Reaction plane determination with the neutron Zero Degree Calorimeters

1. Reaction plane determination with the neutron Zero Degree Calorimeters • Outline • Comparison between Event Plane resolution using 1 or 2 ZN. • Distance between the centroids of spectator neutrons spot on ZN1 and ZN2: • a way to select events with a better Event Plane resolution. III Convegno Nazionale sulla Fisica di ALICE

2. ALICE Zero Degree Calorimeters ZDCs are placed at 116 m from IP at 0º relative to LHC beam axis, where the circulating beams are spatially separated in two different pipes. ZN ZDC will measure the centrality of the nucleus-nucleus collision through the detection of the energy carried by non-interacting (spectators) nucleons. ZP outgoing beam • Two identical sets of ZDCs, one on each side relative to the interaction point (I.P.): • reduce background • improve resolution on centrality Spectator neutrons and protons separated by magnetic elements of the LHC beam line 2 distinct detectors: 1 for protons (ZP) , 1 for neutrons (ZN) III Convegno Nazionale sulla Fisica di ALICE

3. The neutron Zero Degree Calorimeter (ZN) Each ZN is made by 44 grooved W-alloy slabs, each of them 1.6 mm thick, stacked to form a parallelepiped of 7.2 x 7.2 x 100 cm3. The active part is made of 1936 quartz fibers, embedded in the absorber with a pitch of 1.6 mm. The fibers, hosted in the slab grooves, are placed at 00 with respect to the incident particle direction and come out from the rear face of the calorimeter, directly bringing the light to the photomultipliers. III Convegno Nazionale sulla Fisica di ALICE

4. PMT 1 PMT 2 PMT 3 PMT 4 PMT c ZN as rough position sensitive device One out of two fiber is sent to a photomultiplier (PMTc), while the remaining fibers are collected in bundles and sent to four different photomultipliers (PMT1 to PMT4) forming four indipendent towers. This segmentation gives a rough localization of the spectator neutron’s spot on the front face of the calorimeter. III Convegno Nazionale sulla Fisica di ALICE

5. Proton ZDC Neutron ZDC 12 cm 7.04 cm ~ 15 cm Beam exiting IP2 22.4 cm ZDC acceptances No losses of neutrons along the beam line. For the spectator neutrons only the transverse component of the Fermi momentum plays a role in determining the spot size at the ZDC location, which is of the order of 0.6  0.6 cm2 at 1  level. The momentum spread of spectator protons results in a large horizontal dispersion after separator dipole D1: ~30% of protons interact along the beam line Experimentally possible the estimate of thecentroid of the spectator neutrons spot using the responses coming from the fourZN towers. 90% of detected protons hit a 12.6  2.8 cm2 area. III Convegno Nazionale sulla Fisica di ALICE

6. Neutron multiplicity vs impact parameter Pb-Pb 2.7 TeVA Hijing+fragmentation In peripheral collisions many nucleons remain bound in large nuclear fragments, that are not detected by the ZDC. Pb-Pb 2.7 TeVA Hijing+fragmentation Therefore the monotonic correlation between the neutron multiplicity and the centrality variables is partially destroyed. The information provided by two forward e.m. calorimeters (ZEM) will be used to identify very peripheral collisions and to remove the ambiguity. III Convegno Nazionale sulla Fisica di ALICE

7. Reconstruction of the centroid of the spectator neutrons spot Centroid reconstructed by ZN - true Centroid = Centroid Resolution neutron multiplicity = 20 III Convegno Nazionale sulla Fisica di ALICE

8. z Reaction plane y x ZN and the Reaction Plane Reaction Plane defined by beam direction and impact parameter V1 = Directed Flow of spectator neutrons occurs when the spectator neutrons are deflected by the expanding fireball into the reaction plane Thanks to its localizing capability ZN can measure, event by event, the centroid of the spectator neutrons spot, which is sensitive to the sideward deflection (“bounce off”) of the spectator neutrons. The centroid measurement allows to reconstruct the 1st order event plane. III Convegno Nazionale sulla Fisica di ALICE

9. Reaction Plane Estimate • Spectator neutrons (2.76 TeV) on one side of I.P. (OLD STUDY) and on the two sides of I.P. (NEW STUDY) are generated with a momentum distribution taking into account Fermi momentum, transverse Pb beam divergence = 30 mrad, beam transverse size at I.P. = 16 mm. • A random reaction plane azimuth (phiRP) is assignedto each event and a directedflow of spectator neutronsv1 is introduced following standard prescriptions (Poskanzer and Voloshin, Phys. Rev. C58, 1998), same as in AliGenAfterBurnerFlow. • phiZDC0 = event plane azimuth from spectator neutrons true centroid • phiZDC = event plane azimuth from spectator neutrons reconstructed centroid • Two estimators of the event plane resolution: • -The mean cosine of the angular difference <cos(phiZDC – phiRP)> • the variance of the gaussian fit of the distribution phiZDC – phiRP III Convegno Nazionale sulla Fisica di ALICE

10. Reaction Plane estimate forNeutron Multiplicity = 30 on one side of IP v1 = 20% v1 = 5% 680 48.40 III Convegno Nazionale sulla Fisica di ALICE

11. Event Plane Resolution – 1 one side of I.P. Event Plane resolution expressed as <cos(phiZDC – phiRP)> • Neutron Multiplicity up to 60, maximum number of detected neutrons in one ZN, when the production of nuclear fragments in Pb-Pb collisions is taken into account. • The event plane resolutiondepends on • the magnitude of v1 among spectator • neutronsand on a lesser extent on the • neutron multiplicity. III Convegno Nazionale sulla Fisica di ALICE

12. Event Plane Resolution – 2 one side of I.P. Event Plane resolution expressed as σ(phiZDC-phiRP) (from gaussian fit) • Advantages of using the reaction plane • from v1 of spectator neutrons: • direction of the impact parameter vector in • the range 0-2p-> v1 of produced particles, • sign of v2 • less sensitivity to non-flow effects • (originating from jets …) thanks to the large • rapidity gap • less sensitivity to flow fluctuations III Convegno Nazionale sulla Fisica di ALICE

13. LHC beam parameters contribution to event plane resolution V1=20% Transverse Pb beam divergence at IP2 = 30 μrad This value depends on the LHC beam parameters: - transverse normalised emittance εn - twiss function β* - relativistic gamma factor γ - εn = 1.5 μm rad - β* at IP2 = 0.5 m - γ = 2963.5 -> Event plane resolution is dominated by the bias due to beam divergence III Convegno Nazionale sulla Fisica di ALICE

14. Shift of the centroid of spectator neutrons spot on the ZN front face vs Directed Flow v1 V1=0 % V1=20% -> 1.7 mm • reaction plane azimuth = 0. • Neutron multiplicity = 40 • The centroid position does not depend on • the neutron multiplicity III Convegno Nazionale sulla Fisica di ALICE

15. ZN1 ZN2 Event Plane Resolution – ZN1+2 • No Fermi momentum of spectator neutrons • Transverse Pb beam divergence at I.P. = 0 μrad • Beam transverse size at I.P. = 0 • No detector smearing • -> same shift of the centroid of spectator • neutrons spot on ZN1 and ZN2 front face ZN1 ZN2 Fermi momentum of spectator neutrons Transverse Pb beam divergence at I.P. = 30 μrad Beam transverse size at I.P. = 16 μm Detector smearing -> the position of the centroid of spectator neutrons spot on ZN1 and ZN2 is different -> event plane azimuthfrom the mean of the 2 centroids IP2 III Convegno Nazionale sulla Fisica di ALICE

16. Event Plane Resolution – ZN1+2 Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% Single ZN 48,50 event plane azimuth from the mean of the 2 centroids ZN1+2 450 III Convegno Nazionale sulla Fisica di ALICE

17. Event Plane Resolution – ZN1+2 Parametric study as a function of Neutron Multiplicity assumed equal on the 2 ZN. III Convegno Nazionale sulla Fisica di ALICE

18. Distance between the 2 centroids Distance between true centroids No Fermi momentum Beam transverse size at I.P. = 0 Transverse Pb beam divergence at I.P. = 0 μrad Distance between reconstructed centroids No Fermi momentum Beam transverse size at I.P. = 0 Transverse Pb beam divergence at I.P. = 0 μrad (m) (m) Distance between reconstructed centroids Beam transverse size at I.P. = 0 Transverse Pb beam divergence at I.P. = 0 μrad Distance between reconstructed centroids Transverse Pb beam divergence at I.P. = 0 μrad (m) (m) III Convegno Nazionale sulla Fisica di ALICE

19. Distance between the 2 centroids Distance between reconstructed centroids Transverse Pb beam divergence at I.P.= 0 μrad Distance between reconstructed centroids Transverse Pb beam divergence at I.P.= 30 μrad (m) Also with beam divergence=0 the reconstructed distance between the 2 centroids is not zero, because of Fermi momentum of spectator neutrons and detector smearing. But the distance is significantly bigger when the beam divergence is switched on. -> The distance between the 2 centroids can be a tool to select events with a better event plane resolution? theta_div ZN1 (rad) vs reconstructed centroid distance (m) III Convegno Nazionale sulla Fisica di ALICE

20. Event Plane Resolution vs Centroid Distance Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% No cut on the distance between the 2 centroids Distance between the 2 centroids < 5 mm Single ZN 48,50 470 ZN1+2 450 430 III Convegno Nazionale sulla Fisica di ALICE

21. Event Plane Resolution vs Centroid Distance Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% Distance between the 2 centroids < 5 mm No cut on the distance between the 2 centroids Single ZN ZN1+2 III Convegno Nazionale sulla Fisica di ALICE

22. Event Plane Resolution vs Centroid Distance Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% Transverse Pb beam divergence at IP2=0 μrad Transverse Pb beam divergence at IP2=0 μrad No cut No cut No cut III Convegno Nazionale sulla Fisica di ALICE

23. Percentage of events vs cut Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% No cut Percentage of events which survive after the different cuts on the distance between the 2 centroids III Convegno Nazionale sulla Fisica di ALICE

24. Summary • The neutron zero degree calorimeter ZN of ALICE allows to reconstruct the 1st-order event plane from the directed flow (“bounce off”) of spectator neutrons. • The event plane resolution depends on the magnitude of v1 among spectator neutrons and on a lesser extent on the neutron multiplicity. • The event plane resolution predicted for the ALICE ZN is dominated by the bias due to the transverse beam divergence. • The second arm has been inserted and a parametric study as a function of Neutron Multiplicity, assumed equal on the 2 ZN, has been done. • The distance between the centroids of spectator neutrons spot on ZN1 • and ZN2 front face could be a tool to select events with a better event • plane resolution. III Convegno Nazionale sulla Fisica di ALICE

25. Backup Slides III Convegno Nazionale sulla Fisica di ALICE

26. Reconstruction of the centroid of the spectator neutrons spot - 1 Spectator neutrons (2.76 TeV) are generated with a momentum distribution taking into account Fermi momentum and a transverse Pb beam divergence at I.P.2 =30 mrad. A GEANT 3.21 - based simulation code tracks the neutrons in the calorimeter, where the hadronic shower deposits light in the four towers. The centroid of the spectator neutrons spot on the ZN front-face is estimated by means of the relations: where xit and yit are the coordinates of the centre of the i-th tower and Ei is the light in the i-th tower. a and constare free parameters introduced in order to get an accurate reconstructed impact coordinate. III Convegno Nazionale sulla Fisica di ALICE

27. Event Plane Resolution – 3 one side of I.P. STARZDC-SMD simulation (G. Wang, private communication) ALICEZN simulation High segmentation: 8x7 slats but multiplicity shower sampling at only one position (~ 2 li) -> big fluctuations in the signal amplitude in each slat Low segmentation: 2x2 towers but full shower energy measurement -> fluctuations in the signal amplitude in each tower smaller than those in STAR ZDC-SMD III Convegno Nazionale sulla Fisica di ALICE

28. STAR ZDC-SMD • New knowledge of the direction of the impact ~parameter vector • Minimal, if any, non-flow effects • Minimal, if any, effects from flow fluctuations • Worse resolution than from TPC, but that ~disadvantage is minor SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC ZDC side view Scintillator slats ofShower Max Detector Transverse plane of ZDC III Convegno Nazionale sulla Fisica di ALICE

29. PMT 1,3,9,11 PMT 2,4,10,12 PMT 5,7,13,15 PMT 6,8,14,16 PMT c PMT 1 PMT 2 PMT 3 PMT 4 PMT c Comparison between 2x2 and 4x4 ZN segmentation - 1 III Convegno Nazionale sulla Fisica di ALICE

30. PMT 1,3,9,11 PMT 2,4,10,12 PMT 5,7,13,15 PMT 6,8,14,16 PMT c PMT 1 PMT 2 PMT 3 PMT 4 PMT c Comparison between 2x2 and 4x4 ZN segmentation - 2 2x2 segm Full marker 4x4 segm Open marker -> Small difference, why ? III Convegno Nazionale sulla Fisica di ALICE

31. Reconstruction of the centroid of the spectator neutrons spot for different ZN position Centroid reconstructed by ZN and true centroid versus ZN vertical position -> Centroid coordinate, reconstructed by ZN, is accurate up to 1 cm of ZN displacement from the nominal position 80 mrad beam crossing angle 20 mrad beam crossing angle III Convegno Nazionale sulla Fisica di ALICE

32. z Reaction plane y x Event Plane Resolution – ZN1+2 ZN1 ZN2 • No Fermi momentum of spectator neutrons • Transverse Pb beam divergence at I.P. = 0 μrad • Beam transverse size at I.P. = 0 • No detector smearing • -> same shift of the centroid of spectator neutrons spot on ZN1 and ZN2 front face V1=20% III Convegno Nazionale sulla Fisica di ALICE

33. Event Plane Resolution – ZN1+2 • If we take into account: • Fermi momentum of spectator neutrons • Transverse Pb beam divergence at I.P. = 30 μrad • Beam transverse size at I.P. = 16 μm • Detector smearing • -> the position of the centroid of spectator • neutrons spot on ZN1 and ZN2 is different • -> event plane azimuthfrom the mean of the 2 centroids ZN1 ZN2 III Convegno Nazionale sulla Fisica di ALICE

34. Charged hadrons : v1 in 62 GeV Au +Au STAR preliminary directed flow of charged particles is opposite to spectator bounce-off at all centralities, and three methods agree. III Convegno Nazionale sulla Fisica di ALICE