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Part 2 Some details about LHC Some details about the CMS subdetectors QCD measurements

G. Mantovani Some of the first physics analyses with CMS at the CERN-LHC (QCD, UE, multipartonic interactions). Part 2 Some details about LHC Some details about the CMS subdetectors QCD measurements. The collider. To be sensitive to most HE physical processes one needs

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Part 2 Some details about LHC Some details about the CMS subdetectors QCD measurements

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  1. G. Mantovani Some of the first physics analyses with CMS at the CERN-LHC (QCD, UE, multipartonic interactions) • Part 2 • Some details about LHC • Some details about the CMS subdetectors • QCD measurements

  2. The collider To be sensitive to most HE physical processes one needs + more energy : to produce massive particles like the Higgs bosons. A lepton collider would be energetically less convenient. + high intensity and luminosity: The expected processes have cross-sections which differs by ten orders of magnitude

  3. LHC has been built at CERN to profit of the old LEP infrastructures. It is a 27 km-diameter ring 80 m underground on average. The injection system is based on the “old” CERN accelerators PS e SPS (W,Z) 4 experiments were planned: + ATLAS + CMS Two omnipurpose esperiments, designed to guarantee the highest possible discovery potential + LHCb Optimized for B physics + ALICE Optimized for heavy ion collisions

  4. The two proton beams rotate in opposite direction in 2 separate rings and collide frontally (Pztot=0),in 4 interaction points. • The available CM energy will be • √s = 2×Ebeam≈2×pbeam • With fixed-target accelerator√s =√(2×m×Ebeam) m ≈ 1 GeV for proton target • First collisions postponed from 10/2008 • to 11/2009? due to wellknown accident and repairs.

  5. Luminosity The frequency of an event x is R(x) = L*σx where At LHC one expects N ≈1011 ; σ ≈15 μm for CMS (depends on the focusing of the experiment); f = 11 kHz; k = 2808 L = 1034cm-2s-1

  6. Expected rates The CM energy will be √s = up to 14 TeV for proton-proton collisions √s = up to 6 TeV for Pb-Pb ions collisions The Luminosities L = 1034 cm-2s-1 p-p L = 1027 cm-2s-1 Pb-Pb The protons circulate in “bunches” with a time spacing of 25 ns (i.e. 7.5 m) So the bunch-crossing rate will be 40 MHz Considering that the p-p TOTAL cross-section is ~ 100 mb the expected event Rate with L = 1034 cm-2s-1 is R ~ 109 Hz Considering that there will be ~ 25 pp interactions per bunch- crossing one can understand the difficulty of separating an interesting event from the background.

  7. Now the interesting events come from single parton-parton collisions, but all the partons of the initial protons will produce background. Quarks and gluons of the initial state interact strongly and can produce high-pT gluonic radiation. In general the initial state partons carry different momentum fractions so that the laboratory is not exactly a reference system at rest. There is also the Pile Up from collisions of protons belonging to the same bunch which makes the situation even more complex. So, the interesting events have low probability and are submerged by a very high background

  8. Therefore the detectors must have: + very high detection efficiency (eg : 1 “interesting” track in 1000 -> we must be able to see and reconstruct them all) + very high spatial resolution (which among n close interactions is the “right” one? We must be able to recognize n primary and secondary vertices~ ten microns apart) + high time response velocity (40 MHz is the interaction frequency of the LHC bunches) + the most internal detector parts must resist for long time to radiation damages

  9. The CMS detector

  10. Role of the various CMS subdetectors

  11. The tracker SiStrip + Pixel

  12. The ElectroM Calorimeter Is made of PbWO4 crystals + all particle energy is deposited and maily converted il light which is detected by phodiods -triods + high energy resolution + intrinsically tracking thanks to the high granularity a = 2.7%, due to shower development fluctuations b = 0.55%, due to calibration systematics c ~ 0.2 GeV, due to instrumental systematics The Hadronic Calorimeter It is a sampling calorimeter, Cu is the absorberinterposedby plates of active scintillating plastic material. The resolution is

  13. The Muon chambers The momentum resolutions is intrinsically low with  - chambers alone, due to the big quantity of Iron , but it is compensated by the combined use of the internal Tracking System D μ –ch only μ+tracker

  14. What we expect LHC will extend the kinematical region of previous HE machines , with a large phase-space accessible to the final state and an expected high jet multiplicity. One expects mainly gluon-gluon interactions. And, hopefully, to get new information on the proton structure and on the interaction dynamics . A 1034 luminosity -> high pile-up mainly composed by generic soft low pT-interactions Therefore measuring low- and high- pT interactions we aim to + understand the dynamics of HE strong interactions (pQCD, PDFs, αS, new physics…) + the soft part (ie Underlying Event, non-pertubative effects, Multiple Partonic Interactions) to be able to correctly estimate the expected background and to isolate in the best way possible the signals coming both from known processes and “new”ones. 14

  15. Basic QCD measurements The CMS collaboration foresees a wide set of basic QCD measurements related to αSmeasurement and PDF determination: An experimental strategy mainly related to SM measurements Very active CMS working group - https://twiki.cern.ch/twiki/bin/view/CMS/PDFsatCMS - using event shape variables: “central” transverse thrust “central” thrust minor <1-T> α0=0.493, αS(M2Z)=0.119 15

  16. Inclusive Jet production – the legacy from past 1978 – Feynman-Field model predicts a large jet cross section 2006 – CDF 1 fb-1 20XX – LHC 10 pb-1 QCD just born! + PDFs Q2 dependence predicted from QCD Quark & Gluon Fragmentation Functions Q2 dependence predicted from QCD Quark & Gluon X-Sections Calculated from QCD new interactions could be visible

  17. Inclusive Jet production – uncertainty Main experimental and theoretical sources of uncertainty Cross section determination is mainly related to jet counting: + Luminosity + Reconstruction efficiency + Trigger efficiency Energy Scale: + Jet Energy Scale + systematics + Data resolution + Underlying Event‏ Assuming 10 pb-1 -> ~10% JES uncertainty

  18. Jet production – understanding the radiation Why? 1) the radiation dynamics is connected to the transition from hard to soft QCD 2) to understand multi-jet final states (irreducible backgrounds in many LHC searches ) Δφ dijet = π – Exactly 2 jets, no radiation Δφ dijet small deviations from π –Additional soft radiation Δφ dijet as small as 2π/3 – One additional high- pT jet Δφ dijet small – no limit - Multi-jet in the final state pT>800 GeV ! pT>180 GeV D0 CMS 10 pb-1 Df Jet#1-Jet#2 Df Jet#1-Jet#2

  19. QCD First measurements at low- pT 19

  20. Test of Parton saturation With Regge theory (no parton saturation) at η=0, x-sect & multiplicity grow together. With some data at ~900 GeV probably both ratios can be measured , constituting a test of the theory Moreover at LHC parton saturation appears as a distorsion in the low pT spectrum of pp collisions due to gluon recombination (gg->g), justifying the importance of measuring charged particle spectra in the 0-2 GeV range 20

  21. 3. Tracklet 2. Vertex 1. HIT Charged spectra – early analysis Early data-taking stage -> raw methods, low-level physics objects used hits from pixel detector instead full tracking • From pixel hits and primary vertex, particles density and h are estimated layer by layer • Combining the hits from different layers Layer 2 Layer 1 Systematics ~10%

  22. Charged spectra – early analysis With a better control of the tracking system (1-10 pb-1) it is possible to take advantages from the full tracking • pT measurement down to 75 MeV/c • Particle-id (based on cluster shape and charge) • Corrections dependant on particle kind • Secondaries are reconstructed Systematics ~5%

  23. Underlying Event (Perugia) • Minimum-bias & jet events • Measuring the Underlying Event activity • Exploring the dynamics of pp collisions • Calibrating the physics tools • Tuning the Monte Carlo models Main observables: + dN/dηdφ, charged particle density + d(PTsum)/dηdφ, energy density reference is the transverse activity Also in this case the analysis is based completely on the tracker subdetector . All systematics are connected with the tracking uncertainties. 23

  24. Underlying Event – direct measurement + difference among the Monte Carlo models can be exploited + spread at high pT is mainly due to lack of Monte Carlo events + statistical errors are compatible with 10 pb-1 of data-taking

  25. Underlying Event – ratio measurement from the ratio of the observables (built using different tracking thresholds) + lower sensitivity to different Monte Carlo models BUT + inefficiencies from tracking can be mainly reabsorbed (better control of systematics from detector) 25

  26. Double parton scattering Double high PT interactions are evident from AFS, UA2, CDF and now D0 In the simplest models, DS produces a final state which simulates the combination of 2 independent scatterings. σB /(2σeff) is the probability for scattering B to happen together with A and depends on the parton spatial density. σeff contains the information on the parton distribution. Possible analysis strategy: • 1) Same sign W production • 2) 3jets + γ • like Tevatron (CDF e D0) • 3) Count of charged mini-jets pairs in MinBias events m=2 for distinguishable scatterings m=1 for undistinguish. scatterings 26

  27. Double parton scattering Consider the case of same sign W production Same sign W production from Double int. • Cross-section from DS is larger up to a factor ~10 in some kinematical regions respect to SS • Hence one cannot ignore the effect of DS as additional source of background processes like: • HW • W(Z)+jets, W(Z)b+jets, W(Z)bb+jets • tt→llbb, tb →bbln • bb+jets • final state with multi jets (pTmin~20,30 GeV) Same sign W production from Single int. [D.Treleani et al.]

  28. Double parton scattering – 3jet+gamma One has to distinguish between the two topologies : Double Scattering:Single Scattering: The pairs can be isolated using the variable: & considering the ΔΦ(+)between pT1+ pT2e pT3+ pT4 28

  29. Double parton scattering - 3jet + gamma CDF Tevatron: σeff ~12 mb MPI=ON D0 CMS plans the same analysis to estimate σeff

  30. MPI –Direct measurement The basic idea is: + Reconstruct charged minijets in MinBias + Associate minijets in pairs + Use number of pairs as the number of MPI + Determine σeff where σinel = σsoft + σHard “S” = Single Interactions, “D” = Double Interactions, “Hard” = Inclusive 30

  31. Number of Pairs MPI ON Number of Pairs MPI OFF MPI specials - direct measurement + Minijets are reconstructed by tracks clustering and are characterized by a very low pT threshold + The minijets are paired in Δϕ & pT balancing + Number of pairs <-> number of interactions measure σeff(pT) + the measurement is independent of the geometric acceptance + but high dependence on radiation

  32. Conclusion + QCD studies at LHC (xx TeV) are closely connected with the possibility of a correct interpretation of the future experimental data, with the precision of the measurements of “old” physics at higher energies, and with the sensibility for “new” phenomena + the ongoing analysis groups in CMS cover most sectors of QCD, both at low and at high momentum transfer + some of the already performed analyses with simulated data have shown the good potential of a measurement at LHC startup (low_pT) & “early”, 10 pb-1 (high_pT), the possibility of a first tuning of CMS MC at the startup and the sensibility to “new” physics in a “short” time. 32

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