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Strangeness measurements with the Experiment

This presentation discusses the measurements of strangeness in quark matter using the Gábor Veres experiment at Eötvös Loránd University, Budapest, Hungary and Massachusetts Institute of Technology, Cambridge, USA. The presentation includes the identification of strange particles, reconstruction of phi mesons, identified particle spectra in gold and deuteron collisions, and new techniques for reconstructing phi mesons at low transverse momentum.

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Strangeness measurements with the Experiment

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  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 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 LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER 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 pT Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

  5. 70 60 K+ 50 + - K+K mK+K– 40  30 0 5 4 2 3 1 K– p (GeV/c) Particle ID from low to high pT PID Capabilities of PHOBOS PRC 70 (2004) 051901(R) p+p 1/v (ps/cm) ++- Eloss(MeV) p (GeV/c) Stopping particles dE/dx TOF 5.0 0.5 0.03 pT (GeV/c) Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  6. 70 cm 10 cm z -x Particle identification at low pT • 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 • pT > 0.2 GeV/c • track curvature in B field p,charge, • dE/dx in Si mass • ToF • pT = 0.03 – 0.2 GeV/c • low-pparticles stop • in silicon wafers p, mass • B field negligible no charge identification Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  7. F E D C X[cm] B A . . Be pipe 01020Z [cm] Finding very low pT particles Search for particles ranging out in the 5th spectrometer plane: Mass measurement (‘energy-range’ method): • Eloss=dE (kinetic energy) • <dE/dx>Eloss m • (1/2)( m2) • Cuts on dE/dxper plane • ”MASS HYPOTHESIS” p Ek=21 MeV • Cuts on Eloss(Ek=kinetic energy) • ”MOMENTUM HYPOTHESIS” K Ek=19 MeV dE/dx  Ek= 8 MeV • Corrections • acceptance • efficiency • background A B C D E silicon plane Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  8. Au+Au sNN=200 GeV 15% most central PRC 70 (2004) 051901(R) (p,p) <dE/dx> Eloss [10-3GeV2/cm] (K+,K–) (+,) DATA Eloss [MeV] Measuring particle mass at low pT Test of the method: Reconstruction of low momentum MC particles (p,p) <dE/dx> Eloss [10-3GeV2/cm] (K+,K–) (+,) MC Eloss [MeV] Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  9. 15 dE/dx [M.I.P.] 10 dE/dx 5 0 0 0.5 1 1.5 p [GeV/c] Medium pT: PID via dE/dx in the Si • 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) p K d  • 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 Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  10. 60 1/v [ps/cm] 55 50 45 40 35 30 0 1 2 3 4 5 p [GeV/c] • A realistic line-shape is used reflecting the time resolution • the mean positions are given by 1/v=m2/p2+1/c2 • the width: same for all species (time resolution does not depend on mass) • only the amplitudes are free fit parameters Medium pT: 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) p d K  Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

  12. 4 pT [GeV/c] 3 2 1 0 y 1.0 1.5 0 0.5 3 d2N/2pTdpTdy [GeV–2c2] 2.5 2 y 0 0.5 1.0 1.5 Synthesis of dE/dx and TOF data Proton data points on the pT-y plane • Data merged from both detectors and • bending directions • Small but finite rapidity range • Additional error estimated from • difference between • linear and constant fit PHOBOS preliminary PHOBOS preliminary Invariant yields at pT=0.69 GeV/c 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% Npart=294±10 15-30% 160±10 30-50% 78±8 • Smooth centrality dependence • Large baryon/meson ratios at high pT 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 pT and y range in central collisions) Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

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

  17. Low pT spectra of identified particles in Au+Au collisions at 62.4 GeV Fit to the high pT part and extrapolating to low pT d2N/2pTdpTdy [GeV2 c2] Blast wave fit parameters: 0-15%: Tfo = 99 MeV, <T>=0.51 15-30%: Tfo = 98 MeV, <T>=0.51 30-50%: Tfo = 97 MeV, <T>=0.49 d2N/2pTdpTdy [GeV2 c2] d2N/2pTdpTdy [GeV2 c2] Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  18. B-E fit BWF fit T= 229 MeV for (++–) 293 MeV for (K++ K–) 392 MeV for (p + p) mT =pT2+mh2 d2N/2pTdpTdy[GeV–2c2] No enhancement for pions at low pT observed Flattening of (p+p) spectra down to very low pT (consistent with transverse expansion of the system) Low pT identified spectra, Au+Au at 200 GeV • Bose-Einstein fit: d2N/2mTdmTdy=A[exp(mT/T)±1]–1 • Blast Wave fit: • Tfo= 99 MeV • <T> = 0.54 PHOBOS, PR C70, 051901 (R) (2004) PHENIX, PR C69, 034909 (2004) Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  19. d2N/2pTdpTdy [GeV2 c2] Low pT identified spectra, d+Au at 200 GeV • pT range: • (++-) : > 0.03 GeV/c • (K++ K-): > 0.09 GeV/c • (p + p) : > 0.14 GeV/c • Background corrections: • very low-pT: • (++-) : 20% • (K++ K-): 10% • (p + p) : 20% • intermediate pT • (p + p) : 20% • Systematic uncertainties: • very low pT: 30% • intermediate pT : 15% • Event selection: • described in PRL 93, 082301 Blast wave fit: Tfo = 152 MeV <T>=0.37 Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  20. mT 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 mT spectra • are similar d2N/2pTdpTdy[GeV–2c2] Tlocal[GeV/c2] mT-scaling in d+Au collisions at 200 GeV Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

  21. d2N/2mTdmTdy[GeV–2c4] Tloc[GeV/c2] mT scaling in d+Au vs. Au+Au at 200 GeV d2N/2mTdmTdy[GeV–2c4] PRC 70 (2004) 051901(R) Tloc[GeV/c2] • In central Au+Au, larger flattening of p+p mT spectra at small pT • Low pT spectra of p+p can constrain models describing collective transverse expansion of the system Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

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

  24. 2000 100 1500 =8.4 MeV =11.5 MeV 1000 50 500 0 0 1 1.02 1.04 1 1.02 1.04 MK+K–[GeV/c2] MK+K–[GeV/c2]  mass resolution at very low pT (MC study) Single  with 0<pT<0.13 GeV MC MC Table values: M=1.019 GeV/c2 =4.26 MeV/c2 No background, only  mesons With high multiplicity background ( embedded in Au+Au data) Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

  26. Expected signal size, no background Single  mesons, 0.0 < pT < 0.13 GeV/c • Expected signal from all our • Au+Au events at 200 GeV • passing event selection • (67 M events) •  ~ Npart assumed • y distribution shape assumed • to be centrality independent • pT slope vs. centrality • from STAR MC MK+K–[GeV/c2] 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 pT 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. Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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

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