Reaction plane determination with the neutron Zero Degree Calorimeters

Roberto Gemme III Convegno Nazionale sulla Fisica di ALICE

Frascati, 12-14/11/2007

Reaction plane determination with the neutron Zero Degree

Calorimeters

ZP

ZN

ZP

ZN

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.


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)


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 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.


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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


Frascati, 12-14/11/2007

ZDC acceptancesNo 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

7.04 cm

Beam exiting IP222.4 cm

12

cm

~ 15 cm

Proton ZDC Neutron ZDC

90% of detected protons hit a 12.6 2.8 cm2 area.

Experimentally possible the estimate of the centroid of the spectator neutrons spot using the responses coming from the four ZN towers.


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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 TeVAHijing+fragmentation

In peripheral collisions many nucleons remain bound in large nuclear fragments, that are not detected by the ZDC.

Pb-Pb 2.7 TeVAHijing+fragmentation


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Centroid reconstructed by ZN - true Centroid = Centroid Resolution

Reconstruction of the centroid of the spectator neutrons spot

4

1

4

1

i

iti

w

wxconstx E iiw

n

i

n

ii

E

Exx

1

1

neutron multiplicity = 20


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


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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 v1 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


Frascati, 12-14/11/2007

Reaction Plane estimate forNeutron Multiplicity = 30 on one side

of IP

v1 = 5% v1 = 20%

680 48.40


Frascati, 12-14/11/2007

Event Plane Resolution – 1 one side of I.P.

•The event plane resolution depends on the magnitude of v1 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 <cos(phiZDC – phiRP)>


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Advantages of using the reaction planefrom v1 of spectator neutrons:

• direction of the impact parameter vector in the range 0-2 -> 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

Event Plane resolution expressed as σ(phiZDC-phiRP) (from gaussian fit)


Frascati, 12-14/11/2007

LHC beam parameters contribution to

event plane resolution

*n

RMS

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


Frascati, 12-14/11/2007

Shift of the centroid of spectator neutrons spot on the ZN front face vs Directed Flow v1

•reaction plane azimuth = 0.•Neutron multiplicity = 40•The centroid position does not depend onthe neutron multiplicity

V1=0 %

V1=20%-> 1.7 mm


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 neutronsTransverse Pb beam divergence at I.P. = 30 μrad Beam transverse size at I.P. = 16 μmDetector 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


Frascati, 12-14/11/2007

Event Plane Resolution – ZN1+2Neutron Multiplicity = 30 on ZN1 and ZN2, v1 = 20%

Single ZN

ZN1+2 event plane azimuth from the mean of the 2 centroids

48,50

450


Frascati, 12-14/11/2007

Event Plane Resolution – ZN1+2Parametric study as a function of Neutron Multiplicity assumed equal on the 2 ZN.


Frascati, 12-14/11/2007

Distance between the 2 centroids

Distance between true centroids

No Fermi momentumBeam transverse size at I.P. = 0Transverse Pb beam divergence at I.P. = 0 μrad

Distance between reconstructed centroids

No Fermi momentumBeam transverse size at I.P. = 0Transverse Pb beam divergence at I.P. = 0 μrad


Beam transverse size at I.P. = 0Transverse Pb beam divergence at I.P. = 0 μrad


Transverse Pb beam divergence at I.P. = 0 μrad

(m) (m)

(m) (m)


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)


Transverse Pb beam divergence at I.P.= 0 μrad


Transverse Pb beam divergence at I.P.= 30 μrad

(m)


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%

48,50470

450 430


Frascati, 12-14/11/2007


Single ZN

ZN1+2

No cut on the distance between the 2 centroids

Distance between the 2 centroids < 5 mm



Frascati, 12-14/11/2007


No cut

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


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


No cut


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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.


Frascati, 12-14/11/2007

Backup Slides


Frascati, 12-14/11/2007

Reconstruction of the centroid of the spectator neutrons spot - 1

4

1

4

1

i

iti

w

wxconstx E iiw

where xit and yi

t are the coordinates of the centre of the i-th tower and Ei 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:

4

1

4

1

i

iti

w

wyconsty


Frascati, 12-14/11/2007


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


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


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


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 ?


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 angle20 rad beam crossing

angle

Centroid reconstructed by ZN E iiw

4

1

4

1

i

iti

w

wyconsty

and true centroid

n

i

n

ii

E

Eyy

1

1

versus

ZN vertical position


Frascati, 12-14/11/2007


•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%

ZN1

ZN2

Reaction plane

x

z

y


Frascati, 12-14/11/2007


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


Frascati, 12-14/11/2007

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.

Reaction plane determination with the neutron Zero Degree Calorimeters

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