Search for charged massive long-lived particles at $\sqrt{s}\mathbf{=}1.96\mathrm{TeV}$

We present a search for charged massive long-lived particles (CMLLPs) that are pair produced in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the D0 experiment at the Fermilab Tevatron collider. Our result is a combination of two searches where either one or both CMLLPs are reconstructed in the detector. We select events with muonlike particles that have both speed and ionization energy loss ($\mathrm{d}E/\mathrm{d}x$) different from muons produced in $p\overline{p}$ collisions. In the absence of evidence for CMLLPs corresponding to $6.3{\mathrm{fb}}^{-1}$ of integrated luminosity, we set limits on the CMLLP masses in several supersymmetric models, excluding masses below 278 GeV for long-lived gaugino-like charginos, and masses below 244 GeV for long-lived Higgsino-like charginos at the 95% C.L. We also set limits on the cross section for pair production of long-lived scalar tau leptons that range from 0.04 to 0.008 pb for scalar tau lepton masses of 100–300 GeV.

Figure 1

Diagram of the D0 detector showing the locations (blue crosses) where a top squark hadron must be measured as charged to be selected as a CMLLP candidate.

Figure 2

Efficiency for slow-moving gaugino-like charginos of various masses to arrive within the L1 muon trigger gates. All events in this MC sample contain two gaugino-like charginos. (a) The black and red lines show the efficiencies for a pair of charginos, or a single chargino, respectively, to be within the trigger gate. (b) Overall efficiency, which is a product of the trigger gate efficiency and the efficiency for single muon triggers, for the selection of single charginos.

Figure 3

Distributions of $p_T$ of the (a) highest $p_T$ and the (b) second-highest $p_T$ muon in an event for $Z\to \mu \mu$ data and MC, and for simulated gaugino-like charginos with masses of 100, 200, and 300 GeV.

Figure 4

Mean hit time as a function of the Tevatron store number in the forward region A layer for muons from the decay of $Z$ bosons in data (a) before correction and (b) after correction. A store is the time period for which $p$ and $\overline{p}$ beams are circulated in the accelerator after injection. This period is typically 12–24 hours for the Tevatron. When the number of recorded times from $Z\to \mu \mu$ decays for a given store is large, a Gaussian function is fit to those times and the Gaussian mean is used. If the number of recorded times for a given store is small, the median of these times is used. These distributions cover the data taking period between June 2006 and March 2010.

Figure 5

Time distribution for scintillation counter layers A, B, and C in the forward muon system for times from $Z\to \mu \mu$ decays for data and MC events.

Figure 6

Time distributions for scintillation counters in layer C, (a) top and sides and (b) bottom, in the central muon system for times from $Z\to \mu \mu$ decays for data and MC events.

Figure 7

Time distributions for scintillation counters in (a) layer A (top, bottom and sides), (b) layer B sides, and (c) layer B bottom, in the central muon system for times from $Z\to \mu \mu$ decays for data and MC events.

Figure 8

Distributions of $⟨\beta ⟩$ of the (a) highest $p_T$ and the (b) second-highest $p_T$ muons. The distributions are normalized to the same number of events. Background is taken from $Z\to \mu \mu$ MC decays. The selection requirements are identical to those used in the pair CMLLP analysis, as described in Sec. 6, except that the requirement $⟨\beta ⟩<1$ is not imposed.

Figure 9

Speed significance of the (a) highest $p_T$ and the (b) second-highest $p_T$ muons. The distributions are normalized to the same number of events. Background is taken from $Z\to \mu \mu$ MC. The selection requirements are identical to those used in the pair CMLLP analysis, as described in Sec. 6, except that the requirement $⟨\beta ⟩<1$ is not imposed.

Figure 10

(a) Distribution of speed ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ of the highest $p_T$ muon in an event, and (b) speed asymmetry distribution of the highest and the second-highest $p_T$ muons for $Z\to \mu \mu$ data, MC and signal (gaugino-like chargino with masses of 100, 200, and 300 GeV). The distributions are normalized to the same number of events. All events are required to have two isolated muons with $p_T>20\mathrm{GeV}$. For $Z\to \mu \mu$ events, we require $70<M_{\mu \mu }<110\mathrm{GeV}$. The muons are required to not be consistent with cosmic ray muons.

Figure 11

Distributions of the adjusted $\mathrm{d}E/\mathrm{d}x$ (described in Sec. 5b) of the (a) highest $p_T$ and the (b) second- highest $p_T$ muons in data and the simulated chargino events. The distributions are normalized to the same number of events. The background is taken from $Z\to \mu \mu$ MC. The selection requirements are identical to those used in the pair CMLLP analysis, as described in Sec. 6.

Figure 12

Distributions of $\mathrm{d}E/\mathrm{d}x$ significance for the (a) highest $p_T$ and the (b) second-highest $p_T$ muons. The distributions are normalized to the same number of events. Background is taken from $Z\to \mu \mu$ MC. The selection requirements are identical to those used in the pair CMLLP analysis, as described in Sec. 6.

Figure 13

For cosmic ray data taken when there were no $p$ or $\overline{p}$ beams in the Tevatron collider, (a) difference between the A-layer and C-layer times [$t(\mathrm{C})-t(\mathrm{A})$] for single muons. There are two cosmic ray peaks for the two possible directions, away from or towards the $p\overline{p}$ collision vertex. The broad peak at positive $t(\mathrm{C})-t(\mathrm{A})$ values is due to hits from two different cosmic muon tracks in the same cosmic ray shower or from spurious noise, (b) absolute value of the difference between the A-layer times of the two muons in the event. The times shown in these plots are centered at zero for $⟨\beta ⟩=1$ particles. Selection requirements on the time differences are shown with a blue vertical line.

Figure 14

Correlation matrix for different kinematic variables for (a) stau leptons of 300 GeV mass, and (b) background for the search of a pair of CMLLPs in the run IIb data.

Figure 15

BDT-output distributions for simulated gaugino-like chargino masses of 100–300 GeV in 50 GeV steps for the search for a CMLLP pair with the run IIb data. Distributions are normalized to the expected number of events. Selection requirement on the BDT value is shown with a green vertical line. Note that a different BDT was constructed for each mass and the BDT selection requirement optimized separately for each mass.

Figure 16

95% C.L. limits on production cross sections of a pair of stau leptons, top squarks, gaugino-like charginos, and Higgsino-like charginos as a function of their masses from the search for a pair of CMLLPs with run IIb data. “CF” is the scenario without charge flipping. $\pm 1$ SD and $\pm 2$ SD are the 1 and 2 standard deviation bands, respectively, around the expected limit curves.

Figure 17

The transverse mass $M_T$ for single muon events and Higgsino-like chargino events with chargino masses 100, 200, and 300 GeV. The single muon event sample satisfies all of the selection criteria described in Table 11 except the $M_T$ cut, which separates the data and background samples. All the selection criteria described in Table 15 have been applied to the charginos except the $M_T>200\mathrm{GeV}$ cut. The distributions have been normalized to the same number of events.

Figure 18

Adjusted $\mathrm{d}E/\mathrm{d}x$ versus $⟨\beta ⟩$ for (a) data events, (b) background (data events with $M_T\le 200\mathrm{GeV}$), and (c) MC sample for gaugino-like charginos with mass of 300 GeV.

Figure 19

Speed related input distributions to BDTs for the search for single CMLLPs. Background comes from events containing a muon with $⟨\beta ⟩<1$ and $M_T\le 200\mathrm{GeV}$. Signal is gaugino-like charginos of masses 100, 200, and 300 GeV. The distributions are normalized to the same number of events.

Figure 20

Distributions related to $\mathrm{d}E/\mathrm{d}x$ used in the BDT for the search for single CMLLPs. Background comes from events containing a muon with $⟨\beta ⟩<1$ and $M_T\le 200\mathrm{GeV}$. Signal is gaugino-like charginos of masses 100, 200, and 300 GeV. The distributions are normalized to the same number of events.

Figure 21

BDT-output distributions for simulated gaugino-like chargino masses 100–300 GeV in 50 GeV steps in the search for single CMLLPs. The distributions are normalized to the expected number of events.

Figure 22

95% C.L. limits on production cross sections of a pair of stau leptons, top squarks, gaugino-like charginos, and Higgsino-like charginos, as a function of their masses from the search for one or more CMLLPs with run IIb data. $\pm 1$ SD and $\pm 2$ SD are the 1 and 2 standard deviation bands, respectively, around the expected limit curves.

Figure 23

Combined limits at 95% C.L. on production cross sections of a pair of stau leptons, gaugino-like charginos, and Higgsino-like charginos as a function of their masses with run IIa and run IIb data. $\pm 1$ SD and $\pm 2$ SD are the 1 and 2 standard deviation bands, respectively, around the expected limit curves.

Figure 24

BDT-output distributions for stau masses 100–300 GeV in 50 GeV steps for the search for a CMLLP pair with run IIb data. The distributions are normalized to the expected number of events. The selection requirement on the BDT value is shown with a green vertical line.

Figure 25

BDT-output distributions for top squark masses 100–400 GeV in 50 GeV steps for the search for a CMLLP pair with the run IIb data. Distributions are normalized to the expected number of events. Selection requirement on the BDT value is shown with a green vertical line.

Figure 26

BDT-output distributions for Higgsino-like chargino masses 100–300 GeV in 50 GeV steps for the search for a CMLLP pair with the run IIb data. Distributions are normalized to the expected number of events. Selection requirement on the BDT is shown with a green vertical line.

Figure 27

BDT-output distributions for stau masses 100–300 GeV in 50 GeV steps in the search for single CMLLPs. The distributions are normalized to the expected number of events.

Figure 28

BDT-output distributions for top squark masses 100–400 GeV in 50 GeV steps in the search for single CMLLPs. The distributions are normalized to the expected number of events.

Figure 29

BDT-output distributions for Higgsino-like chargino masses 100–300 GeV in 50 GeV steps in the search for single CMLLPs. The distributions are normalized to the expected number of events.