Physics Colloquium
Manuel Calderón de la Barca Sanchez
UC Davis
Outline
• What happens when we take the strongest
force of nature, and turn up the heat?
• Quark-Gluon Plasma:
– What has been established about matter produced in heavy
ion collisions?
• Over a decade of RHIC striking Gold, and making hot QCD matter.
• Cool findings: The matter is hot, dense, and behaves like a fluid!
• Heavy Quark physics in Heavy Ion collisions.
– Charmonium, Bottomonium.
• what can they tell us about Hot QCD?
– How hot does it get? What is this matter made of?
– Bottomonium measurements in STAR.
– LHC and the Lead bullets: Bottomonium in CMS.
• Summary
20/Oct/2011, CSU Colloquium
Manuel Calderón de la Barca Sánchez
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The Strong Interaction in Nuclear Physics
• Nuclei composed of:
– protons (+ electric charge)
– neutrons (no electric charge)
quark
• Does not blow up!?  “nuclear force”
neutron
– overcomes electrical repulsion
– determines nuclear reactions
(stellar burning, fusion…)
• Arises from fundamental strong force :
–
• Acts on color charge of quarks
• Observation: we never see a free quark!
• QCD “confines” the quarks in protons and
neutrons
20/Oct/2011 CSU Colloquium
proton
Manuel Calderón de la Barca Sánchez
3
http://nobelprize.org/nobel_prizes/physics/laureates/2004/illpres/index.html
QCD and confinement: a cartoon
• At high energy and
small distances, the
strength of this force
decreases!
• “Asymptotic
freedom”
• Nobel Prize 2004
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Manuel Calderón de la Barca Sánchez
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Numerical QCD: A proton in the Lattice
• Lattice methods:
Non-pertubative,
numerical
solution 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.
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What we do: Relativistic Heavy Ion Collisions
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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
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Expectations from Lattice QCD
Computer calculations
/T4 ~ # degrees of freedom
confined:
few d.o.f.
deconfined:
many d.o.f.
TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core]
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The phase diagram of QCD.
Temperature
Early universe
quark-gluon plasma
critical point ?
Tc
colour
superconductor
hadron gas
nucleon gas
nuclei
CFL
r0
vacuum
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Neutron stars
baryon density
Manuel Calderón de la Barca Sánchez
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RHIC: Some key results.
• Key Goal of RHIC: Produce matter
in the hot phase of QCD.
– What are its properties?
– What are the right degrees of freedom?
• 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.
Sci Am May 2006. by W. Zajc.
• Matter produced is nearly opaque to passage STAR White Paper:
Nuc Phys A 757 (2005) 102
of color-charged quarks and gluons.
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Manuel Calderón de la Barca Sánchez
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Quenching of away-side Jets
Experiment:
Shoot a controlled probe
through the matter and see
what happens.
What 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
What Happens?
=> Away Side Jet is absorbed
in the Quark Soup!
Quark Soup
produced at RHIC is
the densest matter
produced in the lab!
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Some outstanding questions
• The Heavy Ion experiments have not yet produced direct
evidence for deconfinement.
• 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.
• But they do not establish the detailed relevance of the
lattice calculations to the fleeting dynamic matter
produced in heavy-ion collisions
STAR White Paper:
Nuc Phys A 757 (2005) 102
(see p. 169.)
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Heavy quark bound states
•
•
•
•
Think of Schrödinger equation.
Particles in a potential well.
V(r) ~ kr : Linearly rising.
Bound state of charmanticharm: charmonium
– J/y, y’ (ground state 1s, and excited
state 2s state)
• Bottom-antibottom:
bottomonium
– ’, ’’, ’’’ (1s, 2s, 3s)
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High T: the potential between the
quarks is modified.
• Charmonium
suppression:
longstanding QGP
signature
– Original idea: High T leads
to 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
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O. Kaczmarek, et al.,
Nucl.Phys.Proc.Suppl.129:560-562,2004
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Deconfinement and “screening”
hypothesis
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Goal: Quarkonia states in A+A
Charmonia: J/y, Y’, cc
Bottomonia: (1S), (2S), (3S)
Key Idea: Quarkonia Melt in the plasma
•
•
•
Color screening of static 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 melt?
Tdiss(y’)  Tdiss(cc)< Tdiss((3S)) < Tdiss(J/y)  Tdiss((2S)) < Tdiss((1S))
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Manuel Calderón de la Barca Sánchez
H. Satz, HP2006
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Measuring the Temperature
Quarkonia’s suppression pattern
 QGP thermometer
Lattice QCD Calculations:
Dissociation temperatures of quarkonia states
T/TC 1/r [fm-1]
2
cb(1P)
hep-ph/0110406
• For  production at RHIC and LHC
– 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
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Manuel Calderón de la Barca Sánchez
(1S)
1.2
J/y(1S) ’(2S)
TC
cb’(2P)’’(3S)
cc(1P) Y’(2S)
A .Mocsy, 417th WE-Heraeus-Seminar,2008
• Expectation:
– (1S) no melting
– (2S) likely to melt
– (3S) melts
17
How do you measure quarkonia?
• J/y,  unstable
• Decay into e+e- or m+m• Use detectors to Identify
particles (e or m)
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
(p + p )
1
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2
2
2
æ 2
ö
= ( E1+ E2) - ç p1 + p 2 + 2 p1 ip 2 ÷ = M 2
è
ø
2
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The STAR Detector
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Colloquium
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Sánchez
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The CMS Detector
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Colloquium
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Sánchez
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 : Experimental Pros and Cons
Cons
• Mass resolution pushed to the limit in STAR (but not in CMS)
– Ratio extraction (2S/1S) and (3S/1S) possible, but difficult
• extremely low rate
– BR x ds/dy(1s+2s+3s)~100 pb ( 1 barn = 10-24 m2)
• from NLO calculations.
– Luminosity limited (RHIC II will substantially help)
– pp Run 6 ~ 9 pb-1 (split into 2 triggered datasets)
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.
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STAR  mass resolution
w/ inner material
•High material runs
•Run6 p+p 200GeV
•Run7 Au+Au 200GeV
w/o inner material
•Low material run
•2008 and after
Since 2008: Good opportunity to measure !
high luminosity and low material
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Colloquium
Manuel Calderón de la Barca
Sánchez
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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
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STAR  Trigger
Sample -triggered Event
• 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
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Colloquium
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Sánchez
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ϒ in pp Collisions: Reference Data
L = 7.9 ± 0.6 pb-1
N(total)= 67±22(stat.)
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(1S+2S+3S) cross-section
STAR 2006 √s=200 GeV p+p ++→e+e- cross
section consistent with pQCD and world data trend
Phys. Rev. D 82 (2010) 12004
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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.
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 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
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 35  4(stat.)  5(sys.) nb
Manuel Calderón de la Barca Sánchez
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Nuclear modification factor
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
Cold Nuclear Matter effects
are not large.
d+Au collisions: (mostly)
a superposition of pp.
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Centrality in Heavy Ion Collisions
• “Centrality”: a
useful knob in our
experiments
– Collisions can be
• Peripheral: barely
touching, or
• Central: “head-on”
– Many “Participants”
• Expectation:
– Peripheral collisions: Do not produce hot matter.
– Central collisions: Produce
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ϒ: From Grazing to Head-On Collisions
Peripheral
5.9s
Central
6.7s
5.3s
Background Subtracted
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Compare to pp Reference: ϒ Suppression!
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CMS Dimuons: J/y, ϒ and Z0s in PbPb
Details:
• PRL 106, 212301 (2011)
• PRL 107, 052302 (2011)
• CMS-PAS-HIN-10-006
• CMS is excellently suited for dimuons. PbPb data up to the Z peak.
• pp reference at 2.76 TeV:
– 1 week long run at √s = 2.76 TeV in March 2011
– pp data reconstructed with heavy ion algorithm
– Identical cuts used as in heavy ion analysis
20/Oct/2011, CSU Colloquium
Manuel Calderón de la Barca Sánchez
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ϒ’s in CMS: pp and PbPb at 2.76 TeV
• For comparison to PbPb:
– Use identical analysis for pp
2.76 TeV data
• Note:
𝑝𝑇𝜇 > 4 𝐺𝑒𝑉/𝑐 in both
• 𝑁𝑃𝑏𝑃𝑏 = 86 ± 12𝑠𝑡𝑎𝑡 ± 3𝑠𝑦𝑠
• 𝑁𝑝𝑝 = 101 ± 12𝑠𝑡𝑎𝑡 ± 3𝑠𝑦𝑠
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Manuel Calderón de la Barca Sánchez
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Compare to pp Reference: Suppression!
• ϒ(1S) inclusive yield shows suppression.
– Suppression is constant with centrality within errors.
– Charm bound states (at high momentum) are also suppressed.
• Are the ϒ excited states “melting”?
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• Compare excited states
to ground state
• (2S+3S)/1S
• Compare the above
ratio in pp and PbPb
• “Double ratio”
ϒ: The excited states
Single Ratio:
Pro:
Insensitive to overall
cross section
Note:
Sensitivity to acc & effic.
difference between 1S
and (2S+3S)
pTm> 4 GeV/c
Double ratio:
Acceptance &
efficiency cancels
20/Oct/2011 CSU Colloquium
Match ground state in pp and PbPb
See if excited states are suppressed.
Manuel Calderón de la Barca Sánchez
42
ϒ Excited states suppressed!
PRL 107, 052302 (2011)
T/TC 1/r [fm-1]
¡(2S + 3S)
¡(1S) PbPb
= 0.31+0.19
-0.15 ± 0.03
¡(2S + 3S)
¡(1S) pp
• The ratio in PbPb of ϒ excited-to-ground
state is only 31% of that in pp.
PANIC 2011, 25/July/2011
Manuel Calderón de la Barca Sánchez
2
(1S)
cb(1P)
1.2
J/y(1S) (2S)
TC
cb’(2P) (3S)
cc(1P) Y’(2S)
43
Comparison: STAR and CMS
• Not quite apples
to apples
– Note: Different
collision energy.
– Ground state vs.
sum of 3 states.
• Nevertheless:
– Suppression seen
by both
experiments is
about the same!
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The bottom line…
• Heavy Ion collisions: a lab for heating up the strong force.
– The temperature is hot enough to “melt” particles.
• STAR and CMS are studying these collisions.
• Bottom quarks are very interesting!
– ++→e+e- cross section at RHIC
• pp: Rate is as expected from QCD calculations.
• dAu: Yield scales as expected:  yield looks like a superposition of pp.
– AuAu and PbPb:
• Bottom quark bound states are suppressed in Heavy Ion Collisions.
– Indications that Excited states are suppressed more.
• Can act as a Thermometer for the hottest matter produced in the lab.
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Colloquium
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Sánchez
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Events / ( 0.14 GeV/c2 )
60
50
40
30
20
10
0
7
b)
8
9
10
m
CMS, PbPb, s = 2.76 TeV
m
T
p > 4 GeV/c, |h | < 2.4
T
pU < 20 GeV/c
pp lineshape
PbPb fit
11 12 13 14
mm invariant mass [GeV/c2]
Rocky mountain national park.
Ypsilon Peak
Chapin Peak
Chiquita Peak
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Backup Material
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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
Calderón
de la Barca
Sánchez
20/Oct/2011,
CSU Colloquium
• Tracking
and dE/dxManuel
PID for
electrons
& positrons
48
 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

20/Oct/2011,
CSU
++
Colloquium
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
Manuel Calderón de la Barca
Sánchez
91 pb
49
 Analysis: Electron Id with TPC and EMC
K
p
d
e

π
electrons
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
•
•
•
•
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Colloquium
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Sánchez
preliminary
preliminary
50
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
Manuel Calderón de la Barca
Sánchez
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 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%
STAR Preliminary
Acceptance x
L0 Efficiency x
L2 Efficiency
• Acceptance x Trig Efficiency
~20% at midrapidity.
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STAR Preliminary
52
 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
20/Oct/2011, CSU
Colloquium
preliminary
•geo : geometrical acceptance
•L0 : efficiency of L0
•L2 : efficiency of L2
•(e) : efficiency of e reco
•mass: efficiency of mass cut
0.094±0.01
8
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Sánchez
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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
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 91  28 (stat.)  22 (syst.) pb
Manuel Calderón de la Barca Sánchez
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Heavy quark measurement via electrons
Electrons:
trigger
Final product of semileptonic decay
chain of heavy quarks
Background: photon
conversions, 0 Dalitz decays.
Identification and separation of charm
and bottom
charm
production
use decay topology and
azimuthal angular correlation
- electrons from D/B decays are used
to trigger on charm/bottom quark pairs
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Open Heavy Flavor and Medium Density
PRL98, 192301 (2007)
• We established that matter is opaque to light quarks.
• Do heavy quarks lose energy as the light quarks do?
– Expectation: They should lose less energy, should help determine medium
density.
– Observation: Electrons from heavy quarks are just as suppressed as light
hadrons.
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Separating charm and beauty.
+
eKe
D0
D*0
B-
b
b
Decay kinematics:
D meson ~1.5 GeV
B meson ~ 4.5 GeV
B+
D0
K+
20/Oct/2011, CSU Colloquium
Larger Q value: larger width of
e-h near side angular correlation
for B mesons.
Manuel Calderón de la Barca Sánchez
57
Getting B via semileptonic decays
B/(C+B) Ratio
(C+B) Yields
(AA)/(scaled pp)
•Large contribution of B quarks to electron yield.
•Large suppression of all electrons from heavy quarks.
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B quark suppression at RHIC!
• Statistical analysis of electrons from heavy quarks
– Yield + Correlation :
• 90% confidence interval region for separate B, D energy loss.
– Expect B energy loss < D energy loss: RAA B > RAA D
– Bottom is suppressed at 90% confidence level.
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