QCD Studies with CMS at LHC. Gunther Roland for the Collaboration

Transcription

1 QCD Studies with CMS at LHC Gunther Roland for the Collaboration INT Seattle 5/24/2010

2 First 7 TeV Collisions: March 30th

3 QCD Studies with CMS pp integrated luminosity ~ 10nb -1 n.b. rates for LHC design lumi Reference Rich QCD program in CMS To date, mostly min bias physics

4 Compact Muon Solenoid Muon Chambers 4T Solenoid Total weight t Overall diameter 15 m Overall length 21.6 m

5 Compact Muon Solenoid

6 CMS Tracker Pixels Strips Inside 3.8 Tesla field Hit reconstruction efficiency above 99% >97% of channels operational in 2009 Coverage over η < 2.4 with 3 pixel and 10 strip hits pt coverage from 100MeV/c (ΔpT/pT ~ 10% at 1 TeV/c) 6

7 Tracker Performance Pixel charge Silicon strip PID CMS tracker already well-understood Excellent agreement with simulations. 7

8 QCD Studies in CMS 5 QCD analyses so far: Particle dn/dη, dn/dpt at 0.9, 2.36 and 7 TeV arxiv: , JHEP 1002:041,2010 (0.9 and 2.36 TeV) arxiv: , submitted to PRL (7 TeV) Bose-Einstein correlations at 0.9 and 2.36 TeV arxiv: , submitted to PRL Underlying event at 0.9 TeV (*) Two-particle correlations at 0.9, 2.36 and 7 TeV (*) (*) preliminary

9 Trigger: Min Bias Trigger Single hit in BSC In time with Beam Pickups from OPAL salvage yard Offline: Text 1 tower 3 GeV in each forward calorimeter (HF) Beam halo rejection using BSC timing Beam-background rejection Reconstructed vertex from pixel tracks BSC: 3.23 < η < 4.65 HF: 2.9 < η < 5.2

10 Selection efficiency Generated particles after event selection η < 2.5 Non-single diffractive (NSD) selection efficiency η <

11 Hadron multiplicities and pt spectra Study basic properties of particle production in min bias collisions Tuning of MC models Reference for heavy-ion analysis

12 Three Analysis Techniques - pt > 30 MeV/c - insensitive to misalignment and vertex resolution - pt > 50 MeV/c - data-driven correction for background hits - pt > 100 MeV/c - fake track contribution < 1% 12

13 Cluster-counting method Count pixel hits in single barrel layer Cluster length ~ sinh(η) Short clusters removed (e.g. loopers, secondaries) Correct for loopers, weak decays, secondaries 3 pixel layers independent measurements 13

14 Tracklet method Tracklets are pairs of pixel hits on different layers Sharp Δη correlation used to separate signal Sideband technique: subtracting combinatorial background MC-based corrections: acceptance, weak-decays, secondaries, pixel efficiency, splitting Signal Region 14

15 Tracking method Uses full tracks with hits in all pixel and strip layers Track-finding done in several iterative steps Low fake rate maintained at low-pt by filtering on cluster shapes of hits on tracks Background rejection based on compatibility with primary vertex Requires good understanding of alignment, beamspot 15

16 Tracking Performance tracking efficiency positives π, K, p negatives CMS preliminary 16 24

17 Results: pt Spectra Results well-described by Tsallis fit function. Exponential (low-pt) Power-law (high-pt) As expected, spectrum is harder at higher energy

18 Results: dn/dη Results from three methods weighted by uncorrelated errors, averaged, and symmetrized. 18

19 Results: Energy dependence 19

20 Bose-Einstein Correlations Study enhancement of hadron pairs at low momentum difference Q Information about space-time properties of emission process

21 Bose-Einstein Correlations in pp Small distortions of background Strength Fourier-transform of emission region Exponential and Gaussian fits

22 Comparison to previous data Exponential fits CMS CMS results are consistent with previous measurements at lower energies in various systems

23 Multiplicity dependence n.b. Nch ~ dn/dη * 4 Radius parameter shows strong multiplicity dependence 3D studies needed for HI reference

24 Underlying event in 0.9 TeV collisions Correlation of particle production relative to leading tracks or track jets Relevant to MC tuning for jet analyses Related to di-hadron analyses in heavy-ions

25 Correlation relative to leading track UE activity is correlated with leading track pt over the full Δφ range

26 Particle production in transverse region Strong turn-on of particle production in transverse region as a function of track-jet pt

27 Correlation relative to leading track What is the meaning of ZYAM? Underlying event and hard process are connected

28 2-particle angular correlations Correlations in soft particle production Related to hadronization process Related to flow analyses in heavy-ions

29 Two-particle angular correlations CMS preliminary Familiar structure in angular correlation functions at all energies

30 Energy Dependence pt > 100MeV/c, extrapolated to pt = 0 CMS preliminary model dependent extrapolation to η < 3 CMS preliminary Data fit trend for correlation width/strength Failure of Pythia tunes to describe soft correlations

31 Many more QCD studies to come

32 K 0 s Particle Identification π 0 η Λ 0 ϕ meson

33 Plenty of Jets

34 Some remarks on Multiplicity Particle correlations

35 Some speculation on dn/dη in PbPb? Straight line extrapolation may run into difficulties at high s

36 RHIC: Factorization of dn/dη vs s and N part n.b. these surface defined by AuAu data! dn/dη/<0.5*n part > Energy (GeV) x2.5 Centrality = norm. dn/dη Energy (GeV) norm. dn/dη x1.95 x1.3 x1.3 Centrality x1.95 Armesto, Salgado, Wiedemann hep-ph/ Energy (GeV) Centrality

37 Some speculation on dn/dη in PbPb unlikely 1.6 Compilation by N. Armesto RHIC: AA/pp (per Npart) ~ TeV ~ TeV TeV 1250

38 Some speculation on dn/dη in PbPb λ=0.288 Armesto, Salgado, Wiedemann hep-ph/ Compilation by N. Armesto still doing well

39 What about limiting fragmentation? CMS pp * 1.6 log. extrapolation W. Busza, Zakopane 04 Limiting fragmentation scaling still possible (but probably not a straight line)

40 Mini-jets, correlations and multiplicity from Wei Li

41 Mini-jets and multiplicity PHOBOS 200GeV pp CMS preliminary mini-jet component grows rapidly with s Will we see a breakdown of energy/centrality factorization? n.b. dn/dη (1-α)*Npart + α*ncoll breaks factorization

42 Correlation Functions and Collision Geometry Participant Eccentricity ψ 2 PHOBOS PRL 104, (2010) PHOBOS PRC81, (2010) ε = ( r 2 cos(2φ) 2 + ( r 2 sin(2φ) 2 r 2 PHOBOS PRL 98, (2007) see talk by Constantin Loizides on Friday

43 Correlation Functions and Collision Geometry Participant Triangularity ψ 3 ε 3 = ( r 2 cos(3φ) 2 + ( r 2 sin(3φ) 2 r 2 Burak Alver, GR, arxiv: (PRC in press) see talk by Burak Alver next week Just like elliptic flow reflects event-by-event eccentricity, triangular flow (v3) reflects event-by-event triangularity (ε3)

44 Triangular flow? v = cos( 2(φ ψ )) ε 2 2 ψ 2 v = cos( 3(φ ψ )) ε ψ 3 Burak Alver, GR, arxiv: (PRC in press) Triangular flow in hydro calculations, not just AMPT Luzum, Ollitrault, private communication

45 Correlation Functions and Collision Geometry Inclusive correlations Triggered correlations Burak Alver, GR, arxiv: (PRC in press) Published correlation data (STAR, PHOBOS) show v3 component! arxiv: PRL 104, (2010) Flow contribution to long-range ridge and broad away-side Correlation functions will be a key tool to study collision dynamics at LHC Separation of hard/soft components needs more thinking (ZYAM, flow etc) n.b. Ψ2 and Ψ3 are uncorrelated - triangular flow is not visible in v2 event plane analysis

46 Summary CMS is a remarkable instrument for QCD studies A new frontier of extreme QCD is coming into view Much will be learned