Part 1

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

2. Our present knowledge of HEP is mainly based on the Standard Model , a quantum – relativistic theory The basic simmetry is a local gauge invariance , in which the generators are described by a SU(3)xSU(2)xU(1) algebra Phenomenologically the model includes 3 elements: 1) 3 fermion generations of quarks & leptons ++ a large mass interval 10-10 -> 102 GeV ++ CP violation in the transitions between quark of different generations 2) The vector bosons carrying the interaction 3) The Higgs mechanism giving the mass to fermions & bosons The SM describes very well most of the observed phenomena [with precision in the sector U(1) ~ O(1/10⁸), in SU(2) ~ O(1/10³), in SU(3) ~ O(1/10)]

3. However… the Higgs has not been observed yet More: the neutrino mass (connected to the oscillation observed in solar neutrinos) , the dark matter, the barionic asimmetry of the universe are not foreseen in the SM Also gravity remains outside the formal picture. So, the SM seems not complete and additional theoretical structures valid at the TeV-scale are necessary

4. Why LHC + Hadronic collisions + QCD

5. Present status (briefly) Why LHC + Hadronic collisions + QCD

6. Now the problem was how to accede the ~ TeV energy range and to get sufficient energy (+ luminosity) to perturbate the Higgs field The idea was to build an hadronic collider profiting of the “old” CERN-LEP infrastructures. Why hadronic ? Because it is easier and less expensive to get the same CM energy... but final states are more difficult to disentangle.

7. The structure of interactions at the LHC If the interaction energy is high enough one can “see” the proton with sufficient time resolution to observe the reactions between the gluons and the sea quarks. Partonic cross-section PDF (parton distribution functions): Sum of all initial states leading, at the Q-scale , to pj = xP(proton) Transition from the partonic final state to the final state hadrons (hadronization, fragmentation, jet definition...)

8. S is small at high energies because of theproperty of asymptotic freedom. The role of LEP in determining the size of s hasbeen crucial Most recent result based on inclusive jet x-section measurement from D0 gives S (mz)=0.1173

9. Running coupling costant: At large Q, αs(Q) →0 , so it is possible to use perturbation theory But even at high energies one cannot neglect the problem of confinement, since both the initial state (protons) and the final state detected by experiments (pions, kaons, protons, neutrons, etc) are hadrons • •Separate the dynamics at low Q from that at high Q • •Identify the relevant approximations needed to isolate the most • important contributions in both energy domains, and apply them • separately • This will lead to the study of two dominant classes of processes: • •soft gluon emission • •collinear emission

10. Parton density functions (PDFs) The time-scale of interactions binding the proton is o(1/mP) If a hard proble (Q>>mP) hits the proton there is no time for quarks to communicate,so to study inclusive processes at large Q it is sufficient to consider the interactions between the external probe and a single parton Cross section will depend on Q2 and momentum fraction (x) of partons seen by the probe, D(x, Q2) The evolution is described by the wellknown DGLAltarelliParisi equation: QCDpredicts the scale dependence of fi(x,Q2) through DGLAP evolution equations BUT does not accurately predict the x-dependence which has non perturbative origin

11. Parton density functions (PDFs) From the HERA4LHC workshop: + Combined HERA results (big improvement in to the fit precision) + pdf small x gluon -> fundamental at LHC + theoretical improvements + data -> PDFs at LHC => Working Gr for PDFs “the ultimate fit” Main sources of uncertainty: Theory + perturbative calculations +non-perturbative parametrisations (x-dependence) Experiment + statistical and systematic uncertainties on experimental data inputs

12. Final state We would expect only few partiicles .In effect the experimental hadronic final states contain a large number of particles...not only the 2 or 3 as predicted by perturbative calculations

13. Radiation Soft-hard factorization

14. Parton fragmentation 1+ Parton evolution can be represented as a branching process from higher values of x 2+ DGLAP equation predicts PDFs evolution So, a “shower” Monte Carlo is needed to give a complete description of a hadron scattering 3+ Due to successive branching, parton cascade or shower develops. Each outgoing line is source of new cascade, until all outgoing lines have stopped branching. At this stage, which depends on cutoff scale t0, outgoing partons have to be converted into hadrons via a hadronization model. Eg String fragmentation m. Cluster fragmentation m.

15. Finally contributions to the final state are expected from: + particles arising from recombination of the proton not fragmented part (remnants) + activity due to multiple parton interactions MPI

16. Structure of an interaction at the LHC...complications Main interaction ISR e FSR Jet creation Fragmentation & Hadronisation MPI Beam Remnants ? proton proton 16

17. In order to implement in the models the phenomena due to the “soft” dynamics let us define two useful concepts Minimum Bias events (MB) Underlying Event (UE)

18. Minimum Bias (MB) events • The generic single particle-particle interactions • + Elastic + Inelastic (including Diffractive) • + Soft: low PT, low Multiplicity… • What one would observe with a fully inclusive detector/trigger • Underlying Event (UE) activity • All the activity from a single p-p interaction • superimposed to the hard scattering process • + Initial and final state radiation • + Spectators • + beam-beam remnant • + Multiple Parton Interactions • The UE is related to the hard scattering • + primary vertex sharing • + “pedestal effect”: activity is related to the energy scale • + color and flavor connected • UE ≠ Minimum Bias • but phenomenological aspects are similar

19. Multiple interaction models Observation -> higher multiplicity than expected! Fragmentation and showering are not enough (ISR, FSR, spectators) Experimental evidence for MPI in a single hadronic interaction from SpS & Tevatron IDEA: extend the pQCD to soft regime and PT cut-off is the main parameter which + allows x-sect regularization for PT->0 + is directly related to the number of interactions Pythia is the most used event generator model including MPI: + variable impact parameter + gaussian matter distribution +its parameters can be varied and TUNED

20. Why is important to understand soft dynamics?? ? Exploring Fundamental aspects of hadron-hadron collisions Structure of Hadrons Factorization of interactions Tuning of Monte Carlo Models Discoveries MPI and any additional activity which superimpose the signal can fake any results of “old” and new physics Calibration of major physics tools Jet Energy Photon/Lepton Isolation Vertex Reconstruction Understanding the detector Occupancies Backgrounds etc… There the experience of CDF...

21. Monte Carlo models Why modeling is so important ? + Monte Carlo models summarize our knowledge of strong dynamics + Unfortunately our knowledge is √S-dependant: present models, tuned on SpS or Tevatron, diverge if extrapolated to higher energy + In addition models depend on observables (is there a universal tune?) + A lot of work is going on to understand dynamics from past and be prepared for future How ? For instance from basic observables ?

22. MPI – MinBias observables - different models + Pythia Tune DW OLD MPI model, IP correlations + Pythia Tune DWT DW with default PT cut-off evolution + Pythia Tune S0 New MPI model, more correlations Charged multiplicity dN/d vs  dN/dPt vs Pt <Pt> vs multiplicity All these tunes describe UE@Tevatron, but disagree each other if extrapolated to 14 TeV

23. Tune models using Underlying Event observables From charged jet (using MB and jet triggers) Topological structure of p-p collision from charged tracks Charged jet definition -> JetClustering algorithm with massless charged tracks as input The leading Ch_jet1 defines a direction in the f plane The transverse region is particularly sensitive to the UE Main observables: + dN/dhdf, charged particle density + d(PTsum)/dhdf, energy density From D-Y muon pair production (using muon triggers) observables are the same but defined in all the f plane (after removing the m pairs everything else is UE)

24. CDF: UE in Jet and DY topologies [Rick Field, HERA/LHC, May 08] Z boson or Jet Jet or DY topology: Underlying Event universality Data at 1.96 TeV on the charged scalar PTsum density, dPT/dhdf, with pT > 0.5 GeV/c and |h| < 1 for “Z-Boson” and “Leading Jet” events as a function of the leading jet pT or PT(Z) for the “toward”, “away”, and “transverse” regions. The data are corrected to the particle level (with errors that include both the statistical error and the systematic uncertainty) and are compared with PYTHIA Tune AW and Tune A, respectively, at the particle level (i.e. generator level).

25. Conclusion Part 1 • In describing the p-p interactions expected at the LHC the dynamics has been separated in • + perturbative, high_pT, defined as : • + running alfa_S • + hard scattering • + fragmentation and evolution • + low_pT: • + perturbative approach not always possible • + more connection with phenomenological models • In part 2 we consider the QCD studies foreseen with CMS( in particular those involving the Perugia group) , some details about LHC and the detectors and the possibility of a correct interpretation with CMS of the signals generated by the p-p interactions.