摘要
作为基本粒子的夸克和胶子间的相互作用可以由量子色动力学(QCD)来描述。胶子是部分子(包含夸克和胶子)间强相互作用的携带者,强相互作用由色荷来进行特征化描述。被携带色荷的胶子禁锢的夸克组成了色中性的质子,中子和其他强子(例如π介子)。胶子的质量为零,质子中的夸克质量总和只占其质量的-小部分。这就意味着胶子参与的强相互作用在质子的质量中扮演了很重要的角色。质子(或原子核)的基本组分可由高能物理实验来探测。例如,深度非弹性散射实验(DIS)加速一个轻子或中微子穿越一个质子(或原子核)来探测其中携带纵向动量XBJ和分辨标度Q2。强子(轻子)—强子的硬散射过程可以因子化成部分子分布函数,部分子散射过程以及部分子碎裂函数的卷积。
我们已经对夸克的部分子分布函数有所了解,而胶子的部分子分布函数是由结构函数F2(xBJ,Q2)推导出来的。当Q2的值固定时,胶子的部分子分布函数随χBJ的减少而迅速增加[1,2]。当χBJ减少时,一个胶子会分裂成两个胶子而夸克也会辐射出胶子(该过程就是gluon emission),这些过程导致胶子的部分子分布函数的迅速增长。因为散射振幅要遵守归一定律,所以胶子的部分子分布函数不能无限增长。当胶子的分裂过程与胶子的重组过程(gluon recombination)互相平衡时,质子中的胶子就达到gluon saturation的状态。目前我们还不清楚在小χBJ区域的原子核部分子函数的分布,已有的固定原子靶DIS实验只能推导得到χBJ>0.02区域的原子核的胶子部分子分布函数。RHIC前向粒子的测量可以探测到更小χBJ的胶子。与质子质子(P+p)碰撞相比,STAR实验测量到的前向中性π介子在氘核金核(d+Au)碰撞的产额压低与色玻璃凝聚态(Color Glass Condensate,简称CGC)的描述一致。然而,单粒子末态只能测量一个相对宽泛的χBJ区间内的积分。领头粒子在前向快度区的双强子方位角关联可以测量一个特定小χBJ区。为了探测金核胶子分布函数在小：χBJ区的分布,我们研究了d+Au碰撞中的双强子方位角关联。
RHIC对撞机的STAR实验具有跨越赝快度区-1<η<4将近连续的方位角接收度的电磁量能器系统。本论文涉及的数据分析主要研究RHIC run8运行中质心系能量√s=200GeV的质子质子(p+1))和氘核金核(d+Au)碰撞中,在前向的介子光谱仪(Forward Meson Spectrometer,简称FMS,其覆盖2.5<η<4.0)重建的领头中性π介子和在端盖电磁量能器(Endcap ElectorMagnetic Calorimeter,简称EEMC,其覆盖1.083<η<2.0)重建的中性π介子或jet-like cluster的方位角关联。P+P碰撞中的结果做为氘核金核碰撞中的双强子关联的参考。在d+Au散射中,氘核(d)中大x价夸克可以测量高密度金核中的小x胶子,该过程产生的末态强子可以被FMS和EEMC探测器测量到。FMS-EEMC双强子关联可以测量在0.003<χBJ<0.02范围的金核胶子分布函数。
经过对采集数据的质量保证(Quality Assurance)检查,我开发了一个由能量阈值环绕的cluster finder来寻找EEMC探测器中的光子信号。使用该cluster finder得到的前向-中心快度区π0-π0关联和已发表的结果一致[3],这就证实了该cluster finder的可靠性。我们首先研究了FMS π0-EEMC π0之间的方位角关联[4]。但是FMS π0-EEMC π0的数据统计量不够,考虑到初态的部分子运动方向与散射后产生的强子末态的种类无关,喷注(jet)相对于单强子来说可以更直接的探测初态部分子。因此,我们基于jet cone半径算法在EEMC中重建jet-likecluster。在本论文,我们将详细讨论对EEMC重建的jet-like cluster去除探测器接受度的效应以及如何抑制underlying event对jet-like cluster的影响。我们也将介绍该双强子方位角关联的横动量和碰撞中心度的依赖。为了理解关联峰下部的背景,在d+Au碰撞中通过标记氘核束流方向的中子来近似把该碰撞事件看做质子金核(p+Au)碰撞。本文也讨论了p+Au和d+Au碰撞中FMS-EEMC双强子方位角关联结果的比较。在经过一系列的系统检查后,我们发现FMS π0-EEMC jet-like cluster的背对背方位角关联在d+Au碰撞中的宽度比p+p碰撞中更宽,p+p与d+Au方位角关联的宽度差与underlying event对jet-like cluster的贡献无关。FMS-EEMC方位角关联探测到的是前向—中心快度双强子关联和前向—前向双强子关联测量结果的中间x范围。在STAR实验研究的前向触发的双强子关联证明了从疏松部分子气体到致密CGC态的转换过程是平滑的。
The constituents of the nucleon are discovered to be quarks and gluons. The dynamics of quarks and gluons are described by Quantum Chromodynamics (QCD). The gluons are the force carrier of the strong interaction between partons (quarks and gluons), and the strong force is characterized by color charge. The confined quarks together form protons, neutrons and other hadrons (e.g. pion). The mass of gluons is zero; the sum of quark mass inside a proton only occupies a small fraction of the proton mass. This means the strong interactions propagated by the gluons are predicted to play an important role in forming the mass of a proton. The constituents of a proton can be probed via high energy experiments. For example, Deep Inelastic Scattering (DIS) experiments scatter a lepton or neutrino off a proton (or nucleus) to probe partons with longitudinal momentum fraction xBJ at resolution scale Q2. The hard scattering processes can be factorized into convolutions between parton distribution function, parton scattering and parton fragmentation functions.
The quark distribution function is well determined, while the gluon distribution function derived from the structure function F2(xBJ,Q2) increases rapidity as xBJ decreases at fixed Q2. As xBJ decreases, the proton gluon density increases rapidly as a gluou splits into two gluons or a quark can emit a gluon (gluon emission). The gluon distribution can not increase indefinitely due to the unitarity of the scattering amplitude. When the gluon splitting processes balances the gluon recombination processes, saturation is expected. The nuclear parton distribution is little known at low xBJ region as the current fixed nucleus target DIS experiment only provides constraints to the nuclei gluon distribution function at xBJ>0.02. The forward particle production at RHIC can probe low xBJ gluons. Forward inclusive neu-tral pion production measured at STAR experiment is found to be suppressed in d+Au collisions compared to p+p collisions, which is consistent with a Color Glass Condensate (CGC) description. However, inclusive production is a measure of the integral on a broad range of xBJ value. To select a certain low xBJ region, di-hadron azimuthal correlations with the leading particle triggered in the forward rapidity are further studied in d+Au collisions to probe the gluon distribution function of Au nuclei.
The STAR experiment at RHIC has a nearly continuous electromagnetic system spaning pseudo-rapidity-1<η<4with full azimuthal angle coverage. The anal-ysis in this thesis focuses on the azimuthal correlations between a leading neutral pion triggered in the Forward Meson Spectrometer (FMS,2.5<η<4.0) and an associated neutral pion or jet-like cluster measured in the Endcap ElectorMagnetic Calorimeter (EEMC,1.083<η<2.0) during RHIC run8p+p collisions and d+Au collisions at (?)=200GeV. The correlation studies in p+p collisions are taken as reference for d+Au results. The low x gluons in the dense gold nuclei are scattered by the deuteron nuclear probe, and their fragments can be measured by the FMS and EEMC detectors. The FMS-EEMC correlations provide sensitivity for the gold nuclei gluon distribution function within0.003 A threshold bounded cluster finder is developed to search the photon signal in the EEMC. We first looked at the FMS π0-EEMC π0azimuthal correlations [4]. The statistics of FMS π0-EEMC π0are low. The direction of the initial partons does not rely on the type of final state particles. For this matter, jets are considered as more direct probes than inclusive hadrons. We use the EEMC to reconstruct jet-like clusters based on cone radius algorithm. Details of excluding detector accep-tance effects, suppressing the underlying event contributions to the jet-like cluster are studied. The transverse momentum and collision centrality dependence of the azimuthal correlations are also studied. To understand the pedestal underneath the correlation peak, a p+Au collision is approximated by requiring a neutron to be observed in the deuteron beam direction in d+Au collisions. The comparison between the FMS-EEMC correlation in p+Au collisions and d+Au collisions will be discussed as well.
After a series of systematic check, the back-to-back azimuthal correlations of the FMS π0-EEMC jet-like clusters are found to be broader in d+Au collisions than in that p+p collisions. The width differences between p+p and d+Au colli-sions are not dependent on the underlying event contributions to the jet-like clus-ters. The FMS-EEMC azimuthal correlations probe the intermediate x region for nuclei gluon distribution function the forward+mid-rapidity correlations and the forward+forward correlations. The forward di-hadron correlation studies at STAR prefers a smooth transition from dilute parton gas to CGC state.
引文
[1]C.Adloff, et al., Eur. Phys. J. C 21,33 (2001) 1,9,23
[2]S. Chekanov, et al., Eur. Phys. J. C 21,443 (2001) 1,9,23
[3]E. Braidot, Ph.D. thesis, The University of Netherlands (2011), nucl-ex/ 1102.0931 1,7,25,43,49,50,54,56,109
[4]X. Li, for the STAR Collaboration, J.Phys.Conf.Ser.316,012002 (2011), hep-ex/1106.0621 1,4,7,57
[5]V. N. Gribov, L. X. Lipatov, Sov. J. Nucl. Phys.15,438 (1972) 6,15
[6]L. N. Lipatov, Sov. J. Nucl. Phys.20,95 (1975) 6,15
[7]G. Altarelli, G. Parisi, Nucl. Phys. B 126,298 (1977) 6,15
[8]Y. L. Dokshitzer, Sov. Phys.JETP 46, G41 (1977) 6,15
[9]Y. Balitsky, L. Lipatov, Sov. Phys. JETP 28,822 (1978) 6,15
[10]E. Kuraev, L. Lipatov, V. Fadin, Sov. Phys. JETP 45,199 (1977) 6,15
[11]L. Lipatov, Sov. Phys. JETP 63,904 (1986) 6,15
[12]E. Rizvi, Talk presented at the International Euro Physics Conference on High Energy Physics (2003) 6,12,14
[13]E. Braidot, Nucl. Phys. A 854,168 (2011) 6,25,109
[14]E. Braidot, Suppression of Forward Pion Correlations in d+Au Interactions at STAR (2010), hep-ph/1005.2378 7,25,26,100,101,109
[15]M. Peskin, D.Schroeder, An introduction to Quantum Field Theory (Addison-Wesley,1995) 9
[16]R. D. Field, D. Pines, Applications Of Perturbative Qcd (Addison-Wesley,1995) 9
[17]J. Pumplin, A. Belyaev, J. Huston, et al.,JHEP 0602,032 (2006) 9,15
[18]G. Dissertori, I. Knowles, M. Schmelling, Quantum, Chromodynamics, High Energy Experiments and Theory (Oxford Science Publications,2003) 9
[19]J. D. Bjorken, Phys. Rev.179,1547 (1969) 10
[20]J. C. Collins, D. E. Soper, G. Sterman, Adv.Ser.Direct.High Energy Phys 5,1 (1988), hep-ph/0409313 10
[21]M. Froissart, Phys. Rep.123,1053 (1983) 10,16
[22]K. Nakamura, et al., Journal of Physics G 37,075021 (2010) 11,45,46
[23]M. Gluck, E. Hoffmann, E. Reya, Z. Phys. C 13,119 (1982) 13
[24]L. C. Bland, et al., Eur. Phys. J. C 43,427 (2005), hep-ex/050204014,15, 25,35
[25]F. Gelis, E. Iancu, J. J. Marian, et al. (2010), hep-ph/1002.0333 15
[26]L. Gribov, E. Levin, M. Ryskin, Phys. Rep.100,1 (1983) 16
[27]A. H. Mueller, J. Qiu, Nuclear Physics B 268,427 (1986) 16
[28]Y. V. Kovchegov, Phys. Rev. D 60,034008 (2011), hep-ph/990128 16
[29]L. Gribov, E. Levin, M. Ryskin, Nucl. Phys. B 188,555 (1981) 16
[30]K. Golec-Biernat, M. Wusthoff, Phys. Rev. D 59,014017 (1999) 16,17,18
[31]N. Nikolaev, B. Zakharov, Z. Phys. C 49,607 (1991) 17
[32]N. Nikolaev, B. Zakharov, Z. Phys. C 53,331 (1992) 17
[33]A. H. Mueller, Nucl. Phys. B 415,373 (1994) 17
[34]A. H. Mueller, Nucl. Phys. B 437,107 (1995) 17
[35]A. Mueller, Nucl. Phys. B 335,115 (1990) 18
[36]L. McLerran, R. Venugopalan, Phys. Rev. D 49,2233 (1994) 18
[37]L. McLerran, R. Venugopalan, Phys. Rev. D 49,3352 (1994) 18
[38]J. Pumplin, D. Stump,J. Huston, et al.,JHEP 0207,012 (2002) 18
[39]A. Martin, R. Roberts, W. Stirling, et al., Eur. Phys. J C 28,455 (2003) 18
[40]A. Bohr, B. R. Mottelson, Nuclear Structure (Benjamin, New York,1969) 18
[41]H. Kowalski, D. Teaney, Phys. Rev. D 68,114005 (2003), hep-ph/0304189 18
[42]M. Gyulassy, I. Vitev, X.-N. Wang, et al., Quark Gluon Plasma 3 123-191 (2003), nucl-th/0302077 19
[43]L. McLerran, R. Venugopalan, Phys. Rev. D 49,2233 (1994) 19
[44]L. McLerran, R. Venugopalan, Phys. Rev. D 49,3352 19
[45]L. McLerran, R. Venugopalan, Phys. Rev. D 50,2225 (1994) 19
[46]L. McLerran, Lect.Notes Phys. 583, 291 (2002), hep-ph/0104285 19, 20
[47]F. Gelis, E. Iancu, J. Jalilian-Marian, et al., Ann.Rev.Nucl.Part.Sci. 60, 463 (2010), hep-ph/1002.0333 19, 110
[48]Y. V. Kovchegov, A. H. Mueller, Nucl. Phys. B 529, 451 (1998) 20
[49]A. Dumitru, L. D. McLerran, Nucl. Phys. A 700, 492 (2002) 20
[50]J. Jalilian-Marian, A. Kovner, A. Leonidov, et al., Nucl. Phys. B 504, 415 (1997) 21
[51]J. Jalilian-Marian, A. Kovner, H. Weigert, Phys. Rev. D 59, 014015 (1999) 21
[52]A. Kovner, G. Milhano, H. Weigert, Phys. Rev. D 62, 114005 (2000) 21
[53]E. Ferreiro, E. Iancu, A. Leonidov, et al., Nucl. Phys. A 703, 489 (2002) 21
[54]K. Itakura, Color Glass Condensate and UHECR physics 21
[55]K. Golec-Biernat, J. Phys. G 28, 1057 (2002), hep-ph/0109010 22
[56]A. M. Stasto, K. G. Bicrnat, J. Kwiccinski. Phys. Rev. Lett. 86, 596 (2001) 22
[57]A. Freund, K. R.ummukainen, H. Weigert, et al., Phys. R.ev. Lett. 90, 222002 (2003) 22. 23
[58]E. Iancu, K. Itakura, L. McLerran, Nucl.Phys. A 708, 327 (2002), hep-ph/ 0203137 23
[59]I. Arsene, et al., Phys. Rev. Lett. 93, 242303 (2004) 23
[60]J.Adams, et al., Phys. Rev. Lett. 97, 152302 (2006) 24, 25
[61]A. Dumitru, A. Hayashigaki, J. Jalilian-Marian, Nucl. Phys. A 765, 464 (2006) 24
[62]V. Guzey, M. Strikman, W. Vogelsang, Phys. Lett. B 603, 173 (2004) 24, 25
[63]J. Qiu, I. Vitev, Phys. Rev. Lett 93, 262301 (2004) 25
[64]J. Qiu, I. Vitev, Physics Letters B 632, 507 (2006) 25
[65]D. Kharzeev, E. Levin, L. McLerran, Nucl. Phys. A 748, 627 (2005) 25
[66]C. Marquet, Nucl. Phys. A 796, 41 (2007) 26
[67]C. M. Javier L. Albacete, Phys. Rev. Lett. 105, 162301 (2010), hep-ph/1005. 4065v2 25
[68]A. Stasto, B.-W. Xiao, F. Yuan (2011), hep-ph/1109.1817v2 26
[69]K. Tuchin, Nucl. Phys. A 846,83 (2010) 26
[70]K. H. Ackermann, et al., Nucl. Instr, Meth. Res. A 499,2 (2003) 28
[71]K. Adcox, et al., Nucl. Instr. Meth. Res. A 499,2 (2003) 28
[72]T. Sakuma, M. Walker (2010), hep-ex/1012.117329
[73]M. Anderson, et al., Nucl. Instrum. Meth. A 499,659 (2003) 30
[74]M. Beddo, et al., Nucl. Instrum. Meth. A 499,725 (2003) 31
[75]C. E. Allgower, et al., Nucl. Instrum. Meth. A 499,740 (2003) 33
[76]J. K. for the STAR Collaboration (2005), hep-ex/0501072 38
[77]C. Adler, et al., Nucl. Instrum. Meth. A 470,488 (2001), nucl-ex/0008005v1 39
[78]J. Morris, S. Binello, T. Clifford, et al.,10th ICALEPCS Int. Conf. on Accel-erator and Large Expt. Physics Control Systems proceedings (2005) 41
[79].1. M. Landgraf, M. J. LeVine, A. Ljubicic, et al., Nucl. Instr. Meth. Phys. Res. A 499,762 (2003) 41
[80]F. S. Bieser, H. J. Crawford, J. Engelage, et al., Nucl. Instr. Meth. Phys. Res. A 499,766 (2003) 41
[81]S. van der Meer, CERN-ISR-PO 68,31 (1968) 43
[82]A. Drees, Z. Xu p.3120 (2001) 44
[83]T. Sjostrand, L. Lonnblad, S. Mrenna (2001), hep-ph/0108264 45,50
[84]X. Wang, M. Gyulassy, Phys. Rev. D 44,3501 (1991) 45,50
[85]A. Lednev, Nucl. Instr. Meth. Phys. Res. A 366,292 (1995) 46
[86]Y. Wang, Measurement of Inclusive Forward Neutral Pion Production in 200 GeV Polarized Proton-Proton Collisions at RHIC, Ph.D. thesis, The University of Texas at Austin (2004) 46,47,49
[87]C. P. L. L. Writeup, Detector Description and Simulation Tool (1993) 50
[88]R. Field, AIP Conf. Proc.828,163 (2006) 83,95
[89]M. Strikman, W. Vogelsang, Phys. Rev. D 83,034029 (2011), hep-ph/1009. 6123103,104,105,109,110
[90]E. Braidot, Nucl. Phys. A 830,603 (2009) 109
[91]A. Stasto, B. wen Xiao, F. Yuan (2011), hep-ph/1109.1817 110
[92]F. Dominguez, C. Marquet, B.-W. Xiao, et al., Phys.Rev.D 83,105005 (2011), 1101.0715 110
[93]D. Boer, et al., INT-PUB 11,034 (2011), nucl-th/1108.1713v2 110
[94]D. W. Sivers, Phys. Rev. D 41,83 (1990) 113
[95]D. W. Sivers, Phys. Rev. D 43,261 (1991) 113
[96].7. C. Collins, Nucl. Phys. B 396,161 (1993), hep-ph/9208213 113
[97]T. A. Armstrong, et al., Nucl. Instr.and Meth. A 406,227 (1998) 114
