IN PREPARATION FOR PHD CANDIDACY REVIEW

SKY D ROLNICK10/29/08

Measuring Chiral Symmetry Restoration via low-mass e+e- pairs in Heavy Ion Collisions

22

Outline

Introduction to Heavy Ion PhysicsTheoretical overview of Chiral SymmetryPrevious results at CERES, NA60, PHENIXHadron Blind Detector (HBD)Clustering Algorithms for HBDA look at run7 AuAu dataTimeline for Chiral Symmetry RestorationBig Picture Stuff

33

QCD and the femtoworld

4

What do we know about QCD?

QCD phase diagram.

66

What is Chiral Symmetry?

May be defined as a flavor symmetry of QCD that exist in the limit of vanishing quark masses.

In the limit of chiral symmetry, the “left” handedness and “right” handedness are preserved.

77

Chiral Symmetry Breaking

Think of Chiral Symmety breaking as origin of hadron mass.

• u and d quark masses: 5–10MeVc2

• p and n masses: 950MeVc2

88

QCD Vacuum and Chiral Condensate

Just like the Higgs except pions are the Massless Goldstone modes!

Spontaneous breaking of this approximate symmetry gives very small pion masses; in the limit of exact chiral symmetry these particles would be massless.

For two flavors (Nf = 2) these are Nf^2 Goldstone bosons three pions and the η meson.

There is an explicit violation of the U(1)A symmetry giving mass to one of the Goldstone bosons.

(the η meson for Nf=2)

99

Dielectrons in the fireball?

Dalitz:0e+e-e+e-0e+e-e+e-

Direct:e+e-e+e-e+e-J/e+e-’e+e-

Heavy flavor:cce+e- +Xbbe+e- +X

Drell-Yan:qqe+e-

• Dileptons directly probe the entire space-time evolution of the fireball, since they are continuously emitted during the evolution.

• Since they are not subject to strong interactions, they are not significantly affected by the medium at later stages of the collision and they freely escape from the interaction zone.

• Dileptons should be sensitive to T and B.

10

Experimental Signatures

11

Low Mass Enhancement at CERES

Brown-Rho scaling

m

m 0

qq

qq0

1/3

1

0

Vacuum ρ

Rapp-Wambach

Imem (s) mV

2

gV

, ,

2

ImDV (s)

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NA60 Data & model predictions

Compatible with broadening (RW), no mass shift observed (BR).

All calculations done (before the data were available)

by Ralf Rapp, for ⟨dNch/d⟩ = 140

All the curves (vacuum , dropping mass and

broadening)

Theoretical yields normalized to data for M<0.9 GeV

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RHIC Physics

“If it were possible to experiment with neutrons or protons of energies above a hundred million volts, several charged or uncharged particles would eventulally leave the nucleus or as a result of the encounter; with particles of energies about a thousand million of volts, we must even be prepared for the collision to lead to an explosion of the whole nucleus.”-Niels Bohr, Nature 137 (1936)

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The PHENIX Experiment

Currently, electrons are tracked by drift chamber and pad chamber

The Ring Imaging Cherenkov Counter is primary electron ID device

Electromagnetic calorimeters measure electron energy

e+e

~12 m

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Low-mass e+e- pairs in PHENIX

Typically only 1 electron from a pair falls within the PHENIX acceptance.

The magnetic field bends the pair in opposite directions. Some spiral in the magnetic field and never reach tracking detectors.

~12 m

To eliminate these problems:

Detect electrons in field-free region near beampipe with >90% efficiency.

Need HBD!!

e-

e+

e-

e+

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Results from PHENIX

arXiv: 0802.0050 arXiv: 0706.3034

Au+Au –Large enhancement in low mass region 150MeV < mee < 750MeV.

p+p - Excellent agreement with hadronic

cocktail including Dalitz decays π0, η, Direct decays ρ, ω and φ, and open charm contributions.

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Combinatorial Backgrounds

Very poor S/B ratio, ~1/200! Mostly from Dalitz decay of neutral pions and photon conversions. Main reason for systematic uncertainty

e+ e -

e+ e -

The mass range between 150 and 750 MeV/c2 is enhanced by a factor of 3.4 +/- 0.2(stat.) +/- 1.3(syst.) +/- 0.7(model) compared to the expectation from our model of hadron decays. improved data needed!

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Results from PHENIX

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signal electron

Cherenkov blobs

e+

e-

pair opening angle

~ 1 m

Hadron Blind Detector (HBD)

HBD Gas Volume: Filled with CF4 (LRAD=50 cm)

Cherenkov light forms “blobs” on an image plane(rBLOB~3cm)

Electrons radiate, but hadrons with P < 4 GeV/c do not

Total Radiation Length<5%Dalitz rejection via

opening angle

2020

20 Triple GEM Detectors (10 modules per side)

• Mesh electrode• Top gold plated GEM for CsI• Two standard GEMS• Kapton foil readout plane One continuous sheet per side Hexagonal pads (a = 15.6 mm)

Honeycomb panels

Mylar entrance window

HV panel

Pad readout plane

HV panel Triple GEM module with mesh grid

Very low mass (< 3% X0 including gas)

Design and Construction

2121

Technology of the triple GEM

F. Sauli ,NIM A 386 (1997) 531

Thin (~ 50 μm) Kapton insulator clad with copper on each side

Holes are chemically etched into the GEM

When a voltage is applied between the two sides, the high density electric field causes charged particles to avalanche

140 μm

70 μm

70 m

50 m50 m

5 m

300-500V

Gas gain 10-20

C. Altunbas et al, NIM A, 490 (2002) 177-203

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How to Blind Hadrons

Primary ionization is drifted away from GEM and collected by a mesh

UV photons produce photoelectrons on a CsI photocathode and are collected in the holes of the top GEM

Triple GEM stack provides gain ~ 104

Amplified signal is collected on pads and read out

Primary ionization signal is greatly suppressed at slightly negative Ed while photoelectron collection efficiency is mostly preserved

2323

96 pre-amps/board(1152 per HBD)

Minco heaters to help in H2O evaporation

HV panels

Gas valves

Beampipe

Final Construction

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HBD Installed in PHENIX

HBD West (front side)Installed 9/4/06

HBD East (back side)Installed 10/19/06

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HBD in run7 AuAu data

HBD Performace – Hadron Response•Clear separation between hadrons and electrons (significant improvement in e-id!)

•But lacking a clear separation between single and double electron peaks.

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MC Estimate of HBD Effectiveness

<pe>=15

Number of photoelectrons

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E Simulated detector response.

More detailed analysis of the HBD response allows better separation between the single and double peak than appears to the naked eye. A data set with a fully operational detector will allow this.

More detailed analysis of the HBD response allows better separation between the single and double peak than appears to the naked eye. A data set with a fully operational detector will allow this.

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A look at run7 AuAu data

Invariant mass spectra applying background rejection cuts on HBD data.

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Pattern Recognition & Single Particle Response

Hub n’ Spoke Clustering algorithm.

A novel clustering algorithm designed specifically to identify “single” and “double” electron events in HBD.

The hub can be defined as initial multi-pad cluster associated with track.

The spoke is defined as neighboring multi-pad cluster which can be associated with hub.

Combination of hub and spoke should account for majority of Cherenkov response.

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Hub n’ Spoke Algorithm

Hub radius = 2Photoelectrons <pe> = 100Mean Scint per pad = 0.3Mean Single = 8Mean Double = 8

Both electrons identifiedin hub.

Electron response sharedby hub and spoke.

2nd electron responseidentified in spoke.

Single electron responseidentified in hub.

Doubles Three-tuple – Hub Spoke

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How does this affect the final uncertainty bars?

S = FG -BG

With f the increase in stats, and the electron pair efficiency (same R as before)

Stat

S

1

FG

1

SBG~

1

BG

sys

Sc

BG

S

c stat (likeFG)2 ()2

.12% .2%

HBD will introduce:

BGHBD fRBG

SHBD FGHBD BGHBD fS

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Projections for RHIC: high energy

impact of the HBD will be quite large!

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Timeline for Chiral Symmetry Restoration

2009 – pp run at 500 GeV. High rates and clean signal. Fully test and calibrate HBD. Use to study hadronic cocktail and open charm

contributions.

2010 – auau run at 200 GeV 2 weeks run time gives ~50M events HBD ‘eliminates’ sys. uncertainty electron cooling in RHIC can increase the collision rate by a factor 10 ~500M events in 2 weeks

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