Beam-Energy Dependence of Directed Flow of Λ, \barΛ, K^\pm, K^0_s and φ in Au+Au Collisions
STAR Collaboration, L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, M. M. Aggarwal, Z. Ahammed, N. N. Ajitanand, I. Alekseev, D. M. Anderson, R. Aoyama, A. Aparin, D. Arkhipkin, E. C. Aschenauer, M. U. Ashraf, A. Attri, G. S. Averichev, X. Bai, V. Bairathi, K. Barish, A. Behera, R. Bellwied, A. Bhasin, A. K. Bhati, P. Bhattarai, J. Bielcik, J. Bielcikova, L. C. Bland, I. G. Bordyuzhin, J. Bouchet, J. D. Brandenburg, A. V. Brandin, D. Brown, J. Bryslawskyj, I. Bunzarov, J. Butterworth, H. Caines, M. Calderón de la Barca Sánchez, J. M. Campbell, D. Cebra, I. Chakaberia, P. Chaloupka, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, S. Chattopadhyay, X. Chen, X. Chen, J. H. Chen, J. Cheng, M. Cherney, W. Christie, G. Contin, H. J. Crawford, S. Das, T. G. Dedovich, J. Deng, I. M. Deppner, A. A. Derevschikov, L. Didenko, C. Dilks, X. Dong, J. L. Drachenberg, J. E. Draper, J. C. Dunlop, L. G. Efimov, N. Elsey, J. Engelage, G. Eppley, R. Esha, S. Esumi, O. Evdokimov, J. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, P. Federic, P. Federicova, J. Fedorisin, Z. Feng, P. Filip, E. Finch, Y. Fisyak, C. E. Flores, J. Fujita, L. Fulek, C. A. Gagliardi, F. Geurts, A. Gibson, M. Girard, D. Grosnick, D. S. Gunarathne, Y. Guo, A. Gupta, W. Guryn, A. I. Hamad, A. Hamed, A. Harlenderova, J. W. Harris, L. He, et al. (251 additional authors not shown)
aa r X i v : . [ h e p - e x ] J a n Version A4 (January 17, 2018)
Beam-Energy Dependence of Directed Flow of Λ , Λ , K ± , K s and φ in Au+AuCollisions L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, M. M. Aggarwal, Z. Ahammed, N. N. Ajitanand, I. Alekseev,
15, 26
D. M. Anderson, R. Aoyama, A. Aparin, D. Arkhipkin, E. C. Aschenauer, M. U. Ashraf, A. Attri, G. S. Averichev, X. Bai, V. Bairathi, K. Barish, A. Behera, R. Bellwied, A. Bhasin, A. K. Bhati, P. Bhattarai, J. Bielcik, J. Bielcikova, L. C. Bland, I. G. Bordyuzhin, J. Bouchet, J. D. Brandenburg, A. V. Brandin, D. Brown, I. Bunzarov, J. Butterworth, H. Caines, M. Calder´on de la Barca S´anchez, J. M. Campbell, D. Cebra, I. Chakaberia,
3, 18, 40
P. Chaloupka, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, S. Chattopadhyay, X. Chen, J. H. Chen, X. Chen, J. Cheng, M. Cherney, W. Christie, G. Contin, H. J. Crawford, S. Das, L. C. De Silva, T. G. Dedovich, J. Deng, A. A. Derevschikov, L. Didenko, C. Dilks, X. Dong, J. L. Drachenberg, J. E. Draper, L. E. Dunkelberger, J. C. Dunlop, L. G. Efimov, N. Elsey, J. Engelage, G. Eppley, R. Esha, S. Esumi, O. Evdokimov, J. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, P. Federic, P. Federicova, J. Fedorisin, Z. Feng, P. Filip, E. Finch, Y. Fisyak, C. E. Flores, J. Fujita, L. Fulek, C. A. Gagliardi, D. Garand, F. Geurts, A. Gibson, M. Girard, D. Grosnick, D. S. Gunarathne, Y. Guo, S. Gupta, A. Gupta, W. Guryn, A. I. Hamad, A. Hamed, A. Harlenderova, J. W. Harris, L. He, S. Heppelmann, S. Heppelmann, A. Hirsch, S. Horvat, X. Huang, B. Huang, T. Huang, H. Z. Huang, T. J. Humanic, P. Huo, G. Igo, W. W. Jacobs, A. Jentsch, J. Jia,
3, 42
K. Jiang, S. Jowzaee, E. G. Judd, S. Kabana, D. Kalinkin, K. Kang, D. Kapukchyan, K. Kauder, H. W. Ke, D. Keane, A. Kechechyan, Z. Khan, D. P. Kiko la, C. Kim, I. Kisel, A. Kisiel, L. Kochenda, M. Kocmanek, T. Kollegger, L. K. Kosarzewski, A. F. Kraishan, L. Krauth, P. Kravtsov, K. Krueger, N. Kulathunga, L. Kumar, J. Kvapil, J. H. Kwasizur, R. Lacey, J. M. Landgraf, K. D. Landry, J. Lauret, A. Lebedev, R. Lednicky, J. H. Lee, C. Li, X. Li, Y. Li, W. Li, J. Lidrych, T. Lin, M. A. Lisa, P. Liu, H. Liu, Y. Liu, F. Liu, T. Ljubicic, W. J. Llope, M. Lomnitz, R. S. Longacre, S. Luo, X. Luo, Y. G. Ma, L. Ma, R. Ma, G. L. Ma, N. Magdy, R. Majka, D. Mallick, S. Margetis, C. Markert, H. S. Matis, K. Meehan, J. C. Mei, Z. W. Miller, N. G. Minaev, S. Mioduszewski, D. Mishra, S. Mizuno, B. Mohanty, M. M. Mondal, D. A. Morozov, M. K. Mustafa, Md. Nasim, T. K. Nayak, J. M. Nelson, M. Nie, G. Nigmatkulov, T. Niida, L. V. Nogach, T. Nonaka, S. B. Nurushev, G. Odyniec, A. Ogawa, K. Oh, V. A. Okorokov, D. Olvitt Jr., B. S. Page, R. Pak, Y. Pandit, Y. Panebratsev, B. Pawlik, H. Pei, C. Perkins, P. Pile, J. Pluta, K. Poniatowska, J. Porter, M. Posik, N. K. Pruthi, M. Przybycien, J. Putschke, H. Qiu, A. Quintero, S. Ramachandran, R. L. Ray, R. Reed, M. J. Rehbein, H. G. Ritter, J. B. Roberts, O. V. Rogachevskiy, J. L. Romero, J. D. Roth, L. Ruan, J. Rusnak, O. Rusnakova, N. R. Sahoo, P. K. Sahu, S. Salur, J. Sandweiss, M. Saur, J. Schambach, A. M. Schmah, W. B. Schmidke, N. Schmitz, B. R. Schweid, J. Seger, M. Sergeeva, R. Seto, P. Seyboth, N. Shah, E. Shahaliev, P. V. Shanmuganathan, M. Shao, M. K. Sharma, A. Sharma, W. Q. Shen, S. S. Shi, Z. Shi, Q. Y. Shou, E. P. Sichtermann, R. Sikora, M. Simko, S. Singha, M. J. Skoby, N. Smirnov, D. Smirnov, W. Solyst, L. Song, P. Sorensen, H. M. Spinka, B. Srivastava, T. D. S. Stanislaus, M. Strikhanov, B. Stringfellow, A. A. P. Suaide, T. Sugiura, M. Sumbera, B. Summa, Y. Sun, X. M. Sun, X. Sun, B. Surrow, D. N. Svirida, Z. Tang, A. H. Tang, A. Taranenko, T. Tarnowsky, A. Tawfik, J. Th¨ader, J. H. Thomas, A. R. Timmins, D. Tlusty, T. Todoroki, M. Tokarev, S. Trentalange, R. E. Tribble, P. Tribedy, S. K. Tripathy, B. A. Trzeciak, O. D. Tsai, T. Ullrich, D. G. Underwood, I. Upsal, G. Van Buren, G. van Nieuwenhuizen, A. N. Vasiliev, F. Videbæk, S. Vokal, S. A. Voloshin, A. Vossen, G. Wang, Y. Wang, F. Wang, Y. Wang, J. C. Webb, G. Webb, L. Wen, G. D. Westfall, H. Wieman, S. W. Wissink, R. Witt, Y. Wu, Z. G. Xiao, G. Xie, W. Xie, J. Xu, Z. Xu, Q. H. Xu, Y. F. Xu, N. Xu, S. Yang, Y. Yang, C. Yang, Q. Yang, Z. Ye, Z. Ye, L. Yi, K. Yip, I. -K. Yoo, N. Yu, H. Zbroszczyk, W. Zha, Z. Zhang, J. B. Zhang, J. Zhang, S. Zhang, Y. Zhang, X. P. Zhang, J. Zhang, S. Zhang, J. Zhao, C. Zhong, C. Zhou, L. Zhou, X. Zhu, Z. Zhu, and M. Zyzak (STAR Collaboration) AGH University of Science and Technology, FPACS, Cracow 30-059, Poland Argonne National Laboratory, Argonne, Illinois 60439 Brookhaven National Laboratory, Upton, New York 11973 University of California, Berkeley, California 94720 University of California, Davis, California 95616 University of California, Los Angeles, California 90095 Central China Normal University, Wuhan, Hubei 430079 University of Illinois at Chicago, Chicago, Illinois 60607 Creighton University, Omaha, Nebraska 68178 Czech Technical University in Prague, FNSPE, Prague, 115 19, Czech Republic Nuclear Physics Institute AS CR, 250 68 Prague, Czech Republic Frankfurt Institute for Advanced Studies FIAS, Frankfurt 60438, Germany Institute of Physics, Bhubaneswar 751005, India Indiana University, Bloomington, Indiana 47408 Alikhanov Institute for Theoretical and Experimental Physics, Moscow 117218, Russia University of Jammu, Jammu 180001, India Joint Institute for Nuclear Research, Dubna, 141 980, Russia Kent State University, Kent, Ohio 44242 University of Kentucky, Lexington, Kentucky 40506-0055 Lamar University, Physics Department, Beaumont, Texas 77710 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000 Lawrence Berkeley National Laboratory, Berkeley, California 94720 Lehigh University, Bethlehem, Pennsylvania 18015 Max-Planck-Institut fur Physik, Munich 80805, Germany Michigan State University, East Lansing, Michigan 48824 National Research Nuclear University MEPhI, Moscow 115409, Russia National Institute of Science Education and Research, HBNI, Jatni 752050, India National Cheng Kung University, Tainan 70101 Ohio State University, Columbus, Ohio 43210 Institute of Nuclear Physics PAN, Cracow 31-342, Poland Panjab University, Chandigarh 160014, India Pennsylvania State University, University Park, Pennsylvania 16802 Institute of High Energy Physics, Protvino 142281, Russia Purdue University, West Lafayette, Indiana 47907 Pusan National University, Pusan 46241, Korea Rice University, Houston, Texas 77251 Rutgers University, Piscataway, New Jersey 08854 Universidade de Sao Paulo, Sao Paulo, Brazil, 05314-970 University of Science and Technology of China, Hefei, Anhui 230026 Shandong University, Jinan, Shandong 250100 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 State University of New York, Stony Brook, New York 11794 Temple University, Philadelphia, Pennsylvania 19122 Texas A&M University, College Station, Texas 77843 University of Texas, Austin, Texas 78712 University of Houston, Houston, Texas 77204 Tsinghua University, Beijing 100084 University of Tsukuba, Tsukuba, Ibaraki, Japan,305-8571 Southern Connecticut State University, New Haven, Connecticut 06515 University of California, Riverside, California 92521 United States Naval Academy, Annapolis, Maryland 21402 Valparaiso University, Valparaiso, Indiana 46383 Variable Energy Cyclotron Centre, Kolkata 700064, India Warsaw University of Technology, Warsaw 00-661, Poland Wayne State University, Detroit, Michigan 48201 World Laboratory for Cosmology and Particle Physics (WLCAPP), Cairo 11571, Egypt Yale University, New Haven, Connecticut 06520
Rapidity-odd directed flow measurements at midrapidity are presented for Λ, Λ, K ± , K s and φ at √ s NN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions recorded by theSolenoidal Tracker detector at the Relativistic Heavy Ion Collider. These measurements greatlyexpand the scope of data available to constrain models with differing prescriptions for the equationof state of quantum chromodynamics. Results show good sensitivity for testing a picture whereflow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspiredsum-rule can be a valuable new tool for probing the collision dynamics. PACS numbers: 25.75.Ld, 25.75.Dw
Rapidity-odd directed flow, v odd1 ( y ), is the first har-monic coefficient in the Fourier expansion of the final-state azimuthal distribution relative to the collision re-action plane [1], and describes a collective sideward mo-tion of emitted particles. The rapidity-even component v even1 ( y ) [2] is unrelated to the reaction plane in mass-symmetric collisions, and arises from event-by-event fluc-tuations in the initial nuclei. Hereafter, v ( y ) implicitlyrefers to the odd component. Both hydrodynamic [3] andnuclear transport [4] models indicate that v ( y ) is sensi-tive to details of the expansion during the early stages ofthe collision fireball [5, 6]. To integrate over the rapid-ity dependence, it is common practice to present dv /dy near midrapidity, as in the Solenoidal Tracker at RHIC(STAR) measurements for protons, antiprotons and pi-ons in Au+Au collisions at √ s NN = 7.7 to 200 GeV.Both protons and net protons show a minimum in dv /dy near √ s NN of 10 to 20 GeV [7]. Based on hydrody-namic calculations [8, 9], a minimum in directed flow hasbeen proposed as a signature of a first-order phase tran-sition between hadronic matter and quark-gluon plasma(QGP).There have been several recent v ( y ) model calcula-tions with various assumed quantum chromodynamics(QCD) equations of state [10–15]. The assumption ofpurely hadronic physics is disfavored, but there is noconsensus on whether STAR measurements [7] favor acrossover or first-order phase transition. Models do notproduce any dv /dy minimum over the observed energyrange [10–14, 16], with the exception of one case wherea minimum was calculated near one-third the energy ofthe measured minimum [15]. Moreover, predicted v isstrongly sensitive to model details unrelated to the as-sumed equation of state [17]. Thus, further progress inmodels is needed for a definitive interpretation.Number-of-constituent-quark (NCQ) scaling [18](whereby elliptic flow ( v ) behaves as if imposed at thelevel of deconfined constituent quarks) is an example ofcoalescence behavior among quarks. There is a historyof coalescence observations in heavy-ion collisions, in theformation of nuclei [19–22] as well as in the hadroniza-tion of quarks. The interplay between NCQ scaling andthe transport of initial-state u and d quarks towardsmidrapidity during the collision offers possibilities fornew insights [23]. However, this physics remains poorlyunderstood [24, 25], and these considerations motivate v n versus √ s NN measurements encompassing as manyparticle species as possible.We report the first measurements of directed flow ver-sus rapidity for Λ, Λ, φ , K ± and K s in Au+Au collisionsat eight beam energies √ s NN = 7 .
7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV, where the analyzed samples con-tain 4, 12, 20, 36, 70, 130, 50 and 250 million minimum-bias-trigger events, respectively. These data from theSTAR [26] located at Brookhaven National Laboratorywere recorded in 2010, 2011 and 2014. The STAR TimeProjection Chamber (TPC) [27] was used for charged-particle tracking within pseudorapidity | η | <
1. Thecentrality was determined from the number of chargedparticles within | η | < .
5. For determination of theevent plane [1], two Beam-Beam Counters (BBC, pseu-dorapidity coverage 3 . < | η | < . √ s NN ≤
39 GeV [7, 28], while the STAR Zero-Degree Calorimeter Shower-Maximum Detectors (ZDC-SMD, | η | > .
3) were used at √ s NN = 62 . √ s NN = 7 . p T > . /c ,have a distance of closest approach to the primary ver-tex of less than 3 cm, have at least 15 space points in theTPC acceptance ( | η | < . p T > . /c andmomentum < . /c are identified based on energyloss in the TPC and time-of-flight information from theTOF detector [33]. Λ, Λ and K s within 0 . < p T < . /c and φ within 0 . < p T < . /c are selectedby standard V0 topology cuts using the invariant masstechnique with mixed-event background subtraction [34].Systematic uncertainties arising from event-plane es-timation in essentially the same v analysis for differentspecies are discussed elsewhere [7]. Non-flow is a sourceof possible systematic error that refers to azimuthal cor-relations unrelated to the reaction plane orientation, aris-ing from resonances, jets, strings, quantum statistics andfinal-state interactions like Coulomb effects. Possiblenon-flow effects are reduced due to the sizable pseudora-pidity gap between the TPC and the BBC or ZDC-SMDdetectors [1]. We have studied the sensitivity of dv /dy to all experimental cuts and selections, for both eventsand tracks, and inferred systematic errors are plotted inFigs. 2 and 3.Figure 1 presents v ( y ) at 10-40% centrality for K ± , K s , φ , Λ and Λ. These measurements complementthe corresponding published information for protons, an-tiprotons and charged pions [7]. In the referenced v study, the overall strength of the directed flow signal near − − × Λ − − − − − − − − − − − − − − − − − − − − − − −
10 - 40% Au+Au Λ Λ − + K − Ks K − φ = 7.7 NN s 11.5 14.5 19.6 27 39 62.4 200 GeV d i r e c t ed f l o w rapidity FIG. 1. (Color online) Directed flow as a function of rapidity for the six indicated particle species in 10-40% central Au+Aucollisions at √ s NN = 7 . v scale with the exception of Λ at √ s NN = 7 . v magnitudes areexceptionally large and require the measurements to be divided by five. Examples of linear fits to v ( y ) are shown in the caseof Λ and φ . midrapidity was characterized by the linear term F in afit of the form v ( y ) = F y + F y [7]. This cubic fitreduces sensitivity to the rapidity range over which thefit is performed, but becomes unstable for low statistics,as is now the case for φ and Λ, and to a lesser extentfor Λ. Accordingly, the present analysis uses a linearfit for all particle species at all beam energies. The fitis over | y | < . φ and over | y | < . v ( y ) over the accep-tance of the STAR detector. However, protons [7] showsystematic deviations from linearity and hence the pro-ton dv /dy | y =0 is marginally affected by changing the fitmethod. Hereafter, dv /dy refers to the slope obtainedfrom the above linear fits.Directed flow slope dv /dy versus beam energy for p , p , Λ, Λ, φ , K ± , K s , and π ± is presented in Figs. 2(a)and 2(b). The proton and pion points in Fig. 2 differslightly from those in Ref. [7] in that a new measurementat √ s NN = 14 . dv /dy for Λ and p agree within errors, and the Λ slope changes sign inthe same region as protons (near √ s NN = 11 . √ s NN = 15 to 20 GeV also occurs for Λ. Second, dv /dy for K + and K − are both negative at all energies andare close to each other except at the lowest energy, while dv /dy for K s is everywhere consistent within errors withthe average of K + and K − . It was found previouslythat dv /dy for π + and π − is likewise close over theseenergies and is always negative. Third, the slope for Λ is negative throughout and is consistent within errors with p [7]. Fourth, at √ s NN = 14 . φ slope has much larger magnitude than other mesons(pions and kaons) and is close to p and Λ. At √ s NN =11 . dv /dy for φ increases steeply, althoughthe statistical significance of the increase is poor. The φ -meson v statistics are too marginal to permit a reliabledetermination of dv /dy at √ s NN = 7 . p , Λ and K + receive more contributionsfrom transported quarks ( u and d from the initial-statenuclei) than their antiparticles [23]. “Net particle” rep-resents the excess yield of a particle species over its an-tiparticle. In order to enhance the contribution of trans-ported quarks relative to those produced in the collision,we define v p based on expressing v ( y ) for all protonsas v p = r ( y ) v p + [1 − r ( y )] v p , where r ( y ) is the ratio of observed p to p yield at eachbeam energy. Corrections of r ( y ) for reconstruction inef-ficiency and backgrounds were found to have a negligibleeffect on the net-proton dv /dy and have not been ap-plied. Figure 2(c) presents net-proton dv /dy , and alsoincludes net-Λ and net-kaon dv /dy , defined similarly,except p ( p ) becomes Λ (Λ) and K − ( K + ), respectively.The ten particle species available in the present anal-ysis allow a more detailed investigation of constituent-quark v than was possible in Ref. [7]. We are now ina position to test a set of assumptions, namely that v is imposed at the pre-hadronic stage, that specific typesof quark have the same directed flow, and that the de-tected hadrons are formed via coalescence [18, 23]. In ascenario where deconfined quarks have already acquired
10 100 − − (a) p p Λ Λ φ
10 - 40% Au+Au − − (b) + K − K s0 K + π − π
10 1000.02 − (c) net p Λ net net K = y | y / d v d (GeV) NN s FIG. 2. (Color online) Directed flow slope ( dv /dy ) versusbeam energy for intermediate-centrality (10-40%) Au+Au col-lisions. Panel (a) presents heavy species: Λ, Λ, protons, an-tiprotons and φ , while panel (b) presents K ± , K s and π ± .Note that dv /dy for Λ at √ s NN = 7 . − . ± . ± .
026 (sys), which is far below the bottom of theplotted scale. The φ -meson result at √ s NN = 62 . azimuthal anisotropy, and in the limit of small azimuthalanisotropy coefficients v n , coalescence leads to the v n ofthe resulting mesons or baryons being the summed v n oftheir constituent quarks [23, 35]. We call this assumptionthe coalescence sum rule. NCQ scaling in turn followsfrom the coalescence sum rule [23]. Note that no weightsare involved in coalescence sum rule v calculations, un-like the case of v for net particles.Antiprotons and Λs are seen to have similar v ( y ), andit is noteworthy that these species are composed of threeconstituent quarks all produced in the collision, as op-posed to being composed of u or d quarks which couldbe either transported from the initial nuclei or produced.To test the coalescence sum rule in a straightforward casewhere all quarks are known to be produced, Fig. 3(a)compares the observed dv /dy for Λ( uds ) with the calcu-lation for K − ( us ) + p ( uud ). This calculation is based − − − − Λ )uud (p 31s) + u ( − K
10 - 40% Au+Au (a) − − − (GeV) NN s Λ net + sp 31 − net p net p + s31 − net p )p 31 − − (s = K (b) = y | y / d v d FIG. 3. (Color online) Directed flow slope ( dv /dy ) versus √ s NN for intermediate centralities (10-40%). Panel (a) com-pares the observed Λ slope with the prediction of the coa-lescence sum rule for produced quarks. The inset shows thesame comparison where the vertical scale is zoomed-out; thisallows the observed flow for the lowest energy ( √ s NN = 7 . on the coalescence sum rule combined with the assump-tion that s and s quarks have the same flow, and that u and d have the same flow. The factor arises from as-suming that all u and d quarks contribute the same flow.Close agreement is observed at √ s NN = 11 . √ s NN = 7 . v has been observed in the same energyregion [34, 36].Next, we turn our attention to the less straightforwardcase of coalescence involving u and d quarks. We ex-pect v to be quite different for transported and producedquarks, which are difficult to distinguish in general. How-ever, in the limit of low √ s NN , most u and d quarks arepresumably transported, while in the limit of high √ s NN ,most u and d are produced. In Fig. 3(b), we test two coa-lescence sum rule scenarios which are expected to bracketthe observed dv /dy for a baryon containing transportedquarks. The fraction of transported quarks among theconstituent quarks of net particles is larger than in par-ticles roughly in proportion to N particle /N net particle [37],and therefore we employ net-Λ and net-proton v in thesetests.Figure 3(b) presents the observed dv /dy for netΛ( uds ). The first compared calculation (red diamondmarkers) consists of net protons ( uud ) minus u plus s ,where u is estimated from p , while the s quark flowis obtained from K − ( us ) − p ( uud ). There is no cor-responding clear-cut expression for transported u and d quarks. Here, it is assumed that a produced u quark innet p is replaced with an s quark. This sum-rule cal-culation agrees closely with the net-Λ measurement at √ s NN = 19 . u quark is removedby keeping the term (net p − p ).The second coalescence calculation in Fig. 3(b) cor-responds to net proton plus s (blue circle markers).In this case, it is assumed that the constituent quarksof net protons are dominated by transported quarks inthe limit of low beam energy, and that one of the trans-ported quarks is replaced by s . This approximationbreaks down as the beam energy increases, and thereis disagreement between the black stars and blue circlesabove √ s NN = 7 . √ s NN = 62 . v ( y ) for Λ, Λ, φ , K ± and K s at eight √ s NN values spanning 7.7 to 200 GeV. Wefocus on dv /dy at midrapidity for 10-40% centrality.The directed flow slopes as a function of beam energyfor protons and Λs agree within errors, and change signnear 11.5 GeV. Antiprotons, Λ, kaons and pions havenegative dv /dy throughout the studied energy range.Net-particle dv /dy for p , Λ and K agree at and above √ s NN = 14 . s and s quarks havethe same flow, or a breakdown of the assumption that u and d have the same flow. The energy-dependent mea-surements reported here will be enhanced after STARacquires greatly increased statistics using upgraded de-tectors in Phase-II of the RHIC Beam Energy Scan [25]. We thank the RHIC Operations Group and RCF atBNL, the NERSC Center at LBNL, and the Open ScienceGrid consortium for providing resources and support.This work was supported in part by the Office of Nu-clear Physics within the U.S. DOE Office of Science, theU.S. National Science Foundation, the Ministry of Edu-cation and Science of the Russian Federation, NationalNatural Science Foundation of China, Chinese Academyof Science, the Ministry of Science and Technology ofChina and the Chinese Ministry of Education, the Na-tional Research Foundation of Korea, GA and MSMT ofthe Czech Republic, Department of Atomic Energy andDepartment of Science and Technology of the Govern-ment of India; the National Science Centre of Poland, Na-tional Research Foundation, the Ministry of Science, Ed-ucation and Sports of the Republic of Croatia, RosAtomof Russia and German Bundesministerium f¨ur Bildung,Wissenschaft, Forschung and Technologie (BMBF) andthe Helmholtz Association. [1] J.-Y. Ollitrault, Phys. 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