The Seaquest Experiment - Berkeley Lab Physics Division ...€¦ · The Seaquest Experiment James...

The SeaquestExperiment

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The Seaquest Experiment

James Reed Watson

March 1, 2017


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Table of Contents

1 Quarks and Scattering

2 Mathematical Formalism

3 Problems with the Proton

4 NuSea

5 SeaQuest


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The Proton

Discovered by Rutherford inhis gold foil experiment

Thought to be fundamentalparticle for a long time.

Now understood to be a farmore complicated object.

There are many gaps in ourunderstanding of theProton’s structure, and theSeaQuest experiment isaiming to better understandtwo of them: the EMC effect

and quark flavor asymmetry.

Source:physics.stackexchange.org


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Constituent Quark Model

Anomalous Magnetic moment: µ = 5.58µB . Implies that theproton is a consituent particle.

Color charge: ∆++ Baryon is uuu and spin 32 which makes it

totally symmetric. This means that there must be an SU(3)symmetry to make it overall antisymmetric.

This symmetry is assigned to the ”Color Charge” and isdescribed in Quantum Chromodynamics

In low energy experiments, the proton is treated as being madeof the three quarks uud and nothing else.

At high energies, this model breaks down, as there scatteredparticle interacts with individual quarks.


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Formalism - Particle Scattering

Quantum Field Theorytreats scattering as if a”virtual” particle wasexchanged. (Could be γ, g ,W , Z , etc)4-vectors for scatteredparticle are k,k, and virtualparticle is q = k ′ − k.Q2 = qµq

µ > 0.Momentum transfer: IfE ,E ′ >> ml , thenQ2 ≈ 4EE ′ sin2(θ/2).ν = E − E ′ is the energyloss of the lepton.

y = ν/E is the energyfraction.

W 2 is the squared mass ofthe recoiling system

Source: Particle Data Group,LBL


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Types of Scattering: Coulomb Scattering

At low energies, dominantscattering mode is theelectromagnetic potential.

The Scattering cross sectionis given by Rutherford’sFormula:

dσ

dΩ= (

Z1Z2e2

4πϵ0E sin2 θ/2)2

(1)

At this scale, nothing aboutthe structure of the protoncan be seen.

Spin-1/2 Electron: UseMott scattering:

dσ

dΩ= (

Z1Z2e2 cos θ/2

4πϵ0E sin2 θ/2)2

(2)

Source:wikipedia.org


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Deep Inelastic Scattering

First performed at SLAC in 1969

Electron-Proton Scattering at 7-18 GeV

Elastic Scattering predicts σ falls off as 1Q2 compared to σMOTT,

but SLAC observed almost no dependence on Q2.

Concluded that it must be scattering(incoherently) from pointparticles.

At high energies(> 1GeV), particles may scatter with the quarksthemselves.

At high enough energies, the proton itself may be completelydestroyed afterwards.


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Formalism - Bjorken ”x” Parameter

If a parton(quark or gluon)carries some fraction x ofthe nucleon’s 4-momentum,then the inelastic part of thescattering scales with theparameter

x =Q2

2Mpν(3)

This is the ”Bjorken xparameter” named afterBjorken.Used to parameterize avariety of form factors for

nucleons, due to weakdependence on Q2 forinelastic processes.

James BjorkenSource: fnal.gov


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Formalism - Parton Distribution Function

Using the x parameter, we can identify the structure of thepartons using parton distribution functions (pdfs).

Each flavor of quark gets its own pdf.

Each pdf q(x) describes the number density of quarks of flavor qat momentum fraction x .

Figure: CTEQ Parton Distribution Functions


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Form Factors and Structure Functions

”What is the distribution ofparameter X?”

Can have form factors forspin, electric charge, etc.

F2(x) =∑

i e2i xqi (x) where

qi (x) is the pdf for quarkflavor i .

F (x) is essentially theFourier transform of theelectric charge distributioninto x-space.

So it’s essentially themomentum distributionwithin the nucleon.

Figure: HERA structure functionsarXiv:1409.0397 [hep-ph]


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The Parton Model

DIS Studies of F2 at low xshow more structure thanexpected.

Conclusion: In addition tothe valence quarks andgluons, there are lots andlots of ”sea quarks,” qqpairs residing at low x .

These quarks pop into andout of existence rapidly, andcan only be seen with highenergy(= penetrate deepinto the nucleon) collisions. Source: Halzen and Martin, p.

201


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Proton Spin Crisis

Parton model leads to more questions

Only <1/3 of the spin of the proton is due to the quarksaccording to work by the European Muon Collaboration

EMC (1987): ”total quark spin constitutes a rather smallfraction of the spin on the nucleon.”[1]

1

2=

1

2∆Σ+∆G + Lq + Lg (4)

RHIC is currently probing spin structure of proton to figure outgluon polarization.


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EMC Effect

EMC shot muons at d andFe Targets in 1983.

Measured the F2(x) for theproton in each nucleus.

Found that theF FE2 (x)/F d

2 (x) decreaseswith x .

As F2 is the FT of chargedistribution, this implies thatnucleons are LARGER in Fe! Figure: EMC Effect [2]


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Further EMC effect work

EMC effect was later studied in low x region, where ”shadowing” wasdiscovered”

Figure: EMC effect in Calcium. Open circles: SLAC, closed: NMC Source:CERN


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Quark Flavor Asymmetry

New Muon Collaboration was an upgrade to the EMC that onceagain performed Deep Inelastic Scattering but at 90 GeV and280 GeV at H and D.

Their 1991 results conflicted with theory, either abandon n − pcharge symmetery, or abandon quark flavor symmetry.

Charge symmetry violations not discovered, so∫[d(x)− u(x)]dx = .148± .039 [3].

This is weird, and sparked off a lot of theoretical discussion ofwhy that would be. Ex: Pauli Blocking, Instanton Model, MesonCloud, Hybrid Model, Chiral Quark,...


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Drell-Yan Processes

Leading order process: quarkfrom beam protonannihilated sea antiquark,produces l l pair.Lepton’s won’t interact asstrongly as the quarks, soDY is a good probe of seaquarks.Proton and Deuteron crosssection ratio [4]:

σpd

2σpp≈ 1

2(1 +

d(x)

u(x)) (5)

Figure: Drell-Yan Process


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Why Drell-Yan?

Parton motion through hot nuclear matter is well studied(eg. atthe LHC)

Less is known about cold nuclear matter

Products of DY are leptons, so less energy loss as they exit thenucleon.

Fixed targets simplify math and allow study of the target inisolation.


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E-866/NuSea

800 GeV/c Proton Beam onHydrogen and DeuteriumTargets at Fermilab

Drell-Yan Process, measured2µ flux with magnetic massspectrometer.

Measured at .015 < x < .35

Nothing in theory explainsthe < 1 ratio at larger x .

Figure: Flavor Asymmetry


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Fermilab E-772

E-772 was another Drell-Yanexperiment done atFermilab, which looked atthe structure functions indifferent nuclei

It had inconclusive results,so EMC effect so far has notbeen observed for Drell-Yan/Sea quark experiments.

Figure: E-772 Data showing noEMC effect in Drell-YanExperiments


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E-906/Seaquest Proposal

Want to measure flavor asymmetry at high x . (.1 < x < .45).

Major Changes: Lower energy beam (120 GeV )→ 7x DY σ.

J/Ψ production leads to single µ decays. Lower energy beamhas 7x less single muon rate.

Put together: 50x higher statistics than NuSea.

Figure: Seaquest Spectrometer


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Targets

For EMC effect, several targets are used.

LH2, LD2, Fe, C , W , Empty Flask, and Empty Position

Flasks for LH2 and LD2 correspond to 7− 15% of theinteraction length ( 20”). Made of Steel, with Ti window forbeam to go through.

Arranged on a rotating table that can change targets in 30s.


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Beam Properties

Seaquest receives 5 secondsof every minute ofFermilab’s beam, called a”spill”

Fermilab Main Injector:beam broken into 6 ”trains”

Each ”train” has 83”buckets,” 1-2 ns in lengththat arrive at 53 MHz (every18.9 ns).

Each ”bucket” has 4× 104

120 GeV/c protons[5]. Figure: Fermilab Main Injector [5]


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Cherenkov Veto

A secondary emission chamber and ion chamber monitor beamintensity over the course of a spill.

Bucket level analysis -¿ need faster counter

Beam produces Cherenkov light in Ar/CO2 mixture

Counted with a photomultiplier tuber (PMT).


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Early Seaquest Results

No EMC effect observed in the Sea Quarks so far[6]

Figure: Run II and III SeaquestResults [5]

Figure: Expected Seaquest Results


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Dark Sector Probe

”Dark Sector” is a family ofparticles that don’t couple tothe four fundamental forces.

SQ may produce darkphotons in the iron target[7].

Dark photons go straightthrough the magnetic fieldbefore decaying (with someprobability for dimuons).

Can anaylze the kinematicsto search for peaks inintensity from this”displaced vertex.”

Figure: Parameter Search space forthe coupling ϵ between SM and DS


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Future Goals for Seaquest

Seaquest ran run V in 2016 and expects results out later thisyear.

Future Fermilab Drell-Yan experiments will look intospin-polarization of Sea Quarks.

E-1027 is polarized beam, E-1039 is polarized targets.


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Summary

There are a number of unexplained features of the Protonstructure

The proton seems to have a volume that depends on the nucleusits in (EMC effect)

The sea quarks within the proton exhibit flavor asymmetry

Seaquest builds on previous experiments to explore a largerparameter space for quantifying these asymetries.

Seaquest has had two Physics runs, and is still taking data


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References I

[1] J. Ashman, et al., [European Muon Collaboration], AMeasurement of the Spin Asymmetry and Determination of theStructure Function g(1) in Deep Inelastic Muon-ProtonScattering, Phys. Lett. B206 364, 1988

[2] J.J. Aubert et al., The ratio of the nucleon structure functionsF2N for iron and deuterium, Phys. Lett. 123B, 275 (1983)

[3] The New Muon Collaboration, P. Amaudruz eZ nZ., Phys. Rev.Lett. 66, 2712 (1991); M. Arneodo eZ nZ., Phys. Rev. D 50, R1(1994)

[4] R. Towell,”MEASUREMENT OF THE ANTIQUARK FLAVORASYMMETRY IN THE NUCLEON SEA,” , Thesis, University ofTexas,1999

[5] Shou Miyasaka, ”Probing Flavor Asymmetry of Anti-quarks in theProton by Drell-Yan Experiment SeaQuest,” Thesis, TokyoInstitute of Technology, 2016


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References II

[6] Michelle M. Medeiros, ”The Seaquest Experiment,” Talk given at2016 Fermilab Users Meeting, June 15 2016.

[7] arXiv:1509.00050v2 [hep-ph]

The Seaquest Experiment - Berkeley Lab Physics Division ...€¦ · The Seaquest Experiment James...

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