12009/07/07 1

Improved Measurement of d/uAsymmetry in the Nucleon Sea

Phys. Rev. D 64, 052002 (2001).R. S. Towell et al.

FNAL E866 / NuSea Collaboration

Jinnouchi / Shibata Lab.

06_16830 Ryo Nagai

核子中の sea quark における d/u 非対称の改良測定

Contents1. Introduction2. Experiment3. Analysis4. Results5. Summary

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1. Introduction

valence quark

valence quark

valence quark

gluon splittinggluon splitting

d

dd

u

u

dd

u

u

sea quarks

gluon

proton

— This experiment is effective to This experiment is effective to verify this assumption.verify this assumption.

• This experiment measured dimuon momentum from Drell-Yan process (see next page).

• The goal—

• This paper presents improved analysis results by using the previous data.

The usual assumption• Sea quark-antiquark pair is produced

perturbatively from gluon splitting.

→ The numbers of uu and dd pairs should be approximately equal,because the mass difference of uand d is small.

determine spd/2spp over a wide kinematic rangedetermine d/u in the proton

sea quarks

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Drell-Yan process

• Fig. shows Drell-Yan process in the actual experiment.

– This experiment used proton and deuteron as targets.

– 4-momentum of the muon pair was measured.

– x1, x2 : Bjorken-x of the quarks from the beam and target

Drell-Yan process

(Feynman diagram)

proton-beam

fixed target(proton or deuteron)

At nucleon level

X: hadrons, p: proton, q: quarkg*: virtual photon, m: muon

At quark level

(1-x2) p2

(1-x1) p1

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• Measured—muon 4-momentum p3=(E3, p3), p4=(E4, p4)• The Feynman-x (xF) and dimuon mass (M) are defined as:

s=(p1+p2)2~2E1mp : the total 4-momentum squared of the initial nucleonspg|| : the center-of-mass longitudinal momentum of the virtual photon

Thus, Thus, xx11 and and xx22 are determined by the measurement.are determined by the measurement.

• The p-d Drell-Yan cross section can be expressed as spd~spp+spn.– For x1>>x2, Drell-Yan cross section ratio is expressed

– The right side was simplified further assuming d(x)<<4u(x).

The equation shows thatThe equation shows that sspdpd/2s/2spppp is closely related to is closely related to dd//uu..

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2. Experiment• This experiment used 800 GeV proton beam extracted from the Fermilab

Tevatron accelerator.

The FNAL E866 / NuSea Spectrometer

• SM0, SM12, SM3 (dipole magnets)– SM0 & SM12 … reject hadrons– SM3 … measure the muon

momentum

Detector

• position of muons

• (trigger)

• proportional tubes

• hodoscopes4

• position of muons

• (trigger)

• driftchambers

• hodoscopes

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What to measuredetectorStation

6

3. Analysis

Blank region in Fig.→ cut to remove unwanted events

(ex. U and J/y resonance families …)

The dimuon distribution for x1 versus x2

• The resulting dimuon distributions, x1 versus x2 .

UU resonanceresonanceJJ/y/y resonanceresonance

x1 … Bjorken-x of the beamx2 … Bjorken-x of the target

5 10 15

2500

5000

7500

0

dimuon mass (GeV/c2)

coun

ts /

0.1

GeV

/c2

The figure shows counts as a function of dimuon mass.

The peaks are Uresonances.

ΥΥ

ΥΥ’’JJ/y/y

— These black points in Fig. wereused for the analysis.

beam

targetx2

x 1

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4. Results (1)

The Drell-Yan cross section ratio versus x2

spd

/2s

pp It shows large x-dependenceof the Drell-Yan cross section ratio.

spd/2spp moves up and down along x2.

– Next, convert the data with the following relation,

( x2 … Bjorken-x of the target )

x2

8

d/u versus x2 shown with statistical and systematic uncertainties

4. Results (2)• converted using the relation between spd/2spp and d/u.

large deviation from 1

The expected value was d/u = 1

About 70% difference !!

— There is large asymmetry There is large asymmetry of of uuuu and and dddd pairs.pairs.

← The graph shows d/u increases at 0< x<0.2 and decreases at 0.2 < x.

d >u for x < 0.25.

d/u

x2

99

5. Summary• This paper reports improved analysis of the measurement of the

Drell-Yan cross section ratio per nucleon of p+d to p+p.

• The experiment used 800 GeV proton beam and measured 4-momentum of dimuon.

• From this measurement the large asymmetry of the uu and dd pairs in the nucleon sea is extracted as a function of x.

• This result is different from the expectation, “the same numbers of uu and dd pairs are produced perturbatively”.

• The result suggests possibility that there is a non-perturbative originfor the production of the uu and dd pairs.

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Passive origin of d/u asymmetry• Perturbative origin

– Pauli blocking…less than 1%

• Non-perturbative origin– Meson cloud model

• express the physical proton as the combination of a proton with a symmetric sea and a series of virtual meson-baryon state

– Chiral quark model• Similar to the meson model• Except the virtual meson couples directly to a quark (not to

the nucleon)

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Meson Cloud Model

u

u

d

u

proton

d

du

d

up-meson

valence quark

sea quarks

Light gray region…

pp--meson cloudmeson cloud

u du d

u

d

u

d

u

d

d

dd

u

d

d

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Feynman-x (xF)• Feynman-x (xF) is defined as :

The center-of-mass 4-momentum• The center-of-mass longitudinal momentum is

• Invariant mass (center-of-mass) is

• Using these expressions,

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Dimuon mass (M)• Dimuon mass (M) is defined as:

• The law of conservation of energy and momentum is

• Using the law, M is calculated as

• Invariant mass

• Using the expression of the invariant mass,

p2 = 0 (fixed target)

• m1, m2 ~ mp• mp << E1 (experimental condition)

p1 = (E1, p1) : 4-momentum of the beam

p2 = (E2, p2) : 4-momentum of the target

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Detector• Drift chamber :

– A multiwire chamber – Spatial resolution is achieved by measuring the time electrons need to

reach the anode wire.– measured from the moment that the ionizing particle traversed the

detector.

• Hodoscope : – A combination of multiple detector elements arranged in space and

connected by logic circuitry such that particle tracks can be identified . – used for triggering purposes

• Proportional tube :– drift tube read out without measurement of drift time.

1616

J/y and U resonance

• J/y particle : consist of cc– Mass : 3.1 GeV/c2

• U particle : consist of bb – Mass : 9.5, 10.0, 10.4, 10.5 GeV/c2