Light Axial Vectors, Nuclear Transi6ons, and the 8 Be Anomaly - - PowerPoint PPT Presentation

Light Axial Vectors, Nuclear Transi6ons, and the 8Be Anomaly

Jonathan Kozaczuk (UMass Amherst) U.S. Cosmic Visions: New Ideas in Dark Ma8er 3/23/17

Some References

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Primarily based on JK, D. Morrissey, and S.R. Stroberg, arXiv:1612.01525 [hep-ph] See also: Krasznahorkay et al, PRL 116 (2016) no.4, 042501 Feng et al, PRL 117 (2016) no.7, 071803 PRD 95 (2017) no.3, 035017 Kahn et al, arXiv:1609.09072 [hep-ph] This workshop: I\ah Galon’s slides Subsequent talks by Xilin Zhang, Rafael Lang, and Kyle Leach

The Atomki Experiment in a Nutshell

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Search for internal pair crea6on (e+ e- produc6on) in excited states of 8Be

1.0 µA proton beam from Van de Graaf generator impinge on LiF2, LiO2 targets e+ e- energies and angles determined by 5 plas6c telescope detectors (scin6llator + PMT) and mul6-wire propor6onal chambers

Target Telescope detectors MWPC

Feng et al, 2016 Gulyas et al, 2015

The Atomki Experiment in a Nutshell

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Proton beam energy tuned to excite J=1 8Be states

Detector calibrated using internal pair crea6on in 12C and 16O One week-long experiments at each bombarding energy, targets periodically changed

Feng et al, 2016

The Atomki Results

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Isoscalar transi6on features significant bump-like excess in e+ e-

• pening angle and invariant mass spectrum (6.8 σ)

No corresponding excess in the isovector (8Be*’) transiJon

Feng et al, 2016 Krasznahorkay et al, 2016

The Atomki Interpreta6on

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Interpreta6on put forward by collabora6on: light gauge boson

In the Atomki PRL the collabora6on claimed this to be consistent with a standard dark photon featuring ε2 ~ 10-7 Feng et al (2016) pointed out that explaining the Atomki result actually requires , which is excluded, in par6cular by NA48/2

m = 16.7 ± 0.35(stat) ± 0.5(syst) MeV Γ(8Be0 →8 BeX) Γ(8Be0 →8 Beγ) Br(X → e+e) = 5.8 × 106

Krasznahorkay et al, 2016 NA48/2 Collabora6on, 2015

✏ ≈ 0.011

A Protophobic Vector Explana6on

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Assume more general vector setup The NA48/2 constraint arises from decays. Rate propor6onal to axial anomaly trace factor General setup can work provided X is protophobic,

π0 → Xγ

⌘ ("uqu "dqd)2.

2 |2εu + εd| = |εp| . (0.8 − 1.2) × 103 p Br(X → e+e)

(NA48/2 bound)

|εn| = (2 − 10) × 103 |εe| p Br(X ! e+e−) & 1.3 ⇥ 10−5

(large enough rate; some caveats here) (prompt decays)

Feng et al, 2016 (PRL + PRD)

See I\ah Galon’s slides for more details

L = 1 4XµνXµν + 1 2m2

XXµXµ XµJµ t Jµ = P

f e"f ¯

fµf,

,

An Axial Vector Explana6on

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Another poten6al explana6on: light axial vector Axial anomaly does not contribute to in this case and so X does not have to be protophobic Also, less momentum suppression (L=0 vs L=1 in vector case) Challenge: have to do some nuclear physics

π0 → Xγ

L LA = gA ΛA

8Be GµνF (A) µν

g

Leading term for vector JK, D. Morrissey, S. Stroberg, 2016

M28Be∗→8BeX = 4 3 g2

A

Λ2 M 2m2

X

3 + 2|~ pX|2 m2

X

!

L Xµ X

q

gq¯ qµ5q ,

An Axial Vector Explana6on

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In the vector case, nuclear matrix elements cancel (in the pure isospin limit) Cancella6on does not hold in the axial vector case

Γ(8Be∗ !8 BeX) Γ(8Be∗ !8 Beγ) / h8Be |Jµ

X|8 Be∗i

h8Be |Jµ

EM|8 Be∗i

= (εp + εn)h8Be

• ¯

NγµN

• 8 Be∗i

h8Be

• ¯

NγµN

• 8 Be∗i

= εp + εn Γ(8Be∗ !8 BeX) Γ(8Be∗ !8 Beγ) / h8Be |Jµ

X|8 Be∗i

h8Be |Jµ

EM|8 Be∗i

= a0h8Be

• ¯

Nγµγ5N

• 8 Be∗i

h8Be

• ¯

NγµN

• 8 Be∗i

JX

µ =

X

f

εf ¯ fγµγ5f

Here relates nucleon to quark operators, with current

Need matrix element

a0 = 2(∆u + ∆d)(✏u + ✏d) + 4∆s✏s

An Axial Vector Explana6on

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How large do the couplings have to be?

L Xµ X

q

gq¯ qµ5q ,

hN| X

q

gq¯ qµ5q|Ni = µ

i i X q

gq∆q(N) Γ = k 18π ✓ 2 + E2

k

m2

X

◆ |anh0||σn||1i + aph0||σp||1i|2

Reduced matrix elements of spin operators ac6ng on all nucleons of a given type in the nucleus Quark-level interac6on Nucleon-level operators Decay width for J = 1 0 transi6ons (at leading order in k/mN expansion): Need to compute for the 8Be states of interest 3 h0||p,n||1i.

Matrix Elements

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Full ab-iniFo calcula6on of the matrix elements

U6lizes the In-Medium Similarity Renormaliza6on Group (IM-SRG) with forces derived from chiral effec6ve theory (NN + 3N) and including effects from meson exchange currents (MECs) Fix isospin mixing frac6on from M1 isoscalar transi6on to extract predic6ons for the other matrix elements

Matrix Elements

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Results:

Matrix element Prediction h0+kM1kVi (µN) 0.76(12) h0+kσpkVi 0.102(28) h0+kσnkVi 0.073(29) h0+kσpkSi 0.047(29) h0+kσnkSi 0.132(33)

Things to note:

• Rela6ve sign between the proton and neutron matrix elements for the

8Be*’ transi6on, but not for 8Be*, results in suppression of isovector rate

• Significant error bars (can be improved in the future) but results enough

to begin scru6ny of the axial vector scenario

Implica6ons for the 8Be Anomaly

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Obtain range of couplings required to explain the Atomki result

Requirements depend on precise mass (need more info from experimentalists) Demand that the corresponding isovector transi6on rate is not too large to conflict with null results (Feng et al, 2016)

mX ' 16.7 MeV, Γ8Be∗!8Be X Γ8Be∗!8Be γ ' 5.8 ⇥ 106

mX ' 17.3 MeV, Γ8Be∗!8Be X Γ8Be∗!8Be γ ' 2.3 ⇥ 106 mX ' 17.6 MeV, Γ8Be∗!8Be X Γ8Be∗!8Be γ ' 5.0 ⇥ 107. 14

Γ8Be⇤!8Be X Γ8Be⇤!8Be γ > 5 ⇥ Γ8Be⇤0!8Be X Γ8Be⇤0!8Be γ

From Feng et al, 2016:

Other Constraints

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What about other constraints? And UV comple6on?

Impact of other constraints depend on UV comple6on. Our assump6ons:

• Purely axial genera6on-independent quark couplings
• Both axial- and vector-like couplings to leptons:
• Vanishing couplings to neutrinos
• 100% branching frac6on into electron-positron pairs

These assump6ons can be relaxed if so desired

L ⊃ Xµ X

i

¯ `i

• gV

i µ + gA i µ5

`i

Other Constraints

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Dominant constraints on lepton couplings: Dominant constraints on quark couplings:

Decays inside Atomki detector: Muon (g-2): Electron beam dumps (SLAC E141): Electron-positron colliders (KLOE2): Parity-viola6ng Moller scatering (SLAC E158): η µ+ µ- : Proton fixed target experiments (ν-Cal I) (also depends on coupling to electrons)

p (gV

e )2 + (gA e )2

e & 1.3 × 10−5

⇥ '

• (gA

µ )2 + 9 ⇥ 10−3(gV µ )2

. 1.6 ⇥ 10−9.

p (gA

e )2 + (gV e )2

e & 2 ⇥ 10−4 p (gA

e )2 + (gV e )2

e . 2 ⇥ 103

• gV

e gA e

• . 1 ⇥ 108

gA

µ (gu + gd 1.5gs)

(mX/MeV)2 . 4 ⇥ 1010.

/

See also Kahn et al, 2016 for detailed discussion of constraints on light axial vectors

Puung it all together

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A light axial vector can be a viable explana6on of the 8Be anomaly

Detailed results depend on rela6onship between leptonic and quark couplings Dominant constraints on this scenario depend on leptonic couplings and are highly UV-dependent The Atomki null result for 8Be*’ places the strongest model-independent constraint on the quark couplings Relaxing our ini6al assump6ons could poten6ally

• pen up more parameter space

A UV Comple6on

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Rela6onship of couplings shown can arise in a UV comple6on involving a dark U(1)RH and two Higgs doublets (see Kahn et al, 2016)

Purely axial quark couplings + vanishing neutrino couplings (there’s tuning here) results in: Also require vector-like fermions (+ dark Higgses) to cancel anomalies. LHC limits on “anomalons” yield upper bound on couplings (subdominant to

8Be*’ limit) [Kahn et al, 2016]

gu = 2gd, gA

e,µ = gd,

gV

e,µ = 2gd

Results stress importance of complementarity and of probing both quark and leptonic couplings

Future Experiments

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Many planned experiments should have an impact on the light (axial) vector scenario

Lepton Couplings: VEPP-3, DarkLight, MESA, Belle II, HPS, APEX, PADME… Quark couplings: LHCb, ShiP, SeaQuest … Other nuclear decay experiments (including independent verifica6on of the Atomki result!)

See e.g. Feng et al, 2016; Kahn et al, 2016

Takeaways

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• A light vector axially coupled to quarks can explain the 8Be anomaly

and exist in a viable UV-complete theory

• If the anomaly goes away, we have a new constraint on light

axially-coupled vectors (already have a new constraint from 8Be*’)

• Important to target both lepton and quark couplings
• Are there other nuclear systems that can be useful in discovering or

constraining light new vectors that are otherwise difficult to probe?

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Backup

Ab IniFo Predic6ons for 8Be

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IM-SRG calcula6ons reproduce the observed 8Be spectrum

A UV Comple6on

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Repurpose model from Kahn et al, 2016

RH SM fermions charged under new U(1)RH with gauge coupling gD Kine6c mixing ε with hypercharge Require two Higgs doublets for SM fermion mass terms: Require vector-like fermions (+ two dark Higgs doublets) to cancel anomalies Resul6ng couplings:

LY,2HDM = yuHuQuc + ydHdQdc + yeHdLec + h.c. L = LY,2HDM + yUH0

u UUc + yDH0 dDDc + yEH0 dEEc + h.c.

Field SU(3)c SU(2)L U(1)Y U(1)RH Hu 1 2 + 1

2

+qHu Hd 1 2 1

2

+qHd uc 3 1 2

3

qHu dc 3 1 + 1

3

qHd ec 1 1 +1 qHd U 3 1 + 2

3

+qHu Uc 3 1 2

3

D 3 1 1

3

+qHd Dc 3 1 + 1

3

E 1 1 1 +qHd Ec 1 1 +1 H0

u

1 1 qHu H0

d

1 1 qHd

SM lepton e µ, ⌧ gV

` 1 2gDqHd ✏e 1 2gDqHd ✏e

gA

`

1

2gDqHd

1

2gDqHd

SM quark u, c, t d, s, b gV

q 1 2gDqHu + 2 3✏e 1 2gDqHd 1 3✏e

gA

q

1

2gDqHu

1

2gDqHd

(neutrino coupings set to 0)