Cigdem Issever

Abstract:

Measuring the Higgs trilinear self-coupling λhhh is experimentally demanding but fundamental for understanding the shape of the Higgs potential. We present a comprehensive analysis strategy for the HL-LHC using di-Higgs events in the four b-quark channel (hh → 4b), extending current methods in several directions. We perform deep learning to suppress the formidable multijet background with dedicated optimisation for BSM λhhh scenarios. We compare the λhhh constraining power of events using different multiplicities of large radius jets with a two-prong structure that reconstruct boosted h → bb decays. We show that current uncertainties in the SM top Yukawa coupling yt can modify λhhh constraints by ∼ 20%. For SM yt, we find prospects of −0.8 < 𝜆ℎℎℎ/𝜆SMℎℎℎ < 6.6 at 68% CL under simplified assumptions for 3000 fb−1 of HL-LHC data. Our results provide a careful assessment of di-Higgs identification and machine learning techniques for all-hadronic measurements of the Higgs self-coupling and sharpens the requirements for future improvement.

Abstract:

Cross-sections of associated production of a Higgs boson decaying into bottomquark pairs and an electroweak gauge boson, W or Z, decaying into leptons are measured as a function of the gauge boson transverse momentum. The measurements are performed in kinematic fiducial volumes defined in the ‘simplified template cross-section’ framework. The results are obtained using 79.8 fb−1 of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. All measurements are found to be in agreement with the Standard Model predictions, and limits are set on the parameters of an effective Lagrangian sensitive to modifications of the Higgs boson couplings to the electroweak gauge bosons.

Abstract:

A search for Higgs boson pair production in the b ¯bWW∗ decay mode is performed in the b ¯b`νqq final state using 36.1 fb−1 of proton-proton collision data at a centreof-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of events beyond the background expectation is found. Upper limits on the non-resonant pp → HH production cross section of 10 pb and on the resonant production cross section as a function of the HH invariant mass are obtained. Resonant production limits are set for scalar and spin-2 graviton hypotheses in the mass range 500 to 3000 GeV.

Abstract:

A search for Higgs boson pair production in the bb¯¯bb¯¯ final state is carried out with up to 36.1 fb−1 of LHC proton-proton collision data collected at s√=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260–3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bb¯¯bb¯¯ are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction. Open image in new window

Abstract:

This is the users manual of the black-hole event generator BlackMax (Dai et al., 2008), which simulates the experimental signatures of microscopic and Planckian black-hole production and evolution at proton–proton, proton–antiproton and electron–positron colliders in the context of brane world models with low-scale quantum gravity. The generator is based on phenomenologically realistic models free of serious problems that plague low-scale gravity. It includes all of the black-hole gray-body factors known to date and incorporates the effects of black-hole rotation, splitting between the fermions, non-zero brane tension and black-hole recoil due to Hawking radiation (although not all simultaneously). The main code can be downloaded from Dai et al. (0000).

Program summary

Program title: BlackMax

Program Files doi: http://dx.doi.org/10.17632/p9jg9dypcg.1

Licensing provisions: GNU General Public License version 3

Programming language: C (with Fortran subroutines)

Nature of problem: In the class of models with low scale quantum gravity (known as the “TeV scale gravity models”) collisions of particles at the particle accelerators may lead to novel phenomena, in particular mini black hole production. In order to confirm or exclude this class of models, one needs to calculate the probability of the black hole production in collisions of particles, properties of the formed black holes (mass, spin, charge, momentum), and the signature of the black hole decay (Hawking radiation).

Solution method: BlackMax calculates the probability of the black hole production by utilizing the so-called “geometric cross section” for black hole production. From the energy and quantum numbers of the colliding particles BlackMax calculates the mass, spin, charge, and momentum of the formed black holes. In the next step, BlackMax utilizes the greybody factors that characterize Hawking radiation and calculates the final output. The produced particles are then supposed to leave the signature in particle detectors.

References: Phys. Rev. D 77, 076007 (2008)

Theoretical background summary

Models with TeV-scale quantum gravity offer very rich collider phenomenology. Most of them assume the existence of a three-plus-one-dimensional hypersurface, which is referred as “the brane,” where Standard-Model particles are confined, while only gravity and possibly other particles that carry no gauge quantum numbers, such as right handed neutrinos, can propagate in the full space, the so-called “bulk”. Under certain assumptions, this setup allows the fundamental quantum gravity energy scale, to be close to the electroweak scale. The observed weakness of gravity compared to other forces on the brane (i.e. in the laboratory) is a consequence of the large volume of the bulk which dilutes the strength of gravity. In the context of these models of TeV-scale quantum gravity, probably the most exciting new physics is the production of micro-black-holes in near-future accelerators like the Large Hadron Collider (LHC). According to the “hoop conjecture”, if the impact parameter of two colliding particles is less than two times the gravitational radius, rh, corresponding to their center of-mass energy (ECM), a black-hole with a mass of the order of ECM and horizon radius, rh, will form. Typically, this gravitational radius is approximately ECM /M*2. Thus, when particles collide at center-of-mass energies above M*, the probability of black-hole formation is high.

Once a black-hole is formed, it is believed to decay via Hawking radiation. This Hawking radiation will consist of two parts: radiation of Standard-Model particles into the brane and radiation of gravitons and any other bulk modes into the bulk. The relative probability for the emission of each particle type is given by the gray-body factor for that mode. This gray-body factor depends on the properties of the particle (charge, spin, mass, momentum), of the black-hole (mass, spin, charge) and, in the context of TeV-scale quantum gravity, on environmental properties such as the number of extra dimensions, the location of the black-hole relative to the brane (or branes), etc. In order to properly describe the experimental signatures of black-hole production and decay one must therefore calculate the gray-body factors for all of the relevant degrees of freedom.

Since a black hole can emit particles like quarks and gluons which cannot freely propagate long distance, one has to simulate the process of hadronization. The generator can be interfaced with hadronization generators Herwig and Pythia to obtain the final signature measurable in particle detectors.