marco stratmann
DESCRIPTION
INT workshop, Seattle, 11/19/10 The science case for an EIC. Spin and Flavor Structure of the Nucleon compelling bread & butter physics at an EIC. Marco Stratmann. [email protected]. wiki–page : https://wiki.bnl.gov/eic/index.php/Nucleon_Spin_and_Imaging. - PowerPoint PPT PresentationTRANSCRIPT
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Marco Stratmann
INT workshop, Seattle, 11/19/10 The science case for an EIC
Spin and Flavor Structureof the Nucleon
compelling bread & butter physics at an EIC
wiki–page : https://wiki.bnl.gov/eic/index.php/Nucleon_Spin_and_Imaging
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based on presentations on PDF studies by&
discussions with
S. Alekhin, E. Aschenauer, J. Blümlein, A. Cooper-Sarkar, M. Diehl,
A. Guffanti, K. Kumar, S. Moch, P. Nadolsky, F. Olness, M. Pfeuffer,
R. Sassot, M. Savage, H. Spiesberger, W. Vogelsang
THANK YOU !
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16yrs of data taking leave a rich legacy of knowledge & by now textbook results
(steep rise of F2; small-x gluons, diffraction, e-w effects, photoproduction, spin structure, … )
so, what did we miss which is still of interest in 2020+ ?
spin structure “only” studied in fixed-target regime (HERMES)
only proton beams – neutron structure ? – nuclei ?
L = 500 pb-1 and variation of Ep not sufficient to really study FL
completely unfold flavor & spin structure:
strangeness & s – s asymmetry ? - d/u and the gluon @ large-x ?
concepts/processes introduced but neither fully explored nor understood:
GPDs, unintegrated PDFs, diffraction, role of heavy flavors,
photoproduction, electroweak physics in ep, semi-inclusive processes, … . ..
JLab12? LHC?
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uniqueness
relevance
feasibility
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E. Aschenauer, T. Burton
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find out how low in y we can go
• increase x,Q2 coverage
• more overlap between different energy settings
• more lever-arm for Q2 evolution at fixed x
• upper y cut has much less impact
QED radiative corrections
• known to be significant at HERA
• devise strategies to control them i.e., reconstruct true x, Q2 reliably
• explore different methods to reconstruct x,Q2 (“electron”, “Jacquet-Blondel”, “combined”)
• more relevant for eA (?)
needs to be studied in detail but expected to be under good control
Monte Carlo tools at hand
scatt. electron tracking @ EIC isa big advantage compared to HERA
Aschenauer, Spiesberger
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strategy to quantify impact: global QCD fit with realistic toy data
• DIS data sets produced for stage-1 [5x50, 5x100, 5x250, 5x325] and 20x250, 30x325, …
• DIS statistics “insane” after 1 month of running (errors MUCH smaller than points in plots)
W2 > 10GeV2 W2 > 10GeV2
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x
recall:
RHICpp
DIS&pp
• low x behavior unconstrained
• no reliable error estimate
for 1st moment (entering spin sum rule)
• find
DSSV global fitde Florian, Sassot,
MS, Vogelsang
pos
itiv
e g
pQCD scaling violations
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how effective are scaling violations at the EIC…
DSSV+ includes also latest
COMPASS (SI)DIS data
(no impact on DSSV Δg)
χ2 profile slims down
significantly already
for EIC stage-1(one month of running)
• with 30x325 one can reach down to x ≈ 3×10-5 (impact needs to be studied)
Sassot, MS
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what about the uncertainties on the x-shape …
… wow – cool!
• even with flexible DSSV x-shape we can now determine up to ± 0.07
• work in progress: try weird x-shapes below x = 10-4 to improve/check error estimate
Sassot, MS
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Moch, Vogt, …• in 10+ years the NNLO corrections will be available (needed to match precision of data)
• watch out for surprises at small-x = deviations from DGLAP
(expected to set in earlier than in unpol. DIS; showing up as tension in global fit (?))
Bartels, Ermolaev, Ryskin;Greco, Troyan; …
• strong coupling from scaling violations (needs to be worked out / quantified)
• Bjorken sum rule:
• CBj known to O(αs4) Kodaira; Gorishny, Larin; Larin, Vermaseren; Baikov, Chetyrkin,
Kühn, ...• but not a tool to determine αs (1% change in αs translates in 0.08% change of Bj sum )
• experimental challenge: effective neutron beam (3He), very precise polarimetry, …
• theor. motivation for precision measurement: Crewther relation
non-trivial relation of two seemingly unrelated quantities
Adler function D(Q2) in e+e- Bj sum CBj(Q2) in DISdeviation from
exact conformal symmetry
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• precision data for F2 may help to resolve some issues with old fixed target data
(nice to have, but only “incremental” with little impact; cannot beat HERA at small x)
• longitudinal structure function FL - basically missed at HERA (fixed Ee, Ep)
interesting for several reasons:
• hard to get; recall
contributes mainly at large y (= low x for a given Q2)
• indirect measurement from deviation of σr from “F2 only fit”
• slope of y2/Y+ for different S at fixed x and Q2 strength of an EIC
strategies:
• FL starts only at O(αs) (due to helicity conservation)
this is theLO expression
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best motivation for a precise measurement at the EIC in 10+ years
is not to determine the gluon density but to understand pQCD series
• known up to three loops (NNLO)Moch, Vermaseren, Vogt
• leading small x term
appears first at NNLO
• sensitivity to small x term
at lowish Q2 values (few GeV2)
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E. Aschenauer
5x50 - 5x325 running
TO DO:
refine & test
how well we
can extract FL
with high precision
FL “slopes”(examples)
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strangeness was identified to be one of the least known quantities
– both unpolarized and polarized – where significant progress is unlikely w/o the EIC
DSSV (incl. all latest COMPASS data)
data
• surprise: Δs small & positive from SIDIS data
• but 1st moment is negative and sizable due to “constraint” from hyperon decays (F,D) (assumed SU(3) symmetry debatable M. Savage)
• drives uncertainties on ΔΣ (spin sum)
we really need to determine it ! (as well as their u,d quark colleagues)
NNPDF collaboration
• substantial uncertainties
• known issues with HERMES data at large x
• hot topic:
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at LO:
extra weightfor each quark
actual analysis of data requires NLO QCD where x, z dependence is non-trivial
allows for full flavor separation if enough hadrons are studied
relevant quantities/measurements:
• (un)polarized SIDIS cross sections (we don’t want to study asymmetries anymore at an EIC)
• for u, ubar, d, dbar, s, sbar separation need H = π+, π-, K+, K- (nice to have more)
complications/additional opportunities:
• PDF information entangled with fragmentation functions
• should be not a problem: already known pretty well (DSS), more data (Belle, LHC, …)
• EIC: if needed, can play with x & z integration/binning to reduce uncertainties (needs to be studied in more detail)
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Aschenauer, MS
compute K+ yields at NLO with 100 NNPDF replicas
z integrated to minimize FF uncertainties (work in progress)
PYTHIA agrees very well (despite different hadronization)
--> confidence that we can use MC to estimate yields & generate toy data
actual uncertaintiesmuch smaller than points
one month of running
5×250 GeV
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to do: include also π± ; polarized SIDIS and impact on global fit
next step: assess impact of data on PDFs with “reweighting method” (using full set of stage-1 energies: 5×50 – 5×325) Giele, Keller; NNPDF
how about K- (relevant for separation)
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• neutral currents (γ, Z exchange, γZ interference)
• charged currents (W exchange)
at high enough Q2 electroweak probes become relevant
parameterized by new structure functions which probe
combinations of PDFs different from photon exchange
--> flavor decomposition without SIDIS, e-w couplings
hadron-spin averaged case: studied to some extent at HERA (limited statistics)
hadron-spin difference:Wray; Derman; Weber, MS, Vogelsang;
Anselmino, Gambino, Kalinowski;Blumlein, Kochelev; Forte, Mangano, Ridolfi; …
contains e-w propagatorsand couplings
unexplored so far – unique opportunity for an EIC
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in the parton model (for simplicity)
NC:
CC:
requires a positron beam
• NLO QCD corrections all available
• can be easily put into global QCD analysis
• enough combinations for a flavor separation (no fragmentation)
de Florian, Sassot; MS, Vogelsang, Weber;van Neerven, Zijlstra; Moch, Vermaseren, Vogt
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Ringer, Vogelsang
no y cuty > 0.1
Q2 > 1 GeV2
30×32520×250 HERA
2nd indep. study: Kumar, Riordan, Deshpande, Taneja, Paschke
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Ringer, Vogelsang
20 × 250 GeV
Q2 > 1 GeV2
0.1 < y < 0.9
10 fb-1
DSSV PDFs
very promising! even doable with
5x250 GeV
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Ringer, Vogelsang
20 × 250 GeV
Q2 > 1 GeV2
0.1 < y < 0.9
10 fb-1
DSSV PDFs
TO DO: refine studies & quantify impact on global PDF fits
compare with independent studies by Kumar, Riordan, Deshpande, Taneja, Paschke
NC electron beam
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accessing fundamental electroweak parameters
aq mainly constrained by xF3γZ
vq mainly constrained by F2Z
Can we do better than HERA ? What does it take (energy, luminosity)?
needs to be investigated
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SIDIS through e-w boson exchange
some studies available from “Future Physics at HERA” workshops:
Maul, Contreras, Ihssen, Schafer; Contreras, De Roeck, Maul
(based on PEPSI Monte Carlo)
π, K
TO DO: re-do for EIC kinematics
CC charm production as a probe of strangeness
idea: at O(αs0)
at O(αs1) can potentially spoil sensitivity to strangeness
also, need to keep full dependence on charm mass in EIC kinematics
• NLO available (pol + unpol) Kretzer, MS
• again, studies performed for HERA
• gluon channel suppressed for z > 0.2
in D meson production
Δs < 0
errors assume 500pb-1
TO DO: exhume codes & re-do for the EIC
Δs ≈ 0
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( = getting used to acronyms)
heavy quarks: mQ >> ΛQCD (i.e., charm, bottom, top)
• no mass singularities -> no evolving, genuine heavy quark PDFs
• asymptotically large logarithms in DIS
• zero mass variable flavor-number scheme ZM-VFNS
standard evolution with massless partons above “threshold” Q = mc
different ways to treat heavy quarks in calculations: (use charm as an example)
• fixed flavor-number scheme FFNS only u, d, s, g are active partons; charm produced though
NLO parton-level MC (HVQDIS) Harris, Smith
• general mass variable flavor-number scheme GM-VFNS
attempt to match two distinct theories (nf=3+mc vs. nf=4)
needs some matching & “interpolating” coefficient fcts.
details matter in global fits !
not a priori clear if / where logs matter
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each PDF group has its own favorite scheme:CTEQ: ACOT, ACOT-χ, S-ACOT, S-ACOT-χ; MSTW: TR, TR’; NNPDF: FONLL; ABKM: BMSN
but VFNS must be derived from FFNS: relations between nf and nf+1 partonsBuza, Matiounine, Smith, van Neerven; Bierenbaum, Blümlein, Klein; ….
BMSN construction for F2charm : (used by Alekhin, Blümlein, Klein, Moch)
exact massive part mc ≠ 0
zero mass part mc = 0ln Q/mc
resummed
asymptotic part ln Q/m
mc [GeV]
ABKM 1.43±0.1
MSTW 1.40
CTEQ 6.6
1.30
PDG 1.66+0.09-0.15
another issue: quark masses in PDF fits
• choice of mc part of uncertainty
• all fits use pole mass
• consistently lower than PDG value
• future: use running mass in DIS fits (work in progress Alekhin, Moch)
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long-standing question … (example from ‘94 Glück, Reya, MS)
mc ≠ 0 mc ≠ 0
mc = 0
mc = 0
• even at high Q2 or W2, mc = 0 approx. not effective
• no smooth transition/matching
• existing HERA data described well with mc ≠ 0
• differences more dramatic for FLc
(never measured)
target for an EIC
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ABKM (S. Alekhin)
• FLC is not small
(mc ≠ 0)
• shown:
F2C BMSN
(close to mc ≠ 0)
TO DO:
collect predictions
for F2c, FL
c from
all PDF groups
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E. Aschenauerfor charm (via D mesons)
TO DO:
refine & test
how well we
can extract FLc
5x50 - 5x325 running
bin needs30x325
“FL slopes”for fixed x,Q2
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can we finally settle this?
M. Guzzi, P. Nadolsky, F. Olness (work in progress)
Brodsky, Hoyer,Peterson, Sakai
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• so far safely ignored: << 1% to existing g1 fixed-target data
• relevance at an EIC depends strongly on size of Δg • need massive Wilson coefficients (charm not massless for most of EIC kinematics)
so far only known to LO (NLO is work in progress Kang, MS)
some expectations: (need to be studied in detail)
≈ 2x10-3
≈ 2x10-5
very small (1-2% of g1uds)
10-15% of g1uds
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• make use of bulk of events sitting at low Q2
why should I bother about yet another non perturbative function ?
• access to non-perturbative structure of photons
• needed for consistent factorization in all processes with quasi-real photons
• ILC has a program for γγ physics perhaps even with polarization
• unpolarized photon structure not well known: LEP γ*γ DIS, some HERA data
(a global analysis was never performed; no error estimates)
• polarized photon structure is completely unknown
• non-trivial inhomogeneous Q2 evolution (due to pointlike coupling of photons to quarks)
• pQCD framework more involved than for DIS-type processes
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cross sections consist of two contributions, e.g. at
“direct photon”contribution
need to be added forphysical cross sections
linked through factorization
“resolved photon”contribution
parametrically of
• most processes of interest (charm, hadrons, jets, photons) are known to NLO (pol+unp)
• strategies to enhance sensitivity to resolved part known from HERA:
• single-inclusive: need to look into rapidity dependence
• di-jets: can define resolved sample (LO only)
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• polarized photon structure from 1-jet production
(very similar: 1-hadron production Jäger, MS, Vogelsang)
JägerarXiv:0807.0066
lepton
xγ ≈ 1
probes proton PDFs
proton
xγ << 1
probes unknown photon PDFs
1-jet differentassumptions
about
pTjet > 4 GeV
10x250 GeV
TO DO:work out in detailestimate uncertainties
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H. Spiesberger (ongoing work)
5×325 GeV
3 < pT < 5 GeV
resolved
direct
sum
• unpolarized photoproduction of charm
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photons are also part of protons:
• source for isospin violation
• only one PDF analysis available Martin, Roberts, Stirling, Thorne
• extraction in ep depends on how QED radiative corrections are treated
(defines the factorization scheme)
• needed for consistent factorization of electroweak higher
order corrections (collinear photon radiation)
--> relevant for precision calculations for the LHC Diener, Dittmaier, Hollik; …
interest twofold at an EIC (work in progress)
• step 1: can we extract them at the EIC?
• step 2: important background to Drell-Yan process in ep
(can we control it ?)
C. Pisano; M. Pfeuffer, A. Schafer, W. Vogelsang
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ScienceDeliverable
BasicMeasurement
Uniqueness and
FeasibilityRequirements
spin structure at small x
contribution of Δg, ΔΣ
to spin sum rule
inclusive DIS ✔minimal large x,Q2 coverage
about 10fb-1
full flavor separation
in large x,Q2 range
strangeness, s(x)-s(x)
semi-inclusive DIS
✔ very similar to DISparticle ID
improved FFs (Belle,LHC)
electroweak probes
of proton structureflavor separation
electroweak parameters
inclusive DIS at high Q2
✔some unp. results from HERA
20x250 to 30x325positron beampolarized 3He
beam
treatment ofheavy flavors
in pQCD
DIS (g1, F2, and FL)
with tagged charm
✔some results from HERA
large x,Q2
coveragecharm tag
(un)polarized γ PDFs
relevant for γγ physics
at an ILC
photoproductionof inclusive
hadrons, charm, jets
✔unp. not completely
unknown
tag low Q2 eventsabout 10 fb-1
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looks like we can deliver a pile of new results (but we still need to brush up our appearance)