Raluca Muresan

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Bose-Einstein Correlation Studies at HERA-B

• Raluca Muresan
• NBI
• Copenhagen

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Outline

• HERA-B experiment
• BEC introduction
• Why BEC at HERA-B
• Main ingredients for BEC analysis
• Preliminary results pp in pA with AC, Ti, W
• On-going studies
• Summary

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HERA-B experiment
Located at the proton-electron collider HERA at
DESY. Fixed target experiment 920 GeV proton
collisions on various targets. Large acceptance
at mid-rapidity x?(15, 220) mrad, y ?(15-160) mrad
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BEC

• Symmetric wave functions of bosons, no exclusion
  principle
• Enhanced probability for the identical bosons to
  be emitted with small relative momenta
• Quantum statistical correlations between pairs of
  identical particles.
• Presuming that only particles emitted from
  the same or very close sources exhibit this
  behaviour ? from studies of BEC one can obtain
  information about the size, shape and spacetime
  development of the particle emitting source.

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BEC

• For two identical bosons the Bose-Einstein
  correlation is defined as

probability density of two particles to be
produced with 4-momentum p1 and p2
probability densities for a single particle to
be produced with 4-momentum p1 or p2 ,
difficult to build in practice ? reference
sample
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BEC function parametrisation
invariant four-momentum difference,
related to the fraction of identical bosons
which do interfere,
interpreted as the geometrical radius of the
presumably spherical boson emitting source (just
an approximation),
overall normalization,
linear background.
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Why BEC at HERA-B ?

• Opportunity to measure BEC parameters at
  sqrt(s)41,6 GeV, intermediary value between SPS
  and RHIC
• Possibility of studying the BEC parameters
  dependence on the target material (C, Ti, W).
• BEC can be studied for both pion and kaon pairs
  (RICH selection) .
• Large minimum bias sample(single wire runs)
• 103 million pC events,
• 74 million pW events,
• 28 million pTi events.
• large enough for differential studies
  (directional dependence, multiplicity,
  transverse mass dependence ).

This talk is presenting only a feasibility
study, made on a small sample (few million
events).
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Reference sample

• Mixed sample tracks from different events, some
  other kind of correlations are also disappearing
  (long range correlations , energy-momentum ...)

To correct for the effects introduced by mixing
double ratio.
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Event, track, pair selection
Pion and kaon tracks - RICH likelihood.
Remove from the analysed sample the pairs of
tracks too close in space
abs(tx1-tx2)lt0.0008, txpx/pz
abs(ty1-ty2)lt0.0008, typy/pz and
momentum abs(p1-p2)lt0.5 GeV
pabs( ) to be resolved.
The C2(Q) distributions for pions corrected for
Coulomb interaction (Gamow), only small
variation in the parameter values occured.
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MC studies -pC

• BEC JETSET
• MSTJ(51)2 the shape of correlation function
  Gaussian
• MSTJ(52)9 BEC for p , K , h.
• PARJ(92)1 meaning that particles that can be
  subject to BEC
• are subject to BEC.
• PARJ(93) 2 fm R.
• PARJ(91) 0.020 GeV minimum particle width
  above which the
• particle decays are assumed to take place before
  the stage when BE
• effects are introduced. Particles with broader
  width than 0.020 are
• assumed to have time to decay before BE effects
  are to be considered

Not all the pions are correlated and PARJ(92) and
the measured lambda (obtained by fitting) are
different
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MC study -Influence of the non-belonging pairs-
pC
C2 (Q)
P1N, P2l, P3R, P4d
Non-belonging pairs pairs in which, at least
one particle is not a pion (based on MC
truth). Is a junk, a kaon, a proton etc...
Q(GeV)
The junks and the possible pion contamination do
not affect our result.
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MC studies pC
To test the procedure
C2 (Q)
C input Radius 2 fm All range
l R(fm) 0.2940.039 2.0140.161
Studies of systematic errors are necessary but so
far it seems that the procedure gives the correct
results. The radius used as MC example is bigger
than the radius we measure.
Q(GeV)
P1N, P2l, P3R, P4d
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C2(Q) - Data
P1N, P2l, P3R, P4d
C2 (Q)
C2 (Q)
W
C
Q(GeV)
Q(GeV)
l 0.276 0.016 R 1.0570.056 (fm)
l 0.293 0.021 R 1.3070.076 (fm)
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C2(Q) - data
P1N, P2l, P3R, P4d
C2 (Q)
l 0.253 0.020 R 1.2980.010 (fm)
Ti
Q(GeV)
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Fit results- Dependence on the fit range
Qlt1.2 GeV Qlt0.72GeV Qlt0.36 GeV
C 12 l R(fm) ?2 0.2880.013 1.0030.042 106/95 0.2750.016 1.0570.058 74/55 0.1830.035 1.3470.143 36/25
Ti 48 l R(fm) ?2 0.2070.014 0.9030.050 127/95 0.2530.020 1.2980.097 68/55 0.223 0.032 1.8490.259 24/25
W 184 l R(fm) ?2 0.3050.018 1.1320.055 130/95 0.2930.021 1.3070.076 62/55 0.2420.042 1.5250.177 35/25
About 1.000.000 events for each sample Weak
dependence, if any, on the target material ( same
as Na44- S on S, Ag, Pb). Radius value smaller
than the Na44 one, pPb R2.890.30, but pt larger
at HERA-B, source size decreases.
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LCMS
The spherical source shape just an
approximation, difficult to interpret. LCMS -
the spatial dimension of the source
couples to all components
- the temporal component couples
only to Qt,out.
beam
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Multidimensional correlation function
2-dim
3-dim
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Even more preliminary results 2-dim pp
About 5.5 million pC events
l 0.2610.002
RT(fm) 0.8850.034
RL (fm) 1.0530.040
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On-going studies - Kaons
First results (about 18 million pC events)
indicate, as it was expected that the BEC radius
for kaons is smaller than for pions (about one
half).
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Plans - Multiplicity dependence
Code ready, results on the way
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Plans - mt dependence of the 3-dim BEC parameters
Code ready, results on the way
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Summary

• BEC correlations were observed both for pp and KK
  pairs.
• Preliminary results, pp, were presented for both
  1-dim BEC
• parameters (pA - AC, Ti, W) and two-dim BEC
  (pC).
• Studies on-going pp - 3-dim, multiplicity and
  mt dependence KK.

It still a lot to be done in terms of running
over all the data set, careful study of
systematic errors, undestanding the results and
comparing them with other experimental results.

• HERA-B is the place to do interesting studies of
  BEC correlations
• More results are expected very soon