Directed flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb collisions at √(s(NN))=2.76 TeV

Polarization of Λ and Λ[over ¯] Hyperons along the Beam Direction in Pb-Pb Collisions at sqrt[s]_{NN}=5.02 TeV

The polarization of the Λ and Λ[over ¯] hyperons along the beam (z) direction, P_{z}, has been measured in Pb-Pb collisions at sqrt[s_{NN}]=5.02  TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P_{z} comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P_{z,s2}=⟨P_{z}sin(2φ-2Ψ_{2})⟩, where φ is the hyperon azimuthal emission angle and Ψ_{2} is the elliptic flow plane angle. We report the measurement of P_{z,s2} for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P_{z,s2} is positive similarly as measured by the STAR Collaboration in Au-Au collisions at sqrt[s_{NN}]=200  GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P_{z} in Pb-Pb collisions at the LHC. The comparison of the measured P_{z,s2} with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.

Probing the Effects of Strong Electromagnetic Fields with Charge-Dependent Directed Flow in Pb-Pb Collisions at the LHC

The first measurement at the LHC of charge-dependent directed flow (v_{1}) relative to the spectator plane is presented for Pb-Pb collisions at sqrt[s_{NN}]=5.02  TeV. Results are reported for charged hadrons and D^{0} mesons for the transverse momentum intervals p_{T}>0.2  GeV/c and 3<p_{T}<6  GeV/c in the 5%-40% and 10%-40% centrality classes, respectively. The difference between the positively and negatively charged hadron v_{1} has a positive slope as a function of pseudorapidity η, dΔv_{1}/dη=[1.68±0.49(stat)±0.41(syst)]×10^{-4}. The same measurement for D^{0} and D[over ¯]^{0} mesons yields a positive value dΔv_{1}/dη=[4.9±1.7(stat)±0.6(syst)]×10^{-1}, which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Δv_{1} of charged hadrons and D^{0} mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of dΔv_{1}/dη for both light flavor and charmed hadrons.

Vorticity and particle polarization in heavy ion collisions (experimental perspective)

The recent measurements of the global polarization and vector meson spin alignment along the system orbital momentum in heavy ion collisions are briefly reviewed. A possible connection between the global polarization and the chiral anomalous effects is discussed along with possible experimental checks. Future directions, in particular those aimed on the detailed mapping of the vorticity fields, are outlined. The Blast Wave model is used for an estimate of the anisotropic flow effect on the vorticity component along the beam direction. We also point to a possibility of a circular pattern in the vorticity field in asymmetric, e.g. Cu+Au, central collisions.

Charge-Dependent Directed Flow in Cu+Au Collisions at sqrt[s_{NN}]=200 GeV

We present the first measurement of charge-dependent directed flow in Cu+Au collisions at sqrt[s_{NN}]=200  GeV. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics model, which suggests that most of the electric charges, i.e., quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, 1  fm/c.

Denis RD, Staerz S, Stahlman J, Stamen R, Stamm S, Stanecka E, Stanek RW, Stanescu C, Stanescu-Bellu M, Stanitzki MM, Stapnes S, Starchenko EA, Stark J, Staroba P, Starovoitov P, Staszewski R, Stavina P, Steinberg P, Stelzer B, Stelzer HJ, Stelzer-Chilton O, Stenzel H, Stern S, Stewart GA, Stillings JA, Stockton MC, Stoebe M, Stoicea G, Stolte P, Stonjek S, Stradling AR, Straessner A, Stramaglia ME, Strandberg J, Strandberg S, Strandlie A, Strauss E, Strauss M, Strizenec P, Ströhmer R, Strom DM, Stroynowski R, Stucci SA, Stugu B, Styles NA, Su D, Su J, Subramaniam R, Succurro A, Sugaya Y, Suhr C, Suk M, Sulin VV, Sultansoy S, Sumida T, Sun S, Sun X, Sundermann JE, Suruliz K, Susinno G, Sutton MR, Suzuki Y, Svatos M, Swedish S, Swiatlowski M, Sykora I, Sykora T, Ta D, Taccini C, Tackmann K, Taenzer J, Taffard A, Tafirout R, Taiblum N, Takai H, Takashima R, Takeda H, Takeshita T, Takubo Y, Talby M, Talyshev AA, Tam JYC, Tan KG, Tanaka J, Tanaka R, Tanaka S, Tanaka S, Tanasijczuk AJ, Tannenwald BB, Tannoury N, Tapprogge S, Tarem S, Tarrade F, Tartarelli GF, Tas P, Tasevsky M, Tashiro T, Tassi E, Tavares Delgado A, Tayalati Y, Taylor FE, Taylor GN, Taylor W, Teischinger FA, Teixeira Dias Castanheira M, Teixeira-Dias P, Temming KK, Ten Kate H, Teng PK, Teoh JJ, Terada S, Terashi K, Terron J, Terzo S, Testa M, Teuscher RJ, Therhaag J, Theveneaux-Pelzer T, Thomas JP, Thomas-Wilsker J, Thompson EN, Thompson PD, Thompson PD, Thompson RJ, Thompson AS, Thomsen LA, Thomson E, Thomson M, Thong WM, Thun RP, Tian F, Tibbetts MJ, Tikhomirov VO, Tikhonov YA, Timoshenko S, Tiouchichine E, Tipton P, Tisserant S, Todorov T, Todorova-Nova S, Toggerson B, Tojo J, Tokár S, Tokushuku K, Tollefson K, Tomlinson L, Tomoto M, Tompkins L, Toms K, Topilin ND, Torrence E, Torres H, Torró Pastor E, Toth J, Touchard F, Tovey DR, Tran HL, Trefzger T, Tremblet L, Tricoli A, Trigger IM, Trincaz-Duvoid S, Tripiana MF, Trischuk W, Trocmé B, Troncon C, Trottier-McDonald M, Trovatelli M, True P, Trzebinski M, Trzupek A, Tsarouchas C, Tseng JCL, Tsiareshka PV, Tsionou D, Tsipolitis G, Tsirintanis N, Tsiskaridze S, Tsiskaridze V, Tskhadadze EG, Tsukerman II, Tsulaia V, Tsuno S, Tsybychev D, Tudorache A, Tudorache V, Tuna AN, Tupputi SA, Turchikhin S, Turecek D, Turk Cakir I, Turra R, Tuts PM, Tykhonov A, Tylmad M, Tyndel M, Uchida K, Ueda I, Ueno R, Ughetto M, Ugland M, Uhlenbrock M, Ukegawa F, Unal G, Undrus A, Unel G, Ungaro FC, Unno Y, Unverdorben C, Urbaniec D, Urquijo P, Usai G, Usanova A, Vacavant L, Vacek V, Vachon B, Valencic N, Valentinetti S, Valero A, Valery L, Valkar S, Valladolid Gallego E, Vallecorsa S, Valls Ferrer JA, Van Den Wollenberg W, Van Der Deijl PC, van der Geer R, van der Graaf H, Van Der Leeuw R, van der Ster D, van Eldik N, van Gemmeren P, Van Nieuwkoop J, van Vulpen I, van Woerden MC, Vanadia M, Vandelli W, Vanguri R, Vaniachine A, Vankov P, Vannucci F, Vardanyan G, Vari R, Varnes EW, Varol T, Varouchas D, Vartapetian A, Varvell KE, Vazeille F, Vazquez Schroeder T, Veatch J, Veloso F, Veneziano S, Ventura A, Ventura D, Venturi M, Venturi N, Venturini A, Vercesi V, Verducci M, Verkerke W, Vermeulen JC, Vest A, Vetterli MC, Viazlo O, Vichou I, Vickey T, Vickey Boeriu OE, Viehhauser GHA, Viel S, Vigne R, Villa M, Villaplana Perez M, Vilucchi E, Vincter MG, Vinogradov VB, Virzi J, Vivarelli I, Vives Vaque F, Vlachos S, Vladoiu D, Vlasak M, Vogel A, Vogel M, Vokac P, Volpi G, Volpi M, von der Schmitt H, von Radziewski H, von Toerne E, Vorobel V, Vorobev K, Vos M, Voss R, Vossebeld JH, Vranjes N, Vranjes Milosavljevic M, Vrba V, Vreeswijk M, Vu Anh T, Vuillermet R, Vukotic I, Vykydal Z, Wagner P, Wagner W, Wahlberg H, Wahrmund S, Wakabayashi J, Walder J, Walker R, Walkowiak W, Wall R, Waller P, Walsh B, Wang C, Wang C, Wang F, Wang H, Wang H, Wang J, Wang J, Wang K, Wang R, Wang SM, Wang T, Wang X, Wanotayaroj C, Warburton A, Ward CP, Wardrope DR, Warsinsky M, Washbrook A, Wasicki C, Watkins PM, Watson AT, Watson IJ, Watson MF, Watts G, Watts S, Waugh BM, Webb S, Weber MS, Weber SW, Webster JS, Weidberg AR, Weigell P, Weinert B, Weingarten J, Weiser C, Weits H, Wells PS, Wenaus T, Wendland D, Weng Z, Wengler T, Wenig S, Wermes N, Werner M, Werner P, Wessels M, Wetter J, Whalen K, White A, White MJ, White R, White S, Whiteson D, Wicke D, Wickens FJ, Wiedenmann W, Wielers M, Wienemann P, Wiglesworth C, Wiik-Fuchs LAM, Wijeratne PA, Wildauer A, Wildt MA, Wilkens HG, Will JZ, Williams HH, Williams S, Willis C, Willocq S, Wilson A, Wilson JA, Wingerter-Seez I, Winklmeier F, Winter BT, Wittgen M, Wittig T, Wittkowski J, Wollstadt SJ, Wolter MW, Wolters H, Wosiek BK, Wotschack J, Woudstra MJ, Wozniak KW, Wright M, Wu M, Wu SL, Wu X, Wu Y, Wulf E, Wyatt TR, Wynne BM, Xella S, Xiao M, Xu D, Xu L, Yabsley B, Yacoob S, Yakabe R, Yamada M, Yamaguchi H, Yamaguchi Y, Yamamoto A, Yamamoto K, Yamamoto S, Yamamura T, Yamanaka T, Yamauchi K, Yamazaki Y, Yan Z, Yang H, Yang H, Yang UK, Yang Y, Yanush S, Yao L, Yao WM, Yasu Y, Yatsenko E, Yau Wong KH, Ye J, Ye S, Yeletskikh I, Yen AL, Yildirim E, Yilmaz M, Yoosoofmiya R, Yorita K, Yoshida R, Yoshihara K, Young C, Young CJS, Youssef S, Yu DR, Yu J, Yu JM, Yu J, Yuan L, Yurkewicz A, Yusuff I, Zabinski B, Zaidan R, Zaitsev AM, Zaman A, Zambito S, Zanello L, Zanzi D, Zeitnitz C, Zeman M, Zemla A, Zengel K, Zenin O, Ženiš T, Zerwas D, Zevi della Porta G, Zhang D, Zhang F, Zhang H, Zhang J, Zhang L, Zhang X, Zhang Z, Zhao Z, Zhemchugov A, Zhong J, Zhou B, Zhou L, Zhou N, Zhu CG, Zhu H, Zhu J, Zhu Y, Zhuang X, Zhukov K, Zibell A, Zieminska D, Zimine NI, Zimmermann C, Zimmermann R, Zimmermann S, Zimmermann S, Zinonos Z, Ziolkowski M, Zobernig G, Zoccoli A, zur Nedden M, Zurzolo G, Zutshi V, Zwalinski L. Measurement of flow harmonics with multi-particle cumulants in Pb+Pb collisions at [Formula: see text] TeV with the ATLAS detector

ATLAS measurements of the azimuthal anisotropy in lead-lead collisions at [Formula: see text] TeV are shown using a dataset of approximately 7 [Formula: see text]b[Formula: see text] collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta [Formula: see text] GeV and in the pseudorapidity range [Formula: see text]. The anisotropy is characterized by the Fourier coefficients, [Formula: see text], of the charged-particle azimuthal angle distribution for [Formula: see text]-4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the [Formula: see text] coefficients are presented. The elliptic flow, [Formula: see text], is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, [Formula: see text] and [Formula: see text], are determined with two- and four-particle cumulants. Flow harmonics [Formula: see text] measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to [Formula: see text] measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multi-particle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements.