Rapidity dependence of azimuthal correlations for pp and dAu

Xuan Li (Shandong Uni. &BNL)For the STAR CollaborationWWND 2011 (Winter Park)

Outline

• Motivation– Background– STAR forward-mid rapidity, forward-forward rapidity

correlations.• New results in pseudo-rapidity interval between 1

and 2.– Cluster finder introduction.– π0 candidates searching. – Preliminary correlation results.– Jet-like event correlations.

• Conclusions & Outlook

Xuan Li 2

Motivation• The nucleon gluon density at low x.

Xuan Li 3

The nucleon gluon density is known in the 0.0001<x<0.3.

Can’t increase indefinitely.Saturation?

Fixed Target Experiments

Rapid rise of the gluon density at low-x evident from F2(x)/lnQ2 at fixed x (Prytz relation)

Rizvi E. Talk presented at the “International Euro Physics Conference on High Energy Physics”, July 2003

arxiv:hep-ph/0201195

Proton gluon density

lepton

Nucleon γ*quark

Gluon saturation is expected at low x• Parton gas approach saturation through evolution.

Xuan Li 4

Parton saturation is expected at low x and low Q2. At a given x, nuclei (mass number A) gluon density ≈ A1/3 × nucleon gluon density, leading to the expectation Qs

2≈A1/3 xβ. [hep-ph/0304189] Current fixed target data provides 0.02<x<0.3 range for nuclear gluon density.

€

ln(1

x)

€

ln(PT2)

How sharp is the transition?

How to access low x gluon• Forward inclusive production.

Xuan Li 5

• Large rapidity inclusive production ~4

probes asymmetric partonic collisions. Mostly high-x valence quark + low-x gluon.

pp data is in agreement with perturbative QCD.Suppression of forward inclusive particle in dAu data is better described in Color Glass Condensate (CGC) predictions .

Phys. Rev. Lett. 97.152302

How to access low x gluon• Inclusive π0 to correlated π0-π0.

Xuan Li 6

Phys. Lett. B603 (2004) 173

From inclusive π0 to π0-π0

Forward π0-forward π0 are more sensitive to low x gluon than inclusive production.

Inclusive production measures integral of broad x range.If we measure π0-π0, we can probe limited x range for gluon.

How to measure the sensitivity • Effects on the azimuthal correlations from

different parton distribution for 2->2. scattering.

Xuan Li 7

The away side peak height is correlated with the parton density distribution.

arXiv:hep-ex/1005.2378

Near side peak

Away side peak

Back to back correlations• pQCD 22 process =back-to-back di-jet (Works well for p+p)

• With high gluon density, 21 (or 2many) process = Mono-jet ?

Xuan Li 8

CGC predicts suppression of back-to-back correlation.

Kharzeev, Levin, McLerran  Nucl. Phys. A748 (2005) 627

Color Glass Condensate (CGC) prediction• Test the phase boundary at fixed Q2.

Xuan Li 9

Fix PT , look through different x region

Fix Pt for π0 at forward rapidity, and vary the rapidity of theassociated π0. Vary the Pt to study the boundary.€

ln(PT2)

K. Golec-Biernat, M. Wustoff, Phys. Rev. D 59, 014017

STAR Detectors• The full view of STAR

Xuan Li 10

Forward Meson Spectrometer (FMS)

Endcap electro-magnetic calorimeter (EEMC)

Barrel electro-magetic calorimeter (BEMC)

Proton (Gold) Proton (Deuteron)

East Beam BeamCounter (BBC)

STAR Detectors• The detectors of STAR used for correlations.

Xuan Li 11

Front view of north half of FMSFMS measuring range 2.3<η<4. Δϕ=0.058, Δη=0.1 for large cells.

BEMC measuring range -1<η<1. Tower range Δϕ=0.05, Δη=0.05.

EEMC measuring range 1<η<2 Tower range Δϕ=0.1, Δη=0.057-0.099

Nearly hermetic electro-magnetic calorimeters cover -1<η<4.

Forward-mid rapidity correlation

Xuan Li 12

FMS-BEMC(TPC) correlation

Triggering on the forward rapidity π0, the rapidity of the associated π0 is correlated with the xbj of the soft parton involved in the partonic scattering.

• Probe nuclei gluon density at 0.008 < x < 0.07 .

PYTHIA simulation

Arxiv:hep-ex/0502040

Forward-mid rapidity correlation• FMS-BEMC(TPC) azimuthal correlation.

• Back to back azimuthal correlation peak looks similar in pp and dAu data.• There is no dramatic broadening from pp to dAu.• Forward-mid rapidity correlations are not near the saturation region.Xuan Li 13

arXiv: hep-ex/0907.3473

PT(FMS)>2.5GeV/c1.5GeV/c<PT(BEMC/TPC)<PT(FMS)

Forward-forward rapidity correlation

Xuan Li 14

FMS-FMS correlation

• Look at forward-forward correlation to access lowest x region.

• Probe gluon density at 0.0009 < x < 0.005.

PYTHIA simulation

Forward-forward rapidity correlation• FMS-FMS azimuthal correlation.

• Similarity of near side peak in pp and dAu data.• There is significant broadening from pp to dAu in forward-forward

rapidity azimuthal correlations in the away side peak.Xuan Li 15

PP data dAu dataarXiv:hep-ex/1005.2378

Forward-forward rapidity correlation• Centrality cut on the dAu data.

Xuan Li 16

dAu centrality averaged

dAu peripheral dAu central

J.L. Albacete, C. Marquet arXiv:1005.4065

PP data

(1) The suppression of the height of the away side peak in the central dAu collisions suggests forward-forward correlationsat low x are consistent with gluon saturation in nuclei.(2) The degree of broadening is consistent with multi-parton scattering in central dAu collisions associated with saturation.

Forward-near forward rapidity correlation

Xuan Li 17

FMS-EEMC correlation

• Probe gluon density at intermediate region which is 0.003<x<0.02 .

PYTHIA simulation

FMS π0 triggered event• Within the FMS high tower triggered data, selecting events

where FMS di-photon invariant mass is less than 0.2GeV/c2 and Pt is larger than 2.5 GeV/c .

Xuan Li 18

For example, the invariant massof photon pair in the FMS.

pp data

Introduction to cluster finder• BEMC (EEMC) geometry EEMC η range [1.08,2.0], BEMC η range [-1,1]. Use minimum

bias cluster finder instead of constrained cluster finder to reproduce FMS-BEMC correlations. Then apply the same method to approach FMS-EEMC correlations.

Xuan Li 19

X

Y

ϕ

η

EEMC

Z

X(Y)

ϕ

η

BEMC

Event display• Run 8 pp FMS triggered data. Cluster is energy threshold

bounded group of towers. The threshold for the BEMC tower is 70MeV, and for EEMC is 100MeV. Sorting the tower energy, then add the towers near the high tower to construct cluster.

Xuan Li 20EEMC BEMC

cluster

cluster

BEMC invariant mass with di-cluster

Xuan Li 21

• Take one cluster as photon candidate, then pair two clusters with energy ratio cuts. In addition to π0 selection cuts, we apply Pt cuts for example 1.5GeV/c<Pt<2.5GeV/c.

There are π0 candidates in pp and dAu data.

π0 candidates

π0-π0 azimuthal correlation with new cluster finder (FMS-BEMC)

• FMS photon pair Pt > 2.5GeV/c and mass<0.2 GeV/c2. BEMC di-cluster 1.5GeV/c < Pt < 2.5GeV/c and mass<0.2GeV/c2.

Xuan Li 22

FMS triggered pp data FMS triggered dAu data

Width 0.709± 0.019(stat.) Width 0.754 ± 0.024(stat.)

The FMS-BEMC results with this cluster finder are consistent with preliminary results shown in arXiv: hep-ex/0907.3473 .

New Rapidity (1<η<2)EEMC di-cluster invariant mass

• Take one cluster as photon candidate, then pair two clusters with energy ratio cuts. In addition to π0 selection cuts, we apply Pt cuts for example 1.5GeV/c<Pt<2.5GeV/c.

Xuan Li 23

There are π0 candidates in pp and dAu data.

π0 candidates

π0-π0 azimuthal correlation (FMS-EEMC)• FMS photon pair Pt > 2.5GeV/c and mass<0.2 GeV/c2. EEMC di-

cluster 1.5GeV/c < Pt < 2.5GeV/c and mass<0.2GeV/c2.

Xuan Li 24

FMS triggered pp data FMS triggered dAu data

Width 0.897 ± 0.060(stat.) Width 0.967 ± 0.120(stat.)

The π0 in EEMC

require 2 clusters.

π0-π0 azimuthal correlation (FMS-EEMC) with lower pt cuts

• FMS photon pair Pt > 2.0GeV/c and mass<0.2 GeV/c2. EEMC di-cluster 1.0GeV/c < Pt < 2.0GeV/c and mass<0.2GeV/c2.

Xuan Li 25

FMS triggered pp data(10%) FMS triggered dAu data(10%)

Width 0.833 ± 0.048(stat.) Width 1.032 ± 0.179(stat.)

Initial look at Jet-like events (super-cluster) in BEMC• How to get jet-like event

– Use single cluster as seed, then find a cone with R ( )<0.5.– Require mass of ‘jet’ >0.2GeV/c2 to emphasize ‘jetty events’ .– Distance between jet center and seed less than 3cm to reducebias effects .

• The mass of the jet-like event with 1.5GeV/c<Pt<2.5GeV/c.

Xuan Li 26

€

(Δη)2 + (Δφ)2

pp dAu

π0+jet-like azimuthal correlation (FMS-BEMC)• FMS photon pair Pt > 2.5GeV/c. BEMC jet-like candidate

1.5GeV/c < Pt < 2.5GeV/c.

Xuan Li 27

FMS triggered pp data FMS triggered dAu data

No significant broadening in FMS-BEMC π0+jet-like correlations.

Width 0.761 ± 0.061(stat.) Width 0.832 ± 0.008(stat.)

Initial look at Jet-like events (super-cluster) in EEMC• How to get jet-like event

– Use single cluster as seed, then find a cone with R ( )<0.5.– Require mass of ‘jet’ >0.2GeV/c2 to emphasize ‘jetty events’ .– Distance between jet center and seed less than 3cm to reducebias effects .

• The mass of the jet-like event with 1.5GeV/c<Pt<2.5GeV/c.

Xuan Li 28

€

(Δη)2 + (Δφ)2

pp dAu

π0+jet-like azimuthal correlation (FMS-EEMC)• FMS photon pair Pt > 2.5GeV/c. EEMC jet-like candidate

1.5GeV/c < Pt < 2.5GeV/c.

Xuan Li 29

Significant broadening in FMS-EEMC π0+jet-like correlations.FMS triggered pp data FMS triggered dAu data

Width 0.751 ± 0.042(stat.) Width 0.903 ± 0.014(stat.)

Conclusions• There are π0 and jet-like candidates in the EEMC

tower clusters.• There are hints of broadening in the away side

peak for FMS-EEMC π0+jet-like azimuthal correlation.

• The transition from parton gas to CGC saturation state is not sharp.

Xuan Li 30

Outlook• To add ESMD information for π0 reconstruction.• Use self consistent jet finder for jet-like events. • Get corrected normalized azimuthal correlation.

Introduction to EEMC shower maximum detector

• The SMD helps distinguish photon from charged hadrons.

EM shower

Pre-shower1

Pre-shower2

Tower

Post-shower

Shower Maximum Detector ( SMD )

The electro-magnetic shower passing through EEMC

Front view of SMD U/V plane

Event display of π0 in the EEMC• EEMC has shower maximum detector (ESMD).

Xuan Li 32

Photon 1

EEMC tower E (GeV) ESMD strips E (MeV)

Photon 2

SMD will help define the photon position and the energy sharing between two photons .

Backup

Xuan Li 33

• Current measured nuclei gluon density at low x.

Xuan Li 34

Current fixed target data provide 0.02<x<0.3 range for nuclei gluon density [Phys. Rev. C70 (2004)044905].

The nuclei gluon density prediction• Transverse density of nuclei definition.

• At a given x, nuclei (mass number A) gluon density ≈ A1/3 × nucleon gluon density, leading to the expectation Qs

2≈A1/3 xβ.

• Current fixed target data provides 0.02<x<0.3 range for nuclear gluon density.Xuan Li 35

Transverse density of light nuclei

€

ρA

v r ( ) =

N

exp((r − RA )

δ) +1

, where δ = 0.54fm, RA = (1.12 fm)A1/ 3 − (0.86 fm)A−1/ 3,

A denotes the number of nucleons in nucleus.

The distribution of nucleons in the nucleus,

The transverse distribution is definedas

€

TA (b) = dzρ A b2 + z2( ), where

−∞

+∞

∫

b is the impact parameter.

[hep-ph/0304189]

Partonic scattering for forward π0 production

Xuan Li 36

<z>

<xq>

<xg>

GeVspp 200,8.3,0

NLO pQCD

Jaeger,Stratmann,Vogelsang,Kretzer

FMS coverage

Xuan Li 37

STAR Detectors• In η , ϕ space

Xuan Li 38Nearly hermetic electro-magnetic calorimeter cover -1<η<4.

Forward-forward rapidity correlation• Centrality determination in dAu .

• Multiplicity in dAu measured by the east beam beam counter (BBC) at STAR reflects the centrality. Xuan Li 39

PP data

Peri

pher

al

Cen

tral

dAu data

arXiv:hep-ex/1005.2378

The impact parameter is related with the charge sum in the east BBC by a model.

East BBC charge sum

Average impact parameter (fm)

0 - 500 6.8 ± 1.7

2000 - 4000 2.7 ± 1.3

Centrality cuts in dAu

Xuan Li 40

Motivation

Xuan Li 41

FMS-FMS correlation

FMS-EEMC correlation

FMS-BEMC(TPC) correlation

Triggering on the forward rapidity π0, the rapidity of the associated π0 is correlated with the soft parton involved in the partonic scattering.

• Provide direct sensitivity to nuclei gluon density at 0.001< x < 0.02.

Energy threshold studies• For example, BEMC tower energy deposited (GeV) in pp data.

Xuan Li 42

With energy threshold 35MeV With energy threshold 70MeV

Energy threshold is selected to suppress noise.

Event display• Run 8 dAu fms triggered data.

Xuan Li 43EEMC BEMC

BEMC dAu number of raw tower hits

• P

04/21/23 44

MB data MB simulation

Cluster width definition

• BEMC• Unfold the barrel, and put in the Rϕ, Z plane.

Xuan Li 45

Z

X(Y)

R

ϕ

Z

Rϕ

Large width

Small width

Cluster width definition

• EEMC• In xy plane.

Xuan Li 46

X

Y

Large width

Small width

X

Y

Simulated π0 decay kinematics• Projection on the EEMC, with the π0 Pt in [1.25GeV/c,

2.5GeV/c] and Zγγ<0.7 cuts.

Xuan Li 47

Dγγ VS η of π0

Most of the π0 events are in EEMC single clusters.

For FMS π0 events,Cuts on the single cluster is(1)1.25GeV/c<Pt<2.5GeV/c(2)Zγγ < 0.7

dAu FMS triggered data • FMS di-photon invariant mass.

Xuan Li 48

With FMS photon pair which has mass less than 0.2GeV/c2 and Pt larger than 2.5 GeV/c.

π0 jet-like azimuthal correlation (FMS-BEMC)• FMS photon pair Pt > 2.5GeV/c. BEMC jet-like candidate

1.5GeV/c < Pt < 2.5GeV/c and mass<0.3GeV/c2.

Xuan Li 49

FMS triggered pp data (60%) FMS triggered dAu data(30%)

Width 0.722 ± 0.048 Width 0.728 ± 0.028

No significant broadening in FMS-BEMC π0 jet-like correlations.

BEMC efficiency studies

Xuan Li 50

Ermes Braidot thesis Fig 5.18

• π0 efficiency in the BEMC