Introduction to Hadron Collider Physics. Mark Lancaster. Oct 6 th

Transcription

1 Introduction to Hadron Collider Physics Mark Lancaster Oct 6 th

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3 1974 (J/Ψ) (BNL AGS : pn) 1995 (FNAL Tevatron p-pbar) 1977 (FNAL : pn) 1962 (BNL AGS : ν from pn) 1983 (CERN SPS) : p-pbar 2000 (FNAL Tevatron : ν from pn)

4 Some history - hadron colliders have typically been at energy 10 x electron machines 2005!!! Now :Tevatron/FNAL : CDF/D0 at 1.96 TeV Nearly: the LHC at 900 GeV or 10 TeV or 14 TeV..

5 As well as fundamental particles; hadron colliders were responsible for: - discovery of CP violation in Kaon sector (BNL AGS : 1964) - discovery of heaviest meson (Bc) (CDF : FNAL Tevatron 1998) - first observation of CP violation in B sector (CDF : FNAL Tevatron : 1999) - first observation of Bs oscillations (CDF: FNAL Tevatron : Sep 2006) - first observation of Σ b baryons (CDF: FNAL Tevatron: Oct 2006) And as we will see a wealth of : - electroweak physics - QCD physics - B physics - exotic limits That said - we ALSO need lepton colliders and lepton-hadron colliders - lepton colliders : clean environment for precision measurements - lepton-hadron colliders : precision probes for QCD (PDFs) I will not talk about heavy ion colliders (e.g RHIC at BNL, LHC)

6 Gold on Gold RHIC

7 Why hadron colliders: - easy to get to high energy : less synchrotron radiation - naturally scan in centre of mass energy Synch Rad ~ E 4 /(M 4 R) At LEP2 (100 GeV) beams - 2 GeV per turn was being lost For protons this would not happen until E = 200 TeV. Synch Rad. at LHC is 3 KeV - higher cross sections (factor 3 from color)

8 Ultimate hadron collider proposed by Fermi in 1954!

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10 Since hadron colliders collide composite objects the extraction of the physics is often ''messy'' and not straight-forward. - underlying event, multiple interactions - proliferation of QCD radiation - high event rates - places a premium on - real-time triggering (selection of interesting events) - accurate detectors with some redundancy - understanding QCD

11 Total event rate varies slowly (logarithmically) with CMS E 'Interesting'' physics events (high pt/mass) are enhanced at high CMS E But they can still be at a rate of 11 orders of magnitude below the soft proton-proton scattering events... Event Rates

12 Single W,Z Precision (loop) physics (0.2 fb -1 ) Di-Bosons SM tests : gauge couplings (1 fb -1 ) Single Top Observed now at 4-sigma (2 fb -1 ) Higgs Excluded between GeV (4 fb -1 )

13 What happens when two hadrons collide: 1. ~ 25% ELASTIC collisions hadrons change direction/momenta but there is no energy loss : dull! 2. ~ 75% INELASTIC collisions one or both of the hadrons have a change in energy & direction : rate ~ 1/Q 4 : Q is energy transfer mostly dull! In a collider we have bunches of hadrons circulating the accelerator - each bunch contains ~ protons (anti-protons are lower ~ 10 9 ) We can have more than one collision as the bunches pass through each other at the interaction region : ''Multiple Interaction'' The bunches have a significant size longitudinally (5-20 cm) 30 µm : BUNCH BUNCH : P 15cm

14 P ΔE 1 P ELASTIC : ΔE i = 0 (~ 25 %) P ΔE 2 P P P INELASTIC : NON DIFFRACTIVE (~ 55 %) P ΔE 1 P P ΔE i > 0 P ΔE 2 P P P INELASTIC : DOUBLE DIFFRACTIVE (~ 8 %) INELASTIC : SINGLE DIFFRACTIVE (~ 12 %)

15 Total Cross Section at Tevatron ~ 80 mb At LHC : don t know until we measure it : mb - despite talk of Higgs etc this will be one of the first LHC measurements Need to measure it so we have a prediction of the number of additional events overlapping the interesting physics. Poisson distributed - important at low lumi since skewed, less so at high lumi. LHC crossing interval = 25ns Assuming cross section of 130 mb. How many min bias interactions per crossing at LHC nominal lumi of cm -2 s -1 How many in 2009 at 10 28????

16 HARD & SOFT!! Most of interactions involve a low transverse momentum transfer (pt) from the initial to final state : - these are termed SOFT interactions - in such interactions a few/no particles are produced with significant pt (pt > 2 GeV) In contrast an electron from the decay of a W has a pt of ~ 40 GeV - interactions involving the emission of at least one particle with appreciable pt are termed HARD interactions A given bunch crossing can involve a mixture of separate HARD and SOFT interactions. A given interaction can have HARD and SOFT components (see later). HARD and SOFT terminology is not exact but it is frequently used Why do we care? - HARD interactions have a high scale e.g. mass of W or high pt particle and can be calculated reliably using perturbative QCD - SOFT interactions are NOT easily calculable within QCD and rely on ad-hoc models which are taken from data (with some ''theory'')

17 - first hard hadronic process wasn't seen until early 1970s at BNL & CERN! soft physics model - jets of hadrons were not seen until the SppS in 1980s by UA1 and UA2 (the gluon was only discovered from 3 jet events in e + e - collisions in 1979) - triggers for hard processes invariably involve a pt threshold or the presence of a resonance. - if no trigger then data rate from LHC would be 250 Tb of data per second!

18 Identification of hard components of the event is key to getting to the physics. - Higgs event at LHC with additional soft interactions hard QCD radiation

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20 What is a minimum bias event? - event accepted with the only requirement being some activity in the detector with minimal pt threshold [100 MeV] (zero bias events have zero requirements) - a minimum bias event is therefore most likely to be either: - a low pt (soft) non-diffractive event - a soft single-diffractive event - a soft double diffractive event (some people do not include the diffractive events in the definition!) - it is characterised by: - having no high pt objects : jets; leptons; photons - being isotropic - see low pt tracks at all phi in a tracking detector - see uniform energy deposits in calorimeter as function of rapidity - these events occur in % of collisions. So if any given crossing has two interactions and one of them has been triggered due to a high pt component then the likelihood is that the accompanying event will be a dull minimum bias event.

21 CDF event charged tracks tracks from additional min bias events JET # charged tracks vs rapidity in min bias interactions

22 What is the (soft) underlying event (SUE): - everything else in the event not to do with a hard (high pt) sub-process - sometimes people add into this definition, the concurrent min bias events; such that underlying event is a generic term meaning ''all that's not high pt'' - this is not an exact science neither is the theory! This includes: - the remnants (quarks) of the proton not participating in the hard scatter - the soft (i.e. low pt) particles produced by the colour field (which will radiate) connecting the hard scatter with the remnant - soft gluons (QCD radiation) emitted from the hard scatter quarks - Where soft gluons become hard gluons and not part of the underlying event is not an exact definition if it's of high enough pt to hadronise into a jet then it's generally considered hard but that also depends on your jet algorithm! - soft physics also referred to as infra-red or long-range physics (= low energy)

23 The production of a W in a proton anti-proton collision showing the separate hard and soft components within a single interaction

24 Event Terminology Summary Already... - hard sub-process / hard scatter; (soft) underlying event ; miminum bias event; diffractive / inelastic / elastic - Multiple interaction - generally used to mean a hard scatter process + independent overlapping min bias event from different hadrons in same bunch - Multiple / Double Parton Interaction (DPI) - extremely rare (but have been observed by CDF) process when two partons in the same interacting hadrons undergo two independent hard scatters or more likely have one hard and one soft-ish. - Mini-jets - generally low pt (soft-ish) jets associated with the soft scatter in a double parton interaction. - DPI and mini-jets may be important at LHC since at the LHC there is a high probability of a low-x quark being involved in an interaction

25 - Example double parton interaction

26 Triggering at a hadron collider - this is the key e.g - b quark was discovered at rate of one b event per collisions - top quark was discovered at rate of one top per collisions! - by comparison this is trivial at a lepton collider Needle in a haystack moving at 186,000 miles per second MHz L1 : hardware 5 khz CHALLENGES - ensuring high trigger efficiency & retaining purity - knowing what the trigger efficiency is (use pass-through triggers and rely on pre-scaled triggers with lower thresholds) L2 : firmware 375 Hz Rejection factor of 1:20,000 after level-2 L3 : software 75 Hz Tape Robot ~ few Tb / day disks...

27 Factorisation and PDFs Factorisation is a fundamental theorem of perturbative QCD - it is vital for the theory to have any predictive power i.e be of any use

28 K-factors - factors that account for truncation of hard-scatter cross section e.g. they account for higher order effects. - generally a LO K-factor i.e one taking into account all non leading order diagrams is approx 25-50%. e.g. for Z production at the Tevatron it is 30%. NLO K factor is smaller...

29 Knowledge of PDFs is vital - they determine the rate of processes - we define ''luminosity functions'' to determine what the important partonic sub-processes will be. - this is where HERA measurements are vital

30 gluon-gluon production of ET=500 GeV jet is 4 orders of magnitude larger at LHC than Tevatron Gluon-Gluon Parton Luminosity LHC Tevatron X1 X2 (LHC) = 1/50 X1 X2 (Tevatron) : there are a lot of gluons at low x

31 Tevatron : jet production It works here we have data and perturbative NLO QCD agreeing over 9 orders of magnitude Beware log scales!!!

32 Heavy Flavour Production : Top or Bottom Quarks - for the most part the Tevatron is a quark anti-quark collider and the LHC is a gluon-gluon collider. - - understanding these processes is vital since e.g. bb is the dominant decay mode - of a light Higgs

33 Vector Boson Production Prompt Photon Production

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35 Physics Areas : Past - CERN : SppS / ISR : 1970s and 1980s - discovery of W,Z and first measurement of mass - establishing QCD as a credible theory through jet measurements Physics Areas : Present - TEVATRON : Run-0 ( ), Run-1 ( ), Run-2 ( ) - Electroweak - precision measurements of W properties complementing LEP ( ) - Top Quark - can only be done at the Tevatron. Measurements of mass, xsec, helicity - Bottom - huge cross section - establish existence of oscillations in Bs system - measure properties of heaviest meson (Bc) and compare with lattice QCD - search for rare decays as indicator of new physics - QCD - measure jets at highest energy search for proton sub-structure - understand soft physics / mini-jets as precursor to LHC - hard diffractive processes e.g. diffractive W production; - Exotics - Usual suspects : Higgs, SUSY, extra-dimensions, monopoles, etc, etc

36 Doing Physics at a hadron collider key concepts: - do not measure proton remnant so not possible to add constraints in longitudinal direction. Only constraints are conservation of transverse momentum - e.g in W events we measure ''missing E T '' and a transverse mass. - jet backgrounds are huge & present in every non-jet analysis e.g. rarely a jet may contain only a p 0 and a p + very close together CDF II Preminary CDF II Preminary

37 - To tag/identify b quarks vital for top/new physics - need to measure a displaced secondary vertex. - d 0 is called the ''impact parameter'' - L XY is a pseudo-decay distance (related to lifetime of particle)

38 Silicon Vertex B-tagging efficiency (top) Improving b-tagging efficiency and knowing its efficiency is a key challenge at the Tevatron And it will be at the LHC h bb

39 Identification of taus: - popular decay product in many SUSY models (enhanced coupling) and also owing to its high mass has a non negligible higgs coupling. PRONG TERMINOLOGY - 65% of taus decay hadronically of this 65% : ~70% decays to one particle/ prong and 30% to three ''prong' : draw Feynman diagrams - why only odd # prongs..

40 Hadron Identification - vital for B physics : knowing difference between pion, kaon & proton - use TOF or Cerenkov counter or de/dx

41 Bring these together discover a new particle

42 Why is top interesting - it is by far the heaviest fundamental particle known (175 GeV) - it's mass is at the same scale as W,Z it may offer insights into the nature of electroweak symmetry breaking - its decay is so quick - that it's the only quark that doesn't hadronise - we can thus study its QCD radiation without hadronisation complications e.g. there are effects due to the fact that its massive means that the usual co-linear gluon radiation does not happen.. Need precise W and Top Masses - it has the largest contribution to the radiative corrections to the W/Higgs mass

43 Top events all-jets mode is difficult since backgrounds are large

44 You always do better than you expect (eventually.) LHC New result - Neural Net to remove background - Use matrix element to maximise information - Constrain Jet energy scale using known W mass. Errors: energy scale, PDFs, background, QCD

45 Single Top Production (via weak interaction) Why search for single top? Probe V tb directly / pb / pb < 0.1 pb New physics! s-channel Sensitive to resonances Z t-channel Sensitive to FCNCs Similar topology to Higgs Signature (WH Wbb)

46 Search for Single Top Topology: Somewhere between W+jets and top pair Vtb = 0.91 ± 0.11 (exp.) ± 0.07 (theory)

47 W, Z + Photon u- or t-channel s-channel final-state radiation

48 WW, WZ, ZZ Production WW (SM 12.5 ± 0.8 pb vs pb measured) Trilinear Gauge Coupling - hard to beat LEP (40k WW) Tevatron can produce higher mass than LEP. Important backgrounds to Higgs search (H -> WW)! - WZ established at > 5-sigma - ZZ established at ~ 5-sigma

49 B Physics Tevatron is the only place to produce heavier B mesons : Bs, Bc - - Why is this interesting! - it is calculable - fundamental QM in mixing phenomena : lifetime & mass differences

50 One of the few vindications of Lattice QCD

51 B s, B d, D 0 µ + µ - SM expectations: Br(B s µµ) ~ 3.8 x 10-9 Br(D 0 µµ) ~ SUSY: Br(B s µµ) ~ tan 6 β Can be enhanced by e.g. tanβ ~ 40 for Br ~ % CL µ + µ - Br limits: B s : 2 x 10-7 (unique to Tevatron) B d : 4 x 10-8 D 0 : 2.5 x 10-6 Excludes SO 10 space Large parts of R-parity violating SUSY. Smaller exclusion in msugra MSSM

52 Higgs Search Direct Search Limit : mh > 114 GeV from LEP2.

53 Higgs Search Indirect Limit (95% CL) : mh < 154 GeV 95% CL

54 Higgs Indirect Limit If CDF achieves its aim of 30 MeV then it only takes < 1σ Mw and we exclude the SM Higgs to be below the LEP exclusion at 95% CL Mw (TEV) = ± 25 M H = 75 ± 20 M H = 83 ± 30 (1 sigma - now)

55 Higgs Search

56 Higgs Search Low Mass (115 GeV) Tricky : since requires : - Sacrifical Offering to Likelihood / NN God - Best possible b-tagging efficiency - Understanding of SM backgrounds - Understanding of QCD background - Optimum mass/jet energy resolution High Mass (165 GeV)

57 Higgs Search σ(mssm/sm) = depending on SUSY parameters; Tevatron sensitive to large tanβ

58 Higgs Search Bingo

59 Higgs FNAL

60 Higgs FNAL

61 Typical SUSY Mass spectrum MSSM Higgs Search

62 MSSM Higgs Search Beware statistical fluctuations - one year ago SUSY had been discovered at 160 GeV but now it s gone. 8 fb -1 At high tanβ: enhanced x-sections heavy flavor (b, τ) preferred

63 With 10fb -1 66% chance Tevatron will exclude Higgs in entire predicted region Will this happen before the LHC has enough data???

64 Tevatron performance to date

65 10 fb -1 is very likely before mid 2011

66 More on QCD from Robert Thorne Lecture More on LHC physics & Higgs from Antonella De Santo Lecture