IN PREPARATION FOR PHD CANDIDACY REVIEW SKY D ROLNICK 10/29/08 Measuring Chiral Symmetry Restoration via low-mass e+e- pairs in Heavy Ion Collisions

22 Outline Introduction to Heavy Ion Physics Theoretical overview of Chiral Symmetry Previous results at CERES, NA60, PHENIX Hadron Blind Detector (HBD) Clustering Algorithms for HBD A look at run7 AuAu data Timeline for Chiral Symmetry Restoration Big 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– 10MeVc 2 p and n masses: 950MeVc 2

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:  0   e+e-    e+e-    0 e+e-    e+e- Direct:   e+e-   e+e-   e+e- J/   e+e-  ’  e+e- Heavy flavor: cc  e+e- +X bb  e+e- +X Drell-Yan: qq  e+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 Vacuum ρ Rapp-Wambach

12 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 〈 dN ch /d  〉 = 140 All the curves (vacuum , dropping mass and broadening) Theoretical yields normalized to data for M<0.9 GeV

13 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)

14 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+ ee   ~12 m

15 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+ e-e- e+e+

16 Results from PHENIX arXiv: arXiv: Au+Au – Large enhancement in low mass region 150MeV < m ee < 750MeV. p+p - Excellent agreement with hadronic cocktail including Dalitz decays π 0, η, Direct decays ρ, ω and φ, and open charm contributions.

17 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/c 2 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!

18 Results from PHENIX

19 signal electron Cherenkov blobs e+e+ e-e-  pair opening angle ~ 1 m Hadron Blind Detector (HBD) HBD Gas Volume: Filled with CF 4 (L RAD =50 cm) Cherenkov light forms “blobs” on an image plane (r BLOB ~3cm) Electrons radiate, but hadrons with P < 4 GeV/c do not Total Radiation Length<5% Dalitz rejection via opening angle

20 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 panelTriple GEM module with mesh grid Very low mass (< 3% X 0 including gas) Design and Construction

21 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  m 5  m V Gas gain C. Altunbas et al, NIM A, 490 (2002)

22 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 ~ 10 4 Amplified signal is collected on pads and read out Primary ionization signal is greatly suppressed at slightly negative E d while photoelectron collection efficiency is mostly preserved

23 96 pre-amps/board (1152 per HBD) Minco heaters to help in H 2 O evaporation HV panels Gas valves Beampipe Final Construction

24 HBD Installed in PHENIX HBD West (front side) Installed 9/4/06 HBD East (back side) Installed 10/19/06

25 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.

26 MC Estimate of HBD Effectiveness =15 Number of photoelectrons 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. Estimated Run 7

27 A look at run7 AuAu data Invariant mass spectra applying background rejection cuts on HBD data.

28 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.

29 Hub n’ Spoke Algorithm Hub radius = 2 Photoelectrons = 100 Mean Scint per pad = 0.3 Mean Single = 8 Mean Double = 8 Both electrons identified in hub. Electron response shared by hub and spoke. 2 nd electron response identified in spoke. Single electron response identified in hub. Doubles Three-tuple – Hub Spoke

30 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).12%.2% HBD will introduce:

31 Projections for RHIC: high energy impact of the HBD will be quite large!

32 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 – 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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