Correlations and ridge in ppb collisions in the LHCb Experiment

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1 EPJ Web of Conferences will be set by the publisher DOI: will be set by the publisher c Owne by the authors, publishe by EDP Sciences, 7 Correlations an rige in ppb collisions in the LHCb Experiment Mariusz Witek,a on behalf of the LHCb Collaboration Institute of Nuclear Physics PAS, Cracow, Polan Abstract. he LHCb experiment, besies its main programme concerning flavour physics performs also very well as a general purpose forwar etector, covering the pseuorapiity range from. to 5.. Exploiting the experiment s unique geometry, the LHCb collaboration is pursuing a rich programme of forwar QCD measurements. In particular two-particle angular correlations are stuie in proton-lea collisions at a nucleonnucleon centre-of-mass energy of s NN = 5 ev, with ata collecte by the LHCb etector at the LHC in. he analysis is base on ata recore in two opposing beam configurations, in which either the irection of the proton or that of the lea remnants is analyse. his is the first measurement of a long-range correlation on the near sie in proton-lea collisions in the forwar region. It extens previous observations in the central region. Introuction he two-particle angular correlations are one of the tools to stuy the ynamics of multi-particle prouction in QCD an to probe collective effects arising in the ense environment of a high-energy collision. hey are usually escribe by two-imensional (, )-correlation functions measure in the laboratory system. he two ranges can be istinguishe, the short-range ( ) an longrange ( ) effects. On the near-sie ( ) a short-range jet-peak" at is the ominant structure. It is cause by the fact that in the fragmentation process, the final-state particles are collimate aroun the initial parton. o balance the momentum, the peak is accompanie by a long-range structure on the away sie ( π) cause by particles that fly opposite in azimuthal angle. he near sie long range correlations in the form of the so calle rige effect have been seen at the Relativistic Heavy-Ion Collier (RHIC)in p+au an Au+Au collisions [, ]. Surprisingly, it turne out that in the high multiplicity class of proton-proton interactions the rige has been also foun by CMS []. Later, the rige effect was reporte for p+pb collisions by Alice [], Atlas [5] an CMS [6]. he theoretical interpretation of the mechanism responsible for the rige in small colliing system like proton-proton is still uner iscussion. Various moels have been propose incluing colour reconnection an gluon saturation in the framework of a colour-glass conensate [7 ] or the hyroynamic evolution of a high ensity partonic meium [], multiparton interactions [ ], jetmeium interactions [5, 6], an collective effects [7 ] inuce by the formation an expansion a mariusz.witek@ifj.eu.pl

2 EPJ Web of Conferences of a high-ensity system likely prouce in these collisions. he instrumentation of the LHCb singlearm spectrometer covers a unique rapiity range <η<5 in the laboratory frame [, ]. he measurements are base on proton-lea collisions that are ominate by single interactions (98%). he two beam configurations provie by LHC, p+pb an Pb+ p which iffers by the irection of proton an ion beams can be analyse in the forwar region. he simple ger was configure to accept events with at least one reconstructe track in the vertex etector for non-empty beam bunch crossing. he events were require to have exactly one reconstructe primary vertex with at least five tracks. he tracks were selecte to originate irectly from interaction point base on its impact parameter with respect to primary vertex. he effect of fake tracks after track quality cuts impose an the track reconstruction efficiency is taken into account as a weight assigne to each track. he analysis is base on a subset of the total ata set recore uring the ppb running perio,.6 nb in the p+pb configuration an. nb for the Pb+ p configuration. he rapiity range in the nucleon-nucleon centre-of-mass frame is.5<y<. for p+pb an 5.<y<.5 for Pb+ p. he results of the analysis were publishe in Ref. []. Analysis metho he particles are ivie into three subsets accoring to their p :. GeV/c,.. GeV/c an.. GeV/c, an analyse separately. he analysis metho follows the stanar approach formulate e.g. in Ref. [5]. All particles in a given subset are taken as a ger particle one by one. All other particles are combine with the ger particle in a pair. wo correlation functions are constructe. he signal one S (,) for pairs from the same event an B(,) for pairs combine from mixe events. he correlation function reas: N N pair = S (,) B(, ), () B(,) where N pair is the number of particle pairs foun in a (,) bin an N is the number of ger particles an B(, ) is the normalization factor. By construction, the effects ue to the etector occupancy, acceptance an material are accounte for by iviing the signal by the backgroun istribution, where the latter is normalise to unity aroun the origin. Since the two-particle correlations are strongly epenent on the total number of particles prouce in collision, the analysis is performe for various classes of event activity. he p+pb an Pb+ p minimum bias samples each consist of about. 8 events which are ranomly selecte from about times larger full sample. he high-multiplicity samples contain all available events with at least hits in the vertex etector an amount to about. 8 events in p+pb an. 8 events in Pb+ p collisions. Five event-activity classes are efine as fractions of the hit-multiplicity istributions of the minimum-bias samples, as inicate in Fig.. Since collisions recore in the Pb+ p configuration reach larger hit-multiplicities compare to those in the p+pb configuration, the relative classes are efine separately for each configuration. he 5 % class contains 5% of events with the lowest event activities, followe by the 5% an % classes representing meiumactivity events, an the % an % classes of high-activity events. he event multiplicities seen in the etector for two beam configurations are ifferent. In orer to compare the results, five common absolute activity classes (five bins labele I V) were efine. hey are liste in able. A scaling factor was introuce to achieve the same total event multiplicity in the same bin for p+pb an Pb+ p configurations.

3 XLVI International Symposium on Multiparticle Dynamics Events Activity class 5-% (very low) -5% (low) -% (meium) -% (high) -% (very high) LHCb p+pb s NN = 5 ev 6 8 hit N VELO Events Activity class 5-% (very low) -5% (low) -% (meium) -% (high) -% (very high) LHCb Pb+p s NN = 5 ev 6 8 hit N VELO Figure. Hit-multiplicity istribution in the vertex etector for selecte events of the minimum-bias samples in the (left) p+pb an (right) Pb+ p configurations. he activity classes are efine as fractions of the full istribution, as inicate by colours (shaes). he % class is a sub-sample of the % class. he figures were taken from Ref. []. able. Common absolute activity bins for the p+pb an Pb+ p samples. he activity of p+pb events is scale to match the same activity ranges of Pb+ p events, as explaine in the text. For illustration purposes the average number, N ch MC, of prompt charge particles with p> GeV/c, p > GeV/c an.<η<.9 is liste for events simulate with the HIJING event generator. Common absolute NVELO-range hit p+pb Pb+ p activity bin in Pb+ p scale N ch MC N ch MC Bin I 6.8 ± Bin II ± Bin III ± Bin IV ± Bin V ± Results he correlation for particles with < p < GeV/c is illustrate in Fig. for p+pb configuration an in Fig. for Pb+ p configuration. he near-sie peak aroun== is truncate in the histograms. For low activity events only the jet-peak is observe while for highest activity class a clear rige effect emerges. It is more pronounce in the case of Pb+ p sample. he etaile stuy of the evolution of the long-range correlations on the near an away sies is usually performe in one imensional projections integrate over: Y() N pair N = b N pair. () b a a N he jet-peak area is exclue by averaging the two-imensional yiel over the interval from a =. to b =.9. he non interesting correlations in the form of flat peestals in the yiel are subtracte using the zero-yiel-at-minimum (ZYAM) metho [6]. he subtracte one-imensional yiels for the p+pb (full circles) an Pb+ p (open circles) ata samples are shown in Fig. for all activity classes an p intervals. It can be seen that the correlation strength increases with event activity on the near sie, but ecreases towars higher p where fewer particles are foun. Since more particles are emitte into the acceptance of the etector in the Pb+ p compare to the p+pb configuration, a

4 EPJ Web of Conferences LHCb p+pb s NN = 5 ev. < p <. GeV/c Event class 5-% (a) LHCb p+pb s NN = 5 ev. < p <. GeV/c Event class -% (b) N N N N Figure. wo-particle correlation functions for events recore in the p+pb configuration, showing the (a) low an (b) high event-activity classes. he figures were taken from Ref. []. LHCb Pb+p s NN = 5 ev. < p <. GeV/c Event class 5-% (a) LHCb Pb+p s NN = 5 ev. < p <. GeV/c Event class -% (b) N N N N Figure. wo-particle correlation functions for events recore in the Pb+ p configuration, showing the (a) low an (b) high event-activity classes. he figures were taken from Ref. []. larger offset is observe, as inicate by the ZYAM constants. In the highest-activity class, %, the near-sie rige is strongly pronounce in all p intervals for both Pb+ p an p+pb samples. he rige effect ecreases with ecreasing event activity an isappears for the activity class 5%. For p+pb the near-sie peak is significantly lower. Similar ZYAM-subtracte yiels for common bins of absolute activity are presente in Fig. 5 for < p < GeV/c. he results for the p+pb an Pb+ p samples are compare in five event classes which probe ientical activities in the range of.<η<.9. he measure hit-multiplicities of the p+pb sample are scale to agree with the hit-multiplicities of the Pb+ p sample. It turns out that the observe long-range correlations become compatible for p+pb an Pb+ p. Summary he samples of ppb collisions at s NN = 5 ev collecte by the LHCb experiment were use to stuy two-particle angular correlations for the first time in the forwar region covering the pseuorapiity range.<η<.9 in the laboratory frame. he observation of the rige in the forwar region extens

5 XLVI International Symposium on Multiparticle Dynamics Y()-CZYAM Y()-CZYAM Y()-CZYAM... < p <. GeV/c. < p <. GeV/c. < p <. GeV/c C ZYAM =.6 (p+pb) C ZYAM =.6 (Pb+p) C ZYAM =.8 (p+pb) C ZYAM =. (Pb+p) C ZYAM =.7 (p+pb) C ZYAM =5.78 (Pb+p) C ZYAM =. (p+pb) C ZYAM =.7 (Pb+p) C ZYAM =.56 (p+pb) C ZYAM =.7 (Pb+p) C ZYAM =.8 (p+pb) C ZYAM =. (Pb+p) LHCb s NN = 5 ev p+pb ata Pb+p ata C ZYAM =.8 (p+pb) C ZYAM =. (Pb+p) C ZYAM =. (p+pb) C ZYAM =.6 (Pb+p) 5-% -5% -% Y()-CZYAM. C ZYAM =5. (p+pb) C ZYAM =7.8 (Pb+p) C ZYAM =.9 (p+pb) C ZYAM =.78 (Pb+p) C ZYAM =.9 (p+pb) C ZYAM =.6 (Pb+p) -% Y()-CZYAM. C ZYAM =5.67 (p+pb) C ZYAM =8.6 (Pb+p) C ZYAM =.9 (p+pb) C ZYAM =.7 (Pb+p) C ZYAM =. (p+pb) C ZYAM =. (Pb+p) -% Figure. One-imensional correlation yiel as a function of obtaine from the ZYAM-metho by averaging over.<<.9. he figure was taken from Ref. []. Y()-CZYAM. LHCb Activity bin I C ZYAM =. (p+pb) C ZYAM =. (Pb+p) p+pb ata Pb+p ata s NN = 5 ev.. < p <. GeV/c Activity bin II C ZYAM =. (p+pb) C ZYAM =.6 (Pb+p). Activity bin III C ZYAM =. (p+pb) C ZYAM =.7 (Pb+p). Activity bin IV C ZYAM =.5 (p+pb) C ZYAM =.8 (Pb+p). Activity bin V C ZYAM =.6 (p+pb) C ZYAM =.5 (Pb+p) Figure 5. One-imensional correlation yiel as a function of obtaine from the ZYAM-metho by averaging the two-imensional istribution over.<<.9. he figure was taken from Ref. [].

6 EPJ Web of Conferences previous LHC measurements. he analysis was performe separately for the p+pb an Pb+ p beam configurations probing the rapiities in the nucleon-nucleon centre-of-mass frame of.5 < y <. an 5.<y<.5, respectively. he analysis was performe in various p ranges an for ifferent event activity classes. In high event activity class, a long-range correlation on the near sie (the rige) is observe in both configurations. he near-sie rige is most pronounce in the range < p < GeV/c. For ientical absolute activity ranges in the p+pb an Pb+ p configurations the observe long-range correlations are compatible. Acknowlegements I woul like to express my gratitue to the National Science Centre NCN in Polan, for financial support uner the contract no. //B/S/89. References [] B. Abelev et al., Phys. Rev. C8, 69 (9). [] B. Alver et al., Phys. Rev. Lett., 6 (). [] Khachatryan, V., Sirunyan, A.M. et al., CMS collaboration, J. High Energ. Phys. () 9. [] B. Abelev et al., ALICE Collaboration, Phys. Lett. B79, 9 () [5] G. Aa et al., ALAS Collaboration, Phys. Rev. Lett., 8 () [6] S. Chatrchyan et al., CMS Collaboration, Phys. Lett. B78, 795 () [7] Dusling, Kevin an Venugopalan, Raju, Phys.Rev. D87, 55 () [8] Dusling, Kevin an Venugopalan, Raju, Phys.Rev. D87, 5 () [9] Dusling, Kevin an Venugopalan, Raju, Phys.Rev. D87, 9 () [] Kovchegov, Yuri V. an Wertepny, Douglas E., Nucl.Phys. A96, 5-8 () [] Bzak, Aam an Schenke, Bjoern an ribey, Prithwish an Venugopalan, Raju, Phys.Rev. C87, 696 () [] Alerweirelt, Sara an Van Mechelen, Pierre, Proceeings of the r International Workshop on Multiple Partonic Interactions at the LHC (MPI@LHC ), - () [] Strikman, Mark, Acta Phys.Polon. B, 67-6 () [] Ryskin, M.G. an Martin, A.D. an Khoze, V.A., J.Phys. G8, 856 () [5] Hwa, Ruolph C. an Yang, C.B., Phys.Rev. C8, 9 () [6] Wong, Cheuk-Yin, Phys.Rev. C8, 9 () [7] Avsar, Emil an Flensburg, Christoffer an Hatta Yoshitaka an Ollitrault, Jean-Yves an Uea, akahiro, Phys.Lett., B7,9-97 () [8] Werner, K. an Karpenko, Iu. an Pierog,., Phys.Rev.Lett. 6, () [9] Bozek, Piotr, Phys. Rev. C85, 9 () [] Bozek, Piotr an Broniowski, Wojciech, Phys.Lett. B78, () [] Shuryak, Ewar an Zahe, Ismail, Phys.Rev. C88, 95 () [] Alves Jr., A. A. an others, LHCb collaboration, JINS, S85 (8) [] Aaij, R. an others, LHCb collaboration, Int. J. Mo. Phys. A, 5 (5) [] Aaij, R. an others, LHCb collaboration, Phys. Lett. B (6) [5] Abelev, Betty an others, ALICE collaboration, Phys.Lett. B79, 9- () [6] Ajitanan, N. N. an Alexaner, J. M. an Chung, P. an Holzmann, W. G. an Issah, M. an Lacey, Roy A. an Shevel, A. an aranenko, A. an Danielewicz, P., Phys. Rev. C 7, 9 (5)