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Mean transverse momentum at forward rapidity in STAR
Progress in Particle and Nuclear Physics Progress in Particle and Nuclear Physics 53 (2004) 261–263 www.elsevier.com/locate/ppnp
Short survey
Mean transverse momentum at forward rapidity in STAR F. Simon for the STAR collaboration Max-Planck-Institut f¨ur Physik, F¨ohringer Ring 6, 80805 M¨unchen, Germany Received 18 November 2003
Abstract Recent experiments at the relativistic heavy ion collider (RHIC) with Au + Au collisions at √ s N N = 200 GeV have shown that the charged particle density scales according to a “limiting fragmentation” hypothesis, with an energy dependent universal fragmentation region. Here we present studies of the applicability of this hypothesis to mean transverse momentum using data from the forward time projection chambers (FTPCs) in the STAR experiment. © 2004 Elsevier B.V. All rights reserved. PACS: 25.75.-q Keywords: Heavy ion collisions; Mean transverse momentum; Forward rapidity; Limiting fragmentation
1. Introduction The two radial-drift forward time projection chambers (FTPCs) [1] of the STAR [2] experiment permit the study of charged hadrons at forward rapidity in heavy ion collisions. This extends the acceptance of the spectrometer toward the fragmentation region and gives access to phenomena away from mid-rapidity. √ Around mid-rapidity, Au + Au collisions at s N N = 200 GeV exhibit boost invariance, shown by the fact that many observables such as the density of charged particles dNch /dη, mean transverse momentum pt and elliptic flow v2 are independent of a boost in beam direction. However, contrary to predictions of a broad boost-invariant plateau with a fixed, energy independent fragmentation region [3], boost invariance extends only to |η| ∼ 1. As observed by the PHOBOS experiment at RHIC [4], the charged particle distributions are E-mail address: [email protected] (F. Simon). 0146-6410/$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2004.02.019
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NchTPC Fig. 1. Mean transverse momentum as a function of collision centrality for TPC and FTPC.
found to scale according to a “limiting fragmentation” hypothesis. This implies a universal fragmentation region that grows with increasing collision energy. For all energies, the particle distribution approaches a limiting curve in the fragmentation region near beam rapidity. Here, we present studies of the applicability of “limiting fragmentation” to pt . 2. Mean transverse momentum at forward rapidity From a sample of ∼70 000 minimum bias Au+Au events recorded in the STAR FTPCs, pt is determined for different pseudorapidity and multiplicity bins by applying a powerlaw fit [5] to the particle spectra. Fig. 1 shows the centrality dependence of pt in the pseudorapidity range −0.5 < |η| < 0.5 (TPC) and 3.0 < |η| < 3.2 (FTPC). As expected from a non-boost-invariant scenario, the mean transverse momentum is significantly lower at forward rapidity than at mid-rapidity. The dependence on centrality is the same at both rapidities. The centrality of the events is defined by the number of charged particles Nch in the STAR TPC. To study the applicability of “limiting fragmentation”, pt is plotted as a function of √ η = η − ybeam where ybeam is the beam rapidity, 5.37 at s N N = 200 GeV. This is compared to pt for central Pb + Pb collisions studied by the fixed target experiment √ NA49 [6] at an SPS energy of 158 A GeV ( s N N = 17.2 GeV, ybeam = 2.92). The STAR data are for 10% most central collisions, selected due to the limited statistics, while NA49 datapoints are 5% most central events. Fig. 2 shows that there is reasonably good agreement in the overlapping regions of both experiments, which points to an agreement with “limiting fragmentation”. The FTPC datapoint is uncorrected with the error bar giving only the fit error. A detailed study of corrections and systematic errors will allow to extract pt over an extended range in η, making a comparison of the slopes of the pt curves of both experiments possible.
F. Simon / Progress in Particle and Nuclear Physics 53 (2004) 261–263
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η – ybeam Fig. 2. Mean transverse momentum as a function of η for STAR (Au + Au) and NA49 (Pb + Pb).
3. Conclusion First preliminary measurements of the mean transverse momentum in the FTPCs of √ STAR in Au + Au collisions at s N N = 200 GeV have been presented. When compared to pt obtained by NA49 for central Pb + Pb collisions, agreement with a “limiting fragmentation” hypothesis is found, which has already been observed in charged particle density and recently in anisotropic flow [7]. Further studies to finalize the FTPC results are in progress. In addition, a significant increase of statistics is anticipated in the data taking period in 2004. References [1] [2] [3] [4] [5] [6] [7]
K.H. Ackermann et al., Nucl. Instrum. Meth. A 499 (2003) 713. K.H. Ackermann et al., STAR Collaboration, Nucl. Instrum. Meth. A 499 (2003) 624. J.D. Bjorken, Phys. Rev. D 27 (1983) 140. B.B. Back et al., PHOBOS Collaboration, Phys. Rev. Lett. 91 (2003) 052303. C. Adler et al., STAR Collaboration, Phys. Rev. Lett. 87 (2001) 112303. H. Appelsh¨auser et al., NA49 Collaboration, Phys. Rev. Lett. 82 (1999) 122471. J. Adams et al., STAR Collaboration, nucl-ex/0310029.
Short survey
Mean transverse momentum at forward rapidity in STAR F. Simon for the STAR collaboration Max-Planck-Institut f¨ur Physik, F¨ohringer Ring 6, 80805 M¨unchen, Germany Received 18 November 2003
Abstract Recent experiments at the relativistic heavy ion collider (RHIC) with Au + Au collisions at √ s N N = 200 GeV have shown that the charged particle density scales according to a “limiting fragmentation” hypothesis, with an energy dependent universal fragmentation region. Here we present studies of the applicability of this hypothesis to mean transverse momentum using data from the forward time projection chambers (FTPCs) in the STAR experiment. © 2004 Elsevier B.V. All rights reserved. PACS: 25.75.-q Keywords: Heavy ion collisions; Mean transverse momentum; Forward rapidity; Limiting fragmentation
1. Introduction The two radial-drift forward time projection chambers (FTPCs) [1] of the STAR [2] experiment permit the study of charged hadrons at forward rapidity in heavy ion collisions. This extends the acceptance of the spectrometer toward the fragmentation region and gives access to phenomena away from mid-rapidity. √ Around mid-rapidity, Au + Au collisions at s N N = 200 GeV exhibit boost invariance, shown by the fact that many observables such as the density of charged particles dNch /dη, mean transverse momentum pt and elliptic flow v2 are independent of a boost in beam direction. However, contrary to predictions of a broad boost-invariant plateau with a fixed, energy independent fragmentation region [3], boost invariance extends only to |η| ∼ 1. As observed by the PHOBOS experiment at RHIC [4], the charged particle distributions are E-mail address: [email protected] (F. Simon). 0146-6410/$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2004.02.019
262
F. Simon / Progress in Particle and Nuclear Physics 53 (2004) 261–263 0.6
0.5
0.4
0.3
y
ar
in
0.2
TPC [ η = 0 ± 0.5 ] FTPC [ | η | = 3.1 ± 0.1 ]
e
R TA
pr
S
0.1
0
lim
0
100
200
300
400
500
600
700
800
NchTPC Fig. 1. Mean transverse momentum as a function of collision centrality for TPC and FTPC.
found to scale according to a “limiting fragmentation” hypothesis. This implies a universal fragmentation region that grows with increasing collision energy. For all energies, the particle distribution approaches a limiting curve in the fragmentation region near beam rapidity. Here, we present studies of the applicability of “limiting fragmentation” to pt . 2. Mean transverse momentum at forward rapidity From a sample of ∼70 000 minimum bias Au+Au events recorded in the STAR FTPCs, pt is determined for different pseudorapidity and multiplicity bins by applying a powerlaw fit [5] to the particle spectra. Fig. 1 shows the centrality dependence of pt in the pseudorapidity range −0.5 < |η| < 0.5 (TPC) and 3.0 < |η| < 3.2 (FTPC). As expected from a non-boost-invariant scenario, the mean transverse momentum is significantly lower at forward rapidity than at mid-rapidity. The dependence on centrality is the same at both rapidities. The centrality of the events is defined by the number of charged particles Nch in the STAR TPC. To study the applicability of “limiting fragmentation”, pt is plotted as a function of √ η = η − ybeam where ybeam is the beam rapidity, 5.37 at s N N = 200 GeV. This is compared to pt for central Pb + Pb collisions studied by the fixed target experiment √ NA49 [6] at an SPS energy of 158 A GeV ( s N N = 17.2 GeV, ybeam = 2.92). The STAR data are for 10% most central collisions, selected due to the limited statistics, while NA49 datapoints are 5% most central events. Fig. 2 shows that there is reasonably good agreement in the overlapping regions of both experiments, which points to an agreement with “limiting fragmentation”. The FTPC datapoint is uncorrected with the error bar giving only the fit error. A detailed study of corrections and systematic errors will allow to extract pt over an extended range in η, making a comparison of the slopes of the pt curves of both experiments possible.
F. Simon / Progress in Particle and Nuclear Physics 53 (2004) 261–263
263
0.6
0.5
0.4
0.3
y
ar
in
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lim
e
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STAR AuAu 200 GeV NA49 PbPb 158 GeV
S
–6
–5
–4
–3
–2
–1
0
η – ybeam Fig. 2. Mean transverse momentum as a function of η for STAR (Au + Au) and NA49 (Pb + Pb).
3. Conclusion First preliminary measurements of the mean transverse momentum in the FTPCs of √ STAR in Au + Au collisions at s N N = 200 GeV have been presented. When compared to pt obtained by NA49 for central Pb + Pb collisions, agreement with a “limiting fragmentation” hypothesis is found, which has already been observed in charged particle density and recently in anisotropic flow [7]. Further studies to finalize the FTPC results are in progress. In addition, a significant increase of statistics is anticipated in the data taking period in 2004. References [1] [2] [3] [4] [5] [6] [7]
K.H. Ackermann et al., Nucl. Instrum. Meth. A 499 (2003) 713. K.H. Ackermann et al., STAR Collaboration, Nucl. Instrum. Meth. A 499 (2003) 624. J.D. Bjorken, Phys. Rev. D 27 (1983) 140. B.B. Back et al., PHOBOS Collaboration, Phys. Rev. Lett. 91 (2003) 052303. C. Adler et al., STAR Collaboration, Phys. Rev. Lett. 87 (2001) 112303. H. Appelsh¨auser et al., NA49 Collaboration, Phys. Rev. Lett. 82 (1999) 122471. J. Adams et al., STAR Collaboration, nucl-ex/0310029.