Parity Violation in Deep Inelastic Scattering with the SoLID Spectrometer at JLab

1.2 MB·

by  Y. X. Zhao

APS/123-QED
Parity Violation in Deep Inelastic Scattering with the SoLID Spectrometer at JLab
Y. X. Zhao1,∗
(The SoLID Collaboration)
1Stony Brook University, NY, 11794, USA
For the SoLID Collaboration
(Dated: January 12, 2017)
Measurements of parity-violating asymmetries in DIS region using the SoLID spectrometer at
JeffersonLab(JLab)HallAinthe12GeVeraarepresented. Aproposalwithapolarizedelectron
beam on unpolarized deuteron and proton targets has been approved with an A rating by the
JLab PAC. The deuteron measurement aims to measure the weak mixing angle sin2θ with a
W
precision of ± 0.0006 as well as to access the fundamental coupling constants C with a high
2q
precision. This measurement is ideally suited for testing the Standard Model with the potential to
probe charge symmetry violation and resolve the quark-quark correlations in the DIS region. The
7
proton experiment provides a clean measurement of d/u ratio in the high-x region free of nuclear
1
corrections. Toachievethesegoals,theSoLIDspectrometerwasproposedanddesignedtohandlea
0
high luminosity with a large acceptance. In this article, the details of the approved measurements
2
are discussed, along with new ideas with PVDIS using a polarized 3He target to access new γ−Z

n interference polarized structure functions and a unpolarized 48Ca target to study the EMC effect.
a
J PACSnumbers: 24.80+y,24.85+p,11.30Er,13.60Hb

0
1
I. INTRODUCTION

]
x σ −σ
e Symmetriesplayacentralroleinphysics. Parity, time APV = σR+σL (1)
- reversal, and charge conjugation symmetries etc. were R L
l
c naturally assumed to be conserved until T.D. Lee and with longitudinally polarized electrons and unpolarized
u C.N. Yang first suggested parity violation [1]. C.S. Wu nucleon or nuclear targets. The beauty of the measure-
n
led the first experiment in nuclear β decay which con- ment, especially in the DIS region, is that the interac-
[
firmedtheparityviolation[2]. TheNobelPrizeinphysics tion vertex provides the unique information on effective

1 was awarded to Lee and Yang in 1957 “for their pene- electron-quark couplings while the quarks probed by the
v trating investigation of the so-called parity laws which neutral current reveal the internal structure of the nu-
0
has led to important discoveries regarding the elemen- cleon. It has been a powerful tool since the 1970s to
8
taryparticles.”Inthefollowingdecadeafterthefirstob- access the fundamental quantities of QCD, to study the
7
2 servation of parity violation, many models/theories were nucleon structure, and to search for new physics beyond
0 invented to explain the phenomenon. Among them is the Standard Model (SM). In the JLab 12 GeV era a
. the Glashow-Weinberg-Salam (GWS) theory [3–5] which SolenoidalLargeIntensityDevice(SoLID),showninFig-
1
0 yields the unification of electroweak interaction and pre- ure 1, is proposed to measure the parity violating asym-
7 dicts a new electrically-neutral boson Z0. In GWS the- metryintheDISregion(PVDIS)usingdifferentunpolar-
1 ory, all spin-1/2 particles carry two types of couplings: ized/polarizedtargets[6]. Detailsofthesemeasurements
v: axial and vector couplings. The axial coupling gA de- will be discussed in the following sections.
scribes the difference of the strength of neutral-weak in-
i
X teractionfortheleft-andright-handedstatesofspin-1/2
r particles,whilethevectorcouplinggV describestheaver- II. PHYSICS PROGRAMS
a
ageofthetwo. Forpurevirtualphotonexchange,thereis
no difference for left- and right-handed particles, hence A. PVDIS with longitudinally polarized electrons
only vector coupling exists and it is equal to the elec- and a unpolarized deuteron target
trical charge of the particle. For the W boson, it only
interacts with left-handed fermions. For the Z boson, it In the context of the SM, the PVDIS in a Q2 (cid:28) M2
Z
interacts with both left- and right-handed fermions, for region with one-photon or one-Z0 exchange between the
instance, the electron axial coupling ge = −1 and the
A 2 electron and the target can be expressed as [7]
vector coupling ge =−1 +2sin2θ .
V 2 W
Inelectron-nucleon(nuclei)scattering,parityviolation G Q2 1−(1−y)2
A = √F [a (x)+a (x) ], (2)
isusuallyobservedbymeasuringthenon-zeroasymmetry PV 4 2πα 1 3 1+(1−y)2
where G is the Fermi constant, α is the electromag-
F
netic fine-structure constant, Q2 is the squared momen-
∗ Correspondingauthor: [email protected] tum transfer to the electron, x is the Bjorken variable, y
2
0.244
SoLID (PVDIS)
0.242 Qweak(first)
n-DIS
0.24 E158
EM Calorimeter
(forward angle) 2)Q0.238 APV(Cs)
(W0.236
Baffle electron GEM q2 n PVDIS
GEM si0.234
Target 0.232 LEP
Beamline 0.23 APV(Ra+) Moller P2 Qweak SoLID SLAC
0.228
-3 -2 -1 0 1 2 3
Log Q [GeV]
10
Coil and Yoke
Cherenkov SFoILGI.D2-P. V(DCIoSlor online) The sin2θW projection from SoLID RakithaS.Beminiwattha
project along with other existing or proposed measurements
1 m
[8].
FIG. 1. (Color online) SoLID spectrometer for the PVDIS
program. 2C1u- C1d 2C1u- C1d
0.5
0.4 -0.06 Qweak + APV
is the fractional energy loss of the incident electron. The SLAC-E122
0.3 -0.08 JLab-Hall A
a1,3 terms are all published
0.2 -0.10 SM
FγZ SoLID (proposal)
a (x)=2ge 1 , (3) 0.1 -0.12
1 A Fγ
1 0 -0.14 2C2u- C2d 2C2u- C2d
-0.1 -0.16 10 TeV P
FγZ 20 TeV
a (x)=ge 3 . (4) -0.2 -0.18 o
3 V Fγ 30 TeV
1 -0.3 -0.20 S
40 TeV
TheF1γ,Z3 functionsareγ−Z interferencestructurefunc- -0.4 -0.22 -0.76 -0.74 -0.72 -0.70 -0.68 50 TeV (
tions. Inthepartonmodelattheleadingorder,theycan
-0.5 D
be written as: -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
I
(cid:88)
FγZ = e (g ) (q +q¯ ), (5)
1 qf V qf f f FIG.3. (Coloronline)Thephase-spaceofthelinearcombina- S
f Ftiiognusreof4a:xiaAl-vxeecstoarrealnindevaercctoomr-abxiniaaltieolnesctorfona-xqiuala-rvkecetffoercqtiuvaerk-electron and vector-axial quark-electron
ecfofuepctliivnegccoounpsltianngtscofonrstaexnitsst.inLgemft:eaTsuhreempheanstes-s[p7a]caendofathperoa-xial-vector and vector-axial electron-quark 2
FγZ =2(cid:88)e (g ) (q −q¯ ). (6) ejeffcetciotinveinccoluupdliinnggmcoenasstuarnetsmceanntsbefrocomnsStroaLinIDtsbpyroujesicntg. SoLIDPVDISmeasurementscombinedwithother 0
3 qf A qf f f precisionmeasurements. Presentmeasurementspredictnonzeroelectron-quarkcouplingconstantsandan
f 1
agreement with the SM predictions [11]. Right: A polar plot of the mass scales of new parity violating
The vector couplings gV of quarks and electrons are a pwhhyesricesginAtearancdtiognVsaasrseumthinegaaxniaewl apnhdysvicescctoourpclionugpsltirnegnsgtohfofg2=4π. Thesmallbrowncolorregion 5
function of sin2θW. For an iso-scalar target, such as a sehleocwtsropnressaenntdreuapc/hdoofwthneqmuaasrskss.caIlfesonbeasneedgolencctosmsebainqeudarreksusltsfrom6GeVPVDISandotherprecision )
in the valence region, then
deuteron in the valence region, which carries the same experiments[11] while the large orange color region is the expandedsensitivity assuming final precision
2
amount of u and d quarks, the contributions from PDFs
fromQweak[26]andSoLIDPVDIS. 6
cancel in ratio in a1,3 terms, hence the APV is sensi- a1 = 5(2C1u−C1d),a3 = 5(2C2u−C2d). (9) 3
tive to sin2θ directly: A ≈ 20sin2θ −1. Figure
W PV 3 W 5
2 shows the sin2θW projection from SoLID along with kAintelmaragteicyra,nAgeP.VThisepseronpsiotsiveedttwootdhieffeCr2eqn,tethleectcroonupbleinagmenergies, at11GeVand6.6GeV,will
other existing and proposed measurements. pthroavtidceanra’tngbeeofstQu2diveadlueinsfloorweaecnhexrgy.reactions due to
bjk
In the context of new physics searches, PVDIS can largeanduncertainradiativecorrections. Figure3shows
The cryogenic solenoidal magnet from CLEO-II experiment will be refurbished and imple-
not be described only by the one-boson exchange. The existing and expected results on linear combinations of
effective electron-quark couplings in terms of individual meleecnttraosnt-hqeuaSrokLwIDeamkacgonueptlainftgercocnersttaainntmsfoodrifiexciasttioinngsmtoema-atchSoLIDspecifications. GasElectron
g and g are not valid anymore. Instead, the effective Msuureltmipelinetrsdaentdecatoprrso(jGecEtMions)awftielrlbineculusdedinagsmtreaacskuirnegmdeenttesctors implemented withinthesolenoidal
A V
weak coupling constants C1q,2q are used. In the leading mfroagmneStoicLIfiDeldpraonpdosianl.downstream of the magnet where main particle detectors are located [12].
order of one-boson exchange, they correspond to [9]: ThBeyligmhteagsausrCinegreCnk1qo,v2qa,nodntehecaenlecstertomcoangsntertaicinctsaloonrimneetwerwillprimarilybeusedtoparticleiden-
contact interactions, such as a possible lepto-phobic Z
C1u =2gAegVu, C2u =2gVegAu, (7) tbifiocsoanti.onTaondqrueajencttiftyheapnidoncboamcpkagrreoutnhde[1p2h]y.sTichserperaocphosoefddataacquisition(DAQ)systemwillbe
C =2gegd, C =2gegd, (8) bvaasreioduosnepxippeer-ilmineendtse,leocntreonciacns fqourottreiggmearisnsgliamnditsdawtaitrheiandout toaccommodate veryhighratesof
1d A V 2d V A
above1MHz. Thedetectorswillbetriggeredandreadoutindependently forazimuthallyseparated
sectors to increase the readout rate. An upgraded Compton polarimeter and a superconducting
MøllerpolarimeterwillbothassumedtobereadilyavailablebythetimeSoLIDprogramwillstart
takingdata.
4. Summary
The SoLID apparatus is design to have a broad physics program. The PVDIS program dis-
cussed intheproceeding isonly apartofthis physics program. Thesemi-inclusive deep inelastic
scattering (SIDIS) program using SoLID will measure single and double spin asymmetries to ac-
5
3
Projected 12 GeV d/u Extractions
1
CJ12 - PDF + nucl uncert.
SoLID PVDIS
0.8
0.6
u SU(6)
]C2d d/
− 0.4
C 2u DSE
2 10 TeV 0.2
[ pQCD
20 TeV
Broken SU(6)
0
30 TeV g2 =4π 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
x
40 TeV
50 TeV FIG. 5. (Color online) The projections of d/u ratio from
SoLID proposal, also shown are the calculations based on
PDFs from the CJ12 collaboration [14].
[2C 1 u − C1 d ]
FIG. 4. (Color online) Mass-exclusion plot of the mass
B. PVDIS with longitudinally polarized electrons
scales of new contact interactions assuming a physics cou-
and a unpolarized proton target
plingstrength ofg2 =4π. Thepink (inner)region illustrates
thereachbycombiningthe6GeVPVDISexperimentatJLab
andotherprecisionexperiments[7],theorange(outer)region By measuring A from a unpolarized proton target,
PV
showsthenewreachassumingfinalprecisionfromQweak[11] one can have direct access to the PDF d/u ratio free of
and SoLID PVDIS. nuclear effects. In the SM context at leading order and
leading twist, A in Eq. (2) is a function of d/u in the
PV
x y Q2 valence quark region:
New Physics no yes small
CSV yes small small A ≈− G√FQ2 1+d/u, (11)
High Twist large? no large PV 2 2πα4+d/u
TABLEI.Kinematicdependencefordifferentphysicstopics. where it has been assumed that sin2θ = 1. The tra-
W 4
ditional way of determining d/u relies on comparing the
inclusive DIS cross section on a proton target to that
of a deuteron target. The disadvantages, compared to
compositemodels[10],wherethecouplingsareontheor- the asymmetry measurement, are that the cross section
der of 4π/Λ2 with Λ the compositeness mass scale. The measurement is hard to achieve with high precisions and
limit can be extended to ∼ 20 TeV level with the pro- nuclear corrections in the deuteron target in the large x
posed precision of the SoLID proposal E12-10-007 and region lead to large uncertainties.
other existing measurements, as shown in Figure 4. The projections on d/u from SoLID proposal E12-10-
Otherinterestingtopicsinthehighprecisionmeasure- 007 is shown in Figure 5 along with calculations using
ments of APV in electron-deuteron scattering are the PDFsfromtheCJ12collaboration. ThedatafromSoLID
charge symmetry violation (CSV) and higher twist ef- will be complementary to other proposed experiments
fects from quark-quark correlations. The strategy for at JLab including the one using 3H and 3He nuclei to
the experiment is to have precision measurements over minimize nuclear effects during d/u extraction [12], and
a broad kinematic range in both x and Q2. The data is the BoNuS experiment [13] at JLab Hall B.
fitted with the form
β
A =A [1+ HT Q2+β x2], (10) C. PVDIS with unpolarized electrons and a
measure SM (1−x)3 CSV longitudinally polarized 3He target
where βHT is the asymmetry due to higher twist effects Another attractive physics topic with PVDIS is to use
with a kinematic dependence of (1−x1)3Q2, βCSV is the alongitudinallypolarizednucleartargetandunpolarized
asymmetry due to CSV with a kinematic dependence of electrons. In the DIS region, the single-target parity vi-
x2. The Table I shows the kinematic sensitivities for olation asymmetry is determined by the polarized elec-
different physics topics discussed above. troweak interference structure function gγZ of the nu-
1,5
2
4
%) 102 aa ffrroomm CCBBTT,, 4488CCaa xx//XX ==1122%%,, 6600 ddaayyss,, 8800mm AA 48Ca/ 40Ca Ratios
D A (A 456000 SSIIDDIISS llaarrggee aannggllee ddaattaa with Q2 > 3 GeV2 cut 1 11 a1 CBT 00 1.15 E139 Param
30 0.98 an1aive 1.1 CBT
Our Projections (stat, stat+ pt to pt sys)
48Ca/ 40Ca Ratio (stat + pt to pt sys)
20 0.96 Shared sys. uncert a
C
402, 1.05 Shared systematic
0.94 F
10 /a
86 a10.92 isF48C2, 1
4 m.
0.9 or 0.95
N
2
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.88
x 0.9
0.86
FIG. 6. (Color online) The expected uncertainty on the 3He 0.85
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
single-targetPVDISasymmetry. TheQ2 cutonlyaffectsthe x x
bj
last two data points in x < 0.25 region as illustrated by the
red points.
FIGF.ig7u. r(eC1ol:orAoncloinme)pTahreispornojebcetitownseefonr tthheeatwfounmctieoansurements for this proposal (left) and E12-10-008 (right).
1
using a 48Ca target. The calculations using the CBT model
and PDFs are also shown.
cleon [15]:
σ(+)−σ(−) G Q2 gγZ 2y−y2 gγZ 1nucleaIrntatrrgeotdwuithcattioomnic number A can be written as
A = = √F [ge 5 +ge 1 ],
L σ(+)+σ(−) 2 2πα V F1γ Ay2−2y+2 F1γ 9 12u+−d+
where (12) Within QCa1D≈o5u−r d4essinc2rθipWti−on2s5ouA+Af p+rdoA+Ato,ns and(1n5e)utrons are with the underlying quark and gluon degrees of
freedom. In addition, protons and neutrons also arrange themselves into the more complex objects of nuclei
g1γZ =(cid:88)eqf(gV)qf(∆qf +∆q¯f), (13) awnitdhtthhiesctornavnesnittiioonntbheattwqA±ee=n QqAC(xD)±anq¯dA(nxu).clTeahrerpe-hysics is still out of reach for modern theory. The effective
f fore, the measurement is directly sensitive to differences
theories we have for the description of inter-nucleon interactions have been widely successful in producing
(cid:88) inthequarkflavorswithinanucleus,whichwouldrepre-
gγZ = e (g ) (∆q −∆q¯ ). (14)
5 qf A qf f f dseenttainleewdadnedscimrippotritoannst ionffosrymsatteimonsosnuocuhrausndneursctlaenadr-structure and scattering processes. However, they are based
f ing of the EMC effect. The flavor dependence can also
around the concept that nucleons in the nuclear environment strongly maintain their identities and have few,
beusedtoexaminedifferentmodelsforthescalingofthe
If one assumes sin2θW = 14, the g(cid:80)1γZ function is approx- iEfManCye,ffpercotv[1is6i],ofnosr finosrtahnocwe ththeeiydecahtahnatgeth.e EMC ef-
imately proportional to ∆Σ ≡ (∆q + ∆q¯ ), with
f f f fect sAcanleospweitnhathnedlaimrgepovirrttaunatlitqyuoefstthioenstrfuocrkhnaudclreoonnic physics today is how protons and neutrons are modified
∆q the polarized parton distribution functions. The
f orthosewhichscalewiththenucleon’slocaldensity[17].
g5γZ function is sensitive to the valence quark polariza- whAenmetahseuyreamreentbroeuqnuedstiinnga6n0udcalyesuosf abneadmhtoimweownitehmakes the transition between traditional nuclear physics to
tion ∆qV ≡ ∆q−∆q¯, including ∆s−∆s¯ that can’t be Q80CµAD.bTeahmis cisurerennotrmonouasl4y8Cima ptaorrgteatntwnasotporonployseads a theoretical question, but as a practical one, as measure-
measured from existing experimental techniques. With (PR12-16-006). The predictions for a as a function of
ments often rely on data from bou1nd neutrons to produce “effectively free” neutron data to complement our
the input of the weak mixing angle in the context of x are shown in Figure 7. The prediction from the CBT
the SM, these brand new and yet unmeasured polarized amcocdeesls[i1b8l]eisfraelseopsrhootwonnsin. tFhoerpeloxta.mTphlee,CmBTeamsuodreelments for elastic neutron electromagnetic form factors and
electroweak interference structure functions can be ex- fhoarsmbefeancvtoerryflsauvccoersssfuelpianrarteiporno,duacnindgdteheepquinarekladsitsi-c scattering flavor separation all rely on some level of nu-
tracted, providing independent information on the spin tributions for the EMC effect as well as the measured
structure of the nucleon and SU(3) flavor symmetry test cstlreuacrtumreofduenlcitniogn.s.EIvteins ahlsigohalbyleetxooetxipclasiynspteamrtsofsuthceh neutron stars, where protons and neutrons are compressed
in addition to the g1γ structure function. tNouvToeVluamnoemsasleyv. eral times smaller than their charge radii suggest, frequently use models that maintain their
Aletter-of-intentwassubmittedtoJLabPAC44(LOI-
constituent’s identities.
12-16-007) to carry out the first such measurement with
a longitudinally polarized 3He target at SoLID. The pre- AtIwIIh.atTlHevEelFUalLlLofSOthLeIsDePaRssOuGmRpAtiMons hold have broad reaching effects on complementary processes
dicted relative uncertainties on APV measurements as a that rely on this extracted flavor data. Neutrino deep inelastic scattering is highly quark flavor dependent
function of x are shown in Figure 6.
asInitapdrdoitbioens tdoiftfheerePnVtDcIoSupprloignrgasmthdiasncustsheed satbaonvde,ard electromagnetic probes in the neutral current process,
there are other rich programs based on the SoLID spec-
or changes their flavor in the charged current processes. Parity-violating elastic and neutrino quasi-elastic
trometer [6]. The detector subsystems can be reconfig-
D. PVDIS with longitudinally polarized electrons sucreadttteorianccgomremqoudiraeterSeelmiaib-IlneclnuusicvleeDoenepfoInrmelasftaicctSocratd-ata for both protons and neutrons. Drell-Yan processes and
and a unpolarized 48Ca target tering (SIDIS) with polarized 3He and proton targets to
electron scattering at a future EIC rely on knowledge of the quark and antiquark flavor distributions.
measure Transverse Momentum Dependent Parton Dis-
While we have evidence that modification occurs, the exact mechanism for it is not well understood. It
PVDIS on a heavy nuclear target will provide a direct tributions(TMDs)inmulti-dimensionalkinematics with
measurement of flavor-dependent nuclear medium modi- ihsigchleparrecfisrioomns s[1i9m].plTehmereodiselaslstohaatJt/hψe pFheyrsmicsi mproo-tion of bound nucleons is insufficient to describe observed
fication effects on quarks. The a term in Eq. (2) for a gramtostudythethresholdelectroproductionoftheJ/ψ
1 effects in deep inelastic scattering or nucleon quasielastic scattering. The concept of “nuclear shadowing,”
where a reduction in scattering cross section is observed relative to the naive sum is also important but still
doesn’t offer a complete picture. The “EMC effect” was some of the first data observed that there is a
significantchangeonthe quark levelandthattheidentitiesoftheboundnucleonsaredifferent. Itshoweda
depletionofquarksinthevalenceregionanddespitesophisticatedmodeling, cannotbedescribedbysimple
binding effects. Despite decades of theoretical efforts, a rigorous explanation has been elusive.
5
on the nucleon, which provides a unique opportunity to to the world PDF fit. The SU(3) flavor symmetry could
help understand the low energy struture of the nucleon. also be tested. Furthermore, by using a 48Ca target, one
Recently, there are more ideas to carry out Deeply Vir- canobserveapossibleisovectorEMCeffectthatcanhelp
tualComptonScattering(DVCS)programswithSoLID. understand the NuTeV anomaly. The SoLID spectrom-
The unique feature of combining the large acceptance eter, which is designed to accomodate a high luminosity
and high luminosity of SoLID makes it critical to exploit with a large acceptance, also provides unique opportu-
the full potential of the JLab 12 GeV upgrade to per- nities to various physics programs including SIDIS, J/ψ
formprecisionstudiesofthenucleonstructureandQCD and DVCS experiments. More ideas are coming out in
dynamics. the near future within the collaboration. In summary,
the SoLID project will greatly enhance the physics out-
put of the JLab 12 GeV upgrade in a number of exciting
IV. SUMMARY areas.
TheSoLIDspectrometerwillprovidetheopportunities
to measure the weak mixing angle and effective electron- ACKNOWLEDGEMENTS
quarkweakcouplingsC ,especiallyC ,toveryhigh
1q,2q 2q
precisions. ThereisalsopotentialtoaccesstheCSVand The author is grateful to the inputs from Jian-Ping
high twist effects with quark-quark correlations. With a Chen, Krishna Kumar, Seamus Riordan, Paul Souder,
proton target, the measurement of the d/u ratio can be Xiaochao Zheng for this article. The material is based
achieved in a relatively clean way, free of nuclear correc- upontheworksupportedbytheU.S.DepartmentofEn-
tions. PVDIS with a polarized target opens up a new ergy, Office of Science, Office of Nuclear Physics under
window to access the new, yet unmeasured polarized γZ contract DE-AC05-06OR23177. It is also supported in
interference structure functions, and provides indepen- partbyU.S.DepartmentofEnergy,OfficeofScienceun-
dent inputs on unique combinations of polarized PDFs der contract DE-FG02-84ER40146.
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