soft and hard hadrons hard qcd, the standard model and beyond
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Soft and Hard Hadrons Hard QCD, the Standard Model and Beyond. Mike Seymour CERN/Manchester SPSC meeting on Fixed Target Programme Villars, Switzerland September 24 th 2004. Soft and Hard Hadrons. pQCD, SM and BSM Physics – Mike Seymour Parton Distribution Functions - PowerPoint PPT PresentationTRANSCRIPT
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Soft and Hard Hadrons
Hard QCD, the Standard Model and Beyond
Mike SeymourCERN/Manchester
SPSC meeting on Fixed Target Programme
Villars, SwitzerlandSeptember 24th 2004
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Soft and Hard Hadrons
• pQCD, SM and BSM Physics – Mike Seymour– Parton Distribution Functions– Electroweak measurements– Charged Lepton Flavour Changing Neutral Currents
• Hard exclusive processes – Markus Diehl– Generalized Parton Distribution Functions
• Low energy QCD and spectroscopy – Maxim Polyakov
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
pQCD, SM and BSM Physics
• Parton Distribution Functions– Kinematic coverage– Structure functions at large x ! valence quarks– Drell–Yan at large x ! sea quarks– strange quarks at large x ! strange asymmetry?– charm near threshold ! charm mass?
• Electroweak measurements– sin2w from NC/CC ratio
• Charged Lepton Flavour Changing Neutral Currents– ! e– +N!+X
• Other Miscellanea– Pion structure functions– Minimum bias/underlying event measurements
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Rationale
• What can be done with– a proton beam of » 400 GeV – a muon beam of » 100 GeV – and event samples » 10 times present ?
• Will focus on the physics we would like to do• not whether it is feasible…
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Parton Distribution Functions
• Cross section for deep inelastic scattering:
• Parton model:
• Collinear factorization: Q2-dep.• DGLAP evolution:
• pdf at some x and Q2 point depends on pdf at all lower Q2 and higher x values
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Parton Distribution Functions
• Cross section for deep inelastic scattering:
• Parton model:
• Collinear factorization: Q2-dep.• DGLAP evolution:
• pdf at some x and Q2 point depends on pdf at all lower Q2 and higher x values
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
• High x is the domain of fixed target experiments!
Parton Distribution Functions
• Cross section for deep inelastic scattering:
• Parton model:
• Collinear factorization: Q2-dep.• DGLAP evolution:
• pdf at some x and Q2 point depends on pdf at all lower Q2 and higher x values
x=0.65
x=0.4
x=0.25
x=0.18
x=0.13
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Global Fits
• State of the art: NLO cross sections with NLO evolution– Deep Inelastic Scattering (NNLO starting to be used)
• electron/positron/neutrino/antineutrino
• neutral current/charged current
• proton/deuterium/nuclear targets
• charm-tagged final states
– Drell–Yan production (hh!ll+X)• fixed target (virtual photon)
• W/Z production and forward/backward asymmetry
– Tevatron high–ET jet data
• rapidity–dependence crucial
• cf previous high–ET excess
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Global Fits
• State of the art: NLO cross sections with NLO evolution• Global fits are reperformed » once every two years
– Martin, Roberts, Stirling and Thorne (MRST)– a Coordinated Theoretical and Experimental project for QCD (CTEQ)
• pdfs now come with uncertainties– propagated from experimental errors– uncertainties due to theoretical assumptions?
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Parton Distribution Functions at Large x
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Important Issues
• How much good data do we need at high x?• How much do we trust theoretical framework?
– Large x resummation?– Small W2 » Q2(1-x) power corrections
• ‘Theoretical’ questions (with phenomenological implications)– Are sea quark/antiquark distributions asymmetric?
• u,d ) YES• s ?
– Are valence quarks isospin symmetric?
with thanks to Robert Thorne
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Perturbative Evolution at Large x
• Not very convergent
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Perturbative Evolution at Large x
• log(1-x) terms can be summed to all orders, but ambiguous
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Parton Distribution Functions at Large x
• Summary: more accurate high x data are needed– To pin down leading twist pdfs more accurately
• Q2 as large as possible
– To validate theoretical framework, test resummation in the evolution, measure non-leading twist (“power”) corrections
• Q2 and x range as wide as possible
• Result: more reliable quark pdfs at large x and high Q2 – and also (through sum rules) more reliable quark and gluon pdfs
at smaller x values
• The competition: HERA-III » 6 month run at » ½ current proton beam energy ???
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Prompt Photon Measurements in p–p
• Traditionally thought of as measure of gluon pdf– dominated by q+g!+q
• But perturbation theory out of control– Fragmentation component (final state q! q) poorly constrained– Large renormalization/factorization scale uncertainty at NLO
– Large intrinsic k? needed (non-perturbative?)
– E706 vs WA70?
• May be worth revisiting to learn more about QCD of photon production, but unlikely to constrain pdfs…
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
E706 hep-ex/0407011
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Drell–Yan Production in p–p
• Best way to measure sea quark distributions
• Separate d and u shapes• Fermilab E866/NuSea
experiment
xtarget xbeam
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meetingxtarget xbeam
Drell–Yan Production in p–p
• Best way to measure sea quark distributions
• Separate d and u shapes• Fermilab E866/NuSea
experiment• Future: E906 experiment• Good enough data over wide
enough range ! high x gluon from evolution
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Isospin Violation in Valence Distributions
• Isospin ) fu/p(x,Q2) = fd/n(x,Q2) etc
• Models of explicit isospin violation at low Q2 ! small• Global fit (MRST) ) » 0.5% violation in momentum sum
A.D.Martin, R.G.Roberts, W.J.Stirling and R.S.Thorne, hep-ph/0308087
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Isospin Violation in Valence Distributions
• Isospin ) fu/p(x,Q2) = fd/n(x,Q2) etc
• Models of explicit isospin violation at low Q2 ! small• Global fit (MRST) ) » 0.5% violation in momentum sum• QED effects in evolution equation ) u quarks evolve
faster than d quarks
• Next MRST set will include f/p and hence this source of isospin violation…
) Important to have data on deuterium (isoscalar) targets
(see later…)
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Strange–Antistrange Asymmetry
• Strange quark distribution best measured from CC charm production, eg +s ! +c (CCFR+NuTeV)
• CTEQ fits (Kretzer et al hep-ph/0312322) allow s(x)s(x)-
s(x)-s(x)-x[s(x)-s(x)]-
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Strange–Antistrange Asymmetry
• Strange quark distribution best measured from CC charm production, eg +s ! +c (CCFR+NuTeV)
• CTEQ fits (Kretzer et al hep-ph/0312322) allow s(x)s(x)• Results on x[s(x)–s(x)] stable >0
) important to have sign-selected data
(see later…)
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Charm Production Near Threshold
• EMC dataValidate theoretical
understanding of heavy flavour pdfs (variable flavour number schemes)
Perhaps measure charm mass…?
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
cf HERA reach…
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
PDF Summary
• Fixed target: unique measurements at– large x– medium Q2
• Crucial for LHC ability to make day-1 discoveries– Longer term, probably superseded by in-situ measurements
but always important to validate framework
• Crucial to our understanding of hadron structure– Isospin symmetry– Quark–antiquark asymmetry– Heavy quark parton model
• Charm mass measurement?
• Crucial for electroweak measurement, sin2w…
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
sin2w from isoscalar DIS data (NuTeV)
• Traditional method: (CCFR, CHARM, CDHS, …)– Llewellyn-Smith relation:– Needs many corrections
• / difference
• u/d pdf shape difference
• strange sea contribution
• final state charm effects/mass
) had hit systematic limit
• NuTeV: sign-selected beam:– Paschos–Wolfenstein relation:
• sin2w=0.2277§0.0013(stat)§0.0009(syst)
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
sin2w = (1-mw2/mz
2) ) mw measurement
3 discrepancy
BSM physics? Nothing obvious
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
strange–antistrange asymmetry
isospin asymmetry
new calculation
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
pdf corrections
•
UV=UVp-DV
n
DV=DVp-UV
n
– QV=(UVp+DV
n+DVp+UV
n)/2
– S-=S-S
• MRST: (UV-DV)/QV » –0.5% ! sin2w » –0.0025
• CTEQ: S- » +0.0016 ! sin2w » –0.0016
• i.e. 1–1.5 differences, in the same direction
(each with » 100% uncertainty)
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Radiative corrections
• NuTeV use D.Bardin, V.Dokuchaeva program (1986)• Recently recalculated in K.-P.O.Diener, S.Dittmaier, W.Hollick,
PRD69(2004)073005
Conclusion: uncertainty at least £10 and maybe £100
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
0.00500
My estimates (optimistic?)
More data needed
0.00250
0.00160
0.00580
0.00600
more calculation needed
More data needed
Becomes . 1!(if central value does not shift)
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
sin2w Summary
• Fixed target data ! unique opportunity for independent measure of sin2w
• Theoretical errors almost certainly underestimated– Can be reduced with more calculation– To match experimental error will need better pdf measurements
• Isospin violation
• Strange–antistrange asymmetry
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
BSM Physics — Charged Lepton Flavour Changing Neutral Currents
• Predicted in most Beyond Standard Models– Seesaw mechanism for neutrino masses
• Loop vertex via Majorana mass term
off-diagonal effective Higgs vertex
• Small in SM seesaw
• Sizeable in MSSM seesaw , non-decoupling of heavy particles
– Extra dimension models– Left–right symmetric models
• ! e most stringent limits ) received most attention
• $ mixing » maximal
) $ mixing largest?
with thanks to Apostolos Pilaftsis see talk of S.Gninenko
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Model-Independent Approach
• Low energy effective theory:• Four point vertex:
• Strong coupling: assume C»4• Weak coupling, eg SUSY, C.1,
rescale by p(4/C)
(but remember non-decoupling)
• Scattering experiments favoured: effects grow with E2
• Several operators seem within reach
D.Black, T.Han, H.-J.He, M.Sher Phys.Rev.D66:053002,2002
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
! Summary
• Will never be as stringent as !e• But does it need to be?
– New physics $ third generation?
– Connection with $ mixing?
• If a signal is seen in !e, what next?– Flavour structure
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Miscellanea — Pion Structure Functions
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Miscellanea — Underlying Events
• Understanding the underlying event at LHC is crucial to almost all physics measurements there
• Dynamics crucial: fluctuations and correlations• Can fixed target programme help?
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Miscellanea — Underlying Events
• Understanding the underlying event at LHC is crucial to almost all physics measurements there
• Dynamics crucial: fluctuations and correlations• Can fixed target programme help? Not convinced
Hard-Scattering in Min-Bias Events
0%
10%
20%
30%
40%
50%
100 1,000 10,000 100,000
CM Energy W (GeV)
% o
f E
ven
ts
PT(hard) > 5 GeV/c
PT(hard) > 10 GeV/c
Pythia 6.206 Set A
Ric
k F
ield
, C
DF
tu
ne
of
PY
TH
IA
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pQCD, SM and BSM Mike Seymour Villars SPSC meeting
Summary
• Large x pdfs can be better measured in fixed target than anywhere else– Intrinsic interest: structure of hadrons (and nuclei…)
• isospin violation?
• strange component of hadrons and strange–antistrange asymmetry
– Input into collider physics: precision cross sections and searches• Drell–Yan data on sea quark distributions ! high-x gluon
• Charm quark mass?
• sin2w measurement has a long way to go– Experimental errors dominated by statistics– Theoretical errors need more work
• Charged Lepton FCNC: ! unique opportunity• Other possibilities probably of less interest…