Standard Hadron Colliders I Basics & QCD P.Mättig Bergische Universität Wuppertal

Peter Mättig, CERN Summer Students 2014 Pillars of the Standard Model

Standard model: Comprehensive and precisly tested Peter Mättig, CERN Summer Students years of experimental scrutiny LEP, SLC, HERA, Tevatron, PETRA, PEP, SppS, …. LHC Standard model ‘on the shoulder of giants’ Answers to questions of a century ago!

Standard model pillar I: Matter Twelve kinds of spin ½ fermions to build the world LHC‘s role: Heaviest Standard Model particle - Top Quark: unique at hadron colliders Peter Mättig, CERN Summer Students 2014 Neutrinos not really hadron colliders Most quarks/all charged leptons very deeply scrutinized

Standard Model pillar II: Forces Peter Mättig, CERN Summer Students 2014 realised by three Spin 1 bosons Dynamics known to a precision of – LHC‘s mission: -probe rare processes -probe dynamics at unprecedented energies Three interactions to hold world together

Standard Model Pillar III: Higgs Massive Bosons and Fermions  Basic principles of SM violated  Cross section violates unitarity SM: new elementary boson LHC‘s mission: find it !!!!!!! MISSION ACCOMPLISHED!!!!! (????) Peter Mättig, CERN Summer Students 2014 Spin 0 particle to prevent theory from exploding

LHC‘s quest: ‚New Physics‘ Understanding SM precondition to establish ‚New Physics‘  Standard Model processes background  Tools  Standard Model to the extreme Peter Mättig, CERN Summer Students 2014 LHC entering unchartered territory Our dream for the next years

Today‘s Santa Maria: LHC Peter Mättig, CERN Summer Students 2014 Proton – Proton 14 TeV c.m. energy (up to now 8 TeV) Focus on results from ATLAS and CMS (LHCb  Tim Gershon ALICE  Constantinos Loizides) LHC increases energy range by factor ~ 10 Precision (statistics) by factor ~100 or more Historically almost unprecedented step in energy/precision!

Peter Mättig, CERN Summer Students 2014 How protons interact

Proton interactions: complex events Peter Mättig, CERN Summer Students 2014 More than 1000 particles! Only fraction interesting Experimenters task: - Identify those - Classify events Reveals picture of space - time of m

hard scatter: two in  two out e.g. gluon-gluon  top-antitop Peter Mättig, CERN Summer Students 2014 x 1 * P x 2 * P t t This can be well calculated Incoming: gluons of momenta p 1, p 2 Outgoing: e.g. pair of top / anti-top quarks

Characterising scattering process Peter Mättig, CERN Summer Students 2014 High Q 2  probe of small distances Quark A scattered Quark Quark B outgoing Quark gluon  High resolution, But rare:  ~ 1/M 2

Add-on 1: Parton distribution function Peter Mättig, CERN Summer Students 2014 Parton energy: only fraction x of proton energy Statistical distribution: parton distribution function (pdf) F f For X-section: all combinations (x 1, x 2 )  M contribute Example: M = 0.3 TeV from (x 1 =0.2, x 2 =0.2),(0.25, 0.16), (0.4, 0.1),.... F f (x) depends on -kind of parton f -Q 2

How to derive pdfs Peter Mättig, CERN Summer Students 2014 Parton distribution functions (pdf): Not calculable from QCD, instead derive from data (ep, p,....) Take from theory Not needed for lp Some dependence on -Input data -Calculation -Parametrisation  Several schools of pdfs CTEQ, MSTW, HERApdf ….

Basic Features of pdfs Peter Mättig, CERN Summer Students 2014 High x: mostly quarks Low x: mostly gluons Valence u, d quarks: x  0.2

Significant uncertainties Peter Mättig, CERN Summer Students 2014 G.Watt LHC processes sensitive to pdfs: Specific processes will allow to disentangle contributions  some self – calibration of pdfs High x: few gluons  Large uncertainty low x: many gluons  Theoretical & experimental uncertainties

Add on 2: Interaction disturbed by gluons Peter Mättig, CERN Summer Students 2014 calculable (although in most cases not completely) (N)NLO calculation, i.e. full matrix element with up to two additional partons O(  s 3 ) Beyond this: iterative parton splitting (Markov chain) no interferences considered

Peter Mättig, CERN Summer Students 2014 Quarks and Gluons turn into pions, kaons, protons: hadronisation partons turn to hadrons No fundamental description  model with many parameters

Peter Mättig, CERN Summer Students 2014 Remainung quarks and gluons interact ‚Underlying event‘ Colour flow of proton remnants Requires multidimensional integration  Monte Carlo simulation Several QCD generators: PYTHIA, HERWIG, SHERPA, ….. (see Fabio Maltoni’s lectures)

Noise & Signal

Noise & Signal at the LHC Peter Mättig, CERN Summer Students 2014 Noise: homogeneous distribution of low momentum hadrons: ‘underlying event’ Signal: two high energetic muons Understand and ‘subtract’ noise Muon

Standard Model tests Peter Mättig, CERN Summer Students 2014 Hard scatterpdfs Underlying event Three contributions only mildly related (‘factorisation’) but lead to higher uncertainties

Recurring variable: Rapidity Parton collisions not in rest system  rapidity accounts for boost Peter Mättig, CERN Summer Students 2014 For m  0: y   ‚ pseudo – rapidity‘ Typical for radiative multi-particles Difference of rapidities Lorentz invariant Separation is Lorentz invariant Simpler to measure, but  not Lorentz invariant

Peter Mättig, CERN Summer Students 2014 Soft interactions

No ‚direct‘ parton – parton hit Peter Mättig, CERN Summer Students 2014 Partons see each other ‘from distance’ Proton breaks into pieces  fly along the beam direction low Q 2  large ‘strong coupling’  s Perturbation theory breaks down:  s  a +   s  b …… for  s > 1 divergent Exact calculation not feasible approximate by model assumptions

Examples of model concepts Colour screening P t cut -off Overlap of protons Colour reconnection Peter Mättig, CERN Summer Students 2014 Introduces ‚free‘ parameters to be determined from data Soft processes provide constraints in special conditions Extrapolate to all topologies (and hope it will work) Challenging! Only an approximate description possible!

Separating underlying events Use Z 0  leptons production: Z 0 has no QCD colour no part of soft interaction  Soft contributions = Event - leptons Peter Mättig, CERN Summer Students 2014 Schematic approach Event = hard parton – parton scatter + soft contributions

Particles in underlying events Different models and different parameters  different predictions Model comparison to data  ‚best‘ model to describe underlying event Peter Mättig, CERN Summer Students 2014

99,999% of all LHC events: soft no trigger bias ‚minimum bias events‘ At 7 TeV: 6 charged particles per |  | = 1, mostly low p T Model parameters have to be adjusted Note: per LHC bunch crossing  currently 30 of these events (‚pile –up events‘) the price of high ‚hard‘ event rate Peter Mättig, CERN Summer Students 2014

Repeat notation for soft events Peter Mättig, CERN Summer Students 2014 Underlying Event: -collision of two specific protons -one hard parton interaction -UE: part of event that does not take part in hard interaction Minimum Bias Event: -(overwhelmingly) event with no hard interaction Pile Up Events: -Within same bunch crossing: several proton interactions -One of them leads to hard parton interaction -Others are minimum bias like  pile up events

Peter Mättig, CERN Summer Students 2014 Parton jets ‘hard’ QCD

Hard interaction: Jets Peter Mättig, CERN Summer Students 2014

Jets are universal Jets: bundles of hadrons  representative of quarks and gluons  direction, energy + (sometimes) parton flavour measurable  direct QCD tests possible  Experimental challenge: extract jets from 1000 particles Peter Mättig, CERN Summer Students 2014 e + e - collisions e p collisions

Jets are even more universal Peter Mättig, CERN Summer Students 2014

How to find a jet ? Unambiguous connection to underlying partons  Comparison to theory Anyway..... how many jets? ‘broadness’ of jets arbitrary  jet multiplicity depends on choice  defined according to physics Predefine how broad a jet should be! Peter Mättig, CERN Summer Students 2014

Sequential jet finder ‚Reverse evolution of event‘ 1Select one particle (e.g. most energetic) 2Find ‚most similar‘ particle, (e.g. smallest angle, p t ) 3Is combination smaller than predefined ‚cut off‘ value (e.g. maximum angle, maximum mass....) IF YES: 4 Combine to a new ‚pseudo – particle‘ (e.g. sum 4 – momenta) 5 Go to 2 IF NO: 4 Jet found: sum of all associated particles  Start next ‘jet’ Peter Mättig, CERN Summer Students 2014

Standard jet finding at LHC: ‚Anti – kt‘ © Gavin Salam Select hard particles as ‚seeds‘ for jets: favoured by min(p t -2 ) Hard particles separated in space are disctinct seeds: large  R ij ‚cut off‘ given by d ij (steered by R) Peter Mättig, CERN Summer Students 2014

Standard jet finding at LHC: ‚Anti – kt‘ © Gavin Salam Gradual d ij increase: Associate close by particles: mostly soft ones in neighbourhood (if no hard ones close by) Continue until R max reached Peter Mättig, CERN Summer Students 2014

The final jets © Gavin Salam All particles assigned to jets Close to circular in space good for experimental corrections Note: special treatment of particles close to beam Typical  R ≈ 0.4 – 0.6 Peter Mättig, CERN Summer Students 2014

Probing the strong force

The key test: jet cross section Calculated to Next-to-leading order (NLO) Peter Mättig, CERN Summer Students 2014 Steeply falling 7 orders of magnitude between 20 – 500 GeV  Significant tests require high experimental precision

Experimental challenge I: ‚JES‘ Jet energy determined from calorimeter (+ tracking information) How well is scale known? Jet energy scale (JES) uncertainty  Effects on X-section magnified by steep slope Calibration:  + jets data with p T balance Jet pt Log  True cross section Observed distribution due to wrong energy scale Jet energy scale known to 1 – 3%! 3% at low p T : jet/photon id 2% at high p T : statistics Peter Mättig, CERN Summer Students 2014

Experimental challenge II: JER sensitivity of measured cross section to energy resolution Jet pt Lo g  True cross section Exptl. resolution Jet pt Lo g  Observed due to energy resolution Peter Mättig, CERN Summer Students 2014 Use p T balance in 2 – jet events

Uncertainties: summary Experimental uncertainties dominate at low p T pdf/theoretical uncertainties almost as high at high p T Peter Mättig, CERN Summer Students 2014

LHC QCD studies Peter Mättig, CERN Summer Students 2014  QCD correct at multi – TeV scales?  Constrain basic QCD parameters  Jets understood as tool for high p T physics

Jet cross sections in rapidity and pT Excellent agreement theory data over huge range in phase space: 10 orders of magnitude Jet p T up to 2 TeV Peter Mättig, CERN Summer Students 2014

jet – jet mass Peter Mättig, CERN Summer Students 2014 Excellent agreement theory data Probing masses up to 5 TeV! Forward jets yield higher masses

Determining the strong coupling  s Peter Mättig, CERN Summer Students 2014 Single value less precise, but huge energy range Energy dependence of  s clearly visible ss Measures of  s : Three – jet fraction Jet mass

Measuring jet evolution Peter Mättig, CERN Summer Students 2014 Very good agreement …. at least for some models our view of jet evolution: Sequential radiation of gluons Leading to finer granularities Mimicked in models – how good are these models? Measure, at which d ij jet is split into two, three, ….. I.e. reverse jet finding Into 2 jets …. Into 3 jets …. Ratio 2/3 jets split

High p T Jets I.e. single jet has higher mass Measure mass of high p T jet: Globally: models agree with expectation..... But details differ! Peter Mättig, CERN Summer Students 2014 High p T jets: important to explore TeV scale physics Lorentz BoostSeveral partons merge into a single jet

Peter Mättig, CERN Summer Students 2014 Are quarks composite?

Are partons composite? pp – Q ≈ 0.1 GeV qq – Q ≈ 10 GeV interaction of subconstituents Q ≈ 1000 GeV ? Peter Mättig, CERN Summer Students 2014 Rutherford all over again ….

Jets and BSM: Search for di – jet resonances An excited quark ? q *  q + g (Remember excited atom, nucleus....) Would be strong signal for compositeness Search for resonance: enhancement Peter Mättig, CERN Summer Students 2014

Are quarks composite ? Compositeness  modify angular distribution continous change with higher jet – jet mass No deviation from SM observed:  > 7.8 TeV Peter Mättig, CERN Summer Students 2014 Note: results applicable to other exotic models: black – holes, colour octet quarks, Note: results applicable to other exotic models: black – holes, colour octet quarks,......

Peter Mättig, CERN Summer Students 2014 Strong interactions at core of pp –interactions  Multihadron events (soft interactions) measured  Jet cross sections agree with predictions over a wide range  Probing Multi - TeV range: no sign for physics Beyond Standard Model

Peter Mättig, CERN Summer Students 2014

QCD effects: number of jets Even though not exact matrix element: Good agreement on jet multiplicity Peter Mättig, CERN Summer Students 2014