00:00:01,290 --> 00:00:02,790 Yes, hello, can you see my slides? 2 00:00:02,850 --> 00:00:05,820 Yes, I can see them and I can hear you very well. 3 00:00:07,649 --> 00:00:12,719 It's full screen, yes, please start. Right. Hi, everyone. 4 00:00:12,990 --> 00:00:16,470 It's my pleasure to talk about the recent results on collectivity in small 5 00:00:16,470 --> 00:00:22,320 systems on behalf of the ALICE Collaboration. Let me start from a short reminder on 6 00:00:22,470 --> 00:00:26,610 collective effects. So as you know, the main goal of ALICE experiment is to study 7 00:00:26,610 --> 00:00:31,560 the properties of quark-gluon plasma, or QGP, which is created in heavy-ion collisions. 8 00:00:31,590 --> 00:00:37,080 And so far we've been quite successful in describing the QGP evolution by 9 00:00:37,080 --> 00:00:40,830 hydrodynamics, according to which, as you can see in the sketch on the right, 10 00:00:40,830 --> 00:00:45,660 QGP expansion is driven by pressure gradients that push matter outwards with 11 00:00:45,660 --> 00:00:50,460 collective transverse velocity that we call radial flow. And this is the first effect 12 00:00:50,460 --> 00:00:55,080 that we distinguish when talking about collective effects. Now, the second 13 00:00:55,080 --> 00:01:00,750 one, anisotropic flow, is caused by the anisotropy in the initial geometry of the collision, 14 00:01:01,110 --> 00:01:06,150 which is transformed via low-viscosity QGP into the anisotropy in the 15 00:01:06,150 --> 00:01:10,290 momentum distribution of the final state particles. So these collective effects 16 00:01:10,290 --> 00:01:15,540 are reflected in the measurements of our observables. For example, in the angular 17 00:01:15,540 --> 00:01:19,800 correlation function of the particle pairs that you can see here for lead-lead collisions, 18 00:01:20,070 --> 00:01:24,930 where we observe a famous near-side ridge structure as well as the modulation on 19 00:01:24,930 --> 00:01:30,000 the away side that are attributed to the anisotropic flow. Now, we've been also looking 20 00:01:30,000 --> 00:01:34,260 into the small systems, proton-nucleus and proton-proton collisions, where QGP was 21 00:01:34,260 --> 00:01:38,760 not expected to form, in order to disentangle the cold nuclear matter 22 00:01:38,760 --> 00:01:43,680 effects from QGP-related effects. However, recent observations of collectivity 23 00:01:43,680 --> 00:01:47,640 signs also in high-multiplicity pp and p-Pb collisions led to the change of 24 00:01:47,640 --> 00:01:53,820 this paradigm and raised new questions for us, such as are these observations a 25 00:01:53,820 --> 00:01:57,750 manifestation of collectivity, where the working experimental definition of the 26 00:01:57,750 --> 00:02:01,020 collectivity that we use is "long-range multi-particle correlations", and I will 27 00:02:01,020 --> 00:02:05,730 come back to this later in my slides. If yes, what is the origin of these effects 28 00:02:05,730 --> 00:02:10,890 in small systems and down to which multiplicity do we see it. So, so far we 29 00:02:10,890 --> 00:02:14,520 have few theoretical approaches that try to explain where these effects can come 30 00:02:14,520 --> 00:02:19,320 from, first of them being based on initial state effects, and arguing that collectivity 31 00:02:19,320 --> 00:02:22,980 is present already in the very early stages of the collisions and is 32 00:02:22,980 --> 00:02:27,360 given by initial momentum correlations, at nucleonic or sub-nucleonic level. 33 00:02:28,110 --> 00:02:33,780 The second one is based on the final state effects and states that the collectivity 34 00:02:33,780 --> 00:02:37,710 builds up later in the collision and is given either by the QGP formation or some 35 00:02:37,710 --> 00:02:42,900 other microscopic mechanisms such as string recombination. There are also 36 00:02:42,900 --> 00:02:48,480 hybrid models that combine both of these stages. But so far we don't have a clear 37 00:02:48,480 --> 00:02:54,840 answer on what kind of combination can give a full description of our data in 38 00:02:54,840 --> 00:02:59,250 small systems. So, as experimentalists, we can provide more measurements to constrain 39 00:02:59,250 --> 00:03:05,970 these models and in this talk, I will cover the recent results from ALICE that can 40 00:03:05,970 --> 00:03:10,800 help in this, first focusing on observables sensitive to the anisotropic flow and 41 00:03:10,800 --> 00:03:15,480 then moving to the ones sensitive to the radial flow. So, let me first continue with 42 00:03:15,480 --> 00:03:19,830 the studies of the ridge. Just to remind you, usually we investigate particle 43 00:03:19,830 --> 00:03:24,690 correlations at mid-rapidity using central barrel detectors of ALICE, TPC and ITS. 44 00:03:25,170 --> 00:03:30,060 However, in one of the recent analyses, we also looked at the forward and backward 45 00:03:30,120 --> 00:03:35,940 ranges of the rapidity using forward multiplicity detector FMD. And this way, 46 00:03:36,150 --> 00:03:41,820 one can explore the ridge evolution up to high delta eta values around eight, which is a 47 00:03:41,820 --> 00:03:47,040 unique measurement at the LHC. Now in the bottom plots, you can see the results for 48 00:03:47,040 --> 00:03:50,490 high-multiplicity p-Pb collisions on the left and high-multiplicity pp 49 00:03:50,520 --> 00:03:55,140 collisions on the right, that show the per-trigger yield distributions corrected 50 00:03:55,140 --> 00:04:01,320 for non-flow in both cases, given by FMD-FMD correlations. And, as you can see, the 51 00:04:01,320 --> 00:04:07,440 ridge structure extends up to eight in delta eta in p-Pb case and six in pp 52 00:04:07,440 --> 00:04:12,060 collisions. In order to get more of a quantitative feeling of these results, we 53 00:04:12,060 --> 00:04:17,850 can also extract v2 coefficients as a function of eta, which you can see in 54 00:04:17,850 --> 00:04:24,030 this plot for p-Pb collisions, for four multiplicity classes from zero to five and 55 00:04:24,030 --> 00:04:28,380 up to 20 to 40%, where the data are compared to predictions from AMPT 56 00:04:28,380 --> 00:04:33,960 model with string melting. So, as you can see, we observe an asymmetry in the data, 57 00:04:34,410 --> 00:04:38,190 which is similar to the asymmetry in the charged particle multiplicity distribution 58 00:04:38,190 --> 00:04:43,260 as a function of eta, which is expected due to the asymmetric system. And this 59 00:04:43,440 --> 00:04:48,540 asymmetry seems to be quite well reproduced by AMPT while the multiplicity dependence 60 00:04:48,540 --> 00:04:52,980 that is seen in the results seems to be not that well described by the model. 61 00:04:54,329 --> 00:04:58,319 In this plot, you can see the similar result extracted very recently for pp 62 00:04:58,379 --> 00:05:03,119 collisions as well, in very high multiplicity and with these three curves 63 00:05:03,119 --> 00:05:07,319 you can see the results that correspond to the different non-flow subtraction 64 00:05:07,319 --> 00:05:13,709 methods. But as you can see, the trend remains the same for all of them. Now, to 65 00:05:13,709 --> 00:05:19,349 continue with the latest results on flow coefficients, last year ALICE has published 66 00:05:19,349 --> 00:05:23,489 quite a comprehensive set of measurements not only of v2, but also higher flow 67 00:05:23,489 --> 00:05:28,019 harmonics and in this plot, you can see the results for v2, v3 and v4 obtained 68 00:05:28,019 --> 00:05:31,649 with two-particle cumulants as a function of multiplicity compared in different 69 00:05:31,649 --> 00:05:39,449 systems, Pb-Pb, Xe-Xe, p-Pb and pp collisions and also to predictions from hydro model with 70 00:05:39,449 --> 00:05:45,869 initial state for different systems and PYTHIA8 predictions for pp collisions. Now, the fact 71 00:05:45,869 --> 00:05:51,719 that the data are obtained with the separation in eta means that we observe 72 00:05:51,749 --> 00:05:56,759 long-range two-particle correlations in these results, and this covers the first 73 00:05:56,789 --> 00:06:01,409 part of the definition of the collectivity that I mentioned in first slide. I will 74 00:06:01,409 --> 00:06:05,939 come to the second part in a second. Let me first focus on a few important points 75 00:06:05,939 --> 00:06:11,009 from this plot. So as you can see, we observe quite a pronounced multiplicity 76 00:06:11,009 --> 00:06:16,409 dependence in all vn results for large systems, Pb-Pb and Xe-Xe, which is 77 00:06:16,409 --> 00:06:21,239 however not seen in the small systems, but it's interesting to notice that the 78 00:06:21,239 --> 00:06:28,469 values of all vn are compatible in small and large systems at low multiplicities. 79 00:06:28,979 --> 00:06:33,719 It's also interesting to see that the pp data cannot be described solely by non- 80 00:06:33,719 --> 00:06:38,999 flow effects, which are dominating in PYTHIA8, but it cannot be either described by 81 00:06:38,999 --> 00:06:44,129 the hydro model which does, however, quite a good job for the Pb-Pb and Xe-Xe and is also 82 00:06:44,129 --> 00:06:51,539 able to get qualitatively the data in p-Pb collisions. So, so far, we don't 83 00:06:51,539 --> 00:06:56,249 have a good description of the pp data from the theory side for these results. 84 00:06:56,789 --> 00:07:00,899 Now, coming back to the definition of the collectivity, in order to get this second 85 00:07:01,169 --> 00:07:05,519 part, multi-particle correlations, one can look at the results with multi-particle 86 00:07:05,519 --> 00:07:11,219 cumulants and here you can see v2 results with four, six and eight particle 87 00:07:11,219 --> 00:07:15,659 cumulants. Again, for different systems as a function of multiplicity. What is 88 00:07:15,659 --> 00:07:21,269 important here is that we were able to observe real value of v2{4} for the 89 00:07:21,269 --> 00:07:27,629 first time in pp collisions, which was possible to extract with this correct 90 00:07:27,659 --> 00:07:32,309 non-flow subtraction using the three- eta subevent method. And we also see 91 00:07:32,309 --> 00:07:37,439 non-zero values not only of v2{4}, but also be v2{6} in both pp and p-Pb 92 00:07:37,439 --> 00:07:41,069 collisions in these results, that allows to confirm that we observe not only a 93 00:07:41,069 --> 00:07:45,749 long-range, but also multi-particle correlations. So indeed, this confirms the 94 00:07:45,749 --> 00:07:50,159 observation of collectivity in small systems. However, it's hard to conclude if 95 00:07:50,159 --> 00:07:55,289 the origin of these correlations is the same as in the hevay-ion collisions based only 96 00:07:55,289 --> 00:08:00,419 on vn measurements still. So one can also look at other observables that are 97 00:08:00,419 --> 00:08:05,969 sensitive to the radial flow, one of them being the balance function, which is given 98 00:08:05,969 --> 00:08:09,479 as the combination of charge-dependent per-trigger yields as you can see here, 99 00:08:09,839 --> 00:08:14,789 where the definition of the per- trigger yield can be seen on the 100 00:08:14,789 --> 00:08:19,889 right. So, as you can see, the like-sign part of the correlation is subtracted, 101 00:08:19,949 --> 00:08:25,409 which allows to keep only the charge- dependent part and this means that anisotropic 102 00:08:25,409 --> 00:08:30,539 flow effects are removed, but radial flow effects are still present. On 103 00:08:30,539 --> 00:08:34,649 the right, you can see the example of the two-dimensional balance function distribution 104 00:08:34,769 --> 00:08:40,049 for Pb-Pb collisions in most central class. What we do is we project this on Delta 105 00:08:40,049 --> 00:08:44,999 eta and delta phi and we look at the one- dimensional projections as a function of, 106 00:08:45,929 --> 00:08:51,959 in different centrality or multiplicity classes, and we extract the width of these 107 00:08:51,989 --> 00:08:57,929 projections as a function of multiplicity, so, sigma delta eta and sigma delta phi. Now, 108 00:08:57,929 --> 00:09:01,349 for the purpose of this talk, I will focus only on Sigma Delta eta. 109 00:09:02,700 --> 00:09:07,620 You can see the result of this observable plotted as a function of multiplicity for 110 00:09:07,620 --> 00:09:15,390 Pb-Pb collisions here for charged particles in low-pT range. And what we observe is that we 111 00:09:15,390 --> 00:09:19,590 see a narrowing of the balance function width with increasing multiplicity, which 112 00:09:19,590 --> 00:09:24,690 was mainly attributed to the larger radial flow in central collisions with respect 113 00:09:24,690 --> 00:09:29,640 to peripheral collisions in the Run1 analysis. Now, interestingly, we also 114 00:09:29,640 --> 00:09:37,410 observed the same (qualitatively) behavior in small systems, pp and p-Pb collisions, which 115 00:09:37,410 --> 00:09:42,660 is consistent with the idea of collective phenomena present in these systems as well. 116 00:09:43,650 --> 00:09:48,000 The data in pp collisions were also compared to predictions from PYTHIA8 with 117 00:09:48,000 --> 00:09:52,770 and without color connection. And keep in mind that in this plot, on the x axis, 118 00:09:52,770 --> 00:09:57,210 we see the multiplicity class instead of the multiplicity so the trend is reversed. 119 00:09:57,690 --> 00:10:04,320 Now, you can see here that the tune with color connection is able to get the 120 00:10:04,320 --> 00:10:09,450 trend in the data while the tune without color connection fails to do so. And in 121 00:10:09,450 --> 00:10:14,970 the new analysis these results (this measurement) is extended to the level of 122 00:10:14,970 --> 00:10:20,280 identified hadrons where we expect to see more pronounced narrowing for heavier 123 00:10:20,280 --> 00:10:24,810 particles, if this narrowing is driven by collective phenomena in small systems, due 124 00:10:24,810 --> 00:10:28,770 to the more significant response of the heavier particles to the boost from 125 00:10:28,770 --> 00:10:33,060 collective effects. In this plot, you can see the results, this time for pions and 126 00:10:33,060 --> 00:10:38,400 protons, compared to predictions from PYTHIA8 with and without color 127 00:10:38,400 --> 00:10:43,710 reconnection again, in pp collisions. And it's interesting to notice that the model 128 00:10:43,710 --> 00:10:47,970 with color connection indeed predicts more pronounced narrowing for protons 129 00:10:48,030 --> 00:10:54,480 than for pions, while in the data we see a different trend: while pions show clear 130 00:10:54,480 --> 00:10:59,460 narrowing with increasing multiplicity, protons rather stay flat and this 131 00:10:59,460 --> 00:11:03,780 suggests that there might be some other effects playing a role apart from 132 00:11:03,930 --> 00:11:09,660 collective phenomena. And we can also say that for this particular observable, the 133 00:11:09,660 --> 00:11:13,920 results disfavor color reconnection mechanism implemented in PYTHIA8 at the 134 00:11:13,920 --> 00:11:19,230 level of identified hadrons. Another observable that one can look into 135 00:11:19,230 --> 00:11:23,100 is the pT spectra of identified hadrons. In this plot, you can see the 136 00:11:23,100 --> 00:11:29,010 latest result for p-Pb collisions at 8 TeV for pions, kaons and protons, and these 137 00:11:29,010 --> 00:11:32,790 results, of course, complement the previous studies that ALICE has done for 138 00:11:33,180 --> 00:11:38,910 p-Pb at lower energies and also for pp collisions. What we observe is the behavior that 139 00:11:38,910 --> 00:11:45,060 is qualitatively similar to the one well known from Pb-Pb collisions, where it's 140 00:11:45,060 --> 00:11:50,130 driven by the radial flow. So basically the hardening of pT spectra at higher 141 00:11:50,130 --> 00:11:55,470 multiplicity with respect to lower multiplicity class, which is more 142 00:11:55,470 --> 00:12:00,450 pronounced for heavier particles seen by this more significant flattening at low 143 00:12:00,450 --> 00:12:07,620 pT. And, in order to study better the multiplicity dependence of these results 144 00:12:07,620 --> 00:12:12,420 it's more convenient to use ratios of the particle yields, in particular baryon over 145 00:12:12,420 --> 00:12:17,130 meson ratio. Last year, we published a comprehensive set of measurements for 146 00:12:17,370 --> 00:12:22,890 these ratios in light flavor sector. Here you can see the results (the example) of 147 00:12:22,890 --> 00:12:27,450 the proton over pion ratio compared in small systems and Pb-Pb collisions in two 148 00:12:27,450 --> 00:12:34,020 multiplicity classes of zero to five and 60 to 80%. And if you look at the Pb-Pb results, 149 00:12:34,020 --> 00:12:39,000 you can see an enhancement at intermediate pT which is more pronounced 150 00:12:39,030 --> 00:12:44,130 for central collisions and this is again consistent with the radial flow idea, but 151 00:12:44,130 --> 00:12:48,630 also with quark coalescence at hadronization. And the origin of this 152 00:12:48,630 --> 00:12:56,130 enhancement is still debated...(Alright, thank you.) But it's important to notice that we see 153 00:12:56,370 --> 00:13:01,470 a striking similarity in the trend in these ratios also in the small systems. However, 154 00:13:01,470 --> 00:13:05,550 since these multiplicity classes correspond to very different multiplicities in these 155 00:13:05,550 --> 00:13:11,400 systems, we can compare the values of these ratios and the same multiplicity in 156 00:13:11,400 --> 00:13:16,050 specific pT bins and this is what is done in the bottom plot. You see the ratio 157 00:13:16,050 --> 00:13:20,640 plotted as a function of multiplicity for the low pT range on the left, 158 00:13:20,670 --> 00:13:26,640 intermediate pT range in the middle and high pT range on the right. So, what we 159 00:13:26,640 --> 00:13:31,230 see is the smooth multiplicity evolution from pp up to Pb-Pb collisions of 160 00:13:31,230 --> 00:13:38,160 these ratio seen in all pT ranges and these results suggest common mechanism 161 00:13:38,160 --> 00:13:45,630 driving the multiplicity dependence. And it also further supports the idea of collective 162 00:13:45,630 --> 00:13:50,520 effects present in small systems. We can also check what models have to say about 163 00:13:50,520 --> 00:13:52,800 that: you can see the same ratio 164 00:13:54,210 --> 00:13:58,890 here in the low pT and intermediate pT for pp collisions compared to predictions from 165 00:13:58,890 --> 00:14:03,480 different models. It's easy to see that PYTHIA8 for example is successful in 166 00:14:03,480 --> 00:14:08,040 describing the qualitative features of these data only if color connection is 167 00:14:08,040 --> 00:14:14,250 enabled. HERWIG is not able to reproduce the trend, but this model is being 168 00:14:14,250 --> 00:14:19,380 improved. DIPSY is able to get only the qualitative trend, but not the absolute 169 00:14:19,380 --> 00:14:25,380 values and EPOS-LHC getst the data in low pT, but overestimates it in the intermediate 170 00:14:25,440 --> 00:14:30,270 pT. So these results and also the previous results that I showed in the 171 00:14:30,270 --> 00:14:35,760 slides before provide an important input for improving our models. And since this 172 00:14:35,760 --> 00:14:41,190 brings me almost to the end of my talk, I would like to mention the talk of Livio that you 173 00:14:41,190 --> 00:14:47,070 can check for more interesting details and results on a similar topic. And in the 174 00:14:47,070 --> 00:14:52,470 last few seconds I would like to flesh also the recent results on these ratios in 175 00:14:52,470 --> 00:14:57,270 heavy flavor sector. You can see a lambda_c over D0 ratio plotted on top of lambda 176 00:14:57,270 --> 00:15:03,120 over K0S here for pp collisions for low and high multiplicity classes, and 177 00:15:03,120 --> 00:15:09,930 what is seen here is that we observe a similar trend not only in light flavor 178 00:15:10,320 --> 00:15:17,610 sector but also in heavy flavor sector seen, actually, to be very compatible between 179 00:15:17,610 --> 00:15:20,880 each other at the same multiplicities, and this is a very interesting observation, 180 00:15:21,450 --> 00:15:26,970 but for more details on that, such as the comparison to the models, for example, and 181 00:15:26,970 --> 00:15:32,070 interpretation of this results, I refer you to the previous plenary and parallel 182 00:15:32,070 --> 00:15:38,160 talks in a hard probes section. And this brings me to my conclusions. We see a 183 00:15:38,160 --> 00:15:41,970 similar behavior attributed to collectivity in heavy-ion collisions in many 184 00:15:41,970 --> 00:15:46,230 observables in small systems, which persists down to very low multiplicities. 185 00:15:46,230 --> 00:15:50,910 But most of the times, existing models do not fully reproduce our data in small 186 00:15:50,910 --> 00:15:55,770 systems. So it seems that understanding the origin of collectivity there still 187 00:15:55,770 --> 00:15:58,260 remains a challenging task. Thank you very much. 188 00:15:59,490 --> 00:16:05,100 Thank you Zhanna for this very nice presentation. So now we have time for 189 00:16:05,100 --> 00:16:11,730 discussion. if people have questions, please push, rise hand button. 190 00:16:22,050 --> 00:16:24,390 Okay. I don't see any 191 00:16:27,780 --> 00:16:30,720 slack. Zhanna. Yes. 192 00:16:31,320 --> 00:16:37,200 Could you explain what what this color reconnectivity. Color reconnection 193 00:16:37,230 --> 00:16:42,990 you mean? Yes. Yeah, so this effect basically resembles the radial flow at 194 00:16:42,990 --> 00:16:48,120 microscopic level, it's based on Lund string fragmentation model. So it's basically 195 00:16:48,120 --> 00:16:54,870 strings combining together. and then, since this is somehow connected to the 196 00:16:54,900 --> 00:17:00,000 amount of MPI (due to the fact that there is more MPI's in central 197 00:17:00,000 --> 00:17:04,500 collisions, this leads also to the higher tension between the strings. And in the 198 00:17:04,500 --> 00:17:10,680 end, this will kind of resemble the radial flow mechanism. So you will have more 199 00:17:10,680 --> 00:17:14,880 correlation in the central collisions with respect to peripheral ones, for example, 200 00:17:14,970 --> 00:17:21,000 I'm talking now about the particular example of this balance function results 201 00:17:21,000 --> 00:17:22,050 that I showed here. 202 00:17:22,830 --> 00:17:24,900 Okay, okay. That's that's really helpful. Thank you. 203 00:17:29,280 --> 00:17:35,670 So, given that we are on this slide, I was wondering for the data point points, they 204 00:17:35,670 --> 00:17:40,140 are basically in terms of the precision. This is driven by systematic 205 00:17:40,140 --> 00:17:45,420 uncertainties, right. So what is the main contribution to the systematic uncertainty 206 00:17:45,420 --> 00:17:52,410 on this data points? Let me think. 207 00:17:56,490 --> 00:18:01,470 I'm not sure, now I don't remember this off the top of my head, what was the main 208 00:18:01,470 --> 00:18:06,360 contribution here, because this is the previous analysis that we have done, but 209 00:18:06,360 --> 00:18:09,390 I check this later and let you know maybe offline. 210 00:18:09,660 --> 00:18:16,470 Yeah, I think that's reasonable. Okay. Thank you. I don't see any other 211 00:18:16,470 --> 00:18:23,580 questions. So thank you Zhanna. So now we will switch to a discussion of small 212 00:18:23,580 --> 00:18:27,000 systems from CMS by Miko Miko Ztext/plainUUTF-8_ahttp://mediaarchive.cern.ch/MediaArchive/Video/Public/Conferences/2020/856696c98/856696c98_en.srt   ( ? Q g … ” •`*`5`;  `Ÿ