LHC Collider Phenomenology

Transcription

1 LHC Collider Phenomenology Theorist! You are a theorist working in the CMS experimental collaboration You work on LHC Collider Phenomenology related to CMS By working in the experimental collaboration you will able to have access to data and to do the first class High Energy Phenomenology 1

2 2 Introduction to HERA Physics ARIF AKHUNDOV University of Valencia Spain

3 3 Overview Part 1: What is HERA? Structure Function Formalism Kinematics Structure Function Evolution Part 2: Selected HERA I Results: Structure Functions High Q 2 and EW Jets and the strong coupling α S CONCLUSIONS

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5 5 Part 1

6 What is HERA? 6

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8 HERA: The only ep Collider on 8 the Planet s 320 GeV

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12 12 HERA Highlights: Started operation in Experiments: H1 and ZEUS (ep) HERMES (e) HERA-B (p) s = 300 GeV ( 1997) s = 318 GeV (1998 ) e + and e - beams up to 60% lepton polarization > 200 Mio ep collisions recorded per experiment

13 13 Physics Topics of HERA Proton Structure Parton densities gluon density (xg(x)) Valence quark distrib stributionstions QCD evolution Different evolution schemes (e.g. BFKL) Strangeness and charm α S Perturbative QCD Jets Gluon density α S Multiparticle Observables: Multiplicity distributions Event chapess Multiparticle Correlations Border between pqcd and non-perturbative QCD Photon Structure Diffraction EW BSM and Exotics Leptoquarks Excited Quarks and Fermions FCNC MSSM Searches R-parity violation SUSY Contact Interactions Spectroscopy

14 14 Structure Function Formalism

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16 16 Deep inelastic: W >> M P σ/σ Mott const.

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23 23 HERA Results for F 2 Sample F 2 data Dramatic Scaling Violations!

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25 Kinematics 25 and a guided tour around the HERA phase space

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28 28 Current Jet Scattered e - Scattered e - e - P 27.5 GeV Current Jet 920 GeV Scattered e - Current Jet

29 29 HERA I Kinematic Range Huge extension of kinematic reach: X : 6 orders Q 2 : 6 orders Overlap with previous fixed target experiments

30 30 1

31 Very High Q 2 31 Current Jet e GeV Scattered e - P 920 GeV Very clean events Very high energy electron (> E beam ) Very collimated jet Electron forward Activity around the beam pipe forward (proton remnant)

32 32 2

33 33 Very Low Q 2 e GeV P 920 GeV Little activity in main detector Electron backward seen in special beampipe calorimeter Scattered Electron eenergy - close to E beam No jet structure Activity around the beam pipe forward (proton remnant)

34 34 3

35 35 Medium Q 2 Jet Jet and electron in main detector Well isolated electron Well collimated jet Activity around the beam pipe forward (proton remnant) e GeV Jet Scattered e - P 920 GeV Scattered e -

36 36 Structure Function Evolution

37 37 = 0 in the QPM Z 0 Exchange

38 What QCD tells about 38 F 2 (x,q 2 )? Splitting functions: Can be calculated in pqcd DGLAP Equation: Integral-Differential equation for the dependence of q(x,q 2 ), g(x,q 2 ) on Q 2 Need an initial condition! ( a b)( x) = 1 0 dya x y b( x) y

39 39 QCD Fits Make an ansatz at a fixed value of Q 2 = Q 2 0 Write F 2 simpler: Ignore F 3 and F L F 2 ~ 4/9 (U+U) + 1/9 (D+D) with D = d+s U = u+c U = u+c D = d+s (for the moment)

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42 42 Part 2: Selected HERA Results

43 43 Structure Functions

44 44 F 2 vs Q 2 Note: Enormous range of data (5 orders in Q 2 and 8 orders in x) Approximate scaling at high Q 2 Scaling violations at low Q 2

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46 F 2 Errors 46

47 47 F 2 vs x Bj Dramatic rise a low x Bj Note previous picture:

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50 50 NLO QCD Fits to F 2 Remember the discussion before: Simplify F 2 : F 2 ~ 4/9 (U+U) + 1/9 (D+D) with D = d+s U = u+c U = u+c D = d+s Fit parameters (16 in total)

51 51 Parton Distribution Functions QCD fits to structure functions: F 2 ~ 4/9 (U+U) + 1/9 (D+D) Valence quarks: 2 (U-U) + (D -D) D = d+s U = u+c U = u+c D = d+s % precision except for gluon :

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57 DGLAP and BKFL equations 57

58 The distribution of partons in the transverse plane. 58

59 59 another way to look at it Sample F 2 data Proton Charge radius Hadron-hadron scattering results 0.5 hc/q = fm F 2 x -λ at small x Observation of hadron to parton transition

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61 61 High Q 2 and EW

62 High Q 2 NC (γ,( Z Exchange) HERA Neutral Current at high x 62 σ H1 e - p ZEUS e - p SM e - p (CTEQ6D) x=0.08 (x10000) H1 e + p ZEUS (prel.) e + p SM e + p (CTEQ6D) Note: Handle on F : x=0.13 (x2500) x=0.18 (x500) xf 3 σ σ + 10 x=0.25 (x100) Valence quarks! 1 x=0.40 (x5) 10-1 xg Q 2 = 1500 GeV 2 H x= ZEUS SM (CTEQ6D) Q 2 (GeV 2 ) x γ-z interference: e - constructive / e + destructive

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65 NC vs CC 65 2 ignore F L and xf 3 HERA λ = ± 1 for left/right handed e COUPLING PROPAGATOR NC 2πα 2 1 / Q 4 CC πα 2 /8 sin 4 θ 1 / (M W W 2 +Q 2 ) 2 similar since similar at sin 2 θ W ~ ¼ Q 2 > M 2 W Unification of elm. and weak forces dσ/dq 2 (pb/gev 2 ) ~ 1 / Q 4 ~ 1 / (M W2 +Q 2 ) 2 H1 e + p CC H1 e + p NC H1 e - p NC ZEUS (prel.) e + p NC ZEUS e - p NC SM e + p NC (CTEQ6D) SM e - p NC (CTEQ6D) M 2 W, Z H1 e - p CC ZEUS e + p CC ZEUS e - p CC SM e + p CC (CTEQ6D) SM e - p CC (CTEQ6D) y < Q 2 (GeV 2 )

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68 68 CC Cross Section 2 d 2 σ CC G F dxdq 2 (e ± M ) = 2 (Y 4 πx + F 2 Y - xf 3 -y 2 F L ) M 2 W ± ± ± + Q 2 W 2 ± dσ/dq 2 (pb/gev 2 ) HERA Charged Current H1 e - p ZEUS e - p SM e - p (CTEQ6D) e - σ (e - p) > σ (e + p) H1 e + p ZEUS e + p SM e + p (CTEQ6D) e σ (e - p) ~ x (u+c) + (1-y 2 ) x (d+s) --- σ (e + p) ~ x (u+c) + (1-y 2 ) x (d+s) 10-7 y < Q 2 (GeV 2 )

69 69 Jets and α (Q 2 ) S

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71 71 α (Q 2 ) s QCD fit of F 2 scaling violations : α s = ± (expt+fit) ± (scale) H1 α s = ± (expt+fit) ± (scale) ZEUS Jet cross sections: 1, 2, 3 jets α s = ± (exp) ± (theory) α s ZEUS (82 pb -1 ) (inclusive jet γp) ZEUS (38 pb -1 ) (dijet DIS) ZEUS (39 pb -1 ) (inclusive jet DIS) CDF (87 pb -1 ) (inclusive jet pp) Bethke running α s (E T ) in one experiment μ (GeV)

72 72 α S How do you get Jets? Outgoing partons described by 3 more variables: ξ = z P, i Q x 2 P pi = P q φ : Azimuth + M 2 Q = 2 JJ 1 (1 cosθi ) 2 θ :Scattering Angle in CMS i p 1 p 2 Q x W q 2 Bj 2 = p e = q 2 2 Q = 2P q p 1 x Q x e 2

73 73 η = 1 2 ln tan θ 2 s 300 GeV φ s = 300 η GeV e +/ GeV p 820/920 GeV

74 74 Dijet Cross Section in DIS 1 θ η = ln tan 2 2 Note: Ingredients in these calculations: NLO QCD calculation parton densities strong coupling, α S hadronization corrections Make this a test a test for the strong coupling: take pdfs from inclusive data (F 2 ) Calculate hadronization using an QCD-inspired MC model (like jetset) fit

75 Practical consideration: 75 Use Dijet Fraction R 2+1 has smaller sensitivity to PDFs Some experimental errors cancel Fit: R R = σ σ tot 2 ( α ( M )) = A α ( M ) + A ( M ) 2+ 1 S Z 1 S Z 2 α S Z

76 α S (Q 2 ) from Dijet 76 Production

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78 78 Conclusions

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