CMS experiment

Azimuthal anisotripy in HIC with CMS Tracker

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Draft 2

Date: 5 Feb 2007 From: Stephen Sanders

Stephen Sanders wrote:

Michael has asked me to comment on the azimuthal anisotropy internal > note, without giving guidance on what I should be looking for. Some > of my comments may not, therefore, be relevant when one considers the > purpose of the note.

Overall I thought the presentation was quite clear and the results > interesting, although expected. It would be remarkable, and > somewhat scary, if CMS would be unable to do a good v2 measurement > near mid-rapidity using the Tracker. It is nice, however, to see > expectations confirmed with realistic simulation and to see actual > values for the event plane resolution.

It will be interesting to see how well the experiment can do v4(pt) > measurements near mid-rapidity. More speculatively, I wonder if > there is any hope of a v1(pt) measurement at the most forward > rapidity covered by the tracker: it might be possible for the pt- > integrated v1 signal to be close to zero, but still have a > differential signal. Chen et al. (PRB 605, 95 (2005)) argue that > this differential signal reflects interesting dynamics of the > reaction. A useful v1 simulation probably requires what would be an > unrealistically large number of events. However, if a v4 signal > exists in the HYDJET events at a level similar to what STAR sees at > mid-rapidity, it might be possible to pull this out.

Response

For future study

The assertion in paragraph 3 of the introduction that hydrodynamic > models are unable to reproduce the pseudorapidity dependence of v2 > (eta) is dated. I believe the initial ^Ófailure^Ô to reproduce the > pseudorapidity dependence is now seen as a case of using > inappropriate initial conditions and/or not accounting for hadronic > rescattering. Hirano et al. (PLB 636, 299 (2006)), and the earlier > quoted paper by Chen address this issue. Unfortunately, a reliance > solely on the v2 term may not be sufficient to resolve differences in > the alternative approaches, such as whether the initial conditions > should be taken as purely Glauber, or if a CGC is involved.

There seems to be something missing in the last sentence of the > Introduction (I^Òm having a hard time figuring out what the subject of > the sentence is).

On page 5, ^ÓThe interplay of multiplicity and anisotropic flow^Å^Ô^Å > This sentence suggests that anisotropic flow varies monotonically > with centrality, whereas it is observed at RHIC to fall off somewhat > for more peripheral events, reaching a maximum for mid-central events > (around 50% most central).

In the discussion of v2 event-plane method there should be some > discussion of how the reaction-plane correction factor was obtained > and the magnitude of this correction. In my first reading, I assumed > Psi_R stood for the real event plane, since this is a relatively > common notation. However, on re-reading I see the experimental > event plane angle was used (eqn. 1), which requires application of an > event-plane resolution correction. This makes equation 3 misleading, > since the v2 obtained with the experimental event plane (v2_obs) is > not the same as the real v2, since v2 = v2_obs/(reaction-plane > correction factor). (Again, this led me to initially read the > reaction-plane angle in equation 3 as being the real-event plane > angle, since then equation 3 is correct.) I was hoping to see this > correction factor quoted, allowing a direct comparison of how well > CMS can do flow measurements to what is achieved by the different > RHIC detectors. The same information must be buried in the Gaussian > width analysis, but I^Òve not seen these values quoted as often. > Also, avoiding autocorrelations in determining the event-plane > resolution correction can be tricky, although the symmetry of the CMS > detector should help.

Another analysis that would be interesting would be a comparison of > the four-particle and two-particle cumulant methods using the HYDJET > events, since non-flow correlations might be significant at mid- > rapidity, leading to significant differences in the two results.

Response

For future study

I recognize that many of my comments are areas for future studies. > I offer them just because they come to mind reading through the > analysis note. I do not see anything in the note that would have me > question the results that are shown, except for my question as to how > the reaction-plane resolution correction was obtained and how it was > applied and for the question of whether non-flow correlations were > considered.