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Axion-like particle searches at the LHC

The Standard Model (SM) of Particle Physics is plagued by some longstanding conceptual and practical questions, and we have to go beyond the Standard Model to address them. A guiding principle of the Standard Model is that we should expect to see all processes which are consistent with its symmetries. One such process generates an electric dipole moment for the neutron.  However, experiments have shown it to be at least 10,000,000,000 times smaller than this prediction. The only way this can happen is through an incredible accident, which suggests some unseen mechanism beyond the Standard Model. As this accident occurs only in quantum chromodynamics (QCD), this question is usually referred to as the strong CP problem.
The strong CP problem could be resolved by the presence of a hypothetical new particle, the QCD axion, which is related to a more complex theoretical model governed by the Peccei-Quinn symmetry. These QCD axions acquire their mass from coupling to the QCD condensate and have tight relations between their mass and coupling strengths to SM particles. Removing this relation between mass and coupling strengths motivates the axion-like particle (ALP). Axion-like particles are hypothetical light pseudo-Nambu Goldstone bosons which do not necessarily address the strong CP problem and appear in the spontaneous breaking of a global symmetry. ALPs can interact with all particles of the SM. Their masses and coupling strengths are theoretically free parameters and can span many orders of magnitude. In certain regions of parameter space ALPs can be non-thermal candidates for Dark Matter or, in other regions where they decay, mediators to a dark sector. For large symmetry breaking scales, the ALP can be a harbinger of a new physics sector at such high energy scales that would otherwise be experimentally inaccessible.
Depending on the ALP mass and its couplings to SM particles, the search strategy varies greatly. For masses below twice the electron mass, the ALP can only decay into photons and the corresponding decay rate scales like the third power of the ALP mass. Thus, light ALPs are usually long-lived and travel long distances before decaying. Experiments looking for long-lived ALPs coming from the sun and other distant objects are thus most suited. E.g. energy loss of stars through radiation of ALPs is constrained by the ratio of red giants to younger stars of the so-called horizontal branch (HB). Another strong constraint arises from the measurement of the length of the neutrino burst from Supernova SN1987a, which would have been shorter in the presence of an energy loss from ALP emission as well as from the non-observation of a photon burst from SN1987a due to the decay of emitted ALPs. In addition, a set of cosmological constraints from the modification to big-bang nucleosynthesis, distortions of the cosmic microwave background and extragalactic background light measurements exclude a large region of this parameter space and are sensitive to very small ALP-photon couplings. For subGeV ALPs, beam-dump searches are sensitive to ALPs radiated off photons, which are exchanged between the incoming beam and the target nuclei (Primakoff effect) and decay back to photons outside the target. The proposed ShiP experiment would probe significant parts of the ALP parameter space in the mass range between 1 MeV and 1 GeV, but also ongoing searches at NA62 might be sensitive to such ALPs Dobrich (2015). For ALPs with MeV to hundreds of GeV scale masses, collider experiments become relevant. If the ALP is long-lived, it would leave a missing energy signature in the detector, and BaBar, CLEO, LEP, the Tevatron and the LHC have searched for these signals. ALPs with larger masses and/or larger couplings become short lived and can decay inside the detector. This opens up exciting new possibilities for searches at the LHC and future colliders. Depending on its mass, the ALP could decay into two photons, leptons or jets. ALP couplings to other SM particles are generally less constrained than the ALP-photon coupling.

ALPs searches at the LHC

At the LHC, ALPs can be produced resonantly in the process pp → a, or they can be produced in exotic decays of the Z- or Higgs boson. The relevant processes would be Z → γa, h → Za or h → aa. Once produced, the ALP can then either be long lived and escape the detector as missing energy, or it can decay inside the detector. Long-lived or invisibly decaying ALPs at the LHC have been explored recently in (Mimasu,2014 and Brivio,2017). Projections for missing energy signatures, such as pp → aW(γ), pp → ajj(γ), pp → ha and pp → tta with stable ALPs or invisible ALP decays have been considered in Brivio (2017). For short-lived ALPs, ATLAS and CMS can reconstruct its decay products. The only decay mode for light ALPs is into two photons. For ALP masses larger than twice the electron mass, the decay into two electrons opens up. Similarly, for masses larger than twice the muon or tau mass, ALP decays into two muons or taus are possible. ALP masses larger than three times the pion mass are required for hadronic decays into two jets to be kinematically allowed. All these decay modes can be searched for at the LHC. The left panel of Figure 1 shows a summary of current constraints in the plane ALP mass vs ALP-photon coupling. The coloured regions show the different experimental constraints mentioned above. Collider constraints, in particular, exist for large ALP-photon couplings. The blue area shows bounds from LEP looking for e+e- → Z → γa → 3γ. Decays of on shell Zbosons at the LHC have been discussed Bauer(2017), Mimasu (2014), Jaeckel (2015) and Brivio (2017) and are shown in orange. The purple region is excluded by Tevatron searches for pp → 3γ. 

Figure 1. Constraints on the ALP mass and ALP-photon coupling derived from various experiments. Right: Enlarged display of the constraints from collider searches: LEP (light blue and blue), CDF (purple), LHC from associated production and Z decays (orange), LHC from photon fusion (light orange), and from heavy-ion collisions at the LHC (green). Plot taken from Bauer (2018). 

Exotic decays of Z- or Higgs bosons can be powerful channels to search for axion-like particles. The light green region in Figure 2 shows the expected LHC reach in the channel h → Za → Zγγ with 300 fb-1 integrated luminosity. We see the complementarity to previously excluded regions in parameter space. This is the case because ALPs in the light green region are short-lived enough to decay inside the detector. Projections for h → aa and Z → γa are similar. 

Figure 2. Constraints on the ALP mass and ALP-photon coupling along with the parameter regions that can be probed in the decay h → Za → Zγγ in LHC Run-2 with 300 fb-1 of integrated luminosity (light green region). Plot taken from Bauer (2017). 

These projected bounds for exotic Higgs decays include an intrinsic assumption on the ALPHiggs coupling. Studies on resonant production, pp → a → γγ, avoid these assumptions since the ALP coupling to gluons and to photons is related in many explicit models. Figure 3 shows current and expected bounds in this process. See Mariotti (2017) for more details on this process. 

Figure 3: Shaded regions show constraints from existing collider searches at LEP Adriani (1992) and the CMS (2015, 2017), Sirunyan (2017), and from the bound derived in Mariotti (2017). Lines: projected LHC sensitivities at 8 and 14 TeV. Plot taken from Marriotti (2017). 


ALPs searches in heavy-ion collisions

Ultra-peripheral heavy ion collisions at the LHC present a unique environment to search for ALPs that couple to electromagnetism, owing to a large Z^4 (Z=82 for lead) enhancement to the cross section, and low backgrounds [1]. In fact, the strongest limits on ALPs in the mass range 10 GeV < ma < 100 GeV can be obtained by recasting an ATLAS analysis [2] using the 480 microbarn-1 Pb-Pb data set.

Ultra-peripheral collisions are characterized by an impact parameter that is much larger than the radius of the ion, and very little breakup of the ions (leading to extremely low activity in the detector). ALPs can be produced via photon-photon fusion, where the initial photon flux is high owing to the strong electric field of the ions; decay back into photons provides a `bump-hunt’ channel with low background – see Fig. 1. Dominant backgrounds in this channel are standard model light-by-light scattering [3], and electrons faking photons; both backgrounds become negligible for mγγ >=20GeV.

A recent ATLAS measurement of light-by-light scattering [2] provides an m_gammagamma spectrum in the region 6-30 GeV that can be used to derive a limit on parameter Lambda in the ALP Lagrangian.

The results of this recasting were presented in [4], following the treatment proposed in [1]. They are shown here in Fig. 2, along with projected sensitivities for 1 nb-1 and 10 nb-1 of lead-lead collision data. The limit is set up to 100 GeV, under the assumption that ATLAS sees no events in bins greater than m_gammagamma >30 GeV.

It is an interesting aspect of ALP physics that lead-lead collisions at the LHC can provide more competitive limits than proton-proton collisions, which usually dominate searches for beyond-the-standard model physics.

Figures:  Taken directly from [4]

ALP production via photon-photon fusion in ultra-peripheral lead-lead collisions, with subsequent decay into two photons.

Caption: 95% exclusion limits on the parameter Lambda in eq. (1), as a function of ALP mass. Figure taken from Ref.[4], and using ATLAS results on light-by-light scattering [2]. Exclusion limits are also shown from other LHC searches (ATLAS 2 and 3 photon), LEP 2 (OPAL 2 and 3 photon) and beam dump experiments (for details see Ref. [1]). Projections assuming 1 nb-1 and 10 nb-1 luminosity of Pb-Pb collisions are are shown in green.


A) ALP's LHC searches.

O. Adriani et al., Isolated hard photon emission in hadronic Z0 decays, Phys.Lett (1992)

M. Bauer, M. Neubert and A. Thamm, Collider Probes of Axion-Like Particles, JHEP 12 (2017) 044, [1708.00443].

M. Bauer, M. Neubert and A. Thamm, LHC as an Axion Factory: Probing an Axion Explanation for (g − 2)µ with Exotic Higgs Decays, Phys. Rev. Lett. 119 (2017) 031802, [1704.08207].

M. Bauer, H. Mathias, N. Matthias and A. Thamm, Axion-like Particles at Future Colliders (2018) [arXiv:1808.10323]

B. Dobrich et al., arXiv: 1512.03069v2 [hep-ph], JHEP (2015). 

K. Mimasu and V. Sanz, JHEP 1506, 173 (2015) [arXiv:1409.4792 [hep-ph]]

A. Mariotti, R. Diego, S. Fillippo, T. Kohsaku,  New LHC bound on low-massdiphoton resonances, Phys. Letters, (2018) [arXiv:1710.01743]

J. Jaeckel and M. Spannowsky, Phys. Lett. B 753, 482 (2016) [arXiv:1509.00476 [hep-ph]]. 

I. Brivio, M. B. Gavela, L. Merlo, K. Mimasu, J. M. No, R. del Rey et al., ALPs Effective Field Theory and Collider Signatures, Eur. Phys. J. C77 (2017) 572, [1701.05379].

I. Brivio et al., arXiv:1701.05379 [hep-ph]

ATLAS Collaboration, G. Aad et al., Search for Scalar Diphoton Resonances in the Mass Range 65-600 GeV with the ATLAS detector in pp collisions data at √ s = 8 TeV, Phys. Rev. Lett. 113 (2014) no. 17, 171801, arXiv:1407.6583 [hep-ex].

CMS collaboration, Search for new resonances in the diphoton final state in the mass range between 70 and 110 GeV in pp collisions at √ s = 8 TeV and √ s = 13 TeV. (2017). CMS-PAS-HIG-17-013.

A. M. Sirunyan et al. (CMS Collaboration), Search for Low Mass Vector Resonances Decaying to Quark-Antiquark Pairs in Proton-Proton Collisions at √s=13 TeV, Phys. Rev. Lett. no. 119, (2017) 111802, arXiv: 1710.00159

B) ALPs searches with heavy-ion collisions

[1] S. Knapen, T. Lin, H. K. Lou, and T. Melia, Searching for Axionlike Particles with Ultraperipheral Heavy-Ion Collisions,” Phys. Rev. Lett. 118 (2017) no. 17, 171801, arXiv:1607.06083 [hep-ph]. 

[2] ATLAS Collaboration, M. Aaboud et al., “Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC,” arXiv:1702.01625 [hep-ex]. 

[3] G. Baur and C. A. Bertulani, “ - physics with peripheral relativistic heavy ion collisions,”Zeitschrift für Physik A Atomic Nuclei 330 (Mar, 1988) 77–81.

[4] Simon Knapen (LBL, Berkeley & UC, Berkeley), Tongyan Lin (UC, San Diego & LBL, Berkeley & UC, Berkeley), Hou Keong Lou (LBL, Berkeley & UC, Berkeley), Tom Melia (Tokyo U., IPMU & LBL, BerkeleyUC, Berkeley). Sep 20, 2017. 4 pp. arXiv:1709.07110 Conference proceedings