isolating the genuine radial flow effects in pp collisions

Antonio Ortiz VelasquezAntonio Ortiz Velasquez

Hungarian Academy of SciencesHungarian Academy of SciencesWigner Research Centre for PhysicsWigner Research Centre for PhysicsHEP SeminarHEP SeminarMay 30, 2014.May 30, 2014.

Instituto de Ciencias NuclearesUniversidad Nacional Autónoma de México.

Isolating the genuine Isolating the genuine radial flow effects in pp radial flow effects in pp

collisionscollisions(separation soft-hard)(separation soft-hard)

May 30, 2014 A. Ortiz Velasquez (ICN-UNAM) 2

Outline

1. Motivation.

2. Event shapes

3. Separation soft and hard using the spherocity approach, simulations for pp @ 7 TeV.

4. Predictions for pp @ 13 TeV.

5. Summary.


In pp and p-Pb collisions at the LHC energies, collective phenomena have been discovered: ridge structure, radial and anisotropic flow. But, so far, there are not signatures of jet quenching. Hence, it is important to understand the origin of the collectivity in small systems.

Motivation

May 30, 2014 A. Ortiz Velasquez (ICN-UNAM) 4Slide shown at QM2014 by C.Andrei


Slide shown at QM2014 by C.Andrei


Slide shown at QM2014 by C.Andrei

Larger push of the p/π for pp?


Increase of multiplicity or centrality


Increase of multiplicity or centrality

The Nch reach is smaller in pp than in p-Pb. However, pp gives a larger <β

T>.


A fragmentation bias affects the high multiplicity events

ALICE Collaboration: JHEP 09, 049, 2013 and Eur. Phys. J. C72, 2124, 2012.

How does this effect affect the observables which we measure

at high multiplicity, e.g. pT spectra of id hadrons?

Is the event multiplicity a good reference to compare results

from different systems?

The same Nch value corresponds to a very different collision regime in the three systems (ALICE Collaboration, PLB 727,2013, 371-380).

ALICE Collaboration, Phys. Lett. B 727 (2013) 371-380.


➢ Compare similar event classes in pp and p-Pb collisions.

➢ Study how well theory describes enhanced soft and hard event samples.

➢ Remove jet contributions and study genuine radial flow effects in pp collisions.

Having a tool to separate soft and hard events would allow:


The frameworkPythia 8 was used since it reproduces QUALITATIVELY WELL many features of data, e.g., radial flow

AO et. al., Phys. Rev. Lett. 111, 042001 (2013)


Event Shapes characterize the distribution of the outgoing particle energy from a high energy collision.

IsotropicJetty-like

The tool

Nch=42 Nch=44

E. Cuautle, R. Jiménez, I. Maldonado, AO, G. Paic and E. Perez, arXiv:1404.2372


Sphericity and spherocity● Sphericity.

The following matrix is diagonalized:

Transverse sphericity is defined as follows:

● SpherocityTransverse spherocity is defined as follows:

Where λ1>=λ

2.

Case λ1=λ

2.

x

y

x

yv2

v1

v2

v1

Case λ1>λ

2.

STpherocity

=π2

4min

n⃗=(nx ,n y , 0)

(∑∣p⃗T i

× n⃗∣

∑ pT i

)

2

Sphe

roci

ty A

xis

Andrea Banfi et al., “Phenomenology of the event shapes at hadron colliders”, arXiv:1001.4082


Features at partonic level of low and high S

O events

Event by event, the transverse momentum, , in the rest frame of all the 2 → 2 process, is calculated.

Low SO events: in average low number of MPIs

(<3) are involved, they are hard to semi hard.High S

O events: in average 12 MPI occur, the

partonic interactions are softer. ISOTROPIC(soft)

PENCIL-LIKE(hard)

p̂T


pT spectrum for

inclusive charged particles

Low sphero(i)city, SO<0.1

High sphero(i)city, SO>0.9

Spherical events:Larger low p

T

production w.r.t. Inelastic events.

➢ Using spheri(o)city is possible to split the sample of pp data into enriched soft and hard events sub-samples.

➢ Jetty-like events (low S

T or S

O) have a harder

pT spectrum than the

one obtained for MB. Isotropic events (high S

T or S

O) have a soft p

T

spectrum.


CR effects on the pT spectra

Full markers, are simulations with CR.

Empty points, are simulations without CR. ISOTROPIC

(soft)PENCIL-LIKE

(hard)

Flow patterns are clearly visible in soft events


ISOTROPIC(soft)

PENCIL-LIKE(hard)


Particle ratios

MB + 30 GeV jets This kind of measurement could be done in ALICE.But to see these effects we simulated 3X109 events!RUN II is a good chance to do it.

Flow signature in K/π

ISOTROPICISOTROPIC(soft)(soft)

PENCIL-LIKEPENCIL-LIKE(hard)(hard)


Actually, this is only a modification of the spectral spahes


The effect is not seen if one just cuts on multiplicity

LM, z<2HM, z>7

Selection on multiplicity is a selection on harder processes

High multiplicity starts to be dominated by fragmentation.

Within uncertainties the same trend in seen in ALICE pp data (see slide 6).


ISOTROPIC(soft)

PENCIL-LIKE(hard)

Results for pp @ 7 TeV are shown with solid lines (Pythia 8.180 tune 4C), the amount of reconnection is the same. But remember that flow effects grows with Nmpi.

Predictions for pp @ 13 TeV

Will we be able to see the increase (large nMPI) of flow using only the MB sample?


φ/π p/π

Inel

astic

pp

colli

sion

s The curves cross here.


φ/π p/π

Sof

t pp

colli

sion

s Flow + production of φ

Larger flow effect

The curves cross here.

ISOTROPIC(soft)


φ/π p/π

Har

d pp

col

lisio

ns There is not a crossing point.

PENCIL-LIKE(hard)


Combining event shapes and event multiplicity

jets

Low Q2 processes


Average pT vs Nch


Combining dN/dη and SO

(dN/dη ~ 35)

ISOTROPIC(soft)

PENCIL-LIKE(hard)


ISOTROPIC(soft)

PENCIL-LIKE(hard)


(dN/dη ~ 41)


ISOTROPIC(soft)

PENCIL-LIKE(hard)


(dN/dη ~ 46)


ISOTROPIC(soft)

PENCIL-LIKE(hard)


(dN/dη ~ 53)


Tkin vs <βT>

(Lines are drawn just to guide the eye)

ISOTROPICISOTROPIC(soft)(soft)

PENCIL-LIKEPENCIL-LIKE(hard)(hard)


Tkin vs <βT>(ALICE data)

Plot shown by C. Andrei at QM2014Pythia 8 results are in better agreement with data when high Q2 events are removed.


SummaryThe event multiplicity is not the best reference to compare results which come from different colliding systems. For Pythia 8 events the high multiplicity is produced by means of hard partonic interactions.

Using spherocity I showed that it is possible to have enriched high Q2 and low Q2 event samples.

The pT spectra of identified hadrons which come from two

extreme event classes, high So and low So, were analyzed. The results from the Blast-wave analysis, Tkin vs <βT>, show that jets may increase the average transverse velocity and reduce the temperature. While for soft processes the agreement with LHC data is much better.

E. Cuautle, R. Jiménez, I. Maldonado, AO, G. Paic and E. Perez, arXiv:1404.2372


Backup


➢ The effect is nicely illustrated in the two particle correlation analysis as a function of the event multiplicity: the per-trigger near side pair-yield increases with Nch.

Motivation

ALICE Collaboration, JHEP09(2013)049

Multiplicity is not a good variable to compare the results from different systems.

isolating the genuine radial flow effects in pp collisions

Documents