Recent Jet Results from the CMS experiment Raghav Kunnawalkam Elayavalli ( USA) Nuclear Seminar Monday, February 2nd, 2015 1
Overview https://twiki.cern.ch/twiki/bin/view/cmspublic/ PhysicsResultsHIN Motivations and background QCD phase diagram Experimental overview LHC, CMS HI beams Technicalities Results Nuclear modification factors 2
HI collisions 3
Motivations Production and effects of the QGP (quark gluon plasma) in high temperature and low baryon density environment. Initial (Cold) vs Final (Hot) nuclear matter effects. 4
Phase Diagram µ b = 0: Lattice QCD predicts analytic crossover Large µ b The transition between QGP and hadron gas is expected to be a first order phase transition Critical point expected at the end of the first order line. 5
What we measure 6
Lead Ion beams at the LHC Detlef Kuchler, a physicist in CERN's Beams department, with the container holding the purified sample of lead used to create heavy ions for the LHC http://www.symmetrymagazine.org/breaking/ 2010/11/05/the-skinny-on-the-lhcs-heavy-ions CERN Yellow Report CERN-2010-001, pp. 393-416 7
CMS detector η= -ln (tan θ/2) η=0 η = 2.4 Beam 8
What are jets Collimated stream of particles produced by hadronization of quark or gluon from high impact collisions (evidence for quarks!) Example: quark/lepton/gluon jets CMS is fine-tuned to look at jets using the calorimeters and trackers combined. 9
Jets in CMS Calorimeter (CALO) Jets: Using Calorimeter energy deposits. Particle Flow (PF) Jets: Combines information from all sub detectors to make PF candidates, which are then clustered. 10
Data Sets PbPb 2.76 TeV ppb 5.02 TeV pp 2.76 TeV PRC 84 (2011) 024906 11
Technicalities in Heavy Ions Event classifications Centrality classes & glauber model Background subtraction Iterative pile up Flow modulated Voronoi subtraction PP reference data at 5.02 TeV Identifying B jets 12
Event Classification Heavy Ion collisions are split up in terms of the number of colliding nuclei. Ann.Rev.Nucl.Part.Sci.57:205-243,2007 13
Centrality in Data JHEP 1108 (2011) 141 Shengquan Tuo pa centrality workshop: Feb 14 2014 14
Background subtraction 1. Background (bkg) energy per tower calculated in strips of η 2. Jet finder run on subtracted (sub) towers 3. Background energy recalculated excluding jets 4. Rerun Jet algo on bkg-sub towers without jets -> get the final jets. EPJC (2007) 117. 15
pp Reference spectra @ 5.02 TeV Extrapolated from pp data at 7 TeV. 1. Dependence of the jet radius and the s on the cross section 2. The above effect was extracted from Pythia and compared with NLO (next to leading order) calculations 3. Applied to generated spectra to derive the reference at 5.02 TeV 16
Identifying b-jets 17
b-jet tagging The Simple Secondary Vertex (SSV) tagger is more robust against a combinatorial background due to the secondary vertex requirement 18 template fit to the SV invariant mass distribution in ppb collisions for jets of 90 < p T < 110 GeV/c, where b-jets dominate after 2 GeV/c 2
Results - Nuclear modification factor <N coll > - No of participating nuclei per event σ - cross section <T AA > - Average value of the nuclear thickness function R AA > 1 : Enhancement R AA = 1 : no medium effect R AA < 1 : Suppression/ quenching 19
RpA & RAA tracks vs Jets 20
RpA & RAA: inclusive vs b-jets Nuclear Modification Factor Nuclear Modification Factor 2.5 2.5 2 1.5 1.5 1 2 1 0.5 0.5 ppb -1 CMS Preliminary = 5.02 TeV ppb L = 35 PbPb nb ; PbPb = 2.76 L = 150 TeVµb s NN Inclusive Charged jet particles R (0-100%) R η η < 1< 0.5 pa CM CM Inclusive Charged jet particles R (0-5%) R (0-5%) η < 2 η < 1 AA Nuclear Modification Factor 0 0 0 0 50 100 20 15040200 250 60 30080350 400 100 pa AA p s NN CMS-HIN-14-001 CMS-HIN-12-004 T p[gev/c] T -1 2.5 2 1.5 1 0.5 CMS Preliminary ppb Luminosity Unc. ppb Reference Unc. -1-1 ppb L = 35 nb ; PbPb L = 150 µb b-jet R, (0-10%), η <2 AA b-jet R PYTHIA 0 0 100 200 300 400 pa b-jet p T, -2.4<η <1.6 CM CMS-HIN-14-007 arxiv:1312.4198 [GeV/c] No observable difference between inclusive and b-jets in the explored p T range 21
Missing pt 22
Missing pt- II Sum charged particles for unbalanced A J >0.22 dijets in central (0-30%) PbPb 35 GeV/c of high p T tracks missing from away side jet at ΔR=0.2 Balanced by low p T particles up to very large ΔR=2.0 PbPb-pp : result shows a different p T distribution Take the p T cumulative of all tracks total angular pattern is similar in PbPb and pp Able to recover the lost energy by going to Large ΔR in the away side jet CMS-HIN-14-010 23
Where does the energy go? 24
Jet shape First experimental Jet Shapes in HI Broader jet shapes in PbPb in most central collisions PLB 730 (2014) 243 25
CMS fragmentation function Tracks inside a jet cone of R=0.3 z = p track / p jet High p T no modification Phys.Rev. C90 (2014) 024908 Suppression of intermediate p T in the cone Enhancemen t at low p T 26
Summary Many observables showing independent confirmation of modification of jets in the medium (final state interactions) Jets are heavily quenched in most central PbPb collisions Initial state in ppb collisions can be described by npdf Inclusive jets are not quenched Flavor dependence: So far no glaring differences between tagged and inclusive jets (in the explored p T range). Need results from fully reconstructed D, B mesons (in both PbPb and ppb) Quenched energy recovered by going to higher radii. Lost energy carried away by low p T particles away from the jet cone (Jet+Track measurements) Jet Structure modification: Excess of low p T particles inside the jet cone (A J measurements) Observe quenching of intermediate range p T particles (Jet Fragmentation & jet Shape) 27
Backup slides 28
Evolution of HI collisions https://www.bnl.gov/rhic/news/082807/story3.asp 29
New HF/Voronoi algorithm A Voronoi diagram in the (eta, phi)- plane is used to associate an unique area to each particle such that the UE density can be removed particle-by-particle Voronoi tessellated HYDJET/GEANT particle-flow event (combined tracks and calorimeter towers) before (left) and after (right) subtraction. Non physical negative particle/areas are equalized to maximally approximate to the original (real) jet distribution of radius R. (backup slides) Flow (v2,..,v5) accounted for by projecting the expectation from the HF CMS-DP-2013-018 30