Improving Reach at ~GeV (High) A’ Masses

Philip SchusterPerimeter Institute

with Natalia Toro

New Light Weakly Coupled Particle SessionCSS Conference

July, 2013

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0.001 0.01 0.1 1

10-5

10-4

10-3

10-2

mA' HGeVL

e

A' Æ Standard Model

APEXêMAMITest Runs

U70

E141

E774

am, 5s

am,±2s favored

ae

BaBar

KLOE

Orsay

2

muons+pionsacceptance

+ tolerance to high-current

Vertex precision(higher intensity?)

Directions for Improvement?

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0.001 0.01 0.1 1

10-5

10-4

10-3

10-2

mA' HGeVL

e

A' Æ Standard Model

APEXêMAMITest Runs

U70

E141

E774

am, 5s

am,±2s favored

ae

BaBar

KLOE

Orsay

Why does everyone lose reach at high mA′?

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How do we improve high-mass

sensitivity?

Can we go here with fixed-target?

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This is the biggest effect, and may be avoidable w/ low-Z target

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

mA' HGeVL

Branchingratio

A' Branching Ratios to e and m

Why does everyone lose reach at high mA′?

• σ~1/mA′2 for point-like nucleus

• Falling B.R. for each species

• Acceptance cutoff

• Form factor

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q & qmin ⌘ m2A0

2Ebeam

Form factors at high A′ mass

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0.01 0.02 0.05 0.1 0.2 0.5 1. 2.

0.2

0.5

1.

2.

5.

10.

20.

A' Mass HGeVL

cêZ2H"Lo

g"L

Gold Integrated Form Factors

6 GeV11.1 GeV

1.1 GeV2.2 GeV4.4 GeV

m�4A0

Aʹ′ production cross-section:[BEST]

d�

dx

⇡ 8↵3✏

2x

m

2A0

✓1 +

x

2

3(1� x)

◆Z

2Log

“Z2 Log” is ~ integral of nuclear form factor over γ momenta

Coherence lost for q > 0.4 GeVA�1/3

mA0 &p

GeV EbeamA�1/3

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What about Z-dependence?

• σA′ (low mass) ~Z2 Log

• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)

• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2

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Yield per e– per target thickness in r.l.

is independent of Z for low mA′ and ~1/Z for high mA′.

but

NA0

NeT⇠ Log(mA0)

↵

3✏

2

m

2A0

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What about Z-dependence?

70.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00

0.02

0.05

0.10

0.20

0.50

1.00

A' Mass HGeVL

RHGe

V2 L

R = cX0N0êA "A' production efficiency"

6.6 @11.1D GeV W6.6 @11.1D GeV Al6.6 @11.1D GeV Be

Yield = (Particle Physics Factor) x (Nuclear Physics Factor) WW effective photon cross-section ~ 1/m2

R=(eff. photon flux) x (column density)

How much does lower Z buy you?

R[anything]/R[W] for same mA′ & Ebeam

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R[X]/R[W] for same mA′, Ebeam.

At high mass:

0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00

0.5

1.0

2.0

5.0

10.0

20.0

A' Mass HGeVL

yieldHZLêyie

ldHWL

YieldHZLêYieldHWL @Dashed = 11.1 GeVD

6.6 @11.1D GeV W6.6 @11.1D GeV Al6.6 @11.1D GeV Be C or Be buys up to factor of

12 over W (~ ratio of Z’s)Al buys up to factor of 5

Low-Z better for high mass

What about Z-dependence?Yield = (Particle Physics Factor) x (Nuclear Physics Factor)

WW effective photon cross-section ~ 1/m2

R=(eff. photon flux) x (column density)

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• σA′ (low mass) ~Z2 Log

• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)

• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2

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Yield per e– per target thickness (in r.l.)NA0

NeT⇠ Log

↵

3✏

2

m

2A0

is independent of Z for low mA′, ~1/Z for high mA′.

Practical Issues with low Z?

e–/e+ singles and e+e– pair are mainly from Coulomb & trident processes with σ~Z2

⇒ yield per r.l. approximately indep. of Z

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• σA′ (low mass) ~Z2 Log

• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)

• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2

• σπ ~ A ( σπ+π ~ ?? )

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Yield per e– per target thickness (in r.l.)NA0

NeT⇠ Log

↵

3✏

2

m

2A0

is independent of Z for low mA′, ~1/Z for high mA′.

Yield of pions per r.l. ~ A/Z2 Switching to C, Be, (Al) would raise pion bkgs by factor of 10–15 (~5)

Practical Issues with low Z?

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Summary & Discussion• Low-Z target for 11 GeV beam (6 GeV?) may be

advantageous

• Low Z may increase statistics x 5–10 for mA′>0.5 GeV ⇒ 2–3 in α′/α

– Strategy will be limited by π backgrounds

• Need a better understanding of – pion contrib. to trigger, occupancy, etc. at 6, 11 GeV

π+e– fraction of fixed-target trigger rate?– Sources & effects of π+π– backgrounds– Gains vs. acceptance as function of mass– Engineering issues

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