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ISMD 2004, Sonoma State University
R. Muresan NBI BEC studies 1
Raluca Muresan
NBI
Copenhagen
Bose-Einstein Correlation Studies at HERA-B
ISMD 2004, Sonoma State University
R. Muresan NBI BEC studies 2
Outline
HERA-B experiment BEC – introduction Why BEC at HERA-B Main ingredients for BEC analysis Preliminary results in pA with A=C, Ti, W On-going studies Summary
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R. Muresan NBI BEC studies 3
HERA-B experiment
The HERA-B Experimentat DESY
Ring Imaging Cherenkov Counter
250 mrad
220 mrad
Magnet
Si-StripVertexDetector
Calorimeter
TRD
Muon Detector
TargetWires
0 m5101520
T op V iew
ProtonBeam
ElectronBeam
Vertex Vessel
Inner / Outer Tracker
high-pt
Al BeamPipe
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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Target wires
C, Ti, W, Pd, Al wires inserted in to the halo of HERA proton beam
Mounted in 2 stations separated by 4 cm
Each wire can be inserted independently
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VDS
64 double-sided silicon microstrip detectors aranged in 8superlayers, divided in 4 quadrants
quadrants located in Roman pots allowing the detectorsto be retracted during injection (except SI08)
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OTR
Superlayers (MC’s, PC’s, TC’s) are made of few modules containing several layers with angles of 0 and 5 degrees.The layers are made of honeycomb drift cells.
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RICH
Large vessel filled with C4F10
radiator gas. The photons are reflected, by a serie of mirrors,to the photon detectors placed outside the detector acceptance.
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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 space–time development of the particle emitting source.
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BEC
For two identical bosons the Bose-Einstein correlation is defined as:
))P(pP(p
),pP(p),pC(p
21
2121
),( 21 ppP
)(),( 21 pPpP
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
)1()1()(22
2QeNQ
RQC
21 ppQ
N
R
invariant four-momentum difference,
related to the fraction of identical bosons whichdo interfere,
interpreted as the geometrical radius of the presumablyspherical boson emitting source (just an approximation),
overall normalization,
linear background.
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R. Muresan NBI BEC studies 11
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 ...)
mixMCrecnobe,
MCrecnobe
data,mix
data
dQ
dN
dQ
dN
dQ
dN
dQ
dN
C
2
To correct for the effects introduced by mixing double ratio.
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221
212
2
212 )(
)cos1(*2*pp
ppmppQ
Remove from the analysed sample the pairs of tracks too close in space : abs(tx1-tx2)<0.0008, tx=px/pz; abs(ty1-ty2)<0.0008, ty=py/pz; and momentum abs(p1-p2)<0.5 GeV p=abs( ) to be resolved.
The C2(Q) distributions for pions corrected for Coulomb interaction (Gamow), only small variation in the parameter values occured.
Event, track, pair selection
p
Pion and kaon tracks - RICH likelihood.
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MC studies -pC
BEC JETSET: MSTJ(51)=2 the shape of correlation function – Gaussian MSTJ(52)=9 BEC for , K , 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 BEeffects 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 themeasured lambda (obtained by fitting) are different
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MC study -Influence of the non-belonging pairs- pC
Q(GeV)
C2 (Q)
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...
The junks and the possible pion contamination do not affect our result.
P1=N, P2=P3=R, P4=
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MC studies – pC
C2 (Q)
Q(GeV)
P1=N, P2=P3=R, P4=
C input
Radius 2 fm
All range
R(fm)
0.294±0.039
2.014±0.161
To test the procedure
Studies of systematic errorsare necessary but so far it seemsthat the procedure gives the correct results.
The radius used as MC example is bigger than the radius we measure.
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C2(Q) - Data
C
C2 (Q)
Q(GeV)
P1=N, P2=P3=R, P4=
C2 (Q)
Q(GeV)W
0.276 ± 0.016R = 1.057±0.056 (fm)
0.293± 0.021R = 1.307±0.076 (fm)
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C2(Q) - data
Ti Q(GeV)
C2 (Q)
P1=N, P2=P3=R, P4=
0.253± 0.020R = 1.298±0.010 (fm)
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Fit results- Dependence on the fit range
Q<1.2 GeV Q<0.72GeV Q<0.36 GeV
C
12
R(fm)χ2
0.288±0.013
1.003±0.042
106/95
0.275±0.016
1.057±0.058
74/55
0.183±0.035
1.347±0.143
36/25
Ti
48
R(fm)
χ2
0.207±0.014
0.903±0.050
127/95
0.253±0.020
1.298±0.097
68/55
0.223 ±0.032
1.849±0.259
24/25
W
184
R(fm)
χ2
0.305±0.018
1.132±0.055
130/95
0.293±0.021
1.307±0.076
62/55
0.242±0.042
1.525±0.177
35/25
About 1.000.000 events for each sampleWeak dependence, if any, on the target material ( same as Na44- S on S, Ag, Pb).Radius value smaller than the Na44 one, pPb R=2.89±0.30, but pt larger at HERA-B, source size decreases….
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LCMS
The spherical source shapejust 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,
2,
2sidetouttT QQQ
)exp(1 2222 longlongTT QRQRC
)exp(1 222,,
22,
2,2 longlongsidetsidetouttoutt QRQRQRC
2-dim
3-dim
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Even more preliminary results 2-dim
0.261±0.002
RT(fm) 0.885±0.034
RL (fm) 1.053±0.040
About 5.5 million pCevents
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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 onthe way
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Plans - mt dependence of the 3-dim BEC parameters
Code ready, results onthe way
2
22,
221,
2tt
t
pmpmm
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Summary
HERA-B is the place to do interesting studies of BEC correlations More results are expected very soon
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 comparingthem with other experimental results.
BEC correlations were observed both for and KK pairs.Preliminary results, , were presented for both 1-dim BEC parameters (pA - A=C, Ti, W) and two-dim BEC (pC).Studies on-going: 3-dim, multiplicity and mt dependence; KK.
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Back-up sildes
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HERA-B detector
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Gamow correction
To measure the undisturbed BEC we have to substract all other correlation effects from our distributions. For the correlated distributions of charged particles one has to correct for Coulomb interaction by weighting the Q distribution with the inverse Gamow factor.
1)exp(
lG
Qm /2
)(GeVQ
lG
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Gamow correction
Before:
λ=0.247± 0.012
R=0.958 ± 0.043 fm
After:
λ=0.288± 0.013
R=1.003± 0.042 fm
P1=N, P2=P3=R, P4=
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Cut abs(p1-p2)>0.5 GeV. abs(tx1-tx2)<0.0008 abs(ty1-ty2)<0.0008
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Pion selection
p(GeV)medium soft
The identification probability is not constant over the momenta range in the medium selection. In the soft selection is constant for 5<p<40GeV
p(GeV)
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Only the pairs with 0<Q<1.2 GeV
We must not worry about momenta > 40 GeV and <2.6 GeV
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Purity of the sample –MC
0<Q<1.2 and particles that pass the soft pion selection
Here the truthid of the track is the LUND code of the truth.Tracks for which no MCtruthwas found were considered junk and their truthid was set to 0
We need to look around 2000and in the region whereThe code is smaller than 500
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Purity of the sample
1 pion and
1 junktrack
2 junktracks
1 junk trackand
1 pion
Twopions
Junk tracks= tracks without MCGEN correspondent
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Purity of the sample
Twoprotons
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target all range 5<p<40 GeV
C R(fm)
0.288±0.013
1.003±0.042
0.339±0.020
1.017±0.065
Data