 production in p+p collisions in Manuel Calderón de la Barca Sánchez UC Davis STAR Collaboration 23 d Winter Workshop on Nuclear Dynamics Big Sky, Montana STAR

Goal: Quarkonia states in A+A Charmonia: J/ ,  ’,  c Bottomonia:  (1S),  (2S),  (3S) Key Idea: Melting in the plasma  Color screening of static potential between heavy quarks: –J/  suppression: Matsui and Satz, Phys. Lett. B 178 (1986) 416  Suppression of states is determined by T C and their binding energy  Lattice QCD: Evaluation of spectral functions  T melting (next talk!) Sequential disappearance of states:  Color screening  Deconfinement  QCD thermometer  Properties of QGP H. Satz, HP2006 When do states really melt? T diss (  ’)  T diss (  c )< T diss (  (3S)) < T diss (J/  )  T diss (  (2S)) < T diss (  (1S))

 :Pros for theory interpretation ,  ’,  ’’ sequential suppression   (1S) no melting at RHIC (nor LHC?)  standard candle (reference)   (2S) likely to melt at RHIC (analog J/  )   (3S) melts at RHIC (analog  ’) Pros  co-mover absorption negligible  recombination negligible at RHIC  Both of these affect charmonia, but not bottomonia.

 : Experimental Pros and Cons Cons  Mass resolution pushed to the limit  Ratio extraction (2S/1S) and (3S/1S) possible, but difficult  extremely low rate  BR x d  /dy(1s+2s+3s)=91 pb  from NLO calculations.  Luminosity limited (RHIC II will substantially help)  pp Run 6 ~ 9 pb -1 (split into 2 triggered datasets) Pros  Efficient trigger  ~80%  works in p+p up to central A+A!  Large acceptance at midrapidity  Run VI = Run IV x 4  Small background at M~10 GeV/c 2.  STAR’s strength are the  states

STAR Detectors Used for  Analysis EMC Acceptance: |  | < 1, 0 <  < 2  PID : EMC Tower (energy)  p/E High-energy tower trigger  enhance high-p T sample Essential for quarkonia triggers Luminosity limited for  TPC Tracking and dE/dx PID for electrons & positrons

 Mass Resolution and expected  StateMass [GeV/c 2 ]B ee [%](dσ/dy) y=0 B ee ×(dσ/dy) y=0  nb62 pb  nb17 pb  nb12 pb  +  +  91 pb  STAR detector does not resolve individual states of the   Finite p resolution (B=0.5 T)  e-bremsstrahlung  Yield is extracted from combined  +  +  states  FWHM ≈ 0.7 GeV/c 2 W.-M. Yao et al. (PDG), J. Phys. G 33, 1 (2006); R. Vogt et al., RHIC-II Heavy Flavor White Paper

STAR  Trigger  Fast L0 Trigger (Hardware)  Select events with at least one  high energy tower (E~4 GeV)  L2 trigger (Software)  Clustering, calculate m ee, cos .  Very clean to trigger up to central Au+Au  Offline: Match TPC tracks to triggered towers Sample  -triggered Event  e + e - candidate m ee = 9.5 GeV/c 2 cosθ = E 1 = 5.6 GeV E 2 = 3.4 GeV Offline: charged tracks + EMC tower

 Acceptance in STAR  Simulations Run 6 Conditions  Including detector variations:  Calorimeter crates removed/recovered  Hot towers masked  Two Trigger setups:  Acc = 0.272±0.01 for |y|<0.5 (set 1)  Acc = 0.263±0.019 for |y|<0.5 (set 2) –Set 2 used in results shown today.

 Trigger Efficiency  Simulation of Trigger response  Level-0: Fast, Hardware Trigger, Cut on Single Tower E t  L0 triggered/accepted = 0.928±0.049  Level-2: Software Trigger, Cut on invariant mass of tower clusters  L2 triggered/L0 triggered= 0.855±0.048  Acceptance x Trig Efficiency ~19-21%

 Analysis: Electron Id with TPC and EMC   trigger enhances electrons  Use TPC for charged tracks selection  Use EMC for hadron rejection  Electrons identified by dE/dx ionization energy loss in TPC  Select tracks with TPC, match to EMC towers consistent with trigger preliminary electrons  Kp d preliminary e π

Electron PID Efficiency and Purity  Electron Pair PID+Tracking efficiency= 0.47±0.07 dE/dx cut

 Signal + Background  unlike-sign electron pairs  Background  like-sign electron pairs   (1S+2S+3S) total yield: integrated from 7 to 11 GeV from background-subtracted m ee distribution (0.96 of total)  Peak width consistent with expected mass resolution STAR  Invariant Mass preliminary

 Cross Section and Uncertainties  geo 0.263±0.019  L ±0.049  L ±0.048  2 (e) 0.47±0.07  mass 0.96±0.04  0.094±0.018   =  geo ×  L0 ×  L2 ×  2 (e)×  mass  geo : geometrical acceptance  L0 : efficiency of L0  L2 : efficiency of L2  (e) : efficiency of e reco  mass : efficiency of mass cut preliminary

STAR  Cross Section at Midrapidity NN 48±15(stat.)  0.094±0.018  L dt (5.6±0.8) pb -1 dy1.0 preliminary

STAR  vs. theory and world data STAR 2006 √s=200 GeV p+p  +  +  →e + e - cross section consistent with pQCD and world data trend Only RHIC peeks at √s=200 GeV range prelimina ry

Outlook for Run VII Au+Au  Yield estimate:  17 Week run ~ 100 days  Run 4 Performance  4-20 M events/day  For Run VII:  Assume:  400 – 2000 M events  60  b nb -1   cross section in Au+Au  Using  with  =0.9, (AB)  ~ 13,500  d  AuAu /dy| y=0 =91 pb x = 1.2  b -1   produced at y=0 in dy=1 ~ 73 – 368   after acc. & eff. ~ 7 – 37  Yes, its tough!!! Run IV Au+Au Events sampled per day

Summary  Full EMC + trigger  quarkonium program in STAR  Run 6: first midrapidity measurement of  +  +  →e + e - cross section at RHIC in p+p collisions at √s=200 GeV  BR ee ×(dσ/dy) y=0 =91±28(stat.)±22(syst.) pb  STAR  measurement is consistent with pQCD and world data trend  Next run: Towards a STAR  cross section in Au+Au collisions at √s=200 GeV

18 Extra Slides

STAR J/  Trigger  L0 (hardware)  J/  topology trigger: two towers above E T ≈1.2 GeV  Separated by 60° in φ  L2 (software)  Match EMC high tower to CTB slat  photon rejection  Tower clustering  Cut on m ee =√2E 1 E 2 (1- cosθ)  Cut on cosθ  High background contamination ~1.5 GeV/c  Rejection~100  not sampling full luminosity  Challenging analysis!!!  Efficiency × acceptance ≈ 12% Real Data, p+p Run V preliminary

STAR J/  Signal Signal in 200 GeV p+p from 2006 Tested and working trigger in p+p No trigger for Au+Au until full ToF in 2009 Integrated luminosity in 2006: 377 nb -1 Analysis in progress preliminary