# Presentation: 192704

## The Search for the Critical Point of QCD:

**The Search for the Critical Point of QCD:**
**STAR Capabilities for Low ********s****NN** **Running**

_**OUTLINE**_
-  ** ****Motivation**
** ****Theoretical predictions **
** ****Results from SPS and RHIC**
** ****Low sqrt(s) RHIC running**
** ****RHIC conditions**
** ****RHIC limitations**
** ****STAR Capabilities**
** ****Trigger**
** ****Physics**
** ****Event Rates & requirements**
** ****Summary**

- Workshop:
- Can We Discover QCD Critical Point at RHIC
- Brookhaven National Laboratory
- March 9-10, 2006

- Tapan Nayak
- (for the STAR Collaboration)

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## The phase diagram

**The phase diagram**

-  SPS top energy is below the curve and RHIC data so far are above.
-  It will be worth having a look at the systems produced near the QCD phase boundary.
-  RHIC has a unique possibility to scan the full range from AGS to top RHIC energy.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Lattice predictions

- Gavai, Gupta hep-lat/0412035

- CRITICAL END POINT

**Energy Density**

**Temperature**

- F. Karsch, Prog. Theor. Phys. Suppl. 153, 106 (2004)

- T**C **~ 170 15 MeV
- **C** ~ 0.7-1.5 GeV/fm3

**Lattice predictions**

- Lattice calculations suggest that the Critical Point of QGP phase transition may be located at about  the SPS energies (c.m. energy of 5-20 GeV/nucleon)

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Bjorken estimation of initial energy density

- Boost invariant hydrodynamics:

- Bjorken density as function of #of participants

- Assuming Bjorken energy density is similar to that of the lattice calculation: in order to probe the Critical Point (******C**** ~ 0.7-1.5 GeV/fm****3****) **– the c.m. beam energy need to be between 5-20GeV.

- Raghunath Sahoo,

- Bjorken density as function beam energy

- STAR

- STAR Preliminary

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Kinetic freezeout from AGS->RHIC

- TKFO (RHIC)  =  100 ± 10 MeV

-  Rapid change in freeze-out temperature and flow velocity between 2-20GeV
-  Explosive Transverse Expansion
- at RHIC **** High Pressure

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

##    Elliptic flow: The scaling of v2/ε 

- The scaling of the strength of the elliptic flow v2 with eccentricity shows that a high degree of collectivity is built up at a very early stage of the collision.
- With low energy beam the lower points can be scanned more precisely.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Energy dependence of <mT> and temperature

- Mohanty, Alam, Sarkar, Nayak, Nandi
- PRC 68 (2003) 021901

- Energy dependence of <mT> and temperature

-  Rapid rise in <mT> going from AGS to SPS energies and slowing down towards RHIC.
-  Step like behavior in the inverse slope parameter in heavy-ions around SPS energies, but not in pp.

- Inverse slope parameter (MeV)

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Energy dependence of particle ratio and fluctuations

- <K+/+>

- <K/>

- _Particle Ratio:_
- K/ shows  increasing behavior with energy, whereas a horn structure seen in K+/ +

- Christof Roland (NA49)

- _Fluctuation in Ratio:_
-  K/ fluctuations increase towards lower beam energy.
-  p/ fluctuations are negative, indicating a strong contribution from resonance decays
-  STAR results are being finalized.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Beam energy dependence of pion HBT 

- Debasish Das

- Pion rapidity density is proportional to the freezeout volume => Constant Freezeout Volume (freezeout at a constant density).

- STAR

- S. Voloshin, QM02

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Energy dependence of strange baryon production

- Constant s-Baryon production
- at mid-rapidity.

- s-Baryon production rises
- smoothly at mid-rapidity.

- Sevil Salur, QM2005

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Energy dependence of ratio of strange baryons

- Sevil Salur, QM2005

**Anti-baryon/baryon ratio**

-  The ratio of strange to anti-strange baryons goes up with energy.
-  At RHIC energies the production of strange and anti-strange baryons is equal.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Energy dependence of balance functions

*W* is a normalized measure of the time of hadronization with respect to uncorrelated data sample.
- This is consistent with delayed hadronization at RHIC compared to SPS energies.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

##  <pT> fluctuations

** ****<p****T****> fluctuations**

- Establishing the scale for studying <pT> fluctuations.

- Fractional increase of width of the <pT> distribution w.r.t. to a statistical reference.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Understanding contribution of minijets at low pT

- Understanding contribution of minijets at low pT

-  STAR has the capability to study fluctuations in fine bins of  and .
-  The localized measures of fluctuation is necessary to understand the underlying structure in an event including the contribution of mini-jets.
-  Study of the phase boundary at large sNN has to take the presence of minijets into account.
-  Minijets might help in our understanding of the phase boundary.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## A note on low energy RHIC running

- From: Wolfram Fischer, BNL C-AD   Nov 2005

- No hard limit for the RHIC collision energy down to c.m. energy of 4.6 GeV/n.
- The beam and luminosity lifetimes will decrease gradually with the lower energies.
- Because the ZDC counters will be ineffective – tuning can be done using Beam-Beam-Counters (BBC).
- Low Energy beam runs including cooling has to be tested in advance.
- Details:
- Current regulation of the main dipoles should work even at 50A.
- During RUN-2 with AuAu collisions of 9.8GeV/n beams, luminosity was delivered at a rate of 1 inv. micro-barn/week. The rates drop with about gamma^3 to gamma^4
- Operation near AGS transition energy has to be checked.
- Electron cooling at low energies needs study.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## The STAR experiment

**Magnet**
**Coils**
**Central **
**Trigger**
**Barrel **
**(CTB)**
**ZCal**
**Time **
**Projection **
**Chamber**
**(TPC)**
**Year 2000**

**Barrel EM Cal (BEMC)**
**Silicon Vertex Tracker (SVT)****Silicon Strip Detector (SSD)**
**FTPC****Endcap EM Cal****FPD**
**TOFp, TOFr**
**FPD**
**Year 2001+**

**PMD**

- _Large acceptance: 2____ coverage at mid-rapidity_

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Inclusion of ToF detector

- The STAR experiment (with the inclusion of TOF) is an ideal detection device to search for the Critical Point of QCD and to carry on a systematic study of majority of the physics topics being addressed.

- Fabrication of MRPC based Time-of-Flight (ToF) detector for STAR is in progress. It is scheduled to be installed for RHIC RUN-9.

**ToF**

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Particle production at low sNN energies

**uRQMD**

- Particle production at low ******s****NN** energies

- Pseudo-rapidity distribution of charged particles at 4 different centralities.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Second order 

- event-plane resolution

- Event plane resolution in STAR will be better than what was achieved at NA49.

- STAR
- TPC Acceptance

******s****NN** = 8.75GeV

- Coverage in terms of beam-rapidity increases as we go to lower energies.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## STAR Trigger for 

- Low Energy RHIC Running

- _Trigger Detectors in STAR_

- ZDC

- ZDC

- BBC

- BBC

- BEMC & EEMC
- (only a part of the detectors shown)

- CTB

- FPD

- FPD

- We set up all collision triggers (e.g. ZDC coin., BBC coin., CTB multiplicity, etc) and run them all simultaneously without pre-scales.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Triggering with STAR Beam-Beam Counter (BBC)

- STAR uses two BBCs wrapped around the beampipe: one on either side of the TPC. Each counter consists of two rings of hexagonal scintillator tiles: an outer ring composed of large tiles and an inner ring composed of small tiles. Internally, each ring is divided into two separate sub-rings of 6 and 12 tiles each.

| Centrality
(b in fm) | AuAu Ec.m.= 5 GeV |  |  | AuAu Ec.m.= 8.75 GeV |  |
| --- | --- | --- | --- | --- | --- |
|  | BBC Inner 3.3<<5.0 | BBC Outer 2.1<<3.3 |  | BBC Inner 3.3<<5.0 | BBC Outer 2.1<<3.3 |
| b<3 | 5 | 27 |  | 12 | 54 |
| 3<b<6 | 11 | 30 |  | 21 | 57 |
| 6<b<9 | 22 | 35 |  | 39 | 40 |
| b>9 | 44 | 30 |  | 66 | 8 |

- BBC can effectively be used for triggering for low energy runs.

- Table below gives the #of particles within BBC coverage for
- two c.m. energies and four different centralities:

- BBC is sensitive down to single MIP falling on the detector.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## STAR physics: option-1 

- (for low energy RHIC running)

- Particle spectra (pT, (pseudo)-rapidity distributions)
- Flow (v1, v2, v4 ....) with charged and identified hadrons
- Strangeness production (k/pi ratio)
- Resonance
- HBT Radii
- Fluctuations (net charge, k/pi ratio, baryon number....)
- Correlations
- Formation of Disoriented Chiral Condensates (DCC)
- Using PMD–FTPC and BEMC–TPC
- Long range (forward-backward) correlations

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Required statistics for different observables

**At ********s****NN** = 10GeV

- Limit for a thorough measurement: 20M events

- Very basic measurement: 2M events

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## BBC Coincidence Rates and 

**Beam-time estimation for option-1**

- CuCu 20GeV: beginning of spill: 3000
-                        average over time: ~1000
- AuAu 20GeV: 1000/4 = 250.
- Optimistic view: rates scale as 3

- This energy scan will take a total of 17 weeks for STAR physics Option-1.

| E c.m. | BBC Coin Rate | #of days/1M
(1day=12 hr) | #of events needed | #of days of beam |
| --- | --- | --- | --- | --- |
| 4.6 | 4 | 6 | 10M | 60 |
| 8 | 16 | 1.5 | 20M | 30 |
| 12 | 54 | 0.5 | 20M | 10 |
| 16 | 100 | 0.25 | 20M | 5 |
| 20 | >100 | 0.25 | 20M | 5 |
| 40 | >100 | 0.25 | 20M | 5 |

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## STAR physics: Option-2

- (for low energy RHIC running)

- All STAR Physics Option-1 and in addition:
- v2 of Omega (centrality dependence)
- v2 of phi (centrality dependence)
- Jet quenching, Energy loss (*R**AA** and R**CP*).
- For *R**AA*: pp reference will be needed.

| E c.m. | BBC Coin Rate | #of days/1M
(1day=12 hr) | #of events needed | #of days of beam |
| --- | --- | --- | --- | --- |
| 4.6 | 4 | 6 | 10M | 60 |
| 10 | 30 | 0.75 | 50M | 38 |
| 15 | 100 | 0.25 | 50M | 25 |
| 20 | >100 | 0.25 | 50M | 25 |
| 40 | >100 | 0.25 | 50M | 25 |

- A high statistics energy scan will take total of 25 weeks.
- This option is preferable.

- > 50M events needed at
- each energy.

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Beam-time estimation for a basic energy scan

| E c.m. | B | BBC Coin Rate | #of days/1M
(1day=10 hr) | #of events needed | #of days of beam |
| --- | --- | --- | --- | --- | --- |
| 4.6 | 570 | 3 | 9 | 5M | 45 |
| 6.3 | 470 | 7 | 4 | 5M | 20 |
| 7.6 | 410 | 13 | 2 | 5M | 10 |
| 8.8 | 380 | 20 | 1.5 | 5M | 7.5 |
| 12 | 300 | 54 | 0.5 | 5M | 2.5 |
| 18 | 220 | >100 | 0.25 | 5M | 1.5 |
| 28 | 150 | >100 | 0.25 | 5M | 1.5 |

- 5M events: 12.5 weeks: JUST enough, Errors factor 2-4 better than NA49

**Beam-time estimation for a basic energy scan**

- Workshop:
- Can We Discover QCD Critical Point at RHIC
- Brookhaven National Laboratory
- March 9-10, 2006

-  cleanup
-   vertex
-   efficiency
-  Gunter: 200K 0-5%, minbias:4M

- March 10, 2006

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Summary

**Physics measure **

- AGS  SPS          RHIC

-  The QCD phase boundary is worthy of study, including that of the tri-critical point.
-  STAR experiment with the inclusion of TOF will be the ideal place for this study.
-  **Current investigations don’t indicate any problem carrying out this program with the STAR detector**.
-  The RHIC program looks most promising.

**           ****Energy Density**

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006

## Thanks to the organizers of the Workshop:

- “Can We Discover QCD Critical Point at RHIC”
- Brookhaven National Laboratory
- March 9-10, 2006

- Thanks to STAR Collaborators and in particular:
- Paul Sorensen
- Bill Christie
- Jamie Dunlop
- Nu Xu
- Zhangbu Xu
- Peter Seyboth
- Tim Hallman

- Workshop: Can We Discover the QCD Critical Point at RHIC

- March 9, 2006