The JLab BoNuS Experiment: measurement of the free neutron structure - PowerPoint PPT Presentation

The JLab BoNuS Experiment: measurement of the free neutron structure function at large x and nuclear effects in deuterium via spectator tagging. M. Eric Christy Hampton University n p For the CLAS Collaboration NuInt12 Workshop - Rio de Janeiro, Brazil October 23, 2012

More than 40 years after discovery of the partonic substructure of nucleons, the neutron structure at large-x is still not well determined. In addition, we would like to understand how the simplest nucleus, the deuteron, is built from a proton and neutron.

Why study large- x structure functions? Why study large- x structure functions?  Study pQCD DGLAP evolution  Separation of parton distributions, e.g. , • d/u for x  1, • singlet / non-singlet (valence/non-valence) separation.  Precise PDFs needed to constrain limits on new physics at LHC and Tevatron  Separation and study of perturbative / non-perturbative physics • H igher-twist operators (parton-parton correlations) • Study quark-hadron duality Oct. 23, 2012 M.E. Christy - NuInt12 3

Much can be learned about pQCD from DIS on a neutron target  Need proton and neutron targets to pin down u/d PDFs from DIS F 2 p = x [4/9 u(x) + 1/9 d(x) ] At Leading order n = x [4/9 d(x) + 1/9 u(x) ] F 2 At large x proton dominated by u(x) neutron by d(x) due to charge weighting. → u quark is well determined from proton data → Free neutron target would provide comparable information on d quark Problem: no high density neutron target exists! Oct. 23, 2012 M.E. Christy - NuInt12 4

PDFs Uncertainties PDFs Uncertainties u(x) d(x) PDF’s least well known at large x Gluon comparable in size to d v at x~0.3 not well known here. *Proton + neutron data provides way to Oct. 23, 2012 M.E. Christy - NuInt12 5 separate valence cleanly.

Even F 2 not not well known for x > 0.6 well known for x > 0.6 Even F n n 2 Non-pertabative models J. Arrington et al. arXiv:1110.3362 SU(6) spin-flavor Hard gluon exchange S=0 diquark dominance Oct. 23, 2012 M.E. Christy - NuInt12 6

What about current state-of-the-art What about current state-of-the-art PDF fits including deuterium nuclear PDF fits including deuterium nuclear corrections? corrections? Oct. 23, 2012 M.E. Christy - NuInt12 7

New CJ PDF fits include Large-x + deuterium New CJ PDF fits include Large-x + deuterium including nuclear corrections on D 2 ! Slide from Alberto Accardi Oct. 23, 2012 M.E. Christy - NuInt12 8

Nuclear uncertainties results from CJ11 Nuclear uncertainties results from CJ11 Off-shell model Nuclear Wave-Fn spread spread Still significant uncertainties from nuclear models at large-x Oct. 23, 2012 M.E. Christy - NuInt12 9

Free neutron targets would provide solution Free neutron targets would provide solution → No high density neutron targets exist → Use deuterium and assure that scattering took Place from a 'nearly' on-shell effectively free neutron Oct. 23, 2012 M.E. Christy - NuInt12 10

Method of Spectator Tagging With S( α s , P T ) the nucleon spectral function in the deuteron and F 2 the effective off -shell neutron structure function R=σ L /σ T ν=E − E' α= ( E s − p s z )/ M Tag spectator proton in scattering Q 2 = 4 EE 'sin 2 θ 2 The spectator proton’s four momentum: p μ = -(E s – M D , p p s ) Hadronic W (x) of debris: Oct. 23, 2012 M.E. Christy - NuInt12 11

For free neutron structure must kinematically For free neutron structure must kinematically select to minimize: select to minimize: 1. Off-shell effects 2. Final state interactions 3. Target fragmentation enhancement to proton yield Oct. 23, 2012 M.E. Christy - NuInt12 12

Off-Shell Structure Functions Liuti & Gross PLB 356 (95)157 Melnitchouk et al, PLB 335 (94)11 0.965 0.985 0.93 0.80 • R n decreases with p s or α s • At x*=0.5 and p s =400 MeV/c, R n deviates from unity by 7-20% in BoNuS p s these models detection range Oct. 23, 2012 M.E. Christy - NuInt12 13

Final State Interactions • Struck neutron can interact with the spectator proton (ν, q ) • Proton momentum is enhanced • FSIs are small at low p s and large Θ pq Palli et al, PRC 80 (09)054610 k • Several groups have calculated FSIs • Θ pq > 110 o and p s <100 MeV/c greatly reduces FSIs Θ pq p s Oct. 23, 2012 M.E. Christy - NuInt12 14

Target Fragmentation Enhancement in proton Yield Over PWIA negible for backward protons! Oct. 23, 2012 M.E. Christy - NuInt12 15

Experimental Setup I: CLAS Spectrometer Beam  Detect electrons in CLAS spectrometer  Detect slow protons in radial time projection chamber (RTPC)  Moller electrons bottled up by Solenoid field around target  Solenoid field allows momentum determination Oct. 23, 2012 M.E. Christy - NuInt12 16 June 3 2008

Experimental Setup II: BoNuS RTPC H. Fenker et al., Nucl. Instrum. Meth. A 592 , 273 (2008) 140 µm Fit RTPC points to determine helix of proton trajectory. Momentum determined from beam track curvature in solenoid field. dE/dx along track in RTPC also provides momentum information. Helium/DME at 80/20 ratio to CLAS BoNuS n RTPC p Gas Target To BoNuS RTPC June 3 2008 Moller Catcher Oct. 23, 2012 M.E. Christy - NuInt12 17

RTPC Performance • Upper left: dE/dx vs. p/Z for He target • Lower left: dE/dx vs. p for deuterium target • Below RTPC+CLAS resolution for common e - events σ =8mm z BoNuS vs z CLAS Δz σ =1.4º σ =4 o ΔΘ ΔΦ Oct. 23, 2012 M.E. Christy - NuInt12 18

Kinematic Coverage E = 5.262 GeV E = 4.223 GeV W (GeV) W (GeV) Q 2 Q 2 (GeV 2 ) (GeV 2 ) (MeV) p spec cos(θ pq ) VIPs Oct. 23, 2012 M.E. Christy - NuInt12 19

Kinematic reconstruction with tagged protons µ p n µ + 2([M D -E s ] ν – p n . q) – Q 2 W 2 = (p n + q) 2 = p n ≈ M* 2 +2M ν (2- α s ) - Q 2 W 2 = M 2 +2M ν - Q 2 PWIA: → Backward P is spectator → Neutron is offshell → p p n = - p p p → => correct for neutron momentum Oct. 23, 2012 M.E. Christy - NuInt12 20

Ratio Method First make experimental ratio: N tagged is yield with VIP after accidental subtraction passing: P s < 100 MeV/c, θ p > 110 In terms of structure function ratio: A e : electron acceptance in CLAS ( mostly cancels! ) A p : tagged proton Efficiency * Acceptance I VIP → Integral I vip is largely independent of W* (x*) and Q 2 → Determined from R exp at x=0.3, where nuclear effects are small using F 2 n / F 2 d from CJ PDF fit. d * R exp * I vip Then F 2 n = F 2 Oct. 23, 2012 M.E. Christy - NuInt12 21

Resonance F 2 n results → Clear neutron resonant structure → Compares reasonably well to Bosted-Christy fits to p, d, (extracts neutron using PWIA + Paris potential) → Studies of duality in F 2 n being finalized now. Oct. 23, 2012 M.E. Christy - NuInt12 22

Results on F 2 n / F 2 p → F2n/F2p = F2n/F2d * F2d/F2p Normalization point w ith F2d/F2p from Bosted/Christy fits PRC77(08)065206, [Accardi, et al., PRC81(10)055213 PRD 84(11)014008] → Trend in x consistent with CJ11. → Below normalization point Q 2 less than CJ scale point. → Lower W* cuts reduce stat. uncertainty, but increase resonant contribution at x >0.6 Oct. 23, 2012 M.E. Christy - NuInt12 23

Nuclear effects Studies with BoNuS Can test - spectator model assumptions and - nuclear effect models Using full kinematics coverage for tagged protons. Analysis of S. Tkachenko . Oct. 23, 2012 M.E. Christy - NuInt12 24

Spectator model test: Data/MC cosθ distributions Tkachenko et al. • Analysis of S. Tkachenko utilizes larger kinematic y coverage to test spectator r model. a n • At low p s the data agree with i the spectator model quite m well. i l • At higher p s the distributions e deviate significantly from r P unity, indicating that VIP particles should have p s <100 MeV/c. 25

With n+p and deuterium Structure functions in hand... Study of EMC effect in the deuteron is currently in progress Oct. 23, 2012 M.E. Christy - NuInt12 26

BoNuS Plans for 12 GeV E12-06-113 E12-06-113 Data taking: – 35 days on D 2 – 5 days on H 2 – L = 2 x 10 34 cm -2 sec -1 DIS region: – Q 2 > 1 GeV 2 – W* > 2 GeV – p s < 100 MeV/c – θ pq > 110° – x* max = 0.80 W* > 1.8 GeV: x* max = 0.83

BONUS @ 12 GeV Jlab (BONUS12) d/u F2n Oct. 23, 2012 M.E. Christy - NuInt12 28

∆ transition form factor from neutron with Bonus12 Estimated BONUS12 coverage and uncertainties 29 Oct. 23, 2012 M.E. Christy - NuInt12 29

Summary Spectator tagging method demonstrated to allow → Extraction of free neutron structure. Allows significant reduction in nuclear model uncertainties → On d-quark distribution at x > 0.6. First look at EMC effect extracted for deuterium. → BONUS data allow study of nuclear effects. → Upcoming 12 GeV BONUS will provide F 2 n precision → Comparable to proton data up to x ~ 0.8. Oct. 23, 2012 M.E. Christy - NuInt12 30

Much More to come Thank You! Oct. 23, 2012 M.E. Christy - NuInt12 31

Backup Slides Oct. 23, 2012 M.E. Christy - NuInt12 32 June 3 2008 32

PDFs Uncertainties PDFs Uncertainties u(x) d(x) PDF’s least well known at large x Gluon comparable in size to d v at x~0.3 not well known here. *Proton + neutron data provides way to Oct. 23, 2012 M.E. Christy - NuInt12 33 33 separate valence cleanly.