Have we reached the
bottom at RHIC?
Learning about the strong
interaction with bottom quarks:
The quest for beauty in STAR
Manuel Calderón de la Barca Sanchez
UC Davis
SACNAS National Conference
16/Oct/2009, Dallas TX
Outline
• RHIC and the Quark-Gluon Plasma:
–
– What has been established about matter produced at RHIC.
– What are we aiming to do next.
• Heavy Quark physics in Heavy Ion collisions.
– Charmonium, Bottomonium.
• what can they tell us about Hot QCD?
– Bottomonium measurements in STAR.
• Summary
2
http://nobelprize.org/nobel_prizes/physics/laureates/2004/illpres/index.html
Confinement in QCD: a cartoon
• At high energy and
small distances, the
strength of this force
decreases!
• “Asymptotic
freedom”
• Nobel Prize 2004
3
A proton in the Lattice
• Lattice methods:
Non-pertubative
treatment of
QCD.
• Flux tubes are
visible: vacuum
fluctuations.
• Linear
confining
potential
between
quarks.
– Energy cost of
getting rid of the
vacuum
fluctuations of
the gluon field.
•
D. Leinweber, et al., Center for Subatomic Structure. Physics Dept. U. of Adelaide,
Australia. 2003.
4
Generating a deconfined state
Melting protons and neutrons: Hot
quarks and gluons in (QCD)
• heating
• compression
 deconfined color matter !
Hadronic
Nuclear
Matter
Matter
Quark
Gluon
Plasma
(confined)!
deconfined
5
Expectations from Lattice QCD
Computer calculations
/T4 ~ # degrees of freedom
confined:
few d.o.f.
deconfined:
many d.o.f.
6
Heavy ion collisions = HOT matter.
• Fire: 1000-2000 K
• Sun :
– Surface: 5000 K
– Corona: 5 x 106 K
– Core: 15 x 106 K ~ 1 keV
• Atomic binding energy: ~13 eV
• Nuclear binding energy ~ 8 MeV/A
• Heavy ion collision :
– Tc ~ 173 MeV : 2 x 1012 K
– Tc ~105 Tsun core
7
RHIC: Some key results.
• Key Goal of RHIC: Produce matter
in the hot phase of QCD.
– What are its properties?
– Is the system made up of quarks and
gluons?
• Key results and interpretation.
– Energy density is high.
• All estimates above critical energy density.
– Observation of large momentum
anisotropy.
• Generated at early times: pattern very close
to collective fluid-like behavior of quarks
– Observation of suppression of high
momentum particles.
• Matter produced is nearly opaque to
passage of color-charged quarks and gluons.
Sci Am May
2006. by W. Zajc.
STAR White
Paper:
Nuc Phys A 757
(2005) 102
8
Quenching of away-side Jets
Controlled Probe:
Particle jets measured in
proton+proton collisions
– Property: Jets come in pairs and
are 180 apart.
4.0 < pTtrig < 6.0 GeV/c
2.0 < pTassoc < pT(trig) GeV/c
Experiment:
Shoot a controlled probe through
the matter and see what
happens.
What Happens?
=> Away Side Jet is absorbed in
the Quark Soup!
Quark Soup produced at
RHIC is the densest
matter produced in the
lab!
9
Some outstanding questions
• We know that the matter is very dense, but do we know
• The measurements suggest that matter is formed at initial
temperatures and energy densities at or above the critical values
predicted by LQCD for a deconfinement transition.
– Is there a way to quantitatively measure the temperature of the produced
matter?
• The measurements suggest that the matter at the hottest stage is
consistent with quarks - gluon plasma formation.
– Can we add more convincing evidence that we have deconfined quarks and
gluons?
– Is there a way to connect our measurements to the lattice calculations?
STAR White Paper:
Nuc Phys A 757 (2005) 102
(p. 169.)
10
Heavy quark bound states
• Non-relativistic Quantum
Mechanics
– Schrödinger equation
– Two particles bound by a linearly rising
potential V(r) ~ kr.
• Bound state of charm-anticharm
– Charmonium
– J/y, y’ (ground state 1s, and excited
state 2s state)
– Excited states have different <r>…
• Bottom-antibottom
– Bottomonium
– , ’, ’’ (1s, 2s, 3s)
11
High T: the potential between the
quarks is modified.
• Charmonium
suppression:
longstanding QGP
signature
– Original idea: High T leads to
Debye screening
– Screening prevents heavy
quark bound states from
forming!
– J/y suppression:
• Matsui and Satz, Phys. Lett. B 178
(1986) 416
– lattice calculations confirm
screening effects
• Nucl.Phys.Proc.Suppl.129:560
-562,2004
O. Kaczmarek, et al.,
Nucl.Phys.Proc.Suppl.129:560-562,2004
12
Deconfinement and “screening”
hypothesis
13
Goal: Quarkonia states in A+A
Charmonia: J/y, Y’, cc
(3S)
Bottomonia: (1S), (2S),
Key Idea: Quarkonia Melt in the plasma due to screening of potential
between heavy quarks
•
•
Suppression of states is determined by TC and their binding energy
Lattice QCD: Evaluation of spectral functions  Tmelting
Sequential disappearance of states:
 Color screening  Deconfinement
 QCD thermometer  Properties of QGP
When do states really melt?
Tdiss(y’)  Tdiss(cc)< Tdiss((3S)) < Tdiss(J/y)  Tdiss((2S)) < Tdiss((1S))
H. Satz, HP2006
14
Measuring T, with beauty
Lattice QCD-based potential model:
Dissociation temperatures of quarkonia states
Quarkonia’s suppression pattern
 QGP thermometer
T/TC 1/r [fm-1]
2
cb(1P)
A. Mocsy & P. Petreczky, PRL 99 211602 (2007)
• For the  production at RHIC
– A cleaner probe compared to J/y
• co-mover absorption → negligible
• recombination → negligible
– d-Au: Cold Nuclear Matter Effects
• Shadowing / Anti-shadowing at y~0
• Challenge: low rate, rare probe
– Large acceptance detector
– Efficient trigger
(1S)
1.2
J/y(1S) ’(2S)
TC
cb’(2P)’’(3S)
cc(1P) Y’(2S)
A .Mocsy, 417th WE-Heraeus-Seminar,2008
• At RHIC energy
– (1S) no melting
– (2S) likely to melt
– (3S) melts
15
How do you measure bottomonia?
•  unstable
• Decay into e+e- or
m+m• Use detectors to
Identify particle
Simulation
Electrons are affected
by detector material.
High material runs
2006 p+p 200GeV
2007 Au+Au 200GeV
– (i.e. identify the mass)
• Use spectrometers
Low material run
2008 d+Au 200GeV
2009 p+p 200 GeV
2010 Au+Au 200 GeV
– measure their kinematics
(px, py, pz)
• Obtain two 4-vectors
and combine them
 p1  p2 
2


  E1  E 2   p1  p 2  2 p1  p 2  M 2
2
2
2
16
 : Experimental Pros and Cons
Cons
• Mass resolution pushed to the limit
• extremely low rate
– BR x ds/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)
– Wait 1 year to get ~ 100 counts (after acceptance and efficiency)
Pros
• Efficient trigger
STAR Preliminary
– ~80%
– works in p+p up to central A+A!
• Large acceptance at midrapidity
• Small background at M~10 GeV/c2.
 STAR’s can do  states well
17
How do you trigger on Quarkonia?
• Heavy quarks decay
into stable particles,
in particular e+epairs.
• Use the
ElectroMagnetic
Calorimter (EMC) to
detect the electrons.
• These electrons
should have high
energy, thanks to
E=mc2
18
STAR  Trigger in pp
Sample -triggered Event
• e+e- candidate
• mee = 9.5 GeV/c2
• cosθ = -0.67
• E1 = 5.6 GeV
• E2 = 3.4 GeV
Offline:
charged tracks +
EMC tower
• Fast L0 Trigger (Hardware)
– Select events with at least one  high energy
tower (E~4 GeV)
• L2 trigger (Software)
– Clustering, calculate mee, cos q.
• Very clean trigger up to central Au+Au
• Offline: Match TPC tracks to triggered
towers
19
STAR  in p+p collisions.
preliminary
preliminary
– Signal + Background  unlike-sign electron pairs
– Background  like-sign electron pairs
• (1S+2S+3S) total yield: integrated from 7 to 11 GeV from backgroundsubtracted mee distribution (0.96 of total)
– Peak width consistent with expected mass resolution
20
STAR  vs. theory and world data
preliminary
 '  ''
 ds 
BRee  

 dy  y 0
 91  28(stat.)  22(syst.) pb
STAR 2006 √s=200 GeV p+p
++→e+e- cross section consistent
with pQCD and world data trend
21
Control experiment:  in d+Au
• 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 mee distribution
– Raw Yield: 172 +/- 20 (stat.)
– Strong signal (8σ significance)
– pT Reach up to ~ 5 GeV/c. Differential cross section measurement.
22
 Cross section in d-Au at RHIC
N
172 ± 20

0.15 ± 0.02
Ldt
32.66 nb-1
dy
1.0
Υ  Υ' Υ"
 dσ 
Bee   
 dy  y0
 35  4(stat.)  5(sys.) nb
23
Results : d+Au/p+p
R dAu 
1
s
N bin  dAu
s pp

Υ  Υ'  Υ"
 dσ dAu
B ee  
 dy


 y 0
 dσ pp
B ee  
 dy


 y 0
Υ  Υ'  Υ"
σdAu  2.2b σ pp  42mb
Nbin  7.5  0.4 for Minbias dAu
R dAu  0.98  0.32 (stat.)  0.28 (sys.)
Consistent with Nbin scaling
preliminary
Cold Nuclear Matter effects
(Shadowing) are not large.
24
 in Au+Au: Challenging but doable!
Au+Au
J. Phys. G: Nucl. Part. Phys. 35(2008)104153
Cross section calculation
is in progress
25
 in Au+Au at √s = 200 GeV
QM 2008
RAA< 1.3 at
90% CL
Used QM
2006 p+p
analysis
cuts
S/B can be
improved
Inclusion of
p+p analysis
improvements
in progress
J. Phys. G: Nucl. Part. Phys. 35(2008)104153
26
Summary and Outlook
– And that’s a good thing!
• Measurement of ++→e+e- cross section at RHIC
– pp: BRee×(dσ/dy)y=0=91±28(stat.)±22(syst.) pb
– dAu: Yield scales as expected.:  is a standard candle at RHIC!
– AuAu : Yields and cross sections coming soon.
• Melting of the Excited states?
• Will help use measure the T of
– 2008-2010: x 5-10 in statistics for pp
27
Rocky mountain national park.
Ypsilon Peak
Chapin Peak
Chiquita Peak
28
Backup Material
29
STAR  mass resolution
w/ inner material
•High material runs
•2006 p+p 200GeV
•2007 Au+Au 200GeV
w/o inner material
Runs
Integrated
Lum. (nb-1)
p+p
5600
Au+Au
0.3
d+Au
32
•Low material run
•2008 d+Au 200GeV, 2009 p+p 200 GeV
•2010 Au+Au 200 GeV
2008-2010: good opportunity to measure !
high luminosity and low material
30
 Trigger Efficiency
• Simulation of Trigger
response
– Level-0: Fast, Hardware Trigger,
Cut on Single Tower Et
• L0 triggered/accepted ~ 99%
– Level-2: Software Trigger, Cut
on invariant mass of tower
clusters
• L2 triggered/L0 triggered ~ 80%
• Acceptance x Trig Efficiency
~20% at midrapidity.
STAR Preliminary
Acceptance x
L0 Efficiency x
L2 Efficiency
STAR Preliminary
31
STAR Detectors Used for  Analysis
• EMC
• Acceptance: || < 1 , 0 <  < 2
• PID : EMC Tower (energy)  p/E
• High-energy tower trigger  enhance high-pT sample
• Essential for quarkonia triggers
• Luminosity limited for 
• TPC
• Tracking and dE/dx PID for electrons & positrons
32
 Mass Resolution and expected s
• 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 ≈ 0.4 GeV/c2
W.-M. Yao et al. (PDG), J. Phys. G 33, 1 (2006);
R. Vogt et al., RHIC-II Heavy Flavor White Paper
State
Mass [GeV/c2]
9.46030

10.02326

10.3552

++
Bee [%]
(dσ/dy)y=0
Bee×(dσ/dy)y=0
2.38
2.6 nb
62 pb
1.91
0.87 nb
17 pb
2.18
0.53 nb
12 pb
91 pb
33
 Analysis: Electron Id with TPC and EMC
K
p
d

electrons
e
π
preliminary
 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
preliminary
34
Electron PID Efficiency and Purity
dE/dx cut
dE/dx cut
dE/dx cut
dE/dx cut
• Electron Pair PID+Tracking efficiency= 0.47±0.07
35
 Cross Section and Uncertainties
 ds 
N
BRee  
 
 dy  y 0 dy      Ldt
=geo×L0×L2×2(e)×mass
geo
0.263±0.019
L0
0.928±0.049
L2
2(e)
0.855±0.048
0.47±0.07
mass

0.96±0.04
0.094±0.018
preliminary
•geo : geometrical acceptance
•L0 : efficiency of L0
•L2 : efficiency of L2
•(e) : efficiency of e reco
•mass: efficiency of mass cut
36
STAR  Cross Section at Midrapidity
48±15(stat.)

0.094±0.018
Ldt
(5.6±0.8) pb-1
dy
1.0
preliminary
N
 '  ''
 ds 
BRee  

dy

 y 0
 91  28 (stat.)  22 (syst.) pb
37
STAR  mass resolution
w/ inner material
•High material runs
•2006 p+p 200GeV
•2007 Au+Au 200GeV
w/o inner material
Runs
Integrated
Lum. (nb-1)
p+p
5600
Au+Au
0.3
d+Au
32
•Low material run
•2008 d+Au 200GeV, 2009 p+p 200 GeV
•2010 Au+Au 200 GeV
2008-2010: good opportunity to measure !
high luminosity and low material
38
How do you measure bottomonia?
• J/y,  unstable
• Decay into e+e- or
m+m• Use detectors to
Identify particle
CDF   m m
(1s, 2s, 3s)
– (i.e. identify the mass)
• Use spectrometers
– measure their kinematics
(px, py, pz)
PRL 75 (1995) 4358
• Obtain two 4-vectors
and combine them
 p1  p2 
2


  E1  E 2   p1  p 2  2 p1  p 2  M 2
2
2
2
39