# Presentation: 401971

## EM Probes at RHIC II

**Zhangbu Xu (BNL)**
**For the RHIC-2 EM-Probes Working Group**
**(co-convenors: G. David, R. Rapp, XZB)**

- http://www.phenix.bnl.gov/WWW/publish/david/rhicii_em
- Talks by members at April Workshop
- R. Rapp (Nov. PAC meeting)
- Simulations from individuals
- First Draft of Working Group Document

## Outline

- Chiral Symmetry Restoration
- QGP Radiation -- Temperature
- 2. **Experimental Measurements**
- Vector Meson In-medium Properties
- Thermal Dilepton Spectra
- Direct Photons
- 3. **Challenges: **
- Chiral partners?
- Understanding backgrounds
- What do we learn from SPS results
- 4. **Conclusions**

**Outline**

## Search for Chiral Symmetry Restoration

- scholar.google.com: 4,130www.google.com: 76,700

- Responsible for >90% mass of the visible world

## QCD Phase Diagram

**A thermodynamic state** is specified by a set of values of all
- the thermodynamic parameters necessary for the description of the system. --- statistical mechanics by K. Huang
- Temperature (**T**), chemical potential (****, pressure(**P**) ...

- Chemical/thermal Equilibrium at certain stage of the evolution
- At the predicted QCD phase boundary
- persistent from SPS to RHIC
- Below the phase boundary atAGS and SIS
- Can we put a point above?

## Experimental Signatures

- Vector Meson Properties
- Thermal Dileptons
- Direct Photons

##  EM Emission and Chiral Symmetry

- [Brown-rho, Rapp...]

- nucl-th/0409054

##  Low-Mass Vector Mesons at RHIC

-   **hadron liquid** close to **T****c**: ****, **** “**melt**”, **** more robust?
**  ****baryon+antibaryons** important

- Energy Scan!!

## Dilepton from HI

**smooth transition HG → QGP liquid?**
**Integrated yields from several stages**

##           QGP Temperature and Composition

-  thermal window: ***q******0 ******, M******ee******≈ 2GeV***
-  photons: jet-QGP interactions compromise T-sensitivity
-  less so for dileptons;  charm?

**Direct Photons**

**Intermed.-Mass Dileptons**

- Energy Scan

## Detector Upgrades

- PHENIX
- Hadron Blind Detector (HBD) reject  conversion and Dalitz decay
- Silicon Vertex Tracking DetectorMeasure charm background
- STAR
- Time-of-Flight TPC+TOF+EMC Electron/muon Identification
- Heavy Flavor Tracker (HFT) TPC+HFT reject  conversion, Dalitz decay and HFe
- Data Acquisition System (1KHz minbias rate)

## Measuring Low Mass Electron Pairs in PHENIX 

**   **_**Hardware**_
-  Compensate magnetic field with an **inner coil** to
-   preserve e+e- pair opening angle (foreseen in
-   original design   B0  for r   50-60cm)
-  Compact **HBD**** **in inner region

**    **_**Strategy **_
**  **Identify signal electrons with p > 200 MeV/c from
-    vector mesons in the outer PHENIX  detectors
-  Identify low momentum electrons with p < 200 MeV/c
-   (mainly from Dalitz pairs and conversions) in the HBD
-  Reject pair if opening angle < 200 mrad
-   (for ~ 90% rejection).

- _Requirements__ _
-  * Electron efficiency     90%
-  * Double hit recognition     90%
-  * Modest    rejection  ~ 200

- C. Woody, RHICII EM probes group meeting, 04/30/05

**Notes:**
   This slide should be modified to reflect the new geometry of the HBD

## Resulting Low Mass Pair Spectrum  with the HBD

-  Combinatorial background is
-   reduced by more than two
-   orders of magnitude
-  limited by combinatorial
-  background from open charm
-  A precision measurement of
-   charm using the Silicon
-   Vertex Tracking Detector.

- C. Woody, RHICII EM probes group meeting, 04/30/05

## Rates for Vector Meson Production 

-   ’s produced in || < 0.5
-   e+,e- into **PHENIX** central
-    arm acceptance
-   pT,e > 200 MeV/c

- Consider Run 8 (200 GeV Au x Au, 4x Design Luminosity) :
-   Lpeak = 30 x 1026 cm-2 s-1
-   L ave store = 8 x 1026 cm-2 s-1
-   20 KHz peak min. bias rate
-   5.4 KHz avg min. bias rate
-   10 week dedicated HBD run (central field in  configuration)
-   RHIC x PHENIX = 0.5 x 0.5 = 0.25

- N e+e- = 1.2 x 105  produced in PHENIX acceptance

- 8.2 x 109 min. bias events produced
-  N e+e- = 1.5 x 10-5 / min.bias event

******e****+****e****-**** ********PHENIX********1 : 0.9 : 0.7**

- C. Woody, RHICII EM probes group meeting, 04/30/05

- Lower energy 30GeV2 = 44

## STAR MRPC-TOFr Electron PID

**Electron identification:**
** ****TOFr  |1/ß-1| < 0.03**
** ****TPC dE/dx      electrons!!!**
**2******** in azimuth, |eta|<1 **

***electrons***

- Clean electron PID can be obtained up to PT< 3 GeV/c. 
- This is used to measure the semileptonic decay of open charm.
- Hadron rejection power at 10-5. M. Shao et al., nucl-ex/0505026
- Higher pT: TPC+TOF+EMC

- STAR Collaboration, PRL 94, 2005, 062301

## Reject  conversion

- Reject  conversion

- Background:   e+e-
-  HFT discriminates background !
-  Need low mass detector
-  Also:  DD  e+e-

## Dalitz Decay

- TPC e+e- invariant mass
- γ conversion
- π0 Dalitz decay

- Find the pair from Dalitz,reject both leptons

## Charm Spectrum Measurement

-  Au + Au, 50M central events
-  D0  K + 
-  Stat. uncertainties small

- We can subtract DDbar  e+e-
- Alternative Approach: reject electrons event-by-event
- (~50% for electron pair)
- HFT resolution <10m

## STAR Dilepton Capability

- TOF: vector meson
- TOF+HFT: thermal radiation

## Rate Estimate

- Assume:

- (pp)

- From PDG:

- (AuAu)

- TOF match+PID eff ≈ 80%

- TPC+HFT eff ≈ 60% (?)

| Au+Au | #events for ω
with 3σ signal | #events for Φ with 3σ signal |
| --- | --- | --- |
| TOF+TPC | 7M | 2M |
| TOF+TPC+HFT  | 200K | 100K |

- Preliminary estimation for the requested data

- Acceptance: N e+e- = 8 x 10-4 / min.bias event

## Muon ID at low pT

- 

- e

- 

- STAR Preliminary:
- 0.15<pT<0.25 GeV/c, DCA<3cm

-  less Dalitz decay and  conversion (<x10), Decays from ,K rejected by HFT DCA
- Dilepton mass: 0.2<m<0.6 GeV

## Direct Photons

- Direct Measurement of direct photons
-    All  – known sources
- Virtual photon (via low-mass dilepton)
-  HBT

## VirtualDirect Photons

**Virtual********Direct Photons**

## Comparison to Conventional result

**Comparison to Conventional result**

- Conventional method:all-

##  HBT

- Thermal model for direct photon production
- Assuming =1
- EMC+TPC efficiencies:50%+(5—9%)
- 16 million Au+Au events
- Sandweiss/Chikanian (Yale)

## Challenges

- Lack of measurements of Chiral Partners
- Backgrounds on dilepton, direct photon measurements
- What we learn from SPS Measurements

## Chiral Partner (  a1)

**Chiral Partner (******** ******** a****1****)**

- UrQMD
- minimum bias Au+Au
- at **√**s**NN** = 200 GeV

- h**1** ****** **3 π**    **B.R. = 0.1
- h**1 ******** **ρ π**    **B.R. = 0.9
- Mass = 1.170 GeV/c**2**
- Γ = 360 MeV/c**2**

**** 3σ signal a**1**** ******** ** π**±** **** 54M events ! Interesting but difficult (Type B)

- 
- Cocktail

- P. Fachini

## Vector MesonCSR

**Experimental observation of modification of  Vector Spectral Function significant by itself**
**Model calculation of V-A Spectral Function:**Calculate vector and axialvector spectral functions as a function of temperature and density (including as many of the constraints, Weinberg sum rule) in a chirally invariant model.
**Compare results with Lattice QCD**** **
**Detailed Experimental-Thoery comparison vs energy/density scan**Perform detailed comparisons of the in-medium effects on the vector correlator with dilepton data (centrality, excitation function, mass and q_t-spectra), which requires a realistic expansion model (e.g. hydro/transport).

## Charm Background in PHENIX Dilepton

- Mass (GeV)

- subtracted spectrum
- thickness of line
- represents
- statistical error

- Vacuum Vector
- mesons

**QGP**

- 15% at m=2GeV
- Needs ~+-2%

- R. Seto, RHICII EM probes group meeting, 04/30/05

- crude charm RHIC 2

**Charm Background in PHENIX Dilepton**

## Charm/Dalitz Background in STAR

-  DDbar  e+e- correlations can be measured (<+-10%)
- Dalitz rejection: Inner Detector tracking: SSD+HFT+Vertexin progress

## Direct Photons Temperature

- pQCD+QGP+HG+Jet
- Extract temperature difficult
- Energy Scan important
- Detailed model comparison

## Residual Correlation from  HBT

- Background = 4x signal
- Subtract from known ±± HBT
- Sandweiss/Chikanian (Yale) in progress

- Similar effect for virtual photons?
- Negligible with HBD

## Recent Advances at SPS: Power of Precision  

**[RR+Wambach ’99]**

** **********-meson ****“melting” ****, dropping mass?**
** ****address: absolute norm.,   *****p******t*****-dep.,  *****M > 0.9GeV*****  (****4********→********************!)**
** “****cocktail-********“ (+smooth signal)?  vector dominance? **
** ****precision data essential to rule out models**

**Recent Advances at SPS: Power of Precision**_**  **_

**NA60**

## Electromagnetic Probes at SPS

_**Dileptons**_

**Medium**
**Effects!**
**10% **
**QGP**

** ****[RR+ **
** ****Shuryak ’99]**

**HG: **
**4********→******************
**30% **
**QGP**

## What needed at the SPS?

- _Experimentally:_
- Low-mass e+e: drop. mass vs. broad., energy scan   [run time]
- Cronin enhancement vs. temperature               [systematic p-A]
- Isolation of charm dileptons                              [vertex detector]
- Redundancy
- _Physics:_
- QGP subdominant ?!
- early thermalization (v2) ?!

- RHIC II (dedicated QCDLab) has it all

## Conclusion

**EM probes are directly sensitive** to the **chiral symmetry** properties and to the **temperatures** of the partonic system
- There are **three major measurements** related to the questions above
- Each with **complemental/redundant** techniques
**Challenges:**not only confirm a theory, but rule out incorrect ones requires precision data in M, qt , centrality and √s
**RHIC (detectors) positioned to meet the challenges:****   ****- **Dalitz suppression / charm ID;   redundancy**   - ****energy-scan (006-200GeV!?)**
**Theory Homework: **chiral theory with baryons,  sQGP,  dilepton components, ...

## Remarks

- Am I too negative? 哀兵必胜“pessimistic” troops win
- Continue pushing our detectors to their limits
- Understand the strength and weakness

- Am I too positive?I don’t think of all the misery, but of all the beauty that still remains. – Anne Frank
- Continue exploration of techniques and simulations
- Understand and reduce the background

- CHARM

- DILEPTON

- I believe we can fly, RHIC II will provide us with wings

##  2.2 EM Emission and Chiral Symmetry

-  2.2 EM Emission and Chiral Symmetry

***~ “***********- *****a******1******(1260)”  ***(chiral partners)

**Axial-/Vector in Vacuum**

**pQCD**
** ****cont.**

## Introduction I:  EM-Probes -- The Basic Questions 

**Thermalization at ****RHIC**** ******** study the phase diagram: **
-  **(highest) temperature of the matter**
** ****chiral symmetry restoration (mass generation!)**
** ****medium effects on spectral properties above + below T****c**

- Introduction I:  EM-Probes -- The Basic Questions

**Inevitable consequences of QGP,  link to lattice QCD**_ _

**QCD Phase Diagram**

## (ii) Heavy Partons + Multiple Bound States

- → based on finite-***T*** lattice potentials_**  **_
- approach to “zero-binding line”  ~ stable-mass************-resonance

**[Shuryak,Zahed,**
**     ****Brown, ...]**

-  thermal parton scattering through bound states?
-  composite interactions?      quark scaling?

## Cocktail comparison

-  Data and cocktail absolutely normalized
- Cocktail from hadronic sources
- Charm from PYTHIA
- Predictions are filtered in ***PHENIX acceptance***
- Good agreement in 0 Dalitz
- Continuum:hint for enhancement not significant within systematics
- What happens to charm?
- Single e  pt suppression
- angular correlation???
-  LARGE SYSTEMATICS!