Measurement of BoseEinstein Correlations in pp Collisions at =0.9 and 7TeV
Abstract
BoseEinstein correlations between identical particles are measured in samples
of protonproton collisions at 0.9 and 7 TeV centreofmass energies, recorded
by the CMS experiment at the LHC.
The signal is observed in the form of an enhancement of number of pairs of
samesign charged particles with small relative momentum.
The dependence of this enhancement on kinematic and topological features of the event is studied.
Anticorrelations between samesign charged particles are observed in the region of relative momenta
higher than those in the signal region.
Introduction
Constructive interference affects the joint probability for the emission of a
pair of identical bosons with fourmomenta
.
Experimentally, the proximity in phase space between finalstate particles
is quantified by
the Lorentzinvariant quantity
.
The BEC effect is observed
as an enhancement at low
of the ratio of the
distributions
for pairs of identical particles in the same event, to that
for pairs of particles in a reference sample that by construction is expected to include no BEC effect:
In order to reduce possible biases
in the construction of the reference sample, a Double ratio
is defined,
.
Results
The parameters obtained from
fits using the exponential parametrization for
to the distribution of
are:
at 0.9 TeV
and
at 7 TeV.
In agreement with previous results, an increase of the effective emission radius
with chargedparticle multiplicity in the event is observed, which accounts for the increase of from 0.9 to 7 TeV.
The parameter is nearly independent of the average transverse momentum
of the pair of particles at the lowest multiplicity range,
and decreases with in events with large chargedparticle multiplicity.
For the first time in pp interactions, anticorrelations between samesign charged particles
are observed for values above the signal region, as previously reported with LEP data.
The anticorrelation effects decrease with increasing chargedparticle multiplicity in the event considered in this analysis.
Approved Figures (click to get large size)
Approved Figures 
Abbreviated Caption 


Double ratio at 900 GeV (left) and 7 TeV (right). The reference sample is obtained from samesign charged particles from mixed events with similar multiplicities, and the MC simulation is PYTHIA6.4, tune Z2. The error bars are statistical only. 

Distribution of Double ratio for 3 intervals in and 3 intervals in chargedparticle multiplicity in the event at 7 TeV. The error bars are statistical only. 


Values of the parameters (top) and (bottom), as a function of in three intervals of chargedparticle multiplicity in the event, at 900 GeV (left) and 7 TeV (right). The error bars are statistical only. 

Parameter as a function of the chargedparticle multiplicity in the event, for = 0.9 and 7 TeV. The dotted and solid lines represent the results of the fits described in the text to the 0.9 and 7 TeV data, respectively.The inner error bars represent the statistical uncertainties and the outer error bars the statistical and systematic uncertainties, added in quadrature. The systematic uncertainties are dominating and are pointtopoint correlated. 


Detail of the distribution of the double ratio for 900 GeV (left) and 7 TeV (right). The dotted lines correspond to fits with Aymmetric Levy Eq.(4) and solid lines to exponential fits. Note the enlarged scale on the y axis. The error bars are statistical only. 

Detail of the distribution of the double ratio for 7 TeV using different intervals of chargedparticle multiplicity in the event. The lines are fits to the data with Eq.(4). The error bars are statistical only. 

Depth of the dip in the anticorrelation region, as a function of the chargedparticle multiplicity in the event, for 0.9 and 7 TeV. The inner error bars represent the statistical errors and the outer error bars the statistical and systematic errors, added in quadrature. The systematic uncertainties are dominating and are pointtopoint correlated. 
Responsible:
PaoloChecchia
 PaoloChecchia  12Apr2011