hbt two-pion correlations at lhc

HBT two-pion correlations at LHC

Qingfeng Li (李庆峰 )(Huzhou Teachers College)

Q.Li for NN2012 in San Antonio 2

OutlineOutline

• LHC physics• UrQMD updates (cascade & dynamic modes)• HBT Correlations• Calculation Results at LHC• Summary

Mainly From:Q.Li, G. Graef, M. Bleicher, PRC 85, 034908 (2012);G. Graef, M. Bleicher, Q.Li, PRC, 85, 044901 (2012); G. Graef, Q.Li, M. Bleicher, JPG 39, 065101(2012).


LHC physicsLHC physics

• TeV physics• http://lhc.web.cern.ch/lhc/• To find: Higgs;micro black holes; extra dimensions

(Kaluza–Klein Theory); dark matters

• …a large number of unsolved questions in fundamental physics

CERN


What to do now…What to do now…

• The extracted bulk properties of the high temperature fireball created in such ultrarelativistic collisions have provided unprecedented information for fundamental investigations of the phase diagram of quantum chromodynamics.

• to explore collective features of the strong interaction in high multiplicity pp events;

• to explore expansion properties of the created matter by investigating the spatial shape of the fireball from AA collisions;

• to explore the spatial structure of the source created in collisions of various heavy ions at different energies and centralities to shed light on the observed scaling violation when going from pp to AA collisions at the LHC.


UrQMDUrQMD

• a microscopic many-body approach to p-p, p-A, and A-A interactions at energies ranging from SIS up to LHC.

• It is based on the covariant propagation of mesons and baryons. Furthermore it includes rescattering of particles, the excitation and fragmentation of color strings, and the formation and decay of hadronic resonances.

• At LHC, the inclusion of hard partonic interactions in the initial stage is important and is treated via the PYTHIA model.

• The model can be downloaded from http://urqmd.org……


Main updates in recent yearsMain updates in recent years

1. In cascade mode: the newest version is 3.3 (to include LHC physics); Phys. Rev. C 84, 034912 (2011);

2. In the Boltzmann+hydrodynamics hybrid mode: the newest version is 2.3p1 (can be downloaded from the website); Phys. Rev. C 78, 044901 (2008);

3. In the “mean-field potential” version: based on v2.1, adding mean field potentials for hadrons.

4. V1.3+PYTHIAV2.1+hydrodynamicsV2.3+LHC collisionsV3.3

Q.Li version

SPS RHIC LHC


The HBT correlation and paramerizationThe HBT correlation and paramerization

32

3

31

3

32

31

6

),(

dpNd

dpNd

dpdpNd

KqC 21 ppq 2/)( 21 ppK

The quotient of two-particle and one-particle spectra

The two-particle correlator C(q,K) is related to the emission function s(x,K), which is the Wigner phase-space density of the particle emitting system and can be viewed as the probability that a particle with average momentum K is emitted from the space-time point x in the collision region. the two-particle relative wave function.

222111

2

221121

,,

,,),(

pxsdxpxsdx

pxspxsdxdxKqC

Experimentally:

Theoretically:

The correlator is constructed with the help of the CRAB program

HBT=Robert Hanbury-Brown and Richard Q. Twiss


Cont’dCont’d

• CRAB analyzing program: http://www.nscl.msu.edu/~pratt/freecodes/crab/home.html

• Three-dimensional Gaussian parameterization

• LCMS is employed in usual calculations• Coulomb effect in FSI is considered for charged

two-kaon correlation with a Bowler-Sinyukov method

• non-Gaussian effect can be discussed under the Edgeworth expansion

)2exp(1),,( 2222222LOOLLLSSOOLSO qqRqRqRqRqqqC

))2exp(1)((

)1(),,(2222222

LOOLLLSSOOinvcoul

LSO

qqRqRqRqRqK

qqqC

The fitting work can be done by the ROOT or the ORIGIN software (using -squared method)


Calculation Results at LHCCalculation Results at LHC

1. Q1: p+p collisions at √sNN=7 TeV

2. Q2: : Pb+Pb collisions at √sNN=2.76 TeV

3. Q3: Examination of scaling of HBT radii with charged particle multiplicity


Q1: p+p collisions at √sNN=7 TeVQ1: p+p collisions at √sNN=7 TeV

• It seems like in massive nucleus-nucleus collisions, a strongly interacting medium is created even in pp collisions, that exhibits similar bulk properties such as space momentum correlations and collective behaviour;

• While it is often argued, that the particle emitting system in p+p collisions is too small to create a medium that exhibits bulk properties, this should be different at a center of mass energy of √s= 7 TeV.

• an essential quantity that influences the particle freezeout radii is the formation time in flux tube fragmentation

• the recent LHC data on pp collisions allows to determine the formation time in the flux tube break-up

From JPG 39, 065101(2012)


Formation time in UrQMDFormation time in UrQMD

• For the Lund model the formation times are proportional to the transverse mass of the created hadron and inversely proportional to the string tension.

• For simplicity UrQMD uses a constant formation time of tf = 0.8 fm/c for hard collisions.

The average dNch/d from UrQMD is 15% smaller than ALICE data


Projections of Correlation functionProjections of Correlation function

Non-Gaussian effect is visible in out and long directions and at large q


KT dependence of HBT radii for diff. dN classess and diff. formation timesKT dependence of HBT radii for diff. dN classess and diff. formation times

the present ALICE data allows to constrain the formation timeto values of tf ≈ 0.3-0.8 fm/c.

An additionalmomentum dependence in tf isneeded.


Q2: : Pb+Pb collisions at √sNN=2.76 TeVQ2: : Pb+Pb collisions at √sNN=2.76 TeV

Non-Gaussian effect is stronger at LHC than at lower energiesCalculated non-Gaussian effect is more obvious than data

From PRC 85, 034908 (2012)


KT dependence of HBT radiiKT dependence of HBT radii

1,Strong kT dependencesubstantial expansion of the source2,As RHICLHC, HBT radii (esp. RL) rise.3,At LHC, RL and RO are larger than data, separately & RO/RS ratio is larger than data.


x-t correlationx-t correlation

• even in the cascade calculation, there exists a visibly positive correlation between the emission time and position.

• The most important contribution to RO comes from the emission duration term


Contribution of emission duration to the HBT radiiContribution of emission duration to the HBT radii

• To lead to smaller RO values in all kT bins but leaves RS unchanged;

• Overall it results in an improved agreement with the data of the ratio.


Some hintsSome hints

1. The overestimation of both RO and RL can be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations.

2. A higher pressure would lead to a more explosive expansion, a stronger phase-space correlation, and a faster decoupling of the system, thus leading to smaller regions of homogeneity.

3. A more satisfactory solution is possible in the near future by improving the dynamic processes for both QGP and HG phases.


Q3: Examination of scaling of HBT radii with charged particle multiplicityQ3: Examination of scaling of HBT radii with charged particle multiplicity

• Same: 1) charged particle multiplicity at midrapidity: ||<1.2 for pp; ||<0.8 for other classes. 2) KT bin: 300-400 MeV/c.

• Different solutions:• To change: a) beam energy: Pb+Pb at √s=2760, 200,

130, 62.4 GeV and Elab = 158 GeV; b) centrality: Pb+Pb within 0-5%, 5-20%, 20-50% and 50-80% centralities; c) colliding system: Pb+Pb, Cu+Cu, C+C, p+p.

From PRC, 85, 044901 (2012)


Scaling of the HBT radiiScaling of the HBT radii

1,The scaling is good if the change in Nch is caused by a change of centrality at a fixed energy. 2,A small offset on the order of 2-3 fm is visible for different system sizes, due to the finite size of the nuclei.3,Increasing the center-of-mass energy leads to a reduction of the radii at a given fixed Nch-bin.


Some hintsSome hints

1. The scaling of the source size with (dNch/d)1/3 for different centralities is a hint that the underlying physics, e.g. pion production via resonance decay versus production via string fragmentation, is nearly unchanged by changes in the collision geometry.

2. A change in √s on the other hand results not only in different weights of the production mechanisms, but also in changed expansion dynamics towards a more violent expansion with increased energy.

• Different pp and AA result is attributed to the strongly different particle production mechanisms in AA and pp. I.e., bulk emission vs. string/jet dominated emission.


Freeze-out timeFreeze-out time

• By fitting the hydrodynamic expression:

1, a shorter decoupling time with increased energy2, UrQMD overestimates the source lifetime by a factor of 2–3∼ when compared to LHC data back to the duration time


SummarySummary

• Two-pion HBT correlations at LHC are calculated by using the UrQMD v3.3.

1. the present ALICE pp data allows to constrain the formation time to values of tf ≈ 0.3-0.8 fm/c.

2. The overestimation of both RO and RL from Pb+Pb central collisions can be attributed to the known fact that the pressure in the early stage is not strong enough in the cascade model calculations.

3. The scaling of the source size with (dNch/d)1/3 for different centralities is a hint that the underlying physics, e.g. pion production via resonance decay versus production via string fragmentation, is nearly unchanged by changes in the collision geometry, while change in √s on the other hand results not only in different weights of the production mechanisms, but also in changed expansion dynamics towards a more violent expansion with increased energy.

4. Different pp and AA result is attributed to the strongly different particle production mechanisms. I.e., bulk emission vs. string/jet dominated emission.


Thank you for your attention!

Using the e-mail: [email protected] for more discussions.

Thank you for your attention!

Using the e-mail: [email protected] for more discussions.

hbt two-pion correlations at lhc

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