Latest results from the PHOBOS experiment

Over the past years PHOBOS has continued to analyze the large datasets obtained from the first five runs of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The two main analysis streams have been pursued. The first one aims to obtain a broad and systematic survey of global properties of particle production in heavy ion collisions. The second class includes the study of fluctuations and correlations in particle production. Both type of studies have been performed for a variety of the collision systems, covering a wide range in collision energy and centrality. The uniquely large angular coverage of the PHOBOS detector and its ability to measure charged particles down to very low transverse momentum is exploited. The latest physics results from PHOBOS, as presented at Quark Matter 2008 Conference, are contained in this report.Comment: 9 pages, 9 figures, presented at the 20th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, Feb.4-10, 200

Elliptic flow fluctuations in 200 GeV Au+Au collisions at RHIC

We present first results on event-by-event elliptic flow fluctuations in Au+Au collisions at 200 GeV obtained with the PHOBOS detector. Over the measured range in centrality, large relative fluctuations of 40--50% are found. The elliptic flow fluctuations are well described as being proportional to fluctuations in the shape of the initial collision region, as estimated event-by-event with the participant eccentricity using Glauber Monte Carlo.Comment: 5 pages, 2 figures, QM 2006 proceedings; v2: Corrected a few typo

Collision geometry fluctuations and triangular flow in heavy-ion collisions

We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multi-phase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Using two-particle azimuthal correlations at large pseudorapidity separations measured by the PHOBOS and STAR experiments, we show that this Fourier component is also present in data. Ratios of the second and third Fourier coefficients in data exhibit similar trends as a function of centrality and transverse momentum as in AMPT calculations. These findings suggest a significant contribution of triangular flow to the ridge and broad away-side features observed in data. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.Comment: 8 pages, 8 figures, correction after publication, Fig8b has been corrected: The pt selection in AMPT calculation has been changed to match the selection in STAR dat

Elliptic Flow, Initial Eccentricity and Elliptic Flow fluctuations in Heavy Ion Collisions at RHIC

We present measurements of elliptic flow and event-by-event fluctuations established by the PHOBOS experiment. Elliptic flow scaled by participant eccentricity is found to be similar for both systems when collisions with the same number of participants or the same particle area density are compared. The agreement of elliptic flow between Au+Au and Cu+Cu collisions provides evidence that the matter is created in the initial stage of relativistic heavy ion collisions with transverse granularity similar to that of the participant nucleons. The event-by-event fluctuation results reveal that the initial collision geometry is translated into the final state azimuthal particle distribution, leading to an event-by-event proportionality between the observed elliptic flow and initial eccentricity.Comment: To appear in the proceedings of the Lake Louise Winter Institute 2007. The proceedings of the institute will be published by World Scientifi

Elliptic Flow and Initial Eccentricity in Cu+Cu and Au+Au Collisions at RHIC

We present a systematic study of elliptic flow as a function of centrality, pseudorapidity, transverse momentum and energy for Cu+Cu and Au+Au collisions from the PHOBOS experiment. New data on elliptic flow in Cu+Cu collisions at 22.4 GeV are shown. Elliptic flow scaled by participant eccentricity is found to be similar for both systems when collisions with the same number of participants or the same average area density are compared. This similarity is observed over a wide range in pseudorapidity and transverse momentum, indicating that participant eccentricity is the relevant quantity for generating the azimuthal asymmetry leading to the observed elliptic flow.Comment: 5 pages, 4 figures, the 19th International Conference On Ultra relativistic Nucleus-Nucleus Collisions (Quark Matter 2006), Shanghai China, Nov. 14-20, 200

Trends in multiparticle production and some "predictions" for pp and PbPb collisions at LHC

Based on trends seen at lower energies we "predict" the multiplicities and pseudorapidity distributions of particle density and elliptic flow that will be seen in PbPb and pp collisions at the LHC. We argue that, if these predictions turn out to be correct, either these quantities are insensitive to the state of matter created in high energy heavy ion collisions or the observed simplicity and universality of the data must be telling us something profound about the mechanism of particle production, which to this date is not well understood.Comment: Invited Talk at SQM2007 Conferenc

Phenomenology of the little bang

I review recent selected developments in the theory and modeling of ultrarelativistic heavy-ion collisions. I explain why relativistic viscous hydrodynamics is now used to model the expansion of the matter formed in these collisions. I give examples of first quantitative predictions, and I discuss remaining open questions associated with the description of the freeze-out process. I argue that while the expansion process is now well understood, our knowledge of initial conditions is still poor. Recent analyses of two-particle correlations have revealed fine structures known as ridge and shoulder, which extend over a long range in rapidity. These correlations are thought to originate from initial state fluctuations, whose modeling is still crude. I discuss triangular flow, a simple mechanism recently put forward, through which fluctuations generate the observed correlation pattern.Comment: 10 pages, plenary talk at the International Nuclear Physics Conference (INPC 2010), Vancouver, Canada, July 4-9, 2010. (version 2: minor revision.

Fluctuating initial conditions in heavy-ion collisions from the Glauber approach

In the framework of the Glauber approach we analyze the shape parameters of the early-formed system and their event-by-event fluctuations. We test a variety of models: the conventional wounded nucleon model, a model admixing binary collisions to the wounded nucleons, a model with hot spots, as well as the hot-spot model where the deposition of energy occurs with a superimposed probability distribution. We look in detail at the so-called participant multipole moments, obtained by an averaging procedure where in each event the system is translated to its center of mass and aligned with the major principal axis of the ellipse of inertia. Quantitative comparisons indicate substantial relative effects for eccentricity in variants of Glauber models. On the other hand, the dependence of the scaled standard deviation of the participant eccentricity on the chosen model is weak. For all models the values range from about 0.5 for the central collisions to about 0.3-0.4 for peripheral collisions, both for the gold-gold and copper-copper collisions. They are dominated by statistics and change only by 10-15% from model to model. We provide an approximate analytic expansion for the multipole moments and their fluctuations given in terms of the fixed-axes moments. For central collisions and in the absence of correlations it gives the simple formula for the scaled standard deviation of the participant eccentricity: sqrt(4/pi-1). Similarly, we obtain expansions for the radial profiles of the multipole distributions. We investigate the relevance of the shape-fluctuation effects for jet quenching and find them important only for very central events. Finally, we argue how smooth hydro leads to the known result v_4 ~ v_2^2, and further to the prediction Delta v_4/v_4 = 2 Delta v_2/v_2.Comment: 20 pages, 15 figures, additions include comparison to the CGC result