Midwest critical mass 2010 Measurement of two-particle correlations in pp collisions at sqrt(s) =...

Midwest critical mass 2010

Measurement of two-particle correlations in pp collisions at sqrt(s) = 900 GeV as well as at sqrt(s) = 7 TeV with ALICE

The ALICE group

23. October 2010

Midwest critical mass 2010Toledo, Ohio

Sebastian Huber 2Midwest critical mass 2010

first paper on HBT by ALICE

two particle correlations in pp at sqrt(s) = 900 GeV

1 dimensional


motivation

Sebastian Huber – [email protected]

• measuring space-time dimensions in pp collisions at sqrt(s) = 900 GeV with

ALICE

• investigating collective behaviour in pp collisions at LHC energies

• reference for heavy ion measurements starting in Nov 2010

• using Bose-Einstein enhancement of identical pion pairs to

get access to size of pion emitting source

• dependance of source size as function of event multiplicity dNCH/dη

an transverse momentum kT


HBT – Bose-Einstein correlations


„The only way to get access to space-time properties of the emitting source in elementary particle collisions is through the measurement of Bose-Einstein correlations (BEC) between identical pions“

• building the correlation function (CF)

• transformation in relative momenta q

• correlation function in the experiment

)()(

),(),(

21

21212 pPpP

ppPppC

22221 4)( mmppq Inv

)(

)()(2 qB

qAqC

A(q) is the measured distribution of the pair momentum difference q, whereas B(q) is a reference distribution build by using pairs of particles from different events (event-mixing) or by rotating on particle of the pair in the transverse ebene (rotation)

Reference distribution should be without Bose-Einstein correlations


Extracting the HBT radii


To get the HBT-radii out of the correlations one has to use a propper parametrization

If one asumes that the source function has a gaussian shape also the parametrization has a gaussian shape

A common parametrization is the following

))(exp(1)( 22 RqqC

R is the effective size of the emission region

λ is the coherence parameter measuring the strength of the Bose-Einstein correlation

Bose-Einstein correlation allows to distinguish between collision systems

HBT in pp at sqrt(S) = 7 TeV will be interconnection between small systems (pp) and heavy systems (AA) at lower energies


correlation of non-identical pions at sqrt(S) = 900 GeV


• π+π- correlations in comparisson with

PHOJET simulations

• description of the background of the CF

• rich spectrum of meson resonances

• coulomb effect in the first bins

• using the background of the CF later for the

parametrization of the CF of identical pions

• proper treatment of baseline very important

when studying the dependancy of the radii from

transverse momentum and multiplicity

• parametrization of the background

2)( InvInvInv cqbqaqD

Phys. Rev. D 82, 052001 (2010)


correlation of identical pions at sqrt(S) = 900 GeV


• π+π- correlations in one dimension

• 2009 data not enough statistics for 3D

• 3 bins in event multiplicity

• 5 bins in transverse momentum

• Bose-Einstein effect clearly visible

• fixing the baseline with simulations (see slide

before)

• developing of long range correlations with

increasing kT

• used parametrization

)())))(exp(1()()1(()( 22 InvInvInvInvInv qDqRqKqC


multiplicity dependance


• source radii increase with multiplicity

• same dependance like older measurements

• HBT radii seem to depend more on

multiplicity than on geometry (pp)

• grey shadowed band

Systematic error from:

• baseline assumption

• fitting

• background construction

• UNICOR vs ALIFEMTO


kT dependance - baseline


• source radii independant of kT a

• no sign of collective behaviour (presence of a bulk)

in pp

• very sensitive to basline assumption

• if fitted free (flat baseline) – dependance emerges


results


Parametrization with a gaussian

Parametrization with an exponential

dNCH/dη

dNCH/dη

λ

λ RInv/fm

RInv/fm

3.2

3.2

7.7

7.7

11.2

11.2

0.386 (0.022)

0.331 (0.023)

0.310 (0.026)

0.704 (0.048)

0.577 (0.054)

0.548 (0.051)

0.874 (stat 0.047) (sys 0.181)

1.082 (stat 0.068) (sys 0.206)

1.184 (stat 0.092) (sys 0.168)

0.808 (stat 0.061) (sys 0.208)

0.967 (stat 0.095) (sys 0.206)

1.069 (stat 0.104) (sys 0.203)

)()))(exp(1)()1(()( 22 InvInvInvInvInv qDqRqKqC

)()exp(1)(2 InvInvInvInv qDqRqC


ongoing work

two particle correlations in pp at sqrt(s) = 7 TeV1 dimensional

3 dimensional


motivation


• measuring space-time dimensions in pp collisions at sqrt(s) = 900 GeV and at

sqrt(s) = 7 TeV at ALICE

• investigating collective behaviour in pp collisions at LHC energies

• more statistics makes it possible to go into more than one dimension

• getting deeper into the physics of the system

• using Bose-Einstein enhancement of identical pion pairs to

get access to size of pion emitting source

• dependance of source size as function of event multiplicity dNCH/dη

and transverse momenta kT

• cartesian parametrization in out side and long (Bertsch-Pratt)

• expansion in spheriacal harmonics

• difference between 900 GeV and 7 TeV

• understanding the background – non BE correlations – EMCIC

• no gaussian shape of the CF

• collective behaviour of the pion emitting fireball


1 dim 7TeV – kT and dNCh/dη


•coulomb

•minijets

7 TeV π+π+

7 TeV π+π-


out


parametrization )))(exp(1())()()(exp(1()1(),,( 22222222 lsoBllssoolso qqqRqRqRqRKqqqC

holes due to combination of

single track acceptance pT and

Pair kT cut


side



long


edges due to acceptance of the

detector in η


ROut && RSide && RLong

• measurement of BE of identical pions at 900 GeV (newest results not shown) and 7 TeV with dependance on

multiplicity dNCh/dη and pair momentum kT

• 7 TeV data provide link between multiplicities in pp and AA

• 3D CF not gaussian

• non femtoscopic correlations in 7 TeV as well as in 900 GeV – minijets – simulated with PHOJET - baseline



• radii in 900 GeV and 7 TeV grow with multiplicity

• 900 GeV and 7 TeV behave equaly

• dependance of the radii with pair momentum – very sensitive to baseline

• using spherical harmonics to get a full sight of what is going on (Mikes idea!)

link to the AA data


Backup

Backup

Midwest critical mass 2010 Sebastian Huber – [email protected]

holes and edges

• qlong and qside vanish

• pT is sum of pT1 and pT2

• kT is difference of pT1 and pT2

holes in qout

combination of single particle acceptance and two particle cut leads to holes

edges in qlong

η acceptance of the detector


event and track selectio

UNICOR 7 TeV pass2

900 GeV runs by mistake?


UNICOR 7TeV

HBT – UNICOR

7000 GeV p+p


one dimensional CF in LCMS

UNICOR 7 TeV pass2

• red – Pythia-Perugia LHC10d4

• red line – final parametrization• blue line – peak fit• green line – baseline fit

• no difference between fixed baseline fit and free fit

• baseline not as good fixed as in the 900GeV data

• only parts of the simulation LHC10d4 (Pythia-Perugia) taken


one dimensional CF in LCMS with kt-binning

UNICOR 7 TeV pass2

first and two last bins not usable – kT > 0.7 no HBT correlation!

new kT binning (see slide 10)


UNICOR 7 TeV pass2

one dimensional CF in LCMS with multiplicity-binning

• two highest multiplicity bins missing (by decission)


UNICOR 7 TeV pass2

one dimensional CF in LCMS with kt- and multiplicity-binning

binning in kt (7 bins)

dNCh/dη


UNICOR 7 TeV pass2

three dimensional CF in LCMS – out side long

• baseline fits do not match the Pythia-Perugia simulation

• please ignore the peak fit

• red – Pythia-Perugia LHC10d4

• red line – final parametrization• blue line – peak fit• green line – baseline fit


UNICOR 7 TeV pass2

three dimensional CF in LCMS with kt-binning – out side long


out

side

long


UNICOR 7 TeV pass2

three dimensional CF in LCMS with kt and multiplicity binning – out


dNCh/dη


UNICOR 7 TeV pass2

three dimensional CF in LCMS with kt and multiplicity binning – side


dNCh/dη


UNICOR 7 TeV pass2

three dimensional CF in LCMS with kt and multiplicity binning – long


dNCh/dη


• comparison mixing and rotation

• comparison π+π+ and π-π-

• CMS vs LCMS

• LHC10c vs LHC10b (7TeV)

• comparison of different magnetic field orientations (only in LHC10d)

• different background estimations (fixing the baseline out of simulations or π+π+)

• different fitting ranges

• comparison UNICOR and ALIFEMTO (ALIFEMTO results from Adam and own results)

Systematic error


Paper preparation

• variation of the event cuts

• variation of the track cuts

• variation of the pair cuts

• mixing (vertex binning and multiplicity binning)


dNCH and kT dependancy

Midwest critical mass 2010 Measurement of two-particle correlations in pp collisions at sqrt(s) =...

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