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

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.

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-. No resonance was discovered (not enough statistics) but unexpected angular correlation among identical pions was observed. 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.

Correlation function Used in this analysis Goldhaber parametrization 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

Choice of reference sample The reference sample should be free from Bose-Einstein Correlations (BEC) while possessing other correlations related to e. g. charge, energy-momentum 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 Reference samples derived from data but random choice of pairs eliminates BEC and all other non trivial correlation. 2. PV-mix – pairs of identical charged pions each originating from different PVs in the same event Reference sample derived from data and preserves other correlations but unlike-sign pairs may originate from resonances effects of electric attraction present 3. Unlike-sign – pairs of unlike sign pions from same event

Double Ratio (DR) Double Ratio - an improved correlation function. Features of DR Insensitive to single particle efficiency effects (true also for C2) 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.

Datasets MC - 20 M events DATA - 40 M events MC 10 M events NoBias. Pythia8, Sim08, Reco14, 2011 Bose-Einstein correlations switched off. MC 10 M events Pythia 6.4, Perugia0 tuning 2011 For systematics due to model DATA - 40 M events MinBias, Stripping 20r1, Reco 14 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 NoBias event? No Select all PVs with tracks fired the L0 MinBias trigger. Accept all PVs Remove randomly one of the PVs that fired the MinBias trigger. Accept all remaining PVs

Track selection Relatively loose cuts for tracks IP cut Long track 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.

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

Correlation functions and double ratio: like-sign like-sign π+π+ or π-π- DATA Double Ratio = MC

Correlation functions and double ratio: unlike-sign Not used as reference sample. Only to control the effects at Q range of BEC signal and asymptotic behavior at large Q DATA Double Ratio = MC Coulomb effect Not perfect simulation of resonances

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.

Clones and ghosts Clones and ghosts are expected to appear in the low Q region thus can affect the BEC signal.

Stability cross-checks 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)

Systematic uncertainties Sources of systematics (dominant ones in red): Differences between data and simulation: Dependence on generation model (different tunings of Pythia). Two particle efficiencies at very low Q (ghosts and clones). Residual acceptance effects Reconstruction 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: Lower limit of Q distribution in a fit Upper limit of Q distribution in a fit Q distribution binning

Systematic uncertainties – generation model Dependence on generation model (different tunings of Pythia). Pythia 8 default LHCb tuning Pythia 6.4 Perugia0 tuning ΔR/R ~ (6-9)% Δλ/λ ~ (2-4)% Systematics

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)%

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

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.

Results

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) 3-body correlations for pions extend analysis for: KK, ΛΛ and charm (?) different beam energies: e.g. pp 7,8,13 TeV; pPb 5TeV

Backup slides