Long-lived Massive Particles Research Articles

A search for charged massive long-lived particles at D0

Results are presented on a search for charged massive long-lived particles (CMLLPs), based on 5.2 fb − 1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron $p\!\bar{p}$ collider. This search selects events in which one or more particles are reconstructed as muons but their speed and ionization energy loss (dE/dx) are different from muons produced in beam–beam collisions. CMLLPs have been predicted by several theories of physics beyond the Standard Model. In this search we exclude pair-produced long-lived gaugino-like charginos with masses below 251 GeV and higgsino-like charginos with masses below 230 GeV at 95% CL, as well as long-lived stop quarks with masses below 265 GeV. We place cross-section limits on long-lived staus as 0.04 to 0.006 pb for stau masses in the range 100 to 300 GeV.

Search for Charged Massive Long-Lived Particles

We report on a search for charged massive long-lived particles (CMLLPs), based on 5.2 fb(-1) of integrated luminosity collected with the D0 detector at the Fermilab Tevatron pp collider. We search for events in which one or more particles are reconstructed as muons but have speed and ionization energy loss (dE/dx) inconsistent with muons produced in beam collisions. CMLLPs are predicted in several theories of physics beyond the standard model. We exclude pair-produced long-lived gauginolike charginos below 267 GeV and Higgsino-like charginos below 217 GeV at 95% C.L., as well as long-lived scalar top quarks with mass below 285 GeV.

Resonant annihilation of long-lived massive colored particles through hadronic collisions

Hypothetical long-lived massive colored particles (MCPs or $Y\mathrm{s}$) would be confined in colorless exotic strongly interacting massive particles (SIMPs) at a color confinement temperature of ${T}_{\mathrm{C}}\ensuremath{\sim}200\text{ }\text{ }\mathrm{MeV}$. Two long-lived MCPs form a bound state ($Y\overline{Y}$) at collisions of two SIMPs. We study sensitivities of MCP annihilation to decay properties of resonances ($Y\overline{Y}$), and binding energies or energy levels of exotic SIMPs. The ($Y\overline{Y}$) formation is assumed to dominantly proceed through resonances of ($Y\overline{Y}$) in this paper. We make a toy model of the effective cross section for $Y\overline{Y}$ annihilation. Abundances of SIMPs are then calculated for different sets of parameters specifying properties of ($Y\overline{Y}$) resonances, binding energies of SIMPs, the initial abundance and the mass of MCP. Calculated relic abundances for respective SIMP species are $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}--3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ times that of baryon. They can be much higher but cannot be much smaller than the previous estimate. The abundances can be consistent depending on parameters with the possible scenario that SIMPs bind to nuclei and subsequent exotic nuclear reactions reduce the primordial abundance of $^{7}\mathrm{Li}$ or enhance those of $^{9}\mathrm{Be}$ and/or B in the early Universe. A unique information on the quark-hadron phase transition in the early Universe may become available in future by elaborated studies on the annihilation process with light element abundances as observables.

Search for stable hadronising squarks and gluinos with the ATLAS experiment at the LHC

Hitherto unobserved long-lived massive particles with electric and/or colour charge are predicted by a range of theories which extend the Standard Model. In this Letter a search is performed at the ATLAS experiment for slow-moving charged particles produced in proton–proton collisions at 7 TeV centre-of-mass energy at the LHC, using a data-set corresponding to an integrated luminosity of 34 pb−1. No deviations from Standard Model expectations are found. This result is interpreted in a framework of supersymmetry models in which coloured sparticles can hadronise into long-lived bound hadronic states, termed R-hadrons, and 95% CL limits are set on the production cross-sections of squarks and gluinos. The influence of R-hadron interactions in matter was studied using a number of different models, and lower mass limits for stable sbottoms and stops are found to be 294 and 309 GeV respectively. The lower mass limit for a stable gluino lies in the range from 562 to 586 GeV depending on the model assumed. Each of these constraints is the most stringent to date.

(Standard model) universe dominated by the right matter

We analyze the phenomenology of a prolonged early epoch of matter domination by an unstable but very long-lived massive particle. This new matter domination era can help to relax some of the requirements on the primordial inflation. Its main effect is the huge entropy production produced by the decays of such a particle that can dilute any possible unwanted relic, as the gravitino in supersymmetric models, and thus relax the constraints on the inflationary reheating temperature. A natural candidate for such a heavy, long-lived particle already present in the standard model of the electroweak interactions would be a heavy right-handed neutrino. In this case, we show that its decays can also generate the observed baryon asymmetry with right-handed neutrino masses well above the bound from gravitino overproduction.

Big-bang nucleosynthesis and hadronic decay of long-lived massive particles

We study the big-bang nucleosynthesis (BBN) with the long-lived exotic particle, called $X$. If the lifetime of $X$ is longer than $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{sec}$, its decay may cause nonthermal nuclear reactions during or after the BBN, altering the predictions of the standard BBN scenario. We pay particular attention to its hadronic decay modes and calculate the primordial abundances of the light elements. Using the result, we derive constraints on the primordial abundance of $X$. Compared to the previous studies, we have improved the following points in our analysis: The JETSET 7.4 Monte Carlo event generator is used to calculate the spectrum of hadrons produced by the decay of $X$; the evolution of the hadronic shower is studied taking into account the details of the energy-loss processes of the nuclei in the thermal bath; we have used the most recent observational constraints on the primordial abundances of the light elements; in order to estimate the uncertainties, we have performed the Monte Carlo simulation which includes the experimental errors of the cross sections and transferred energies. We will see that the nonthermal productions of D, $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, and $^{6}\mathrm{Li}$ provide stringent upper bounds on the primordial abundance of a late-decaying particle, in particular, when the hadronic branching ratio of $X$ is sizable. We apply our results to the gravitino problem, and obtain an upper bound on the reheating temperature after inflation.

Search for stable and long-lived massive charged particles in e+e− collisions at [formula omitted] GeV

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or fractional charges of 2/3, 4/3, and 5/3 is reported using data collected by the OPAL detector at LEP, at centre-of-mass energies from 130 to 209 GeV. These particles are assumed to be pair-produced in e+e− collisions and not to interact strongly. No evidence for the production of these particles was observed. Model-independent upper limits on the production cross-section between 0.005 and 0.028 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM), this implies a lower limit of 98.0 (98.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged heavy leptons and charginos are excluded for masses below 102.0 GeV. For particles with fractional charge ±2/3, ±4/3 and ±5/3, the upper limit on the production cross-section varies between 0.005 and 0.020 pb. All mass and cross-section limits are derived at the 95% confidence level and are valid for particles with lifetimes longer than 10−6 s.

Primordial nucleosynthesis and hadronic decay of a massive particle with a relatively short lifetime

In this paper we consider the effects on big bang nucleosynthesis (BBN) of the hadronic decay of a long-lived massive particle. If high-energy hadrons are emitted near the BBN epoch (t{approx}10{sup -2}--10{sup 2}sec), they extraordinarily interconvert the background nucleons with each other even after the freeze-out time of the neutron to proton ratio. Then, the produced light element abundances are changed, and that may result in a significant discrepancy between standard BBN and observations. Especially on the theoretical side, we can now obtain a lot of experimental data on hadrons and simulate the hadronic decay process, executing the numerical code of the hadron fragmentation even in the high energy region where we have no experimental data. Using the computed light element abundances in the hadron-injection scenario, we derive a constraint on the properties of such a particle by comparing our theoretical results with observations.

Radiative decay of a massive particle and the nonthermal process in primordial nucleosynthesis

We consider the effects on big bang nucleosynthesis (BBN) of the radiative decay of a long-lived massive particle. If high-energy photons are emitted after the BBN epoch ($t \sim 1 - 10^3$ sec), they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between standard BBN and observation. Taking into account recent observational and theoretical developments in this field, we revise our previous study constraining the abundance of the radiatively-decaying particles. In particular, on the theoretical side, it was recently claimed that the non-thermal production of $^6$Li, which is caused by the photodissociation of $\hefour$, most severely constrains the abundance of the radiatively-decaying particle. We will see, however, it is premature to emphasize the importance of the non-thermal production of $^6$Li because (i) the theoretical computation of the $^6$Li abundance has large uncertainty due to the lack of the precise understanding of the $^6$Li production cross section, and (ii) the observational data of $^6$Li abundance has large errors.

SUSY long-lived massive particles: Detection and physics at the LHC

We draw a possible scenario for the observation of massive long-lived charged particles at the LHC detector ATLAS. The required flexibility of the detector triggers and of the identification and reconstruction systems are discussed. As an example, we focus on the measurement of the mass and lifetime of long-lived charged sleptons predicted in the framework of supersymmetric models with gauge-mediated supersymmetry (SUSY) breaking. In this case, the next-to-lightest SUSY particle can be the light scalar partner of the tau lepton\((\tilde \tau _1 )\), possibly decaying slowly into a gravitino. A wide region of the SUSY parameters space was explored. The accessible range and precision on the measurement of the SUSY breaking scale parameter √F achievable with a counting method are assessed.

Search for stable and long-lived massive charged particles in e+e− collisions at =130−183 GeV

A search for stable and long-lived massive particles of electric charge | Q / e |=1 or 2/3, pair-produced in e + e − collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge ±2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10 −6 s.

Neutrino astronomy and massive long-lived particles from the big bang

We consider the neutrino flux from the decay of long-lived big-bang particles. The red-shift ztr at which the neutrino transparency of the universe sets in is calculated as a function of neutrino energy: ztr ≅ 1 × 105 for TeV neutrinos and ztr ≅ 3 × 106 for 10 MeV neutrinos. One might expect the production of detectable neutrino flux at z ⪅ ztr, but, as demonstrated in this paper, the various upper limits, most notably due to nucleosynthesis and diffuse X- and gamma-rays, preclude this possibility. Unless the particle decay is strongly dominated by the pure neutrino channel, observable neutrino flux can be produced only at the current epoch, corresponding to red-shift z ≈ 0. For the thermal relics which annihilate through the gauge bosons of SU(3)×SU(2)×U(1) group, the neutrino flux can be marginally detectable at 0.1 < Ev < 10 TeV. As an example of non-thermal relics we consider gravitinos. If gravitinos are the lightest supersymmetric particles (LSP) they can produce the detectable neutrino flux in the form of a neutrino line with energy Ev = 12MG, where MG is the gravitino mass. The flux strongly depends on the mechanisms of R-parity violation. It is shown that heavy gravitinos (MG ⪅ 100 GeV) can make up the dark matter in the universe.

Search for long-lived massive neutrinos in Z decays.

We search for events in the Mark II detector at SLAC Linear Collider with the topology of a {ital Z} boson decaying into a pair of long-lived massive particles. No events that are consistent with the search hypothesis are found. Interpreting the long-lived particle as a sequential Dirac neutrino {nu}{sub 4} of the fourth generation, we exclude at the 95% confidence level a significant range of mixing-matrix elements of {nu}{sub 4} to other-generation neutrinos for a {nu}{sub 4} mass from 10 to 43 GeV/{ital c}{sup 2}.

Search for Long-Lived Massive Particles and Time Structure of Electrons in Extensive Air Showers

An experiment to observe accompanied showers delayed relative to the bulk of extensive-air-shower particles near the core was carried out by using plural detectors set at separate places. Four delayed events hitting two or more detectors at the same time were observed. It is difficult to explain these events by accidental coincidence or conventional processes. However, these events can be explained, provided that a parent particle producing the delayed shower has a lifetime of the order of 1 ×10 -6 s and is heavier than a nucleon. The thickness of air shower disk was also measured at core distances less than 50 m.

Search for Long-Lived Heavy Particles

We have performed a high-sensitivity search for massive long-lived particles produced at 2.5 mrad by 400-GeV/c protons on a beryllium target using time-of-flight, Cherenkov, and calorimetric techniques. A total of ${10}^{11}$ light particles (${\ensuremath{\pi}}^{\ensuremath{-}},{K}^{\ensuremath{-}},\overline{p}$) was sampled at 70 GeV/c. This experiment places a limit of 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}37}$ ${\mathrm{cm}}^{2}$/${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} nucleon on the invariant cross section for the production of stable particles in the mass range of 4 to 10 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$.

Search for New Massive Long-Lived Neutral Particles

We have carried out a search for neutral hadrons with masses approximately-greater-than2 GeV/c/sup 2/ and proper lifetimes approximately-greater-than10/sup -7/ sec in the M4 neutral beam at Fermilab. The particle masses were determined from their flight time and kinetic energy. Our upper limits for the production cross section of such particles are 10/sup -1/ to 10/sup -3/ times those for the production of psi's by protons for comparable kinematic conditions. This is the most sensitive search to date for quasistable neutral particles with masses approximately-greater-than2 GeV/c/sup 2/. (AIP)

Further search for massive long-lived particles produced by 400-GeV protons

We report the negative result of a search for the decay products of long-lived particles having lifetimes in the range 50 milliseconds to 10 hours. The upper limit for the "cross section for production and capture" of such particles in a 33-cm aluminum target irradiated with 400-GeV protons is set at less than ${10}^{\ensuremath{-}36}$ ${\mathrm{cm}}^{2}$.

Search for massive long-lived particles in aluminum targets irradiated by 300- and 400-GeV protons

A directional gas Cherenkov counter, which employed six phototubes to sample Cherenkov light from single particles having a gamma greater than ~10 was used to achieve accidental rates of less than one per day if operated near targets with surface radiation levels of as high as 10/sup +3/ r/hr. The cosmic ray background measured by the apparatus was reduced to less than one per day by mounting our directional Cherenkov counter above the irradiated targets and facing toward the earth. Two searches of a few days duration, after bombardments at energies of 300 and 400 GeV with >10/sup 16/ protons at NAL, were made in four inch thick targets of aluminum, mounted just down stream from another aluminum target. No long-lived particles were observed with cross sections for production and capture of approximately less than a micro-micro barn in a lifetime range of a few to a few thousand hours. (auth) from the decay of a long-lived intermediate state. (auth)