Study of Bose-Einstein correlations for pions
at √s = 7 TeV
Marcin Kucharczyk, Tadeusz Lesiak, Bartosz Małecki, Mariusz Witek
Institute of Nuclear Physics PAN, Kraków, Poland
Outline
• Introduction
• Analysis method
• Data selection
• Results
• Summary
Alice, Atlas and CMS already published Bose-Einstein correlation papers at
central rapidities.
LHCb can add measurements in the forwad region.
Twiki analysis page
https://twiki.cern.ch/twiki/bin/viewauth/LHCbPhysics/BEC2011
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HBT interferometry
In 1950s Robert Hanbury Brown an Richard Q. Twiss
(radio astronomy engineers) found
correlations between photons from different sources
Initially this observation was not accepted.
• Opponents were quoting Dirac: “Interference between two different photons
can never occur.”
• Laboratory experiments quoted conclusion:”…in agreement with quantum
theory, the photons of two coherent light beams are independent from each
other .”
It turned out that the observed effect does
not contradicts Quantum Mechanics but
oppositely, it is its consequence related to
Bose-Einstein statistics.
Intensity interference (second order
interference) to measure the
diameters of radio sources.
By changing the distance between
detectors, one can measure the diameter
of emitting source.
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From interferometry to particle physics
1959 - Goldhaber, Goldhaber, Lee and Pais.
•
Experiment at the Bevalac/LBL, in Berkeley to look at the resonances by comparing Q
distribution of unlike-sign pairs π+π- to like sign π+π+ or π-π-.
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No resonance was discovered (not enough statistics) but unexpected angular correlation
among identical pions was observed.
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Counterpart of the HBT effect in particle physics
Quantum interference effect
between indistinguishable particles
emitted by a finite source.
Useful tool to probe the spatial and temporal structure of the hadron emission volume.
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Correlation function
Q distribution for like-sign pair in data – includes qunatum correlations
Q distribution for reference sample – no quantum correlations
Goldhaber parametrization
+ for bosons
- for fermions
R - radius of the spherical source
λ - chaoticity parameter
λ=0 fully coherent source
λ=1 completely chaotic source
Alternative, Levi parametrization to include shape of long range correlations
Used in this analysis
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Choice of reference sample
The reference sample should be free from Bose-Einstein Correlations
(BEC) while possessing other correlations related to e. g. charge, energymomentum conservation, particle decays, event topology etc.
Three reference samples considered
Used in this analysis
1. Event-mix – pairs of identical charged
pions each originating from different events
2. PV-mix – pairs of identical charged pions
each originating from different PVs in the
same event
3. Unlike-sign – pairs of unlike sign pions
from same event
Reference samples derived from data but
• random choice of pairs eliminates BEC and all
other non trivial correlation.
Reference sample derived from data and
preserves other correlations but
•
•
unlike-sign pairs may originate from resonances
effects of electric attraction present
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Double Ratio (DR)
Double Ratio - an improved correlation function.
Features of DR
•
Insensitive to single particle efficiency effects (true also for C2)
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spatial acceptance
selection cuts
PID ( contamination of other particles without BEC)
Insensitive to two-track efficiency effects if properly simulated.
Reduces imperfections of reference sample.
•
MC correlation function contains similar pattern of distortions as correlation function for data.
•
By dividing them, one can eliminated second order effects to large extend and imperfections of
the reference sample.
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Datasets
MC - 20 M events
•
•
NoBias. Pythia8, Sim08, Reco14, 2011
Bose-Einstein correlations switched off.
DATA - 40 M events
•
MinBias, Stripping 20r1, Reco 14
MC 10 M events
•
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Pythia 6.4, Perugia0 tuning 2011
For systematics due to model
To study BEC we are interested in proton-proton interactions i.e. single PV.
Data MINIBIAS.DST contains NoBias and MinimumBias with multiple PVs
MinimumBias introduces a certain PT(ET) cut which biases PV
PV unbiasing procedure (DATA only)
Yes
Accept all PVs
NoBias event?
No
Select all PVs with tracks fired the
L0 MinBias trigger.
Remove randomly one of the PVs
that fired the MinBias trigger.
Accept all remaining PVs
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Track selection
Relatively loose cuts for tracks
Long track
IP cut
MC resampling method was chosen for correcting simulated PID
• Standard binning
•
Λ0→pπ calibration sample added for protons.
• Corrected ProbNNs are drown according to true MCID.
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Three multiplicity bins
•
R, radius of the source depends on total multiplicity Ntot of the event
•
Nch – VELO multiplicity of charged tracks is good probe of the Ntot
•
To check the R(Nch) dependence the data were split into 3 multiplicity bins
Bin
VELO Nch
Activity class
1
0-10
(52-100)%
2
11-20
(15-52)%
3
20-60
(0-15)%
Nch bins were chosen to have similar number
of pairs in each beam.
Activity classes were calculated according to Nch bins
They are useful to compare between experiments.
For example: activity class (0-15)%
corresponds to 15 % of all events with
highest VELO multiplicity.
On next slides pictures of correlation function or double ratio will be shown for bin 2.
Full set of plot is available in the note on the analysis Twiki page
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Correlation functions and double ratio: like-sign
DATA
like-sign π+π+ or π-π-
Double Ratio
=
MC
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Correlation functions and double ratio: unlike-sign
DATA
unlike-sign π+π-
Not used as reference sample.
Only to control the effects at Q range of
BEC signal and asymptotic behavior at
large Q
Double Ratio
=
MC
Coulomb effect
Not perfect simulation
of resonances
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Coulomb effect correction
Final state interactions:
•
•
Strong – independent of charge combination
Electromagnetic (Coulomb effect)
• like-sign – repulsive, BEC effect decreased
• unlike-sign – attractive, BEC effect increased
Coulomb effect has been calculated.
It can be removed using Gamov penetration factor.
Residual acceptance
effects included in
systematics.
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Clones and ghosts
Clones and ghosts are expected to appear in the low Q region thus can
affect the BEC signal.
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Stability cross-checks
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Variation of single track reconstruction
efficiency/acceptance
Fitting procedure (ToyMC fits)
Variation of IP cut
Magnet polarity
Data taking period (before/after technical stop)
Contamination from beam-gas interactions
ToyMC fits – pull distributions
Slightly overestimated errors σpull = 0.95
Artificial introduction of additional inefficiency for single
track reconstruction. Linear decrease of efficiency from
100% at η=2 down to 80% at η=5.
Strong prove of insensitivity to single track inefficiency
and detector acceptance
Data taking period (before/after technical stop)
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Systematic uncertainties
Sources of systematics (dominant ones in red):
Differences between data and simulation:
•
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Dependence on generation model (different tunings of Pythia).
Two particle efficiencies at very low Q (ghosts and clones).
Residual acceptance effects
Reconstruction
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Number of PVs in an event ( 1, 2, 3-4 )
Particle identification (ProbNNpi cut)
Particle identification (MC recalibration)
PV reconstruction efficiency
Resolution of Q variable
Technical:
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Lower limit of Q distribution in a fit
Upper limit of Q distribution in a fit
Q distribution binning
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Systematic uncertainties – generation model
Dependence on generation model (different tunings of Pythia).
Pythia 8 default LHCb tuning
Systematics
Pythia 6.4 Perugia0 tuning
ΔR/R ~ (6-9)%
Δλ/λ ~ (2-4)%
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Systematic uncertainties – number of PVs
Number of PVs in an event ( 1, 2, 3-4 )
Distributions almost identical but
some systematic differencies
affect the fit results
ΔR/R ~ (4-6)%
Δλ/λ ~ (4-6)%
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Systematic uncertainties - summary
Summary of systematic uncertainties for the 3 bins of PV VELO track multiplicities
Bin 1 - low
Bin 2 - medium
Bin 3 - high
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Results
Bin 1 - low
Bin 2 - medium
Bin 3 - high
Some systematic patterns can be observed in pulls.
Phenomenological Levi parametrization is only
approximation derived with certain assumptions.
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Results
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Summary
Bose-Einstein correlations for pions produced in p-p collisions at √sNN = 7 TeV
have been measured in the forward region.
The analysis showed that LHCb is able to perform BEC measurements. It
opens the possibility for other studies.
•
different collisions: pp, pPb, PbPb(?)
•
3D analysis for pions (LCMS system)
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3-body correlations for pions
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extend analysis for: KK, ΛΛ and charm (?)
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different beam energies: e.g. pp 7,8,13 TeV; pPb 5TeV
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Backup slides
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