D. Cebra

July 18-24, 2002

QM2002 Nantes

1

QM2002 Poster - Version updated 13-Jul-02

Daniel CebraUniversity of

California, Davis

For the STAR Collaboration

Rapidity (dN/dy) and Transverse Mass (mt) Disributions for +/-,K+/-, Proton and Anti-Proton Production from AuAu Collisions at

sqrt(sNN)= 19.6 GeV

D. Cebra

July 18-24, 2002

QM2002 Nantes

2

Abstract

Rapidity (dN/dy) and transverse Mass (mt) distributions for+,-, K+, K-, p, p- from central AuAu collisions at sqrt(sNN) = 19.6 GeV are studied as a function of rapidity for -0.4 <y <0.4 using the Solenoidal Tracker At RHIC. The mt-m0 spectra for these particles are used to determine the temperature and radial flow of the the emitting source and the yield of each particle species. Using the mid-rapidity p-bar/p ratio, and also the K-/K+ ratio which is sensitive to the valence quark content of the kaons, one can infer the net-baryon density. These data are of particular interest since this energy and baryon density regime is comparable to that for fixed target PbPb collisions at 158 AGeV (sqrt(sNN) = 17.2 GeV) at the CERN Super Proton Synchrotron (SPS). The systematic trend of the data from the baryon rich environment in 40 AGeV fixed PbPb collisions at the SPS (sqrt(sNN) = 8.8 GeV) to the baryon depleted region at RHIC (AuAu at sqrt(sNN) = 130,200 GeV) will be examined. Mid-rapidity pion ratios are used to image the residual positive charge of the pion emitting source and comparison is made to corresponding results from the the SPS, AGS, SIS, and RHIC (AuAu collisions at sqrt(sNN) = 130,200 GeV). The shapes and magnitudes of the rapidity density plots for +,-, K+, K-, p, p- are contrasted with those at higher and lower energy, and the scaling behavior for these distributions is examined in the context of the gluon saturation model.

D. Cebra

July 18-24, 2002

QM2002 Nantes

3

A Typical 19.6 GeV event.

Note that the track multiplicity is generally lower than STAR events at 130 or 200 AGeV.

D. Cebra

July 18-24, 2002

QM2002 Nantes

4

The STAR Au+Au 19.6 GeV Data Set

• The 19.6 GeV data set was taken on the final day of year-two heavy-ion running (09:15 25-Nov-01 to 06:26 26-Nov-01). The detector was run at HALF FIELD.

• 12 data runs were recorded (2329088,...91-94,...100-101, 2330002-2330005)

• “Minimum bias” trigger (ZDC coinc.&CTB>15 .or. No ZDC&CTB>600)

•The production data is the pico-DSTfrom:

/auto/stardata/starspec3/flow_pDST_production/reco/minBias22GeVZDC/ReversedHalfField/P02ge/2001/

• With no event selection cuts, we have: Nevents = 175466.

• Selecting on collisions with vertices within 30 cm the center of the detector reduces this to 42412.

• Total number of good top 10% centrality events = 5106.

D. Cebra

July 18-24, 2002

QM2002 Nantes

5

Event Vertex Cuts

Vertex Z location Vertex X and Y Location

Bef

ore

Cu

tsA

fter

Cu

ts

Note: The beampipe is

evident.

Note: The beam

diamond is quite

broad.

25.1 % of events pass

this cut96.3 % of events pass

The beam diamond for the 20 GeV run was fairly large. We imposed event vertex cuts to restrict our analysis to events with vertices within the center 30 cm of the detector, where the acceptance is the most uniform. We selected events with vertices within the center of the beampipe to eliminate contributions from triggers on secondary interaction with the beam pipe material.

D. Cebra

July 18-24, 2002

QM2002 Nantes

6

Triggering for the 19.6 GeV dataset

ZDC Trigger(147953)

CTB Trigger(27513)

Nch

Co

un

ts

The STAR trigger was not optimized for 19.6 GeV Collisions. The correlation betweenZDC and CTB that had been observed in the 130 and 200 GeV data sets was not seen in the lower energy collisions. To approximate a minimum bias trigger, events were selected if the satisfied either a minimum ZDC threshold or a minimum CTB threshold.

D. Cebra

July 18-24, 2002

QM2002 Nantes

7

Centrality Determination

5%

10%20%30%

80% 60%

50% 40%

The raw mid-y (||<0.5) charged particle multiplicity is shown on the left for the three RHIC collision energies. On the right the same data are scaled both horizontally and vertically to estimate the best positions for the centrality bias cuts. The cut lines correspond to percentages of the total minimum bias cross section. (Note the 20 GeV trigger had inefficiencies for collisions in the impact parameter range corresponding to 10-30%)

D. Cebra

July 18-24, 2002

QM2002 Nantes

8

An Alternate Approach to Centrality Determination

Put 10% Cut Here

Limit to Correlation

The Multiplicity distributions are scaled to thepoint where the correlation between mid- and higher- particle yield starts to break down.The Cut locations determined with this methodare indistinquishable to those determined in the previous method.

D. Cebra

July 18-24, 2002

QM2002 Nantes

9Fit Points

68.2% Pass

Track Quality Cuts

DCA (cm)

100% Pass

Tracking Percentage

98.8% Pass

Chi squared

91% Pass

In addition to event quality cuts, individual track quality cuts were also imposed on the data. The distance of closest approach between the track and the main event vertex was limited to 3 cm. The 2 of the helix fit to the found points was limited to 2.5. The total number of points used in the fit was required to be greater than 25 (out of a maximum of 45). And the fraction of points used in the fit was required to be greater than half the possible number to avoid split tracks.

D. Cebra

July 18-24, 2002

QM2002 Nantes

10

Particle Identification

protons

Kaons

pions

electrons

Particle identificationis achieved through relative ionization in the TPC gas. The plot to the right shows a scatter plot of the <dE/dx> of a track versus the log of its total momentum.

D. Cebra

July 18-24, 2002

QM2002 Nantes

11

Fits to the dE/dx distributions

u = 2.8 ln (dE/dx) + 1.5

We use a Bethe Bloch parametrization with three free parameters to account for detector effects. We determine these values with the well IDed particles and fine tune the parametrization for eachcondition.

Fix the centroids and wdiths rigorously. Onlyfree parameters in the fits are the amplitudes.This extends the fitting range beyond previous analyses. Fit for e, , K, p.

pions

Kaonsand

protons

electron

e K p

D. Cebra

July 18-24, 2002

QM2002 Nantes

12

Still need to consider proton background!

Particle Ratios - Au+Au @ 19.6 GeV

p-/p+

R = 0.611 +/- .013 stat R = 0.093 +/- .003 stat +/- .004 sys

Pion Ratio has structureas a function of mt-m0.

Ratio is independent of mt-m0

The first observables are the particle ratios. Theseoften require the fewest corrections.

STAR PRELIMINARY RESULTS

D. Cebra

July 18-24, 2002

QM2002 Nantes

13

Pion Ratios - Imaging the Source

A comparison of the pi+/pi- ratios at different energies. From this comparison, we can find the Coulomb potential assuming a spherically symmetric geometry. As can be seen, the potential decreases as the collision energy increases. This is evidence of the depletion of the initial baryons in the central region.

E866 PRC57 (1998) R446KaoS PLB420 (1998) 20

D. Cebra

July 18-24, 2002

QM2002 Nantes

14

Kaon and Proton Ratios

Results are consistent with expectations

STARPreliminary

results

PRL04778

Publicationin prep

J. Berger

Proton and kaon ratios for central events both increase with collisions energy. This illustrates the reduction in the net baryon content and/or the baryon chemical potential across this collision energy range from the AGS to the top energies at RHIC. At 19.6 GeV the collisions produced Baryon-rich matter.

E866/917 nucl-ex 0008010WA97 JPhys G25 (1999) 171NA44 JPhys G23 (1997) 1865NA49 NP A661 (1999) 45c

D. Cebra

July 18-24, 2002

QM2002 Nantes

15

Estimated Efficiency Corrections

TPC efficiency is influenced by many effects, including event multiplicity, vertex location, and detector configuration. We determined our efficiency for this analysis by simulating tracks in GEANT and then embedding those tracks into full events.

These events are then analyzed and found tracks are associated with those that were embedded. This allows us to accurately determine the efficiency for a given detector configuration. In these figures, we show the embedded track distribution for midrapidity pions and in red the found track. The ratio of found to embedded is then the efficiency. Similar analysis is done for all other particle species and rapidity bins.

D. Cebra

July 18-24, 2002

QM2002 Nantes

16

Efficiency Corrected Mid-rapidity Spectra

-

-

p-

+ + p+

A blast wave model simultaneousfit to these spectra yields:

Tfo = 104 +/- 4 MeV r = 0.47 +/- 0.12

STAR PRELIMINARY RESULTS

D. Cebra

July 18-24, 2002

QM2002 Nantes

17

dN/dy near mid-rapidity

- +

-

+

p+

NA49 ResultsCERN Beam Request

p

The yields of the various particle species near mid-rapidity are consistent with similar measurements at the SPS. The pions and positive protons demonstrate yields which are constant as a function of rapidity near mid-rapidity. The yields of the kaons and p-bars are peaked at mid-rapidity.

19.6

GeV

dat

a

D. Cebra

July 18-24, 2002

QM2002 Nantes

18

Mid-rapidity K to ratios

19.6 GeV Preliminary STAR resultFigure fromNA49 Beamrequest

R=0.187 +/- 0.004 stat +/- 0.031 sys

R=0.111 +/- 0.003 stat +/- 0.029 sys

D. Cebra

July 18-24, 2002

QM2002 Nantes

19

dN/d as a Function of Centrality

Top 5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-80%

19.6 GEV STAR PRELIMINARY RESULT

The yields of charged particles as a function of centrality and rapidity show a dip at mid-rapidity for central events and are flat for peripheral events.

300

200

100

D. Cebra

July 18-24, 2002

QM2002 Nantes

20

dN/d Collision Energy Systematic

The charge particle yield at mid-rapidity for central events (top 5 or 6%) is consistent with the trends established by AGS, SPS, and higher energy RHIC data.

Phobos PRL 85 (2000) 3100Phobos nucl-ex 018009 (2002)PHENIX PRL 86 (2001) 3500STAR(130) nucl-ex 106004 (2001)BRAHMS QM2001NA49WA98 nucl-ex 0008004 (2000)WA97/NA57 CERN-EP-2000-002E866/917 PRC59 (1999) 2173E877 PRC51 (1995) 3309

WA98

WA97/NA57Phobos

NA49

E917/866

STARPRELIMINARY

RESULT

E877

PHENIX

BRAHMS

D. Cebra

July 18-24, 2002

QM2002 Nantes

21

v1 vs. Eta

Most Peripheral Semi-Peripheral

Semi-Central Most Central

Directed Flow results for the 19.6 GeV Dataset

STAR Preliminary

Results

D. Cebra

July 18-24, 2002

QM2002 Nantes

22

v2 vs. Eta

Most Peripheral Semi-Peripheral

Semi-Central Most Central

Elliptic Flow Results for the 19.6 GeV dataset.

STAR Preliminary

Results

D. Cebra

July 18-24, 2002

QM2002 Nantes

23

Conclusions

• Studies of Pion ratios demonstrate the reduction of source charge consistent with a transport of the incident baryons beyond mid-rapidity.• Kaon and Proton ratios are consistent with SPS trends. The 19.6 GeV collisions create baryon-rich matter.• The temperatures and radial flow are consistent with other RHIC results, but suggest temperatures below those previously reported for SPS collisions.• dN/dy’s show magnitude and shape consistent with SPS results. The protons exhibit an elongation along the beam axis consistent with incident baryon transport. The produced kaons and p-bars are peaked at mid-rapidity. The pions, which have a strong delta resonance contribution are flat at mid-rapidity similar to the protons.• Kaon to pion ratios consistent/low wrt SPS. • dN/d are consistent wrt SPS and RHIC results. • Preliminary Flow results.

• Generally - agreement with SPS results.