Femtoscopic measurements in $p$+Pb collisions at $\sqrt{s_{_{\text{NN}}}}= 5.02$~TeV with ATLAS at the LHC

0.26 MB·

by  Markus K. Koehler

Nuclear
Physics A
NuclearPhysicsA00(2016)1–4
www.elsevier.com/locate/procedia
+
Femtoscopic measurements in p Pb collisions at
√
=
s 5.02 TeV with ATLAS at the LHC
NN
Markus K. Ko¨hler
6
1 (on behalf of the ATLAS Collaboration)1
0
2 WeizmannInstituteofScience,Israel

n
a
J

1
2
Abstract

x] Recent measurements in two-particle correlations in p+Pb collisions suggest collective behavior reminiscent of that
e observedinPb+Pb. Femtoscopicmeasurementsmayprovideusefulinsightonthisbehaviorbecausetheyimagethe
- spatio-temporalsizeoftheparticleemittingregion.Thisproceedingpresentsidentical-pionHanburyBrownandTwiss
l
c (HBT)measurementsfromATLASusingone-andthree-dimensionalcorrelationfunctions. Pionsareidentifiedusing
u dE/dxmeasuredinthepixeldetector.CorrelationfunctionsandtheresultingHBTradiiareshownasafunctionofpair
n momentum(k )andcollisioncentrality.
T
[
Keywords: ATLAS,p+Pbcollisions,Femtoscopy
1
v
2 1. Introduction
3
6 Longrangetwo-particlecorrelationfunctionsinrelativepseudorapidityandrelativeazimuthalanglein
5 ppand p+PbcollisionsmeasuredattheLHCareconsistentwithapossiblecollectivebehaviour[1,2,3],
0
reminiscenttothatobservedinPb+Pbcollisions,seee.g.[4].
.
1 Femtoscopic measurements in pp and p+Pb collisions [5, 6] allow the investigation of the time evolution
0
ofsmallcollidingsystems. Thisproceedingreportsthefirstmeasurementofthecentralityandmomentum
6 √
dependence of same- and opposite-sign pion pair correlations in p+Pb collisions at s = 5.02 TeV
1 NN
: measuredbytheATLASexperiment.
v
i
X
2. Dataanalysis
r
a
Thisanalysisusesadatasamplewithanintegratedluminosityof28.1nb−1ofp+Pbcollisions,measured
in2013bytheATLASdetector[7]. ThePbbeamhadanenergyof1.57TeVpernucleonandtheopposing
√
p beam had an energy 4 TeV, resulting in a center of mass energy of s = 5.02 TeV. The centrality is
NN
determinedusingthetotaltransverseenergymeasuredintheforwardcalorimeteronthePb-goingside,see
e.g. Ref.[8]formoreinformation.
1AlistofmembersoftheATLASCollaborationandacknowledgementscanbefoundattheendofthisissue.
2 M.K.Ko¨hleretal./NuclearPhysicsA00(2016)1–4
m] 7 m] 7
[fRinv 6 ATLAS Preliminary 70-80% 40-50% [fRinv 6 00..25 << kkT << 00..36 GGeeVV
5 10-20% 0-1% 5 T
p+Pb 2013, s = 5.02 TeV
NN
L = 28 nb-1
4 4 int
3 3
2 2
L = 28 nb-1
1 int 1
p+Pb 2013, sNN = 5.02 TeV ATLAS Preliminary
0 0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 2 2.5 3 3.5 4
k [GeV] <dN /dh >1/3
T ch
Fig.1. InvariantradiiRinvasafunctionofkTindifferentcentralitybins(leftpanel)andasafunctionofthecuberootoftheaverage
chargedparticlemultiplicity,(cid:104)dNch/dη(cid:105)1/3,intwokTintercepts(rightpanel).Figuresfrom[9].
Reconstructedtracksarerequiredtohaveatransversemomentum p >0.1GeVandtobewithinthepseu-
T
dorapidity2range|η|<2.5. PionsareidentifiedusingthedE/dxinformationfromthesiliconpixeldetector.
Pionpairsarerequiredtohave|∆φ| < π/2andtobewithinthepairrapidityregionof|η | < 1.5. Opposite-
k
signpairsthathaveaninvariantmassclosethemassesofρ0,K0 andφarerejected.
S
The two-particle correlation function is defined as C(q) = A(q)/B(q), where q is the relative momentum
q = pa − pb, A(q)=dN/dq| is the same-event distribution and B(q)=dN/dq| is the mixed-event
same mixed
distributioninthesameeventclass.
In three dimensions, a longitudinally co-moving frame is chosen, such that pa = −pb. The coordinates
z z
aredefinedintheBertsch-Prattconvention[10,11], whereq showsinthedirectionofthepairmomen-
out
tum, q shows in the direction of the beam and q is perpendicular to the q and q . A graphical
long side out long
visualizationofBertsch-Prattcoordinatesisshowne.g. inRef.[12]. UsingtheBowler-Sinyukow[13,14]
parametrization,thecorrelationfunctioncanwrittenas
C(q)=(cid:2)(1−λ)+λK(q)C (q)(cid:3)Ω(q), (1)
BE
where λ, K(q), C (q) and Ω(q) is the correlation strength, a correction factor for final-state interactions,
BE
theBose-Einsteinenhancementfactorandthecontributionfromnon-femtoscopiccorrelations,respectively.
TheBose-EinsteinfactorC (q)inthecorrelationfunctionisfittoanexponentialfunction
BE
(cid:16) (cid:17)
C (q)=1+exp −Rˆqˆ , (2)
BE
where Rˆ and qˆ are the invariant radius R and the invariant momentum q in the one-dimensional case
inv inv
and a diagonal matrix with the entries R = diag(R ,R ,R ) and (cid:126)q = (q ,q ,q ) in the three-
out long side out long side
dimensionalcase,respectively.
Thenon-femtoscopiccontributiontothecorrelationfunctionΩ(q)isestimatedbyafittotheopposite-sign
pairdistribution
Ω(qinv)=N(cid:104)1+λbkgexp(cid:16)−(cid:12)(cid:12)(cid:12)Rbkgqinv(cid:12)(cid:12)(cid:12)αbkg(cid:17)(cid:105), (3)
whereN isanarbitrarynormalizationfactor,λ istheexperimentalcorrelationstrength,R isaparam-
bkg bkg
eterforthewidthandα isashapeparameter. Theoriginofthisbackgroundisidentifiedtobefromhard
bkg
2ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalinteractionpoint(IP)inthecentreofthedetectorand
thez-axisalongthebeampipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispointsupward.Cylindrical
coordinates(r,φ)areusedinthetransverseplane,φbeingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms
ofthepolarangleθasη=−lntan(θ/2).
M.K.Ko¨hleretal./NuclearPhysicsA00(2016)1–4 3
m] 7 m] 7
[fRout 6 ALinTt L=A 2S8 Pnrbe-1liminary 70-80% 40-50% [fRside 6 ATLAS Preliminary 70-80% 40-50%
5 10-20% 0-1% 5 10-20% 0-1%
4 4
3 3
2 2
1 1 Lint = 28 nb-1
p+Pb 2013, s = 5.02 TeV p+Pb 2013, s = 5.02 TeV
NN NN
0 0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.2 0.3 0.4 0.5 0.6 0.7 0.8
k [GeV] k [GeV]
T T
[fm]Rlong 67 ATLAS Preliminary 70-80% 40-50% RoutRside 11..46 ATLAS Preliminary 70-80% 40-50%
5 10-20% 0-1% 10-20% 0-1%
1.2
4
1
3
2 0.8
1 Lint = 28 nb-1 Lint = 28 nb-1
p+Pb 2013, s = 5.02 TeV 0.6 p+Pb 2013, s = 5.02 TeV
NN NN
0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.2 0.3 0.4 0.5 0.6 0.7 0.8
k [GeV] k [GeV]
T T
Fig.2.TheradiiRout(upperleftpanel),Rside(upperrightpanel)andRlong(lowerleftpanel)asafunctionofkTindifferentcentrality
bins.ThelowerrightpanelshowstheratioofRout/RsideasfunctionofkT.Figuresfrom[9].
processes, such as particles from jet fragmentation. A mapping of the parameters λ and R between
bkg bkg
opposite-sign and same-sign is extracted from Monte Carlo simulations. It should be noted, that all pa-
rametersdescribingthebackgroundareconstrainedbythismethod. Systematicuncertaintiesareestimated
by taking into account the hard process background description, particle identification, effective Coulomb
correctionsizeReff,chargeasymmetry,andtwoparticleeffects. Moredetailsonthedataanalysisprocedure
canbefoundinRef.[9].
3. Resultsandconclusions
Figure1showstheinvariantradiiR asafunctionofthepairmomentumk andasafunctionofcube
inv T
rootoftheaveragechargedparticlemultiplicity(cid:104)dN /dη(cid:105)1/3. Themeasuredradiiareobservedtodecrease
ch
withincreasingk ,whichisconsistentwithcollectiveexpansion. Thisbehaviorislesssevereforperipheral
T
collisions.
Figure2showstheresultsfortheradiiR ,R andR andtheratioR /R asafunctionofthepair
out long side out side
momentumk fordifferentcentralities. Theradiishowadecreasingtrendwithincreasingk . Inthemost
T T
centralevents0−1%, theradiiareaboutafactor2.5largerthaninperipheralcollisions70−80%. The
ratio R /R falls significantly below 1, indicating a very rapid and explosive expansion of the fireball.
out side
Thisbehaviorcanbeexplainedbyacombinationofpre-thermalizedacceleration,astifferequationofstate,
andaddingviscouscorrections[15].
In Fig. 3, the product R R R , which scales linearly with the volume, is shown as a function of the
out side long
4 M.K.Ko¨hleretal./NuclearPhysicsA00(2016)1–4
3]m 90 3]m 90
R RR [foutlongside 56780000 pLi+ntP =b00 2..22580 <<1n 3bkk,-TT1 <<s N00N.. 36= GG5ee.0VV2 TeV R RR [foutlongside 56780000 GGGlGGaCCubFFe wwr ss(w ==s 00=.. 1201)
40 40
30 30
ATLAS Preliminary
20 20
0.2 < k < 0.3 GeV
10 10 T
ATLAS Preliminary p+Pb 2013, s = 5.02 TeV
NN
0 0
0 10 20 30 40 50 60 0 5 10 15 20 25 30
<dN /dh > <N >
ch part
Fig.3.TheproductRoutRsideRlongasafunctionof(cid:104)dNch/dη(cid:105)fortwodifferentkTintervals(leftpanel)andasafunctionof(cid:104)Npart(cid:105)for
threedifferentmodelsdescribingtheinitialgeometry.Figuresfrom[9].
average multiplicity (cid:104)dN /dη(cid:105) for two intercepts of the pair momentum in the left panel. The volume is
ch
linearlyincreasingwithincreasing(cid:104)dN /dη(cid:105),indicatingaconstantsourcedensityatthemomentoffreeze-
ch
out. Theproductisalsoshownasafunctionof(cid:104)N (cid:105)fortheGlauberModelandfortheGlauber-Gribov
part
ColorFluctuationmodel(GGCF),includingdifferentvaluesω forthemagnitudeofthecolorfluctuations.
σ
Theextractionofcentralitydependentvaluesof(cid:104)N (cid:105)isdescribedinRef.[8].
part
4. Acknowledgement
ThisworkwassupportedbyU.S.DepartmentofEnergygrantDE-FG02-86ER40281.
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