Probing the anomalous γγγZ coupling at the LHC with proton tagging

Journal of High Energy Physics, Jun 2017

The sensitivities to the anomalous quartic gauge boson coupling γγγZ are estimated via γZ production with intact protons in the forward region at the LHC. Proton tagging proves to be a powerful tool to suppress the background, which allows consideration of the hadronic decays of the Z boson in addition to the leptonic ones. We discuss the discovery potential for an integrated luminosity of 300 fb−1 and 3000 fb−1. The sensitivity we obtain at 300 fb−1 goes beyond the one expected from LHC bounds on the Z → γγγ decay by about three orders of magnitude. The γZ channel provides important discriminatory information with respect to the exclusive γγ channel, as many particles beyond the Standard Model (such as a radion or Kaluza Klein gravitons) predict a signal in the latter but not the former.

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Probing the anomalous γγγZ coupling at the LHC with proton tagging

Revised: June Probing the anomalous with proton tagging C. Baldenegro 0 1 2 S. Fichet 0 1 G. von Gersdor 0 1 C. Royon 0 1 2 Lawrence 0 1 Kansas 0 1 U.S.A. 0 1 R. Dr. Bento Teobaldo Ferraz 0 1 S~ao Paulo 0 1 Brazil 0 1 0 Z channel provides important discrimi- 1 Rio de Janeiro , Brazil 2 University of Kansas , USA The sensitivities to the anomalous quartic gauge boson coupling timated via Z production with intact protons in the forward region at the LHC. Proton tagging proves to be a powerful tool to suppress the background, which allows consideration of the hadronic decays of the Z boson in addition to the leptonic ones. We discuss the discovery potential for an integrated luminosity of 300 fb 1 and 3000 fb 1. The sensitivity we obtain at 300 fb 1 goes beyond the one expected from LHC bounds on the Z ! decay by about three orders of magnitude. The natory information with respect to the exclusive Standard Model (such as a radion or Kaluza Klein gravitons) predict a signal in the latter but not the former. Phenomenology of Large extra dimensions - HJEP06(217)4 1 Introduction sensitivities are then given in section 6 and 7. 2 Z process have been rst computed in ref. [22]. This process will be neglected in our study as, just like for SM light-by-light scattering, it is greatly reduced in the acceptance of the forward detectors (see sections 4.2, 5.2). The rare SM decay Z ! It has a branching ratio predicted to be BSM(Z ! have beeen computed in refs. [23, 24] and the W loop contribution in [25], the latter is found to be subdominant. In the presence of New Physics with a mass scale heavier than the experimentally accessible energy E, all New Physics manifestations can be described using an e ective Lagrangian valid for E. In this low-energy e ective eld theory (EFT), the Z interactions are described by two dimension-eight Z operators is another process sensitive to the anomalous Z interaction. ) = 5:4 10 10. The fermion loops L Z = O O Z + ~ ~ Z = F F F Z + ~F F ~ F ~ ; Z (2.1) with F~ The O physics. = 12 F . The O2 Z = F F F Z operator can sometimes be encountered in the literature. It is related to the above basis via the identity 4O2 Z ; O~ Z provides a somewhat clearer mapping onto the properties of the underlying Z = 2O Z + O~ Z . These operators can be seen as arising from a SU(2) U(1)Y e ective Lagrangian with operators such as B B B B , where B denotes the hypercharge gauge eld. The SU(2)L U( 1 )Y e ective Lagrangian contains ten such operators, see e.g. [3, 4, 26, 27]. The coe cients of these operators, once expressed with the same Lorentz structures as shown in eq. (2.1), must be positive to avoid superluminal excitations in the theory [28]. The SU(2) U( 1 )Y e ective Lagrangian also generates 4 , ZZ interactions, as described in [4]. Because of the large number of e ective operators in the SU(2) U(1)Y Lagrangian, anomalous interactions in the broken phase can be considered as independent. The operators of eq. (2.1) induce an anomalous Z ! decay [29], with a partial width that in our notation reads NP(Z ! ) = m9Z (2 2 + 2 ~2 ! An anomalous Z reaction is also induced, which is the focus of this work. We nd the unpolarized di erential cross section to be1 d NP d ! Z = Using the well-known partial wave analysis [30] we can estimate for what values of , ~ and s the theory remains unitary. By imposing unitarity on the S-wave of the EFT amplitudes and neglecting the Z boson mass one nds the conditions (see [4] for details on similar ~)2m2Z stu ; m2Z =s for the Z (2.3) nal (2.4) (2.5) amplitudes) As most of the recorded unitary for couplings up to j + ~js2 < 4 ; Z events have p j j ~ s2 < 12 5 : s below 1 TeV, we expect the EFT to remain 1It has been noted in [29] that the operators O = O Z a cross check of our result eq. (2.3), as in this basis we get = 4( 2 + ~2 ~) = +2 + 3 2 , hence a vanishing interference. O~ Z do not interfere. This property provides ~, (3 2 + 3~2 2 ~) = +2 + 2 2 and { 3 { with where and generate the Z couplings by tree-level exchange as cosine of the Weinberg angle by sw and cw and labeling the SU(2)L representation by its dimension d, we can write [4] ( ; ~) = (cs; c~s) swcw m4 em 2 d c where s denotes the spin of the heavy particle running in the loop.2 Beyond perturbative contributions to , ~ from charged particles, non-renormalizable interactions of neutral particles are also present in common extensions of the SM. Such theories can contain scalar, pseudo-scalar and spin-2 resonances, respectively denoted ', '~, h , that couple to the photon as L = ' + h 1 f0+ f 1 2 F f0+Z F 1 F (F )2 + Z + '~ F ~ F + f 1 0 1 f Z 2 1 0 f Z F ~ Z + (F )2=4 + ( F Z + F Z =4) ; ( ; ~) = 1 fs fs Z m2 (ds; d~s) ds = < 0 8 >1 > >>: 14 s = 0+ s = 0 ; s = 2 d~s = < 1 8 >0 > >>: 14 s = 0+ s = 0 : s = 2 Interestingly, the f Z couplings vanish if the neutral particle couples universally to the W I and B kinetic terms of the SU(2)L U( 1 )Y Lagrangian. This happens in particular when the neutral particle couples to gauge bosons via the stress-energy tensor. It is the case of the Kaluza Klein (KK) graviton present in models of warped extra dimensions with gauge elds on the IR brane, as well as the radion and the KK graviton in bulk gauge eld scenarios with small IR brane kinetic terms [4, 6]. This peculiar feature of the Z coupling becomes very interesting when put together with the measurement of the 4 interaction. Indeed, if a signal was observed, the Z channel would then provide a clear test whether or not the underlying exchanged particle is universally coupled to the gauge kinetic terms. 2The coe cients cs, c~s have been determined ref. [29] in a speci c case. { 4 { 4.1 Photon coherent ux We use the equivalent photon approximation [31, 32] to describe the pp ! photon exchanges. In this approximation, the almost real photons (with low virtuality Q2 = Z pp process via fusion q2) are emitted by the incoming protons producing a state X, pp ! pXp, through photon ! X. The photon spectrum of virtuality Q2 and energy E is proportional to the ne-structure constant em and reads: dN = em dE dQ2 E Q2 1 E E 1 Q2min Q2 FE + E2 2E2 FM (4.1) where E is the energy of the incoming proton of mass mp, Q2min = m2pE2=[E(E photon minimum virtuality allowed by kinematics and FE and FM are functions of the electric and magnetic form factors GE and GM . In the dipole approximation the latter read 2 FM = GM over Q2 FE = (4m2pG2E + Q2G2M )=(4m2p + Q ) 2 G2E = G2M = 2p = (1 + Q2=Q02) 4 : The magnetic moment of the proton is 2p = 7:78 and the tted scale Q20 = 0:71 GeV2. Since the electromagnetic form factors fall steeply as a function of Q2, the cross section can be factorized into the matrix element of the photon fusion process and the two photon uxes. In order to obtain the production cross section, the photon uxes are rst integrated f (E ) = Z 1 dN Q2min dE dQ2 dQ2: = Z dL !X dW dW The result is given for instance in ref. [1]. These uxes can then be used to introduce an e ective di erential luminosity dL =dW obtained by integrating f (E 1) f (E 2) dE 1 dE 2 (W 2pE 1E 2) where W is invariant mass of the diphoton system. Using the e ective photon luminosity, the total cross section for the pp ! pXp process reads where !X denotes the cross section of the sub-process ! X, dependent on the invariant mass of the two-photon system. In addition to the photon exchange, there might be additional soft gluon exchanges that might destroy the protons. To take into account this e ect, we can introduce the so-called survival probability that the protons remain intact in photon-induced processes [33, 34]. In this paper, we assumed a survival probability of 90%. 4.2 Proton detectors The pp ! p Zp proccess can be probed via the detection of two intact protons in the forward proton detectors at CT-PPS or AFP [20, 35] and the detection of the Z boson decay and the photon in the respective central detector. The forward detectors are located symmetrically at about 210 m from the main interaction vertex and cover a range { 5 { (4.2) (4.3) (4.4) of 0:015 < 1;2 < 0:15, where 1;2 is the proton fractional momentum loss which is reconstructed by the forward proton detector. This leads to an acceptance in the central mass m Z between 300 and 1900 GeV. The average number of multiple proton-proton collisions per bunch crossing, , sets a huge background environment on the search for exclusive events. Intact forward protons arising from the pile-up together with an uncorrelated non-exclusive process in the central detector can mimic the signal exclusive events. Usually ranges from 30 to 50 interactions per bunch crossing at the current LHC luminosity. The pile-up is expected to go up to = 200 interactions at the High Luminosity LHC which will pose a challenge on the search for New Physics. The detection and characterization of the outgoing protons provides the HJEP06(217)4 complete kinematic information on the event, which in turn allows us to exploit the fourmomentum conservation via rapidity and mass matching methods. These methods provide a strong background rejection which is the key feature of the forward proton detectors in exclusive processes. Further background rejection can be achieved with the use of timing detectors. Timing detectors are expected to be installed and operating in both CT-PPS and ATLAS to measure the time-of- ight of protons with a precision of 15 ps, which would allow to determine the interaction vertex of the protons with a 2.1 mm precision, thus allowing a large background rejection by a factor of 40 [36]. In this work, we will not make use of this potential future improvement. 4.3 Central detection of Z In ATLAS and CMS, photons can be reconstructed in the central detectors instrumented with electromagnetic calorimeters which cover the pseudorapidity range j j < 2:5 and provide excellent resolution in terms of energy (less than a percent at pT > 100 GeV) and position (0.001 units of and 1 mrad on the azimuthal angle ) for photons and leptons with pT ranging from a few GeV up to the TeV scale. Photon identi cation e ciency is expected to be around 75% for pT > 100 GeV. In addition, about 1% of the electrons and jets are misidenti ed as photons [37]. The decay of the Z boson is widely studied in CMS and ATLAS and is used for calibration of the detectors [38]. In this study we consider both leptonic (electrons and muons) and hadronic decays. For ATLAS, the ducial acceptance corresponds to leptons with tranverse momenta p`T > 25 GeV and absolute rapidity ` < 2:5. For Z boson production, the dilepton invariant mass m`` is required to be between 66 < m`` < 116 GeV. Similar requirements are made in CMS. For the Z boson decay into hadrons, the jet is reconstructed by clustering particles deposited in the electromagnetic and hadronic calorimeters. The energy of photons is obtained directly from the electromagnetic calorimeter measurement. The energy of a charged hadron is determined from a combination of the track momentum and the corresponding electromagnetic and hadronic calorimeter energies. The energy of a neutral hadron is obtained from the calibrated energies in electromagnetic and hadronic calorimeters. The typical jet energy resolution is between 5{10% for jets with pT > 200 GeV. Commonly, the anti-kT jet clustering-algorithm with a radius parameter of R = 0:5 is used in CMS and ATLAS [39, 40]. { 6 { ! The anomalous Z process with intact protons in the nal state has been implemented in the Forward Physics Monte Carlo (FPMC) generator [41]. Contributions and simulations of the various backgrounds are discussed in the subsections below. In order to model systematic uncertainties on nal states, we have included Gaussian smearings on the photon, lepton and hadron energies of 1%. In addition, we apply Gaussian smearings of 15% in the individual reconstructed jet energy, as well as 0.1% for the pseudorapidity and 1 mrad for the azimuthal angle. 5.1 Pile-up backgrounds The largest background to the p Zp nal state originates from Z detection in the central detectors simultaneously with the detection of two intact protons from pile-up. Background contribution in the jj channel is dominated by W and Z in association with pile-up protons if we restrict ourselves to two-jet nal states. Around 1% of the electrons are misidenti ed as photons, thus the background qqe in association with pile-up is also considered in our study. The non-exclusive background processes were simulated in PYTHIA8 [42] at leading order. Jets are reconstructed with the anti-kt clusteringalgorithm with FastJet [43] using R = 0:5 and pT min = 10 GeV at the hadron level, which is close to the standard CMS and ATLAS parameter choice for jet reconstruction. We assign a 15% resolution to the reconstructed jets energy, and apply smearings on the j , pT j and j on top of the gaussian smearings applied to the individual particles that form the jet. In the `` channel, the dominant background is the leptonic decay of the non-exclusive Z production in association with pile-up protons, as can be seen in gure 2. We also consider misidenti cation of jets and electrons as photons as part of the background. The pile-up events were simulated as follows. For each non-exclusive background event generated on PYTHIA, the number of pile-up interactions in the event is drawn from a Poisson distribution with mean = 50; 200 respectively for the low and high luminosity scenarios. We draw the tag probability for the protons arising from the pile-up interactions from a uniform distribution and compare it with the single, double or no tag probabilities, which were computed by propagating single and double di ractive protons generated on PYTHIA8 along the beamline up to the proton detectors within their acceptance (see [36, 44] for more details). We assign the fractional momentum loss = p=p, which would be reconstructed by the forward detectors in AFP or TOTEM, by randomly sampling the distribution f ( ) = 1= (which roughly resembles the distribution measured by the proton detectors) de ned in 0:015 < < 0:15 via its inverse transform. When the forward p detectors have at least one proton tagged in each arm, we compare the diproton mass 1 2s and rapidity 12 log( 1= 2) with the central mass and rapidity of the Z nal state, and select the best match in such observables. Only events with two proton tags pass through the selection cuts quoted in tables 1, 2 and 3. { 7 { HJEP06(217)4 an integrated luminosity of 300 fb 1 and pT jj > 100 GeV] m Z > 700 GeV pT jj =pT 1631 358.9 with an essentially background-free measurement in this channel. + pile-up + pile-up + pile-up Cut/Process [0:015 < 1;2 < 0:15, pT > 200 GeV pT `` > 200 GeV] m Z > 1100 GeV pT =pT `` > 0:95, p j jypp j < 0:01 1 2s = m Z 3% y Z j < 0:025 Signal ( ~ = 0) background events after all the selection cuts with a signal e ciency of 50%. The SM predicts Z production at one-loop with two intact protons via two mechanisms: elastic gluon fusion [45], with the exchange of an extra gluon to ensure the process is colourless and the protons stay intact, and photon fusion. SM !Z The elastic gluon fusion contribution can be neglected at high mZ (within the proton taggers acceptance) since the soft gluon emission in the gluon ladder has to be suppressed in order to get an exclusive di ractive event with intact protons. In practice, a Sudakov form factor is introduced to suppress this emission which greatly reduces the cross section at high mZ [45]. This is true in general for central exclusive processes in QCD. The SM cross section for is related to the cross section roughly as ! Z within the mass acceptance (see for instance gures 4 and 5 in [46]). We can use this to extrapolate the results from the diphoton studies in ref. [14] (from 0:04) for an integrated luminosity of 300 fb 1 after selection cuts, which is found to be less than 10% of the total background in the exclusive channel as seen in section 6. For 3000 fb 1, we expect less than 0:25 background events in `` , less than 12% of the total background estimated in section 6. Thus, in this study we assume that the QCD and QED exclusive Z contribution is small and focus on the non-exclusive background contribution, which constitutes the dominant background in this search. Event selection 6 j The (jj); (``); nal states from the New Physics signal are typically back-to-back and have similar transverse momenta. In the dijet (dilepton) case, this translates to cuts on jj j < 0:02 (j `` j < 0:02) and pT =pT jj > 0:90 (pT =pT ``) > 0:95). As can be seen in gures 2 and 4, the signal events appear in the high mass region, allowing for further background rejection by asking mjj > 700 GeV (m`` > 600 GeV). The probability to detect at least one proton in each of the forward detectors is estimated to be 32%, 66% and 93% for 50, 100 and 200 additional interactions respectively. The pile-up background invariant mass within 10% (5%) resolution, m jj = p 1 2s is further suppressed by requiring the proton missing invariant mass mpp to match the Z 10% (m `` = p 1 2s 5%) and the Z system rapidity and the rapidity of the two protons ypp = 12 ln( 1= 2) to be the same within jy jj yppj < 0:10 (jy `` yppj < 0:03) units in rapidity for the hadronic (leptonic) channel. This is the key background rejection tool provided by the forward detector information. The number of expected signal and background events passing their respective selections can be seen in table 1, (table 2) for the jj channel (`` channel) for an integrated luminosity of 300 fb 1 ( pile-up interactions 3 years of data-taking at the LHC) and moderate Exploiting the full event kinematics with the forward proton detectors allows us to suppress the non-exclusive background in both channels with a high signal selection e ciency of 70% (for = 50 average pile-up interactions), as can be seen in tables 1 and 2 and gures 5 and 3. For a coupling value of = 4 10 13 GeV 4, the signal cross section within the proton taggers acceptance is 1:1 fb. We expect about 25 events in jj and 10 { 9 { Z mass distribution for the signal in the `` channel for two coupling values ( = 10 12; 10 13 GeV 4) for events within the 0:015 < < 0:15 proton detectors acceptance and the requirement on transverse momenta pT ; pT `` > 100 GeV. The main contribution to the background is the SM Z production in association with protons arising from the pile-up. The plot assumes an integrated luminosity of 300 fb 1 and an average pile-up of V 102 10−1 10−4200 103 102 0 / s t 10−1 10−20 102 10 5 2 0 / s t 10−1 −1.5 1 2s to central mass ratio distribution (top) and rapidity di erence distribution (bottom) in the `` channel for signal and background within the acceptance 0:015 < 1;2 < 0:15 considering two di erent coupling values after applying the requirements on the acceptance, pT , invariant mass m Z , pT ratios and angle separation according to table 2. The width of the signal is due mainly to the 1;2 resolution. The integrated luminosity is 300 fb 1 and the average pile-up is Z mass distribution for the signal in the jj channel for two coupling values ( = 10 12; 10 13 GeV 4, ~ = 0) for events within the 0:015 < 1;2 < 0:15 proton detectors acceptance and after the transverse momenta requirement as in table 1. The plot assumes an integrated luminosity of 300 fb 1 and an average pile-up of V 103 e G 10−4100 104 103 102 4 s t v E en 1 10−1 10−2 10−3 103 102 0 / s t 10−1 10−−21.5 ∫Ldt = 300 fb-1 1 2s to central mass ratio distribution (top) and rapidity di erence distribution (bottom) in the jj channel for the signal and background within the acceptance 0:015 < 1;2 < 0:15 considering two di erent coupling values after applying the requirement on pT , invariant mass mZ , pT ratios and angle separation according to table 1. The integrated luminosity is 300 fb 1 and the average pile-up is = 50. The signal width is due to a combined e ect of the reconstructed jet energy low resolution ( 15%) and the 1;2 resolution from the proton detectors. The asymmetry on the mpp=mZ distribution is due to the resolution on the jet energy. p |ypp0­yZγ| Coupling (GeV 4) Luminosity Pile-up ( ) Channels `` jj jj L `` Coupling Luminosity Pile-up Channel `` channels with their respective selection cuts. = 200 average pile-up interactions, which corresponds to the High Luminosity LHC. Sensitivities for the jj channel are not quoted in this scenario, due to the high number of background events which compromises signi cantly the signal e ciency. events in `` channels for an integrated luminosity of 300 fb 1 for this coupling value after selection cuts. In addition, we include a similar study at a higher number of pile-up interactions per bunch crossing = 200 and an integrated luminosity of 3000 fb 1 . We consider the `` nal states in this scenario, since we do not obtain much improvement in the hadronic channel in comparison to the 300 fb 1. We optimized the cuts for this case to increase the background rejection even further, as shown in table 3. Let us note that a signi cant fraction of the Z boson hadronic nal states are reconstructed as a single jet, since the dijet system is boosted in the high mass regime where the signal is enhanced. The QCD background from q and g nal states is very large, and contributes to O(103) background events after selection cuts. For this reason, we restricted ourselves to the dijet nal states in this study, We stress however that a more evolved jet substructure analysis can in principle e ciently discrimate between large-radius jets from a Z and from QCD [47] but this goes beyond our study. 7 Expected sensitivities Expected sensitivities to the , ~ coe cients are shown in tables 4, 5 and in gure 6. The sensitivities are roughly of 2 10 13 GeV 4 for both 300 fb 1 at low luminosity and 3000 fb 1 at high luminosity. The reach at high luminosity is limited by the large pile-up. ζ − − be probed at 5 , 3 , and 95% C.L. using proton tagging at the LHC. The leptonic nal state turns out to be the cleanest channel for this search with nearly background-free events without the use of timing detectors. Interestingly, it turns out that a good sensitivity is also obtained in the hadronic channel at moderate pile-up, with only three background events after selection cuts. The remaining background events that may pass the selection cuts can be further rejected by a factor of 40 with the timing detectors to be in operation in AFP and CT-PPS, as discussed in section 4. The B(Z ! ) branching ratio has been constrained at LEP [48{50]. More recently ) < 2:2 10 6. This bound translates as a limit a stronger bound from ATLAS using 8 TeV data has been quoted in ref. [51], B(Z ! s ~ 2 2 + ~2 < 1:3 10 9 GeV 4 (95% C:L:) : (7.1) Imagining the same search is done at 13 TeV data with 300 fb 1 in the same conditions, we expect very roughly an improvement by an order of magnitude of the bound of eq. (7.1). In addition, the current number of pile-up interactions at 13 TeV sets a challenge to the measurement of 3 nal states. This remains far away from the expected sensitivities obtained in the exclusive channel at the same luminosity by roughly three orders of magnitudes. Finally, let us compare the Z channel sensitivity to exclusive channel sensitivity estimated in refs. [6, 14]. We do so for the case of heavy neutral particles described in section 3. Using the Z, sensitivities at 300 fb 1 with no form factors, which are given respectively in table 4 of this paper and table 3 of ref. [14], we obtain that the neutral particle can be detected with 5 signi cance if m < 2:3 TeV m < 4:5 TeV 1 TeV pf f Z 1 TeV f Depending on the relative strength of the f obtained in the two channels. in the Z channel ; in the channel : (7.2) (7.3) and f Z couplings, a similar reach can be We would like to stress that in order for these bounds to be the most sensitive probe of HJEP06(217)4 a new particle, one has to assume that the couplings to gluons are somewhat suppressed, as otherwise gluon fusion processes can provide stronger bounds. For instance, in the case of the KK graviton coupling universally to all SM elds, our 5 sensitivity in eq. (7.3) translates to mKK < 1:4 TeVp =0:1, where This bound is slightly weaker than standard searches. is the universal KK graviton coupling strength. However, our method becomes more sensitive at large coupling, both because the resonance becomes too broad for standard searches, and also because we are sensitive to mass regions outside the kinematic reach of the LHC. Also note that, even if our method did not provide the primary discovery channel, Equations (7.2) and (7.3) show that the Z channel could e ciently determine whether the underlying particle couples universally to (B )2, (W I; )2 (see discussion in section 3). Finally, let us comment on electroweak production modes. Because of gauge invariance, the and Z couplings also imply nonzero W W and ZZ coupings. Therefore, inelastic photon fusion as well as W W and ZZ fusion are always present. We have checked that of the latter, inelastic photon fusion has the largest cross section.3 However, all of these production modes have to compete with a much larger background, which becomes particularly problematic when the width of the resonance is large. The sensitivity to charged particles is fairly weak, unless large d; Y or a large multiplicity are taken. For a vector in the SU(2)L adjoint for example, one has d = 3; Y = 0 and the reach on the mass is found to be m 120 GeV. For N vectors, the bound would increase as N 1=4. On the other hand, this bound on charged particles is very generic, as it depends only on the quantum numbers of the underlying particle (see eq. (3.1)). This measurement is thus quite complementary to direct searches for charged particles, which are very model-dependent. 8 Conclusion The forward proton detectors recently installed at the LHC provide the opportunity of measuring the anomalous Z coupling with unprecedented sensitivity, providing another high precision probe into the SM gauge sector. We have estimated the discovery potential 3For instance, in the case of the 750 GeV diphoton excess it was found that the inelastic photon fusion cross section is about 15{20 times the elastic one, while the correction to this from ZZ and W W contributions amounted to about 10% [17]. Notice that the irreducible weak boson couplings have a di erent tensor structure than for instance in the case of the SM Higgs. for exclusive Z production at a center-of-mass energy of 13 TeV, for both low and high luminosity scenarios. ! We have computed the anomalous Z rate induced by New Physics in the e ective eld theory framework. Contributions to the Z couplings from tree-level exchange of neutral particles and from loops of particles with arbitrary electroweak charge have been calculated. Prospects for both hadronic and leptonic channels have been evaluated. The tagging of the protons drastically reduces the background in all cases, and allows studying the Z decay in the hadronic channel in addition to the leptonic channel, which is usually very challenging in the standard searches. For 300 fb 1 of integrated luminosity, similar sensitivities are found in both channels, and their combination amounts to reach the anomalous couplings down to 2 10 13 GeV 4 with 5 statistical signi cance. This sensitivity goes beyond the one expected from the Z ! roughly three orders of magnitude. decay searches at the LHC by The combined sensitivity in the Z channel turns out to be roughly comparable to the sensitivity expected in the exclusive channel. Combining the information from both channels would provide a powerful way to pin down New Physics scenarios. In particular, a number of New Physics candidates such as the radion or Kaluza Klein gravitons contribute to the nal state and not to the Z one. Acknowledgments We thank O. Kepka and M. 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C. Baldenegro, S. Fichet, G. von Gersdorff, C. Royon. Probing the anomalous γγγZ coupling at the LHC with proton tagging, Journal of High Energy Physics, 2017, 1-19, DOI: 10.1007/JHEP06(2017)142