Helen Caines The Ohio State University MSU – Nucl. Seminar June 2001 A Strange Perspective – Preliminary Results from the STAR Detector at RHIC Science is a wonderful thing if one does not have to earn one's living at it – Einstein (1879—1955)

Helen Caines MSU The STAR Collaboration Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP- Protvino U.S. Labs: Argonne, Berkeley, Brookhaven National Labs U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale Brazil: Universidade de Sao Paolo China: IHEP - Beijing, IPP - Wuhan England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt Poland: Warsaw University, Warsaw University of Technology Institutions: 36 Collaborators: 415 Students: ~50 Spokesperson: John Harris

Helen Caines MSU QCD Quarks confined within hadrons via strong force v(r) =  /r +  *r At large r -second term dominates At small r -Coulomb-like part dominates However  function of q( mtm transfer) and  -> 0 faster than q (or 1/r) -> infinity (called asymptotic freedom) This concept of asymptotic freedom among closely packed coloured objects (q and g) has led to one of the most exciting predictions of QCD !! The formation of a new phase of matter where the colour degrees of freedom are liberated. Quarks and gluons are no longer confined within colour singlets. The Quark-Gluon Plasma!

Helen Caines MSU most dangerous event in human history: - ABC News – Sept ‘99 Don’t Panic!!! "Big Bang machine could destroy Earth" -The Sunday Times – July ‘99 the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99 - New Scientist Will Brookhaven Destroy the Universe? – NY Times – Aug ‘99 No… the experiment will not tear our region of space to subatomic shreds. - Washington Post – Sept ‘99 Apocalypse2 – ABC News – Sept ‘99

Helen Caines MSU The Phase Space Diagram TWO different phase transitions at work! – Particles roam freely over a large volume – Masses change Calculations show that these occur at approximately the same point Two sets of conditions: High Temperature High Baryon Density Lattice QCD calc. Predict: T c ~ MeV  c ~ GeV/fm Deconfinement transition Chiral transition

Helen Caines MSU T fireball < T c (170MeV)  Hadron gas Hard to make S  0 particles  + N   + K (E thresh  530MeV)  + K   +  (E thresh  1420MeV) Mtm phase space suppressed Need to create 3 qq pairs (initially there are no q) with similar momenta in a region already containing many quarks. T fireball >T c (170MeV)  QGP Easy to make s quarks E=2m s (  300MeV) Free gluons g-g fusion - dominate ss creation faster reaction time than qq Pauli blocking may aid creation of ss quarks ( probably not true at high T, too many states). _ _ _ _ _ _ _ Why are we interested in Strangeness?

Helen Caines MSU Introduction When is Strangeness Produced – Resonances Chemical content – Yields Thermal Freeze-out – Radii and Inverse slopes Flow – How much and when does it start? Chemical Freeze-out - Ratios

Helen Caines MSU Previous Strangeness Highlights WA97 Evidence of strangeness enhancement between pA and AA collisions at the SPS – Not reproducible by models SPS  s=17GeV Enhancement  >  >  > h |s|

Helen Caines MSU Strangeness Highlights (2) AGS and SPS > 1 Need to consider p absorption Multi-Strange Particles appear to freeze out at a cooler temperature/ earlier or have less flow SPS AGS _

Helen Caines MSU The CERN announcement Strangeness was one of the corner stones of the CERN announcement. Have numerous pointers that there is evidence of a new state of matter even at SPS energies so why RHIC? Still a large baryon number so need models to understand what’s going on. Those models can probably be tuned to reproduce the experimental data but would require more “knobs” So want to go to “cleaner” system Less baryon number (only look at produced particles) Closer to the region where QCD predictions work – Definite theory not models Higher energies – further across phase transition boundary – In new regime for longer and more frequently

Helen Caines MSU Welcome to BNL- RHIC!

Helen Caines MSU installation in 2003 Endcap Calorimeter Year 2000, The STAR Detector (Year-by-Year) ZCal Time Projection Chamber Magnet Coils RICH * yr.1 SVT ladder TPC Endcap & MWPC ZCal Central Trigger Barrel FTPCs (1 + 1) Silicon Vertex Tracker * Vertex Position Detectors year 2001, + TOF patch Barrel EM Calorimeter year-by-year until 2003,

Helen Caines MSU STAR Pertinent Facts Field: 0.25 T (Half Nominal value) (slightly worse resolution at higher p, lower pt acceptance) TPC: Inner Radius – 50cm (p t >75 MeV/c) Length – ± 200cm ( -1.5  1.5) Events: ~300,000 “Central” Events –top 8% multiplicity ~160,000 “Min-bias” Events L3-Real time display

Helen Caines MSU Needle in the Hay-Stack! How do you do tracking in this regime? Solution: Build a detector so you can zoom in close and “see” individual tracks Good tracking efficiency Clearly identify individual tracks high resolution P t (GeV/c)

Helen Caines MSU Particle ID Techniques - dE/dx dE/dx PID range: ~ 0.7 GeV/c for K /  ~ 1.0 GeV/c for K/p dE/dx 6.7%Design 7.5%With calibration 9 %No calibration Resolution: Even identified anti- 3 He !

Helen Caines MSU Particle ID Techniques - Topology Decay vertices K s   + +  -   p +  -   p +  +  -   +  -  +  +  +    + K -   “kinks”: K     + VoVo

Helen Caines MSU Finding V0s proton pion Primary vertex

Helen Caines MSU High P t K + & K - Identification Via “Kinks”  +/- K +/- 

Helen Caines MSU Particle ID Techniques Combinatorics K s   + +  -   K + + K -   p +  -   p +  + Combinatorics  from K + K - pairs K + K - pairs m inv same event dist. mixed event dist. background subtracted dn/dm Breit-Wigner fit Mass & width consistent w. PDG K* combine all K + and  - pairs (x ) m inv (GeV)

Helen Caines MSU STAR STRANGENESS! K0sK0s  K+K+ (Preliminary) ̅̅   ̅̅  ̅̅ 

Helen Caines MSU Triggering/Centrality “Minimum Bias” ZDC East and West thresholds set to lower edge of single neutron peak. ~30K Events |Z vtx | < 200 cm “Central” CTB threshold set to upper 15% REQUIRE: Coincidence ZDC East and West REQUIRE: Min. Bias + CTB over threshold

Helen Caines MSU The Collisions The End Product

Helen Caines MSU Baryon Stopping/Transport Anti-baryons - all from pair production Baryons - pair production + transported B/B ratio =1 - Transparent collision B/B ratio ~ 0 - Full stopping, little pair production Measure p/p,  / , K - /K + (uud/uud) (uds/uds) (us/us) _ _ __

Helen Caines MSU p/p Ratio _ Phys. Rev. Lett March 2001 Still finite baryon number Ratio is flat as function of p t and ySlight fall with centrality Ratio = 0.65 ±0.03(stat) ±0.03(sys)

Helen Caines MSU Strange Baryon Ratios Ratio = 0.73 ± 0.03 (stat) ~0.84  /ev, ~ 0.61  /ev Reconstruct: _ STAR Preliminary Ratio = 0.82 ± 0.08 (stat) _ ~0.006   /ev, ~0.005   /ev

Helen Caines MSU Preliminary  ̅ /  Ratio  = 0.73  0.03 (stat) _ Ratio is flat as a function of p t and y Central events |y|<0.5

Helen Caines MSU  and  ̅ from mixed event Studies Good cross-check with standard V0 analysis. Low p t measurement where there is no V0 analysis High efficiency (yields are ~10X V0 analysis yields) Background determined by mixed event STAR preliminary The ratio is in agreement with “standard” analysis  = 0.77  0.07 (stat) _

Helen Caines MSU ¯ _ _ _ _ _ _ _ Anti-baryon/Baryon Ratios versus  s STAR preliminary Baryon-pair production increases dramatically with  s – still not baryon free 2/3 of protons from pair production, yet pt dist. the same – Another indication of thermalization Pair production is larger than baryon transport

Helen Caines MSU Particle Freeze-out Conditions time 3. freeze-out 1. formation Chemical Freeze out: inelastic scattering stops 2. hot / dense Kinetic Freeze out: elastic scattering stops

Helen Caines MSU K + /K - Ratio - N ch dE/dx Kinks K + /K - = 1.08±0.01(stat.)± 0.06(sys.) (dE/dx). (The kink method is systematically higher.) STAR preliminary K + /K - constant over measured centrality

Helen Caines MSU K - /  - Ratios K - /   ratio is enhanced by almost a factor of 2 in central collisions when compared to peripheral collisions STAR preliminary SP S

Helen Caines MSU K 0 * and K 0* Identification Short lifetime (c  =4fm) – sensitive to the evolution of the system? _ First measurement in heavy ion collisions

Helen Caines MSU K 0 */h - Strangeness Enhancement? Represents a 50% increase compared to K 0* /  measured in pp at the ISR. Also look at K*/K From spin counting K*/K = vector meson/meson = V/(V+P) =0.75 e + e - (LEP)K*/K = 0.32 ±0.02 pp (ISR)K*/K = 0.6 ±.09 ±.03 Au-Au (STAR)= 0.42

Helen Caines MSU Comparing to SPS K + /K - (kink) = 1.2 ± K + /K - (dE/dx) = 1.08 ±0.01 (stat.) ± 0.06 (sys.) K - /    = 0.15 ± 0.02 (stat.) K*/h - = 0.06 ± (stat.) ± 0.01 (sys.) K*/h - = ± (stat.) ± 0.01 (sys.) p/p = 0.6  0.02 (stat.)  0.06 (sys.) ¯  /  = 0.73 ± 0.03 (stat.)  ± 0.08 (stat.) ¯ ¯ ¯

Helen Caines MSU Simple Model Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models D=1.12 No free quarks so all quarks have to end up confined within a hadron Predict D=1.08± 0.08 Measure System consistent with having a de-confined phase

Helen Caines MSU Particle Ratios and Chemical Content  j = Quark Chemical Potential T = Temperature E j – Energy required to add quark  j – Saturation factor Use ratios of particles to determine  T ch and saturation factor

Helen Caines MSU Chemical Fit Results Not a 4  -yields fit!  s  1  2  1.4 Thermal fit to preliminary data: T ch (RHIC) = 0.19 GeV  T ch (SPS) = 0.17 GeV  q (RHIC) = GeV <<  q (SPS) = GeV  s (RHIC) < GeV   s (SPS)

Helen Caines MSU P. Braun-Munzinger, nucl-ex/ Chemical Freeze-out Baryonic Potential  B [MeV] Chemical Temperature T ch [MeV] AGS SIS LEP/ SppS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restauration Lattice QCD atomic nuclei

Helen Caines MSU Kinetic Freeze-out and Radial Flow If there is transverse flow Look at m t =  (p t 2 + m 2 ) distribution A thermal distribution gives a linear distribution dN/dm t  e -(mt/T) mtmt 1/m t d 2 N/dydm t Slope = 1/T Slope = 1/T meas ~ 1/(T fo + 0.5m o 2 ) Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

Helen Caines MSU T  = 190 MeV T  = 300 MeV T p = 565 MeV mid-rapidity m t slopes vs. Centrality Increase with collision centrality  consistent with radial flow.

Helen Caines MSU Radial Flow: m t - slopes versus mass Naïve: T = T freeze-out + m   r  2 where   r  = averaged flow velocity  Increased radial flow at RHIC ß r (RHIC)  ß r (SPS/AGS) = 0.6c = c T fo (RHIC)  T fo (SPS/AGS) = GeV = GeV

Helen Caines MSU No evidence of mass modification  Identification STAR Preliminary

Helen Caines MSU Inverse slope for  Hyperons T=352±6(stat) MeV 15% Most Central As  /  ratio is flat as a function of p t can infer that the  slope is the same – backed up by fitting to corrected spectrum Some evidence that a single exponential fit is not the best fit to the data e (-mt/T) Same slope as 

Helen Caines MSU Radial Flow and Strange Particles Do not follow “radial flow systematics”  early kinetic freeze out? STAR Preliminary Neither the  or the proton are corrected for feed-down. Correction would drive the p slope up. What about p absorption/annihilation? Lower momentum  more collisions  more absorption/annihilation. _

Helen Caines MSU D: overall rough “size” KK R out R side Measuring the Source “Size” (HBT) ~5 fm x1x1 x2x2 y1y1 y2y2 ~1 m 3D decomposition of relative momentum provides handle on shape and time as well as size

Helen Caines MSU K 0 s -K 0 s Correlations  = 0.7 ±0.5 R = 6.5 ± 2.3 No coulomb repulsion No 2 track resolution Few distortions from resonances K 0 s is not a strangeness eigenstate - unique interference term that provides additional space-time information K 0 s Correlation will become statistically meaningful once we have ~10M events

Helen Caines MSU What have we “learnt” so far Mapping out “Soft Physics” Regime  Net-baryon  0 at mid-rapidity! (  y = y 0 -y beam ~ 5 )  Chemical parameters Chemical freeze-out appears to occur at same ~T as SPS Strangeness saturation similar to SPS  Kinetic parameters Higher radial flow than at SPS Thermal freeze out same as at SPS  Strange Particles The  and  do not seem to flow with the other particles. Reduced rescattering for the kaons from  decay and/or  feel less flow More than we ever hoped for after the first run !!!

Helen Caines MSU This Year – RICH,TOF Patch,SVT,FTPC RICH and TOF: Increase K identification in p t over a limited geometric acceptance Centered at mid-rapidity they provide complimentary pt coverage TOF patch 0.3< p t <1.5 GeV/c RICH 1.1 < p t < 3.0 GeV/c Overlaps with the TPC kink and dE/dx measurement kink p t < 5 GeV, dE/dx p t < 0.8 GeV SVT: Increased efficiency for all strange particles and resonances due to improved tracking Should measure spectra for all particles this year. HBT with strange particles Exotica FTPC: Strange particles at high y

Helen Caines MSU The Silicon Vertex Tracker Radii – 6,10 15 cm Length ±12.4 cm ± 18.6 cm ± 21.7cm (-1 <  < 1)

Helen Caines MSU SVT STAR detector gets new silicon heart – CERN Courier SVT installed and operational April 2001!! 91% live (out of 103,680 channels)

Helen Caines MSU SVT being Assembled Radiation Length 1.5%/layer (including Electronics+Cooling) 216 wafers on 36 Ladders 0.7m 2 Silicon Half assembled. Fully assembled

Helen Caines MSU Principle of Operation SDD gives unique position in X- Y * 6.3 cm x 6.3 cm area * 280  m thick n-type Si wafer * 20  m position resolution Ionizing particle X-position from drift time Electron cloud X SDD Y-position from readout anode number

Helen Caines MSU SVT Performance 1ch=2mV Noise Hits from Au-Au Event Cosmic Ray Event–L3 Trigger Threshold at 4mV  6% live

Helen Caines MSU Conclusion Lots done Lots still to be done The future looks bright and exciting

Helen Caines MSU Comparison of AGS, SPS, RHIC AGS SPSRHIC Energy density 1GeV/fm 3 5.3GeV/fm 3 17GeV/fm 3 Multiplicity 1,0003,000 10,000 Baryon chemical potential  b 520MeV 167MeV 47MeV Freeze-out temperature (T) 120MeV 130 MeV 160MeV

Helen Caines MSU How a TPC works 420 CM Tracking volume is an empty volume of gas surrounded by a field cage Drift gas: Ar-CH 4 (90%-10%) Pad electronics: amplifier channels with 512 time samples –Provides 70 mega pixel, 3D image

Helen Caines MSU Calibration - Lasers Using a system of lasers and mirrors illuminate the TPC Produces a series of >500 straight lines criss-crossing the TPC volume Determines: Drift velocity Timing offsets Alignment

Helen Caines MSU Calibration – Cosmic Rays Determine momentum resolution  p/p < 2% for most tracks

Helen Caines MSU K + /K - vs p t

Helen Caines MSU K - Inverse Slope Results Kink dE/dx h - mid rapidity dN/d  Increasing centrality

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Helen Caines MSU Large h - multiplicity Nearly Boost invariant

Helen Caines MSU The central rapidity region Excited Vacuum hadrons Almost net-baryon free dN net-B /dy ~ 30 Large particle multiplicity dN/d  ~ 800 C. Bernard et al, PRD 55, 6861 (1997)

Helen Caines MSU

Helen Caines MSU In case you thought it was easy… Before After