1. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 1 Strangeness measurements with the Experiment Gábor Veres Eötvös Loránd University, Budapest, Hungary Massachusetts Institute of Technology, Cambridge, USA for the Collaboration Strangeness in Quark Matter ’06 UCLA, California, March 27, 2006

2. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 2 Collaboration (March 2006) Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

3. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 3 Outline  Identification of strange particles in PHOBOS slow particles stopping in the active part of the detector dE/dx in the Si spectrometer Time Of Flight measurement reconstruction of  mesons  Identified particle spectra and ratios in Au+Au at 62.4 GeV  Identified particle spectra and ratios in d+Au at 200 GeV  Connections to net baryons and baryon transport  New techniques to reconstruct the  meson at low p T

4. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 4 The PHOBOS detector 137000 Silicon Pad Channels 1m Spectrometer Octagon Vertex Ring Counters Paddle Trigger Counters Čerenkov Counter Proton Calorimeter Time of Flight Spectrometer trigger

5. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 5 PID Capabilities of PHOBOS p T (GeV/c) 0.03 0.5 5.0 Stopping particles dE/dxTOF Particle ID from low to high p T E loss (MeV) 1 23 4 50 p (GeV/c) 30 40 50 60 70 1/v (ps/cm) PRC 70 (2004) 051901(R) p+p K +K + - ++-++- p (GeV/c)  K+K+ K–K– mK+K–mK+K–

6. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 6 Particle identification at low p T 10 cm z -x 70 cm PHOBOS Spectrometer: 16 layers of silicon wafers fine pixelization, precise deposited energy measurement collision vertex close (10 cm) to spectrometer near mid-rapidity coverage dipole magnetic field of 2T at maximum, but small at the first layers p T > 0.2 GeV/c track curvature in B field p,charge, dE/dx in Simass ToF p T = 0.03 – 0.2 GeV/c low-p particles stop in silicon wafers p, mass B field negligible no charge identification

7. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 7 Mass measurement (‘energy-range’ method):  Cuts on dE/dx per plane ”MASS HYPOTHESIS” Search for particles ranging out in the 5 th spectrometer plane: A B C D E dE/dx  E k = 8 MeV p E k =21 MeV K E k =19 MeV  Cuts on E loss (E k =kinetic energy) ”MOMENTUM HYPOTHESIS” E loss =  dE (kinetic energy)  E loss  m (  1/  2 )  (  m  2 )  Corrections acceptance efficiency background silicon plane Finding very low p T particles 0 10 20 Z [cm] X[cm] A B C D E F Be pipe..

8. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 8 Test of the method: Reconstruction of low momentum MC particles E loss [MeV] E loss [10 -3 GeV 2 /cm] (+,)(+,) (K +,K – ) (p,p) MC Measuring particle mass at low p T E loss [MeV] (+,)(+,) (K +,K – ) (p,p) Au+Au  s NN =200 GeV 15% most central DATA PRC 70 (2004) 051901(R) E loss [10 -3 GeV 2 /cm]

9. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 9 Medium p T : PID via dE/dx in the Si 0 0.5 1 1.5 p [GeV/c] 5 10 15 0 dE/dx [M.I.P.] dE/dx Calibrated and well studied dE/dx measurement in the Spectrometer Particles separated in the 1/  2 region of the Bethe-Bloch function Track-by-track identification:  –K: up to p  0.4 GeV/c K–p: up to p  0.8 GeV/c Yields can be extracted using fits up to p  1.5 GeV/c for protons (PID in the statistical sense) A realistic line-shape is used: natural tail to higher dE/dx (Landau-fluctuations) the mean positions are given by the Bethe-Bloch formula the width follows the empirical relation:  dE/dx ~ (dE/dx) 0.9 only the amplitudes are free fit parameters  K p d

10. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 10 Medium p T : PID via Time of Flight Improved time measurement (new T0 detectors, better cables and correction for timing drifts) Particles separated: 1/  vs. p (even better constraint than dE/dx) Track-by-track identification:  –K: up to p  1.3 GeV/c K–p: up to p  2.3 GeV/c Yields can be extracted using fits up to p  5 GeV/c for protons (PID in the statistical sense) 0 1 2 3 4 5 p [GeV/c] 50 45 40 35 30 60 55 1/v [ps/cm]  K p d A realistic line-shape is used reflecting the time resolution the mean positions are given by 1/v=  m 2 /p 2 +1/c 2 the width: same for all species (time resolution does not depend on mass) only the amplitudes are free fit parameters

11. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 11 Acceptance of the PID Time of Flight + tracking Very low p T particles dE/dx in the Si Spectrometer p=const. lines different bending directions

12. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 12 Synthesis of dE/dx and TOF data Data merged from both detectors and bending directions Small but finite rapidity range Additional error estimated from difference between linear and constant fit 00.5 1.01.5 y 0 1 2 3 4 p T [GeV/c] 00.51.01.5 y 3 2 2.5 d 2 N/2  p T dp T dy [GeV – 2 c 2 ] Proton data points on the p T -y plane Invariant yields at p T =0.69 GeV/c PHOBOS preliminary PHOBOS preliminary

13. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 13 Identified spectra in Au+Au at 62.4 GeV Corrections: Acceptance, efficiency Occupancy in the spectrometer Feed-down from weak decays (DCA fits and estimates) Ghosts (fakes), secondaries Dead channels in the detector Momentum resolution Centrality bins: 0-15% N part =294±10 15-30%160±10 30-50% 78±8 Smooth centrality dependence Large baryon/meson ratios at high p T

14. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 14 Antiparticle to particle ratios Results at 62.4 GeV fit smoothly into the energy evolution of the antiparticle/particle ratios (ratios are integrated over the accessible p T and y range in central collisions)

15. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 15 Fraction of protons among all hadrons p>  + +K + line At p T  2.5–3 GeV/c, baryons become dominant in central Au+Au collisions at 62.4 GeV PHOBOS Preliminary Au+Au 62.4 GeV (p>  – or p>K – ) line

16. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 16 Open symbols: not feeddown-corrected central A+A collisions data from: E802 (4%) NA44 (3.7%) NA49 (5%) STAR (5% and 6%) PHENIX (5%) PHOBOS (15%) PHOBOS Preliminary PHOBOS Preliminary p/p p/  + ‘crossing’ is there at all measured energies ‘crossing’ p T value increases with energy contribution of ‘pair produced’ protons grows with energy p/p at high p T grows with energy since p T spectra of p and p are similar p T where p/  + =1, as a function of  s

17. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 17 Low p T spectra of identified particles in Au+Au collisions at 62.4 GeV Blast wave fit parameters: 0-15%: T fo = 99 MeV, =0.51 15-30%: T fo = 98 MeV, =0.51 30-50%: T fo = 97 MeV, =0.49 d 2 N/2  p T dp T dy [GeV  2 c 2 ] Fit to the high p T part and extrapolating to low p T

18. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 18 Low p T identified spectra, Au+Au at 200 GeV No enhancement for pions at low p T observed Flattening of (p+p) spectra down to very low p T (consistent with transverse expansion of the system) Blast Wave fit: T fo = 99 MeV = 0.54 T= 229 MeV for (  + +  – ) 293 MeV for (K + + K – ) 392 MeV for (p + p) m T =  p T 2 +m h 2 B-E fit BWF fit d 2 N/2  p T dp T dy [GeV –2 c 2 ] d 2 N/2  m T dm T dy=A[exp(m T /T)±1] –1 Bose-Einstein fit: PHOBOS, PR C70, 051901 (R) (2004) PHENIX, PR C69, 034909 (2004)

19. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 19 Low p T identified spectra, d+Au at 200 GeV p T range: (  + +  - ) : > 0.03 GeV/c (K + + K - ): > 0.09 GeV/c (p + p) : > 0.14 GeV/c Background corrections: very low-p T : (  + +  - ) : 20% (K + + K - ): 10% (p + p) : 20% intermediate p T (p + p) :  20% Systematic uncertainties: very low p T :  30% intermediate p T :  15% Event selection: described in PRL 93, 082301 d 2 N/2  p T dp T dy [GeV  c 2 ] Blast wave fit: T fo = 152 MeV =0.37

20. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 20 m T -scaling in d+Au collisions at 200 GeV m T spectra of (  + +  – ), (K + +K – ) and (p + p) have similar shape (K + + K - ) yield is smaller than the other species by a factor of 2 (strangeness suppression) Local slopes of m T spectra are similar T local [ GeV/c 2 ] d 2 N/2  p T dp T dy [GeV –2 c 2 ]

21. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 21 m T scaling in d+Au vs. Au+Au at 200 GeV T loc [GeV/c 2 ] d 2 N/2  m T dm T dy [GeV –2 c 4 ] PRC 70 (2004) 051901(R) In central Au+Au, larger flattening of p+p m T spectra at small p T Low p T spectra of p+p can constrain models describing collective transverse expansion of the system T loc [GeV/c 2 ] d 2 N/2  m T dm T dy [GeV –2 c 4 ]

22. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 22 Net proton yield at midrapidity, Au+Au 62.4 GeV: PHOBOS Preliminary 200 GeV: PHENIX PRC 69, 024904 (2004) (correlated errors assumed: underestimated errors) Net protons: p–p Their yield is proportional to N part within errors! Really strange result: Number of protons ‘transported’ to midrapidity per participant pair is independent of number of collisions per participant! PHOBOS Preliminary

23. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 23  meson studies Tracking modified for  -s: ● Si layers 0-5 (no mag. field) straight lines ● Si layers 8-9 (low mag. field) 3D-, 4D-fits extends the PID (strangeness scope) of PHOBOS in a special direction low p T  is an important tool to study the hot medium created 0 0.5 1 p T [GeV/c] 0 0.5 1 y 0 0.5 1 1.5 0 100 200 Single  meson acceptance (MC) p T [GeV/c] Kaons in different spectrometer arms Kaons in the same arm MC

24. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 24  mass resolution at very low p T (MC study) No background, only  mesons With high multiplicity background (  embedded in Au+Au data) Single  with 0<p T <0.13 GeV  =8.4 MeV M K + K – [GeV/c 2 ] 1 1.02 1.04 2000 1500 1000 500 0  =11.5 MeV M K + K – [GeV/c 2 ] 1 1.02 1.04 100 50 0 Table values: M=1.019 GeV/c 2  =4.26 MeV/c 2 MC

25. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 25  signal level estimation BRAHMS (nucl-ex/0403050) K – rapidity distribution: Gaussian, width  2.14 Total # of  's per 10% most central event:  36 STAR (nucl-ex/0406003) dN/dy  6.65 for |  |<0.5, T=357 MeV Assumptions for  's: Rapidity: Gaussian, width=2.14 p T : rapidity distributions: NA49 (nucl-ex/0305017) K+K+ K–K–  pTpT exp ( ) T –  p T +M  2 2

26. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 26 Expected signal size, no background Single  mesons, 0.0 < p T < 0.13 GeV/c M K + K – [GeV/c 2 ] Expected signal from all our Au+Au events at 200 GeV passing event selection (67 M events)  ~ N part assumed y distribution shape assumed to be centrality independent p T slope vs. centrality from STAR MC

27. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 27 Preliminary  meson studies A new method of  meson reconstruction was developed. Acceptance of the PHOBOS detector for low p T  was studied. The method was tested on single MC  single MC  embedded into real data events The method is being applied on real data and its performance is being studied.

28. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 28 Summary PHOBOS developed various PID techniques: stopping low p T particles, dE/dx, TOF, resonance spectroscopy No evidence for enhanced production of very low p T pions in the most central Au+Au collisions at 62.4 and 200 GeV Flattening of p+p m T spectra at low p T in the 15% most central Au+Au collisions at 62.4 and 200 GeV Approximate m T scaling of particle spectra for d+Au collisions at low and intermediate p T At high p T, baryons become dominant over mesons. Net baryon yield per participant is centrality independent Extensive MC studies on sensitivity to  mesons

29. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 29 backups

30. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 30 PHOBOS magnetic field Phobos Si SpectrometerPhobos Magnetic Field (Gauss) 0 5000 10000 15000 20000

31. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 31 PID Measurement in PHOBOS Spectrometer p T > 0.2 GeV/cp T = 0.03 - 0.2 GeV/c E tot =  dE i, i=A,...,E M p i = E i dE i /dx M p =  /K separation: p T < ~0.6 GeV/c p(p) separation: p T < ~1.2 GeV/c  K p p+p K +K + - ++-++-

32. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 32 d+Au, Model Comparison

33. Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006 33