Physics at Hadron Colliders Part II Marina Cobal
Physics at Hadron Colliders Part II Marina Cobal Università di Udine 1
The structure of an event One incoming parton from each of the protons enters the hard process, where then a number of outgoing particles are produced. It is the nature of this process that determines the main characteristics of the event. Hard subprocess: described by matrix elements 2
An event: resonances The hard process may produce a set of short-lived resonances, like the Z 0/W± gauge bosons. 3
Resonances • In this range the momentum scale is known at the permill level. • it is a cross-check of the detector performance in particular for the lepton energy measurements 4
The structure of an event: ISR One shower initiator parton from each beam may start off a sequence of branchings, such as q → qg, which build up an initial-state shower. Initial state radiation: spacelike parton shower 5
The structure of an event: FSR The outgoing partons may branch, just like the incoming did, to build up final-state showers. Final state radiation: timelike parton showers 6
An event: Underlying events • Proton remnants ( in most cases coloured! ) interact: Underlying event, consist of low p. T objects. • There are events without a hard collision ( dependent on p. T cutoff)
An event: Underlying events Underlying event: • Multi-parton interaction • Beam-beam remnants • Initial/final state radiation
Underlying Event • Studying underlying event is crucial for understanding high p. T SM events at LHC. • ingredient for many analyses. In fact they affect: the jet reconstructions and lepton isolation, jet tagging etc. . • One can look at charged track multiplicities Nch in transverse regions which are little affected by the high p. T objects. • Reasonably described by models 9
The structure of an event: Pile up In addition to the hard process considered above, further semi-hard interactions may occur between the partons of two other incoming hadrons. ‘Pile-up’ is distinct from ‘underlying events’ in that it describes events coming from additional proton-proton interactions, rather than additional interactions originating from the same proton collision.
Pile up 2012 ATLAS event; Z in with 25 primary vertices Z in event with 25 vertices 11
• Multiple interactions between partons in other protons in the same bunch crossing – Consequence of high rate (luminosity) and high proton-proton total cross-section (~75 mb) Pile up without pile-up Et ~ 81 Ge. V Et ~ 58 Ge. V • Statistically independent of hard scattering – Similar models used for soft physics as in underlying event Prog. Part. Nucl. Phys. 6 0: 484 -551, 2008
• Multiple interactions between partons in other protons in the same bunch crossing – Consequence of high rate (luminosity) and high proton-proton total cross-section (~75 mb) Pile up with design luminosity pile-up Et ~ 81 Ge. V Et ~ 58 Ge. V • Statistically independent of hard scattering – Similar models used for soft physics as in underlying event Prog. Part. Nucl. Phys. 6 0: 484 -551, 2008
Challenge Pile up: example ET Important for quantities, affected by soft hadrons, for example; ETmiss = -| Σ p. T | miss without PU suppression with PU suppression Use data! • Requirements on track vertexing • Number of reconstructed vertices proportional to the pile-up • Measure pile-up density event by event: Use it to subtract from the jets energy a pile-up term. do the same with isolation cones. 14
Minimum bias events The underlying event ¡ • Inelastic hadron-hadron events selected with an experiment’s “minimum bias trigger”. • Usually associated with inelastic non-single-diffractive events (e. g. UA 5, E 735, CDF … ATLAS? ) ¡ Need minimum bias data if want to: 1) Study general characteristics of proton interactions 2) Investigate multi-parton interactions and the structure of the proton etc. 3) Understand the underlying event: impact on physics The “soft part” associated with hard scatters ¡ In parton-parton scattering, the UE is usually defined to be everything except the two outgoing hard scattered jets: Beam-beam remnants. 1) Additional parton-parton interactions. 2) ISR + FSR ¡ Can we use “minimum bias” data to model the “underlying event”? Ø At least for the beam-beam remnant and multiple interactions?
Minimum bias • Non head-on collisions, with only low p. T objects. Those are the majority of the events in which there is a small momentum transfer Δp ~ h/Δx • Distributed uniformly in η: d. N/dh = 6 • On average the charged particles in the final state have a p. T~500 Me. V Not well described by models! Shape is sort of OK Normalisation is off 16
Minimum bias • It is interesting by its own to study such events. Also an ingredient for many analyses you will see. • A necessary first step for precision measurements (such as top-quark mass) • A key ingredient to modelling pile-up • As can be seen most of the events do have quite low p. T • Anyhow those events constitute a noise of few Ge. V per bunch crossing 17
Monte Carlo Simulations • Attempt to simulate all physics and experimental aspects as well as possible in MC • Examples shown here: – Pile-up – Jet response – Electron acceptance on detector level – Corrections from quark to jets • Use data ('data-driven' techniques) to verify that MC is correct w. r. t all relevant aspects 18
Monte Carlo Simulations • MC contains two aspects – description of detector response → efficiency, resolutions – description of shapes (physics model) → acceptance • This allows to translate the cross section measurement into a determination of a correction: N. B. assuming good description of efficiency and acceptance by MC – uncertainty ? 19
Monte Carlo for Processes with jets
Parton shower
MC simulation of LHC event Detector simulation Particles Hadronisation QCD and QED radiation Hard partonic scattering Incoming parton distributions Additional partonic scatters
A Monte Carlo Event Hard Perturbative scattering: Usually calculated at leading order in QCD, electroweak theory or some BSM model. Modelling of the soft underlying event Multiple perturbative scattering. Perturbative Decays calculated in Final QCD, State EW or Initial and parton showers resum the large QCD Finally the unstable hadrons are decayed. some theory. logs. BSM Non-perturbative modelling of the hadronization process.
Uncertainties • • Statistical uncertainties, due to finite number of events Systematic uncertainties, due to errors and biases in the analysis • Simplest, most-often-used approach: assume that systematic errors are mutually independent, i. e. uncorrelated – – • • make list of all sources of systematic uncertainties remove those that are correlated with others repeat analysis for variation of each uncertainty separately add variations up in quadrature More complex treatment of systematics not addressed today Most analysis work goes into dedicated studies aiming to minimize the systematic uncertainty 24
Table of uncertainties Example: CMS top pair production in di-lepton channel Experimental aspects Theory uncertainties backgrounds
SM processes • No hope to observe light objects ( W, Z, H) in the fully hadronic final state! • We need to rely on the presence of an isolated lepton! • Fully hadronic final states can be extracted from the backgrounds only with hard. O(100 Ge. V) p. T cuts-> works for heavy objects! 26
QCD Sector
Snapshot of QCD
QCD vertices
Colour factors
QCD Potential
Jets from quarks and gluons • Quarks and gluons cannot exist as free particles -> hadronization • Collimated stream of charged and neutral hadrons -> QCD jets
Where do Jets come from at LHC? inclusive jet cross-section • Fragmentation of gluons and (light) quarks in QCD scattering • Most often observed interaction at LHC
Multi-jet events at LHC
Jet multiplicity • Another possible test of QCD is obtained by checking the jet multiplicity • Tests also the modelling of the radiation 35
Where do Jets come from at LHC? • Decay of heavy Standard Model (SM) particles Prominent example: top mass reconstruction
Where do Jets come from at LHC? Associated with particle production in Vector Boson Fusion (VBF) E. g. , Higgs
Where do Jets come from at LHC? • Decay of Beyond Standard Model (BSM) particles – E. g. , SUSY missing transverse energy jets ele s n o r ct o ns o u rm
What is a jet?
How to identify jets? Jet algorithm should collect all particles in the same way for: • • Leading order partons Partons+gluon emission Parton shower (soft) Hadrons-> detector
Jets • Definition (experimental point of view): bunch of particles generated by hadronisation of a common confined source – Quark, gluon fragmentation • Signature – Energy deposit in EM and HAD calorimeters – Several tracks in the inner detector • Calorimeter energy measurement - Gets more precise with increasing particle energy - Gives good energy measure for all particles except ’s and ’s - Does not work well for low energies - Particles have to reach calorimeter, noise in readout 41
jet algorithms
Jet Reconstruction Task
Jet Reconstruction • How to reconstruct the jet? – Group together the particles from hadronization – 2 main types • Cone • k. T 44
Jet reconstruction algorithms: cone
Jet reconstruction algorithms: Kt
Di-jet quark flavours ar. Xiv: 1210. 0441 v 3
Jet physics: jet energy scale Before looking at jet physics be aware of few issues, first of all when we have steeply falling cross sections-> we have a sensitivity of its measurement from the energy scale -Jet energy determined from calorimeter (+tracking information) -Sophisticated calibration procedure Different contributions to JES error. (jets reconstructed with the Anti-k. T alogrithm cone 0. 6 that is used in ATLAS)
Jet production • NLO QCD works over ~9 orders of magnitude! • excellent exp. progress: jet energy scale uncertainties at the 1 -2% level • for central rapidities: similar exp. and theo. uncertainties, 5 - 10% • inclusive jet data : starts to be important tool for constraining PDFs, eg. also by using ratios at different c. o. m. energies 50
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