Physics Letters B 718 (2013) 795–814 Contents lists available at SciVerse ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Observation of long-range, near-side angular correlations in pPb collisions at the LHC ✩ .CMS Collaboration � CERN, Switzerland a r t i c l e i n f o a b s t r a c t Article history: Received 19 October 2012 Received in revised form 7 November 2012 Accepted 9 November 2012 Available online 13 November 2012 Editor: M. Doser Keywords: CMS Physics Heavy ions Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon– nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, η, and full azimuth, φ, as a function of charged particle multiplicity and particle transverse momentum, pT. In high-multiplicity events, a long-range (2 < |�η| < 4), near-side (�φ ≈ 0) structure emerges in the two-particle �η–�φ correlation functions. This is the first observation of such correlations in proton–nucleus collisions, resembling the ridge-like correlations seen in high- multiplicity pp collisions at √ s = 7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of pT = 1–1.5 GeV/c and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These observations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is significantly larger in pPb collisions. © 2012 CERN. Published by Elsevier B.V. All rights reserved. 1. Introduction This Letter presents measurements of two-particle angular cor- relations in proton-lead (pPb) collisions at a nucleon–nucleon center-of-mass energy √ sN N = 5.02 TeV, performed with the Com- pact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). Two-particle correlations in high-energy collisions provide valuable information for characterizing Quantum Chromodynamics and have been studied previously for a broad range of collision energies in proton–proton (pp), proton–nucleus (pA), and nucleus– nucleus (AA) collisions. Such measurements can elucidate the un- derlying mechanism of particle production and possible collective effects resulting from the high particle densities accessible in these collisions. Studies of two-particle angular correlations are typically per- formed using two-dimensional �η–�φ correlation functions, where �φ is the difference in azimuthal angle φ between the two particles and �η is the difference in pseudorapidity η = − ln(tan(θ/2)). The polar angle θ is defined relative to the coun- terclockwise beam. Of particular interest in studies of collective effects is the long- range (large |�η|) structure of two-particle correlation functions, which is less susceptible to known sources of correlations such ✩ © CERN for the benefit of the CMS Collaboration. � E-mail address: cms-publication-committee-chair@cern.ch. as resonance decays and fragmentation of energetic jets. Measure- ments in high-energy AA collisions have shown significant mod- ifications of the long-range structure compared with minimum bias pp collisions [1]. Novel correlation structures extending over large �η at |�φ| ≈ 0 and |�φ| ≈ 2π/3 were observed in az- imuthal correlations for intermediate particle transverse momenta, pT ≈ 1–5 GeV/c [2–10]. In AA collisions, long-range correlations are interpreted as a consequence of the hydrodynamic flow of the produced strongly interacting medium [11] and are usually characterized by the Fourier components of the azimuthal parti- cle distributions [12]. Of particular importance are the second and third Fourier components, called elliptic and triangular flow, as they most directly reflect the medium response to the initial col- lision geometry and its fluctuations [13], and allow the study of fundamental transport properties of the medium using hydrody- namic models [14–16]. In current pp and pA Monte Carlo (MC) event generators, the dominant sources of such long-range correlations are momentum conservation and away-side (�φ ≈ π ) jet correlations. Measure- ments in pp collisions at 7 TeV have revealed the emergence of long-range, near-side (�φ ≈ 0) correlations in a selection of colli- sions with very high final-state particle multiplicity [17]. A large variety of theoretical models have been proposed to explain the origin of these so-called ridge-like correlations (see Ref. [18] for a recent review). The proposed mechanisms range from color con- nections in hard scattering processes and collective effects in the initial interaction of the protons to hydrodynamic effects in the 0370-2693/ © 2012 CERN. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physletb.2012.11.025 http://dx.doi.org/10.1016/j.physletb.2012.11.025 http://www.ScienceDirect.com/ http://www.elsevier.com/locate/physletb mailto:cms-publication-committee-chair@cern.ch http://dx.doi.org/10.1016/j.physletb.2012.11.025 796 CMS Collaboration / Physics Letters B 718 (2013) 795–814 high-density system possibly formed in these collisions. It is natu- ral to search for the possible emergence of related features in pPb collisions, where a similar range of final-state multiplicities can be explored. The first comparison of pp and pPb measurements as a function of charged particle multiplicity and particle transverse momentum, presented in this Letter, should provide valuable in- formation for understanding the origin of the long-range, near-side correlation signal seen in high-multiplicity pp collisions. 2. Experimental setup This analysis uses a pPb data set collected during a short run (lasting for about 8 hours) in September 2012. The beam ener- gies were 4 TeV for protons and 1.58 TeV per nucleon for lead nuclei, resulting in a center-of-mass energy per nucleon pair of√ sN N = 5.02 TeV. Due to the energy difference, the nucleon– nucleon center-of-mass in the pPb collisions is not at rest with re- spect to the laboratory frame. Since the higher energy proton beam traveled in the clockwise direction, i.e. at θ = π , massless particles emitted at ηcm = 0 in the nucleon–nucleon center-of-mass frame will be detected at η = −0.465 in the laboratory frame. A detailed description of the CMS experiment can be found in Ref. [19]. The main detector subsystem used for this analysis is the tracker, located in the 3.8 T field of the superconducting solenoid. It measures charged particles within the pseudorapid- ity range |η| < 2.5 and consists of 1440 silicon pixel and 15 148 silicon strip detector modules. It provides an impact parameter res- olution of ∼15 μm and a transverse momentum (pT) resolution of about 1.5% for 100 GeV/c particles. Also located inside the solenoid are the electromagnetic calorimeter (ECAL) and hadron calorime- ter (HCAL). The ECAL consists of more than 75 000 lead-tungstate crystals, arranged in a quasi-projective geometry and distributed in a barrel region (|η| < 1.48) and two endcaps that extend up to |η| = 3.0. The HCAL barrel and endcaps are sampling calorimeters composed of brass and scintillator plates, covering |η| < 3.0. Iron forward calorimeters (HF) with quartz fibers, read out by photo- multipliers, extend the calorimeter coverage up to |η| = 5.0. The detailed MC simulation of the CMS detector response is based on Geant4 [20]. 3. Event and track selection The relatively low pPb collision frequency (about 200 Hz) pro- vided by the LHC in this pilot run allowed the use of a track-based minimum bias trigger. For every pPb bunch crossing, the detec- tor was read out and events were accepted if at least one track with pT > 0.4 GeV/c was found in the pixel tracker. In the offline analysis, a coincidence of at least one HF calorimeter tower with more than 3 GeV of total energy on both the positive and nega- tive sides of HF was required to select hadronic collisions. Events were also required to contain at least one reconstructed primary vertex within 15 cm of the nominal interaction point along the beam axis (zvtx) and within 0.15 cm transverse distance to the beam trajectory. At least two reconstructed tracks were required to be associated with the primary vertex. Beam related background was suppressed by rejecting events with a high fraction of pixel clusters inconsistent with a single collision vertex [21]. Based on simulations using the hijing event generator, the event selections have a total acceptance of about 96.2% for hadronic inelastic pPb interactions. A total of 2 million events passed all selection cri- teria, corresponding to an integrated luminosity of about 1 μb−1, assuming a pPb interaction cross section of 2.1 barns. The angular correlation functions were obtained using the CMS highPurity [22] track selection. Additionally, a reconstructed track was only considered as a primary-track candidate if the Table 1 Fraction of the full event sample for each multiplicity class. The last two columns show the observed and corrected multiplicities, respectively, of charged particles with |η| < 2.4 and pT > 0.4 GeV/c. Systematic uncertainties are given for the cor- rected multiplicities, while statistical uncertainties are negligible. Multiplicity class (Noffline trk ) Fraction (%) 〈Noffline trk 〉 〈Ncorrected trk 〉 Minimum bias 100.0 40.6 53.4 ± 2.9 Noffline trk < 35 50.4 17.1 23.5 ± 1.3 35 � Noffline trk < 90 41.9 56.3 75.6 ± 4.1 90 � Noffline trk < 110 4.6 98.2 114.3 ± 6.2 Noffline trk � 110 3.1 128.2 149.1 ± 8.1 significance of the separation along the beam axis, z, between the track and the primary vertex, dz/σ (dz), and the significance of the impact parameter relative to the primary vertex transverse to the beam, dT /σ (dT ), were each less than 3. The relative uncer- tainty of the momentum measurement, σ(pT)/pT, was required to be less than 10%. To ensure high tracking efficiency and low fake rate, only tracks within |η| < 2.4 and with pT > 0.1 GeV/c were used. To match the analysis used for high-multiplicity pp colli- sions [17], the events were divided into classes of reconstructed track multiplicity, Noffline trk , where primary tracks with |η| < 2.4 and pT > 0.4 GeV/c were counted. The fraction of events falling into each of the four multiplicity classes is listed in Table 1. The table also lists the average values of Noffline trk and Ncorrected trk , the event multiplicity of charged particles with |η| < 2.4 and pT > 0.4 GeV/c corrected for detector acceptance and efficiency of the track reconstruction algorithm, as discussed in the following section. 4. Calculation of the two-particle correlation function The analysis of two-particle correlations was performed in classes of track multiplicity, Noffline trk , following the procedure estab- lished in [7,8]. For each track multiplicity class, “trigger” particles are defined as charged particles originating from the primary ver- tex within a given pT range. The number of trigger particles in the event is denoted by Ntrig. In this analysis, particle pairs are formed by associating every trigger particle with the remaining charged primary particles from the same pT interval as the trigger particle (a minimum of two particles is required in each pT bin from each event). The per-trigger-particle associated yield is defined as 1 Ntrig d2Npair d�η d�φ = B(0,0) × S(�η,�φ) B(�η,�φ) , (1) where �η and �φ are the differences in η and φ of the pair. The signal distribution, S(�η,�φ), is the per-trigger-particle yield of particle pairs from the same event, S(�η,�φ) = 1 Ntrig d2Nsame d�η d�φ . (2) The mixed-event background distribution, used to account for ran- dom combinatorial background and pair-acceptance effects, B(�η,�φ) = 1 Ntrig d2Nmix d�η d�φ , (3) is constructed by pairing the trigger particles in each event with the associated particles from 10 different random events in the same 2 cm wide zvtx range. The symbol Nmix denotes the number of pairs taken from the mixed event, while B(0,0) represents the mixed-event associated yield for both particles of the pair going CMS Collaboration / Physics Letters B 718 (2013) 795–814 797 in approximately the same direction and thus having full pair ac- ceptance (with a bin width of 0.3 in �η and π/16 in �φ). There- fore, the ratio B(0,0)/B(�η,�φ) is the pair-acceptance correction factor used to derive the corrected per-trigger-particle associated yield distribution. The signal and background distributions are first calculated for each event, and then averaged over all the events within the track multiplicity class. Each reconstructed track is weighted by the inverse of an effi- ciency factor, which accounts for the detector acceptance, the re- construction efficiency, and the fraction of misreconstructed tracks. Detailed studies of tracking efficiencies using MC simulations and data-based methods can be found in [23]. The combined geometri- cal acceptance and efficiency for track reconstruction exceeds 50% for pT ≈ 0.1 GeV/c and |η| < 2.4. The efficiency is greater than 90% in the |η| < 1 region for pT > 0.6 GeV/c. For the multiplicity range studied here, little or no dependence of the tracking efficiency on multiplicity is found and the rate of misreconstructed tracks re- mains at the 1–2% level. Simulations of pp, pPb and peripheral PbPb collisions using the pythia, hijing and hydjet event generators, respectively, yield ef- ficiency correction factors that vary due to the different kinematic and mass distributions for the particles produced in these gen- erators. Applying the resulting correction factors from one of the generators to simulated data from one of the others gives asso- ciated yield distributions that agree within 5%. Systematic uncer- tainties due to track quality cuts and potential contributions from secondary particles (including those from weak decays) are exam- ined by loosening or tightening the track selections on dz/σ (dz) and dT /σ (dT ) from 2 to 5. The associated yields are found to be insensitive to these track selections within 2%. 5. Results Fig. 1 compares 2-D two-particle correlation functions for events with low (a) and high (b) multiplicity, for pairs of charged particles with 1 < pT < 3 GeV/c. For the low-multiplicity selec- tion (Noffline trk < 35), the dominant features are the correlation peak near (�η,�φ) = (0,0) for pairs of particles originating from the same jet and the elongated structure at �φ ≈ π for pairs of parti- cles from back-to-back jets. To better illustrate the full correlation structure, the jet peak has been truncated. High-multiplicity events (Noffline trk � 110) also show the same-side jet peak and back-to- back correlation structures. However, in addition, a pronounced “ridge”-like structure emerges at �φ ≈ 0 extending to |�η| of at least 4 units. This observed structure is similar to that seen in high-multiplicity pp collision data at √ s = 7 TeV [17] and in AA collisions over a wide range of energies [3–10]. As a cross-check, correlation functions were also generated for tracks paired with ECAL photons, which originate primarily from decays of π0s, and for pairs of ECAL photons. These distributions showed similar features as those seen in Fig. 1, in particular the ridge-like correlation for high multiplicity events. To investigate the long-range, near-side correlations in finer detail, and to provide a quantitative comparison to pp results, one-dimensional (1-D) distributions in �φ are found by averag- ing the signal and background two-dimensional (2-D) distributions over 2 < |�η| < 4 [7,8,17]. In the presence of multiple sources of correlations, the yield for the correlation of interest is commonly estimated using an implementation of the zero-yield-at-minimum (ZYAM) method [26]. A second-order polynomial is first fitted to the 1-D �φ correlation function in the region 0.1 < |�φ| < 2. The minimum value of the polynomial, CZYAM, is then subtracted from the 1-D �φ correlation function as a constant background (con- taining no information about correlations) to shift its minimum to be at zero associated yield. The statistical uncertainty on the Fig. 1. 2-D two-particle correlation functions for 5.02 TeV pPb collisions for pairs of charged particles with 1 < pT < 3 GeV/c. Results are shown (a) for low-multiplicity events (Noffline trk < 35) and (b) for a high-multiplicity selection (Noffline trk � 110). The sharp near-side peaks from jet correlations have been truncated to better illustrate the structure outside that region. minimum level of 1 Ntrig dNpair d�φ obtained by the ZYAM procedure as well as the deviations found by varying the fit range in �φ give an absolute uncertainty of ±0.0015 on the associated yield, inde- pendent of multiplicity and pT. Fig. 2 shows the results for pPb data (solid circles) for various selections in pT and multiplicity Noffline trk , with pT increasing from left to right and multiplicity increasing from top to bottom. The results for pp data at √ s = 7 TeV, obtained using the same proce- dure [17], are also plotted (open circles). A clear evolution of the �φ correlation function as a function of both pT and Noffline trk is observed. For the lowest multiplicity se- lection in pp and pPb the correlation functions have a minimum at �φ = 0 and a maximum at �φ = π , reflecting the correla- tions from momentum conservation and the increasing contribu- tion from back-to-back jet-like correlations at higher pT. Results from the hijing [24] model (version 1.383), shown as dashed lines, qualitatively reproduce the shape of the correlation function for low Noffline trk . For multiplicities Noffline trk � 35, a second local maximum near |�φ| ≈ 0 emerges in the pPb data, corresponding to the near-side, long-range ridge-like structure. In pp data, this second maximum is clearly visible only for Noffline trk > 90. For both pp and pPb col- lisions, this near-side correlated yield is largest in the 1 < pT < 2 GeV/c range and increases with increasing multiplicity. While the evolution of the correlation function is qualitatively similar in pp and pPb data, the absolute near-side correlated yield is signifi- cantly larger in the pPb case. In contrast to the data, the hijing calculations show a correlated yield of zero at �φ = 0 for all multiplicity and pT selections. The 798 CMS Collaboration / Physics Letters B 718 (2013) 795–814 Fig. 2. Correlated yield obtained from the ZYAM procedure as a function of |�φ| averaged over 2 < |�η| < 4 in different pT and multiplicity bins for 5.02 TeV pPb data (solid circles) and 7 TeV pp data (open circles). The pT selection applies to both particles in each pair. Statistical uncertainties are smaller than the marker size. The subtracted ZYAM constant is listed in each panel. Also shown are pPb predictions for hijing [24] (dashed curves) and a hydrodynamic model [25] (solid curves shown for 1 < pT < 2 GeV/c). long-range, near-side enhancement is also absent in simulated pp collision events with the pythia [27,28] event generator (version 6.4.24) and in simulated pPb collisions with the ampt [29] model (version 1.25/2.25). Long-range correlations in pPb collisions have been quantita- tively predicted in models assuming a collective hydrodynamic ex- pansion of a system with fluctuating initial conditions [25]. The correlation resulting from the predicted elliptic and triangular flow components for pPb collisions at √ sN N = 4.4 TeV are compared to the observed correlation in Fig. 2 for the 1 < pT < 2 GeV/c selec- tion (solid line, second column). The magnitudes for elliptic and triangular flow of v2 = 0.066 and v3 = 0.037 correspond to those given in Ref. [25] for the highest multiplicity selection and the av- erage value of pT ≈ 1.4 GeV/c found in the data. The same v2 and v3 coefficients were used for all multiplicity classes, show- ing the multiplicity dependence of the correlated yield assuming a constant flow effect. While this provides an indicative and useful illustration of the magnitude of the observed near-side enhance- ment, a detailed quantitative comparison of the model and data will need to include the additional non-hydrodynamical correla- tions from back-to-back jets, as well as the effects of momentum conservation, which suppress the correlation near �φ ≈ 0 relative to �φ ≈ π . The ridge-like structure in pPb collisions was also predicted to arise from initial state gluon correlations in the color-glass conden- sate framework, where the contribution of collimated gluon emis- sions is significantly enhanced in the gluon saturation regime [30]. This model qualitatively predicts the increase in the correlation strength for higher multiplicity pPb collisions, although it remains to be seen if the large associated yield seen in the highest mul- tiplicity selection can be quantitatively reproduced in the calcula- tion. The strength of the long-range, near-side correlations can be further quantified by integrating the correlated yield from Fig. 2 over |�φ| < 1.2 using 12 classes of multiplicity. The resulting inte- grated “ridge yield”, normalized by the width of the pT interval, is plotted as a function of particle pT and event multiplicity in Fig. 3 for pp (open circles) and pPb (solid circles) data. The error bars correspond to statistical uncertainties, while the shaded boxes in- dicate the systematic uncertainties. Fig. 3(a) shows that the ridge yield for events with Noffline trk � 110 peaks in the region 1 < pT < 2 GeV/c for both collision CMS Collaboration / Physics Letters B 718 (2013) 795–814 799 Fig. 3. Associated yield for the near-side of the correlation function averaged over 2 < |�η| < 4 and integrated over the region |�φ| < 1.2 in 7 TeV pp collisions (open circles) and 5.02 TeV pPb collisions (solid circles). Panel (a) shows the associated yield as a function of pT for events with Noffline trk � 110. In panel (b) the associated yield for 1 < pT < 2 GeV/c is shown as a function of multiplicity Noffline trk . The pT se- lection applies to both particles in each pair. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties. systems. However, while the yield in pp collisions is consistent with zero for the 0.1 < pT < 1 GeV/c selection, it remains greater than zero in pPb data even for the 0.1 < pT < 0.5 GeV/c range. For higher pT, the ridge yield in pp collisions is consistent with zero for pT > 3 GeV/c, while the pPb results only approach zero at pT ≈ 4–7 GeV/c. The multiplicity dependence of the ridge yield for 1 < pT < 2 GeV/c particle pairs is shown in Fig. 3(b). For low-multiplicity collisions, the ridge yield determined by the ZYAM procedure is consistent with zero, indicating that ridge-like correlations are ab- sent or smaller than the negative correlations expected due to, e.g. momentum conservation. At higher multiplicity the ridge-like cor- relations emerge, with an approximately linear rise of the ridge yield observed for Noffline trk � 40, which corresponds to Ncorrected trk � 53. While the multiplicity dependence is qualitatively similar for pp and pPb collisions, a significantly larger yield per trigger parti- cle is seen in pPb than in pp at a given multiplicity. When interpreting the differences in the correlation structure between the two collision systems, it is important to consider the relative contributions of different particle production mechanisms to the observed particle yields. While very high multiplicity pp col- lisions should mainly arise from rare multiple hard-scattering pro- cesses, the high-multiplicity pPb events should mostly result from particle production in multiple soft proton–nucleon scatterings. This will in particular affect the correlations due to back-to-back jet fragmentation in the �φ ≈ π region. A simultaneous descrip- tion of the measurements in pp and pPb should provide significant constraints on models of the underlying physics processes. With such an improved understanding of the smaller systems, compar- isons to the PbPb data will also provide insights in understanding the similarity of the ridge-like correlations in all three systems. 6. Conclusion The CMS detector at the LHC has been used to measure an- gular correlations between two charged particles with |η| < 2.4 in pPb collisions at √ sN N = 5.02 TeV. Azimuthal correlations for 2 < |�η| < 4 in high-multiplicity pPb collisions exhibit a long- range structure at the near side (�φ ≈ 0). This ridge-like struc- ture is qualitatively similar to that observed in pp collisions at√ s = 7 TeV and in AA collisions over a broad range of center-of- mass energies. The effect is most evident in the intermediate trans- verse momentum range, 1 < pT < 1.5 GeV/c. The near-side ridge yield obtained by the ZYAM procedure is found to be consistent with zero in the low-multiplicity region, with an approximately linear increase with multiplicity for Noffline trk � 40 (corresponding to Ncorrected trk � 53). While the multiplicity and pT dependences of the observed effect are similar to those seen in pp data at √ s = 7 TeV, the absolute ridge yield in pPb is significantly larger than in pp collisions of the same particle multiplicity. Acknowledgements We congratulate our colleagues in the CERN accelerator depart- ments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS in- stitutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centres and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construction and operation of the LHC and the CMS detector provided by the following funding agencies: BMWF and FWF (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MEYS (Bulgaria); CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); MoER, SF0690030s09 and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NKTH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Republic of Korea); LAS (Lithuania); CINVESTAV, CONACYT, SEP, and UASLP- FAI (Mexico); MSI (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MON, RosAtom, RAS and RFBR (Russia); MSTD (Ser- bia); SEIDI and CPAN (Spain); Swiss Funding Agencies (Switzer- land); NSC (Taipei); ThEP, IPST and NECTEC (Thailand); TUBITAK and TAEK (Turkey); NASU (Ukraine); STFC (United Kingdom); DOE and NSF (USA). 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Paolucci a,2 a INFN Sezione di Napoli, Napoli, Italy b Università di Napoli “Federico II”, Napoli, Italy CMS Collaboration / Physics Letters B 718 (2013) 795–814 805 P. Azzi a, N. Bacchetta a,2, M. Biasotto a,32, A. Branca a,b,2, R. Carlin a,b, P. Checchia a, T. Dorigo a, F. Gasparini a,b, A. Gozzelino a, M. Gulmini a,32, K. Kanishchev a,c, S. Lacaprara a, I. Lazzizzera a,c, M. Margoni a,b, G. Maron a,32, A.T. Meneguzzo a,b, J. Pazzini a,b, N. Pozzobon a,b, P. Ronchese a,b, F. Simonetto a,b, E. Torassa a, M. Tosi a,b, S. Vanini a,b, P. Zotto a,b, G. Zumerle a,b a INFN Sezione di Padova, Padova, Italy b Università di Padova, Padova, Italy c Università di Trento (Trento), Padova, Italy M. Gabusi a,b, S.P. Ratti a,b, C. Riccardi a,b, P. Torre a,b, P. Vitulo a,b a INFN Sezione di Pavia, Pavia, Italy b Università di Pavia, Pavia, Italy M. Biasini a,b, G.M. Bilei a, L. Fanò a,b, P. Lariccia a,b, G. Mantovani a,b, M. Menichelli a, A. Nappi a,b,†, F. Romeo a,b, A. Saha a, A. Santocchia a,b, A. Spiezia a,b, S. Taroni a,b a INFN Sezione di Perugia, Perugia, Italy b Università di Perugia, Perugia, Italy P. Azzurri a,c, G. Bagliesi a, J. Bernardini a, T. Boccali a, G. Broccolo a,c, R. Castaldi a, R.T. D’Agnolo a,c,2, R. Dell’Orso a, F. Fiori a,b,2, L. Foà a,c, A. Giassi a, A. Kraan a, F. Ligabue a,c, T. Lomtadze a, L. Martini a,33, A. Messineo a,b, F. Palla a, A. Rizzi a,b, A.T. Serban a,34, P. Spagnolo a, P. Squillacioti a,2, R. Tenchini a, G. Tonelli a,b, A. Venturi a, P.G. Verdini a a INFN Sezione di Pisa, Pisa, Italy b Università di Pisa, Pisa, Italy c Scuola Normale Superiore di Pisa, Pisa, Italy L. Barone a,b, F. Cavallari a, D. Del Re a,b, M. Diemoz a, C. Fanelli a,b, M. Grassi a,b,2, E. Longo a,b, P. Meridiani a,2, F. Micheli a,b, S. Nourbakhsh a,b, G. Organtini a,b, R. Paramatti a, S. Rahatlou a,b, L. Soffi a,b a INFN Sezione di Roma, Roma, Italy b Università di Roma, Roma, Italy N. Amapane a,b, R. Arcidiacono a,c, S. Argiro a,b, M. Arneodo a,c, C. 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Song Chonnam National University, Institute for Universe and Elementary Particles, Kwangju, Republic of Korea S. Choi, D. Gyun, B. Hong, M. Jo, H. Kim, T.J. Kim, K.S. Lee, D.H. Moon, S.K. Park, Y. Roh Korea University, Seoul, Republic of Korea 806 CMS Collaboration / Physics Letters B 718 (2013) 795–814 M. Choi, J.H. Kim, C. Park, I.C. Park, S. Park, G. Ryu University of Seoul, Seoul, Republic of Korea Y. Choi, Y.K. Choi, J. Goh, M.S. Kim, E. Kwon, B. Lee, J. Lee, S. Lee, H. Seo, I. Yu Sungkyunkwan University, Suwon, Republic of Korea M.J. Bilinskas, I. Grigelionis, M. Janulis, A. Juodagalvis Vilnius University, Vilnius, Lithuania H. Castilla-Valdez, E. De La Cruz-Burelo, I. Heredia-de La Cruz, R. Lopez-Fernandez, J. Martínez-Ortega, A. Sánchez-Hernández, L.M. Villasenor-Cendejas Centro de Investigacion y de Estudios Avanzados del IPN, Mexico City, Mexico S. Carrillo Moreno, F. Vazquez Valencia Universidad Iberoamericana, Mexico City, Mexico H.A. Salazar Ibarguen Benemerita Universidad Autonoma de Puebla, Puebla, Mexico E. Casimiro Linares, A. Morelos Pineda, M.A. Reyes-Santos Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico D. Krofcheck University of Auckland, Auckland, New Zealand A.J. Bell, P.H. Butler, R. Doesburg, S. Reucroft, H. Silverwood University of Canterbury, Christchurch, New Zealand M. Ahmad, M.I. Asghar, J. Butt, H.R. Hoorani, S. Khalid, W.A. Khan, T. Khurshid, S. Qazi, M.A. Shah, M. Shoaib National Centre for Physics, Quaid-I-Azam University, Islamabad, Pakistan H. Bialkowska, B. Boimska, T. Frueboes, M. Górski, M. Kazana, K. Nawrocki, K. Romanowska-Rybinska, M. Szleper, G. Wrochna, P. Zalewski National Centre for Nuclear Research, Swierk, Poland G. Brona, K. Bunkowski, M. Cwiok, W. Dominik, K. Doroba, A. Kalinowski, M. Konecki, J. Krolikowski, M. Misiura Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Warsaw, Poland N. Almeida, P. Bargassa, A. David, P. 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Shreyber, V. Stolin, E. Vlasov, A. Zhokin Institute for Theoretical and Experimental Physics, Moscow, Russia A. Belyaev, E. Boos, A. Ershov, A. Gribushin, V. Klyukhin, O. Kodolova, V. Korotkikh, I. Lokhtin, A. Markina, S. Obraztsov, M. Perfilov, S. Petrushanko, A. Popov, L. Sarycheva †, V. Savrin, A. Snigirev, I. Vardanyan Moscow State University, Moscow, Russia V. Andreev, M. Azarkin, I. Dremin, M. Kirakosyan, A. Leonidov, G. Mesyats, S.V. Rusakov, A. Vinogradov P.N. Lebedev Physical Institute, Moscow, Russia I. Azhgirey, I. Bayshev, S. Bitioukov, V. Grishin 2, V. Kachanov, D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, A. Sobol, L. Tourtchanovitch, S. Troshin, N. Tyurin, A. Uzunian, A. Volkov State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, Russia P. Adzic 35, M. Djordjevic, M. Ekmedzic, D. Krpic 35, J. Milosevic University of Belgrade, Faculty of Physics and Vinca Institute of Nuclear Sciences, Belgrade, Serbia M. Aguilar-Benitez, J. 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Swanson University of Wisconsin, Madison, USA * Corresponding author. † Deceased. 1 Also at Vienna University of Technology, Vienna, Austria. 2 Also at CERN, European Organization for Nuclear Research, Geneva, Switzerland. 3 Also at National Institute of Chemical Physics and Biophysics, Tallinn, Estonia. 4 Also at Universidade Federal do ABC, Santo Andre, Brazil. 5 Also at California Institute of Technology, Pasadena, USA. 6 Also at Laboratoire Leprince-Ringuet, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France. 7 Also at Suez Canal University, Suez, Egypt. 8 Also at Zewail City of Science and Technology, Zewail, Egypt. 9 Also at Cairo University, Cairo, Egypt. 10 Also at Fayoum University, El-Fayoum, Egypt. 11 Also at Helwan University, Cairo, Egypt. 12 Also at British University in Egypt, Cairo, Egypt. 13 Now at Ain Shams University, Cairo, Egypt. 14 Also at National Centre for Nuclear Research, Swierk, Poland. 15 Also at Université de Haute-Alsace, Mulhouse, France. 16 Also at Joint Institute for Nuclear Research, Dubna, Russia. 17 Also at Moscow State University, Moscow, Russia. 18 Also at Brandenburg University of Technology, Cottbus, Germany. 19 Also at The University of Kansas, Lawrence, USA. 20 Also at Institute of Nuclear Research ATOMKI, Debrecen, Hungary. 21 Also at Eötvös Loránd University, Budapest, Hungary. 22 Also at Tata Institute of Fundamental Research - HECR, Mumbai, India. 23 Now at King Abdulaziz University, Jeddah, Saudi Arabia. 24 Also at University of Visva-Bharati, Santiniketan, India. 25 Also at Sharif University of Technology, Tehran, Iran. 26 Also at Isfahan University of Technology, Isfahan, Iran. 27 Also at Shiraz University, Shiraz, Iran. 28 Also at Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran. 29 Also at Facoltà Ingegneria Università di Roma, Roma, Italy. 30 Also at Università della Basilicata, Potenza, Italy. 31 Also at Università degli Studi Guglielmo Marconi, Roma, Italy. 32 Also at Laboratori Nazionali di Legnaro dell’INFN, Legnaro, Italy. 33 Also at Università degli Studi di Siena, Siena, Italy. 34 Also at University of Bucharest, Faculty of Physics, Bucuresti-Magurele, Romania. 35 Also at Faculty of Physics of University of Belgrade, Belgrade, Serbia. 814 CMS Collaboration / Physics Letters B 718 (2013) 795–814 36 Also at University of California, Los Angeles, Los Angeles, USA. 37 Also at Scuola Normale e Sezione dell’INFN, Pisa, Italy. 38 Also at INFN Sezione di Roma, Roma, Italy. 39 Also at University of Athens, Athens, Greece. 40 Also at Rutherford Appleton Laboratory, Didcot, United Kingdom. 41 Also at Paul Scherrer Institut, Villigen, Switzerland. 42 Also at Institute for Theoretical and Experimental Physics, Moscow, Russia. 43 Also at Albert Einstein Center for Fundamental Physics, BERN, Switzerland. 44 Also at Gaziosmanpasa University, Tokat, Turkey. 45 Also at Adiyaman University, Adiyaman, Turkey. 46 Also at Izmir Institute of Technology, Izmir, Turkey. 47 Also at The University of Iowa, Iowa City, USA. 48 Also at Mersin University, Mersin, Turkey. 49 Also at Ozyegin University, Istanbul, Turkey. 50 Also at Kafkas University, Kars, Turkey. 51 Also at Suleyman Demirel University, Isparta, Turkey. 52 Also at Ege University, Izmir, Turkey. 53 Also at School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom. 54 Also at INFN Sezione di Perugia; Università di Perugia, Perugia, Italy. 55 Also at Utah Valley University, Orem, USA. 56 Now at University of Edinburgh, Scotland, Edinburgh, United Kingdom. 57 Also at Institute for Nuclear Research, Moscow, Russia. 58 Also at University of Belgrade, Faculty of Physics and Vinca Institute of Nuclear Sciences, Belgrade, Serbia. 59 Also at Argonne National Laboratory, Argonne, USA. 60 Also at Erzincan University, Erzincan, Turkey. 61 Also at Mimar Sinan University, Istanbul, Istanbul, Turkey. 62 Also at KFKI Research Institute for Particle and Nuclear Physics, Budapest, Hungary. 63 Also at Kyungpook National University, Daegu, Republic of Korea. Observation of long-range, near-side angular correlations in pPb collisions at the LHC 1 Introduction 2 Experimental setup 3 Event and track selection 4 Calculation of the two-particle correlation function 5 Results 6 Conclusion Acknowledgements Open access References CMS Collaboration