L-T Separated Kaon production Cross Sections from 5-11 GeV
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L-T Separated Kaon production Cross Sections from 5-11 GeV. P. Bosted, S. Covrig, H. Fenker, R. Ent, D. Gaskell, T. Horn* , M. Jones, J. LeRose, D. Mack, G.R. Smith, S. Wood, G. Huber , A. Semenov, Z. Papandreou, W. Boeglin, P. Markowitz , B. Raue, J. Reinhold, F. Klein,

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L-T Separated Kaon production Cross Sections from 5-11 GeV

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L t separated kaon production cross sections from 5 11 gev

L-T Separated Kaon production Cross Sections from 5-11 GeV

P. Bosted, S. Covrig, H. Fenker, R. Ent, D. Gaskell, T. Horn*, M. Jones,

J. LeRose, D. Mack, G.R. Smith, S. Wood, G. Huber, A. Semenov,

Z. Papandreou, W. Boeglin, P. Markowitz, B. Raue, J. Reinhold, F. Klein,

P. Nadel-Turonski, A. Asaturyan, A. Mkrtchyan, H. Mkrtchyan, V. Tadevosyan, D. Dutta, M. Kohl, P. Monaghan, L. Tang, D. Hornidge, A. Sarty,

E. Beise, G. Niculescu, I. Niculescu, K. Aniol, E. Brash, V. Punjabi,

C. Perdrisat, Y. Ilieva, F. Cusanno, F. Garibaldi, M. Iodice, S. Marrone,

P. King, J. Roche

JLab, Regina, FIU, CUA, Yerevan, Mississippi, Hampton, Mount Allison, Saint Mary’s, UMd, JMU, California State, CNU, Norfolk, W&M, South Carolina, INF, Ohio

Hall C User’s meeting

30 January 2009


Meson reaction dynamics

Meson Reaction Dynamics

e’

e

  • Meson production can be described by the t-channel exchange meson pole term in the limit of small –t and large W

    • Pole term is dominated by longitudinally polarized photons

    • Meson form factor describes the spatial distribution of the nucleon

Q2

π, K, etc.

W

t

N(p’)

N(p)

t-channel process

  • At sufficiently high Q2, the process should be understandable in terms of the “handbag” diagram

    • The non-perturbative (soft) physics is represented by the GPDs

      • Shown to factorize from QCD perturbative processes for longitudinal photons [Collins, Frankfurt, Strikman, 1997]

π, K, etc.

hard

pointlike

handbag


Form factors and gpds

Form Factors and GPDs

  • Form factors and GPDs are essential to understand the structure of nucleons, which make up nucleons and mesons (q-q systems)

-

π, K, etc

  • But measurements of form factors and GPDs have certain prerequisites:

    • Before we can start looking at form factors, we must make sure that σL is dominated by the meson pole term at low -t

    • Before we can learn about GPDs, we must demonstrate that factorization applies

φ

φ

Fπ,K

π,K

π, K, etc.

φ

  • A comparison of pion and kaon production data may shed further light on the reaction mechanism, and intriguing 6 GeV pion results

Hard Scattering

GPD


Context

Context

  • Understanding of the kaon production mechanism is important in our study of hadron structure

    • GPD studies require evidence of soft-hard factorization

  • Flavor degrees of freedom provide important information for QCD model building and understanding of basic coupling constants

  • K+Λ and K+Σ° have been relatively unexplored because of lack of the necessary experimental facilities

    • There are practically no precision L-T separated data for exclusive K+ production from the proton above the resonance region

  • Limited knowledge of L/T ratio at higher energies limits the interpretability of unseparated cross sections in kaon production

  • Relative σL and σT contributions are also needed for GPD extractions since they rely on the dominance of σL


Transverse contributions

Transverse Contributions

Hall C 6 GeV data (W=1.84 GeV)

  • In the resonance region at Q2=2.0 GeV2σT is not small

  • In pion production, σT is also much larger than predicted by the VGL/Regge model [PRL97:192001 (2006)]

  • Why is σT so large? Difficult to draw a conclusion from current data

    • Limited W and Q2 range

    • Significant uncertainty due to scaling in xB and –t

K+Λ

K+Σ˚

σL

0.5<Q2<2.0 GeV2

VGL/Regge

σT

K+Λ

K+Σ˚

High quality σL and σT data for both kaon and pion would provide important information for understanding the meson reaction mechanism


R l t kaon form factor prerequisite

R=σL/σT: Kaon form factor prerequisite

  • Meson form factor extraction requires a good reaction model

    • Need high quality data to develop these models

  • Current knowledge of σL and σTabove the resonance region is insufficient

    • Role of the t-channel kaon exchange in amplitude unclear

  • Not clear how to understand reaction mechanism through current models

VGL/Regge (Λ2K=0.68 GeV2)

Nucleon resonance data scaled to W=1.84 GeV

L/T separations above the resonance region are essential for building reliable models, which are also needed for form factor extractions


High q 2 q n scaling of l and t

High Q2: Q-n scaling of σL and σT

Hall C p+ production data at 6 GeV

  • The QCD scaling prediction is reasonably consistent with recent JLab π+σL data, BUTσT does not follow the scaling expectation

  • To access physics contained in GPDs, one is limited to the kinematic regime where hard-soft factorization applies

Q2=2.7-3.9 GeV2

Q2=1.4-2.2 GeV2

  • A test is the Q2 dependence of the cross section:

    • σL~ Q-6 to leading order

    • σT ~Q-8

    • As Q2 gets large: σL >> σT

σL

σT

T. Horn et al., Phys. Rev. C78, 058201 (2008)

Kaon production data would allow for a quasi model-independent comparison that is more robust than calculations based on QCD factorization and present GPD models


L t separated kaon production cross sections from 5 11 gev

Bonus: Fπ,K - a factorization puzzle?

T. Horn et al., Phys. Rev. Lett. 97 (2006) 192001.

T. Horn et al., arXiv:0707.1794 (2007).

  • The Q2 dependence of Fπ is also consistent with hard-soft factorization prediction (Q-2) at values Q2>1 GeV2

  • BUT the observed magnitude of Fπ is larger than the hard QCD prediction

    • Could be due to QCD factorization not being applicable in this regime

    • Or insufficient knowledge about additional soft contributions from the meson wave function

A.P. Bakulev et al, Phys. Rev. D70 (2004)]

Comparing the observed Q2 dependence of σL,T and FFmagnitude with kaon production would allow for better understanding of the onset of factorization


Motivation summary

Motivation Summary

  • The charged kaon L/T cross section is of significant interest to the study of GPDs and form factors at 12 GeV

    • Can only learn about GPDs if soft-hard factorization applies

    • If transverse contributions are large, the accessible phase space may be limited

    • If σL not dominated by the K+ pole term at low -t, we cannot extract the form factor from the data and interpretation of unseparated data questionable

  • Our theoretical understanding of hard exclusive reactions will benefit from L/T separated kaon data over a large kinematic range

    • Constraints for QCD model building using both pion and kaon data

    • Understanding of basic coupling constants (Σ°/Λratio)

    • Quasi model-independent comparison of pion and kaon data would allow a better understanding of the onset of factorization


L t separated kaon production cross sections from 5 11 gev

Experiment Overview

  • Measure the separated cross sections at varying –t and xB

    • If K+ pole dominates σL allows for extraction of the kaon ff (W>2.5 GeV)

  • Measure separated cross sections for the p(e,e’K+)Λ(Σ°) reaction at two fixed values of –t and xB

    • Q2 coverage is a factor of 2-3 larger compared to 6 GeV at much smaller –t

    • Facilitates tests of Q2 dependence even if L/T ratio less favorable than predicted

Q2=3.0 GeV2 was optimized to be used for both t-channel and Q-n scaling tests


L t separated kaon production cross sections from 5 11 gev

Cross Section Separation

  • The virtual photon cross section can be written in terms of contributions from transversely and longitudinally polarized photons.

  • Separate σL, σT, σLT, and σTT by simultaneous fit using measured azimuthal angle (φK) and knowledge of photon polarization (ε)


Separation in a multi dimensional phase space

Separation in a Multi-Dimensional Phase Space

Low ε

High ε

  • Cuts are placed on the data to equalize the Q2-W range measured at the different ε-settings

  • Multiple SHMS settings (±3° left and right of the q vector) are used to obtain good φ coverage over a range of –t

    • Measuring 0<φ<2π allows to determine L, T, LT and TT

SHMS+3°

SHMS-3°

Radial coordinate (-t),

Azimuthal coordinate (φ)


Kaon pid

Kaon PID

Heavy Gas Cherenkov and 60 cm of empty space

  • π+/K+ separation provided by heavy gas Cerenkov for pSHMS>3.4 GeV/c

  • For reliable K+/p separation above 3 GeV/c an aerogel Cerenkov is essential

    • Provision has been made in the SHMS detector stack for two threshold aerogel detectors

TOF

  • Four sets of aerogel would provide reliable K+/p separation over the full momentum range (2.6-7.1 GeV/c)

  • Alternate PID methods (such as RICH) are also possible

Kaon 12 GeV Kinematics

π/K Discrimination power

Heavy Gas Cerenkov

Momentum (GeV/c)


Expected missing mass resolution

Expected Missing Mass Resolution

  • Missing mass resolution (~30 MeV) is clearly sufficient to separate Λ and Σ° final states

  • Acceptance allows for simultaneous studies of both Λ and Σ° channels

    • Total effect of the Λ tail and possible collimator punch-through to K+Σ° projected to be <1/10 of the size of the tail

e+p→e’+K+Λ(Σ°)

Λ

SHMS+HMS

Σ°

Simulation at Q2=2.0 GeV2 , W=3.0 and high ε


Projections of r l t

Projections of R=σL/σT

  • Empirical kaon parameterization based on Hall C data was used in rate estimates

    • Conservative assumptions on the evolution of L/T ratio

    • Projected Δ(L/T)=28-60% (10-33% using VGL/Regge) for typical kinematics

  • PR12-09-011 may indicate larger values of R, with associated smaller uncertainties

    • Reaching Q2=8 GeV2 may ultimately be possible

VGL/Regge calculation

Hall C parameterization

VGL/Regge

Fπ param


Projected uncertainties for l and t

Projected Uncertainties for σL and σT

σL

σT

  • High quality kaon L/T separation above the resonance region

  • Projected uncertainties for σL and σT use the L/T ratio from Hall C parameterization

PR12-09-011:

Precision data for W > 2.5 GeV


L t separated kaon production cross sections from 5 11 gev

Projected Uncertainties for the Kaon FF

  • If the K+ pole dominates low -t σL, we would for the first time extract FK above the resonance region (W>2.5 GeV)

  • Projected uncertainties for σL use the L/T ratio from Hall C parameterization


L t separated kaon production cross sections from 5 11 gev

Projected Uncertainties for Q-n scaling

p(e,e’K+)Λ

xB=0.25

  • QCD scaling predicts σL~Q-6and σT~Q-8

  • Projected uncertainties use R from the Hall C parameterization

1/Q4

1/Q6

Fit: 1/Qn

1/Q8

Is onset of scaling different for kaon than pion?

Kaons and pions together provide quasi model-independent study


Pr12 09 011 summary

PR12-09-011 Summary

  • L/T separated K+ cross sections will be essential for our understanding of the reaction mechanism at 12 GeV

    • If transverse contributions are found to be large, the accessible phase space for GPD studies may be limited

    • Basic coupling constants in kaon production (Σ°/Λ ratio)

    • If t-channel exchange dominates σL, we can perform the first reliable extraction of the kaon form factor above the resonance region

  • L/T separated K+ data over a wide kinematic range will have a significant impact on our understanding of hard exclusive reactions

    • Constraints on QCD model building using both pion and kaon data

    • Quasi model-independent comparison of kaon and pion data would allow better understanding of the onset of factorization

Request 47 days to provide first precision L/T separated kaon production data above the resonance region. Excellent candidate for early running.


L t separated kaon production cross sections from 5 11 gev

Backup material


Pr12 09 011 beam time

PR12-09-011 Beam Time


Systematic uncertainties

Systematic Uncertainties


L t separated kaon production cross sections from 5 11 gev

Overlap with pion data

  • No significant improvement in statistics through overlap with the approved p(e,e’π+)n experiments

    • Covers only a small region at very high –t, which is not interesting for studies of form factors or GPDs

    • Most events are off the focal plane

      • exclusive K+ peak lies at -5.5%<δSHMS<-2%

    • Missing Mass tail cut off by SHMS acceptance

  • Would require that aerogels installed during pion experiments as well

Proposed kaon experiment

Approved pion experiments


Transverse contributions in pion production

Transverse Contributions in Pion production

  • In pion production, magnitude of σT has been controversial for a long time

    • VGL/Regge model systematically underestimates σT, for which it seems to have limited predictive power

VGL σL

VGL σT

T. Horn et al., Phys. Rev. Lett. 97, 192001 (2006)


Bonus interference terms

Bonus: Interference Terms

K+Λ(Σ°) as calculated in VGL/Regge model

Q2=0.4 GeV2

  • In the hard scattering limit, these terms are expected to scale:

    • σLT ~ Q-7

    • σTT ~Q-8

  • Additional information about the reaction mechanism may be obtained for free if one performs a full cross section separation

Q2=3.5 GeV2


Significance of multiple epsilon points

Significance of multiple epsilon points

  • Additional epsilon settings require additional beam time

  • Resulting benefit in systematic uncertainty must be weighted against the increased statistical uncertainty


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