{'fraction_non_alphanumeric': 0.0417067076686527, 'fraction_numerical': 0.0200845665961945, 'mean_word_length': 4.237544036235531, 'pattern_counts': {'":': 0, '<': 14, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Soft probes of p+Pb and Pb+Pb collisions in the ATLAS experiment at the LHC

Collective flow and azimuthal correlation measurements of particles in the pT region ≲ 10 GeV provide valuable information in constraining the initial conditions and expansion dynamics of the system produced in heavy-ion collisions. ATLAS collaboration has recently measured correlations between flow harmonics in Pb+Pb collisions at = 2.76 TeV, using an event-shape selection procedure, and has separated the components arising from linear and non-linear hydrodynamic evolution in higher order harmonics (v4 and v5). A brief overview of these results are presented in these proceedings. Recently, azimuthal correlations extending to large pseudorapidity differences (“ridge”), similar to that seen in heavy-ion collisions, was observed in p+Pb collisions at LHC. These proceedings also present a measurement of the Fourier harmonics associated with the ridge correlations (v1 to v5) in p+Pb collisions at = 5.02 TeV by ATLAS collaboration. The results are compared with vn in Pb+Pb collisions at = 2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average pT of particles produced in the two collision systems, consistent with a recent hydrodynamic calculation.


Introduction
The particles produced in heavy-ion collisions at RHIC and LHC show azimuthal anisotropy that extend over large pseudorapidity (η) [1,2]. In AA collisions these long-range correlations are well described by a hydrodynamic expansion resulting from the anisotropic initial density distribution [3,4]. The azimuthal anisotropy in particle production is often expanded in a Fourier series, dN dφ ∼ 1 + ∞ n=1 2v n cos(n(φ − Ψ n )). The second and third harmonics in the Fourier expansion arise from a linear response to the corresponding initial eccentricities, n , i.e v n ∝ n [5]. However higher-order harmonics contain non-linear contributions from lowerorder harmonics [5,6]. Using an event-shape selection procedure, ATLAS collaboration has measured the correlations between flow harmonics in Pb+Pb collisions at √ s NN = 2.76 TeV. The correlation between higher and lower-order harmonics show non-linear correlations, consistent with expectations from hydrodynamics. The linear and non-linear components are separated from the measured correlations for different centrality intervals. A brief discussion of these results are presented in these proceedings.
Recently, azimuthal correlations extending to large pseudorapidity differences (referred to as "ridge"), similar to that observed in heavy-ion collisions, was observed in p+Pb collisions at LHC [7,8,9] and d+Au collisions at RHIC [10]. The theoretical understanding of the origin of these correlations is still a matter of debate [11,12]. ATLAS collaboration has done a detailed measurement of the first five Fourier harmonics (v 1 − v 5 ) associated with the ridge correlations in p+Pb collisions at √ s NN = 5.02 TeV. These proceedings discuss the ATLAS v n results in p+Pb collisions. A comparison between v n from p+Pb collisions and Pb+Pb collisions at √ s NN = 2.76 TeV with similar event multiplicity is also included. Reasonable agreements are observed after accounting for the difference in the average p T of particles produced in the two collision systems, consistent with a recent hydrodynamic calculation, thus suggesting a similar or perhaps common origin of these correlations in both systems.

Flow correlations in Pb+Pb collisions
Correlation between flow harmonics in heavy-ion collisions can be studied using an event-shape analysis. The event-shape selection is done using the transverse energy (E T ) distribution in the ATLAS [13] forward calorimeter (FCal) in 3.3 < |η| < 4.9. For each event, the energy distribution is expanded in a Fourier series, 2π dE T dφ = ( E T )(1 + 2 ∞ n=1 q n cos(n(φ − ψ n ))). The reduced flow vector, q n , represents the E T weighted observed flow coefficients. In each centrality class, events are classed into different ellipticity bins according to the value of the observed elliptic flow, q 2 . The v n values are then calculated for each of these q 2 classes using tracks in the Inner Detector (ID, |η| < 2.5), using a two-particle correlation method.
The left panel in Figure 1 shows the v 4 values measured in the ID, as a function of the v 2 values measured in the ID for different q 2 classes. The results for different centrality classes are shown. The observed non-linear correlation between v 4 and v 2 is found to be well described by a two component fit, motivated by hydrodynamics [5], of the form v 4 , as a function of the number of participants. The values from a similar decomposition using the results from event-plane correlation measurements [14] are also shown. Good agreement can be seen between both, suggesting that the correlations between flow magnitudes arise from the correlations between the flow angles. Right panel shows a similar decomposition for v 5 , from a fit of the form v 5 = c 2 0 + (c 1 v 2 v 3 ) 2 to the measured correlation between v 5 and v 2 . More results related to the analysis along with a detailed description of the procedure and systematics can be found elsewhere [15].

Ridge in p+Pb collisions
The long-range correlations in p+Pb are studied using two-particle correlation constructed using the charged particle tracks reconstructed in the ID. A recoil subtraction procedure, similar to that described in [9], is applied at the per-trigger yield level by subtracting the per-trigger yield above uncorrelated pairs in a "peripheral" event class − defined as having E T < 10 GeV in the FCal on the Pb-going side. The recoil subtracted distribution is then expanded in a Fourier series, and the two particle harmonic coefficients v n,n in the expansion are used to compute the single particle coefficients v n , assuming factorization: v n (p a T ) = v n,n (p a T , p b T )/ v n,n (p b T , p b T ). A detailed description of the analysis procedure and systematic uncertainties along with a complete set of results can be found in [16].
The v n values extracted from the subtracted distribution, for n = 2, 3, 4 and 5, as a function of the transverse momentum are shown in the left panel of Figure 2, for events with 220 ≤ N rec ch < 260. The v n values increase with p T in the low p T region, reach a maximum around 3 − 5 GeV and then decrease with further increase in p T , but remain positive in the measured range. This is similar to the p T dependence of v n harmonics measured in heavy-ion collisions [17]. The magnitude of v n is found to decrease with increasing harmonic number n. The second panel of Figure 2 shows the first order harmonic, v 1 , as a function of p a T . The values are obtained assuming factorization, but accounting for the sign change of v 1 at low p T . Good agreements are found between the v 1 values extracted using three different p b T ranges. The v 1 is negative at low p T , crosses zero ∼ 1.5 GeV and then increase to 0.1 in the 4 − 6 GeV range. A similar p T dependence for v 1 (with v 1 (p T ) crossing zero ∼ 1.1 GeV) was observed in Pb+Pb collisions at √ s NN = 2.76 TeV [17]. T for associated particles in the range 1 < p b T < 3 GeV and 2 < |∆η| < 5 for events with 220 ≤ N rec ch < 260. (Right) v 1 as a function of p a T for different associated p T selections for events with 220 ≤ N rec ch < 260. The error bars and shaded boxes represent the statistical and systematic uncertainties, respectively [16].
The left-panels of Figure 3 compares the v n values from p+Pb collision in the event-activity class 220 ≤ N rec ch < 260 with those from Pb+Pb collisions in 55−60% centrality from [17]. The two event classes are selected such that they have similar efficiency corrected number of tracks with p T > 0.5 GeV and |η| < 2.5. The larger v 2 values in Pb+Pb collisions can be attributed to the elliptic collision geometry of the Pb+Pb system, while the larger v 4 values could be due to the non-linear coupling between v 2 and v 4 in the collective expansion [15]. The v 3 data for Pb+Pb collisions are similar in magnitude to those in p+Pb collisions which could be expected if v 3 arises from density fluctuations in both systems. A scaling relation between the v n of p+Pb and Pb+Pb systems was proposed recently in [18]. It is argued that the v n (p T ) in p+Pb and v n (p T /K) in Pb+Pb systems should be proportional to each other at a given p T , with K = 1.25 being the ratio of mean p T in the two collision systems. The right panels of Figure 3 show the comparison between v n (p T ) in p+Pb and v n (p T /K) in Pb+Pb. The v 2 (p T /K) and v 4 (p T /K) in Pb+Pb are also scaled vertically by an empirical factor of 0.66. The two sets of values are found to agree well with each other after the scaling of p T axis.

Conclusions
Event-shape selection are used to measure correlations between flow harmonics and to separate linear and non-linear components in higher harmonics (v 4 and v 5 ). The linear and non-linear  . v 2 (top row), v 3 (middle row) and v 4 (bottom row) data compared between p+Pb collisions with 220 ≤ N rec ch < 260 in this analysis and Pb+Pb collisions in 55-60% centrality from [17]. Left column shows the original data with their statistical (error bars) and systematic uncertainties (shaded boxes). In right column, the same Pb+Pb data are rescaled horizontally by a constant factor of 1.25, and the v 2 and v 4 are also down-scaled by an empirical factor of 0.66 to match the p+Pb data [16].
components are extracted as function of centrality and show good agreement with the linear and non-linear components evaluated from event-plane correlation measurements. The first five Fourier harmonics associated with the ridge correlations in p+Pb collisions are measured. The v n values show similar p T dependence as the v n from heavy-ion collisions. The v n (p T ) values from p+Pb collisions (for n =2, 3 and 4) are compared to those from Pb+Pb collisions for event class with similar average multiplicity. After applying a scale factor of K = 1.25, that accounts for the difference of mean p T in the two collision systems, the v n (p T /K) from Pb+Pb is found to have similar shape as the v n (p T ) from p+Pb collisions. This could suggest that the long-range correlations in p+Pb and Pb+Pb systems are driven by similar dynamics.