- Email: [email protected]
Overview of the latest d + Au results from RHIC
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 261 (2007) 1061–1062 www.elsevier.com/locate/nimb
Overview of the latest d + Au results from RHIC Ming X. Liu
*
P-25, Physics Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Available online 29 April 2007
Abstract The latest results of high pT particle production in the asymmetric d + Au collisions at RHIC is reviewed. We discuss the roles of the cold nuclear matter effects such as gluon shadowing and saturation, parton energy loss and coherent final states interactions on the light pffiffiffiffiffiffiffiffi and heavy flavor particle productions in d + Au collisions at sNN ¼200 GeV. Published by Elsevier B.V. PACS: 25.75.q Keywords: Heavy ion collisions; Parton distribution; Nuclear medium effects
Early heavy ion results from RHIC showed that high pT particle production is strongly suppressed in central Au + Au collisions, which is explained in terms of parton energy loss in dense medium created at the collision volume. However, there were also other models predicted that such suppression could be due to other mechanisms, such as gluon saturation. To investigate whether the observed suppression is due to initial state effects, a control experiment with d + Au was conducted in 2003. Instead of suppression, an enhancement has been seen in high pT particle production at middle rapidity in minimum biased d + Au collisions. This observation has clearly shown that RHIC has produced a dense medium in heavy ion Au + Au collisions that significantly modifies the particle production via final state interactions. It has also led some people to believe the absence of strong shadowing and other cold nuclear effects in d + Au collisions. However, people quickly realized that physics in d + Au collision is much rich than they thought before. Results from BRAMHS [1] and PHENIX [2] on high pT particle production in d + Au collisions at large rapidity have shown a significant suppression (enhancement) in particle
*
Tel.: +1 505 667 7125; fax: +1 505 665 7920. E-mail address: [email protected]
0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2007.04.255
yield in the forward (backward) rapidity in d + Au collisions where ‘‘forward’’ is defined by the deuteron going direction in the laboratory frame (center of mass frame at RHIC). To quantify such nuclear medium effects, people normally use the nuclear modification factor, which is defined as the particle yield per nucleon–nucleon collisions relative to the yield in the baseline elementary p + p collisions. RdA ðpT ; gÞ ¼
d2 rdþAu dpT dg 2 pþp
N dþAu ddpr dg coll
:
T
This comparison with p + p results is based on the assumption that the production of high transverse momentum particles scales with the number of binary collisions N dþAu in coll the initial states, where N dþAu is estimated to be 7 for coll minimum bias d + Au collisions. Without any nuclear medium effects, RdA(pT, g) 1. It is also important to note that in general, particles produced at a large forward (backward) rapidity are mostly likely coming from collisions of partons of small (large) ‘‘x’’ values inside the Au nucleus in d + Au collisions. Figs. 1–3 show RdA of light and heavy hadrons as a function of transverse momentum pT and rapidity measured by the BRAHMS and PHENIX experiments at RHIC at central and forward rapidities.
1062
M.X. Liu / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1061–1062
Fig. 1. Charged hadron RdA as a function of pT at central and forward pseudo-rapidities measured by the BRAHMS experiment at RHIC. Particle production at the forward rapidity is suppressed.
Fig. 3. RdA as a function of pT for prompt muons (mostly from heavy quark decay) at the forward and backward rapidities measured by the PHENIX experiment at RHIC. Results are very similar to what have been observed in high hadron sector.
Fig. 2. RdA as a function of pseudo-rapidity of high pT charged hadrons and J/W measured by the PHENIX experiment at RHIC. Particle yields are suppressed (enhanced) at the forward (backward) rapidity.
pression pattern. To understand the experimental data, several models have been implemented to study light hadron and charm yields at the forward rapidity, including gluon shadowing model [3], color glass condensate [4], coherent multiple scattering and initial state parton energy loss [5] and final state recombination [6]. The current data are statistically limited to distinguish different models. More precise d + Au measurements in various channels are needed to understand the cold nuclear medium effects as a baseline for understanding the hot dense matter produced in Au + Au collisions. References
These results have generated much interest and led to intense debate on the underlying physics of the forward rapidity processes in d + Au collisions at RHIC. The conventional models of multiple scattering based on Glauber theory predicted an enhancement of Cronin effect in the forward rapidity region, while the data clearly show a sup-
[1] [2] [3] [4] [5] [6]
BRAHMS Collaboration, Phys. Rev. Lett. (2004) 242303. PHENIX Collaboration, Phys. Rev. Lett. 94 (2005) 082302. K.J. Eskola, V.J. Kolhinen, R. Vogt, Nucl. Phys. A 696 (2001) 729. L. McLerran, R. Venugopalan, Phys. Rev. D 49 (1994) 2233. I. Vitev, M.B. Johnson, J.W. Qiu hep/0605200. R.C. Hwa, C.B. Yang, R.J. Fries, Phys. Rev. C 71 (2005) 024902.
Nuclear Instruments and Methods in Physics Research B 261 (2007) 1061–1062 www.elsevier.com/locate/nimb
Overview of the latest d + Au results from RHIC Ming X. Liu
*
P-25, Physics Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Available online 29 April 2007
Abstract The latest results of high pT particle production in the asymmetric d + Au collisions at RHIC is reviewed. We discuss the roles of the cold nuclear matter effects such as gluon shadowing and saturation, parton energy loss and coherent final states interactions on the light pffiffiffiffiffiffiffiffi and heavy flavor particle productions in d + Au collisions at sNN ¼200 GeV. Published by Elsevier B.V. PACS: 25.75.q Keywords: Heavy ion collisions; Parton distribution; Nuclear medium effects
Early heavy ion results from RHIC showed that high pT particle production is strongly suppressed in central Au + Au collisions, which is explained in terms of parton energy loss in dense medium created at the collision volume. However, there were also other models predicted that such suppression could be due to other mechanisms, such as gluon saturation. To investigate whether the observed suppression is due to initial state effects, a control experiment with d + Au was conducted in 2003. Instead of suppression, an enhancement has been seen in high pT particle production at middle rapidity in minimum biased d + Au collisions. This observation has clearly shown that RHIC has produced a dense medium in heavy ion Au + Au collisions that significantly modifies the particle production via final state interactions. It has also led some people to believe the absence of strong shadowing and other cold nuclear effects in d + Au collisions. However, people quickly realized that physics in d + Au collision is much rich than they thought before. Results from BRAMHS [1] and PHENIX [2] on high pT particle production in d + Au collisions at large rapidity have shown a significant suppression (enhancement) in particle
*
Tel.: +1 505 667 7125; fax: +1 505 665 7920. E-mail address: [email protected]
0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2007.04.255
yield in the forward (backward) rapidity in d + Au collisions where ‘‘forward’’ is defined by the deuteron going direction in the laboratory frame (center of mass frame at RHIC). To quantify such nuclear medium effects, people normally use the nuclear modification factor, which is defined as the particle yield per nucleon–nucleon collisions relative to the yield in the baseline elementary p + p collisions. RdA ðpT ; gÞ ¼
d2 rdþAu dpT dg 2 pþp
N dþAu ddpr dg coll
:
T
This comparison with p + p results is based on the assumption that the production of high transverse momentum particles scales with the number of binary collisions N dþAu in coll the initial states, where N dþAu is estimated to be 7 for coll minimum bias d + Au collisions. Without any nuclear medium effects, RdA(pT, g) 1. It is also important to note that in general, particles produced at a large forward (backward) rapidity are mostly likely coming from collisions of partons of small (large) ‘‘x’’ values inside the Au nucleus in d + Au collisions. Figs. 1–3 show RdA of light and heavy hadrons as a function of transverse momentum pT and rapidity measured by the BRAHMS and PHENIX experiments at RHIC at central and forward rapidities.
1062
M.X. Liu / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1061–1062
Fig. 1. Charged hadron RdA as a function of pT at central and forward pseudo-rapidities measured by the BRAHMS experiment at RHIC. Particle production at the forward rapidity is suppressed.
Fig. 3. RdA as a function of pT for prompt muons (mostly from heavy quark decay) at the forward and backward rapidities measured by the PHENIX experiment at RHIC. Results are very similar to what have been observed in high hadron sector.
Fig. 2. RdA as a function of pseudo-rapidity of high pT charged hadrons and J/W measured by the PHENIX experiment at RHIC. Particle yields are suppressed (enhanced) at the forward (backward) rapidity.
pression pattern. To understand the experimental data, several models have been implemented to study light hadron and charm yields at the forward rapidity, including gluon shadowing model [3], color glass condensate [4], coherent multiple scattering and initial state parton energy loss [5] and final state recombination [6]. The current data are statistically limited to distinguish different models. More precise d + Au measurements in various channels are needed to understand the cold nuclear medium effects as a baseline for understanding the hot dense matter produced in Au + Au collisions. References
These results have generated much interest and led to intense debate on the underlying physics of the forward rapidity processes in d + Au collisions at RHIC. The conventional models of multiple scattering based on Glauber theory predicted an enhancement of Cronin effect in the forward rapidity region, while the data clearly show a sup-
[1] [2] [3] [4] [5] [6]
BRAHMS Collaboration, Phys. Rev. Lett. (2004) 242303. PHENIX Collaboration, Phys. Rev. Lett. 94 (2005) 082302. K.J. Eskola, V.J. Kolhinen, R. Vogt, Nucl. Phys. A 696 (2001) 729. L. McLerran, R. Venugopalan, Phys. Rev. D 49 (1994) 2233. I. Vitev, M.B. Johnson, J.W. Qiu hep/0605200. R.C. Hwa, C.B. Yang, R.J. Fries, Phys. Rev. C 71 (2005) 024902.