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The Large Hadron Collider Machine, Experiments, Physics SM Physics (at the LHC)

The Large Hadron Collider Machine, Experiments, Physics SM Physics (at the LHC). Johannes Haller Thomas Schörner-Sadenius Hamburg University Summer Term 2009. SM MEASUREMENTS: OVERVIEW. Heavy-flavour physics (c,b) B S , B C hadrons.

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The Large Hadron Collider Machine, Experiments, Physics SM Physics (at the LHC)

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  1. The Large Hadron ColliderMachine, Experiments, PhysicsSM Physics (at the LHC) Johannes HallerThomas Schörner-Sadenius Hamburg UniversitySummer Term 2009

  2. SM MEASUREMENTS: OVERVIEW Heavy-flavourphysics (c,b)BS, BC hadrons QCD / jets, strong coupling,Underlying event,diffraction, forward physics, QGP, ET,miss Top physics Prompt photons • W,Z cross-sections • EW parameters (asymmetries, sin2θ) • MW and Γ(W) • Boson couplings • anomalous quark couplings? • PDFs? Further gauge bosons? • Lepton universality • Zll • Z’ll QCD Not shown: Importance of particles for BSM measurements (as signal or background)!Mostly results from CMS simulations (newer “Physics TDR” than ATLAS)!

  3. QCD MEASUREMENTS: JET YIELD AT LHC Remember: Cross-section for production of jetsfrom quarks and gluons in pp collisions: Remember: parton-parton lumi at LHC and Tevatron: CDF data Huge pT values are reachable  Test of pQCD to highest scales! Large sensitivity to new physics!

  4. JETS: ALGORITHMS, EXAMPLE EVENTS jet e– p remnant jet jet jet neutrino jet jet Either “cone” algorithms or clustering a la kT algorithm.

  5. JETS: EXAMPLE EVENTS

  6. JETS: EXAMPLE EVENTS

  7. QCD MEASUREMENTS: UNCERTAINTIES Unfortunately (or luckily?): large theory uncertain-ties for predictions (up to 100%?): – scale μR of strong coupling αS(μR). Effect due to truncated perturbative expansion in powers of αS: Often dominates the theory! – PDF uncertainty Other view on PDF uncertainties: Uncertainty when using best HERA PDFs, and possible improvements due to all HERA data:

  8. UNCERTAINTIES: HERA VERSUS TEVATRON Tevatron goes to high ET, but with large uncertainties – especially jet energy scale (yellow) and PDF (red lines). HERA: much smaller ET, but with small uncertainties less than 10% (depending on observable).

  9. QCD MEASUREMENTS Also at LHC: Large experimental uncertainties. Dominated by jet energy scale determination. Remember that (multi)jets have high cross-sections(clearly higher than W,Z production) triggering no bigissue – but need to keep rate under control – also for background subtraction! Example CMS QCD background to(high-mass) SUSY!

  10. QCD MEASUREMENTS: PDFS? M = 10 TeV M = 1 TeV M = 100 GeV LHC covers much larger range in x and Q2– but can this be used to learn more about the PDFs f(x,Q2)? No simple answer – depends dramatically on experimentaland theoretical precision (remember that PDFs are ex-tracted in comparisons of (N)NLO theory with data). Lots of work done especially by Oxford fitters within theATLAS collaboration (M. Cooper-Sakar et al.). Lots of physics at the LHC play at (very) low x! But new heavy resonances require (very precise) high x! Jets at the LHC might help themselves – but depends critically on uncertainties (jet energy scale to 1%???)

  11. QCD MEASUREMENTS: PDFS? Might also use prompt photons (cleaner than jetsbecause no hadronisation involved – but large backgrounds): • Then try to distuingish predictions for these processes using different PDF parametrisations (two very close blue curves in plot below): • There is some potential, but it requires extremely good photon ID, fake photon rejection, and a photon selection efficiency of above 90%. Alternative ideas are to use Z+b events  access to b PDF: ppb+gZ+b! Note that also determination of αS possible from jets!

  12. QCD MEASUREMENTS: MINIMUM BIAS Porting these findings to the LHC requires know-ledge of the energy behaviour of minimum bias: • At the LHC, up to 25 proton-proton collisions will takeplace in one bunch-crossing (every 25 ns) – pile-up! • Most of these events will be soft – will not involve a hard QCD scattering, but a rather soft distribution ofparticles with low transverse momenta – minimum bias! • Difficult to model (no hard scale for pQCD!), they are • an important background for all studies • interesting in themselves • … and a good tool to monitor detector performance! • Tevatron: charged particle flows and pT spectra: • Much larger discrepanies! So what is the real particle flow at the LHC? • Will be among the first questions to be investigated at the LHC! … but it gets worse: Underlying events!

  13. QCD MEASUREMENTS: UNDERLYING EVENTS

  14. A FULL EVENT

  15. QCD MEASUREMENTS: UNDERLYING EVENTS • The underlying event (UE) is defined as everything in the event except the hardest scattering: • Minimum bias. • Proton remants. • Initial- and final-state radiation. • multiple parton interactions (MPI) • Investigation: transverse regions! Experience from Tevatron: UE can be described! But extrapolation to LHC fails drastically!

  16. QCD MEASUREMENTS: UE AT HERA Example: 3- and 4-jet cross-sections in photoproduction With UE model Without UE model

  17. QCD MEASUREMENTS: PROMPT PHOTONS Prompt photons from QCD events are – powerful QCD test (cross-section calculations similar to jet cross-sections)  PDFs?– difficult to measure: high backgrounds from QCD jets, and neutral mesons (π0).– Background to (and playground for) photons in Hγγ events. Jet rejection based on HAC energy, shower shape: But we have high statistics and efficiency: d2σ/dηdpT [pb/GeV]

  18. QCD MEASUREMENTS: NEW PHYSICS Contact interactions and alike: expect modification of dijet cross especially at high scales / masses – where the uncertainties are large  need high theoretical and experimental precision! Also taking other observables than just ET helps! Dijets from decay Xjetjet for BSM searches! Sensitivity depends on mass and cross-section:  Difficult to separate using only mass! Need very precise modelling of QCD background (NLO theory, control of uncertainties)! Example: Z’qq SUSY QCD δΦ(pT,miss,jet2) δΦ(pT,miss,jet2)

  19. ELECTROWEAK PHYSICS: W,Z XSECTIONS O(as) q W q W g g q’ q’ q’ • W and Z production ppW/Zll/ν is a high-rate, clean environment for • Luminosity determination (precise NNLO σ!)- PDF information. • - tests of couplings (consistency with SM?).- tests of higher-order QCD corrections About 100 W/s at the LHC (1/s at the Tevatron!)

  20. ELECTROWEAK PHYSICS: W,Z AND PDFs Theory predictions for W and Z boson productionare available in NNLO! How precise are thesepredictions? So NNLO gives a rather precise (1%) answer – but the PDFs give a problem: So sometimes more difference between differentmeans than errors would allow – and newer results(CTEQ6.5) make it even worse – 8% difference! Study: Inclusion of W,Z data in PDF fits would changethese substantially! Electron rapidity spectrumfrom W decay: pseudodataversus prediction. No ATLAS data With ATLAS data

  21. ELECTROWEAK PHYSICS: Z XSECTIONS Wlν Zll Relatively clear signatures: *A ~ 10% Purity 98% Tevatron results: σZ = 264.9 ±3.9 stat ±9.8 syst ±17.2 lum (pb) (e) σZ = 261.3 ±2.7 stat ±6.3 syst ±15.1 lum (pb) (μ)

  22. ELECTROWEAK PHYSICS: Zqq COUPLINGS However, compared to LEP, the reach is rather limited: Extract, from ppZee events, the couplings of the Z to u and d quarks:  SM fits!

  23. ELECTROWEAK PHYSICS: W XSECTIONS Measurement in small part of CDF data (by nowmore precise measurements out): So far all results are compatible with expectationsfrom the SM: data compared to NNLO theory!  Also couplings according to SM expectations! *A ~ 23(10)% Purity 97/90% CDF: = 2719 ± 10stat ± 53sys ±165lum (pb)

  24. ELECTROWEAK PHYSICS: W XSECTIONS CDF data compared to NNLO theory! Comparisons of various Wlν cross-section measurements: • NNLO theory works very well at the Tevatron • Good tool for studies at the LHC!

  25. EW PHYSICS: W MASS + WIDTH TEVATRON Relevance of MW: – SM makes clear predictions for MW and its connection to Z mass: So measurements of MW and MZ are stringent tests of SM! – But MW also sensitive to higher-order effects due to vacuum fluctuations: These effects are entering mainly through rW.  Sensitivity to MH (especially when combined with top mass measurement) and to SUSY! Plot on the right: Connection of masses of top, Higgs, and W. Current measurements prefer ratherlow values for MH! • Method for MW extraction: “Template method”: • Final state neutrino  no direct MW reconstruction.- Use correlation of MW with transverse mass: • Fit distributions of MW,T(MW), pT,l etc. to data  measurement on statistical basis only!

  26. EW PHYSICS: W MASS + WIDTH TEVATRON Example of fit to MW,T: from CDF. Overview on various MW determinations: Current world average of MW: Aim LHC: Errors less than 15 MeV !

  27. EW PHYSICS: W MASS + WIDTH TEVATRON Example of fit to MW,T: from CDF. Overview on various ΓW determinations: Aim of Tevatron Run 2: dGW < 40 MeV per experiment

  28. W WIDTH INDIRECT TEVATRON Also indirect determination of W mass: Measure R: This allows extraction of CMK parameter Vcs: pQCD LEP SM EW CDF: R(e+μ) = 10.92 ± 0.15stat ± 0.14syst DØ : R(e) = 10.82 ± 0.16stat ± 0.28syst ΓW = 2078.8 ± 41.4 MeV (CDF) ΓW = 2118 ± 42 MeV (World) ΓW = 2.0921 ± 0.0025 GeV (theory) Vcs = 0.967 ± 0.030

  29. W MASS + WIDTH LHC Also MW and ΓW at the LHC: Huge statistics! 10-100 W bosons produced per second  aim for precision measurement! Remember Tevatron: Aim at LHC: <15 MeV errors. Needed (if top mass error <2 GeV) so that MW does not dominate error on Higgs mass determination! Systematics in electron, muon channels largelyuncorrelated; dominated by energy scale+resolution, and muon pT resolution. That’s what a signal could look like! Achieve 20 MeV precision with first 10 fb-1? Assumed electron ETspectrum in Weν

  30. ELECTROWEAK PHYSICS: ASYMMETRIES (1) Remember AFBin e+e- collisions: Asymmetry allowsimportant tests of couplings and Weinberg angle! The asymmetry is caused by the V-A structure of the EW Z interactions: eeγ/Zff

  31. ELECTROWEAK PHYSICS: ASYMMETRIES (2) l- p p θ l+ Remember AFBin e+e-: The measurement was best done in eebb events because of the slow variationof Ab with sin2θ  Best way to access Ae and thus the effective Mixing angle for electrons! Tevatron not really able to compete here – mainlybecause of statistics – would need 10 fb-1! How about the LHC? Study the phenomenon in Drell-Yan process ppZll: AFB - Sensitive to new physics via new terms or interference.- Possible access to PDFs (sea!)

  32. ELECTROWEAK PHYSICS: ASYMMETRIES (3) AFB Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) uuee Mee [GeV/c2] • Especially at Mee > LEPII AFB is very interesting – sensitivity to multi-TeV resonances (like Z’!). • Example here: different Z’ couplings, masses! • Precise measurement of AFB will reveal structure beyond SM! First results from Tevatron Run II  so far it looks like SM!

  33. ELECTROWEAK PHYSICS: ASYMMETRIES (4) xf(x,Q2) u d – d – u log(x) Alternatively: Consider W charge asymmetry: … with rapidity y: Predictions for two different PDF sets: These differences can be explained in terms of the different valence quark parametrisations in the two PDF sets: A A y y

  34. ELECTROWEAK PHYSICS: ASYMMETRIES (5) CTEQ and MRST valence distributions at two different values of Q2 = MW2:  This indicates potential to further constrain PDFs! However, we only measure the lepton asymmetry, not directly the W charge asymmetry: Convolution of production asymmetry with V-A structure

  35. ELECTROWEAK PHYSICS: ASYMMETRIES (6) • CDF measurement in small data sample: • Consider large data errors at high lepton rapidities! Can be improved by LHC! • Also PDFs least constrained for high y  Potential to constrain the PDFs there!!! Newer measurement does not confirm discrepancybetween data and NLO theory (with full PDF uncertainty indicated as band):  Is there something interesting hidden? Only further data from Tevatron or input from LHC can clarify!

  36. EW PHYSICS: MULTI-BOSONS VERTICES SM: triple and quartic gauge boson couplings (con-sequence of non-abelian structure of underlyinggauge group SU(2)LxU(1)Y):… but no γZZ vertex! In pp reactions, the following diagram with WZ final state is unique, since it allows separation of the WZZ and WWγ vertices! All this can be seen from the Lagrangian: Write this in more generic way  allow for SM extensions (only W vertices, simplest extension): SM: g, κ, λ=1.  measurement of κγ, λ,gZ is powerful test of SM. Also: Relation W couplings and static W features:

  37. EW PHYSICS: MULTI-BOSONS EVENTS Non-SM! For studies of multi-boson vertices (TGC, triple gauge coupling) select events with two gauge bosons, especially via leptons (W,Z). Examples (signals + backgrounds): At LHC: mainly WZ and ZZ measurements!

  38. EW PHYSICS: MULTI-BOSONS: TEVATRON Example 2: Zγ final states with Zll. Question: Do we observe a non-SM ZZγ contribution? Answer: No! So far: – TGC observed and measured. – Limits on non-SM couplings derived. – Cross-sections for diboson production measured with good accuracy. Example 1: W+Z selection in about 2 fb-1 (CDF):

  39. EW PHYSICS: MULTI-BOSONS: LHC Also photon channels will be included! Very precise samples can be selected already in verysmall data sets at the LHC (5σ statistical significancealready for 150 pb-1 of data in the WZ channel, includingsystematic uncertainties!)  LHC will allow for detailed analyses of the TGCs!.

  40. LEPTON UNIVERSALITY Remember V-A coupling of leptons in the SM with coupling strength g: Question: Do all leptons e, μ, τ have the samevalue of g – as predicted by the SM? Measurablefor example in Zll or Wlν events – possibly withcorrections for τ mass etc. CDF preliminary results (note BR(Wlν ~ g2(ν)): CDF preliminary from R(μ)/R(e)=g2(μ)/g2(e): Nature seems to obey lepton universality!

  41. FUTURE OF EW SM MEASUREMENTS

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