energy dependence of transverse momentum fluctuationsif.pw.edu.pl/~kperl/evo/kg_cm_5_2008.pdf ·...
TRANSCRIPT
Katarzyna GrebieszkowWarsaw University of Technology
Nuclear Physics DivisionHeavy Ion Reactions Group
Energy dependence of transverse Energy dependence of transverse momentum fluctuationsmomentum fluctuations
status of the draft
NA49 Collaboration MeetingMay 26th – 28th, 2008
Kraków
Draft10Draft10 of publication: of publication:/afs/cern.ch/user/k/kperl/group/ENERGY_PUBLICATION/pt_fluct_energy_draft10.pshttp://www.if.pw.edu.pl/~kperl/EVO/evo3.html
● Introduction and Motivation – shorter version suggested by Peter Seyboth● Measures of fluctuations M(pT) distribution, ΦPT correlation measure, twoparticle
correlation plots using cumulative variable 'x' (definitions, properties, etc.)● Experimental Setup – description, plot● Data selection and analysis
– Data sets– Event and particle selection – Corrections and error estimates
● Results and Discussion ● Summary
● Old Appendix – what was the origin of increased pT fluctuations at lower energies (eventbyevent impact parameter fluctuations). Appendix has been removed completely, instead a reference to K.G. “Influence of Impact Parameter Fluctuations on Transverse Momentum Fluctuations”, Phys. Rev. C76, 064908 (2007) has been added.
● Data selection and analysis
Event and track selection criteriaEvent and track selection criteria
● Cut on x, y, z position of the fitted vertex
● ntf/nto > 0.25
● z_first < 200 cm
● |bx| < 2 cm, |by| < 1 cm
● nmp > 30, np/nmp > 0.5
● 0.005 < pT < 1.5 GeV/c
● 1.1 < y* < 2.6 forward rapidity
Kinematic + quality cuts and common acceptance ==> we are looking at 5% of all charged particles produced in central Pb+Pb
pT φ example for 2.0 < y* < 2.2
pT =A
2−B
beam rapidity
“at lower energies the NA49 TPC
acceptance extends to the projectile
rapidity domain and the selected particles
may be contaminated by
elastically scattered or diffractively
produced protons”
therefore additional track cut: therefore additional track cut: yy**pp< y< y**
beambeam 0.5 0.5
Corrections and error estimates
Points for 40A and 158A GeV – removed to make plots readable
Systematic error – from ΦPT
stabilities
dotted – 20A GeVdashed – 30A and 80A GeV
New analysis: ΦPT for different beam intensities and magnetic field configurations at top SPS energy only (158A GeV). 7.2% of the most central events selected using Benjamin's class. The horizontal axis (production type):
1 00M (min. bias, STD+), 15 921 events, multip.(all)=76.7 ,2 00N (min. bias, STD), 7 053 events, multip.(all)=76.7,3 01J (min. bias, STD+, low intensity), 8 595 events, multip.(all)=76.2,4 00B (10% central, STD+), 228 485 events, multip.(all)=76.8,5 01I (20% central, STD+, 256TB), 498 319 events, multip.(all)=75.6.
Kinematic (forward rapidity) and acceptance cuts included (see multiplicities). Azimuthal angle versus pT as in our current publication (socalled "common acceptance" for all energies). Points are NOT corrected for TTR.
The final version in the draft:
bias due to tracking inefficiency – lower than 0.51.0 MeV/c
● Results and Discussion M(pT) for data and M(pT) for
mixed eventsall charged particles
No evidence of increased eventbyevent fluctuations; the same shape of histogram for data and for mixed events
● Results and Discussion M(pT) data / M(pT) mixedall charged particles
No evidence of increased eventbyevent fluctuations; the same shape of histogram with data and mixed events
← new color scale
● Results and Discussion
Comparison to the UrQMD model (version 1.3)
(an old sample of 40A GeV interactions – replaced by a new one )
Stephanov, Rajagopal, Shuryak: fluctuations due to the critical point should be dominated by fluctuations of pions with low pT, for example pT ≤ 500 MeV; suggestion to do analysis with several upper pT cuts.
new: lines for UrQMD
Magnitude – M. Stephanov, K. Rajagopal, E. V. Shuryak (Phys. Rev. D60, 114028 (1999)) + private communication (see remark above)Size (width of the critical region) – Y. Hatta, T. Ikeda (Phys. Rev. D67, 014028 (2003))Position of CEP (µB) and the NA49 value of µB– Z. Fodor, S. D. Katz (JHEP 0404, 050 (2004)) & F. Beccatini, J. Manninen, M. Gaździcki (Phys. Rev. C73, 044905 (2006))
Remark: predicted fluctuations at the critical
point should result in ΦPT
≅ 20 MeV/c, the effect of limited
acceptance of NA49 reduces them to Φ
PT ≅ 110 MeV/c0 MeV/c
Comparisons with other experiments – new plots
The reason of significant difference between lowest RHIC (19.6 GeV) and top SPS (17.3 GeV) energy
difference
Fragment added by M. Gaździcki:
“The approximately linear dependence of ΦPT on the fraction of accepted particles in the limit lA << lC suggested an introduction of fluctuation measures ΣpT (%) and σpT,dyn, which for large particle multiplicities are proportional to sqrt(ΦPT/<N>).”lA – acceptance lengthlC – correlation scale
The end of the draft
The new kind of analysis – suggested to included in the draft
The latest CERES paper (arXiv:0803.2407):(x1,x2) for several bins of angular separation ∆φ =|φi – φj| (∆φ =|φ1 – φ2|)
Small ∆φ short range correlations (BoseEinstein, Coulomb), vanish for higher ∆φClose to ∆φ = 900 – small anticorrelation at high pT
Large ∆φ positive correlation due to jets (high pT corresponds to high x)CERES
Pb+Au 158A GeV05% most central interactions
0.1 < pT < 1.5 GeVmidrapidity 2.2 < < 2.7
Twoparticle correlations as function of (x(pT)1,x(pT)2) in different regions of ∆φ. No charge selection has been applied.
Twoparticle correlations in 00 < ∆φ < 300 (upper row) and 1500 < ∆φ < 1800 (lower row). Results for positive, negative andunlikesign particle pairs are shown separately.
Correlation for ++ and on diagonal → the effect of HBTCorrelation of unlikesign particles (+) restricted to lower pT as expected from Coulomb interaction and contaminations from e+e pairs
CERES results for different charge combinations
Our results for 20A and 158A GeV (7.2% most central) Still no cuts on ∆φ
BE correlations visible on diagonal for (++) pairs and () pairs, Coulomb effects not visible in (+) figure
1. here statistics can be only slightly increased, for 20A GeV from 160 k events to 200220 k events or 2. we can use different color scale
Should be included in the draft??
For a comparison – results from my PhD thesis (5% most central Pb+Pb at 158A GeV)
Maximum for Coulomb effect (+ pairs) visible because the azimuthal angle acceptance much better than in the case of the energy scan
Taken into analysis: 20% of charged particles produced in Pb+Pb
(instead of 5% as in the previous slide)
Additional test – azimuthal acceptance curves removed
...and another scale:
Now, the Coulomb maximum is visible
Results for several ∆φ
NA49 results for 158A GeV (7.2% most central,00B)∆φ =|φi – φj| (∆φ =|φ1 – φ2|)
Procedure:
1. Selection of accepted particlesNormally (STD+) positively charged particles are concentrated around 00, whereas negatively charged ones populate region around 1800. In order to define acceptance curves we transform (reflection + shifting) azimuthal angle of negatively charged particles (see the draft for details)
Our azimuthal angle – example for both positively and negatively charged, but the negatively charged ones has been transformed
pT φ example for 2.0 < y* < 2.2
pT =A
2−B
2. Very important: having defined a list of accepted particles we must come back to the original definition of azimuthal angle in order to calculate ∆φ =|φi – φj| for each pair
3. When 1800 < φi < 1800 and 1800 < φj < 1800, their difference ∆φ =|φi – φj| will vary between 00 – 3600. Therefore:
∆φ = |φi – φj|if (∆φ > 1800) then∆φ = ∆φ 1800
looks.... strange!
Very fast test of my program:
Our “normal” procedure for 158A GeV:
Results of program where additional cut on ∆φ =|φi – φj| (∆φ =|φ1 – φ2|) is applied but = 00 < ∆φ < 1800 what should result in the same plot as in “normal” procedure
The same results so the program works properly! The reason of “strange” structure for different ∆φ – our azimuthal angle in NOT flat
If it is indeed a problem of azimuthal angle acceptance than mixed events* should exhibit a very similar structure of (x1, x2) plots. Histograms for mixed events:
mixed events (30 k events for each energy) – constructed on the basis of original ones; the same multiplicity and inclusive pT distributions but each particle in a mixed event taken at random from different real event. All particle correlations are washed out.
very similar to the real data!
Additional test for mixed events at 158A GeV:
Results of the program where additional cut on ∆φ =|φi – φj| (∆φ =|φ1 – φ2|) is applied but = 00 < ∆φ < 1800 what should result in the same plot as in “normal” procedure
⇒ everything is OK.
(no particle correlations for mixed events, uniformly populated plot)
Let's divide data by mixed events:
Important remark: CERES (x1, x2) histograms were divided by those for mixed events in order to get rid of “residual pT correlations caused by detector acceptance effects. Such correlations may occur in differential analyses, if local deviations from the inclusive pT distributions are present.”Note: CERES “residual” azimuthal acceptance problems, NA49 – a real problem NA49 results for
158A GeVNow, it looks much more reasonable!
Evidence of shortrange correlations (small ∆φ),
Jets not observed (high ∆φ), due to pure azimuthal acceptance? Different rapidity region?
Maybe we will be able to improve the situation when we forget about acceptance curves (pT, φ)
Possible problems: acceptance description
Results with no azimuthal angle acceptance curves applied
Real events
Results with no azimuthal angle acceptance curves applied
Mixed events
Results with no azimuthal angle acceptance curves applied
Tests: results of the program where additional cut on ∆φ =|φi – φj| (∆φ =|φ1 – φ2|) is applied but = 00 < ∆φ < 1800 what should result in the same plot as in “normal” procedure
Real events Mixed events
Results with no azimuthal angle acceptance curves applied
Real events divided by mixed events:
Evidence of shortrange correlations (small ∆φ),
Jets still not observed (high ∆φ),
Conclusions, plans:
1. Probably due to the azimuthal angle acceptance of the NA49 and/or different (than in CERES) rapidity region (forwardrapidity instead of midrapidity) I was not able to reproduce CERES results at high 2. One can analyze lower energies, but since the acceptance is even worse at lower energies we should not expect to observe jet peak.3. We should probably publish results without analysis for different , but we can still add new plots for ++, and + pairs.