Event Structure and Color Reconnection in Heavy Ion Physics
GuyPaić
Instituto de Ciencias Nucleares
In c ollaboration with A. Ortiz, P. Christiansen, I.Maldonado and E. Cuautle,
Simulation from
D. B. Leinweber, hep-lat/0004025
gluon action density: 2.4 x 2.4 x 3.6 fm
The
message
•
the LHC has allowed us to investigate the pp collisions in great
details: an opportunity we have not yet fully exploited
•
In spite of the good statistics we remain steadfastly in the averaging
process
•
applying the analysis of the event structure of pp events in function
of multiplicity may have relevance to heavy ion results on parton
energy loss
•
The hydrodynamic flow confronted with partonic level interactions
like color reconnection in pp sharpens the debate
Guy Paic
2
25/09/2013
Underlying event in Pythia:
MPI (CR), ISR, BR
Guy Paic
25/09/2013
The Inelastic Non-Diffractive Cross-Section
Guy Paic
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!
From Rick Field
25/09/2013
Chriss Quigg: arXiv:1004.0975v1 [hep-ph]
from
earlier experiments:
how wrong we can
be
5
Guy Paic
25/09/2013
Guy Paic
Introducing the event
structure variable
transverse velocity
6
25/09/2013
The transverse sphericity
from a pencil to a hedgehog
7
Guy Paic
25/09/2013
Guy Paic
Guy Paic
25/09/2013
Multiparton interactions and
multiplicity distribution
Guy Paic
From A. Ortiz
peripheral
central
What can be expected in the tail of the MPI/multiplicity? Collectivity?
A clear dependence on
multiplicity
9
25/09/2013
Guy Paic
http://home.fnal.gov/~skands/leshouches-
plots/lhc07m-19-1.png
The multiplicity is strongly
dependent on the MPI
At low multiplicity the effect is
negligible.
We need to study the high
multiplicities in detail!
Note: the MB is dominated by
the low multiplicities.
Not seeing the high multiplicities
occults the importance of MPI
Guy Paic
25/09/2013
The sphericity spectrum in bins of
multiplicity
The general trend with multiplicity is trivial: more multiplicity – > sphericity rises
Detail: do the generators reproduce the trend? Actually not!
The generator
sphericity spectra CROSS the experimental spectrum at ST of ≈
0.5 at low multiplicity and 0.7 at the highest multiplicities
A
L
I
C
E
C
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.
P
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s
.
J
.
C
7
2
(
2
0
1
2
)
2
1
2
4
“We show here that the higher sphericity of high-multiplicity events compared to
MC predictions is due to an apparent reduction of the relative jet yields in such type of events”
(
from properties in low and high multiplicity events in p-p collisions at 7 TeV CMS Collaboration
CMS-PAS-FSQ-12-022
Questions
•
Can we talk of “ fluctuations” in the pp events or we do have two
very distinct mechanisms which may be connected with the number
of multiparton interactions and or centrality dependence of the pp
collisions (like in heavy ions)?
•
Guy Paic
12
25/09/2013
The next big question: is flow
existing in pp collisions?
Ortiz et al. Phys. Rev. Lett. 111, 042001 (2013)
Guy Paic
25/09/2013
From pp to PbPb not much change!
Guy Paic
Can the theory coherently
explain???
14
25/09/2013
In Pythia, the final step at parton level before the
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R
,
its aim is to describe the hadronization of a
many parton system in a single event with
multiple hard sub collisions.
G. Gustafson, Acta Phys.Polon.B40:1981-
1996,2009
Color reconnection
Toy model
of (non-perturbative) color
reconnections
,
applicable to any final state
at hadronisation time, each string piece
has a probability to interact with the
vacuum / other strings:
P
reconnect
= 1 – (1-
χ
)
n
χ
=
strength parameter: fundamental
reconnection probability (free
parameter)
n =
number of multiple interactions
in current event ( ~ counts # of
possible interactions)
15
Guy Paic
25/09/2013
Effect of multiparton interactions and
color reconnection on observables with
identified hadrons
Guy Paic
25/09/2013
p/
π vs
p
T
and MPI
Dependence of the non-strange
baryon-to-meson ratio with the
number of MPI's, N
mpi
.
•
For with N
mpi
>5 the ratios exhibit
the peak.
•
The peak is enhanced w.r.t.
reference when N
mpi
increases,
N
mpi
>10. Also the peak is pushed
up to higher
p
T
(characteristic of
flow).
Guy Paic
25/09/2013
Λ
/K
0
S
vs
p
T
and MPI
Same for Lambda-to-K
0
S
ratio as a
function of
p
T
Guy Paic
25/09/2013
4/17/2013
guy paic
19
Effect of CR on the
p/
π ratio vs
p
T
The peak on p/
π is due
to color reconnection
Guy Paic
Guy Paic
25/09/2013
Is this just a baryon effect?
T
e
s
t
w
i
t
h
φ
No! We have a
m
a
s
s
e
f
f
e
c
t
=
f
l
o
w
-
l
i
k
e
Is this just a baryon effect?
Guy Paic
25/09/2013
4/17/2013
guy paic
21
Other observables with
identified hadrons
The behavior of the
double ratios indicates
that we have a mass
effect since the φ/π and
the p/π ratios exhibit a
much larger “bump”
than the K/π ratio.
We know that Pythia
underestimates the
strange particle
production, specially at
intermediate
p
T
. To
avoid this problem we
can look at the <
p
T
> vs
Nch.
Guy Paic
Guy Paic
25/09/2013
<
p
T
> as a function of
multiplicity
Pythia describes the data when CR is
implemented,
i.e.
a flow-like effect is
necessary.
So far we know that MC generators are
too hard, very jetty-like. See transverse
sphericity as a function of the event
multiplicity. More MPI activity best
describes data -> more isotropic -
>larger flow-like effect.
CMS Collaboration, Eur. Phys. J. C 72 (2012) 2164
Guy Paic
25/09/2013
Conclusion
•
For the first time (unknown to the author of Pythia 8)
we have proven that color reconnection does
produce effects similar to flow but only up to a limit in
pt.
•
The effect of the CR does weaken at higher
momenta as also requested by the data,
•
The fact of having some kind of collectivity without
the introduction of often complicated hydrodynamics
arguments (thermalisation, temperature time
sequence etc) seems interesting and its application
to heavy ion collisions could be full of surprises!!
Guy Paic
25/09/2013
speculations
•
we seem to have pinpointed a mechanism that can
generate flow like patterns. However a priori I do
not see a mechanism that prevents having
something similar – a color reconnection that
throws particles in eta.
•
Ridge? The ridge spectra lie in the same pt range
where CR is present
Guy Paic
24
25/09/2013
A short excursion in heavy
ions
•
The similarities observed in the p/pi and T-beta
behavior asks an obvious question: is all what we
have seen and liked so far in heavy ion already
present in pp!?
•
One of the “sacred cows” of the field is the parton
energy loss…
•
If one takes a view from the side of pp one may say
that the effect is already seen in pp!!
25
Guy Paic
25/09/2013
Comparison of high and low
sphericity spectra in Pythia 8 4C
B
y
n
o
w
i
t
i
s
w
e
l
l
e
s
t
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l
i
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H
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o
n
s
We have made a simulation
with Pythia 8. The simulations
show the same trend although
in real data it is yet more
marked…
26
Guy Paic
25/09/2013
Ratio of low sphericity to high
sphericity pt spectra
The ratios show an interesting
pattern that bears some similarities
to what is observed in Heavy ion
collisions and is attributed to parton
energy loss
Are we witnessing two different
mechanisms of suppression or we
witness the same in pp and heavy
ions???
27
Guy Paic
25/09/2013
The elliptic flow – could it be
MPI dependent.
One should take in account that also the initial state is different with respect to the
number of interaction if a collective effect like MPI and color reconnection takes
place it should be taken into account.
28
Guy Paic
25/09/2013
Long-range near-side angular
correlations
29
Peaks are
truncated !
Guy Paic
25/09/2013
Final comments
•
We have seen that the pp collisions should be
studied not only in minimum bias and/or
multiplicity – the event shapes should be taken into
account
•
Some features of the heavy ion collisions can be
observed in the pp collisions at high multiplicity
•
The pp collisions with a good statistics should be
planned for the next LHC run in 2015 in good
conditions with low pileup and high stats top reach
higher multiplicities
30
Guy Paic
25/09/2013
25/09/2013
Guy Paic
31
Investigating event structures in proton-proton collisions sheds light on potential relevance for heavy ion physics, especially in understanding parton energy loss and hydrodynamic flow dynamics. The study discusses phenomena like color reconnection, underlying events in Pythia, and multiple-parton interactions. The analysis of event structures and transverse sphericity provides valuable insights into particle collisions and the intricacies of partonic interactions. Collaboration between researchers continues to explore the event structure at a detailed level to uncover new facets in high-energy physics.
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Presentation Transcript
Event structure and color reconnection - possible relevance for heavy ion physics GuyPai Instituto de Ciencias Nucleares In c ollaborationwith A. Ortiz, P. Christiansen, I.Maldonadoand E. Cuautle, Simulation from D. B. Leinweber, hep-lat/0004025 gluon action density: 2.4 x 2.4 x 3.6 fm
The message the LHC has allowed us to investigate the pp collisions in great details: an opportunity we have not yet fully exploited In spite of the good statistics we remain steadfastly in the averaging process applying the analysis of the event structure of pp events in function of multiplicity may have relevance to heavy ion results on parton energy loss The hydrodynamic flow confronted with partonic level interactions like color reconnection in pp sharpens the debate 25/09/2013 Guy Paic 2
Underlying event in Pythia: MPI (CR), ISR, BR 25/09/2013 Guy Paic
The Inelastic Non-Diffractive Cross-Section Occasionally one of the parton-parton collisions is hard (pT > 2 GeV/c) From Rick Field Proton Proton Majority of min- bias events! Semi-hard parton- parton collision (pT < 2 GeV/c) + Proton Proton Proton Proton + + Proton Proton Proton Proton the pp collision is ~ an average of all processes Multiple-parton interactions (MPI)! Guy Paic 25/09/2013
from earlier experiments: how wrong we can be CDF Chriss Quigg: arXiv:1004.0975v1 [hep-ph] Albajar, C., et al. (UA1 Collaboration). Analysis of the Highest Transverse Energy Events Seen in the UA1 Detector at the SppS Collider. Z. Phys. C36 (1987), UA1 25/09/2013 Guy Paic 5
Introducing the event structure variable transverse velocity 25/09/2013 Guy Paic 6
The transverse sphericity from a pencil to a hedgehog 25/09/2013 Guy Paic 7
25/09/2013 Guy Paic Guy Paic
Multiparton interactions and multiplicity distribution A clear dependence on multiplicity From A. Ortiz central peripheral What can be expected in the tail of the MPI/multiplicity? Collectivity? 25/09/2013 Guy Paic 9
The multiplicity is strongly dependent on the MPI At low multiplicity the effect is negligible. http://home.fnal.gov/~skands/leshouches- plots/lhc07m-19-1.png We need to study the high multiplicities in detail! Note: the MB is dominated by the low multiplicities. Not seeing the high multiplicities occults the importance of MPI 25/09/2013 Guy Paic Guy Paic
The sphericity spectrum in bins of multiplicity The general trend with multiplicity is trivial: more multiplicity > sphericity rises Detail: do the generators reproduce the trend? Actually not! The generator sphericity spectra CROSS the experimental spectrum at ST of 0.5 at low multiplicity and 0.7 at the highest multiplicities ALICE Collaboration Eur.Phys.J. C72 (2012) 2124 We show here that the higher sphericity of high-multiplicity events compared to MC predictions is due to an apparent reduction of the relative jet yields in such type of events (from properties in low and high multiplicity events in p-p collisions at 7 TeV CMS Collaboration CMS-PAS-FSQ-12-022
Questions Can we talk of fluctuations in the pp events or we do have two very distinct mechanisms which may be connected with the number of multiparton interactions and or centrality dependence of the pp collisions (like in heavy ions)? 25/09/2013 Guy Paic 12
The next big question: is flow existing in pp collisions? Ortiz et al. Phys. Rev. Lett. 111, 042001 (2013) 25/09/2013 Guy Paic
From pp to PbPb not much change! Can the theory coherently explain??? 25/09/2013 Guy Paic 14
Color reconnection In Pythia, the final step at parton level before the hadronization is the color reconnection CR, its aim is to describe the hadronization of a many parton system in a single event with multiple hard sub collisions. Toy model of (non-perturbative) color reconnections, applicable to any final state at hadronisation time, each string piece has a probability to interact with the vacuum / other strings: Preconnect = 1 (1- )n = strength parameter: fundamental reconnection probability (free parameter) n = number of multiple interactions in current event ( ~ counts # of possible interactions) G. Gustafson, Acta Phys.Polon.B40:1981- 1996,2009 25/09/2013 Guy Paic 15
Effect of multiparton interactions and color reconnection on observables with identified hadrons Guy Paic 25/09/2013
p/ vs pT and MPI The dependence with MPI (multiplicity) resembles the evolution of the p/pi vs pT from peripheral to central Pb-Pb collisions. Dependence of the non-strange baryon-to-meson ratio with the number of MPI's, Nmpi. For with Nmpi>5 the ratios exhibit the peak. The peak is enhanced w.r.t. reference when Nmpi increases, Nmpi>10. Also the peak is pushed up to higher pT (characteristic of flow). ALICE Collaboration, http://arxiv.org/abs/1303.0737 25/09/2013 Guy Paic
/K0 S vs pT and MPI Same for Lambda-to-K0 function of pT S ratio as a 25/09/2013 Guy Paic
Effect of CR on the p/ ratio vs pT The peak on p/ is due to color reconnection 4/17/2013 25/09/2013 guy paic Guy Paic 19 Guy Paic
Is this just a baryon effect? Is this just a baryon effect? No! We have a mass effect=flow-like 25/09/2013 Guy Paic
Other observables with identified hadrons The behavior of the double ratios indicates that we have a mass effect since the / and the p/ ratios exhibit a much larger bump than the K/ ratio. We know that Pythia underestimates the strange particle production, specially at intermediate pT. To avoid this problem we can look at the <pT> vs Nch. 4/17/2013 25/09/2013 guy paic Guy Paic 21 Guy Paic
<pT> as a function of multiplicity Pythia describes the data when CR is implemented, i.e. a flow-like effect is necessary. So far we know that MC generators are too hard, very jetty-like. See transverse sphericity as a function of the event multiplicity. More MPI activity best describes data -> more isotropic - >larger flow-like effect. CMS Collaboration, Eur. Phys. J. C 72 (2012) 2164 25/09/2013 Guy Paic
Conclusion For the first time (unknown to the author of Pythia 8) we have proven that color reconnection does produce effects similar to flow but only up to a limit in pt. The effect of the CR does weaken at higher momenta as also requested by the data, The fact of having some kind of collectivity without the introduction of often complicated hydrodynamics arguments (thermalisation, temperature time sequence etc) seems interesting and its application to heavy ion collisions could be full of surprises!! 25/09/2013 Guy Paic
speculations we seem to have pinpointed a mechanism that can generate flow like patterns. However a priori I do not see a mechanism that prevents having something similar a color reconnection that throws particles in eta. Ridge? The ridge spectra lie in the same pt range where CR is present 25/09/2013 Guy Paic 24
A short excursion in heavy ions The similarities observed in the p/pi and T-beta behavior asks an obvious question: is all what we have seen and liked so far in heavy ion already present in pp!? One of the sacred cows of the field is the parton energy loss If one takes a view from the side of pp one may say that the effect is already seen in pp!! 25/09/2013 Guy Paic 25
Comparison of high and low sphericity spectra in Pythia 8 4C By now it is well established by LHC experiments that in high multiplicity events the jets are suppressed at high multiplicities in pp collisions We have made a simulation with Pythia 8. The simulations show the same trend although in real data it is yet more marked 25/09/2013 Guy Paic 26
Ratio of low sphericity to high sphericity pt spectra The ratios show an interesting pattern that bears some similarities to what is observed in Heavy ion collisions and is attributed to parton energy loss Are we witnessing two different mechanisms of suppression or we witness the same in pp and heavy ions??? 25/09/2013 Guy Paic 27
The elliptic flow could it be MPI dependent. coordinate space Less MPI y More MPI More MPI x One should take in account that also the initial state is different with respect to the number of interaction if a collective effect like MPI and color reconnection takes place it should be taken into account. Guy Paic 25/09/2013 28
Long-range near-side angular correlations 7 TeV Peaks are truncated ! new ridge-like structure at ~ 0 25/09/2013 Guy Paic 29
Final comments We have seen that the pp collisions should be studied not only in minimum bias and/or multiplicity the event shapes should be taken into account Some features of the heavy ion collisions can be observed in the pp collisions at high multiplicity The pp collisions with a good statistics should be planned for the next LHC run in 2015 in good conditions with low pileup and high stats top reach higher multiplicities 25/09/2013 Guy Paic 30
25/09/2013 Guy Paic 31
