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J. Milošević, Measurement of Two- and Four-particle Correlations in pPb and PbPb Collisions at CMS

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J. Milošević, Measurement of Two- and Four-particle Correlations in pPb and PbPb Collisions at CMS

  1. 1. Measurement of Two- and Four-particle Correlations in pPb andPbPb Collisions at CMSJ. MiloševićUniversity of Belgrade andVinča Institute of Nuclear Sciences,Belgrade, Serbiaon behalf of the CMS Collaboration28.04.2013 BW2013, Vrnjacka Banja, Serbia 1
  2. 2. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   2  Outlinev  CMS experimentv  Azimuthal anisotropyv  Two- and four-particle correlations as a tool to study azimuthal anisotropyv  Resultsv  Comparison to the theoretical predictions and results from other experimentsv  Conclusions
  3. 3. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   3  Schematic view of the CMS detectorv  Higgs particlev  supersymmetryv  dark matterv  extra dimensionsAlthoughdesigned tostudy ppcollisions, CMSis well suited tostudy heavy ionphysics tooA multipurpose detector to search for:
  4. 4. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   4  x!y!z!Elliptic flow in a non-centralsymmetric heavy ion collisionv The elliptic flow is one of, but mostfamous, of collective movementsv  converts spatial anisotropymomentum anisotropyv  Experimentally it is measured withrespect to the event plane (EP)v  But, there are methods which do notrequire knowledge about the EP!!px > !pydNd!~ [1+ 2 vn cos(n!)n! ]ηΔ-4-2024φΔ024φΔdηΔdpairN2dtrigN11.61.71.8110≥trkoffline= 5.02 TeV, NNNsCMS pPb< 3 GeV/cT1 < p(b)2D Δϕ-Δη correlation functionin asymmetric pPb collisionPhys.Lett.B718(2013)795v For the first time seen in pPb collisionsv Long range structure in ϕ over widerapidity range is characteristic forhydrodynamically driven flowv  Does the effect seen in pPb has hydroor Color-Glass-Condensate (CGC) origin?v  How does it compare with the effectseen in PbPb collisions?
  5. 5. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   5  Azimuthal Anisotropy Harmonics from Two-particle Correlations1NtrigdN paird!!=Nassoc2"1+ 2Vn! cos(n!!)n"#$%&(Correlation:1Ntrigd2N paird!!d!"= B(0,0)S(!!,!")B(!!!")S(!!,!") =1Ntrigd2Nsamed!!d!"B(!!,!") =1Ntrigd2Nmixd!!d!"1D correlation is fitted withAnisotropy harmonics vn{2, !! > 2}(pT ) =Vn!(pT, pTref)Vn!(pTref, pTref)!! = !!trigg" !!assoc!! = !!trigg" !!assoc
  6. 6. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   6  Four-particle correlations – Q cumulant methodAdvantage wrt 2-part.corr.:Four-particle correlationsremove two- and three-particle non-flowcorrelationein(!1+!2!!3!!4 )! ein(!1!!3)ein(!2!!4 )! ein(!1!!4 )ein(!2!!3)cn 4{ }= 4 ! 2" 22Reference flow: v2ref4{ }= !c2 4{ }4 Differential flow: v2 4{ } pT( )=!d2 4{ } pT( )(v2ref4{ })3where in d2 4{ } pT( ) one of four reference particles is replaced with a particlefrom a particular pT region.
  7. 7. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   7  gen-levelchN50 100 150 200{4}2c-0.03-0.02-0.010.000.010.020.03-310×= 5.02 TeV, Gen-levelNNspPb HIJINGbin width of 2offlinetrkNbin width of 5offlinetrkNbin width of 30offlinetrkNofflinetrkN50 100 150 200{4}2c-0.03-0.02-0.010.000.010.020.03-310×= 5.02 TeVNNspPb datav  Hydro-flow is not incorporated in the HIJING MC model – c2{4} consistent withzero for small bin width (2 or 5), while becomes nonzero for big bin width (30)v  The effect becomes larger going to more peripheral collisionsv  In pPb data, c2{4} crosses zero and becomes negative at certain multiplicity. Thisis an indication of the onset of multi-particle correlation effectv  A bin width of 5 is chosen for the v2{4} analysistobesubmittedinPhys.Lett.Bh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  8. 8. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   8  Examples of 2-dimensional (2D) dihadron correlationsv  PbPb collisions:the ridge – structure at Δϕ around zero andelongated in Δηv  Surprisingly seen in high-multiplicity pPband even in high-multiplicity pp collisionsv  Is the ridge in pPb collisionsrelated to hydrodynamicalflow like in PbPb collisions orit is connected with CGC?ηΔ-4-2024φΔ024φΔdηΔdpairN2dtrigN12.42.62.8N < 260≤= 2.76 TeV, 220NNsCMS PbPb< 3 GeV/ctrigT1 < p< 3 GeV/cassocT1 < pηΔ-4-2024φΔ024φΔdηΔdpairN2dtrigN13.13.23.33.4N < 260≤= 5.02 TeV, 220NNsCMS pPb< 3 GeV/ctrigT1 < p< 3 GeV/cassocT1 < ptobesubmittedinPhys.Lett.Bh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  9. 9. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   9  φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4 = 2.76 TeVNNsCMS PbPb|>2ηΔ|< 1 GeV/ctrigT0.3 < p< 260offlinetrkN≤220< 20offlinetrkN≤0Fourier fitφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8|<1ηΔ|φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 2 GeV/cassocT1 < p< 2 GeV/ctrigT1 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 4 GeV/ctrigT2 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 6 GeV/ctrigT4 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 12 GeV/ctrigT6 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8Averaged projection of 2D correlations onto Δϕfor 2<|Δη|<4 (top) and |Δη|<1 (bottom)PbPb case:v  Cutting on |Δη|, jet contributionlargely suppressed in the long-rangeregion (2<|Δη|<4) w.r.t. the short-range (|Δη|<1)tobesubmittedinPhys.Lett.Bv  Low-multiplicity events: no near-side correlations are observed inthe long-range regionv  Fourier fits are shown by linesh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  10. 10. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   10  φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4 = 5.02 TeVNNsCMS pPb|>2ηΔ|< 1 GeV/ctrigT0.3 < p< 260offlinetrkN≤220< 20offlinetrkN≤0Fourier fitφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8|<1ηΔ|φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 2 GeV/cassocT1 < p< 2 GeV/ctrigT1 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 4 GeV/ctrigT2 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 6 GeV/ctrigT4 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8φΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.10.20.30.4< 12 GeV/ctrigT6 < pφΔ0 2 4ZYAM-CφΔdpairdNtrigN10.00.20.40.60.8Averaged projection of 2D correlations onto Δϕfor 2<|Δη|<4 (top) and |Δη|<1 (bottom)pPb case:tobesubmittedinPhys.Lett.Bv  In order to compare with the PbPbresults, 1D correlation function,constructed in the same multiplicityrangev  The yields in pPb case show asimilar structure as those fromPbPb collisionsv  As in PbPb, no near-sidecorrelations is observedh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  11. 11. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   11  The near-side associated yield vs trigger pTin PbPb and pPb eventslong-range short-range – long-rangev  The jet yield increases with pTv  Explanation: higher jet energy –more associated particlesv  Seen in both pPb and PbPbcollisions with similar magnitudev  The ridge yield rises with pTreaching maximum at pT ≈ 2-3GeV/cand then decreases toward zerov  The jet yield is obtained bysubtracting the ridge yield (from long-range region)tobesubmittedinPhys.Lett.B(GeV/c)Ttrigp0 5 10AssociatedYield/(GeV/c)00.20.40.6 CMS Preliminary|>2ηΔ(a) | < 260offlinetrkN≤220< 2 GeV/cassocT1 < p= 5.02 TeVNNspPb= 2.76 TeVNNsPbPb(GeV/c)Ttrigp0 5 10AssociatedYield/(GeV/c)00.20.40.6|>2ηΔ|<1 minus |ηΔ(b) |h;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  12. 12. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   12  offlinetrkN0 100 200 300AssociatedYield/(GeV/c)0.00.20.40.6CMS|>2ηΔ(a) | < 2 GeV/cassocT, ptrigT1 < ppPb CGC= 5.02 TeV, 2013NNspPb= 2.76 TeVNNsPbPb= 5.02 TeV, 2012NNspPb= 7 TeVspp2(proton)=0.336 GeV20Q2(proton)=1.008 GeV20Q2(proton)=1.680 GeV20QofflinetrkN0 100 200 300AssociatedYield/(GeV/c)0.00.20.40.6|>2ηΔ|<1 minus |ηΔ(b) |The near-side associated yield vs multiplicityfor 1<pTtrig,pTassoc<2GeV/c in PbPb and pPb eventslong-range short-range – long-rangev  The ridge yield: monotonic rise withmultiplicityv  At a given multiplicity, the ridge yielddecreases going from PbPb trough pPb tothe smallest pp systemv  The CGC qualitatively describesthe ridge yieldv  The jet yield: moderate rise withmultiplicityv  There is a slight, nearly nosignificant difference between jetyield in PbPb and pPb collisionstobesubmittedinPhys.Lett.B(arXiv:1302.7018)h;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  13. 13. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   13  (GeV/c)Tp2 42v0.00.10.20.3 = 2.76 TeVNNsCMS PbPb< 150trkofflineN≤120|>2}ηΔ{2, |2v<20 sub.offlinetrk, N|>2}ηΔ{2, |2v{4}2v(GeV/c)Tp2 42v0.00.10.20.3 = 5.02 TeVNNsCMS pPb>80 GeVPbTEΣATLAS,|>2}ηΔ{2, |2v{4}2v(GeV/c)Tp2 42v0.00.10.20.3< 185trkofflineN≤150(GeV/c)Tp2 42v0.00.10.20.3 (GeV/c)Tp2 42v0.00.10.20.3< 220trkofflineN≤185(GeV/c)Tp2 42v0.00.10.20.3 (GeV/c)Tp2 42v0.00.10.20.3< 260trkofflineN≤220(GeV/c)Tp2 42v0.00.10.20.318≥part= 4.4 TeV, NNNspPb Hydro2-nd Fourier harmonics vs pT in different multiplicity rangesv  Hydrodynamics predicts the long-rangecorrelations (Phys.Rev.C 85(2012)014911)v  Elliptic flow from1D Δϕ correllations(v2{2,|Δη|>2})v  To reduce non-flow correlations: v2from 4-particle cumulant method (v2{4})v  The v2 is larger in PbPb than in pPbv  The difference between v2{2,|Δη|>2}and v2{4} could come from event-by-eventfluctuations in the flow signalv  Hydro describes v2{4}. It does notincludes event-by-event fluctuationsv  Comparison to the ATLAS results(arXiv:1303.2084)tobesubmittedinPhys.Lett.BLong-range
  14. 14. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   14  (GeV/c)Tp2 43v0.000.050.10= 2.76 TeVNNsCMS PbPb< 150trkofflineN≤120|>2}ηΔ{2, |3v<20 sub.offlinetrk, N|>2}ηΔ{2, |3v(GeV/c)Tp2 43v0.000.050.10= 5.02 TeVNNsCMS pPb (GeV/c)Tp2 43v0.000.050.10< 185trkofflineN≤150(GeV/c)Tp2 43v0.000.050.10(GeV/c)Tp2 43v0.000.050.10< 220trkofflineN≤185(GeV/c)Tp2 43v0.000.050.10(GeV/c)Tp2 43v0.000.050.10< 260trkofflineN≤220(GeV/c)Tp2 43v0.000.050.1018≥part= 4.4 TeV, NNNspPb HydrotobesubmittedinPhys.Lett.B3-rd Fourier harmonics vs pT in different multiplicity rangesLong-rangev  Triangular flow, v3{2,|Δη|>2}, isextracted in the same way as v2{2,|Δη|>2}v  Similar magnitude for v3{2,|Δη|>2} inboth, PbPb and pPb collisionsv  If jet-induced correlations areindependent of multiplicity in pPbcollisions, they could be removed bysubtracting low-multiplicity results fromfrom high-multiplicity eventsv  The low-multiplicity-subtractedv2{2,|Δη|>2} results are somewherebetween v2{2} and v2{4}, while thetriangular flow remains unchangedunder such a subtraction
  15. 15. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   15  offlinetrkN0 100 200 3002v0.000.050.10= 2.76 TeVNNs(a) PbPb< 3 GeV/cT0.3 < p|>2}ηΔ{2, |2v<20 sub.offlinetrk, N|>2}ηΔ{2, |2v{4}2vofflinetrkN0 100 200 3002v0.000.050.10= 5.02 TeVNNs(b) pPbofflinetrkN0 100 200 3000.20.40.60.8422+v222v422-v222vofflinetrkN0 100 200 3000.20.40.60.8tobesubmittedinPhys.Lett.B2-nd Fourier harmonics vs multiplicity for 0.3<pT<3GeV/cfluctuationv  In the PbPb case, the v2 coefficientsincreases moderately with multiplicity,while in pPb case remain relativelyconstant at high multiplicityv  The PbPb data show a larger v2 thanthose in the case of pPb collisionsv  The low-multiplicity-subtracted v2gives almost the same values asv2{2,|Δη|>2} in high-multiplicity eventsv  40% in PbPb and 50-60% in pPb areupper limits on the flow fluctuations
  16. 16. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   16  offlinetrkN0 100 200 3003v0.000.010.020.03|>2}ηΔ{2, |3v<20 sub.offlinetrk, N|>2}ηΔ{2, |3v= 2.76 TeVNNs(a) PbPb< 3 GeV/cT0.3 < pofflinetrkN0 100 200 3003v0.000.010.020.03= 5.02 TeVNNs(b) pPbtobesubmittedinPhys.Lett.B3-rd Fourier harmonics vs multiplicity for 0.3<pT<3GeV/cv  The magnitude of v3 in pPb collisions is similar to the PbPb casev  The procedure of subtracting low-multiplicity results doesn’t changev2 (2,|Δη|>2)v  The v3 coefficient is largely determined by the event-by-eventgeometry fluctuationsh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  17. 17. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   17  offlinetrkN0 100 200 3002v0.000.050.10 |>2}ηΔ{2, |2v<20 sub.offlinetrk, N|>2}ηΔ{2, |2v{4}2v= 5.02 TeVNNsCMS pPb< 5 GeV/cTATLAS, 0.3 < p< 3 GeV/cT0.3 < p, 50-100% sub.|>2}ηΔ{2, |2v{4}2vtobesubmittedinPhys.Lett.BComparison to the ATLAS resultsv  The v2{4} values from the ATLAS are higher than those from the CMSalthough they agree within the errorsv  The applied cuts on compared ATLAS and CMS data are not preciselyequalh;ps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN13002  
  18. 18. 28.04.2013   BW2013,  Vrnjacka  Banja,  Serbia   18  Conclusionsv  The near-side features in pPb collisions at =5.02TeV are studied vstransverse momentum, pT, and multiplicityv  The near-side ridge yield increases with multiplicity. It has similar shape asthe one from PbPb collisions but with smaller magnitudev  The short-range jet yield shows a weak multiplicity dependece in both pPband PbPb systemsv  The ridge yield rises and then falls, while the jet-yield rises monotonicallywith pTv  The extracted v2 and v3 values has pT dependence similar with the ridge-yield,and a weak multiplicity dependencev  In both systems, the v3 has comparable magnitude and similar dependenceon pT and multiplicityv  As the data are studied over a broad range in multiplicity and in pT it couldprovide significant constraints on theoretical predictions of the origin of thelong-range ridge phenomenasNN

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