1. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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.

2. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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. Spectator neutrons and protons separated by magnetic elements of the LHC beam line Two identical sets of ZDCs, one on each side relative to the interaction point (I.P.):  reduce background  improve resolution on centrality ZDC will measure the centrality of the nucleus-nucleus collision through the detection of the energy carried by non- interacting (spectators) nucleons. outgoing beam ZP ZN 2 distinct detectors: 1 for protons (ZP), 1 for neutrons (ZN)

3. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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 cm 3. 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 0 0 with respect to the incident particle direction and come out from the rear face of the calorimeter, directly bringing the light to the photomultipliers.

4. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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. PMT 1 PMT 2 PMT 3 PMT 4 PMT c

5. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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 cm 2 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 7.04 cm Beam exiting IP2 22.4 cm 12 cm ~ 15 cm Proton ZDC Neutron ZDC 90% of detected protons hit a 12.6  2.8 cm 2 area. Experimentally possible the estimate of the centroid of the spectator neutrons spot using the responses coming from the four ZN towers.

6. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Neutron multiplicity vs impact parameter 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. 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

7. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Centroid reconstructed by ZN - true Centroid = Centroid Resolution Reconstruction of the centroid of the spectator neutrons spot neutron multiplicity = 20

8. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 ZN and the Reaction Plane Reaction Plane defined by beam direction and impact parameter The centroid measurement allows to reconstruct the 1st order event 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. V1 = Directed Flow of spectator neutrons occurs when the spectator neutrons are deflected by the expanding fireball into the reaction plane Reaction plane x z y

9. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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  rad, beam transverse size at I.P. = 16  m. A random reaction plane azimuth (phiRP) is assigned to each event and a directed flow of spectator neutrons v 1 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 - the variance of the gaussian fit of the distribution phiZDC – phiRP

10. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Reaction Plane estimate for Neutron Multiplicity = 30 on one side of IP v 1 = 5% v 1 = 20% 68 0 48.4 0

11. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – 1 one side of I.P. The event plane resolution depends on the magnitude of v 1 among spectator neutrons and on a lesser extent on the neutron multiplicity. 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. Event Plane resolution expressed as

12. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – 2 one side of I.P. Advantages of using the reaction plane from v 1 of spectator neutrons: direction of the impact parameter vector in the range 0-2  -> v 1 of produced particles, sign of v 2 less sensitivity to non-flow effects (originating from jets …) thanks to the large rapidity gap less sensitivity to flow fluctuations Event Plane resolution expressed as σ(phiZDC-phiRP) (from gaussian fit)

13. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 LHC beam parameters contribution to event plane resolution V 1 =20% Transverse Pb beam divergence at IP2 = 30 μrad This value depends on the LHC beam parameters: - transverse normalised emittance ε n - twiss function β* - relativistic gamma factor γ - ε n = 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

14. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Shift of the centroid of spectator neutrons spot on the ZN front face vs Directed Flow v 1 reaction plane azimuth = 0. Neutron multiplicity = 40 The centroid position does not depend on the neutron multiplicity V 1 =0 % V 1 =20% -> 1.7 mm

15. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – ZN1+2 ZN1 ZN2 IP2 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 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 azimuth from the mean of the 2 centroids ZN1 ZN2

16. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – ZN1+2 Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% Single ZN ZN1+2 event plane azimuth from the mean of the 2 centroids 48,5 0 45 0

17. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – ZN1+2 Parametric study as a function of Neutron Multiplicity assumed equal on the 2 ZN.

18. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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 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)

19. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Distance between the 2 centroids 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) 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)

20. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution vs Centroid Distance Single ZN ZN1+2 No cut on the distance between the 2 centroids Distance between the 2 centroids < 5 mm Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% 48,5 0 47 0 45 0 43 0

21. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution vs Centroid Distance Single ZN ZN1+2 No cut on the distance between the 2 centroids Distance between the 2 centroids < 5 mm Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20%

22. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution vs Centroid Distance No cut Transverse Pb beam divergence at IP2=0 μrad Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% Transverse Pb beam divergence at IP2=0 μrad No cut

23. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Percentage of events vs cut Percentage of events which survive after the different cuts on the distance between the 2 centroids Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20% No cut

24. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Summary The neutron zero degree calorimeter ZN of ALICE allows to reconstruct the 1 st -order event plane from the directed flow (“bounce off”) of spectator neutrons. The event plane resolution depends on the magnitude of v 1 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.

25. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Backup Slides

26. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Reconstruction of the centroid of the spectator neutrons spot - 1 where x i t and y i t are the coordinates of the centre of the i-th tower and E i is the light in the i-th tower.  and const are free parameters introduced in order to get an accurate reconstructed impact coordinate. 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  rad. 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:

27. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – 3 one side of I.P. STAR ZDC-SMD simulation (G. Wang, private communication) ALICE ZN simulation High segmentation: 8x7 slats but multiplicity shower sampling at only one position (~ 2 i ) -> 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

28. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 STAR ZDC-SMD SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC 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 ZDC side view Scintillator slats of Shower Max Detector Transverse plane of ZDC

29. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Comparison between 2x2 and 4x4 ZN segmentation - 1 PMT 1 PMT 2 PMT 3 PMT 4 PMT c PMT 1,3,9,11 PMT 2,4,10,12 PMT 5,7,13,15 PMT 6,8,14,16 PMT c

30. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Comparison between 2x2 and 4x4 ZN segmentation - 2 PMT 1 PMT 2 PMT 3 PMT 4 PMT c PMT 1,3,9,11 PMT 2,4,10,12 PMT 5,7,13,15 PMT 6,8,14,16 PMT c 2x2 segm Full marker 4x4 segm Open marker -> Small difference, why ?

31. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Reconstruction of the centroid of the spectator neutrons spot for different ZN position -> Centroid coordinate, reconstructed by ZN, is accurate up to 1 cm of ZN displacement from the nominal position 80  rad beam crossing angle 20  rad beam crossing angle Centroid reconstructed by ZN and true centroid versus ZN vertical position

32. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 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 V 1 =20% ZN1 ZN2 Reaction plane x z y

33. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Event Plane Resolution – ZN1+2 ZN1 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 azimuth from the mean of the 2 centroids ZN2

34. Roberto GemmeIII Convegno Nazionale sulla Fisica di ALICE Frascati, 12-14/11/2007 Charged hadrons : v 1 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.