Underlying Event measurements in pp collisions at $\sqrt{s}$ = 0.9 and 7 TeV with the ALICE experiment at the LHC

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  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-PH-EP-2011-20409.12.2011 Underlying Event measurements in pp collisions at √  s  =  0.9 and 7TeVwith the ALICE experiment at the LHC The ALICE Collaboration ∗ Abstract We present measurements of Underlying Event observables in pp collisions at √  s  =  0 . 9 and 7TeV.The analysis is performed as a function of the highest charged-particle transverse momentum  p T , LT  inthe event. Different regions are defined with respect to the azimuthal direction of the leading (highesttransverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collectthe fragmentation products of the hardest partonic interaction. The Transverse region is expectedto be most sensitive to the Underlying Event activity. The study is performed with charged particlesabove three different  p T  thresholds: 0.15, 0.5 and 1.0 GeV / c . In the Transverse region we observe anincrease in the multiplicity of a factor 2-3 between the lower and higher collision energies, dependingon the track   p T  threshold considered. Data are compared to P YTHIA  6.4, P YTHIA  8.1 and P HOJET .On average, all models considered underestimate the multiplicity and summed  p T  in the Transverseregion by about 10-30%. ∗ See Appendix A for the list of collaboration members   a  r   X   i  v  :   1   1   1   2 .   2   0   8   2  v   3   [   h  e  p  -  e  x   ]   1   6   A  u  g   2   0   1   2  Underlying Event measurements with ALICE 3 1 Introduction The detailed characterization of hadronic collisions is of great interest for the understanding of the under-lying physics. The production of particles can be classified according to the energy scale of the processinvolved. At high transverse momentum transfers (  p T  2GeV / c ) perturbative Quantum Chromodynam-ics (pQCD) is the appropriate theoretical framework to describe partonic interactions. This approach canbe used to quantify parton yields and correlations, whereas the transition from partons to hadrons is anon-perturbative process that has to be treated using phenomenological approaches. Moreover, the bulk of particles produced in high-energy hadronic collisions srcinate from low-momentum transfer pro-cesses. For momenta of the order of the QCD scale, O  (100 MeV), a perturbative treatment is no longerfeasible. Furthermore, at the center-of-mass energies of the Large Hadron Collider (LHC), at momentumtransfers of a few GeV/  c , the calculated QCD cross-sections for 2-to-2 parton scatterings exceed the totalhadronic cross-section [1]. This result indicates that Multiple Partonic Interactions (MPI) occur in thisregime. The overall event dynamics cannot be derived fully from first principles and must be modeledusing phenomenological calculations. Measurements at different center-of-mass energies are required totest and constrain these models.In this paper, we present an analysis of the bulk particle production in pp collisions at the LHC bymeasuring the so-called Underlying Event (UE) activity [2]. The UE is defined as the sum of all theprocesses that build up the final hadronic state in a collision excluding the hardest leading order partonicinteraction. This includes fragmentation of beam remnants, multiple parton interactions and initial-and final-state radiation (ISR/FSR) associated to each interaction. Ideally, we would like to study thecorrelation between the UE and perturbative QCD interactions by isolating the two leading partons withtopological cuts and measuring the remaining event activity as a function of the transferred momentumscale ( Q 2 ). Experimentally, one can identify the products of the hard scattering, usually the leading jet,and study the region azimuthally perpendicular to it as a function of the jet energy. Results of suchan analysis have been published by the CDF [2, 3, 4, 5] and STAR [6] collaborations for pp collisions at  √  s  =  1 . 8 and 0.2TeV, respectively. Alternatively, the energy scale is given by the leading charged-particle transverse momentum, circumventing uncertainties related to the jet reconstruction procedure atlow  p T . It is clear that this is only an approximation to the srcinal outgoing parton momentum, the exactrelation depends on the details of the fragmentation mechanism. The same strategy based on the leadingcharged particle has recently been applied by the ATLAS [7] and CMS [8] collaborations. In the present paper we consider only charged primary particles 1 , due to the limited calorimetric accep-tance of the ALICE detector systems in azimuth. Distributions are measured for particles in the pseudo-rapidity range | η | <  0 . 8 with  p T  >  p T , min , where  p T , min  =  0.15, 0.5 and 1.0GeV / c , and are studied as afunction of the leading particle transverse momentum.Many Monte Carlo (MC) generators for the simulation of pp collisions are available; see [9] for a recentreview discussing for example P YTHIA  [10], P HOJET  [11], S HERPA  [9] and H ERWIG  [12]. These pro-vide different descriptions of the UE associated with high energy hadron collisions. A general strategyis to combine a perturbative QCD treatment of the hard scattering with a phenomenological approachto soft processes. This is the case for the two models used in our analysis: P YTHIA  and P HOJET . InP YTHIA  the simulation starts with a hard LO QCD process of the type 2 → 2. Multi-jet topologies aregenerated with the parton shower formalism and hadronization is implemented through the Lund stringfragmentation model [13]. Each collision is characterized by a different impact parameter  b . Small  b values correspond to a large overlap of the two incoming hadrons and to an increased probability forMPIs. At small  p T  values color screening effects need to be taken into account. Therefore a cut-off   p T , 0 is introduced, which damps the QCD cross-section for  p T ≪  p T , 0 . This cut-off is one of the main tunablemodel parameters. 1 Primary particles are defined as prompt particles produced in the collision and their decay products (strong and electro-magnetic decays), except products of weak decays of strange particles such as  K  0 S   and Λ .  4 The ALICE CollaborationIn P YTHIA  version 6.4 [10] MPI and ISR have a common transverse momentum evolution scale (calledinterleaved evolution [14]). Version 8.1 [15] is a natural extension of version 6.4, where the FSR evolu- tion is interleaved with MPI and ISR and parton rescatterings [16] are considered. In addition initial-statepartonic fluctuations are introduced, leading to a different amount of color-screening in each event.P HOJET  is a two-component event generator, where the soft regime is described by the Dual PartonModel (DPM) [17] and the high-  p T  particle production by perturbative QCD. The transition between thetwo regimes happens at a  p T  cut-off value of 3 GeV / c . A high-energy hadronic collision is described bythe exchange of effective Pomerons. Multiple-Pomeron exchanges, required by unitarization, naturallyintroduce MPI in the model.UE observables allow one to study the interplay of the soft part of the event with particles produced inthe hard scattering and are therefore good candidates for Monte Carlo tuning. A better understanding of the processes contributing to the global event activity will help to improve the predictive power of suchmodels. Further, a good description of the UE is needed to understand backgrounds to other observables,e.g., in the reconstruction of high-  p T  jets.The paper is organized in the following way: the ALICE sub-systems used in the analysis are describedin Section 2 and the data samples in Section 3. Section 4 is dedicated to the event and track selection. Section 5 introduces the analysis strategy. In Sections 6 and 7 we focus on the data correction procedure and systematic uncertainties, respectively. Final results are presented in Section 8 and in Section 9 we draw conclusions. 2 ALICE detector Optimized for the high particle densities encountered in heavy-ion collisions, the ALICE detector is alsowell suited for the study of pp interactions. Its high granularity and particle identification capabilitiescan be exploited for precise measurements of global event properties [18, 19, 20, 21, 22, 23, 24]. The central barrel covers the polar angle range 45 ◦ − 135 ◦  ( | η | <  1) and full azimuth. It is contained in the L3solenoidal magnet which provides a nominal uniform magnetic field of 0 . 5T. In this section we describeonly the trigger and tracking detectors used in the analysis, while a detailed discussion of all ALICEsub-systems can be found in [25].The V0A and V0C counters consist of scintillators with a pseudorapidity coverage of  − 3 . 7  <  η  < − 1 . 7and 2 . 8  <  η  <  5 . 1, respectively. They are used as trigger detectors and to reject beam–gas interactions.Tracks are reconstructed combining information from the two main tracking detectors in the ALICEcentral barrel: the Inner Tracking System (ITS) and the Time Projection Chamber (TPC). The ITS is theinnermost detector of the central barrel and consists of six layers of silicon sensors. The first two layers,closely surrounding the beam pipe, are equipped with high granularity Silicon Pixel Detectors (SPD).They cover the pseudorapidity ranges  | η | <  2 . 0 and  | η | <  1 . 4 respectively. The position resolutionis 12 µ  m in  r  φ   and about 100 µ  m along the beam direction. The next two layers are composed of Silicon Drift Detectors (SDD). The SDD is an intrinsically 2-dimensional sensor. The position alongthe beam direction is measured via collection anodes and the associated resolution is about 50 µ  m. The r  φ   coordinate is given by a drift time measurement with a spatial resolution of about 60 µ  m. Due todrift field non-uniformities, which were not corrected for in the 2010 data, a systematic uncertainty of 300 µ  m is assigned to the SDD points. Finally, the two outer layers are made of double-sided Siliconmicro-Strip Detectors (SSD) with a position resolution of 20 µ  m in  r  φ   and about 800 µ  m along the beamdirection. The material budget of all six layers including support and services amounts to 7.7% of aradiation length.The main tracking device of ALICE is the Time Projection Chamber that covers the pseudorapidity rangeof about | η | <  0 . 9 for tracks traversing the maximum radius. In order to avoid border effects, the fiducial  Underlying Event measurements with ALICE 5 Collision energy: 0.9 TeVEvents % of all Offline trigger 5,515,184 100.0Reconstructed vertex 4,482,976 81.3Leading track   p T  >  0 . 15 GeV / c  4,043,580 73.3Leading track   p T  >  0 . 5 GeV / c  3,013,612 54.6Leading track   p T  >  1 . 0 GeV / c  1,281,269 23.2 Collision energy: 7 TeVEvents % of all Offline trigger 25,137,512 100.0Reconstructed vertex 22,698,200 90.3Leading track   p T  >  0 . 15 GeV / c  21,002,568 83.6Leading track   p T  >  0 . 5 GeV / c  17,159,249 68.3Leading track   p T  >  1 . 0 GeV / c  9,873,085 39.3 Table 1:  Events remaining after each event selection step. region has been restricted in this analysis to | η | <  0 . 8. The position resolution along the  r  φ   coordinatevaries from 1100 µ  m at the inner radius to 800 µ  m at the outer. The resolution along the beam axis rangesfrom 1250 µ  m to 1100 µ  m.For the evaluation of the detector performance we use events generated with the P YTHIA  6.4 [10] MonteCarlo with tune Perugia-0 [26] passed through a full detector simulation based on G EANT 3 [27]. Thesame reconstruction algorithms are used for simulated and real data. 3 Data samples The analysis uses two data sets which were taken at the center-of-mass energies of  √  s  =  0.9 and 7TeV.In May 2010, ALICE recorded about 6 million good quality minimum-bias events at √  s  =  0.9TeV. Theluminosity was of the order of 10 26 cm − 2 s − 1 and, thus, the probability for pile-up events in the samebunch crossing was negligible. The  √  s  =  7TeV sample of about 25 million events was collected inApril 2010 with a luminosity of 10 27 cm − 2 s − 1 . In this case the mean number of interactions per bunchcrossing  µ   ranges from 0.005 to 0.04. A set of high pile-up probability runs ( µ   =  0 . 2 − 2) was analysedin order to study our pile-up rejection procedure and determine its related uncertainty. Those runs areexcluded from the analysis.Corrected data are compared to three Monte Carlo models: P YTHIA  6.4 (tune Perugia-0), P YTHIA  8.1(tune 1 [15]) and P HOJET  1.12. 4 Event and track selection 4.1 Trigger and offline event selection Events are recorded if either of the three triggering systems, V0A, V0C or SPD, has a signal. The arrivaltime of particles in the V0A and V0C are used to reject beam–gas interactions that occur outside thenominal interaction region. A more detailed description of the online trigger can be found in [20]. Anadditional offline selection is made following the same criteria but considering reconstructed informationinstead of online trigger signals.For each event a reconstructed vertex is required. The vertex reconstruction procedure is based on tracksas well as signals in the SPD. Only vertices within ± 10 cm of the nominal interaction point along the
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