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RHIC Spin Goals and How to Get There. Reminder of RHIC Spin scientific goals Baseline measurement program to reach the goals Remaining machine and detector needs to carry out the measurements Ruminations on the beam-time dilemma Frequently asked questions

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slide1

RHIC Spin Goals and How to Get There

  • Reminder of RHIC Spin scientific goals
  • Baseline measurement program to reach the goals
  • Remaining machine and detector needs to carry out the measurements
  • Ruminations on the beam-time dilemma
  • Frequently asked questions
  • A few remarks on possible programs beyond the baseline, a decade hence

LO pQCD two-spin asymmetries for all 22 partonic processes

S. Vigdor Open Meeting on RHIC Planning BNL, Dec. 3-4, 2003

slide2

RHIC Spin Major Goals: pQCD Hard Scattering Probes Non-pQCD Spin Structure

  • Does preferential spin orientation of gluons account for a major portion of the “nucleon spin puzzle”?
  •  Either answer interesting! If not gluon spins, then Lorbital !
  • Do sea antiquarks have a substantial and flavor-dependent helicity preference in a polarized nucleon?
  •  Illuminates the relative roles of gluon splitting vs. pseudoscalar meson clouds in generating the “sea”.
  • 3) Unravel the contributions to transverse spin asymmetries (an area of intense recent theoretical development) from:
  • a) quark transverse spin preferences in a transversely polarized proton (p)
  •  “transversity”  quark property decoupled from gluons
  • b) quark transverse motion preferences in p
  •  spin-kT correlation related to quark orbital ang. mom.
  • c) explicit chiral symmetry breaking from mq terms in LQCD.

slide3

RHIC Spin Goals Among NP “Milestones” for Coming Decade

Hadronic Physics Mile-stones

NSAC Sub- committee on Per-formance Measures, Nov. 2003 Report

slide4

What We Measure:

Two-spin helicity asymmetry:

Can be large in pQCD hard scatter.

Stat. Unc. ~ (P12P22 L dt )1/2 One-spin helicity asym. ALviolates parity if non-vanishing, but can be large in weak processes like W prod’n.

N++/L++ N+/L+

1

ALL

P1P2

N++/L++ + N+/L+

vs.

Single-spin transverse asym.

Detected particle momentum

N/L N/L

1

AN

P1

N/L+ N/L

where  () are defined with respect to reaction plane, is suppressed by chiral symmetry in pQCD hard scatter, but can occur via non-pert. aspects of initial and final-state spin dynamics.

Proton spin vector

vs.

Stat. Unc. ~ (P12 L dt )1/2

slide5

Gluons and the Nucleon Spin

Gluons  ~ half the nucleon mass, half the momentum; do they also make important contribution to the spin?

RHIC sensitivity via hard scat. processes:

Blümlein & Böttcher, 2=4 GeV2

plus gluon-gluon fusion producing heavy quarks – will compete with/com-plement studies of photon-gluon fusion in HERMES & COMPASS exp’ts

NLO analyses of scaling violations in DIS  weak constraints on G, with large stat. + syst. uncertainties, little info. on shape of G(x).

slide6

G Constraints from Abundant Processes @ RHIC

  • Inclusive  0 cross sections at RHIC energies are quite consistent with NLO pQCD calcs. down to pT ~ 1-2 GeV/c.
  • Inclusive  0and jet production arise from pT-dependent mixture of qq, qg, and gg scattering. The mixture fractions are also sensitive to fragmentation functions.

pp  0X, s = 200 GeV, ||  0.38

GRSV-max ( )

Calcs. by Jäger, Schäfer, Stratmann & Vogelsang

pp  jet+X, s = 200 GeV, 0    1

Calcs. by de Florian, Frixione, Signer & Vogelsang

  • RHIC run delivering 5-10 pb1 will allow significant tests of G models & parameterizations
  • Info on x-dependence of G is limited by convolution with pT – dependence of qq vs. qg vs. gg fractions and sensitivity to fragmentation functions.

ALLjet

slide7

PHENIX and STAR Have Begun Jet/Hadron ALL Measurements in 2003

PHENIX Preliminary

STAR projected 2003 statistical uncert. from ongoing jet analysis

  • These measurements made with more than an order of magnitude less statistical Figure Of Merit than predictions on previous slide!
  • Both collaborations are clearly ready to make high-impact ALL measurements as soon as machine performance and available beam time permit!
slide8

“Rare” Processes Needed to Provide Full Map of G(x)

  •  production advantages: single dominant LO diagram (qg  q);  - jet coinc. permits selection of x-range, internal consistency checks on pQCD interpret’n
  •  disadvantages: needs sizable  L dt; remaining problems in understanding cross sections (esp. for fixed target exp’ts)
  • Error bars shown for STAR simulations are stat. only. Subtraction of  0 bkgd. remaining after cuts will increase point-to-point errors by factor of 1.5 - 2.0.
  • Shortfall in P4L dt can be partially compensated by relaxing cuts (here on pT  10 GeV/c, max{x1,x2}  0.2), but a competitive measurement will require at least P  0.6, recorded  L dt  100 pb-1 at 200 GeV,  250 pb-1 at 500 GeV.
slide9

Flavor Dependence of Sea Antiquark Spin Preferences

B. Dressler et al. CQSM

¯

¯

2 = (5 GeV)2

¯

¯

x(ud)

x(du)

¯

¯

u(x) vs. d(x) will provide critical clues to the non-perturbative nature of the nucleon sea:

  • Many models of nucleon spin structure (like chiral quark sol-iton model at right) predict larger polarized than known un-pol’d sea flavor asymmetry.
  • Latest (contro-versial) HERMES results do not support this picture!

HERMES SIDIS results & analysis

slide10

Two Proposed Methods for Probing Polarized Flavor Asymmetry in qq Sea Have Quite Different Sensitivities

Semi-Inclusive DIS: e + N  e’ + h + X

W ± Production at RHIC

¯

SIDIS sensitivity reduced by fragment- ation functions and eq2 weighting

B. Dressler et al. Predictions

{

{

s = 500 GeV

s = 500 GeV

slide11

RHIC W Production Allows ~Direct Determination of u/u and d/d :

¯

¯

¯

¯

  • Measure single-spin parity-violating asym. AL for p + p  W  + X with respect to helicity flip of each beam.

  • Detect W± via isolated high-pT daughter e± or ±, unaccompanied by away-side jet
  • 2 asyms.  2 charges  pol’n of valence q and sea q separately for u,d.
  • Separation of quark vs. antiquark pol’n sensitivity is kinematically cleanest for  > 1, esp. for W.
  • Measurement requires 500 GeV and as much P2L dt as we can get! Can still make significant measurement with P  0.6 and 250 pb-1.

¯

slide12

Transverse Spin Measurements Have Stimulated Rapid Development of Theory

  • Large AN seen by STAR for forward 0 (cross section consistent with NLO pQCD), and sizable azimuthal asyms. in HERMES SIDIS, can arise from various contributions.
  • Unraveling these requires complementary measurements for a number of channels, involving transverse analyzing powers, spin correlations and polarization transfer.
  • Potential payoff: mapping out transverse spin vs. transverse motion preferences of partons in transversely polarized proton.

pQCD

Non-pert.

Non-pert.

Factorization:

Hard hadronic ampl. = PDF’s  hard partonic ampl.  fragment’n fcn.

AN0 can arise, in principle, from (e.g.):

 q in p  aNpartonic~ (mq/pT)s  0 for light quarks

 q in p  dNNpartonic pol’n xfer  Collins fragment’n

sq(pqkTfragment)  0

sp(ppkTquark)  0

 Sivers correl’n in p  kT / pT effect on  partonic

 q in p + sq(ppkTquark) in unpol’d p aNNpartonic + kT / pT effect on  partonic

slide13

Examples of Future Transverse Spin Measurements

p

q

+

g

p

Jets with 2 hadrons detected

p

q

+ …

q

p

  • Attempt to determine transversity by measuring transverse pol’n xfer from incident proton to final-state quark. For latter, rely on fragmentation “analyzing powers” (Collins or interference fragment’n fcns.) calibrated at e+e collider.
  • Probe transverse quark and gluon motion in incident protons by measuring (leading-twist) AN with respect to kT direction, inferred from misalignment of not quite back-to-back dijets.

pp  dijet + X s = 200 GeV 8  pT(1,2)  12 GeV |(1,2)|  1.0

p spin

AN

  • Measure quark transv’ty2 via (pQCD-allowed) ANN for qq-dominated high-pT dijets.
  • AN for W± prod’n sensitive only to transverse quark motion preference in pol’d p, can be used in principle to map out flavor-dependence.

D. Boer & W. Vogelsang, PRELIMINARY predictions



Measurements will progress among channels and observables as L,P improve and theory provides better guidance!

slide14

Machine Progress Toward RHIC Spin

(Pol)

A good store at RHIC, 5/15/03

0.5

BBC MinBias

Background Blue beam

Background Yellow beam

30000

0.4

0.3

Polarization

0.2

20000

0.1

0.0

10000

L = 2 x 1030 cm-2 s-1

0

06:00 08:00 10:00 12:00

Time of Day

  • RHIC is first polarized collider: an enormous technical achievement!
  • Significant technical challenges remain to reach pp design goals, and there is little relevant experience at other facilities for guidance.



  • To reach design goals on which spin program was based – P=0.70, L = 6  1031 @ 200 GeV  factor of ~1000 improvement in P4L– requires additional equipment and substantial beam development time!

¯

Measured beam polarization extracted from AGS generally well understood from simulations including known intrinsic depolarizing resonances

slide15

What is Needed to Realize the “Baseline” Spin Program?

  • Machine aspects:
  • New AGS partial Snakes to improve P (anticipated for runs 4 & 5)
  • NEG coating on RHIC warm beam pipes, to ameliorate vacuum breakdown at high bunch intensity (completed for run 6?)
  • New RHIC working point + reduction of quad triplet nonlinearity, to ameliorate beam-beam tune spread limits on L (1st try, run 4)
  • Polarized jet target + abs. P calibration experiment (runs 4 & 5)
  • At least 2 200 GeV runs of substantial length to build up, and find new limits on L, P (factor 1000 gain likely to come in stages!) – also for important meas’ments for jet and light-quark hadron prod’n!
  • Pol’d beam acceleration/calibration & L development at 500 GeV
  • Detector aspects:
  • STAR EMC completion for  prod’n (anticipated for run 5)
  • PHENIX Si tracker upgrade enhances  - jet reconstr’n capability
  • STAR Forward hadron calorimeter for Collins transverse asym. meas.
  • STAR inner/fwd tracking upgrade to enhance W, heavy quark programs
  • PHENIX  trigger upgrade to enhance W prod’n triggering
slide16

RHIC Spin Community Consensus (Oct.-Nov., 2003)

  • Outstanding scientific opportunities at “enhanced” design L
  • Realization of program highly uncertain at 27 weeks/year. Creative “constant-effort” scenarios help, but also bring problems.
  • Modest increase to 32 weeks/year can make qualitative difference.
  • Adequate and timely access to beam development requires pp beam time in every RHIC run for next few years.
  • Efficient L development requires “long” runs 15 weeks (incl. 5 weeks commissioning).
  • First long pp run needed in FY05 in order to yield significant G results and demonstrate path to full design L, P on time scale appropriate to next NP LRP and worldwide competition.
  • FY05 run yielding ~10 pb-1 would produce high-impact physics constraining G from jet and pion prod’n + progress on origin of large transverse spin asymmetries
  • At least one more long 200 GeV run, ASAP after FY05, needed to approach P4Ldt required for mapping G(x). Long 500 GeV runs needed later for G at lower x, plus W production.
slide17

Frequently Asked Questions: I

  • Can one make good measurements for  and W production in less than 320 (200 GeV) and 800 (500 GeV) pb-1?
  • Can probably survive factor ~4 reduction in PnL dt, but not much more.
  • Can one achieve the needed integrated luminosities “in our lifetime”?
  • A plan consistent with NP “milestones” list (200 GeV G ~complete by 2008, W prod’n by 2013), with ~1/3 RHIC time devoted to pp, is possible in 32-week run scenarios, but it does require hard choices!
  • Why isn’t it sufficient to devote 1 of the next 5 RHIC runs completely to pp, optimizing the efficiency of beam usage?
  • All experience suggests that several beam development periods will be needed to improve stat. FOM by needed 3 orders of magnitude. Physics program lends itself naturally to staging as L, P improve.
  • Why not postpone most pp running until after LHC startup?
  • RHIC-II L increases for heavy ions + STAR/PHENIX incremental detector upgrades will set in after LHC startup, increasing the demand for HI running time at RHIC.
slide18

Frequently Asked Questions: II

  • Why does one need G constraints from jets and pions, as well as from direct photons?
  • Different channels have complementary advantages/disadvantages with respect to interpretation. Convergence on gluon spin contribution is likely to require coherent understanding of all these channels, plus heavy quark prod’n and photon-gluon fusion.
  • Why not concentrate all efforts on 500 GeV pp?
  • Very inefficient to probe xgluon  0.05 at 500 GeV, hence lose overlap with COMPASS exp’t @ CERN. Beam development time likely to be even greater at 500 GeV. HI program needs baseline 200 GeV pp spectra to higher pT.
  • Why not accept lower L and plan a commensurate program?
  • Program outlined has been rated “must do” by HENP PAC. RHIC, STAR, PHENIX, many institutions and funding agencies are heavily invested in the program’s success. First-rate institutions continue to be attracted by the outstanding science opportunities. The larger NP community eagerly awaits the results. The need for substantial development time in opening up a new regime should not be confused with encountering fundamental barriers.
slide19

RHIC Spin Beyond the Baseline

Examples of additional physics opportunities opened by potential future RHIC upgrades beyond “enhanced” design L and/or 500 GeV:

QCD investigations: single-spin transverse asymmetries (incl. final-state baryon polarization) for b-quark jet production, to probe effects of quark mass-dependent terms in QCD Lagrangian

Nucleon spin structure: AL for W +- n (ZDC) coincidences, to probe polarization of anti-down quarks in |n+-like configurations within the proton

Beyond the Standard Model: parity-violating two-spin asymmetries for very hard (qq-dominated) jet production, to search for new interactions and their chiral structure (e.g., by comparing LL to TL PV asymmetries).

slide20

Conclusions

  • Outstanding science opportunities in RHIC spin program form an essential intellectual complement to strong interaction questions addressed in HI collisions, e.g.
  • RHIC is a unique facility for hadronic spin investigations in the pQCD regime. Tremendous technical strides have been made, but significant challenges remain to realize anticipated performance.
  • It is a major challenge to provide for the healthy progress of both programs in parallel, under presently envisioned budgetary and beam time constraints. The health of the overall program depends on our working together to find creative solutions.

How is chiral symmetry breaking realized in the qq sea inside a nucleon?

How is chiral symmetry restored in high-T strongly interacting matter?

¯

slide24

Comparison to Other Channels, Other Facilities

Slide 4

STAR 200GeV +jet 320 pb-1

Projected

gluon pol’n results

from several exp’ts vs. param- etrizations consistent with DIS

W. Vogelsang et al.

  • ALL for inclusive jet production also has substantial sensitivity to G, but must understand relative contributions from qq, qg and gg scattering in the presence of possible bias from trigger and reconstruction algorithms.
  • Results from  + jet to be com-pared to other RHIC channels (inc-lusive , jet prod’n, charm prod’n) and to polarized photon-gluon fusion exp’ts carried out at present & future polarized lepton facilities
slide27

Flavor Asymmetry in the Nucleon Sea

  • FNAL E866 compared Drell-Yan for p+d to p+p, to reveal sizable unpolarized flavor asymmetry d(x)u(x).
  • Results are qualitatively consistent with pion cloud models, instanton models, chiral quark soliton models, etc.
  • Chiral quark soliton model is appropriate in large-Nc limit of QCD: Dirac quarks bound in collective pion field to model SB.

¯

¯

B. Dressler et al., Chiral Quark Soliton Model Predictions

  • Is there a large flavor-dependence of q polarizations in the proton?
  • Most quark-based models predict 01[u(x)d(x)]dx  01[d(x)u(x)]dx most meson-based models disagree

2 = (5 GeV)2

¯

¯

¯

x(ud)

2 = (600 MeV)2

¯

¯

¯

x(du)

¯

¯

¯

slide28

New HERMES SIDIS Results…

…do not appear to support either large positive ud or negative s

But, error bars are large and questions have been raised regarding analysis details.

See:

A. Airapetian et al., hep-ex/0307064

A.N. Sissakian et al., hep-ph/0307189

¯

¯

slide29

STAR Simulations for W Production

Slide 6

  • Dominant charged hadron background can be adequately suppressed by cuts on e/h dis-crimination, e isolation and dijet rejection
  • Different W+ vs. W decay patterns  quite different  distributions for daughters
  • Quark vs. antiquark pol’n sensitivity are separated most cleanly for  > 1, esp. for W
  • x-values of quark, antiquark can be de-termined event-by-event, with some ambi-guity, from  and pT of detected daughter
slide30

STAR Spin Results: Forward Pion Asymmetry and Cross Section

Slide 18

p + p  “p0”+ X , s = 200 GeV

STAR PRELIMINARY

  • Measured cross sections consistent with pQCD calculations.
  • Large spin effects observed for s = 200 GeV pp collisions, qualitatively consistent with models extrapolating from FNAL E704 data at s = 20 GeV.
  • Still have large normalization uncertainty on measured AN , to be reduced when Pbeamcalibration exp’t is done.
slide31

Long-Term Physics Goals III: Transversity

Slide 7

Strong interaction phase shifts

Bin +

p

q

+

g

p

Jets with 2 hadrons detected

Bin -

p

q

+ …

q

P. Estabrooks and A.D. Martin, Nucl. Phys. B79 (1974)301

p

  • Measure transverse spin preference for quarks in transversely pol’d p via transverse polarization transfer from proton to final-state quark.
  • Determine final quark polarization via transverse (chiral-odd) hadron asymmetry in jet fragmentation, either: Collins Fragmentation Fcn: asym.  sp  (pjet  kThadron) or: Interference Fragmentation Fcn: asym.  sp  (p+  p)
  • Rely on other experiments (e.g., BELLE @ KEK B factory) to calibrate jet fragmentation analyzing powers for transversely pol’d quarks

sp  p spin

}

J. Tang (MIT thesis 1999) prediction of maximum possible RHIC asymmetries from interference fragment’n

Jaffe, Jin & Tang: near  mass, - interference allows asymmetry

s = 200 GeV

=0.0

500 GeV

slide32

More Speculative Spin-Flavor Structure Determinations via W Production

S

Initial state interaction

  • Single-spin transverse (non-PV) asymmetry AN for W± production at pT (W)  0 can aid in distinguishing transverse motion of partons from transverse spin alignment in a transversely polarized proton: both contribute to AN in hadronic processes, but W’s do not couple to transverse quark spin, can map flavor-dep. of q,q kT preference.
  • PV helicity asymmetry AL for W+  e+ detected in coinc. with spectator n in ZDC can probe dominance of Goldstone bosons (0  unpolarized d) in |n meson configurations of proton: this measurement requires well beyond “enhanced” pp design luminosity!

¯

Sivers-effect mechanisms for transverse single-spin asyms. in SIDIS and RHIC W prod’n:

from T-odd Sp  (pp  kT)  0, allowed by FSI or ISI phases – related to SL, anomalous magnetic moment in proton

slide33

Transverse Spin Asymmetries in Heavy Quark Production

Hard hadronic ampl. = PDF’s  hard partonic ampl.  fragment’n fcn.

Non-pert.

Non-pert.

pQCD

Factorization:

Chiral symmetry of QCD (for mq0)  no helicity flip 

aNpartonic  0 at leading twist

^

Observed hadronic AN0 (generally @ moderate pT) usually attributed to T-odd spin-kT correlation in the 1st[e.g., Sivers effect: sinc(pinc  kTparton) ] or 3rd[Collins effect: sq(pjet  kTfragment) ] non-perturbative factors above.

However, LQCD contains (explicit chiral symmetry breaking) terms  mqqq for mq  0  expect:

aNpartonic ~ (mq / Eq ) strong

¯

^

Negligible for hard processes and light quarks (u,d,s), but not necessarily for c or b quarks!

 Measure 1-spin transverse spin asymmetries for prod’n of charmed or bottom hadrons, to look for mq-dependent effects of perturbative origin.

slide34

Caveats & Rates

¯

  • Dominant prod’n mechanism likely gg  cc, bb. But g doesn’t share p transverse spin, so unlikely to generate AN  0 of pQCD origin.
  • Measure either PN of outgoing c+ or b0, via their self-analyzing weak decay to lighter ’s + leptons (polarized beams not necessary for this!), and compare to 0ormeasure AN for kinematics chosen to emphasize qq  cc, bb (e.g., high pT, asymmetric collisions: one jet in EEMC, one at mid-rapidity)

No simulations yet performed to flesh out these ideas!

¯

¯

¯

¯

Total cross section for bb prod’n within STAR, pT = 10-20 GeV/c:

bb ~ 10 nb (R. Vogt calcs.)  ~ 2  106 pairs produced in 200 pb-1

Interesting sensitivity level to AN = ± 0.01  need ~ 2  104 events analyzed  need:

Trigger (high-pT e in EMC) eff.  APS/inner tracker vertex acceptance  reconstruction eff.  relevant kinematic cut acceptance ~ 1%

¯

slide35

What is Needed to Attain These Goals?

Slide 8

RHIC:

  • polarized proton collisions up to s = 500 GeV  Siberian Snakes
  • luminosities ~ 1032 cm2s1
  • beam polarizations ~ 70%
  • local control of spin orientation at STAR, PHENIX
  • ~25% of RHIC running time
  • reliable, quick relative polarimeters, plus ~5% absolute calibration of beam polarizations
  • rapid spin flip of stored beams, for systematic error assessment
  • STAR:
  • detection/triggering capability for high-energy , e, jets  EMC upgrades
  • jet reconstruction capability
  • reliable local polarimeter(s)  forward 0 and charged part. detection
  • accurate relative luminosity monitoring
  • improved forward tracking for W daughters
  • possible forward hadron calorimetry for transversity studies

A lot of hard work by many people has brought us part way along these ambitious parallel paths and part way up the steep learning curve in this virgin territory! Progress report…

slide36

Relative Luminosity Monitoring

Slide 17

Yellow

Blue

STAR BBC Coinc. Rate

Spin Down

Beam Crossing Number

Spin Up

Unpolarized

  • RHIC stores up to 120 bunches per ring
  • Different bunches injected with diff. spin orientation
  • Collision luminosity can vary significantly with spin combination
  • Precision of relative luminosity monitoring critical – demonstrated better than 10-3 in 2002 run
  • Special problem for ALL measure-ments: asymmetry -independent, shows up only as yield change per integrated luminosity unit
  • Must demonstrate that L monitor reaction does not have its own ALLof magnitude comparable to physics of interest  comparisons of diff. L monitors in 2003 (first ZDC/BBC results encouraging!)

Example of Relative Luminosity

and time dependent!

slide37

New Opportunities to Study the Partonic Structure of Cold Matter

S. Vigdor, LRPWG, Santa Fe, 2001

  • pp, pA, ep, eA collisions and RHI collisions represent complementary aspects of High Energy Nuclear Physics:
  • Intellectual Interdependence
  • Both seek understanding of strong interactions, hadron structure at the parton level (e.g., what is the origin of confinement and hadron mass? How are constituent quarks that carry the mass and spin of a nucleon related to partons? How do gluons behave at high density?)
  • p-p and p-A collisions form essential baseline in search for new collective behaviors in A-A collisions at high energy
  • Common experimental techniques needed to detect high-pThadrons, leptons, photons
  • Shared Facilities
  • Polarized pp and pA collider opportunities at RHIC, at energies and momentum transfers where pQCD is applicable, are unprecedented – greatly expand the utility of hadron beams in probing partonic structure