Highlights from the ATLAS experiment

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1 Nuclear Physics A Nuclear Physics A (28) 7 XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 28) Highlights from the ALAS exeriment Iwona Grabowska-Bold on behalf of the ALAS Collaboration a, a AGH University of Science and echnology, Kraków, Poland Abstract his reort rovides an overview of the new results obtained by the ALAS Collaboration at the LHC, which were resented at the Quark Matter 28 conference. hese measurements were covered in 2 arallel talks, one flash talk and osters. In this document, a discussion of results is groued into four areas: electromagnetic interactions, quenching, quarkonia and heavy-flavour roduction, and collectivity in small and larger systems. Measurements from the xenon-xenon collisions based on a short run collected in October 27 are reorted for the first time. Keywords: ALAS exeriment, quark-gluon lasma, hoton-induced rocesses, quenching, quarkonia roduction, heavy-flavour roduction, collectivity in small systems, xenon-xenon collisions AL-PHYS-PROC July 28. Introduction In addition to roton-roton () hysics, the ALAS Collaboration [] articiates in the heavyion (HI) rogramme which has been carried out at the LHC since 2. Lead-lead (Pb+Pb) and rotonlead (+Pb) collisions were rovided at the centre-of-mass energies of 2.76 ev, 5.2 ev for Pb+Pb, and 5.2 ev for +Pb. In October 27 a short eriod with xenon-xenon (Xe+Xe) collisions was taken. his oened u an oortunity of studying imact of different geometries on a broad range of observables. Moreover, the HI rogramme is sulemented with measurements in the system, which serve as a reference to disentangle initial- from final-state effects. 2. Electromagnetic interactions in the QGP he strong electromagnetic field associated with highly boosted nuclei at the LHC can be utilised to study the scattering of the quasi-real hotons emitted coherently from the nuclei as they ass by next to each other. hese are so-called Ultra-Periheral Collisions (UPC). In the revious measurements of exlusive roduction of di-muon airs [2] and light-by-light scattering [3], ALAS already demonstrated that hoton Coyright 28 CERN for the benefit of the ALAS Collaboration. Reroduction of this article or arts of it is allowed as secified in the CC-BY-4. license.

2 2 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) 7 fluxes emerging from Pb beams are well modelled by the SARlight generator. Also in those measurements, an acolanarity distribution (α = φ π, where φ is a distance in azimuth between final-state articles) was measured and roved to be owerful in discriminating between signal and background rocesses. Very recenty ALAS has exlored a otential of observing events from exclusive roduction of γγ µ + µ contributing to a samle of inelastic minimum-bias Pb+Pb collisions [4]. After evaluating and removing the contribution from background sources, the azimuthal angle ( φ) and transverse momentum ( ) correlations between the muons are measured as a function of collision centrality. Figure shows the background-subtracted acolanarity distribution in different centrality intervals. Each distribution is normalised to unity over its measured range. he > 8% distribution with a significant contribution from UPC events is lotted in each anel for comarison. A clear, centrality-deendent broadening is seen in the acolanarity distributions when comared to the > 8% interval. he corresonding distribution from the γγ µ + µ MC samles is also shown. he MC α distributions show almost no centrality deendence, indicating that the broadening evident in the data is notably larger than that exected from detector effects. One otential source of modification is the final-state interaction of the roduced letons with the electric charges in the QGP. Assuming that the broadening of the α distribution results from transfers of a small amount of to each muon, in the % centrality interval that scale, assumed to be the RMS momentum transfer to each final-state muon in the transverse lane, is evaluated to amount to 7 ± MeV. Fig.. Background-subtracted acolanarity distribution (α) in four centrality intervals in Pb+Pb data [4]. A comarison to the SARlight calculation for γγ µ + µ is also shown. he distributions are normalised to unity over their measured range. 3. Jet quenching Using the large statistics of the 25 Pb+Pb data, ALAS finalised measurements of inclusive nuclear modification factor ( ) [5], as well as fragmentation functions [6]. hose results rovide very detailed studies of inclusive roduction as a function of, raidity (y) and centrality in comarison to the reference data collected at 5.2 ev. he evaluated as a function of for two centrality intervals % and 3 4% is resented in the left anel of Fig. 2. he value is obtained for s with y < 2. and with between 8 GeV. A clear suression of roduction in central Pb+Pb collisions relative to collisions is observed. In the -% centrality interval, is aroximately.45 at = GeV, and is observed to grow slowly (quenching decreases) with increasing, reaching a value of.6 for s with around 8 GeV. In the same figure, the values at 5.2 ev are comared with the revious measurements at s NN = 2.76 ev. he two measurements agree within their uncertainties in the overlaing region. he aarent reduction of the size of systematic uncertainties in the new measurement is ossible thans to large samles of and Pb+Pb data collected during the same LHC running eriod. Further insight in quenching can be obtained by studying fragmentation functions. he right anel of Fig. 2 resents a ratio of transverse fragmentation functions D( ) in Pb+Pb to those extracted in collisions as a function of fragment for three intervals. he R D( ) is above unity (enhancement) for low fragments, dros below unity for intermediate fragments (suression) and becomes larger than unity again for fragment around 5 GeV. here is no significant difference between R D( ) for three intervals. A comarison between the data and the hybrid model calculation is also shown.

3 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) 7 3 ALAS anti-k t R =.4 s y < ALAS y < 2. anti-k t R=.4 s - %, s NN = 2.76 ev [PRL 4 (25) 7232] - %, s NN = 5.2 ev 3-4%, s NN = 2.76 ev [PRL 4 (25) 7232] 3-4%, s NN = 5.2 ev and luminosity uncer. AA ) R D( < 2 < 36 < Hybrid Model, R res 26 < 2 < 36 < < 58 GeV < 25 GeV < 398 GeV = 3 < 58 GeV < 25 GeV < 398 GeV [GeV] - Pb+Pb, s NN = 5.2 ev,.49 nb, -% -, s = 5.2 ev, 25 b 2 [GeV] Fig. 2. (Left) Inclusive as a function of for s with y < 2. in % and 3 4% centrality intervals comared to the same quantity measured in 2.76 ev Pb+Pb collisions [5]. (Right) R D( ) ratios for three ranges: GeV (circles), 2 25 GeV (diamonds) and GeV (crosses) comared with calculations from the hybrid model with R res = 3 [6]. he model is able to describe the intermediate- and high- regions for fragments, while it fails in the low- region. he ALAS Collaboration erformed a reliminary measurement of the angular distribution of charged articles around the axis in 5.2 ev Pb+Pb and data [7]. he measured yields are defined as: R D( ) = d 2 n ch (r), () N 2πr drd where N is the total number of s, 2πrdr is the area of the annulus at a given distance r from the axis (r = η 2 + φ 2 with η and φ being the relative differences between the charged article and the axis, in seudoraidity and azimuth resectively), dr is the width of the annulus and n ch (r) is the number of charged articles within a given annulus. Results are resented as a function of Pb+Pb collision centrality, and both and charged-article in the left anel of Fig. 3. Ratios of D(, r) distributions in Pb+Pb to those measured in collisions as a function of r for six charged-article intervals sanning values between GeV in % centrality, and for between 2 25 GeV are shown. he R D(,r) is above unity for all r values for charged articles with less than 4 GeV. For these articles, R D(,r) grows with increasing r for r <.3 and is aroximately constant for.3 < r <.6. For > 4. GeV, R D(,r) is below unity and decreases with increasing r for r <.3 and is aroximately constant for.3 < r <.6. he observed behaviour inside the (r <.4) agrees with the measurement of the inclusive fragmentation functions [6], where yields of the low- fragments are observed to be enhanced and yields of charged articles with intermediate are suressed. he measured deendence of R D(,r) suggests that the energy lost by s through the quenching rocess is being transferred to articles with < 4. GeV with larger radial distances. ALAS also measures inclusive mass (m) divided by the transverse momentum [8]. his fullyunfolded measurement of the structure is sensitive to the angular and momentum correlations of the constituents. hese correlations can be used to study modifications of s in HI collisions, where they rovide comlementary information to reviously measured fragmentation functions. he right anel of Fig. 3 resents as a function of m/ in % centrality for between GeV. For all centrality bins, these values have no significant deendence on m/. hey are also observed to be consistent with the inclusive. A reliminary measurement of the balance between isolated hotons and inclusive s in in 5.2 ev Pb+Pb and data is erformed. Photons with γ > 63. GeV and η γ < 2.37 are aired inclusively with all s that have > 3.6 GeV and η < 2.8 in the event. he transverse momentum balance given by the -to-hoton ratio, x Jγ, are measured for airs with azimuthal oening angle φ > 7π/8. Distributions of the er-hoton yield (/N γ )(dn/dx Jγ ) are corrected for detector effects via a two-dimensional unfolding rocedure and reorted at the article level for the first time.

4 4 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) 7 Fig. 3. (Left) Ratios of D(, r) distributions in % Pb+Pb collisions to collisions as a function of angular distance r for of 2 to 25 GeV for six selections [9]. (Right) Jet as a function of m/ in % centrality for between GeV [8]. Figure 4 shows the measured x Jγ distribution in five centrality intervals of Pb+Pb collisions for γ = GeV in comarison to the x Jγ distribution from collisions. he x Jγ distributions in Pb+Pb collisions evolve smoothly with centrality. For eriheral collisions with centrality 5 8%, they are similar to those measured in collisions. However, in increasingly more central collisions, the distributions become systematically more modified. he x Jγ distribution in the most central % events is so strongly modified that it is monotonically decreasing over the measured x Jγ range and no eak is observed. (/N γ )(dn/dx Jγ ) (/N γ )(dn/dx Jγ ) % 3-5% ALAS Preliminary 5.2 ev, 25 b - Pb+Pb,.49 nb 2-3% -2% -% x Jγ x Jγ x J γ γ - = GeV (same each anel) Fig. 4. Photon- balance distributions (/N γ )(dn/dx Jγ ) in events (blue, reroduced on all anels) and Pb+Pb events (red) with each anel denoting a different centrality selection [7]. Pb+Pb o robe hysics of quenching in HI collisions with nuclei lighter than Pb, the transverse momentum asymmetry of di airs and roduction rates of charged articles are measured by ALAS with Xe+Xe collisions collected in October 27 []. Figure 5 resents charged-hadron as a function of for three centrality and N art intervals along with the measurement from the Pb+Pb system. he is comared between Xe+Xe and Pb+Pb data at 5.2 ev. Even though they have different centralities, the N art for the same intervals are comarable. he Xe+Xe data shows more suression than the Pb+Pb data in more central collisions, and less suression in more eriheral collisions..3 - ALAS Preliminary, 25 b η <2.5 s = 5.2 ev (extraol. to 5.44 ev) Xe+Xe, N art 5-5%, %, %, 24 - Xe+Xe, 3 µb s NN = 5.44 ev - Pb+Pb,.49 nb s NN = 5.2 ev Pb+Pb, N art 2-3%, %, %, [GeV] Fig. 5. Charged-hadron as a function of measured in Xe+Xe collisions 5.44 ev (closed markers) and in Pb+Pb collisions at 5.2 ev (oen markers) [].

5 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) Quarkonia and heavy-flavour roduction he ALAS Collaboration finalised a detailed study on romt and non-romt J/ψ and ψ(2s ) roduction and suression at high in 5.2 ev Pb+Pb and collisions []. he measurements of er-event yields, nuclear modification factors, and non-romt fractions are erformed in the di-muon decay channel for 9 < µµ < 4 GeV in di-muon transverse momentum, and 2. < y µµ < 2. in raidity. Strong suression is found in Pb+Pb collisions for both romt and non-romt J/ψ, as well as for romt and non-romt ψ(2s ), increasing with event centrality. he suression of romt ψ(2s ) is observed to be stronger than that of J/ψ, while the suression of non-romt ψ(2s ) is equal to that of the non-romt J/ψ within uncertainties, consistent with the exectation that both arise from b-quarks roagating through the medium. Desite romt and non-romt J/ψ arising from different mechanisms, the deendence of their nuclear modification factors on centrality is found to be similar. he left anel of Fig. 6 shows a -deendence of the nuclear modification factor for romt J/ψ mesons reconstructed via the muon channel at 5.2 ev in the 2% centrality bin. he is at the level of 5 at = 9 GeV and tends to increase slowly with. he ALAS measurement at high nicely comlements the ALICE results for < 2 GeV that are also shown on the same figure. Recently the ALAS Collaboration also evaluated ellitic flow of J/ψ with resect to the event lane in 5.2 ev Pb+Pb collisions and resented reliminary results as a function of transverse momentum, raidity and centrality [2]. It is observed that romt and non-romt J/ψ mesons have non-zero ellitic flow. Promt J/ψ ν 2 decreases as a function of, while non-romt J/ψ ν 2 is flat over the studied kinematical region. here is no deendence on raidity or centrality observed. he right anel of Fig. 6 shows results for the ν 2 as a function of for romt and non-romt J/ψ as measured by ALAS comared with inclusive J/ψ at < 2 GeV, as measured by ALICE at 5.2 ev, and romt J/ψ at 6.5 < < 3 GeV, by CMS at 2.76 ev. Desite different raidity selections, the ALAS data is found to be in reasonable agreement with the ALICE and CMS data in the overlaing region..4 ALAS.2.8 s NN = 5.2 ev Centrality -2% + - J/ψ µ µ ALAS, Promt, y < 2 ALICE, Inclusive, 2.5 < y < 4 Correlated systematic uncer. v 2 5 ALAS Preliminary ALAS, Promt J/ψ, 5.2 ev, y < 2, - 6% ALAS, Non-romt J/ψ, 5.2 ev, y < 2, - 6% ALICE, Inclusive J/ψ, 5.2 ev, 2.5 < y < 4, 2-4% CMS, Promt J/ψ, 2.76 ev,.6 < y < 2.4, - 6% CMS, Promt J/ψ, 2.76 ev, y < 2.4, - 6% [GeV] [GeV] Fig. 6. (Left) Comarison of romt J/ψ measured in 5.2 ev Pb+Pb collisions by ALAS with the inclusive J/ψ measured by ALICE []. (Right) ν 2 as a function of for romt and non-romt J/ψ as measured by ALAS comared with inclusive J/ψ at < 2 GeV, as measured by ALICE at 5.2 ev, and romt J/ψ at 6.5 < < 3 GeV by CMS at 2.76 ev [2]. he ALAS Collaboration also finalised a measurement of the roduction of muons from heavy-flavour decays in 2.76 ev Pb+Pb and collisions [3]. Results are rovided in the muon transverse momentum range 4 < < 4 GeV and for five centrality intervals. Backgrounds arising from in-flight ion and kaon decays, hadronic showers, and mis-reconstructed muons are statistically removed using a temlate-fitting rocedure. he heavy-flavor muon differential cross-sections and er-event yields are measured in and Pb+Pb collisions, resectively. Figure 7 resents the heavy-flavour muon as a function of. he does not deend on within the uncertainties of the measurement. he decreases between eriheral 4 6% collisions, where it is about.65, to more central collisions, reaching a value of about.35 in the % centrality interval. In Ref. [3] the azimuthal modulation of the heavy-flavor muon yields is also measured and the

6 6 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) 7 associated Fourier coefficients ν n for n=2, 3 and 4 are given as a function of and centrality. hey vary slowly with and show a systematic variation with centrality which is characteristic of other anisotroy measurements, such as that observed for inclusive hadrons. ALAS s NN = 2.76 ev ALAS s NN = 2.76 ev - Pb+Pb,.4 nb -, 57 nb -% 2-3% 4-6% - Pb+Pb,.4 nb -, 57 nb η < [GeV] -2% 3-4% η < [GeV] Fig. 7. Heavy-flavuor muon as a function of for five centrality intervals in 2.76 ev Pb+Pb collisions [3]. 5. Collectivity in small and large systems One active area of ongoing research is investigation of the nature of the long-range ridge observed in two-article correlations in small collision systems such as and +Pb. o understand the multi-article nature of the long-range collective henomenon in those systems, the ALAS Collaboration erformed a measurement of symmetric cumulants sc n,m {4} and asymmetric cumulants ac n {3} which robe four- and three-article correlations of two flow harmonics ν n and ν m in 3 ev, 5.2 ev +Pb, and 2.76 ev eriheral Pb+Pb collisions [4]. he large non-flow background from di roduction resent in the standard cumulant method is suressed using a method of subevent cumulants. Fig. 8. he N ch deendence of sc 2,3 {4} (left), sc 2,4 {4} (middle) and ac 2 {3} (right) in < < 5 GeV obtained for collisions (solid circles), +Pb collisions (oen circles) and low-multilicity Pb+Pb collisions (oen squares) [4]. Figure 8 shows a comarison of cumulants for the three collision systems. he three anels resent the results for sc 2,3 {4}, sc 2,4 {4}, and ac 2 {3} for charged articles with.3 < < 3 GeV. hese results indicate a negative correlation between ν 2 and ν 3 and a ositive correlation between ν 2 and ν 4. Such correlation atterns have reviously been observed in large collision systems, but are now confirmed also in the small collision systems, once non-flow effects are adequately removed in the measurements. In the multilicity range covered by the collisions, N ch < 5, the results for symmetric cumulants sc 2,3 {4} and sc 2,4 {4} are comarable among the three systems. In the range N ch > 5, sc 2,3 {4} and sc 2,4 {4} are larger in Pb+Pb than in +Pb collisions. he results for ac 2 {3} are similar among the three systems at N ch <, but they deviate from each other at higher N ch. he results for data are aroximately constant or decrease slightly with N ch, while the +Pb and Pb+Pb data shows significant increases as a function of N ch. he similarity between different collision systems and the weak deendence of these observables on the range and N ch, largely free from non-flow effects, rovide an imortant inut for understanding the sace-time dynamics and the roerties of the medium created in small collision systems.

7 I. Grabowska-Bold for the ALAS Collaboration / Nuclear Physics A (28) 7 7 Using a data set of Xe+Xe collisions collected at 5.44 ev, ALAS measured flow harmonics with the scalar method (SP) and correlation techniques involving 2, 4 and 6 articles [5]. Centrality and -deendence of the ν n are studied. Figure 9 shows the ν n harmonics integrated over the < < 5 GeV range. he values are comared to those obtained for Pb+Pb and are shown as a function of centrality. he small differences are related to differences in the initial-collision geometry and subtle differences due to the system size. Detailed studies of scaling of ν n and cumulants with N art confirm the main source of ν 2 to be the initial geometry, while geometry fluctuations to be the origin of differences for ν n with n > 2. v n {SP}. ALAS Preliminary < <5 GeV - Xe+Xe s NN =5.44 ev, 3µb η <2.5 - Pb+Pb s NN =5.2 ev, 5µb v2 v3 v v4 5 Solid: Pb+Pb Oen: Xe+Xe Centrality [%] Fig. 9. he ν n n=2-5 measured with the SP method in Xe+Xe and Pb+Pb collisions as a function of centrality ercentile [5]. he Pb+Pb data oints are shifted along the centrality axis, for clarity. ALAS also measured the modified Pearson s correlation coefficient to quantify correlations between flow coefficients and mean of charged articles in the event using 5.2 ev Pb+Pb data [6]. It can be used in further exerimental studies to understand the underlying mechanism of QGP dynamics and constrain theoretical models attemting to describe them. 6. Summary he ALAS Collaboration resented many new results covering Pb+Pb, +Pb,, and also data from the new Xe+Xe system collected for the first time at the LHC. hese measurements rovide new information on electromagnetic interactions, the quenching, quarkonia and heavy-flavour suression, as well as comrehensive results which rovide further insight into the collectivity henomenon of small collision systems. his work was suorted in art by Polish National Science Centre grant DEC-26/23/B/S2/49, by the AGH US statutory tasks No...22./4 within subsidy of the Ministry of Science and Higher Education, and by PL-Grid Infrastructure. References [] ALAS Collaboration, JINS 3 (28) S83. doi:.88/748-22/3/8/s83. [2] ALAS Collaboration, htt://cdsweb.cern.ch/record/257689, ALAS-CONF [3] ALAS Collaboration, arxiv: [he-ex], Nature Physics 3 (27) 852. [4] ALAS Collaboration, arxiv: [nucl-ex], submitted to PRL. [5] ALAS Collaboration, arxiv: [nucl-ex], submitted to PLB. [6] ALAS Collaboration, arxiv: [nucl-ex], submitted to PRC. [7] ALAS Collaboration, htt://cdsweb.cern.ch/record/238869, ALAS-CONF-28-. [8] ALAS Collaboration, htt://cdsweb.cern.ch/record/239867, ALAS-CONF [9] ALAS Collaboration, htt://cdsweb.cern.ch/record/238868, ALAS-CONF [] ALAS Collaboration, htt://cdsweb.cern.ch/record/238588, ALAS-CONF [] ALAS Collaboration, arxiv: [nucl-ex], submitted to EPJC. [2] ALAS Collaboration, htt://cdsweb.cern.ch/record/239788, ALAS-CONF [3] ALAS Collaboration, arxiv: [nucl-ex], submitted to PRC. [4] ALAS Collaboration, arxiv:87.22 [nucl-ex], submitted to PLB. [5] ALAS Collaboration, htt://cdsweb.cern.ch/record/23887, ALAS-CONF-28-. [6] ALAS Collaboration, htt://cdsweb.cern.ch/record/238589, ALAS-CONF-28-8.