samedi 26 septembre 2015

The 2TeVbump@LHC chronicle : what possible echoes from high intensity and cosmic frontiers?

Synergies of high intensity, high energy and cosmic frontiers to probe some TeV scale Left-Right symmetric models
A number of recent resonance searches with the √s=8 TeV LHC data have observed excess events around an invariant mass of 2 TeV... Of course, these excesses should be thoroughly scrutinized in light of possible subtleties in the analysis [6] and must be confirmed with more statistics at the LHC run II before a firm conclusion on their origin could be deduced. Nevertheless, given the lucrative possibility that it could easily be the first glimpse of new physics at the LHC, it seems worthwhile to speculate on some well-motivated beyond SM interpretations. 
One class of models that could simultaneously explain all these anomalies is the Left-Right Symmetric Model (LRSM) of weak interactions based on the gauge group SU(2)L×SU(2)R×U(1)B-L [7], with the right-handed (RH) charged gauge boson mass MWR≈2 TeV and the SU(2)R gauge coupling strength gR≈0.5 at the TeVscale [8, 9a,9b,9c]. The main objective of this paper is to study the implications of this scenario for various low-energy experiments searching for lepton number violation (LNV) and lepton flavor violation (LFV), which are complementary to the direct searches at the LHC. 
For concreteness, we work in the Type-II seesaw ... dominance, where the hitherto unknown RH neutrino mixing matrix and mass hierarchy can be directly related to the light neutrino sector. This scenario is known to give potentially sizable contributions to the low-energy LNV and LFV observables [11, 12], apart from its novel LNV signatures at the LHC [13, 14]. We show that for the RH gauge boson mass and coupling values required to explain the LHC anomalies as indicated above, the LRSM parameter space for the RH neutrinos is already constrained by neutrinoless double beta decay (0νββ) experiments for relatively low (MeV-GeV) RH-neutrino masses, whereas the heavier (TeV) RH-neutrino masses are constrained by the searches for LFV processes, such as µ→eγ. Most of the remaining parameter space could be accessible in the next-generation 0νββ and LFV experiments at the intensity frontier. We derive a novel correlation between the 0νββ and µ→eγ rates, which clearly illustrates the testability of this scenario. We further show that future information on the absolute light neutrino mass scale from precision cosmology [with the future Euclid project] could make it even more predictive, irrespective of the neutrino mass hierarchy and uncertainties in the neutrino oscillation parameters or nuclear matrix elements ... 
Our predictions for Br(µ → eγ) ... are shown in Fig. 3 [below] for both normal hierarchy [NH] and inverted hierarchy [IH] three benchmark values of M> [the third and heaviest right handed neutrino] where the band in each case is due to the 3σ uncertainties in the neutrino oscillation parameters. The horizontal shaded region is ruled out at 90% CL by the MEG experiment [36], while the horizontal dashed and dotted lines show the MEG-II [37] and PRISM/PRIME ... sensitivities, respectively. It is evident that the µ→eγ searches could be more effective in probing the relatively heavier M> values, as compared to the 0νββ searches. Also note that the Planck upper limit on the lightest neutrino mass effectively puts a lower limit on the µ→eγ branching ratio in the quasi-degenerate (QD) region; for instance, for M>=1 TeV, this lower limit is (0.5 − 1.9) [(0.5 − 1.7)]×10−14 for NH [IH], as shown in Fig. 3 [below]. However, for NH with M>/MWR 1, there is a destructive interference between the two heaviest neutrino contributions for certain values of the Dirac CP phase, which leads to a cancellation region, unless the third RH neutrino contribution is sizable, as demonstrated in Fig. 3 for M>=9 TeV. On the other hand, smaller M> values lead to a suppression in the µ→eγ rate, pushing it well below the future sensitivity even for M>=100 GeV, which is however accessible to 0νββ experiments. Thus, a combination of the low-energy probes of LNV and LFV is crucial to probe effectively the entire LRSM parameter space in our case. This is complementary to the direct searches at the LHC [5, 39], which can probe RH neutrino masses from about 100 GeV up to MWR [40] using the same-sign dilepton plus dijet channel... Similarly, the GeV-scale RH neutrinos can also be searched for in the proposed SHiP experiment [41a, 41b].

vendredi 18 septembre 2015

The 2TeVbump@LHC chronicle : turning the degree of lepton number violation knob

A "Majorana-Dirac tuned" inverse seesaw minimal model as a straight route to grand unification and successful leptogenesis without susy
This post carries on my personal exploration of the non supersymmetric proposals to explain the anomalous bump in data from the Large Hadron Collider at CERN. I focus today on a phenomenological article which shows how interesting is this bump for triggering works with quite unexpected results like a seemingly consistent and quite minimal no-susy grand unified model with only one intermediate scale around a few TeV. 
Recently, a number of resonance searches at the √s=8 TeV LHC have reported a handful of excess events around invariant mass of 1.8 – 2 TeV. The most significant ones are: (i) a 3.4σ local (2.5σ global) excess in the ATLAS search [1] for a heavy resonance decaying into a pair of Standard Model (SM) gauge bosons V V (with V = W, Z); (ii) a 2.8σ excess in the CMS search [2] for a heavy right-handed (RH) gauge boson WR decaying into an electron and RH neutrino NR, whose further decay gives an eejj final state; (iii) a 2.2σ excess in the CMS search [3] for W' → WH, where the SM Higgs boson H decays into b and W→ν (with =e,µ); (iv) a 2.1σ excess in the CMS dijet search [4]. These excesses of course need to be confirmed with more statistics at the LHC run II before any firm conclusion about their origin can be derived. Nevertheless, taking them as possible indications of new physics beyond the SM, it is worthwhile examining whether all of them could be simultaneously explained within a self-consistent, ultra-violet (UV) complete theory that could be tested in foreseeable future. 
One class of models that seems to broadly fit the observed features in all the above-mentioned channels is the Left-Right Symmetric Model (LRSM) of weak interactions based on the gauge group SU(2)L×SU(2)R×U(1)B-L [5], with the RH charged gauge boson mass MWR∼2 TeV and with gR<gL at the TeV-scale [6], gL(R) being the SU(2)L(R) gauge coupling strength. Within this framework, the eejj excess [2] can be understood [6,7,8,9] as pp → WR → eNR  → eejj [10] (for a review, see e.g. [11]) and is related to the type-I seesaw mechanism [12] for neutrino masses. The W Z excess [1] and WH excess [3] can be understood [6, 13, 14] in terms of WR → WZ, WH, since these couplings naturally arise in these models from the vacuum expectation values (VEVs) of the bidoublet field used to generate quark and lepton masses [5]. Finally, the dijet excess [4] can simply be understood in terms of WR → jj. 
However, a particular aspect of the observations in the eejj channel, namely, a suppression of same-sign electron pairs with respect to opposite-sign pairs [2], cannot be explained within the minimal LRSM with type-I seesaw mechanism. This is because of the fact that for a type I seesaw interpretation of the eejj excess, one expects equal number of same and opposite-sign dileptons due to the purely Majorana nature of the RH neutrinos [an exception is when the interference of two non-degenerate RH Majorana neutrinos with mixed flavor content and opposite CP parities can partially suppress the same-sign dilepton signal [9]]. Thus, a heavy pseudo-Dirac neutrino, as naturally occurs in the inverse seesaw mechanism [15], seems to be the simplest possibility to explain the suppression of same-sign dilepton events in both CMS [2, 16] and ATLAS [17] searches. 
The main result of this paper is that if the difference between same and opposite-sign dilepton signal becomes statistically significant, it more likely suggests an inverse seesaw interpretation rather than a type-I seesaw. Note that in the original inverse seesaw proposal [15], the lepton number violation is small, being directly proportional to the light neutrino masses, and hence, it is rather unlikely to observe any same-sign dilepton events in this scenario. In this paper, we show that there exists a class of inverse seesaw models where the heavy neutrinos are still Majorana fermions with non-negligible lepton number violation, without affecting the inverse seesaw neutrino mass formula. In this class of models, it is possible to accommodate a non-zero same-sign dilepton signal, while being consistent with the suppression with respect to the opposite-sign signal. In particular, a statistically significant non-zero ratio of same and opposite-sign dilepton signal events could be used to test the relative strength between the Dirac and Majorana nature of the heavy neutrinos at the LHC.
Another important result of this paper is that our TeV scale LRSM with inverse seesaw unifies to an SO(10) Grand Unified Theory (GUT) at a high scale MU∼1017GeV without introducing any other intermediate scales, which is remarkable for a non-supersymmetric theory. This is achieved with a minimal TeV-scale particle content, which predicts the value of gR0.51 at the TeV scale, thus naturally satisfying the requirement gR<gL to explain the excess events mentioned above. Moreover, such a single-step unification without introducing supersymmetry (SUSY) also requires the existence of SU(2)- triplet fermions [at TeV scale which can play the role of Dark Matter [18and TeV-scale color-octet, SU(2)L-singlet scalar fields which can lead to new signals [37] at the LHC and future colliders]. Finally, this model could also explain the observed baryon asymmetry of the Universe via leptogenesis through the out-of-equilibrium decay of the heavy Majorana neutrinos, while avoiding the stringent leptogenesis bounds on MWR [19] due to suppressed WR -induced washout in this case [20].

(Submitted on 10 Aug 2015)

jeudi 10 septembre 2015

Can LHC2 phenomenology meet some unified left-right symmetric Pati-Salam (Spectral-like) models?

Welcome back to a blog which has not waited the most popular string theorist blogger** to get interested in and to talk about the provisory signature at the LHC of a potential left-right symmetry in Nature ;-)
Today I report on an article that might be the very first post-higgs phenomenologists' work on the spectral models investigated by noncommutative geometers:
... we feel that [the noncommutative geometrization (NCG) of the Standard Model] may have the potential to develop into a full-fledged paradigm. In particular, from the phenomenological standpoint, the necessity to enlarge the gauge symmetry (via an enlargement of the underlying NCG) to accommodate the Higgs mass can be considered a strength rather than a weakness. It tells us that the approach is restrictive enough for the models to be confronted by experiment, and point us in new directions to explore. Indeed, in a recent paper, Chamseddine, Connes, and van Suijlekom have proposed a new formulation of an NCG based unified left-right symmetric Pati-Salam model, which comes in three different versions differing in Higgs content. In all three, the gauge theory which emerges from the underlying NCG at the unification scale, which we will call MU, is that with gauge symmetry G224 = SU(2)L×SU(2)R×SU(4)C with unified couplings: gL(MU) = gR(MU ) = g4(MU ). In one version, the symmetry is actually G224P = G224 ×P, where P denotes parity which maintains left-right symmetry. G224 or G224P is assumed to break down to G213 = SU(2)L ×U(1)Y ×SU(3)C of the SM at scale MR...For all three versions, which differ in particle content, [Chamseddine et al.] argue that both boundary conditions can be satisfied if MU∼10^15 GeV and MR∼10^13 GeV.
In this paper, we will not attempt to review or justify the derivation of these models, but look at their consequences purely phenomenologically. From that viewpoint, the high value of MR is problematic in light of recent hints of a WR with a mass of around 2 TeV at the LHC. If the LHC signal is indeed the gauge boson of the SU(2)R group, then MR on the order of a few TeV would be more compatible with that possibility... We address the question whether MR for Chamseddine et al.’s NCG models can be lowered by the addition of intermediate breaking scales between MU and MR at which the symmetry breaks down from G224P /G224 to G213 via several intermediate steps. In other words, is any symmetry breaking pattern compatible with a unified left-right symmetric Pati-Salam model at MU , and the SM below MR ∼ few TeV?
(Submitted on 4 Sep 2015)

Summary of Results
... we have looked at whether the IR conditions MR = 5 TeV and gR(MR) = 0.4 could be realized within left-right symmetric, and unified left-right symmetric Pati-Salam models in which the unification/emergence scale is below the Planck mass. The left-right symmetric Pati-Salam demands the unification of gL and gR, while the unified left-right symmetric Pati-Salam demands further unification of gL and gR with g4. The requirements that these couplings unify at a single scale, and the matching conditions between g1, gBL, and gR at MR, and that between gBL, g3 and g4 at MC , place conflicting demands on the various symmetry breaking scales, and it is found that the target IR conditions cannot be realized so easily. In particular, if the Higgs content at various energy intervals is determined based on the Extended Survival Hypothesis (ESM), MR and gR(MR) tend to be much larger than our target values. Lowering these values requires the breaking of ESH. The most promising cases are Models B and C of [Chamseddine et al] with the colored ∆3R field surviving below MC . We note that this may put the ∆3R particles within reach of the LHC. But even for those cases gR(MR) cannot be made as low as 0.4. In all cases, the optimum conditions for minimum MR and/or minimum gR(MR) requires degeneracies of some of the symmetry breaking scales.

While our analysis could suggest that the NCG motivated unified left-right PatiSalam model is not favored phenomenologically by the current LHC data, we note the possibility that the current approach of grafting the NCG spectral action to RG evolution of standard QFT at the GUT scale may not capture the true nature and predictions of NCG theories*... In our view the approach based on NCG (and its related proposal based on the superconnection approach 22) offers a new and, phenomenologically, almost completely unexplored view on the rationale for the SM and also for its natural completion. This approach also offers a possibly exciting relation with the fundamental physics of quantum gravity, thus relating the infrared physics of the current exciting experimental searches conducted at the LHC to the hidden ultraviolet physics of quantum theory of space and time.
//addendum 12 /09/15
Blogger's comments 
One must not let oneself be fooled by the purported resonances in the 2 TeV range reported  by ATLAS and CMS collaborations in the data from the first run of LHC. Their hypothetical theoretical understanding as a signature of a heavy right handed gauge boson WR is exciting and might be motivated by a quite "natural" extension of the Standard model (or theory) but their statistical significance is low. Thus the real interest in this article is - from my biased view - the efforts done by Aydemir et al. to give a simple discussion (in the vernacular language of model builders) about some basic phenomenological aspects like energy scales, gauge couplings and symmetry breaking sequence with the corresponding Higgs spectrum derived from the spectral models recently proposed by the leading group of noncommutative geometry applied to high energy physics (in a frustratingly too brief recent article).

Needless to say, all subtle aspects of the spectral models theoretical motivation can not be addressed in one work as the one quoted above. This might be the reason for some precautions of speech (*). For instance one could expect a discussion of the relevance of the (extended) survival hypothesis in the spectral models context. Indeed as far as I understand it, the usefulness of the noncommutative framework is it does not require such hypothesis since the full Higgs spectrum is not introduced by hand but computed from the vacuum solution of the spectral action derived itself from first principles. Said it differently there is almost no flexibility in the scalar spectrum of these standard model extensions (if the spectral models come into three distinct ways in the last Chamseddine et al. article this seems to me essentially a matter of assumptions on the most general form of the hermitian matrix parametrizing the inverse of the Euclidean propagator of fermions in the discrete noncommutative fine structure of spacetime).

Last but not least there is no offense in my introduction (**) towards string theorists. I just regret that the blogger Lubos Motl has not informed yet his readers from the resilience of the noncommutative geometric inspired physical models after having proposed a review of them in the past. More importantly I am glad to see a former PhD student of Joseph Polchinsky like the second author Djordje Minic get involved in the phenomenological assessment of spectral models! Our common horizon for all physicists is naturally to find the most appropriate M(eeting or Mathematical?) theory to better understand and further explore Nature I guess.