaboration author list and acknowledgments, see Appendix “Collaboration” of this volume
http://dx.doi.org/10.1016/j.nuclphysa.2016.04.004 0375-9474/© 2016 Elsevier B.V. All rights reserved.
418
R. Petti / Nuclear Physics A 956 (2016) 417–420
2. Experimental techniques for photon identification at PHENIX The PHENIX detector was specifically designed to make good measurements of photons and electrons. Since PHENIX has multiple techniques for identifying photons, it is particularly well suited to making the direct photon measurements described here. The central region of the PHENIX detector is a two arm spectrometer. The details of the detector are discussed elsewhere [9]. The main subsystems used for the analysis of direct photons are the electromagnetic calorimeters, the drift chambers and pad chambers, and the ringimaging Cherenkov detector (RICH). The beam-beam counters (BBC) and the reaction plane detector are also used to provide information about the event vertex, collision centrality, and reaction plane orientation. PHENIX employs three techniques for identifying photons in the detector. Two of these techniques involve measuring di-electrons that come from a photon conversion, either from a real photon via an external conversion [2], or a virtual photon via an internal conversion [1]. Electrons are identified primarily with the RICH detector. The third photon identification technique identifies photons that directly deposit their energy into the calorimeters [8]. The external conversion and calorimeter techniques will be briefly described here, as they form the basis for the latest measurements. For a more detailed explanation on the methods, see [2, 8]. The external conversion technique relies on measuring e+ e− pairs from photons converting in material in the experimental aperture. We focus on conversion in the back of the Hadron Blind Detector (HBD) as this portion of the detector provides a localized source of photon conversions at a radius of about 60cm. At the year of data taking, PHENIX performed all of its charged particle tracking outside of the magnetic field in the drift chambers and pad chambers and thus could not track a particle to its vertex, but rather assumed the event vertex to be the origin for all tracks. This has the effect of giving conversion pairs at the HBD an artificial mass of 12MeV. The momentum of each track is reconstructed with two different vertex assumptions, with origins at the event vertex and at 60cm. A strong correlation in the e+ e− pair mass calculated from the two vertex assumptions is observed for physical pairs. We place a two-dimensional cut on the reconstructed masses to isolate the conversions in the HBD. In an alternate identification technique, photons are identified in the electromagnetic calorimeter directly. Cuts are placed on the shower shape of the cluster in the calorimeter, ensuring that the shape is consistent with an electromagnetic shower, rather than with a hadron. A charged track veto cut is also placed, where any cluster in the calorimeter is rejected if a charged track is projected to point to that cluster based on the tracking in the DC and PCs. Remaining hadron contamination is corrected for by Monte Carlo simulations. 3. Data analysis for direct photon signal extraction A first step in the measurement of direct photons is to establish that there is a signal of direct photons produced in the collisions. This is quantified with the ratio known as Rγ which is defined as the ratio of the inclusive photon yield to the yield of photons from hadron decays. A value above one means that there is a direct photon signal while a value equal to one indicates the absence of direct photons. A value below one is unphysical. As a more detailed example, we consider the external conversion analysis chain. In this analysis, we directly measure the inclusive photon yield (the di-electron pairs from conversions) as well a tagged subset of those inclusive photons as coming from a π0 decay. This is done by reconstructing π0 s as one converted photon and one photon in the calorimeter. Rγ is expressed as a double ratio in Eqn. 1 to reduce systematic uncertainties. In Eqn. 1, γ represents the true yield produced by nature, N represents the experimentally observed yield, and εγ f represents the correction for inclusive photons that we fail to tag as coming from a π0 decay due to detector dead area or inefficiency. Note that each yield is expressed as a function of the converted photon pT . The corrections for the di-electron acceptance and reconstruction efficiency explicitly cancel in the double ratio, leaving only the corrections dealing with the partner photon reconstruction in the calorimeter. The numerator of the double ratio (labeled ”data”) is the ratio of the number of inclusive photons (di-electron pairs) to the number of inclusive photons that are tagged as coming from a π0 decay measured in the data. The denominator of the double ratio (labeled ”sim”) accounts for the decay photons from hadrons other than π0 s (η, η , ω) as calculated in a decay cocktail simulation based on published results.
R. Petti / Nuclear Physics A 956 (2016) 417–420
419
√ Figure 1: Left: The invariant yield of direct photons in sNN = 200GeV collisions in Au + Au and p + p for the minimum bias centrality selection. Various data points represent results from various measurements as indicated in the legend. A fit to the p + p data represented by the dashed black line is shown, as well as the Ncoll scaled fit as the green line. The shading around the line represents the estimated systemic uncertainty on the fit. Center: The invariant yields from Au + Au collisions in four centrality bins as measured with the external conversion method. The error bars represent the statistical uncertainty, the error boxes represent the correlated systematic uncertainty. Right: The integrated yield of the isolated excess after subtracting the estimated hard scattering component as a function of Npart . The integration is done with different lower pT bounds, shown by the various colors. All plots reproduced from [2].
Rγ =
γdirect γhadron
εγ f =
Nγinc
0 Nγπ tag
γhadron γπ0 sim
data
(1)
The measured Rγ is used to extract the direct photon yield and the direct photon v2,3 via Eqns. 2 and 3. The hadron decay photon yield, γhadron , and the decay photon vdec 2,3 are calculated in a decay photon cocktail based on previously published measurements. γdirect = Rγ − 1 γhadron vn =
− Rγ − 1
Rγ vinc n
vdec n
(2) (3)
4. Results and discussion The left panel of Fig. 1 shows a comprehensive set of measurements from four different data sets (2004, 2005, 2007, 2010), two different collision systems (p+ p, Au+Au) and three photon identification techniques (internal conversions, external conversions, direct calorimeter) for the minimum bias selection. In all cases, the different measurement techniques agree very well where the measurements overlap, which is a great success and gives confidence in the result. Also shown in the figure is a fit to the p + p data (dashed black line) that is used to quantify the expected contribution from hard scattering. This fit is scaled by the number of binary collisions (Ncoll ) for each corresponding centrality bin, as hard processes are expected to scale by this number. At a pT greater than 4GeV, there is good agreement between the scaled p + p fit and the data. The center panel of the figure shows the invariant yield as measured with the external conversion method in four centrality bins as labeled in the figure. In the right panel of the figure, the integrated yield of the isolated excess (after subtracting the estimated hard scattering component) is shown as a function of the number of participants, Npart in a given centrality bin. The plots in the figure are referenced from [2].
R. Petti / Nuclear Physics A 956 (2016) 417–420
420
v2
0.3 0.2
(a) 0-20% Direct photon vn Conversion method Calorimeter method
(b) 20-40% PHENIX 2007 and 2010 Au+Au 200 GeV
(c) 40-60%
(d) 0-20%
(e) 20-40%
(f) 40-60%
0.1
0.15
v3
0.1
0.05
0
0
1
2
3
4
0
1
2
3
p GeV/c
4
0
1
2
3
4
T
Figure 2: The direct photon v2 (panels (a), (b), (c)) and v3 (panels (d), (e), (f)) shown as a function of pT in three centrality bins indicated in the columns. Two different measurements are shown. The green circles represent the conversion method, the black squares represent the calorimeter method. The error bars represent the statistical uncertainty, the error boxes represent correlated systematic uncertainties. All plots reproduced from [3].
Fig. 2 shows the results of the direct photon v2,3 as a function of pT in three centrality bins as labeled in the figure (reproduced from [3]). The top row shows the v2 measurement, the bottom row shows the v3 measurement. The figure shows the results from two different methods (external conversions and direct calorimeter). The two methods agree well where they overlap. 5. Summary The results presented in these proceedings provides important differential measurements of the direct photon yield, v2 , and v3 . These measurements provide critical constraints to theoretical models which seek to describe simultaneously both the production as well as the dynamic time evolution of the heavy ion collision. Currently, models still fail to simultaneously describe the observed yield and v2 , although much progress has been made as calculations have become more detailed and rigorous. PHENIX has observed a large excess of direct photons above what is expected from hard processes which persists in all centrality bins. Looking at the excess further, it is observed that the shape of the excess is the same in all centrality bins, while the yield drops by two orders of magnitude in going from most central to most peripheral collisions. Additionally, the excess is observed to follow a power law as a function of the number of participants in the collision (N part ). A large v2 for direct photons is observed, which is at a similar magnitude to that for hadrons in this low pT range. A sizable non-zero v3 is also observed. √ In the near future, PHENIX is expecting to release results for sNN = 62GeV and 39GeV, which will provide further constraints to models. References [1] [2] [3] [4] [5] [6]
A. Adare, et al., Phys. Rev. Lett. 104 (2010) 132301. A. Adare, et al., Phys. Rev. C 91 (2015) 064904. A. Adare, et al., submitted to Phys. Rev. C (2015). arXiv:1509.07758. H. van Hees, C. Gale, R. Rapp, Phys. Rev. C 84 (2011) 054906. O. Linnyk, W. Cassing, E. L. Bratkovskaya, Phys. Rev C 89 (2014) 034908. C. Gale, Y. Hidaka, S. Jeon, S. Lin, J.-F. Paquet, R. D. Pisarski, D. Satow, V. V. Skokov, V. Vujanovic, Phys. Rev. Lett. 114 (2015) 072301. [7] C. Shen, U. Heinze, J.-F. Paquet, C. Gale, Phys. Rev. C 89 (2014) 044910. [8] A. Adare, et al., Phys. Rev. Lett. 109 (2012) 122302. [9] K. Adcox, et al., Nucl. Inst. Methods Phys. Res. Sect. A A499 (2003) 469–479.