Rapidity dependence of azimuthal correlations for pp and dAu Xuan Li (Shandong Uni. &BNL) For the STAR Collaboration WWND 2011 (Winter Park)
Jan 19, 2016
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