Oct 9, 2014

"Measurement of long-range pseudorapidity correlations and azimuthal harmonics in √sNN=5.02 TeV proton-lead collisions with the ATLAS detector" is published in PRC 1409.1792, a work led by Sooraj. This paper presented the most detailed measurement of harmonics v1-v5 using a recoil subtraction technique to remove the short-range correlations. For the first time the data show significant ridge at pT>10 GeV, possibly reflecting the existence of jet-medium interaction. The magnitudes of v2,v3 and v4 are surprisingly similar to Pb+Pb collisions with the same multiplicity (after removing the average geometry component), suggesting a similar mechanism for the observed harmonics in the two collision system.

Oct 1, 2014

Three papers from Peng and Soumya have been published in the last two months (PhysRevC.90.024910 ,PhysRevC.90.034905 ,PhysRevC.90.034915). For the first time, the rapidity de-correlation of the harmonic flow is shown to be caused by the forward-backward fluctuations of participating nucleons and their transverse shape. Several method have been developed to measure such longitudinal flow flucuation and twist effects.

Aug 20, 2014

Thomas's work on W boson production in Pb+Pb collisions is submitted for publication ( He is primarily responsible for the W->muon analysis. This paper addresses the benchmarking properties and modification to the PDF by measuring W bosons in Pb+Pb collisions at 2.76 TeV with the ATLAS detector.

May 31, 2014

In Quark Matter 2014, both Soumya's and Sooraj's results are highlighted. Link

Apr 16, 2014

Congratulation to Yi Gu's graduation! Our group member Yi Gu has successfully passed his PhD thesis defence. His work is focused on hadron flow measurements at PHENIX, particularly flow signals from correlation analysis and higher order flow harmonics studies. We had a dinner party after the meeting and here is the photo.

Mar 3, 2014

One of Soumya's thesis work " Measurement of event-plane correlations in √s_NN = 2.76 TeV lead–lead collisions with the ATLAS detector" has been submitted for publication! This paper show detailed centrality dependence of 8 two-plane and 6 three-plane correlators,  the first measurement of this kind. It should provide strong constraint on non-linear mode-mixing effects in collective expansion of the Quark-Gluon matter created in lead-lead collisions. A job well done!

Nov 25, 2013

"Measurement of flow probability distribution in Pb+Pb collisions" is published in JHEP: JHEP11(2013)183. In this work led by Soumya, we obtained the first EbyE probability distribution of the elliptic flow v2, triangular flow v3 and quadrangular flow v4 in heavy ion collisions. These probability distributions p(vn) are obtained by unfolding the raw vn distributions with a data-driven response function that accounts for the smearing of the vn due to finite number of tracks in the detector acceptance. This is the first time unfolding method is successfully applied to the flow analysis. One key advantage of p(vn) distribution is that it allows direct and precise understanding on the role of the initial geometry and hydrodynamic responses. For example the measured p(v2) and p(v3) can be used to directly constrain the initial state eccentricity fluctuations p(ε2) and p(ε3). Details can be found at

Aug 8, 2013

Soumya defended his thesis "Measurement of the azimuthal anisotropy for charged particle production in Pb+Pb collisions at √s_NN = 2.76 TeV and in p +Pb collisions at √s_NN = 5.02 TeV with the ATLAS detector at LHC".his work already results in 3 ATLAS publications. He is now a Research Associate in the Columbia University.

Dec 19, 2012

Observation of symmetric-ridge in proton - lead collisions: Studies of two-particle correlations in high-multiplicity proton–proton and proton–lead collisions at the LHC have shown a phenomenon frequently referred to as the "ridge". The ridge is a result of correlated production of particles at small relative-azimuthal angle (Δφ) over a wide range of relative pseudorapidity (Δη). Using data from the highly successful pilot proton–lead run on 12 September 2012, ATLAS has shown that the ridge has an identical twin resulting from correlated production of particles that are back-to-back in azimuth. The double-ridge structure extends over +/–5 units of Δη, and the strength of ridge increases with the transverse momentum and persists to at least 6 GeV in pT. The data analysis was carried out by soumya and Jiangyong.
Read more at:
Link to the  paper and auxiliary plots.