Mid-rapidity anti-baryon to baryon
ratios in pp and Pb-Pb collisions
Michal Broz
•
•
•
•
•
p/p, Λ/Λ, Ξ+ Ξ− , Ω+ Ω−
Footsteps of the analysis
pp at 0.9, 2.76 and 7 TeV
Energy dependence in pp
Multiplicity dependence in pp
Comparison of pp and Pb-Pb at 2.76 TeV
2
Baryon models
Constituent quark model
D. B. Lichtenberg and L. J. Tassie, Phys. Rev.
155, 1601 (1967)
•
•
•
•
Baryons are described as quarkdiquark pairs
Valence quarks carries baryon
number
BN of valence quark = 1/3
Baryon produced in collision is
formed around the diquark
String junction model
M. Imachi, S.Otsuki, and F. Toyoda, Prog. Theor.
Phys. 52, 341 (1974)
• Valence quarks are connected via
nonperturbative configuration of
gluon field called string
junction(J)
• Baryon number is carried by
string junction
• BN of valence quark = 0
• BN of string junction = 1
• Out coming baryon is formed
around string junction with
presence of several sea quarks
3
Baryon number transport
•
•
•
•
In inelastic pp collisions at very high energy, the incoming projectile breaks
up into several hadrons that typically emerge, after the collision, at small
angles close to the original beam direction.
Deceleration of the conserved BN of the beam = BN transport.
Most of the baryons in central rapidity are from pair production
Any excess of baryons in central rapidity comes from the BN transport
▫ BN asymmetry in central rapidity is a consequence of BN asymmetry of colliding
particles
▫ Reasonable measurement only in BN asymmetric collisions
Amount of transported BN can be quantified as asymmetry or ratio
𝑅 = 𝑁B 𝑁B
• Probability of BN transfer
α≈
thorough rapidity interval is
•
 1
 ( s)
e
• α depends on configuration in
which BN is transported
▫
▫
▫
▫
Diquark
-1/2
SJ accompanied by diquark
-1/2
SJ accompanied by quark
(Rossi&Veneziano)
1/2
SJ itself
1/2
SJ itself (Kopeliovich)
1
( 1) y
Diquark (CQM)
SJ accompanied by diquark
SJ accompanied by quark
SJ itself
B.Z.Kopeliovich, Sov. J. Nucl. Phys. 45,
(1987) 1078
G.C.Rossi, G.Veneziano, Nucl. Phys.
B123, (1977) 507
4
Motivation for the mid-rapidity ratio
measurements at LHC energies
• Probability of BN transfer thorough rapidity interval is ≈ exp (α-1).Δ y
α≈
SJ itself (Kharzeev)
1/2
SJ itself (Kopeliovich)
1
• If α for SJ itself is about ½ then we should see vanishing asymmetry
(ratio -> 1) at high energies
• If α is close or exactly 1 then the non-zero asymmetry is predicted at high
energies
• If α is exactly 1 then asymmetry have to converge to constant non-zero
value
• Difference between various models is on the order of few percent
• We need high precision measurement!
𝐩/𝐩
𝚲/𝚲
Kopeliovich (αJ = 1)
QGSM (αJ = 0.9)
𝚵+ 𝚵−
𝛀+ 𝛀−
0.958
0.958
0.93
0.946
0.945
5
Event and track selection
[email protected]
• Trigger on minimum bias events
• Pb-Pb in 0-80% centrality - based on V0
multiplicity
• Track quality cuts – number of TPC clusters
• Cuts on ITS clusters to decrease
contamination in the primary proton sample
• Particle identification based on dE/dx in TPC
gas – Selecting particles in 3σ area around
the expected value
Phase space
√s (TeV)
p
0.9
Λ
Ξ
Ω
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.8
0.5 < pT < 4.0
-
2.76 pp
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.8
0.5 < pT < 4.5
|y|< 0.8
1.0 < pT < 4.5
2.76 Pb-Pb
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.6
0.5 < pT < 5.5
-
7
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.8
0.5 < pT < 5.5
|y|< 0.8
1.0 < pT < 5.5
6
Reconstruction of
strange baryons
[email protected]
• Strange baryons are reconstructed
using their weak decay topology in
charged particles only.
• Signal is clearly visible and can be
easily distinguished from the
background
• Several methods were used to
extract the signal
i.
Background areas fitted simultaneously
with polynomial functions
ii. Background areas averaged by simply
counting the number of entries
 Difference between results using various
methods is included into systematic
uncertainty
Source
Λ (%)
Ξ (%)
Signal/Background extraction
0.5 – 0.6
0.4 – 0.2
Λ
7
Reconstruction of
strange baryons
[email protected]
• Strange baryons are reconstructed
using their weak decay topology in
charged particles only.
• Signal is clearly visible and can be
easily distinguished from the
background
• Several methods were used to
extract the signal
i.
Background areas fitted simultaneously
with polynomial functions
ii. Background areas averaged by simply
counting the number of entries
 Difference between results using various
methods is included into systematic
uncertainty
Source
Λ (%)
Ξ (%)
Signal/Background extraction
0.5 – 0.6
0.4 – 0.2
Ξ
7
Reconstruction of
strange baryons
[email protected]
• Strange baryons are reconstructed
using their weak decay topology in
charged particles only.
• Signal is clearly visible and can be
easily distinguished from the
background
• Several methods were used to
extract the signal
i.
Background areas fitted simultaneously
with polynomial functions
ii. Background areas averaged by simply
counting the number of entries
 Difference between results using various
methods is included into systematic
uncertainty
Source
Λ (%)
Ξ (%)
Signal/Background extraction
0.5 – 0.6
0.4 – 0.2
Ω
8
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
9
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• This mechanism is directly
related to the interaction cross
section
• Knowledge of proper cross
sections is crucial
Source
Sys. uncert.
Material budget
0.5%
Inelastic cross-section
p
0.8%
Λ, Ξ
0.5%, 1.0%
9
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• This mechanism is directly
related to the interaction cross
section
• Knowledge of proper cross
sections is crucial
Source
Sys. uncert.
Material budget
0.5%
Inelastic cross-section
p
0.8%
Λ, Ξ
0.5%, 1.0%
9
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• This mechanism is directly
related to the interaction cross
section
• Knowledge of proper cross
sections is crucial
Source
Sys. uncert.
Material budget
0.5%
Inelastic cross-section
p
0.8%
Λ, Ξ
0.5%, 1.0%
9
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• This mechanism is directly
related to the interaction cross
section
• Knowledge of proper cross
sections is crucial
Source
Sys. uncert.
Material budget
0.5%
Inelastic cross-section
p
0.8%
Λ, Ξ
0.5%, 1.0%
9
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• This mechanism is directly
related to the interaction cross
section
• Knowledge of proper cross
sections is crucial
Source
Sys. uncert.
Material budget
0.5%
Inelastic cross-section
p
0.8%
Λ, Ξ
0.5%, 1.0%
10
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• p and Λ contaminated by
material produced secondaries
• Effect visible in Distance of
Closest Approach or Cosine of
pointing angle
• Fit of data distributions with a
sum of MC templates
▫
Difference between results using
various methods is included into
systematic uncertainty
Source
p (%)
Λ (%)
Contamination
0.4 - 1.4
0.5 - < 0.1
10
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• p and Λ contaminated by
material produced secondaries
• Effect visible in Distance of
Closest Approach or Cosine of
pointing angle
• Fit of data distributions with a
sum of MC templates
▫
Difference between results using
various methods is included into
systematic uncertainty
Source
p (%)
Λ (%)
Contamination
0.4 - 1.4
0.5 - < 0.1
10
Corrections
[email protected]
• The ALICE TPC is symmetric around mid-rapidity => many
detector effects cancel out in the ratio
• Leading effects are:
▫ Absorption
▫ Contamination by
secondary particles
• p and Λ contaminated by
material produced secondaries
• Effect visible in Distance of
Closest Approach or Cosine of
pointing angle
• Fit of data distributions with a
sum of MC templates
▫
Difference between results using
various methods is included into
systematic uncertainty
Source
p (%)
Λ (%)
Contamination
0.4 - 1.4
0.5 - < 0.1
[email protected]
[email protected]
11
p/p at 2.76 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
Rapidity distribution
pT distribution
11
Λ/Λ at 0.9 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
Rapidity distribution
pT distribution
11
Λ/Λ at 2.76 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
Rapidity distribution
pT distribution
11
Λ/Λ at 7 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
Rapidity distribution
pT distribution
11
Ξ + Ξ− at 0.9 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
pT distribution
11
Ξ + Ξ− at 2.76 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
pT distribution
11
Ξ + Ξ− at 7 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
Rapidity distribution
pT distribution
11
Ω+ Ω− at 2.76 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
pT distribution
11
Ω+ Ω− at 7 TeV
[email protected]
• Results show no sign of rapidity or transverse momentum
dependence
• Data points are well described by Pythia: Perugia 2011 and Hijing/B
pT distribution
12
Mid-rapidity ratios
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• Results at √s = 7 TeV are compatible with unity for all baryons
• Stringent limit on amount of baryon transport over 9 units in rapidity
[email protected]
14
[email protected]
Energy and rapidity dependence parametrisations
• An approximation of the 𝑦beam and y dependences of the ratio R can be derived in
the Regge model, where baryon pair production at very high energy is governed by
Pomeron exchange
• The asymmetry between baryons and anti-baryons can be expressed by the stringjunction transport and by an exchange with negative C-parity (e.g. ω exchange)
𝑅=
1 + 𝐶1 × exp αJ − αP 𝑦beam × cosh αJ − αP 𝑦
1 + 𝐶2 × exp αJ − αP 𝑦beam × cosh αJ − αP 𝑦
• If C1 = 0 equation counts only for exchange of string-junction
𝑅=
1
1 + 𝐶 × exp αJ − αP 𝑦beam
ALICE: Phys. Rev. Lett. 105, 072002 (2010)
15
[email protected]
Energy dependence of the mid-rapidity ratio
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
15
[email protected]
Energy dependence of the mid-rapidity ratio
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
15
[email protected]
Energy dependence of the mid-rapidity ratio
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
15
[email protected]
Energy dependence of the mid-rapidity ratio
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
15
Rapidity dependence of the ratio
[email protected]
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
15
Rapidity dependence of the ratio
[email protected]
• αP and αJ intercepts are assumed to be 1.2 and 0.5
• Fit to the p/p data gives C2 = - C1 = 3.9 ± 0.3
• Reggeon with negative C parity and αJ ≈ 0.5 is sufficient for describing B/B data
[email protected]
16
Multiplicity dependence in pp
• Multiplicity was estimated using
combination of global tracks +
complementary SPD tracklets
• Was verified that this estimate is
proportional to dNch /dη
• Study of multiplicity dependence was
performed in words of relative charged
particle multiplicity density
• <dNch /dη> is value measured for inelastic
pp collisions with at least one charged
particle in |η| < 1
[email protected]
17
Multiplicity dependence p/p
[email protected]
17
Multiplicity dependence Λ/Λ
[email protected]
17
Multiplicity dependence
Ξ+
Ξ−
[email protected]
[email protected]
18
p/p at 2.76 TeV
Rapidity distribution
• Results show no sign of rapidity
or transverse momentum
dependence
• Data points in pp at √s = 2.76
TeV are well described by
Pythia: Perugia 2011
• Results from pp and Pb-Pb
collisions are compatible
pT distribution
𝐩/𝐩
𝚲/𝚲
𝚵+ 𝚵−
18
Λ/Λ at 2.76 TeV
Rapidity distribution
• Results show no sign of rapidity
or transverse momentum
dependence
• Data points in pp at √s = 2.76
TeV are well described by
Pythia: Perugia 2011
• Results from pp and Pb-Pb
collisions are compatible
pT distribution
𝐩/𝐩
𝚲/𝚲
𝚵+ 𝚵−
18
Ξ + Ξ− at 2.76 TeV
[email protected]
• Results show no sign of
transverse momentum
dependence
• Data points in pp at √s = 2.76
TeV are well described by
Pythia: Perugia 2011
• Results from pp and Pb-Pb
collisions are compatible
pT distribution
𝐩/𝐩
𝚲/𝚲
𝚵+ 𝚵−
19
Mid-rapidity ratios
• Results from pp and Pb-Pb collisions are compatible
• Results at √s = 2.76 TeV are very close to unity
[email protected]
22
Summary
[email protected]
• Baryon number transport can be investigated via measurement of
antibaryon-to-baryon ratios
• I was working on the analysis of antibaryon-to-baryon ratios in pp
and Pb-Pb collisions measured by ALICE
• No sign of rapidity or transverse momentum dependence
• Multiplicity dependence of the ratio was analyzed in terms of
relative charged particle multiplicity density – no sign of such a
dependence
• Ratios goes to unity for all measured baryons
• Energy and rapidity dependences can be described by exchanges
with Regge trajectory intercept of αJ ≈ ½ which are suppressed
with increasing rapidity interval
▫ Any significant contribution of an exchange not suppressed at
large Δy, reached at LHC energies (αJ ≈ 1) is disfvoured
• Ratios in Pb-Pb are within uncertainties (even stat.) compatible
with pp for p/p, Λ/Λ and Ξ+ Ξ−
[email protected]
23
[email protected]
Event and track selection
Primary protons selection
Event Selection
Min. Bias trigger
Yes
|Vz|
< 10 cm
Hyperons selection
Dcaxy (cm)
< 0.2
Topological selections
TPC PID
< 3σ
PID on daughter tracks
Number of ITS clusters
2
M(KS0) rejection for Λ
Hit on SPD1 || SPD2
Yes
(Daughter) Track Selection – Quality cuts
Number of TPC clusters (dE/dx)
80
Centrality Selection in Pb-Pb
V0 Multiplicity
0-80 %
Phase space
p
Λ
Ξ
pp
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.8
0.5 < pT < 4.5
Pb-Pb
|y|< 0.5
0.45 < pT < 1.05
|y|< 0.6
0.5 < pT < 5.5
24
Previous experimental results
[email protected]
Results from ALICE
Results from STAR
ALICE: Phys. Rev. Lett. 105, 072002 (2010)
STAR: Phys. Rev. C 75 (2007) 64901
•
•
•
•
Increase of mid-rapidity ratio
with strangeness measured on
STAR (and others)
Flat pt dependence of ratio
Scenario with αJ=1 disfavored by
ALICE results
Energy dependence of mid
rapidity ratio goes to unity for
high energies
25
[email protected]
Feed down for Ξ -> Λ + π
• MC part
▫ Extract Ξ invariant mass spectrum from Data
▫ Extract fractions of “primary”, “from Ξ” ,(“from Ω”) and “other
secondary” in selected Data Λ from MC stack – using labels of
daughters
• Data part
▫ Extract Ξ invariant mass spectrum
▫ Extract Λ invariant mass spectrum
• Get signal from invariant mass spectra using standard
signal/background extraction
Feed down fraction
pp 0.9 TeV
pp 2.76 TeV
Pb-Pb 2.76 TeV
pp 7 TeV
Λ
0.22
0.241
0.213
0.224
antiΛ
0.21
0.238
0.211
0.225
N from Sig DT
FD 
Sig MC Sig DT
26
[email protected]
dNch /dη
ALICE Collaboration, Eur. Phys. J. C68, 89 (2010).
27
Systematic uncertainties
[email protected]