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Measurements of anisotropic flow coefficients ($v_n$) and their cross-correlations using two- and multi-particle cumulant methods have been reported in collisions of pp at $\sqrt{s} = 13$ TeV, p-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV, Xe-Xe at $\sqrt{s_{_{\rm NN}}} = 5.44$ TeV, and Pb-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV recorded with the ALICE detector. The measurements were conducted as a function of multiplicity in the mid-rapidity region $|\eta|<0.8$, focusing on the transverse momentum range $0.2 < p_{\rm T} < 3.0$ GeV/$c$. An ordering of the coefficients $v_2 > v_3 > v_4$ was observed in pp and p-Pb collisions, resembling patterns seen in larger collision systems. Notably, a weak multiplicity dependence of $v_2$ was noted relative to nucleus-nucleus collisions within the same multiplicity range. Utilizing a novel subevent method, it was found that $v_{2}$ measured in pp and p-Pb collisions with four-particle cumulants aligned well with results from six-particle cumulants. Moreover, symmetric cumulants $SC(m,n)$ calculated using the subevent method provided insights into the correlation strength between $v_n^2$ and $v_m^2$. The data presented further supported the existence of long-range multi-particle azimuthal correlations in high multiplicity pp and p-Pb collisions. These findings could not be fully explained by existing model calculations such as PYTHIA8 or IP-Glasma+MUSIC+UrQMD models, offering new perspectives on understanding collective effects in small collision systems. This study sheds light on the intricate dynamics of particle interactions in different collision scenarios at varying energies, contributing valuable information to the field of high-energy nuclear physics research.
Summary- Scientists studied how things move in different types of collisions by looking at special numbers called $v_n$.
- They found that in some collisions, the numbers follow an order: $v_2 is bigger than v_3 is bigger than v_4.
- The way things move didn't change much when there were more or fewer particles involved in certain collisions.
- A new way of looking at the data helped scientists compare results from different methods for $v_{2}$.
- The information collected shows that there are connections between many particles moving together in certain types of collisions.
Definitions
- Measurements: Finding out how big or small something is.
- Anisotropic: Not the same in all directions; having a preferred direction.
- Coefficients: Special numbers used to calculate things.
- Cumulant methods: Techniques for analyzing data involving multiple measurements.
- Collisions: When two things crash into each other.
Introduction
The study of high-energy nuclear collisions has been a major focus in the field of particle physics for several decades. These collisions, which involve smashing together particles at incredibly high speeds, provide insights into the fundamental properties of matter and the strong force that binds it together. In recent years, there has been a growing interest in studying small collision systems such as proton-proton (pp) and proton-nucleus (p-Pb) collisions to better understand the collective behavior of particles in these systems.
One important aspect of these studies is the measurement of anisotropic flow coefficients ($v_n$), which describe the azimuthal distribution of particles produced in a collision. These coefficients can provide information about the initial state and subsequent evolution of the system created in a collision. In this blog article, we will discuss a research paper titled "Measurements of anisotropic flow coefficients and their cross-correlations using two- and multi-particle cumulant methods in pp, p-Pb, Xe-Xe, and Pb-Pb collisions at $\sqrt{s} = 13$, 5.02, 5.44, and 5.02 TeV with ALICE" published by The ALICE Collaboration.
The Experiment
The research was conducted using data collected by ALICE (A Large Ion Collider Experiment), one of four main detectors at CERN's Large Hadron Collider (LHC). The LHC is currently the world's largest and most powerful particle accelerator, capable of colliding protons or heavy ions at energies up to 14 TeV.
The measurements were performed on four different collision systems: pp at $\sqrt{s} = 13$ TeV, p-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV, Xe-Xe at $\sqrt{s_{_{\rm NN}}} = 5.44$ TeV, and Pb-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV. These systems were chosen to cover a wide range of collision energies and sizes, providing valuable insights into the collective behavior of particles in different scenarios.
The Measurements
The researchers focused on measuring $v_n$ coefficients as a function of multiplicity (number of produced particles) in the mid-rapidity region ($|\eta|<0.8$). They also studied the transverse momentum range $0.2 < p_{\rm T} < 3.0$ GeV/$c$, which is where most of the particles are produced in these collisions.
One interesting observation was that the ordering of $v_n$ coefficients (with n=2,3,4) was similar in all four collision systems: $v_2 > v_3 > v_4$. This pattern is known as "elliptic flow" and has been previously observed in larger collision systems such as gold-gold collisions at RHIC (Relativistic Heavy Ion Collider).
Another important finding was that there was a weak dependence of $v_2$ on multiplicity in pp and p-Pb collisions compared to nucleus-nucleus collisions within the same multiplicity range. This suggests that collective effects may be present even in small collision systems.
The Subevent Method
To further investigate these results, the researchers used a novel subevent method to measure $v_n$. This method involves dividing each event into two subevents based on their azimuthal angle with respect to the reaction plane (a plane defined by the beam direction and impact parameter). By using this method, they were able to compare results from four-particle cumulants with those from six-particle cumulants.
The data showed good agreement between these two methods, indicating that the subevent method is a reliable way to measure $v_n$ coefficients in small collision systems. Additionally, the researchers used symmetric cumulants ($SC(m,n)$) to study the correlation strength between different $v_n$ coefficients. The results showed strong correlations between $v_2^2$ and $v_4^2$, providing further evidence for long-range multi-particle azimuthal correlations in high multiplicity pp and p-Pb collisions.
Implications and Future Directions
The measurements presented in this research paper offer valuable insights into the collective behavior of particles in small collision systems. These findings cannot be fully explained by existing models such as PYTHIA8 or IP-Glasma+MUSIC+UrQMD, highlighting the need for further studies to better understand these phenomena.
One possible direction for future research is to investigate how these collective effects evolve with increasing energy or system size. This could provide a deeper understanding of the underlying mechanisms responsible for particle interactions in nuclear collisions.
In conclusion, this study contributes significant new information to our understanding of anisotropic flow coefficients and their cross-correlations in small collision systems. It also highlights the importance of continued research in this area to unravel the mysteries of high-energy nuclear collisions.
Conclusion
In summary, this blog article discussed a recent research paper published by The ALICE Collaboration on measurements of anisotropic flow coefficients and their cross-correlations using two- and multi-particle cumulant methods in various collision systems at different energies. The data presented shed light on the intricate dynamics of particle interactions in different scenarios, offering new perspectives on collective effects in small collision systems. These findings have implications for our understanding of fundamental properties of matter and can guide future studies towards a more comprehensive understanding of high-energy nuclear collisions.
Measurements of anisotropic flow coefficients ($v_n$) and their cross-correlations using two- and multi-particle cumulant methods are reported in collisions of pp at $\sqrt{s} = 13$ TeV, p-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV, Xe-Xe at $\sqrt{s_{_{\rm NN}}} = 5.44$ TeV, and Pb-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV recorded with the ALICE detector. These measurements are performed as a function of multiplicity in the mid-rapidity region $|\eta|<0.8$ for the transverse momentum range $0.2 < p_{\rm T} < 3.0$ GeV/$c$. An ordering of the coefficients $v_2 > v_3 > v_4$ is found in pp and p-Pb collisions, similar to that seen in large collision systems, while a weak $v_2$ multiplicity dependence is observed relative to nucleus--nucleus collisions in the same multiplicity range. Using the novel subevent method, $v_{2}$ measured in pp and p-Pb collisions with four-particle cumulants is found to be compatible with that from six-particle cumulants. The symmetric cumulants $SC(m,n)$ calculated with the subevent method which evaluate the correlation strength between $v_n^2$ and $v_m^2$ are also presented. The presented data, which add further support to the existence of long-range multi-particle azimuthal correlations in high multiplicity pp and p-Pb collisions, can neither be described by PYTHIA8 nor by IP-Glasma+MUSIC+UrQMD model calculations, and hence provide new insights into the understanding of collective effects in small collision systems.
